23417867 Mould Theory

INTRODUCTION TO PLASTIC MATERIALS

OBJECTIVE: After end of the session/lesson trainees should be able to

- Explain the different type of material - Use of the the different different material. material. - Identify the different material. material. - Explain the necessity necessity of mould - Explain the working principle - Explain the functional requirement of each parts.

LESSON 1

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MOULD THEORY 1

INDO-ZIMBABWE TRAINING PROGRAMME

INTRODUCTION TO PLASTIC MATERIALS 

   

Plastics is a unique class of materials, came into existence by the virtue of their superior properties and cost performance balance over to conventional materials like wood, ceramics and metals. Plastic materials based on their characteristic performance were classified as thermoplastics and thermosetting materials. It is well known that the former class of materials distinguished from the latter by their processing behavior and characteristic characteristic properties. Example for thermoplastic includes includes PS, PP, PVC, PET, PA,POM Examples of thermo-sets include PF, UF, and MF etc.

Based on applications, plastics are classified as   

Commodity plastics Engineering plastics Specialty plastics.

COMMODITY PLASTIC

These are low performance, high volume resins, characterized by: HDT ? 100°c and tensile strength, <50MPa. Two of them, PE and PP are semi crystalline, thus their properties depends on nucleation and annealing. The glossy resins (PS, PVC, and PMMA) are available as either rigid or impact modified varieties, viz. GPPS, PVC-R, and PMM or HIPS, PVC-F, and HI-PMMA. All these polymers can be formulated as filled and reinforced grades excellent for film blowing or blow molding. ENGINEERING PLASTIC

Five types of resins belongs to this category polyamides (PA), polyesters (PEST), polycarbonates (PC), poly-oxy-methylene (POM) and modified poly-phenyleneether (PPE). By definition, the resins are characterized by HDT £ 100°c and tensile strength >40MPa. They can be formed to precise and stable dimensions. Their consumption is about 12% of total plastic. There is a great diversity of performance within each of this type, especially considering the great chemical verity within the PA, PEST & PC type as well as profusion of blends, filled, and reinforced systems comprising co mprising engineering engineering resins. SPECIALITY PLASTIC

To this category belong low volume, high performance, high temperature, and high cost polymers. Their consumption amount to less than 0.1% of total plastics. By definition, the resins are characterized by high modulus, tensile and impact strength. They can be formed to precise and stable dimensions. Their continuous use temperature is CUT £ 150°c. Most of specially polymers are available as GF or CF filled grades. grades.

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MOULD THEORY 2


WHY PLASTICS Why

we use plastic material in the first place instead of traditional and familiar material such as metal. In general plastic offers impressive advantages over metals. Some of it is listed li sted below

       

They are not subjected to corrosion They are light in ;weight with good strength to weight ratio Very cost effective The speed with which they can be produced. They give design freedom They provide with good electrical insulation property They are available in wide range of colures Reduced assembly time. In addition to it each plastic material offers some special property which serves a particular application or can be made to do so by the incorporation incorporation of suitable additives with the plastic materials.

THE CASE FOR PLASTICS

The successful use of plastics usually derives from a combination of cost savings and improvement in performance or appearance, but often the cost saving alone is sufficient to justify the choice of a plastics material. Plastics can offer the following technical advantages. LIGHT WEIGHT

All plastics have low densities, generally in the range 830 to 2500Kg/m3.These figures can be extended upwards or downwards. For example, foamed materials can have densities as low as 10 Kg/m3, and filled plastics as high as 3500Kg/m3. In comparison the density of aluminum is about 2700 Kg/m3 and that of188 stainless steel about 7900 Kg/m3.There are two consequences of the low densities of plastics. Large volume of material for unit weight that can be obtained with metal.

TOUGHNESS

Some plastics are extremely tough, and objects made from them are difficult to destroy by mechanical treatment. Other plastics are less tough, and still other is fragile. RESILIENCE

Plastics show some of the behavior associated with rubbers in accommodating relatively large strains without fracture and in recovering their original shape and dimensions when the stress is removed. VIBRATION DAMPING

The quietness in use of plastics gear trains and bottle crates depends on the inherently high degradation of mechanical energy to heat.

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MOULD THEORY 3


RESISTANCE TO FATIGUEE

Some plastics appear to perform remarkably satisfactorily in situations involving dynamic stresses or strains. LOW COEFFICIENT S OF FRICTION

Plastics / plastics and plastics metals combinations have low coefficients of friction and can often perform un-lubricated without fear off seizing. THERMAL INSULATION

Plastics are good insulators, their thermals conductivities being many orders of magnitude lower than those of metals. This low conductivity may be exploited in handles for utensils and in the design of pipes for carrying hot fluids, where a lagging effect may be built in. CORROSION RESISTANCE

In general, plastics are complementary to metals in their chemical resistance, in that they are resistant to weak acids, weak bases and aqueous salt solutions, although strong oxidizing acids may cause some attack, leading to discoloration and possible embitterment. On the other hand, organic solvents to which metals are generally inert, may cause swelling, deterioration of properties and eventual dissolution. The plastics material of the environment and on the temperature. COLOUR POSSIBILITIES

Some plastics are transparent, some are translucent and a few are opaque. Acrylics, polystyrenes, methyl pentene polymers, polycarbonates and certain grades of PVC can be very transparent indeed to visible light. All plastics can be colored by incorporating a wide range of dyes or pigment s, thus avoiding the need for painting. However, subsequently painting or plating is possible with some, if required

MANUFACTURING METHODS

A variety of automatic and semi-automatic techniques allows easy, economical and reproducible fabrication of articles and components. Although often unnecessary, further finishing operations are easy to carry out on most plastics.

WHY IDENTIFICATION OF PLASTICS   

To know the material. Used in any product For processing of scrap material. For product indigenization

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MOULD THEORY 4


IDENTIFICATION TECHNIQUES             

Appearance Method of fabrication Penetration to hot rod and cutting with knife Floatation Test Scratch resistance Colour Odour Tear Solubility Burning characteristics Melting Point Melting Point Confirmation test

APPEARANCE:      

Transparent - PS, SAN, PMMA, PC, PVC, PET Translucent - PE, PP Glossy - ABS, PC, POM Milky White- PTFE Dark colors- Phenolics Yellowish - Epoxy Resins and unsaturated polyesters

METHOD OF FEBRICATION   

Injection Moulding - Thermoplastics and injection mould able thermo-sets (Gate marks, Flash Marks) Without any mark - Nylons (Polyamides), HDPE, PTFE. Films - PE, PP, PVC, PET, CELLULOSICS

PENETRATION TO HOT ROD AND CUTTING WITH KNIFE:    

If a shaving can be pared of with a knife it is probably thermoplastics. If the material s rigid and difficult to pare off it is probably thermo-set PMMA and polystyrene are difficult to pare off When a hot electric soldering iron is kept on the material. Thermoplastics melt and sink but thermo-set do not melt but degrade



FLOATING TEST IN WATER: 

Drop a piece of sample in water, if it sinks then it is PC, PS, SAN, CA, PVC, NYLON, PTFE, PMMA, Poly-acetals If it floats then it is PE or PP but mineral filled grades sink in water

SCRATCH RESISTANCE   

Low gloss easily scratch able - LDPE, LLDPE High gloss can be scratched - HDPE High gloss can not be scratched - PP

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MOULD THEORY 5


SOUND:  

Take the sample and drop on the hard surface Metallic Sound ± PC, PS, SAN, PPS Dull sound ± CA, PVC, NYLON, PTFE, PMMA, POLYECETAL

ODOUR: 

Camphor smell ± Cellulose Nitrate Plasticizer - Plasticized PVC smell

TEAR:    

Tough, stretches before tearing - Polyethylene Tough stretches a lot before tearing - Polypropelene Tears Straight - Polystyrene Stretches then tears raggedly - PVC Tear easily and straight - Cellulose Acetate

BENDING:      

Bends and tends to remain unbend - PE, PP Cracks but retain unbend - Polystyrene Bends and tend to remain - ABS Bends easily and spring back ± PVC(Rigid) Cracks and splinters difficult to bend - PMMA Spring Back - Nylon

SOLVENTS FOR SOME IMPORTANT PLASTICS:           

Thermoplastics - solvents Cellulose Esters - Ketones, Esters PVC - Cyclohexane one dimethyl formamide Polyacrilic Acid Esters - Aromatic Hydrocarbons, Acetone Polymethyl Acid Esters - Chloroform, Dioxin, Aromatic hydrocarbon Polyesters - Ketones PET - Cresol, Conc. H2SO4 PC - Chlorinated Hydrocarbons, Dioxan PE - At elevated temp. Dichloroethelene, Tetralene, Decalene PS -Toluene, other aromatic hydrocarbons, Ethyl Methyl Ketone, Polyvinyl Acetate - Acetone, Methanol, Aromatic Chlorinated Hydrocarbons.

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MOULD THEORY 6


BEHAVIOR OF PLASTICS IN FLAME: Plastic Matl.

PE

Flame Characteristics

Kind of

smoke

Blue base yellow tip burns Continuously

Matl. behavior

Odour

No smoke

Drips

W axy

smell

PP

Blue base and yellow tip

No smoke

Drips

Lubricating oil smell

PS

Orange yellow flame, fast and continuously burns

Black soot

No dripping

Merry Gold Smell

HIPS

-do-

-do-

-do-

Burning Rubber smell

ABS

-do-

-do-

-do-

-do-

SAN

-do-

-do-

-do-

Merry Gold Smell

. NOTE: ABS and HIPS can be differentiated by detection of nitrogen by

elements analysis NOTE : SAN and PS can be differentiated by detection of presence of

extra elements

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MOULD THEORY 7


PRILIMINARY MATERIAL CONSIDERATIONS FOR PRODUCTS Thermosetting Type

Thermoplastic Type

Thermo-sets undergo a chemical Thermoplastics are softened by heat, and become change during moulding, becoming solid on cooling. This process can be repeated solid, and therefore, they Cannot many times. be melted. Overall chemical resistance of thermo-sets in Unequalled by any thermoplastic. Dimensional stability is excellent.

Chemical resistance of thermoplastics is limited to fewer compounds than is the case with thermo-sets.

Creep over a prolonged time period is well below even glassreinforced engineering thermoPlastics. This means better dimensional stability and better resistance to property deterioration.

Creep is one of the major problems with the thermoplastics When subjected to long term loads. This means sizable Dimensional changes and Strength degradation. Creep Resistance is improved by filler s and reinforcements.

Moulded-in stresses with compression moulded parts are parts are lower than in other moulded parts , and where minimal distortion is a factor, it can be a deciding consideration. Toughness is a property that can be found in Thermo-sets at a considerable cost; by using reinforcing materials such as fabrics and/or Fiberglass mat.

Since thermoplastic respond more readily to the influence of heat, they also tend to fluctuate in dimensions. Some polymers vary more than others, but as a group they would Rank second to thermo-sets.

Injection moulded thermoplastics have moulded in Stresses in varying degrees that are caused by part design, Processing parameters and mould design.

Most thermoplastics are inherently tough materials and for this reason one used whenever this requirement is needed. They provide good toughness at low cost.

Colors in thermosetting compounds Thermoplastic materials can be coloured to any are limited in variety and their desired shade of appearance and normally maintain stability is not Satisfactory. The the colour throughout the life of a product. resins tend to discolour over Prolonged periods of time. Very few thermo-sets are available in clear, see Through materials.

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The selection of clear material in thermoplastics is quite large.

MOULD THEORY 8


ASSIGNMENT:-

1. Plastics are generally --------material. 2. As per their characteristic performance plastic materials are classified as -----------and -------------material. 3. Give some examples of thermoset and thermoplastic material? 4. What do you mean by commodity plastic? 5. What do you mean by engineering plastic? 6. What is specialty plastic or modern plastics? 7. What are the benefits or characteristics of plastic? 8. What are the technical advantages of plastic? 9. How plastic materials can be identified? 10. -------, -------, ------- are transparent materials. 11. ------, ------are glossy material. 12. What is floating test in water of plastics? 13. ------flames appear in case of PP. 14. What is the behavior of plastics in flame? 15. Differentiate thermoset and thermoplastic material. 16. ------ materials can be reused and ------materials cannot. 17. What is the full form of ABS, PP, PE, LDPE, HIPS & PVC? 18. Plastic materials based on their characteristics performance were classified as ««.. & «««. Material. 19. Based on applications plastics are classified in «.. , ««.. & «« plastics. 20. Write two identification techniques to identify the plastic material. 21. ««« & «« are used to produce transparent components. 22. Which plastic undergoes chemical change during demoulding ? 23. ««.. gives metallic sound on hitting. (PC, SAN, PS) 24. Cellulose nitrate produces««. Smell. 25. ------plastics can not be reused. 26. A mould is an assembly of ««« & «««. 27. ««gives external shape of components. 28. ««. gives internal shape of component.

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MOULD THEORY 9


MOULD:-

It is an assembly consists of an Impression, which is a recess or gap similar to the Component formed by the two Mould members termed as µCORE¶ and µCAVITY¶ , into which the molten Plastic Material is Injected under Pressure an is allowed to be cooled either by water or air till the component gets hard.

CAVITY: -

It is female part of the mould, which gives the external shape of the Component. Pockets, Slots, holes are considered as Cavities. These are highly polished to a mirror finish, requires better finishing appearance on the outer surface of the component.

CORE: -

It is male part of the mould, which gives the internal shape of the Component. All Projections are considered as Cores. These are not required high polish as the Component is to be sticks on to core.

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MOULD THEORY 10


Terminology of M ould Base:Top Clamping Plate / Top plate:  Location Ring or Retainer Ring:  Cavity Plate :  Core Plate :  Core back plate :  Bottom Clamping Plate / Bottom plate:  Spacer/ Riser :  Ejector Plate:  Ejector back plate :  Feed Buttons:  Guide pillars / Leader pins:  sprue Bush :  Sprue Puller :  Return Pins / Push back pins:  Guide bush :  Cavity:  Core:  Parting Surface: 

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MOULD THEORY 11


Top clamping Plate:

Holds the stationary part of the mould to the stationary platen of the injection machine. Location Ring: Fits into a counter bore in the top clamping plate and is used to locate the mould on the platen of the press so the nozzle and sprue bushing are aligned. Cavity Retainer Plate: Part of the stationary section of the mould into which the leader or guide pins are mounted. Also used to hold core, cavity blocks, and sprue bushings. Core Retainer Plate: Top plate of the movable section of the mould. Forms the parting line of the mould with cavity retainer plate. Used to hold the leader pin bushing as well as core and cavity blocks. Core back Plate: Mounted behind the core retainer plate to keep this plate from bending under the high pressure used in injection molding: Bottom Clamping Plate: Holds the moving portion of the mould to the movable platen of the injection machine. Spacers: Mounted on the bottom clamping plate under the support plate to form a space which allows the ejection bar to move when the piece parts are ejected.

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MOULD THEORY 12


Ejector Retainer Plate:

Counter bored for the heads o0f ejector pins, ejector return pins, and spur puller pin. Ejector back Plate: Bolted together with the ejector retainer plate to form a unit. Acts as a back up plate for the pins in the ejector retainer plate. Feed Buttons: Pressed into the bottom clamping plate, they are lands for the ejector plate. Pillars : Round bars placed between the support plate and bottom clamping plate . The same height as the parallels. Bolted to the bottom clamping plate, they are used as additional support for the core retainer plate. Sprue Bushing: Butted up against the nozzle of the injection machine. Has a conical-shaped hole through which the material is forced into the mould runner. Sprue puller Pin: Pin located directly under the opening of the spur. Used to pull the moulded spur out of the bushing after shot has been made. Push back pins: Located in the ejector retainer plate. Force the ejector plate and ejector retainer plate, and therefore the ejector pins, to the bottom position as the mould closes. Leader Pins /Guide Pillars : Hardened and ground steel pins pressed into one of the plated. Align the two halves of the mould base. Guide bushes:

Cavity: Core:

Hardened and ground steel bushings which are pressed into one of the plates. Serve as bearing surfaces for the leader pins. Some injection mould bases are manufactured with the parallels welded to the bottom clamp plate. The unit thus formed is called the Ejector housing. The female portion of a mould which gives to the moulding its external form.

The male portion of a mould which forms the internal shape of the moulding. Parting Surface : That part of the mould plate, adjust to the impression, which butt together to form a seal and prevent loss of plastic material from the impression.

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MOULD THEORY 13


DIFFERENT MOULDING METHODS

OBJECTIVE: After end of the lesson/session trainees should be able to

- Distinguish the different moulding processes - Explain the individual process. - Distinguish different types of mould - Explain the application of different types of mould & its limitation, advantages & disadvantages.

LESSON 2 IZTC HARARE

MOULD THEORY 14


CLASSIFICATION OF MOULD

COMPRESSION MOULD

TRANSFER MOULD

INJECTION MOULD

BLOW MOULD

EXTRUCTION MOULD

THRRMOFORING MOULD

ROTO MOULD

COMPRESSION HAND

FLASH

POSITIVE Open flash

LANDED POSITIVE

Semi +ve

Semi +ve

vertical flash

horizontal flash

DIRECT POSITIVE

TRANSFER MOULD

POT TYPE

PLUNGER TYPE

INJECTION MOULD

ACC. TO CONS.

BASED ON FUNCTION

-HAND -SEMI AUTOMATIC -AUTOMATIC

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-2- PLATE -3- PLATE

MOULD THEORY 15

ACC. TO RUNNER SYSTEM

-COLD RUNNER -HOT RUNNER INSULATION RUNNER


BLOW MOULD

EXTRUCTION

INJECTION

STRETCH

THRRMOFORING MOULD

VACUM

PRESSURE

EXTRUCTION MOULD FILM EXTR.

PIPE EXTR.

DIFFERENT MOULDING MEATHODS-:

Plastics are shaped by a Varity of methods such as molding, calendaring, extruding, blow molding, vacuum forming, etc. For thermo-setting materials hot compression molding, Transfer±molding, laminating and castings are the most common methods. COMPRESSION MOULDING: 



The hot compression molding method is the most commonly used method of shaping a thermo-setting plastic to some desired form. The basic principle in molding is that plastic material softened by heat and pressure takes the shape of cavity due to pressure acting on it and it gets hardened by the heat and pressure causing a chemical change and the desired shape is obtained In general, compression and transfer moulds are made of High Carbon and High chromium of hot die steel. The core and cavity should be heat treated to the required hardness for maintaining the dimensional accuracy of product and also the life of mould. There are different types of compression moulds in use.

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MOULD THEORY 16


INJECTION MOULDING:-

Thermo plastic materials are used in injection moulding. The thermo plastics soften and become plasticized in state by the application of heat and harden when cooled. There is no chemical reaction taking place during the process of heating and cooling. These materials have linear molecular chains that flow over each other when heated and solidify into a new shape when cooled without significant chain breakage.

BLOW MOULDING 



Basically, blow molding is intended for use in manufacturing hollow plastic products: a principal advantage is its ability to produce hole ±low shapes without having to join two or more separately molded parts. Although there is considerable difference in the available processes, as described below, all have in common production of a parison (precursor). Enclosing of the parison in a closed female mold, and inflation with air to expand the molten plastic against the surface of the mould, where it sets up into the finished product. Differences exist in the way that the parison is made (i.e., by extrusion or by injection molding); in whether it is to be used hot as it comes from the extruder or injection molding machine (as in conventional blow molding), or stored cold and then reheated (as in cold perform molding); and in the manner in which the parison is transferred to the blow mold or the blow mold is moved to the parison.

The basic process steps remain the same however :

1. 2. 3. 4. 5. 6. 7.

Melt the material. Form the molten resin into a tube or parison. Enclose the parison in the blow mould. Inflate the parision inside the mould. Cool the blow moulded part. Remove the part from the mould. Trim flash, as needed.



In many cases, all theses steps can be carried out automatically, with the finished products conveyed to downstream stations for secondary operations and packaging.



Although there are many variations, the two basic processes are extrusion processes injection blow molding. Extrusion processes are by far the more widely used, but injection blow molding and injection stretch blow molding have captured significant market segments. While reviewing these methods, the reader is urged to refer to Chapters 4 and 5 additional background material

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MOULD THEORY 17


ROTATIONAL MOULDING 



In rotational molding, the product is formed from liquid or powdered thermoplastic resin inside a closed mold or cavity while the mold is rotating biaxial in a heating chamber. To obtain this mold rotation in two planes perpendicular to each other, the spindle is turned on a primary axis, while the molds are rotated on a secondary axis Rotational molding (also popularly know as roto-molding) is best suited for large, hollow products requiring stress-free strength, complicated curves, a good finish, a variety of colors, a comparatively short (or very long) production run, and uniform wall thickness. It has been used for products such as fuel tanks, furniture, tilt trucks, industrial containers, storage tanks, portable outhouses, modular bathrooms, telephone booths, boat hulls, garbage cans, light globes, ice buckets, appliance housings, and toys .The technique is applicable to most thermoplastics but is most widely used with polyethylene.

Rotational molding offers a number of a advantages :

1. Virtually unlimited design possibilities (parts as small as a golf ball to a 22,500- gallon agricultural tank). 2. Relatively low machinery cost. 3. Low tooling costs. 4. Economical prototyping. 5. Strong outside corners in virtually stress-free parts. 6. Part finish from matte to high gloss. 7. Simultaneous processing of multiple colors. 8. Simultaneous processing of different parts. 9. Quick mold changes. 10. Possibility of moulding in metal inserts. 11. Molded-in multicolor graphics. 12. Multilayer moulding for chemical resistance or strength. 13. Double-walled parts molding for additional rigidity. 14. Possibility of minor undercuts. 15. Virtual 100% usage of material (no scrap).

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MOULD THEORY 18


How It Works 

There are essentially four basic steps in rotational molding: loading, molding or curing, cooling, and unloading. In the loading stage, either liquid or powdered plastic is charged into a hollow mold. The Mold halves then are clamped shut and moved into an oven where the loaded mold spins biaxial. Rotation speeds should be infinitely variable at the heating station, ranging up to 40 rpm on the minor axes and 12 rpm on the major axes. 4:1 rotation ratio generally is used for symmetrically shaped objects, but a wide variety of ratios are necessary for molding unusual configurations.



In the oven, the heat penetrates the mold. Causing the plastic, If it is in powder form, to become tacky and stick to the mold surface, or if it in liquid form, to start to get. On most units, the heating is done either by air (as in a units, gas fired hot-air oven) or by a liquid of high specific heat, such as molten salt; where jacketed molds are used (see below), heating is cone with a hot liquid medium, such as oil

THERMOFORMING MOULDING . 

Thermoforming is the process of heating a plastic material in sheet form to its particular processing temperature and forming the hot and flexible material against the contours of a mold by mechanical means (e.g., differentials in air pressure created by pulling a vacuum or using the pressures of compressed air). When held to the shape of the mold and allowed to cool, the plastic retains the shape and detail of the mold. Because softening be heat and curing by the removal of heat are involved, the technique is applicable only to thermoplastic materials and not to thermo-sets.



Examples of thermoformed products are plastic or foam dinnerware, cups, meat and produce trays, egg cartons, refrigerator liners, computer housings, interior and exterior automotive parts, blisters for packaging, and countless others. These common and often taken-for-granted products are not usually thought of as the result of detailed tooling design, precise controlled heating and forming, expert material technology, and trimming/ finishing operations. Clearly, the thermoforming process is an important link in the plastics industry.



Advantages of thermoforming over most other methods of processing plastics include lower tooling and machinery costs, high output rates, the ability to use pre decorated plastic sheet, and good- quality physical properties in finished parts.



Its disadvantages include the need to begin with sheet or film rather than less costly basic resins, trimming material used to clamp sheet for forming, and the problem of trim scrap reclamation.

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MOULD THEORY 19


Compression Moulds are classified into three main categories

Hand moulds Semi-automatic moulds  Automatic moulds  Hand Moulds Hand Moulds are usually small in size and do not weight more than about 10kg for ease of handling. The material is put into the cavities and the two halves of the mould are assembled and placed between the platens of the press. After the press has been closed and parts molded, the mould is removed from the press closed and the parts molded, the mould is removed from the press and the parts ejected from the mould on a conveniently located bench. The process is then repeated. Hand moulds are used for making sample parts, or when a limited number of small parts are to be produced. Hand moulds, being removed from the press, facilitate the loading of inserts. 

Semi-Automatic Moulds

Vary greatly in size. These moulds are mounted on to the platens of the press by clamps. The mould is loaded with material and the two halves close. After the pieces have been molded some sort of ejection mechanism pushes the parts out of the cavity or off the plunger. The press operator then places the molded parts in a suitable container or on a bench for flash removal etc. The operator cleans the mould of the excess material and the process is repeated.

