Thin Wall Technology

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Technology Guide

Advanced Technology For THINWALL DESIGN & PROCESSING

Thinwall

SM

GE Plastics

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Contents Introduction

Material Selection

About GE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii About GE Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii About engineering Thermoplastics . . . . . . . . . . . . . . . .iv

Thinwall Technology What is Thinwall Technology . . . . . . . . . . . . . . . . . . . .1-2 Thinwall Technology Benefits . . . . . . . . . . . . . . . . . . .1-2 Thinwall Technology Market Opportunities . . . . . . . .1-3 Standard Molding vs. Thinwall Molding . . . . . . . . . . .1-3 Thinwall Nomenclature . . . . . . . . . . . . . . . . . . . . . . . . .1-4 Total Systems Approach . . . . . . . . . . . . . . . . . . . . . . . .1-5

Design Considerations Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . .2-2 Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-3 Mode of Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-4 Design Strategies for Load Absorption . . . . . . . . . .2-5 Design Strategies for Load Transfer . . . . . . . . . . . . .2-5 Basic Impact Analysis . . . . . . . . . . . . . . . . . . . . . . . .2-6 Advanced Impact Analysis . . . . . . . . . . . . . . . . . . . .2-6 Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-6 Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-7 Part Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-9 Ribs, Bosses and Gussets . . . . . . . . . . . . . . . . . . . . .2-9 Sink Marks and Voids . . . . . . . . . . . . . . . . . . . . . . . .2-10 Component Assembly . . . . . . . . . . . . . . . . . . . . . . . .2-11 Snap Fits/Press Fits . . . . . . . . . . . . . . . . . . . . . . .2-11 Manufacturability . . . . . . . . . . . . . . . . . . . . . . . . . . .2-13 Shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-14 Warpage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-17 Mold Filling and Gating . . . . . . . . . . . . . . . . . . . . . . .2-18 Flow Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-19 Melt Flow Index . . . . . . . . . . . . . . . . . . . . . . . . . .2-20 Fill Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2-20 Mold-Filling Analysis . . . . . . . . . . . . . . . . . . . . . . . . .2-20 COVER: Thinwall technology is broad reaching and applies to a variety of markets including telecommunications, computers, business equipment, appliances and automotive. SM

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-2 Materials Requirements . . . . . . . . . . . . . . . . . . . . . . . .3-3 Resin Flow Length . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-3 Spiral Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-3 Melt Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-4 Impact Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-4 Low Temperature Impact . . . . . . . . . . . . . . . . . . . . . .3-4 Aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5 Stiffness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5 Heat Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-5 Flame Retardance . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-6 Mechanical Integrity . . . . . . . . . . . . . . . . . . . . . . . . . .3-6 LEXAN® PC Resins . . . . . . . . . . . . . . . . . . . . . . . . . . .3-7 CYCOLAC® ABS Resins . . . . . . . . . . . . . . . . . . . . . . . .3-7 CYCOLOY® PC/ABS Resins . . . . . . . . . . . . . . . . . . . . .3-7 Property Considerations . . . . . . . . . . . . . . . . . . . . . . . .3-8 Flow vs. Impact . . . . . . . . . . . . . . . . . . . . . . . . . . .3-8 Aesthetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3-9 Stiffness vs. Impact . . . . . . . . . . . . . . . . . . . . . . . .3-9 Material Evaluation . . . . . . . . . . . . . . . . . . . . . . .3-10 Materials Portfolio . . . . . . . . . . . . . . . . . . . . . . . . . .3-10

Processing Molding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-2 Tooling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-6 Machinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4-10

Finishing Operations Screws and Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-2 Ultrasonic Welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-3 Shielding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5-5

Appendix UL 1950 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-2 Key Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6-3

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-2 Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-8 Sales Office . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7-9

All statements in this guide are subject to the disclaimer contained on page 7-7.

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© Copyright 1998 General Electric Company


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Introduction About GE The General Electric Company has its roots in the age of invention when, more than 120 years ago, it was founded by pioneering inventor Thomas Edison. Closely following its founder’s philosophy of innovation and the creative application of technology, GE has grown to become one of the largest and most diversified companies in the world. Today, GE products and services make a positive contribution to virtually every sector of commerce and industry. From jet engines to financial services, from lighting and medical systems to factory automation, power generation, transportation and construction.

About GE Plastics Of all GE businesses, one of the fastest growing is GE Plastics. Today, GE Plastics has emerged as the leading producer of engineering thermoplastics. Through application development centers around the world, customers can access data from GE designers, engineers, and tooling, processing and finishing experts, utilizing the most sophisticated equipment and systems available. Working closely with customers is at the core of the GE Plastics’ business culture. Today’s customers need to get the job done better, more costeffectively and within tighter schedules. Having a concentration of molding equipment, testing laboratories and product specialists close to the action permits a cross flow of information that can lead to important breakthroughs and exciting new product developments. At the nucleus of this unmatched global technical network are the worldclass facilities at GE Plastics’ headquarters in Pittsfield Massachusetts. Realizing that speed is the key to profitability today, these support services are backed up by production plants in several locations in the U.S., Europe, Australia, Japan and Mexico.

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Introduction About Engineering Thermoplastics The advantages of high performance engineering thermoplastics have grown dramatically both from new material developments and through a new generation of design engineers. Today, designers who have learned to “think” in plastics can take full advantage of their inherent benefits, rather than just simply translate metal components into plastic. Some of the many potential benefits offered by plastics include: • • • • • • • •

Consolidation of parts Integrated system assembly Molded-in assembly features Unprecedented strength to weight ratios Thinwall technology Elimination of painting and other operations Outstanding impact resistance Excellent chemical resistance

Through re-thinking and re-design, many traditional metal assemblies can be produced in dimensionally stable plastics: with 50% fewer parts, engineered for automated assembly and offering a full range of impact, heat, electrical and chemical properties. This Thinwall Guide is one example of how designers can access and utilize the knowledge and experience available from GE Plastics.

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Technology

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Thinwall Technology This guide has been created to help address problems commonly associated with design, tooling, and molding of plastic parts in wall sections thinner than today’s nominal wall sections. Basic information about products and fundamentals of designing with engineering thermoplastics utilizing “Thinwall ” technology have been provided. With this basic knowledge, proper resin selection, coupled with good design practice, should result in the development of a successful part. SM

What is Thinwall Technology? Thinwall Technology, or Thinwall, gets its name from one of the end results that it provides, a thin wall section. The difficulty is deciding a wall section at which a part goes from being “standard” or “conventional” wall thickness to “Thinwall.” The portable electronics and notebook computer industries have established themselves as being rich in Thinwall applications. With wall thicknesses often less than one half of a millimeter, there is no question that these applications qualify as Thinwall. The smaller the part, typically, the easier it is to fill parts with these small wall thicknesses. Parts that have different geometries, materials, and longer flow lengths, may not be able to be manufactured at these low wall thicknesses, even with current Thinwall technology. The benefits associated with decreasing wall thicknesses below their current values are still measurable and desired even if the final wall thickness is nowhere near those of the aggressive portable electronics industry. The techniques suggested within this guide can be applied to a wide range of markets and injection molded applications. Rather than setting a cut-off value between “standard” and “Thinwall” thicknesses, use of this guide can help attain thinner wall sections.

Thinwall Technology Benefits Reduction of wall thickness has always been important and is an enabling technology in a variety of markets. For small hand held parts, thinner wall sections help reduce weight and overall part size where these traits are critical. For all size parts, Thinwall technology can also help enhance productivity by providing opportunities to reduce costs and cycle times. The rapid growth in demand in the consumer electronics market for smaller, lighter, faster cycling parts created a need for advancements in thinner wall-section applications and technology. Requirements continue to increase for lighter, more compact products, necessitating high-performance housing designs with thinner wall sections – a critical requirement that is challenging engineers, resin suppliers, toolmakers, processors, and original equipment manufacturers (OEMs) alike.

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Thinwall Technology Thinwall Technology Market Opportunities A popular market for Thinwall applications and engineering thermoplastics has been portable electronics where demand for new products has increased an average of 30% each year since the 1980s. Technology advances in the industry are frequent. New technologies and products tend to make current products obsolete quickly. The fast pace has necessitated short product design and life cycles. Thinwall technology has been advancing at an equal pace. With virtually each new product offering has come a decrease in wall thickness. This has brought the wall thicknesses of many injection molded portable electronic applications down to levels previously thought to be impossible. An equally fast paced market with similar goals of light weight and smallest possible package has been the computer notebook industry. It too has relied on Thinwall technology to not only meet these goals, but also to provide lower cost parts. Here, the parts have been larger and the machines to produce them have been more traditional than those used in the portable electronics market. The technology developed within both of these markets is directly translatable to many markets. The trend has been to not only continue to look for ways to drive wall thicknesses down in established markets, but also towards allowing thinner walls on larger and different parts.

Standard Molding vs. Thinwall Molding As an example, today’s injection molding market innovators are typically molding hand held applications at wall sections between 0.030 and 0.060 in (1.0-1.5 mm), with the rest of the market molding at 0.0650.100 in (1.7-2.5 mm). However, the gap between these two groups possibly will increase in the near future as Thinwall technology becomes better understood and standardized. At that time, the market will likely be molding between 0.060-0.080 in (1.5-2.0 mm), while the market innovators will be molding between 0.020 and 0.040 in (0.5-1.0 mm). It is important to note that gains in wall-section reduction don’t always occur without investment – in this case, in tooling and machinery upgrades. Equally important is the fact that productivity and performance benefits of reduced material usage, faster cycle times, and lighter weight can often outweigh most added costs.

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Thinwall Technology Thinwall Nomenclature While it may be difficult to apply a thickness at which an application goes from “standard or conventional” wall thickness to “Thinwall”, for the purposes of discussion three sets of terms with dimensional references will be used:

Standard Wall Thickness Technology [>0.080 in (2.0 mm)] Standard or conventional technology is represented by those applications with wall thickness between 0.125 in (3.2 mm) and 0.080 in (2.0 mm), where conventional design rules apply. Standard engineering thermoplastics are usually sufficient for these applications. In addition, processing and tooling are well understood and standardized. Much information related to wall thicknesses in this range is contained in GE Plastics Injection Molding Processing Guide, PBG-135 and related product line publications. As parts become larger (longer flow lengths), Thinwall processing methodology applies, but if the standard wall thicknesses are used, conventional design rules should be applied.

One Step Thinner Wall Thickness Technology [0.080 and 0.050 in (2.0 and 1.2 mm)] Current Thinwall technology is heavily represented by applications with wall thicknesses between 0.080 and 0.050 in (2.0 and 1.2 mm); where design rules are transitional, where higher flow resins are required, and where processing favors high pressures and fill speeds that necessitate tooling changes. A majority of today’s Thinwall applications fall into this category.

Dedicated Thin Wall Thickness Technology [<0.050 in (1.2 mm)] The most technically challenging thinner wall thickness technology applications are those with wall thicknesses below 0.050 in (1.2 mm). Such dedicated applications require new and unique design considerations. In many cases, applications with wall thicknesses <0.050 in (1.2 mm) involve the use of high pressure, high velocity processing techniques that in turn often require changes not only in tooling, but in the molding machine itself. To address the requirements of Thinwall applications in this wall thickness range, GE Plastics offers engineering thermoplastic resin grades with very high flow and outstanding physical and mechanical performance.



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Thinwall Technology Total Systems Approach Experience has proven that risk increases when thinner wall applications tare approached independently from a design, processing, or tooling point of view. Hence, it is important to treat all aspects of Thinwall technology in an integrated systems approach that does not separate design, tooling, or manufacturing. When the inter-relationship between these functions is clearly understood, the chances for success are the greatest. GE Plastics welcomes the opportunity to work with industry players – OEMs, designers, engineers, tool builders, and processors – on expanding Thinwall technology from an art into a science. When key players are involved at the outset of the product development cycle, both the team and the application benefit from a wide range of real-world experience. Understanding current market challenges and anticipating future changes within Thinwall technology, GE Plastics has formulated special Thinwall resins and worked on new design and processing techniques to minimize the risks of Thinwall applications.



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Considerations

Design

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Design Considerations Design Considerations Design is a key component of successful product development. When working on applications with thinner walls, depending on the market and the application, three main issues or goals will typically be of concern to the engineer: • Impact • Stiffness

}

• Manufacturability,

both of which relate to the mechanical performance of a component in its end-use environment; and, which relates to mold filling and cooling performance and the part’s dimensional stability.

As engineers move to increasingly challenging designs with wall thicknesses of less than 0.050 in (1.2 mm), many “normal” design guidelines and approaches change. Achieving required design features may be more challenging as wall thickness decreases. Figure 1 reviews some of the design issues and options that are considered. Figure 1. Thinwall Design Issues and Options.

Design Issues High Impact Energy Appearance Concerns

Design Options Geometry Effects

Flow Length Limitations Large Flat Surfaces Tight Tolerances Large Static Loads

Assembly Techniques

Impact Modified Materials Multiple Gates Processing Techniques Reinforced Materials

Extremely Minimum Minimum Extremely Tough Weight Size Rigid

The following information generally applies to all applications where wall thicknesses are typically between 0.080-0.050 in (2.0-1.2 mm). In many instances, information provided for these wall thicknesses can be translated from applications with wall thicknesses from 0.0800.125 in (2.0-3.2 mm). Special notations for applications with wall thicknesses less than 0.050 in (1.2 mm) will be noted specifically.



