Herbert Rees Bruce Catoen
Selecting Injection Molds Weighing Cost vs Productivity
Hanser Publishers, Munich • Hanser Gardner Publications, Cincinnati
IV The Authors: Herbert Rees, 248386-5 Side Road (Mono), RR#5, Orangeville, Ontario, Kanada L9W 2Z2 Bruce Catoen , 21 Hamilton Crescent, Ontario, Kanada L7G 5J4 Distributed in the USA and in Canada by Hanser Gardner Publications, Inc. 6915 Valley Avenue, Cincinnati, Ohio 45244-3029, USA Fax: (513) 527-8801 Phone: (513) 527-8977 or 1-800-950-8977 www.hansergardner.com Distributed in all other countries by Carl Hanser Verlag Postfach 86 04 20, 81631 München, Germany Fax: +49 (89) 98 48 09 www.hanser.de The use of general descriptive names, trademarks, etc., in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. Library of Congress Cataloging-in-Publication Data Rees, Herbert, 1915Selecting injection molds : weighing cost versus productivity / Herbert Rees, Bruce Catoen. p. cm. ISBN-10: 1-56990-389-1 (hardcover) ISBN-13: 978-1-56990-389-6 1. Injection molding of plastics. 2. Injection molding of plastics—Equipment and supplies. I. Catoen, Bruce. II. Title. TP1150.R446 2005 668.4‘12—dc22 2005023027 Bibliografische Information Der Deutschen Bibliothek: Die Deutsche Bibliothek verzeichnet diese Publikation in der Deutschen Nationalbibliografie; detaillierte bibliografische Daten sind im Internet über abrufbar. ISBN-10: 3-446-40308-6 ISBN-13: 978-3-446-40308-6
All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying or by any information storage and retrieval system, without permission in wirting from the publisher. © Carl Hanser Verlag, Munich 2006 Production Management: Oswald Immel Typeset by Manuela Treindl, Laaber, Germany Coverconcept: Marc Müller-Bremer, Rebranding, München, Germany Coverdesign: MCP • Susanne Kraus GbR, Holzkirchen, Germany Printed and bound by Appl, Germany
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Preface When I retired in the early 1980s, from my position as VP of R&D and Engineering at Husky Injection Molding Systems, Ltd., I had been in the plastics field since almost from the beginning of the technology, from Compression Molding of Thermosets, and then worked through the gradual shift to Injection Molding, after the Second World War. In 1985, I was asked by a Canadian non-governmental organization that supplies technical assistance to industries in developing countries to join them. At their request, I traveled to countries in East Asia, North Africa, South and Central America, and worked with a number molders and mold designers of small and medium sized operations in the plastics industry. The time spent there was very rewarding, and I was able to help them to improve their designs, methods and, ultimately, their productivity. These experiences abroad, but also many previous events throughout my career pointed out a general need for easily understood technical (theoretical and practical) education. As a result, I started putting my thoughts and experience first into a book “Understanding injection Molding technology” (1988) and followed it up by other books on Injection Mold Design and Engineering, as well as on Product Design for Injection Molding. But still missing was an easily understood book about the relationship between Productivity, Production and Mold Costs. I was fortunate that my friend Bruce Catoen, who joined Husky in 1987 as development engineer and who is, at this time, VP of Packaging and Systems at Husky accepted my invitation to co-author the book I had in mind. The purpose was, partly, that Bruce should review what I had written so far, but mainly, to update it where necessary and add the latest developments, where they are germane to the subject of this book. Injection molds are always expensive to make, but unfortunately, without a mold there cannot be a molded product. Every mold designer will have his or her own approach to the design of a new mold, and there are many different ways a mold can be designed and built. A frequently asked question is then how to get the lowest cost mold. But this is the wrong question. The question to ask must always be: “How can I get the best molded product at the lowest cost, for the expected production?” Whenever talking with molders, mold makers and mold designers I have been asked many times how to decide which features a mold should have. (Number of cavities, methods of injection, type of runners, methods of gating, methods of ejection, machine selection, etc). I have also been frequently asked how one can reasonably estimate the mold cost. As will be shown, mold cost, mold quality and cost of product are inseparable. The often-quoted saying: “The devil is in the details” applies clearly to molds, and the effect of many such details are illustrated and discussed. “Productivity and Cost of Injection Molds” is not a design manual, although there are a number of suggestions for the mold and product designers how to select certain design features to build the most suitable mold for the job. The authors highlight some of the critical decision areas for the construction and the operating details for the most economical mold for the job on hand in an easily understood language, with a minimum of theory or complicated formulae. The book tries to explain to the “decision makers”, i.e. the persons given the responsibility of deciding what kind of mold to design and build, (or to purchase, if the mold is to be built elsewhere,) how they should examine the product design and its specifications, and to highlight the significance of some of the features of the product design on the expected productivity. Such examinations often result in suggestions for practical product design changes that will make it easier to build the best-suited mold at the lowest cost. I have used some examples of molds I have been involved with, and tried to show how even little details can significantly affect the mold cost, the cost of the product, and the productivity of the mold. For the actual mold engineering process I have referred occasionally to my earlier books “Mold Engineering” (ME) and “Understanding Product Design for Injection Molding” (UPDIM.)
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Preface An event (from the early 1960s) will illustrate the importance of getting the right mold for the purpose. A friend, starting up as a custom molder with a few small machines, came one day, and told me of a prospective customer, requiring 100,000 each of three very similar, simple, round containers, who had approached him with the request to quote 3 single cavity molds, to be used on his 100 ton machines. We quoted these molds at $3,000 each, (runnerless, fully automatic,) and estimated a cycle time of 10–12 s. Based on these figures, the molder submitted a quotation to his customer who liked the price of the products, but objected to the “high” price of the molds. He said he could get these molds for $1,000 each. The molder, glad to get an order for his machines, accepted that the customer would supply the molds. When they were delivered, they were of very poor quality, with a sprue to be cut, with only token cooling, and the mold ran no better than at a 60 s cycle, or 60 pieces per hour, also, it needed an operator to cut the sprue and to scrape flash where the stripper joined the core. The molder had based his pricing on a 12 s cycle, or 300 pieces per hour. At a machine hour cost of $25.00, this would be $0.08 per piece machine hour time. However, the machine hour cost with the supplied molds would be $0.42 per piece. The mold cost per piece based on our proposal was $0.03, for the 100,000 pieces, and with the cheaper molds only $0.01, so there was little difference ($0.02) in mold costs per piece, but a huge difference in the machine hour costs. In order not to lose his shirt on this deal, the molder thereupon asked us to supply the molds, and paid for them out of his own pocket. He would have lost 0.42 – 0.08 = $0.34 per piece shipped, and his total loss would have been in the order of $100,000.00 for machine time and the unforeseen labor! The example above shows that a mold is not “just a mold”! When ordering a mold it must be clearly specified what is expected from it. The “cheap” molds would have been all right for very small requirements, but were very expensive for the expected production. Bruce and I would like to thank all those companies that contributed illustrations and photos to the book. We would also like to give special thanks to Elaine Lafontaine for her administrative assistance during the writing of this book. H. Rees
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Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V 1 Introduction and Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.2 Oversimplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Is Injection Molding the Right Choice for this Product? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.4 The Injection Molding Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.4.1 The Right Machine for the Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.5 The Injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.1 What Is an Injection Mold? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.5.2 Elements of an Injection Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2 The Plastic Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.1 The Product Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 2.2 Product Drawings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.1 Product Shape: How Can the Product Best Be Molded? . . . . . . . . . . . . . . . . . . . . . . 13 2.2.2 Parting Line (P/L) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.2.3 Side Cores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3 Accuracy and Tolerances Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.3.1 General and Specific Tolerances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.3.2 Are Special Fits with Matching Products Required? . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.3 Tolerances for the Filling Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.3.4 Stacking of Products and Free Dispensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.3.5 Mismatch (Deliberate) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.4 Tolerances, Mold Alignment, and Mold Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2.5 Heat Expansion, Alignment, and Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.6 Surface Finish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.6.1 Finish of Molding Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.6.2 Texturing of Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.6.3 Fitting Surfaces of Mold Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.7 Engravings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.7.1 Engravings Versus Applied Labels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 2.7.2 Two-Color and Two-Material Engraving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.7.3 Depth of Engravings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
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2.7.4 Font Style and Size of Artwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.7.5 Polarity of Engraving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 2.7.6 Are the Locations Selected for Engraving Practical? . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.7.7 Engravings in the Walls and Bottoms of Products . . . . . . . . . . . . . . . . . . . . . . . . . . 34 2.8 General Appearance of the Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.8.1 Flatness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.8.2 Sinks and Voids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.8.3 Witness Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 2.8.4 Weld Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.8.5 Surface Defects (Flow Marks, Splay, Record Grooves, Haze, Jetting, Hooks, and Ripples) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.8.6 Identification of the Molded Piece . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 2.9 Product Strength Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 2.9.1 Gate Location to Increase Product Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2.10 Special Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2.10.1 Holes and Counter Bores for Assembly Screws or Rivets . . . . . . . . . . . . . . . . . . . . . 47 2.10.2 Hinges and Snaps for Assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3 Cost Factors Affecting Productivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.1 Where Will the Mold Be Operated? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.1.1 Condition of Ambient (Shop) Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 3.2 Coolant Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? . . . . . . . . . . . . . . . . . . 51 3.2.2 Is the Cooling Water Clean? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 3.3 Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 3.4 Will the Mold Run in a Variety of Machines or a Single Machine? . . . . . . . . . . . . . . . . . . . 53 3.5 Is the Mold Planned to Run in a Newly Created Operation? . . . . . . . . . . . . . . . . . . . . . . . . 54 3.6 Projected Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.6.1 Making Prototype or Experimental Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 3.6.2 Production Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 3.7 Forecasting the Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 3.7.1 Type of Plastic Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.7.2 Wall Thickness of Product . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 3.7.3 Mold Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.7.4 Efficiency of Cooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.7.5 Venting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 3.7.6 Effect of Molding Machine on Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 3.7.7 Ejection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.7.8 Ambient Temperatures and Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
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3.7.9 Comparing Molding Cycles of the Same Product in New Molds . . . . . . . . . . . . . . 94 3.8 Number of Cavities Required . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 3.8.1 Available Operating Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 3.8.2 The Minimum Number of Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 3.8.3 Machine Hour Cost per Unit Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 3.8.4 Mold Cost per Unit Molded . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.8.5 Calculation of the Required Clamp Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 3.8.6 Shot Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.8.7 Plasticizing Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.8.8 Preferred (Practical) Number of Cavities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 3.8.9 Business Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 3.8.10 Preliminary Estimate of Product Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 4 Mold Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.1 Selection of an Appropriate Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.1.1 Dedicated Mold, Universal Mold Shoe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 4.1.2 “One-Product” Molds or “Family” Molds? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.1.3 Where to Gate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 4.1.4 Gate Size and Runner Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 4.1.5 Hot Runner Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 4.1.6 Single Cavity Molds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.1.7 Two and More Cavities, Cold or Hot Runner Molds . . . . . . . . . . . . . . . . . . . . . . . 135 4.1.8 Single- or Multi-Level Molds? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 4.1.9 Semi or Fully Automatic Operation? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154 4.1.10 Insert Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 4.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 5 Mold Cost, Mold Price and Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 5.1 Mold Cost and Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 5.1.1 Spare Parts for the Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 5.1.2 The Basic Elements of the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 5.1.3 Cost of the Preparation of a Mold Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 5.2 Overhead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 5.3 Mold Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 5.3.1 Risk Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 5.4 Mold Cost Is Absorbed by the Molder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 5.5 Arriving at Mold Cost and Delivery Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 5.5.1 Calculating the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 5.5.2 Estimating the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
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Contents
5.5.3 “Guesstimating” the Mold Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 5.5.4 “Ball Parking” the Mold Price . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 5.5.5 Mold Price Catalogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 5.6 The Quotation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 5.6.1 Delivery Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 5.6.2 Confirmation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 5.7 In-House Mold Making Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 6 Warranties, Patents, and Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 6.1 Warranties and Guaranties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 6.1.1 Guaranteed Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 6.1.2 Guaranteed Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 6.1.3 Guaranteed Cycle Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.1.4 Guaranteed Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.2 Patents and Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 6.2.1 Patents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.2.2 Ethical Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Appendix 1: Suggested Contents of a Mold Manual . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Appendix 2: Mold Set-up Guide Blank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Appendix 3: Example of Light-Weighting a Product and Increasing Productivity . . . . . . . . 197 Appendix 4: Buying a Mold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Appendix 5: Suggested Format of a Confirmation of Order . . . . . . . . . . . . . . . . . . . . . . . . . . 204 Appendix 6: Molding Properties of Injection-Grade Plastics . . . . . . . . . . . . . . . . . . . . . . . . . 208 Appendix 7: General Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 210 Appendix 8: Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 Appendix 9: Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Appendix 10: Typical Mold Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220 Appendix 11: What Characterizes a Good, High-production Mold? . . . . . . . . . . . . . . . . . . . . 222 Appendix 12: Advice for the Mold Designer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Appendix 13: Surface Finishes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
191
References [1] Rees, Herbert, Understanding Product Design for Injection Molding (1996) Carl Hanser Publishers, Munich, Section 4.4.3.2.5 [2] Malloy, Robert A., Plastic Part Design for Injection Molding, 1994, Carl Hanser Publishers, Munich [3] Tres, Paul A., Designing Plastic Parts for Assembly, 5th ed., 2003, Carl Hanser Publishers, Munich [4] Belofsky, Harold, Plastics: Product Design and Process Engineering, 1995, Carl Hanser Publishers, Munich [5] Rees, Herbert, Mold Engineering, 2nd ed. Chapter 8, 2002, Carl Hanser Publishers, Munich [6] Rees, Herbert, Mold Engineering, 2nd ed. Chapter 14, 2002, Carl Hanser Publishers, Munich
1
1
Introduction and Planning
1.1
Introduction
Today, injection molding is probably the most important method of processing plastics in the production of consumer and industrial goods, and is performed everywhere in the world. Once the decision has been made to use injection molding for a new product, a number of difficult choices are ahead which will be addressed later in more detail:
Number of cavities
Stack design, which is the purely technical aspect of how to mold the product. It is important to understand the design of that portion of the mold that is actually in contact with the plastic (the “stack”), i.e., the cavity, core, and any other mold components, which determine how the final product will be shaped and how the plastic will enter the cavity space
Method of ejection, i.e., how the product will be ejected from the mold
What machine should be used?
Automation – will it be required?
With new, possibly difficult shapes, these decisions are usually left to the ingenuity of a mold designer. More frequently, precedents from earlier molds are used and reapplied. However, the mold designer must be aware of and evaluate new ideas, new methods, and developments, which when applied would lead to better quality, higher productivity, simpler molds, and savings in the cost of the molded products.
Mold designers must not be stuck in a comfortable rut
After the design of the basic stack and before proceeding with any mold design, the mold designer must understand what kind of mold should be selected; in other words, which features will be most suitable for the application to achieve the most economic overall manufacturing method for the product. This means not just to specify the number of cavities that will be required for the expected output, but also the selection of mold materials and the degree of sophistication of the mold. Any planned automation, especially in product handling after molding, can affect the mold layout, particularly spacing and orientation of the stacks. The mold designer must never lose sight of the ultimate goal: The cost of the product must be the lowest possible, while still achieving all specified requirements. The most important information is to know beforehand the quantities to be molded, a piece of information, particularly with new products, often very difficult to obtain.
The ultimate goal of a mold is to produce an acceptable quality product at the lowest possible cost
It should also be pointed out that of the total cost of almost all plastic products, the cost of the plastic material alone constitutes the greatest component.
Often, the most important information is the most difficult to obtain
2
1 Introduction and Planning The most sophisticated, best designed mold will not lower the cost of the product by as much as the reduction of just a few percent of the amount of plastic material, if it could be removed from the product without affecting its quality or serviceability. Most often, unnecessarily heavy wall thickness and ribbing affects the cost more than anything else. Chances are that the lowest weight will be achieved with the highest quality molds. We must not forget in dealing with the customers who require a new mold that it is not what they want but what they need
In my long experience, I have had numerous occasions when the client insisted on having his way. When I strongly believed it was the wrong thing to do, I suggested to them to have this mold built somewhere else. Almost all came back sooner or later for other business, and acknowledged that they should have listened to me. The foremost intent of this book is to present various alternatives available to the mold designer or decision maker when planning a mold for a new product or planning to increase the productivity for a product for which a mold exists. It raises many of the questions that must be asked by anybody who needs a mold built. Some of these “questions” may appear obvious and not worth mentioning, or their pursuit may be thought a waste of time, but I like to point out that any input could significantly affect the productivity as well as the cost of a mold. For an experienced mold designer, the answers for many of these questions often come automatically, without him or her being aware of the fact that a decision has been made. But even the most experienced mold designer can gain important information by systematically investigating all areas that can affect the design and the complexity of the mold and even the most experienced designers overlook some obvious facts. In this book an attempt has been made to explain why certain mold features should be selected, considering the planned productivity and expected costs. There will also be occasionally references to other books on this subject, such as “Mold Engineering” [5] and “Understanding Product Design for Injection Molding” [1].
Figure 1.1 Typical mold-making factory using automated equipment
Since in many mold shops the mold designer is also involved in estimating the cost of the mold to be quoted, the book also intends to discuss various ways of how to estimate mold costs. Properly estimating mold cost is probably the most difficult part of running a successful mold making operation. Regardless of how well a shop is equipped with machine tools and other mold making equipment, and how high the level of experience is of the machinists and mold technicians (mechanics), if the mold cost is not adequately quoted it will be impossible to stay in business. We must never forget that the primary purpose of any business is to make money, and there is nothing easier than to lose money by poorly estimating and quoting. There is no magic formula to estimate a mold cost, but good understanding of the principles will lead to better cost estimates.
3
1.2 Oversimplification
1.2
Oversimplification a b
In the early 1950s, when I was an R&D engineer at a large electrical manufacturer, I had just submitted a request of appropriation for a mold for a new product when the vice-president of sales stormed into my office, and said: “Why do you always need so much money for a mold? What is a mold? Isn’t it just an upper and a lower half?” This was in the heydays of compression molding, before the injection molding technology gained importance dramatically. A compression mold was exactly what the VP implied: a lower half with one or more cavities, and an upper half with the matching cores (see Fig. 1.2). The plastic was hand-fed into the lower (open) cavity; there was no or little sophistication with heating (these molds were processing thermosets and therefore needed to be heated, not cooled). Often, there was no ejection mechanism at all, or it was relatively simple. Of course, what the VP failed to understand was the complexity and accuracy of the work required to build the various components of these “halves”, the strength required to resist the high molding forces, the time required for machining and good polishing, and other features required for even a “simple” mold. Unfortunately, even today, many years later, this attitude of oversimplification is frequently encountered when discussing a required mold and its cost. Since that time, thermosets (“compression”) molding has become quite sophisticated, and is using injection molding technology occasionally, but is still mostly using the vertical machine arrangement, because of the original loading method of the plastic material by gravity. This was also the time when injection molding took over the molding market from small beginnings. But for a number of reasons it soon became more convenient to use horizontal machines, although today again, some vertical injection molding machines are used for certain applications. But regardless of the type machine used, the most important part of the molding system is still the mold.
1.2.1 Definitions Before continuing, here is a list the various terms used:
Product: an injection molded plastic piece
End product: an assembly, of which the product is a part
User (end user): persons using the product or end product
Customer: the person or company interested in buying the injection mold
Mold maker: the person or company engaged in making injection molds
Mold designer: the person responsible for designing the mold
c d e f g h
a b c d e f g h
Upper platen (stationary or moving) Heating platen Upper mold half Core Cavity Lower mold half Heating platen Lower platen (stationary or moving)
Figure 1.2 Schematic of a compression mold for a plate
4
1 Introduction and Planning
Product designer: the person responsible for designing the product to be molded
Molder: the person or company engaged in injection molding plastic products
1.3
Is Injection Molding the Right Choice for this Product?
Before proceeding, we must ask: “why was injection molding selected for the job?” The molder may have a financial or other interest in preferring to have the product made by injection molding, but we must keep an open mind.
Figure 1.3 Typical injection molded parts
Have alternative methods or product designs been considered or investigated, employing other manufacturing processes using the same or a similar materials, or using other materials which may permit a similar end product, possibly even with better quality, and/or at lower cost? A few typical examples of possible manufacturing alternatives for injection molding:
Thermoforming
Foam molding
Coining and die stamping (blanking)
Extrusion blow molding
Machining, forming of sheets
Some other, maybe yet to be developed methods and materials
Another possibility is not to use plastics at all, but rather use:
The relatively high capital cost of a mold is often almost negligible when evaluated on a per-molded-part basis
Paper (cardboard), wood, cloth
Metals (steel, aluminum, etc.)
Injection molding has many advantages, particularly low mass, achievable accuracy, good strength-to-weight ratio, good appearance and surface definition, and numerous specific physical properties. But injection molded products always suffer from the fact that the initial capital outlay for molds and machines can be very high. But we must never forget that on a per unit basis, especially whenever large quantities are considered, the contribution of the cost of the equipment (mold, machine, etc.) to the cost of the product is small and often almost negligible.
5
1.4 The Injection Molding Machine
1.4
The Injection Molding Machine
We will not discuss the advantages and disadvantages of the various injection molding machines that are on the market, but rather introduce the reader who is not too familiar with this industry to the various terms that will be used from time to time if a subject under discussion will have special reference to a machine element or feature. The accuracy of molding, and especially when molding so-called thin-walled products, is very dependent on the quality of the molding machine, its mechanical rigidity, accuracy of alignment, parallelism of platens, the quality of its controls, and the state of maintenance.
The mold designer who believes that the product considered could be made better by other methods has a duty to discuss this with the customer, even if it could mean lost business, this time
Every good injection-molding machine consists of these basic elements
Even the best machine – if poorly maintained – will not perform as it should
1. A rigid base 2. A rigid clamping unit, consisting of two platens, for the mounting of the mold halves and provisions for guiding the platens (tie bars or ways) 3. Provision for moving the platens, preferably fast, relative to each other, for opening and closing the mold; the speed of motion is usually adjustable 4. Provision for clamping, i.e., holding the mold shut against the force of the injection pressures within the mold (in some machines, provisions 3 and 4 are combined) 5. Provision for ejecting the molded product(s) from the mold 6. Provision to transform the raw plastic (pellets, etc.) into an injectable melt (the plasticizing unit) 7. Provision for injecting the melt into the mold (in most machines, provisions 6 and 7 are combined in one unit) 8. Provision for heating the plastic in the plasticizing unit 9. Cycle controls (sequencing logic, timers, etc.) and a command post for manual operation and for mold setup 10. Heat controls for all heaters in machine and molds. Some machines have a limited number of heat controls and additional controls could be required for the molds, especially with larger hot runner systems. This point must be considered when estimating the mold cost. 11. Safety gates to protect operators and bystanders from all hazards when operating the machine 12. Mechanical safety drop bar(s) to prevent closing the machine when gates are open, in case of failures of the other (electric and hydraulic) safety measures. 13. Provision for cooling water distribution to the mold 14. Provision for compressed air, for auxiliary actions required in the mold
6
1 Introduction and Planning There are other features available, e.g., for the convenience of quick mold installation, setting up and operation of the mold and machine; these features are often offered as options which can be bought with the machine or added on later.
1.4.1 The Right Machine for the Mold Often, the mold cost will surpass the cost of the machine. It does not matter how ell a mold is built if the machine cannot meet the molding requirements to produce quality products. While considering the purchase of an injection mold, it is always important to make sure the machine can do the job. Some of the basic considerations are:
There is no point in buying a premium priced mold to run it in an out-dated machine
Tie bar spacing
Stroke and shut height
Injection speed (average and peak)
Available injection pressure
Recovery rate capability (throughput)
Platen rigidity (are the platens rigid/robust enough to carry the mold weight?)
Available clamp tonnage
Platen parallelism
Clamp speed requirements
Shut-off nozzles
Screw design
Accuracy and repeatability of controls
Operator access
Mold protection capability
As the machine and mold act together as a system, it is fair to say that the system will perform only as well as its weakest component. If an existing machine is to be used, the machine should match the machine's capability. The mismatched machine can easily destroy the new mold in a matter of months, resulting in costly rework. To determine the right machine, the following information on the mold is required:
Mold width, length, and height
Opening stroke required (usually 2.5 × part height)
7
1.5 The Injection Mold
Ejector rod locations
Locating ring size
Part dimensions, including wall thickness
Flow length (length of flow from gate to longest flow path)
Part weight
Runner weight (if cold runner)
Cavitation
Nozzle radius
Material (including color and additives, viscosity)
1.5
The Injection Mold
To the customer or entrepreneur not familiar with the problems of molding and mold making who wants to make a new product, the price of a mold may seem to be high, occasionally even outrageous; it is often difficult to convey that the mold price constitutes only a very small portion of the product (piece) cost, and depends much on the expected production of the mold.
1.5.1 What Is an Injection Mold? A (plastics) injection mold is a permanent tool, i.e., a tool that, if properly designed, constructed, and maintained will have a life expectancy (useful life) well beyond the time where the product itself becomes obsolete. This differentiates it from a “one-time use” mold such as a sand-casting mold, as used in metal foundries. A mold can be used to make products in a virtually infinite variety of shapes, made from injectable plastics. Common to all molds is the condition that it must be possible to remove the product after molding, without the need to destroy the mold (as is the case in sand-castings). There is an exception to this, the so-called “lost-core molding”: There are injection molds for intricate products, such as intake manifolds for internal combustion engines, previously made from cast iron, which have an outside shape that can be molded with conventional (permanent, “open and close”) molds but where the intricate inside shape is made from a molded, low melting point metal composite which is inserted into the mold before injection, and then ejected together with the molded product; the metal is then removed by heat at a temperature above the melting point of the insert, but of course below the melting point of the plastic used for this product; the molded metal insert is thereby destroyed, but the metal will be reused. A basic mold consists of two mold halves, with at least one cavity in one mold half, and a matching core in the other mold half. These two halves
It is important to understand that it is not the mold cost but the piece (unit) cost of the product, which is important
8
1 Introduction and Planning meet at a parting plane (parting line). As the mold opens – after the injected plastic (now in the shape of the desired product) is sufficiently cooled and rigid – the product can be removed by hand or be automatically ejected. Because injection-molding machines are mostly built with the injection on the stationary platen side, there is, typically, no built-in ejection mechanism on this side. If ejection from the injection side should be required – always the case in stack molds, and occasionally required in single level molds – any required mechanism must be added to the mold, and occasionally to the machine; in either case, this adds complexity and increases costs. Only molds designed for using only air ejection do not require any external ejection mechanism. Most products are removed (ejected) from the core. There are also many molds, which need special provisions to allow the products to be removed from either the cavity or the core. This is the case with products having severe undercuts or recesses on the inside and/or the outside of the product, such as screw threads, holes, ribs or openings in the sides of the product, etc., or molds for insert molding. Almost any shape can be molded – but at what cost?
Some of these design features of the product may require moving side cores, which are either inserts or whole sections of the cavity that move at an angle which is 90° to the “natural opening path” of the mold. Others may require special unscrewing mechanisms, either in the core or in the cavity side. The mold may require split cavities (or “splits”), i.e., the cavity consists of two or more sections, which are mechanically or hydraulically moved in and out of position, and then clamped together during injection. In some cases, the mold may require collapsible cores, or retractable inserts, which are all quite complicated (and expensive) methods. Any of the above special features can add considerably to the mold cost when compared to a simple “up and down” mold where the products can be readily ejected with the machine ejectors during the mold opening stroke or after the mold is open, without the need for any of these complicated mold features. Note that in this book, the term (simple) “up and down” molding is used, which comes from the earlier vertical molding machines, even though, today, most general-purpose injection molding machines are horizontal and the mold opens and closes in a horizontal motion.
Example 1.1 To illustrate how different mold features affect the mold cost, we assume that a single face mold with air ejection of the products costs X dollars. A similar mold, but with mechanical ejection, costs about 1.2 times X. A similar, air- ejected 2-level stack mold will be about 1.8 times X. An unscrewing mold for a similar size mold and product will cost about 2 times X.
9
1.5 The Injection Mold
1.5.2 Elements of an Injection Mold Most readers will expect to see some illustrations (photos or schematics) of injection molds at this point. However, we must not forget that this is not a book about mold design, but about the relationship between productivity and cost of molds, as well as the cost of the products to be made. There will be, however, a number of photos of molds accompanying the text where deemed useful. There are books that show designs of numerous, specific molds but it is virtually impossible to show every possible configuration that may be required. It is more important for the designer, and any person requesting a new mold, to understand that a mold consists essentially of a number of elements from which to choose for the most appropriate design for the purpose. Every injection mold consists of the following basic elements: 1. One or more matching cavities and cores, defining the cavity space(s) (today, there are molds with anywhere between 1 and 144 cavities). 2. A method, or element, to duct the (hot) plastic from the machine nozzle to the cavity spaces: There is a choice between – Cold runners (2-plate or 3-plate systems) – Hot runners (various systems) – Insulated runners, through shooting – Sprue gating (cold or hot) 3. Provision to evacuate air from the mold (venting): There is a choice between – Natural venting – Vacuum venting 4. Provision to cool the injected hot plastic sufficiently to allow ejection of the molded product 5. Provision to eject the molded product: There is a choice between – Manual product removal – Ejector pins and sleeves – Stripper s (stripper rings or bars) – Air ejection – Random ejection – Various methods of in-mold product removal methods – Robotic product removal 6. Provision to attach (interface) the mold to the molding machine: There are several methods to consider – Mold is for one machine only
Product quality, productivity, and mold cost depend heavily on the proper selection of the runner system
10
1 Introduction and Planning – Mold to be used on several, different machines – Quick mold change methods (various designs) 7. Method of alignments of cavities and cores: There are several methods to consider – No alignment feature provided in the mold – Leader pins and bushings (2, 3, or 4) – Leader pins and bushings between individual cavities and cores – Taper fits between individual cavities and cores – Taper fits between plates – Any combination of the above Figure 1.4 Mold maintenance in the press is important. Here, the operator is changing a nozzle tip while the mold is in the press (Courtesy: Husky)
8. Any number of (mold) plates to provide the necessary for carrying and backing the above elements But molds could have additional features, which will also be discussed in the following. Each of these features can add (often considerable) costs to the mold but in many cases can increase the productivity of the mold and reduce the cost of the product. They may or may not all be necessary and must be carefully considered when deciding on the type of mold most suitable (and most economical) for the job on hand.
Easy serviceability of the mold is important but often overlooked. It adds some mold cost, but saves much more in future servicing costs and downtime
Features such as serviceability of the mold may affect the mold cost; for example, the access to hot runners for cleaning plugged gates or making minor repairs, such as changing a nozzle, a burned-out heater, or a faulty thermocouple at a hot runner drop will cost more in the initial mold, but this will be easily recouped by reducing the down time necessary to accomplish such repairs. By designing easy access to these components in the machine (without the need to remove the whole mold, or part of it, to the bench), such repairs can be made in less than an hour, instead of taking several hours. This work can also be done by the mold setup staff rather than getting the (expensive) mold makers involved. Another area where valuable maintenance time can be saved is to design and provide easy access from the parting line to screws holding modular mold parts to their mounting plates, while the mold is in the machine. On the other hand, in my experience, many molds, particularly molds for lower production quantities, have been vastly over-designed and much money has been wasted.
Even minor changes to the part can dramatically lower or increase mold costs
The main purpose of this book is to discuss the various elements or features listed above and to facilitate the selection and the decision making. Defining what is really required considering the shape and complexity of the product and the required production quantities will enhance mold productivity. In addition, the book should facilitate investigating if, even minor, changes to the product shape could lower the mold cost and improve the productivity of the mold or the whole system.
11
2
The Plastic Product
Plastics have evolved to be a very useful material. Today, plastics are used in almost every area, from small bottle caps, disposable cutlery, and packages for dairy products, to large containers, such as laundry baskets and garbage pails. Plastics have transitioned from a “cheap” substitute for metal and glass to the material of choice providing almost unlimited design freedom, unique properties, and significant cost savings. Figure 2.1 shows various industrial containers and house wares that create durable products in cycles from 10–30 seconds.
Figure 2.1 Molded products of various sizes (Courtesy: Husky)
Figure 2.2 shows various thin-walled containers are typically used in the dairy industry and are molded with wall sections typically less than 0.7 mm with cycles of 20 shots per minute. Figure 2.3 shows a collection of PET bottles for water, soft drinks, etc. and some of the preforms used for blowing these bottles. Today, more than 500,000 tonnes annually of plastic are converted into bottles. Cycle times for molding these parts have been reduced from 35 to 8 s in the last 20 years. In addition, cavitations have increased from 8 to 144 cavities, resulting in significantly lower product costs.
Figure 2.2 Various thin-walled containers (Courtesy: Husky)
Figure 2.4 shows a sampling of “stadium cups” with printed or in-mold labelled decorations. Figure 2.6 shows samples of small, thin-walled technical products made from engineering plastics such as ABS, Acrylic, and PC.
Figure 2.3 PET bottles for water, soft drinks, etc. and some of their preforms (Courtesy: Husky)
Figure 2.5 Small and large technical (engineering) products, heavy-walled jars for cosmetics, and tubular containers with integral, hinged lids (Courtesy: Husky)
Figure 2.4 Stadium cups
12
2 The Plastic Product
Figure 2.6 Small, thin-walled technical products made from engineering plastics
2.1
The Product Design
The following contains suggestions for the product design and how it may impact the mold design and the productivity of the mold A new mold is usually required
For a new product
After the redesign of an existing product
To increase the productivity and the output of the production facilities already in place. This usually provides a good opportunity to reevaluate and improve the product, and to reduce manufacturing costs, particularly through the reduction of the plastic mass of the product.
The mass of the plastic accounts for a significant portion of the cost of every product. Reducing wall thickness and reduction of unnecessarily heavy cross sections will not only reduce the cost of plastic material for the product, but will also result in – sometimes significantly – faster molding cycles. The result is that more of the products can be made per hour at lower cost than was possible with the preceding design. In such a case, important considerations are
The output of the plasticizing unit and the dry cycle of the machine manufacturing the product before the planned changes
If there was special handling equipment (product removal, stacking, printing, etc.) with the old mold, will it be able to handle the greater output, or will it need improvements as well
The above will be discussed in more detail later in this book.
13
2.2 Product Drawings
Product Drawings
This is also the moment when the designer has the greatest opportunity to decide on the most suitable design for the mold, and/or to make suggestions on how the product design might be modified to improve the productivity, to simplify the mold design, and to reduce mold costs. This is also the time to consider any ancillary equipment required for this production. An opportunity graph (Fig. 2.7) shows symbolically the value of planning a project. At the outset of the project, the opportunity to make improvements, revisions, and selections is highest to affect the final outcome of the project, while the costs are lowest. After concept analysis, once the elements of the project have been agreed upon and as engineering of the mold progresses, the opportunity to make conceptual changes or improvements diminishes, and any costs associated with it will increase. By the time the project reaches completion and gets into testing and production, the opportunity to make changes is low, and any costs could be very high. To confirm that the part drawing is acceptable to all parties it should always be signed off in writing as acceptable. Appendix 12 provides some general advice for the designer on how to critique a part drawing.
It is critical that complete product drawings are available for the mold designer before any mold design is started
Opportunity
Costs
Occasionally, only samples or CAD models of a new product are available. This may be of some advantage to better visualize the product, but it is absolutely necessary, to minimize risk for all parties involved in the final decision, to have a complete detail drawing of the product, showing all features, tolerances, and specifications.
Opportunity
2.2
Costs Time Period of evaluation of product, opportunity for changes is high, changes are easy to obtain, and low in cost. During engineering, opportunity for revisions is still fairly high. Changes are still relatively inexpensive During manufactoring, there is little opportunity to make revisions. Changes can be quite costly. Mold tests and production:
Figure 2.7 Opportunity graph
2.2.1 Product Shape: How Can the Product Best Be Molded? Here, even an experienced, conscientious designer may want to consult with another (knowledgeable) colleague, and/or with anyone else who is familiar with the type of product for which the mold is to be built, and discuss problems of making and of operating such a mold, to get their input regarding the proposed product design. In the following, some of the most important areas to be contemplated are discussed.
2.2.2 Parting Line (P/L) Is There an Obvious Location for the (Main) Parting Line? In many products, the location of the parting plane (parting line, P/L) is obvious. It is along the largest cross-sectional dimension of the product, at right angles to the motion of the opening and closing of the mold, and should preferably be in one plane. This is the least expensive, and fortunately, the most frequent case. However, there are many cases where the P/L cannot be
The old proverb “a stitch in time saves nine” applies here too: Spend more time at the beginning of the project, to save much time later on
14
2 The Plastic Product located there, and requires special consideration. A few examples are listed below:
Figure 2.8 Examples of straight, simple parting lines (top: at the opening; bottom: at the largest diameter)
Simple parting lines (Fig. 2.8)
Sometimes, the P/L must be offset because of the shape of the product (Fig. 2.9).
It may be of advantage to place the P/L at a level, which is not at the largest cross section, to force the product to stay on the side from where it will be ejected, as can be the case with flat products. This would not affect the mold cost; however, flat products often cause trouble at ejection, because they do not always stay reliably with the side from where they are ejected. Additional mold features, such as sucker pins, or grooving in the side of the product (“pull rings”) may be required to hold the product on the ejection side to make sure that the mold can operate automatically, without interruptions (Fig. 2.10).
The P/L is curved. This is sometimes unavoidable because the product shape will not permit a straight P/L; for example in some toys, but occasionally also in technical products. A typical example is the P/L for plastic forks or spoons. In all these cases, the matching of the P/L is difficult and expensive. It may need special, costly grinding equipment or expensive fitting by hand (“bluing”) (Fig. 2.11).
Figure 2.10 Typical flat piece with undercut below parting line
Figure 2.9 Example of simple mug handle, using offset P/L
Figure 2.11 Typical mold profile for cutlery
15
2.2 Product Drawings
2.2.3 Side Cores Is There a Need for Side Cores, Splits, or for Other Methods to Release Severe Undercuts or Threads? Any of these features will add considerable cost to the mold (and to the cost of the product), not only because of the added complexity of the stack but also because each stack requires much more space than a simple stack without side cores. For the same number of cavities, a much larger mold and therefore often also a larger machine size may be required just to accommodate the mold in the available platen area, even though the clamping forces required would be little more than for the mold without side cores or splits. Such side cores, splits, etc will lengthen the cycle time and reduce productivity compared to molds that do not have such features.
Figure 2.12 4-cavity handle mold with 3 side actions per cavity (Courtesy: Topgrade Molds)
Could a Redesign of the Product Avoid the Need for Side Cores? In some cases, round holes or “odd shape” openings generated by using side cores or split cavities could be redesigned without sacrificing the usefulness of the product, and possibly without significantly changing the appearance, by creating such holes or openings in the side walls (or even in ribs inside the product) with a design method where core and cavity meet on a “shutoff ”. This may require the use of special inserts in either or both of cavity and core, which may necessitate a change in the shape (or in the draft angle) of the side wall of the product, or require an opening in the bottom of it. In many cases, this could be acceptable for the end use of the product and allow a much simpler, less costly mold [1]. By just giving a bit more thought to the product design before planning and designing a mold, and by understanding the application for which the product is used, a little redesign can often result in spectacular savings in mold and product costs. Selecting Other than the Conventional Parting Line Occasionally, the choice of the obvious placing of the parting line would require a side core, while by slanting the P/L, the product could be molded with a simple up-and-down mold. An example is a simple louver (Fig. 2.13), but the principle applies to any similar case. The cost of a mold with a “slanted” P/L is somewhat higher than that of a mold with an ordinary P/L, but much lower than a mold with a side core.
Figure 2.13 Example of louver; top: needs side core; bottom: tilted – it becomes an “up-and down” mold
Investigate Shape of Threads and Undercuts Often, a design specifies threads or undercuts, on the inside of the product (Fig. 2.14). Is the specified shape of thread or undercut designed with molding in mind? Many such threads or undercuts could be molded without unscrewing, or the need for collapsible cores, by changing the shape of the undercut so that the product can be stripped off the core, i.e., the undercuts can easily slip out of the grooves that created them when pushed by ejectors or a stripper.
Figure 2.14 Typical bottle cap with tamper-proof ring and stripped thread for simpler ejection (no unscrewing mold required). This product is outside-gated, using a hot runner hot tip gate
16
2 The Plastic Product
Types of closures
Stationary ratchet ring
Unscrewed thread Stripper ring
Rachets
Top of thread almost flat, less than 15°. If stripped will be greatly deformed.
Core Rotating core
Figure 2.15 Schematic of difficulties of a typical unscrewing mold.
Figure 2.16 Mold where thread can be stripped
Stripped thread Angle on top of thread allows thread to be stripped off the core.
Figure 2.17 Change in flank angle allows thread to be stripped
Figure 2.15 shows the difficulties of a typical unscrewing mold. The core must rotate out of the cap before it can be ejected. This makes core cooling more difficult and results in 30% longer cycle times than a stationary core. Unscrewing molds are much more complicated than “bump-off ” (stripped) closure molds. Figure 2.16 shows a schematic of a much simpler mold, where the thread (and the cap) can be stripped. Here, core cooling can be very efficient. The cycle time for a typical (28 mm) bottle cap made from HDPE MFI 19, weighing less than 3 g, molded in a 24-cavity mold running in a 90 t (1,000 kN) machine is in the order of 4.0 s, equaling a productivity of 21,600 caps per hour. Figure 2.18 72-cavity unscrewing mold (Courtesy: Stackteck)
Figure 2.17 exemplifies of how a small change in the angle of the flank of the thread can allow a thread to be stripped from the core, rather than requiring an unscrewing mold. Small changes like this can have a major impact on product cost because mold cycle, cost, and maintenance will be significantly improved with a stripped product. Need for Two-Stage Ejection or Moving Cavity This applies to a shape or design feature of a product consisting of
Figure 2.19 Stripped closure mold
Deep ribs on the cavity side, as is often the case with containers with “false” bottoms. Such ribs could also be specified on technical enclosures, etc., as illustrated in Fig. 2.20. The depth of the rib F and the ratio of the thickness of the rib t, as well as the draft angles of the rib are critical considerations, or
Deep ribs (often circular) on the core side; typically, the underside of an over-cap, as illustrated in Fig. 2.21 (even without the thickening at the end of the rib as shown in this illustration).
In both cases, if the ratio of F/t > 2, or if there is any thickening at the end of the rib (as in Fig. 2.21), either a “two-stage ejection” or a “moving cavity” are necessary, which will increase the mold cost by about 15–20%. In both cases, it is important to provide especially good venting at the end of the ribs to ensure proper filling. Failure to use these methods will make it very difficult
17
2.2 Product Drawings
Figure 2.20 Schematic of a moving cavity in two halves; top: mold is closed; bottom: mold opens and follows core for a limited distance to ensure that the rib becomes free
Figure 2.21 A product with deep ribs and (with or without) thickening at the end is ejected in two stages; 1: Sleeve and stripper lift product off the core; 2: Stripper continues to push product off the sleeve
to withdraw (eject) the products, and increases the risk of breaking portions of the rib in the mold. A 2-stage mold will cost about 15–20% more than a comparable mold without this feature. Also, because the sleeve is usually rather thin, it is very difficult to get cooling into it; the mold will cycle much slower than a similar product without this complication, and the maintenance cost of such molds is much higher. Moving cavities are more complicated and cost about 10% more than a mold without this feature. Some molders use it despite its higher cost for products even without a false bottom, because they can cycle even faster than a mold with a conventional cavity. Post-Molding Operations Sometimes, molds can be much simplified by doing additional work to the product after molding. Post-molding operations are of particular importance whenever relatively small quantities are to be made. For example, one or a few simple holes or openings in the side wall of a product would require a side core in the mold, but such holes or openings could also be drilled or die-stamped after molding. Such additional operations may require a drilling fixture or a stamping die. The actual time (direct labor) for such post-molding operations and any costs for tools or fixtures would have to be added to the
Always keep in mind: It is possible to mold almost any shape, but at what cost?
18
2 The Plastic Product total cost of the product. But such post-molding operations could also take place later at the assembly line, where the product is assembled or packed, without any additional labor cost if properly integrated in the process. Again, it is the overall cost of the end product that is important, not just the cost of the mold or the molded piece itself. In many cases, the savings in the mold cost achieved by eliminating a side core (or some other complications of the mold) can be substantially greater than the combined additional cost for fixtures or tools, plus the cost of the additional direct labor to finish the product.
Figure 2.22 Lugs with holes
A typical example for this would be the need for small holes for a hinge pin (for a hinged lid), located in two lugs projecting from the bottom of a product (see Fig. 2.22). The plastic melt is injected into the bottom of the product, near the lugs. It is of course feasible to mold these holes, but it could be quite difficult to arrange the side cores required for such holes as well as the actuation for such side cores, without interfering with the gating and the cooling layout in this area. It would be, however, quite easy to just mold the lugs as projections from the container bottom, and then drill the holes, using a simple drilling fixture.
2.3
Accuracy and Tolerances Required
Next, the mold designer should look at the specifications relating to accuracy and tolerances. How is the product to be used? What is really required?
Unfortunately, often, after a product has been conceived, the design has been either just sketched by the inventor or an artist, or a model has been created. This information has then been passed on to a draftsman to be put “on paper” (by computer or pencil drawing). This may result in a good visual description of the new product, but to be practical for manufacturing, any drawing must be fully dimensioned, and intelligently toleranced. To design a product for injection molding requires certain knowledge of this technology. A design which may be suitable for one method of processing plastics (or other materials) may be unsuitable or impractical for another process, even though the end use is the same. For example, a disposable drinking cup of a specified capacity could be made from paper, styrofoam, be thermoformed from sheets, be injection molded, or made by another, entirely different, new method or material. The final product design for each of the above cited materials and methods would most likely look different to suit the method of manufacturing and the selected material. Also, while the dimensional accuracy of the product for its final use (i.e., as a drinking cup) may be of little importance, its actual dimensions will require high accuracy because of demands not related to its use, such as stacking height (e.g., for packaging), ease of releasing of the individual cups from the stack as required in automatic vending machines, and mainly because even
19
2.3 Accuracy and Tolerances Required small variations in wall thickness may have a great effect on the mass of plastic used for each unit and on the molding cycle. A design for a metal product is different from the design for a similar product made by injection molding, even though the products may be fully interchangeable in their use. This applies especially for design features such as
Radii and sharp corners,
Flow path for injection (if applicable),
Wall thickness,
Ribbing and reinforcements,
Openings (round or shaped),
Others.
These features, by their presence or absence, not only affect the making of the mold (and its cost) but also affect the speed of the molding operation itself. I refer the reader to the many books on product design for injection molding, which go into much detail on this subject [2, 3, 4]. It is very important to understand that it is relatively easy to achieve close tolerances for the mold parts usually made from metal; however, the plastic products made by the mold do not solely depend on the mold dimensions. The designer must be aware that the final size of the product is greatly affected by variations in the shrinkage of the plastic (see Appendix), which in turn is caused by variations in molding conditions (pressures, temperatures, and timing) and by variations in the composition of the plastic not only from batch to batch, but also from manufacturer to manufacturer. All this makes it very difficult to mold products dimensioned within close tolerances. But even the above statement “relatively easy to produce the mold parts to close tolerances” must be qualified. Using unsuitable, old, and/or poorly maintained machine tools makes it more difficult to make mold components to close tolerances; the accuracy of the work depends much on the skill of the machinists, and even with good checking equipment can become time consuming, because it requires frequent measuring of the closely toleranced dimensions. The alternative is to use good machine tools, or even machines specially designed or adapted for certain steps in the manufacture of the mold parts, requiring much higher investments by the mold maker. Either one of these conditions affect the cost of machining and explain why close tolerances can be expensive too achieve. Note also that dimensions are affected by the ambient temperature of the machine shop and that even when cooled by cutting fluids, the work pieces heat up during machining; they will measure larger when warm immediately after cutting than after cooling to room temperature. Of course, the larger the dimension, the larger the dimensional differences caused by heat expansion.
Many millions of dollars are squandered annually because of demands for unnecessary tight tolerances
Next Page
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2 The Plastic Product As can be seen in Fig. 2.23, the mold cost increases exponentially with the tightness of the tolerance. Without giving actual cost figures, the curve just shows how costs can increase, as the tolerances get tighter. The cost to achieve a 0.005 mm (0.0002″) tolerance can be 3 times the cost of a 0.03 mm (0.0012″) tolerance.
Figure 2.23 Relationship between tolerances and mold cost
Other points that should be clarified when looking at product dimensions with close tolerances: how will these dimensions (or the entire product) be checked (measured) on the finished product? With Vernier, micrometer, gages, measuring machines, fits with other products? Also, when will they be checked? Immediately after ejection, one hour later, 24 hours later? Will there be 100% inspection or statistical (random) inspection? To clarify all this ahead of time can avoid much future unpleasantness or arguments.
2.3.1 General and Specific Tolerances Always remember that tighter tolerances mean higher mold costs, maintenance, and inspection
All tolerances must be specified on the product drawing and must be looked at by the mold estimator or designer when starting the project to see if they are reasonable. The Society of Plastics Industry (SPI) has a suggested list of practical general tolerances for injection-molded products. For more information, go to the SPI website www.socplas.org. In most cases, these tolerances are satisfactory and achievable. Specific, closer tolerances may require that experiments be made with cavity and core sizes, and under various molding conditions, to achieve the required sizes. This can mean considerable added costs for the mold maker and a higher mold cost. The following tolerances are suggested to be used on plastic product drawings (radii are not toleranced): Product weight:
± 10% on projected weight (range ± 2%)
Wall thickness:
± 0.03 mm (in special cases 0.013 mm)
Fit diameter:
up to 75 mm ∅ up to 106 mm ∅ up to 160 mm ∅ up to 300 mm ∅
Overall height:
± 0.5% or 0.13 mm minimum
Stack height:
± 0.5% or 0.13 mm minimum
→ ± 0.20 mm → ± 0.25 mm → ± 0.30 mm → ± 0.64 mm
Note that the steel size requirements, and thus the difficulty of manufacture, are dependent on the plastic tolerances on the product drawing.
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2.3 Accuracy and Tolerances Required
2.3.2 Are Special Fits with Matching Products Required? Often, certain dimensions of a product are specified with unnecessary close tolerances, when all the designer wanted to convey is that the product should fit suitably on another product (tightly or loosely), typically, a container and a matching lid. This requirement must be clear. Especially, when molding plastics with high shrinkage factors (e.g., PP or PE), it can be difficult to arrive at the proper “steel” dimensions, and some experimenting may be required to achieve the required fit. Specifying the matching diameters with standard, loose tolerances may yield pieces correct in size, but wrong because the fit is not as desired. The alternative – providing closer tolerances – could be unreasonable, because the dimension of the molded product depend not solely on the steel dimensions of the stack parts but also on the molding parameters. In such cases, it is of advantage to complete the more complicated mold first and test it in actual molding conditions until the best cycle time is established. The critical mold parts of the matching product (e.g., the lid) should be finish-machined only after having established what the actual molded container dimensions are. This could require completing the lid mold with only one cavity, using assumed suitable dimensions, testing the unfinished mold until the best cycle is achieved, and then adjusting the assumed dimensions so that the proper fit can be achieved. All lid mold parts can then be finished. For more information on this subject see [5].
2.3.3 Tolerances for the Filling Volume This applies specifically – but is not restricted – to containers into which a more or less viscous product will be filled by volume to within closely specified limits (typically, containers for margarine, paint, etc.). In their end use, it is important for the seller that a minimum amount must be filled into the package without shortchanging the buyer, but also they should not be overfilled, which would mean a loss for the seller. There should be clearly defined fill lines (usually inside the container) to mark the minimum and maximum volumes. This can be a problem with plastics with large shrinkage factors such as PE and PP. It requires special consideration when dimensioning the cavity and core because of the unavoidable variations in shrinkage values, as the plastic flows away from the gate and slowly cools and as the injection pressure within the mold decreases. The same considerations apply to measuring cups or vials which have the various levels (or volumes) indicated by lines on the sides of the product. It may be necessary to first test the mold to find the best cycle times, and then establish the location of the measuring lines.
Prototyping is often used to verify the required dimensions or fits of a part after shrinkage
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2 The Plastic Product
2.3.4 Stacking of Products and Free Dispensing Any product stacked for shipping must have a clearly defined stacking height, which is usually created by resting the outside or the bottom of one piece on the inside stacking provision of the following piece. These provisions for stacking can be “stacking lugs”, or clearly defined steps in the product. The purpose of these lugs (or steps) is
The products must not jam when pushed together, which would make it difficult to separate them where required by the user, and
They will ensure a total stack height of a certain, specified number (e.g., 20, 25, 40, etc.) of the products when stacked. The stack height should be suitable for the size of boxes or containers (preferably, standard size cartons) in which stacks will be shipped.
If special cartons are to be provided, it may be necessary to investigate if their size will suitable for standard rail or sea shipping containers, for best use of the available space inside these containers.
Figure 2.24 View of stacked lids
Stacking is more difficult if the angle of the sidewalls is small. Obviously, a cylindrical container (0° draft) cannot be stacked at all. A typical disposable drinking cup has approx. a 7° angle. Larger angles stack easily. Figure 2.25 View of stacked products
Figure 2.26 View of products stacked on lugs
Problems can also arise when parts are used in an assembly line or in a dispensing mechanism (e.g., vending machine) where it is important that the parts will release easily, without fail, from the stack, i.e., not being “hung up” by vacuum or by friction because the gap between two stacked containers is too small, even though they are properly stacked as designed. When the gap between two sidewalls is very close, static electric charges may also prevent the lowest part from falling from the stack when desired. Some dispensers have mechanical separators and don’t depend on gravity, but it is preferable not to depend on having such separators (added costs). It is highly recommended to make sure that any stacking height dimensions are carefully checked before beginning to build a mold. If they are wrong, the mold has to be changed after finishing, or the packaging (carton size) has to be redesigned after the height of the stack was not as originally planned. Occasionally, a mold maker may decide to make slots in the mold cavity for the stacking lugs by EDM into the core only after the mold is finished, rather than do it before and then have to increase the height later. The disadvantage of this method is that the mold has to be dismantled to be able to machine the cores (costs!). The advantage is that a minimum stacking height can be achieved. Also refer to Appendix 12 for more advice for mold designers.
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2.3 Accuracy and Tolerances Required
2.3.5 Mismatch (Deliberate) Trying to produce a “perfect match” between two surfaces is not only difficult to achieve but also very costly. Designers often create deliberate mismatches for ease of manufacturing. There are two areas of “deliberate mismatch” to consider, and two typical examples are shown. There are many variations of matching parting lines, or between lids and covers, but the basic principle applies to all of them. Mismatch at the Parting Line, Between Cavity and Core First, it must be clarified, whether a rounded edge is really necessary for the product. In many cases, the product designer may not be aware of the possible additional cost involved to produce a round edge as in Fig. 2.27, (a) or (c), and will often agree that a simple, “sharp” edge (b) or (d) would be just as acceptable for the application. Figure 2.28 shows just one of several designs of a round edge, with the ideal case (1) having a perfect match at the parting line. However, due to the buildup of manufacturing tolerances of the mold parts, such ideal case is not practical. In reality, the nominal diameter D of the cavity, or of the core, will be either larger or smaller than the matching one, and create either a hook (2), which is generally not tolerable, or small step (3), which in most cases is perfectly acceptable. Note that the actual differences caused by the tolerances of the diameters are small, usually less than 0.1 mm (0.004 in.), so that a step would not be more than about half this amount. However, a step is much less noticeable than a hook. In fact, a mismatch can be corrected by very time consuming handwork, by grinding or stoning (polishing), but this should be avoided because of the high cost. The suggested proper (and most economical) approach is to dimension the matching diameters so that there is always a step, as shown in Fig. 2.28, item (3), of a magnitude between 0 and 0.1 mm (0–0.004 in.). Mismatch Between Two Matching Pieces, such as Box and Lid The conditions are similar when designing and building molds for “matching” boxes and lids. Here, deliberate mismatch (2) is even more important, because the products may come from different cavities and even molds, made under varying molding conditions, and the mismatch due to build up of many tolerances (in cavities for both products) could be much larger. Figure 2.29 shows the ideal condition (1), which is difficult to achieve, and a way to minimize the effect of a mismatch between matching parts (2). There is also another way shown by adding a “decorative” band to the larger part (3).
Always consider: 1. Is the rounded edge really necessary? 2. Is the sharp edge really necessary?
