Plastic Product Material and Process Selection Handbook Part 14 ppt

To be reviewed arc the basic compression molding process Figure 14.1, the transfer molding process, resin transfer molding process Chapter 15, compression-transfer molding process, and o

COMPRESSION MOLDING

Introduction

Compression molding (CM) encompasses different techniques in processing plastics To be reviewed arc the basic compression molding process (Figure 14.1), the transfer molding process, resin transfer molding process (Chapter 15), compression-transfer molding process, and other molding processes These compression molding methods provide different capabilities to fabricate products to meet performance requirements using different materials (Tables 14.1 and 14.2)

Figure 14,1 Schematics of compression molding plastic materials

Compression molding is an old and common method of molding thermoset (TS) It now processes TS plastics as well as other plastics such as thermoplastics (TP), elastomers (TS and TP), and natural rubbers (TS) By this method, plastic raw materials are converted into finished products by simply compressing them into the desired shapes

4 4 0 Plastic Product Material and Process Selection Handbook

Table 14.1 Example of applications for compression molded thermoset plastics

Phenol-formaldehyde

General-purpose Durable, lowcost Small housings

Electrical grade High dielectric strength Circuit breakers

Heat resistant Low heat distortion Stove knobs

Melamine formaldehyde Hard surface Plastic dinnerware

Diallyl phthalate High dielectric strength Multipin connectors

components

by using molds, heat, and pressure This process can mold a wide variety of shapes ranging from parts of an ounce to l O0 lb or more.26s, 469,484

The process requires a press with heated platens or preferably heating in the mold Basically a two-part mold is used (Figure 4.1) (Chapter 17) The female or cavity part of the mold, when using a molding com- pound, is usually mounted on the lower platen of the press, while the male or plunger part is aligned to match the female part and is attached

to the upper platen (Figure 14.1) If a plastic impregnated material (sheet, mat, etc.) is used the female or cavity part of the mold is usually mounted on the upper platen of the press, while the male or plunger part is aligned to match the female part and is attached to the bottom platen (Figure 14.1)

The plastic molding material is weighed out and is usually preheated before charging (transferring) to the cavity part of the heated mold After charging the mold, the press is closed bringing the two parts of the mold together This allows the molding material to melt and flow through filling the cavity between the two parts of the mold, and at the same time pushing out any entrapped air ahead of the melt so as to fill the mold cavity completely After holding the plastic in the mold for the time specified for a proper cure under the required temperature and pressure, the pressure is released, the mold opened, and the solid molded plastic part discharged In a modern high-speed automatic compression press all the operations are performed automatically

The necessary preheating and mold heating temperatures and mold pressure may vary considerably depending upon the thermal and theological (refers to the deformation and flow properties of the plastic) properties of the plastic (Chapter 1) For a typical compression molding

Ta!)~e ~ 4_~ Cor~par!qg comp~essi3n m3!ded pro=erties with o-_her ;rocess~s

Compressive Tensile Fle~,,ral Reintnrccm~n t Stremgl~ Modulus Strength

Heat

MPa ksi J/m ft Ibf/fl C |: kV/cm kVfin, f~lycJt~r

4~ 4~

4 4 2 Plastic Product Material and Process Selection Handbook

thermoset material preheat may be at 200F (93C) and mold heat and pressure may be near 250 to 350F (121 to 177C) and 1000 to 2,000 psi (6.9 to 13.8 MPa) A slight excess of material is usually placed in the mold to insure it being completely filled and this excess is squeezed out between the mating surfaces of the mold in a thin, easily removed film known as flash As shown in Figure 14.2 flash can form in different positions based on how it is to be removed Different methods are used

to remove flash such filing, sanding, a n d / o r tumbling There are systems where parts arc frozen (dry ice) malting it easier for certain types of plastic parts to be &flashed

Figure 14,2 Examples of flash in a mold: (a) horizontal, (b) vertical, and (c) modified vertical

In the case of a thermoplastic, the molding temperature cycle is from heating to plasticizing the plastic, to cooling in the mold under pressure, the pressure released, and the molded article removed When TS is used the mold need not be cooled at the end of the molding operation or cycle, as the plastic will have hardened and can no longer flow or distort (Chapter 1 )

The molding cycle takes anywhere from a few minutes to an hour depending upon the type of plastic used and the size of the charge The cycle steps are

1 charging;

2 closing the press;

3 melting the plastic;

4 applying full pressure and heat;

5 curing for TSs or cooling for TPs;