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MOULD THEORY 20


Automatic Moulds 

Are similar to semi-automatic moulds but do not need and operator once the mould has been set up for production run. The automatic moulds, through the use of combinations of loading devices, timing devices positive ejector systems, sweeps, micro-switches, safety devices and cleaning apparatus, lend themselves well for molding many articles. There is a purpose for each type of mould, and many factors such as the size of the piece part and the production requirements, determine which of the three is used.

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MOULD THEORY 21


ASSIGNMENT:-

1.

««««. &«««««. are two main members of a mould.

2.

««««. is a female part of mould, which gives the external shape of the component.

3.

«««. is a male part of mould which gives the internal shape of the component.

4.

«««« is used to located the mould into the moulding machine platen during loading a mould

5.

«««. Act as a channel through which molten material flows from the machine nozzle to the cavity.

6.

««..forces the ejector assembly to its original position.

7.

Bottles are manufactured by «« moulding process.

8.

Food packaging products are manufactured by ««« moulding.

9.

Pipes are manufactured by------ moulding process.

10.

Big size plastic products like water tank are manufactured by ------process.

11.

««««.. is the most commonly used method for thermoset plastic materials.

12.

«««.. is a semi solid form of plastic material enclosed in blow moulding process.

13.

««. Materials are used in injection moulding.

14.

Incase of bottles having thicker cap area or having variation in thickness at cap portion are manufactured by «.. blow moulding process

15.

......, «.., «.. & «.. are four basic steps in rotational moulding.

16.

Thermoforming moulding method is applicable only for «.. «.. material.

17.

««. are usually smaller in size and operated manually.

18.

«««. & «««. are two methods of thermoforming moulding process.

19.

««.. pins are used to pull the moulded sprue out of the bushing after processing.

20.

««« & «««.. are used to align the two halves of the mould base.

21. 22. 23.

««. & ««.. are two methods of blow moulding process. The taper angle of sprue bush generally varies from «« to «««« r «« gives a proper landing to the ejector assembly and provides provision for easy assembly of mould ejector.

24.

Generally the component shrinks on to the ««. after opening.

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MOULD THEORY 22


INTRODUCTION OF COMPRESSION MOULD

OBJECTIVE:-

After end of the lesson/session trainees should be able to

- Explain the types of compression moulds - Distinguish different types of compression moulds.

LESSON 3

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MOULD THEORY 23


INTRODUCTION OF COMPRESSION MOULD:

Compression and Transfer Molding Processes are mainly used for the manufacture of thermosetting plastic products such as Electrical Switches, Cooker Handles, Miniature Circuit breakers, Ceiling roses, Switch gears, Switch boxes and also various consumer oriented products. To some extent this process is also used for thermoplastics components which are otherwise not possible to produce by other conventional processing methods due to processing limitations. Example includes PTFE liners, Gears and Washers, UHMHDPE products etc. Several technological advancements have taken place in these areas which are mainly aimed at improving the quality of the product and enhancing the production rate Robotics and process automation with microprocessor based on control systems have revolutionaries, the entire compression and transfer molding line and specially designed thermo set injection molding process.



The quality of products largely depends upon the machine and the mould. In this context, the requirement of precision mould can not be less emphasized. Factors that Influence Compression Molding:

Three important factors that must be controlled in compression molding are:   

Temperature Pressure Cure Time

Temperature :-

Thermo-setting materials which are used in compression molding are cured by heat and pressure. Heating is and important phase in the molding operation. Heat softens the material sufficiently to allow it to flow under the influence of the press pressure into any openings in the mould to the desired shape. Heat causes a chemical change or polymerizes the material into its hard infusible finished state. Temperatures for molding thermo-setting materials vary from 270°F to 350°F. Temperatures for molding the various materials can be determined by experimentation or by getting the information from the manufacturer of some particular material. Temperatures the finishe4d article. Temperatures which are too low do not allow the material to flow properly and result in incompletely filled cavities and insufficiently cured piece parts of poor consistency. Mould temperatures must be maintained within 5°F. For best results molding temperatures not only vary with the material used, but with the geometry of the molded article, the type of mould, and whether loose powder or pre-heated performs are used.

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MOULD THEORY 24


Methods of Heating:

The two methods most commonly used in heating press platens and moulds using thermo-setting materials are steam and electricity. Steam is favored by many molders because of its economy and heating qualities. Electric heating of moulds is replacing steam in many instances because of its cleanliness and lower maintenance costs. Pressure:-

Pressure as well as temperature must be regulated in order to produce satisfactory parts economically. Pressure needed to mould a particular article depends on the flow characteristics of the material, the cavity depth, and the projected are of the piece part. Generally, It is recommended that minimum molding pressure of 225kg/cm² of projected area be used. However, in practice, about 375kg per Cm² of projected area is used to compensate for any variables that may be encountered. Cure time:-

Varies with the materials use. The size and shape of the molded article, and the method of molding. In compression and transfer molding, cure time is the time elapsed when the movement of the press stops until the pressure on the molded part is released. For smaller and thin wall pieces, cure time may be only a minute or two. On larger pieces, and pieces with thick sections, the cure time may be as high as 15 minutes.

Vents:

During the process of compression molding, gases are formed as the chemical reaction takes place in the material. Some provisions must be made to get rid of the gases otherwise poor quality piece parts will be the result. Gas pockets can cause incomplete shots or blistered piece parts. One method of getting rid of the gases is to allow the mould to breathe that is the mould is Vent placement

Vent depth

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closed and then opened again for about 3mm to get rid of the gases and then closed again. Other methods used are in small flats ground on the periphery of the plunger that telescopes ground on the periphery of the plunger those telescopes into the cavity. Small grooves 0.050 to 0.25 mm deep and about 3mm wide ground into the top portion of the cavity wall. This type is used on large cavities.

MOULD THEORY 25


Loading of Material in Compression Molding:

To prevent the wastage of material for efficient molding, it is important to load the proper amount of material into the mould. This is done by one of the following three ways: 1. Volumetric 2. Weight 3. Pre-determined or Pre-forms 





The volumetric method is most efficient when used with ferrous materials. A containner of predatermined volume is filled with material and the contents poured into the cavity. The weight method is used with materials in which the amount of material cannot be controlled rea-dily by volume. A predetermined amount of material is weigh-ed on a balance and then placed into the mould. The third method used is a predetermined amount of material which has been compressed into pre-form of desired shape size and weight. The use of performs is a very efficient method of loading moulds.

Hand compression mould   

Smaller in size and weight not more than 12kg for Easy Handling. Production is limited to small parts, short runs or Prototype Work. Overall height of the molding is controlled by land areas on mating surfaces of the top force and cavity Maximum Density of the molding may be maintained by clearance between the side wall of the top force and the cavity. Used as a single cavity mould on a Rotary

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MOULD THEORY 26


Press and molding pressure is controlled at each station. Flash must be removed in the land area otherwise it will result in damage or breakage of the lands. It is advisable to provide additional pressure pads outside the cavity. Positive Mould (Direct & Landed) 

The POSITIVE type of compression mould. The plunger telescopes within the cavity compressing the material and molding the piece as shown. There is very little clearance between the plunger and the cavity wall. In the positive mould, almost all the pressure is exerted on the material and very little material is allowed to escape, the clearance between the plunger and cavity varies between 0.036mm to 0.130mm per side, depending on the size of the mould and the material to be molded. The amount of material that escapes through the clearance between the plunger and cavity is called flash. The flash is formed vertically on the type of mould shown. The disadvantage of the positive type of mould is that after frequent operation the cavity walls become scored and ejection of piece parts is difficult. Flash is formed on every piece part molded by the compression method. The thickness and position of this flash depends on the design of the mould, type of material being molded, and accuracy of the mould. Flash is removed by filling, sanding, and tumbling. The positive mould is used primarily with material containing coarse fillers. The amount of material placed into the mould cavity must be measured accurately as there is very limited means for the excess material to escape.



Used for high bulk materials and large deep draw parts when maximum density is required. It is single cavity mould and uses and accurately weighed charge of material. All the applied pressure is exerted on the material. Flash between the top & bottom force can be produced in the direction of pressure. Flash thickness varies according to the clearance between the loading chamber and top force.

   

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MOULD THEORY 27


Semi-positive Moulds:

The principle of the semi ± positive type of mould. As the two halves of the mould begin to close, the mould acts much like a flash mould as the excess material is allowed to escape. As the plunger telescopes into the cavity, full pressure is exerted on the material and produces piece parts of maximum density. The mould becomes a positive mould for the distance X as shown on the illustrations. The distance X varies with the size of the mould and material used. There is very little clearance between the plunger and side walls of the cavity, which results in a very thin vertical flash being formed. This type of mould takes the advantages of the free flow of material in a flash mould and the quality of producing dense parts of the positive mould.

Semi-positive, vertical flash type 

It controls maximum density and critical dimensions as related to cavity and top force.



Easy removal of flash on large parts and leaves no flash line scar on the side of the parts

Semi-positive Horizontal flash type 

It is less costly and more popular.



It is recommended for close tolerance parts and assures minimum flash finish.

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MOULD THEORY 28


Flash Moulds (Open Flash) 

The FLASH type of mould. The cavity is filled with material and the excess is squeezed out over the lands which are about 3mm. Wide. External landing bars are provided so the plunger does not crush the top of the cavity when the mould is completely closed. Clearance between 0.050mm and 0.125 m is provided so excess material can escape and the cavity is not damaged. Fig. shows the flow of the material and illustrates the thin horizontal flash formed with this type of mould. The flash type mould is not generally used with coarse-filled materials or for pieces which require a high density. The flash type mould lends itself well to shallow-depth articles such as dinner-ware plates and saucers.

Split wedge Mould:

Another type of mould construction is the split wedge or split cavity mould. This mould is used primarily for articles that have undercuts, such as spool shaped pieces. These projections or under cuts prevent the piece part from being removed from a conventional mould cavity. In order to overcome this difficulty, the cavity is constructed in to two or more sections. When these sections are together, the inside surface has required shape, and the exterior of these sections has a wedge like shape, fig

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A & B - Conical Cups C ± plunger D ± lower heating plate E ± upper heating plate F - mould casing L ± heating bandage G ± location dowel & CAM combination K ± knock out pin

MOULD THEORY 29


illustrates one type of split wedge mould in

closed position. Fig; illustrates the semi-automatic type of construction with the mould mounted on the platens of the press, fig; shows the two sections of the cavity split, the projections on the piece part released from cavity, and the piece part ready for removal from the mould. As the mould is closed, the spring loaded pins ± heads of which are in T-slots in the wedges ± pull the wedges up the wear plates on the angle shown and the two halves of the cavity close. By proper timing of the knock out system which controls the action of the pins, the cavity is closed before the plunger enters the cavity. In the type of mould shown in fig; per forms are generally used and are placed on the top of the plunger. After the piece part has been molded, the plunger is withdrawn and the pins are activated to push the wedges down the wear plates, opening the two halves of the cavity. The wedges are held along the wear plates by T-slots and keys. Landing buttons or bars are provided for the wedges to make certain that there is no gap between the two halves of the cavity at the parting line when the mould is closed. This type of mould, some times called a basket mould, is mounted on the press as shown because of the ease of removal of molded pieces and ease of cleaning the mould. Moulds of this type can be of single as well as multiple cavity variety

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MOULD THEORY 30


ASSIGNMENT:-

1. Compression and transfer moulding is used for ------materials. 2. ------ are used for articles having undercuts. 3. ------plates are used on the headed block or split to resist wear and can be removed. 4. ------, ------ & ------are three main factors that affect the compression moulding process. 5. Temperatures for moulding thermosetting material vary from ------ to ------ rF. 6. ------ Or ------are generally used raw materials for compression moulding. 7. The overall area of the component top view is known as ------. 8. The time taken to get the thermoset material permanently asset is known as ------. 9. As the thickness of the moulding increases the area time of the process------. 10. ------ are given in the mould to escape out the air from the cavity on the moulding. 11. Generally the width and depth of the vents are ------ & ------ respectively. 12 as the projected area increases the pressure ------.. 13. What is positive type compression mould? 14. What is semi positive type compression mould? 15. What is the difference between semi positive vertical flash type and horizontal flash type mould? 16. What is the use of knockout plate in compression moulding.

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MOULD THEORY 31


TYPES OF TRANSFER MOULDS


- Explain the functional feature of compression mould & transfer mould. - Explain the application & limitation of compression mould & transfer mould. - Select the mould & moulding process for a particular given component.

LESSON 4

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MOULD THEORY 32


TYPES OF TRANSFER MOULDS

 

Transfer moulds are of the semi automatic type and are made in single and multiple cavities, There are two basic types of transfer mould according to construction; pot transfer mould plunger transfer mould

Pot Transfer Mould:

The pot transfer is used in the conventional compression moulding press. Figures illustrate the pot type transfer mould and the functioning of this type of mould. Fig. Shows the mould in the position in which the pre-form is loaded into the pot or loading chamber. The pre-form is shown in place. The arrow to the right indicates the movement of the mould in the press. Procedure:

The material is heated by the hot mould and combined with the pressure 0f the pot plunger, the material becomes fluid and is forced into the sprue, runners, gates, and into the cavity to form the piece part. This is illustrated in Fig. Pressure is kept on the material until the piece parts are cured. Excess material forms a cull at the bottom of the pot and also forms a dovetail shape in the plunger called the cull pickup. After the parts have been cured, the mould opens as shown in Fig. As the plunger comes out CONCEPT DRAWING of the pot, the sprue is broken at the small diameter of the taper. The cull pickup on the plunger carries the cull out of the pot and the spure out of the spure bushing. As the press continues to open the down ward movement of the plunger plate is halted, while the rest of the mould below the parting line continues to move. The mould is constructed as that the molded pieces remain in the cavities. Continuous movement of the press activates the knockout bar and the molded parts are ejected from the cavities. The mould in the completely open position is shown in Fig.

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MOULD THEORY 33


To protect the edges of plunger, a soft headed mallet is used to drive off the cull pickup and cull from the plunger. The piece parts are removed from the mould and the runner are scrap and cannot be re-used. The piece parts are set aside for further finishing if necessary. All excess material is removed from the cavities pot and plunger. The mould is closed until the parting line is mated and the mould is again in loading position. (See Fig.) The pot is then loaded and sequence is repeated. Fig illustrates another version of the pot transfer type of mould. In this design, the material flows directly from the sprue into the cavity; no runner or gates are used. If the material is forced directly into the cavity as shown in Fig. , the small diameter of the sprue should be constructed as shown in Fig., A break point is furnished so the sprue does not pull material from the surface of the piece part. The operation of the mould follows the same sequence as described for the previous transfer mould. Another variation has two sprues feeding the material into a single cavity.

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MOULD THEORY 34


Plunger Transfer Mould: 

The plunger type of transfer mould in the closed position with the pre-forms in the target area and the plunger on its down ward stroke. The combination of the heat of the mould and the pressure of the plunger on the pre-forms causes the material to become fluid and to flow through the runners and gates into the cavities. The plunger transfer differs from the pot transfer in that the plunger is part of the molding press and not a part of the mould itself. By the use of this plunger, the sprue is eliminated and very thin cull of small are is formed above the target are, thus reducing the loss of material. The top clamp plate is fastened to the stationary platen of the press. The arrow at the right indicates the movement of the mould base at the parting line. This type of mould is loaded in one of two ways.



The pre-heated pre-forms are placed or stacked in the target are. As the mould is closed, the pre-forms are lifted into the transfer sleeve. After the mould is completely closed, the plunger is activated for its downward stroke.

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MOULD THEORY 35




The mould is closed, and the pre-forms are loaded into the closed mould through the opening at the top of the transfer sleeve. The plunger is then activated, forcing the material throughout the mould.



Fig. Illustrated two other methods used in feeding the material into the runners in a plunger type transfer mould. The view at the left shows the construction when the round type of runner is used. In order to maintain a constant volume flowing into the runner system, the runners are machined at an angle in the runner plate. See sections A-A and B-B. On the right is shown the construction used for trapezoidal runner. See section C-C Distance X should be two or three times the width of the runner. The type of loading depends on the height and size of the press used.



It is recommended that a clamping pressure 700 to 800 Kg per square Cm be used to keep the mould together at the parting line. Pressure on the transfer plunger is generally about 550 kg per cm². All transfer moulds must be vented to allow air to escape from the cavities.

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MOULD THEORY 36


DESIGN OF POT AND PLUNGER:











 



Both the pot and plunger are made from a good grade of wear resistant tool steel which is heat treated and ground. Pots and plungers are made square, rectangular or round in shape. The shape is determined by the shape of the piece part, number of cavities, and available space in the mould base. Round pots and plungers are preferred because less machining difficulties are encountered. A clearance of 0.025 to 0.075 mm per side is provided between the pot and plunger. The area of the pot should be 20% to 30% greater than the area of all the cavities and runners. The dimensions of the pot, if it is round or square, can be calculated once the area is known. To determine the volume of the pot, the total volume of all the piece part, the runner and the sprue, plus a small amount for a 0.375 to 0.75 mm thick cull, is approximately calculated. At least twice this volume is to be used for the pot volume. Knowing the area and the volume, one can easily arrive at the depth of the pot by dividing the volume by the area. The additional volume in the pot is provided to compensate for the bulk factor of the pre-forms used and to allow the plunger to enter the pot a short distance before exerting pressure on the material. High bulk factor materials are generally not used in transfer moulding. The bulk factor of the pre-forms used in transfer molding is approximately 1 to 2.Figs. illustrate some of the construction details of the pot and plunger. For sufficient strength, horizontal distance ³Y´ should be equal to the depth of the pot ³Y´, - A1.5 to 3mm radius is provided at the top edge of the pot. A1.5 mm radius is machined at the bottom of the pot to facilitate the flow of the material and to simplify the machining of the corner. A 2.5 to 3mm radius is machined at the bottom of the plunger. The difference in the radii on the plunger and the bottom of the pot results in a clearance so that the plunger will not wedge in the pot but will land on the flat surface of the pot. In assembly there is a small clearance between the plunger and the transfer pot. In practice it was very difficult to maintain the clearance for long time. Fitting the plunger to the cold pot size ± the plunger staying relatively cold during operation but the pot having to heated ± the clearance widened between them allowing material outflow. Adjusting the plunger to the chamber when expanded by heat prevented its entrance in the cold state was difficult. So it frequently occurred that the plunger was forced into the pot strongly on clamping the mould and there was seizing the cracking and other damage occurred. This problem is solved by adjusting the plunger to the cold pot with running fit, but permitting material flow around the plunger so as to form a collar when solidified according to the hot pot size, thus preventing material flow out, during molding. A sealing groove approximately 2.5 mm wide and 0.8 mm deep is cut into the perimeter or periphery of the plunger. During the operation of the mould this groove fills with the molding material and acts as a natural sea, allowing very little material to escape past the bearing surface of the plunger. Flats or grooves are ground on the bearing surface of the plunger for venting purposes. A clearance of 0.75 mm per side is machined above the bearing surface of the plunger. This clearance keeps the bearing surface narrow to prevent galling, and allows flash and excess material to

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MOULD THEORY 37


escape. The sprue and the interior or the pot is polished so the material can flow easily. The sprue has a taper of 2 to 3 per side. The large diameter of the sprue varies in size from 9 to 12 mm with a 1.5 to 3 mm radius at the entrance of the sprue. The small diameter (at the runner or piece part) varies from 3 to 6 mm depending on the size of the piece part. Wedge-shaped slots called cull pickup are machined in to the plunger. The thick or heavy section of the cull pickup is located directly above the sprue as shown in fig. The length of the cull pickup varies from the width is generally 2 to 3 times the diameter of the sprue.

CALCULATION OF POT DIMENSION IN TRASFER MOULD

The clamping pressure provide by the chamber is an important consideration,  If the total cavity area is greater than the total pot area , the hydraulic pressure exerted by the plastic compound would tend to open the mould at the parting line.  So insure perfect mould locking, the area (Ap) should be 25% to 30% greater than the combined area of the molding surface and the area of all runners and sprues. The dimension of pot, If it is round or square can be calculated, once the area is known. Total area of pot Ap = total projected area of cavities, runners and sprues + 25% to 30% of total projected area. 

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MOULD THEORY 38


Volume of pot Vp

= total volumes of all the piece parts, runners and sprues + Approximate volume of a small amount of .5 to 1mm Thick cull multiplied by bulk factor of the compound. Depth of pot = Vp/Ap

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MOULD THEORY 39


ASSIGNMENT:-

1. What is the use of transfer mould? 2. How many types of transfer moulds are there? 3. What do you mean by cull and what is its purpose? 4. What is plunger transfer moulding? 5. What is the use of transfer sleeve in plunger transfer mould? 6. The area of the pot should be ------greater than the area of component and runner. 7. How the volume of the pot can be calculated. 8. What is bulk factor? 9. Why a clearance is provided in the bearing surface of the plunger. 10. ------to ------taper angle given in the sprue. 11. Depth of the pot is calculated by ------. 12. What is champing pressure?

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MOULD THEORY 40


MOULD DESIGN CONCEPTS FOR PLASTICS

OBJECTIVE:-

After end of the lesson/session trainees should be able to - know the injection mould - know the injection moulding process - know the injection moulding machines - know the injection mould parts materials Flow chart for designing injection mould - Different steps for designing injection mould

- Know the calculations point of view - Know the different parts of moulding machine - Explain the mechanism & functional feature of moulding machine. - Specify the moulding machine according to production requirement. - Explain the safety precautions to be taken during operation

of machine.

LESSON 5

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MOULD THEORY 41


MOULD DESIGN CONCEPTS FOR PLASTICS INJECTION MOULD DESIGN 

Injection moulding is one of the most versatile processing methods by means for manufacturing manufacturing small clips to large l arge industrial crates. Technological advancement advancement was taken place over the last two decades. Sophisticated Micro processed Control of injection moulding; Structural From Molding, TWO COLOUR INJECTION MOULDING , Gas Assisted injection Moulding etc., are gaining more and more popularity in developing countries. Precision moulds are essential to meet the stringent quality requirements of the end product. With this point in view the necessary in put data required for the mould manufacture in the area of design and fabrication of moulds has been provided in this chapter.



It is not for being true to say that there are probably as many design for injection mould as there are different plastics products, each product to be molded has its own peculiar problems which most be considered when designing the moulds.



From a practical point of view, a classification of injection moulds should be based on the main design features and manner of operation.

These includes    

The type of gating and means of de-gating The type of ejection The presents or absence of external & internal under-cuts on the products The manner in which the product is released form the moulds

The different types of in of in jection moulds are;      

Standard moulds (two or three plate moulds) Split cavity moulds Stripper plate moulds Stack moulds Hot runner moulds Insulation runner mould

of in jection moulding: Principle and process of in 





The process of injection moulding essentially consists of plasticize the raw material in a cylinder by the application of heat and then injecting it under pressure through a nozzle by means of a ram into a closed mould, where it is allowed to cool, and then opening the mould and removing the moulded component. There are two methods employed to inject the plastic material into the mould: and another by means of a one by means of plunger of plunger and of a screw which transports the materials during which time it is plasticized and then it is injected. Shows a simple injection mould in the closed position after a shot has been made shot is a term used to describe the total amount of material that has been injected into the mould in one cycle, including piece parts, runners, gates, and sprues.The material material is placed in a hopper, in the form of a granules or pallet; the

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MOULD THEORY 42








hopper is located at the end of and electrically heated cylinder. To distribute the heat evenly throughout the material The spreader causes the material to flow against the wall of the heating cylinder. The material heated in this cylinder softens and the plunger forces the molten material (when has the consistency of thick syrup) through a nozzle into a sprue and fills any opening in the closed mould. The material cools in the relatively cold mould and hardens to a solid state. The injection plunger is retracted, the mould is opened at the parting line, and the piece parts are ejected from the mould. As the mould opens, the sprue puller pulls the sprue out of the bushing. The sprue is separated from the molten material at the small diameter at the nozzle as shown in Fig. The piece parts, runners, gates, and sprue are ejected from the mould as a unit. The place piece parts are then removed from the sprue and runners at the narrow gates. With the mould in the open position and the injection plunger retracted, material is fed into the heating cylinder, the mould closed and the sequence repeated. Illustrate the simple gravity type of feeder arrangement. Other more practical and efficient methods of feeding material into the heating cylinder include volumetric, weighs type, and pre-plasticizers. pre-plasticizers.

List of mold of mold parts and corresponding steel to be used

SL.NO

PARTS NAME

1 2

Top plate Cavity plate

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

MATL.