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Design Considerations Impact Impact events take place when two or more bodies strike each other at speed (See Figure 2). Drops or falls where a component strikes the ground are the most common impact requirement for plastic parts or assemblies. The response of an engineering thermoplastic part to such an impact is affected by three main factors: • Loading Conditions (Impact Velocity) • Temperature • Geometric Complexity of Part (Stress State)

Figure 2. Impact Performance. ε = 5 × 10-1 1/S

ε = 10-2 1/S ε=

0

0.02

10-4 1/S

0.04

0.06

Impact Performance vs. Temperature Tensile Strength Relative to Room Temperature Strength

ε = 5 × 10-2 1/S

80 70 60 50 40 30 20 10 0

0.08

1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0˚

50˚

Strain

Impact Performance vs. Part Complexity and Stress Rate

Biaxial

Uniaxial

100˚

150˚

200˚

250˚

Temperature (F˚)

Impact Performance

Stress, MPa

Impact Performance vs. Loading Rate

Impact Performance – Brittle vs. Ductile Failure Mode Bri

ttle

Fai

lure Ductile

Failure

Triaxial Loading Rate: Low

High



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Design Considerations All three of these factors are interrelated such that a complex part with sharp notches may fail in a brittle mode at a high impact velocity and low temperature, but may fail in a ductile mode if temperature is increased or the velocity is decreased. Accurate design for impact performance requires the use of appropriate analysis techniques and a good understanding of material properties.

Mode of Failure When a part is loaded to failure, it will fail either in a brittle or ductile manner. The only other option is a combination of the two modes. Figure 3 illustrates the ductile and brittle behavior in biaxial instrumented impact tests. Brittle failure occurs when there is a rapid drop in load carrying capability after crack initiation, and little or no additional energy is needed to propagate the crack. Brittle failure occurs at a stress level that is lower than the yield strength of the material and with minimal plastic deformation (yielding). Ductile failure occurs when there is a relatively slow drop in load carrying capability as the material tears, and additional energy is required to continue to propagate the crack. Plastic deformation is always present with ductile failures, which occurs beyond the yield strain of the material.

Figure 3. Ductile and Brittle Behavior.

Displacement (in.) 0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

4000 Brittle Failure

Load (N)

3000

800 Ductile Failure

700 600

2500

500

2000

400 1500

300

1000

200

500

100

0

0 0

5

10

15

Displacement (mm)


20

25

Load (lb)

3500


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Design Considerations Design Strategies for Load Absorption Applications utilizing unreinforced thermoplastics typically use a strategy of load absorption during an impact event. In the case of a plastic housing it tends to absorb most of the energy of the impact through deformation. This design strategy requires more package space, since the housing needs to deflect without contacting the internal components. A part is generally considered to have failed if significant unrecoverable deformation is present after the impact event – even if internal components are still functioning. When opting for this design strategy, it is important that internal components are secured tightly and do not move during the initial impact event or this could result in a second impact event of these components into the housing. Snug fits and effective fastening can often be used to attach the internal components to the housing and still provide sufficient room for the housing to deflect during impact. All sharp corners and stress concentrators should be avoided in this type of design to increase the impact performance of the unit and to reduce the chance of brittle failure.

Design Strategies for Load Transfer In other applications, reinforced thermoplastics will typically use a strategy of load transfer during the impact event. In this case, the plastic housing deflects very little, transferring the impact energy directly into the internal structures and components. Such a design requires virtually no additional package space, because the housing should not deflect significantly during the impact. In fact, it is important to minimize deflection of the housing to assure that strains remain low and reduce the chance of failure of the housing. When using this strategy, it is important to remember that the housing merely has to be stronger than the weakest internal component. It is often employed with notebook computers where some components, such as hard drives or LCDs, are not exceptionally robust. Internal components should be fastened snugly so that secondary impacts between internals and the housing are minimized. Stress concentrators and sharp notches should be avoided to increase performance. Generally, in load transfer designs, internal components fail before the housing.



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Design Considerations Basic Impact Analysis As a quick estimate of a design’s impact performance, simple hand calculations can be used to determine how much energy must be absorbed during the impact event. A common and useful assumption is that all kinetic energy in the system prior to impact is converted into strain energy during impact. This assumption allows a designers to calculate deflections and stresses for simple parts rapidly and is often useful in preliminary design evaluation. Figure 4. Advanced Impact Simulation. Ductile

Increasing Time

Brittle

Advanced Impact Analysis Non-linear finite element analysis (FEA) techniques can be used to characterize the impact performance of a thin wall part. When material behavior is properly modeled, these techniques can provide correlation with both simple test parts (like an impact tup) and complex real-world parts (like telecom enclosures and laptops). The load/ deflection response, including ductile and/or brittle failure, can be predicted, helping designers assess design feasibility before tooling is cut and actual parts are tested. (See Figure 4.)

Stiffness The need to achieve a desired deflection under a known load is a common requirement for thermoplastic components. To determine a suitable approach for a given application, consideration must be given to the overall goals and limitations of the design, such as minimizing weight or package space. For example, in an application where part volume or package space is limited, an engineer may not be able to



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Design Considerations achieve higher stiffness by manipulating part geometry alone. A stiffer material or tighter assembly techniques may also be necessary to achieve the design goals within its limitations. As wall thicknesses decrease the need to design in stiffness again increases. For many applications significant use of ribbing is not feasible. Part stiffness must therefore be achieved in other ways. One generally effective method to increase stiffness is to modify the part geometry. For example, using curved surfaces instead of flat plates will help limit deflection. Part stiffness can also be enhanced through careful design of assembly features in each piece of a larger component. As wall sections become even thinner, in the range of less than 0.050 in (1.2 mm), the need to design-in stiffness increases even more. Currently, many of these applications are very compact hand held products and do not feature designs with large unsupported spans, therefore, reaching the target stiffness value is achievable. By combining geometric features and assembly techniques, required stiffness can usually be attained without using reinforced materials. For larger parts, stiffness can be improved through proper material selection, design, and assembly.

Material Selection One way to increase part stiffness with standard injection molding applications is to select a resin with a higher elastic (tensile) modulus. Glass or carbon-fiber reinforced thermoplastics provide 2 to 6 times the modulus of unreinforced resins. (See Figure 5.) Typically, the material selection decision must be made early in the design phase because it will influence not only what stiffness strategy is employed, but also which strategy is used to achieve the necessary impact performance. There is a direct relationship between an increase in a material’s elastic modulus and an increase in part stiffness. It is important to remember that fiber-reinforced resins may have anisotropic (directionally dependent) mechanical properties, behave in a more brittle fashion, increase tool wear, and have lower aesthetics than unreinforced plastics. Use of these materials may require more care during component design to ensure maximum performance. Applications with thinner wall sections in which reinforced resins are often chosen tend to feature large unsupported spans and must meet stringent compactness requirements. A good example of such an application is found on the back of LCD panels for a notebook computer. In this case, common geometric stiffening features such as ribs and gussets cannot easily be incorporated due to packaging requirements. So, stiffness is generally sought through the use of a reinforced resin.



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Design Considerations Figure 5. Stress/Strain Curves for Reinforced and Unreinforced LEXAN ® Resins.

Material Selection (Continued) 90 12000

80 Reinforced LEXAN® SP Resin 70

10000

8000 50 6000

40 Unreinforced LEXAN® SP Resin 30

Stress (psi)

Stress (MPa)

60

4000

20 2000 10 0

0 0

1

2

3

4

5

6

Strain (%)

Unreinforced materials have been used successfully in many applications with wall thicknesses ranging from 0.080-0.050 in (2.0-1.2 mm). If there are no large, unsupported spans, or if part geometry can be used to impart stiffness (e.g., clothes iron), stiffness targets may be easily achieved. In parts with large spans, required levels of part stiffness are typically obtained through the general geometry of the part (e.g., using a curved part as the back panel on the LCD display), and by effectively attaching this part to the rest of the components in the assembly. With smaller hand held applications, where wall thicknesses are less than 0.050 in (1.2 mm), unreinforced thermoplastics are usually selected, because stiffness is attained through size, geometry, and assembly rather than material reinforcement. For larger applications, reinforced materials can often be used to quickly increase the part stiffness. The engineer must be cautioned once again that reinforced materials may have anisotropic (directionally dependent) mechanical properties. Time spent in designing in stiffness is well worth the effort if an unreinforced product can be used.



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Design Considerations Part Geometry One of the most effective ways of improving part stiffness is through part geometry. By using curved surfaces or including such reinforcing features as ribs, bosses, and gussets, substantial increases in stiffness are possible. Since flexural stiffness is proportional to the cube of the height of a rib or of wall thickness, even the addition of small ribs or of small increments in wall thickness can provide a significant increase in overall part stiffness.

Ribs, Bosses and Gussets Ribs, bosses, and gussets are common design features in applications with wall thicknesses ranging from 0.125-0.080 in (3.2-2.0 mm) as shown in Figure 6. Ribs can be used to greatly increase part stiffness. Bosses have many uses, including facilitating assembly, accepting inserts, and helping to position a part relative to a larger component. Gussets can be used to increase the overall part stiffness, or to increase the stiffness of particular features such as bosses or ribs. Gussets can also be used to help local adjacent parts in a larger component. The design guidelines for standard applications are well known and provide options for designing with these features. Thinwall applications bring their own set of guidelines

Wall Thickness

Ribs

+

Conventional: 0.080 to 0.125 in (2.0 to 3.2 mm)

Thinwall: 0.050 to 0.080 in (1.2 to 2.0 mm)

< 0.050 in (1.2 mm)

trib ≤ 0.6 • twall h ≤ 4 • twall

trib ≤ 0.6 • twall ➝ twall h ≤ 4 • twall

trib ≤ twall h ≤ 4 • twall

r ≥ 0.375 mm θ ≥ 1/2˚

r ≥ 0.375 mm ➝ 0.6 • twall θ ≥ 1/2˚ ➝ 1˚

r ≥ 0.6 • twall θ ≥ 1˚

tboss ≤ 0.6 • twall h ≤ 4 • twall

tboss ≤ 0.6 • twall ➝ twall h ≤ 4 • twall

tboss ≤ twall h ≤ 4 • twall

r ≥ 0.375 mm θ ≥ 1/2˚ OD ≈ 2 • ID

r ≥ 0.375 mm ➝ 0.6 • twall θ ≥ 1/2˚ ➝ 1˚ OD ≈ 2 • ID

r ≥ 0.6 • twall θ ≥ 1˚ OD ≈ 2 • ID

tgusset ≤ 0.6 • twall h ≤ 4 • twall

tgusset ≤ 0.6 • twall ➝ twall h ≤ 4 • twall

tgusset ≤ twall h ≤ 4 • twall

r ≥ 0.375 mm θ ≥ 1/2˚

r ≥ 0.375 mm ➝ 0.6 • twall θ ≥ 1/2˚ ➝ 1˚

r ≥ 0.6 • twall θ ≥ 1˚

Figure 6. Features Design Guidelines for Thinwall Applications.

Bosses

Gussets



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Design Considerations Sink Marks and Voids To reduce sink marks and voids, ribs, bosses and gussets generally need to be thinner than the nominal wall-section in conventional applications. In addition, sides of these features should have draft to facilitate removal from the mold. To promote complete filling of the drafted features, the maximum height of the feature should be kept below fourtimes the nominal wall thickness. Sharp notches should be avoided to reduce stress concentrations at the base of the feature. See a standard GE Plastics design guide for additional information on designing with these features. Increasing a part’s moment of inertia by adding features like ribs, bosses, and gussets is a traditional means of adding stiffness. However, since maximum compactness of the final product is often a design limitation, and the addition of ribs and gussets may not be possible, stiffness may be increased by using the overall part shape. For instance, with Thinwall parts, stiffness can be increased by selecting a curved surface instead of a flat one or by incorporating small ribs to accentuate styling or functional elements (raised or recessed features), such as the edge of a product logo or the border of a system component. Design guidelines, for Thinwall applications where thickness is less than 0.050 in (1.2 mm), are different than those for standard wall thickness applications above 0.080 in (2.0 mm). Refer to Figure 6 for Thinwall design guidelines. For Thinwall applications between 0.050-0.080 in (1.2 mm-2.0 mm) these parts represent a transition in feature design between standard design guidelines and Thinwall guidelines. At the upper limits of wall thickness, in general, design guidelines for wall thicknesses ranging from 0.0800.120 in (2.0-1.3 mm) generally apply when designing features such as ribs, bosses, or gussets. As wall thicknesses decrease to 0.05 in (1.27 mm), the greater design freedom characteristics indicated for dedicated Thinwall applications can typically be used. Due to weight and ergonomic issues for small hand held Thinwall applications, compactness is of great concern. With these Thinwall parts, ribbing strategies may take up too much space to be effective for increasing stiffness. Fortunately, hand held parts are small, so requirements for part stiffness can be usually met using key stiffening features. For instance, by adding ribs to component features such as buttons, screens and logos, stiffness can be substantially improved. Stiffness is also improved by selecting curved rather than planer surfaces.



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Design Considerations For larger Thinwall applications, stiffness should be improved wherever possible. Styling lines and curved surfaces can be used to improve the aesthetics as well as the stiffness of the part. Internal components can also be used to further enhance the rigidity of the housing by carefully tying parts together. While achieving the desired stiffness with large parts can be challenging, it is, for most applications quite possible. For parts where wall thicknesses are less than 0.050 in (1.2 mm), the design suggestions for ribs, bosses, and gussets change since high injection and packing pressures are used during molding. Part shrinkage, especially near the gate, is reduced, helping to limit sink marks and voids. This, in turn, can allow design features to be made as thick as the wall-section. When these features are located far from the gate, however, traditional design guidelines relating to the thickness of design features to walls should usually be applied, because the pressure at that location will not generally be as high as at the gate. Also, to facilitate ejection of the part, draft angles on ribs, bosses, and gussets should be increased, because they will generally shrink less than they will at traditional wall thicknesses. To help minimize stress concentrators, radii at the base of the features can be increased.

Component Assembly Another way the stiffness of a part can be improved is through assembly into a larger component. By fixing two parts of a component together, such as a shoe box and its lid, the stiffness of the overall component is increased versus the stiffness of the individual parts. In the case of the shoe box, if the component did not include the lid, or the lid merely rested on the box without overlapping its sides, it would be significantly more flexible. This same concept can be translated into many thermoplastic parts, and especially Thinwall parts, to greatly enhance the stiffness of the overall component.

Snap Fits With traditional design, snap fits are a good example of assembly features that improve overall component stiffness by tightly linking two parts together. Snap fits also simplify assembly by reducing or eliminating screws or bonding operations and facilitating subsequent disassembly and recycling. The same considerations as ribs and gussets with respect to thickness, height, width and draft apply to snap fits. With conventional and Thinwall applications alike, properly designed snap fits are engineered so no permanent plastic deformation of the snap finger results from its use. Typically, maximum strain of the snap finger should not exceed 6% for unfilled resins and 1.5% for filled resins, although this value will vary depending on the specific grade of resin



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Design Considerations Figure 7. Snap-Fit Design.