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2 The Plastic Product
Figure 2.27 Typical round edges where a “sharp” edge could be considered
Figure 2.28 Round edge: ideal (1), with “hook” (2), and with “step” (3)
Figure 2.29 Mismatch avoidance between box and lid
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2.4 Tolerances, Mold Alignment, and Mold Costs
2.4
Tolerances, Mold Alignment, and Mold Costs
The relationship between: (1) product tolerances, (2) machining tolerances of mold parts, (3) resulting requirements for alignment of the mold halves (cavities and cores), and (4) the mold cost could be the subject of another book. Here, we will try to condense the subject, by outlining some major points when making the decision of which method of alignment to select. The main reason for any alignment method between cavity and core is to keep the centerlines of cavities and cores in line. Any deviation from the actual centerlines of cavity and core from the “true” centerline will result in thickness variations of the sidewalls of the product. This is true for any cupshaped product. With flat products, including lids, usually we do not have this concern; in such cases, alignment of the mold halves using only the machine tie bars could be sufficient, even without leader pins. But don’t forget: leader pins (even if not used for alignment) on the core side are also meant to protect the (projecting) cores from damage. They should always be higher than the cores.
Figure 2.30 1+1 cavity mold requires only leader pin alignment to keep mismatch to an acceptable level (Courtesy: Stackteck)
There are, basically, four methods of alignment used: 1. Use only the machine tie bars to align cavity and core. This can be done in some cases where the alignment between cavity and core is not very important; it can be used for experimental and prototype molds, or even for limited-production molds. This case will not further be discussed here. 2. Alignment of the mold plates with leader pins. This is the oldest and most common method used, for any size of mold, and for any number of cavities. This is the lowest cost method of alignment. 3. Alignment with taper pins between mold plates, and occasionally between cavities and cores, and taper locks between the individual sets of mold stacks, whether in single- or multi-cavity molds. This method usually also requires at least two or more “loosely” fitting leader pins (with or without bushings), not for real alignment purposes, but to protect the core(s) from damage and to facilitate handling of the mold outside of the machine. This method is more expensive than leader pin alignment. With taper locks, we also have to chose between
Figure 2.31 This lid mold has leader pins and round taper lock alignment, while the modules have no alignment mechanism. This works well for shallow parts (Courtesy: Husky)
(a) round tapers (less expensive), or (b) wedges (adjustable) 4. There are also combinations of these two methods of alignment, such as where the mold plates are “loosely” aligned with usually 2 (sometimes 3, rarely 4) leader pins, but the final alignment is achieved with tapers between each cavity and core stack, in single- or in multi-cavity molds.
Figure 2.32 Lid stack module with flat (no) alignment on the stack. Mold alignment is typically accomplished with round taper locks on plates
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2 The Plastic Product
B
C
A F
Figure 2.33 Modular mold for container (round taper interlock) (Courtesy: Husky)
A D E
Figure 2.34 8-cavity mold for rectangular containers (Photo: Courtesy Dollins Tool Corp., USA)
Figure 2.33 shows an example of a modular mold for a container with circular alignment tapers (A). Note that the cavity (B) is set into the cavity retainer plate (not shown), while the core (C) is mounted on top of the core backing plate (not shown) to ensure proper alignment. Note the absence of a stripper ring: this product is air-ejected from the core, making for a much simpler mold. For best cooling efficiency, there is a beryllium-copper alloy (BeCu) core cap (D), and a BeCu gate insert (E) in the cavity bottom plate (F). Note the intricate venting channels in both cavity and core to ensure fast filling of the cavities. This type of stack usually produces at 6.0 s or less. Figure 2.34 shows an 8-cavity modular mold for rectangular containers. The cavities (A) are set into the hot runner plate (B), the cores (C) are mounted with float on the core backing plate (D). Each core is aligned with its cavity with wedges (E); the containers are ejected by air.
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2.4 Tolerances, Mold Alignment, and Mold Costs
D
E
C B
A
Figure 2.35 Modular single-cavity mold for large thin-walled container (square lock alignment) (Courtesy: Husky)
Figure 2.35 shows a modular single-cavity mold for a very large, thin-wall container, with wedge-lock alignment. This mold too operates with air ejection only, and is simple in construction. For the most effective cooling, there is a BeCu core cap (A) and a BeCu gate insert (B). The jaws (C) for the wedge lock are easily adjustable. The two leader pins (D) are basically only for mold handling and for protection of the cores and fit only loosely in the leader pin bushings (E). If the sidewall tolerances are large, which is often the case with heavy-walled products, a possible misalignment between cavity and core is usually insignificant, and alignment with leader pins is perfectly viable. The average clearance between leader pins and leader pin bushings (standard hardware) is about 0.04 mm (0.001 in.). If, e.g., a wall is 1.5 mm thick (0.060 in.) and the tolerance is ± 0.1 mm (± 0.004 in.), any misalignment falls within the permissible limits, and leader pins are perfectly acceptable for the mold. Note: In theory, only two leader pins are ever required to ensure proper alignment. The fact that many molds use 4 pins is mainly to protect the cores during servicing the molds. If the walls are thinner than in the above example, say, in the order of 1.0 mm or less, and the tolerances are tighter, alignment with leader pins may not be good enough to ensure that the variations fall between the allowable limits. In these cases, individual ”taper locks” (of various designs) are required.
The tolerances of the product decide which method of alignment to use
Round tapers are relatively easy to manufacture, but require high accuracy to ensure concentricity with the center of the cavity, and to ensure that the proper preload is achieved. The basic requirement of any taper fit is the preload between the matching faces.
Without preload, a taper is useless for alignment
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2 The Plastic Product The importance of preload is discussed in detail in [5], Chapter 30. The biggest problem with round taper fits is that the tapers wear with time, and need to be reset or replaced, which is often quite expensive. But it is still the most economical method of alignment. The “wedge lock” method is a very good, efficient method, used mainly for molds where accuracy is very important and the higher cost can be easily justified over long periods of use. It consists of two opposing pairs of matching wedges, at 90 degrees The advantage is that the wedges are easily accessible and can be adjusted (by shimming and/or grinding) or replaced with little cost. The main disadvantage of the wedge lock design is that more space is required to accommodate the wedge lock than the space required for a round taper lock, thereby making the mold larger and more expensive.
2.5 Usually, molds are designed with fixed cavities and floating cores
Heat Expansion, Alignment, and Mold Cost
Heat expansion [6, Chapter 14], is another area that must be taken into account. It is always necessary to have both mold halves at the same temperature; particularly the mold plates carrying the alignment elements. The plates on the cavity side in a hot runner mold can easily become hotter than the plates on the core side of the mold. For example, a temperature difference of 20 °C between two plates, on a distance of 400 mm, causes an expansion of 0.091 mm (0.004 in). This can result in a serious misalignment. If we depend on leader pins for alignment, they will deflect and/or wear rapidly, as will the bushings. If taper elements are used, they too will wear out rapidly and lose their usefulness. There are basically only two ways to avoid misalignment caused by heat and/ or manufacturing variations 1. Make sure that the cooling channels are laid out so that the temperatures of the plates are kept the same; this has little effect on the mold cost. 2. For molds with more than one cavity, allow the cores to “float”: the cavity side consists usually of a ”cavity retainer plate” into which the individual cavities are set in. These locations are fixed but subject to manufacturing variation (tolerances). The mold can be designed so that in the individual stacks, the cores (with their taper alignment) can ”float” on their mounting surface (plate) to “find” the matching taper in the cavity. There are two methods commonly used to achieve this: – The cores are screw-mounted to the backing plate, with the screws accessible from the parting line. The mold is assembled completely,
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2.6 Surface Finish but these screws are, at first, not tightened fully so that the mold, the first time it is closed, will push the cores into proper relation to the cavities. After the mold is opened again, the screws can be fully tightened to be ready for production. This method is satisfactory as long as the temperature difference between the two mold halves is kept low, at about 5 °C or less. – A better, but more expensive method is to make the cores really floating, regardless of the temperature differences, as shown in [6]. Note that the amount of float is limited and only in the order of 0.1 mm (0.004 in.)
2.6
Surface Finish
The finish of the mold parts, the molding surfaces, and the fitting surfaces where mold parts meet, are important cost factors. The finer the machining finish, and the more hand finishing is required, the higher is the mold cost. This appears to be obvious but is often overlooked or neglected. The relationship between surface finish and costs and the relationship between tolerances and costs (as shown in Fig. 2.23) are very similar and apply here too.
2.6.1 Finish of Molding Surfaces Molding surfaces (the areas in contact with the plastic product) are finished
The costs rise exponentially with finer finish
1. To provide the required appearance or function of the product 2. To ensure that the product can be easily ejected from the mold, however: – Occasionally, a relatively rough surface in specific areas may be beneficial to keep the product on that side of the mold, from where it will be ejected. – On the other hand, sometimes, a high polish could also be detrimental to easy ejection, depending on the design of the product. In such cases it is the decision of an experienced mold designer to specify the proper finish in these locations (refer to Appendix 16 for list of surface finishes commonly used). Especially with very thin-walled products, the surface finish of the cavity space affects the plastic flow over the molding surfaces. Better finish results in faster filling and shorter cycle time. In some cases (notably with PS), flash chrome plating over a highly polished area can increase the productivity of the mold by up to 10%.
Figure 2.36 The etched cavity wall gives this tumbler a frosted look
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2 The Plastic Product Finishing (polishing, etc.) the mold parts is generally an expensive activity in the mold making process because much handwork is required, and should be limited to those areas that really require it. Most mold makers today utilize hand-operated mechanical and some fully automatic methods to finish a surface, but there is still much need for hand finishing wherever the shape of the product does not allow easy access for mechanical or automatic equipment. Figure 2.37 Typical PS tumblers
Figure 2.38 PS tumblers and core/cavity show the highly polished finishes required to achieve the glass-like look of the molded cup
Figure 2.39 Polishing area in a shop
The purpose of finishing, in general, is to remove the tool marks remaining on the surface of a work piece. In many cases, the rough, “as machined” finish after chip removing operations (turning, milling, etc.) could be quite satisfactory for the appearance of the product, for example on the inside surface of a technical product (enclosures, boxes, television cabinets, etc.), but this may not always be satisfactory for the ejection of the product, because the plastic will not easily (or not at all) slide over too rough a surface. It is also important to consider in which direction the rough machining grooves are lying: to be in line with the ejection could be satisfactory, but across it is usually not acceptable. Also, the draft angle of a wall (or of the sides of a rib) is important. With little draft (a small draft angle), the surface finish must be much better, whereas with a large angle (approx. 5° or more), a much rougher finish, such as “as machined”, could be permissible. With the need to design for less and less mass, the draft angles, especially of ribs, must be kept small, and these walls therefore need a good finish, but not necessarily a polish: a good finish in line with the ejection motion (“draw stoning”) will usually be good enough. If ejectors can be placed under such ribs, the finish becomes even less of a problem. We must always consider what would happen if a piece of plastic breaks off inside a rib: it may save time in the making of the mold but can become expensive later, when the service personnel are frequently required to remove some broken-off bits of plastic from the mold causing severe delays in production. Grinding and electric discharge machining (EDM) leave smaller tool marks on the worked surface; such surfaces may not need any further finish, except polishing where required for appearance. EDM finish can be from rough to very fine, which may not require any polishing at all. Rough finish is the result of high currents and faster cutting speed and therefore requires less time. In addition, with today’s new methods of finish turning and milling hardened surfaces, the achieved finish is often as good as a ground finish and no further polishing is required.
2.6.2 Texturing of Surfaces Be specific as to where dimensions point to; for example, to the peaks and valleys of the finish
There are also other surface finishes for appearance, such as texturing, to create leather, basket weave, or other patterns. If it is a deep pattern, it should be clear if any related dimensions apply to the highest point of the pattern or to the base where it is applied to. A rough EDM finish is a good and inexpensive solution for a good-looking, matte surface.
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2.7 Engravings
2.6.3 Fitting Surfaces of Mold Parts This applies to all surfaces of mold parts that abut on other mold parts, but are not in touch with plastic. Usually, grinding or fine machining surfaces are required where the dimensions stack up and their sum must be held to close tolerances. Otherwise, ordinary turning and milling surfaces are sufficient. We have dwelled on the finishing of mold parts to highlight the importance of properly specifying how and where a mold (mold part) needs to be finished (polished) because of the cost. The mold designer should analyze whether the finishing specifications shown on the product design are realistic and really necessary for the functioning or use of the product and discuss it with the product designer. This can result in great savings, reduced delivery time and improved productivity (output of the mold). All agreed-upon finish specifications must be shown on the finally approved product drawing. SPE (Society of Plastics Engineers, www.socplas.org) provides a series of standard finishing specifications, which can also be translated into finish in microns (thousands of a millimeter). They are a good method of specifying finishes, but additional information may be required on the drawing to clearly specify for which areas these specifications apply. The mold designer should never accept a general finish unless it is easy to produce, or the cost of it will be factored in the mold cost.
2.7
Engravings
The term “engravings” covers lettering, lines, ornaments, logos, and others.
2.7.1 Engravings Versus Applied Labels Engravings in the mold represent a one-time cost; therefore, in the long run, the cost of the finished product is less than the cost of applying labels made from paper or plastic film to the molded product. If the labels are applied in a separate operation, this cost must be added to the cost of the product. In some operations, the application of labels could be done “on-line”, with an automatic applicator, in which case only the equipment and maintenance costs need to be considered. In either case, the cost of the labels must be added. We must not forget that the same product could be used for different end user applications (for example, different chemicals are sold in containers of the same size) and/or for different end users (manufacturers). In either case, labels applied after molding would make more sense than changing mold components for a different engraving. Whether to use all engraving, labels alone, or part engraving and part labeling must be decided in view of the quantities of pieces to be produced and the flexibility needed in each case. There are other methods of applying information on a plastic product such as printing, hot stamping, and others.
Figure 2.40 Engraved products
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2 The Plastic Product
As a general guideline we can assume that the cost of the molded product increases approximately
Little, when engraving
Approx. 10% with printing
Approx. 50–100% with labeling
Also factored into the considerations should be other methods of manufacturing, such as in-mold automatic insert molding of printed labels and some other molding methods that from time to time have appeared on the market. These specialized techniques should not be ruled out, especially if the production quantities are such that the special equipment for such methods can be economically justified. Although these types of molding will not be discussed in this book, Section 4.1.10 provides illustrations of systems for automatically inserting labels into molds.
2.7.2 Two-Color and Two-Material Engraving Buttons (typewriter keys, pushbuttons, etc.) with two materials or colors molded in one molding setup (quite complicated) are another method of marking molded surfaces. Originally, these buttons or keys were molded with (depressed) engraved “text” (alphabet, symbols) and the thus created molded recesses were then filled with paint. This was expensive hand work; in addition, raised engraving is very expensive to make in the mold (see below). Today, most mass-produced keyboard keys for computers, etc. are printed by various methods. Two (and more) color molds will not be discussed here, because they are rarely used today in lieu of engraving. However, two-color molding for many other products (mostly automotive) is still much in use. The general principles of anything discussed in this book do also apply to these molds. Figure 2.41 Printed keyboard keys
2.7.3 Depth of Engravings
Figure 2.42 Hot-stamped logos on cosmetic cases give a multi-material look to the products
It is important to understand that engravings which are to appear depressed (appearing engraved) in the surface of the product are created by raised features in the mold. Conversely, engravings depressed (engraved) in the mold appear as raised features in the product. It is amazing how many product designers do not realize that it is fairly easy to engrave into a steel surface, but very time-consuming (and costly) to create engravings projecting from a surface.
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2.7 Engravings Many designers, when confronted with these facts, confessed they did not know that it makes such a difference, and readily changed their design to “raised in the product”. The only time when it may be really necessary to have the engraving depressed in the product is when the lettering will be filled with paint, after molding, for better readability or special effects or for special, artistic designs, usually associated with high-quality products, such as technical enclosures for hand held devices (cell-phones, etc.) containers for cosmetics (compacts) and so forth. Occasionally, when the raised lettering in the product is objectionable, there is always the possibility of depressing a “panel” and have the engraving on this panel, so that the top of the engraving is level or slightly below the main surface, see Fig. 2.43.
Figure 2.43 Upper view: raised engraving on top of product. Lower view: raised engraving in depressed panel; t = wall thickness of the product, H = height (depth) of engraving
2.7.4 Font Style and Size of Artwork For general applications, such as cavity marking or manufacturer’s identification, the style (font) or size the lettering may not be very important. The lettering should be (pleasantly) proportional to the size of the product and be easily readable. The mold maker may have only a certain range of styles and sizes available; using these will be less expensive. If the engraving has special requirements, the product designer must supply the artwork from which the necessary templates or models are made for machining. The mold designer and product designer must agree on the form of artwork best suitable for the mold maker, as there can be costs involved in preparing such artwork, in the size (photo-enlargement), and material (Mylar film, etc.) required. The smallest acceptable size of engraving should be considered. A suggested minimum size is 8 pt, to be readily legible, but 6 pt could be required in exceptional cases. In all cases of engravings, it is also important to consider the cost of removing the burrs (by hand or mechanically) after cutting the steel, to prevent unsightly, fuzzy outlines of the engravings on the molded products.
2.7.5 Polarity of Engraving We shall define positive engraving as any engraving such that will appear “readable” to the user. Negative engraving is the inverted image, e.g., as ordinary lettering would appear in a mirror. This may seem obvious but it still does require some comments. Most engravings are viewed from the outside of the product (top, side, or bottom), regardless of whether the plastic is opaque, transparent, or translucent. In all these cases, the engraving must be negative to appear in the molded piece as “readable” (positive). This is also important where it may not appear as obvious, such as in the case of logos or trademarks, which may appear to the casual observer to be symmetrical but may have some asymmetrical features, which must be seen by the user in the proper orientation (polarity).
Figure 2.44 Picture of artwork
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2 The Plastic Product In some products molded from transparent or translucent plastics, the required lettering or marking could be molded on the inside of the product, so as to be read by the user through the plastic. In these cases, the engraving must be positive in the mold steel. This is the case in measuring cups, if the engraving is on the core.
2.7.6 Are the Locations Selected for Engraving Practical? The product designer usually places the lettering, lines, or symbols at locations where they are best suited for the end user, but occasionally such engravings could be difficult to produce by the mold maker in the location specified. This could be the case where engraving inside a pocket in the mold would be difficult or even impossible, and would require inserts or EDM requiring special electrodes. In some cases, the engraving could be too close to the edge of the mold steel, thereby increasing the risk of early failure of the mold steel due to stress cracks. A minimum of 2 mm between any engraving and the edge of the mold steel is suggested. Here again, the mold designer and the product designer must work together to find the most suitable compromise between product requirement and mold cost.
2.7.7 Engravings in the Walls and Bottoms of Products Engravings can be either on the cavity wall or on the core (they could also be on inserts in either cavity or core). Engraving on the Outside of the Product (Engraved Cavities) Containers usually require markings on the outside of the sidewalls or in the bottom. Markings in the bottom are often required to show trademarks, patents, product identification, batch identification, dates of manufacture, or others. Engravings in the sides are occasionally required (usually with transparent or translucent plastics) to indicate liquid levels inside a container.
Figure 2.45 Gate pad engraving (bottom of container)
Engraving into the bottom of a cavity is usually not difficult, especially if most of the bottom of the cavity is an insert in the cavity block. Alternatively, it is not too difficult or costly if inserts with the required engravings are placed either in the solid cavity bottom, or within a large cavity bottom insert (“inserts within an insert”). Serious problems can arise when laying out the cooling circuits in such complex cavity bottoms. Good cooling in the gate area is very important for fast molding cycles; inserts make it more difficult to lay out efficient cooling channels. A poorly cooled cavity bottom, especially near the gate, will result in a longer molding cycle. In this case, the preferred method is to have a solid insert for much of the cavity bottom. If there are
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2.7 Engravings changes required in the engraving, it is not too difficult or expensive to change the bottom. This may result in having and storing a number of different bottoms for the cavity for the various applications or end users of the product, which are also costs to be considered. Mechanical engraving in the bottom of deep cavities is always difficult, because long unsupported engraving cutters will by necessity operate at a slower speed for the required accuracy and cleanliness of cut. Long EDM electrodes can be used, although they are slow and expensive; however, this method has the advantage that it can be done even after the cavity is finished. A method not much used today is the hobbing of the engraving into the bottom of a cavity. This method was used extensively in molds built about the middle of the last century (both for small compression and injection mold cavities). This method can be used only in soft steels and requires special heat treatment (carburizing and hardening) of the steel after hobbing. It is still occasionally used today. The injected plastic, as it cools inside the mold, shrinks away from the cavity wall and, provided the depth of engravings into the cavity walls is not too deep, there is usually no problem with ejection. As the product shrinks toward the core, it will not “hang up” in the cavity as the mold opens. However, the clean withdrawal of the molded piece from the cavity depends also very much on the draft angle of the sidewall, on the wall thickness of the product in this area, and on the type of plastic used. There is no easy formula to indicate what is possible and what is not, but as a general rule it can be stated that
Any engraving (by chip removing or EDM) in the sidewall inside a cavity, especially in a small one, is very difficult and can be very expensive. Shallow engravings “burnt” with EDM are easier to achieve; but there is the problem of matching the engraved electrodes to the shape (curvature) of the cavity wall so that the depressions created with EDM are uniform both in depth and appearance and do not exceed the critical depth beyond which the product can not pull out of the cavity. The suggested maximum depth is in the order of 0.1 mm (0.004 in.) or even less for difficult cases, such as explained in the following points.
Walls with heavier thickness allow deeper engravings because they shrink more and let the product withdraw more away from the cavity.
The greater the shrinkage factor, the easier the engraved portion pulls away from the cavity.
The greater the taper of the sidewalls, the easier will the product pull out of the cavity. Engravings in sidewalls with tapers of less than approx. 5° are more difficult to withdraw than from walls with larger tapers.
Engravings into the sidewall of the cavity are always difficult and expensive
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2 The Plastic Product
Hard plastics such as PS will offer more resistance if they were “caught” by the edge of too deep a depression than would be more flexible plastics, such as PP and PE. However, there are many molds successfully producing even thin-walled PS products with decorations on their outside walls.
The angle and shape of the sides of the engraving within the sidewall of the cavity must be so that it offers little resistance as the mold opens and the edge of the engraved projection in the product slides past the engraved depression in the sidewall.
Any deeper engravings in the side walls, or where there is not enough draft angle, will require to place the engravings either on moving side cores in the cavity or on split cavities. Both methods would require more space, much larger molds, and add considerably to the mold cost; such molds will usually also potentially produce more scrap, require longer molding cycles, and thereby increase the cost of the product even more. Figure 2.46 shows heavy-walled tumblers engraved with an artistic pattern on the outside, produced by engraving (texturizing) the inside of the cavity. This engraving is not deep enough to require a split cavity. Note the stacking lugs visible through the plastic. They are used to stack the parts in a dense pattern. Figure 2.46 Tumblers engraved on the outside
Engravings Inside of the Product (Engraved Cores) The following comments apply to engravings into the top or the sides of the core. Engravings in the sides are often required with transparent or translucent plastics, e.g., to indicate liquid levels inside a container (measuring cups, vials, etc.). The markings are usually lines indicating the proper height and lettering to identify the values. Such products are made mostly from clear polystyrene (PS), SAN, Acrylic, or polycarbonate (PC) that have low shrinkage factors. This makes it relatively easy to calculate the dimensions where the measuring lines should be located. If such products are made from highshrinkage materials, such as PE or PP, the high shrinkage factor makes it more difficult to predetermine the proper location for the level markers. In such cases, especially if the accuracy of the measuring lines is important, it may be necessary to finish the mold first, complete with the lettering, but to engrave the measuring lines only after the mold has been tested and runs on an optimal cycle, because the volume of the container can vary substantially when operating at different operating conditions of the mold. Except for very stiff plastics, such as PS, SAN, or PC, and sometimes with air ejection of even softer plastics, lines and lettering on the core present fewer problems, because the plastic will stretch during ejection and let the plastic slide out of the engravings. This is possible because at the time of ejection, the cavity has already moved away from the product and there is ample room for the plastic to stretch during ejection. However, the deeper the engraving, the
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2.8 General Appearance of the Product more important it is to make sure that the sides of the engravings are tapered and/or rounded sufficiently to allow easy sliding out of the engravings. The draft of container sidewalls can be quite small; a 1° taper could be acceptable as long as the engraving is not too deep and the side of the engraving in the direction of the ejection is smooth and chamfered or rounded. Engraving into the side of a core is usually not difficult to achieve. The depth should be in the order of 0.1 mm, but less is recommended for small draft angles of the core. While it is feasible to produce raised “engraving” on the core, this is extremely difficult to machine and then to finish the molding surface of the core, and would therefore make for a very expensive mold. The top of the core can be a good location to engrave the cavity number; it is easy to produce and is frequently done in technical products and enclosures. The designer must be sure that it can be easily read. If it is to be read from the inside, the engraving must be negative, if it is to be read from the outside (through the plastic), the engraving must be positive.
2.8
General Appearance of the Product
2.8.1 Flatness It is usually easy to machine a flat surface; however, where very high polish is required, common polishing practices can result in waviness of the surface, which may not be acceptable for products requiring near-perfect flat areas with optical clarity. In such cases it may be necessary to provide the mold with inserts for the areas requiring the optical finish; they can then be polished separately, on appropriate lapping equipment, which can guarantee flatness. A typical example is the top surface – both on the core and the cavity side – of Petri dish bottoms and lids made from crystal PS. Flat surfaces may be easy to machine but molding them can be a problem, particularly when materials, such as PE or PP, with high heat content and low thermal conductivity are used (see Appendix). Taking this into consideration is especially important when the products are to be ejected as early as possible to achieve fast molding cycles, i.e., while the products are still warm but rigid enough to allow ejection without damage. A flat, relatively large area in the mold is usually easier to cool than corner areas or heavy rims or intricate sections in the product. However, the surrounding, often thicker and almost always poorer cooled areas stay hot longer and will continue to shrink after ejection and thereby tend to deform the already cold, flat areas. Typical examples are rectangular flat trays or other flat shapes surrounded by heavier rims; such rims stay warm longer and distort the flat areas while they cool down to room temperature. There are several approaches to solve this problem, but as always, they needs full cooperation between the mold designer and the product designer. The following are some typical examples of these approaches:
Figure 2.47 Flat parts can look like potato chips if the mold and part are not designed properly. A stepped ring was added to the part to eliminate warpage
Figure 2.48 Petri dishes require optical clarity and flatness
Figure 2.49 Flow leaders are used to aid in even filling and to avoid warpage
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2 The Plastic Product steel dimensions
plastic after shrinkage
Note that with typical small containers, such as drinking cups or cottage cheese containers, even with good cooling and equal thickness walls, the bottom, when molded in a flat bottom mold, will pull towards the center and deform (pull) the sidewalls inwards as the product continues to cool outside the mold. This deformation may be objectionable. In such cases, the arching of the bottom of the mold is absolutely necessary.
Figure 2.50 Schematic of cup with arched bottom.
A flat surface of a lid can be modified by adding some steps or “expansion loops” so that, as the top of the lid shrinks, the steps or loops will bend due to the pull from the shrinking and prevent warping of the lid. This is of special advantage with large lids as for pails, etc. (see Fig. 2.51).
Large, especially rectangular trays or lids that must be flat are always difficult to keep from warping (“potato chipping”). It is very important that an equal wall thickness throughout the tray is maintained so that there are no warmer pockets of plastic, which will take longer to cool and shrink after the rest of the molded piece is cooled. If the rim must be thicker, more emphasis must be given to the cooling of the thick areas so that all the plastic in the mold is cooled evenly. If this is not the case, longer cycle times will be required to achieve flatness, or costly shrinking fixtures may have to be planned.
It is also important that the flow lengths from the gate(s) to the rim are as symmetrical as possible to permit the plastic to arrive to all parts of the rim at the same time. This depends also on the thickness of the product, where heavier sections permit easier and faster flow. This can affect the selection of the hot runner system (e.g., using more than one drop) and adds costs to the mold.
Figure 2.51 Lids with added steps or loops
Figure 2.52 Tray with added flow leaders
The flat surfaces at the bottom of a container can be designed in the mold as “curved” (or arched) so that the plastic, as it cools outside the mold, will shrink to a less arched shape or even become flat (see Fig. 2.50). If it does not matter to the appearance and/or the usefulness of the product, this is a preferred solution. The curvature of the arch must be selected to suit the anticipated cycle time. It is suggested to ask the product designer for a wide tolerance on the curvature of the arch so that it will be still acceptable for the purpose of the product, regardless of the actual shrinking experienced, which may change with changes in the molding conditions and with the plastic batches.
Figure 2.52 shows how the flow in a tray can be improved by machining so-called “flow leaders” into the cavity or core, which are slight thickening in the wall thickness in those areas which should flow faster to equalize the filling pattern in a mold. Such thickening will add a very small amount of plastic that can hardly be seen but will ensure better, less warping trays. Flow analysis of such a product will show where such flow leaders are required. In Fig. 2.52, T2 may be 10% greater than the wall thickness T1 and the width of the flow leader would range from approx. 10–20 mm (0.38–0.75 in.).
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2.8 General Appearance of the Product
2.8.2 Sinks and Voids Sinks (“sink marks”) are surface flaws (imperfections) of the product resulting from either incomplete filling during injection, excessive local shrinkage, or a combination of both. During injection, the hot plastic flows through the cavity space in contact with the cooled mold walls. This causes the plastic layer near the walls to solidify, thereby reducing the passage for the flow; it requires more “effort” (higher pressure, higher melt, and/or higher mold temperature) to completely fill the subsequent portions of the cavity space.
Cooling lines Melt front
Frozen layers
Velocity profile
Thickness Fountain flow
Figure 2.53 Plastic flow through the cavity space
Figure 2.32 shows the plastic flow through the cavity space. The frozen plastic layers close to the cold walls reduce the passage through which the plastic has to flow on its way to fill the cavity. The shrinkage factor must also be considered: To avoid poor quality products (voids) and/or unsightly shrink marks caused by the shrinkage as the plastic cools, pressure must be kept on the plastic already in the cavity space with the so-called “injection hold” pressure to add more plastic into the cavity space and make up the “lost” volume due to cooling. This is useful only as long as the gate is not frozen, i.e., as long as plastic can still pass through the gate. The hold time adds to the cycle time and adds cost to the products. Ideally, for best flow, the cross section through which the plastic flows away from the gate should be largest near the gate and from there gradually diminish toward the end of the flow. However, this is not practical because a lot of plastic would be wasted. The next best thing is to make sure that, at least, the same cross section is maintained throughout the mold; this is not always possible because of the requirements of the product, but it should be attempted.
Figure 2.54 Automotive grill molded with 8 gates. The left side shows a filled part and the right side a short shot. Venting was required where the flow fronts meet to resolve filling issues
The possibly worst condition is if a heavy area must be filled after the plastic has passed through a long, narrow path and has suffered a large “pressure drop”. Such remote heavy sections (typically, the rim of the product), even when they are completely filled, see much lower injection pressures and because the amount of shrinkage is greatest where the pressure is the lowest, these areas will experience much shrinkage and result in sink marks or voids (more about rim shapes in Section 3.8.7.1). Sinks and voids appear often at the intersection of ribs and walls or in general at any localized thickening of the plastic required for functional reasons, such as hubs, and so forth. Because the thick section of the plastic remains hot longer than the thinner sections, the plastic will continue to shrink there. While the plastic is still relatively soft, it will pull the already more or less cooled surface towards the center of the heavy, hot section, thereby creating dips in the nearest surfaces. In many applications, a sink on a surface visible to the user may be acceptable, but it should be agreed upon before designing the mold how much of a sink is acceptable as well as its probable location.
Figure 2.55 Intersections of ribs and thick sections can cause sinks or voids
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2 The Plastic Product The alternative is either to increase the injection pressure, which may not always be possible, especially with older injection molding machines (or the machine may not have enough clamp force to keep the mold closed against the higher pressures), or to raise the temperatures of the melt or/and the mold and use longer injection hold pressure cycles, all of which will result in longer molding cycles and higher product costs. If the plastic surface is already so stiff that it cannot be pulled in (or “sink”), the still hot plastic will shrink away from the center toward this stiffer outer skin and will create a “void”. A void is a hollow space inside the plastic and contains a vacuum. In opaque plastics a void cannot be seen, but it can be undesirable because it weakens the plastic, similar to porosity. Such a weak spot, e.g., in a hub designed to receive a screw, would not be as strong as expected. Figure 2.56 Creation of a sink or void
The possibility of voids or any potential defects caused by heavy sections in a product must be discussed at the time a job is started, and not after the mold is completed
If a transparent or translucent plastic contains a void, it is visible and can look like a chain of round or elongated bubbles near the center of the heavy section. To eliminate this defect, the molding conditions must be changed to ensure that injection pressure is maintained in this critical area, often requiring higher temperatures and resulting in longer cycle times. To remedy this problem, the product design should be modified to eliminate any thick spot(s). Voids can be easily seen by cutting the suspect section with a saw or by drilling a small hole into it from the nearest surface while holding the product under the surface of a pail of colored water. As the drill breaks into the void, the water is sucked past the drill into the void and can be seen as the colored fluid fills it. This is especially important if the customer has been quoted a specific cycle time (a more detailed discussion about this subject can be found in [1] or in any book on product design with plastics).
2.8.3 Witness Lines Witness lines appear on the product wherever mold parts or inserts join on the molding surface. No matter how good the fit of the mold parts and how well polished the surface is at this spot, there will always be a more or less fine line visible on the product. When the gap between the mold parts is too large, it will flash, i.e., the plastic will enter the gap during injection, and if it can pull out during ejection, it will be an unsightly thin projection from the surface of the molded piece. At best, it may not affect the overall appearance or serviceability of the product, but it is still the sign of poor workmanship. Figure 2.57 The gate insert witness line can clearly be seen on this worn insert for a specimen cup lid
In general, gaps in the mold of less than 0.01 to 0.03 mm (0.0004 to 0.0012 in.) will not flash, depending on the type of plastic, the melt temperature, and the injection pressure.
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2.8 General Appearance of the Product Parting Line Flash Good molds have venting around most of, if not the whole length of the parting line. As long as the stack and the vents are designed properly, there should never be flash on the parting line. If flashing does occur despite a properly designed and built mold, there could be a number of other causes:
The selected molding machine has not enough clamp force (tonnage) for the projected area of the cavity and the required injection pressure or was not adjusted properly. This is a rather common occurrence, especially when the mold is installed in a low-tonnage machine (clamping force) because a suitable machine was not available.
Flashing at the parting line could be caused by a poorly maintained and out of alignment molding machine, which could be the result of poorly adjusted tie bars, worn tie bar bushings, worn or bent machine platen surfaces, poor hydraulic system, and so forth. Only too often a mold is blamed for flashing when the real reason is a poorly maintained or a too low tonnage machine.
There may be (hard to see) plastic caught between the mold faces.
A common misconception is that if a mold fits a machine it should be able to run on it
Gaps Between Cavity and/or Core Parts and Inserts Mold parts and inserts can be dimensioned for locational clearance fits (Standard Geometrical Tolerancing); that is, they will probably always have a clearance, but this clearance must never exceed a size where plastic could flash into it. This means that the so-called standard fits are not always suitable with larger mold components and inserts. Smaller tolerances than given with “standard fits” must be specified for some mold dimension. This modified method of tolerancing fits is quite suitable for the assembling of molds. Because gates wear out in time and get easily damaged, often during startup of the mold, molders prefer that the gate be in an insert, see Fig. 2.58, that can be replaced when necessary, rather than rebuilding the entire cavity. Welding a worn gate is sometimes successful, if done by experienced welders, but in most cases, even the best weldment will be visible on the product, especially on a highly polished surface. This kind of repair is generally not recommended. Any gate insert will leave a circular witness line. It must be pointed out to the customer before beginning the mold design that the mold would be less expensive to build without an insert, but the costs of upkeep will be much higher, especially with high production requirements, which in the long run will affect the cost of the product. Figure 2.59 shows a modular cavity (A), the matching cavity bottom (B), and the beryllium-copper alloy gate insert (C) for the cavity bottom The witness line will be a small circle around the gate. It is also important to realize that a properly designed gate insert permits better cooling around the gate which is very important for achieving a faster molding cycle. The added cost of the insert can be easily justified with the
Good cooling Witness line
Figure 2.58 Typical cavity with and without gate inserts
Cavity
Gate insert
Figure 2.59 Modular cavity
Cavity bottom
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2 The Plastic Product increased productivity of the mold. The cost of a gate insert may range between $200 and $1,000 each. Clearance of Ejector Pins and Ejector Sleeves Consider the locations and sizes of the ejector pins carefully. In many molds, fewer pins of appropriate size in more suitably placed locations could save a lot on mold costs and improve operation
Ejector pins are always produced by mold part supply manufacturers with specified, standard, close tolerances so that when the mold is built with the bore dimensions for these pins, as suggested by the pin manufacturer, they will not flash and they will always leave a witness line. The clearance between ejector pin and bore is actually desirable, because it creates a good, selfcleaning vent. Ejector pins (and sleeves) and the bore wherein they slide will wear over time. Especially when abrasive plastics are molded, but also, frequently, when the mold is poorly designed or built, the pins are allowed to rub on the side of the bores. If the ejector plates are properly aligned, with their mass suitably supported by guide pins (see Fig. 4.51 in Section 4.1.7.1) and the ejector pins (or ejector sleeves) are allowed to float freely in their retainer plates, there is little wear to expect. But even under the best conditions, after the mold is in operation for a long time, the clearance in the bores will increase, and the plastic will begin to flash around the pin. Such flash may also create points of hang-up and prevent fully automatic ejection. The proper way to repair the mold is to increase the bore size and replace the ejector pins with a larger size. Standard oversize pins are commercially available, usually 0.13 mm (0.005 in.) larger in diameter than the original, nominal standard sizes. Allowing space for oversize bores should be considered when designing the mold; this is sometimes difficult with small technical products, where small pins barely clear the space between walls or under ribs. Note that some molders use double or even multiple ejection strokes to overcome the problem of hanging up due to flashing; however, this adds considerable time to the cycle, adds wear to the machine and mold, and is not recommended. Hiding the Gate Mark
Figure 2.60 Gate mark hidden in the letter Y of this Playdoh lid
Occasionally, to improve the appearance of the product, it may become necessary to use certain “tricks” to hide the gate marks. A pinpoint gate mark (vestige) from a 3-plate or a hot runner mold can be hidden inside an ornament or inside the closed portion of a letter appearing on the same surface (typically, inside a letter O, P, D, etc.); in other cases, an ornament could be created around a gate, such as several concentric circles, with the gate as the center. In some cases, where the vestige of a valve gate is on the visible top surface near one end of the product, a similar “fake vestige” near the opposite end can be created with an inserted round pin, which serves no other purpose than to make the product appear symmetrical, which is usually more pleasing to the eye.
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2.8 General Appearance of the Product
2.8.4 Weld Lines Weld lines are created when two semi-molten streams of plastic within the cavity space intersect. They appear on the surface as more or less pronounced lines. These streams may originate from two separate nozzles in the same cavity, or when a projecting pin or other obstacle breaks the flow into two separate streams. In some products, these weld lines can be objectionable for aesthetic reasons; in others, they may be acceptable. When the plastic surface is to be printed after molding, such lines could present an unacceptable visible flaw. Note that a weld line also weakens the product, because the plastic streams – by the time they rejoin – are already colder and may not fuse sufficiently. Higher mold and melt temperatures can sometimes improve or even eliminate a weld line; however, this means longer cycle times (less productivity). When molding pigmented (colored, particularly metallic) plastics, weld lines can show up very distinctly. Plastic always finds the easiest path (“path of least resistance”) within the mold. Typically, a heavier rim or ribs allows more and faster flow than the adjoining thinner walls, so that the plastic, after rim or ribs are filled, will fill the remainder of the cavity space from there (“back-fill”) and create weld lines where these flows meet. This is an important consideration for locating the gate(s). If weld lines are created by such conflux of plastic it is important to provide vents at these spots to let any trapped air escape. These features add cost to the mold.
Figure 2.61 Weld lines on cosmetic case. The plastic flows from the gate through the grill and rejoins afterwards, forming the weld line
Location of Weld Lines Usually, the location of weld lines can be quite accurately predicted by plain logic or with the help a flow modeling computer program. If the area where the weld line is expected to form is critical for strength, a change in the gate location may be a way to change the location of a weld line. In addition, it may be possible to facilitate the plastic flow to a certain path (e.g., by slightly increasing a portion of the cross section of the flow path) so that the plastic reaches the critical point and flows around it before the other stream is met, thereby relocating the weld line to an area where it will do less harm to the strength of the product. Such slight increase in the thickness may be hardly noticeable and, while increasing the amount of plastic insignificantly, it ensures a stronger product. Since this is a change to the product, the approval of the product designer may be necessary. In many cases, a weld line is unavoidable. Only its location can be influenced by the location of the gate and flow leaders/ribs and changes in thickness of the part. To improve the appearance and strength of a weld line, typically a molder will increase mold temperature, injection speed, melt temperature, or venting.
2.8.5 Surface Defects (Flow Marks, Splay, Record Grooves, Haze, Jetting, Hooks, and Ripples) Figure 2.63 shows four examples of typical surface defects that are essentially defects caused by improper settings of the molding machine.
Figure 2.62 This lid for an ice cream container has a cardboard insert and the plastic is molded in a ring around it. The weld line is formed opposite the gate location
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2 The Plastic Product The surface may show unsightly flow marks as the plastic flows away from the gate. This is often the case when injecting into the open surface of a product. A simple remedy is to ensure that the plastic coming from the gate is immediately broken up by “shooting” against the core if that is possible. This is always the case with hot runner molds and with cold runner, 3-plate molds where the gate is opposite the core. With 2-plate molds, if there is no sidewall of the product, it is best to shoot against a core pin located in the cavity space (a pin or projection either in the cavity or in the core). Excessive sink marks
Products with many thick and thin sections may also show such flow defects. Slower injection and adjusting the temperatures and pressures may be the only remedy. This will increase the cycle time and affect the cost of the product.
2.8.6 Identification of the Molded Piece
Record grooves
Every molded piece should – and in some cases must – have certain identifications molded into it. The product designer will sometimes specify the location of some of the engravings, but often leave it to the mold maker to decide their location, size, and style. The mold designer must know before starting, which engravings will be required. Some of the markings could be created by special, often standard inserts (e.g., the date – week, month, year – when the product was molded); they should be located so that the date can be changed easily without major work on the mold. These markings are often mandatory for food packaging, pharmaceutical, and other products. Typical markings, in addition to any specific instructions, are:
Drool
Identification of the seller of the product
Figure 2.63 Typical surface defects
Identification of the molder, if not the same as above
Resin type identification for recycling
Cavity (and mold) numbering (position, location)
Patent number, if applicable
Made in ______ (identification of country of origin)
Date of manufacture
Special information: “Food grade”, “Microwave safe”, “Dishwasher safe”, etc.
Others as could be required by law
Logos
Product identification
Bar code
Figure 2.64 Identification of a molded piece
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2.9 Product Strength Requirements In some countries, some of these markings are mandatory, and it is up the product designer to specify what is obligatory and what is optional in the countries where the product will be used and/or sold. Occasionally, there is resistance to have cavity numbers shown, but it is absolutely necessary for the molder to know from which cavity, and from which mold a piece has originated to be able to find the cause of any defects. If consecutive numbers are not desired, some code could be used for identification of the stack from which a faulty piece has been ejected.
Every molded part should have a unique identification such as a cavity number
It is always difficult to add engravings to a mold or to make revisions to existing engravings. If it is anticipated that changes are likely to come, the mold designer should design the mold with the necessary inserts so that changes can be made easier, or put it on an inexpensive part of the stack. It may add some costs at the time of building of the mold, but will be much less expensive than making changes to a finished mold. This is often the case when the same mold could be used for products sold to different customers. Every mold insert should be marked with the mold and cavity number to ensure that after disassembling the mold for maintenance or repairs, the various mold parts are assembled in the same relationship as before disassembly.
2.9
Every insert should be marked with mold and cavity number
Product Strength Requirements
Plastic products must not only “look good,” but they must also be strong enough and suitable for the application for which they are intended. The molding material (the plastic) is usually specified by the product designer to create a product with all the desired properties. This material selection can affect the selection of features to be built into the mold. Not every mold can be used for every type of molding material. A mold that is ideal for one type of plastic may not be suitable for another, or at least not perform as well. For example, air ejection could be used for a product made from PP, but the same product in PS would require a stripper ring ejection. A different plastic from what was originally intended to use may also present difficulties in filling and packing. The addition of fillers to the plastic can increase the strength of the product but will also affect the shrinkage factors, usually reducing them. This means that the size of the product will be affected when molding a material different from the one for which the mold was built. Fillers can be abrasive and wear parts of the mold sooner than unfilled plastics. This can affect the mold design and construction, therefore mold parts subject to erosion should be easily replaceable when worn. It may also require the use of better wearing steels. This is particularly important for gates in hot runner molds and all other areas where the hot, abrasive plastic flows at high speed over the molding surface, but this kind abrasive action can also be the case in molds with cold runner gates. These provisions will increase the cost of the mold.
Typical wear points usually can be foreseen and the affected mold parts made easily accessible for servicing and replacement
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2 The Plastic Product
2.9.1 Gate Location to Increase Product Strength Gate location is important for the ability to fill the mold. Some large cavities may require two or more gates to properly fill, which of course affects the cost of the hot runner system.
Figure 2.65 Typical elongated products, gated at or near the end for maximum strength
The gate location is also important because it determines the direction in which the plastic molecules will be oriented within the molded product to a large extent. In general, the strength of plastic in the direction of flow is greater than in the direction across the flow. Near the gate, when the plastic flows in all directions, the strength is relatively low and can be compared with the low strength at a weld line. Therefore, if maximum longitudinal strength is required in an elongated product, gating near the end of the product is very important (see Fig. 2.65). Thin-walled products are especially vulnerable to this difference in strength, because the thinner plastic cools faster and does not have a chance to re-align the molecules randomly for better strength in all directions, as is the case with heavier-wall products where the plastic stays hot longer between the cooled outer layers and gives the inside core of the plastic time to re-align. It is important to recognize the importance of the gate location. It may be easier (and cheaper) to locate the gates so as to employ the shortest runner length (cold or hot runner), but this could result in lower strength of the product, particularly with elongated products, which are subjected to bending forces in use. There are also other issues that affect the gate location.
Figure 2.66 Gate protection rings are used to increase a pail’s impact resistance
The gate needs to be hidden or recessed, so that any protruding gate mark cannot hurt (scratch) anyone using the product (this is very important in medical products)
A protruding gate mark could also be in the way of a paper label, which will be applied there.
Some protruding gate marks are protected with gate rings for strength, typically at the bottom of large containers (see Fig. 2.66).
2.10 Special Features A big advantage of molding products from plastic is that different parts of a product can be consolidated into one molding and that many features can be molded in. Whether this is done for better appearance or for better functioning and lower assembly costs of the final product, this makes it sometimes more difficult for the design of the cavities and cores (see Fig. 2.68). In addition, some designers, when redesigning a product which was originally made from metals, do not appreciate or understand the intricacies and problems of mold design and molding, especially if they are used to designing for metals or other materials. It is therefore always suggested to scrutinize the drawing for the proposed product to see if its design is appropriate for plastics.
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2.10 Special Features
2.10.1 Holes and Counter Bores for Assembly Screws or Rivets In general, it makes for more pleasant appearance if mounting screws, used for example to fasten a cover to a base, are recessed, i.e., the screw head will be below the surface of the cover. If the counter-bore for the screw head is deep and its diameter is small, the core pin creating this counter-bore becomes very difficult to cool efficiently and a redesign may be necessary in this area (see [1] Section 5.2.4 for suggestions in this area). Plastics lend themselves well for the use of self-tapping screws. The design (dimensions) of screw holes must be to the specifications provided by the manufacturer of such screws and the screws must be selected to suit the plastic material with which they are used. The holes for the (usually small size) screws are molded with very small core pins; these pins are very difficult to cool or cannot be cooled at all. Because the hubs or walls where the screw holes are located are much heavier than the adjoining product walls, the cooling time will be longer than expected for the rest of the product. This problem may result in grossly underestimating the molding cycle time and lead to a mold with less productivity than expected. Many molds have been built with excellent cooling in areas for which it was easy to provide cooling. However, because there was no effective cooling at all near critical points, such as some small core pins, they were a waste of effort and money. In other words, despite the expensive (and unnecessary) good cooling for most areas, such molds ended up having a very slow molding cycle because of poor cooling in one area.
Concentrate on the mold parts that are difficult to cool. They determine the productivity of the mold
2.10.2 Hinges and Snaps for Assembly Snapping two molded pieces together (or snapping one molded piece over a metal piece) is an ideal application for plastics products.
Never locate a gate in a position that will cause a weld line on a hinge
Typical examples are containers for dairy products using a PE snap-on lid over a PP container, a PE lid on metal can (coffee can), cell phone assemblies, automotive applications, DVD cases, and many more. Some of these snaps have a long, successful history and the dimensioning is well documented. However, the designer is cautioned that such “snaps” depend on the amount of shrinkage experienced (a) in the mold during cooling and (b) after ejection, while cooling outside the machine. Also, from experience, a mold can produce perfect (as expected) products in one machine, but not in another, which could result in wrong snapping (holding) forces. More modern machines can use higher injection pressures and speeds and can therefore also cycle faster; in doing so, the shrinkage values will change. For example, when receiving a repeat order for a mold (e.g., a lid), it will be correct to assume that the steel dimensions of cavities and cores for the new mold will be the same provided that the mold will run on the same or a similar machine. However, if a more recent or better injection-molding machine will be used, it will most likely be necessary to establish new core and cavity dimensions.
Figure 2.67 DVD cases with hinges
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2 The Plastic Product This can be costly and must be considered in estimating the mold cost. In the long run, faster cycles can save a lot of money. Figure 2.68 shows several samples of small and large over-caps for containers with integral, hinged snap-on lids that snap close over the opening in the over-cap. Some of these over-caps are produced with simple “up and down” molds (A), some need intricate side cores (B) and much more complicated molds. Also shown is a very large box (C) with snap-on lid, with an integral, molded hinge connecting the two parts of the product.
A
B
C
Figure 2.68 Examples of over-caps for containers
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3
Cost Factors Affecting Productivity
A mold’s productivity is ultimately measured by how many good parts it can produce over time. A mold that runs very fast but is frequently down for maintenance or repair will not produce at lowest part cost and highest productivity. Conversely, a mold that runs too slow but produces parts consistently over time is not optimal. The same can be said for each component in the injection molding system. One way to quantify productivity is to measure the total equipment productivity (TEP): TEP =
Production hours (auto cycling) Parts made Scrap parts × × Available hours Hour Parts produced
Figure 3.1 Total equipment productivity (TEP)
A good custom molder can achieve TEP’s greater that 80% and good dedicated systems achieve values higher than 90%.
3.1
Where Will the Mold Be Operated?
3.1.1 Condition of Ambient (Shop) Air We tend to assume that the mold will be operated under “ideal” conditions, but this is typically not the case. The environment in the molding shop can vary from very cold to very hot, from dry to very humid, from clean to dusty and dirty. With sudden changes in any of these conditions, a molding operation can be affected significantly. High humidity will affect the mold itself (corrosion) and will affect the cycle time (productivity) of the mold. Rapid temperature changes may even affect the operation of a machine and mold and lead to breakdowns and loss of production. A typical example (A): a molding shop operated eight identical machines in each of two parallel rows; all were molding the same or very similar products with the same type of mold. They all worked fine, except the last machine in one row, which stopped frequently, without apparent reason. After checking for machine problems, such as possible power fluctuations, poor cooling water supply, etc. it was noted that this last machine was close to an emergency exit door, which was supposed to be closed all the time; however, on some days, the workers kept the door jammed open to improve the shop ventilation. The draft from the entering cooler outside air was enough to affect to operation. After ensuring that the door stayed closed at all times, there were no more problems reported with this machine.
Figure 3.2 Plan of molding plant in Example A
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3 Cost Factors Affecting Productivity
Another example: a mold and machine worked perfectly, but occasionally, for several hours, produced pieces with surface blemishes that looked like blisters. Investigation showed that it happened only on very humid days. This particular operation required a rather long mold open cycle. On humid days, the water in the air condensed in tiny droplets on the cold mold cores during the few seconds the mold was open and the cavities and cores were exposed to the shop air; the droplets appeared as blisters on the surface of the product. After slightly increasing the cooling water temperature to bring it above the dew point the problem disappeared. The “penalty” was a slightly longer cycle time, but it ensured continuous production of quality products. Corrosion Prevention It is important to decide how the mold will be protected from corrosion if it is evident that the mold is operated and stored in a humid environment. This can affect the mold cost. A common approach in many shops is to protect the molding surfaces before the mold is put into storage by using silicon spray (“Mold Saver”) or to just apply plain, clean machine oil. Many shops paint the outside of the mold shoe with a permanent oil paint to protect the outside of the mold against corrosion. Mold shoe material options: Pre-hardened plate steel, painted Plate steel with ENP Stainless steel
Another approach is to flash chrome plate the stack parts or to make them from stainless steels; both methods will of course add to the mold cost. For the mold shoe (the mold plates) itself, instead of using oil paint, it can be protected against corrosion with electro-less nickel plating (ENP), which has the additional advantage that it also protects some of the inner surfaces of the mold shoe, which would not normally be covered when the mold is just painted on the outside. ENP also enters the cooling channels to some extent and protects them against corrosion caused by the coolant, but the penetration is limited and does not cover the walls of the channels deep inside the plates. ENP is hard (70Rc) but thin and not resistant to scratches and wear.
Always consider the total costs when comparing mold material costs
The best method may be to make the entire mold shoe from stainless steel (SS). The basic cost of SS is higher than the cost of mild steels or pre-hardened machinery steels. However, when SS is bought in large quantities, the cost difference can be much less. When molds are expected to run for a long time, the advantage of SS over other steels can justify the higher cost. We must not forget that chrome plating or ENP also cost money. We must also consider the costs of transportation to and from the nickel or chrome plater, the additional time required for these operations, the lack of control over the transport, and the dependence on an outside supplier. Another problem with chrome plating is that any change (requiring remachining) of a chromed surface requires that the chrome must first be removed from the steel part. This requires shipping the part to the plater for removing the coating by a process similar to plating. After re-machining, the changed part must again be shipped to be plated. This is an expensive and time-consuming procedure.
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3.2 Coolant Supply Note that corrosive plastics such as rigid PVC always require chrome plating or, better yet, SS for the stack parts. The use of full-hardened (or pre-hardened) SS for cavities, cores, and inserts is quite common today, even though the steel cost is higher. When considering the expenses and risks with chrome plating of mold steels and the time saved, the total cost could be more than using SS. Another solution for all these issues is to provide the molding plant and the mold storage facilities with air conditioning or at least with controlled, low humidity air. Some modern molding plants have this equipment, although this means added expense and may not be needed or cannot be justified economically unless in cases where delicate products are mass-produced. Occasionally it can be useful to surround the machine with a shroud to keep the environment immediately around the mold and machine at a desired low humidity with a portable dehumidifier.
3.2
Coolant Supply
The available cooling water supply (quantity, quality, and pressure of the coolant) must also be considered. Also, remember, for water-cooling to be effective, the water must flow fast enough to establish turbulent flow. Turbulent flow removes significantly more heat per liter (or gallon) and can be calculated (see [5], Chapter 13).
3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? There is no point to design a mold with an expensive, elaborate cooling system if there is not enough coolant flow and pressure available to take full advantage of it. I have seen some mold plants that developed from only a few to a high number of machines, but neglected to increase the cooling water supply to grow with the rest of the operation. This resulted in the molds running much slower than they could if the cooling water supply had been sufficient. Good cooling of a mold depends not only the coolant temperature but also on the volume of coolant that flows through the mold, measured in liters or gallons per minute. This volume depends essentially on the pressure differential between IN and OUT of the cooling channels in the mold and on the method of distribution through the mold (see [5], Chapter 13).
3.2.2 Is the Cooling Water Clean? Cooling water must be clean, i.e., free from contaminants and/or oxidizers, which corrode the inside of the cooling channels. This is where stainless steel
Table 3.1 Calculating Chiller Requirements
Resin
Chiller lb/h/ton
HDPE
30
LDPE
35
PMMA
35
PP
35
PA
40
PPE
40
ABS
50
PS
50
Acetal
50
Tons required = Resin lb/h/ton × lb/h of resin consumed
For highest productivity ensure that the cooling channels in the mold are free from sediments (lime, rust, etc.)