HRC

MS MS, Tool steel (P20, EN8-31-24) Core plate MS, Tool steel (P20, EN8-31-24) Core back plate Ms/ EN8 Spacer or Riser Ms/ EN8 block Ejector plate Ms/ EN8 Ejector back plate Ms/ EN8 Cavity back plate Ms/ EN8 Core P20,H11,H13,EN24 P20,H11,H 13,EN24 Cavity P20,H11,H13,EN24 P20,H11,H13,EN24 Locating ring Ms Sprue bush P20,OHNS,H11,H13 P20,OHNS, H11,H13 Pillar EN36,31,24 EN36,31,2 4 Bush EN36,31,24 EN36,31,2 4 Spure puller Carbon steel, D2 Ejector pin Carbon steel, D2 Pushback pin Carbon steel, D2 Or Return pin Rest button Carbonsteel,D2,En31 Carbonst eel,D2,En31

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MOULD THEORY 43

REMARKS

50-52 50-52 45-50 48-50 48-50 48-50 48-50 48-50

STD STD STD

48-50


FLOW CHART FOR METHODICAL INJECTION MOULD DESIGN . PART DESIGN , REQUIDE QUENTY ORDER, NO. OF PICES / TIME

NO OF CAVITY

SELECTION OF MOULDING M/C

TYPE OF MOULD THREE PLATE MOULD

TWO PLATE MOULD HOT RUNNER

HOT RUNNER

COLD RUNNER

COLD RUNNER

CONVENTIONAL SPURE

LAY OUT OF CAVITY STAR ARRANGEMENT

SYMMETRICAL

IN LINE

TEMP. CONTROL SYSTEM

CORE/CAVITY

SURFACE OIL

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OIL

WATER

MOULD THEORY 44

WATER


TYPE OF EJECTION PIN

STRIPPER

RING/PLATE

AIR

SLEEVE

VALVE

TYPE OF VANTING PARTING LINE

CORE INSERT

EJECTOR PIN

MOULD MATERIAL

MOULD BASE

INSERT / MOULD PL .

CORES

ANTICIPATED SHRINKAGE

PART DESIGN

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GATE

MOULD THEORY 45

PROCESSING CONDITION


SIMPLE 13 STEPS FOR PERFECT MOULD DESIGN 

STUDY OF PLASTIC COMPONENT

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STUDY OF MOULDING M/C TO BE USED



METHOD OF CONSTRUCTION OF MOULD



DETERMINATION OF NO. OF CAVITY



SELECTION OF PARTING SURFACE



TYPES OF LAYOUT OF CAVITYS



SELECTION OF RUNNER



SELECTION OF GATE



SELECATION OF TYPE OF MOULD



COLLING SYSTEM

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TYPES OF EJECTER GRID

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EJECTION SYSTEM

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MOULD VENTING

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MOULD THEORY 46


STUDY OF PLASTIC COMPONENTS

1. material, 2.shrinkage, 3. quantity, 4. density, 5. accuracy, 6. shape and size  

Material - material with which the component is to be moulded. Shrinkage ± after cooling how much mm. per mm length will shrink. MATERIAL

SHRINKAGE

Acrylonitritle butadiene styrene (ABS)  High impact Heat resistance   Medium impact Acetal Acrylic  Easy flow  Heat resistance  High impact Cellulose acetate  Hard /medium / soft  Cellulose acetate butyrate

DENSITY

0.005 - 0.007 1.01 - 1.04 0.004 - 0.005 1.06 - 1.08 0.005 1.04 - 1.07 0.020 - 0.035 1.42 0.002 - 0.007 1.09 - 1.14 0.002 - 0.009 1.11 - 1.18 0.003 - 0.010 1.09 - 1.14 0.004 - 0.008 1.09 0.002- 0.005 0.002 - 0.005

1.22 - 1.34 1.15 - 1.22

0.007 ± 0.016 0.010 ± 0.025 0.010 ± 0.025 0.010 ± 0.025 0.008 ± 0.020 0.004 ± 0.006 0.005 ± 0.010

1.12 ± 1.14 1.07 1.08 ± 1.14 1.04 ± 1.05 1.01 ± 1.02 1.23 1.34 ± 1.42

0.015 ± 0.035 0.015 ± 0.030 0.010 ± 0.030

0.92 ± 0.925 0.941 ± 0.965 0.860 ± 0.960

Polystyrene (PS)  General purpose  Heat resistant  Toughened

0.002 -0.008 0.002 -0.008 0.003 -0.006

1.040 ± 1.100 1.040 ± 1.100 1.040 ± 1.100

Poly tetraflourethylene (PTFE)

0.050 -0.100

Poly vinyl chloride (PVC)  Unplasticized  Rigid  Semi rigid  Flexible Styrene-acrylonitrile (SAN) Poly carbonate (PC)

0.002 -0.004 0.002 -0.004 0.005 -0.025 0.015 ± 0.030 0.002 ± 0.006 0.006 ± 0.007

nylon

Type ± 6 Type ± 6.1  Type ± 6.6  Type ± 11  Type ± 12  Transparent  Glass filled Polyethylene (PE)  Low density  High density  Polypropylene  

1.35 ± 1.45 1.16 ± 1.35 1.00 1.075 ± 1.100 1.20

Quantity ± as per requirement Density ± depends upon resine used( to be decided form table ± 1) Accuracy - as per components used Shape and size ± to be studied from component

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MOULD THEORY 47


STUDY OF MOULDING MACHINE TO BE USED In jection Moulding Machines: 

The injection moulding machine essentially consists of two units namely the mould clamping unit and the injection unit. The mould clamping unit performs the function of closing the mould, locking the mould and opening it, the injection unit. The mould clamping unit performs the functions of closing the mould, locking the mould and opening it. The injection unit plasticize the material and injects it in to the moulds. Fig. Shows an injection moulding machine. The left side is the clamping unit and the right side the injection unit.



The mould clamping unit comprises of the fixed, movable and end platens. The cover half of the mould is mounted on the fixed platen and the ejector half of the mould is mounted on the moving platen. The fixed and end platens are held by four tie bars and the movable platen moves over the four tie bars. There are, in general, two methods employed to move the movable platen and to apply the locking force these are direct hydraulic locking system and the mechanical toggle system.



The injection unit is required to plasticize the material and inject it into the mould cavity and exert a holding pressure on the molten material in the mould cavity. Almost all of the injection moulds are of the semi ± automatic or automatic type. Depending on the size of piece part and the size of press available, moulds are made in single or multiple cavity. A variety of ejector systems, which are generally an integral part of the mould, are used to eject the piece parts, runners etc., for the mould. All injection moulds are cooled by having water circulate through channels drilled into the various part of the mould. Injection moulding offers vary efficient and economical method of

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MOULD THEORY 48


moulding pieces from a wide range of thermoplastic materials. Parts moulded by this method possess very little or no flash, thus eliminating secondary operation of sanding, filing, and tumbling. Very high production rates are possible because of shorter moulding cycles than in moulding thermo ± setting materials of comparable size. There is no wastage of material in the injection moulding of the thermo ± plastic materials, as the scrap, runners, sprues, and incompletely ± moulded pieces are reground and remoulded.

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MOULD THEORY 49


SHOT CAPACITY

The plunger type machine is often rated in terms of maximum SHOT W EIGHT (in gm) with Polystyrene. The rating in terms of another material can be determined as follows: Shot capacity with material B

= (Shot capacity with material A) X Density of B X Bulk factor of A Density of A Bulk factor of B W here,

A = Polystyrene B = Plastic to be used

The screw type machine is normally rated in terms of S W EPT VOLUME (in cc) of the injection cylinder. Shot capacity = Swept volume (cc) X V (gm /cc) X c W here,

V = Density of plastic material at normal temp. In gm/cc c = 0.85 for crystalline Material. = 0.93 for Amorphous Material

Mater ial

Bulk f actor

Mater ial

Bulk f actor

ABS, A ACET AL, AC ACRYLIC

1.8 - 2.0

PP

1.9 - 1.96

S AN AN

1.9 - 2.5

POLYTHENE

1.8-2.3(LD) 1.7-1.9(HD)

NYLON

2 .0 - 2 .1

POLYSTYRENE

1.9 ± 1.96

POLYC AR ARBON AT ATE

1.75

PVC

2.3 (Rigid) 2.2 (f lexible)

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MOULD THEORY 50


Plasticizing capacit y

Plasticizing rate of B (m/hr)

=

Plasticizing rate of A (gm /hr) X QA QB

W here,

A - Polystyrene B - Plastic to be used. QA -Thermal Capacity of A in cal/gm Specific Heat in (cal /gm) X moulding temp. Of A QB - As same as A for B =

Plasticizing rate (gm /hr) =

wt of moulding (gm) X (number of mouldings / hr)

For maximum efficiency machine should not operate above 80% of its rated shot weight or plasticizing capacity

Cycle time (tc) when press is limited by plasticizing capacity is given by

tc W here,

MATERIAL

=

m×3600 P×K

m = weight of shot (g) P = plasticizing capacity with polymer to be moulded and K = 1000 (metric unit). SP. HEAT

MOULDING TEMP.

MOULDING TEMP.

SP. HEAT

MATERIAL

ABS

0.35

225

250

0.38

NYLON-6

AC ACRYLIC

0.035

225

300

0.30

PC

ACET AL

0.35

225

240

0.55

PE

PP

0.46

250

180

0.24

PVC

PS

0.32

200

200

0.35

C AB AB

S AN

0.33

220

195

0.36

C A

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MOULD THEORY 51


PLATEN SIZE

This includes  Distance between tie ± bars  Maximum Day lights  Size of mould plates  And other details of platen from machine manual CLAMPING FORCE

The clamping force required to keep the mould closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners. For plunger type Machines

Clamping force (tons) Pro jected area of mouldings Including runners in cm2 X 1/2 to 1/3 of in jection pressure in kg / cm2 =

For Screw type machines 2/3 to 1/2 of in jection

pressure should be taken.

INJECTION PRESSURE

The injection pressure may be obtained from manual or may be calculated as follows: In jection pressure (kg / cm2) Hydraulic line gauge pressure (kg / cm2) x (In j. Cylinder Dia in cm) ² (Heating cylinder ram dia in cm) ² =

(Di²) / (Dp²)

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=

20 on many plunger type machines

MOULD THEORY 52


ASSIGNMENT:-

1. ------& ------ are mainly two main units of an injection moulding machine. 2. Ejector half of the mould is loaded in the------ of the machine and the other half is located on fined plate. 3. ------ and ------ systems are two methods used for clamping system. 4. Plastic granules are fed into the injection unit through-----5. The process of pulling the ejector assembly of a mould during closing a mould is known as ------. 6. ------ and ------ Are used for cooling the mould during running. 7. The left hand side of an injection moulding machine is the------ and right hand side is ««.. . 8. ------is used to plunge the method from the injection chamber to the mould.

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MOULD THEORY 53


METHOD OF CONSTRUCTION OF MOULD


-Know the method of construction of mould for a given component -Determine the no. of cavities for a specific component as per the requirement -Know

the designing of guide pillars & guide bushes

-Explain the use & different types of guide pillars &guide bushes -Know the designing of locating ring -Explain the functions & use of different types of locating ring -Know the designing of sprue bushes -Explain the use of sprue bushes in moulds -Know the suitable layout of cavity for a specific component -Know the selection of appropriate runner for a specific component

LESSON 6

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MOULD THEORY 54


METHOD OF CONSTRUCTION OF MOULD Integer Method When

core and cavity is made in a single block of steel without any bolstering is

called integer method. In these method two halves contains two solid blocks only. USE 

Single impression mould.



For more strength.



Big component.

Disadvantages: 

Difficult to make multi-cavity mould because even if one part of cavity and core becomes misaligned then the total block is rejected.



Higher machining cost because big machine with high accuracy is required.



High material cost.

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MOULD THEORY 55


INSERTED METHOD

In this method core and cavity is made from a small block of steel Known as core insert and cavity insert respectively and fitted in core holding cavity holding plate. USE   

Multi impression mould. Small components. Replacement of insert easy.

DISADVANTAGES;

To machine recess for collar of insert machining cost increases and mould become little weak.

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MOULD THEORY 56


DETERMINATION OF NUMBER OF CAVITIES By Shot capacity

Ns = 0.85 W m

Based on 85%of rated shot capacity

By Plasticizing capacity

Np = 0.85 X P X Tc 3600m

Based on 85%of rated plasticizing capacity

By clamping capacity

Nc = W here,

C Pc X Am

.

Ns, Np & Nc = Number of cavities based on shot, plasticizing and clamping capacity respectively. W

= Rated shot capacity for polymer in gm

m = Moulding wt per cavity in gm. (Volume* Density) P = Rated plasticizing capacity for polymer (gm/hr) Tc = Over all cycle time (sec) C = Rated clamping capacity in tons Pc = Clamping pressure in Tons/cm²of projected area (0.630T/cm²) Am = Projected area of moulding including runner (cm²) *Moulding weight µm¶ is to be multiplied with a factor to add up

spur and runner weight

Molding Wt (g)

Factor

Molding Wt(g) Factor

0.3-0.5

1.5

5-10

1.15

0.5-1.0

1.4

10-20

1.10

1.0-3.0

1.3

Above 20

1.05

3.0-5.0

1.25

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MOULD THEORY 57


DESIGN OF GUIDE PILLAR

Diameter of guide pillar to be used depends on the size of the mould and side thrust. Mould having deep and heavy cross sectional cores exerts more side thrust. Ps = Stem. dia of pillar ( P0 + 7 to 8mm) Pc = Collar dia of pillar (Ps +2 ×tcp) Tcp = Width/thickness of collar of pillar (from table)

Pillar s r stem. Dia .(mm) Ps

Collar t /Width(mm) r thickness /W

Up to 20

3

20-40

4

40-60

5

60-100

6

100-200

8

tcp // Wcp

Pl Length of pillar (can be found out from assembly drawing i.e. Perpendicular distance from guide pillar holding plate to guide bush =

holding plate)

SIZE OF MOLD (Length x Width) 100x100 100x150 150x200 200x250 250x300 300x400 400x600 600x700

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WORKING DIA. OF PILLAR (In mm) 10 13 16 19 22 25 32 38

MOULD THEORY 58


DESIGN OF GUIDE BUSH

Bi = Guiding dia of bush = Guiding dia of pillar Po Bo = Outside dia of bush = stem dia of pillar Ps. BC = Collar dia of bush = collar dia of pillar Pc Bl = Length of bush = Thickness of mounting plate.

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MOULD THEORY 59


DESIGN OF LOCATING RING

D1d8 = Fixed platen hole dia ± 0.1mm (say 125) d1

= Dsp = Collar dia of sprue bush +0.1mm

D2

= 7mm (for M6 screw)

D3

= Dia of screw head (10mm)  = 20°,  = 45°

H1

= 10mm for small and 12mm for big mould

PCD = 70 ± 90mm H

= 6 ± 8mm

h

= 4mm

K

= 6mm

TYPES OF LOCATING RINGS AND ITS USES 

Reduced diameter type



Constant diameter type



Increased diameter type

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MOULD THEORY 60




Increased depth type

DESIGN OF SPRUE BUSH  

The pact of mould having the tapered channel, connecting the cavity and machine nozzle is known as sprue bush. The taper channel is called as sprue.

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MOULD THEORY 61


W here,

Dsp= Collar dia. of spur bush (From table ± 1)

dsp= Outside dia. of spur bush based on spur dia (From table±1) tsp =Collar thickness of spur bush based on spur Dsp (From table ± 1) Lsp= Length of sprue bush   

As minimum as possible. Normally 45 to 100mm Equal to height from top surface of cavity plate to the top surface of top plate.

d = R = = Dy = di = Di =

Depth of radius (1.5 - 4mm) Approx Nozzle sitting radius (From table ± II) Nose radius of machine nozzle + 2mm Smaller dia. of sprue (Nozzle orifice dia + 0.5-1mm) Bigger dia. f sprue (calculate) (table ± II) 2 LSP tan/2+dy

W here,

Lsp= 60mm usually

= 2° to 5° Normally µdi¶ is kept equal to dia of runner. TABLE 1

d i (mm) 2.5 - 3.5 3.5 - 5.5 5.5 - 7.5

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d sp (mm) 12 16 20

Dsp (mm)

24 32 40

MOULD THEORY 62

t sp (mm) 16 18 22

d i (mm) 2.5 - 3.5 3.5 - 5.5 5.5 - 7.5


TABLE 2 Machine capacity Nozzle or if ice Dia In tons. In mm.

30 80 130 180 300

Nozzle Radius In mm.

3 3 5 5 5

10 10 10 10 10

TYPES OF LAYOUT OF CAVITIES  



Cavities should be placed in such a way that when they are connected with sprue through runner and gate will ensure the uniform filling of impressions. In general multi-cavity mould having same shape and size of impressions are connected through runners and gates of similar cross sectional area and length and cavity should be placed at equidistance from the centre of the sprue. For multi cavity mould having differently shaped impressions should be connected through following system:

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MOULD THEORY 63


SYMMETRICALLLY AT SAME PCD LAYT-OUT OF A FOUR IMPRESSIONS MOULD .

R= Distance between sprue centre and cavity insert or distance at which cavity insert to be placed from the centre of cavity holding plate

R = (CLd+5)/ (2Sinx/2) CLd = Collar dia. of cavity insert. C I = I.D. of cavity insert. GL = Gate length (0.5 ± 1mm) CE = External dia of cavity insert. X = 360°/Number of cavities. L = Runner length = R-(CI/2+GL)

Runner of different cross-sectional area but same length.   

Gate should have similar cross-sectional area. It is required when dissimilar cavities are placed at equidistance centre of the sprue. It is called runner balancing.

from the

Runners of same cross sectional area and length.   

Gate should have different cross sectional area This is done when dissimilar cavities are placed at equidistance from the centre of the sprue. This is called gate balancing.

Runners of different cross sectional area and length.  

Gate should have same cross sectional area. W hen cavities not placed at equidistance from sprue centre.

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MOULD THEORY 64


SELECTION OF RUNNER

   

Runner is a channel in the mould which connects the spur with gate For a single day-light mould runner is made on the parting surface. Runner may also be placed below the parting surface for complex mould. Runner cross sectional shapes should be so design that it provide maximum cross sectional area from the pressure transfer point of view and at the same time it should have minimum contact on the periphery from maximum heat transfer point of view. Keeping the above in view the following cross

section of runner are usuall y used: - Round - Trapezoidal - Modified Trapezoidal. 

 

Out of this round runner is most efficient from heat transfer point of view, but cutting half of the runner in fixed half and other half in movable half matching of two halves to get full round runner is difficult. Instead of round runner if half-round runner is used it leads to problem of ejection of component and feed system. Round and Modified Trapezoidal runners are mostly used.

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MOULD THEORY 65




Runner length is kept as short as possible for rapid filling of cavity & to reduce pressure loss. Corners formed at the junction of main & secondary runner should be rounded off to reduce flow resistance.

CALCULATION OF RUNNER DIAMETER = W here,

¥W X 4¥L 3.7

w = wt. of comp. in gm, (volume X density) L = runner length in mm Runner efficiency Area / Perimeter Area is important to ensure the delivery of particular volume molten plastic  material into the cavity in specific time duration i.e. within injection time. Perimeter of the runner profile is important as less perimeter is preferred for less  heat loss from the molten plastic material . There by maintaining proper melt condition for smooth flow of the molten plastic through spure, runner , gate and finally into the impression =

Size of runner depend on flowing factors   

Runner length W t. of moulding MFI of the moulding For high viscose material the runner size will be higher than the calculated value. As the behavior of the plastic material has the restriction for easy flow .in addition to the this care May taken in the flow path to maintain high surface finish there by reducing the frictional flow restriction during the filling of the cavity

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MOULD THEORY 66


RECOMMENDED MAIN AND SECONDARY RUNNER DIAMETER Weight of c f component

in g m Up to 15

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Runner l r length

in mm Under 50

Runner Dia

in mm 2 .5

Up to 15

Over 5 r 50

4

15-50

Under 50

5

15-50

Over 5 r 50

6

50-200

Under 8 r 80

6

50-200

Over 80

8

Over 200

Under 100

2

Over 2 r 200

Over 1 r 100

10

MOULD THEORY 67


SELECTION OF GATES


-Select the suitable gate for a specific component -Know the different types & use of gates -Use of different sprue puller.

LESSON 7

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MOULD THEORY 68


SELECTION OF GATES Direct Sprue Gate Application: for temperature-sensitive

& high viscose materials, high-quality parts and those with heavy section. Advantage : results in high quality &

exact dimensions. Disadvantage: post-operation for sprue

removal, visible gate mark

Rectangular edge gate        

If,

The cross section being simple cheap to machine. Gate dimensions can easily be modified. All common moulding materials can be moulded by this. Witness mark is left on the visible surface of component. For polystyrene, acetal; it is more clear. Use of soap case, Instrument box etc. Normal size is (3 X1) mm Approx. for components up to 30gm wt. Large components with hard flow material size may (10 X 8) mm. Components can be de gated at machine by operator.

h = Depth of gate in mm Then, A = Surface area of cavity (mm²) t = Thickness of component. Lg = Land length in mm. (1mm to 1.5mm). n = Material constant

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MOULD THEORY 69

h nt W n¥A =

=

30


Mater ial

Value of µn¶

Polyethylene, Polystyr ene

0.6

Poly acetal, Polycar bonate, Polypr opylene

0.7

Cellulose acetate Ac , Acr ylic Nylon

0.8

Pvc

0.9

Overlap gate 



In this type of gate the material coming from the gate is forced to impinge on an opposing face of the impression and then the material progressively fills the impression displacing the air. If a gate is provided at the centre of on end of a solid rectangular block then the material will enter the cavity with a jet form and will be solidified when comes in contact of cool mould walls. After this the material required for filling the4 cavity will flow around the original jetted material giving rise to a flow line.

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MOULD THEORY 70




It is used for block type component, radio knob etc. Land length Gate width Gatelength Gate Height

L1 W L2

= = =

0.5 - 0.75mm n¥A/30 (n = Material constant) n+W/2

h nt (t = component thickness) =

Fan gate    

This is a modified rectangular edge gate. The width of the gate increases with the decrease of depth towards impression to get a constant cross ± sectional area through out the gate length. Land length is little more than rectangular gate. Used for the component having large volume and thin wall thickness like scale, jeweler box etc. W idth

of gate at bigger end, W n¥A =

30

Minimum depth of gate, h1 nt =

Maximum depth of gate, h2 Wh1 =

W here,

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Land length, L

=

D 1.3 mm

N = Material constant A = Area of cavity surface (mm²) t = Thickness of component (mm)

MOULD THEORY 71


Diaphragm gate    

It has a circular runner which is connected with the sprue centrally. A gate is cut to connect the impression with runner. If bore is not important gate is cut in the core which is easy to cut. This is used for tubular shaped moulding in a single cavity mould (baby cycle wheel, lamp shade).

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MOULD THEORY 72


Ring gate    

In this case as usual runner is used with a gate all round the external periphery of the component. It ensures uniform feeding around the core pin. Ring gated components are normally provided with stripper plate ejection. The gate is in the form of a concentric film between the runner and impression. Used in producing tubular mouldings like body of a float valve, water gun, plastic bangles with multi impression two-plate mould. L= Length of gate h= Depth of gate n= Material constant t= Thickness molding

Film gate    

Long rectangular type edge gate is called film gate. Due to longer length wrap age of the component reduced. Depth of gate is kept lesser than rectangular edge gate. Runner is extended beyond the end of component. It is used for thin walled large components like compact disc cover, motor Cycle wind screen etc

W here,

h=Depth of gate, L= Land length of gate, n= Material constant t= Thickness of component

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MOULD THEORY 73


Subsurface /Tunnel /submerged gate     

W hen

a gate is provided below the parting line is called a subsurface gate It is a circular or oval gate The runner is terminated at distance X from impression. A secondary runner usually of conical form is machined at an angle ø to the impression wall and is stopped short of the impression wall by a distance L. It is used for moldings of injection syringe

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L(Land length)

=

1.9mm (minimum)

Ø

=

30° - 45°

MOULD THEORY 74

Angle subtended by centre line of Secondary runner and impression wall


Application: primarily for smaller parts in

multy-cavity molds and for elastic materials. Advantages: automatic gate removal. Disadvantage: for simple parts only

because of high pressure loss.

Pin-point gate Application: for multi-cavity moulds &

center gating.

Advantage: automatic gate removal. Disadvantage: large volume of scrap &

higher mould cost.