Snap Fits (Continued)

l y t

ε=

3yt 2l 2

F=

yBt 3 E 4l 3 s

F1 = F

( 1 – µ tan φ)

B

Friction Normal

F1

θ

φ

Cantilever Force

Normal Force

µ + tan φ

ε = Strain y = Beam Deflection t = Beam Thickness l = Beam Length B = Beam Width Es = Secant Modulus F1 = Insertion Force F = Cantilever Force µ = Coefficient of Friction φ = Insertion Angle

Use secant modulus to calculate beam stiffness and insertion and pullout forces.

used. The maximum bending strain of a snap finger can be calculated using standard linear beam equations, as long as the secant modulus, not the elastic modulus, is used (See Figure 7). See a standard GE Plastics Design Guide, PBG-130, for additional information on designing with these features. Assembly features of a Thinwall part can also be used to greatly enhance component stiffness (See Figures 8 and 9). For example, a shoe box is always more flexible than the same box with a lid. Stiffness of that component can be further increased by more rigidly attaching the lid. A lid that is snapped down on the box will be much stiffer than one that is merely resting in place. And a lid that is screwed into the box will result in a component that is even more rigid. Snap fits, screws, ultrasonic welding, bonding, and the use of molded-in interlocks can also be used during assembly to effectively stiffen a Thinwall component. The design goal here is to have two or more parts act as if they are one part – i.e., to constrain all degrees of freedom at the interface between the parts. This design approach may require the part to hold tighter dimensional tolerances, since mating parts must fit tightly together. Also, snap fits in Thinwall applications will usually appear more flexible due to deflection of much thinner sidewalls. In conventional applications these same sidewalls are assumed to be rigid.



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Design Considerations Press Fit Interlock

Figure 8. Press Fit and Spade Interlocks for Thinwall Assemblies.

Spade Interlock

By using molded-in features to interlock pieces, component stiffness can be increased significantly. Tying into the stiff, internal components can also enhance rigidity. When moving to thinner wall sections engineers need to be aware that design strategies change and become more complex, and are further influenced by processing limitations and materials selection. A critical challenge in Thinwall design is to compensate for thinner wall sections by increasing component stiffness. This is accomplished through a combination of materials selection, use of geometric features, and careful selection of assembly features. Boss Interlock

Figure 9. Boss and Snap-Fit Interlocks for Thinwall Assemblies.

Snap-Fit Interlock

Back

Front

Manufacturability Ensuring that a part can be manufactured economically is the designer’s responsibility. With adequate forethought and early input from manufacturing, tooling, and the materials supplier, design engineers can help reduce potential manufacturing problems and develop component designs to improve the results from the injection molding process. The design engineer must consider how the part geometry will affect, and be affected by mold shrinkage. Also, the designer must consider how



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Design Considerations Manufacturability (Continued) the part will be filled, and where it is to be gated. Mold-filling analysis can be used early in the design phase to help assess part geometry and processing conditions. As wall thicknesses are reduced to the range of 0.080-0.050 in (2.01.2 mm), the injection pressures required to fill parts increases. And as injection pressure increases, so does the need for careful part design. To help maintain good dimensional performance, guidelines for good molding practices should be followed and observed very stringently. In addition, the use of mold-filling analyses becomes even more important in thinner wall-section applications to help reduce the risk of creating an unmoldable design. For applications where wall sections are less than 0.050 in (1.2 mm), processability becomes a greater concern. While there has been good success in some markets at driving the average wall thickness down to these levels, and while it is usually still possible to predict mechanical performance, it is strongly advisable to assemble all stakeholders in the design together early in the development process. By bringing the part designer, materials supplier, processor, and tool builder together, it is more likely to achieve a better design, faster, and with much less trial and error. This concurrent engineering approach can also reduce the risks of molding at thinner walls, helping to provide a design that will be manufacturable and a part that will perform.

Shrinkage With all applications to promote proper function, quality and aesthetics, thermoplastic parts must achieve and hold well-defined dimensional tolerances. The dimensional performance of the part is largely controlled by the mold shrinkage. The mold shrinkage applied to both conventional and Thinwall applications is commonly thought of as solely a material property and is actually a system property driven by processing conditions, part geometry and material properties. This can be demonstrated by considering the shrinkage of a part as the sum of local shrinkages from all over the part. These local shrinkages are controlled by the response of the material to the temperature and pressure history at that point. This history, in turn, is controlled by the process conditions set on the machine and the geometry of the part. So, the part geometry and process conditions set the temperature and pressure history at every point in a part to which the material responds, causing local shrinkage. When these shrinkages are summed up over the entire part, an overall mold shrinkage can be determined.



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Design Considerations One of the key process conditions that influences shrinkage is the packing pressure magnitude and the packing time as illustrated in Figure 10. As the material cools in the mold, it shrinks. Packing pressure is used to add more material to the old cavity to help compensate for this shrinkage. If the packing pressure is higher or the packing time is longer, more material can be pumped into the cavity to help compensate for shrinkage, which decreases.

Injection Molding Time


Filling

Filling

Packing

Pressure

Pressure

Packing

Figure 10. Effects of Packing Pressure on Shrinkage.

Length

Length



Filling

Filling

Packing

Pressure

Pressure

Packing

Length

Length

Part geometry can play a profound role in the dimensional performance of a part. The geometry on the flow path between any local point in the gate helps determine the pressure history at that point. For instance, if the gate is located in a thin section of the part that freezes off quickly, packing pressure will not be maintained in thicker sections of the part located further from the gate, causing increased shrinkage in these thicker sections. Another example is molding a center-gated disk and achieving a flat part. The tendency in this application is to overpack the center (near the gate) and underpack the periphery. In that case, the perimeter of the part can shrink more than the center, warpage can occur, and the part can become bowl-shaped.



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Design Considerations Shrinkage (Continued) The response of the material can be seen using PVT diagrams, which plot changes in specific volume (1/density) with temperature for various pressures. Both the compressibility and the thermal expansion can be seen on these diagrams. Since the process and geometry dictate the pressure and temperature history at every point in the mold, the material response can be roughly estimated locally using a PVT diagram. Engineering thermoplastic materials typically used for computer and telecommunications devices (such as LEXAN® polycarbonate resin, CYCOLAC® ABS resin, or CYCOLOY ® polycarbonate/ ABS resin) are amorphous polymers that have relatively low and predictable shrinkage rates (0.005-0.007 in/in [5.0-7.0 mm/m]). Unfilled amorphous resins also tend to shrink isotropically (independent of direction). Because these materials do not tend to shrink extensively when processed correctly, the large shrinkage differentials that can lead to warpage are more easily avoided. Fiber-reinforced thermoplastics exhibit anisotropic (directionally dependent) shrinkage based on orientation of the filler, which tends to line up in the direction of flow. Therefore, gate placement, which defines flow and cross-flow directions in the part, can have a significant impact on shrinkage as well as part stiffness. Orientation of the reinforcing filler in the flow direction will reduce shrinkage values in that direction versus the cross-flow direction. Care should be taken to place gates properly to balance not only structural but also dimensional considerations when working with fiber-reinforced thermoplastics. See standard GE Plastics data-sheets for shrinkage values on fiber-reinforced thermoplastics. As wall sections decrease to where thicknesses are less than 0.050 in (1.2 mm), the injection velocities required to fill these parts increase. The higher velocity combined with the thinner wall sections increase the shear rate seen by the material during filling. In turn, this higher shear rate will tend to enhance molecular orientation with reinforced and unreinforced materials, and fiber orientation with reinforced materials, leading to anisotropic shrinkage, where the material will shrink more in the cross-flow direction than the flow direction (See Figure 11).



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Design Considerations Figure 11. Shrinkage in Thinwall Geometries.

Mold Wall

Direction of Flow

3 mm Wall

Molten Core

Frozen Skins

Mold Wall High Shear Layers Mold Wall Molten Core

Flow Front

1 mm Wall

Mold Wall

To help reduce anisotropic shrinkage, parts should be adequately packedout. Since Thinwall parts freeze-off very quickly, it is important to have short injection times in order to allow for adequate packing while the core is still molten. This is especially critical for applications where wall thickness is less than 0.050 in (1.2 mm). It may also be necessary to use large gates (where gate thickness is greater than wall thickness) to promote sufficient flow of plastic through the gate during packing. If traditional gates (sized to 80% of wall thickness) are used, the gate can freeze-off very quickly and reduce packing of the part.

Warpage Warpage of parts is caused by differential shrinkage throughout the mold cavity. During the filling phase, molten plastic is injected into the mold under high pressure at the gate and the melt front is at or near atmospheric pressure. As the part fills, this pressure gradient remains, although it lessens during the packing phase. Higher pressure sections of the part will tend to shrink less than sections held under low pressure, which can lead to warpage. Use of a sufficient packing phase as well as proper part and gate geometries will help reduce this phenomenon.



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Design Considerations Mold Filling and Gating Mold and runners should be designed and sized such that parts fill from thicker to thinner sections to reduce large, uncontrolled shrinkage variations, which can lead to voids, sink marks, and excessive warpage. A single constant nominal wall is generally suggested, but when that is not possible, parts should be gated into thicker sections to avoid trapping molten material that, uncompensated by packing pressures, will shrink more than the material surrounding it once the gate has frozen off. Gates should be large enough (greater than the nominal wall) that the gate does not freeze off prematurely, before proper packing can be accomplished. Gates should be positioned to achieve balanced filling of the mold, which helps to reduce shrinkage differentials and warpage, and reduce injection pressure requirements (Figure 12). Figure 12. Gate Placement and Flow Balancing.

20 18

2500

Unbalanced

14

2000

12 1500

10 8

Balanced

1000

6 4

500

2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

Time (sec.)

Unbalanced Flow

Gate

Balanced Flow


0.7

0.8

0.9

01.0

Injection Pressure (psi)

Injection Pressure (MPa)

16


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Design Considerations Flow Length The maximum length a material will flow into a mold will help determine both the number and location of gates in that mold. It can also set a lower bound on wall thickness if, for instance, a single gate is desired and the material must reach the extremities of the mold. The maximum flow length is driven primarily by part geometry, process conditions and material properties. Typically, the flow properties of particular resins have been indicated by spiral flow data found in product literature. While these values may be somewhat useful for comparing different materials, they cannot be used directly for design purposes with either conventional or Thinwall applications. Actual part flow patterns are typically a combination of three configurations – channel flow, (e.g., spiral flow geometries); radial flow (e.g., a center-gated disc); or planer flow (e.g., an end-gated rectangular plate) – with most parts having predominantly planer and radial flow (Figure 13). Using radial flow length estimates to determine maximum flow length, therefore, can be quite useful for the design engineer. Figure 13. Basic Flow Geometries.

Radial Flow Geometry

Planer Flow Geometry

Channel Flow Geometry

Radial flow lengths at suggested processing conditions and varying wall thicknesses can be obtained from a GE Plastics’ representative or by consulting the Engineering Design Database. Typically, as a rule of thumb radial flow lengths are approximately 70% of maximum spiral flow lengths for the same melt temperature and wall thickness. However, this value can vary with material, wall thickness and processing conditions.



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Design Considerations Melt Flow Index Melt Flow Index (MFI (ASTM D1238)) is a form of low-shear qualitycontrol testing that has also been used to indicate resin flowability. However, since this value represents only a small portion of the complex rheological behavior of engineering thermoplastics, it is not, standing alone, a good measure for use either in design or for material comparison – especially when comparing resins from different families (e.g., LEXAN polycarbonate and CYCOLAC ABS resins).

Fill Times For Thinwall applications, the length of flow of a material into a part can be improved by injection time. For applications less than 0.050 in (1.2 mm), this injection time will typically be between 0.1 to 0.5 seconds. Such short injection times require the use of high pressure, high speed injection molding equipment. If longer times are used because of machine limitations due to speed or pressure, expected flow lengths will probably not be achievable and molded in stress will be extreme. This situation can render an otherwise good design useless.

Mold-Filling Analysis Since maximum resin flow length and mold filling are dependent on many factors – e.g., material, part design, and processing conditions – mold-filling analysis tools can be very useful for assessing/evaluating part design and processing conditions. The use of these tools can help eliminate expensive trial and error tool design and improve part quality. Multiple configurations (e.g., part geometry, gating configuration, process conditions, material, etc.) can be electronically prototyped early in the design phase while changes are easier and less costly to make. Mold-filling analyses can be used to help assess the processability of Thinwall parts. While accurate thin wall filling simulation presents some challenges due to the very rapid injection phase, good correlation can often be achieved between simulations and molding trials. Some of the commercially available software even have Thinwall specific material models to more accurately predict filling pressure. However, these



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Design Considerations simulations will not currently predict material degradation during processing. If the process contains excessive residence time, melt temperatures or shear – all of which can cause material degradation – the simulation will probably not accurately represent the magnitude of the injection pressure. This is because degradation causes a viscosity change, which in turn changes the pressure requirements. However, filling and cooling patterns and pressure trends will not be affected. When moving to thinner wall sections engineers need to be aware that design strategies change and become more complex, and are further influenced by processing limitations and materials selection. A critical challenge in Thinwall design is to compensate for thinner wall sections by increasing component stiffness. This is accomplished through a combination of materials selection, use of geometric features, and careful selection of assembly features. In many cases, when designing at the upper limits of Thinwall where wall thicknesses are greater than 0.070 in (1.7 mm), conventional design guidelines can be applied to features such as ribs, bosses or gussets. As wall thickness decreases to 0.060 in (1.5 mm), the greater design freedom characteristics of Thinwall design guidelines can often be used.