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3 Cost Factors Affecting Productivity is of great advantage. The coolant must also be free from lime and dirt, which will gradually settle in corners of the cooling system and plug the cooling channels, especially if the channels are small and in elaborate circuits, as is often required in high production molds to cool small mold parts. Under such bad conditions, a mold will probably run satisfactorily and produce as planned for the first few months, but because of buildup of dirt in the cooling channels, the mold will gradually lose its cooling efficiency and run slower than it could with good, clean coolant. Dirt in the water will also require more mold maintenance, as the channels will have to be cleaned from time to time. Such mostly unnecessary costs are often overlooked while worrying about the high initial mold cost.
Figure 3.3 Rusted mold components
Rust is an insulator and will eventually slow the molding cycle as it builds up.
3.3
Power Supply
Electric power supply is not always as stable as required, especially outside the larger industrial areas of North America and Europe. In many parts of the world, especially in developing countries, there are often considerable voltage fluctuations because of weak and overloaded power lines; molders experience occasional, and sometimes even daily, “brownouts” (periods of lower voltage) and are often plagued with complete power failures (blackouts) lasting anywhere from just minutes to many hours. To say the least, these stoppages are annoying, but they can also be very expensive if a mold stops frequently just because of failure of the machine controls. Voltage fluctuations affect molding operations for two main reasons.
A drop of 10% in voltage will reduce heat output by 20% if not thermostatically controlled
Logic controls are sensitive to voltage fluctuations and may require voltage stabilizers. Although this is a machine requirement, it needs to be pointed out. Every time the machine stops, the mold also stops producing. In general, electronics are quite sensitive to high ambient temperature.
Melt temperature. Virtually all heaters in molds and molding machines today are electric resistance heaters. The heat output of a resistance heater is proportional to the square of the voltage applied. A drop of just 10% in voltage will reduce the heat output by 20%. While the barrel heaters of the extruder are always thermostatically controlled, a transformer, without feedback, often controls the machine nozzle heaters. With heat controls, any reduction in voltage (and temperature) will be automatically compensated by having the heaters ON for longer time periods. In hot runner molds, the hot runner manifold heaters are always equipped with thermocouples; however, because of the high initial costs (in the mold, and for the associated external controls required) many molds do not have heat controls on the nozzle tip heaters and can therefore experience
3.4 Will the Mold Run in a Variety of Machines? major temperature variations as the voltage varies. This will lead to trouble in the mold’s performance. Even so, today, about 80% of the highproduction hot runner molds are equipped with thermostatically controlled nozzles as the added costs can be easily justified with the increased productivity. Cold runner molds: With such molds, power interruptions, while annoying, are not serious. If an interruption is only of short duration – in the order of a few minutes – the plastic in the injection unit is probably still hot enough so that production can resume immediately, without causing problems. If the interruption takes longer, it will take again the time necessary to heat up the injection unit before resuming production after purging. Hot runner molds: With these molds, power interruptions can be more serious. Short interruptions of a minute or two can be tolerated without problems, but any longer stoppage will cause the plastic in the manifold and the hot runner nozzle
to degrade, especially heat-sensitive plastics in the still hot manifold, and
it will freeze sooner, because the masses of the manifold are much smaller than the masses of the extruder. It takes time to heat up the whole system to operating temperatures, and the plastic both within the injection unit and in the hot runner system must be first purged before resuming operation.
Note that well designed and built hot runner systems require less time for restarting than poorly designed systems. A good hot runner system should be ready for resuming production in about 10–15 minutes after any interruption. These details are important to understand before deciding on the kind of runner system to select for the mold. A hot runner system may be more suitable than a cold runner mold for a certain application, but may cause endless grief if the power supply is poor. All the well-known and proven advantages of a hot runner system can be lost because of the frequent stoppages due to power supply problems.
3.4
Will the Mold Run in a Variety of Machines or a Single Machine?
The mold will often be required to operate in different models of molding machines. This may result in quite some complications in the mold layout and will certainly increase the mold cost. In particular, different locations of the machine ejectors can affect the ejection and the cooling layout of the mold and the overall size of the mold.
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3 Cost Factors Affecting Productivity
The mold must be equipped with all features that are compatible with these various (existing or planned future) machines. This applies to several areas of the mold:
Shut height
Any downstream automation
Mold mounting (including any systems for quick mold changing)
Locating ring size
Sprue bushing size and shape
Machine ejector pin locations
Cooling- and air-circuits
Hydraulic functions
Electrical connectors
If a mold is to be designed for one machine only, in one location only, it can result in a simpler mold. For example, there would be no need to provide for various sizes of locating rings and the ejector mechanism and the mold mounting provisions could be designed for the pattern of the selected machine only.
3.5
Is the Mold Planned to Run in a Newly Created Operation?
It is a very desirable condition for the mold designer when a mold (or a series of molds) are planned to be operated in a new factory (or in a separate section of an existing factory), because it creates an opportunity for close cooperation of the mold designer with the planning of the whole project. It provides an opportunity to participate in the selection of the most suitable machine for the product to be made, but also to take part in the plant layout, power distribution, cooling water system, and so forth. This is also a good time to introduce standardization of many of the mold elements and mold sizes, of mold mountings (including quick mold changes), power and cooling connections, and any other feature that will affect not only the mold(s) now under consideration, but also future molds for this location. Standardization of mold components, molding machines, and ancillary equipment will not be further discussed here, but they are an important field where savings both in investment (costs of equipment) and increase in productivity can be made.
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3.6 Projected Requirements
3.6
Projected Requirements
How many pieces of the product will be made from the planned mold? This could be the most important question to ask before deciding on the type of mold required for any job. But this is also often the most difficult question to answer, particularly if the product is new on the market. It is nearly impossible to foresee if the product will find the hoped-for acceptance and increase in sales, or if the product will not be accepted as expected. Also, assuming a total quantity is known, what is the time frame when these quantities are required?
Projecting the number of molded pieces is often the most important and difficult question to answer
If 1,000,000 pieces of a new product are to be molded, the question is:
Is this a limited production run, say within four months (usually as soon as possible) or
Is this quantity needed every year, for a unspecified number of years, or
Is this quantity needed over the expected life of the product, e.g., 5 years, in which case the annual requirement is only 200,000 pieces.
3.6.1 Making Prototype or Experimental Molds 3.6.1.1 Prototype Molds Prototype molds are required to make samples of a new product for evaluation of a newly developed shape, to see how the product appeals to the eye and/or to the touch. Molded samples can be subjected to the expected stresses and wear and the results are better than testing a hand made (machined, or assembled) model. The result also could be more accurate (and possibly cheaper) than a computer simulation. Because it is only important to mold the overall shape of the product, without worrying about productivity of the mold, shortcuts can be taken everywhere: mold materials such as mild steel, aluminum, even plastics (epoxy, etc.) can be selected, as long as they are sufficiently strong and resistant to the heat and the pressure of the injected plastic. Working to close tolerances is usually not necessary. Generally, there will be no need to worry about surface appearance (polish, engraving, even flashing). There is no need for cooling channels; it will take just a little longer to cool the plastic before being able to remove the molded sample from the mold. In many cases there is also no need for an ejector mechanism. An air jet directed against the edge of the product at the parting line, or a few simple ejector pins that can be manually pushed to eject, may be all that is required. Other features of the product, such as internal or external threads, can be produced by using loose inserts in the mold that can be ejected with the product and then unscrewed by hand. Loose inserts can also be used for odd shapes in the sides of the product, which would otherwise require side cores. Round holes or simple openings in the sidewalls could be machined after the molded piece is cold. These are just some of the mold features that can
Figure 3.4 Typical prototype mold for a lid, capable of 4 in to 8 in lid prototyping (Courtesy: Husky)
Figure 3.5 Single-cavity prototype mold for production 2×2 system. In this case, the prototype stack was used as a spare in the production mold (Courtesy: Husky)
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3 Cost Factors Affecting Productivity be omitted to simplify the stack and to reduce the cost of the prototype mold. If prototypes are frequently required, the stacks could be mounted in a common mold shoe, thus saving even more costs. The runner system would normally consist of a simple sprue gate directly into the product or a sprue and short runner could be used for edge gating. The gate will then be cut manually.
3.6.1.2 Experimental Mold
Figure 3.6 Typical 4-cavity experimental mold that will emulate the behavior of the production mold (48 up to 144 cavities) (Courtesy: Husky)
This type of mold is different from the prototype mold: it will be used mostly to establish the behavior of the plastic in a newly developed product during injection. Some of the above cited shortcuts to save costs can be used, but in general, the mold would be closer to a simple, single-cavity production mold. The gate should be located as planned for the production mold. The mold could also be used to establish the most suitable location of the gate and the method of gating for the product. Such a mold would normally require the proper finished appearance of the product. Note that especially in thin walled products, the finish affects the flow of plastic through the cavity space. Cooling efficiency is not as important as in a production mold, but some cooling should be provided to maintain a stable mold temperature. Because the quality of a molded piece depends very much on the accurate repetitiveness of cycle time, an ejector mechanism should be provided rather than manual product removal to eliminate any operator-created variations in ejection (and cycle) time. An important feature of an experimental mold is often the facility with which some stack parts can be changed. This adds costs but will make experimentation easier. Experiments with such molds can also determine the effect on molding cycles when areas of the mold are not cooled, little cooled, or well cooled. Such information can be valuable before an expensive, multi-cavity production mold is designed. The difference between “ordinary” and “exceptional” cooling could mean much in engineering the production mold. Reduction in cycle time achieved by exceptional cooling could be insignificant and not worth the additional costs and complications to the mold.
3.6.1.3 Combination of Prototype and Experimental Mold This applies when an inexpensive mold is required to establish the shape of the product, but at the same time it is planned to explore market acceptance of such product by manufacturing a few hundred or even thousand of samples for field testing. Typically, such molds should run “fully automatic,” but there is no need to achieve maximum efficiency in molding, as with better cooling, better runner system, etc., and without special finish or most engravings. Such molds can also be used to establish shrinkage conditions. I remember a case where a client wanted a very simple prototype mold to see how a newly designed LDPE cover would fit as a shield over a metal product he had been selling for years. The prototype mold was supposed
57
3.6 Projected Requirements
to produce about 100 samples. We made a mold with a very simple cavity and core, all mild steel, with a few ejectors and a simple through-shooting gate right into the center of the product; some cooling channels, no polish, no engraving. There was hardly a simpler mold possible. The client promised that if the new idea was accepted in the field, he would buy a production mold. After a few months, I called to ask him how the idea took on, and he told me that the mold has already produced several thousand pieces and was still in perfect condition, and that he wont need another mold. A properly designed production mold would surely run faster – i.e. produce more pieces per hour – but with really small quantities this is not worth the extra cost.
3.6.2 Production Molds Production molds are any type of molds other than prototype and experimental molds. At this point in the planning for a new mold it becomes necessary to have basic information on
How many pieces will be required?
What will be the molding cycle?
Once these data are available, there should be not much difficulty to proceed, but both these data are usually difficult to ascertain. Since the mold type and number of cavities will depend primarily on the quantities required to be molded, we must first differentiate between the various possibilities as they present themselves, before deciding on the kind of mold that will be most appropriate.
3.6.2.1 New Products The new, untried product is a common case and can be part of a new “invention” or an existing product previously made from a different material. Will the market accept it as is in its new shape, made from injection-molded plastics? Will it require modifications after complaints or suggestions from the field after it was launched on the market? Or will it be a disappointment for the seller, and soon disappear? Unfortunately, the “entrepreneur” takes all the risk when investing in the required mold. Of course it would be convenient to keep mold cost as low as possible, but we know that this may increase the cost of the products in the long run. The cost of a high cavitations mold may also affect the timing of the launching of the product. Should a large production be anticipated, which will require a multi-cavity mold of high quality? In this case, if the product is not accepted in the field, the loss could be substantial. But there is also another, just as serious problem when launching a new product: the investor was overly cautious and is waiting for
Conclusion: There is no clear answer to the above questions. It may depend on the expected life of the product, which is often just as difficult to estimate. Some products are seasonal and the demand finds an early saturation point. Some products increase in demand until some competitive, similar or even better product comes along, in which case demand for the original product could sooner or later disappear. One possible advice is to build a mold for the initially estimated volume, and add 25% for surge demands, unanticipated stoppage, and some growth
58
3 Cost Factors Affecting Productivity the acceptance in the field. If the product is a great success, the first mold was probably not designed for the unanticipated, high demand. What would be the best strategy at this point? Make another mold (or even several molds) similar to the first one and run them side-by-side? This approach may have the advantage of lower additional investment while providing more flexibility. It is easier to find several smaller machines than larger machines. However, a larger system, using a high-production mold, with more cavities, better runner systems, better cooling, better ejection, more automation, and therefore higher up time, will result in the lowest cost of the products.
3.6.2.2 Existing Product, Large Quantities
Figure 3.7 High volume production mold for a stadium cup (2×12 cavities, air eject, modular construction) (Courtesy: Husky)
Some products are “timeless”, meaning that their annual quantities are more or less constant and known. Their use may vary within seasons and even with the economy in general, but they remain essentially unchanged. This applies to many technical articles, as well as to many packaging products, such as food containers and to medical products. In these cases, it is not difficult to establish annual requirements and a projection for how long the product will be in demand. In addition, it is always important to consider the whole system, i.e., machine, mold, and any after-molding operation (automation, product handling, packaging, assembling, etc.,) that will yield the lowest-cost product. With long and high production runs, even high mold cost is insignificant per unit produced and helps lower the product cost, provided it runs faster, longer, and with higher quality products.
3.6.2.3 Limited Quantities
Conclusion: Investing in the best possible mold is usually the key to a successful operation
Sometimes, a product is required in a limited quantity or for a one-time occasion only. This may be the case where a molded piece is designed for a special occasion or application. The quantities are relatively small but usually known. Frequently, a molded piece will be required as a promotional item, such as giveaway items to retail customers. Such promotions are usually limited in time and the requirements are stipulated at the beginning of an advertising campaign. Usually, such promotion needs fast delivery of the molded pieces, and the total amount of pieces in a very short time span. A decision will have to be made: Should the order be produced on a large, multi-cavity mold? This will yield the best piece cost but will require a larger machine, which may not always be available at the time the mold is ready for production. The mold cost will be higher but the cost per molded piece is probably insignificant. The problem is that such larger molds will take longer to build and there may not be enough time. Also, it leaves the molder vulnerable, in case of machine or mold breakdowns, in which case there could be no production at all. As an alternative, several smaller, identical molds could be built which are simpler and can be made faster by contracting out to more than one mold maker if necessary. These (smaller) molds can be built faster and then be run on smaller machines, which are also usually easier to locate; if necessary,
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3.7 Forecasting the Cycle Time even at different custom molders. This will probably increase the piece cost, (a) because the smaller and less expensive molds are not as productive as a larger mold, and (b) because of the added cost of dealing with more than one source. However, this approach will also ensure that any breakdown will be less serious to the customer. In all these cases we assume that the molds are complete, self-contained molds.
Conclusion: Go small and use several sources
3.6.2.4 Short Runs, Small Production For short runs or for very small total production, when products are relatively simple and small, and when it is known that the annual requirements are also small, there are two alternatives:
Individual molds, with the least amount of “high productivity” features (especially good cooling, hot runners, etc.) or
Making inserts for so-called “universal mold shoes” which are listed in most of the mold supply house catalogues.
Such mold inserts for universal mold shoes usually do not cost much more than the mold stack for a regular mold. They can be mounted in the “shoe” and run by itself, in pairs, or in multiples, or even in combination with inserts for another product. Mold changing is usually simple and fast.
Figure 3.8 Standard mold base for a stripped closure application (Courtesy: Husky)
There is also the alternative of using a regular mold shoe, such as would be used for a conventional (”designated”) mold, and solely changing the stack or the inserts. This is quite practical if the molding shop personnel are familiar with mold work or where a mold shop is connected with the molder, otherwise there is always the danger of damage to delicate stack parts while changing. Larger products, which will not fit a standard universal mold shoe, or which are too large to fit into a common mold shoe, will have to be built as designated molds, but can be using any shortcuts available, as mentioned above in Section 3.6.1.1, to keep the mold cost as low as possible while still getting good quality products.
Conclusion: Use standard mold shoes if possible, or use simple, dedicated molds
3.7
Forecasting the Cycle Time
After we are clear on the question of how many of the products will be required, our next step is to arrive at an estimate of how fast the piece can be molded. Some molders like to indicate the number of seconds (cycle time) to mold a piece (or shot); others prefer the number of shots per hour or pieces produced per hour. Either method is suitable. The cycle time of any molding operation depends on a number of parameters, which will be discussed in detail in the following sections. It is important to understand these dependencies before settling on a reasonable figure for the cycle time.
3,600 s (1 hour) divided by the cycle time (in seconds) equals the number of shots per hour. Shots per hour times the number of cavities equals the number of pieces produced per hour
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3 Cost Factors Affecting Productivity
3.7.1 Type of Plastic Molded There are several issues to consider:
Figure 3.9 Resin to be molded can significantly affect the cycle time (such as this PS)
Melt temperature required to be able to inject and to fill the cavity space. Higher melt temperatures require longer cooling times before the pieces can be ejected.
Thermal conductivity of the plastic. With lower heat conductivity it takes longer for the heat within the melt to travel to the cooled mold walls than with higher conductivity. However, with very thin-walled products, without heavy sections, the difference in conductivity can be insignificant because of the very short distance the heat travels to the cooled walls.
Injection speed, especially through the gate(s). Some plastics are sensitive to shear stresses caused by high injection speeds (especially in the gate area) and exhibit degradation of the plastic flowing into the cavity space. In this case, lower injection speed (and pressure) will be required, which will affect the molding cycle. Some plastics, especially the largest groups (“commodity plastics” such as PS, PP, and PE) used for many products, are little affected by shear stresses, but many heat sensitive plastics can be damaged (degraded) by high injection speeds.
Crystallinity of the plastic. Crystalline plastics such as PE and PP require more heat input than amorphous plastics such as PS to reach the required melt temperature. The resulting higher heat content in the plastic will then require more time to cool before ejection is possible. Occasionally, a mold may be planned to produce both types of plastics: Not only will the shrinkage be different but also the molding cycle.
Fillers. Filling of the plastic with inert materials (fibers, talcum, etc.) can also adversely affect the cycle time. In addition, the shrinkage will be smaller than when using the same but unfilled plastic. We must also consider that many fillers tend to erode the mold materials, especially the gate; when selecting the materials for the gates and the mold in general, this must be taken into account because of its effect on the mold cost. For example, a high strength, wear resistant steel for a gate insert is good to resist abrasion but because it is also a poor heat conductor, the gate area is more difficult to cool. This will result in a slower molding cycle. This is especially important with fast cycling molds.
3.7.2 Wall Thickness of Product The wall thickness plays a significant role in the cooling process and thus influences the cycle time. Ideally, the walls should be uniform throughout the whole surface of the product; however, this is rarely achievable or practical, except in some lids (covers), some containers, and some flat products. Most products have thicker sections in design features such as hubs for fasteners, but even with otherwise uniform walls, there will be heavier sections at the
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3.7 Forecasting the Cycle Time junctions of walls and at the base of ribs. It is not the uniform wall thickness that governs the cycle time but these heavier sections. This becomes even more complicated, because these heavier sections are usually more remote from the areas that are easily provided with good cooling and therefore depend on longer paths for the heat to travel to the cooling channels. Better heat conducting materials are occasionally used to help to remove the heat faster from these “hot spots”. There are charts, nomograms, and computer programs prepared to relate the wall thickness to the cooling time; they are mostly based on the simplest but often unlikely cases, namely perfectly uniform wall thickness throughout, and equally well cooled surfaces throughout the mold (see Fig. 3.10).
Figure 3.10 Schematic relationship between material, uniform wall thickness, and cooling time. These graphs are only shown to demonstrate that there is a definite advantage in designing with thinner walls and with uniform thickness
3.7.3 Mold Materials The selection of mold materials for the stack parts also has an effect on the molding cycle. There is some, but relatively little, difference in (the rather poor) heat conductivity between the various hardened alloy steels (“mold steels”) commonly used. The conductivity of pre-hardened machinery steels that are often used for larger stack parts is somewhat better but still poor. So-called “mild” steels have a still better conductivity, but are rarely used for stack parts because of their low physical strength, poor polishing quality, and the frequent dirt enclosures. Metals with much higher conductivity, such as aluminum and copper, are not used because of their softness; certain aluminum alloys are easily machinable and relatively inexpensive and are used in blow molds where pressures are much lower and occasionally in injection molds in areas of low stresses, and even in prototype molds.
Figure 3.11 This 2-cavity lid mold uses BeCu inserts (copper color) on the cavity ring and the gate inserts to significantly cut cooling time (Courtesy: Topgrade Molds)
Beryllium copper (BeCu) alloys have aheat conductivity about four to seven times better than steel. Their use in mold stacks (usually as inserts in cavities and cores) is often of advantage in areas that require the highest heat removal rate possible. In fast running molds, the difference between steel and BeCu can be a few seconds, or even just a fraction of a second. It may not appear to be much, but a saving of 0.5 s can translate into a large increase in production. For example, a mold with steel cavities runs at a 4 s cycle, it will produce 3600 ÷ 4 = 900 shots per hour. By using BeCu for the cavities it may run at a 3.5 s cycle; 3600 ÷ 3.5 = 1,028 shots per hour, an increase of 14% in productivity! The reason that BeCu is not used more in molds is that it is much more expensive than steel and not as strong. Because BeCu is softer than hardened steels, it is usually inserted in steel (cavities or cores) and it should never be used on the parting line or on alignments or shut-off tapers. With larger pieces, there is the danger of porosity. Smaller parts can usually be machined from forged or drawn rods and bars, but for larger parts, pressure casting of the blanks for mold parts is required.
Note: BeCu requires special precautions in the machining operations because of hazardous gases created when working with machine tools
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3 Cost Factors Affecting Productivity
3.7.4 Efficiency of Cooling The purpose of mold cooling is to remove, in the shortest possible time, the heat energy that entered into the cavity space during the injection of the hot plastic melt. The higher the efficiency of removing the heat, the higher the productivity of the mold. Molds for Small Production (Fewer than Approx. 1,000 Pieces) With any injection mold, over time, the heat will dissipate through the mold into the surrounding air and into the machine platens. This could be considered as sufficient in cases where only a few pieces are required and where the cycle time is not important. In this case, there would be no need to provide any mold cooling at all. Molds for Large Production of Thin-Walled Products
Figure 3.12 Mold cooling schematic
On the other end of the scale for cooling efficiency is the cooling for a mold for fast running, thin-walled products. The mass of each product may be relatively small, but because the cycle times are short, large amounts of heat (in the hot plastic) per unit of time enter the mold, which must be well cooled to ensure that its temperature is kept stable and at an optimal (low) temperature. This will require the best possible cooling methods, which are more costly to design and to manufacture. These molds use the most suitable (and sometimes expensive) mold materials to facilitate the rapid removing of the heat. The higher costs incurred will usually be worthwhile, because they result in a mold with higher productivity and in lower costs per molded piece. Molds for Large Production of Heavy-Walled Products As the plastic cools during molding, it shrinks onto the core, away from the cavity walls. After losing contact with the cavity walls, even the best cavity cooling will not do much in removing heat from the product. But the relatively short time the plastic is in contact with the cold cavity walls is enough to create a rather thin, rigid but still warm surface, while the core cooling continues to remove heat from the inside of the plastic walls. While the rigid outer skin allows early opening of the mold and to pull the product (while still on the core) out of the cavity without risk of damage, it would not be possible to eject it at this time, because the outside of the product could be damaged despite the rigid skin. It is usually fairly easy to provide adequate cooling to the cavity but often not easy to cool the core, because
The volume of the core is usually much smaller than the volume of the cavity,
There may be ejector pins going through the cores, and
There are sometimes air channels in the core.
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3.7 Forecasting the Cycle Time The latter two conditions reduce the space to accommodate effective cooling lines. But there are a number of possibilities to increase the productivity:
The first method going back to the earliest manufacture of injection molded product is simple and still being used occasionally for very heavy, simple, and not particularly closely dimensioned products. The mold opens and the outwardly cool but inwardly still hot and soft (and therefore easily damaged) products drop into a container with cooled, circulating water, from which they are then removed and dried either by hand or a conveyor carries them through an air cooling tunnel for drying. This is a rather crude method, but can be quite efficient in some cases. For nylons and other materials that require high water content to reach their physical properties, this water immersion may even be an advantage. For other materials, water absorption could be damaging and therefore this method should not be used.
Another typical method is the handling of flat products such as trays, but also other – usually larger – shapes that tend to warp after being removed early from the mold. The still hot (and easily damaged) product is placed into a cooling fixture (some are simple, others quite elaborate), where it is held by weights, clamps, or in a mechanically locked frame or any other suitable method, until it is cool enough and keeps its proper (as molded) shape without warping. There could be a small number of such fixtures beside the machine where pieces are successively placed as they are ejected and then packed once they are cold. This method too is rather crude and labor-intensive, but can shave quite a few seconds from the cycle time. The alternative is to keep the mold closed until the molded product is cold enough and will not warp after ejection.
Hot products can be held in actively cooled fixtures (post-mold-cooling). Typical examples for post-mold cooling are the heavy walled preforms used for the manufacture of blown PET bottles. These products are required daily by the millions world wide, and every fraction of a second saved amounts to huge savings over the years. The biggest problem is the intense cooling and the long time required to cool the very thick walls which, in the next step of operation, will be reheated and blown up to the final bottle shape in special machines. The usual wall thickness is in the order of 3–4 mm (1/8–5/32 in.). Cooling must be very intense and efficient to prevent crystallization of the plastic as it cools, requiring very cold water with large flow. But while it is relatively easy to cool both the cavity and the core, the cycle could be still in the range of 30–35 s or even more. The problem of efficiently cooling the preforms has been, and still is, the subject of much research and many improvements are developed in this narrow field of injection molding. The simplest approach to increase productivity is to increase the number of cavities; while this obviously has improved the productivity, the main target of research is how to reduce the cooling time (note: from the early beginnings of these
Figure 3.13 A part is cooled very efficiently when dropped into cold water
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3 Cost Factors Affecting Productivity molds, the number of cavities has increased from 4 to 192 in many production molds). The key to reducing the cycle time is to increase the time of the heat transfer from the plastic to the cooling media (water or air), either in special cooling fixtures or by removing the products earlier from the cavities but leaving them longer on the cores, as explained below:
B
A
– The still very hot products can be ejected from the core into watercooled, tightly fitting sleeves, advanced by a robot into the molding area. The whole array of still hot preforms is then transported out of the molding area and a new injection cycle can begin. There are several (mostly patented) executions of this method and the cycle times have been reduced by 1/2, 2/3, or even better. This significant increase in productivity is not inexpensive, but the cost of the necessary equipment can be written off in a very short time by the savings achieved. Figure 3.14 shows a 48-cavity mold for PET preforms from the rear of the machine. It shows (right) the array of cores (A) on the moving platen. The side-entry robot (B) carries 3 × 48 cooling receptacles (C), their position timed so that every time after the machine ejects an array of still hot preforms into the cooling receptacles, the robot plate shifts so that by the next cycle, another (empty) set of cooling receptacles faces the cores. Before the third cycle of unloading the mold, the now cold preforms of the first unloading cycle are ejected. For every ejection, the plate swings 90° so that the now cold preforms drop onto a conveyor (D) for removal to a shipping crate. At a 12– 14 s cycle time, this system yields between 14,400 and 12,340 preforms per hour.
C
D
Figure 3.14 48-cavity mold for PET performs with cooling receptacles (Courtesy: Husky)
– A more recent development is the use of identical sets of cooled cores mounted on a rotary moving half of the machine, either in sets of 2 or of 4 arrays. This requires a special machine; however, it does not require the above-described robot. The plastic is injected and the mold opens as soon as the cool skin has formed on the outside of the preform and permits the products still on the cores to be pulled out of the cavities. As soon as the first array of cores is in the open position, the core carrier rotates and a new array of cold cores enters the cavity for the next shot. The preforms stay on the cores until they are ready to be safely ejected. A core side with two arrays of cores will rotate 180° every cycle and eject as it rotates, while the products are pointing downwards, before reaching the position where they are again in line with the cavity for re-closing the mold. A core side with four arrays will rotate 90° at every cycle, and will eject when the products are pointing downwards before the rotation that brings the empty cores into line with the cavity and closing the mold. These are very sophisticated systems, but they can be easily justified, especially because they require much less floor space than the systems using robots.
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3.7 Forecasting the Cycle Time Figure 3.15 shows a portion of the special indexing machine, with a 96-cavity mold for PET preforms. The mold consists of one array of 96 cavities, shown on the right (A). There are 4 arrays of 96 cores (B) on the indexing section of the machine. The machine is timed so that after the first injection, as soon as the preforms are cool enough that they can be pulled out of the cavities, the mold opens, the indexing clamp (C) rotates 90°, and the mold closes again for the next injection cycle. The injection repeats. When the second shot is ready to be removed from the cavity, the indexing repeats for another 90° turn. The first molded preforms arrive now in a position opposite the injection unit. The same step is repeated for the third injection cycle. After the clamp rotates another 90°, the first array of preforms (from the first injection), which has been on the core through more than 3 complete injection cycles, has arrived in the position facing downwards. By this time, the preforms are cold enough to be ejected safely onto a conveyor (D) located below the clamp. The now unloaded (empty) array is ready to enter the cavities again for the next cycle. At a 11 s cycle time, this system yields more than 31,400 preforms per hour. These indexing machines can be used for any very-heavy wall product, wherever large quantities are required, such as cosmetic products jars. For more details on the economics of PET preforms, see Section 3.4.10.
Drive pinion
Movin platen assembly
Tension wheel
Timing drive belt
B
C
A
D
Figure 3.15 A special indexing machine with a 96-cavity mold for PET preforms (Courtesy: Husky)
Index clamp unit (Operator side)
Shutter cylinder
Turret gear
Stationary platen
Tiebar nut Tension plate Runner block
Mold stroke cylinder (4)
Guide rail
Clamp base
Tiebar (4)
Figure 3.16 Schematic of the indexing clamp (Courtesy: Husky)
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3 Cost Factors Affecting Productivity Molds for Most Other Products Molds for most other products are equipped with a cooling system somewhere between these extremes. Often, in molds for intricate shapes requiring many stack inserts (usually technical products), the cavities and cores are almost impossible to cool close to the cavity space. Cooling can only be achieved by conducting the heat through the stack walls and the inserts to the cooled mold plates located immediately behind them and/or surrounding them. This results in slower heat removal than cooling the cavity or core walls directly, but is often the only way to keep the mold at a stable temperature. Similarly, in some products, which have unavoidable hot spots (thick sections), there is not much point in providing excellent cooling for the areas that could be easily cooled just because there is enough room for such cooling, unless it is possible to provide better cooling to these hot spots. The slowest cooling area of the mold always governs the molding cycle
The slowest cooling area of the mold always governs the molding cycle. It is amazing how many mold designers and mold makers overlook this point and then wonder why “despite the massive cooling provided” in the mold they cannot achieve a better cycle time. As already pointed out, especially in molds for containers or other cup-shaped products, some mold parts, such as cavity blocks, have ample space for cooling circuits, while the core, which really should have more cooling, does not provide enough space, because it is much smaller than the cavity block and has to accommodate the ejector mechanism as well, which is encroaching on the available space for cooling and air channels. Unfortunately, many mold designers don’t understand this and see the large available space in the cavity, provide more cooling than is necessary for the job in non-critical areas, and thereby waste money.
3.7.5 Venting Venting is another feature that can affect the molding cycle; it is important to evacuate the air in the mold in front of the inrushing plastic during injection. Venting is important for any mold to ensure that the plastic can enter freely into all areas of the cavity space and must be properly specified. Venting is especially important with fast running molds. Although some ribs and bosses may fill without venting, all points where the plastic finishes filling should be vented. Regarding vent sizes, it is best to consult the material suppliers for their recommendation
A typical example is an experiment with a 4-cavity mold for a small disposable container. With standard, yet ample venting, the mold ran at 17 shots per minute. It had continuous vent gaps along the rim; by simply providing more vent channels (8 instead of 4) to allow the air to escape easier into the open, the production could be increased to 20 shots per minute, an increase in productivity of more than 17%! Figure 3.17 shows a 4-cavity mold for a 4 lb (PP) margarine tub.
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3.7 Forecasting the Cycle Time
B
A
D
C
E
D
Figure 3.17 4-cavity mold for a 4 lb (PP) margarine tub (Courtesy: Husky). The mold exhibits modular construction, floating cores, and cavity lock. Air ejection (absence of an ejector box) allows very short shut height of the mold. Air jets (A) (4 per cavity) and the 2 blow-down air jets (B) on top assist fast removal of products from the molding area. Note the intricate system of continuous vent gaps (C), venting grooves (D), and channels (E) to permit fast filing of the cavities. The productivity of the mold at 6.0 s cycles yields 2,400 tubs per hour.
3.7.6 Effect of Molding Machine on Cycle Time Several features of the molding machine affect the mold productivity and will be discussed in the following. It is important to be familiar with the machine for which the mold is to be built in order to arrive at a more accurate estimate of the probable cycle time. In case of similar or even identical products and molds, the cycle time can vary considerably when run on different make and size machines. Machine factors affecting the mold productivity are dry cycle, injection speed/pressure, tonnage, and recovery time. The molding cycle time is the dry cycle time plus the time required to inject and cool the molded piece(s) sufficiently for ejection, plus any added mold open (MO) time.
3.7.6.1 Dry Cycle The dry cycle is probably the most significant variable from machine to machine and the feature that can influence the cycle time more than other factors. Dry cycle is defined as the time (in seconds) it takes the moving platen to move over the length of the stroke, from the mold open (MO) position to close, clamp up, unclamp, and then return to the MO position. Obviously, the larger the masses (platen and moving mold half) to be moved, the more power will be required to accelerate, to move, and to decelerate them for a soft stopping in both the Mold Closed (MC) position and in the MO position. Therefore, smaller machines can have shorter dry cycles than larger machines, but also, better machines have shorter dry cycles than lower performance machines. Machines in the 500 kN to 10,000 kN (50–1,000 ton) range can have dry cycles from 1.5 s up to about 10 s. But there are also other, mainly older machines with dry cycles up to 20 s! It obvious that for large production
Molding cycle (s) = Dry cycle + Injection + Cooling + Ejection + Mold open
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3 Cost Factors Affecting Productivity and a mold with a short molding cycle, the length of the dry cycle is much more important than with a mold that requires a long molding cycle.
Figure 3.18 Schematic showing the closing motion of the clamp
Figure 3.18 depicts the approximate motion of the closing of the moving platen. It is typically an S-curve. The speed at the start of closing is rather slow; then, a fast speed is reached for much of the closing stroke, until the platen slows down again and finally creeps the last few millimeters before reaching the closed position, when the stretching of the tie bars begins as the clamping force is created. Similarly, but in reversed order, the mold is first “unclamped”; then starts opening slowly, accelerates and moves fast until it is slowed down again for a gentle stopping in the MO position. Note that the following illustrations (Fig. 3.19 and following) show the speed (velocity) of the moving platen as a straight line that really represents the average speed from start to stop (see Fig. 3.18). The opening speed is not necessarily the same as the closing speed. In most machines, segments of the speeds in either direction can be adjusted to best suit the molding conditions, but molds are often run at the maximum available speed.
Figure 3.19 Schematic showing a shorter and a longer dry cycle
Note also that the straight line representing the average speed terminates sooner than at the end of the tie bar stretch time, i.e., at the moment when the mold halves meet and the tie bars begin to stretch. This fact allows the injection to start earlier; as soon as the mold closes, just at the moment the mold halves “kiss off ” and the tie bars begin to stretch. Any significant forces inside the mold will commence only when the cavity spaces are almost completely filled. The filling takes usually longer than the time required for the final clamp-up. Starting the injection sooner, even by only a second or even a fraction of a second, will result in a significant gain of cycle time and productivity, especially with molds running at short cycles. The tie bars’ stretch provides the necessary clamping force (preload). This force must be greater than the force created by the injection pressure inside the cavity space, which tends to crack the mold open at the parting line. The difference between the time required for the shorter and the longer dry cycle is wasted time. For example, if the estimated combined injection and cooling time is 3 s and the dry cycle is 3.5 s; because MO = 0, the molding cycle is 6.5 s; this corresponds to 554 shots per hour. If the dry cycle were 5 s, the molding cycle would be 8 s, and the mold would yield only 450 shots per hour, a considerable loss of production. This is certainly enough to seriously consider the choice of a faster machine, especially when the expected production is high.
Mold Open time of zero (no mold open time) is an ideal condition
Production with zero mold open time (MO = 0) has been achieved on many smaller molds, even with short dry cycles (in the order of 2 s), but can also be achieved with molds equipped with automatic unloading equipment where the action of the “take-out”, i.e., the mechanism that reaches into the mold
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3.7 Forecasting the Cycle Time A B
C
D
to remove the products while the mold is opening and closing, is mechanically linked with the mold open and close motion. Figure 3.20 shows a 2 × 8-cavity stack mold for dairy tubs with modular construction, built for a machine equipped for stack molds. Air ejection, cam (A) operated swing arms (B) with suction cups (C) to pick the products from the cores and deliver them into the chutes (D) – shown in blue – on the side of the cavity plate. This mold operates with zero mold open time (MO = 0) and has a productivity at 4.5 s cycle of 12,800 tubs per hour. In the early 1990s, I observed several 8,000 kN machines producing large products requiring an injection and cooling cycle in the order of 10 s. The dry cycle of these (then new) machines was 18 s (!), resulting in a molding cycle of 28 s and yielding 128 shots/h. On a comparable size machine, but with a reasonable 6 s dry cycle, the total cycle would have been 16 s, or 225 shots/h, or an increase of 76% in production. At the time, I asked the factory mechanic who installed these machines, if there was any possibility to decrease the dry cycle time and was told that the machines were designed to run so slow to save on the expensive hydraulic and electrical components needed for higher speeds. The owner had bought these eight new machines mainly because their price was much lower than comparable size good machines. Did he really save money? After 3 years of running these machines, this molder got rid of them all by selling them as scrap iron. Nobody else wanted the machines even though they were in “good” running condition. This is probably an extreme case but it highlights how important it is to consider the dry cycle when buying a molding machine.
Figure 3.20 Modular 2 × 8-cavity stack mold for dairy tubs, built for a machine equipped for stack molds (Courtesy: Husky)
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3 Cost Factors Affecting Productivity 3.7.6.2 Required Shot Size of the Mold Shot size is an important characteristic of the molding machine, which affects the molding cycle. But first, what is the difference between “shot size” and “rated shot size”? “Rated shot size” is the amount (in grams or ounces) of polystyrene (PS) that the injection system (ram screw or two-stage) can inject at every cycle, and is indicated in all machine specifications. “Shot size” depends on the mass (W) of the molded product (in grams). There are several common methods to determine the mass:
Weigh a sample (or a handmade model). If the sample (or model) is of the same material as the desired product, this is the mass of the product. Otherwise, divide by the specific weight of the sample (model) and multiply with the specific weight of the desired plastic.
Establish the volume, by completely immersing the sample (or model) in a graduated container, filled partly with water. The difference in filling levels gives the volume.
Calculate the volume from the drawing dimensions (this can be very cumbersome and time consuming)
Multiplying the established volume with the specific weight of the desired plastic gives the estimated mass (weight) of the product (see Appendix for charts of average specific densities (weights) for various plastics). At this point it should be determined, which runner system would be most suitable for the planned mold. If a cold runner system is selected, the mass of the cold runner must be added to the shot size. And finally, the number (N) of cavities should be determined. All these calculations will have to be repeated several times, with different assumptions, before settling on the final selections for the planned mold. For 2- and 3-Plate Molds Only: Shot weight SW = (N · W) plus the mass of the cold runner The runner size (mass) is a very important. The mass of a cold runner R can be small and represent only a few percent of the shot weight. But it can also be quite large; in 3-plate molds, and in some 2-plate molds, R could be as much, or even more than N · W, particularly if the products are very small and a large network of runners is required.
Figure 3.21 Runner system for a 32-cavity 2-plate mold for caps
Figure 3.21 shows a runner system for a 32-cavity 2-plate mold for caps. The mass of the runner system is about 25% of the mass of all the caps per shot. The runner system is fed in the center by a hot sprue (not shown) to avoid an otherwise large, cold sprue. The cavities are tunnel-gated, and the products and runners are separated after ejection.
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3.7 Forecasting the Cycle Time Figure 3.22 (a) shows a runner for a 72-cavity 3-plate mold for a small cap. The runner takes almost as much plastic as the products themselves. The cycle time is controlled by the cooling time of the (clearly visible) heavy distribution runners. A hot runner system for such a mold would reduce the injected mass by half and the mold would cycle twice as fast, but would be more expensive to build in the first place. This is a typical example where both methods must be considered, in view of the total requirements of the product. Figure 3.22 (b) shows the runner and products from a very simple 4-cavity 2-plate mold for a simple but heavy product. Note the heavy runners and the cold sprue. This too is a candidate for hot runners (valve gated into the product) provided the quantities justify the greater expense.
In many cases, a hot runner makes excellent economic sense
For both products in Figure 3.22 (a) and (b), the amount of plastic in the runner system is large in relation to the mass of products molded (25% in one, almost 100% in the other case). What could we gain by using hot runners? The answer is mostly a matter of economics. We must consider the price of about $750 – $1,000 per hot runner drop, plus the price of the manifold at approximately $5,000 or more. On the other hand, the gain in cycle time and productivity by using hot runners can be considerable. A mold as shown in Figure 3.10 (b) could cycle about twice as fast with hot runners! Also, the cost of recycling the runners and the percentage of plastic lost during recycling must be considered. A smaller injection unit will be required with hot runners, because all of the injected plastic will be converted into products. There is also savings in energy, because less plastic material is processed for the same output. The mold sizes are about the same, whether a 3-plate or a hot runner mold is used. Therefore, it is just a question of the quantities to be produced. If the quantities are large, the savings in the cost of the product can easily justify the higher cost of the hot runner mold.
Figure 3.22 Runner for a 72-cavity 3-plate mold for a small cap (a), and a runner for a simple but heavy product (b)
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3 Cost Factors Affecting Productivity As a rule, the shot weight SW should not be more than about 80% of the rated shot capacity of the machine. This allows for leakage of plastic in the screw check valve, and for wear in the screw and barrel. If a material other than PS is used, it is necessary to convert the masses into volumes, which is really what the machines inject. For the same mass, PE has about 10% more volume than PS; therefore, if the mass is known, the volume will be about 10% more than for PS. For the same mass, the shot capacity will be about 10% less than that for PS. This must be properly calculated to avoid surprises when the new mold cannot be filled on the selected machine. Hot Runner Molds Only: Figure 3.23 shows a schematic of a typical hot runner system consisting of a manifold to distribute the melt to the gate and one of each of two types of nozzles: one an open type nozzle, the other a valve gate type nozzle. The various elements are clearly labeled. Normally, these two types of nozzles would not be used in the same manifold. One of the major advantages of hot runner molds is that they do not have runners to be molded (and ejected) with each shot and therefore make full use of the shot capacity of the machine, therefore, Shot weight SW = N · W These molds used to be called “runnerless,” which is a misnomer. There is a runner, but it remains (molten) in the mold and is not ejected at every cycle. Hot runner molds have the additional advantage of not needing to cool a runner. In a cold runner mold, the runner, and in particular the sprue, are the cycle-limiting factors. With a hot runner, the cycle time depends mostly on the wall thickness of the product and the quality of cooling of the mold.
a
Manifold backing plate
b c Wire groove Manifold T/C Center insulator Nozzle housing Manifold heater Insulating air gap
Figure 3.23 Schematic of a typical hot runner system (Courtesy: Husky)
Guide pin
Plate bolt Back up insultor pad Sprue bushing Locating ring Piston cylinder Manifold Alignment pin Plate cooling
3.7 Forecasting the Cycle Time The injection capacity can also be too large for a required (small) shot size. The practical lower limit for the shot size (the distance the screw retracts for the shot) should not be smaller than 0.5 times the screw diameter. If the shot size is smaller than this value, injection will be inconsistent because some stroke is required to reset the check ring or ball check. If this is the case, as smaller machine or a smaller injection unit should be selected.
3.7.6.3 Plasticizing Capacity of the Machine The plasticizing capacity is defined as the amount (mass) of plastic an injection unit can convert per hour from cold pellets into a homogeneous, thoroughly heated and mixed plastic melt, at the required temperature, ready for injection. Today, practically all machines use an extruder to “plasticize” (or “plasticate”) the material. The extruder consists of a plasticizing screw of appropriate design, rotating (for plasticizing) inside an externally heated barrel. In most machines, the screw is driven by a hydraulic drive. Today, in more and more machines the screw is driven by an electric motor, which is more efficient and saves energy costs. As the screw rotates, the raw plastic, which enters usually near the drive end, is pushed against the inside wall of the barrel. The friction generated between the plastic in the rotating screw flights and the barrel heats the plastic and the “melt” gradually moves forward toward the end (“tip”) of the screw, where it accumulates while the screw retracts, pushed back by the pressure exerted by the plastic. The screw stops when the desired shot volume is reached. At the “injection” signal, in a ram screw, the plastic accumulated in front of the screw is pushed out through the machine nozzle into the mold. In 2-stage injection machines, the extruder is used to fill an injection cylinder (the ”shooting pot”). Note that as a rule, the heaters surrounding the barrels contribute less than 10% to the plasticizing process. The heaters are there mainly to allow starting up again after a shut down, when the screw and barrel are cold and filled with cold plastic. Heating the screw from the outside and melting the frozen plastic enables the drive to turn the screw again. As a rule, only the mechanical energy of the drive generates the (frictional) heat for plasticizing the cold pellets. The amount of plastic an extruder can convert depends essentially on, and is limited by, the size (or power) of the drive motor (kW or HP), torque, screw speed and screw diameter, and on the design of the screw (screw design is a specialized area of engineering and not within the scope of this book). Many machines are equipped with so-called “general purpose” (GP) screws, which do a fairly good job for most materials, but do not work as efficiently as a screw designed for a specific material. This is an important consideration when planning production. A mold in a machine equipped with the most suitable screw will perform better (i.e., deliver better quality melt faster and with less power) than when using a GP screw. Today, most machines are equipped with easy screw-change features. Note also that the condition of the screw is very important. A worn screw (and/or
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3 Cost Factors Affecting Productivity barrel) will have greater clearances than when new, and therefore will produce less than the original specifications indicate. Plasticizing capacity is typically given in the machine specifications in kg/hr polystyrene (PS), using a universal screw, running continuously
As with shot capacity, the effect of specific gravity of the selected material must be considered and used to adjust the rated figures. In addition, allowances for the type of plastic are necessary: some plastics require a different L/D ratio (the ratio of the active length of screw over the screw diameter) and a different compression ratio (ratio of height of screw flights from the feed zone to the final “metering” zone, near the screw tip). If molds for plastics other than the most common ones are used, this should be discussed with suppliers of the plastic intended to be used, with the machine designer, or with a plasticizing screw design specialist. Here, we are mostly concerned with the data provided for plasticizing capacity. All machine specifications rate the capacity in kg/h, but that really means the amount the extruder can plasticize if it runs continuously! But no reciprocating screw or ram screw (RS) machine can run continuously. The screw cannot turn when pushed forward by the high injection forces. As we have described earlier, while the screw turns, the plastic in the barrel is melted and moves forward toward the screw tip, past the check valve and accumulates there until enough plastic is made up for the next shot. When the desired shot volume is ready, the screw stops and waits for the signal to inject. At this moment, the screw is pushed forward to inject the plastic into the mold. During the time when the screw is stopped (and waiting before injecting), while injecting (injection cycle), and while holding the screw forward (low pressure hold cycle), the screw does not plasticize. The sum of these times must be subtracted from the total available time; therefore, less time is available for plasticizing. The concept of “plasticizing per hour” is really a guide only. Note: Usually, the higher the back pressure, the better is the quality of the melt, but at the same time, more power (kW, hp) is drawn from the screw motor and less melt is pushed ahead of the screw. The amount plasticized is directly proportional to the speed of the screw (in RPM), its diameter, and design. There is a limit to the available power of the motor and to the screw speed (per machine specifications), which limits the amount plasticized per unit of time. It is important to have an idea of how long the screw will be stopped for each molding process. The low-pressure hold time is usually not required for thinwalled products, which freeze so fast that there is no possibility for the plastic to enter the cavity space after the original injection. But for thicker walled products, which are likely to shrink after the cavity space is first filled, it is important to keep the flow coming from the machine nozzle by keeping the pressure on the screw. Another important consideration is the injection speed, as discussed in more detail in Section 3.7.6.7. With faster injection, less time is required to keep the screw stopped. This is of particular importance with large shots, which could take several seconds to fill the mold. But the designer must also be aware that not all plastics are suitable for very fast injection; they may suffer excessive shear stresses and lose some of their physical characteristics, which could reduce the quality of the product.
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3.7 Forecasting the Cycle Time
Example 3.1 A 6-cavity mold for a product with a mass of 50 g is estimated to run at a 10 s cycle. We calculate: 3600 s/h ÷ 10 s/shot = 360 sh/h 360 sh/h · 50 g · 6 cav. = 108,000 g/h or 108 kg/h Assuming that the injection and hold time required will total 1.5 seconds, the plasticizing capacity of the machine must at least ensure that it can prepare the required mass per shot in 8.5 seconds, or 108 ÷ 8.5 · 10 = 127 kg/h. This will require a machine rated at least 130 kg/h (PS). If the planned product is, e.g., made from PE, we must still convert for the different specific gravity and add about another 10%, which demands a plasticizing unit yielding at least 145 kg/h (PS). Note that these figures are only achievable with a shut-off machine nozzle. With an open nozzle, a much larger extruder would be needed. For explanation, see later examples for open and shut-off nozzles. What happens if there is no such larger machine available and we do not have sufficient plasticizing capacity? The mold will be able to run, but it will not run at the expected speed, because the system will have to wait for the shot size to be made and will therefore have less output than planned. Light-Weighting the Product and Mold Improvements The importance of understanding the plasticizing capacity of a machine becomes clear when planning to redesign a product for less mass (“lightweighting”). The reduction of mass not only saves plastic, but also decreases (in most cases) the cooling time thus increasing productivity of the mold. The preliminary questions to ask are: will the new mold require more plastic per hour? Is the existing injection unit large enough? But even if no plastic can be saved by light-weighting, better mold design, and particularly better cooling, better ejection methods and other improvements can result in a substantial decrease in cycle time; in other words, more pieces per hour. Is the plasticizing capacity now large enough for this new mold, on the same machine as before?
Example 3.2 A product has a mass of 40 g and can be molded in a 12-cavity mold, running at 4 shots per minute. This product is redesigned to have a mass of 35 g and will be able run at 6 shots per minute. The old design of the product required 40 g · 12 cav. · 4 sh/min · 60 min = 115,200 g (or 115 kg/h). Because the extruder in a common ram screw injection unit cannot run during injection, we will assume that an extruder of 150 kg output will be required.
Always make sure that the machine is not the limiting factor when trying to improve a mold’s productivity
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3 Cost Factors Affecting Productivity
With the redesigned product, we now require 35 g · 12 cav. · 6 sh/min · 60 min = 151,200 g or 151 kg/h, which means that the machine will require a much larger plasticizing unit. But it is also important to ensure that the dry cycle of the machine is capable of allowing the faster cycle. Example 3.2 highlights important consequences: 1. The productivity can be increased by 50%: Before, 12 cav. · 4 sh/min · 60min = 2,880 pieces per hour were produced, after redesign, 12 cav. · 6 sh/min · 60 min = 4,320 pieces per hour can be produced, requiring much fewer machine hours, thus also reducing the product cost. The increase in productivity is significant. However, the cost savings could be somewhat less if a larger machine is required. 2. The product cost has been greatly reduced by using less plastic, the difference being 5 g per unit or 5 kg per 1,000 pieces. At an estimated cost for a commodity plastic of approx. $1.00 per kg and an annual requirement of 10,000,000 pieces, this amounts to a saving of $50,000.00 per year. If it were an expensive engineering plastic, the savings would be spectacular. 3. If, at the same time, we also consider to switch from a cold runner to a hot runner system in the new mold, we must remember that for cold runners, the plasticizing unit must not only provide melt for the products but also for the runners. By selecting a hot runner system, we are in fact increasing the usable plasticizing capacity by the no longer required mass of the cold runner system. This means that, especially if the runner was large, the existing injection unit could possibly be sufficient for the new mold. With hot runners, there is also the cost of material per unit affected, because we don’t have the mass of runners to consider. Even if all plastic runners could be recycled, there will always be some losses of material during recycling. Also, there is no cost of recycling
Heavy
Thinned out
4. If the injection unit is not capable of supplying the new required melt quantities, the whole effort of redesigning for better productivity would have been economically useless and the money wasted. The machine would have to continue to cycle at the old, lower speed. All the above must be considered and the calculations must be done every time a redesign is contemplated.
Figure 3.24 Sections through a molded tumbler
Figure 3.24 shows sections through a molded tumbler; on the right, the wall thickness before redesign, on the left, after redesign. This resulted in a reduction of plastic of 20% and at the same time, in a decrease in cycle time of about 20%. At the same time, it eliminated many molded defects caused by the thick to thin transitions.
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3.7 Forecasting the Cycle Time 3.7.6.4 Open Nozzles All molding machines come standard with “open” nozzles, i.e., the tip of the machine nozzle is open and will let plastic pass through freely, from the end of the screw either into the open air or into the sprue bushing of the mold, while injecting. In order to achieve good plasticizing, we must provide some controlled low backpressure in the injection cylinder, acting on the injection piston. This back pressure is usually only in the order of about 5–10% of the injection pressure, but the pressure is high enough at the tip of the screw (while the screw is plasticizing) to push the plastic through the open nozzle. With an open nozzle, the screw must not turn (plasticize) unless it is blocked, because:
With cold runner molds, the nozzle is pressed against the mold sprue bushing. The plastic in the sprue acts as a stopper and the screw can start rotating and producing as soon as the injection (or injection hold) pressure ends. But as soon as the mold opens, the screw rotation and the back pressure must stop, otherwise, the plastic will be pushed into the now empty sprue bushing and into the open mold.
With hot runner molds, the nozzle is also pressed against the sprue and the screw can start rotating as soon as the injection (hold) pressure ends. The mold could be opened safely even with the screw plasticizing, but only if all gates were frozen sufficiently to stop the plastic from drooling out of the gates; otherwise, plastic will drool into the open cavities, which is of course unacceptable. In these cases, the screw must also be stopped as soon as the mold opens. With most valve-gated molds, the gates are mechanically closed after injection, and a shut-off nozzle (see below) would not be required.
In both these cases, the time available for plasticizing is limited to the “cooling” cycle. While the mold is open, the screw is stopped. In molds with long cooling cycles, there is usually sufficient time for plasticizing the next shot volume.
Example 3.3 Let us assume a mold and machine with a 4 s dry cycle, an injection and hold cycle of 2 s, and a cooling cycle of 6 s. The total cycle is therefore 12 s. When using an open nozzle, the maximum time the extruder can run is 6 s, which is the same length of time as the cooling cycle. If the amount of plastic needed for the shot to be injected can be plasticized in 6 s or less, there is no problem with an open nozzle. Note that in this example, the screw can run only 50% of the time; therefore, the extruder is used only 50% of its rated capacity. Figure 3.25 Graphic illustration of Example 3.3
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Example 3.4 Let us assume the same machine and mold conditions as in Example 3.3, but this time not enough plastic can be plasticized in the 6 s between the end of injection and the end of cooling. If we assume we need 9 s to generate the melt for the next shot, we must increase (unnecessarily) the “cooling” time by 3 s (from 6 s to 9 s), for a total cycle of 15 s. This represents a severe loss of productivity (4 versus 5 shot/min). This extruder is used 60% of its rated capacity We should therefore look for an alternative, either the use of a shut-off nozzle, or find a machine with a larger extruder.
Figure 3.26 Graphic illustration of Example 3.4
It is also important to understand that by adding unnecessary cooling time, the products will eject cooler than necessary and shrink less (they will be larger than expected). This can be significant when molding plastics with high shrinkage factors. Also, products cooled too much inside the mold may become overstressed in some areas as they shrink onto the core and therefore fail early in use. To overcome both problems, the melt temperature should be higher to ensure that the product will not be “overcooled” in the mold. This adds to the product cost, because it requires not only more energy for heating the plastic higher than necessary, but it will also require more energy for cooling it. The use of a shut-off nozzle (see below) eliminates the extra cooling time.