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MOULD THEORY 75


TAB GATE     

A projection or tab is moulded on the side of the component. At right angle a rectangular gate is provided to join the tab and runner. This is an alternative to overlap gate. This was designed specifically for use with acrylic material to obtain stress free high optical clarity. This is used for solid block or thick type job where mark is allowed on side of the component only, like clock glass, lens etc. Tab width: Y= D Tab depth: X = 0.9t Tab length: Z = 1.5D W here, D = Runner diameter, t = Wall thickness of plastic component

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MOULD THEORY 76


Application: for high-quality

technical parts, independent of cycle time, also suitable for material difficult to process Advantage: no material loss

from runner system & automatic gate separation. Disadvantage: expensive

moulds especially due to control equipment.

Application: for materials with large

softening & melt temperature range& rapid sequence cycles Advantage: automatic separation

material loss from only after shut down Disadvantage: danger of cold material

gating into cavity after interruption

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MOULD THEORY 77


DESIGN OF SPRUE PULLER REVERSE TAPER TYPE:

d = 5-8mm D = 2-20mm Dc=D+5mm Tc=0.25D-0.4D L1=8-10mm L2=5-8mm = 4° -6°

Types of sprue puller & its uses

Reverse taper cold slug well sprue puller Grooved slug well  Z ± type  Mushroom headed  Reverse taper 

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MOULD THEORY 78


EJECTION SYSTEM


- Know different Ejections system - Explain functions & use of different Ejections system - Know the function of Knockout rod, - Knockout bushes, - Auxiliary guide pillar - Auxiliary guide bushes

LESSON 8

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MOULD THEORY 79


EJECTION SYSTEM

Plastic material shrinks on solidification this property compels plastic components to sit tightly on the core and makes the removal difficult. The mechanism of removal of moulded part from the core is called ejection system. Injection moulding machines are provided with an automatic activation of an  ejector system which is situated behind the moving platen. Ejector Grid: Ejector grid is the portion of the ejection system which provides a space into  which the ejector plate assembly is mounted. This grid allows to and fro movement of the ejector plate assembly within the grid. The grid normally consists of back plate and spacers  

TYPES OF EJECTOR GRID In line Ejector grid: 

Inline type of grid consists of two support block and a back plate. Usually on top of spacers core back plate is mounted. This type of system is suitable for small mould when the distance between two riser blocks increase additional block may be incorporated in between in stead of increasing the thickness of core plate or core back plate. Sometimes additional local support pillars may be used

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MOULD THEORY 80


Frame type e jector grid:   

Some times grid is made by enclosing all the four sides which is called frame type ejector grid. It prevents foreign particles to enter and there by ensures smooth and accurate functioning of ejector assembly movement. It provides better support than inline grid.

Circular support pillar grid: 

Ejection grid can be made by mounting circular support pillars on to the back plate and enclosing these circular pillars by thin metal plates to prevent foreign particles in ejection system used for large moulds.

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MOULD THEORY 81


Ejector plate assembly:







  



It consists of retainer plate which holds ejector element like pins, sleeve etc and ejector plate, which supports the ejector pin etc and helps in actuating pins. An ejector rod is screwed with ejector plate and other end of ejector rod is free which is pushed by the actuating rod of the injection moulding machine. This assembly system shall depend mainly on the size of the mould. For smaller mould and ejector rod bush is fitted into the back plate in which the ejector rod slides. For heavy moulds two or four bushes are fitted in the ejector plate assembly itself. This assembly with the help of guide pillars mounted on the back plate is made to slide to eject the component. Length of the ejector rod depends on the ejection stroke which is again dependant on the max depth of hole of the plastic component. After placing the ejector elements in the retainer plate, ejector plate is placed at the back of retainer plate and screwed together tightly. Thickness of retainer plate is governed by the thickness of head of ejector elements length and breadth depends on the distance between ejector element placed at max distance in the direction of x and y. Thickness of ejector plate depends on the ejection force required to eject component. Length and breadth is same as retainer plate.

Ejector plate assembly return system:   

In general two systems are used for the purpose of returning of ejector plate assy. For the next shot. One is push back system and other spring return system. In push back pin system four pins called push back pin are fitted at four corners in the ejector plate assy. Just opposite to these pins four more pins called returning pins are fitted in the fixed half of the mould. When mould starts closing these returning pins pushes the push back pins there by the ejector plate assy. Returns to rear most position.

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MOULD THEORY 82




In spring return system the spring is fitted to the ejector rod supported by cap or washer screwed at rear end of ejector rod so that it is slightly at compression between ejector back plate and cap or washer. This system is used for smaller mould. For larger mould more than one spring are used between retainer and core back plate.

Push Back Return System: 

Push back pins (return pins) are basically large diameter ejector pins fitted close to the four corners of the ejector plate assembly. A cross section through part of a mould which illustrates a push back pin is shown in Figure 5-13. In the molding position as shown at (a) the push back pins are flush with the mould plate surface. In the ejected position the push back pins protrude beyond the mould plate surface (b). Thus when the mould is in the process of being closed, the push back pins strike the fixed mould plate and progressively return the ejector plate assembly to its original position (a).

Push-back return system in close condition(a)

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push-back return system in open condition(b)

MOULD THEORY 83


Spring Return System:

For small mould, where the ejector assembly is of light construction, a spring or stack of µBelleville¶ washers can be used to return the ejector plate assembly. A typical arrangement of the spring actuating method is shown in Fig. 5-14. In this design the spring is fitted on the ejector rod. A cap is attached to the end of the ejector rod to hold the spring in position under slight compression. In operation when the ejector assembly is actuated, the spring is compressed further. Immediately the mould closing stroke commences, the spring applies a force to return the ejector assembly to its rear position. The Basic Ejection Techniques:Pin ejection  - Stepped pin ejection - D ± shaped ejection Sleeve ejection  Blade ejection  Stripper plate ejection  DESIGN OF PIN EJECTION SYSTEM:

Cut = Dep = Cd = Bs =

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Thickness of plastic component Diameter of ejector pin Depth of hollow portion of plastic component. Bearing surface of plastic component for

MOULD THEORY 84


Side wall contact on the core.

=

W here,

TCid × Cd (cm2 ) Thumb rule

Cid = Inside diameter of component = core dia

For ever y 100sqcm of bearing surface are 1sqcm of e jector pin is recommended

Total area of ejector pins = Bs 100 Max. Diameter of pin = Thickness of plastic component. Nep = Number of ejector pins required =

Bs×4 100× T(Dep)2

=

Bs 25 × T(Dep)2

.

.

Ejector pin should be placed at thicker portion of the plastic component and should be placed in such a way that they do not get bend during e jection. PIN EJECTION:

This is the most common type of ejection as in general it is the most simplest incorporated in a mould with this particular technique the moulding is ejected by the application of pressure a circular rod called as ejector pin. The ejector pin headed to its attachment to ejector plate assembled, the working diameter of ejector pin is must be good slide fit. In matching hole in the mould plate, if it is not the plastic material with creep through the clearance and the mass of material will progressive of build of bending the mould plate. The rare part of ejector pin is fitted into a suitable hole which is bored and counter-bored in retainer plate. The rare case of ejector pin is back up of the ejector plate. The accommodation provides must allow the ejector pin float way the feature is necessary. As taken above, the ejector pin must be slide

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MOULD THEORY 85


fitted in the hole in the mould plate. The direction of movement of ejector pin is therefore controlled by this hole should not be bored absolutely in right angle when the ejector unity assembled. STEPPED EJECTOR PIN 

Consider the case of where the small diameter (3mm) ejector pin are required the stepped ejector pins, now slender long length diameter ratio in ejector pin have the tendency to construct in use. It is desirable to keep the working length to such as ejector pin to minimum this is achieved by designing ejector pins. A stepped ejector pin manufactured from a solid rod, alternating. It could have to be steel. The small diameter portion being fitted as suitable hole matching in large diameter portion. The two parts will brazed together .this later method has advantages that should the ejector pin break only the smaller diameter portion be remade. The stepped ejector pin is normally used face pin for ejection of moulded bushes and ribs, note that the main ejection is provided the standard plain type ejector pin. The length should be kept as short as possible. This length is need only equal length in contact with the mould plate is kept into minimum by in corpora ting a clearance diameter hole in the mould plate .A suitable length the small diameter ejector pin is 5 to 6 times the diameter.

D- SHAPED EJECTOR PIN 

   

This is the name given to a flat sided ejector pin. It is the made quite simply by machining. A flat on to a standard ejector pin. It is used primarily for the ejection for the thin walled box type the procedure adapted for producing µD¶ shaped hole as shown in figure. Mark required position of ejector pin. Bore the required diameter hole in the bolster. Machining out the recess to accommodate the insert. Fit the inserts and hold back with screws.

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MOULD THEORY 86


BLADE EJECTION 







The main purpose of blade ejector is for the ejection of vary slender such as ribs and other projection. Which can not satisfactorily be ejected by the standard type of ejector? The blade is basically a rectangular ejector pin, while the blade ejector is machined solid rods It is more usually fabricated the element in which case a blade of steel is inserted into a slot machine into a standard type of ejector pin. The blade may be pinned or alternated it may be brazed .the advantage of two part construction is that the blade can easily replaced should become damaged. The blade ejector element is fitted to the ejector assembled in a manner to that described for the standard ejector pin. The rectangular plate ejector accommodated in a completely shaped hole in the mould part.

THE SLEEVE EJECTION

The sleeve ejector is mounted into the ejectors pin plate like a conventional ejector pin. The core pin which fits into the sleeve is mounted into the clamping plate. As the ejector unit is activated, the sleeve pushes the piece part off the core pin. Sleeve ejectors are used to push bosses and knoblike piece parts off core pins. It is undesirable to allow the sleeve to be in contact with the core pin over its entire length. To reduce frictional wear, to facilitate fitting and to lesson the possibility of scoring, the surface contact between the two parts is kept to a minimum.

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MOULD THEORY 87


Stripper Plate Ejection:

Stripper plate ejection is generally used where ejector pin marks would be objectionable on the piece parts and where maximum ejection surface is required. Stripper plates are used on single and multiple cavity moulds. An angle of approximately 5° is machined in the stripper plate and on the plunger, as shown in fig. 5-18. This prevents scoring

of the plunger as the stripper plate moves in and out over the plunger. The illustration shows two methods of keeping the stripper plate from coming completely off the plungers and out of the mould. View at shows the use of a stripper bolt to limit the travel of the stripper plate. View at B shows the return pin held to the stripper plate by a screw. This allows the stripper plate and ejector plate to operate as a unit. In more complicated designs, pull rods mounted in the stationary portion of the mould are used to activate the stripper plate.

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MOULD THEORY 88


DESIGN OF KNOCK-OUT ROD

L = Length of the ejector rod. = Height of ejector rod bush + Ejector stroke length +10mm Ejection stroke length = Max. Depth of component +8mm H = Thread diameter length = Ejector plate thickness ± 3mm = 15 to 20mm in general D = Ejector rod diameter (Table-1) d = Threaded diameter (Table-1)

Design of Knock-Out Rod & Bush Table µL¶ in mm µD¶ in mm µd¶ in mm Up to 150 15 M10 150-200 20 M12 250-300 25 M16 DESIGN OF KNOCK-OUT BUSH:

H = Height of the ejector rod bush = Thickness of back plate H = Thickness of collar = 4to 5mm D1 = Internal diameter of bush= Outside diameter of ejector rod. D2 = outside diameter of bush = D1 + 8 to 10mm D3 = Diameter of collar of bush =D2 + 6 to 8mm

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MOULD THEORY 89


COOLING SYSTEM


- Use of different Cooling system. -Select the suitable cooling system for a mould

LESSON 9

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MOULD THEORY 90


COOLING SYSTEM 

One of the most important acceptation of mould design is the provision of suitable and adequate cooling arrangement in all injection moulding. Even though it involves having a heated mould, the purpose of mould is to cool the molten plastic. The means of cooling and insulated to prevent any escape of heat by conduction or radiation. It would quickly cool the material will be mould and would no longer fulfill its function. The cooling system is an essential mould feature, requiring special attention in mould design. It should ensure rapid and uniform cooling of the moulding. In design of mould component and layout of the guides and ejectors. The allowance should be proper size and position of the cooling system. Rapid cooling improves process economy, while uniform cooling product by different shrinkage, internal stresses and mould will relies problem. In addition uniform cooling ensure a shorter moulding cycle. A rapid and uniform cooling is achieved by a sufficient number of properly located channels. The location of this channel should be consist with shape of moulding and should be as close as cavity will allowed by strength and rigidity of the mould.



Increasing the depth of cooling line from the moulding surface reduces the heat transfer efficiency and to wide a pitch gives a non-uniform surface. A straight drill line are preferred to bubblers they should be designed so that cross-sectional area remains constant for entire circuit for tube bubblers area in the both side of the tube should be equal material with higher thermal conductivity should be used. If all the heat cannot be removed with a steel mould.



The described location of this heating-cooling phase is the mould close to where most of the heat decapitated that is where the most of the material is located.

DESIGN OF COOLING HOLE :

dT= Diameter of cooling hole C = Distance between centre of the cooling hole and the surface of the plastic component. = 2 to 3 dt t = Thickness of the plastic component b = Centre distance between two adjacent holes = max. 3dt t in mm

dT in mm

Up to 2 2-4 4-6

8-10 10-12 12-15

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MOULD THEORY 91


STRAIGHT SECTION STROKE ROUND FLOW  

These are the most common cooling channels found in the mould. They are normally in series of drilling package of series and that are round in diameter.

STRAIGHT SECTION STROKE RECTANGULAR FLOW 

This type of cooling is used commonly found in backup plate rushed in mould cooling.



These are the usually series of package but a rectangular shape that compromises the cross-sectional area.

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MOULD THEORY 92


CIRCULAR SECTION ROUND FLOW 

Circular sections are used to cool round plate, cylindrical stroke, cone shape cores and cavity, etc. this arrangement, when properly interfered makes it possible to follow close radius of the round core or cavity so that the distance of channel is kept as a uniform depth.

CIRCULAR SECTION RECTANGULAR FLOW 

These have the rectangular cross-section area for easy of machining.

BUBBLERS 

This channel component are normally connected with straight section round flow. They are used in cooling of pins, cores and deep draw area. Epical two channels are parallel the surface of the back plate of the different depth. In bottom channel tubes are screwed go to that top area to be cooled (pin and core) the inlet water groove & lower channel fills the tube, and then overflow of the outlet each core tube receives same cooling with maximum velocity.

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MOULD THEORY 93


BAFFLES 

These constitute an alternating method for cooling pins, cores, and deep draw areas, unlikely baffles they are tied together in series triple straight section stroke the coolant extruded section that intersect with all of the baffles of the channels, each baffle is a round drill section, with blade to divided in cross-section area in half. The coolant flowing in straight section runs baffles blades, and makes it bends to the baffles. In as much as the blade does not extend all the way of the end of the baffle. If they run blow the back side of the plate, make another so turn back into straight into as goes next baffle. This is an expectable method for smaller number of cavity from side to side or very large diameter channels and baffles.

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MOULD THEORY 94


Mold venting

As molten plastic enters the mold, it quickly displaces air in the tightly scaled mold, although some air escapes through the parting line or loose-fitting ejectors or slides, most molds need strategically placed vents for rapid and complete air removal. This section discusses vent design and placement. Parting -line vents As a first choice, place vents along the mold parting line. Typically easy to cut and keep clear of material, vents in the parting line provide a direct pathway for air escaping the mold. Figure shows standard parting-line vent guide lines for thermoplastic resins. To prevent material form flowing into the vent during filling, the depth of the first 0.150 inch to 0.300 inch of vent length must be small, typically less than 0.0020 inch for amorphous resins and less than 0.0015 inch for semi-crystalline resins. Your resin selection and processing conditions determine the vent's maximum depth. The ranges given in figure apply to typical molding conditions. Other rules of thumb for venting:   

The amount of venting needed increases with part volume and filling speed; Add more vents or widen existing ones to increase venting; and To avoid flash, do not increase vent depth beyond the guidelines.

For the vast majority of resins and part geometrics, more vents are better. The exceptions are resins with components - usually flame retardants or other additives ± that can boil to the surface at the flow front and deposit on the mold surface and vents. These resins rely on pressurized air in front of the flow front to hold volatiles in the materials. Over-venting can prevent the flow front from generating the required pressure. Add vents sparingly in molds for these materials. Carefully review product information bulletin for specific venting recommendations, particularly for flame-retarded materials.

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MOULD THEORY 95


Vent placement

Vents should be placed at various locations along the runner system and part perimeter, but they are especially needed at the last areas of the mold to fill. Typically these areas are located on the parting line and lie farthest from the gate. When the last area to fill is not vented, air may become trapped in the mold, preventing complete filling of the cavity and causing a gas burn on the part. The trapped air is supper heated during compression and in severe cases can pit or code the mold steel. W hen feasible, move gates or vary part thickness to change the filling pattern and direct air to parting-line vents. If air-trap areas persist, consider using ejector pins modified with flats for venting (see figure). Ejector -pin vents usually self clean with each ejection stroke. Air trap areas not accessible by ejector-pin vents may require vents placed along mold inserted or splits in the mold. This type of vent usually required periodic type of vent usually requires periodic disassembly for cleaning. Porous metal inserts can also provide venting for difficult airtrap areas but may require periodic cleaning. Part features produced by blind holes in the mold, such as posts and bossed, require venting at the last area to fill, usually the tip or end. Bosses can usually vent along the core insert forming the inside diameter of the boss. Posts usually require ejector-pin vents at the tip of the post. Other venting issues you should address: 



Direct mold filling along the length of the rib so gasses can escape at the ends; and Round or angle the ends of standing ribs to prevent air entrapment (see figure).

Air trapped in unvented pockets or recesses in the mold can exit these areas behind the flow front and lead to splay or teardrop-shaped surface defects. Severe weld lines often form where flow streams meet head on, especially at the end of fill. You can often improve the strength and appearance of these weld lines by installing overflow wells (see figure). Overflow wells are modified vent features that provide an extra-deep vent channel, usually about one- third the part thickness, that empties into a cylindrical well .Venting air escapes the well around a shortened ejector pin fitted with a 0.002-inch clearance. Cool material at the leading edge of the advancing flow fronts merges and enters the overflow well leaving hotter material to mix and fuse at the weld line. The overflow well is ejected with the part and clipped off after moulding. Overflow wells can also provide ejector pin locations for parts such as clock faces or instrument lenses that cannot be tolerate ejector pin marks on the part surface.

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MOULD THEORY 96


ASSIGNMENT

1.

What

do you mean by shrinkage?

2.

What

do you mean by shot capacity?

3.

What

do you mean by swept vol. of an injection cylinder?

4.

What

do you mean by platen size?

5. Define clamping force and write its unit. 6. Define injection pressure and write its, unit. 7. Plasticizing shot and clamping capacity is required to determine ««.. . 8. State different types of runner. 9. Define runner length. 10. What are the factors determine size and type of runner? 11. Define rectangular edge gate, overlap gate, fan gate diaphragm gate. 12. Define subsurface and tab gate. 13. Write one advantage of tunnel gate. 14. What is a sprue bush? 15. Define working dia of guide pillar. 16. What is nozzle sitting radius? 17. What do you mean by ejection system and ejector grid? 18. Why locating rings are used. 19. Why push back pins are used in a mould. 20. What is feed button. 21. What is day light? 22. Why riser blocks are used. 23. Write down different types of ejectors & their uses. 24. Why cooling is provided in a mould. 25« use to locate the mould on the platen so that the nozzle and sprue bush are aligned. 26«« are used to bring back the ejector plate & ejector back plate to bring back its original position as the mould closes. 27. The surface at which core & cavity are separated is known as ««. 28.«« moulding is most commonly used method of shaping a thermosetting plastic.

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MOULD THEORY 97


29.«««. Plastics are used in injection moulding. 30. For manufacturing hollow plastics ««.. moulding method is used. 31« Moulding is a process of manufacturing components from plastics in sheet form. 32. Injection moulding machine consists of three units ««., ««.. & ««« 33. Why sprue bush consist of angular hole. 34. What is Z-type cooling? U=-type cooling? 35. How to determine the type of mould to be used/ 36. What are the considerations made before designing as mould? 37. What are the materials used for guide pillar & guide bush 38. How many types of sprue pullers are there and what are those? 39. What is the use of sprue puller? 39. ««sprue puller is widely used. 40 . . . . ««is the portion of the ejection system which provides a space into which the ejector assembly is mounted. 41. Give some examples of different ejector grid. 42 . . . . ««and. ««are two methods of ejection return system. 43. What is push back ejection system? 44. What is mould heating? 45. What is a baffle? 46. What is the use of cooling?

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MOULD THEORY 98


DIFFERENT PARTS OF INJECTION MOULD


-Know the designing of core & cavity inserts -Know the designing of Cavity holding plate, - Core holding plate, - Ejector plate, - Ejector back plate - Riser block

LESSON 10

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MOULD THEORY 99


DESIGN OF CAVITY INSERTS:

CI = Internal dia of cavity insert = OD of plastic component + Shrinkage value of plastic. Ce = External dia of insert of width of cavity insert. = ID of insert + (wall thickness ×2) [Refer Table: 1] t = Wall thickness of insert (in mm) CT = Collar Thickness of cavity Insert in (mm) [Refer Table: 2] Cw = Collar width of cavity insert CLd = Collar Dia or Width of cavity insert = Outer diameter or Width + collar width of insert TABLE 1: Inter nal dia /length Round of cavity in mm cavity

30 30-50 50-75 75-100 100-125 125-175 175-225 225-300 300 & Ab Above

08 10 12 12 15 16 19 22 25

Rectangular cavity

12 12 15 16 18 21 25 35 45

TABLE2: OD /W /Width of cavity

Thickness /W /Width of

inser t in mm Up to 20 20-40 40-60 60-100 100-200

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collar in mm 3 4 5 6 8

MOULD THEORY 100


DESIGN OF CORE INSERTS W here,

D= Diameter or width or length of Core inserts. I = Height of core insert without collar. b= Bearing length of core insert. T = Core Holding plate thickness. Tc= Thickness of collar. Ts= Stripper plate thickness.

W

= Collar width.

L= Depth of hollow portion of plastic component. Total Height of core insert

= [(L+ Shrinkage value of plastic on depth) + b + Tc + Ts (if stripper plate is used) Diameter or width or length of core

insert

= Inside dia or width or length of component+ Shrinkage value of plastic Height of core insert

= Depth of hollow portion of component +shrinkage value of plastic + bearing length of core in core holding plate + stripper plate thickness (if used) + Collar thickness of the core. Collar dia. Of the insert = Core dia + 2(collar width) Collar width = collar thickness.

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MOULD THEORY 101


Table used in cavity insert collar dia and thickness is also applicable here. TABLE OF BEARING LENGTH ; (b) D in mm

/D I /D

b in mm

Remar k

Up to 10

Up to2 Ab Above2

1 .5 D & 1 0 2 to 3D & 10

Lesser I/D r atio Lesser b r bear ing length

Up to 20

Up to 2 Ab Above2

1.25D D to 2D & 10

Ä

20 - 30

Up to 2 Above2

0.75D - D D - 1.5D

Ä

30 - 40

Up to 2 Above2

0 .7 D - D D - 1.5D

Ä

40 - 60

Up to 2 Above2

0 .6 D - D D - 1.3D

Ä

60 - 100

Up to 2 Ab Above2

0.5D - 0.75D 0 .8 D - D

Ä

DESIGN OF CAVITY HOLDING PLATE

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MOULD THEORY 102


Lchp

= Length of cavity holding plate.

W chp

= Width of cavity holding plate (similar calculation of Lchp tn ydir)

CE

= External dia of cavity insert.

CI

= Internal dia of cavity insert.

dT

= Dia of cooling hole.

DP

= Dia of guide pillar.

CDp

= Collar dia of guide pillar = DP + (2 x collar width of pillar)

Ctd

= Thickness of cavity holding plate = Height of cavity inserts.

Lchp

= [R + 1.5dp + 0.5dp + 5 + dT + 5 + 0.5CE] × 2

For symmetricall y at same PCD lay out Lchp

=

Wchp

DESIGN OF COREHOLDING PLATE

L = Length of core holding plate = same as cavity holding. W

= Width of core holding plate = Same as cavity holding.