Notes

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Page 33

Selection

Material

GE Plastics

Thinwall

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SM



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Material Selection Introduction New products are frequently vital parts of a business strategy. Bringing the right product to the market at the right time, at the right price is an enviable goal. Knowing the process for new product development and executing the steps flawlessly can provide a substantial edge. Manufacturing with engineering thermoplastics continues to enable many new product developers to propose innovative solutions. Proper materials selection is a critical step in the product development process. A properly specified material can enhance a product’s design and complement manufacturing. An improperly specified material can hamper proper function of the design and can render the product difficult to process. Therefore, proper understanding of the material’s behavior, processing constraints, and the end-use requirements are necessary for material selection. In its purest sense, materials selection is a process of elimination. The easiest way to begin is to define and rank key application requirements, e.g. it must be tough, rigid, and have good aesthetics. Next, these requirements need to be translated into physical properties, e.g. it must have good impact resistance, relatively high tensile modulus, offer a wide processing range, and provide a low gloss appearance. Once key physical properties have been identified, a broad material selection can be performed based on general performance of various resin families. Finally, a more detailed selection is performed based on the specific requirements of the part design and end-use environment. Engineering calculations will be useful to help define and quantify key material properties required and to help further refine materials selection. Actual molding of parts is always required to confirm final part design and performance and finalize materials selection. Resins should not be selected based solely on datasheet values alone, although this resource can be useful for comparing various materials.



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Material Selection Understanding how important proper material selection is to successful Thinwall design and processing, GE Plastics has developed a wide range of materials to specifically address the requirements of a variety of applications. GE Plastics’ engineering thermoplastic resins have all been used successfully in various standard applications. For Thinwall applications with wall thicknesses below 0.080 in (2.0 mm), higher material performance is often required. For these applications, specific LEXAN polycarbonate resin grades offer property profiles that include high strength, durability, and good processing characteristics, while selected CYCOLAC ABS or CYCOLOY PC/ABS resin grades offer a balance of cost-effectiveness and performance. Many other GE Plastics’ products have been chosen for use in various Thinwall applications. Contact your local GE Plastics representative or call 1-800 845-0600 for additional information.

Material Requirements Engineering thermoplastic materials used for Thinwall applications must provide both processing freedom and the performance to withstand abusive end-use environments. The following properties are critical:

Resin Flow Length Resin flow length, is the distance an injected resin will flow before the melt front is stopped by freeze-off. It is a critical property for Thinwall applications. However, the need for increased flow length must be balanced against the need to maintain other mechanical properties, such as impact strength. (For more details on flow length, see Design section on pages 2-2 to 2-21.) Maximum flow-length requirements for typical Thinwall applications in the range of 0.080-0.050 in (2.0-1.2 mm), such as small appliance products, are about 6 in (15 cm). For notebook computers flow-length requirements might be in the range of 10 in (25 cm). Flow requirements can be reduced by adding more gates, but doing so can create cosmetic problems with additional knitlines. Sequential valve gating can be used to reduce these issues.

Spiral Flow Spiral flow however, is the most common method of comparing material flow length. Because spiral flow does not accurately model flow in most applications, it should not be used to establish the minimal wall thickness of a part. (For more details, see Design section on pages 2-2 to 2-21.)



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Material Selection Melt Flow Rate Viscosity (melt flow rate) has also been used to compare materials, but because of the nonlinear behavior typical of the test, and plastic materials in general, results can be very misleading. Generally, CYCOLAC resins have the highest flow lengths, followed by CYCOLOY resins and then LEXAN resins.

Relative Flow Length

Figure 14. Relative Flow Length vs. Wall Thickness (Typical).

A C Y C OL

ins C Res

esins low R F h g i YH sin s C Y C OLO SP Re N A X E L O LOY & St and a r d C Y C ins XAN Res S tanda r d L E 0.04

0.06 Wall Thickness (in.)

0.08

Impact Strength Impact strength is very often a critical property in Thinwall applications. Notched Izod values (ASTM D256A) of 12 ft-lb/in (640 J/m), or more, are typically used to meet toughness requirements. In general, experience has shown that notched Izod values are not always sufficient to properly characterize a material’s impact performance in Thinwall-section applications. Notched Izod and Instrumented Impact values for the same material can vary significantly due to sample geometry and test methods. Performance is more accurately measured by load/displacement curves (similar to stress/strain curves) generated from instrumented impact testing. These load/displacement data found in GE Plastics’ Engineering Design Database can help product designers and engineers better understand and evaluate a material’s impact performance. Actual part molding and testing is necessary to determine whether a particular material satisfies the needs of a particular application.

Low Temperature Impact Low temperature impact performance can also be a critical property. Otherwise ductile materials can transition into brittle behavior based on temperature for a given geometry and impact speed. In most cases with Thinwall popular LEXAN resin and CYCOLOY resin, an impact modifier is used to achieve ductile behavior at temperatures below -20°F (-29°C).



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Material Selection Aesthetics Aesthetics of Thinwall applications are often important for consumer appeal. Blemishes of any kind (sink marks, splay, blush, knitlines, etc.) are typically not acceptable. Lower gloss levels are generally preferred for these products, and acceptable color shifts due to ultraviolet exposure typically range between 1 to 2 ∆e. Painting is an option, but it can be expensive.

Stiffness Stiffness of a molded plastic component is a function of both material properties and design geometry and plays an important role in providing a high-quality feel. Typically, the unreinforced engineering thermoplastics used in Thinwall applications have a flexural modulus (ASTM D790) of roughly 300,000 psi (2,000 Mpa). For reinforced products, flex modulus can range from 500,000 to 2,000,000 psi (3,500 to 13,800 Mpa). Typically, reinforced products have less impact strength, so a tradeoff between stiffness and impact must be made.

Heat Resistance Heat Resistance is another important property in many applications for two reasons. First, in the case of an electronic device, it can generate heat during normal operation, the plastic selected must not distort, sag or suffer thermal aging during service. Typically, a UL* relative thermal index (RTI [UL746B]) of 167-194°F (70-95°C) is sufficient to provide protection, depending on the heat generated by the internals. Second, because the device could also be exposed to external heat (such as a closed car on a hot day) the material might have to also withstand thermal excursions without damage. Typically, a deflection temperature under load (DTUL [ASTM D648]) of 180°F (82°C) is sufficient to resist such temperatures. Data on these properties are in the UL Component Directory. Data on other properties are in UL files for GE Plastics. Note that the DTUL test is geometry dependent and the value can change with wall thickness. While DTUL may be a useful property for comparing materials in the same resin family, it is not directly useful in any engineering equation. Therefore, engineers may wish to consult dynamic mechanical analysis (DMA) curves to determine a more accurate description of a material’s thermal performance. DMA curves on GE resins and use of DMA information can also be found in GE Plastics’ Engineering Design Database. *UL is a Registered Trademark of Underwriters Laboratories, Inc.



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Material Selection Flame Retardance Flame retardance is mandated for many electronic applications based on their power, voltage or current levels (see UL 1950 information in Table 9 in Appendix A on page 6-2). Examples are cellular products which require materials with a UL HB listing, while notebook computers require materials with UL 94* V-0 or V-1 listings. The industry standard for most stationary electronics is typically a material listing of UL V-0 and 5V.

Mechanical Integrity Mechanical integrity of a part is related to the end-use requirements, material properties, the part design, assembly layout and manufacturing methods. An appropriate assembly method will often help reduce the total weight of a portable unit. Snap fits, small screws and ultrasonic welding are the typical methods of joining Thinwall parts. Material characteristics that can impact the assembly method used include knitline strength, strain-to-failure, notch sensitivity, flexural strength, screw pull-out/strip-out strength and weldability. (For further information, see the Secondary Operations section of this guide on pages 5-2 to 5-5.) *Underwriters Laboratories, Inc. Standard 94 describes a vertical burning test to be performed under laboratory conditions. In this test, specimens are placed in the flame of a laboratory burner, and the ability of the substance to sustain a flame over a specified period of time, upon removal of the source of the flame, is determined. The claims, representations, and descriptions regarding the flammability of the products described in this brochure are based on a standard, small-scale laboratory test and as such are not reliable for determining, evaluating, predicting, or describing the flammability or burning characteristics of the products under actual fire conditions, whether the products are used alone or in combination with other products. Each potential user should determine for himself/herself whether a particular test procedure is meaningful for a particular application and should run independent tests to determine whether the products mentioned in this paper are suitable for a particular application.



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Material Selection Material Selection GE Plastics offers several resins which are commonly used in Thinwall applications. (See Figure 15.)

LEXAN ®PC Resins LEXAN SP7000 series resins are a family of glass- and carbon-fiber-reinforced mate rials combining high stiffness with the same easy-flow characteristics of unreinforced LEXAN SP resin products. By compensating for the loss in part stiffness due to reduced wall thicknesses, these higher modulus materials are often good candidates for consideration in notebook computer applications. LEXAN SP resins are polycarbonate (PC) based polymers that combine high resin flow with the high-impact performance and outstanding mechanical properties of standard LEXAN resins. LEXAN SP resins are also available with impact modifiers (LEXAN ML6339R and ML6018R resins) for improved low-temperature performance. These materials can offer new opportunities for intricate Thinwall part designs. Standard grades of LEXAN resin include tough and very versatile engineering thermoplastics. A combination of processing and performance properties can provide design engineers with exceptional freedom to create functional, attractive, and cost-effective products.

CYCOLAC ® ABS Resins CYCOLAC ABS resins can provide engineers the precise properties needed for Thinwall applications without over- or under-specifying. They offer physical and economic advantages compared to other conventional design materials. They are often used by manufacturers where high impact, excellent flow characteristics and well as heat resistance and modulus are desired.

CYCOLOY ® PC/ABS Resins CYCOLOY HF resins provide high-flow capabilities for enhanced Thinwall processing. Coupled with the good impact strength, heat resistance, UV stability and cost/ performance value of standard CYCOLOY PC/ABS resins, this combination of properties makes the materials good candidates for consideration in small hand held parts applications. Standard Grade CYCOLOY PC/ABS resins are versatile thermoplastic materials offering low-temperature ductility, excellent impact resistance, practical thermal performance, excellent aesthetics, and good flow characteristics. These materials can offer improved productivity with higher yields and better cost efficiency.



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Ultrasonic Weld Strength

Heat Resistance

Stiffness

Flow Length

Impact Strength

Aesthetics

Figure 15. Thinwall Materials Comparison.

Low-Temp Ductility

Material Selection

LEXAN SP7000 Resins (reinforced) LEXAN SP Resins (impact modified) LEXAN SP Resins LEXAN Resins CYCOLAC Resins CYCOLOY HF Resins CYCOLOY Resins Excellent

Good

Poor

Relative Performance

Property Considerations All of the GE resin families can offer some advantages for Thinwall applications, but no one group of resins can meet every requirement. In selecting materials for particular applications, it is necessary to weigh advantages and make the best overall choice in order to balance stiffness, impact, and flow properties. Here are some tradeoffs to consider.

Flow vs. Impact For each standard LEXAN resin grade, there is a LEXAN SP resin grade with higher flow and equivalent impact strength (Figure 16). CYCOLOY HF resins will provide even higher flow than LEXAN SP resin materials, but there can be a compromise in impact strength.

Impact Strength

Figure 16. Impact Strength vs. Flow Length.

LEXAN 101 Resin LEXAN 141 Resin

LEXAN ML6339R Resin Improved impact similar flow Improved flow same impact

LEXAN SP1210R Resin LEXAN SP1010 Resin

LEXAN 121 Resin CYCOLOY C1200 Resin

CYCOLOY C1000 Resin

Flow Length


CYCOLOY C1200HF Resin

CYCOLOY C1000HF Resin


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Material Selection Aesthetics Flow-length requirements on a material can be reduced by adding more gates to the tool, but this will create additional knitlines that are typically not acceptable for cosmetic as well as strength issues. Sequential valve gating can be used to reduce knitlines. For better aesthetics, use of higher flow materials with a single gate is suggested. Materials with high-flow capabilities, such as LEXAN SP resins, can often pick up the finer texture needed for lower gloss applications – a result that typically cannot be achieved solely through processing techniques. LEXAN SP resin grades also create a more resin-rich surface, which in many cases allows for the successful use of the 10%-glass-reinforced LEXAN SP7602 resin grade without painting. This can be an important advantage, since most reinforced materials require additional secondary operations than their unreinforced counterparts to achieve optimal surface appearance.

Stiffness vs. Impact A decision should be made early in the concept phase of product development as to whether to use an unreinforced material (which will typically provide better impact resistance but greater deflection, necessitating a design approach for stiffness), or a reinforced material (which will typically provide better stiffness but require a design approach for impact). The decision is important because of the distinct advantages and consideration associated with each material type and their effect on design. Key points are highlighted in Table 1. Unreinforced Resins

Reinforced Resins

Advantages

• Higher Impact Strength • Cosmetics

• Greater Stiffness • Higher Static Strength

Considerations

• Lower Stiffness/ Higher Deflection

• Brittle Impact Failures • Possible Aesthetic Issues (painting may be required)

Table 1. Unreinforced vs. Reinforced Materials Comparison.

Unreinforced plastics can usually absorb and dissipate a tremendous amount of energy as they deform during impacts. Therefore, this feature can be exploited during the design process to help protect internal electronics from damaging loads or shocks. Conversely, higher modulus, reinforced products typically provide greater stiffness and resist deflection – critical features at thinner wall sections. Components using reinforced resins must therefore be designed to minimize localized deflections and quickly transfer impact load throughout the rest of the unit.



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Material Selection Material Evaluation While proper materials selection is always a very important step in product development, it is especially critical in Thinwall applications. As wall sections become thinner and processing becomes more challenging, it is vital to select resins that have been specially formulated for high flow impact strength, adequate heat resistance, required flame retardance, assembly integrity and aesthetics. GE Plastics has developed several materials specifically for use in Thinwall applications, which can provide designers and processors a balance of necessary properties at an economical price as shown in Tables 2 through 5.

Materials Portfolio for Use in Thinwall applications Table 2. LEXAN Standard Resin Grades – Typical Property Values.