Example 3.5 Let us assume the same machine and mold conditions as in Example 3.3, but here the necessary cooling time is 9 s, for a total cycle of 15 s. In this case, the screw has sufficient time for plasticizing, up to 9 s out of a 15 s cycle. The extruder can be used up to 60% of the cycle time. This illustrates that with longer cooling cycles, a simple open nozzle is adequate.
Figure 3.27 Graphic illustration of Example 3.5
3.7.6.5 Shut-off Nozzles For short cycle times, a shot-off nozzle can greatly increase the productivity of mold and machine. The basic principle of the shut-off nozzle is to provide a mechanical stop within the machine nozzle, which closes and opens the flow path of the plastic from the extruder to the nozzle tip. Shut-off nozzles come in various executions, such as shuttles, rotary cocks, or pins and are usually operated by compressed air or by hydraulic pressure oil.
B A C
Figure 3.28 Shut-off nozzle
Figure 3.28 shows a photo of a complete shut-off nozzle. In this design, the lever (A) pushes a pin inside the nozzle to close the nozzle opening (B). When injecting, the plastic pressure pushes the pin to the right, thus opening the nozzle opening to let the plastic enter the mold. The lever is operated by link (C) connecting it with a hydraulic or air actuator (not shown).
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3.7 Forecasting the Cycle Time Even though the shut-off nozzle represents an added one-time expense, their use is very desirable, especially with short cycle times because it allows the screw to plasticize while the mold is open.
Example 3.6 Let us again assume a mold and machine with a 4 s dry cycle, an injection and hold cycle of 2 s, and a cooling cycle of 6 s. The total cycle is therefore 12 s. Using a shut-off nozzle, the time the extruder can run now is 6 s (the cooling cycle) plus 4 s (the dry cycle) equals 10 s. The screw has now enough time to plasticize 10 out of the 12 seconds full cycle, or 83% of the rated capacity. This is a tremendous improvement over the use of an open nozzle. Even a smaller extruder (or machine) could be used for this job.
Figure 3.29 Graphic illustration of Example 3.6
Example 3.7 This example illustrates an extreme case: A machine with a 2 s dry cycle runs a mold with a 1 s injection cycle (no hold time). The cooling cycle is 1 s, for a total cycle of 4 s (15 shots per minute). With an open nozzle, the screw would have only 1 s time to make up for the next shot or can run at most 25% of the rated capacity. With a shut-off nozzle, the screw could plasticize for 3 s (adding the dry cycle time and the cooling time). The screw could therefore plasticize during 3 out of 4 seconds, or at 75% of the rated capacity.
Figure 3.30 Graphic illustration of Example 3.7
Example 3.8 A molder planned to operate a mold in a machine with a 4 s dry cycle, at a 9 s total cycle, or 400 shots/hour. However, there was not enough time for the extruder to deliver the required shot size in time for the next shot. The cooling cycle had to be lengthened from 3 to 6 s and the total cycle increased from 9 to 12 seconds; in other words, there was about 25% less production than expected.
When I got involved, I suggested the use of a shutoff nozzle. The time available to extrude could be raised to 7 s, plenty of time for this job.
Figure 3.31 Graphic illustration of Example 3.8
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3 Cost Factors Affecting Productivity
Example 3.9 Here the revised conditions in Example 3.8 are demonstrated. Figure 3.32 shows that using a shutoff nozzle increases the time available for plasticizing.
Example 3.10
Figure 3.32 Graphic illustration of Example 3.9
I then suggested moving the mold to another, same-tonnage machine, but with a shorter dry cycle time of only 3 s. Fig. 3.33 shows that the overall cycle could be reduced by another second, to a total of 8 s, thus increasing the production to 450 sh/h, or 11% over the original estimate.
3.7.6.6 Two-Stage Injection Units Figure 3.34 shows the difference between an RS (reciprocating screw or ram screw) injection system (above) and the two-stage injection system (below) In the RS system, the plasticizing and injection functions are combined into one unit. While the screw rotates, the plastic is transformed into a melt, which accumulates in front of the check valve. A shut-off nozzle (A) keeps the plastic locked, while the rotating screw retracts to prepare the shot volume for the next shot. The screw stops when the shot size is ready. The check valve at the tip of the screw prevents the melt to flow back into the extruder. Figure 3.33 Graphic illustration of Example 3.10
In the two-stage system, the two functions are separated. A rotating valve (B) between the nozzle (C), the shooting pot (D), and the duct (E) coming from the extruder direct the plastic flow as required: In one position, the valve lets the plastic melt coming from the extruder enter the shooting pot and drive the shooting piston back until the pot is filled for the next shot. In the alternate position of the valve the plastic can be injected into the mold, without pushing it back into the extruder (this position of the valve is shown). The extruder is also reciprocating, i.e., during the injection cycle, the screw can continue to rotate and plasticize while retracting. There is no screw check valve required. Two-stage injection is a special method, in which the plasticizing unit (with a reciprocating extruder) can run continuously if needed. The melt is fed into an injection cylinder (“shooting pot”) where it is stored and from where it is injected into the mold with a piston. The molding machine system is more complicated and expensive than an RS system, but it can be easily justified economically whenever large shot sizes are injected because a much smaller, much lower cost, extruder can be used. In addition, the extruder can be running continuously, i.e., without stopping at every cycle. This can also be of advantage for the quality of the melt, because the melt temperature will be more constant.
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3.7 Forecasting the Cycle Time
RS Injection
Check value
A Shutoff value
Two-Stage Injection E
B
C D Plunger sized for shot volume Figure 3.34 Reciprocating screw (or ram screw) injection system (above) and the two-stage injection system (below)
Another advantage of the two-stage injection is that it allows for very small or very large shot sizes, in the same machine. A small size (diameter) shooting pot will also permit extremely high injection pressures where needed under exceptional conditions.
3.7.6.7 Injection Speed and Pressure Injection speed, in this context, is the amount of plastic per second that can be pushed out of the machine nozzle into the mold as the extruder screw (or piston, in two-stage systems) moves forward to inject. It is usually specified in grams or ounces per second. Assuming that there is no loss (due to leakage or poor design of the check valve in front of the screw), the entire plasticized volume of melt waiting to be injected will be pushed into the mold. This volume V can be easily calculated as the cross sectional area A of the barrel (with a bore D), multiplied by the stroke S of the screw. V (mm3) = A (mm2) · S (mm) or V (mm3) = [D (mm2) · π ÷ 4) · S (mm)]
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3 Cost Factors Affecting Productivity For this discussion, we will ignore the fact that plastic expands as it is heated and therefore the volume of the melt is – in some plastics – actually up to 30% greater than the volume of the cold plastic. We also ignore the fact that the usable volume is actually somewhat smaller because of the “cushion” left at the tip of the screw, after every injection.
Figure 3.35 Typical temperature/viscosity relationship
Figure 3.36 Typical shear/viscosity curve for plastics
A (the screw area) is a constant; therefore, the (shot) volume V is directly proportional to the stroke S. Each extruder screw (or shooting pot) has a specified, maximum stroke Smax. The stroke is the same as the stroke of the (hydraulic) injection cylinder, which pushes the screw (or piston) forward during injection. In most injection molding machines, for the “standard size” screw (or piston), the ratio of area of the screw (piston) and of the injection cylinder is 1 : 10, so that the oil pressure of the hydraulic oil in the injection cylinder can be simply multiplied by 10 to arrive at the pressure of the plastic at the machine nozzle. For example, for an indicated oil pressure of 14,000 kPa (2,000 psi), we get a plastic injection pressure of 140,000kPa (20,000 psi). In most machines, the extruders can be easily changed (within limits) for smaller or larger screws and barrels; however, the hydraulic injection cylinder remains unchanged. Therefore, using the above formula, a 25% larger screw diameter will result in increasing the maximum shot volume by about 56%, at the same time reducing the maximum possible injection pressure by 36%. On the other hand, reducing the screw diameter by 20% will result in about 36% less volume, but with an increase of 56% of the injection pressure. This can (and should) always be calculated, and it may often permit to run a mold that does not need high injection pressures on a smaller machine by just changing the extruder screw and barrel to a larger diameter The injection speed is essentially dependent on four factors: (1) The viscosity of the injected plastic (which varies as a function of the injection rate and the temperature) (2) The available injection pressure (3) The resistance against flow within the runner system, the gates, and the cavity space and (4) The volume of hydraulic oil flowing into the injection cylinder. Points (1) and (2) above are closely related: Plastic at higher temperatures (with lower viscosity) will require lower pressures, and with lower temperatures (higher viscosity) will require higher pressures to fill the cavity space. This fact may appear obvious, but it has some implications on the selection of the mold, especially its construction. In thin-wall products, in order to push the more viscous (or colder) plastic through the narrow flow paths between cavity and core, higher pressure will be required than for similar, but thicker-walled products. This higher pressure means that more forces are generated inside the mold, which affects the stresses on some of the mold components, but more importantly, it will require higher clamping forces to ensure that the mold will not open up during injection and cause flashing.
3.7 Forecasting the Cycle Time The parting line (surface) area must be large enough to support the higher clamp forces without being damaged. This could mean that a larger mold is required. To prevent “hobbing” and damage to the mold when steel meets steel under very high clamp forces, it is recommended that the clamping pressure at the parting plane be less than 42,000 kPa (6,000 psi). The alternative, to make the plastic less viscous by raising its melt temperature, is not generally a good solution, although some operators who do not understand the implications use it frequently. Higher melt temperatures can make the mold flash easier, but more importantly, raising the melt temperature means adding more energy (costs!) to the plastic; the hotter injected plastic needs more cooling before ejection, requiring additional (cooling) energy (more costs!). Since it will take more time to cool the hotter plastic, it also will require longer molding cycles resulting in lower productivity. It does take more energy to generate the higher pressures, but much less than the energy required for the added heating and cooling. Older machines were often limited by injection pressures to a maximum of about 140,000 kPa (20,000 psi); most modern machines can deliver injection pressures of 200,000 kPa (29,000 psi) and even more by providing a selection of extruder barrels and screws, or shooting pots, of smaller sizes. All this explains why it is usually an advantage to use high injection pressures when molding thin wall products of any size, whether disposable goods or not. While the increased strength may add costs to the mold, the savings in energy and the increased production readily pays this additional cost back. As for the third limiting factor for injection speed, much can be done to improve the plastic flow within the mold, in the runners and in the cavity space by optimizing the gate location and size, occasionally by thickening the walls in certain areas, and by making sure that the finish of the molding surfaces is best suited for the plastic injected. As stated earlier, flash chrome plating a highly polished surface can greatly improve the flow and reduce the molding cycle. Also, hot runner systems, especially the channels inside the manifolds, must be properly sized for optimal conditions, and have all channels smoothly finished. The injection force is generated by the injection cylinder. The piston within the injection cylinder is driven forward by the hydraulic pressure oil as soon as the hydraulic valve admits oil to the cylinder. The speed of the piston motion is therefore proportional to the amount (volume) of oil flowing into the injection cylinder. This volume depends on the source of the pressure oil, but also on the flow restrictions in piping and valving between the source and the cylinder. In many injection-molding machines, the source of the pressure oil is a hydraulic oil pump, either a single pump, supplying the whole machine (clamp and injection, and any accessories requiring pressure oil), or a pump dedicated to the injection system, or a combination of pump(s) and hydraulic accumulators.
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3 Cost Factors Affecting Productivity Hydraulic pumps are rated by their output, in liters or gallons per minute. The relation between the pump output and the volume of the injection cylinder limits the injection speed. If we have, e.g., an injection cylinder with a diameter of 150 mm and a stroke of 150 mm, the volume is 2.65 liters. A pump (in this example) may have an output of 40 liters high-pressure oil per minute (60 seconds). It will therefore take 60 s × 2.65 liters ÷ 40 liters = 3.98 s to fill the cylinder. This translates into a maximum injection speed of 150 mm ÷ 3.98 s = 37.74 mm/s (or about 1.5 in./s). This would be the speed by which the plastic can be pushed into the mold. For some production this may be adequate, for others, especially thin-walled products, this may not be fast enough. If we want to increase the injection speed, we have one of two options:
Figure 3.37 Injection unit equipped with an accumulator to increase injection speed
Equip the machine with a larger pump, also requiring a larger drive motor (and larger controls), or
Equip the machine with an accumulator system, i.e., adding one or more pressure vessels (accumulators) where hydraulic pressure-oil can be stored during the time when it is not needed during a molding cycle. During injection, the pressure oil coming from the accumulator is added to the pressure oil coming from the pump. Note that this method is only viable if there is enough time available during each cycle to recharge the accumulator. If the molding cycle is very short, there may not be enough time for it and a larger pump and motor will be required.
It is therefore important for the decision maker to ascertain that the machine for which the mold is planned has sufficient injection speed to take advantage of the higher capability of the mold that is planned. There is no point in having a mold designed that could run, e.g., 15 shots per minute if the injection system of the planned machine is capable of providing only 10 shots. Note: there is some difference in injection time when filling a mold with more cavities as compared to a mold for the same product with fewer cavities, because of the longer distances the plastic must flow (greater pressure drop) and the greater total amount of plastic that must be injected, but at this point, a conservative estimate can ignore such differences, which may amount only to a fraction of a second. Once a mold layout is available, these differences in injection time could be calculated.
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3.7 Forecasting the Cycle Time
3.7.7 Ejection We will discuss two important areas affecting the productivity of a mold:
The actual ejection method, and
The timing of ejection.
3.7.7.1 The Selected Ejection Method Selecting the ejecting method is really a problem of mold design, outside the scope of this book, but a good understanding of the principles will help in arriving at the most suitable mold, sometimes with the help of the product designer. Note that the examples shown (Figs. 3.25 to 3.30) cover many of the rim designs found in today’s products; however, the comments do not only apply to rims but to other features of a molded product as well. First some basics:
In plastics, as in all materials, the shear strength is about half the tensile strength.
Compressive strength is about the same, or even higher than the tensile strength.
Tensile (and shear) strength vary with temperature; the strength is lower at higher temperatures. In other words, cold plastic is stronger than hot plastic. This seems obvious, but is often forgotten when planning for rapid ejection.
If an ejector (stripper, pin, sleeve) pushes on still warm (weak) plastic, it could damage it or not eject at all, or pierce the plastic, or shear it off. If we push rather than pull, we take advantage of the higher compressive strength (instead of the lower shear strength) and thereby can gain valuable cycle time by being able to eject sooner.
It is always better to compress plastic than to shear it during ejection
For example: most plastic products are “cup-shaped”, i.e., they have a bottom and sidewalls. Typical examples are boxes, lids, cups, baskets, flat containers, such as audio or video cassettes, or any cup shape, whether perfectly round or rectangular. Figure 3.38 shows two typical shapes. One with almost straight sidewalls (little or no draft), the other with good draft (7° or more). Not determined at this time is the shape of the rim, which will be discussed in detail later. The ejection force (Fe) to eject the piece from the core depends on the friction (and stickiness) between the plastic and the side wall of the core, on the finish of the core or any undercuts where the plastic could “hang up”, and to some extent on the force created by the shrinking of the plastic onto the core. It will depend heavily on the angle of the sidewalls (less force is required with good draft); in addition, more force is required if there is vacuum trapped between the core and the molded piece.
Figure 3.38 Two typical cup (or box) shapes
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3 Cost Factors Affecting Productivity The choices are to eject either from the rim or from the bottom, as indicated with arrows. Ejecting from the rim (with stripper or with pins) has the advantage that the force is directed against the bottom of the sidewall, i.e., Fe is pushing, and the sidewall is loaded in compression. This allows the product to be ejected sooner, but we must make sure that there is provision to break the vacuum, especially with no or little draft angle. Ejecting from the bottom means that the product is pulled off the core; the force of the ejector pin acts on the bottom wall and loads it in shear (the pins try to pierce the plastic). We must therefore make sure that the plastic is strong (and cold) enough not to be pierced, which means a longer cycle. There is usually no problem with vacuum, because ejectors pins or sleeves provide ideal, self-cleaning vents. It is obvious that the largest possible ejector pin diameters should always be selected to increase the shear length created at the circumference of the pin. The actual time difference between this method and ejecting from the rim could be in the order of one or more seconds, which may not amount to much (in percentage) in an otherwise slow cycle or with low production, but can make a huge difference with large quantities. To pull is quite common with any product with well-tapered sides and when requiring only low production. Another disadvantage of this method, especially with large, deep products is the required great length of ejector pins passing through the whole depth of the core. There is also the danger of the product hanging up on the extended pins at the end of the ejection stroke. Ribs and Hubs Figure 3.39 shows that the ejection of deep ribs is related to ejecting cupshaped products. The proper way, where possible, is to eject the ribs from the deepest point of the rib for two very important reasons:
Better
Worse
Figure 3.39 Examples of ejecting ribs and hubs
The plastic will fill any rib better if the bottom is well vented, as it would through the clearance of any ejector pins (in some cases the rib would not fill at all if not properly vented).
During ejection, the rib will be pushed, which is good because the plastic is under compression. The smaller the draft angle, the more important it is to push, rather than pull. There is less force required to push on a rib because the rib will usually shrink away from the mold wall. However, the smaller the draft angle, especially with thin ribs, the more difficult it is to provide the necessary fine finish for easy ejection. Even though the plastic may shrink away from the wall, it can still hang up there. When molding an extended hub, the plastic will shrink onto the core pin and will require more force and therefore should be ejected (pushed) from the bottom, e.g., with sleeve ejectors (higher costs!). The alternative of pulling the rib (or the hub) from the top is not recommended, because (1) there is the danger of piercing the top surface and possibly leaving the plastic in deep, inaccessible spots and (2) it could also restrict the space available for good cooling in the core just where it is most required.
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3.7 Forecasting the Cycle Time Rim Shapes In general, for containers, lids, etc., a stripper ring is the preferred method of ejection. It is effective, relatively low cost, and allows a fast cycle because it pushes on the sidewalls that can be still quite warm but are already strong enough to take the ejection force. With today’s technology, it is possible to produce odd-shaped stripper outlines (almost any shape that can be generated on CNC milling machines and grinders and/or with wire EDM); however, we must never forget that while it is possible to make these shapes, they are quite expensive to make, as well as to provide the proper preload and to maintain it. Unless it is absolutely necessary for the product appearance or function to insist on odd-shaped strippers, it is much better (easier, less expensive) to use round strippers, stripper bars, or conventional, inexpensive ejector pins.
Figure 3.40 Simple rims and stripper
Figure 3.40, items 1 to 6 show simple rim configurations as are often found on utility shapes, ejected with strippers. The corner of the stripper must be about 0.1 mm away from the core (1). If a radius is specified there (2), it should be smaller than the wall thickness to make sure to push under the wall. There will be a witness line from the stripper all around the rim. If there is a radius larger than the wall thickness t (see Fig. 3.41), a design change should be requested, explaining how it will affect the molding operation (particularly, the cycle time). If the radius is larger than the wall thickness, there is the danger that the stripper (or ejector pins) will act like a shear and cut off (or pierce) the rim rather than eject unless the plastic is well cooled and stiff. A smaller radius should be selected, so that at least part of the stripper (or the pins) will be under the sidewalls.
Figure 3.41 Large radius in corner
Figure 3.42, items 7 to 12, show similar rims as are shown in Fig. 3.25, but this time ejector pins are employed. Note that the pin diameter is much larger than the wall thickness, and will push with a segment of the face of the pin. This has the advantage of not only using a large, rather than a delicate, small diameter pin, but it also covers a relatively large area of plastic which will therefore see a much lower specific ejection force with less risk pushing into, or shearing the plastic when ejecting. Such large ejector pins will leave an ark-shaped witness line. Note also that small ejectors wear easier (more upkeep), and that the bores for small pins are more difficult to produce than those for larger pins. Wherever possible, pins should be at least 6 mm (1/4 in.) in diameter. In case (9), it will be necessary to prevent the pins from turning, which adds costs. The methods shown using ejector pins are suitable for any shape of rim.
Figure 3.42 Simple rim and ejector pins
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3 Cost Factors Affecting Productivity
Figure 3.43 Ejector pins with shape of rim
Figure 3.44 Stripper with shape of rim
Figure 3.45 Beaded rim, with void and sinks
As said above, a portion of the ejector pin should be under the sidewall. If the radius is not smaller than the sidewall, we can consider shaping the pin in line with the rim, (see Fig. 3.43) but the pin should never end in a featheredge, which is difficult to maintain in production. In both cases, the pins must be secured from turning. It should be made clear to the customer that it would be much better to have the design changed so that it is not necessary to resort to cases as shown in Fig. 3.43. Similarly, a stripper can have a sharp edge as shown, but the stripper must never end in a featheredge as shown in Fig. 3.44. This is also not advisable, and a design change is suggested.
Figure 3. 46 Section of the rim of a pail with a large void
Figure 3.45, items 13 to 15, shows typical rims found frequently specified. Rim 13 is a poorly conceived shape. Being at the end of the flow path, the injection pressure is greatly reduced and such a rim is difficult to fill. The result is either a void, usually with amorphous plastics such as PS which freeze rapidly on the surface, or a sink, usually with crystalline plastics which freeze slower and pull the still soft plastic skin toward the center, as shown (exaggerated) in 13a and 13b. Any good design will avoid such thickenings at the end of any flow path and the mold maker should explain it to the product designer and request a change as suggested in 14 or 15 which will cycle much faster and not have the risk of sinks; it also uses less plastic, and permits faster cycles (lower product cost!).
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3.7 Forecasting the Cycle Time 3.7.7.2 Timing of Ejection and Stroke Required The cycle time depends much on the timing of the ejection. The proper timing of the molding machine to suit the mold is often overlooked. Clear instructions specifying when ejection should start are most important to achieve the best cycle time. The mold must of course open up sufficiently to allow the products to eject, before closing again safely, without trapping pieces on their way out of the molding area. But any unnecessarily long stroke represents a waste of time and loss of production. There is, however, the need to service the mold, usually during startup of the mold. There must be sufficient space between the open mold halves, at least 150 mm (6 in.) for small molds, and more for larger ones to permit access, as may be required for cleaning a frozen gate or removing a broken piece of the product which failed to eject. In the case of a multicavity mold for a flat product, such as a lid for containers, there could be an opening stroke of as little as about 2.5 times the height of the product, provided the products will clear the molding area fast enough so they will not be trapped by the closing mold half. In some machines the stroke can be adjusted very easily so that the mold can be set up for production with a very short stroke, but can be quickly opened up wider when servicing is required. The relation between stroke length and cycle time can be easily seen in Fig. 3.47. Figure 3.48 shows a 2 × 4 cavity family stack mold for rectangular flat boxes and cover, equipped with guide rails. As the mold closes, the guide rails (A) are pushed out of the way of the closing cavities and cores by the action of the cams (B). As the mold opens, the guide rails close in as the products are pushed in between them. When the ejector pins retract, the molded pieces are free, but oriented as they come out of the cavities, to fall onto conveyors (not shown) that transfer them to assembly stations. There is no mold open time required. B
A
Figure 3.47 Schematic showing relation between stroke length and cycle time
Figure 3.48 A 2 × 4 cavity family stack mold for rectangular flat boxes and cover
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3 Cost Factors Affecting Productivity
Figure 3.49 Schematic of ejection timing without MO time
Mold Open (MO) Time (General Observation) The time that the mold stays open (Mold Open or MO) time is an important factor to consider, because it adds time to the cycle without contributing to the molding process; it should be kept to a minimum or better yet, not be used at all (MO = 0). The need for MO time depends very much on the product and the mold design, the size of the mold, the type of machine, and the selected ejection system. The ideal arrangement would be (a) to use no MO time (to run the machine “on the fly”), or (b) to use as short a mold opening stroke as possible, or (c) both (a) and (b) In many molds it is possible to start ejection shortly after the product has partly pulled out of the cavity and by the time the mold has reached the fully open position, the products have cleared to molding area making it safe to re-close the mold. In some cases, air assist (air curtain or air blasts, see Fig. 3.17) will assist the product in going out of the way of the closing mold. Figure 3.34 shows a schematic of a mold cycle not requiring MO time.
Figure 3.50 To reduce cycle time, closures are ejected before the mold is fully open
In Fig. 3.49, the point denoting the start of ejection has been selected arbitrarily. If ejection starts too soon, there is the danger that the product will be pushed back into the cavity and will fail to eject. If starting too late, time is wasted. The “time for ejection” is shown ending at the end of the molding cycle. This is not absolutely necessary; it could extend into the next cycle, as indicated by an (arbitrarily placed) vertical line L shown in the graph. When this additional time ends depends on the shape of the product, on the reliability of the ejection method, and the speed with which the products clear the molding area to avoid trapping some of them between the closing mold halves. It also depends on the layout of the cavities in the mold, which affects the time of the piece ejected from the uppermost cavity to clear the bottom edge of the mold. For example, products falling from a four-cavity mold laid out with four cavities in a row will take longer to clear the molding area if the row is in a vertical rather than in a horizontal layout.
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3.7 Forecasting the Cycle Time
Figure 3.51 Schematic of ejection timing with added MO time
Multi-Cavity Molds Especially with multi-cavity molds, it may not be possible to clear all molded pieces out of the molding area in time and some MO time will have to be added. The need for this MO time should be closely scrutinized; while it is sometimes not possible to avoid MO time, it is quite common that the setup persons add extra MO time “just to be sure” that the pieces have cleared, rather than to ensure that the ejection, including air assist, is properly adjusted and timed as intended. The length of the ejection stroke, when to start ejection, when to start and to stop blowing, etc. must be recorded for the next time the mold will be used in the instructions for the setup of this mold, but also in the job file for future reference. Figure 3.51 shows the same cycle as in Fig. 3.50 but with the addition of MO time, providing more time available for ejection but also lengthening the cycle time. Here again it would be possible to use a portion of the closing time of the next cycle to extend the time available for ejection (line L). Following is a list of reasonably achievable MO times to be added for various ejection methods:
Air ejection only
Machine ejector (hydraulic) 1.0–2.0 s
Robot
none, or 0.5–1.0 s
1–2.5 s
Figure 3.52 Large machine with 2-cavity mold, molding two large industrial pails (pail weight 800 g, cycle time 35 s, productivity 205 pieces/h)
Multiple Ejection Strokes Most molding machines are equipped with the feature of double or multiple ejection strokes. The idea behind this system is that if the original (first) ejection stroke has failed to eject some of the molded pieces, additional strokes will shake these pieces loose from where they hang up so that they fall free to clear the mold before the next cycle will start. A well-designed mold has no need to use this system: the molded pieces must eject on the first stroke. Additional strokes not only add to the cycle time but are also noisy and wear out machine and mold.
Don’t use multiple ejection just because it is available.
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3 Cost Factors Affecting Productivity Automatic Product Removal
Cavities in the mold can be laid out so that the products are in the proper attitude (and relative relation to each other) for subsequent operations. With suitable take-offs, they can be ejected and transferred to ancillary equipment for stacking, assembling, etc. and save much on unnecessary handling such as unscrambling and rearranging
Molds with integrated mechanized product removal, which act in mechanical or timed synchronization with the mold opening and closing stroke, will remove the moldings as soon as the mold has opened far enough to allow the take-off member to enter the space opposite the core or cavity. The products are then grabbed (mechanically or with suction cups) and the takeoff arm withdraws ahead of the re-closing mold. This method has been used successfully even in multi-cavity molds, cycling as fast as 3 s or 20 shots per minute. Such special ejection methods should be considered, especially when it is required to maintain the orientation of the products outside the mold. These methods also allow shorter strokes thus reducing the time for the products to clear the molding area (swing chutes, guide rails, and other systems). Typically, there is no MO time required. While these integrated systems could add considerably to the mold cost, this is quickly recovered by saving substantial cycle time and by the increase in productivity. It also eliminates the need for subsequent orientation of the molded pieces for stacking, assembling, decorating, etc., often requiring expensive orienting devices further down the line. Robots Robots can often be used to unload large products, or for very large multicavity molds for smaller product, especially with relatively long molding cycles. The principle is similar to automatic product removal, except that for safety reasons the mold will have to stay open longer. As a rule, the robot’s “end-of-arm tooling” will not enter the molding area until the mold has stopped in the MO position and the ejection will not start before the end-ofarm tooling is in front of the cores (or cavities) from where the products will be removed. Similarly, the mold will not close before the products and all robot mechanisms are safely out of the molding area. Typically, MO time is 1–2 s, sometimes even longer. Figure 3.54 shows a typical robot (from the rear of the machine), removing very large industrial pails from a single cavity mold. The end-of-arm tooling (A) grabs the pail with 4 suction cups (B) as it is ejected from the mold. The pail is then raised and moved out of the molding area, where the pail is rotated into a vertical attitude and stacked, opening down, onto the previously ejected pieces (C). Large machines are not only used for large single cavity molds; for example, Fig. 3.52 shows a two-cavity mold for large products with a high production rate; it shows a setup, with a large machine molding two large industrial pails. The pails are ejected randomly, and drop into “pail catchers” below the molding area, from where they are moved by conveyor to stacking and packing stations.
Figure 3.53 High-speed side entry robot for DVD case removal (0.7 s part removal time)
Figure 3.55 shows a machine with a 2 × 16 cavity stack mold for small containers. After the mold is open, two parallel arms (A) enter the mold between the two levels (B) to face the cores from where the products are
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3.7 Forecasting the Cycle Time ejected into two arrays of 16 receptacles (C) on robot arms, held there by vacuum. The arms retract, and the mold can start the next cycle. Once outside the mold, the plates with the receptacles (C) rotate 90° to deposit the products onto a conveyor (D) that guides them, still oriented as they came out of the cavities, to the next station in the process.
A
B
Figure 3.54 Typical robot (from the rear of the machine), removing very large industrial pails from a single cavity mold (mass 2.0 kg (PP), 40 s cycle, productivity: 90 pieces/h)
C
B
A C
D
Figure 3.55 Machine with a 2 × 16 cavity stack mold for small containers
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3 Cost Factors Affecting Productivity Ejection Stroke, Air Assist The ejection method used in the mold will also affect the time needed for ejection. The mold must be properly designed with sufficient stroke to ensure that the pieces will not hang up on the core (ejector side) or on the ejectors themselves. On the other hand, poor adjustment during setup of the mold will have the same effect as a poorly designed mold. The ejection stroke does not necessarily have to be long, if other methods are used, such as moldintegrated take-offs, robots, or if compressed air is used to eject from the mold. Combined with a short ejection stroke, which will push the molded pieces off the cores just far enough to break a vacuum or its hold on the cores (e.g., products with undercuts, or with very little draft), air assist will blow the product out of the mold.
Figure 3.56 Air ejected medicine cups (2×32) into a high-speed robot (Courtesy: Stackteck)
Any “air only” method can be quite tricky and needs experience in designing and setting up to prevent hanging up caused by a vacuum created by the air blowing out of the mold, between the product walls, and the core. With some materials and some shapes, pure air ejection (without any mechanical ejectors) allows the construction of a better, and at the same time, lower cost mold. This is often used for PP and PE containers; these molds can be smaller (no need for an ejector system), they run quieter, faster, and they last longer because there is no wear of moving parts, such as stripper rings, etc. However, the mold maker also needs experience in this design and not all product shapes are suitable for this system.
3.7.8 Ambient Temperatures and Humidity Both ambient temperatures and humidity have an influence on the molding cycle. As already shown in Section 3.1.1, we must mold in an atmosphere above the dew point. In a lower humidity environment, colder coolant could be used and thereby cycle time saved. If the ambient temperature is high, more energy will be used to keep the mold at the preferred, low temperature. Air conditioning or at least humidity control will improve the molding conditions and the productivity.
3.7.9 Comparing Molding Cycles of the Same Product in New Molds Having the cycle times recorded from a mold for the same product, which ran in a machine and environment other than planned for the new mold provides accurate production figures (shots per hour). However, the influence of all the factors which may be different for the new mold must be considered. The new mold may have more (or fewer) cavities; the molding machine may have a different dry cycle or faster injection; the ambient shop air may be different, and so on. The new conditions could be better or worse and must be considered. Any improvements in ejection, runner system, and cooling, etc. can make dramatic improvements in productivity.
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3.8 Number of Cavities Required Conclusion Many factors affect the cycle time. We have learned what affects it in a positive and in a negative way, but we still are left with these questions:
What is the expected cycle time?
How can we proceed in selecting the proper size mold?
What is the proper number of cavities?
To state it bluntly, there is no definite, correct answer; we must rely heavily on the memory and opinion of experienced molders and on records of the operation of earlier, similar molds. All we can say is that compared with a known mold for a similar product, and by knowing how such mold was constructed and operated and on which machine it has been producing, we will be able to claim that we can do better (or differently) by using different features and equipment in the planned mold that will improve the cycle time (and productivity) of the mold at a reasonable cost.
3.8
Number of Cavities Required
To determine the number of cavities required the following data or values based on assumptions are needed:
The annual production quantities required of the mold, or the output of the mold if required only for a shorter period
A conservatively estimated cycle time. Factors affecting cycle time have been discussed in Section 3.3.
3.8.1 Available Operating Time There are 8,760 hours in a full year (365 days × 24 hours). Obviously, there will be non-productive (“lost”) hours of inactivity, required for maintenance, or caused by breakdowns. There are two commonly used approaches to determine the actually used hours of a machine, but molders also often approach to this issue their own way. A more conservative approach (often by a custom molder) may select the following method: 50 weeks of 5 days at 24 hours equals 6,000 hours. By allowing 10% for lost time, there will be 5,400h/year available per machine. This gives a “buffer” of 8,760 – 5,400 = 3,360 hours, in case that working more hours should be required. This buffer can come in handy when deciding on the number of
A good estimate for available hours per year is 5,400
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3 Cost Factors Affecting Productivity cavities, e.g., when, by working a few extra days or even weeks, a smaller mold (fewer cavities) could be satisfactory for the expected production requirements. A more aggressive approach (often used by “dedicated molders”) plans on using more hours: Many dedicated molders use 7,560 hours/year
50 weeks of 7 days of 24 hours = 8,400 h/year, and by allowing about10% for shutdowns, will assume 7,560 h/year available per machine The examples of production shown later in this book are based on the more conservative number, 5400 h/year
3.8.2 The Minimum Number of Cavities The calculation for the number of cavities is simple:
Establish the productivity of the planned mold per hour (shots per hour), based on the estimated cycle time (test) in seconds (s). 3,600 s (1 hour) divided by test (s) equals the number of shots per hour (shots/h).
Multiply the number of shots per hour (sh/h) with the available number of hours per year (5,400) to get shots/year. This is the number of molded pieces produced by one cavity
Divide the projected annual quantity Q by the number of shots/year. This gives the approximate minimum number of cavities required for the job.
Example 3.11 For example, if 2 million pieces per year are needed and the cycle time is estimated to be 10 s: 3,600 s/h ÷ 10 s/sh = 360 sh/h. This will result in 5,400 h · 360 sh/h = 1,944,000 sh/year. This is just shy of the required 2 million and one cavity could be satisfactory, provided other considerations justify a single cavity mold. However, if for example, this requirement of 2 million pieces must be delivered in 4 months, we have only 5,400 h ÷ 12 · 4 = 1,800 h available. We now calculate 1,800 h · 360 sh/h = 648,000 shots during this shorter time span, and by dividing 2,000,000 ÷ 648,000 we get 3.09, that means we need at least 3 cavities. Since we have a buffer (by working more than 5,400 hours), and the difference is small (0.09), the choice of three cavities could be acceptable, provided the product could be laid out practically in a mold. In some cases, a 3-cavity mold is practical; otherwise, a 4-cavity mold would be the preferred choice. This provides also some provision for future growth.
3.8 Number of Cavities Required These calculations give us the minimum number of cavities required for a job, but they do not tell us whether it is the most economical number of cavities. For this, we must bring in another important consideration: the machine hour cost.
3.8.3 Machine Hour Cost per Unit Molded Before proceeding, we must define and recognize the term “machine hour cost” and understand how the cost of the molding machine will affect the cost of the product we intend to mold on the machine considered for the production. Machine hour cost is the total of the actual cost of the machine, its accessories, the cost of installation of the machine in its location in the plant, and the connection to the services, plus the cost of money (interest of loans for the purchase of the machine etc.) spread over the number of hours (or years) the machine is expected to produce. There is really no definite life expectancy (in years) of a machine, as long as it is well maintained and as long as the molding technology does not change. For practical reasons, and as permitted by tax legislation, the cost of the machine is usually written of in 5, 7, or 10 years, even though most machines can (and do) work satisfactorily for much longer time periods before being replaced. We should also consider the case where a machine could be acquired for a one-time special molding application and would not be suitable for other molding operations. This machine could possibly be required for a much shorter time and should in fact be considered as an extension of the mold. That means the machine should be depreciated entirely over this single project and this special product should then be priced accordingly. For a standard type injection-molding machine, we must first know the actual total price paid and then assume the number of years for writing it off. For example, if the total cost (as defined above) is $200,000 and we plan to write off the machine in 7 years, we calculate: $200,000 ÷ (7 years · 5,400 h) = $5.29/h of depreciation. This is the actual machine hour cost per hour for this machine. Obviously, by writing it off in fewer years, the cost will be higher. By writing it off over a longer period of time or by using it more hours/year, the machine-hour cost will be lower. The amount is relatively small and for simplicity of cost accounting, groups of machines of various sizes and costs or even the total of all machines in a plant are lumped together into one machine hour cost. To this cost, the costs of overhead (cooling plant, compressed air supply, power, plant space, etc.) are added to define a plant-wide machine hour cost to be used for costing of any product made there. The machine hour cost
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3 Cost Factors Affecting Productivity used for cost calculations may be $25.00, $40.00, $60.00, or more, even though the actual machine hour cost may only be in the order of $5.00. Note that the machine hour cost should really not include the cost of power, compressed air, and cooling water for machine and mold, because these costs are directly proportional to the number of pieces molded (the amount of plastic “converted”). However, for simplicity of accounting it is usually included in the overhead. This (rather artificial) machine hour cost must now be apportioned to the cost of the product. Typical custom molding rates can be found in trade magazines. They can range from $30–$100/hour, depending on machine size and location
If we have a single cavity mold, running at a 10 s cycle, we produce 3,600 s ÷ 10 s = 360 pieces per hour. Assuming a machine hour cost of $40.00, the machine hour cost per unit molded is therefore $40.00/h ÷ 360 pieces/ h = $0.111 per unit. By selecting a 2-cavity mold instead, the production doubles to 720 pieces per hour, while the machine hour cost per unit is reduced by half, to $0.0555. If another machine could operate the same mold at an 8 s cycle, the figures would be different: 3,600 s/h ÷ 8 s/sh = 450 sh/h, and with the same machine hour cost of $40.00, the machine hour cost per unit for a 1-cavity mold would be $40.00 ÷ 450 = $0.0889 per unit. Similarly, a 2-cavity mold cycling at the faster speed would result in a machine hour cost per unit of $0.0444. Therefore, the more cavities are in the mold, the lower is the machine hour cost per unit molded; but also, the faster the machine cycles, the lower is the machine hour cost. This change in machine hour cost per unit can be charted as follows, based on an assumed machine hour cost of $40.00 The incremental gain, i.e., the reduction in machine hour cost per cavity, becomes smaller as the number of cavities increases and as the cycle time decreases.
With large quantities, the deciding factor will usually be the largest cavitations approach
For example, with a 10 s cycle, increasing the number of cavities from 1 to 2, we reduced the machine hour cost from $0.11 to $0.055. By selecting 4, 8, or 16 cavities, this cost will reduce to $0.0275, $0.0138, and $0.0069, respectively. With larger number of cavities, these incremental amounts are very small and would in most cases not influence the choice of the number of cavities. With large quantities, (100 millions or more), the deciding factor will usually be the highest productivity required of the mold. Table 3.1 Cost of Machine Hours in $ per Unit Molded
Cycle time
shots/ hour
1 cavity
2 cavities
4 cavities
8 cavities
16 cavities
12 s
300
$0.1333
$0.0667
$0.0333
$0.0167
$0.0083
10 s
360
$0.1111
$0.0556
$0.0278
$0.0139
$0.0069
8s
450
$0.0889
$0.0444
$0.0222
$0.0111
$0.0056
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3.8 Number of Cavities Required
3.8.4 Mold Cost per Unit Molded The productivity of a mold increases in proportion to the number of cavities. But a mold with more cavities will be more expensive and will require a larger machine with more clamp force and more injection and plasticizing capacity. The cost of two identical stacks will be about twice as much as the cost of one stack, but this relation can change. The more identical stacks are made at the same time, the more attention can be given to better “massproduction” mold manufacturing methods by providing special tooling, jigs, and fixtures. Also, programming for CNC machines is the same whether one or any number of cavities are produced, so that the first cavity will cost more than additional cavities. Obviously, the mold shoe will be larger as the number of cavities increases, but not necessarily proportional to the number of cavities. Only a tentative layout will make it possible to indicate how much larger the mold shoe will have to be. For the purpose of this discussion, we will assume that a 2-cavity mold will be 1.8 times as expensive as a 1-cavity mold for the same product and we will use the same increase for larger molds. The factor of 1.8 has been arbitrarily chosen from experience and would apply to a fairly complicated stack; this factor cannot and must not be used as a universally applicable factor. It is used here only to indicate that doubling the number of cavities does not mean that the mold will be twice as expensive. With simpler stacks, this factor could be smaller, such as 1.6 or 1.5. When the cavities and cores are very simple and can be machined right out of the cavity and core plates and when cold runners are planned, there could be only little difference between the cost of making 1, 2, 4, or even more cavities. Figure 3.57 shows a simple cold-runner, multi-cavity layout for simple, small cavities and cores that can be cut right into the cavity and core plate. In Table 3.2, if a single cavity mold costs $20,000, we assume that a 2-cavity mold will cost $36,000, and a 4-cavity mold could cost 1.8 · $36,000, or $64,800, and so on.
Table 3.2 Relationship Between Mold Cost per Unit and Total Production Requirements
# of cavities
Mold cost
1
Estimated total production 10,000
100,000
1,000,000 10,000,000
$20,000
$2.0000
$0.2000
$0.0200
$0.0020
2
$36,000
$3.6000
$0.3600
$0.0360
$0.0036
4
$64,800
$6.4800
$0.6480
$0.0648
$0.0065
8
$116,640
$11.1600
$1.1160
$0.1116
$0.0112
Figure 3.57 Simple cold-runner, multi-cavity layout A: balanced; B: not balanced
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3 Cost Factors Affecting Productivity The mold cost per unit is calculated by dividing the mold cost (price) by the total number of pieces that are expected to be made from this mold
Pail Molder
Molds 7%
Machines 6% Automation 1% Labor Direct 3%
Resin 51%
Labor Indirect, Admin & Insurance 22%
Maintenance 1% Building and Sub-Systems Electricity & Water 6% 3%
Figure 3.58 The cost of plastic usually constitutes the largest portion of the piece cost
It can be easily seen that the larger the number of pieces to be produced, the smaller will be the mold cost per unit. A simple table based on the above assumptions and with assumed total quantities of 10,000, 100,000, 1,000,000, and 10,000,000 will show this relationship. Table 3.2 shows that the mold cost per unit increases dramatically when the requirements are relatively small and that in such cases a multi-cavity mold does not make much sense, unless the total requirement must be delivered in a very short time. On the other hand, for large quantities, the actual mold cost per unit is very small and a mold with more cavities should be selected. These simple calculations, which take only minutes to perform, must always be made before deciding on the number of cavities planned for a new mold. In general, it can be stated that for very large quantities, the best quality mold (most solid construction, best cooling, best injection system, best ejection method, etc.) and the largest number of cavities possible is the best choice for a new mold and will yield the lowest product cost, even though the initial investment is high. As we mentioned earlier, in most products, the cost of plastic usually constitutes the largest portion of the piece cost. If, for example, the molds in the above example produce pieces with a mass of 100 g and the cost of the plastic is $1.00 per kg, the cost of material would be $0.10 per unit. In most cases, it would not make much difference whether the mold cost per unit of $0.002 or $0.0064 was added to the material cost, but we gain much in productivity of the mold with higher cavitation and at the same time dramatically reduce the total cost by using fewer machine hours. More about the total estimated product cost will follow.
3.8.5 Calculation of the Required Clamp Size We must calculate the projected area of the product and then make an assumption of the average pressure the cavity space will see during injection. This is difficult and can be calculated by a computer (mold flow) analysis of the product. But from past experience we know that thick-walled products need lower pressure to fill the mold, and thin-walled products need higher pressures. Also, if good surface definition is required, maximum pressures will be needed. For the purpose of quick but conservative estimating, the following values can be used:
Thick-walled products: Use 28,000 kPa pressure (3,000 to 4,000 psi)
Thin-walled products: Use 35,000 to 55,000 kPa (5,000 to 8,000 psi)
Note that we speak here of the average injection pressure within the cavity, not the indicated injection pressure at the machine nozzle, which could be in the order of 140,000 kPa (20,000 psi) or more. If the machine cannot provide these required high pressures, the product cannot be molded successfully on
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3.8 Number of Cavities Required it, unless the plastic is injected with very low viscosity (very hot), which, as we have already seen, is as a rule not desirable, mainly because it increases the cycle time. For the next step, we take the above assumed pressure and multiply it with the projected area of the product to arrive at the clamp force required to hold the mold closed during injection. For example, assuming a projected area of 64.5 cm2 (10 in.2) and an assumed injection pressure of 35,000 kPa (5,000 psi) necessary to fill the cavity, the required clamping force will be 64.5 cm2 · 35,000 kPa ÷10,000 = 225 kN (or 10 in.2 · 5,000 psi = 50,000 lb = 25 tons) We require at least 225 kN (or 25 tons) per cavity and the number of cavities selected will define the minimum clamp size of the machine selected. Conversely, we can determine the maximum number of cavities possible in a machine by dividing the available clamp force of the planned machine by the force required to keep the cavities closed during injection. For this example, for a 100 t clamp, we obtain 100 t ÷ 25 t = 4, which means a maximum of 4 cavities.
In the above example we have omitted the impact of the projected area of the cold runner system (hot runners do not add to the projected area, as they are fully contained within the hot runner manifold).
With 2-plate (cold runner) molds, the projected area of the runner system must be added to the sum of the projected areas of the cavities. For relatively large products, the area of the runners is usually small in comparison to the area of the cavities, but with small products, the area of the runners must be seriously considered; it could be as large or even larger than the area of the cavities (see also Fig. 3.9).
With 3-plate molds, it is very rare that the projected area of the runner system is larger than the projected area of the sum of all cavities and therefore only the sum of the projected area of the cavities needs to be considered. If the projected area of the runner system is larger than the sum of the cavity areas (which could be the case for very small products), the projected area of the runners must be used to calculate the clamping force.
Whenever possible, attempt to inject with lower melt temperature and higher injection pressure for highest productivity and lowest cost
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3.8.6 Shot Size Shot size was discussed earlier in more detail, here only the essentials will be repeated: Once the actual or estimated mass of the planned product is known, we must consider the machine on which the mold will be run. But first, we must again differentiate between hot runner molds and cold runners molds (2plate or 3-plate). With hot runners, only the mass of each product, multiplied by the number of cavities planned, needs to be injected. With cold runners, in addition to the mass of the sum of products to be molded we must inject the estimated mass of the runners, which can be considerable in the case of small and very small products (it can amount to more than the total mass of the products). Do not use more than 80% of the machine’s rated shot capacity
We can now decide how many cavities the machine intended for the job can inject. It is suggested not to use more than approx. 80% of the rated shot capacity, after the shot capacity of the machine (in PS) has been adjusted for the plastic for the planned product. The safety allowance of approx. “20% less than the maximum shot capacity” will allow for the required cushion ahead of the extruder screw check valve and for possible leakage of the check valve.
3.8.7 Plasticizing Capacity For molds running > 10 shots/min, use no more than 20% of the machine’s shot size to provide enough time for the plastic to melt
We must consider the hourly throughput adjusted for the type of plastic used, not the figure specified in the machine specifications, which is for PS. We must then include in the calculation the estimated injection time and the cycle time to arrive at the percentage of time during which the extruder can plasticize. Also, it is prudent not to use the total adjusted value for plasticizing capacity, but rather use only about 80 to 90% of it, to allow for loss due to wear in the extruder, but also to have the possibility to increase the output if the mold can be cycled faster than anticipated. If the output of the extruder is less than required, it means that although the mold can be run on the machine, it will run slower and therefore with lower productivity than planned.
3.8.8 Preferred (Practical) Number of Cavities Once the minimum number of cavities has been established, we will select a number that is also practical for laying out the mold and for the mold manufacturing process, and which will provide the best flow of plastic to the cavities, whether in a cold runner or hot runner mold. For example, if the above calculations suggest that seven cavities are required, we will select the next larger preferred number, in this case, eight. We will have to recalculate to see if the first selected machine will be able to handle the new requirements (clamp force, plasticizing, and injection capacity) for the new total of cavities.
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3.8 Number of Cavities Required Listed below are the commonly preferred numbers of cavities, although under certain conditions, other numbers are also possible. Essentially, we consider either rectangular or circular layouts. Each has its own advantages and disadvantages. For symmetrical, rectangular layouts, we consider the following number of cavities: 2, 4, 6, 8, (10), 12, 16, (20), 24, 32, 36, 64, 72, 96, 144 cavities. For circular layouts, symmetrical layouts usually use 1, 3, 5, or 9 cavities, but virtually any number of cavities is possible. However, it could be more difficult to arrange for good balanced cooling and injection. The main advantage of a circular layout is that the runners can be kept very short, especially in 2-plate molds. There are also family molds in which a number of different products are molded in one shot, as is often the case for toy components, but also, occasionally, for technical products. In these cases, it is recommended to arrange the cavity layout so that the various projected areas are arranged as symmetrically as possible around the center of the mold (see Fig. 4.1, Section 4.1.2). These jobs are usually not high-volume items and a slower cycle can be easily tolerated.
Figure 3.59a Family mold layout
Figure 3.59b Floppy disk, family mold parts
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3.8.9 Business Decisions The considerations in Sections 3.4.1 to 3.4.8 must always be kept in mind before deciding on a suitable number of cavities for a mold. All points are equally important and none can be ignored. But there are also considerations, which are outside the technical field. It may require repeated calculations to get the best answer.
3.8.9.1 Uncertainty Factor The uncertainty factor reminds us that essential design input is often based on assumptions.
Will the product be successful and are the assumed quantities realistic?
Should we, at the launching of a new product, be more conservative in our assumptions?
It is purely a business decision, whether to spend a lot of money for a first class multi-cavity mold, before we even know whether the new product will be accepted. Over the years, I have seen cases where the original mold was much too large and the product not successful, resulting in large loss of investment. But I have also often seen that the cautious approach became a problem, when the first mold, in view of the uncertainty, was deliberately selected for a smaller output. When the product appeared to be in demand, it was a question of getting either additional, small molds similar to the first one, or select much larger and better molds that would give lower-cost products and higher productivity. If the new product is an early success and the demand cannot be satisfied with a low production mold, it could invite competitors to enter the market sooner
There is another risk with the cautious approach: If the new product is an early success and the demand cannot be satisfied with a low production mold, it could invite competitors to enter the market sooner.
3.8.9.2 Flexibility of Production, Design Changes Another business decision affecting the selection of the number of cavities is flexibility of operation. Building a mold with a large number rather than with fewer cavities can have great economic advantages (fewer machines, less plant space, lower unit cost) but leaves the production more exposed to problems, such as mold or machine breakdowns. Design changes, which are always a possibility, particularly after a new product has been introduced, are also easier to plan and execute if more, smaller molds are in production, and the changes (improvements) can be gradually incorporated, without interruption of production. It is also often easier to find machines suitable and available for smaller molds than for the larger ones when looking for the proper machine size for the molds, especially when using custom molders rather than molding “in house”.
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3.8 Number of Cavities Required
3.8.10 Preliminary Estimate of Product Cost We must always be aware of our goal, which is to arrive at the best quality product, at the lowest cost to be able to make a profit. There are many ways to arrive at the right number of cavities. For comparison, we shall assume that the cost of the plastic per unit is always the same. The cost of a molded product in general consists of these components 1. The machine hour cost per unit 2. The cost of the mold, per unit 3. The cost of plastic 4. Any additional costs, after molding, attributable to the molded piece, such as direct labor, handling, and post-molding operations, such as printing. Because both items 3 and 4 above are constants and have no direct effect on the selection of the cavity numbers, they are omitted in the following tabulations. Remember that items 1and 2 (the mold and machine hour costs) usually represent only a minor fraction of the total product cost. Tables 3.1 and 3.2 demonstrated the effect of the number of cavities on the machine hour time per unit and the mold cost per unit. We can combine these two tables, for each specific estimate; this comparison will greatly help in making a decision on the number of cavities for the planned mold. Table 3.3 Unit Costs of Product with 12 s Cycle
# of cavities
Cost per unit
1
2
4
8
Estimated total production 10,000
100,000
1,000,000 10,000,000
Mold cost Machine cost
$2.0000 $0.1333
$0.2000 $0.1333
$0.0200 $0.1333
$0.0020 $0.1333
Total
$2.1333
$0.3333
$0.1533
$0.1353
Mold cost Machine cost
$3.6000 $0.0667
$0.3600 $0.0667
$0.0360 $0.0667
$0.0036 $0.0667
Total
$3.6667
$0.4267
$0.1027
$0.0703
Mold cost Machine cost
$6.4800 $0.0333
$0.6480 $0.0333
$0.0648 $0.0333
$0.0065 $0.0333
Total
$6.5133
$0.6813
$0.0981
$0.0398
Mold cost Machine cost
$11.1600 $0.0167
$1.1160 $0.0167
$0.1116 $0.0167
$0.0112 $0.0167
Total
$11.1767
$1.1327
$0.1283
$0.0279
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3 Cost Factors Affecting Productivity Table 3.4 Unit Costs of Product with 10 s Cycle
# of cavities
Cost per unit
1
2
4
8
Estimated total production 10,000
100,000
1,000,000 10,000,000
Mold cost Machine cost
$2.0000 $0.1111
$0.2000 $0.1111
$0.0200 $0.1111
$0.0020 $0.1111
Total
$2.1111
$0.3111
$0.1311
$0.1131
Mold cost Machine cost
$3.6000 $0.0556
$0.3600 $0.0556
$0.0360 $0.0556
$0.0036 $0.0556
Total
$3.6556
$0.4156
$0.0916
$0.0592
Mold cost Machine cost
$6.4800 $0.0278
$0.6480 $0.0278
$0.0648 $0.0278
$0.0065 $0.0278
Total
$6.5078
$0.6758
$0.0926
$0.0343
Mold cost Machine cost
$11.1600 $0.0139
$1.1160 $0.0139
$0.1116 $0.0139
$0.0112 $0.0139
Total
$11.1739
$1.1299
$0.1255
$0.0251
Table 3.5 Unit Costs of Product with 8 s Cycle
# of cavities
Cost per unit
1
2
4
8
Estimated total production 10,000
100,000
1,000,000 10,000,000
Mold cost Machine cost
$2.0000 $0.0889
$0.2000 $0.0889
$0.0200 $0.0889
$0.0020 $0.0889
Total
$2.0889
$0.2889
$0.1089
$0.0909
Mold cost Machine cost
$3.6000 $0.0444
$0.3600 $0.0444
$0.0360 $0.0444
$0.0036 $0.0444
Total
$3.6444
$0.4044
$0.0804
$0.0480
Mold cost Machine cost
$6.4800 $0.0222
$0.6480 $0.0222
$0.0648 $0.0222
$0.0065 $0.0222
Total
$6.5022
$0.6702
$0.0870
$0.0287
Mold cost Machine cost
$11.1600 $0.0111
$1.1160 $0.0111
$0.1116 $0.0111
$0.0112 $0.0111
Total
$11.1711
$1.1271
$0.1227
$0.0223
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3.8 Number of Cavities Required To illustrate the points raised, we have assumed production requirements over a wide range and three possible cycle times for a planned product. The same figures for mold costs per unit and machine hour cost per unit as in Tables 3.1 and 3.2 were used. Unit costs of product attributable to mold cost and machine hour cost (referring back to mold costs shown in Table 3.1 and based on a machine hour cost of $40.00) are shown in Tables 3.3 to 3.5. Instead of the unit values given in Tables 3.3 to 3.5, we could also consider the sum-total of the estimated mold cost plus the cost of producing the total number of products required by taking the mold cost and then adding the machine hour cost per unit multiplied with the total number of pieces required (in this example, we will only consider the 10-second cycle values). Table 3.6 Total Costs of Production versus Numbers of Cavities
# of Cost of the total cavities production 1
2
4
8
Estimated total production 10,000
100,000
1,000,000 10,000,000
Mold cost Machine cost
$20,000 $1,111
$20,000 $11,110
$20,000 $20,000 $111,100 $1,111,000
Total
$21,111
$31,110
$131,100 $1,131,000
Mold cost Machine cost
$36,000 $556
$36,000 $5,560
$36,000 $55,600
$36,000 $556,000
Total
$36,556
$41,560
$91,600
$592,000
Mold cost Machine cost
$64,800 $278
$64,800 $2,780
$64,800 $27,800
$64,800 $278,000
Total
$65,078
$67,580
$92,600
$342,800
Mold cost Machine cost
$116,640 $139
$116,640 $1,390
$116,640 $13,900
$116,640 $139,000
Total
$116,779
$118,030
$130,540
$255,640
To calculate the cost of the product, we must now add the cost of plastic, the cost of handling directly chargeable to the product, and the cost of any postmolding operations.