T = Thickness of core holding plate = tc + b Tc= collar thickness of core. b = Bearing length of core holding plate. D = dia of core or width (max) I = Length of core (Height) = tc+b+L L = Depth of cavity. Wc

= collar width of core = tc

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MOULD THEORY 103


DESIGN OF EJECTOR PLATE: Lrp

Length of retainer plate

=

= Perpendicular distance between two ejector element to be placed at opposite extreme ends of the retainer plate in X axis + collar diameter of ejector element + 10 to 15mm Wrp

=

Width of retainer plate

= Perpendicular centre distance between two ejector elements to be placed at opposite extreme ends of the retainer plate in Y axis + collar diameter of ejector element + 10 ± 15mm Trp

Thickness of retainer plate

=

Collar thickness of ejector element such as ejector pin, sleeve etc + 5 to 8mm (In general thickness is taken 12 to 15mm)

DESIGN OF EJECTOR BACK PLATE:

Lep = Length of ejector plate = same as retainer plate. W ep = W idth of ejector plate = same as retainer plate. Tep = Thickness of ejector plate (cm) =

 6MP 8LS

Where,

M =Perpendicular centre distance between two core inserts placed at opposite extreme end of the core holding plate in x axis (cm) P = Ejection force (kgf) L = Length of ejector plate (cm) S = Permissible working stress (840 kg/cm²)

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MOULD THEORY 104


Ejection force µp¶ = (s.t.E.A.U) d d - dy 2t 4t W here,

s = Thermal contraction of plastic across diameterµd¶ = cte ×dT ×d Cte = Coefficient of thermal expansion of the plastic material used (Table) dT = Temperature difference between softening and ejection = (T soft ±T eje.) d = Dia of circle of circumference equal to length of perimeter of molding surrounding male core (cm) E = Elastic modulus (Kgf/cm²) of plastic material (Table) A = Total area of contact between molding and mould faces (cm²) =  dh (if circular) W here, H = Depth of component (cm) Y = Poisson¶s ratio of plastic (0.4 ± 0.5 on average) U = Coefficient of friction between plastic and steel (table) Mater ial

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µu¶

Cte

Tsof t

Te je je

E

AB ABS

0.5

6-13

85

20

0.1-0.3

Acr ylic PC LDPE HDPE PP PS

0.4 0.55 0.4 0.25 0.33 0.4

5-9 7 16-18 11-13 11 6-8

90 165 86 125 150 90

20 20 20 20 20 20

3.14 2.3 0.02 0.1 1.05 1.8

MOULD THEORY 105


DESIGN OF TOP AND BOTTOM PLATE  

These are two plates on which two halves of moulds are built and finally these two are mounted on m/c platens. These must be robust enough to hold all parts of mould. Lt/b = Length of top and bottom plate = Length of cavity holding plate. t /b top plate / bottom plate W t/b = Width of top and bottom plate = Width of cavity plate + 50 mm Tt/b = Thickness of top and bottom plate (From table) =

Lt/b

Up to 250

250-350

350-450

450-550

Tt/b(mm)

22

27

37

47

DESIGN OF RISER BLOCK:

Hrb = Height of Riser Block = Retainer Plate thickness + Ejector plate thickness +Depth of max hollow portion of the plastic component + Shrinkage on this depth + 20mm

Lrb = Length of Riser block = Length of cavity or cavity holding plate W rb

=

W idth

of Riser block

= (Width of cavity or cavity holding plate) - (width of Ejector plate + 2mm) 2

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MOULD THEORY 106


SPLIT MOULD


- Explain the classification & know different types of Split mould - Determine the split movements of a given component by using different calculation methods

LESSON 11

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MOULD THEORY 107


SPLIT MOULD

SPLIT MOLD

SLIDING SPLITS

CAM

SPRING

NGLE-LIFTED SPLITS

HYDROULIC

CAM TRACK

SPRING

FINGER CAM

DOG LEG CAM CAM TRACK

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MOULD THEORY 108


Split movement actuation (finger pin actuation)    

   

M = L sin  ±

C Cos  L = M + 2C Sin  sin 2 M = split movement  = angle of finger cam (10 to 25 maximum) L = working length of finger cam C = clearance D = delay movement D= C Sin 

.

Spring actuation

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MOULD THEORY 109



M = ½ H tan J



M = movement of each split



H = height of locking heel



J = angle of locking heel (20 r to 25r)



M = 0.2 H


Cam track actuation   

      

M = La tan J ± c La = M + c tan J D = Ls + c + r 1 ± 1. tan J tan J sin J M = movement of each split La = angle length of cam track Ls = straight length of cam track J= cam track angle c = clearance D = delay r = radius of boss

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MOULD THEORY 110


Dog leg cam actuation  



      

M = La tan J ± c La = M + c tanJ D = (Ls ± e ) + c . tan J M = movement of each split La = angle length of cam Ls = straight length of cam J= cam track angle c = clearance D = delay e = length of straight position of hole

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MOULD THEORY 111


ASSIGNMENT:-

1.

What

is split mould.

2. Type of split mould. 3. How split moulds are classified. 4.

What

is a finger pin?

5.

What

is a cam track?

6.

Why

7.

What

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dog leg cam are used. is a slider in a split mould?

MOULD THEORY 112


THREE PLATE MOULD


-Explain the functional feature & uses of three plate mould -Know the different parts of three plate mould

LESSON 12 IZTC HARARE

MOULD THEORY 113


Three Plate Mould: 

Figures illustrate the principle of one of the numerous varieties of the 3 plate mould construction. The third plate or floating plate is located between the top clamping plate and the cavity back up plate. The material from the nozzle is forced through the primary sprue, into the runner system, into the secondary sprues, and into the cavity. Fig. shows the mould in the closed position with sprues, runners, and cavity filled with material.



The mould opens first at parting line 1. The compressed spring (S) starts the cavity back up plate and the cavity plate moving with the plunger portion of the mould. The floating plate stays with the top clamping plate. As the mould opens, the runner is held to the floating plate by pins (P) containing undercuts. This allows the gates to break at the piece part. The mould then opens at parting line 2, pulling the piece part and plunger out of the cavity.

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MOULD THEORY 114






At a predetermined distance, the pull rod A pulls the cavity plate and attached back up plate, thus freeing the secondary sprues from the cavity block. When the secondary sprues are clear of the cavity back up plate, the head of the stripper bolt B engages the shoulder of the counter bored hole in the cavity plate, pulling the floating plate away from the top clamp plate at parting line 3. This movement frees the runners from the undercut pins and breaks the primary sprue at the nozzle. Stripper bolt C limits the travel of the floating plate. As the mould continues to move back, the stripper plate ejects the piece parts from the plungers. The stripper plate, which rides on guide pins mounted in the support plate, is activated by return pins. Pin D limits the travel of the stripper plate. Both ejector pins and stripper plates are used to remove piece parts from the plunger. Often a poppet valve (through which a stream of compressed air is released) is installed in the plunger to facilitate the removal of piece parts from plungers. The 3 plate mould construction lends itself well to automatic molding, as the piece parts are ejected from the mould free of runners, sprues etc. These moulds, generally built with multiple cavities, are a very efficient method of molding bowl ± or tumbler ± shaped articles.

The main uses of the three plate mould are as follows:

1. Moldings

produced on multi-

impression moulds can be centre fed. 2. Off

centre

feeding

can

be

achieved for both single and multi-impression moulds. 3. Multi-point feeding can be accomplished on single impression moulds and on multiimpression moulds

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MOULD THEORY 115


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MOULD THEORY 116


HOT RUNNER MOULD


-Explain the detail process of hot runner system -Know the advantages, disadvantages & economics of hot runner system -Explain the concept of expansion theory

LESSON 13

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MOULD THEORY 117


INTRODUCTION OF HOT RUNNER MOULD 

All injection molds in variable in corporate ³Runner´ which are flow paths for carrying molten plastics materials form its entry point into the mold to its ultimate destination i.e. the impression. Molten material enters into the impression through a narrow opening at the end of the runners called gate. In a conventional 2 plate or 3 plate injection mold , the entire network of runners and gate along with the molded component has to be some how ejected out of the mold ,in order to clear the passage for the next cycle. The material that has solidified as runner is the necessary evil .It is finally the molded component that is desired. But the runners can not be eliminated altogether. Sometimes incase of multi-cavity injection mould especially, it happens that the volume of plastic material solidified in the runner is more than the volume of moulding themselves. More volume in the runner means more wastage. However this wastage is minimized by reusing the runner material. In parts, still the economics of moulding is adversely affected.



A great deal can be saved upon moulding, if somehow, the ejection of the runners could be eliminated. This is what exactly is achieved in a hot-runner system. This is also called runner-less moulding. Because what we get finally out of the mould, is the moulding without runners. The runners are not eliminated together in a runner-less system. These are simply not ejected out the mould. In a hot runner system, the plastic material is kept at the nozzle temperature right up to its entry point into the impression i.e., the gate. During the ejection stroke only the moulding is degated and ejected out the mould. How, all this is achieved, may be point of interest at this stage. But before we proceed any further, let us understand why we should opt for the hot runner system. W hat are the relative advantages and disadvantages of hot runner moulds over conventional moulds?

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MOULD THEORY 118


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MOULD THEORY 119


Hot Runner Moulds: 

Figure gives and example of one type of hot runner mould. Hot runner moulds are more efficient than other designs when molding rather large articles. The cavity can be filled uniformly and quickly as the material material is kept molten in the hot runner block located close to the sprue feeding the cavity. There is not need for the material to go through a long runner system before it reaches the cavity. The hot runner block is insulated from the press and the rest of the mould by a series of spacers and buttons which form and air gap (See µG¶ in Fig. 90) the material is constantly kept molten in the hot runner block by a number of heating elements. See Fig. for a section view of the hot runner and heating elements. elements.



The pin located at the end of the hot runner is ground at approximately 30° to divert the flow of material into the sprue. The hot runner is made approximately 16 mm in diameter. The cavity in Figure is filled through a hole in the piece part from the runners and gates connected to the sprue. There is no pressure on the material as the mould opens at the parting line. The plunger pulls the piece part out of the cavity c avity and the under-cut in the plunger, acting as a sprue puller, causes the sprue to break at the small diameter where the material is still molten. Continued movement of the press activates the ejector system and the piece part is removed from the plunger by ejector pins or a stripper. The ejector pin located opposite the sprue pushes the material out of the under-cut as the part is being ejected (See figure) Variations:

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MOULD THEORY 120


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MOULD THEORY 121




Hot runner moulds are also constructed so that runners are eliminated and the sprue feeds the material directly into the cavity. Multiple sprues fed by the hot runner can be used to fill a single cavity producing a large part. Slides or Splits: Figures illustrates the principle of using slides to produce under ± cuts in spoolshaped pieces. Fig. 90 shows the mould in the closed position. The positive locks push against the slides to keep the surfaces of the two slides firmly together to the vertical parting line (Parting line µ3¶ ) to prevent the formation of any flash as the mould opens at a parting line µ1¶, the part stays with the core pin and the angle pins force the slides out of the undercuts. The slides move along parting line µ2¶ at a 90° angle to the movement of the molding machine. When the slides have completely cleared the undercuts, the sleeve ejector pushes the piece part free of the core pin (see Figure). The angle pins are generally set at an angle of 20 ° to 25 °. The distance which the slides will travel depends on the angle and the length of the angle pin. The positive lock is machined at an angle approximately 5° greater than the angle of the angle pin.



The slide movement of the splits must be prevented to ensure that the split halves always come together in the same place.

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MOULD THEORY 122


ADVANTAGES AND DISADVANTAGES OF HOT RUNNER SYSTEM TECHNOLOGICAL ADVANTAGES

DISADVANTAGES

Omission of demoulding the runner system: permits the simpler automation of process.

Hazard of thermal degradation of sensitive polymers.

Heated runners allow the realization of long flow paths: option to place the gate at an optimum location.

Area near the gate may show blemish or difference in appearance.

Diameters can be kept larger: resulting in easier machining and less pressure loss.

Changing the material or colour may take more time.

Longer holding pressure is made possible.

ECONOMICAL DISADVANTAGES

ADVANTAGES

Savings in materials for runners and cost for regrind. No regrinding required. Shorter cycles:

Time for demoulding runner system is omitted.  Shot size is reduced by volume for runner.  Larger shot volume is available for filling larger number of cavities.  Smaller opening stroke than for a 3-plate mould.  Cooling time only for the moulding and not for thicker runner. standard parts for hot runner system are availa ble such as:  nozzles  Manifolds, readymade as well as tailor made.  heaters  thermocouples 

Less moving parts than in 3-plate mould.

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More work necessary in moulding design: higher experienced designers required. Higher mould cost: Installation of auxiliary equipments like heaters, temperature sensors, control units, etc.

Costlier in maintenance and repair.

More difficult assembly of the mould and handling. Better trained operators are required

MOULD THEORY 123


MANIFOLD

The temperature of the manifold is controlled by means of one or more thermocouples. The manifold is situated between the top plate/cavity back plate and cavity plate. The contact points with these mould plates are kept as small in area as possible, in order to keep the heat losses low. Hot runner blocks are made of hot die steel e.g. H12.The manifold receives material from the machine nozzle through a sprue bush, which is generally heated by a band heater or a hollow pipe heater. It is firmly attached to the manifold and made of hardened steel. It may have its own thermocouple. The heart of hot runner systems manifold which, infect is a heated block, housing the runner network distributing the material to various cavities. Depending upon the number of cavities it may have

Types of manifold:-

Rectangular forms H forms Cross forms Circular forms

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MOULD THEORY 124


Manifolds are heated by means of (Types of heaters) : 

Cartridge heaters Tubular heaters Induction Band Zigzag

Cartridge heaters :-( placed in the drilled holes)

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MOULD THEORY 125


The most common method of manifold heating is the one with cartridge heaters they are placed parallel to the runner. Their advantages lie in ready availability. One can form individual heating circuits for each nozzle or can couple them together. The holes for them are required to be very accurate; otherwise there is risk of their burning out. The design shown here depicts a quick method of replacement.

Tubular heaters :-( placed in milled slots)

Tubular heaters have the advantage that they can be bent and placed conforming to the form of cavities. However, they form only one circuit for all cavities; therefore they cannot be regulated for individual nozzles. They have a longer life as compared to cartridge heaters, owing to less wattage density.

Induction :-( heating coil)

Heavy gauge copper wire is wound onto a former and then bound with a tape to form a coil of convenient size. The induction heating coil for this application is normally made to specification and is generally, not available as standard part. The coil is fitted into an accommodating recess machined into the sides of the manifold block. A cover plate secures the coil in position. As noted previously, when a current is passed through a coil, eddy currents are induced into the surface of the manifold block and heat is transferred from the immediate vicinity of the coil to the melt flow-way by conduction. All of the above external flow-way heating techniques related specifically to the rectangular cross section manifold block. We now consider the methods adopted for heating cylindrical manifold blocks. Basically, two types heater are applicable, the band heating elements (and variations) and the low voltage coil heater. 

Zigzag: - (low voltage heating element)

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MOULD THEORY 126


A length of high-resistively wire is formed into a zigzag configuration, the adjacent parallel lengths being relatively close together. Terminals are fitted to either end of the wire to facilitate its connection to low voltage supply. The overall length of the wire required depends upon a number of factors including the receptivity value of wire, the voltage used and the power input required. The zigzag element is fitted into a recess in the side wall of the manifold block and secured by a cover plate in a similar manner to that described for the flat heating element. Band :-(heating elements) The main voltage resistance type of element is enclosed within a casing which is in the form of split hollow cylinder. This is mounted on the external surface of the cylindrical manifold block and secured in position by clamp screws. The band heater is available as standard part in a wide variety of diameters, width and wattages. In many composite cylindrical manifold block designs, use of the conventional band heater necessitates dismantling the unit each time a heater has to be replaced. To overcome this limitation, µhalf shoe¶ heating elements may be used. These half shoes can be clamped round the manifold to form what is, effectively, one continuous heating element. The band heating element has other applications in hot runner unit design, such as the heating of secondary nozzles, manifold bushing, etc.

Economics of Hot runner molds

It is difficult to quantify the pros and cons of a hot runner gate mould in items of cost without having a particular job in mind. For that matter, let¶s consider the case of a four cavity injection moulding in a

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MOULD THEORY 127




3-plate version, with a cycle time of 45sec



Hot runner version, with a cycle time of 30sec

Let¶s assume that:

Total quantity to be produced

= 25, 00,000pcs. Per yr.



Additional cost for hot runner system = Rs.3,50,000



Machine cost for 3-plate mould (with operator) = Rs.150 per hr.



Machine cost for hot runner mould (automatic) = Rs.130 per hr.



Time for moulding 100pcs. in 3-plate mould



Time for moulding 100pcs. in hot-runner mould =100(30/4) = 750sec



Cost of 100pcs. in case of 3- plate mould =150×1125/3600 =Rs. 46.875



Cost of 100pcs. In case of hot runner mould =130×750/3600 =Rs.27.08



We

= 100(45/4) = 1125sec

have saving from 3-plate to hot runner = Rs 46.875 ± Rs 27.08 Rs19.79/100pcs Pay back quantity=3500000 100 19.79 =





We

have saving in case of hot runner =1768570pcs



Rest quantity 2500000 ± 1768570=731430pcs



Profit on rest quantity=731430×19.70/100 =Rs144750

(Saving on regrinding the runners as well as electricity have not yet been included)

Energy required for the manifold:-



Wattage



Size of manifold

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per kilograms weight 200-300 200×76×36

MOULD THEORY 128




W eight

of the manifold

4.27kg--------(1)



Maximum energy required

300×4.27=1281watt---- (2)



Number of heaters

2



Effective length

200-20 =180mm



Assumingµd¶ as heater diameter.



Surface area of heaters



Permissible watt density



Total wattage

3.14×d×18 2=113.1d cm2 10/cm2 = 113.1×d ×10 =1131d -------- (3) =1281 from ----- (2)

d



=

1281 1131

=

1.113cm

«««.(4)

Diameter selected =12.5mm--------(5)

Required for molding 25,00,000pcs = (2500000 750) ==5208.3hrs. (1003600) 

With



With

3 shifts and 80% efficiency = 5208.3 = 271 days (say 1 year) (240.8)

3-plate mould one needs 50% more that is another machine, thus investment on a new machine is saved.

EXPANSION THEORY

The value for the thermal expansion of steel is in the region of 12-13*10 -6 mm/mm º

c. The précised value depending upon the specific type of steel used. In this case,

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MOULD THEORY 129


in the manufacture of manifold block. The general equation for calculating expansion is as follows: e L E  (T =

Where, L

=

Length dimension (mm)

e

=

Expansion (mm)

=

Coefficient of thermal expansion (mm /mm  C)

E

(T Increase in temp. ( C) =

Example Q.

A manifold block is to be heated from 200 ºC to 230º C. Calculate the increase in dimensions between the secondary Nozzles situated 635 mm apart. Use a value of the thermal expansion of steel of 13 10-6 mm/mm  C Ans.

e L E  (T =

L= 635mm

E = 1310-6 mm/mm  C (T 200ºC to 230º C. = (230-20) =

=210 e = 635  (13 10-6) (210) =1.73 mm Such a large increase in the dim. Between the secondary nozzle centers and the impression Centers could not be ignored. `

ASSIGNMENT:-

1.

W hat

is a manifold?

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MOULD THEORY 130


2. Different parts of hot runner 3.

What

is a heater

4.

What

is a tabular heater?

5. Different types of manifold. 6. Among is-plate mould and hot runner mould which has higher efficiency. 7.

What

do you mean by cartridge heater?

A GUIDE TO TROUBLE SHOOTING IN INJECTION MOULDING 1. DROOLING AT NOZZLE:

Clear shut off valve start injection later, adjust nozzle pressure check radius of nozzle spure bushing, reduce nozzle temperature.

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MOULD THEORY 131


2. SHORT SHOT:

Increase injection pressure, increase cylinder temperature, increase nozzle temperature, clear nozzle, increase injection speed , increase back pressure, enlarge nozzle size, increase mold temp., increase size of gates, provide vents in mold, increase feed. 3. SCREW DOES NOT RETURE:

Increase screw r.p.m. decrease back pressure, fill hopper or clear, and reduce temperature of rear zone. 4. SINK MARKS:

Increase holding pressure and time, increase nozzle temperature, decrease injection speed, decrease back pressure, enlarge nozzle, decrease mold temperature increase size of gates, increase feed , decrease material temp. 5. Burning of Burn marks :

Decrease stock temperature, decrease injection speed, enlarge nozzle, increase size of gates, and provide vent in mold.

6. Surface Blemishes :

Increase stock temperature, increase nozzle temperature, Increase screw r.p.m., decrease injection speed, increase back pressure, increase mold temp. Or decrease, polish mold, increase size of gates, enlarge cold slug well.

7. Flashing: Decrease injection pressure, decrease stock temp., decrease holding pressure and time, increase clamping pressure, decrease mold temp.¶ rework mold. 8. Dull surface :

Increase stock temp., increase screw r.p.m., decrease injection speed, increase back pressure, enlarge nozzle, increase mold temp., polish mold, and increase gate use dry material. 9. lamination:

Increase stock temp., increase screw r.p.m decrease injection speed, increase back pressure, enlarge nozzle, increase mold temp., increase gate use dry material.

10. Part sticks in Mold:

Decrease stock temp., decrease holding pressure, decrease injection speed, increase or decrease mould temp., polish mould, rework mold, use mold release, provide air for ejection. 11. Runner breaks: Decrease holding pressure & time, decrease mold temp., polish sprue, runner & gates, check radius of nozzle, lengthen cooling and mold open. 12. Parts Distort :

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MOULD THEORY 132


Decrease injection pressure, increase stock temp., decrease holding pressure, decrease injection speed, increase back pressure, increase or decrease mold temp., and lengthen cooling. 13.. Flow Lines: 13 Increase injection pressure, increase stock temp., increase injection speed, increase bad pressure, enlarge nozzle increase mold temp., increase size of gate, provide vents in mold, use dry material, and increase feed. 14. Brittle Parts: Increase stock temp., decrease holding pressure, increase screw r.p.m. decrease injection speed, increase back pressure, increase mold temp or decrease, increase gate, and use dry and uncontaminated materials. 15.. Wavy Surface : 15 Increase injection pressure, Increase stock temp., increase holding pressure, decrease injection speed of increase, enlarge nozzle, decrease mold temp., increase feed length cooling. 16.. Worm Tracks on Part: 16 Increase cylinder temp., increase nozzle temp., increase screw r.p.m. decrease screw speed, increase back pressure, enlarge nozzle, increase gate, and enlarge cold slug well, use dry material. material. 17.. Matl. Temperature too high: 17 Decrease stock temp., decrease holding pressure and time, decrease screw r.p.m. decrease back pressure, increase mold temp., shorten cooling and mould open. 18.. Streaks on Part: 18 Increase stock temp., or decrease, decrease screw r.p.m., tighten nozzle temp., decrease injection speed, increase back pressure, use dry material, length cooling and mold open time. 19. Voids in Part: Decrease stock temp., increase holding pressure and time, increase nozzle temp., decrease injection speed, increase mold temp. of decrease, increase size of gate, increase feed. 20. Discoloration: Decrease cylinder temp., decrease nozzle temp., decrease screw speed, decrease back pressure, decrease cycle, vent mould, increase gate, decrease residence time, and check material and pigments. Errors in mould and product design with possible consequence for process for process or moulded or moulded part: part: Faults

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possible problems

MOULD THEORY 133


W rong location

of gate:

Gates and/or runner too narrow: Runners too large

:

Unbalanced cavity lay ± out

:

Non-uniform mold cooling

:

Poor or no venting

:

Spure insufficient tapered

:

Sprue too long

:

cold weld lines, flow lines, jetting air entrapment, entrapment, venting problems warping, stress concentrations, voids and sink marks. Short shots, Plastics overheated, premature, free Zing of runners, sink and voids. Longer molding cycle, waste of plastics and Pressure losses. Unbalanced Unbalance d pressure build up in mould, mould, dimensional variation between Products, poor mould release, flash and stress. Longer molding cycle, high after shrinkage, Stress (warping), poor mold release, irregular surface finish and distortion of part during Ejection. Need for higher injection injecti on pressure, burned Plastics, poor mould release, short ± shot, and flow Lines. Poor mold release higher injection pressure, mold wear. Poor mold release, Pressure losses, longer moulds Cycle, and premature freezing of spure.

No round edge at end of sprue:

Notch sensitivity (cracks, bubbles) stress concentrations. Bad alignment and locking locki ng of cores: distortion of components, air entrapment, Dimensional variant, uneven stresses, poor mold Release. Mold movement due to insufficient: insuffici ent: Part flashes, dimensional variations, poor mold Mold support release, pressure losses Radius of sprue bushing too small : Plastic Plasti c leakage, poor mold release, pressure Losses. Mold and injection cylinder out of : Poor mold release, Plastic leakage, cylinder pushed Alignment back, pressure losses. Draft of Molded part too small Sharp transitions in part wall Thickness and sharp corners

:Poor mold release, distortion of molded part, dimensional variations. : Parts unevenly stressed, dimensional dimensional variations, air entrapment entrapment notch sensitivity mold wear. ASSIGNMENT:-

1. To avoid flash-----flash----------- the clamping pressure and --------- the injection pressure.

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MOULD THEORY 134


2. Why burning mark occurs. 3. Why short shot occurs? 4. Due to high stock temperature and injection speed ---------occurs. ---------oc curs. 5. Why dull surface occurs? 6. How flow lines can be be controlled? 7. Define streaks and its remedies? remedies? 8. How discoloration occurs? 9. What is purging and what is its requirement? 10. How wavy surfaces of a part can be controlled? 11. How a part sticking problem can be solved? s olved? 12. What are the problems of high melt temperature? 13. What is jetting? 14. --------- runner length increases i ncreases moulding cycle. 15. What do you mean by cavity lay-out? 16. What are the disadvantages of non-uniform cooling? 17. ---------runner ---------runner size causes short shot. 18. Small draft of moulded parts causes poor mould release. 19. Improper nozzle radius causes plastic leakage, poor mould release. 20. What are problems of low melt temperature?