LEXAN ML6018R resin 8,300 (57)

UNITS ENG (S1) psi (MPa)

LEXAN 141 resin 9,000 (62)

Flexural Modulus, 0.125" (3.2 mm)

ASTM D 790

psi (MPa)

340K (2,345) 310K (2,135) 266K (1,830)

Izod Impact, notched, 73˚F (23˚C)

ASTM D 256 ft-lb/in (J/m)

Tensile Strength, yield, Type 1, 0.125" (3.2 mm)

DTUL, 264 psi, (1.82 MPa) ASTM D 648 0.250" (6.4 mm)

15.0 (801)

15.0 (801)

10.6 (566)

°F (°C)

270 (132)

231 (110)

240 (115)

Specific Gravity

ASTM D 792

—

1.20

1.18

1.18

Mold Shrinkage flow, 0.125" (3.2 mm)

ASTM D 955

in/in E-3

5-7

5-7

5-7

UL 94

in (mm)

0.045 (1.14)

0.043 (1.09)

—

UL 94 HB Flame Rating*

Table 3. LEXAN SP Resin Grades – Typical Property Values.

LEXAN ML6339R resin 8,800 (61)

TEST METHOD ASTM D 638

PROPERTY

TEST METHOD

UNIT ENG (S1)

LEXAN SP1210R resin

LEXAN SP1010 resin

LEXAN SP7700 resin

LEXAN SP7602 resin

LEXAN SP7604 resin


ASTM D 638

psi (MPa)

8,700 (60)

8,700 (60)

15,100 (104)

10,800 (74)

13,300 (92)


ASTM D 790

psi (MPa)

Izod Impact, notched, 73˚F (23˚C)


PROPERTY

345K (2,375) 345K (2,375) 967K (6,660) 570K (3,925 883K (6,085) 16.0 (854)

12.0 (641)

1.8 (96)

1.1 (59)

1.0 (53)

DTUL, 264 psi (1.82 MPa) ASTM D 648 0.125" (3.2 mm)

°F (°C)

—

—

—

199 (92)

198 (92)


°F (°C)

230 (110)

225 (107)

212 (100)

—

—

Specific Gravity

ASTM D 792

—

1.18

1.18

1.25

—

—


ASTM D 955

in/in E-3

5-7

5-7

1-3

2-4

1-2


UL 94

in (mm)

0.045 (1.14)

0.042 (1.07)

—

—

—

UL 94 V-0 Flame Rating*

UL 94

in (mm)

—

—

0.048 (1.22)

0.030 (0.76)

0.030 (0.76)

* These ratings are not intended to reflect hazards presented by any material under actual fire conditions.



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Material Selection TEST METHOD ASTM D 638

UNITS ENG (S1) psi (MPa)


ASTM D 790

psi (MPa)

Izod Impact, notched, 73˚F (23°C)


PROPERTY Tensile Strength, yield, Type 1, 0.125" (3.2 mm)

DTUL, 264 psi, (1.82 MPa) ASTM D 648 0.125" (3.2 mm)

CYCOLAC GPM5600 resin 5,500 (38)

CYCOLAC GPM6300 resin 6,400 (44)

Table 4. CYCOLAC Resin Grades – Typical Property Values.

CYCOLAC DSK resin 6,100 (42)

310K (2,135) 360K (2,480) 370K (2,550) 6.5 (347)

4.5 (240)

4.0 (214)

°F (°C)

176 (80)

178 (81)

174 (78)

Specific Gravity

ASTM D 792

—

1.03

1.04

1.05


ASTM D 955

in/in E-3

5-8

5-8

6-8

UL 94

in (mm)

0.062 (1.57)

0.062 (1.57)

0.059 (1.50)

TEST METHOD

UNITS ENG (S1)

CYCOLOY C2800 resin

CYCOLOY C6200 resin

CYCOLOY C1200HF resin

CYCOLOY C1000HF resin

CYCOLOY C1200 resin


ASTM D 638

psi (MPa)

8,500 (59)

9,700 (67)

8,300 (57)

8,400 (58)

8,800 (61)


ASTM D 790

psi (MPa)

—

—


ASTM D 790

psi (MPa)

390K (2,685)

390 (2,685)

—

—

—

Izod Impact, notched, 73°F (23°C)


8.0 (427)

10.0 (53.4)

11.0 (587)

10.0 (53.4)

12.0 (641)

°F (°C)

165 (73)

190 (87)

235 (112)

210 (98)

235 (112)


PROPERTY


Table 5. CYCOLOY Resin Grades – Typical Property Values.

340K (2,345) 360K (2,480) 340K (2.345)

Specific Gravity

ASTM D 792

—

1.17

1.18

1.15

1.12

1.15


ASTM D 955

in/in E-3

4-6

4-6

5-7

5-7

5-7


UL 94

in (mm)

—

—

0.063 (1.60)

—

0.063 (1.60)

UL 94 V-0 Flame Rating*

UL 94

in (mm)

0.060 (1.52)

0.060 (1.52)

—

—

—

* These ratings are not intended to reflect hazards presented by any material under actual fire conditions.



Notes

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Page 45

Processing

GE Plastics

SM

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Thinwall



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Processing With the move to thinner wall-section designs, three key aspects of processing need to be considered: molding, tooling, and machinery. With thinner wall sections, processing requirements change with respect to the use of higher pressure and speeds, faster cooling times, part ejection and gating scenarios. Typically, the acceptable range for each of these parameters becomes narrower and more precise at thinner walls, providing processors with greater challenges but also greater opportnities. In this section, specific processing techniques and equipment for processing Thinwall parts are contrasted with those of more conventional wall thicknesses. Some of the significant differences are noted (See Table 6). Parts with nominal wall thicknesses between 0.080 and 0.125 in (2.03.2 mm), typical with stationary electronics housings and a variety of other applications including automotive parts, represent “standard” applications that are generally well understood and easily handled by OEMs and experienced processors in the plastics injection molding industry. As the wall section of a part decreases (0.080-0.050 in [2.01.2 mm]), the ability of the injected material to easily fill the mold cavity is also reduced. Extra measures are therefore required to help the material reach the end of the cavity before freeze-off and, at the same time, retain enough molten core to pack out sink marks. To accomplish this, typically injection pressures are increased and time-to-fill is decreased. As an example, if a part with a 0.120 in (3.0 mm) wall section is filled in 4 sec, a 0.060 in (1.5 mm) part may need to be filled in less than 2 seconds to achieve the same degree of pack-out at the end of fill. The trend in small hand held parts housings has been to much thinner walls than have traditionally been utilized in injection molding. Wallsection thicknesses from 0.030-0.050 in (0.75 to 1.2 mm) are becoming commonplace, and wall sections below 0.020 in (0.5 mm) are even being witnessed.

Molding Quite often, higher hydraulic pressure and shorter fill times are required to drive the molten thermoplastic materials into thinner cavities at a sufficient rate. Where fill pressures of 12,000-14,000 psi (83-97 MPa) are commonly sufficient for midsize parts with 0.12 in (3.0 mm) wall thicknesses, 0.06 in (1.5 mm) wall thickness parts of the same size may require pressures between 16,000-20,000 psi (110-138 MPa) to achieve necessary injection rates. (See Figures 17 and 18 on page 4-4.)



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Processing Table 6. Comparison of Standard vs. Thinwall Processing.

Key Factors

Conventional Applications

Typical Wall

0.080 - 0.120 in (2.0 - 3.0 mm)

0.050 to 0.080 in (1.2 - 2.0 mm)

< 0.050 in (< 1.2 mm)

Standard

High-end

Customized high-end equipment

9,000 - 14,000 psi (62 - 97 MPa)

16,000 - 20,000 psi (110 - 138 MPa)

20,000 - 35,000 psi (138 - 241 MPa)

Standard

Standard

Accumulators on injection & clamp units, servo valves

Standard

Closed loop, microprocessor control over: injection speed, hold pressure, decompression speed, screw RPM, back pressure, all temps, (including feed throat & oil temperature)

Microprocessor controlled with the following resolutions: speed-0.40 in./sec (1 mm/sec), pressure-1 bar, position-0.004 (0.1 mm), time-0.01 sec, rotation-1 RPM, clamp force-0.10T (0.10t), temperature-2°F (1°C)

Compression ratio: 2.0:1 to 2.5:1, L/D = 20:1 to 24:1 flights 5/10/5: Nitriding not suggested

Compression ratio: 2.0:1 to 2.5:1, L/D = 20:1 to 24:1 flights 5/10/5: Nitriding not typically used

Compression ratio: 2.0:1 to 2.5:1, L/D = 20:1 to 24:1 flights 5/10/5: Nitriding not typically used

> 2 sec

1 to 2 sec

0.1 to 1 sec

40–60 sec

20–40 sec

6–20 sec

Dew point of -20°F to -40°F (-29°C to -40°C); hoppers sized for material throughput



Standard

Better venting, heavier construction, more ejector pins, better polish

Extreme venting, very-heavy molds, mold interlocks, precise mold surface preparation, extensive ejection features, mold costs 30–40% higher vs. standard

Machinery

Inj. Pressures

Hydraulic system Control system

Screw design

Thinwall Applications

Processing Time to fill Cycle time Drying

Tooling



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Processing Figure 17. Injection Pressure and Fill Time vs. Wall Thickness. Pressure 3

Time 20

2

10

1

Time Required to Fill (Seconds)

Injection Pressure (x 1000 psi)

30

0

0 0

1mm

2mm

3mm

Wall Thickness

Figure 18. Typical Cycle Time Range.

70 60

Seconds

50 40 30 20 10 0 0

1 mm

2 mm

3 mm

Wall Thickness

Theoretically, as wall-sections drop in size, cycle times also drop, because there is less material to cool. Careful management of runner and sprue, through redesign or the use of hot drops, may permit a reduction in total cycle time. Typical cycles can then be in the range of 20 to 40 seconds. At these wall thicknesses, closed-loop control over injection speed, transfer pressure, and other process variables can help to control the process for the filling and packing required to produce consistent quality parts.



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Processing As with conventional molding, proper drying and consideration of material residence time and temperatures in the barrel are required. Because the shot size for these applications may be smaller than is typical with conventional molding, material property degradation may be accentuated due to overly long residence times for the material in the barrel. With wall-sections of less than 0.050 in (1.2 mm), during processing, the total cavity time-to-fill values may need to be reduced even further. For most applications, fill times between 0.1-0.5 seconds are suggested. If fill times are longer than this, the material may simply “freeze-off” before the cavity is filled and packed. To drive the material at sufficient speeds, injection units may need to generate pressures of 20,000-35,000 psi (140-241 Mpa). With proper heat management and due consideration to the sprue and runner, cycle times can often be greatly reduced with wall-sections of less than 0.050 in (1.2 mm). Total cycle times of 10-20 seconds should be expected; these values can be even lower with other specialized equipment. Use of very fast cycle times and premium tooling make it prudent for molders to select mold safety equipment such as video systems to ensure there is no part hang-up in the mold. The use of robots may also be appropriate, since they can remove parts faster than gravity will allow them to clear the mold’s surfaces. As wall thicknesses continue to drop below 0.050 in (1.2 mm), additional care must be taken in processing. Melt temperatures for these applications should not exceed GE Plastics’ suggested processing temperatures. It is often tempting to exceed these temperatures in order to fill the cavity, however, this can be counterproductive. Too high a melt temperature and/or too long a material residence time in the barrel can cause a significant loss of the material’s physical properties and/or create aesthetic issues in the part. With even the thinnest of walls, proper drying of the material is no more or less critical than it is with thicker walls. CAUTION: With much smaller shot sizes, material through-put should be such that the resin does not sit in the drying hopper for extended periods of time. Therefore, hoppers of suitable size or leveling switches installed in the hoppers should be used to match projected material through-put.



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Processing Tooling Tooling requirements for conventional applications are well documented and understood by molders, tool designers and tool builders. P-20 steel is used extensively in the industry, and usually works adequately. For more detailed information about tool steels, refer to the GE Plastics’ Injection Molding Guides. Significant changes must occur in tooling when moving to molding Thinwall applications. Because of the higher pressures used to mold at thinner walls, the tooling for Thinwall applications is usually stiffer and more solid than that used for standard molding. For instance, the use of heavier mold plates and support pillars is common (See Figure 19). Mold interlocks are sometimes incorporated into the tooling to help prevent flexing, misalignment, or other mold movement. Use of cores that telescope into the cavity helps to reduce core shifting and breakage. Materials injected at high speeds may cause additional mold wear, which should be planned for when selecting mold materials. With wall-sections of less than 0.050 in (1.2 mm), accuracy in tool build is a critical factor. Due to the very high pressures generated during processing, there is a greater susceptibility of the mold to flash and a higher possibility of mold plate flexing. Tools for Thinwall molding applications must therefore be built heavier than those for standard molding. Mold support plates 2 to 3 in (5 to 8 cm) thick are often required, with support pillars under the cavities and sprue. Other desirable modifications might include: • Preloading of the support pillars (typically 0.005 in [0.127mm]) • Extensive use of mold interlocks for better alignment and lateral support • Application of #2 diamond polish to cores and ribs to help eliminate the problem of part sticking • Improvements in part release via mold surface treatments such as nickel-polytetrafluoroethylene (Ni-PTFE) • Use of more and larger ejector pins than with conventional molds to reduce pin pushing • Strategic placement of sleeve and blade knockouts With the very short fill times, typical of applications with wall-sections of less than 0.050 in (1.2 mm), makes venting more critical as well. Evacuation of gases to reduce burning is usually facilitated by extensive venting of core pins, ejector pins and along the parting line (up to 30% of the edge of the part). Some processors have successfully sealed the parting line with an “O” ring in order to pull a vacuum on the cavity and help with gas evacuation.



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Processing Because of the reduced cycle times typical of Thinwall molding, adequate cooling of mold cores and cavities becomes more critical and challenging. Some general guidelines to keep in mind are: • Size and placement of cooling lines should be considered from the beginning when the tool is designed, not as the last item added into the mold. • Non-looping cooling lines should usually be located directly in the core and cavity blocks to help keep the mold surface temperatures as consistent as possible. • Instead of decreasing coolant temperature to maintain the desired steel temperature, it is generally better to increase the amount of coolant flow through the tool. As a rule of thumb, the difference in temperature between the delivery coolant and the return coolant should be no more than 5-10°F (3-5°C). • For better heat management within the tool, the use of thermal pins, beryllium copper, and other specialty metals is also suggested. Figure 19. Typical Thinwall Tooling. 4

4 1

3

2

5

10 11

1 12

7

Cavity Side, Notebook Application 1. Mold Interlocks 2. Extensive Venting 3. Hardened Cavity Blocks 4. Non-Looping Cooling Lines 5. Extra-Thick Mold Plates

6. 7. 8. 9. 10.