Example 3.12 Assuming a mold is required for a product with a mass of 40 g, made from PP, at $1.00 per kg. The estimated cycle time is 10 s. The cost of plastic 40 g · $1.00 ÷ 1,000 g/kg = $0.04 per unit
The cost of the product is a function of mold cost, number of cavities, cycle time, and plastic weight, plus any post-molding handling and operations
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3 Cost Factors Affecting Productivity Table 3.7 Cost of a Product Made with Different Numbers of Cavities with 10 s Cycle
# of cavities
Cost per unit
2
Mold cost Machine cost Plastic
$0.3600 78.9 $0.0560 12.3 $0.0400 8.8
Total
$0.4560
$0.1320
$0.0996
Mold cost Machine cost Plastic
$0.6480 90.5 $0.0278 3.9 $0.0400 5.6
0.0648 48.8 $0.0278 21.0 0.0400 30.2
$0.0065 8.7 $0.0278 37.4 $0.0400 53.9
Total
$0.7158
$0.1326
$0.0743
Mold cost Machine cost Plastic
$1.1160 95.4 $0.0139 1.2 $0.0400 3.4
0.1116 67.4 $0.0139 8.4 0.0400 24.2
$0.0112 17.1 $0.0139 21.4 $0.0400 61.5
Total
$1.1699
$0.1655
$0.0651
Mold cost Machine cost Plastic
$2.0088 97.7 $0.0070 0.3 $0.0400 2.0
0.2009 81.0 $0.0070 2.8 0.0400 16.2
$0.0201 29.9 $0.0070 10.4 $0.0400 59.7
Total
$2.0558
$0.2479
$0.0671
4
8
2×8 (stack mold)
For an estimated total prodction of 100,000 % of $1,000,000 % of 10,000,000 % of total total total
0.0360 27.3 0.0560 42.4 0.0400 30.3
$0.0036 3.6 $0.0560 56.2 $0.0400 40.2
To arrive at the actual cost of the product, we add the mold cost/unit and machine hour cost/unit from Table 3.6. Assuming a requirement of 20,000, 200,000, and 2,000,000 units per year, over 5 years, the total cost per unit will be: For simplicity, we assume that all overheads are included in the machine hour cost of $40.00/hr and are the same for all machines. We also assume that there is no direct labor involved with this product. Conclusion:
With low quantities, the mold cost represents a substantial portion of the product cost; therefore don’t over-build the required capacity. It cannot be justified economically to make a multi-cavity mold when the total quantities are small, except where such small quantity is required in a very short time with not enough molding capacity to supply the required quantities on time. Also, two smaller molds are usually faster to build than one larger one.
3.8 Number of Cavities Required
With large quantities (often disposable items), the cost of plastic represents a major portion of the product cost. The incremental increase of the mold cost will be paid back easily when the quantities are high. Therefore, build to the anticipated capacity. However, there are some additional considerations.
The machine time cost can be substantial with a low number of cavities, especially with products with low mass. The cost of the product from a 2 × 8-cavity stack mold ($0.0671) is about the same as the cost of the same product coming from a single-level 8-cavity mold; however, the productivity of the mold-machine system is doubled. This means fewer machines, less plant space, etc.
The tables above apply to relatively complicated molds. However, if the mold stack is simple, and especially if the product is small, it is usually not much more work to make 2, 4, or even more cavities and cores without substantially increasing the mold cost, especially with 2-plate cold runner molds (see Fig. 3.39). In such cases, a multi-cavity solution is preferable, because such mold will use much less machine time while giving much higher production. For symmetry reasons in the mold construction, and to avoid unbalanced clamping, single cavity molds are usually avoided, unless they can be centergated to ensure proper balancing of the clamping forces. In such cases, a 2-cavity mold would often be the preferred choice. Let us now apply what we have learned so far: What are the economics of a setup as shown in Fig. 3.14? Examples 3.13 and 3.14 go through the calculations step-by-step:
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3 Cost Factors Affecting Productivity
Example 3.13 In this example, we base our calculations on the following (reasonable) assumptions:
Figure 3.60 48-cavity perform mold (ca. 1990’s) (Courtesy: Husky)
72-cavity mold with cooling robot system
Cycle time: 14 s (used to be in the order of 40 s for molds with standard cooling methods)
Mass: 26 g (used to be in the order of 40 g)
Cost of PET: US$1.40/kg (Year 2005)
Production: 7,200 h/year
Write-off time (very aggressive): 5 years for mold, machine, and drying system for PET
Productivity: 3,600 s/h ÷ 14 s/sh · 72-cav. = 18,510 pieces/h · 7, 200 hr = 133,300,000 preforms/year, or 666,500,000 over 5 years
Cost of mold, including upkeep for 5 years: $1,500,000
Mold cost, per preform: 1,500,000 ÷ 666,500,000 = $0.00225
Cost of machine, cooling robot and drying system, including upkeep for 5 years: $2,000,000
Machine hour cost: $2,000,000 ÷ (5 · 7,200) = $55.56.
To this we add an assumed other $80.00 for overhead (power, water, services, etc.) to get a net machine hour rate of $135.56. If we add to this also a profit of $50.00 per hour, we get $185.56.
Selling price (machine hour, overhead, and profit) per preform is therefore: $185.56/h ÷ 18,510 pieces/h =$0.0100/preform To this we must add the cost of the plastic. Cost of plastic per preform: 26 g/piece · $1.40/kg PET ÷ 1,000 g/kg = $0.0364/prefom Total cost of preform: PET Mold cost Machine incl. profit Total selling price
$0.0364 $0.0023 $0.0100 $0.0487
75% 5% 20% 100%
From the above it can be seen that: (a) The mold cost is only 5% of the total (b) One way to reduce the cost of the preforms, besides reducing the cost of the plastic and/or reducing the mass of the perform, is to improve the performance of molds and machines, which is an ongoing process. (c) By operating 7,200 h/year, over 5 years, there will be a profit of 5 years · 7,200 h/year/$50/h = $1,800,000 on an investment of $3,500,000.
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3.8 Number of Cavities Required
Example 3.14 The following example compares two molds for the same product; the lower priced mold has poor cooling arrangement (does not use BeCu inserts), resulting in a slow cycle time. In addition, the ejection mechanism fails frequently and causes downtime and scrap. Cost of molds Cycle time Up-time Scrap Machine time cost Material cost Product weight Productivity (pieces per hour) Machine hour cost per unit (A) Plastic cost per unit (B) Scrap cost per unit (C)
Mold 1 $10,000 10 s 80% 5% $40 $1/kg 100 g 1,152 $0.0347 $0.1000 $0.0050
Mold 2 $20,000 6s 90% 2% $40 $1/kg 100 g 2,160 $0.0185 $0.1000 $0.0020
$0.1000 $0.0010 $0.2397 $0.1407 $23,970 $1,407,000
$0.2000 $0.0020 $0.3205 $0.1225 $32,050 $1,225,000 $182,000
Mold cost per unit: At total production 100,000 (D) At total production 10,000,000 (E) Unit cost A+B+C at (D) Unit cost A+B+C at (E) Total cost, at (D) Total cost at (E) Savings at 10,000,000 production
Conclusion: For low production, a less costly mold can be justified, unless the quantities must be available in a short time and the shorter cycle becomes significant. With the larger quantity, the savings in the long run are significant
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4
Mold Selection
4.1
Selection of an Appropriate Mold
Once a good product design has been achieved and it is decided where the product will be made and how many cavities are required, we must consider the available alternatives for the molds.
4.1.1 Dedicated Mold, Universal Mold Shoe “Dedicated mold” means a complete mold that is used for one purpose only. After use, the mold is put into storage until it is used again. This is the most common type of mold. Occasionally, especially with molds with 2–8 cavities, the same mold shoe can be and often is used for more than one set of cavities and cores. In principle, there is nothing wrong with this concept, provided the molding shop is well organized (good record keeping and proper storage facilities for the loose stack parts) and the personnel is capable of making the switch from one product to another without the need for high-priced mold makers. It may take a few hours to switch from one set of stacks to another and there is always the risk of damage to the mold components in handling and during assembly. The question is whether it is worthwhile to switch molds, especially if it is done frequently. If the mold shoe is quite simple, it would be better (safer and more economical) to have a dedicated mold. But there are cases where the mold shoe is large, complicated, and relatively expensive; if the stacks for the various (preferably similar) products are designed from the beginning so that they can be easily interchanged, this is a very good and economical solution.
Figure 4.1 A 4-level mold designated to quickly switch to different sets of inserts (Courtesy: Stackteck)
Dedicated molds are usually preferred. However, the use of a common mold shoe with different sets of stacks can often be more economical
Typical examples are 4- or 6-cavity molds for a series of round containers, with none or only small differences in diameters, but with large differences in height, as would be the case with small tubs for dairy products, e.g., in sizes from 0.25 liter to 1 liter capacity. Such molds can be designed and built with all the advantages of a dedicated mold, but saving the cost of several mold shoes. “Universal mold shoes” are used mainly for low production runs, for which only small numbers of cavities are required. They are based on the principle that stack inserts can be easily and quickly interchanged by the molding technicians or setup personnel, often even without removing the mold shoe from the machine. The stacks do not necessarily have to be for the same or even similar products. They are usually designed for one cavity per insert. If there is space, two or more cavities and cores could well be placed within one insert. The disadvantage is that, because the stacks are designed for easy interchangeability in the mold shoe, it may not be possible to provide them with the best cooling layouts (facilitating faster cycles) of a dedicated mold. In addition, the product requiring the longest cooling time governs the cycle
For small products and low quantities, universal molds can often be the most economical solution
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4 Mold Selection time; however, for low production, short cycle times are not as significant for the unit cost as is the lower mold cost.
4.1.2 “One-Product” Molds or “Family” Molds? “One-product mold” is a mold built for one specific product. The best layout for minimum mold size, space (stack location), cooling, ejection, etc., can be achieved with a dedicated (one-product) mold.
Figure 4.2 Schematic of symmetrically balanced cavities in relation to the centerline of the clamp
A “family mold” is a dedicated mold, in which more than one shape of product is made during the same injection, which will be of the same material and color. A very serious disadvantage of all family molds is that the cycle time of the mold is governed by the product (and the mold stack) that is most difficult to cool. This difference can be substantial, and should be seriously considered, particularly with products as described in Section 4.1.2.1 and 4.1.2.2. For all family molds producing pieces of different size, we must make sure that the mold is laid out so that the clamp forces are balanced as well as possible, i.e., that the sum of all projected areas is about equal in each of the 4 mold quadrants. In other words, the projected areas of the cavities above and below the horizontal center line of the mold must be nearly equal and so must be the sum of the projected areas to the right and the left of the vertical center line of the mold (see Fig. 4.2).
4.1.2.1 Family Molds for Composite Products
Figure 4.3 Stack family mold for container and lid (Courtesy: Husky)
For composite products, such as toys and games, it may be desirable to make all the components of the toy in one shot. Often, the various pieces are kept on the runner system of a 2-plate mold and are packed and shipped together with the runner; it is left to the user to take the pieces off the runner during assembly of the toy. The production runs are usually relatively small; therefore, this is a most effective method of producing with low cost molds (don’t forget to include the cost of the runners in the cost of the product). Occasionally, a product, e.g., a toy car, may have two or more colors. It could be a car with a blue body, red wheels, and yellow bumpers, etc. By molding equal production runs of first blue, then red, then yellow parts, 3 sets of cars can be produced, in the three combinations of colors. In this case, the runners are not shipped with the product. This method is also used occasionally for technical products.
4.1.2.2 Family Molds for Small or Medium-Sized Technical Products
Figure 4.4 Cavity view of 72-cavity cutlery mold; 24 forks, 24 spoons, and 25 knives are molded every shot (ca. 8–10 s)
Family molds for small or medium-sized technical products are used when a number of different sizes of similar, rather small products, such as washers or seals, are molded in one mold. But such molds can also be used for larger products, which are required as a “set” in production, as they are used, e.g., for home appliances, among others. Any type of mold can be used (hot runner
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4.1 Selection of an Appropriate Mold or cold runner, 2-plate or 3-plate). There are two main disadvantages of this type of mold: (1) Except for edge-gated 2-plate molds, the products fall out of the mold all mixed together and must be separated before storage or use. (2) Stock and production control can have serious problems when some of the products are used up (e.g., wear) faster than others, and must be available as spare parts. It may then be necessary to run the mold to produce the full shots while only some of the items are required. This problem can be overcome by blocking off the runner system ahead of the unwanted cavities and running the mold only for the products required; this means to run the mold less efficiently.
Figure 4.5 View of ejected array in robot end of arm tooling
4.1.2.3 Family Molds for Perfect Color Matching Any plastic, and especially colored plastic, whether colored in-house or bought already colored from the supplier, comes in batches. Within each batch, the plastic can be considered uniformly mixed and colored. These batches are supplied in bags, or in large carboys, or in truckloads, etc. Even though the specifications to make the batches were identical, there are mixing tolerances in manufacturing and small variations from batch to batch are unavoidable. It is better to work with large batches, which will yield large numbers of matching-colored pieces, but this is not always practical or economical. If a product pair (or assembly) must have a perfect color match, the answer is to make the matching pieces in one shot, which is of course supplied by the same injection unit, at the same time. A typical application for this is a “lady’s compact”, consisting of a base (for the face powder) and a matching lid (for the mirror). But there are other applications, some of them in the technical field. It is quite common to build molds that have the same number of each of the products that require the perfect color match. If the pieces are required in pairs and their projected areas are about the same, a mold layout is rather easy and the stacks can be laid out symmetrically. A problem is that the pieces are ejected together and must be separated after molding; also, they must also be stored so that the matching colors are kept together and are not mixed with products from another color batch (this can also add costs to the product).
4.1.2.4 Family Molds for In-Mold Assembly Family molds for in-mold assembly are more sophisticated molds, usually for very high production volumes and are only rarely used. A multiple of two different but matching pieces is molded in the same mold; they are assembled during the ejection time, using special motions, which are part of the mold or during the mechanical removal (with synchronized take-offs or robots), so that already assembled pieces are ejected to a conveyor or carried away under controlled conditions. Such assembly methods may require longer ejection times but can save subsequent assembly equipment, and time.
Perfect color matching can easily be achieved with family molds
Figure 4.6 Color matched parts for personal care products
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4 Mold Selection Figure 4.7 Petri dish system (Courtesy: Husky) A
B C D
E
4.1.2.5 Family Molds Using Controlled Ejection for Subsequent Assembly
Figure 4.8 Petri dishes and CD jewel boxes are typically molded using family molds
This method is almost exclusively used for products where the required annual quantities are very large and virtually no changes are expected for years. In these cases, a number of pairs of matching pieces, usually of the same projected area or with only small difference in area, are molded in one (single level or stack) mold and then removed either by take-off or by other methods, which maintain the orientation of the matching pieces so that they can be easily assembled in a specially designed machine or mechanism, usually adjacent to the molding machine. Typical examples are Petri dishes, video and audio- cassettes, CD “jewel boxes,” and so forth. Figure 4.7 shows a Petri dish system taken from the rear of the clamp, which is protected by guards (A). The bottoms and the tops of the Petri dish are molded on each face of a 2 × 4, 2 × 6, or 2 × 8 stack mold. Guide rails transport the molded parts by conveyor (B) to an assembly station (C); from there the assembled dishes move to a stacker (D) and the stacks of assembled Petri dishes are then moved to an (open) “sleeving” station (E) where plastic sleeves are manually pulled over the stacks for boxing and shipping to a sterilizer; cycle time: 3.5 s, productivity (with 2 × 8 mold): 8,200 assembled dishes/hour.
4.1.3 Where to Gate
Figure 4.9 Cutlery is also often molded in family molds
The next issue to consider is the location of the gate. The gate is the point where the plastic enters the cavity space. In some cases, the product designers will indicate where they believe the gate should be. They may select this location because of the function and strength of the product and in some
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4.1 Selection of an Appropriate Mold cases, because any projecting gate vestige may be bad for appearance or even harmful to the user. However, such suggested location may not always be the best for filling the cavity space or for the best strength properties of the product. At this point of the development, the input by a molder or the mold designers could be very valuable and a dialogue between the product and mold designers should be encouraged to find the best location for the gate. These days, computer aided mold filling simulation packages can accurately predict the fill patterns of any part. This allows for quick simulations of gate placements and helps finding the optimal location.
4.1.3.1 Cup- or Box-Shaped Products
Figure 4.10 Mold filling analysis is very useful for finding the best gate location
In general, for cup- or box-shaped products, outside center gating is most desirable, because it ensures more evenly distributed flow from the gate towards the rim or edge. However, center gating (except for single-cavity molds) implies the use of either 3-plate or hot runner molds, both of which are more expensive than 2-plate molds. Note that the gate area is always an area of inherent weakness; molding conditions such as higher melt temperatures, longer molding cycles, and higher cooling temperatures can improve the strength there and this must be considered as a factor affecting the cycle time and cost of the product. It should also be noted here that hot runner valve gating reduces the stresses in the gate area. The foregoing does not imply that 2-plate molds cannot be used for cup- or box-shaped products; in fact, 2-plate molds are used for many such products, but usually only those with larger wall thickness.
4.1.3.2 Flat Products “Flat” in this context means relatively flat, as opposed to “cup-shaped.” It includes really flat pieces (in one geometric plane) but also curved products, such as automotive panels, trays, etc. of all shapes. Flat products are preferably gated from the edge of the product, because the flow away from the gate (or gates) will result in a stronger product; it also ensures that there are no unsightly gate marks in the middle of the product. Here also, it is much better if the incoming stream of plastic will be directed against a solid portion of the core or at least a projection of the core and not to flow into an open space, such as a rib or an open surface. Thin-walled, round products, such as lids for containers and trays, should be center-gated for faster filling and to reduce possible distortion when ejected early to gain cycle speed; however, they can also be edge-gated when the center of the lid must not show a gate mark. Figure 4.12 shows a selection of typical automotive products. The quantities are usually small compared with the huge numbers molded for packaging and medical products and the molds are usually small cavitations (1 or 2). But even so, most of these products are molded with hot runners, because it is easier and more effective to control the quality of the products and there is often less labor required than with cold runner molds. Also, the use of regrind is sometimes not possible, which makes the justification of a hot runner easier.
Figure 4.11 Typical bottom center-gated parts
Figure 4.12 Selection of typical automotive products
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4 Mold Selection
Figure 4.15 Schematic of large, single-cavity mold with hot runners feeding cold runners; Figure 4.14 Balancing of mold clamp- (a) product (a large panel); (b) sprue; (g) hot ing forces; (left) added second cavity; runner channel; (h)”drop” to cold runner; (j) cold runner; (i) gate (right) added balancing pressure pad
Figure 4.13 Two examples of gating into the center of an open product. The sprue could be a cold sprue or a hot (runner) sprue
Because in most molding machines the injection unit is in line with the center of the machine platens, it is not possible to edge-gate a single-cavity mold, unless either there is a large enough opening near the center of the product, from where a cold or hot runner system can feed one or more edge gates (see Fig. 4.13), or a hot runner system is used with drops outside of the profile of the product, feeding into cold runners (see Fig. 4.15). An alternative is to have the cavity located completely to one side of the centerline of the machine, which could be possible for any product small enough to fit there. This however could leave to a severe unbalance in the clamp. This is not recommended. A Fill time = 0.9408 [s]
B
[s] 0.9408
A
A
0.7056
0.4704
0.2352
0.0000
Figure 4.16 Flow model of an automotive panel
There are ways of balancing the clamp forces, e.g., by doubling the size of the mold and providing a second, similar cavity if the cavity is not too complicated and expensive, or by adding a pressure pad in a location on the platen symmetrically opposed to the cavity (see Fig. 4.14). For more details on this subject, refer to [5] Chapter 6. If the product is very large, edge-gating into a single cavity can be achieved with a 3-plate mold (now rarely used for this purpose) or by using a hot runner system, which enters one or several cold runner systems outside the edge of the product. From there, cold branch runners can lead to edge or tunnel gates into the side of the product, just like in a regular 2-plate mold. This method is used for large, mostly flat products, such as automotive panels, and so forth (see Fig. 4.15). Figure 4.7 shows a flow model of a large automotive panel with three gates (A) from a hot runner system (B), but without the use of cold runners as in the schematic of Fig. 4.6. The runners are shown schematically, superimposed over the photo. Note that here again, the gates are near the edge of the panel for greater strength.
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4.1 Selection of an Appropriate Mold 4.1.3.3 Deep, Large Cup-Shaped Products Products in this category are large pails, boxes, garbage containers, large crates, children’s bathtubs, and so forth. It is always desirable to use one center gate, if the L/t ratio is low enough (200 or less.) Today’s machines with high injection pressures have made it even possible to mold large industrial pails with an L/t ratio of up to 500 with only one gate. However, most large products (tubs, boxes, etc.) have two or more gates in the bottom for faster, lowerstress filling and to reduce the L/t ratio for each gate. Large industrial containers, crates, pallets, etc. may have four or more gates. It is important to provide venting where the streams coming from the gates are expected to meet to avoid the risk of air enclosures or even holes at the predicted weld lines. Such molds with one gate can use a cold sprue (simplest mold) or they can have a hot sprue. If two or more gates are required, a hot runner system must be used. 3-plate molds, although theoretically possible, are almost never used in this arrangement, because of the large size and mass of the cavity block that would have to move (float) between the moving and stationary platens to allow ejection of the runner. Figure 4.17 shows a typical heavy crate (A) for bottles with separators (B) for individual bottles. This design requires a mold with side cores for the deep engravings (C) and the openings (D) in the sides. There are also two baskets (E) with openings (F) in all 4 sides. Because the sides are angled, the openings can be produced by so-called “shut-offs” between core and cavity contacting in each opening, thus not requiring side cores. Such a mold is much less expensive and can cycle much faster than the mold with side cores. The other picture illustrates a large box (G) with matching, flat snap-on lid (H).
A
D
B
C E
F
G
H
Figure 4.17 Typical crates and baskets
Figure 4.18 shows 10 and 20 Liter industrial pails. Depending on the ratio of flow length to wall thickness (L/t ratio), they use either a single gate in the center or three gates near the rim to facilitate filling.
4.1.3.4 Elongated Products For maximum strength it is always better to gate near the end of the product (cold runners) or on the top surface (A) near the end of the product (3-plate or hot runners). Gating into the top may be undesirable for appearance, but proper function of the product should always be the first consideration. A gate mark at the top surface can often be hidden, for example, inside a letter or and ornament on such surface, or by creating a “fake vestige” in a location symmetrically opposite the gate (see also Section 2.8.3, Witness Lines). Figure 4.19 shows typical elongated products (tooth brush, safety razor handle), which must be gated near the end for maximum strength. Similarly, other products (not shown), such as cutlery (disposable or not), must also be gated at the end. If these parts were to be gated in the middle they would break at the gate.
Figure 4.18 Very large industrial pails
A
A
A
A
Figure 4.19 Typical elongated products
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4 Mold Selection 4.1.3.5 Inside Center Gated Parts Cold Runner 3-Plate and Hot Runner Molds In all gate locations mentioned so far, the gate is always located on the outside (top or side) of the product, i.e., in the hollow (concave) portion of the cavity. This is good practice because It is the shortest path for the plastic between the machine nozzle and the gates
The product will stay with the core from where it can be easily ejected by any conventional method
The best cooling is on the core where the product shrinks on, to ensure proper ejection
However, there are cases where a gate on the outside of the product is not desirable, mostly because of required esthetic appearance. Typically, this is the case with high-quality closures (for perfume bottle caps, some in-mold labeled products, etc.) or some spray bottle or over-caps, where a gate vestige on top would “cheapen” the appearance of the package. But there are also some technical products and enclosures for which inside gating is preferred.
A
B
B
Figure 4.20 Over-caps (A) with inside center gating (Courtesy: Husky)
Figure 4.20 shows over-caps (A) with inside center gating. It requires long nozzles (B) and, as can be seen, there is not much space for core cooling. These molds cycle 2–3 times longer than outside center-gated molds, but have no gate vestige on the outside. This example shows clearly how little space there is to provide good cooling, a gate insert, and good heat insulation from the nozzle tip. There are some basic drawbacks with inside center gating (ISCG) (see Fig. 4.21):
Figure 4.21 Outside center gated (top), and inside center gated mold (bottom)
The sprue (or drop of a hot runner system) is much longer than with outside gating to reach the bottom of the product.
Because the cavity is on the moving mold half, the product cannot be ejected easily. The product will most likely shrink onto and stay with the core from where it is injected; therefore, an ejection system must be incorporated into the injection side of the mold and, if necessary, into the stationary platen of the machine. Ejection by air would be best, but is often not possible because of the shape of the product and/or the plastic processed. Therefore, mechanical ejectors (strippers or ejector pins) must be on the injection side of the mold, which also carries the cores and the runner system. This ejector system is either air actuated or driven by mechanical links connected to the moving platen. Most likely, this ejection mechanism adds still more length to the sprues or drops. These difficulties are even greater with unscrewing molds, with the cores on the injection side and the sprue inside the cores.
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4.1 Selection of an Appropriate Mold
The cooling on the inside of any core contributes always more than 60% of the cooling efficiency of a mold. But with ISCG, inside the core is also the sprue of a 3-plate system or the drop of a hot runner system. For shortest molding cycles, we need to cool the core efficiently to remove the heat both from the product and the sprue in 3-plate molds; however, in hot runner molds, we must remove the heat from the product while keeping the hot drop well insulated from the cooled core so that the plastic in the drop will not freeze. Both these conditions mean that there is very little space to provide good cooling for the product and much slower cycles will be unavoidable compared to a similar product gated from the outside.
The need to provide an ejection system on the injection side makes it difficult to position the runners and cooling lines in either 3-plate or hot runner mold. While the moving mold half with the cavities becomes very simple and relatively small, the injection side with the cores will be very complicated and large.
With these problems, an ISCG mold is always considerably more complicated and about 25% more expensive to design and build and will cycle two to three times slower than a comparable mold with outside gated cavities.
4.1.3.6 Slender Products Round, thin-walled products such as vials, syringes, etc. are best (outside) center-gated, using either 3-plate molds or, preferably, hot runner systems, Fig. 4.22. Core Shift Slender products (length over diameter ratio of more than 2.5 : 1) have the problem of “core shift”; in fact, the core is not shifting but a bending of the core is caused by differences in the plastic injection pressure. It is practically impossible to gate exactly concentric between cavity and core; even with the closest practical tolerances, there will always be some minute misalignment between the center of the gate (at the closed end) and the center of the cavity space between the cavity and the core, Fig. 4.23. Such misalignment will allow more plastic to flow into one side of the core than into the opposite side and as the cavity space fills, the core will deflect because of a pressure differential in the plastic between the sides and remain deflected to some extent until the product is ejected. After ejection, the core returns (elastically) to its original straightness. The effect of such core deflection can be measured in the wall thickness but can also be seen easily by rolling the molded piece on a flat surface; its easily recognizable “banana shape” is caused by the different shrinkage conditions of the thicker and the thinner side of the vial (the thicker side takes longer to cool and thus bends the product after ejection). The thinner the walls are, the worse is the problem and the more precision in mold making will be required, adding to the cost
Figure 4.22 Gating for vials; (top): center gating (hot runner or 3-plate); (bottom): cold runner 2-plate gating. A and B show top view of gating at 180° and at 120°
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4 Mold Selection of the mold. On the other hand, thicker walls require more plastic (cost!) and longer cooling (more cost!). When center-gating, there is no problem with venting because the plastic flows toward the parting line where good venting is easy to achieve. To overcome core deflection, there are various methods (some of them patented) of stabilizing the core inside the cavity and/or selecting a stiffer core material with a greater modulus of elasticity (E) than mold steel to reduce the deflection of the core. Some tungsten-carbide alloys exhibit a modulus of elasticity 2.5 times greater than steel; however, they have little shock resistance and are expensive to manufacture. Figure 4.23 Schematic of effect of misalignment of gate and center of the core
Figure 4.24 Typical long slender products
There are mold makers specializing in these products (vials, syringes, pen barrels, etc.) who have the experience and skills to overcome the problems and provide good molds. An older method for making these products is to use cold runner gates (either self-degating or not) into the side of the barrel (see Fig. 4.22) at or near the open end, and to use two gates located at 180°, or three gates at 120° around the circumference of the barrel, or to provide a continuous ring gate all around the opening. The ring gate will then be machined off. The advantage of any of these methods is that the plastic enters the cavity space from two or more symmetrically opposed gates and flows in parallel streams, which tend to hold the slender core in center. One serious problem with this method is that the cold runner from the sprue is located in the same plane as the stripper. Using floating stripper rings is not possible because of the gap required for floating between the rings and the stripper plate and positioning the stripper rings exactly in line with the core is very difficult to achieve and very costly. Still, there are many multi-cavity molds built this way. The second serious problem with this method is the venting of the air as it is pushed ahead of the inrushing plastic. With vials, there is no opening at the closed end and vent pins at the top (dome) are absolutely necessary. A composite cavity, with a separate part forming the dome will permit vent gaps between the dome and the cavity portion forming the sides of the vial. Products such as syringes have a hole in the dome and can be vented there. Note that uneven filling because the center of the gate is not in line with the centers of the cavity and the core cannot only happen with slender products as schematically shown in Figs. 4.22 and 4.23. Figure 4.25 shows a fairly stubby container with ribs and outside center gating. The lower part of the photo shows a complete shot. Because the gate is (unintentionally) off-center, the cavity space fills unevenly, as shown clearly with the 6 short shots, from right to left. By progressively increasing the shot size it can be clearly seen how the plastic gradually fills the cavity space and produces an air entrapment that causes much difficulty when molding. Figure 4.26 shows the short shot of a thin-walled cap with a “corrugated” sidewall. The corrugations have slightly different radii in the cavity and on the core so that the outer tips of the corrugations are somewhat thicker than the sidewalls, permitting the plastic to flow easier through the thicker tips
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Figure 4.25 Stubby container with ribs and outside center gating
while filling the cavity space. This can be easily seen by the plastic being father advanced at the tips in the short shot. Note that the mold is almost perfect, with the advance practically equal all around. This is the result of very close tolerancing and good workmanship. Figure 4.27 depicts an over-cap with shallow, flat ribs (A). This is another example of how the plastic advances faster through heavier sections in the flow path. To demonstrate and check the filling pattern, the mold was first injected with clear plastic until it ran at optimal conditions. Then, some yellow colorant was added to the extruder. When the colored plastic reached the mold, the front of the incoming melt was still “clear” but the following melt was already yellow. The plastic advances much faster in the sidewall with the shallow ribs than in the wall sections between the ribs. These different wall sections could cause some molding difficulties, if the plastic in the thicker sections fills so fast that it causes an air entrapment.
Figure 4.26 Short shot of a thin-walled cap with a “corrugated” sidewall
4.1.4 Gate Size and Runner Systems A gate presents a serious restriction to the flow of the plastic; the larger the gate the easier the cavity can be filled. However, the larger the gate, the more unsightly will be the gate mark (“vestige”) left on the product. Valve gating would avoid this problem. In some cases, with certain heat insensitive plastics, a very small gate could be of advantage by creating so much resistance to the flow that the shear created will heat the plastic above the melt temperature and thereby reduce the viscosity of the plastic; this can sometimes help filling an otherwise difficult-to-fill cavity space. The gate size can be determined by calculations (see [5], Chapter 10) or by using past experience with similar products and materials.
Figure 4.27 Over-cap with shallow, flat ribs
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4 Mold Selection The various shapes of gate designs are discussed in detail in [5]. Here, we will discuss the advantages of certain gates and their influence on productivity.
4.1.4.1 Edge and Fan Gates (Cold Runners)
(a)
(b)
Figure 4.28 Schematics of edge gate (a) and fan gate (b)
Both edge and fan gates have been used from the beginning of the injection molding technology and are still used today. Both gates keep the product attached to the runner during ejection; the runner must be severed before using the products. Edge gates, when properly designed, will break easily at the product and leave a clean vestige, usually a slightly rough (matte) area the size and shape of the cross section of the gate. Fan gates leave a very narrow, long vestige that can be almost invisible. Large edge gates could be required for very large products or for products that must be molded with a long low-pressure hold cycle to ensure that the cavity will be fully filled without visible sinks or voids. The large gates are milled or sawed off, if clipping with pliers is not acceptable for appearance or if the plastic is too brittle. The cost of this extra operation must be added to the cost of production. Also, the cost of any jigs or fixtures required for this purpose must be added to the mold cost. There are a few occasions where the products should stay connected with the runners:
The products will be shipped with the runners, e.g., with family molds, when it is of advantage to have the end user remove the products from the runner when needed.
Very small and delicate products, needing 100% inspection. In this case, it may be easier to handle the whole array of runners and products from molding to inspection. This handling and the inspection can also be automated. Products are degated after inspection.
Oriented packaging into boxes is easier from complete arrays, where the products (e.g., cutlery) are still in the attitude as they were molded, rather than being randomly ejected.
4.1.4.2 Self-Degating Cold Runner Gates
Figure 4.29 Typical tunnel gate
The most frequently used self-degating method is tunnel gating (Fig. 4.29). If the runners and the gates are properly designed and sized, the products are severed from the runner as the mold starts opening. The molded pieces and the runners fall out together and must then be separated. There are automatic separating machines on the market. The gate vestige is small, usually a round or oval, slightly rough (matte) mark in the side of the product. If the product is deep enough, there is usually no problem for locating the gate; if the product is rather shallow, the steel remaining between the gate and the parting line can be small and fragile and easily be damaged; in this case, steel selection is very important, but even so, this is an area requiring frequent repairs. Placing an insert in this area when the mold is built will save much cost and downtime when the gate is damaged or breaks.
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4.1 Selection of an Appropriate Mold Self-degating is somewhat more expensive than edge gating and while an insert for the gate area adds some cost, it may be necessary for long running molds to avoid expensive repairs later. The somewhat higher mold cost may be well worth it in the long run. There is a number of different designs for tunnel gates [5]. The size of the tunnel gate is determined the same way as the size for any other gate; however, there is a limit to the size that the can be sheared off cleanly. If the gate is too large, hard and brittle plastics may shear poorly (very rough and uneven) and can easily damage the cavity wall where the gate is located. Submarine gating is another method of self-degating, if the piece is shallow and no gate vestige is permitted either on top or on the sides [5]. Submarine gates are more difficult to build than tunnel gates and are therefore more expensive to manufacture; the mold will also cycle slower. The vestige is similar to that of a 3-plate mold gate, but is on the underside of the product. Note: Dirt in the plastic can easily plug any cold runner gate, but such dirt is molded into the plastic as it freezes; it will be ejected with the runner so that the following cycle will see again a clean, open gate. As the runner is ground up before reusing the plastic, hopefully, the dirt will also be ground up; otherwise, sometimes later it could again plug a gate.
4.1.4.3 Inherently Self-Degating Molds 3-plate and hot runner molds are inherently self-degating, i.e., the gates break off as soon as the cavities and cores (with the products held on them) separate, provided the product stays reliably on the core as the mold opens. Gates for 3-plate molds are usually very small (“pin point gates”) and often take advantage of the fact that the plastic will heat up due to shear as it passes through the gate. Gates as small as 0.5 mm in diameter are quite common for small bottle caps, among others. There is an upper limit to the size. If too large, the vestige can become very rough and unsightly or too hot, causing stringing. The gate may even break the top of a thin product as the mold opens.
4.1.5 Hot Runner Molds Today, hot runners are a fully accepted technology and the preferred method of gating; they are replacing more and more of the older runner methods, especially the 3-plate systems. In fact, older, existing 3-plate molds can often be quite easily rebuilt into hot runner molds. In the earlier years of the hot runner technology – occasionally even today – mold makers design their own hot runner systems, either based on their own ideas or by copying other systems that gradually came on the market (see Fig. 4.30). Today, there are many well established companies specializing in hot runners, who sell either the basic hardware (manifolds, nozzles, heaters, etc.) or assembled hot runner systems, complete with all associated plates and other
Figure 4.30 Typical hot runner, section view (Courtesy: Husky)
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4 Mold Selection hardware ready to be joined to an otherwise complete mold built by the mold maker. All that is required for a purchase order is to specify the important interface dimensions and any production data such as plastic to be used and the mass of the product. These hot runner suppliers mass produce the hardware items and use specialized methods and equipment to produce better quality system parts at lower costs. Such hot runners are then guaranteed to work in the new mold and eliminate the need for the mold maker to experiment and waste time and money trying to get a “home made” system to work. Make sure to use clean plastic when operating hot runner systems with open gates
There are still some molds for which the advantages cannot be justified economically, especially for low production items. In these cases, the older systems, especially cold runner 2-plate molds, are still much in use.
4.1.5.1 Degradation of the Plastic in Hot Runner Systems Another important consideration is the amount (mass) of plastic injected into each cavity. Each plastic has a limited total time that it can remain exposed to high temperatures before it will start to degrade and lose at least some of its properties. A “temperature and time” graph can be obtained from the materials suppliers. Some plastics, and most of the commodity plastics, such as PS, PE, and PP, have a high tolerance for heat and can stand long exposure to high temperatures, much longer than many other so-called “heat sensitive” plastics. But even the commodity plastics will sooner or later degrade. If they are exposed too long to high temperatures, they too must be purged from the injection unit before good quality products can be produced again. When injecting into single cavity molds, there is usually no problem, because the runner system (the sprue) is relatively small compared to the mass of the product; the plastic inside the hot machine nozzle and inside the sprue are replaced at every shot. With multi-cavity molds, where we need a heated distribution system – the hot runner manifold and the drops through which the plastic must flow to the cavities – the plastic can reside there for a considerable time. A good rule of thumb is to have less than three shots of plastic in a hot runner
If the products are large and the channels in the manifold are relatively small, there is little concern, because the plastic residing in the manifold (the “inventory”) is replaced quickly (after one or a few shots), especially if the molding cycles are short. But small channels in the manifold cause a large pressure drop, especially if high pressure is required to fill the cavities. A compromise – an optimal condition – must be reached. The hot runner manufacturer uses computer calculations, based on the information on the product, its mass and shape (wall thickness), the cavity spacing, the plastic, etc., so that a manifold with the optimal channel sizes can be supplied. Manifolds for multi-cavity molds for very small products with little mass are more difficult, because the amount of plastic required at every shot is small and the plastic advances only slowly through the manifold channels, especially with longer molding cycles. This means that the plastic is exposed to the melt temperature within the hot runner system for a long time. Any, even minor, stoppage can bring the plastic close to or over the permissible
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4.1 Selection of an Appropriate Mold temperature/time limit at which it starts to degrade. If this is expected to be a problem, a cold runner mold should be selected or a better hot runner system may have to be developed, maybe together with a specialist in this field.
A hot runner should not consume any more than 25% of the available injection pressure
4.1.5.2 Open Hot Runner Gates There are two distinct styles of open gates, the circular pinpoint and the annular ring gate. Both gates function on the same principle, by (1) freezing off at the end of the injection cycle to avoid drooling while the mold is open for ejection, and by (2) opening up again, triggered by the pressure of the hot plastic as it is injected during the next cycle. The functioning of both styles of these gates depends entirely on (a) the operating conditions such as melt temperature, the nozzle temperature, the injection pressure, the timing, and (b) on the design characteristics of the system used, such as the cavity cooling, the size and shape of the gate, and the design of the hot runner system. Because of the small size of the gate (a small round hole in one case, a very narrow, circular gap in the other case), a serious drawback of the open gate hot runner system is its sensitivity to “dirt” (paper, wood, tobacco, metal chips, etc.) in the plastic. Unlike with the cold runner gates, any dirt fully or even only partially blocking the small passages will cause the cavity not to be fully filled. Additional dirt will remain there until the mold is stopped and the dirt removed by opening and cleaning the hot runner system. With good mold designs, the cavity plate can be pulled while the mold is in the machine and the obstructed gate or gates can be cleaned. There will be an interruption in molding, resulting in lost production of maybe one hour or maybe a whole day, which may nullify any savings from utilizing the system.
Figure 4.32 Schematic of circular open gate. The gate opens and closes based on thermal cycling and control of temperature and pressure in the gate (Courtesy: Husky)
Figure 4.33 Schematic of annular open gate. This gate also opens and closes based on thermal cycling and control of temperature and pressure in the gate (Courtesy: Husky)
Note that molding at lower melt temperatures is of greater advantage even though it requires higher injection pressures to fill the cavities
Figure 4.31 Hot runner maintenance while in the press is an important feature
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4 Mold Selection 4.1.5.3 Valve Gates (Hot Runner) This is a method to control the opening and closing of the gate, either by mechanical means or by electrically controlling the melt temperature within the gate. The mechanically controlled gates use a moving pin that opens and closes the gate as required during the cycle. Today, the valve pin is actuated mostly by compressed air and occasionally by hydraulic pistons.
A
Figure 4.34 shows a schematic of one of several typical valve gates. The principle here is that a pin (A) is mechanical moved into and out of the gate (B) on every cycle to open and close the gate. It allows for faster cycles and higher quality gates. The main advantages of valve gates are:
B
The gate can be much larger than the openings possible with pinpoint or circular open gates. Gate diameters of 4 mm (5/32 in.) or even larger are quite common.
The vestige of the gate is a circular mark, similar to that of an ejector pin mark. Because of the large opening, dirt is much less of a problem. Most dirt will pass through the gate when open and end up being encapsulated into the product, which may – or may not – be acceptable.
The cycle time is shorter than with a comparable open gate diameter because of the longer time required to freeze a larger open gate There is an exception to this: with very thin-walled products, the gate area with a valve gate would be hotter than with an open gate and could slow down the otherwise faster molding cycle possible, because the thin walls cool faster.
Figure 4.34 Typical valve gate (Courtesy: Husky)
The main disadvantages of this system are
Larger costs compared to open hot runner gates (approx. 40%)
Possibly added space requirement for the valve actuating mechanisms. This can affect the spacing between the stacks, especially if the products are small.
4.1.5.4 Combination of Hot and Cold Runners In some molds, often for smaller products and with a large number of cavities, but also with larger ones, as the example shown in Section 4.1.3.2, a combination of cold and hot runner systems can be of great advantage:
It can eliminate a large portion of the cold runner and thereby significantly reduce the mass of plastic to be reground or lost.
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There is much less pressure drop between the machine nozzle and the gates, because the pressure drops in the hot runner manifold is smaller than in a (long) cold runner.
It can be used when very small cavities cannot be located very close to each other, at a “pitch” (distance) for which there are no standard-spaced hot runner nozzles available, or where it is not possible or practical to use hot runner gates. Typically, a cluster of 2–6 (or even more) very small products can be gated from a small runner or a disk, which is fed from a hot runner drop.
It will shorten the cycle time. Large distributing (cold) runners take much longer to cool than the final runners feeding the cavities. Especially if the products cool rapidly, such heavy runners significantly slow down the molding cycle. The cold runner portion in such cases can be treated as any cold runner mold; it could be a 3-plate arrangement (rarely used) or a 2-plate system with edge or tunnel gates into the product.
4.1.6 Single Cavity Molds Many products, and in particular large products, are molded in single cavity molds. This allows the use of the simplest mold construction, with simple injection methods as well as simple methods of ejection. In addition, large products are often not required in very large quantities, and if they are, it is usually more economical to use two or more machines, each with single cavity molds. Such machines can later be used for other molds and give the plant more flexibility. Very large machines (15,000 kN or 1,500 tons and over) are usually dedicated for molds for specific, very large products, which cannot be fitted into a smaller machine because of the physical size of the mold, the clamping force required, and the required large shot capacity and plasticizing capacity of the injection system.
Figure 4.35 Typical 16-cavity mold; top: cold runner layout; bottom: hot runner supplying 4 drops to small cold runner, each supplying 4 cavities
4.1.6.1 Single Cavity Cold Runner Molds Single cavity molds have been used since the beginning of the injectionmolding era, for any product size from small containers to large pails. As explained in Section 4.1.3.2, edge gating a single cavity is often not practical or even possible; therefore, we will consider only gating into the outside on top of the flat or deep product. In a typical mold, the plastic enters the cavity space from a sprue inside a sprue bushing. The machine nozzle presses against the sprue-bushing seat while injecting; the sprue bushing must be well cooled to ensure that the plastic within is stiff enough for ejection before the mold opens up. Unfortunately, this cooling time for the sprue is often longer than the cooling time required for the product and will unnecessarily increase the cycle time. The mass of the tapered sprue increases as the length of the sprue increases (Fig. 4.36). This can be easily improved by using a shortened sprue bushing,
Figure 4.36 Cold sprues (top) standard length, (bottom) shortened, into top of product or into runner system
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4 Mold Selection and providing the molding machine with a longer nozzle. Shortening the nozzle length by half will reduce the mass by almost 75% of the mass of the longer one, resulting in shorter cycles. In addition, the gate vestige, after the sprue is trimmed from the product, is much smaller. This type of gate is used frequently for large parts, as long as they can be filled from only one gate.
4.1.6.2 “Through Shooting” System Through shooting is a better method than using a sprue; however, this method is only applicable, if (a) the cycle time is short and (b) if the L/t ratio is such that one nozzle alone will be sufficient to fill the cavity space (this method is really a hot runner, in its simplest form). It can also be called “single cavity insulated runner method” (see Fig. 4.37). The principle of this sprue is simple and the method can be used for most types of plastic. It is particularly suitable for plastics, such as PE and PP, but also for PS and other plastics if the cycle is short enough. The machine nozzle seats on a sprue bushing, with a large (approx. 15 to 18 mm diameter) “well”, deep enough to reach the short, open, circular, tapered gate, which leads to the cavity or to a runner system. The well can be as deep as 75 mm (3 in.), and even longer wells have been used successfully, but it is recommended to keep it shorter (approx. 25–30 mm). As the plastic is injected the first time, the well is filled with a plastic “slug”. The outer surface of the slug, in contact with the cooled cavity steel, will freeze, but because of the good heat insulating properties of most plastics, the melt around the axis of the slug stays hot long enough that even though the gate will freeze, the plastic injected during the next shot will easily traverse the still hot center of the slug. The heat of the incoming plastic will then melt the frozen gate and the cavity will be filled for the next shot. Figure 4.37 Two examples of the through shooting system; long slug (top) and short slug (bottom)
This system of runner and gate works very well for most plastics if the cycle time is 15 seconds or less; with PE and PP, molds with cycles as long as 30 s are running successfully. If a stoppage is long enough to completely freeze the plastic slug, it can be easily removed by pulling back the injection unit about 15–20 cm (6–8 in.) and then pulling the slug out with a heated, hooked wire or some special tool made for the purpose. It is suggested to provide the walls of the well with a draft of at least 3° per side and good polish so that the slug slides out easily. As soon as the slug is removed, the injection unit can move forward into the molding position, and production can resume. This through shooting system is very inexpensive to make and very reliable. Color changes are easy: As soon as one color is finished, the slug can be removed and the new color finds a clean mold. The gate vestige is small; it looks similar to the vestige of a 3-plate gate or an open hot runner gate. Other benefits of this method are: (a) there is no sprue to regrind or to scrap and (b) even though any dirt in the plastic larger than the diameter of the gate will plug the gate, this dirt is easily removed together with the slug and the interruption in production is just the short time required to remove the slug with the molded-in dirt and to restart.
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4.1 Selection of an Appropriate Mold The size of the gate is determined like any other gate, from a small pin point gate up to a gate of about 6 mm diameter. Larger gates are possible but they might not freeze and the gate could drool while the mold is open for ejection of the product. If a larger gate is required because of the large amount of plastic that must enter the cavity space, either a cold sprue as described in Section 4.1.6.1 or a single cavity, hot runner system (“hot sprue”) will be required, as described in the following.
4.1.6.3 Single Cavity Molds for Large Products There are several options for single gating a large product, apart from the old fashioned standard cold sprue (for multiple gates into one product see Section “Single cavity mold, multiple gated” see p. 134). (1) Use a short, cold sprue, as described in Section 4.1.6.1 Since the product is large, the cycle will probably be long (25 s or more). The sprue can be cut after molding, e.g., during stacking, assembling or packing, without adding labor cost. If the gate is large and needs to have a good appearance, it may have to be milled in a fixture. Many molders and designers overlook this simple, and inexpensive solution when making such large products and select more expensive methods. (2) Use the through shooting method, as described in Section 4.1.6.2 This method will work well, as long as – The cycle time fits in the time frame described in Section 4.1.6.2, – The gate size is smaller than approx. 2 mm diameter. This method could yield an even lower product cost because (a) there is no gate cutting required, and (b) the cycle time can be less than with a cold sprue, because the sprue may take longer to freeze than the product thus controlling the cycle time. (3) Use a hot sprue, as described in Section 4.1.6.4 below A “hot sprue” is essentially a heated cold runner sprue. The melt within the sprue is kept hot with electric heaters. The gate could be an open gate as shown in Figs. 4.33 and 4.34 or a valve gate as shown in Figs. 4.41 and 4.42. Hot Sprue, Center Gating The hot runner suppliers sell hot sprues of various designs as standard hardware, ready for incorporating into the mold. It is important to follow exactly the interface dimensions and tolerances specified by the manufacturers, as well as the operating instructions to ensure trouble-free operation. Hot sprues with an open gate (annular or circular) or with a valve gate can be purchased in a variety of lengths. They are most suitable when it is necessary to gate inside a deep product, too deep to reach for even a very long extended machine nozzle.
Figure 4.38 Single-cavity pail mold with a hot sprue (Courtesy: Topgrade molds)
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4 Mold Selection Figure 4.39 shows a hot sprue with heater bands for the top (A) and for the nozzle extension (B). The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume. Figure 4.40 depicts the schematic of the open hot sprue. There are heater bands for the top (A) and for the nozzle extension (B). The nozzle insert (C) can have different configurations, determined by the requirements of the mold, the type of plastic and the shot volume. A great advantage of a valve gated hot sprue is that the gate can be of any reasonable size, 4 mm diameter and even larger; the gate vestige is circular. Hot sprues are quite expensive (in the order of $3,000.00) and need heat controls, wiring, and air pressure lines and controls for the actuating mechanism. However, in some molds, these expenses are warranted. Hot Runner Manifold with Offset Gate(s) Figure 4.39 Hot sprue with heater bands
In some single cavity molds it may not be acceptable, or even possible, to use a center gate into the product; often, for appearance reasons. Two typical examples:
A
B
The cavity must be edge-gated, as shown in Section 4.1.3.2, where several drops from a hot runner manifold are located to feed one or more cold runner systems outside the circumference of the product.
The gate could be positioned away from the center, but still within the outline of the parting line (or the circumference) of the product (see examples in Fig. 4.45).
C
Figure 4.40 Schematic of the open hot sprue
Figure 4.41 Valve gated hot sprue
Figure 4.42 Schematic of the valve gate hot sprue (all: Courtesy: Husky)
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4.1 Selection of an Appropriate Mold Molds for Very Large Products and Limited Production Single-cavity molds, such as for large automotive products, can weigh many tons, and because the products change almost yearly with the model changes, it is important that all possible shortcuts be taken to keep the mold cost low, while still guaranteeing the best quality of the product. Because of the large mass of plastic entering the mold, today, all such molds use several valve gates, located usually where suggested by flow analysis. This may appear to be expensive but it is necessary to ensure the quality of the product and for achieving a reasonable molding cycle. On the other hand, great savings can be achieved by proper selection of the mold steels used. The cavities and cores are often cut right out of steel blocks, each of which could weigh several tons. “Conventional” mold plates are rarely used. There is no need for expensive mold steels or pre-hardened steels, especially if the specifications of the product do not require high gloss finish. Mild steels are often acceptable, but there could be inserts required for places where wear is expected. However, expensive beryllium-copper inserts are used frequently in locations where it is important to provide better cooling to reduce cycle time. Other good mold making practices, such as cross drilling for cooling channels, are replaced by the use of flexible hose connections from channel to channel. Simple horn pins, wedges, or hydraulic actuators can move side cores. Alignment is provided with leader pins and bushings.
Figure 4.43 Large automotive mold for bumper fascia (Courtesy: Accurate Molds)
Even the parting line match of the usually complicated shape of the products need not be perfect. Excessive but reasonable gaps can provide good venting for the large flow of plastic entering the mold, and ensure proper filling. Any unwanted flash occurring could be scraped off by hand. The large, sometimes unwieldy products are often removed from the molding area by robots, but additional handling is usually done by hand and, if necessary, any excessive or unsightly flash can be removed at that time. Figure 4.44 shows a mold for a rear door of a car. The mold weighs more than 17,000 kg and runs on a 2,000 ton machine. The wall thickness of the panel (in TPO) is approx. 3 mm and the molding cycle is approx. 40 s for a productivity of 700,000 pieces per year. Injection is with two Synventive sequential valve gates. The parting line matches perfectly, therefore no scraping is required. Finish is SPI #4 on the inside, and SPI #2 on the visible outside. Ejection is by ejector pins, with hydraulic actuators inside the mold. The panels are removed by robot, directly to a conveyor. In these cases, either a standard size (listed in a catalogue) hot runner manifold can be selected or a specially designed manifold will be required. The drops (nozzles) to the product or the cold runners could be open or valve gates. Note that today, many hot runner manufacturers offer standard sizes of many manifolds and all other hot runner hardware, from stock, at lower cost and faster deliveries, than special sizes.
Figure 4.44 Mold for a rear door of a car (Courtesy: Accurate Molds)
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4 Mold Selection There is an often overlooked and much less expensive solution for products such as the one shown in Fig. 4.45 (top). This solution is depicted schematically in Fig 4.46: Instead of using an expensive offset hot runner, this large product with an opening in the center can be easily filled from a cold sprue as shown in Fig. 4.36 (or a throughshooting sprue as in Fig. 4.37) that both feed into a cross-shaped or multispoke runner or even into a shallow disk in order to edge-gate or to feed a continuous gate all around the inside of the opening. Now we will turn our attention to the (small) cold runner and the problems possibly associated with it: (a) To make sure that the runner stays with the product from where it will be broken off or cut by an operator, (b) We will have some scrap, which may represent a very small percentage of the product weight, depending on the size of the opening.
Figure 4.45 Schematic illustrations of two large, single-cavity molds with offset hot runner gates within the outline of the products
Economically, both the cost of the runner and the labor of removing it, and regrinding the scrap, may still be less than the cost of the otherwise necessary hot runner system. This can be easily calculated. If the total number of products from the mold is small, the cold runner system is preferable. If the number is very large, the added cost of a hot runner can be easily justified Single Cavity Mold, Multiple Gated (Hot Runner or 3-Plate Mold) The following applies mainly to hot runner molds, but also to 3-plate molds. In a large single-cavity mold, several gates are often used to provide better plastic flow into the cavity. This approach must be selected if the L/t ratio for a single center gate would be too high or if one gate would not allow enough plastic to flow into the cavity. By choosing suitable locations for two or more gates, far enough spaced from each other, the L/t ratio per gate can be much reduced. A product that otherwise could not be filled at all, or only with very low viscosity (very hot) plastic and with very high pressures, can be filled much easier from several gates. Special care must be taken to ensure that the cavity space is well vented where the streams from the various gates are expected to meet, e.g., by placing ejector pins or vent pins there. One problem with multiple gating with open hot runner gates is that if one of them freezes more solidly than the other(s), the incoming stream at the next cycle will not be able to dislodge the frozen slug in that gate. The plastic will then not use all gates as intended; this can result in unfilled products. The most effective solution in such cases is to use valve gates. The advantages of valve gating have been described earlier. The disadvantage is the added cost of the valve gate systems and more controls in the machine.