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MOULD THEORY 135


MOULD MATERIALS AND ITS SELECTION:


- Explain the properties of Different mould materials - Know the application different mould materials in different standards - Know the recommended fits & tolerances for different parts of a mould

LESSON 14

MOULD MATERIALS AND ITS SELECTION: 

It is well known that without proper mould material proper tooling cannot be achieved. A wide range of mould materials are used for fabrication of moulds and

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MOULD THEORY 136


        

dies for plastics which includes Mild steel Alloy steel, Carbon Steel, Case hardened steel etc. Mould material selection will have to be carried out based on the requirements from product/Mould designer, Mould maker and Molder. Essential requirements of steel for moulds and dies: Excellent machinability machinability Excellent heat treatability treatability Good polishability Good Compressive strength High wear resistance Sufficient corrosion ± resistance The selection of steel s teel for a given mould is governed by the above requirement. requirement. The different grades of steel, characteristics hardening methods, surface treatment methods and relative merits and demerits are given in the chapter.

MOULD MATERIAL AND ITS APPLICATION

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MOULD THEORY 137


SL.NO

ISI Specif ication

Application

1

T35Cr 5Mo1V30

2

T35Cr 5Mo1 W1 V30 T35Cr 5Mo1 V1

Cavities, Cor e, E jector pins, guides, Wear pads of mould. Cor e and cavities of d f die casting die and moulds.

3 4

Cavities cor e E jector pins, Guides and wear pads. High tensile load applications max. str ength when har dened to 58-60 HRC used f or cavity housing cor e and cavity back plate shoulder s r scr ew and clamps etc. - do -

5

40Ni2Cr 1Mo28

6 7 8 9

13Ni3 Cr 80 T105Cr 1 T103

10

C10 and C14

Bolster , suppor t block, plates, Backing plates, holding plate etc.

11 12

C35Mn75 40Cr 1

Moulds with a shor ter r un and on accur ate cavities. Pillar , Bush, Spr ue bush, Locating Ring, Bigger dia e je jector p r pin.

13 14

50Cr 1 T55Ni2Cr 65Mo30

Used f or coil and plate spr ing Cor e and cavities.

Cavity and cor e Br ight Steel, used f or e jector pin, dowels etc Used f or g r guide pillar , Bush and wear ing pur poses Used as a wear plate, backing pad, delicate cor e pins used in moulds and die casting dies.

GENERAL RECOMMENDATION OF STEEL FOR CORE AND CAVITY Pr ocess In jection Mould

Ther moplastics

Compr ession Ther mo-set Mould Thr u har dened Pr ocess

Ther moplastics

P20 H11 H13

40Cr MnMo7 X38Cr MoV5 1 X40Cr MoV51

H13

Pr e har dened Steel Thr u har dened steel X40Cr MoV51

1.2311 1.2344

1.2344

SKD61

4550HRc

AISI

DIN

W .Nr

JIS

Har dness

MOULD MATERIALS ± EQUIVALENT STANDARDS .

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MOULD THEORY 138


SL.NO

ISI

BS

DIN

AIS / I /SAE

1

T35Cr 5Mo1V30

BH11

X38Cr MoV5 1

H11

2

T35Cr 5Mo W1 V30

BH12

X37Cr MoW 5 1

H12

3

T35Cr 5Mo W1

BH13

4

W.Nr (Ger many)

JIS

H13 Impr oved

1.2344

SKD6 1

420 Modif ied

1.2083

SUS4 20

H13

En110

34Cr NiMo6

5

40Ni2Cr 1Mo28

En24

30Cr NiMo4

*9840

***6

13Ni3Cr 28

En36

14NiCr 14

*3318

BF 1

120W A4 A4

F1

**7 (a)8

T105Cr 1

En31

100Cr

13

(b)9

T103

BW18

C105W 1

W1

(c)10

C10and C14

En2 A

*1006

11

C35Mn75

En8

*1040

12

40Cr 1

En18

*5140

(d)13

50Cr 1V23

En47

(d)14

50Cr 1V23

En48

15

T55Ni2Cr 65Mo30

(e)16

En560

50Cr 84

6152 5152

X40Cr 13

*51420

17 18

P20 X40Cr MoV51

19

*SAE Specification **Silver Steel ***case Hardening a) Bearing Steel b) Carbon Tool Steel c) Mild steel d) 1% Chromium Spring steel e) Corrosion Resistance, direct quenching steel for moulds used for corrosive plastics.

RECOMMENDED FITS AND TOLERANCES IN MOULD DESIGN

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MOULD THEORY 139


SL.N O 1

Descr iption

Natur e of Fit

Main Guide pillar ± r ± Main Guide bush

Slack r unning

***

H7/e7

2

Main Guide pillar ± r ± Main Guide bush

Close r unning

H7/g6

***

3

Main Guide pillar and Bush Housing

Light Keying

H7/k6

H7/k6

4 5

Spr ue Bush ±C ±Cavity inser t or H r Housing Cor e pin ±± Cavity inser t

Medium Dr ive Sliding

H7/m6 H7/h6

*** H7/h6

6 7

Side moving Cor e or sliding block ±Guiding slot Cavity inser t Housing

Close r unning Light Keying

H7/g6 H7/k6

H7/g6 H7/k6

8

Finger Cam Housing

Light Keying

H7/k6

H7/k6

9

Push Back pin ±± Cavity inser t or C r Cavity Nest

Close r unning

H7/g6

H7/g6

10

E je jector p r pin ±± Cavity inser t

Close r unning

H7/g6

H7/g6

11

Cavity inser t ±± Cavity inser t

Sliding

H7/h6

H7/h5

12

Al Alignment bush ±± Housing

Close r unning

H7/g6

H7/g6

13

E jector Guide Pin ±± Housing

Medium Dr ive

H7/m6

H7/m6

14

E je jector G r Guide pin ±± E je jector G r Guide bush

Close r unning

H7/g6

***

15

E jector Guide pin ±± E jector Guide Bush

Slack r unning

***

H7/e7

16

E je jector G r Guide pin ±± E je jector R r Retainer p r plate

Light Keying

H7/k6

H7/k6

17

E je jector G r Guide pin ±± E je jector p r plate

Running

H7/f 7

H7/f 7

18

Rest button ±± Housing

Heav y Dr ive

H7/m6

H7/m6

19

Register Ring ±± Machine Platen

Running

H7/f 7

***

20

Register R r Ring

Running

H7/f 7

***

COMPOSITION OF HSS STEEL AI DIN. C. Si . Mn. AISI

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P.

S.

MOULD THEORY 140

Cr .

Mo.

In jection

V.

W.

Compr ession

Ni.


P20+S P20+Ni ST AV A K R AM A XS AI AISI42 0

40Cr MnMo S86 0.35 0.45 0.40 X42Cr 13 0.38 0.45 X36Cr Mo17 0.33 0.43 X45 0.40 NiCr MO4 0.50

0.30 0.50 0.30 0.50 1.00 M AX 0.10 0.40

1.40 1.6 1.50 0.20 0.4 1.00 M AX 0.150 0.45

0.03 0

0.50 0.10

0.03 0 0.03 0 0.03 0

0.03 0 0.03 0 0.03 0

1.80 2 1.90 12.50 13.50 15.00 17.00 1.20 1.50

0.15 0.25 0.20

1.00

1.00 1.30 0.15 0.5

1.00 M AX 3.80 4.30

COMPOSITION OF E n MATERIALS:-

Gr ade

C.

Si .

Mn.

Cr .

Ni.

EN1 A

0.07/0.15

0.10Max

0.80/1.20

EN8

0.35 /0.45

0.05 /0.35

0.60 /1.00

EN8D

0.40/0.45

0.05/0.35

0.70/0.90

EN8M

0.35/0.45

0.25Max

1.00/1.30

EN8 AM AM

0.33/0.38

0.25Max

0.90/1.30

EN9

0.50 /0.60

0.05 /0.35

0.50 /0.80

EN18

0.35/0.45

0.10/0.35

0.60/0.95

0.80/1.10

EN19

0.35/0.45

0.10/0.35

0.50/0.80

0.90/1.50

EN19C

0.40/0.45

0.10/0.35

0.50/0.80

0.90/1.20

EN24

0.35 /0 /0.45

0.10 /0 /0.35

0.45 /0 /0.70

0.90 /1 /1.40

EN31

0.90 /1.20

0.10 /0.35

0.30 /0.75

1.00 /1.60

EN36b

0.12 /0.18

0.10 /0.35

0.10 /0.35

EN36c

0.12/0.18

0.10/0.35

EN41b

0.35/0.45

EN110 EN354

Mo.

V.

0.20/0.35 1.30 /1 /1.80

0.20 /0 /0.35

0.30 /0.60

0.60 /1.10

3.00 /3.70

0.10/0.35

0.30/0.60

0.60/1.10

3.00/3.70

0.10/0.25

0.10/0.45

0.65Max

1.40/1.80

0.40Max

0.10/0.25

0.35/0.45

0.10/0.35

0.40/0.80

0.90/1.40

1.20/1.60

0.10/0.20

+ Al.0.90/ 1.30

0.20Max

0.35Max

0.50/1.00

0.75/1.20

1.50/2.00

0.10/0.20

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MOULD THEORY 142


PRETO STANDARD MOULD BASES Preto

W

L

A1

A2

A3

B1

B2

B3

C

E

RD D1/D2 T1&

code

T2&T3

T4

T6 T7

26 to 56 26 to 76 26 to 76 26 to 86 26 to 86 27 to 86 26 to 86 26 to 86 26 to 106

66

130

38

90 31

10 24/16 20

P-P 1620 156 196 120 160 112

66

170

38

90 31

10 24/16 20

P-P 1625 156 246 120 210 162

66

220

38

90 31

10 24/16 20

P-P 2020 196 196 158 158 98

97

168

58

120 36

12 28/20 26

P-P 2025 196 246 158 208 148

97

218

58

120 36

12 28/20 26

P-P2030 196 296 158 258 198

97

268

58

120 36

12 28/20

26

P-P2035 196 346 158 308 248

97

318

58

120 36

12 28/20

26

P-P2040 196 396 158 358 298

97

368

58

120 36

14 28/20

26

P-P2525 246 246 200 200 136

94

218 132 150 46

14 32/24 26

P-P2530 246 296 200 250 184

94

268 132 150 46

14 32/24

P-P2535 246 346 200 300 234

94

318 132 150 46

P-P2540 246 396 200 350 284

94

368 132 150 46

P-P2550 246 496 200 450 384

94

468 132 150 46

26 26 to 106 26 to 14 32/24 26 106 26 to 14 32/24 26 106 26 to 14 32/24 26 106

P-P3030 296 296 250 250 184

144

268 182 200 46

14 32/24 26 66 to 146 46

P-P3035 296 346 250 300 234

144

318 182 200 46

14 32/24 26 66 to 146 46

P-P3040 296 396 250 350 284

144

368 182 200 46

14 32/24 26 66 to 147 46

P-P3045 296 446 250 400 334

144

418 182 200 46

14 32/24 26 66 to 146 46

W

L

A1

A2

A3

B1

B2

B3

C

E

RD D1/D2 T1&

code

26 26 36 36 36 36 36 36 36 36 36 36

T4

T5

T8

P-P3050 296 496 250 450 384

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T2&T3

26

46 to 66 46 to 66 46 to 66 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106

144

468 182 200 46

14 32/24 26 26 to 146 46

MOULD THEORY 143

66 to 106

Screws SMBe

EPL

10 12

M8

M6

10 12

M8

M6

10 12

M10

M6

12 16

M10

M6

12 16

M10

M6

12 16

M12

M8

12 16

M12

M8

12 16

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

16 26

M12

M8

T8

P-P 1616 156 156 120 120 72

Preto

T5

T6 T7

Screws SMBe

EPL

16 26 M12

M8


P-P3060 296 596 250 550 484

144

568 182 200 46

14 32/24

26 26 to 146 46

P-P3535 346 346 300 300 234

186

318 230 250 46

14 32/24

26 26to 146 46

P-P3540 346 396 300 350 284

186

368 230 250 46

14 32/24

26 26 to 146 46

P-P3545 346 446 300 400 334

186

418 230 250 46

14 32/24

26 26 to 146 46

P-P3550 346 496 300 450 384

186

468 230 250 46

14 32/24

26 26to 146 46

P-P3560 346 596 300 550 484

186

568 230 250 46

14 32/24

26 26 to 146 46

P-P4040 396 396 340 340 232

206

360 256 278 56

18 42/34

36 36to 186 56

P-P4045 396 446 340 390 282

206

410 256 278 56

18 42/34

36 36 to 186 56

P-P4050 396 496 340 440 332

206

460 256 278 56

18 42/34

36 36 to 186 56

P-P4060 396 596 340 540 432

206

560 256 278 56

18 42/34

36 36 to 186 56

P-P4545 446 446 390 390 282

256

410 306 328 56

18 42/34

36 36 to 186 56

P-P4550 446 496 390 440 332

256

460 306 328 56

18 42/34

36 36 to 186 56

P-P4560 446 596 390 540 432 256

560

306 328 56

18 42/34 36 36 to 186 56

P-P4570 446 696 390 640 532 256

660

306 328 56

18 42/34 36 36 to 186 56

P-P5050 496 496 440 440 332 306

460

356 328 56

18 42/34 36 36 to 186 56

P-P5055 496 546 440 490 382 306

510

356 328 56

18 42/34 36 36 to 186 56

P-P5060 496 596 440 540 432 306

560

356 328 56

18 42/34 36 36 to 186 56

P-P5070 496 696 440 640 532 306

660

356 328 56

18 42/34 36 36 to 186 56

P-P5555 546 546 490 490 382 356

510

406 428 56

18 42/34 36 36 to 186 56

P-P5560 546 596 490 540 432 356

560

406 428 56

18 42/34 36 36 to 186 56

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MOULD THEORY 144

66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 106 66 to 126 66 to 126 66 to 126 66 to 126 66 to 126 66 to 126 66 to 126 66 to 126 66 to 146 66 to 146 66 to 146 66 to 146 66 to 146 66 to 146

16 26 M12

M8

16 26 M12

M8

16 26 M12

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M10

16 26 M16

M12


P-P5570 546 696 490 640 532 356

660

406 428 56

18 42/34

P-P6060 596 596 532 532 424 390

560

440 462 64

18 50/42 36 36to 186 56

P-P6070 596 696 532 632 524 390

660

440 462 64

18 50/42 36 36 to 186 56

P-P6080 596 796 532 732 624 390

760

440 462 64

18 50/42

36 36 to 186 56

P-P6090 596 896 532 832 724 390

860

440 462 64

18 50/42

36 36 to 186 56

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MOULD THEORY 145

36 36 to 186 56

66 to 146 66 to 146 66 to 146 66 to 146 66 to 146

16 26 M16

M12

16 26 M16

M12

16 26 M16

M12

16 26 M16

M12

16 26 M16

M12


ASSIGNMENT:-

1. What are the required properties of mould materials? 2. ---------materials are widely used for core & cavity material. 3. ---------&---------are used for top plate and bottom plate. 4. ---------materials are used in compression moulding. 5. ---------is used for guide pillar & guide push material. 6. ---------, ---------materials are generally used for back plats, support block. 7. List the different parts of mould and write down the specific material for the parts. 8. List the different mating parts and the recommended fit for that. 9. For guide pillar and guide bush ---------fit is used. 10. For sprue bush and cavity insert ---------fit is used. 11. For register ring and machine platen ---------fit is used. 12. Why groove provided in the pillar. 13. Why stepped guide pillar is mostly used. 14. What do you mean by standard mould base? 15. How standard mould base are specified? 16. Give some example of standard mold base. 17. List the design check list. 18. What do you mean by parting surface of mold? 19. What is shot height of mould? 20. What is daylight of mould? 21. What is double day light? 22. Define floating stripper in case of three plate mould? 23. What is overflow well? 24. Define weld line. 25. Define shrinkage.

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DESIGN CHECK LIST:

Latest issue parts drawing used



with mould-fit press for which cut ended are press ejector specified



Are day-light and stroke of press sufficient for trouble for ejection



Are reverse views correct



Are one guide-pin and one return-pin upset



Do guide pin enter before any part of mould



Can mould assembled and disassembled easily



Has allowable been indicated (draft angle is per side)



Is plastic material and shrinkage factor is considered or specified



Are more plate heavy enough



Are mould parts be hardened and clearly specified



Are sufficient pillar are located specifically



Are water line , steam line, thermo-couple holes specified



This water in stroke out location clear press tie bars and clamp location



Is ejector trouble sufficient



Are stop bottoms under ejector bars specified



Are ejector pins sufficient



Is the steel type for mould parts specified



Have eye-bottle holes been provided



If stripper type, this stripper plate ride on guide pin for full stroke



With



Do lose parts fit in one way only (make fool-proof)



Can mould parts be ejected properly



Have trade mark and cavity number be specified

mould parts stay on ejector side of the mould

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

Have engraving been specified



Has mould identification been specified



Has plating special finish been specified



If there is provision for clamping position in the press



Are runner-gates and vents are shown are specified

DIE CASTING DIES

OBJECTIVE After end of the lesson/session trainees should be able to

-Explain the different die casting methods -Know the different die casting machines -know the different die casting materials, its properties & its applications -Know the die casting faults & its remedies

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LESSON 15

CLASSIFICATION OF DIE CASTING DIES .

SAND DIE CASTING

INVESTMENT DIE CANSTING

GRAVITY DIE CASTING PRESSURE DIE CASTING

HOT CAHMBER

SINGLE CAVITY

COLD CHAMBER

COMBINATION DIE

DOUBLE CAVITY

HORIZONTAL M/C

VERTICAL M/C

DIE CASTING DIES Introduction:

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Casting of molten metal in moulds is to shape metals into objects. The process involves the filling of a mould with molten metal and allowing the metal to harden. A great deal of this type of casting or molding was and still is done in moulds made of sand. Sand moulds require a new mould for each piece to be cast for more efficiency the permanent steel moulds were developed. The molding of non-ferrous material and their alloys (with relatively low melting temperature) in permanent steel moulds under pressure is called Die Castings.

DIE CASTING DIES 









Any die-casting die must be split into two sections so that the casting can be removed after it has been formed. These two sections are called the µcover die¶ and the µejector die¶. The cover die is fastened to the stationary platen on the casting machine and does not move during the casting cycle. The ejector half is mounted on the movable platen of the machine. A cavity that is a reproduction of a section of the part that is so be cast is formed or machined in each of these dies. It is conceivable that half of the casting could be formed in the cover die and half in the ejector die, but this is seldom the case because the parting plane, i.e., the mating surface of the die is usually the plane having the greatest cross-section al area on the part, and parts to be die-cast are seldom symmetrical. Although it is desirable to have the parting line in one plane, design consideration sometimes requires that it be irregular, curved, or slanted Both surfaces forming this parting line must be smooth and finished so that the die halves fit closely together. Otherwise a gap would exist through which molten metal could escape when forced into the cavity under pressure. It is also apparent that the two halves of the die must be in exact register when the die is closed, and the usual method of accomplishing this is to use dowel pins, as is done in stamping or drawing dies. These dowel pins are always placed in the stationary or cover-die member. Other die components include the die base, an ejection plate and ejector pins, and surface pins. The die base is a cast-iron or cast ±steel base on which the ejector die is mounted and in which is provided means of ejecting the casting. In the simple type of die, ejector pins are used to p0ush the part from the ejector cavity after the casting machine opens. These pins are mounted in a plate called the µejector plate¶, and slide through holes in the ejector-die half. When the machine opens this plate is pushed forward by some means, usually by a rack and pinion that may be tied in with the machine cycle so that it is forced forward automatically at a predetermined point. Since the ejector pins must be flush with the parting surface of the ejector die when the casting machine closes, surface pins and stop pins are incorporated in the die to ensure accurate location of the ejector plate. In addition to these die members, there are other components and considerations that must be investigated by the die designer prior to detailing a die: die gates and runners, which are the paths through which the metal flows to all portions of the cavity; vents, which provide a path of escape for entrapped air and released gases, cores and slides which form the hollow part or under cut sections of the

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casting locks that hold the die halves in register when an irregular parting line is specified: mounting holes land brackets for all die components: and of course a means of actuating the movable-die components.

Classification of Die Casting :

Die-casting can be classified broadly into two types as follows:Gravity Die Casting  Pressure Die Casting  Gravity Die Casting: 

The metallic mould is generally made into two halves to enable the casting to be taken out. The mould is closed and the liquid metal is poured from the top. In this process, liquid metal flows into the die entirely under its own weight and hence it is known as gravity die casting.

Pressure Die Casting: 

Here the molten metal is injected into the closed mould under pressure, thereby producing accurately dimensioned, sharply defined, smooth surfaced parts.

Advantages of the Pressure Die Casting Process:

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      

More complex shapes can be produced by the pressure die casting process than gravity die castings e.g. carburetor. Since the dies are filled under pressure, castings with thinner walls, greater length to thickness ratio and greater dimensional accuracy can be produced. Production rates are higher in pressure die casting. The castings are produced as almost completely finished parts. Dies for pressure die casting can produce many thousands of castings without significant change in casting dimensions. Many die castings can be plated with minimum surface preparation. Die castings produce fine grained structure there by having better mechanical properties.

Limitations of Pressure Die Casting Process :

Casting size is limited. Depending on the casting contours and gating, difficulty may be encountered with  air trapped in the die, which may cause porosities in the component. Die casting equipments are quite costly and hence large quantities are required  for the process to be economical. Commercial production is limited to nonferrous metals.  Application of Die Castings: They find extensive use in thousands of consumer and industrial products such as automobiles, house appliances. Electrical goods, photographic equipments, toys etc. 

Die Casting Alloys:

Only non-ferrous alloys are currently suitable for commercial castings.

production of die

These alloys are classified into the following three groups.   

Low melting point alloys (heavy metal alloys) ± Zinc, Lead and Tin High melting point alloys (light metal alloys) ± Aluminum, Magnesium. High melting point alloys (Heavy metal alloys) ± Silver, copper, Brass, Beryllium copper.

Die Casting Machines:

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There are two general types of die casting machines as identified by the placement of the injection chamber or metal pumping systems.  

Hot chamber (Goose neck) Machine. Cold Chamber Machine (a) Horizontal (b) Vertical.

Hot chamber machine.

In the hot chamber machine the goose neck containing the pressure cylinder and plunger is submerged in the molten alloy. The molten metal is injected under pressure of 280 ± 700 Kgs/cm². Only low melting point alloys are processed by this machine since these alloys do not pick up iron at higher temperatures.

Hot Chamber Process: 

The basic components of a hot-chamber die-casting machine and die are illustrated in Fig1 in the hot-chamber process; the plunger and cylinder are submerged in the molten metal in the holding furnace. The power to pump zinc into the die cavity is provided by a hydraulic accumulator. Oil is supplied to the

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accumulator by a hydraulic pump at a rate that will bring the accumulator pressures up to the desired operating level each time a casting (shot) is to be made. 

The casting sequence in the hot-chamber die-casting process is illustrated in Fig. 2. When a shot is made, the control valve opens causing the shot cylinder to force the plunger down and force molten metal though the nozzle, past the sprue pin, through the runners and gates, and into the die cavity. The gases that were in the system and some of the molten metal from flow through the die cavity and out through vents and / or into overflows. After the cavity is filled, the metal is allowed to solidify, the casting is ejected, and the cycle is repeated. Since the gooseneck and plunger are submerged in the molten metal, the system refills automatically when the plunger is withdrawn. Die is closed and hot chamber (i.e. goose neck) is filed with molten metal Plunger pushes molten metal through gooseneck and nozzle and into the die cavity. Metal is held under pressure until it solidifies. Die opens and cores, if any, retract. Casting stays in ejector die Plunger returns, puling molten metal back through nozzle and gooseneck. Ejector pins push casting out of ejector die. As plunger uncovers filling hole, molten metal flows through inlet to refill gooseneck

Cold Chamber Machine 

The cold chamber machine is similar to hot chamber machine except the injection side. Here the alloy is melted in a separate pot and the required amount of molten metal is loaded into the shot sleeve from where the injection plunger forces the molten metal into the die cavity. High melting point alloys are processed by the

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cold chamber process in order to keep the iron pick up by these alloys at higher temperatures to a minimum. W hen the shot chamber is in a horizontal position it is called horizontal cold chamber machine and when it is vertical it is called vertical cold chamber machine. Advantages of Hot Chamber Machines : Metal can be injected into the die cavities in minimum time.  The decreased in metal temperature is minimum.  Advantages of Cold Chamber Machines :  

Relative freedom from attack of molten metal on equipment. High injection pressure.