3 6

5 7 13 8

Core Side, Battery Cover Application

Hot Drop to Cold Runner 11. Tapered Walls Release Coating 12. Telescoping Cores #2 Polish on Ribs and Cores 13. Hardened Gate Inserts Extensive K.O. Features Valve Gate on Appearance Surface (Opposite Gate Well)



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Processing All types of conventional injection sprues, runners, and gates can be used with most Thinwall applications. Large sprues and runners can help minimize pressure drops. However, they may also contribute to longer cooling and total cycle times. When sucker pins and sprue pillar pins are used, they should be made as non-restrictive as possible to reduce pressure loss and excessive material shearing. When gating directly onto a thin wall with a sprue, pin points or hot drop, etc., gate wells should be used to help reduce stress at the gate, aid in filling, and also help reduce part damage when degating. And because of the very high injection speeds that are often seen in Thinwall molding, gate inserts with a Rockwell (Rc) hardness of greater than 55 are typically used. Larger gates are generally better than smaller gates for material flow purposes and they also can help reduce gate wear. Use of gates thicker than the nominal wall is also common to aid flow and help prevent freeze-off before proper packing takes place. Figure 20. Area of Material Through-Put: Valve Gate Examples.

6.3 mm (0.250") Dia. Gate

1.0 mm (0.040") Wall Section

1.5 mm (0.060") Dia. Gate

Total area for material to flow through: 19.8 mm2(0.0314"2)

4.7 mm2(0.007"2)

Thinner wall-sections of less than 0.050 in (1.2 mm), in general, reduce a material’s flow length and a single gate may not be sufficient to fill the cavity (Figure 20). It may need to be relocated to a more central region on the part, or multiple gates may be necessary. Multiple gates, however, will give rise to knitlines in the part. Knitlines are not as strong as surrounding regions of polymer and they may be undesirable from an aesthetic standpoint. Sequential valve gating can often be used to minimize knitlines.The location and number of gates are critical factors when designing Thinwall tooling and must be considered carefully before part and tooling designs are finalized.



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Processing With thin wall sections of less than 0.050 in (1.2 mm), hot manifolds can help reduce pressure loss seen in cold runner systems. When hot runners are used, cycle times are not controlled by the cooling rate of the sprue and runner. However, hot runners need careful consideration. The additional material that is held in the manifold and hot drops between cycles adds to the total residence time for the molten material. In order to minimize pressure drops hot runners often require a minimum of 0.500 in (13 mm) diameter inner passages with no sharp corners or dead zones where material can get hung up. They also require the use of external heaters. Internal heaters should not be considered as an option. Because of the high pressures found in Thinwall molding, careful consideration must be given to the design and construction of the manifold. Finally, valve gates must be constructed for high pressure capability and be non-restrictive. These suggested features for Thinwall tooling are generally add-ons rather than standard issue with the mold. Therefore, costs for Thinwall tooling may in fact run 30-40% higher than for more traditional mold designs. However, these costs are often offset by increased productivity. Tool steels harder than P-20 should be used in most applications with thin wall sections, especially when high wear and erosion are expected. (H-13 and D-2 materials have been used successfully for gate inserts.) Faster injection rates will make gas venting even more critical to prevent gas-entrapment burning. Additional vents are generally required, especially in areas where flow fronts converge and trap gases. Vent dimensions are typically 0.0008-0.0012 in (0.02-0.03 mm) deep by 0.200-0.400 in (5-10 mm) wide. It may also be useful to vent core pins that shut off, as well as ribs, bosses, and ejector pins. Larger gates are often helpful at these higher injection speeds by reducing material shear and allowing for better packing.

Thinwall Ejector Layout

Conventional Ejector Layout

Figure 21. Typical Thinwall Part Ejectors.

Ejector Blades

Sleeve Ejector

Thinwall Parts Typically Require More and Larger Ejector Pins and Careful Attention to Placement



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Processing In applications with thin wall-sections part ejection may also be more challenging due to: 1. minimal shrinkage across the thickness which tends to make ribs and other features stick; 2. faster injection rates, which make controlling packing more challenging; 3. thinner walls and ribs, which are more easily damaged; and 4. higher pressures, which cause higher packing and less shrinkage. To reduce the likelihood of pin punching and sticking of ribs, it is common practice to use more and larger ejector pins (up to twice as many pins that are twice as large) than with conventional molding (See Figure 21). Draw polishing of all internal features, such as ribs, bosses, etc., is strongly suggested. Minimum draft angles should generally be 1°/side + 1°/ 0.001 in (0.025 mm) depth of texture.

Machinery Most molding machines found in the custom molding shops today are adequate to meet production requirements for most standard wall-section thicknesses applications in the range of 0.125-0.080 in (3.2-2.0 mm). Based on the total projected area of the part and runners, machine tonnage in the range of 2-5 T/in2 (28-70 MPa) will usually provide sufficient clamp pressures. Injection units will typically be run using 35-70% total shot capacity. Proper drying of material is important in order to maintain published physical properties and to promote uniform viscosity of the material during processing. Dryers should typically be sized for the material through-put of the job being run and provide -20 to -40°F (-29 to -40°C) dew point air at suggested temperature. Screw design is an important and often overlooked factor for molding of engineering thermoplastic resins. For LEXAN PC resin and CYCOLOY PC/ABS resin properly designed screws generally should have the following characteristics: • Compression ratio of 2.5:1 or less (2.0:1 - 2.5:1 are suggested). • Flight configuration of 5/10/5 (Feed/Transition/Metering). • 20:1 minimum length/diameter (L/D) ratio (suggested between 20:1 and 24:1).



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Processing • Nitriding is not suggested for screw, barrel, screw tip and components. Some nitrided components have led to degradation, discoloration and black specks in molded parts. • For the screw tip, use of full-flow sliding check rings is suggested. Standard microprocessor-controlled machines with closed-loop functions are often suitable for most Thinwall-sections applications ranging from 0.080-0.050 in (2.0-1.2 mm). Because the high injection pressures used to mold these parts necessitate the use of clamp tonnages in the range of 4-6 T/in2 (55-83 Mpa), larger presses are generally required than are typical for conventional molding. But these larger machines are usually fitted with larger barrels which must be properly sized to the application. The injection presses required to mold most Thinwall applications are more specialized than those typically found in custom-molding facilities. In these Thinwall applications, tight control over injection of the material becomes critical. Injection fill times below 0.5 seconds do not leave much time for injection speed profiling or pressure cutoff. Also, at these speeds and pressures, overpacking of the cavity can easily occur. A microprocessor-controlled machine with tight resolution is therefore virtually mandatory. In addition, the very high injection pressure used in most Thinwall applications require machines with accumulators on the injection unit and a typical line pressure of 3,000 psi (21 MPa), which can generate fill pressures in excess of 30,000 psi (200 MPa). Requirements for very rapid clamp movement also makes accumulators helpful in the clamp unit. Effective control of the clamp and injection functions at these high rates can usually best be handled with servo valves and very good microprocessor control units. Overall, tight resolution and control of times, speeds, temperatures, and pressures will typically be necessary in these machines to achieve consistent Thinwall parts with desired tolerances and appearance. The machine clamp unit for most Thinwall parts will be required to provide a minimum of 5-7 T/in2 (69-97 MPa) force per unit of the projected area of the part and runner. Extra-heavy platens are usually needed to reduce flexure below 0.005 in (0.127 mm). Machines with tie-bar-distance to platen-thickness ratios of 2:1 or less have been used successfully.



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Processing Small shot sizes used to mold Thinwall parts can lead to material degradation due to excessive residence time in the barrel. The barrel size of the machine should have a shot capacity that is not too large for the Thinwall application being molded. There are 200 T (181 t) molding machines with 5 oz (142 g) barrels that are currently being used to mold portable electronics Thinwall applications. A minimum shot-size to barrel capacity of 40% is suggested. Since total cycle times are often greatly reduced in Thinwall molding, it may be possible to reduce the shot size minimum down to 20-30%. However, such an approach should be taken with caution, and parts must be thoroughly tested for property loss due to possible material degradation. If the barrel capacity falls below this, processors risk significant material degradation due to excessive residence times. (This 20-30% range takes into account that the residence time will be shortened by the reduced cycle time.) Special screw tips and check rings are usually not required for Thinwall applications. In non-worn barrels, standard free-flow check rings have performed quite well at elevated pressures. For Thinwall applications, processors and designers need to be aware that there are critical tooling and machine changes that are required. It is important to note that while these changes rarely occur without additional investment, the productivity benefits of reduced material usage, faster cycle times, and greater yield can often outweigh the added cost. Under these circumstances, the added investment can be amortized quickly.


Page 57

Operations

Finishing

GE Plastics

SM

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Thinwall Molding



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Finishing Operations With the exception of assembly techniques, secondary operations for both conventional and Thinwall molded parts will be similar. Like conventionally molded parts, typical Thinwall assembly techniques include the use of welding, screws, inserts or adhesives. As with conventionally molded plastic parts, typical assembly techniques for Thinwall components include the use of mechanical (welds, screws, inserts) or chemical (adhesives) fastening systems. When mechanical assembly of Thinwall parts is selected, several special considerations are required for screws or inserts, or of ultrasonic welding. For more specific information on standard secondary finishing operations, please consult GE Plastics Design Guide and Product Line Guides for individual resin being used in a particular application.

Screws and Inserts The use of screws and inserts remains the primary assembly method for most Thinwall parts, as it does for conventional molded plastic parts. The main advantage to this assembly technique is secure and costeffective fastening and easy disassembly. Even with the use of smaller screw sizes in some Thinwall sections, pull-out strengths of 100 lb (46 kg) or more can often be achieved. With some Thinwall parts, however, there can be a issue with low strip torque of screws – especially in the case of smaller screws. Bosses for screws and inserts put in with heat or ultrasonic welding are a viable assembly option for many Thinwall applications (See Figure 22). However, the wall thicknesses of those bosses should not be less than the outer wall thickness, and it is possible to use outer diameters that are significantly less than twice the inner diameters. Strip torques as low as 2 in-lb (0.24 N-m) will require the use of precision drivers that can be accurately set to these values. Further, care should be taken in defining the gate location to reduce knitline weakness in the boss walls.



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Finishing Operations Figure 22. Typical Boss Design. 2 to 2.5d

.3 to .5d

d hole dia.

d = nominal diameter of the screw hole diameter = .75d - .9d depending on the material

Ultrasonic Welding The use of ultrasonic welding as an assembly technique offers the advantages of greater strength and stiffness in the assembled part, as well as reduced weight due to the elimination of threaded fasteners. In many Thinwall applications, ultrasonic welding is now increasingly used in the assembly of products which do not require disassembly for maintenance. Ultrasonic welding of many Thinwall parts is different from that of thicker wall parts. Relative weld strengths for the same material in thick and thinner wall sections can be significantly dissimilar, with much greater weld energies/times being required to achieve a full weld in Thinwall parts – especially when walls are somewhat flexible. Like conventional applications, weld strength is highly designdependent. Other important factors affecting performance include: stiffness geometry (both as-molded and when under pressure and vibration), usage of near-or far-field welding mode, horn design and amplitude, and joint design.



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Finishing Operations In a laboratory welding comparison using 2.25 by 3.75 in (57 by 95 mm) boxes with a 0.040 in (1.0 mm) wall, the following results shown in Table 7 were obtained. Table 7. Welding Strength Comparison for Thinwall Materials.

Material

Welding Strength [lbs]* [kg]*

LEXAN ML6339R/ML6018R Resin LEXAN SP7602 Resin CYCOLOY C1000HF/C1200HF Resin LEXAN SP1210R Resin LEXAN 920 Resin CYCOLOY C2950HF Resin LEXAN 121 Resin LEXAN SP1010 Resin

750-800 500 350-400 450 400 250 250 200

340-360 225 160-180 200 180 110 110 90

*The quantified weld strengths represent a material comparison, and not an end-use performance standard such as that determined by impact loading from drop tests. At the same time, a relatively low weld strength does not necessarily mean that an ultrasonically welded material cannot function in a real-world application. It does however, confirm the need for end-use testing.



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Finishing Operations Shielding When properly molded, shielding techniques used for most Thinwall components are much the same as those used for thicker wall parts (See Table 8). However, it is important to note that stress levels can be substantially higher in Thinwall designs unless care is taken during both the design and processing stages. For more specific information, consult the EMI/RFI Shielding information available from GE Plastics. Table 8. EMC Coating Technologies (Shielding Systems). Conductive Paints

Electroless Metallization

Vacuum Metallization

Copper/Nickel Alloy Multilayer Coating Available in Single-Sided and Double-Sided Systems

Pure Aluminum Coating

Immersion Coating (Double-Sided) or Spray/Immersion Coating (Single-Sided)

Radially Dispersed Evaporated Metal in Vacuum Chamber

Excellent Shielding Characteristics

Very Good Shielding Effectiveness

0.05-1.0 Ohms/Square Surface Resistivity, 25-75 Microns Thick

0.03-0.04 Ohms/Square Surface Resistivity, 3-4 Microns Thick

0.3-0.4 Ohms/Square Surface Resistivity, 5 Microns Thick

Good Abrasion Resistance

Excellent Abrasion Resistance

Very Good Abrasion Resistance

Double-Sided Electroless Plating Requires Aesthetic Outer Finish but Can Coat Parts Independent of Complexity

Line-of-Sight Coating Process Limits Part Complexity

• Form Metal-filled Paint System (Silver, Copper, Nickel)

Applied by Pressurized Spray Robotic or Human Application

• Performance Variable Shielding Effectiveness Dependent Upon Metal Formulation, Good-Excellent (Ni
• Limitations Human Application Limits Consistency

Line-of-sight Coating Process Limits Part Complexity

• Aesthetics Internal Coating Only, Molded-In Color/Texture

Single-Sided Electroless Plating is Dependent Upon Line-of-Sight Application Limiting Part Complexity Single-Sided Electroless Plating Coats Internal Surfaces Only, Molded-In Color/Texture Possible

Internal Coating Only, Molded-In Color/Texture Possible

Double-Sided Electroless Plating Requires Decorative Color Coat, Textured Paints Available



Notes

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Page 63

UL 1950 Agency Requirements

Appendix

GE Plastics

Thinwall

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6-2 • Thinwall Technology Guide Electrical Electric strength test 1 min.at operational voltage

Mechanical Adequate mechanical strength for application

Mechanical Adequate Mechanical Strength

Flammability** V-2 Minimum or device test or needle flame

Thermal 1. RTI ≥ Maximum Operating Temp. 2. If hazardous voltage component is mounted to plastic material directly and is subject to: - Primary voltage; (Ball Pressure Test at 125˚C for 1 hr.) - Secondary voltage; (Ball Pressure Test at 40˚C > Maximum Operating Temperature for 1 hr.)