Figure 4.46 Large product with center opening, with cold runner gating
When using multiple gating for 3-plate molds, typically when very small (pin point gates) are desirable for appearance, it is important to use very clean plastic; if one gate gets plugged, the other gate(s) will not be large enough to fill the cavity space. There is also a method of sequentially programming
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4.1 Selection of an Appropriate Mold the time when each of two or more valve gates open (by Synventive Corp.). This can be of advantage for properly filling cavities of complicated shape and to prevent the formation of weld lines where they are visible or where they could weaken the product’s strength.
4.1.7 Two and More Cavities, Cold or Hot Runner Molds Figure 4.47 shows typical cross sections for (A) 2-plate, (B) 3-plate, and (C) hot runner molds with two or more cavities. The 2-plate mold (A) is shown with a simple edge gate, but the layout would be similar with any other cold runner gate into the side (fan, tunnel) or underside (submarine) of the product. The 3-plate mold is shown in its most frequently used arrangement, i.e., outside center-gated. The layout for inside center-gating is similar, except that the gate is placed through the core, which is on the “hot” side of the mold and the cavity on the in moving half of the mold (see Section 4.1.3.5). The hot runner mold (C) is also shown in its most common arrangement, but it could also have inside center gating at a higher cost and with much less productivity. The example shows a simple open gate. The layout for a valvegated system would be similar.
4.1.7.1 Modular Molds or Retaining Plates? From the earliest days of molding technology, in multi-cavity molds (unless they could be cut right into the plates) both cavities and cores were inserted into pockets or bores of matching plates, the so-called “cavity-and coreretainer plates”. Retainer Plate Design Advantages of retainer plate design are
The parting line of the stacks is level with the surface of the two plates. This makes it possible to have runners for the cavities without leaking into the open
The mold can be shallower, which is of advantage with machines with little shut height.
Stacks can often be placed closer together, because the cavity walls can be thinner and still resist against the forces created by the injection pressure, but only if the cavities are forced into the plates with properly calculated press fit.
The mold is usually less expensive, because cavities, cores, and plates are smaller than corresponding size modular molds.
Figure 4.47 Three typical cross sections of conventional multi-cavity molds (A) 2-plate mold; (B) 3-plate mold; (C) hot runner mold
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4 Mold Selection There are several disadvantages with retainer plate design, even when precision (“jig-”) boring the locations
Accuracy of position and size of these bores or pockets
Accuracy (fit) of cavity and core inserts into these pockets
Problems with cooling channel layouts. There is usually less cooling possible and longer cycles will result
There are more problems with heat expansion differentials (see Section 2.5).
Molds with retainer plates are usually only recommended if they are small and the productivity is less important than the mold cost. All cold runner 2-plate molds, many 3-plate molds, and some hot runner molds have been, and are being, built with the retainer plate design and are still very common. Modular Molds
Figure 4.48 Modular mold construction for a thin-walled container. Note that the cavities are pocketed into a plate while the cores “float” (Courtesy: Dollins)
A more recent method of designing multi-cavity molds is to consider molds that are assemblies of a number (2, 3, 4, or more) of single-cavity molds (modules), mounted on common backing plates. Each such module has its own cooling circuits, but they share a common injection (hot runner, or, rarely, cold runner 3-plate) system and a common ejection mechanism. This design can be quite crude, especially with a larger number of cavities; the number of cooling water supply hoses can become very unwieldy. At a trade shows, I have seen such molds with more than 100 hoses for the IN and OUT of coolant and still more hoses for air supply. Such a mold may be less costly to build, but is expensive for set-up and service. There is also the problem of unequal heat expansion of the backing plates, resulting in misalignment. From these rather crude molds, the present system of modular molds was developed, where the (modular) cavities are inserted into jig-bored pockets of the cavity retainer plate that is part of the hot runner system plates, while the (modular) cores are (preferably floatingly) mounted on top of the core backing plate, which may, at the same time, be the back of the integral ejector box. Advantages of the later-design modular molds are:
Cooling water for all cavity modules is supplied through drilled channels in the cavity retainer plate and the cooling (and any air channels for ejection) for the cores are provided through drilled channels in the core backing plate. This makes for a simple, well-cooled mold with a minimum of hoses encumbering the space around the mold. However, it requires more thought in designing such mold and more elaborate machining, and is somewhat more expensive than the earlier designs.
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4.1 Selection of an Appropriate Mold
Because of the better cooling, the, mold will cycle faster for higher productivity
Interchangeability of components is easier to achieve and repairs are less costly.
Heat expansion is not a problem; by having the cores mounted to float a limited amount to self-align with the cavities, regardless of any temperature difference between the plates, the size of the mold (number of cavities) is not limited.
If this design is used for molding products that can only be ejected with air, it becomes a really simple mold, with very few mold shoe components. This method is of special advantage in stack molds, which are then much simpler.
Disadvantages of the modular mold design are:
The cavity spacing is larger than a comparable retainer plate design, because the wall thickness of the cavity must be strong enough to withstand the injection pressures. Therefore, the mold will be larger, for the same number of cavities and more expensive.
Because the mold will be larger than a comparable mold with retainer plate design, the cost of the mold will be somewhat higher, but this additional cost can be easily recovered by the increased productivity and lower maintenance cost.
Modular molds cannot be used for 2-plate molds, because the cold runners would be interrupted between the modules. However, they can be used for 3-plate molds.
Today, most molds for small containers (dairy, etc.) are of modular construction
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4 Mold Selection
N A B C
G O
O
F
K A L
H
M
E
I
Figure 4.49 Modular 6-cavity mold (Courtesy: Husky)
G
U P P
V L S
T R
Figure 4.50 Underside of core backing plate (Courtesy: Husky)
Figure 4.49 shows a modular 6-cavity mold for a container with core lock design. The cavities (A) are set into the cavity retainer plate (B), the hot runner system is in the plate (C) behind the cavities. In front is a cavity (A) with cavity bottom (E). The core assemblies (F) are mounted with float, on top of the core backing plate (G) that is also the frame for the ejector mechanism shown in Fig. 4.35. The core assembly (F) consists of the core (H), a BeCu core tip (I), the core retainer block (K), and the stripper ring (L). All water supply to the cores and the cavities is cross-drilled in the cavity retainer plate and the core backing plate. Some of the water connections can be seen at the bottom (M). The three leader pins (N) are just for mold handling and to protect the cores from damage; the final alignment is by the individual tapers (O) between cavities and cores. Note the numerous venting groves and channels, cavities, and stripper rings.
Figure 4.50 shows the underside of core backing plate (G) in Fig. 4.34. Note the substantial supports (P) under the cores and the center of the mold. Note also the three guide pins (R) and bushings (S) for supporting the stripper ejector plate. The stripper rings (L) are driven by four pins (T) each, which are guided in bushings (U) in the plate (G). There is no need for a mold mounting plate. The mold can be mounted directly by screws into the tapped holes of the machine platen or by clamps, entering the recesses (V).
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4.1 Selection of an Appropriate Mold 4.1.7.2 Cavity Spacing Note that the cavity spacing in 2-plate molds for most products is usually greater than in either a 3-plate or a hot runner mold because of the space required for the runners between the cavities. This means that for the same number of cavities, a somewhat larger mold shoe will be required; in addition, more clamping force will be needed because of the additional projected area of the runner system. Very small cavities, flat or with little depth, can often be located very close together and can take less space than hot runner molds, even when adding the space required for the runners. But hot runner technology has considerably advanced by providing drops to feed into more than one gate, therefore the products can be spaced much closer together.
4.1.7.3 Hot Runner Edge Gates There are two other gate configurations not mentioned earlier in the section on gates, because they are used only in hot runner systems with two or more cavities. The hot runner edge gate (HREG) is an open hot runner gate into the sidewall of the product. The principle (and construction) of the HREG is simple and explained in detail in [5]. Gates from one drop from the manifold can feed into just one cavity, two cavities (most frequently used), three, or even four small cavities. There are standard HREG nozzles commercially available. It is very important that the design suggestions by the manufacturers are closely followed. The system is very reliable and trouble-free, but must be used with absolutely clean, preferably virgin, plastic material to avoid plugging the small gates by dirt. Molds with HREG are frequently used in cases where gating into the top surface of the product is not acceptable, because the end use of the product or appearance reasons prohibit it. Typical examples are earlier molds for Petri dish bases and covers that need top surfaces with optical clarity, but also for clear small boxes for packaging delicate items, such as jewelry, cosmetics, and so forth. As stated above, drops can feed more than one cavity. For example, for an 8-cavity mold, rather than using a standard hot runner system with eight drops (one per cavity), a HREG mold can be selected with four drops only, each feeding two cavities. This could represent a considerable saving in mold and hardware cost. Note: HREG molds can be more expensive to design, build, and maintain. Today, Petri dishes are often molded using valve gates, placed near the edge of the top surface. The problem of hot runner gating small products, or where two products are placed closely side by side, can also be solved by using hot runner drops and nozzles as shown in Fig. 4.53. This method is less expensive than the use of hot runner edge gates, but will leave a gate vestige on top of the product, near its edge. There are also similar nozzles with three or even four gates.
Figure 4.51 Schematic of hot runner edge gate
Figure 4.52 Typical hot runner edge gate vestige (A) on the side, near the bottom of the product
Figure 4.53 One “drop” feeding more than one gate. This picture shows two gates, but there are also drops for 3 or 4 gates
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4 Mold Selection 4.1.7.4 Insulated Runner Molds Insulated runner molds are a further development of the through shooting principle. It is based on the insulating properties of plastics, as we have already seen in the through shooting nozzle (Section 4.1.6.2). Using large diameter flow channels, in the order of 16–19 mm diameter, the plastic layers closer to the walls will freeze, but the center of about 5–10 mm diameter remains molten to allow an effective flow of hot plastic toward the gates.
Figure 4.54 Schematic of cross section through a two-cavity insulated runner mold; (a) cavity plate; (b) core plate; (c) runner plate; (d) sprue; (e) insulated runner; (f ) gate
C D
B
A
Figure 4.55 Section through a 2-cavity insulated runner “slug”
D E
C B
A
Figure 4.56 Partial section through an 8-cavity insulated runner “slug”
While the principle of operation is simple (Fig. 4.54), the start-up of an insulated runner mold needs a certain amount of skill. At the first shot into the empty, clean mold, the sprue (d) and the rather large runner (e) are filled, together with some or all of the cavity space. If the first shot is insufficient to fill all cavities, after quickly removing the incomplete shot, a second shot immediately following will be usually sufficient to have the whole system filled. After removing this second, by now probably complete shot, the mold is ready for production. If not, a third shot may be required The main problem with insulated runner systems is the startup of the mold: after the first injection that usually does not fill the runner and all the cavities immediate action is of the essence. The incomplete first shot must be removed if it has not properly ejected, and the mold must be started again quickly so that the plastic in the sprue and runner does not have enough time to freeze. If the plastic does freeze, the mold must be opened between the plates (a) and (c) to remove the plastic in the runner; after closing and locking these plates together, the startup will be repeated. By then, the plates have warmed up a bit and will make the following startup shot(s) easier. Preferably, the cooling water is turned off during startup. Once the mold can run on cycle automatically, the molding conditions (pressures, temperatures, and times) can be adjusted for best productivity. With PS, the time frame available is approx. 15 s, for PP and PE it is approx. 30 s, before the runner will freeze. The main requirement for successfully running these molds is that the ejection method must be absolutely reliable to avoid any delays due to failure to eject. Once the mold runs on cycle, it could run without stopping until the production run is completed. Molds for 2, 3, 4, 6, 8, and even 12, or 16 cavities can be quite easily made and operated. A typical example is a 16-cavity mold for PE chair leg protectors that runs on a 25 s cycle; once started, the molder did not stop it at all for two consecutive years. This is not necessarily practical with other molds that need interruptions (time for maintenance), but it shows how well these molds can run. The advantages of this system are
Very simple and inexpensive construction
There is no need for any heaters in the system, although there are variants to the “true” insulated runner mold where pointed nozzle heaters inside the drop keep the gates from freezing. This is used with plastics such as PS that tend to freeze easier because of their better heat conductivity. Both Fig. 4.55 and 4.56 show the use of such heater probes with insulated runners.
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4.1 Selection of an Appropriate Mold Figure 4.55 shows a section through insulated runner “slug” of a 2-cavity mold for nylon gears (A). Note the flow of the last injection (blue, B) inside the white, older plastic (C). Note also the cylindrical hollows (D) where electric heated “torpedoes” keep the melt hot in each drop, especially with its pointed torpedo tip extending right into the gate. Such heated torpedoes are suggested for all molds requiring longer cycles and with plastics other than PP or PE. Figure 4.56 shows the removed insulated runner of an 8-cavity mold for PVC products (A). The cut-open section of one runner branch clearly depicts the flow of the dark plastic (B) within the surrounding, colder plastic (C). For illustration purposes, the color of the plastic was changed from white to dark. Note the pockets (D), where the heated torpedoes are located (see also note with Fig. 4.55).
Insulated runner molds are ideal for molds requiring multiple color changes a day
Color changes are easy; there are two methods: 1. Stop the mold, remove the runner, close the now clean mold, and restart with fresh new color after the extruder has been purged. This takes some time and effort. 2. An even easier method is to remove the old color plastic from the machine hopper but to continue to run the extruder while the machine is producing pieces of the old color. When the new color is fed into the hopper; it will gradually replace the old color in the extruder and after 15–20 shots products with the clean new color should be ready. The production of the shots during the changeover will have a color mix and will have to be scrapped, unless the plastic can be reground and used where the color mix does not matter. The runners inside the mold will change their hot inside core (where the plastic flows) to the new color, while the frozen plastic near the outside of the runners is still of the old color. This can be clearly seen in the two photos above. Note that with conventional hot runners, color changes are always done by gradually purging through the manifold while the mold is producing. A good hot runner design will permit changing from a lighter to a darker color in about 50 shots, and longer when changing from a darker to a lighter color. Safety Considerations with Insulated Runner Molds With all these substantial advantages of insulated runner molds, why are they not used more frequently? As these molds are built today, they are inherently unsafe as will be explained below, and with the rapid development of reliable hot runner systems and the associated standard hardware, the insulated hot runners have been put on the sidelines and have been either completely forgotten or are being avoided. Any “regular” mold can be operated from the operator’s side of the machines, without the need to open a safety gate, except during startup when the front
Insulated runner molds seem to be ideal – so why are they not used more frequently? The reason is that these molds need special skills for start-up and are inherently unsafe, as explained in the text
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4 Mold Selection gate must be opened to access the mold for servicing. The rear gate is usually either screwed locked, or does not have safety features such as drop bars. Opening the rear gate often stops the operation of the machine completely, even shutting down the hydraulic pumps. Over the years, with these safety features in molding operations, the number of personal injury-accidents has greatly decreased. Referring back the Fig. 4.54, plates (a) and (c) must be locked together solidly even though they are clamped together during injection by the clamping force of the machine. If plastic is injected (inadvertently or deliberately) while the clamp is open, these plates could see a large separating force; the plastic could escape through a gap between these two plates and spray into the open, causing injury to bystanders. With any such molds, to ensure that these plates are held together securely, pairs of large, solid screws or clamps are provided on the operator’s side of the mold and at the rear of the mold. To pull the plates (a) and (c) apart is relatively easy; after unlocking them, cavity plate (a) can be latched to the core plate (b); the clamp is then opened so that plate (a) moves with the core plate (b). As seen above, for startup of an insulated runner mold it is necessary to open the mold between plates (a) and (c), remove the runner, re-close and lock the plates together, and restart after closing the safety gates. This presents a serious problem. Since the screws or clamps are on both sides of the mold and this whole “operation” of unlocking, opening, cleaning, and re-locking must be accomplished within about 15 seconds; therefore, operators are required on each side of the machine. This can be very unsafe, unless other safety measures for the rear gate than the ones mentioned above are provided. There is always a risk of having more than one operator starting up any machine, but here, they are also being rushed, while only one of the operators is at the controls of the machine. The challenge is to find a method to allow locking the two plates (a) and (c) together from the operator’s side only, with only one operator used for startup. Once a method of safe operating conditions is created, this very economical system can be used for many applications.
4.1.8 Single- or Multi-Level Molds? “Stack” (multi-level) molds have a long history. Back in the 1950s, a stack mold was used for making matched color toilet seats and covers. This was an ideal application: both pieces are very heavy-walled and require similar, very long cooling times. In addition, the projected area of each piece was about the same and the color match is (inherently) perfect. Later, stack molds were used for smaller products, using elaborate systems of hot nozzles and cold runners; today, mostly thin-walled products are made on stack molds and practically every stack mold uses a hot runner system. The cycle times are often in the order of 5–8 s and the output of these molds
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4.1 Selection of an Appropriate Mold can be enormous. The theory and design features of stack molds are explained in much detail in [5]. Here, we are more concerned with why and when to use them. Understanding the following will allow the decision maker to make the right choice between single- and two-, or more-level molds.
4.1.8.1 Two-Level Stack Molds for High Production A 2-level stack mold consists of two essentially identical, conventional molds, placed back to back, but with one, common hot runner system feeding the two cavity plates. The two core sections are mounted on the stationary and moving platen, respectively. The center section of the mold, also called the “floating” mold section, (two cavity plates and the hot runner assembly in between them) is supported (directly or indirectly) on the machine base or on the tie bars. It is moved, usually half the length of the machine stroke, by levers, gears, or other mechanical methods in synchronization with the clamp motion. Alternatively, hydraulic cylinders could move the center section, independently of the clamp motion. The core half on the moving platen can be exactly like it would for a single-level mold and can use the existing machine ejection mechanism. The core half mounted on the stationary platen presents two problems:
For the same product, and the same number of cavities per level, a stack mold essentially doubles the output of single level mold. The stack mold requires only one rather than two molding machines, provided the shot- and plasticizing capacity of the machine can provide for the increased number of products made on the stack mold
(1) In most stack molds, the sprue supplying the center hot runner system passes through the center of the (stationary) core half, so that there must be a passage large enough for a long, heated sprue bushing to move through so that the cavity stacks closest to the center may have to be spaced farther apart. With the above mentioned toilet seat and cover mold, this problem was resolved by placing the open gap of the horseshoe-shaped seat on the side nearer the injection side of the machine, so that the sprue bar could pass through it and the seat could fall freely without hanging up on the sprue bar Figure 4.57 shows a 2 × 1 cavity stack mold for a large container. This mold is built for a machine equipped with the actuating mechanism and with special supports for the floating center section. Note the sprue bar (A) of the hot runner system (only partly shown) is located at the side of the cavity to engage in a special, offset hot runner manifold behind the core plate near the stationary platen (not visible in this photo).
A
Figure 4.57 A 2 × 1 cavity stack mold for a large container
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Figure 4.58 Typical air eject stack mold (2 × 4) for a round container (Courtesy: Topgrade Molds)
If the sprue bar cannot be in the center of the mold, as, e.g., in Fig. 4.57, the solution is to relocate the sprue bar outside the molding area. This requires an additional hot runner system in the mold half, located on the stationary platen. The machine nozzle enters at the usual center of the mold into the (additional) hot runner manifold leading to outside of the molding area where the long sprue can supply the melt to the center section from outside the molding area. In this arrangement, the main hot runner system is supplied near its end instead of the usual point in the center of the manifold. This method will allow molding large products even on that side of the mold where the sprue bar is located. This method is also useful for example for a 2 × 2-cavity mold or a 2 × 6-cavity mold, with the center cavities arranged one above the other, for free-fall ejection, so that the upper products will not hit the hot sprue bar when falling down. This off-set sprue bar arrangement is also useful when extracting the molded products with robots, should more clearance for the path of the robot arm be required. Injection-molding machines, as a rule, don’t have ejection systems on the injection side; actuators must be added either to the core section located on the stationary platen, or to the stationary machine platen. The ejection system can also be mechanically linked to the moving platens or to the operating mechanism moving the floating section. This is sometimes done (very crudely) by connecting the ejector mechanism (stripper plate or ejector plate) with chains or (somewhat better) with lost-motion links to the center section of the mold. A better solution is to connect the ejector mechanism with links to the “propeller” that actuates the center section. One disadvantage of these methods is that the machine stroke must be closely controlled to avoid damage to the links. In addition, the ejection takes place only close to the end of the opening stroke, and MO time is necessary to allow the products to clear the molding area. A newer design avoids these problems. Figure 4.59 shows a stack mold for lids. The profile of the rocker arm (A) is designed so that, as soon as the roller (B) on the cavity-side is engaged, the shorter arm of the rocker pushes on roller (C) to move the stripper plate (D) forward to eject. This can take place early during the opening stroke so that the products have enough time to clear the molding area before the mold closes again. Usually, no MO time is required and the mold will cycle faster, for higher production. Note that springs (not visible) inside the core plate return the stripper plate as soon as the rocker (A) leaves the roller (B). This method also makes the mold independent of an accurate opening stroke of the machine, because there are no fixed links between the mold sections. The rocker arm system is also less expensive (by about $10,000) than an independent ejection system on the mold half mounted on the injection side of the machine. The ideal solution (where possible) is to use only air ejection from the cores, requiring no mechanical ejection system at all; but this only is possible with certain product shapes and plastics, see [5]. Figure 4.60 shows an early design (year 1975) of a self contained stack mold with center section supported and aligned on the upper tie bar (A), actuated by levers (B) and (C).
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C
D
A
E
Figure 4.59 Self-contained 2 × 32-cavity stack mold for threaded lids. The lids are stripped off the cores. The mold is equipped with rocker arms (A) in each corner of core plate and the matching rollers (B) and (C) for the actuation of the arms and motion of the stripper plate. Note the BeCu core caps (E); random ejection; product mass: 14 g; cycle time: 11.0 s; productivity: 21,000 pieces per hour
B
A
C
C
B
Figure 4.60 Self contained stack mold (Courtesy: Husky)
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C
Figure 4.61 Self-contained 2 × 4 stack mold for dairy containers
E
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Figure 4.61 shows a recent self-contained 2 × 4 stack mold for dairy containers (modular construction). It features special supports for the center section (A) and rack and pinion actuation (B) for its motion. The mold is equipped with servomotors (C) and suction cups (D) to remove the air-ejected containers. The mold runs without MO time, at a 2.75 s cycle, for a productivity of 10,473 pieces/h. Figure 4.62 shows a recent self-contained 2 × 8 stack mold (modular construction) for fairly heavy (55 g) stadium cups. The mold features rack and pinion actuation (A), air ejection, random free-fall. Note the center sprue (B) and how the machine tie bars (C) are supported (D) on the base to take the mass of the heavy center section (E). The mold has a cycle time of 10 s, for a productivity of 5,760 cups/h.
A E
Figure 4.62 Self-contained 2 × 8 stack mold (modular construction) (Courtesy: Husky) D
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4.1 Selection of an Appropriate Mold C
A
Figure 4.63 A 2 × 4 family stack mold for two different rectangular tubs (Courtesy: Husky) B B
D
A stack mold can make identical products in both levels or have stacks for matching products in each level. This offers the possibility for perfect color match, for assembling the products inside or near the mold, and/or for controlled ejection for subsequent assembly or other post-molding operations, as explained in Section 4.1.2.2. Figure 4.63 shows a 2 × 4 family stack mold for two different rectangular tubs (modular construction). Air ejection and cam-operated actuators with suction cups (A) remove the products and drop them into chutes (B) on the side of the center section. This mold is built for a machine specially equipped for stack molds. On the side of the center section (C) are provisions (D) to mount the center section to the machine supports. Cycle time 6 s; no MO time; productivity: 4,800 tubs/h. Figure 4.64 shows a 2 × 8 stack mold for containers (modular construction). Air ejection and cam-operated actuators with suction cups (A) remove the products and drop them into chutes (B) on the side of the center section. This mold is built for a machine specially equipped for stack molds. The side of the center section has provisions to mount the center section to the machine supports. Cycle time 6 s; no MO time; productivity 9,600 pieces/h. Notable advantages of stack molds are as follows:
For the same output there is only about one half of the plant space required, compared with two machines making the same product on single-level molds.
The same clamp size can be used for twice the number of cavities. In fact, it is required to have about 10% more clamping force than for a similar, one-level mold, as explained [5]. However, it is important that the machine has at least twice the shot capacity than that required for a single
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A
Figure 4.64 A 2 × 8 stack mold for containers (Courtesy: Husky)
B
B
level mold for the same product. If not, the mold cannot be filled. Also, the plasticizing capacity must be large enough to supply at least twice the amount of plastic.
The mold usually costs approx. 1.8 times of two single-level molds.
There is less handling of materials and products, because there is only one point of production instead of two; also, fewer services are required.
Disadvantages of stack molds include:
Any breakdown of mold or machine can have twice the effect on production. If one of two single-level molds stops producing, there will be at least the output from the other mold.
The molding machine may have to be modified for longer tie bars, more stroke, to provide good support for the tie bars and the floating center section of the mold, and more travel to retract the injection unit. It may even need a larger injection unit. The safety gates protecting the point where the machine nozzle sits on the sprue bushing may have to be extended to protect bystanders.
Sometimes, more heat controls are needed, because now there is the equivalent of two molds controlled from one control panel.
When planning ahead for new installations with the intention of using more stack molds in the future, we must consider two possibilities. 1. A planned stack mold can be a complete (self-contained) mold, with the supports for the floating (center) section and the actuating mechanisms (racks and pinion, or levers) as part of the mold. This mold can be operated in any suitable machine, but is more expensive because of the
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4.1 Selection of an Appropriate Mold added components; at today’s rates (year 2005) approx. US$15,000 per stack mold. There is a safety concern, which must be considered. While the floating plate can sometimes be well supported during operation with (especially long and strong) mold leader pins, there is the danger of dropping the center plate, if it is not safely secured during handling and installation. It is better to have the usually heavy mass of the center section independently supported on the machine frame (using the well supported tie bars or ways on the machine bed) to ensure alignment with the other mold plates. 2. The supporting and operating components (standard accessories) can be added to the molding machine as a one-time machine investment of about US$25,000. The cost of these accessories is readily recovered after the second mold used in such a modified machine. The following figures show how much simpler the mold can be if the machine is already equipped with all the necessary hardware to support and move the center section of the mold. Figure 4.65 shows a 2 × 2 stack mold for large flower pot trays, installed in a machine modified for use with two-level stack molds. The machine features random air-ejection. Carrier (A) for center section (B) glides on lower tie bars (C), which, in turn, are well supported (D) on the rigid base. This machine uses a different design of rack and pinion drive (E) for moving the center section. The racks are part of the machine. The pinions are hidden, above left, behind the safety gate. Cycle time 8 s; productivity: 1,800 trays/hour. E
Figure 4.65 A 2 × 2 stack mold for flower pot trays (Courtesy: Husky) B
A
C
D
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Figure 4.66 A 2 × 12 stack mold for stadium cups (Courtesy: Husky) F
E
D
A
B
C
Figure 4.66 shows a 2 × 12 stack mold for heavy containers, installed in a machine equipped for 2-level stack molds. The mold features random airejection. Carrier (A) for center section (B) glides on tie bars (C), which are well supported (D) on the rigid base. This machine uses “propellers” (E) and levers (F) for the actuation of the center section of the mold. The levers are part of the machine. Cycle time 10–12 s; productivity: 7,200–8,640 pieces/h. One machine today can produce the output of 7–10 machines that are 30 years old
Figure 4.67 Typical growth pattern of productivity of molds and machines
Figure 4.67 shows a bar chart for a typical growth pattern of productivity of molds and machines. The increase is the result of the development of better molds and better performing machines used for this product, a “409” (4–9/ 16 in. diameter) series container. Note that this is the productivity of one machine and that the footprint of the 1,000-ton machine is only about 1.5 times that of the 225-ton machine.
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Figure 4.68 A 4 × 8 stack mold for lids (Courtesy: Husky) B
A
D
C
4.1.8.2 Multi-Level Molds for Very High Production Rates Theoretically, there is no limit to the number of levels. In compression molding of thermosets, the use of up to ten levels is old technology and quite common, e.g., for rubber floor mats. Figure 4.68 shows a 4 × 8 stack mold for lids, installed in a machine equipped for 4 level stack molds with random air-ejection. The (standard) small propellers (A) and links (B) are part of the mold; the large propellers (C) and links (D) are part of the machine. Cycle time 5.5 s, productivity: almost 21,000 lids/h. For practical reasons, the limit in injection molding today is four levels, mainly because of the size (the length) of the mold and machine required for multiple stacked levels, but also because of the required output and injection capacity of the injection unit. Dozens of four-level molds have been built and are in production since the early 1990s. The following illustrations are of 4-level molds built for machines specially equipped for these molds. Note that the red colored parts are parts of the machine, not the mold, and can be used for other molds on the machine. Figure 4.69 shows a 4 × 8 stack mold for containers, installed on a machine equipped for 4 level stack molds with air ejection and swing chute design: Servomotors (A) move swing arms (B) with suction cups over the cores to receive and remove the cups, which are then dropped into the chutes (C) when the mold is closed. The actuation of this mold is of a different design, with one propeller (D) and different levers (E) and (F) for the motion of the three moving sections. All red parts are part of the machine. Cycle time: 4.7 s; productivity: 24,500 containers/h.
For the same product and the same number of cavities per level, a 4-level stack mold essentially quadruples the output of single-level mold. The stack mold requires only one rather than four molding machines. A specially built machine provides the required high shot and plasticizing capacity
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B
C
Figure 4.69 A 4 × 8 stack mold for containers F
E
D
4.1.8.3 Provisions to Remove Molded Products This is an area often overlooked when planning for a new and better production method. Even when molding with single-level molds, handling the quantity of products coming from the mold can become a serious problem. I remember in the 1960s, the first 2-cavity disposable drinking cup mold running at 30 shots per minute. Because of the large number of cups falling out of the mold (60 cups per minute) we had to provide conveyors out and away from the molding area, toward automatic stackers. If this handling system stopped for any reason, the molding machine was literally covered with cups within minutes and had to be stopped to prevent damage to mold and machine. The problem is: “What to do with the molded products?” This problem must be solved before even considering any high-production mold. How will we get rid of the products in time to clear the molding area?
Example 4.1 A 2 × 4 stack mold for a margarine tub will run at a 5 s cycle and produce 2 × 4 · 60 s/min ÷ 5 s = 96 tubs per minute. Even when properly stacked, they represent quite a volume, but being ejected at random will create quite a mountain. One of the 4-level molds used was for a PE lid of 150 mm (6 in.) diameter with 8 cavities per level, at a 5.5 s cycle, it yielded 4 × 8 · 60 ÷ 5.5 = 349 lids per minute, or 21,000 per hour!
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4.1 Selection of an Appropriate Mold When considering the use of stack molds, especially for fast cycling products, special measures must be planned to remove the huge amounts of products. Depending on the shape of the product, these measures could be any of a number of controlled ejection methods, conveyors, stackers, packing machines etc. Figure 4.70 shows a typical cup stacker. The stacker is positioned at the rear of the molding machine. It consists of an unscrambling vibratory conveyor (A) that receives the randomly ejected, scrambled containers from under the mold. The cups are then elevated to the orienting section (B) and moved to the stacking section (C), where they are counted to the required stack length. The stack is moved to the staging table (D). From there the stacks are removed by hand and packed into boxes. Production, depending on product size, ranges from 2,400 to 12,000 pieces/hour. Figure 4.71 shows a typical lid stacker for round lids, in a range from 3 to 6.5 in. diameter. The randomly ejected lids are moved by in-press conveyors onto a twin-disc unscrambler (A). The lids are then single-filed from each disc and oriented ready for stacking in two lanes up the elevating conveyor (B), to the stacking system “waterfall” (C). Next, the flow is split again and four lanes (D) enter the spin bar stacking section (E). Stacks are cut to length ready for transfer to the tray loading section (F). The automatic tray feeder (G) moves the cardboard trays under the stacked lids, four stacks at a time, and moves the loaded boxes to the unloading station (H) from where they are hand packed into cartons.
Figure 4.70 Typical cup stacker (Courtesy: CBW)
Figure 4.72 shows a (CHSV) retrieval, transfer, and stacking system, seen from the side, where the molding machine would be located. Arms enter between the open 2 × 8 container stack mold. Eight suction cups face the cores in each level. The ejected containers are held in these cups, which then retract out of the molding area. Another array of suction cups in the
A
E
B C D H F G
Figure 4.71 Typical lid stacker for round lids (Courtesy: CBW)
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Figure 4.72 CHSV retrieval, transfer, and stacking system (Photo courtesy: CBW, USA)
“out-side” position picks up the cups from the transfer arm. After two shots, the receiving station rotates and stacks the cups to a predetermined number. From there, the 16 stacks per level move on a conveyor to the packing station.
4.1.9 Semi or Fully Automatic Operation? With extremely rare exceptions, every mold can run automatically. “Automatic” in this context means that the mold cycles without an operator’s intervention, such as opening the safety gate to remove the products, or lubricating (spraying) the molding surfaces, and so forth. The following reasons make it desirable for the mold to run automatically:
Safety considerations. Every time an operator needs to open the gate and reach into the molding area, there is a safety hazard, even with all the safety features on today’s machines. Safety features have been known to fail and, worse, they have been deliberately tampered with or voided to save costs to repair a problem. The best and safest policy is to have no operator near the operating parts of the machine. Note that in general, mold operators employed to “run” a machine are often not educated in machine operations and are therefore more difficult to train in safe procedures around a machine. Usually, well-trained setup personnel are employed to install the molds and to get the machine up to production. They are better educated and well trained on the job and are less likely to run the machine in an unsafe condition or to condone unsafe practices.
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Quality of product. Uniformity of the quality of product depends much on the uniformity of the cycle time. Some operators are more skilled than others, but even with the same operator, the time to open the safety gate, to remove the product, and to close the gate before starting the next cycle can vary greatly, depending on the time of the day or night, the length of time working on the machine, and is affected by fatigue and boredom. If the product tends to hang up in the mold, or worse, if a portion of the piece sticks in the core or the cavity and must be removed with a tool (e.g,. with a pointed piece of brass wire), it may take a while before the mold can be restarted. These undesired stops and variations in the molding cycle affect the melt quality and the cooling temperatures in the mold and can result in unequal quality of the product. Also, if such stoppages are too long, the extruder and any plastic in the mold may have to be purged, before the mold can be restarted.
Absenteeism. This is often a serious problem, especially with the available labor force required to operate the molds. To be operated most economically, molding machines should be run uninterrupted, 24 hours a day, for at least 5 days a week. Many plants operate 7 days a week and never stop a machine, except for mold changes and scheduled mold and machine maintenance. The problem is that absenteeism can create labor shortages, which are difficult to control and affect the continuous operation of a machine
Labor cost. The elimination of machine operators is often quoted as the reason for running fully automatic, but in fact this reason is much less significant than the others given above. With very few exceptions, every mold can be designed so that it will run fully automatically. The problem is sometimes that the mold has been designed correctly to run automatically, but was installed and set up poorly so that it will not eject properly, as intended.
4.1.10 Insert Molding There are many applications where “inserts” must be used in a mold. Here, we will highlight a few examples (see Fig. 4.73). The real challenge is to design these molds to run automatically. Inserted labels at the bottom of products. There are different methods of adding labels to the mold before it closes. Label insertion is always done automatically, often with multi-cavity molds. They use automatic label dispensers reaching into the molding area and placing the label into the cavity or on the core while the mold is open for ejection Inserted labels at the sidewalls of products. More difficult is the application of labels on the sidewalls of containers, which are usually tapered, with little or large draft angles. Figures 4.74 and 4.75 show a typical system for applying labels on the side of containers. Figure 4.74 shows a view (from the rear of the machine) of an automatic inserting mechanism, attached to a standard
Figure 4.73 Variety of plastic lids with labels applied automatically in the molds
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A
B
D
C
E
Figure 4.74 View (from the rear of the machine) of an automatic inserting mechanism (Courtesy: Hekuma)
Figure 4.75 Schematic of the system shown in Fig. 4.74 (Courtesy: Hekuma) 1000
Inserting labels
Cavities arrays: cups 2 cavities 2 cavities 4 cavities 8 cavities Inserting-head Takeout-head of EDAT for of EDAT for cups Taking out cups labels
Fixed plate
Movable plate
663
Operator side
890
Ferromatic K 155 D
Non-operator side
Taking over labels
Label separation
Horizontal stackingstation
Overhead cable trunking
560 6388
1420
High speed side entry robot HELI 1-1700
3087
Pivoted chute for manual packaging
Label magazine refilling position Electric cabinet
3750
Cup stacking
Label magazine refilling position Label preforming Push and wind device (Patent depending)
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4.1 Selection of an Appropriate Mold molding machine. The mold (A) can be seen behind the tie bars. In the foreground, left, is the dispensing mechanism (B) for the pre-formed cupshaped label blanks (C). Vacuum in the transfer arm (D) picks up the inserts, ready for the next cycle. After the mold arrives in the open position, the arm moves the inserts into a position opposite the cavities (left) and the vacuum changes to air pressure to push the inserts into the cavities. At the same time, the opposite face of the transfer arm receives the finished products as they are ejected from the cores and holds them with vacuum. When the transfer arm arrives in the OUT position, the finished products are then air ejected and stacked to the right (E).
A
C B D
Figure 4.75 schematically shows the same system as in Fig. 4.74, including other features, which are not visible in the photo. It shows the label magazines, the label separating, and the label pre-forming devices, and the stacking of the finished products. The top right corner gives layouts of molds suitable for this operation with 2, 4, 6, or 8 cavities. Printed strips. Some molds feed printed strips through the cavity side of the mold and cut the label as the mold closes. This too is performed in a fully automatic mold. Paper and plastic containers. Some products (e.g., low-cost boxes) consist of a printed cardboard blank in a shape that, when folded, forms a box. As the mold closes, the core folds the cardboard while pushing it into the cavity. Plastic is then injected so that the cardboard along the open joints and corners is sealed by narrow, thin, molded plastic. These molds run also fully automatically, with a dispenser supplying the blanks to the open mold.
Figure 4.76 Molded box
Figure 4.76 shows a molded box (A), consisting of printed cardboard bottom and sides, but with a molded plastic rim (B) and plastic edges (C). Also shown is the flat cardboard blank (D), which is fed automatically into the mold. Metal inserts. The inserts can be loaded with independent robots or with dispensing and inserting attachments to the mold. Unfortunately, there are still molds for which the inserting is done by hand or with suitable loading boards, requiring an operator at the machine. Wire inserts. Molding plastic over electrical connectors and electrical wires is a special industry. This is done manually by using shuttle molds with at least two cavities (or cores), depending on the design of the product and the mold. While one mold half with the inserts is in the molding position, an identical mold half is outside the molding area, where an operator places the inserts into their respective locations. This is done mostly in vertical clamp machines, so that the shuttle can slide horizontally in and out of the molding area. This is relatively safe, because the (sitting) operator never needs to reach into the molding area. The molding cycle is usually fairly long and the operator has ample time to place the inserts before the next shot is ready; the finished products are removed when the other shuttle reaches the OUT position, before reloading.
Figure 4.77 Metal insert over-molded parts
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4.2
Summary
Up to this point, we have learned about the things to be considered by the decision maker in order to make the proper selection of a mold, i.e., how to look critically at the product, the importance of the expected production and productivity, and how to select the type of mold (and molding machine) most suitable for the job. All this applies to the technology of injection molding as it stands today, but is also valid for any future developments in this field. The most important principles are always common sense, simplicity and safety, and the target to produce the best product, at the best quality and the lowest cost.
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Mold Cost, Mold Price and Delivery
5.1
Mold Cost and Records
Some of the following may be obvious to the reader, but it is surprising how many mold makers and their staff do not keep proper records. The decision makers must be aware of the methods used by mold makers to arrive at a mold cost and mold price. They will then better understand what is involved when negotiating the purchase of any mold or special equipment used to increase production and to improve productivity. The mold cost is the total of the actual costs incurred to make a mold, which include general plant overhead, but does not include any addition for risk and profit as will be discussed later. It is the sum of all moneys spent for raw materials, hardware, and services, plus the total of the time (hours) used by designers, machinists, assemblers, and other technicians involved in building a mold and any additional equipment required for this mold, multiplied by the hourly wage (or salary) rates, which will include all employee benefits provided. Added to this is the plant overhead rate. Hourly rates and/or overhead rates used in these calculations can be different for different employees or groups of employees and for different equipment used; or, there can be a single, plant-wide rate. Overhead costs can also be included in the hourly rates, so that the actual rate used to arrive at the mold cost consists of, for example, $18.00 (actual hourly pay to the employee), plus $12.00 (employee’s benefits), plus $25.00 (overhead rate) for a total rate of $55.00 per hour to be used when calculating the mold cost. In order to arrive at an accurate mold cost it is therefore important (1) To have a complete and clearly understood Bill of Materials (BoM), and component drawings, (2) To record all the funds spent for purchased items and services, (3) To have a method of recording conscientiously the actual times spent on this job by each employee, e.g., by filling out time cards, and (4) To have clearly defined hourly rates The old-fashioned method was to have the BoM shown on the assembly drawing. This used to be satisfactory with simple dies or molds, but is not practical for today’s molds, which often consist of many components. It is also not convenient for a buyer or accountant to work with an often unwieldy, large drawing.
Keep proper records of any job!
Major costs to make a mold: Raw materials Direct labor (hours) Machining costs (hours) Purchased components Equipment depreciation Testing costs (hours) Overhead Plant costs
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5 Mold Cost, Mold Price and Delivery The mold maker must make sure that only materials and times necessary for the mold as designed (originally planned) are included in the actual mold cost. Neither time, nor materials required to make changes to correct any design or manufacturing errors while building the mold, nor any costs which were found necessary to be added after the mold was tested before shipping should be included in the actual mold cost, if these costs were not anticipated when estimating. These additional costs will unfortunately eat into the profit margin, because they cannot be billed to the customer. These changes and corrections, as well as their reasons, must be properly recorded, so that they will be remembered when using this record for a repeat order of the same mold or for future estimates of similar molds. Always create job files for every project (molds, etc.)
All mold makers should keep a special “job file” for every mold built, in numerical order of jobs, by subject, and possibly also by customer, and keep it in one central location rather than in the memory or in the files of individual employees. Changes in personnel are unavoidable and it is quite common that much important data is lost when an employee leaves. These records must include the original cost estimate, a copy of the purchase order or sales contract, the confirmation of order, the BoM, the actual cost data, and the test report of the mold (or the system). Any significant information gathered about the performance of the mold at the molder after delivery and any complaints or changes that were necessary to make and any other information that could be valuable for future, similar molds can then be added to the file at later dates, as they occur.
5.1.1 Spare Parts for the Mold This is an often-overlooked area. Today, especially with high production molds, there are some delicate, easily damaged mold parts such as mold pins. Other parts, such as gate inserts, stripper rings, ejector pins and sleeves, and other moving parts are subject to wear. In addition, there are electrical heaters and thermocouples that may have a limited service life. It is also a good idea to have the proper size O-rings on hand when dismantling and re-assembling a mold, to make sure there won’t be any water leaks. It is always a good idea to order spares at the time of the mold. The cost to produce another piece will be much lower
Mold spare parts will be much less expensive to make while the mold is being built in the first place than to have them built when needed. It is good practice to have such spare parts on hand when buying a mold and this should always be included in a mold order. While they add to the mold cost, the will save much time when the mold needs repairs and ensure minimum downtime.
For multi-cavity molds, a rule of thumb is to order 5% spare parts and round up
It is up to the mold maker to suggest the spare parts to be recommended to the buyer at the time of the mold order.
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5.1 Mold Cost and Records
5.1.2 The Basic Elements of the Mold Cost The total mold cost consists of a number of costs directly attributable and necessary for the making of a mold, as detailed in the following sections.
5.1.2.1 Designing the Mold, Including Checking of Drawings Design of a mold can sometimes require considerable time. The time required is not necessarily related to the size, but always to the complexity of the product and to the performance features expected of the mold. Design time can range from 10 to 15% of the total hours estimated for the mold, but could be as high as 20% or even more. But this time is not wasted: always remember it is easier to change the designs at an early stage than to change the steel once cutting has started. CAD (Computer Assisted Design) has revolutionized design practices, but is really most useful when applied in conjunction with standardization of designs of mold details. The more hardware, mold features, or entire molds are standardized and in the memory of the computer, the less design time will be required. But this does not imply that there is not ample room for the ingenuity on the part of the designers and it must be understood that there is no progress without change. It may be necessary to deviate from established standards and sometimes to change them partly or altogether, if a better design can be found. But the overall target must never be forgotten: While a proposed design change could improve one area, e.g., the mold performance, it could negatively affect another area, such as mold life or the manufacturing process, so that the final cost of the product could increase.
Figure 5.1
Mold designer working at a CAD station
Another area that must not be overlooked is delivery time. Mold deliveries are always urgent and critical. Molds are usually required “as soon as possible” and for this reason alone it is often better to stick with existing, reliable standards. They assist not only during the design phase but also in machining operations that may also have standard procedures, and even specially equipped machines and fixtures to facilitate the making of mold parts. Checking of drawings is often overlooked or ignored, but it is very important; at least 10% of the design time should be dedicated for checking. Checking is much simplified with CAD, because the computer performs most calculations, but it is important to make sure that the data input was correct. Checking for dimensions is only part of the process. The checker must also make sure that the final mold design covers all design features required by the product design, all fits and clearances for the mold, and much more. Any error can be very costly. Having a responsible checker, preferably another experienced designer (never the original designer!) looking at the drawings before they reach the manufacturing phase will be much less expensive than having to fix errors later. Figure 5.2 shows the various design and manufacturing steps in relation to the time needed for completion (usually weeks). The description on the graph is self-explanatory.
From experience, it is practically impossible to check one’s own designs and drawings
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5 Mold Cost, Mold Price and Delivery
Sample Mold Project Timeline Time step
QUOTING Obtain project information (part drawing, etc..) Develop rough concept Quote
ORDER CLEAN UP Order received Send out order confirmation Obtain complete design information Finalize concept Quote any modifications to original concept
DESIGN Stacks or inserts Mold Shoe Hot runner Assemblies BOM
MANUFACTURE Inserts Process planning Programming Rough machining Hardening Final machining and polishing Shoe and Hot Runner Process planning Programming Machining (Gun drill, mill, bore and grind) ASSEMBLY TESTING Adjustments Re-testing Acceptance Prepare for shipping
Figure 5.2 Time line
Shipment
5.1.2.2 Materials The cost of material is usually about 10–15% of the mold cost, and may be as high as 20% or even more, in large but otherwise simple molds. Raw Materials The deciding factor when selecting steels for mold shoe and stacks is usually a policy matter, whether to make mold shoes or plates in house or purchase them from a mold maker supply house, either as (listed) catalogue items or have them made to order. It is essentially a simple question of economics: there are considerations, such as the shipping distance and time from a steel supplier and the need to carry steel inventories and invest much money in them.
5.1 Mold Cost and Records When selecting mold materials, there are many considerations: for the mold to last a long time and to perform best, the following characteristics must be considered for suitability:
Compressive strength
Wear resistance
Corrosion resistance
Toughness (against impact)
Thermal conductivity
Resistance to high temperature
In addition to the above, the following are also very important:
Machinability
Hobbability
Polishability
Dimensional stability in heat treat
Weldability
Nitriding ability
Cost of the raw material
Availability
Materials must be selected to prevent galling and seizing; materials which slide across each other should have different molecular structures (unless one of the materials is nitride plated). As a general rule of thumb, material suppliers recommend that even for dissimilar materials in a contacting condition, the difference in hardness should be 6 Rc if possible. For similar materials the difference should be 10 Rc. When selecting material for a mold, there are several material properties to be considered. The following list contains the most important properties:
Ultimate and yield strength
Impact strength
Modulus of elasticity
Hardness
The effect of temperature on these properties
Thermal conductivity
Thermal expansion
Wear resistance
Corrosion resistance
The quality of the materials (cleanliness) is an important consideration.
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5 Mold Cost, Mold Price and Delivery Some mold makers (usually larger operations) prefer to keep a selection of steels for the mold plates and for stack parts in plates and bars in their stock; this may require heavy lifting equipment, large machine tools, a large inventory, and much plant space. Other mold makers, with easy access to steel suppliers, can contract with them to supply small and large plates already cut to size, rough finished, or even ground to size. Still other mold makers (usually smaller operations) specialize in making only the stacks and to buy the completed mold shoe from mold supply houses and then to assemble and test the completed mold. In all these cases, standardization to a limited number of sizes can have a significant influence on the mold cost. When buying steel, contract for large quantities (e.g., estimated annual requirements) for lowest price
Contracting large annual volumes of steel can lower the price of steel significantly. The larger the pieces of steel, and the larger the annual quantities committed to purchase from the supplier, the lower will be the cost per mass (kg). In the case of very large mold parts, it is often worthwhile to buy the steel in forgings, with only little machining allowance. The forging process incurs separate cost, but saves not only on steel costs but also on machining time. Other raw materials used in molds are BeCu, aluminum, bronze, plastics (for wear), among others. When buying BeCu, it is important to inquire before ordering how the blanks will be made. Blanks cut from rolled bars are usually free from enclosures or voids but are not available in larger sizes. If larger pieces are required, they should be pressure-cast to ensure homogeneity. It is not unusual for small voids inside the work piece to become visible after many hours had been spent on machining BeCu parts and the pieces have to be scrapped at high cost to the mold maker. Standard Hardware Today, no mold maker even thinks of making hardware items such as screws, dowels, and others. However, it is important that hardware items are used as they are provided by the supplier. Any modification can diminish their rated strength characteristics, and therefore they must not be modified for safety reasons. If they absolutely need to be modified, they must be called up in the BoM as a special part, so that they are easily identified as “special” in assembly and service. Mold Hardware Mold makers used to make many mold hardware items, such as leader pins, bushings, ejector pins, and even hot runner parts, among others. Today, it is often much less expensive to use standard hardware as supplied from mold supply houses and other specialists. They are also easier to replace for service. They are made in large quantities, on special equipment and with specially selected materials, heat treatment, and surface finishes, and are usually of much better quality than the homemade varieties. The cost of standard and mold hardware in a mold (not counting any hot runner systems) could be estimated as about 5% of the total cost.
5.1 Mold Cost and Records 5.1.2.3 Production Planning Cost for production planning is usually included in the overhead. It includes the costs of deciding the best way of routing the work piece through production, the selection of the appropriate machine tools, cutting speeds, the need to prepare jigs, fixtures and gages for the job (and to design them), and the cost of programming software for machining each of the required mold parts. Obviously, standardization will be of great advantage. In some CAD/CAM system, some of the programming is done as the mold part is designed. At smaller mold making enterprises, it is usually the boss or the foreman who does the production planning.
5.1.2.4 Machining Costs Machining costs includes the total of all costs incurred when transforming a “blank”, i.e., a raw piece of steel, into a finished mold part, ready for assembly. These transforming methods may include: Rough machining. Cutting blanks from rough plates, sizing and squaring these blanks, and machining these or purchased plates by milling, drilling, deep hole drilling, boring, rough grinding, or other methods. Rough turning. Rough turning from rods or cut blanks. Stress relieving. This is often required after large amounts of material have been removed (cut) from a pre-hardened work piece, which may cause it to shrink and/or warp. Stress relieving will ensure that the finished part will retain its shape after the final machining. Finish machining. Milling, drilling, turning, finish grinding, jig boring, jig grinding, honing, lapping, EDM, etc. Heat treating (hardening) (see Section 5.1.2.9) Hobbing of cavities. The method of producing cavities by hobbing is rarely used today. For details of this technology see [5]. Finishing, engraving, texturing, polishing (see Section 2.1)
5.1.2.5 Cost of Special Gages This includes the cost for any gages, fixtures, or other devices that are required to assist in machining and/or checking of this particular mold. It includes the cost of gages required to check special tapers and the cost of any special tools used to facilitates machining, which will save significant time when making any number of identical mold parts. If the cost is included in the mold cost, the customer should have the right to these manufacturing aids, but they could also be kept at the mold maker for future use for similar molds for this customer.
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5 Mold Cost, Mold Price and Delivery 5.1.2.6 Cost of Electrodes for EDM Electrodes to produce the necessary shapes are usually machined from special carbon compounds or longer lasting, special copper alloys. For a large number of identical shapes, electrodes can also be molded or cast from special alloys by specialists in this field. Which type of material to use, or whether to use machined, molded, or cast electrodes is a question of economics and the required accuracy. In any case, the cost of making or buying the electrodes must not be overlooked; in addition, the cost of the special wire for wire EDM must be included in the mold cost.
5.1.2.7 Cost of Plating Hard chrome, flash chrome, or electroless nickel-plating and other surface finishes are best left to specialists. The cost is considered part of the mold cost. Note that if any change to a part already chrome-plated is required, the plating must first be removed (by a method similar to the plating) before remachining. After the changes, the part must be plated again before assembly. Plating and other surface treatments are rarely done in-house. The time delay and the transport costs can be substantial and the decision maker should consider selecting a different mold material, even if it is more expensive or more difficult the machine. Typically, chrome-plating can be avoided by choosing a suitable stainless steel.
5.1.2.8 Cost of Transportation Between Shop and Service Providers Include the cost of transportation to and from specialists of any of the machining operations subcontracted by the mold maker. Operations and services performed by outsiders may include any job beyond the capacity of the mold maker’s equipment, or not within his expertise, or they may be less costly than doing them in-house. Also, the shop could temporarily be overloaded and it would be preferable to purchase the necessary service elsewhere to avoid delaying the job. The cost of transport is easily overlooked. Some mold makers consider this cost as overhead, but it should really be charged to the mold.
5.1.2.9 Cost of Heat Treatment Many mold makers use outside sources for heat treatment of the mold parts. Most heat treatment shops have regular (often daily) pickup and delivery services and this cost is usually included in the price for their service. Because the heat treatment for most mold or tool steels is different from each other and varies to achieve different hardness and toughness conditions of the steel, it is good practice for the mold designer to specify (wherever possible)
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5.1 Mold Cost and Records only a carefully selected, small number of different steel types and range of hardness. Occasionally, the designer can select from a small number of most frequently used “standard” steels, which can be treated to a condition almost as good as the most suitable but rarely used steel. This allows simplifying the handling of the heat treatment and shortening the time for the work pieces to be returned.
Standardize on the types of steel selected if possible
Some mold makers do their own heat treatment on a selected, relatively small number of steels and subcontract only those parts that are outside the capability of their own facility, e.g., if the parts are too large for their furnaces or because it is a steel for which they have no facilities or experience in heat treating.
5.1.2.10 Cost of Inspection Inspection of mold parts between the various manufacturing steps and after completion of a part is essential. The more often the work pieces are inspected, the less is the likelihood that a “spoiled” part continues through more operations on its way to assembly, where it will be discovered to be defective. However, inspection can be very expensive. Most mold shops have their own methods and practices to minimize the cost of inspection; some have inspection stations located near certain machine tools or between the various groups of machine tools; some have central inspection stations, to avoid duplication of the usually very expensive testing equipment; some educate the machinists to check their own handiwork, some depend on independent inspectors. There is high cost incurred with checking; this could be charged to the mold cost, but it is often considered part of the general overhead.
Inspection is expensive but saves money in the long run
5.1.2.11 Cost of Assembling In some mold making shops, a “lead mold maker” will do the assembly himself or supervise the assembly process by technicians or the machinists themselves, but in larger mold shops, the mold assembly is mostly done independently of the manufacturing of the mold parts. Ideally, if all drawings are correct and the parts are made to specifications, assembling a mold should not be more difficult than assembling any clean, delicate machine or apparatus. The parts are supposed to fit and nothing more than standard or torque wrenches, a soft hammer, and lubricants should be required to assemble a mold. The ancient method of fitting mold parts by filing or grinding should not be used. If parts do not fit as they should, they must be returned to the design office, to establish why they don’t fit and who was at fault: the designers, the machinists, or the suppliers. Any corrective action must originate with the responsible mold designer and be properly recorded for the future, especially if repeat orders are expected. To make changes to mold parts without this practice will only perpetuate the error. Any costs incurred should be charged against the department from which the error originated and not recorded as part of the mold cost.