Disadvantages of Cold Chamber Machines:   

Need for auxiliary method of feeding. Longer cycle time is needed. Possibility of metal defects due to loss of super heat.

Cold- Chamber Process: 



The casting sequence for the clod-chamber process is illustrated in figure. In this process, molten metal is ladled into the cold chamber and then the plunger advances to force the metal into the die. Except for the manner in which molten metal is fed into the shot system and injected into the die, the casting sequence for the two processes is similar. Die is closed and molten metal is ladled into the cold chamber Plunger pushes molten metal into die cavity. The metal is held under pressure until it solidifies Die opens and plunger advances to insure casting stays in ejector die Cores, if any, retract. Ejector pins push casting out of ejector die and

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MOULD THEORY 155


plunger return to original position.

Process Variables: 1 MATERIAL

a) Alloy composition b) Metal Temperature c) Shot Weight 2 MACHINE

a) Accumulator pressure b) Injection Line Pressure c) Intensification d) Plunger Speed e) Locking Force f) Mode of Injection g) Plunger Diameter h) Timing of Stages in the casting cycle i)Lubrication 3. DIE a) Die Temperature b) Filing rate Alloy Composition:

MATERIALS:

To ensure durable casting, the alloy composition must conform to specifications. If the alloy composition is outside the specification range, immediate difficulties are unlikely to be experience but its performance in services can be greatly impaired. Satisfactory performance in casting is, therefore, no Indication that the alloy conforms to the specification. Metal Temperature : Aluminium:

Zinc:

Most die-casting alloys, for example LM2, 4,6 and 24. Contain the alloying elements silicon, iron and manganese, which for compounds that settle out when a melt is held that too low a temperature. These metallic segregates form hard spot inclusions, which can cause considerable difficulties when machining the castings. The temperature, temperature of the shot sleeve, mode of injection and delay between pouring and the start of injection. Metal temperature has relatively little effect on casting quality within the normal practicable limits of 395-450 C. If the metal by operating outside the normal casting range of 410-450 C. If the metal temperature is too high, it shortens the life of the plunger and gooseneck and it has been reported that the magnesium

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MOULD THEORY 156


loss increases substantially at 450C. Too low a metal temperature often results in a build-up of metal on then plunger stem unless heating is used in this area. This build up can reduce the injection speed by mechanical interference. Shot Weight:      

Variations in shot weight can influence the quality of castings by affecting. The volume of gases in the shot sleeve (%filling of the shot sleeve); The average metal temperature entering the die; The size of slug, which in turn influences the transmission of force from the plunger to the casting; The position of the metal at the changeover from slow to fast injection; The start of intensification on machines when it is initiated from a position switch. In some cases and excessive shot weight can be dangerous because very long slugs can burst and injure the operator as the die opens Die-casting materials

Alluminium: 

Alluminium and its alloys are used for products with intricate contour which are normally processed by die casting dies. Ex-cylinder heads, carburetor, crank casing, transmission housing (gear box), cast alluminium wheel, vacuum cleaners, electrical irons, utensils handles.

Copper & its alloys:      

Copper in its original state possesses some properties like ± Excellent resistant to corrosion Non magnetic properties Moderate to high hardness & strength High thermal & electrical conductivity Good appearance Good machinability Resistant to fatigue, corrosion

Uses:-

    

Electrical parts, heat exchangers, screw machine products, household utensils, for making alloys. When Cu is mixed with other metals, it gives the alloy of following properties:Excellent corrosion resistance Electrical & thermal conductivity High strength & Nice appearance Cu alloys are mainly formed by addition of Aluminium, Zn, Tin, Barium, Ni, Si, Pb with copper. Some of the alloys are Brass, Bronze, gun metal, bearing materials etc.

Magnesium & its alloys:-

Magnesium is a silvery white metal having the lowest density other than common structural materials. It has a specific gravity of 1.74 & a specific wt of 1.73 gm/cc. Mg has the melting point of 650 c Mg is often coated or painted or given some

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surface finish to avoid corrosion. It is used in aircraft industry due to its light weight. It makes alloy with Al, Zn, Mn, Zr etc Magnesium alloys possess following properties    

Uses        

High strength to weight ratio Good fatigue strength Good damping capacity High thermal conductivity High electrical conductivity.

airframes, engines, gear boxes, flooring, seating for aeroplanes, Helicopters, missiles and satellites. Body panels of ground transportation vehicles. For material handling equipments such as hand trucks, barrel skids For storage tanks ;and hopper Moving parts of textile machines. Furniture, lawn movers Typewriters, calculator etc. Binocular and camera bodies

The types of alloys are

Dow metal,

y

cast alloys of mg,

y

wrought mg alloys etc.

Lead and its alloys:-

Lead is the oldest among the softest of heavy metals. It is poisonous and should not be brought into contact with food. PropertiesLow melting point of 327C and density 11.34 kg/dm3  Resistant to corrosion except against aqua-regain.  It is poisonous.  Low strength and less hardness.  High density and heavy weight  soft and malleable  Good lubricating properties  High co-efficient of thermal expansion  Low electrical conductivity.  UsesManufacturing of storage batteries  Tank lining for corrosion protection.  pipe and drainage fitting¶  Low melting solders  Lead sheathing of electric cable.  Radiation protection.  Lead is availa ble commerciall y on the following forms 

sheet and food extrusions

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wool castings  powder  lead alloys  Zinc and Zinc base alloysZinc is a blue to gray metallic element. It has the following characteristicslow melting point, 419.5C  resistance to corrosion  Solubility in copper  Inherent ductility and malleability  

Application-

Stamping, anodes for electro galvanizing, coating on steel Die-casting, shells for dry batteries, engraver¶s plates MACHINE: Accumulator Pressure:

Accumulators are used to store up energy from the hydraulic pump during parts of toe cycle when the demand from the pumps is low. This enables a fast injection speed to be achieved with pumps of low output. Modern machines generally use pistons to separate the hydraulic fluid from the nitrogen. For any particular accumulator pre-charge pressure, the range of injection pressures, which can be used, is small. Therefore, if a substantial change is made in the injecting pressure, the accumulator pre-charge pressure must be adjusted. It is very important to have the correct pre-charge. Too high a pre-charge in relation to the injection pressure can result in the separator not being lifted, with the result that the accumulator wills not operate. Too low pre-charge results in a low injection pressure and a late build up to the full injection pressure.

In jection Line Pressure:

This (when no intensification is used) controls the maximum force on the plunger and it also influences the speed of injection. It cases where intensification is used, the line pressure influences the maximum injection force. On some machines the hydraulic pressure control valve of the locking circuit is situated upstream of the valve controlling the injection pressure, with the result that lowering the locking pressure can also lower the injection pressure. Intensification: The pressure build-up at the end of injection, the intensification stage can have a large effect on the porosity of castings. Intensifiers enable large changes to be made in injection force without changing the line pressure and accumulator precharge pressure. The rate of pressure build-up should not be too fast; otherwise it may cause the die to flash. Alternatively, too long a delay in the pressure buildup results in a low pressure being applied during solidification. Plunger Speed:

The plunger speed is usually set by a valve which simply restricts the flow of hydraulic fluid from the accumulator to the injection cylinder or air receiver to the injection cylinder. Small variations in plunger speed can occur due to fluctuations

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in the injection line pressure and also with large variations in die temperature when producing thin section castings. If a large change is made in injection pressure, this can produce a significant change in the injection speed. Fast injection can increase the amount of flash because of the heavy impact and the metal¶s more fluid condition at the completion of fill. Locking Force: 





Locking force is dependent upon the locking pressure, tie bar nut setting and temperature the die and surroundings. As the die heats up, the locking force increases and unless allowance has been made for this increase, the machine will stall when locking. For a given tie bar nut setting, increasing then pressure to the locking ram does not increase the locking force. Locking force, injection pressure and projected area of the casting together with die distort are the main factors controlling flash. For the best results, the die should, where possible, designed so that the center of pressure produced in the cavity acts in the center of the within the tie bars, i.e. the impressions should be positioned centrally within the tie bars. On some machines the hydraulic pressure control valve for the locking circuit is situated stream of the valve controlling the injection pressure. Little is to be gained by lowering the lock pressure. If one wished to reduce wear in the toggle linkages etc. When producing castings, the tie bar nuts should be slackened. For this purpose a plot of locking force against tie bar nut position is useful

Mode of In jection:

The use of two-stage injection with a slow first stage allows the gases more time to escape from the cavity during the initial fill period of the nozzle and in some cases of the run. Two-stage injection often also influences the cavity filling speed. Depending on the injection system and where the changeover occurs, it can either increase or decrease the filling speed. Plunger Diameter : 





Large diameter plungers and sleeves are used to increase the shot capacity. Increasing plunger diameter also reduces the final force of the metal and tends to reduce the plunger speed during injection. If the resistance to metal flow offered by the die is low, the change injection speed can be very small which results in a significant increase in metal velocity therefore reduced fill time. If the die offers a large resistance it may be possible for plunger speed to be lowered so, such that the metal velocity is also rescued thus increased the fill time. A change in the plunger diameter also alters the injection change over position of the ««« Increasing the plunger diameter makes the actual injection changeover position (Position metal) occurs later. In order to minimize time spent in changing shot sleeves, and also to reduce the number spare parts required, it is often convenient to use the same size plunger for the whole ran castings produced on a give machine. The size of the plunger is the governed by the last shot weight from the given range of castings.

Timing of Stages in the Casting Cycle:

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Some machines have an overall cycle timer which in effect controls the die open time. Usually the die open time is controlled by the opening and closing speed of the machine and, in the case of semi-automatic machines, by the speed of the operator. During the die open period the cavity surface and sub-surface have a slower cooling rate than at any other time during the cycle (apart from the short time interval between the die closing and injection). The injection timer and the die cooling timer usually run consecutively, with the result that both influence the solidification time of the casting. The fastest method of operating the die, from the heat extraction point of view, is to have the die open time at a minimum. This increase the temperature of the die fact at injection which is an advantage, especially in the case of a thin section casting requiring a good surface finish. Lubrication: Correct lubrication of the die and of shot sleeve and plunger is very important because it affects the slenderness and surface finish of the casting and the life of the shot sleeve and plunger. DIE: Die Temperature : 









  

No one die temperature can be recommended for producing acceptable casting s because not only does the correct temperature vary from die to die by also it varies with time within the die both perpendicular to the surface and across the surface. Surface finish is largely controlled by the die face temperature at injection. The casting dimensions are dependent on the temperature of the casting at ejection and to a lesser extent by the die temperature. Too high an ejection temperature can also cause blisters in the castings. Correct die temperature distribution and cooling can increase soundness by improving the freezing pattern, In some cases, is sounder castings are produced during the warm ± up period than when the die is up to full working temperature. This is because the extremities of the casting tend to freeze faster relative to the feed area (the runner / gate area) during the early shots. Die temperature is governed by the casting rate, cooling water flow and temperature. The maximum heat extraction rate is obtained by having the highest ratio possible of the time when the casting is in contact with the die compared with the non-contact period, i.e. approximately, die closed time/die open time should be maximum. The die is cooled by the cooling water, radiation and conduction to the surrounding machine and atmosphere. Generally the cooling effect of the die lubricant is negligible. For example, in the case of a 2-impression zinc door handle casting, the following heat input and extraction figures were recorded and calculated on the assumption that the lubricant extracted the maximum amount of heat, i.e. the lubricant was turned into dry steam: Ratio heat extract by cooling water/heat input from castings = 0.84 Ratio heat extracted by die lubricant/heat input from castings=0.06 The die lubricant used was diluted 5:1 with water. Even in cases where excessive lubricant is used, sometimes with the object of cooling the die, it is

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surprising how little cooling is achieved by the water/lubricant because only a thin layer on the die is cooled and then rapidly heated by the bulk of the sub-surface material. Filling Rate:    

Filling rate can be considered in terms of Cavity fill time, u its seconds Cavity filling rate or volume flow rate during fill, units: cm3/s or in 3/s Gate velocity, units m/s or ft/s For a given die, all three can be related by simple mathematical relationships. None is the all-important factor for surface finish or soundness, but gate velocity is important from the point of view of die wear, and soldering problems in the gate area. TYPES OF DIES

Die-casting dies can be roughly classified into five types,     

single impression, multiple impression, combination dies, unit dies, and Adjustable dies for different lengths and widths of castings.

SINGLE-IMPRESSION DIES:

A single-impression die is probably the most universal type used by all die casting plants. Since it contains only one cavity which is machined or hobbed in the mating halves, it is basically the least simplex and easiest to build. For small, simple castings, it is inexpensive and therefore often is used when production requirements are low; for medium-or large size parts, it may be the only practical unit that can be used because of the size limitation of the machine. For all practical purposes it may be considered as the standard die-casting die, all the others being modification of it to achieve faster and more economical production. COMBINATION DIE:

Combination dies and multiple dies both have the same purpose: to permit the casting of two or more parts simultaneously. Thus, both contain two or more die cavities. The difference between them is that the multiple die is used for casting that are similar (i.e. for

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the casting of several parts of the same shape), while the combination die is used for castings that are different (i.e. for the casting of two or more parts of unlike shape). Also, the no. of cavities in the combination dies often is limited, while a greater no. is usually are employed in the multiple dies. Combination dies usually are used for different parts of the same assembly whose production requirements- either large or small- always are similar. Multiple dies, on the other hand, are confined almost entirely too small parts that must be produced in quantity, such as certain business- machine casting. UNIT DIES 

For the die casting of moderately sized parts, especially when the desired production is relatively low, the use of unit dies is of considerable advantage. The difference between this and a multiple die is that in the later the impressions are machined or hobbed in the die halves, while in the unit die the cavities are formed in replaceable die units which then are set in machine location in the die, and to which the charge of metal is simultaneously delivered.



Each of the replaceable units constitutes a separate part. It may or may not differ from the others in respect to the size and shape of the casting impression, but it must have similar outer dimensions so that it can be fitted into the locations in the die. Thus, one unit die may be used to die cast a number of different parts for a number of different products.

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All unit dies have a common supporting mechanism, termed as µmaster holding die¶ that contains the machined locations for the unit blocks. The master holding die is permanently fastened to the die casting machine, from which the units can be quickly and easily interchanged without interrupting casting operation for more than several minutes at a time.

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The holding die is gated in order to distribute the casting alloy to each of the units from a single gate opening, and each unit as an individual ejector mechanism that is coupled with the ejector mechanism of the holding die.

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Units are made in various sizes and may be of round, square or rectangular shape. The master holding die may be constructed for multiples of two, four, six, or eight units and thus may range in overall size from quite small to quite large.

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The primary purpose if a combination dies is, of course, to reduce cost- to use on the die block instead of two, and to produce two or more parts per casting cycle

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

instead of one. The precaution that must be observed is to balance the die thermally. The parts to be die-cast may vary considerably in shape and size. For this reason it is sometimes quite; difficult to obtain a thermal balance in the die, but when the die is designed the impressions should be disposed with the view of obtaining the maximum uniformity of heat input.

Multiple-Impression Dies: 

Multiple ±impression dies are used where large and rapid production of a part is required. The number of impressions in such a die depends upon the size and shape of the part and the rate of production needed. For some small simpleshaped parts the number of impressions may run as high as 32 or 48, such dies usually are made for only one type of casting. They may be used, however, to cast two parts that differ from each other only by slight change in orientation, such as the casting of right-and lefthanded parts that go into one assembly.

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The greater the number of impressions used in a die, the greater the productivity in parts per hour, and consequently the lower the labour cost per piece. On the other hand, the more impressions used, the greater will be the die cost.

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The impressions in a multiple die should be arranged symmetrically and uniformly each impression being equidistant from the other. Impressions may be disposed in a straight line of in circular form. If arranged in a line, the total amount of impressions should be of an even number since it is desirable to have the same number of impressions is necessary to obtain uniformity of heat distribution in the die.

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Sufficient space should always be provided between the edge of the impressions and the outer edge of the Impressions and the outer edge of the die block. This are, usually termed µthe metal seal¶, generally should measure not less than 3 in (75mm). If less than this minimum, the molten metal may shot out through the parting line, especially if the die blocks ³blow´ or part is slightly under the pressure of the metal entering the die.

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DIE CASTING FAULTS AND REMEDIES

The various faults commonly encountered in die-castings can be classified into three groups: 1. Cold type defects     

Lack of fill Cold shut Severe chill Chill Flow lines

2. Hot-type defects     

Soldering Cracks Broken part Bend part Heat marks or shrinkage pits

3. Miscellaneous defect      

Scale Blisters Porosity Excessive flash Mechanical defects Ejector pin marks When correcting faults during a casting run, it is important that the right corrective action is taken as soon as possible. A fault correcting procedure, intended for shop floor use is given below:

Lack of fill:

This condition has three basis causes. The first cause is inadequate metal in the gooseneck or cold chamber. The metal level n the holding furnace of a hot chamber machine ma=must be maintained above the gooseneck inlet pots. For cold chamber machines, the correct size ladle must be used, and care must be taken to insure that the ladle is full for each shot. When a automatic ladle is being used, it must be properly adjusted to ladle exactly the correct amount of metal into the cold chamber. Secondly, this defect may be caused by cold metal, cold die or both. The temperatures should be checked and adjusted ad necessary. Finally, lack of fill may be the result of slow shot speed. The shot control hydraulic valves should be opened the proper amount. Cold shut:

Cold metal, slow shot or low die causes cold shut, like lack of fills, temperatures. If air vents and/or overflows clogged with flash they may also contribute to the problem. All these factors must be checked and correct as necessary.

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Severe Chill:

Severe chill similar to cold shut. But it will cover a large surface of the casting instead of being a single line. Excessive release material as well as cold, metal, slow shot, low die temperature, or clogged air vents may be the cause. Severe chill usually appears when shot are made into a cold die, but will rarely occur during normal operation. Chill:

This defect has the same appearance and is cause by the same conditions as severe chill, but is less noticeable. Slight or fain chill lines on the surface may not be cause for rejection of certain types of castings. However, castings used for ornamental parts will usually require a chill-free surface. Low die temperature, low metal temperature, is low shot speed, or excessive die release materials are al cause of chill. Flow lines:

Flow line defects are similar to chill and clod shut. Flow lines can usually be reduced or eliminated by increasing the die temperature, metal temperature, or both. Like chill, the defect may be corrected by higher shot speed or less die release material. Soldering: 

This condition is the result of the cast metal bonding to the die surface. Upon ejection the casting tears away, leaving a layer that has bonded to the die. When soldering occurs, it may create additional problems such as cracking or bending of the casting, out-of-tolerance dimensions depressed ejector pin marks in the casting and/or porosity within the casting.



Soldering may be cause by excessive metal temperature, incorrect die temperature (too hot or too clod) or insufficient die release material. If the condition is severe or when other methods fail to remove the solder, it must be cleaned from the die. Caustic solutions provided for this purpose may be use, or the material may be polished out of the die. It is recommended that a die maker or other person with special training polish the die cavity. If the polishing is not done properly, a rough die surface may be created that will increase the soldering condition. Care must also be exercised when increasing the amount of die release material applied to the soldering area. Excessive amounts of this material may create other defects such as porosity, chill, or blisters. When adjustments to the above conditions do not eliminate the soldering condition, it is likely that the problem may have been cause by the metal alloy being cast or it may result from the die construction.

Sink Marks:

Shallow smooth depressions on the casting¶s surface are called sink marks. Such marks usually appear on the casting surface opposite any heavy section such as rib or boss and are caused by uneven shrinkage of the casting. Reduced

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die temperature in; the area of the sink mark, reduced metal temperature, and sometimes increasing the temperature of the other die half will minimize these defects. Sometimes increased injection (shot) pressure coupled within higher die temperatures between the defect and the gate will help to reduce sink marks. Scale :

Build-up of die release materials (or their oxides) on the die results in an irregular rough surface on the casting. The material must be removed with a caustic solution or by polishing the die. The same care is needed as described for soldering. After cleaning the die, the amount of release material applied for each shot must be reduced. Sometimes it may be desirable to change the type or release material and/or the ratio of release ;material to solvent(or water) Blisters:

Bubble-like bumps on the casting surface are caused by air or other gases trapped inside the casting. Slower shot speed, clean vents, reduced die temperature, or less die release material will usually eliminate blisters. Porosity:

Large holes in the casting are called porosity. Low die temperature (particularly in the runner and gate area), low shot pressure, clogged vents, or excessive release material can cause porosity. Porosity is also often related to lack of fill, cold shut, heat marks, and blisters. When such a relationship exists, the correction for the related defect will often improve or eliminate the porosity.

Excessive Flash:

Excessive flash results from material such as flash sticking to the die faces and holding the die open, excessive injection pressure or speed, or insufficient clamping force. The first problem is corrected by cleaning the die faces. Flash that has become embedded into the die face must be scraped off. Corrections to injection speed and pressure must be made by adjusting the appropriate hydraulic valves. The clamping force is increased by adjusting the tie-bar nuts. Very slight adjustments to these nuts are usually al that is required. Flash indicates that an extra thick casting is being made. Extra thickness causes extra thickness causes extra heat input to the die, and may result in additional problems. Cracks : 

Castings may crack from internal stress or from abnormal pressure during ejection. The first cause, internal stress, is created from excessive metal or dies temperatures. If the condition persists after several temperature adjustments have been made, it may be necessary to increase the shot or machine timer settings. The timers should be adjusted only after everything else has failed.



Cracks from abnormal ejection pressure may be indirectly the result of soldering. The operator should carefully inspect the casting for signs of soldering. If soldering exists, the appropriate corrective measure should be taken. Insufficient draft or a rough cavity finish in the die can also cause abnormal ejection

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pressure. Additional die release material may help, but will not correct this situation. In severe instances, die repair may be necessary Broken Part:

A portion of the casting may stick in the die during ejection and the rest of the casting break away and eject normally. The cause I the same condition as described for cracks: excessive metal or die temperature, insufficient die release material, or soldering. Bent Part:

Castings may bend instead of breaking when part of the casting sticks in the die. This is a different result from the same conditions that cause a broken part. Heat Marks: 



Have the appearance of surface pits and are causes by excessive di3 temperature or excessive metal temperature. Depressed areas in the casting and sharp inside corners are the most susceptible to this defect. Sometimes die temperature µbalance¶ can be adjusted to eliminate heat marks. For example, the flow of cooling water to the area affected may be increase, and decreased to the other die half in the same area. Clogged vents or excessive die release may cause large volumes of gas to become entrapped in the die. Such gases will increase the size of heat marks and may cause the pits to become rounded & smooth on the inside.

Wave or Lake:

Irregular lines or slight steps on otherwise smooth casting surfaces are usually caused by excessive die temperature in the area of the defect. Increasing the flow of die cooling water of increasing the water (or solvent) dilution of the die release material will reduce such defects.

Mechanical Defects:

The operator should be aware of all moving and fragile parts of the die cavity, which are subject to wear, breakage, or other failures that could cause defective castings. Small cores or thin blades of the die forming deep narrow holes or slots in the casting can bee easily broken or bent. Ejector pins and moving cores can wear, break, or not seat properly. In any of these situations, the die will not make the part to the correct shape.

Ejector Pin Marks:

High and low ejector pin marks. Ejector pins may push into the casting when solder and/or a rough cavity surface result in the casting sticking in the cavity. High casting temperature at the time of ejection also can let the pins push in to the part. Increases flow of cooling water or more liberal application of release material will sometimes reduce low ejector pin marks.

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ASSIGNMENT

1.

W hat

do you mean by casting?

2.

W hat

are the types of die casting?

3. In «.. method molten metal is poured from the top. 4. In ««. Method molten metal is injected i9nto the mould under pressure. 5.

W hat

is die casting alloys.

6. Name some die casting alloys.

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ALIGNMETN OF MOULDS . CHANGING OF MOLDS .