Electrical None

Flammability** 94 V-2 Minimum

Thermal RTI≥ Maximum Operating Temperature

Supplied by Non-Limited Power Source

*This flow chart is a broad interpretation of the UL 1950 Agency Requirements. There are many requirements that have not been shown on this chart; specific and independent review of all requirements is mandatory. This chart is intended to show the general flow of the UL 1950 process. This chart is not recognized or endorsed by UL. This chart represents some of the minimum UL requirements, specific customer requirements may be different. **These ratings are not intended to reflect hazards presented by materials under actual fire conditions.

Mechanical Adequate Mechanical Strength

Mechanical 1. Impact -5 ft-lbs ball impact 2. Steady Load - Operator access area 30N (6.6#) for 5 sec. - External enclosure 250N (55.5#) for 5 sec.

Electrical 1. If metalized coating used for EMI/RFI shielding; UL QMRX2 required 2. Plastic < 13mm (1/2") from arc in a device; HAI PLC2 3. Plastic < 13mm (1/2") from source of temperature ignition; HWI PLC3

Electrical 1. If metalized coating used for EM/RFI shielding, then UL QMRX2 required 2. Plastic < 13mm (1/2") from arc in a device, HAI PLC2 3. Plastic < 13mm (1/2") from source of temperature ignition then HWI PLC3

- 5 ft-lbs ball impact - 3 drops, 1 meter 2. Steady Load - Operator access area 30N (6.6#) for 5 sec. - External enclosure 250N (55.5#) for 5 sec.

Flammability** 5VB minimum (holes allowed) or device test 5" flame (A V-0 may pass a 5V device test)

Flammability** V-1 minimum at minimum wall thickness or device test 3/4" yellow flame or needle flame (A V-2 material may pass the device test)

Flammability** 94-HB Minimum

Mechanical 1. Impact

Thermal 1. RTI ≥ Maximum Operating Temperature (95˚C) 2. Mold stress relief heat soak - 7 hr. at 10˚C > Maximum Operating Temperature (70˚C Min.) 3. If hazardous voltage component is mounted to plastic material directly and is subject to: - Primary voltage; (Ball Pressure Test at 125˚C for 1 hr.) - Secondary voltage; (Ball Pressure test at 40˚C > Maximum Operating Temperature for 1 hr.)

Thermal 1. RTI ≥ Maximum Operating Temperature (MOT must not exceed 95˚C due to skin burn consideration) 2. Mold stress relief heat soak - 7 hrs. at 10˚C > MOT (70˚C Min.) 3. If hazardous voltage component is mounted to plastic material directly and is subject to : - Primary voltage; (Ball Pressure Test at 125˚C for 1 hr.) - Secondary voltage; (Ball Pressure Test at 40˚C > M.O.T. for 1 hr.)

Thermal RTI ≥ Maximum Operating Temperature

Supplied by Limited Power Source

Internals (e.g. Electrical Insulation)

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Electrical None

Stationary/Fixed Device weighs more than 18Kg (40 lbs.) - Stationary - cord set, movable - Fixed - hard wired, not easily movable (Desktop Computer, Monitor, Printer, etc.)

Portable Device weighs less than 18 Kg (40 lbs.) and has cord set (Laptop, Palmtop, Notebook etc.)

Non Limited Power Source: Supplied by Non-Limited Power Source Hazardous Voltage (Vac 4.24V Peak; Vdc .60V); Hazardous Energy (VA > 100) see para 2.11 and 4.4.5.2 for details

Global Information Technology Equipment Agency requirements UL1950 (IEC950) (CSA 22.2-950)

UL 1950 Stationary/Fixed Equipment* for Thinwall Portable

Portable or Stationary/Fixed (keyboards, calculators, cellular phones)

Supplied by Limited Power Source

Externals (e.g. Enclosures)

Table 9. UL1950 Agency Requirements.

27299 Thinwall Inside.qxd Page 64

UL Appendix


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UL Appendix Overview of UL1950 Portable/Stationary/Fixed Equipment Key Definitions High-current Arc Ignition – Number of arc ruptures applied to the surface of a material at a specified rate and distance that causes the material to ignite. Hot Wire Ignition – Number of seconds it takes for a material to ignite when wrapped with wire with a specified resistance subject to a high level current. Limited Power Source – Non-Hazardous Voltage, Non-Hazardous Energy (Vac ≤ 42.4V Peak; Vdc ≤ 60V; VA ≤ 100; See para 2.11 and 4.4.5.2 for details). UL 94 5V* – A 5" flame is applied to a test sample (flame bars & plaque specimens) for five seconds on, five seconds off, for a total of five applications. In order to be classified 5V the sample must not burn with the flame or glowing combustion for more than 60 seconds, after last application of flame. For plaque flame ratings: 5VA* no holes allowed in plaque, 5VB* hole allowed. UL1950 accepts a 5VB flame rating. UL 94 HB* – A 1" flame is applied to the free end of a specimen for 30 seconds and then removed. The travel time of the flame from the front is measured and a burning rate calculated in inches per minute. For specimens 0.120" to 0.500 burn rate not to exceed 1.5" per minute over 3.0”. For less than 0.120" not to exceed 3.0" per minute over 3.0”. UL 94 V-0* – A 3/4" flame is applied to five specimens for two ten second applications each. No single specimen can burn for more than 10 seconds or glow for more than 30 seconds and the total flaming combustion time for the group must not exceed 50 seconds. The UL 94 V1* and V2* tests are conducted in a similar manner with the major difference being longer allowable burn times. In the case of V2, dripping particles that ignite cotton are allowed. UL Device Flame Test For Stationary/Fixed Equipment – Similar testing protocol to UL 94 5V specimen testing described above except flame applied to actual enclosure (end-product testing). *This rating is not intended to reflect hazards presented by this or any other material under actual fire conditions.



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UL Appendix UL Device Test For Portable Equipment - 3/4" Flame (yellow) Test – The enclosure shall not flame for more than one minute after two 30 second applications of test flame, with an interval of one minute between applications of the flame. The results are not acceptable if the sample is completely consumed. 12mm (needle) Flame Test – Similar testing protocol to the 3/4" Flame (yellow) Test described above except a 12mm (needle) flame is applied to the sample. Note: A 5V rating does not guarantee a V-0 and below rating and vice versa. The 5V test is designed more for quick high heat and flame electrical failures while the V-0 tests are designed for more of a smoldering type failure. If a material has a V-0 rating it will pass the V-1 and V-2 requirements. The foregoing is a summary only and is not intended to be a substitute for review of applicable standards. For more information call: 1-800-845-0600 and request the following: GE Plastics Reference Guides: • EMI/RFI Shielding Guide (CCS-002) • UL Recognition (PBG-UL6)


Page 67

Glossary

GE Plastics

SM

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Thinwall



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Glossary Amorphous resins – Thermoplastics in which the solidified molecular chains exist in a random configuration. The overall structure is characterized by a general absence of any regular three-dimensional arrangement of molecules. Amorphous thermoplastics, in contrast to crystalline resins, tend to offer higher impact strength, greater warp and creep resistance, and lower shrinkage. Amorphous resins do not have a distinct melting point, but rather have a softening range that represents the transition between solid and liquid. Anisotropic material – A material having properties (i.e., shrinkage, stiffness, strength, etc.) that are dependent upon direction. Typically, injection molded fiber-reinforced thermoplastics have different properties in the direction and perpendicular to the direction of flow (cross-flow). Check rings – Sliding rings on reciprocating screw tips that prevent injected melt from flowing back into the screw flights. The rings function much like check valves in a hydraulic system, which allow fluid to pass in one direction only. Cooling pattern – Temperature gradients in the part or mold as the molten melt solidifies and cures. This pattern affects the residual stresses in the finished plastic part, and therefore its resultant mechanical and physical properties. Cross-flow direction – Direction perpendicular to the direction of flow. The cross-flow direction of a center-gated disk, for example, is always aligned with the circumference, perpendicular to the radial flow direction. In anisotropic materials, stiffness and strength properties are usually lower and shrinkage rates are usually higher in the cross-flow direction. Crystalline resins – Thermoplastics with molecular structures arranged in a very regular repeating lattice structure. In contrast to amorphous thermoplastics, crystalline thermoplastics tend to exhibit greater chemical resistance, lubricity, greater flow length, and higher shrinkage. Crystalline resins have a distinct melting point that marks the transition from solid to liquid.



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Glossary Cycle time – In plastics molding, the period of time required for the complete sequence of operations on a molding press to produce one set of parts from the mold. For the injection molding process, the sequence includes: • closing and clamping of the mold • injection of molten resin into the mold • holding mold closed under pressure while plastic cools or cures • opening the mold and • ejection of the part(s). Degrees of freedom – Direction of possible movement. There are 6 degrees of freedom (DOFs): three in translation (x, y, and z) and three rotations (about x, y, and z). Various assembly methods are used to constrain different DOFs between mating parts. Maximum stiffness is attained by fully constraining all 6 DOFs between these mating parts. Dew point – The level to which air temperature must be reduced to cause water condensation. In preparing thermoplastic resins for use in injection molding, dryer units should provide at least -20°F (-33°C) dew point air at recommended temperature. Engineering Design Database (EDD) – An interactive, customeraccessible computerized database generated by GE Plastics that provides a matrix of dynamic (multipoint) property data with interpolation/ extrapolation routines. Engineers can use these data directly in engineering equations to estimate material properties at actual end-use conditions. To access EDD within the U.S.A., call GE Plastics at (800) 845-0600. Far-field welding – Ultrasonic welding together of plastic parts in assembly, in which the joint created is more than 0.250 in (6.4 mm) from the area of horn contact. Filling pattern – The path of the thermoplastic melt as it fills the injection mold. Mold-filling analysis software can be used to predict these filling patterns. Gussets – A geometric feature used to selectively stiffen another feature. Typically, these are rib-like structures that can be placed between features to help tie them together and stiffen the overall structure.



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Glossary Hot drops – Melt delivery units that bring the plastic melt from hot manifolds into gates. During the injection cycle, plastic in hot drops and hot manifolds will stay molten and will not freeze-off. Isotropic – A material having properties (e.g. shrinkage, stiffness, strength, etc.) that are not directionally dependent. Injection molded unreinforced thermoplastics typically have similar properties in all directions. Knitline – A line created wherever two flow fronts meet and freeze in the mold. Knitlines may not only be aesthetically undesirable in some parts; they also represent an area of lower strength and potential part failure. Linear beam equations – Simple, text-book equations that rotate the geometry and material of a beam to its stress, strain, and deflection. These equations are usually accurate for deflection not greater than the beam thickness. Load absorption design – A design strategy for electronic products in which the aim is to have the plastic housing absorb most of the energy generated by accidental impact events through elastic (reversible) deformation. This strategy requires increased package space, since the housing needs to deflect without contacting the internal electronics. Load/displacement curve – Chart that can be used to display impact performance. It is useful in determining the impact failure mode (e.g. ductile or brittle) and maximum load during impact. It can also be converted into stress/strain data. Load transfer design – A design strategy for electronic products in which the aim is to minimize deflection and thus transfer loads generated by accidental impact events from the plastic housing directly into internal structures and components. Mold-filling analysis – Computerized simulation of the injection molding process that can be useful for evaluating the part and process design before the design is finalized through prototype manufacture and testing. Mold shut-off – Areas within the mold where moving steel meets steel and does not allow molten plastic to flow past. Also referred to as “kiss off,” the parting line steel is an example where the mold halves “shut off” against one another tightly and do not permit material to flow between the halves.



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Glossary Notched Izod strength – An impact value and test method that shows the material’s relative ability to resist triaxial stresses created by impact loads. Although the most widely used impact test for plastics, Notched Izod values cannot be used directly in engineering equations to predict part performance. P-20 steel – A common tool-grade steel widely used to build tooling for conventional plastic injection molding applications. Packing Phase – The portion of the injection molding cycle, after the mold cavity is filled, when additional material is added to compensate for shrinkage during cooling and to help equilibrate cavity pressures to promote consistent, high quality parts. Pin punching – In plastics injection molding, cases in which pins used to eject the finished plastic part from the mold penetrate the part and damage it. Plastic deformation – Permanent deformation where the part does not return to its original shape even after the loading has been removed. This is in contrast to elastic deformation, which will reverse upon removal of the load. Pressure cutoff – The transfer point when the first stage (fill) hydraulic injection pressure is stopped and the second stage (pack/hold) pressure is initiated. This is usually the result of a signal from the mold or machine to activate certain hydraulic valves. The signal can be from a timer, pressure transducer, or position sensor, or may be computer generated. Relative Thermal Index (RTI) – Underwriters Laboratories’ relative thermal index – which provides guidelines as to how a plastic can be expected to perform at temperature over time. In testing, a heated product is allowed to cool. A rating is then assigned based on how well the cooled item retains its shape following subjection to a certain amount of stress and pressure. Residence time – The length of time a given amount of material resides in the barrel (and sometimes the hot manifold) before it is injected into the mold. Lengthy residence time exposure should be avoided, since it can lead to material degradation in the form of surface defects, and a reduction in molecular weight and property performance. Rheological behavior – The complex response of a material during flow. Engineering thermoplastics typically exhibit non-Newtonian rheological behavior, meaning that their viscosity is not constant with shear rate. These materials are shear thinning in that viscosity drops as shear rate increases. The viscosity is also strongly dependent on the temperature of the material.