Figure 5.3 Mold assembly area (Courtesy: Topgrade Molds)
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5 Mold Cost, Mold Price and Delivery 5.1.2.12 Cost of Mold Testing Today, many mold makers have at least one molding machine for mold testing. If the machines in-house are not suitable for the mold to be tested, the mold maker will have to test the machines at the molder who ordered the mold. If this is too difficult because of distances and possible need for crossings of national borders (with all the problems of custom duties, paper work, etc.,) or because the customer does not want to interrupt his production just for testing a mold, the mold maker could buy machine time from a (close-by) molder. One way or another, testing a mold adds to the cost of the mold. Where and how the mold will be tested must be discussed at the time of ordering and testing time and machine time should be included in the mold cost. The actual cost of testing, i.e., the time of mounting the mold in the machine, startup, getting the mold on cycle and running it “hands-off ” (i.e., without any further adjustments) for a specified length of time (typically 2–4 hours, sometimes more) and then shutting it down and removing it from the machine is only part of the test. The molded products from each cavity must be inspected for visual defects, measured for accuracy, and possibly tested for their intended performance. This of course takes time, and the mold maker may have to wait days or even weeks until the product, and the mold, is accepted. But there is also another, very important, purpose of testing: can the productivity be improved by changing molding parameters or even by changing some features of the mold or its auxiliary equipment to reduce the cycle time? This can take much longer than the planned original testing. In some cases, the customer or molder will make these tests in his own plant, after receiving the mold. An unfortunate but quite frequent result of testing is the discovery that one or more dimensions of the product are outside specifications, either in all cavities (design or concept error), or in only some of the cavities (manufacturing or concept error). In this case, the mold needs to be removed from the machine and dismantled, the “offending” mold parts altered or replaced, or in the worst case, the mold design needs to be changed. This also means retesting of the mold. Part of a proper testing program is the issuance of a setup guide, detailing all molding parameters used for the final, nonstop test run, which produced the test samples. It should be mandatory that the responsible mold designer be present during all testing procedures of “his” or “her” mold to see first hand and learn from any problems as they show up, during the installation, and the running of the mold. This person is then also responsible for any remedial action, for recording any problem, and to make sure that they do not occur again in the future, not only in this type of mold, but in any other mold having similar features. Even if several identical molds are made at the same time, for the same customer, they all must be individually tested.
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5.1 Mold Cost and Records Molds are often tested several times before the mold is finally accepted. While the first (and sometimes the second round) of testing are expected and often included in the mold cost, additional testing is probably the fault of the mold maker, not of the customer. Retesting is expensive, but is not really part of the mold cost; rather, it must be charged to the learning experience of the mold maker and should not be added to the mold cost. If the same mold will be required again later, the error causing the trouble should not be repeated, if good records have been kept of all the errors and the remedial actions. In some cases, a representative of the customer is present at the tests and can accept the mold right then and there. More often, the product samples must be shipped to the customer for approval, which means that, while waiting, the mold will have to be removed from the machine to make room for other uses of the machine.
5.1.2.13 Cost of Errors Costs incurred after testing because of necessary adjustments and rework due to mold maker’s or designer’s errors or inexperience must never be charged towards the mold cost; however, they can be foreseen in the “risk factor” used in determining the mold price (see Section 5.3.1).
5.1.2.14 Cost of Product Design Changes Product design changes while the work on the mold progresses are, unfortunately, quite common. It is important to establish (estimate) any expected cost increases caused by such changes at the time the change is requested and to have the customer to agree to pay for it. A simple change while the mold is still being designed could have a negligible effect and its additional cost could be ignored. A more complex change could be quite significant, even requiring redoing one or more mold parts. It could even affect the planned mold design, the estimated mold cost, performance, and any quoted delivery time. The further the job has progressed, the more expensive can even a small change, such as the diameter of a core pin, turn out. Many unpleasant discussions or arguments between a mold maker and a customer can be avoided, if the effect of a change on the quoted mold cost is discussed and clarified at the time of the request for a change and not when the mold is already finished and approved by the customer.
If the mold maker makes a mistake, he cannot increase the mold price but must absorb it. Similarly, if the customer made a mistake in the product drawing or changed his mind regarding a feature of the product after the mold job is started, he too will be responsible for any costs caused by the requested changes
5.1.2.15 Cost of Crating and Shipping Depending on the sales contract, costs of packing and shipping are often borne by the customer; if not, they must be included in the mold cost. These costs can be substantial, especially if the mold must be packed in a seaworthy crate or shipped by air. This point must also be clarified at the beginning and confirmed in writing.
Note that everything agreed upon must be confirmed in writing
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5.1.3 Cost of the Preparation of a Mold Manual When buying an automobile, a kitchen stove, an electronic appliance, or an electric shaver, we always expect to receive an instruction manual to tell us how to properly use it, even if we know how to drive or how to use any of these gadgets. Every user expects from the manual to explain safe use, the necessary upkeep and maintenance schedules, how to trouble shoot it, and where to get help, and so forth. The same applies when buying a mold. A proper mold manual is expected not only to describe the mold (mold drawings, bill of materials, wiring diagrams, timing sequences, etc.) and how to operate and maintain the mold properly. It will also contain data on products used in the mold that were supplied from vendors (valves, actuators, etc.). It should also contain a test report from the final testing at the mold maker that should give the optimal settings and conditions at that time. This does not mean that the molder will not have ample opportunity to change the suggested settings to further improve the mold’s productivity, but it means that the suggested conditions and settings were optimal, at the time of testing, and give a good starting point for the customer starting up the mold. A table of contents of such a mold manual is shown in the Appendix. It is obvious that to assemble a good, comprehensive mold manual as suggested is time-consuming and costly, but it will serve the customer well, especially when operating the mold for years to come.
5.2
Overhead
This is not meant to be a book on accountancy practices and we are not presuming to suggest how to determine the overhead rates that will be used in the calculations of the mold costs. Each plant has its own management and accountancy methods and will decide how to arrive at an appropriate overhead rate. Here, we would like to show what is usually included in overhead:
Buildings and land: rent (mortgage), heating and air conditioning, cleaning, maintenance Power, water, other supplies not attributable or chargeable to a project in progress Interest on moneys owed Insurance (public liability, fire, etc.) Indirect labor: salaries and benefits required to run the offices and plant, except for those whose time is directly chargeable to the customer Cost of administration (management, personnel, accounting, purchasing, sales, security, etc.) Communication (telephone, etc.)
5.3 Mold Price
Transportation (cars, trucks, travel, etc.) Office computers, furniture, and supplies Plant supplies, oil, cutting fluids, small tools Production planning Investment in checking (inspection) equipment, usually spread over several years Other costs associated with the smooth running of the plant
The total of all these expenses over each year is divided by the number of hours of all those employees (engineers, designers, machinists, assemblers, mold testers, etc.), whose time is charged directly against the project they are working on, and which in turn will be paid by the customer. By increasing the productivity and the quality of the shop equipment, e.g., by buying better (or newer) machine tools, the overhead will go up. However, the time required to make a mold part should decrease, for example, by being able to cut more steel per hour, or by reducing the setup time of the work piece, or by having one machinist operate two or more (automatic) machines. This can greatly reduce the direct labor portion of the mold cost. However, this type of investment in plant and machinery will only be economical, if the new machines will work enough hours per year so that the hoped-for savings in labor are greater than the increase in overhead.
5.3
Mold Price
Mold “price” is the amount quoted by a mold maker to the prospective customer, either directly, but quite frequently indirectly, through the molder who will (possibly, but not for sure) make the products. The problem for the mold maker is that in many instances, a customer will approach three or more different molders, who in turn will then approach three or more mold makers, so that it is quite possible that nine or more mold makers will be requested to quote for the same job! I remember a time, when within one week inquiries for molds for the same product from three different molders landed on my desk. It is obvious that in such a case, only one mold maker will get the job, but it is also possible that none of them will get the job, because it could be just an early inquiry to find an approximate mold cost to assist in establishing the cost for a new product, which may or may not, come into production. There are exceptions to this, if, e.g., a customer needs a few identical molds in a hurry and could ask several mold makers for quotes and then lets some or all of them build these molds. There is another serious problem with this method of approaching many mold makers for the same quote. Although this should not be the case, unfortunately, the mold maker with the lowest bid often (probably) did not have the experience with this type of product or mold and has greatly
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5 Mold Cost, Mold Price and Delivery underestimated the work to be done. This could be bad for the buyer, who may then get an inferior mold, but also for the mold maker, who will probably lose money on the job. A conscientious mold maker will try to build a good mold, even if he or she will lose money on it. After a number of under-priced mold orders, a molder who keeps losing money will eventually go out of business. On the other hand, deliberately quoting high is not helpful either, because there will be no buyer to purchase at the quoted high price. Buying on price alone is a very risky proposition
There is, however, a leveling influence in the trade: a shop that always bids low will get a lot of jobs, which will fill the shop and require extended deliveries for new prospective orders. Because there are only a limited number of mold makers that are easily accessible to a molder, shops with higher prices will then also get busy, and the shop with the lowest prices will gradually learn from the gained experience to make better estimates in the future. A short but significant story from an earlier experience: In the 1950s, I was designing mostly tools and dies for metal parts, but also plastics molds, at a large electrical manufacturer. We had our own large tool room, but sometimes there was so much work that we had to subcontract the excess. At one occasion, we needed 35 blanking and forming dies for a rush project very quickly and had all designs detailed and ready. To get sooner into production, we planned to place orders with several suppliers. I invited twelve tool and die makers to quote. We had previously worked with nine of them; three we did not know. I told them that nobody was going to get all 35 jobs and grouped them so that each group was for one specific product. Everybody picked up the already prepared drawings and within days, most quoted on just two or three groups according to their preference, but all 12 die makers quoted on one specific group of 6 dies. Before, I had already estimated the cost for this group at $6,500. The quotes came in as follows: $2,650, $4,000, eight were between $6,200 and $6,800, one was $8,500, and one at $10,250!!! (Note that at that time, $1,000 was a lot more money than it is today). I went to see the lowest bidder whom I did not know. He was a farmer, outside the city, who had in his barn a lathe, a small milling machine and a small surface grinder, all second hand, with virtually no overhead. He told me he wanted to become a tool and die maker, but had so far only experience in jigs and fixtures. I also talked to the boss of the firm that quoted $10,250, whom I knew quite well, and asked him how he arrived at such a high price. The answer was that he had lost money the last time he built some dies for us, and wanted to recuperate his loss! Needless to say that neither of these two got the job. The moral of this little story is that (1) Experience is needed when quoting (2) One can never recuperate in one job losses suffered on a previous job. (3) It also points out the importance of knowing the suppliers you work with, by visiting them, and by getting references, if possible.
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5.3 Mold Price Our company had a policy that stated that when letting a contract we must make sure that the supplier had not made a mistake in quoting and that we must never select a supplier based on the lowest bid alone. This is mainly in the company’s interest. If the supplier loses money, he will go under; if he makes money, he could become be a good and reliable supplier. Unfortunately, many buyers, often at larger companies, used to place all their orders with the lowest bidder, regardless of other considerations. And all too often, the lowest bidder was the one that made mistakes in estimating or was not familiar with the subject. The result was that many tool and die makers (and this applies to mold makers as well) always lost money with these customers, and in future just refused to bid for them at all, or only quote jobs where they expected not to get into trouble. Once the proper mold cost has been estimated, we must add profit to it, to arrive at the mold price to be quoted. Never forget that in order to stay in business, the mold maker must make a profit, or he will eventually be forced to close down.
5.3.1 Risk Factor When quoting any new mold, it is very important to consider the possible risk taken, based on our familiarity (or lack thereof) with the product or the type of mold; however, there are other factors, which are all purely business decisions. The first question must always be: is the product similar or, better yet, identical to one for which we have built molds before? In this case, there should be little or no risk. If the product is similar, but a different number of cavities is required or, possibly, the runner system is different from one we have done before, or if the wall thickness is smaller than we had experience with, or if there is some other point where there may be need for some (costly) learning experience, it may be wise to add a risk factor of approx. 10–25% or even more to the estimated cost. This depends on how certain we are that the planned mold will work as expected. If the product is different but still within the range of products for which the mold maker has already built molds, the same or even more “risk insurance” may have to be added. If we believe that we are entering unknown territory, we should consider either to decline quoting at all, or adding a high enough risk factor so that, if the quote is accepted despite a high price, there will be enough money to learn on the job and build a good mold. Most customers understand that there is nothing gained if the mold maker is not familiar with the type of mold required. I have often refused to bid and suggested to the prospective customer some other mold maker more familiar with the type of products for which he wanted a mold, not because
There is a risk with every quotation
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5 Mold Cost, Mold Price and Delivery we could not have made it, but because the customer would be better served by a specialist in that field who could provide not only a better mold but probably at a lower price and with shorter delivery. Such customers were always grateful for this approach and came back when they had a job in our line of expertise.
5.3.1.1 Development Design Sometimes, a mold maker may consider a new or unfamiliar product and/or mold as a challenge. If technologically solved successfully, it could open up a new field of expertise with the potential for manyfuture, profitable sales. In this case, the mold maker could just add his ordinary markup to a relatively low, “guesstimated” cost, and absorb any cost above this as the cost of “learning experience” and consider it as “product development cost”. When working on new developments, have the customer participate and share the development and design costs
There is also another approach when “charting unknown waters”. It works well if there is a good relationship and trust between customer and mold maker and if the mold maker has a reputation for making good molds and for developing new, or improving existing technology. In such case, the customer will readily agree to pay a quoted price for engineering and design time to develop a new mold design (or molding method) and even for any costs of prototypes and experiments, in order to arrive at a mold price based on such preliminary work. The customer can then, depending on the terms of such a “research” program, retain ownership of the tentative designs and hardware. If the job goes ahead, the finally estimated cost of the design time for the mold or project will be reduced by the amounts already paid. In my own experience, most such design contracts have resulted in orders. The molds and any connected equipment could always be reasonably well estimated and quoted and the customer was better off this way. If a job did not proceed it was mostly because of economical rather than technical reasons. In injection molding, there are very few things that are impossible to do and if a project was dropped, the reasons were usually either very high mold and/ or system costs or the final cost of the product could not be economically justified.
5.3.1.2 Deliberately Low Quoting to Get a Job This is sometimes done if business is slow, so that rather than laying off and losing skilled, hard-to-find workers, the mold maker could decide to forego adding profit to the estimated mold cost or even go below cost just to keep the operation busy until more work comes in. This, however, can be risky: at the time, it may appear to be a good move and the mold maker may be able to fill up the shop with marginally profitable or even money-losing jobs, but the shop may not be able to accept (for reasonable delivery time) other, profitable jobs.
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5.5 Arriving at Mold Cost and Delivery Time
5.4
Mold Cost Is Absorbed by the Molder
In the following, we will consider the question: Who pays for the molds? Ordinarily, a customer buys a mold from the mold maker, either directly or through the molder, at previously established price and delivery terms. But there is a notable exception to this: whenever the molder has control over an in-house mold making operation, but also, if the molder must purchase the mold from an independent mold maker, the molder could decide to absorb some or all of the mold costs, particularly if there is a contract for a large production run at stake, possibly spread over several years. In such cases, the mold cost per unit of the product could be negligible. However, a problem could arise with the ownership of the mold. This should be settled at the time of the contract for the product order. It is very tempting for the customers not to have to spend much or any money up front for a mold, but the mold is then locked in with the molder and the customer cannot remove the mold, e.g., when planning to change the molder. Also, if the molder goes into receivership, the customer must be able to prove that the mold belongs to him before being able to get possession of the mold. Another, somewhat similar, scenario is when the molder has universal mold shoes and buys or makes a suitable number of stack inserts, which are probably only suited for the mold shoe for which they were built. This arrangement is less expensive for the customers but could also lock in the stack inserts with the molder.
5.5
Arriving at Mold Cost and Delivery Time
Over time, experience has taught mold makers (and this applies mainly to those for whom the business is to build molds as their product) that only a fraction of inquiries for mold prices and requests for quotations will ever result in an order. The problem is therefore how to arrive at a reasonable estimate – not too high and not too low – and with a reasonable delivery time, without spending too much time on estimating. In the following are some hints about how to go about this issue. To my knowledge, there is no definite or perfect method of estimating a mold cost. Don’t forget that the very term “estimate” implies an uncertainty and risk, both for the vendor and the buyer. The least risky case is when the mold maker specializes in only one or just a few selected types of products and mold sizes and can standardize them to such a degree to be able to even make a sales catalogue and a published price list for these molds, for example, by size of (similar) products and a certain, limited number of cavities. In this case, there is virtually no risk involved at all. There are many mold makers who specialize exclusively in certain molds and research ever better methods of molding to assist the customer in getting
Ownership of the mold must be clearly established
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5 Mold Cost, Mold Price and Delivery better productivity. They invest in better and more economical methods to manufacture the mold components with shorter deliveries. These mold makers specialize, e.g., in molds for bottle caps, disposable thin wall containers, toys, large pails, pallets, crates, PET preforms, small gears, and many more. It is very difficult for a “general” mold maker, who builds any kind of mold for any shape of product, to compete successfully with the specialists.
5.5.1 Calculating the Mold Cost Calculating the mold cost accurately is impossible
It is possible to calculate correctly the machining times required for any and each (mold) part by breaking it down into the various elements (steps) of manufacturing from raw material to the finished part, including all handling, setups, machining, checking, and so forth. This is done quite commonly in any industry where large numbers of the identical or similar parts are made, such as in the automotive industry, using time study methods and by determining for every operation the optimal machining methods, thus arriving at an accurate manufacturing time (and cost) for each part. Obviously, such method requires a lot of time and is based on the availability of detailed part drawings.
5.5.2 Estimating the Mold Cost Typically, at the time of estimating,
We don’t have detailed part drawings for every mold part,
Even if there were detail drawings, economically we could not devote that much time to estimating by breaking manufacturing down to its single step operations and timing, and
We would still have to guess assembly times and all the other times, such as designing and testing.
In fact, at the time of estimating, usually, we don’t even know how the mold will look or how large it will be, to determine the cost of materials, hardware items, and other costs. Proper estimating is based on past experience. Without experience (records, personal memory, and understanding the facts), every estimate is just a guess. As an oversimplified example, let us examine the case of a mold maker with enough money not to go out of business after building a number of molds that were underestimated because of lack of experience. The first mold was estimated much too low, resulting in a large loss. The next one was estimated too high and no sale resulted. When the next mold (for a similar product as in the first mold) is quoted, this time, already prepared with some experience from the first molds quoted, the mold maker arrives at a better estimate. The mold could be quoted deliberately low to get a better chance of getting the
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5.5 Arriving at Mold Cost and Delivery Time job. After building this mold, the mold maker has now gathered more real experience, but just in this type of mold, and hopefully begins to establish some reputation of building good molds in this line of products. This could lead to more lucrative mold orders. But if a new inquiry for a mold for a different type of product arrives, the same uncertainty will repeat itself. Slowly, over many months and even years, the mold maker will get to the stage where he can refer to a fairly large number of precedents and his estimating becomes more accurate. The problem was (at least in the past) that many mold makers have jumped from job to job and have accepted just about any mold inquiry that came their way, regardless of their familiarity, or lack thereof, with the product. They had to “guess” many times and probably lost money on many of these molds. The trend today is to specialize in a limited field of products and refuse jobs outside one’s expertise. This benefits both the mold makers and their customers. There is no substitute for trust and good, positive relationship and cooperation between the mold makers and their customers. Once the customers are aware that the mold maker is on their side and helps them to reach their goals by improving the quality of their products and the productivity of their machines, there is usually little objection, even if the mold price is higher than that of competitors who are just interested in selling a mold. After all, as we have stated earlier, in mass production, the mold price per unit represents only a very small component of the cost of the product.
5.5.2.1 Past Experience We always learn from our mistakes, as well as from our successes; but we must never believe that because the last (similar) job seemed to have worked well, we should blindly repeat that design. When looking at an older design, we should always ask ourselves:
Was it really the best we could have done?
Were there any problems with it?
Could we improve on it?
But, even if we could change the design for a better mold, is it worth the risk?
Over the second half of the 20th century, the plastics molding technology has advanced in great strides. A large variety of new plastics have given us the choice of many new molding compounds, some of them requiring new molding techniques, as well as new molding machines with more and better features, thus resulting in new approaches to mold design. A mold, which was on the cutting edge of technology only a few years ago, probably would not be built today the same way. But considering the stack parts or the mold shoe, there could still be many similarities with an older mold. Machining times have also much improved over the years, with the use of CNC and
There is nothing that replaces past experience, especially if the experience was bad
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5 Mold Cost, Mold Price and Delivery automated machine tools. Even though standardization and CAD/CAM has reduced the design and work preparation times, we can still use at least some of the old machining times as reference for an estimate.
5.5.2.2 The Importance of Accurate Records The most important assets of a mold maker are his records of earlier molds. These records, kept in the “job file”, consist not only of the drawings and Bills of Materials (“BoM”) of a mold made earlier, but also any records showing the actual times worked on the various steps, such as designing, machining, assembling and testing, and any test reports generated at the time. It is still better for these records to be also kept in computer files, because it will be easier and faster to retrieve old references for estimating, without the need to go manually through filing cabinets, and the danger of losing hardcopy in the process, or misfiling when returning the files after use.
5.5.2.3 Anticipated Mold Design Before proceeding with any estimate, we must have some idea how the mold will look. This is easy if we have a good precedent. In this case, we will probably also have good data of the cost of the precedent mold(s) and there is not much risk involved when quoting the new mold. If we have no precedent, or just similarities with an earlier mold, it will be up to the estimator (maybe with the help of a mold designer) to make a few sketches, primarily of the stack and then of the overall mold. The sketch of the stack can often easily be imposed on top of a significant cross section of the product drawing, provided we have such drawing to work from. We can then also check how any specified tolerances will affect the mold cost. If working from a sample or model, a sketch of the significant cross section will have to be made by the estimator before starting to plan the stack believed to be most suitable for the job. There is always a great risk when working from a sample or model, because of the absence of dimensions and, more importantly, specified tolerances
There is never just one possible solution in mold design, but unfortunately, an estimator usually does not have enough time allotted to an estimate to come up with several ideas, and to select the most promising one. In addition, if two or more estimators work on the same project, each one will probably come up with a different layout. This is the risk of estimating. Only if the job results in an order, the designer eventually assigned to the job, will have enough time to consider the product from all angles, before arriving at a suitable mold design. This may ultimately look quite different from the design sketch used to estimate the mold cost. It is good practice to have the sketches made by the estimator passed on to, and discussed with, the mold designer.
5.5.2.4 Times and Materials Required Once a reasonable design for a stack (with all its appendages such as actuators, ejectors, etc.) and the overall sizes of the mold has been sketched, the estimator
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5.5 Arriving at Mold Cost and Delivery Time can proceed to determine the approximate times required to make one stack and any special features and the mold shoe and the expected time for designing and mold testing, based on identical precedents or on similar jobs. In the case of multi-cavity molds, the estimate for one stack can usually be multiplied by the number of stacks, while allowing for any possible savings (cost reduction) by using fixtures etc. or other mass production helps, so that making N pieces will usually cost less than N times the cost of one piece. These estimates assume the time (hours, days, or weeks) required for designing, machining, assembling, and testing of the whole mold. The planned mold, for which there are no precedents, can also be broken down into smaller entities such as cavities, cores, the ejection mechanism, the separate mold plates, and so forth. To this must now be added the cost of raw materials, hardware items, standard mold components, such as hot runners, heat treatment, and any other foreseen subcontracted costs. The more detailed the records of previously made molds and mold parts are, the closer will the estimate be. After multiplying the hours with the plant rates and overhead rates as they apply (at the time of the estimate) and adding the estimated costs of materials and all other costs, the risk factor should be considered and must be factored in, if deemed necessary. This estimates the total cost of the mold, (labor and materials) ready for quoting, but before any profit has been added, which is usually outside the responsibility of the estimator, but rather in the hands of a sales organization or the management of the mold maker, who will consider all other factors to arrive at the mold price, as already discussed earlier. Most mold makers use this method. In smaller organizations, a top mold maker or designer may be involved. Larger plants often employ one or more dedicated estimators, usually experienced mold designers or machinists, whose full-time job is to estimate any mold inquiries that could result in orders. Obviously, this can result in higher overhead. While some estimates could require less than one hour, e.g., with a perfect precedent, others could take several hours or even days to arrive at a reasonable cost. In some operations, it is practice to have management involved even before the estimator gets a chance to see any inquiry, to establish if it is worthwhile to spend time on it. Large manufacturing companies often send out inquiries for molds just to find out how much they would cost. They often have their own mold making facilities and have no intention to place any order with somebody else. It could be a complete waste of time to estimate such jobs. Also, inquiries could come from companies with poor credit ratings, or poor payment records, and management could decide not to waste valuable time on them.
An estimator makes or breaks a mold-making business
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5.5.3 “Guesstimating” the Mold Cost Because accurately calculating the mold cost is practically impossible, most mold makers use the method of estimating described in Section 5.5.2. However, there is another method that is simple and inexpensive, but quite risky, and should not be used unless with ample, long experience in the type of mold to be estimated. It requires essentially two people, both with good experience. Both estimators visually inspect the sample or drawing to evaluate its complexity (shape, size, accuracy required) and jot down on a slip of paper their estimate of the cost to build the mold. This may take maybe five minutes or even less. They then compare these figures; if they are in agreement, a mold price will be quoted. If there is little difference, the higher one will be selected. If there is a substantial difference, they will discuss the reason for their lower and the higher estimate. It is possible that one estimator saw something that the other overlooked. If both agree on a new figure, the quote will be on this basis. If there is still a doubt, sketches will be made as explained in Section 5.5.2, which of course takes much longer.
5.5.4 “Ball Parking” the Mold Price Quite often, a customer asks the mold maker for a “ball park estimate”, i.e., a preliminary, approximate estimate for a mold or a whole project. “Ball parking” should never be given without a written understanding that it is not a binding quotation. It is meant solely to give the customers an idea for a mold price that they in turn can use for estimating a product cost. A ballpark estimate can be arrived at the same way as a guesstimate (see above) and it only means that the final (quoted) mold price could be somewhere around ± 20% of the “ball ballpark” quote. Most mold makers ball park high to make sure the customer will not be shocked by the final price.
5.5.5 Mold Price Catalogue The easiest method of estimating, with the smallest risk, is associated with specializing in one narrow range of products so that there are established mold prices, which can be used and priced by any sales person, even without mold or estimating experience or it can be determined by the customers themselves. As long as the new product fits within a listed, usually limited range of sizes and for a specific, listed number of cavities, regardless of the actual dimensions and shape of the product, a mold price is listed. This method is used by mold makers specializing in a number of different products, from small bottle caps to large industrial pallets. The risk for the mold makers is that the mold prices, which are ordinarily kept confidential, are open to everyone, which makes it possible for an aspiring competitor to underbid. On the other hand, the customer should
5.6 The Quotation know that buying from an established specialist means usually getting the benefit of long experience, better quality molds, higher productivity, and faster delivery.
5.6
The Quotation
Any quotation (“quote”) to a customer should contain at least the following details 1. Date of quotation 2. Specification of the quoted mold, and any other specifications applying to the use of the quoted mold, such as the type of molding machine and ancillary equipment. It should also indicate certain details about the planned mold, e.g., type of gating, estimated cycle, responsibility for shrinkage, and so forth (explored in detail in the foregoing text) 3. The quoted mold price (explored in detail in the foregoing text) 4. Time period, for which the quote is valid (important, especially, if there is much activity, many inquiries and heavy shop loads) so that any long delay in accepting the quote could seriously affect the quoted delivery time 5. Delivery data to testing the mold, from receipt of order and complete drawings (will be discussed in Section 5.6.1) 6. Shipping method (subject to negotiation between customer and supplier) 7. Payment terms and conditions (subject to negotiation between customer and supplier) 8. Warranties and disclaimers (will be discussed in detail in Sections 6.1 and 6.2) 9. There could be other items included in the quotation. See sample of a quotation in the Appendix.
5.6.1 Delivery Time Delivery time is an important part of any quotation. To determine a delivery time, the estimator uses the same data as for the costs, the time that is required to perform the necessary work (designing, machining, assembling, and testing); however, a delivery promise is also dependent on the available time in any of these areas and on how soon the job can be started. It is common to quote “delivery to testing” rather than “delivery to shipping”, to make the customer is aware that testing could reveal unforeseen problems, which will have to be solved before shipping.
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5 Mold Cost, Mold Price and Delivery Plant loads and conditions change constantly. Everybody could be busy today but may be looking for work in a month or two. Quotations usually indicate deliveries that would be possible, if the order is placed now or within a short period of time. The delivery is based on the shop load at the time of quoting or expected to be within the next few weeks. Because the shop load changes continually, every quotation must clearly limit the time during which the quotation is valid. However, this also bears a risk for the mold maker, who is continuously submitting quotations, usually in the order as they are requested, but knowing full well from past experience that only a fraction of them will ever result in purchase orders. While it would appear to be a great success for the mold maker, if the backlog of orders increases, i.e., when more orders are received than can be shipped, the wishes of the customer (for early delivery) must not be ignored. The obvious answer would be to increase the number of hours available for design, manufacturing, etc., but this is not a practical solution, because more space, capital for equipment, and most of all, more skilled personnel would be required to make use of such an immediate expansion. Working overtime is usually a simple solution, but in some localities, the number of overtime hours is legally limited. Adding shift work in some areas could help, provided the necessary qualified personnel could be found in time. One solution available to the mold maker is to subcontract some of the work. It is quite common with smaller mold making operations to subcontract their design work and many mold makers subcontract some machining and finishing operations, as has been discussed earlier.
5.6.2 Confirmation
There is nothing worse than breaking a delivery promise
Any quotation should always contain the cautionary phrase “subject to confirmation”. With these words, the mold maker reserves the right to have another look at the quotation and to see how it will affect the shop load, before finally committing to a delivery time with a formal “confirmation of order”. The formality of confirmation also gives the mold maker another opportunity to look at the quoted price: since the customer is now obviously serious about ordering the mold, it is wise to make sure that the price was quoted properly. At this time, there could be some serious discussions between mold maker and customer, if an extension of the originally quoted delivery time, or even a higher price should be asked by the mold maker, but it is better to have an argument now, even risking to lose the order, rather than losing money on the job or having unpleasant arguments, or even law suits, later. Once the customer accepts the confirmation, the mold maker must do everything to ensure that the delivery date is kept. There is nothing worse than promising a delivery date that cannot be kept. It is good practice for the mold maker to keep the customer regularly informed (weekly or monthly) about the status of an ordered mold and the mold maker must advise the customer as soon as something occurred that could affect the delivery date.
5.7 In-House Mold Making Operations The mold maker must never forget that the customer too has a responsibility to the people who expect the molded products. Every mold maker would be happy if the volume of work passing through the shop were constant. Unfortunately, this is never the case and it is usually “feast or famine”. Mold work is always seasonal, e.g., much work in Fall and Winter and then tapering off towards Summer, but there are also other factors affecting these fluctuations, which makes it always difficult to accurately forecast the shop load. No mold maker wants to lay off skilled personnel when there is little work; many workers, and especially machinists, prefer to work overtime during “feast” periods and regular or even shortened hours during a “famine”. The best way for planning has always been to have a good backlog of orders of a few months, usually from customers who don’t mind waiting longer, because they know from past experience that they will get a good job. At the same time, the mold maker should always keep some shop time open for unforeseen problems. See sample of a confirmation in the Appendix.
5.7
In-House Mold Making Operations
There is really little difference between independent mold making operations that make their living from making molds and an in-house mold making shop, which is part of a molder’s operation or at least under his control. Today, almost every molder has some facilities (a few selected machine tools and some skilled mechanics) to service molds and fix minor mold problems, often right in the molding machines; some also can make major repairs and perform regular and major maintenance functions on the molds. From there, it is only a small additional step to make use of these facilities and the mold maintenance personnel, especially during periods when they are not otherwise busy, to build stack parts, stacks, or even complete molds. Over time, some additional equipment may be installed and mold designers are engaged, so that the molders could eventually be in a position to build their own molds. These are usually molds similar to those that are in operation in their molding plants and with which they are familiar. These molds will then often not be estimated or valued on their real cost, but much lower, because the machining and assembly times are not considered at all, or maybe only partly, because the machinists and technicians are there anyway and would be idle if they would not be working on a new mold. In addition, the molder could save by accepting shortcuts in mold design and construction, which otherwise would not be acceptable. This can (and often does) permit a molder to quote to a customer prices well below those quoted by an independent mold maker. Problems with this approach are:
These molds will be more or less copies of existing technology and there will be few new ideas or improvements over the old methods so that the molder loses out on any advances in technology, which are constantly developed by independent mold makers.
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Because the molders will be competing with mold makers, there will be less active cooperation between them, and the mold makers could be reluctant to quote or build any new molds, which the molders may require.
There is an exception to this: a mold making operation which is located (usually) within or next to the parent molding plant could be considered an independent organization (profit center) and will serve the molder just as an independent mold maker would, by quoting on actual costs and delivery and could even build molds for competitors of the parent company. The advantage to the molder is that (1) the mold making shop is close by and will also look after any repairs and mold maintenance, and (2) the molder could have some influence of the pricing policy of the mold maker since they are closely related companies.
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6
Warranties, Patents, and Ethical Considerations
6.1
Warranties and Guaranties
When a mold maker builds a mold, it is expected to perform as quoted and to last without breakdowns for a long time. It is also expected to make the product to specifications and dimensionally correct, and at a productivity that has been estimated to the best knowledge of the designer and mold maker.
6.1.1 Guaranteed Quality Normally, a mold maker will guarantee the quality of the mold for six months or for one year, covering materials and labor. In fact, if problems did not arise within the first few weeks or months (or the first few thousand molding cycles) after start of operation, a properly designed and built mold, well maintained, will virtually last forever, except for unavoidable wear on tapers, gates, etc., or when abusing the mold, e.g., by improper setup, or physical damage due to negligence. Occasionally, the mold maker may be asked to guarantee shrinkage, cycle time, or delivery time.
6.1.2 Guaranteed Shrinkage Experienced designers or mold makers can usually foresee the shrinkage, especially if they are familiar with the product. Shrinkage affects not only the dimensions of a product but sometimes also its shape by distorting the product. This can be difficult to foresee, in particular with non-symmetrical and odd-shaped products. In many cases, this requires experimentation before finalizing the mold or testing after some or the entire mold is finished. It could require experimenting with cycle times, even changing some steel dimensions, or the cooling provisions in all or in certain areas of the mold. Often, the molder will provide steel sizes to the mold maker, eliminating the need and cost to guarantee the plastic sizes. There are also usually only one or a few dimensions that are critical and these should be the only ones guaranteed. The possibility of the need to experiment to arrive at proper sizes should be foreseen by the estimator and should be included in the mold price by selecting a higher risk factor. As an alternative, such testing and necessary rework could also be quoted separately as addition to the mold price, e.g., by quoting labor and materials required to achieve the desired result.
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6.1.3 Guaranteed Cycle Time An experienced molder, designer, or mold maker will have a good idea what cycle time to expect. A mold quote can include the estimated, approximate cycle times, e.g., “estimated 10–12 s cycle” (or “estimated 5–6 shots per minute”), together with the specified physical characteristics of the mold. Some customers (not necessarily molders) want to have the cycle time guaranteed. It is quite risky for the mold makers to accept such a demand. The estimated times are always given to the best of their knowledge and experience and the mold makers will do their best to achieve the quoted cycle time. If the customer insists on a written guarantee for a certain cycle time, an experienced mold maker may consider accepting it, provided that there will also be a guaranteed premium if the cycle time is better than promised. Such an arrangement could be, for example, that the customer will get a percentage of the mold price reduced for each second or fraction of a second that the cycle is longer, and will pay a premium of the same amount for every second or fraction of a second the cycle is shorter than the guaranteed cycle, up to a certain limit. There is always the problem that, in some cases, higher speed may affect the quality of the product. This must be recognized before and discussed with the molder, before agreeing to any, especially shrinkage, warranty.
6.1.4 Guaranteed Delivery Here too, it is a question of the mold maker’s experience with the mold and their understanding and control of the mold shop load. If the customer wants to have the mold within a certain, reasonably short period and wants to negotiate penalties if the mold is not ready as promised, this could be accepted provided that the customer is also willing to pay a premium equal to the penalty if the mold is ready before the promised delivery date. All guarantees must be agreed to in writing, before the project starts
I have seen both guaranteed cycle times and guaranteed deliveries and most often the jobs were concluded to the mold maker’s benefit. Note that the customers were never unhappy to pay the premium, because they too gained by being in production sooner and/or gaining higher productivity.
6.2
Patents and Ethical Considerations
While patents and ethics may not directly or immediately affect the cost of a mold, everyone in this chain should be aware of the risks and problems of ignoring potential patents or unethical behavior. The following highlights some of the basics that anybody considering a new product and/or a new mold design should be aware of.
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6.2 Patents and Ethical Considerations
6.2.1 Patents Patents are issued by the patent office. They describe in detail the patented features (“improvements”) of parts of or of whole products, or machines (“apparatus”), or of methods of manufacturing. The features to be patented must satisfy all three of the following. They must be
“novel”, i.e., new and “surprising” to anyone familiar with the subject,
“useful”, i.e., explain at least one useful purpose, and
“feasible”, i.e., the patent application must show at least one possible way of how the improvements can be achieved and how they function.
“Claims” define precisely, which features are claimed to be patented. A claim can cover a portion or a whole machine, a product or part of a product, or a method, but most often it covers just an “improvement” of a certain (often very small) area or feature of a product, machine, or process. An issued patent does not protect the inventors, but gives them the right to legally attack anybody who is “infringing”, i.e., “making”, “selling”, or “using” anything covered by the claims. A patent is valid (effective) only in the country where the patent was issued and only for a limited time, usually 17 years from the day of issue. A patent, issued in the USA, does not give the right to attack someone in other countries from making, selling, or using it, but it gives the right to attack anyone who wants to use or sell this infringing product in the USA. An issued patent is an asset, and can be sold or licensed. In assessing the value of a company, banks and investors look favorably at a company who owns patents. “Design patents” cover certain special shapes and patterns of (usually) consumer items. Design patents are much easier to obtain, however, their value is often questionable. “Prior art”. When applying for a patent, the inventors must solemnly declare that, to their best knowledge, they are not aware of anyone who has ever used or published the claimed feature or design anywhere else in the world; they must also indicate all the fields where the “invention” applies. The patent office will then do a perfunctory search of similar features in existing patents, not necessarily only in the indicated fields, and if none are found, can issue a patent. This process from application to issuance can take one or more years and is quite expensive, especially because a patent attorney should handle it to comply with the proper format and wording of the application. There are no patent rights while the application is being considered, but there are some advantages for the inventor between the date of application and the date of issue of the patent. Prior art is the weak spot of most patents. If it can be proven that the “claimed” features have been used anywhere in the world, whether patented or not,
“A patent is a Sword, not a Shield”
188
6 Warranties, Patents, and Ethical Considerations prior to the date of the application, even an issued patent can be declared invalid. A worldwide search can be very expensive. It covers only issued patents in some countries, some trade magazines, articles or books, but not all magazines nor all published catalogues, or designs or practices which are or were in public use. For most patents that have been declared invalid it was discovered after issue that the claims were not “novel”. The most frequent reason for declaring a patent invalid is that “prior art” has been unearthed
If an inventor decides not to apply for a patent, he or she can select to publish the product in which the invention is clearly used, e.g., in a sales catalogue, advertisements, or trade magazine. While thereby forgoing any right for a patent, it also prevents anyone else to get a patent after the date of publishing. It is often a question of simple economics to decide whether it is worthwhile to apply for a patent. A patent is only of value to the owner as long as anyone infringing it is found out by the owner of the patent, first warned to “cease and desist” and eventually, if needed, prosecuted. These legal proceedings are very expensive and can draw out over years. It is often better and much cheaper to settle such disputes amicably. Also, if the owner of a patent does not use a patent within a stipulated time after issue (typically 5 years), or does not prosecute a known “infringer”, the patent can become invalid. If a product is wholly or partly covered by one or more patents, the relevant patent number(s) must be disclosed, either on the product itself by engraving, printing, or on an attached nameplate. If the product is too small, or if, for other reasons, it is impractical to show the number(s) on the product itself, they can be shown on the packaging for the product. The frequently found words such as “patent protected”, “patent issued”, “patent pending”, or any abbreviation thereof have no value. The country granting the patent should also be shown, e.g., “US pat. #9,876,543”. The patent numbers put the buyer or user of the product on notice that there are patented features to consider which may be used to prevent any copier from proceeding. But on the other hand it also opens the door to an attack by a copier to declare the patent(s) invalid because of possible prior art that has not been found during the original patent application. How Do Patents Affect the Decision Maker and the Mold Maker? Both parties should be aware that any patents they do not own or are licensed to use could be used to prosecute either the customer, the molder, the mold maker, or even the end user of the product, or any or all of them. Anyone making, selling, or using a product (and that includes the mold) that is covered by a patent is subject to prosecution by the rightful owner of the patent. Anyone who wants to have a mold built for any product must be asked to assure the molder and the mold maker in writing that, to their knowledge, there is no valid patent covering all or some of the features of the product for which a mold needs to be built and also agree (in writing) that they will
6.2 Patents and Ethical Considerations assume full responsibility to keep the other involved parties harmless in the event of prosecution because of patent infringement (“hold-harmless agreement”). A similar agreement may be necessary between the mold maker and the molder that the mold does not contain any patented features which could be attacked by an owner of a valid patent. This could happen to the molder or even the end user after the product is sold to the public. Typically, this applies when copying patented mold features, such as hot runner hardware, ejection and handling methods, and others. When finalizing a purchasing contract for a new mold or molding system, it is good practice to discuss the ownership of any patentable features that may be the result of designing the molds or ancillary equipment by the mold designer or the mold maker. There could be an agreement that any issuing patent(s) will be owned by the purchaser (customer) of the mold, who will also assume the costs of obtaining the patent, but naming the actual designer or designers as inventors, and maybe including the names of the product designers of the customer. It could also stipulate that the mold maker will have the right to use such patentable features in other molds, but not in molds that would compete with the patent owner.
6.2.2 Ethical Considerations Not all copying problems apply to patented features only. For example, let’s assume a new, but not patented, plastic product is launched by an entrepreneur and turns out to be a great success. The mold maker, who built the original mold, will probably be asked to work on additional, better molds with more output, to cover the expected increased demand. At this moment, some other entrepreneur may see an opportunity to make and sell the same or a very similar product and approach the same mold maker to build a similar mold for this competing entrepreneur. Often, such “copiers” don’t even bother with a product drawing but just bring a sample of the original product, from which the mold maker should build a mold. In this case, the problem could be solved simply by the mold maker, by flatly refusing to get involved. It would be neither ethical, nor fair to their existing customer to help a competitor. Another frequent scenario is a prospective customer, or a molder, presenting the mold maker with a sample of a product made by someone else and the mold maker was not originally involved with it. In such cases, it is better to ask the customers to come back for quotations when they have their own completely detailed, toleranced, and dimensioned drawing ready, complete with assurances as indicated above regarding patent infringement. Under those conditions a job can be accepted.
189
193
Appendix Appendix 1: Suggested Contents of a Mold Manual Description (possibly a photo) of the mold Date of building Serial number, size, weight List of patents applying to the mold. List of third-party trade marks of components used. Table of contents 1. Safety of operation 2. Specifications: What is required to operate the mold: Molding machine type, space, plant floor, hoist capacity, air supply, cooling water, lubricants, power, etc. 3. Safe handling of the mold 4. Installation, startup, and operation 5. Troubleshooting 6. Mold storage 7. Mold maintenance (frequency and procedures) 8. Setup guide from the original testing of the mold 9. Mold drawings and bill of materials Assembly drawing(s) Detail drawings of stack components Detailed drawings of all other mold parts, optional Drawings or specifications of special, commercial products used in the mold (valves, heaters, seals, etc.)
194
Appendix
Appendix 2: Mold Set-up Guide Blank Mold Wizard Inc. SET UP GUIDE Customer:
_____________________
Mold No: __________________
Job No.: _________________
Project Engineer: _____________________
Tested by: __________________
Date: _________________
MOLDED PART Part Description: _____________________
Part Wt (g): __________________
Shot Wt (g): _________________
Overall Cycle:
Avg Wall Thick.: ______________
Flow Length: _________________
___ (sec) ___ (shots/min)
MATERIAL Type:
_________
Manufactured by:
Number:
_________
Batch No:
__________________________
_________
Color:
_________
Pre-Dry Temp & Time:
________
% Concentrate:
________
MOLD No. of Cavities:
__________
Nozzle Bushing:
__________
Locating Ring (mm):
Shut Height (mm): __________ __________
Mounting Bolts: Metric
Type: Single Face
Ejection System:
__________
Ejector Stroke (mm):
__________
Runner System:
Stack Inch
______________
MOLD PROCESS TEMPERATURES AND PRESSURES Core:
_____ (°C) _____ (°F)
Pressure in (psi): Flow Rate:
Cavity:
_____________
___________________
_____ (°C) _____ (°F)
Pressure out (psi):
Gate Pad:
_____________
_____ (°C) _____ (°F)
Air Pressure (psi):
(US gal/min or Litres/min) Cooling Tonnage Required:
_____________
__________________
MACHINE Type:
______________
Extruder Dia. (mm): ______________
Clamp Stroke (mm):
______________
Serial No.:
______________
Inj. Pot Dia (mm):
No. of Accumulators:
______________
Auxiliary Equipment:
______________
______________________________________________________________________________
HEAT PROFILE Machine
NOZ
DIS1
DIS2
S/P2
S/P1
B/HD
___ °C
___ °C
___ °C
___ °C
___ °C ___ °C
___ °C
___ °C ___ °C
___ °C ___ °C
___ °C
S/Point
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
Mold
N/T1
N/T2
N/T3
N/T4
S/BAR
X/MA
H/R1
H/R2
H/R3
H/R4
Start
___ °C
___ °C
___ °C
___ °C
___ °C ___ °C
___ °C
___ °C ___ °C
___ °C ___ °C
___ °C
Auto
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
___ °F
EXT6
EXT5
EXT4
___ °F
NOTES (For different zone temperatures and/or special start-up procedures see notes below)
EXT3
EXT2
___ °F
EXT1
195
Appendix 2: Mold Set-up Guide Blank
CLAMP CLAMP CLOSE Timer(s)
Clamp Tonnage
Set Points (mm)
Close
Velocity (%)
(%)
(t)
(%)
Mold Break (mm)
(t)
Clamp Pos. (mm)
CLAMP OPEN Timer(s) Open
Mold Pos. (mm) Set Points (mm) Mold Protection
Velocity (%)
Pressure (%)
Set Point (mm)
EJECTOR & AIR HYD. EJECTOR
AIR FUNCTION Set
Set Point (mm)
Hyd. Ejec. Start S/P (mm)
1
Hyd. Ejec. Back (mm)
2
Hyd. Ejec. Stroke (mm)
3
Hyd. Ejec. Fwd. Timer
4
Hyd. Ejec. Counter
5 6 7 8 9 10
Set Timer (t)
196
Appendix
INJECTION FILL
Cycle Time(s)
Pull Back
Shot Size
Inj. Time(s)
Set Points (mm)
Trans. Time(s) Injection Pressure
Velocity (%)
(%)
(psi)
Piston Pos.
(mm)
TRANSITION Time
(s) Position
(mm)
Cavity Press
(psi)
HOLD
Inj. Pres.
SCREW
Hold Time (t)
Back Press. (%) (psi)
Speed (%)
CARRIAGE Hold Press.
(psi)
(psi)
Fwd. Set P.
(rpm)
Current (amp)
EXTRUDER Stroke
Back Pos.
(%)
TIMERS & COUNTERS CLAMP
Set
INJECTION
Mold Close Timer
Exc. Inject. Timer
Mold Open Timer
Exc. Cycle Timer
Hyd. Ejec. Fwd Timer
Hold Zone #3 Timer
Hyd. Ejec. Stroke Ctr.
Hold Zone #2 Timer Hold Zone #1 Timer
PRODUCT HANDLING
Shooting Pot Pack Time
Index Conveyor Counter
Extended Decompression
Parts Drop Off Delay
Shut-off Noz. Del. Tmr.
Tilt Forward Time
VALVE GATE Delay Valve Gate O/Tmr. Ext. Valve Gate O/Tmr.
Set
Appendix 3: Example of Light-Weighting a Product and Increasing Productivity
Appendix 3: Example of Light-Weighting a Product and Increasing Productivity A 2 × 4 stack mold for DVD cases (Fig. A1) is a good example of the results of clever product design, weight reduction by thin-walling, and using advanced mold and handling technology. Amongst other good design features, this mold is using 2 valve gates per cavity located on the spine of the DVD case. The productivity has increased enormously, and reduced the product cost considerably. Box and cover are now one unit. After ejection into a high-speed side-entry robot, the two case halves are flipped closed and are then held in a buffer storage to control shrinking and warping. Figure A2 shows the side entry robot. The tooling arms entering the mold take a total of 0.75 s to enter, grab the products, and retract. The mass of the product has been reduced from the original 84 g, which amounted to 75% of the product cost, to 60 g. At the same time, by selecting a stack mold, the number of cavities has been increased from 2 to 8. Cycle time is now 6 s, for a production of 4,800 cases per hour, or more than 34 million per year. As a result of redesigning (light-weighting) the product and getting better molds, the product cost has been reduced by 40%. While the mold cost is considerably higher than before and the handling equipment was added, the capital cost per unit molded amounts to only a few percent of the total piece price. These products run continuously for 5 years, using the best quality molds with high cavitations, resulting in the lowest product cost.
197
198
Appendix
Figure A1 Side entry robot for the removal of 8 DVD cases from the mold. The cases are then folded and put in a buffer storage to prevent warping while cooling and shrinking outside the mold.
Figure A2 A 2 × 4 self-contained mold for DVD cases. This mold shows another method of dragging and spacing the mold sections, using pull rods (A); (Courtesy: AWM Inc., Switzerland)
Appendix 4: Buying a Mold
Appendix 4: Buying a Mold If you were planning to buy a “dream house” for you family, you would first inquire to find some recommended architects and/or builders that specialize in private homes. There would be no point to contact builders that specialize in industrial or commercial buildings. You would probably approach one (or maybe 2 or 3) and discuss with of them at length what you have in mind regarding size, shape, number of rooms, type of construction, and so forth. They will then prepare a quote for you, including a sketchy outline of your planned house, the size and shape of rooms, the types of walls, height of ceilings, inside and outside finish, the type of floor finish, heating, air conditioning, and so on. You also expect a complete set of detail drawings of the construction on completion. The quotations must have a time line to completion of the job and the price and payment conditions. After such quotations have been received from the builder(s), there is still time to negotiate terms and to change, add, or remove features, before the sales contract will be finalized. It can be very expensive to make changes once the building has started. This is really not different from getting a quotation for a mold. But it is amazing how many people request quotations for molds, by just specifying the number of cavities and the type of machine in which the mold is to be run, without specifying what they expect of the mold, in the area of performance and mold life. No wonder that quotations for such “unspecified” molds can run from very low to very high prices. An example of this from real life was shown in the preface of this book. The following pages show a typical quotation and a confirmation of order for a mold, as used by one of the major mold makers. The quotation is only released after thorough discussion with the customer to understand what he really needs. This procedure may appear overly detailed and costly, but it ensures that the customer is well served, and that there will be no misunderstandings when the mold is finished.
199
200
Appendix
Company XYZ Mold quote Budgetary Mold Quote Number XXXX REV X CUSTOMER:
ABC Molding
CONTACT:
Mr. Smith
DATE:
Product Description:
Widget part number 123456
Machine:
Description of the machine in which the mold will be run
Jan 22, 2003
PRICES, $US Mold Description
Mold Price
Options
Electrical Cables
Water/Air Kit
Machine
2 × 8 cavity
$150,000
$36,000
Included
Included
300T machine
Exclusions: Embossing not included, if required will be quoted separately Re-cuts for part sizing and fit DELIVERY TO TEST Based on current workload, 12–14 weeks from receipt of purchase order, deposit, and approval of final part drawing. Test date to be confirmed at time of order receipt. Required mold test results: • Establish optimal processing conditions to achieve acceptable part quality • Optimize cycle time • Determine if re-cuts (lid fit, ensure proper filling) are required Estimated mold test duration: 5 days (longer if re-cuts required) INCLUDED FEATURES • 2 hour acceptance run with set-up report • Mold technical data manual • Mounting bolts, lifting bar, latches MOLD MATERIALS Item
Material
Hardness R.C.
Finish3
Cavity Gate Insert Gate Pad Core Base Core Insert Stripper Ring Shoe
420SS 420SS 420SS 420SS BeCu S7 SS
49–51 49–51 49–51 49–51 Pre-hard 52–54 28–36
Draw-Stone 0.08, Buff Draw-Stone 0.08, Buff Draw-Stone 0.08, Buff Draw-Stone 0.08 Draw-Stone 0.08 Ground Ground
Surface treatments such as ENP or flash chrome plating are not included, and can be quoted separately if desired. GATING Outside center hot tip
201
Appendix 4: Buying a Mold
EJECTION Stripper ring, stripper plate actuated by internal pneumatic pistons ESTIMATED CYCLE TIME: 7 seconds Estimated cycle is based on customer information and mold maker experience – this is not a cycle guarantee. The mold maker will, on a best-efforts basis, optimize the molding process during the scheduled mold test to achieve optimal cycle time, shrinkage, and weight. Notes on the product quality: • The thick sections in the stacking area may show a degree of sinking • Steel-safe re-cuts are not included and will be quoted as a separate item. PRODUCT DESIGN This quotation is based on the customer supplying a completely detailed plastic product design and shrinkage values. Part drawing assistance is available, not included in this mold price, quoted separately at a rate of $US XX/hour. Lead time for the project starts only after approval of final product drawing. Changes to the product design after the order is placed will extend lead time. MOLD TESTING AND ACCEPTANCE The mold will be tested in mold makers test room. Please send 1000 kg of PP test resin two weeks prior to test date. INTERRUPTION CHARGE If the order is placed on hold, canceled, or if machine or mold design changes are requested after commencement of engineering and/or manufacturing, please understand that the mold maker may be faced with considerable expenses including production rescheduling, additional set-ups, handling, storage, inventory costs, obsolescence, etc. Accordingly, an interruption charge may be required on all or part of the order. RELOCATION In the event the mold maker’s equipment is relocated from its original installation, a review of applicable safety regulations and services (i.e., electrical voltage, frequency) and operating environment (i.e., temperature/humidity) must be completed. Additional equipment modifications may be required to operate equipment safely and optimize equipment service. Always contact the mold maker prior to relocating equipment and we will assist in assessing any required equipment modifications. TERMS XX% With Order, YY% 30 Days prior to Test Date, ZZ% Net 30 days from Date of Shipment, Payable in U.S. Funds EX WORKS Mold maker’s location, any local taxes (such as sales) applicable shall be paid by the customer The Customer is responsible for insurance from the Mold maker’s facility to the point of delivery. VALIDITY This quotation is valid for a period of thirty (30) days and is subject to our confirmation of the order and credit approval. WARRANTY For full details, please refer to the enclosed Standard Warranty Conditions. Per: _______________________________ Estimator Mr. Jones
202
Appendix Standard Warranty and Hold Harmless Agreement Injection Molds, Hot Runners, and Spare Parts The mold maker provides molds to produce articles, which meet customer specifications. The mold maker assumes no responsibility whatsoever for the design or manufacture of any products made using this mold, but warrants the mold to be free from defects in material and workmanship, for a period of XXXX (XX) months from date of shipment. The mold maker’s obligation under this warranty is limited to repair or replacement of mold parts. This warranty does not cover the following:
Repair or replacement of the mold due to normal wear or damage caused during routine maintenance.
Damage to mold components whose fragility is determined by product design.
Damage to the mold or to molded parts from not following recommended maintenance and operating procedures, as outlined in this Mold Manual.
Consequential loss, personal injury, damage to goods and/or equipment, work stoppages, delays, or slowdowns in production associated with any breach of this warranty on the mold.
Damage arising from mold modifications not included in the procedures in the Mold Manual.
Damage resulting from the use of any unauthorized parts, supplied, manufactured, or modified by procedures not included in the Mold Manual and the Parts List (Bill of Material).
Transportation charges and local sales taxes for replaced or repaired mold parts.
Expenses resulting from meeting local codes and standards.
Any claims for loss, costs, or damages for personal injury or property damages arising out of the manufacture of molded products using this mold.