Functions of alignment injection of moulds are mounted onto the platens of the clamping unit of the injection. Moulding machine. The clamping unit opens and closes them during the course of the molding cycle. The molds have to be guided in such a way that all inserts are accurately aligned and the mold halves are tightly closed. Without proper alignment moldings would exhibits deviations in wall thickness; they would not have the required dimensions because guiding moulds with the clamping unit alone is generally not sufficient. Injection moulds also need a so-called internal alignment. It aligns both mold halves with the necessary precision and prevents their convolute joining. Alignment with the axis of the plasticating unit

Precise alignment is mandatory here otherwise the nozzle could interrupt operation. Therefore alignments concentric with the sprue bushing are used almost exclusively. For this purpose the mold is equipped with a flange-like locating ring, which keeps the sprue bushing in the mold and matches the corresponding opening in the machine platen. Locating rings are either machined into the mold clamping plates or most of the time, a separate part mounted onto the mold. They are readily available from producers of mold standards and are machined from case-hardening steel or water-quenched unalloyed tool steel. Of course, a mould has usually only one locating ring. If both mold halves a re equipped with locating rings, then a loose fit is needs on the movable side to better align both sides. This is only a help for setting up the mold. The locating ring is slightly press- fitted into the mold plate and fit the machine platen with a close sliding fit the figure. 505 shows a two piece locating ring. This design is particularly useful for adding an insulating sheet to the mold, this is done for processing thermosets or thermoplastics if a high mold temperature is needed for molding parts. Internal alignment and interlocking.

The mold halves themselves have to be guided internally, besides by the tie bars of the machine, to obtain the needed accuracy. In small molds this is done with leader pins. They are pins which protrude from one mold half of the opened mold and slide into precisely fitting bushing in the other mold half during mold closing. This ensures a constant and accurate alignment of both surfaces of flat moulds without shifting during injection and production of moldings. In molds with deep cavities, especially those with long and slender cores, a shifting of the core can occur during injection in spite of exact alignment with leader pins this has

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already been discussed in chapter 11. " shifting of core" . design examples for such molds were presented with figure . 405 Figure shown an example of positioning and mounting a leader pin and the appropriate bushing. Four leader " units" ( pin and bushing) are usually required for proper alignment .to facilitate the assembly and to make sure that the mold is always correctly put together, one leader pin is offset or made in different dimension. The latter method may cause fewer difficulties. If two leader pins, one diagonally opposite the other, are made longer. It is easier to slide the two halves together while placing the mold into the machine or during assembly. The leader pins are positioned as close to the edge as possible to gain space for the cavity and an adequate number of cooling lines. Effective alignments is possible only if close tolerances are kept between leader pins and corresponding holes. This however, causes considerable wear. Therefore it is not prudent to let the pins slide directly into the respective holes of the mold plates. As a matter of principle, leader bushings should be used to counteract wear and to enable worn out parts to be exchanged easily. Leader bushings. Like leader pins are made of case hardened steel with a hardness of 60 to 62 Rc. They are commercially available in various sizes and shapes. Wear can furthermore be reduced by lubricating the pins with molybdenum disulfide. For this purpose pins or bushings have oil grooves, leader pins without lubrication (fig.510) should only be used for rather small molds or special application in sliders or with ball bushing.(fig.512) Leader pins and bushing are commercially available in various designs . attention should be paid to the recommended fits . the length of leader pins depends on the depth of the cavity. Leading has to begin before the mold halves are engaged. Therefore a sufficient length must be selected. Shoulder leader pin can at the same time. Pin mold plates together. Commercial availability is treated in chapter 17. The length of leader bushings depends on their diameter. It should be 1.5 to 3 times the inside diameter. the corresponding holes in the mold plate have to be drilled according to instructions. Figure 510 and 511 demonstrate the assembly leader pins and bushing [1]. Fig.510 shows the system with shoulder leader pins. Bores for pin and bushing can be drilled in a single operation. ( equal diameters). The system of fig. 512 is not used very often . it is carefree and ensures a precise and low-friction movement but at additional expenses. With

a leader sleeve, mold plates can be aligned and connected with one another which would otherwise not be engaged by leader pin or bushing . The diameter of the sleeve is kept the same as that of the bushing or the shoulder of a shoulder leader pin. Therefore all holes can be machined in one pass. The internal diameter is adequately large to permit unrestricted entering of pins.

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Among the multitude of leader pin systems, those shown in figure 514 and 515 should also be mentioned . both are based on components already described: leader pin, bushing and leave. The system of Fig. 514 has pins and bushings with threaded holes and tightening crews, which are propped on the opposite side by head supports. Plates not engaged by pin and bushing are lead by sleeves. This design is more expensive, however, than the one shown in fig 506 but has some decisive advantages. The assembly of the individual plates is not done with screws, and additional drilling of holes is unnecessary. The plates are kept together with the tightening screws. At the same time the mold plate can be utilized better for accommodating cavity and cooling lines. The system the system in fig 515 shows a very different design a leader bushing and sleeves and their assembly in mold. The bushings consist of three parts : the bushing proper , a retainer ring and a ring nut. There are two locating for the retainer ring. The bushings can be mounted flush with the plate surface or protruding by 5 mm and tightened with the nut. In the protruding position any number of plates cab be connected to one another. This kind of bushings can take over the job of bushing with or without retainer ring and can also be used as leader sleeve. In this case no ring nut is used. The nut connects the bushings reliably to the mold plates. This could save additional plates. W hich are needed in other systems for supporting the bushings . the height of the mold is lower. This may compensate the cost for the more elaborate design. To ensure proper operation no lateral forces should act upon the leader system, if there is no lateral load, the required cross-section of the leader pins does not have to be computed. For oblique pins, especially those acting on slides. The necessary cross section should be calculated. Alignment of large molds

Occasionally no leader pins are used in a large and deep molds such as those for buckers and boxes. Guiding is left to the tie bars of the molding machine during opening and closing until short of complete engagement. Since this accuracy is insufficient for proper alignment , special arrangements become necessary . they are all characterized by the fact that the alignment does not begin sooner that shortly before the mold is closed. Both mold halves brace one another when closed. Of special advantage is the "pot" design( fig.516) and its variations and because it also reacts against forces from cavity expansion. The inserted ledges in the variations are easily replaceable after they are worn-out. More variation re presented with Fig 519. Frequently bolts are used as aligning interlocks fitted into both mold halves their center line is not in the plane of the parting line. Thus, both mold halves are braced against one another after clamping. Fig.521 shows such a mold. Instead of cylindrical alignment bolts rectangular interlock made of shock-resistant tool steel can be employed. Alignment of molds by such interlocks calls for high accuracy of machining, because later corrections are not practical. Frequently, tapered interlocks according to fig. 522 are finally used.

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Chain molds: Systems for a quick change of molds for thermoplastics.

Injections molds are usually mounted to the machine platens by mechanical clamping devices( conventional mold clamps with bolts) and connected to power- and watersupply lines. To of this the mold is either horizontally or vertically brought into the machine by lifting device. Depending on size and weight of he mold and the number of connections this leads to shutdown times. Which may last from an hour to several days( Fig 523) such secondary times affect the productivity and better economics lead by necessity to systems for a quick mold change. In spite of this , such systems have hardly prevailed so far. There are two reasons for this. One is the lack of compatibility among the various systems on the market today[7,8]. The second one is the need for a change of almost all molds used in a machined and associated high cost. A quick change system consists of several components which allow to change injection molds either fully automatically or semi-automatically, controlled by an operator. Such components serves the function of - detaching and fastening the mold at he machine platens. - disconnecting and connecting the supply lines. - bringing the mold into the clamping it or taking it out. From this follows the need for these means of quick-change system: -quick-clamping devices. -quick-connection couplings - changing equipments. Besides this some more components are required for an automation of mold changing, which have to be combined to one system. Only the combined action of all component permits flexible and automated injection molding[7]. Mold design is mostly affected only by quick-connection couplings. Two solutions for quick ±clamping devices have prevailed on t eh market. One can distinguished between adaptive and integrated clamping systems, which are usually actuated hydraulically. They can easily be inserted into a concept of flexible automation. The adaptive clamping system has hydraulically actuated locking cylinders or ledges with integrated collects[6] mounted to the clamping platens of the machine, into which the precisely machined clamping plate of the mold is inserted. They are mostly chamfered or provided with a groove( fig. 525 &526) During clamping , piston or ledge, which is also chamfered, is moved against a corresponding counter chamfer of the mold( fig.525 &526). The counter chamfer is about 5r. This angle causes self-locking ( as long as oil has dripped on it). For reasons of safety, clamping elements are therefore equipped with a proximity switch as a standard[11].

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The integrated clamping system has a hydraulic locking device integrated into the clamping platens, which hydraulically locks a bolt mounted in the base plate of he mold. The equipment to automate the mold change is so multifarious today, that not everything can be presented here. The references [12 to 18] should be consulted. All systems presented so far operate basically with the same locking mechanism. As soon as they are mounted to the clamping platen of he machine, they have to be looked at as rigid system, which determines the size of he mold plate independent of the mold size. Thus, the base plates of small molds may become disproportionately large and for large molds one eventually has to witch to a machine with greater clamping force. The solutions presented so far assume a rectangular clamping plate of the mold. Fig 528 present a design with hydraulic clamping sides acting on locating rings and integrated into the clamping platens of the machine. This system is suited for rectangular as well as for circular molds[19]. A manually operated quick-clamping device is on the market as a supplement composed of standards[8]. The quick-clamping systems for changing molds already results in a considerable shortening of the setup time but it doesn't yet ensure a fully automatic change of molds. This is only made possible by employing quick-coupling systems for energy supply and for sensors. Required are the following connections for - ejectors - heat-exchange medium( oil, water) - hydraulic connections for heating (hot runners) - connections for thermocouples and pressure sensors. Coupling systems are designed as modular systems and consist, depending on size. Of individual couplings for energy supply and sensor connections, guide pins. Lock and docking cylinders. Thus the system cab be assembled as demanded by the application. They are supplied as standards for manual and automatic operation. Accurate assembly of these systems is mandatory for their trouble free function. A small inexactness during assembly can lead to inadmissible displacement, which results in leakage and premature wear. Therefore coupling elements are floating in the carrier plates in order to eventually allow an adjustment. Thermal expansions cab also be captured this way especially in large molds[ 6,7]. A mold-change equipment affects mold design, if at all, only insignificantly. They are accessories, which change molds and, depending on make-up, can accept the function of a mold- conveying system. . they are illustrated in even more detail in numerous publications [ 6 to 8,10,13 to 17, 20 to 26]. The complete mold is always exchanged. Depending on the standard of the equipment , molds are immediately ready for operation and are already brought up to working temperature during the change procedure.

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Starting with a special mold concept[27], which only provides for an exchange of shapeproviding mold insets, an exchange device was developed[ fig. 530]. It is machinebound and attached to the mould inserts are located above the clamping unit. An exchange mechanism feeds the inserts can be locked by the already described quickclamping devices and connected to supply lines by quick-connectors. This system can not be transferred to all types of molds, but offers significant cost advantages for small lots and with simple molds. Suitable mold bases are offered in the US and Japan as standards [29].

Standard elements for molds. Standard elements in general

Mould prices and delivery deadlines are always the critical points in production planning and sales negotiations. The price of the mold has a major effect on the unit price of the part to be produced. Other factors having a decisive effect on successful sales are exact calculation and rapid launching of the new products onto the market. These requirements put the mold maker under constant pressure. For quotation purposes high-precision molds must be exactly calculated and then manufactured at short notice. The mold maker is forced to limit his activities to the essential, i.e., to the machining of cores and cavities. Everything else is a necessary evil and must be achieved in the simplest manner possible. The most economical way is to use standard elements. Standard elements are standardized mold part, which are available ex-stock. Even during the stages of mold calculation and planning. Standard elements at fixed prices can be widely considered. Today standard elements are used to a large extend as bases for molds and dies, as well as for connections to machines and other equipment

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and for special purposes. The use of standard elements helps to keep within the bounds of production capacities. To minimize the calculation and production risk and to simplify the procurement of spare parts. The possibility of buying standardized elements at short notice considerably reduces stock keeping and shut down times of production capacities. The first standard elements for molds were known in the U.S. even before World War II. In Germany, too, the first step in this direction was made at a very early stage: "turnwald standardization" of mold bases for compression molds can be regarded as the first positive result. In the late 1950's standardized mold bases were introduced on the German market for the first time, thus completing the range of standardized mold component which had already been available for a considerable time. Mold making today is moving towards more specialization. Various machine operators on milling, lathe, EDM or jig grinding machines manufacture parts of the finished mold separately at the final stage all these parts are brought together and assembled by the mold maker. He has the overall responsibility for the functioning of the mold, which is also a result of teamwork. Standard elements are designed exactly for this type of manufacturing process. Figure shows a selection of standard elements available from the catalogue for economical and efficient mold making and production. In principle almost all molds consist of the same basic elements: mold plate for the inserts, intermediate plates for supporting the cores and insets in the mold plate on the ejector side, risers to limit the working distance of the ejector plates, and clamping plates to clamp the mold to the machine. Comprehensive studies have shown that the total hour required for mold making can be split up as shown in fig. 2. at least 25% of the required capacity can be saved by using standard elements. In addition, special machining requirements can also be handled by the manufacturer of standard elements. In total this adds up to some 40% of the capacity required for the production of a mold. Thus in the example shown some 25% of the machine capacity is required for rough machining. Only partial utilization is possible, since the contour parts require more time on small special-purpose machines. A mold making shop that replaces rough machining capacity by specialized machining capacity for the production of counterparts utilizes manpower and machines more efficiently. Calculations can be made with lower hourly rates, since the rate or utilization is better. Mass production of standard elements for molds on large, highly efficient special machinery guarantees high quality and purchase without risk because of fixed prices and reliable delivery dates. Standard elements for mold construction.

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Basically two types are available as standard elements: assembled mold bases, referred to a basic mold, and the modular systems for individual combinations.

Basic molds

Basic molds as shown in Fig.3 are assembled as ready-made mold bases, available in a relatively small range only. Their application is far more limited because the designer is confined by being kept to the assembled mold base. Modular systems Modular standard elements consist of individual places. Which allows the designer and mold maker the flexibility to build molds to their own individual versions. 1. ready to use interchangeable jig-bored plates , so-called standard elements" with holes for guide pins, bushes and screws 2. Unbored plates, all surfaces machined, so called "P standard elements" in the same dimension and steel grades as K standard elements. These tow versions of modular elements can be combined in any way to solve every mold construction problem.

The K standard elements are based on a patented modular system. Figure 6/A shown the guide and doweling elements of this system. They fit in uniform holes passing through all plates, guiding and doweling all the plates of the mold. The guide pins(l) , guide bushes(2) and centering sleeves(3) can be arranged as required , depending on the combination of plates used. In addition , P standard elements can be used to advantage for the construction of jigs, special machinery and also as mold inserts if made of suitable steel grades. The round K standard plates are available with either four, three or two guide bores. Guide elements as shown in fig.6/B can be used for simpler molds where not all of the plates need to be guided through the guide system. They do not have a spigot at the rear and are particularly suitable when no clamping plate is used on the stationary mold half. This illustration also shows the possibility of combining K and P standard elements. The guide pin shown in Fig 6/C is designed for special applications. It has tow differently tolerance fits, ensuring that one side of the pin has a press fit in the H7 bores of the cavity plates , while the other side has a slide fit in the H7 bores of the guide bushes. The location holes for the guide and centering elements( diameter 1) are identical for all plates. The only exception are the bore of the K 25 cavity plates. In order to eliminate problems arising from distortion during heat treatment of machining , these plates have grinding allowances in the locating holes ( fig. 7).

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MOULD THEORY 178


Reworking these holes form dia 2 to dia 1 ensures interchangeability. This is of importance for stock keeping and replacing the guide elements. The guide dia( dia 3) of the pins and bushes are different in order to prevent incorrect assembly of the stationary and moving half. Apart from the finished guide bores , the K 20 cavity plates have all side faces ground to close tolerances. They can be used as reference faces. Unlike assembled mold bases, which are ground in this state, these plates can be interchanged at any time. The so-called "Euro- system" was developed for special-purpose mold designs. The individual plates are not screwed together separately. The guide elements are specially designed also for holding the plates together, and more space for cavities is available. These standard elements are also designed on the basis of he modular system, and they can be combined and interchanged as required. If guide bolts or bushes of this system are fitted in one plate only with out holding other mold elements together, a split clamp ring such as that shown in fig. 8/B is used the counter sunk socket head screw,(l), is used to press the tapered ring.(2) into the split clamp, ring(3), thus fastening the guide element in the hole. The sizes of modular standard plates are steeped on a 50 mm scale, and such plated are available ranging in size form about 95 x 95 mm to about 800 x 1000 mm. the round dimensions range form a diameter of about 95 to 400 mm. Molds can be designed in various ways using modular standard elements. Figure 9 shows a variety of possible combinations (l) using only K standard elements(2)using a combination of K and P standard elements and (3) using only P standard elements. Thus what is offered is not a ready-made solution but a set of possibilities for individual use and maximum flexibility of design. If premachining is required on standard plates, e.g. for slide guides or inserts, the supplier of the standard elements should at least carry out the rough machining according to the mold maker's specification. This will avoid any distortion , which could otherwise have a negative effect on the function of the guide elements.

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MOULD THEORY 179


CESSING PARAMETERS FOR INJECTION MOULDING OF THERMOPLASTICS

bol

Polymer

Predrying Temp. C

S

M

Moulding Barrel Temperatures C Feed Hour Temp c zone R ear Middle Front Nozzle emp c

Acrylontrle ButadienceStyrene

70-80

24

cellulose Acetate

70

3

Polyacetal

110

23

A Polyacetal Methacrylate

70

4

Polyphenylene Oxide

100

2

6

Polyamide 6

80

24

/6

Polyamide 6 6

80

24

Polycarbonate

120

3

S

Ployether Sulfone

150

3

T

Polybutylene terephthalate Polyphenylene Sufflone

120

34

150

6

130

10

T

Polyester (linear) Polystyrene Polystyrene HI

E

70

13

High Density Polyethylene

N

Hard Polyvny Choirode Soft Polyvny Choirode Styrene Acrylonitrile

U

Thermoplastic

80

Polyurethane

100

C

220-

70

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23

<200

50-60 135 185 200 200 180 200 250 250 40-120

Low 1050 -1750 Low

60-80

180275

60 40 70-80 60-80

60-80

60-80 20-30 20-30 20-30

20-30 30-40 40

40

40

AMO

High 750 - 1500 Low<100

240276 340370 260280 220280 220270 220280

180200 2401 3 270

0.10.7

40-120

60-80

190 240 200 230 265 235 270 320 390 220 260 290 310 205 220 150 180 150 200 160 230 150 210

230 270 240 260 280 285 315 350 410 250 270 300 320 220 240 180 230 180 230 200 260 210 250 165 180 150 175

250 290 250 280

240 275 240 270

80- 100

280 285 315 360 430

270 280 315 360 420

60-120

260 275 310 360 230 250 210 230 210 260

255 270 305 320 230 250 210 230 220 280

220 280 240 290

High 100- 1500 Low<100 <35 high Torque High 750 -1500 Low<100

<150

High 300 -1500 Low<100 Low< High 1200 -2000 100Torque

<150 <300

AMO

SEMI.CRYS

90-140

Low 1200 - 150 Low

<50

30-80

High 800 -2000 High

<150

30-50

High

800-1500 High

<150

210 270 240 300

20-60

High 1200-1820 High

<200

20-80

High 1500-1800 High

<200

180 210 150 200 210 250

20-60

Low 1000-1700 Low<80

20-60

High

30-80 15- 65

190

190

200

210

220

25-200

SEMI.CRYS

1

180

Low 1200 -2000 Low<100

SEMI.CRYS

<50

170

70-140

SEMI.CRYS

High 1000 -1500 Low<100

150

<400

0.82.5 0.82.5 0.70.8

AMO

<400

200 250

130-200 High 1200 -2000 Low<60

0.40.7

SEMI.CRYS

AMO

180 205 160 200 210 260

180

80-120

<400

2 0.40.8

0.6 12.2

135 160 125 150 150 200

MOULD THEORY

60-90

750-1500 Low<100

AMO

<150

180 200

222 280 185 200 195 215

Structure

< 250

Low 300 - 1500 low

140 185

210 280 165 200 190 200

Back 0.40.7

40-120

250290 230290 290300 260310 330400

180 230

rpm

Screw Pressure Bar

High

30-40

150 180 135 165 150 180

Injection

30-80

40

250275 170190

Polypropylene C

35-40

260

O

S

220 260 170250 195245

Mould Injection Pressure Speed Bar

1.31.5 0.4.06 0.40.6 1.52

SEMI.CRYS

SEMI.CRYS AMO AMO SEMI.CRYS

<50

1.22.2 0.50.02

AMO

<250

1.2

AMO

High 750 -1000 Low

<150

AMO

Low

<15

0.5 11.5

800-1200

SEMI.CRYS

High

750-1200 Low<100


SEMI.CRYS

POSSIBLE REMEDIES

Increase injection pressure Decrease Injection pressure Increase cylinder temperature Decrease cylinder temperature Increase holding pressure time Decrease Holding pressure time Increase nozzle temperature Nozzle seal or nozzle blocked Nozzle seal is jammed Increase screw rotation speed Decrease screw rotation speed Nozzle seal or nozzle loose W rong type of nozzle seal Inject with rotating screw Increase moulding locking force Injection pressure starts too early Decrease Injection speed Increase injection speed Increase screw back pressure Decrease screw back pressure

e l z z o n d n a e s r u o s n e e w t e b t u o s e l b b i r d l a i r e t a M

. d e l l i f y l e t e l p m o c n e e b t o n s a h d l u o m e h T

*

. s t r a p d d n e e r d d l u l t u u e r o o t m m o n i n e h s s t e k n o r i d a g n w m i e n r r k n c i u S S B

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g n i k a l F

s e i t i v a c n i g n i k c i t s s t r a p d e d l u o M

e u r p s e h t f f o g n i r a e T

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s t r a p d e d l u o m e h t f o s s e n e l t t i r B

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e c a f r u s e h t f o n o i t i d n o c y v a

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t r a p d e d l u o m e h t n i s e n i l m r o

W

h g i h o o t e r u t a r e p m e t l a i r e t a M

. s t r a p d e d l n i u l o a i m r e n t i a s m l e d b n b a u s b r o i b a . m i e l . i k s a d e r i o t S V

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Decrease mould temperature Polish mould and

e u r p s e h t t a n o i s t n a r r e u c o l s o c w s l o i D F

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s t r a p d e d l u o M e h t f o g n i p r a

) g n i n r u b (

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MOULD THEORY 181

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

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*

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*

round off edges Moulds requires maintenance Polish sprues runners and channels Sprues and runners should be increased Introduce mould venting Introduce cold slog pocket Pre-dry material Materials is contaminated Hopper is empty or throat blocked Metering is insufficient Use mould release agent Check on nozzle setting Check on nozzle/ sprue bush radiuses Temperature too high increase nozzle contact time Decrease cylinder temp. at intake zone ept nylon) Bridging in hopper/ vent attachment Check for equal fitting (multi ±cavity tools) Introduce air ejector Increase cooling and pause times Decrease cooling and pause times

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 

 quantity

*

*

* * *

* *

k k

*

* * * *

*

*

* *

 shape and size

SHRINKAGE (Mm /mm) Acr ylonitrile Butadiene styrene (ABS)

High Impact 0.005 - 0.007 Heat resistance 0.004 - 0.005 Medium impact 0.005 Acetal Acr ylic 0.020 - 0.035 Easy flow 0.002 - 0.007 General purpose 0.002 - 0.009 Heat resistant 0.003 - 0.010 High Impact 0.004 - 0.008 Cellulose Acetate Hard/medium/soft 0.002 - 0.005 Cellulose Acetate Butyrate 0.002 - 0.005 Nylon Type-6.6 0.010 - 0.025 Type-6 0.007 - 0.016 Type-6.1 0.010 - 0.025 Type-11 0.010 - 0.025

IZTC HARARE

*

*

MATERIAL

k

*

*

TABLE ± 1

k

*

*

- Material- Material with which the component is to be moulded - Shrinkage- After cooling how much mm. per mm length will shrink

k

*

*

STUDY OF PLASTIC COMPONENT  material  density  shrinkage

 accuracy

*

MOULD THEORY 182

D ensity (G/cc)

1.01 - 1.04 1.06 - 1.08 1.04 - 1.07 1.42 1.09 - 1.14 1.11 - 1.18 1.09 - 1.14 1.09 1.22 - 1.34 1.15 - 1.22 1.08 - 1.14 1.12 - 1.14 1.07 1.04 - 1.05


23417867 Mould Theory

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