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Glossary Secant modulus – Idealized modulus derived from a secant drawn between the origin and any point on a nonlinear stress/strain curve. The secant modulus is the line drawn from zero stress point to the curve for a specific designated strain. It represents the stiffness of the non-linear thermoplastic at a given stress/strain condition and leads to a more accurate force and deflection calculation. Servo valves – Components of a motion control system that use a feedback device to precisely control injection molding functions such as mold clamping and melt injection. Skin – The outer surface of the wall section of a thermoplastic part, formed during the filling stage of the injection molding process. As the molten plastic is injected into the mold cavity, it is quenched and solidifies first on contact with the cooler mold surface, forming the skin of the part. Spiral flow length – A test used to predict the distance a thermoplastic melt will flow under specified pressure and temperature along a spiral runner in a mold. The test can define the “flow length” of a given material, but is not an accurate predictor of actual material performance in a specific injection molding application. Stress state – The measure of the complexity of the stress in a part under load. A uniaxial stress state, found in a tensile bar under load, is the most simple form. A biaxial stress state, found in a Dynatup-style impact test setup, is more severe. A triaxial stress state, found in a Notched Izod test setup, is the most severe state. Stress state can be used to determine the severity of a stress concentrator. Stress/strain curve – A graphical representation of the relationship between stress and strain, traditionally displayed using data from tensile tests. The elastic modulus can be seen as the slope of the linear portion of the curve at low strains. The “yield” stress can be seen as the maximum value of the stress.



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Glossary Total shot capacity – For any given injection molding machine, the maximum volume of melt (in3 or cm3) that can be injected by a single stroke of the injection screw. Also commonly measured in ounces of polystyrene in the U.S.A. Ultrasonic welding – An assembly method used for mechanical fastening of plastic parts. The welding is accomplished through the application of vibratory mechanical pressure at ultrasonic frequencies. This vibratory pressure in the sealing area generates frictional heat that melts the thermoplastics and allows them to bond. Weld energies – In ultrasonic welding, microprocessors can control the time of the weld by calculating the energy (in joules) that has been provided to the weld.

DISCLAIMER: THE MATERIALS AND PRODUCTS OF THE BUSINESSES MAKING UP THE GE PLASTICS UNIT OF GENERAL ELECTRIC COMPANY, ITS SUBSIDIARIES AND AFFILIATES (”GEP“), ARE SOLD SUBJECT TO GEP‘S STANDARD CONDITIONS OF SALE, WHICH ARE INCLUDED IN THE APPLICABLE DISTRIBUTOR OR OTHER SALES AGREEMENT, PRINTED ON THE BACK OF ORDER ACKNOWLEDGMENTS AND INVOICES, AND AVAILABLE UPON REQUEST. ALTHOUGH ANY INFORMATION, RECOMMENDATIONS, OR ADVICE CONTAINED HEREIN IS GIVEN IN GOOD FAITH, GEP MAKES NO WARRANTY OR GUARANTEE, EXPRESS OR IMPLIED, (I) THAT THE RESULTS DESCRIBED HEREIN WILL BE OBTAINED UNDER END-USE CONDITIONS, OR (II) AS TO THE EFFECTIVENESS OR SAFETY OF ANY DESIGN INCORPORATING GEP MATERIALS, PRODUCTS, RECOMMENDATIONS OR ADVICE. EXCEPT AS PROVIDED IN GEP‘S STANDARD CONDITIONS OF SALE, GEP AND ITS REPRESENTATIVES SHALL IN NO EVENT BE RESPONSIBLE FOR ANY LOSS RESULTING FROM ANY USE OF ITS MATERIALS OR PRODUCTS DESCRIBED HEREIN. Each user bears full responsibility for making its own determination as to the suitability of GEP‘s materials, products, recommendations, or advice for its own particular use. Each user must identify and perform all tests and analyses necessary to assure that its finished parts incorporating GEP materials or products will be safe and suitable for use under end-use conditions. Nothing in this or any other document, nor any oral recommendation or advice, shall be deemed to alter, vary, supersede, or waive any provision of GEP's Standard Conditions of Sale or this Disclaimer, unless any such modification is specifically agreed to in a writing signed by GEP. No statement contained herein concerning a possible or suggested use of any material, product or design is intended, or should be construed, to grant any license under any patent or other intellectual property right of General Electric Company or any of its subsidiaries or affiliates covering such use or design, or as a recommendation for the use of such material, product or design in the infringement of any patent or other intellectual property right.



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Literature Design Guide

Provides comprehensive property profiles and descriptive information on GE Plastics’ entire product portfolio with data on testing methods utilized to evaluate engineering materials.

The Internet

Processing Guides We’re pleased to offer our customers an extensive library of product, processing, technical and general literature. From our popular Design Guide to our in-depth Product and Processing Guides, our literature is designed to act as a critical reference tool for our full product portfolio. To order a particular guide or a series of multiple guides, please call 1-800-845-0600 or access our online order center at www.geplastics.

General Guides

A comprehensive 380-page guide that provides in-depth information to assist engineers design state-ofthe-art equipment and components with GE engineering thermoplastics. Includes explanations of product groupings and grades, application development assistance, procedures and data relating to material selection, design assistance, prototyping and processing considerations, assembly and finishing details. Also included is a glossary of commonly used engineering terms that relate to plastics.

Product Guides

Over 200 pages of information contained in 17 brochures to assist processors fabricating components using injection molding processes. Includes general information on converting engineering thermoplastics together with specific processing details and molding parameters for each GE resin family and grade. Additional processing publications containing information on Thinwall, Gas Assist, Thermoforming and Engineering Structural Foam processing techniques are also available.

Complete access to over 3,000 pages of technical information is available on-line on the Internet. Users can navigate within GE Plastics’ www.geplastics.com address to access the literature described above as well as GE Select™, an engineering database containing product data sheets for over 500 grades of GE engineering resins.

SM

GE Product Families A broad range of valuable information can be found in a host of general literature from GE Plastics. From our Global Resources and Six Sigma Quality brochures, to The Weatherables™ and new Polymer Processing Development Center brochures, we offer you in-depth coverage of our key business and technological initiatives.

A series of brochures containing information to assist designers and processors understand the characteristics of various polymer chemistries within GE Plastics’ resin families. Includes information to help select engineering thermoplastics for specific applications.

Key product family products, markets and services are featured in a variety of new product-line brochures. Includes property and application information on our Crystalline, Engineered Styrenics Resins, LEXAN®, ULTEM® and NORYL® product lines. ® CYCOLAC, CYCOLOY, LEXAN, NORYL, ULTEM and VALOX are Registered Trademarks of General Electric Company. ™ The Weatherables and GE Select are Trademarks of General Electric Company.

7-8 • Thinwall Technology Guide 98923


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Sales Offices AMERICAS United States GE Plastics One Plastics Avenue Pittsfield, MA 01201 USA Telephone: (413) 448-7110 Cable: GEPLASTICS California ★ Western Business Region 4160 Hacienda Drive Pleasanton, CA 94588 Telephone: (510) 734-0161 Georgia ★ Commercial Development Center 205 Scientific Drive Norcross, GA 30092 Telephone: (770) 662-1000 Illinois Suite 100, One Corporate Lakes 2525 Cabot Drive, Lisle, IL 60532 Telephone: (630) 505-2500 Massachusetts ★ One Plastics Avenue Pittsfield, MA 01201 Telephone: (413) 448-7110 Michigan P.O. Box 5011 Southfield, MI 48086-5011 ★ 25900 Telegraph Road Southfield, MI 48034 Telephone: (248) 351-8000 Ohio Suite 660, 6000 Lombardo Center Seven Hills, OH 44131 Telephone: (216) 524-2855 Texas Suite 930, 5430 LBJ Freeway Dallas, TX 75240 Telephone: (972) 458-0600 Puerto Rico General Computer Building P.O. Box 2010 Bayamon Puerto Rico 00960 Road 174, No. 101 Minillas Industrial Park Bayamon Puerto Rico 00959 Telephone: (787) 288-2340 Telefax: (787) 288-2348

Brazil GE Plastics South America S/A Av. das Nacoes Unidas, 12995 -20 andar Brooklin Novo 04578-000 São Paulo, SP Brazil Telephone: (55) 11-5508-0500 Telefax: (55) 11-5505-1757

★ Application Development Center

Canada GE Plastics – Canada General Electric Canada Inc. 2300 Meadowvale Blvd. Mississauga, Ontario L5N 5P9 Canada Telephone: (905) 858-5700 Telefax: (905) 858-5798

Mexico GE Plastics Mexico S.A. de C.V. Av. Prolongación Reforma #490 4o. Piso Colonia Santa Fe 01210 Mexico, D.F. Telephone: (525) 257-6060 Telefax: (525) 257-6070

EUROPE European Headquarters ★ General Electric Plastics B.V. Plasticslaan 1 4612 PX Bergen op Zoom The Netherlands Telephone: (31) 164-292911 Telefax: (31) 164-292940 General Electric Plastics B.V. P.O. Box 117 4600 AC Bergen op Zoom The Netherlands Telephone: (31) 164-292911 Telefax: (31) 164-291725 Austria GE Plastics Austria Pottendorfarstrasse 47 A-2700 Wiener Neustadt, Austria Telephone: (43) 2622-39070 Telefax: (43) 2622-39047 France ★ General Electric Plastics France S.a.r.L. Z. I. de St. Guénault Boite Postale No. 67 F-91002 Evry/Cedex, France Telephone: (33) 1-60796900 Telefax: (33) 1-60796922 Germany ★ General Electric Plastics GmbH Eisenstraße 5 65428 Rüsselsheim Postfach 1364-65402 Rüsselsheim, Germany Telephone: (49) 6142-6010 Telefax: (49) 6142-65746 India ★ GE Plastics India Limited 405-B, Sector 20 Udyog Vihar Phase III Gurgaon 122016 (Haryana) Telephone: (91) 124-341-801 Telefax: (91) 124-341-817

Italy ★ GE Plastics Italia S.p.A. Viale Brianza 181 20092 Cinisello Balsamo Milano, Italy Telephone: (39) 2-61834-301 Telefax: (39) 2-61834-305 Spain ★ GE Plastics Iberica, S.A. Avinguda Diagonal 652-656 08034 Barcelona, Spain Telephone: (34) 93-252-1606 Telefax: (34) 93-280-2619 United Kingdom ★ GE Plastics Ltd. Old Hall Road Sale, Cheshire M33 2HG United Kingdom Telephone: (44) 161-905-5000 Telefax: (44) 161-905-5106

PACIFIC Pacific Headquarters GE Plastics Pacific Pte. Ltd. #09-00 GE Tower 240 Tanjong Pagar Road Singapore 0208 Telephone: (65) 220-7022 Telefax: (65) 326-3290 Australia ★ GE Plastics (Australia) Pty. Ltd. 175 Hammond Road Dandenong, Victoria 3175 Australia Telephone: (61) 3-9703-7200 Telefax: (61) 3-9794-8563 GE Plastics (Australia) Pty. Ltd. 57/2 O’Connell Street Parramatta, New South Wales 2150 Australia Telephone: (61) 2-9689-3888 Telefax: (61) 2-9689-3530 GE Plastics (Australia) Pty. Ltd. Legal and General Building 206 Greenhill Road Eastwood, South Australia 5063 Australia Telephone: (61) 8-8272-5044 Telefax: (61) 8-8272-2479 China GE Plastics – Beijing General Electric (USA) China Company, Ltd. 3rd Floor, CITIC Building No. 19 Jian Guo Men Wai Ave. Beijing 100004, P.R. China Telephone: (86) 10-6500-6438 Telefax: (86) 10-6500-7476

GE Plastics – Shanghai GE China Company 9th Floor, Shartex Center 88 Zunyi Road (S) Shanghai 200335, P.R. China Telephone: (86) 21-6270-9623 Telefax: (86) 21-6270-9973 Hong Kong GE Plastics Hong Kong Limited Room 1008, Tower I, The Gateway 25 Canton Road, Tsimshatsui Kowloon, Hong Kong Telephone: (853) 2629-0853 Telefax: (853) 2629-0804 Indonesia GE Plastics – Indonesia KH Mas Mansyur Kav. 126 Jakarta 10220, Indonesia Telephone: (62) 21-574-4980 Telefax: (62) 21-574-7101 Japan GE Plastics Japan, Ltd. Tokyo Office Nihombashi Hamacho Park Bldg. 2-35-4, Nihombashi-Hamacho Chuo-ku, Tokyo 103, Japan Telephone: (81) 3-5695-4861 Telefax: (81) 3-5695-4859 Korea ★ GE (USA) Plastics Korea Co., Ltd. 231-8, Nonhyundong Kangnamku Seoul 135-010, Korea Telephone: (82) 2-510-6250/6000 Telefax: (82) 2-510-6666/6224 Singapore GE Plastics – Singapore Sales & Marketing Office 80 Anson Rd., #38-00 IBM Towers Singapore 079907 Telephone: (65) 223 -7022 Telefax: (65) 223 -7033 Taiwan GE Plastics, Taiwan 8F -1, 35 Min Chuan E. Road Sec. 3 Taipei, Taiwan, R.O.C. Telephone: (886) 2-509-2124 Telefax: (886) 2-509-1625 Thailand GE Plastics – Thailand 21st Floor Thaniya Plaza Bldg. 52 Silom Road Bangkok 10500, Thailand Telephone: (66) 2-2312323 Telefax: (66) 2-2312322

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GE Plastics We bring good things to life.

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General Electric Company One Plastics Avenue Pittsfield, MA 01201 800.845.0600 http: / / www.geplastics.com

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