This warranty applies to the original purchaser only, and is non-transferable. If you require mold parts under warranty, the mold maker may require damaged or defective parts to be returned to its closest office. A purchase order will be required to cover the part(s) in question. Upon return of the part(s), or on advice of the mold maker’s representative in the field, a credit note will be issued. Mold Spare Parts Only Warranty The mold maker warrants the parts to be free from defects in workmanship and material under normal operating conditions. The mold maker’s obliga-
Appendix 4: Buying a Mold tion under this warranty is limited to repair or replacement of any parts, which within XXXXX (XX) months of shipment to the original purchaser are deemed by the mold maker to have been defective at the time of shipment. This warranty does not apply to any components which, in the mold maker’s opinion, have been subjected to misuse, negligence, or operation in excess of limits recommended by the mold maker or which have been altered by other than the mold maker’s personnel. Replacement of parts or material would be F.O.B. – Our Plant. This warranty does not cover any consequential damages nor any labor charges for replacement of parts, adjustment, or repairs. Hold Harmless Agreement In the event that suit is filed against the mold maker or any subsidiary or affiliate thereof charging the mold maker with patent infringement by virtue of the mold maker’s sales of molds or sales by Purchaser of any product molded therefrom; Purchaser will defend and hold the mold maker harmless with respect to any count therein. This shall include adding the mold maker or any affiliate or subsidiary thereof to any existing litigation and shall also include any and all counts in said litigation.
203
204
Appendix
Appendix 5: Suggested Format of a Confirmation of Order Confirmation of Order February 9, 2002 ABC Plastics 123 North Street Jones, NY 90120 Attention:
Mr. John Smith, Purchasing Manager
Reference:
Job Number 123456 Purchase Order Number AB3214
Dear John: We are pleased to acknowledge your Purchase Order Number PO#B5678, covering the following: 1 only
6 Cavity Widget MOLD MATERIALS
Item
Material
Hardness
Cavity Core Base Core Insert Gate Insert Stripper Ring Wear Ring Lock Ring Gate Pad Wear Ring Shoe
420SS/H13 420SS/H13 BeCu25 BeCu25 S7/A2 A2 A2 420 SS/H13 A2 SS
49–51 Rc 49–51 Rc 36–41 Rc 36–41 Rc 54–56 Rc 52–54 Rc 52–54 Rc 52–54 Rc 28–32 Rc STANDARD SHOE FEATURES
• • • • • • • • • • • •
Tested mold complete with set-up guide Mold Technical Data Manual Stainless Steel Mold Base Mounting Bolts Heater Cables and Plugs Lifting Bar and Mold Feet Nozzle Tip Wrench Machine Stroke Limiter Multi-pin Connectors Safety Blocks Rack and Pinion Alignment Bearing blocks for Center Section Support
Finish
Ground Ground
Appendix 5: Suggested Format of a Confirmation of Order
INCLUDED OPTIONAL SHOE FEATURES • • • • • • •
Horizontal Center Section Support Water and Air Manifolds Water and Air Connection Kit Product Blow down system with Venturi Nozzles Cable Holder Leader Pin Shields Customer Specified air and water location INCLUDED OPTIONAL CONTAINER STACK FEATURES (CAVITY LOCK)
• • • • • • •
Solid, Adjustable, Front Mounted Cavity Lock Cores Fixed Core Cap Blow-off Tube Flush rear-mounted cavity inserted in a cavity plate BeCu replaceable gate insert Replaceable core wear ring Moving Cavity GATING
Outside Center, Hot Tip EJECTION Stripper rings TOTAL PRICE $60,000 DELIVERY SCHEDULE Please note that we are not able to formally accept this order or place it in our production schedule until we receive a fully dimensioned, finalized product drawing along with resin shrinkage to be applied to the part. On the basis of your order, we have scheduled the mold into engineering. However, we must have any missing information before March 31, 2002 or your order may lose its place in the engineering schedule, moving it back to the next available opening. Providing that all outstanding information is received by the above date, we can schedule your mold to be ready for delivery to our test room on or before July 31,2002, 16 weeks from receipt of final part drawing. You should allow approximately two weeks for testing, acceptance of samples, and preparation of the mold for shipment. A formal confirmation of the delivery will follow at a later date, once all the information is received. If this order is placed on hold or product design changes are requested after commencement of engineering and/or manufacturing, please understand that we may be faced with considerable expenses; including production rescheduling, additional set-ups, handling, storage, inventory costs, etc. Accordingly, an interruption charge may be required on all or part of the order. Changes to the product design after test, such as stacking, lid fit or wall thickness, will be chargeable to ABC Plastics.
205
206
Appendix
MOLD TESTING The mold will be tested in our test room on a xxxx machine. Please send 1000 pounds of PP 2345 by July 20, 2002. TERMS As agreed, the terms applying to this order are as follows: 40% with order 40% 30 days prior to test 20% net 30 days from date of shipment F.O.B. our plant Payable in U.S. Funds Any local taxes payable by the customer. Since the initial deposit is now due, we have forwarded our Deposit Invoice Number AX2345, in the amount of $24,000 to your accounts payable department and would appreciate receiving your formal Purchase Order and deposit cheque at your earliest convenience. John, we thank you for your valued order and we look forward to working with you on this program. Should you have any questions or comments, please do not hesitate to contact either me or David Jones. Best regards,
Jack Brown Project Engineer
cc: David Jones Accounts Payable
208
Appendix
Appendix 6: Molding Properties of Injection-Grade Plastics General properties of plastic materials typical values a
b
Density (g/cc) D-792
Specific Volume (cc/g)
Specific Volume 3 (in. /lb)
Water Absorption D-570
AB S ABS 20% GR Acetal copolymer Acetal 20% GR
1.08 1.2 1.41 1.55
0.926 0.833 0.709 0.645
25.7 23.1 19.6 17.9
0.25 0.12 0.22 0.45
Acetal homopolymer Acetal 20% GR Acrylic Modified acrylic MMA
1.42 1.55 1.2 1.16
0.704 0.645 0.833 0.862
19.5 17.9 23.9 24.0
0.25 0.25 0.2 0.3
Cellulose acetate Cell.acet.butyrate Cellulose proprionate Ethyl cellulose
1.22 1.15 1.17 1.13
0.820 0.870 0.855 0.885
22.7 24.1 23.7 24.5
2.6 0.9–2.2 1.2–3 1.3
Chlorinated polyether CTFE Polyvinylidene fluoride FEP Ionomer
1.4 2.1 1.75 2.12 0.93
0.714 0.476 0.571 0.472 1.07
19.8 13.2 15.8 13.1 29.8
0.01 0.00 0.04 0.01 0.1–1.2
Nylon 6 Nylon 6 30% GR Nylon 6/6 Nylon 6/6 30% GR Nylon 1 1 Nylon 6/10 Nylon 6/10 30% GR Polysulfone Modified phenylene oxide PPO
1.16 1.35 1.14 1.35 1.4 1.09 1.35 1.24 1.08
0.862 0.741 0.877 0.741 0.962 0.917 0.741 0.806 0.926
23.9 20.5 24.3 20.5 19.8 25.4 20.5 22.3 25.6
1.6 1.3 1.5 0.7 0.4 0.4 0.2 0.22 0.066
PPO 30% GR Polyaryl ether Polyaryl ether 30% GR Polyarylsulfone
1.29 1.14 1.37 1.36
0.775 0.877 0.730 0.735
21.5 24.3 20.2 20.4
0.06 0.25 1.8
209
Appendix 6: Molding Properties of Injection-Grade Plastics
a
a
b c
b
Density (g/cc) D-792
Specific Volume (cc/g)
Specific Volume 3 (in. /lb)
Water Absorption D-570
Polycarbonate Polycarbonate 30% GR Polyethylene LD Polyethylene MD
1.2 1.43 0.910-.925 0.926-.940
0.833 0.699 1.10–1.08 1.08–1.06
23.1 19.4 30.4–30.0 30.0–29.4
0.16 0.07 < 0.015 < 0.01
Polyethylene HD Polyethylene 30% GR Polyethylene High MW Ethylene vinyl acetate (EVA) Polypropylene
0.941–0.97 1.18 0.94 0.935 0.9
1.06–1.03 0.847 1.06 1.07 1.11
29.4–28.6 23.5 29.4 29.6 30.8
< 0.01 0.04 0.09 < 0.01
Polypropylene 30% GR Polycopolymer Polyallomer 4-Methyl pentene-1 Polystyrene GP Polystyrene GP 30% GR Polystyrene HI PTMT Styrene-acrylonitrile (SAN)
1.12 0.9 0.9 0.83 1.07 1.33 1.07 1.31 1.07
0.893 1.11 1.11 1.20 0.935 0.752 0.935 0.763 0.935
24.7 30.8 30.8 33.3 25.9 20.8 25.9 21.1 25.9
< 0.01 < 0.01 < 0.01 0.01 0.03–0.1 0.05–0.1 0.05–0.6 0.08 0.25
Styrene-acrylonitrile 20% GR Styrene-butadiene Rigid polyvinyl chloride (PVC) Chlorinated PVC
1.22 0.93–1.1 1.4 1.5
0.820 1.07-.909 0.714 0.666
22.7 29.6–25.2 19.8 18.4
0.08 0.2–0.4 0.07–0.4 0.02–0.15
PVC/PP Vinylidene Chloride Urethane elastomer Phenol-formaldehyde (no filler)
1.4 1.7 1.2 1.3
0.714 0.588 0.833 0.769
19.8 16.3 23.1 21.3
0.07–0.4 0.1 1.6
To obtain density as ounces per cubic inch, multiply density (g/cc) 0.578l; and as pounds per cubic inch, multiply density (g/cc) by 0.03613. Water absorption; 1/8 in. thick, percentage increase in weight – 24 hr. to obtain tf/in3, multiply density (g/cc) by 3.613 and cost of material(5/lb).
210
Appendix
Appendix 7: General Properties Thermal properties of plastic materials (these are typical values and vary with the different grades of material) Deflection Temp (°F)
Deflection Temp (°C)
264 psi D 648
66 psi
18.6 2 kg/cm
4.64 2 kg/cm
AB S ABS 20% G R Acetal copolymer Acetal 20% GR
200 230 230 320
210 240 316 325
93 110 310 160
Acetal homopolymer Acrylic Modified acrylic MMA Cellulose acetate
255 180 221 140
330 190 239 150
Cellulose acetate butyrate Cellulose proprionate Ethyl cellulose Chlorinated polyether
160 170 140 210
170 200 150 285
CTFE Polyvinylidene fluoride FEP Ionomer
Thermal Conduca tivity
Thermal Expansion (in.) (in.) –5 (°C) ⋅ 10
99 116 158 163
7 5.5 6
8 3 8.5
124 82 105 60
115 88 116 66
5.5 7 5 6
8.1 7 5.4 13
71 77 60 99
77 93 66 141
6 6 5.5 3.1
14 14 13 8
38
126 149 71 43
5 3 6 5.8
6 8.5 9 12
100
258 300 160 110
Nylon 6 Nylon 6 30% GR Nylon 6/6 Nylon 6/630% GR
150 390 185 480
330 420 395 490
66 199 85 249
166 216 202 254
6 6 6 5
8.3 4 8 2
Nylon 1 1 Nylon 6/10 Modified phenylene oxide PPO Modified phenylene oxide 30% GR Polyaryl ether Polyarylsulfone Polycarbonate
180 180 240
300 300 260
82 82 116
149 149 127
7 5 5
15 9 5.2
300 300 525 275
310 320
149 149 274 135
154 160
4 7 4.6 4.6
2.2 6.5 4.7 6.6
195
280
91
138
211
Appendix 7: General Properties
Deflection Temp (°F)
Deflection Temp (°C)
264 psi D 648
66 psi
18.6 2 kg/cm
4.64 2 kg/cm
Polyethylene LD Polyethylene MD Polyethylene HD Ethylene vinyl acetate (EVA)
100 115 120 93
112 142 165 146
38 46 49 34
Polypropylene Polypropylene 30% GR Polyropylene copolymer Polyallomer
132 280 127 130
215 310 206 175
4-Methyl pentene-1 Polystyrene GP Polytetramethylene terephthalate (PTMT) Polystyrene GP 30% GR
175 122 223
Polystyrene HI Styrene-acrylo-nitrile (SAN) SAN 20% GR Styrene-butadiene Rigid polyvinyl chloride (PVC) Chlorinated PVC PVC/PP Vinylidene Choride Phenol formaldehyde (no filler) a
Thermal Conduca tivity
Thermal Expansion (in.) (in.) –5 (°C) ⋅ 10
44 61 74 63
8 9 11.5
17 15 12 18
57 138 53 54
102 154 96 80
2.8
8
3 3
8.5 9
239 195 302 231
80 50 106
115 91 150 111
4 3
12 3 9 4.5
200 205 230
207 215 240
93 96 110
97 102 116
2 2.9
160 218 160 140 250
165 232
71 102 71 60 121
74 111
3.6 4. 3.3
4–20 3.7 2.7 13.5 7 7.2
127
3. 4.5
19 5
Thermal conductivity [(cal) (cm)/(s) (cm2) (°C)] · 10–4
260
212
Appendix
Appendix 8: Mechanical Properties Mechanical properties of plastic material (these are typical values) 2
Tensile Strength D-638 (1000 psi)
(kg/cm )
Tensile Modulus 5 (10 psi) D-638
Percentage Elongation at Yield D-638
ABS
6–8
422–562
3–4.2
5–60
ABS 20% GR
8–19
562–1336
5.9–10
3
Acetal copolymer
10
703
4.1
70
Acetal homopolymer
8.8
619
5.2
25
Acetal 20% GR
11
773
10
3
Acrylic
7–11
492–773
3.5–4.5
2–7
Modified acrylic MMA
10
703
4.6
3
Cellulose acetate
2–9
141–633
0.7–4
6–10
Cellulose acetate butyrate
2.5–7
176–492
0.5–2
40–90
Cellulose proprionate
2–8
146–562
0.6–2
30–100
Ethyl cellulose
2–8
141–562
1–3
5–40
Chlorinated potyether
6
422
1.6
60–160
CTFE
4.5–6
316–422
1.5–3
80–230
Polyvinylidene fluoride
5.5–7.5
387–527
1.2
100–300
FEP
3
211
0.5
250–330
Ionomer
2–4
141–281
Nylon 6
7–12
492–844
1.1–3
100–400
Nyton 6 30% GR
21
1476
10
2.2
Nylon 6/6
9–12
633–844
1.8–4
60–300
Nylon 6/6 30% GR
20–28
1406–1968
10–12.8
5–10
Nylon 11
8
562
1.9
300
Nylon 6/10
8.5
597
3.8
85–300
Nylon 6/10 30% GR
18
1125
9
2.5
Polysulfone
10.2
717
3.6
50–100
Modified phenylene oxide PPO
8–10
562–703
3.7
20–30
PPO30%GR
17
1195
9–12
6
Polyaryl ether
7.5
527
3.2
25–90
150–450
213
Appendix 8: Mechanical Properties
2
Flexural Strength D-790 (1000 psi)
(kg/cm )
6–13 16–27
a
Flexural Modulus 5 (10 psi) D-790
Izod D-256
Rockwell Hardness (L, R, M Scales)
(Shore-D Scale)
422–914
2–4
3–8
R
75–115
1125–1898
9–13
1–2.4
M
65–100
13
914
3.8
1.5
R
120
14
984
4.1
1.4
M
80
15
1055
8.8
0.8
M
80
13–14
914–984
4.3
0.4
M
100
12
844
5
0.3
M
105
2–16
141–1125
0.4–5
R
35–125
2–9
141–633
0.5–12
R
30–116
3–11
211–773
1–6
R
10–122
4–12
281–844
2–8
R
50–116
5
352
1.3
0.4
R
100
9.3
654
2.2
2.6
R
85
7
492
2
4
R
80
1
NB
R
25
6–1l
R
55
R
103–120
1.5–3.4
7–15
492–1055
1
2
26
1828
10
3
1.8
1–2
R
108–120
10–12
2
R
110–120
NB
1.4
1.8
R
110–120
NB
1.6
1.2
R
110–120
NB 30
2109
23
1617
8
1.6
R
120
15.4
1083
8–18
1.3
M
100
13
914
16
1.8
R
118
20
1406
20
1.5
L
108
11
773
8
R
117
214
Appendix
2
Tensile Strength D-638 (1000 psi)
(kg/cm )
Tensile Modulus 5 (10 psi) D-638
Percentage Elongation at Yield D-638
Polyarylsulfone
13
914
3.7
13
Polycarbonate
8–9.5
562–668
3.3
100–130
Polycarbonate 20% GR
9
633
8
110
Polyethylene LD
0.6–2.3
42–162
0.14–0.38
90–800
Polyethylene MD
1.2–3.5
84–246
0.25–0.55
50–600
Polyethylene HD
3.1–5.5
218–387
0.6–1.8
30–1000
Polyethylene 30%GR
8
562
8.0
3
Polyethylene high MW
2.5–3.5
176–246
0.2–1.1
300–500
Ethylene vinyl acetate (EVA)
1.5–2.8
105–197
0.02–1
750–900
Polypropylene
4.3–5.5
302–387
1.6–2.3
200–700
PTMT
8.2
576
Polypropylene 30% GR
7.3
513
0.8
3
Polypropylene copolymer
2.5–4.5
176–316
1–1.7
200–700
Polyallomer
34
211–281
4-Methyl pentene-1
4
281
2.1
15
Polystyrene GP
5–12
352–844
4–6
1–2.5
Polystyrene GP 30% GR
9–15
633–1055
12
1.3
Polystyrene HI
1.5–7
105–492
1.5–5
2–80
Styrene-acrylo-nitrile (SAN)
9–12
633–844
4–6
3
SAN 20% GR
9–20
633–1406
4–14
3
Styrene-butadiene
0.6–3
42–21 1
0.01–0.5
300–1000
Rigid polyvinyl chloride (PVC)
5–9
352–633
3.5–6
100–1000
Chlorinated PVC
7.5–9
527–633
3.6–5
5–65
PVC/PP
5–8
352–562
4
100–140
Vinylidene chloride
3–5
211–352
0.6
10–250
Urethane elastomer
6.5–8
457–562
Phenol-formaldehyde (no filler)
7
492
250
400–500
600 7–10
1.5
215
Appendix 8: Mechanical Properties
Flexural Strength D-790 (1000 psi)
2
(kg/cm )
Flexural Modulus 5 (10 psi) D-790
a
Izod D-256
Rockwell Hardness (L, R, M Scales)
(Shore-D Scale)
17
1195
5
M
110
12.5
879
3.4
12–18
M
70
13.5
949
3.4
16
M
91
0.08–0.6
NB
D
45
4.8–7 9.5
337–492 668
0.6–1.2
0.5–15
D
55
1–2.6
0.5–20
D
65
7
2
R
70
1.3
NB
0.01–0.2 6–8
422–562
12
844
10
703
5–7
352–492
9–14
NB
D
20
0.5–2
R
85–110
3.1
NB
R
117
7.6
2
R
110
1–20
R
50–95
R
50–85
0.8
L
70
633–984
4.5
0.3
M
72
11–20
773–1406
8–10
4
M
80
5–12
352–844
3.8
0.5–11
R
50–100
14–19
984–1336
6
0.4
M
85
22–26
1547–1969
8–18
4
M
100
0.4–20
D
70
NB
0.4–1.5
NB
10–16
703–1125
15–17
1055–1195
3.4–6
1–6
R
120
11–15
773–1055
3.5–5
0.4–32
R
110
4–6
281–422
1
M
60
D
52
M
125
9 a
633
10
Izod impact – ft lb/in. of notch (½ × ½ bar).
0.25
216
Appendix
Appendix 9: Thermal Properties Properties chart for injectlon molding grade plastics ASTM
D792
D648
Transp. (TP) Amorphous Transluc. (TL) Crystalline Opaque (O)
Shrinkage Drying Req’d (%) (time@temp) (hours@°C)
O,TP
Amorphous
0.4 to 0.6
O,TP
Amorphous
0.5 to 0.8
2–3@88–77
Acronym
Full Chemical Name
Solid Density 3 (g/cm )
ASA
Acrylonitrile-Styrene-Acrylate
1.07
ABS
Acrylonitrile-Butadine-Styrene
1 to 1.2
CA
Cellulose Acetate
1.3
Transparent
0.3 to 0.7
2–3@60–70
CAB
Cellulose Acetate Butyrate
1.2
Transparent
0.3 to 0.7
2–3@60–70
CAP
Cellulose Propionate
1.2
Transparent
0.3 to 0.7
2–3@60–70
EVA
Ethylene-Vinyl-Acetate
0.94 avg
0.8
TL,TP
HDPE
High Density Polyethylene
0.93 to 0.97
0.81
O,TP
Crystalline
1.2 to 2.2
none
HIPS
High Impact Polystyrene
1 to 1.1
0.9
O,TP
Amorphous
0.4 to 0.7
none
Ionomer
Ionomer
0.94 to 0.96
Transparent
Semi-Cryst.
LCP
Liquid Crystal Polymer
1.5 to 1.7
TP,TL
Liq. Crystal
0.2 to 0.8
4–8@150
LDPE
Low Density Polyethylene
0.91 to 0.93
TP,TL
Crystalline
1.5 to 3
none
LLDPE
Linear Low Density Polyethylene 0.90 to 0.92
TP,TL
Crystalline
1.5 to 5
none
MDPE
Medium Density Polyethylene
0.93 to 0.95
O,TP
Crystalline
1.2 to 2.2
none
PA66
Polyamide (Nylon 6/6)
1.1 to 1.4f
1.0 to1.2
Translucent
Crystalline
1 to 2.2
3–4@71–60
PA6
Polyamide (Nylon 6)
1.1 to 1.4
1.2 to 1.3
Translucent
Crystalline
0.8 to 2.1
3–4@71–60
PAEK
Polyaryletherketone
1.3 to 1.5f
Opaque
Semi-Cryst.
0.1 to 0.6
2–4@175–150
PAI
Polyamide-Imide
1.4 to 1.6
Opaque
Amorphous
0.1 to 0.2
5@175
PAR
Polyarylate
1.2
Opaque
Amor./Cryst. 0.6 to 0.9
3–4@175–150
PAS
Polyaryl-Sulfone
1.36 avg
Opaque
Amorphous
0.6
3–4@175–150
PBT
Polybutylene-Terephthalate
1.3 to 1.6f
Translucent
Semi-Cryst.
1.5 –2 (0.5f) 3@150
NOTES: f = FILLED PROPERTY MPa · 145 = PS1 Deg F = (Deg C – 32) / 1.8
Melt Density 3 (g/cm )
D1003
0.9
0.77
1.1 to 1.2
none (8@60)
217
Appendix 9: Thermal Properties
ASTM Acronym
Melt Temp. (°C)
Mold Temp. (°C)
Injection Speed
Shear Rate or Temp. Sensitive
Resid. Time Sensitive
Max L/t No Flow Ratio Temp (1mm thick) (°C)
Tonnage Required 2 (tons/in )
Typical Inj. Pres. (MPa)
ASA
230 to 260
40 to 90
ABS
195 to 240
38 to 93
slow, even
NO
YES
30–150 : 1
4 to 6
120 to 140
CA
180 to 230
50 to 80
various
YES
130–170
69 to 180
CAB
180 to 230
50 to 80
various
YES
130–170
69 to 180
CAP
180 to 230
50 to 80
various
YES
130–170
69 to 180
EVA
200 to 210
20 to 60
HDPE
200 to 280
10 to 70
fast
NO
NO
250 : 1
HIPS
180 to 280
10 to 85
fast
NO
NO
Ionomer
210 to 260
5 to 50
slow, mod. YES
LCP
400 to 430
240 to 280
medium
LDPE
170 to 240
10 to 50
LLDPE
170 to 200
MDPE
135–150
100 2 to 2.5
100 to 200
200–250 : 1 130
2 to 4
100 to 200
NO
100
2 to 3
70 to 110
YES
NO
200–300 : 1 370
2 to 3
80 to 120
fast
NO
NO
275 : 1
1.5 to 2
80 to 120
10 to 50
fast
NO
NO
250–300 : 1 90
1.5 to 2
80 to 120
190 to 260
10 to 70
fast
NO
NO
250 : 1
2 to 2.5
100 to 150
PA66
270 to 320
20 to 100
fast
YES
YES
140–340 : 1 240–260
4 to 5
100 to 150
PA6
260 to 310
20 to 100
fast
YES
YES
140–340 : 1 190–200
3 to 5
90 to 150
PAEK
370 to 400
160 to 220
slow
NO
NO
170–200 : 1 370
3–4(6f)
160 to 200
PAI
305 to 370
205 to 220
fast
YES
MILD
140–340 : 1
5 to 6
160 to 200
PAR
260 to 380
65 to150
moderate
NO
YES
30–100 : 1
3–5 (4–6f)
138 to 200
PAS
340 to 370
120 to 155
moderate
NO
NO
140–170 : 1
PBT
240 to 270
50 to 100
mod., fast
YES
YES
160–200 : 1 220–250
NOTES: f = FILLED PROPERTY MPa · 145 = PS1 Deg F = (Deg C – 32) / 1.8
120–130
100–110
110
138 to 200 3 to 5
80 to 120
218
Appendix
ASTM
D792
D1003
D648
Acronym
Full Chemical Name
Solid Density 3 (g/cm )
Melt Density 3 (g/cm )
Transp. (TP) Amorphous Transluc. (TL) Crystalline Opaque (O)
Shrinkage Drying Req’d (%) (time@temp) (hours@°C)
PC
Polycarbonate
1.2 to1.5f
1.1
Transparent
Amorphous
0.4f to 0.7
PEI
Polyether-Imide
1.3 to1.5f
Opaque
Amorphous
0.5–0.7 (0.2f) 4–6@150–130
PEK
Polyetherketone
1.3 to1.5f
Opaque
Semi-Cryst.
0.1 to 0.6
2–4@175–150
PEEK
Polyether-ether-Ketone
1.3 to1.4f
Opaque
30% Cryst.
0.1 to 1.4
2–4@175–150
P ES
Polyether-Sulfones
1.2 to1.6f
Transparent
Amorphous
0.3f to 0.6
3–4@150–135
PET
Polyether-Terephthalate
1.4 to 1.7f
O,TP
Cryst./Amor. 0.2 to 2
2–4@180–160
PETG
PET (Co-polymer)
1.2 to 1.3
O,TP
Amorphous
4@66
PMMA
Poly-Methyl-Methacrylate(Acrylic)1.1 to 1.2
Opaque
Amorphous
0.4 to 0.8
2–3@77–93
Polyester
Thermoplastic Polyester
1.3
Opaque
Amorphous
1.5 to 1.8
2–4@120–77
POM
Polyoxymethylene (Acetal)
1.4 to1.6f
1.2
Opaque
80% Cryst.
0.8f to 2
none
PP
Polypropylene
0.9 to 0.92
0.77
TP,TL
Semi-Cryst.
1 to 2.5
none
PPO
Polyphenylene-Oxide
1.1 to1.2f
0.96 to 1.0 Opaque
Amorphous
0.2f to 0.7
2–4@115–107
PPS
Polyphenylene Sulfide
1.3 to 1.9f
1.5 avg
Opaque
65% Cryst.
0.1 to 0.5
3–4@150
PS
Polystyrene
1 to 1.1
0.9
O,TP
Amorphous
0.4 to 0.7
none
PSU
Polysulfone
1.2 to1.6f
O,TP
Amorphous
0.7
5@120
PVC
Polyvinyl-Chloride
1.2 to 1.4
1.2 to 1.3
Transparent
Amorphous
0.2 to 0.5
None (2@60)
SAN
Styrene-Acrylonitrile
1.1 to 1.3
0.9
O,TP
Amorphous
0.3 to 0.7
2–3@80–70
TPUR
Thermoplastic Polyurethane
1.2 to 1.3
O,TL
Amorphous
0.8 to 2
2–3@110–104
O,TP
Crystalline
1.2 to 2.2
none
UHMWPE Ultra High Molecular Weight PE 0.93 to 0.94
NOTES: f = FILLED PROPERTY MPa · 145 = PS1 Deg F = (Deg C – 32) / 1.8
1.2
1.0 to 1.1
0.9
3–4@120
219
Appendix 9: Thermal Properties
ASTM Acronym
Melt Temp. (°C)
Mold Temp. (°C)
Injection Speed
Shear Rate or Temp. Sensitive
Resid. Time Sensitive
Max L/t No Flow Ratio Temp (1mm thick) (°C)
Tonnage Required 2 (tons/in )
Typical Inj. Pres. (MPa)
PC
270 to 325
80 to 110
fast
NO
YES
30–100 : 1
3–5 (4–6f)
138 to 200
PEI
340 to 425
65 to 175
med.-fast
NO
YES
230
PEK
370 to 400
160 to 220
slow
NO
NO
170–200 : 1 370
3–4 (6f)
160 to 200
PEEK
370 to 400
160 to 220
fast
NO
NO
200 : 1
2–4 (6f)
160 to 200
P ES
340 to 380
140 to 160
fast
YES
YES
60 to 120 : 1 300
5 to 10
160 to 200
PET
260 to 300
7 to 80
slow, even
YES
YES
80 to 200 : 1 240–250
2 to 6
70 to 160
PETG
190 to 275
20 to 30
slow-fast
YES
YES
80 to 200 : 1
PMMA
200 to 260
38 to 60
various
YES
NO
130–150 : 1 160–170
2.5 to 3
100 to 200
Polyester
230 to 260
40 to 100
slow, even
YES
YES
80 to 200 : 1 70–80
3 to 5
80 to 100
POM
180 to 230
80 to 100
med.-fast
YES
YES
100–200 : 1 160–170
3.5 to 5
100 to 170
PP
230 to 275
15 to 65
fast
NO
NO
200–300 : 1 170–180
2 to3
100 to 130
PPO
250 to 315
82 to 110
fast
YES
YES
150–200
2.5 to 5f
120 to 180
PPS
300 to 360
40 to 150
slow
YES
NO
150 : 1
260–280
2 to 3
50 to 140
PS
180 to 280
10 to 85
fast
NO
NO
200–250 : 1 130–160
2 to 4
100 to 200
PSU
310 to 400
100 to 170
slow
NO
NO
PVC
180 to 204
20 to 40
slow-mod.
YES
YES
SAN
220 to 270
5 to 60
YES
YES
TPUR
190 to 220
30 to 65
YES
UHMWPE 200 to 280
10 to 70
NOTES: f = FILLED PROPERTY MPa · 145 = PS1 Deg F = (Deg C – 32) / 1.8
fast
NO
NO
100 : 1
150 : 1
195
370
100 to 160
80 to 100
120
70 to 140
130–170
35 to 140
120
.5 to 2
70 to 140
120
3 to 4
130 to 200
220
Appendix
Appendix 10: Typical Mold Materials Metric Units Prehardened Steels Property
Temp (C)
Chemical comp.
Tool steels
P20
4140
NAK55
H13
H13
S7
0.33 C
0.40 C
0.15 C
0.38 C
0.38 C
0.3 Si
0.3 Si
0.3 Si
1.0 Si
1.0 Si
1.4 Mn
0.8 Mn
1.5 Mn
0.3 Mn
1.8 Cr
1.0 Cr
0.1 S
5.2 Cr
0.8 Ni
0.2 Mo
1.0 cu 3.0 Ni
0.2 Mo
Stainless steels CPM9V CPM10V
Ultimate strength (MPa)
Yield strength (MPa)
Elongation (%) Imp. str. (J) (V-notch)
Elastic modulus (GPa)
Therm exp. (10E-6/C)
Thermal cond. (W/M C)
Corrosion resistance Applications
20
28–32 28–32 40 (270–300 (270–300 (375 HB) HB) HB)
420 ESR
416T BECU3 BECU25 (prehrd) HT
Aluminum
Titanium
6061 T6
6A1-4V
A2
D2
A10
O6
0.50 C
1.0 C
1.55 C
1.35 C
1.45 C
1.78 C
2.45 C
0.38 C
0.33 C
0.38 C
0.15 C
0.4 Be
1.9 Be
97.6 Al
6.0 Al
0.3 Si
0.2 Si
0.3 Si
1.2 Si
0.9 Si
0.9 Si
0.9 Si
0.3 Si
0.4 Si
0.9 Si
1.0 Si
1.8 Ni
0.6 Co, Fe, Ni
0.6 Si
4.0 V
0.3 Mn
0.7 Mn
0.6 Mn
0.3 Mn
1.8 Mn
1.0 Mn
0.5 Mn
0.5 Mn
1.1 Mn
1.4 Mn
0.5 Mn
1.2 Mn
0.28 Cu
0.3 Fe
5.2 Cr
3.2 Cr
5.3 Cr
12.0 Cr
1.9 Ni
0.3 Mo
5.3 Cr
5.3 Cr
16.0 Cr
16.7 Cr
13.6 Cr
13.0 Cr
1.0 Mg
1.3 Mo
1.3 Mo
1.4 Mo
1.1 Mo
0.8 Mo
1.5 Mo
1.3 Mo
1.3 Mo
0.1 S
0.3 V
0.2 S
0.2 Cr
1.0 V
1.0 V
0.2 V
0.8 V
9.0 V
9.7 V
42–44
49–51
54–56
54–56
56–58
58–60
58–60
53–55
60–62
49–51
26–36
18–26 (95–102 Rb)
38–41
(95 HB)
30–34
(2448 comp)
(2454 comp)
1917 (2765 comp)
1917 (3599 comp)
(3392 comp)
1896
1096
1765
945
924
1517
310
896
1000
1731
896
1310
689
717
310
572
807
1344
786
1172
620
648
241
483
5–25
3–10
1.0 Al Hardness (RC)
DIN. 420F 1.2316 (prehrd) (prehrd)
Copper alloys
0.1 S
20
1007
965
1262
1386
1696
200
951
910
1041
1303
1744
400
793
751
882
1158
1469
20
800
758
1020
1117
1331
200
751
710
841
1048
1262
400
627
586
683
931
1062
20
17
18
1–15
15
13
20
73
51
8
30
27
200
81
60
12
37
30
(2034 comp)
(2061 comp)
1475 (2123 comp)
1737 (2275 comp)
(2744 comp)
28–36 36 (270–340 (340 HB) HB) 1076
924
1620
3–9 27
910
1469
820
1172
758
3–10
8
16
17
12
31
200
41 (Cnotch)
47 (unnotch)
58 (unnotch)
30 (unnotch)
73 (Cnotch)
35 (Cnotch)
50
193
207
221
221
221
200
200
214
193
193
11.0
11.0
10.5
11.5
11.5
400
98
76
11
41
34
20
207
207
207
200
200
200
193
193
200
200
200
200
186
186
193
186
186
275
12
827
11 19
30
207
179
138
131
110
117
70
114
400
186
186
179
172
172
179
172
172
200
12.6
12.8
12.6
11.5
11.5
12.1
11.7
11.0
11.7
11.2
11.2
11.2
11.0
62
48
17.6
17.5
400
13.5
13.5
12.2
12.2
12.4
11.3
10.8
11.3
12.6
11.7
11.7
12.1
24
9.0
20
29.1
42.7
24.7
25.9
25.9
29.1
26.1
20.1
25.9
21.4
21.4
16.9
23.0
23.0
24.9
249.1
103.9
167.0
6.9
200
29.6
42.2
25.8
25.9
26.1
29.7
27.1
21.1
27.0
19.0
23.9
23.9
28.7
400
31.1
37.7
26.8
26.5
26.5
31.0
28.7
23.0
28.5
24.7
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Good
Good
Good
Good
Good
Good
Fair
Good
Plates, misc. comp.
Plates, misc. comp.
Mflds
Nozs, H.R. comp.
Cores, cavs, gates
Gates, stripr rings
Lock rings, stripr rings
Wear parts
Sliding surf.
Sliding surf.
High wear gates
High wear parts
Mold plates, slides
Mold plates, misc. comp.
Cores, cavs, gates
Misc. comp.
Noz tips
Core caps, gate insers
Cooling manif., T.O. comp.
Insulators
9.4
8.6
24.9
(Note: Values shown are approximate depending on supplier and quality level of material.)
11.4
221
Appendix 10: Typical Mold Materials
English Units Prehardened Steels Property
Temp (F)
Chemical comp.
Tool steels
P20
4140
NAK55
H13
H13
S7
0.33 C
0.40 C
0.15 C
0.38 C
0.38 C
0.3 Si
0.3 Si
0.3 Si
1.0 Si
1.0 Si
1.4 Mn
0.8 Mn
1.5 Mn
0.3 Mn
1.8 Cr
1.0 Cr
0.1 S
5.2 Cr
0.8 Ni
0.2 Mo
1.0 cu 3.0 Ni
0.2 Mo
Stainless steels CPM9V CPM10V
Ultimate strength (KSI)
Yield strength (KSI)
Elongation (%) Imp. str. (Ft-lbs) (V-notch)
Elastic modulus (PSIE06)
Therm exp. (10E-6/F)
Thermal cond. (BTU/ FTHRF)
70
Applications
416T BECU3 BECU25 (prehrd) HT
Titanium
6061 T6
6A1-4V
D2
A10
O6
0.50 C
1.0 C
1.55 C
1.35 C
1.45 C
1.78 C
2.45 C
0.38 C
0.33 C
0.38 C
0.15 C
0.4 Be
1.9 Be
97.6 Al
6.0 Al
0.3 Si
0.2 Si
0.3 Si
1.2 Si
0.9 Si
0.9 Si
0.9 Si
0.3 Si
0.4 Si
0.9 Si
1.0 Si
1.8 Ni
0.6 Co, Fe, Ni
0.6 Si
4.0 V
0.3 Mn
0.7 Mn
0.6 Mn
0.3 Mn
1.8 Mn
1.0 Mn
0.5 Mn
0.5 Mn
1.1 Mn
1.4 Mn
0.5 Mn
1.2 Mn
0.28 Cu
0.3 Fe
5.2 Cr
3.2 Cr
5.3 Cr
12.0 Cr
1.9 Ni
0.3 Mo
5.3 Cr
5.3 Cr
16.0 Cr
16.7 Cr
13.6 Cr
13.0 Cr
1.0 Mg
1.3 Mo
1.3 Mo
1.4 Mo
1.1 Mo
0.8 Mo
1.5 Mo
1.3 Mo
1.3 Mo
0.1 S
0.3 V
0.2 S
0.2 Cr
1.0 V
1.0 V
0.2 V
0.8 V
9.0 V
9.7 V
42–44
49–51
54–56
54–56
56–58
58–60
58–60
53–55
60–62
49–51
26–36
(355 comp)
(356 comp)
278 (401 comp)
278 (522 comp)
(492 comp)
275
159
256
137
145
251
214 (308 comp)
252 (330 comp)
(398 comp)
132
213
119
170
0.1 S
70
146
140
183
201
246
400
138
132
151
189
253
750
115
109
128
168
213
70
116
110
148
162
193
400
109
103
122
152
183
750
91
85
99
135
154
70
17
18
1–15
15
13
(295 comp)
(299 comp)
28–36 36 (270–340 (340 HB) HB) 156
134
235
3–9
70
54
38
6
22
20
400
60
44
9
27
22
750
72
56
8
30
25
70
30
30
30
29
29
29
400
29
29
29
27
27
28
20
110
3–10
8
16
13
9
23
29
35 (unnotch)
43 (unnotch)
22 (unnotch)
54 (Cnotch)
26 (Cnotch)
37
28
28
28
30
32
32
32
29
29
27
27
31
28
28
6.1
6.1
5.8
6.4
6.2
41 (Cnotch)
18–26 (95–102 Rb)
38–41
(95 HB)
134
220
45
130
190
30–34
130 100
104
45
117
195
83
114
170
90
94
35
70
5–25
3–10
40
12
120
11 14
22
750
27
27
26
25
25
26
25
25
400
7.0
7.1
7.0
6.4
6.4
6.7
6.5
6.1
6.5
6.2
6.2
6.2
6.1
750
7.5
7.5
6.8
6.8
6.9
6.3
6.0
6.3
7.0
6.5
6.5
6.7
70
16.8
24.7
14.3
15.0
15.0
16.8
15.1
11.6
15.0
12.4
12.4
400
17.1
24.4
14.9
15.0
15.1
17.2
15.7
12.2
15.6
750 Corrosion resistance
28–32 28–32 40 (270–300 (270–300 (375 HB) HB) HB)
420 ESR
Aluminum
A2
1.0 Al Hardness (RC)
DIN. 420F 1.2316 (prehrd) (prehrd)
Copper alloys
30
26
9.8
13.3
13.3
14.4
11.0
13.8
13.8
16.6
14.3
20
19
16
17
9
7
9.8
9.7
10
13.3
16.5
5.0 5.2
144.0
60.1
97.0
4.0 5.0
18.0
21.8
15.5
15.3
15.3
17.9
16.6
13.3
16.5
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Poor
Good
Good
Good
14.4 Good
Good
Good
Fair
Good
Plates, misc. comp.
Plates, misc. comp.
Mflds
Nozs, H.R. comp.
Cores, cavs, gates
Gates, stripr rings
Lock rings, stripr rings
Wear parts
Sliding surf.
Sliding surf.
High wear gates
High wear parts
Mold plates, slides
Mold plates, misc. comp.
Cores, cavs, gates
Misc. comp.
Noz tips
Core caps, gate insers
Cooling manif., T.O. comp.
Insulators
(Note: Values shown are approximate depending on supplier and quality level of material.)
6.6
222
Appendix
Appendix 11: What Characterizes a Good, High-production Mold? The difference between a well-designed and solidly built mold (A), planned to produce reliably good quality products at the lowest cost, and a low-cost mold (B) intended for the same product, can usually be seen right from the start, by visually comparing the two molds, and becomes obvious after operating the mold for a relatively short time.
Subject
Mold (A)
Mold (B)
1. Concept and design
Conceived from start with high productivity and lowest cost product in mind. Much time is spent on the mold concept before starting the actual design, or beginning to cut steel.
Little if any thought has been given to the mold, except getting the shape of the product right, at the lowest possible mold cost.
2. Runner systems
Much thought is given to decide on the most suitable runner system, the gating, cooling, and ejection of the products.
As long as the mold can produce the desired product, even only for a short length of time, it is considered acceptable.
3. Servicing of hot runners
In hot runner molds, access to the hot runner system for minor servicing (changing heater bands or nozzle tips, etc.) is possible right in the machine, without removing the mold.
The mold must be removed from the machine for any service to the hot runners.
4. Strength of materials
The strength of materials used in the mold is properly engineered. This applies typically to the strength of cavities and cores to contain the high injection pressures and the size of mold plates to support both cavities and cores.
Cavities and cores are often not strong enough and expand or deflect when subjected to high injection pressures. The mold plates are thin, just strong enough to hold the components in position.
5. Strength of supports
Support plates are adequately heavy and sufficient supports are provided under the cavities and cores to ensure minimum deflection of the plates under load by clamping and during injection, thus guaranteeing quality products.
Support plates are too thin and supports under the cavities and cores are too small and often too far apart, resulting in excessive deflection of these plates, misalignment between cavities and cores and causing excessive wear in the alignment features.
6. Mold steel selection
Mold steel selection is appropriate for the expected life and performance of the mold and often, the best quality steels and other materials are selected, without too much concern about the added cost. (After all, with large production runs, the additional cost per molded piece is infinitesimally small).
Any grade steel is used for the plates and cost of materials and mold components is a serious consideration.
223
Appendix 11: What Characterizes a Good, High-production Mold?
Subject
Mold (A)
Mold (B)
7. Mold hardware
Careful consideration is given to select the best quality of (purchased) mold components to ensure trouble-free operation.
Leader pins are often too short and too slender, just long enough to engage to LP bushings. They deflect easily and do not ensure proper alignment of the mold halves. Tapers are usually too short, wear soon, and do not hold the cores aligned with the cavities.
8. Mold alignment
Alignment feature (leader pins, tapers). Long, especially unsupported leader pins are heavy to ensure minimum deflection. They are long enough to protect the exposed cores from damage. Tapers for alignment are sized proportional to the length of the core.
Leader pins are often too short and too slender, just long enough to engage to LP bushings. They deflect easily and do not guarantee proper alignment of the mold halves. Tapers are usually too short, wear soon, and do not hold the cores aligned with the cavities.
9. Tapers
Alignment tapers are properly engineered, with specified amounts of preload, to ensure holding the matching parts in alignment securely.
Tapers are matched without proper preload, and therefore do not provide alignment. This can be easily seen just after a few days of operation when both matching surfaces are seen dirty or corroded. Properly preloaded tapers are always shiny.
10. Mold cooling
Cooling channels are carefully planned for optimal size and location to provide maximum cooling efficiency and ensure highest productivity. Cross drilling of the plates (which is possible because they are thick enough) simplifies the installation of the mold and improves the accessibility of the mold for servicing during startup.
Cooling channels are frequently placed almost as an afterthought, rather than planned from the beginning, and often omit difficult-to-cool hot spots. They are then connected outside the plates and often require an excessive number of hoses to be connected; they are making setup and servicing more difficult and invite errors.
11. Moving plates
Moving plates, such as ejector plates, are properly guided and supported to minimize wear of delicate pins, etc. Proper, often automatic, lubrication of wear points is provided.
Moving plates, such as ejector plates, are properly guided and supported to minimize wear of delicate pins, etc. Lubrication of wear points is left to the operators of the machine.
12. Mold mounting
Mounting the mold to the machine is considered from the beginning and the most suitable mounting methods are provided.
Usually, only mounting ledges are provided for the use of rather unsafe mold clamps to hold the mold in position.
13. Testing of mold
The mold is thoroughly tested at the mold maker, not just for product sizes and fits but also for productivity of the mold, before delivery.
The mold is only tested to check the sizes of the products, and then shipped.
224
Appendix
Subject
Mold (A)
Mold (B)
14. Cost of extras
Cost of extras are included in the mold price. These extras consist of all necessary hardware to store, install, start up and run the mold. To have these costs included makes it possible to compare fairly all potential vendors. The following list shows some but not necessarily all of these items: (Note that when requesting a quotation, it is good practice to request that any such foreseen item or feature is included in the quotation)
Mold making is a very competitive business, and by omitting to commit oneself on expected productivity and to consider necessary extras, the quoted mold price could become lower, thus more interesting to the unsuspecting buyer, who will later be faced with many additional expenses, or lack of productivity of the mold. The difference could be as much as 20% of the mold price.
• Materials used for construction • Coatings and finishing including plating (molding surfaces) • Pipe fittings and hoses • Electrical cables • Mold feet, latches, and lift bars • Suggested spare parts • Crating and preparation for shipping • Cost of re-cuts • Cost of re-tests • Guarantees on product size • Expected productivity • Payment terms and conditions • Extent of warranty 15. Mold manual
The customer is supplied with a “Mold Manual”, which documents all that is to be known about the mold and its accessories and vendor’s products. It includes mold drawings, test reports, safety tips, and mold maintenance and trouble shooting instructions.
There is usually no such mold manual supplied.
16. Productivity
The molder smiles when he sees the mold producing large quantities of good quality products, without breakdowns. The higher mold cost is soon forgotten.
With every of the frequent breakdowns the molder loses money, and any savings on the mold are soon surpassed by the costs of downtime and repairs. Low productivity and higher costs per unit produced will cost the molder much more than was saved in the first place, particularly with large production runs.
Appendix 12: Advice for the Mold Designer
Appendix 12: Advice for the Mold Designer Product Drawings Although it is the responsibility of the customer to provide a fully dimensioned and approved product drawing, before doing any mold design work always check these drawings to make sure they do not contain errors or omissions.
A12.1 Product Drawing Critique 1. Dimensions Are they plastic (final product) or steel (mold) dimensions? (Normally, a plastic product drawing shows the dimensions after shrinkage.) 2. Artwork/surface finish The artwork and surface finishes must be supplied by the customer. 3. Wall thickness Can product fill easily and completely as it is drawn by the customer? Are the radii sufficient to help filling or are there any pinch points? Plastic must always flow from thick to thin. 4. Drafts Question any negative or no draft and determine if the product can be ejected. Air ejection requires a minimum of 1° draft, but better 1.75° or more. Product drawings must show all drafts and mismatches. 5. All Geometry defined? Make sure that all radii, angles, etc. are defined by the customer. Note areas where a minimum radius of 0,08 mm is required. 6. Parting line/split line The locations of the parting line and all split lines must be shown on the product drawing and approved by the customer. Printing areas and/or cosmetic requirements for the product may require that these locations be reconsidered and moved. The customer must approve all final parting line and split line locations. 7. Sink marks Notify the customer of areas were sinks may occur (thick sections or ribs) and get his approval.
225
226
Appendix 8. Gate location and gate vestige Determine if the gate vestige will be an issue. If the bottom of the container is flat or the customer will print over the gate, the gate position could become a problem. A recessed gate should be considered in these cases. It is recommended to always use a dimple! A dimple is used to prevent gate cracking in PP and PS parts. It also aids filling of the products. 9. Stacking Will the products stack and de-nest readily? 10. Shrinkage/warpage Shrinkage data must be provided by the customer. Avoid any flat bottoms. A domed bottom will prevent warpage. If you cannot dome the bottom, suggest ribs or a foot to aid in stability. ALWAYS question possible warpage in rectangular products. Intensive cooling in strategic areas and thicker walls can reduce the problem. 11. Features to be left off mold until after first mold test? Normally, pull rings, which aid in part transfer are left off for the first test. In some cases, undercuts (on lids, etc.), and occasionally engravings are left off until after the mold test. 12. Leave stock for lid fit? Normally, stock is left on the lid undercut for the first test to determine shrinkage and lid fit requirements. 13. Ribs Any ribs must be shown in plan, top and side view. The rib must have draft in every direction. The rib should be no thinner than 0.5 mm and be no greater than 75% of the nominal wall section to avoid sinks.
A12.2 Stacking of Products A critical design festure is the ability to stack the product. Some products are not meant to stack except for saving space in shipping. There are several common methods to stack containers and lids, where stacking is an important feature. Containers are often stacked using a step (a so-called stacking shoulder) near the lip or the base. Alternatively, stacking lugs or ribs can be used at the lip area or in the bottom of the cup. Flat products most commonly use a stacking bead for alignment. A12.2.1 Stacking Clearances Stacking clearances are required to separate the products after stacking. This air gap should allow them to separate with gravity. However, if the clearance is too large, the containers or lids can shift and slip off the stacking ledge.
Appendix 12: Advice for the Mold Designer Clearance is dependent on height, diameter, wall thickness, draft, tackiness of the material, surface finish, and method of stacking (rib, shoulder, bead). A12.2.2 Stacking Engagement Stacking engagement ensures that stacked containers or lids will not collapse and telescope together. In the case of lids, it prevents the parts from sliding off of one another when stacked. Note that on the plastic part drawings, stacking engagement is dimensioned in two ways. One method is the distance from the intersecting edge of the part to the intersecting edge of the shoulder or rib. Another is the actual horizontal contact between the parts. The guidelines given here are based on the first method because the second method sometimes suggests there is no minimum engagement.
A12.3 Draft Angles Drafts are required both for stacking and for ease of ejection. Question any negative draft and determine if the product can be ejected. There are certain cases where negative drafts can be acceptable. Mechanical means (stripping) are normally used to remove products with zero draft, although in some exceptional cases removel is accomplished with air ejection. The use of air ejection is preferred for several reasons: the mold is easier to manufacture, maintain, install and requires far less maintenance. The minimum draft angles should be 1° (1.75 degrees or more are better.) Mechanical ejection should be considered when draft angles are less than 1°.
Many factors must be considered when deciding whether to use air or mechanical ejection, or a combination of both. Surface finish (polished can be very difficult to air eject), type of resin, temperature at ejection, presence of static electricity, and product geometry all affect air ejection.
A12.4 Wall Thickness There is tremendous pressure on the plastics industry to reduce the amount of plastic in the product. The reasons are twofold. First, a lighter product is a more cost effective product. And second, the public wants to reduce the amount of plastic being sent to landfill sites. Due to this pressure there is a trend to reduction in product weight. There are, however, limitations on how thin we can go. The limitation on wall thickness is a function of the distance the plastic has to travel from the gate to the parting line, the resin used, and the number of cavities in the mold.
227
228
Appendix
Appendix 13: Surface Finishes The following table can be used to determine some typical molding surface finishes. Micron
Micro inch
SPI #
Symbol
Manufacturing method
Applications
0–3
1
A-1
0.025 ∇
Lapping, 8000 diamond
Petri dishes
5
2
0.05 ∇ and buff
900 stone, 8000 diamond polish
Test tubes
8
3
0.08 ∇ and buff
900 stone, 3000 diamond polish
Styrene tumblers
10
4
A-2
0.1 ∇ and buff
600 drawstone, 3000 diamond polish
Opaque Shiny surface
10–20
4–8
C-3
0.1–0.2 ∇
900 drawstone
Opaque surface (stack shoulders)
D-2
0.1–0.2 ∇ and v. hone
900 drawstone, vapour hone
Matte surface (cores, gate pad)
15–20
4–6
0.15–0.2 ∇ v.hone, lt. buff
900 drawstone, vapour hone, light buff
Semi-opaque (‘clear’ cores)
10–15
4–6
0.1–0.15 ∇ v.hone and buff
900 drawstone, vapour hone, buff
Semi-opaque surface (cavities)
20
8
0.2 ∇ and buff
600 drawstone, 3000 diamond buff
Preform inserts
20–30
8–12
4
0.2–0.3 ∇
400–600 stone
Technical parts
D-3
0.2–0.3 ∇ and sand blast
400–600 stone, sandblast
Texture finish
0.4 ∇
220–300 stone
40
16
Notes:
A tool steel finish kit is available through the D-M-E Company as an official SPE/SPI kit.
The newest SPE/SPI codes were achieved using the specified media (diamond paste) as a guide and hand operations.
Frosting (e.g., on tumbler sidewalls) should never be specified with any depth since it is a surface finish only (i.e., 21–24 Charmille).
229
Index A
G
accumulator system 84 air assist 94 air only 94 alignment of the mold halves 25 annular ring gate 127 artwork 33 automatic product removal 92 automotive products 133 available operating time 95
guaranteed cycle time 186 guaranteed delivery 186 guaranteed quality 185 guaranteed shrinkage 185
B back pressure 74 beryllium copper 61 Bills of Materials 178 bluing 14
H haze 43 heat expansion 28 heat sensitive 126 hobbing 35, 83 hooks 43 hot runner edge gates 139 hot runners 125 hot sprue 131 hubs 86 hydraulic pumps 84
C I
D
J
deflection temperature 210, 211 delivery time 181 density 208, 209 draw stoning 30 dry cycle 67
jetting 43 job file 160
edge and fan gates 124 elongation at yield 212, 214 end product 3 engravings 31 estimating 176
F family molds 114 fillers 45 filling volume 21 flexural modulus 213, 215 flexural strength 213, 215 flow leaders 38 flow marks 43
N negative engraving 33 number of cavities 102
O open gates 127 open nozzles 77 overhead 97, 170
P
circular pinpoint 127 collapsible cores 15 compression molding 3 confirmation of order 199, 204 core shift 121 corrosion prevention 50 crystallinity 60
E
mold project timeline 162 mold set-up 194 molder 4 molding properties 208 moving cavity 16 multi-level molds 142 multiple ejection strokes 91
injection mold 7 injection speed 60, 74 insert molding 155 inside center gated parts 120 inventory 126 Izod 213, 215
L L/t ratio 119 labels 31 light-weighting 75, 197 locational clearance fits 41 lost-core molding 7
parting line 13, 83 patents 187 path of least resistance 43 PET bottles 11, 63 Petri dish system 116 plant-wide machine hour cost 97 plasticizing capacity 73 positive engraving 33 post-mold-cooling 63 post-molding operations 17 potato chipping 38 prior art 187 product 3 product designer 4 production planning 165 pull ring 14
Q quotation 181, 199
M machine hour cost 97 mechanical properties 212 methods of alignment 25 mismatch 23 modular molds 136 mold cost per unit molded 99 mold designer 3 mold maker 3 mold manual 170, 193 mold open time 68
R record grooves 43 required clamp size 100 retainer plate 135 ribs 86 rim shapes 87 ripples 43 risk 173 robots 92 Rockwell hardness 213, 215
230
Index S safety considerations 141 screw design 74 self-degating 125 self-tapping screws 47 shot size 70 shut-off nozzles 78 side cores 15 sinks 39 spare parts 160 special fits 21 specific volume 208, 209 splay 43 splits 15 stack molds 142 stacking of products 22
stadium cups 11 Standard Geometrical Tolerancing standardization 54 submarine gating 125 surface finish 29
T tack height 22 temperature and time graph 126 tensile modulus 212, 214 tensile strength 212, 214 texturing 30 thermal conductivity 60, 210, 211 thermal expansion 210, 211 thermal properties 216 threads 15
41
tunnel gating 124 two-stage ejection 16
U undercuts 15 universal mold shoes 59, 113 unscrewing 15
V venting 66 voids 39
W water absorption 208, 209 witness lines 40