6 discharging or ejecting the molded part

Most of the time is consumed in the cure or cooling stage, while some

of the other stages could take only a few seconds

14 Compression molding 443

Compression molding was the major method of processing plastics worldwide during the first half of the last century because of the development of phenolic plastics (TSs) in 1909 By the 1940s this situation began to change with the development and use of thermo- plastics (TPs) in injection molding (IM)

CM originally processed about 70wt% of all plastics, but by the 1950s its share of total production was below 25%, and now that figure is about 3% of all plastic products produced internationally Worldwide

350 million l b / y r arc estimated to be consumed This change does not mean that CM is not a viable process; it just does not provide the much lower cost-to-performance benefit of TPs that are injection molded, particularly at high production rates In the early 1900s plastics were almost entirely TS (95wt%) used in different processes, but that proportion had fallen to about 40% by the mid-1940s and now is about 10%

TSs has experienced an extremely low total growth rate, whereas TPs have expanded at an unbelievably high rate Regardless of the present situation, CM is still important, particularly in the production of certain low-cost products as well as heat-resistant and dimensionally precise products CM is classified as a high pressure process Some TSs may require higher pressures while others require lower pressures of down

to 50 psi (0.35MPa) or even just contact/zero pressure

The advantages that keeps the compression process system popular are due primarily to the simple operation that defines the system The heated cavity is filled directly and then pressurized for the duration of the cure cycle Examples of advantages arc as follows:

1 Tooling costs are low because of the simplicity of the usual molds

2 Little material is wasted since there are usually no sprucs or runners [when not compared to runnerless injection molding (Chapter 4)]

3 TSs when compared to TPs are not subject to retaining internal stresses after being cured

4 Mechanical properties remain high since material receives little mastication in the process and when using reinforcements they are literally not damaged or broken

5 Less clamping pressure required than in most other processes

6 Capital equipment is less costly

7 Wash-action erosion of cavities is minimal and mold maintenance is low since melt flow length is short

4 4 4 Plastic Product Material and Process Selection Handbook

Limitations of the method include"

Fine pins, blades, and inserts in the cavity can become damaged as the press closes when cold material is used in the cavities

Complex shapes may not fill out as easily as by the transfer or injection molding processes

Extremely thick and heavy parts will cure more slowly than in trans- fer or injection molding, but preheating preforms or powder can shorten these cures

Thcrmosets with their low viscosity will produce flash during their cure that has to be removed

Mold

Three general types of molds are used for CM In the positive mold (Figure 14.3a) all the material is trapped in the mold cavity The pressure applied compresses the material into the smallest possible volume Any variation in the weight of the charge will result in a variation in part thiclmess In multicavity molds, if one cavity has more material than the others, it will receive proportionately greater pressure Multiple cavities, therefore, can result in density variations between parts if loading is not done with some degree of precision control 1, 278-284

A flash mold (Figure 14.3b) has a narrow land or pinchoff area around the cavity Material is compressed in the cavity to a density that will match the force applied Excess material escapes across the pinchoff line

as flash Immediately beyond the pinchoff line, the surface is relieved to allow the flash to fluff out rather than to cure in a hard skin that would adhere tightly to the metal surface

The semipositive mold (Figure 14.3c) is by far the most popular It combines the best features of the positive and the flash molds Since its design includes a plastic material well of larger diameter, with a tight fitting force above the cavity, the material is trapped fairly positively and the plastic is forced to flow into all corners of the cavity As the material picks up more heat and becomes fluid, it escapes between the force and cavity sidewalls as flash, allowing the force to scat on the land area Clearance between the sidewall of the cavity and the OD of the force generally is about 0.004 in Variation in this clearance will vary the density of the molded part The gases that are released from curing certain TS plastics, as well as the air in the cavity, must be allowed to

[ : i g u r e 1 4 , 3 Exampie of mn : b/pes: I:a) positive :omF, ressioq "note, (b) flas k compression mold, and (c} semipos[tiv¢ compression mold

4 4 6 Plastic Product Material and Process Selection Handbook

escape They will, in some cases, filter through the flash a n d / o r the clearance around the ejector pins Usually vents are also included in the mold to permit release of these gases When processing TPs there should not be a problem in flash occurring However air in the cavity has to be released so vents in the mold are used that is a take off of injection molding molds 3

The TS gases are more of a problem with urea and melamine than with phenolic To ensure they do not become entrapped in the molding material during compression molding, and in turn weaken the molded part or cause surface blemishes, it often is advisable to open the mold to allow gases to escape This is called breathing or bumping It amounts

to sufficient reduction in clamp pressure to allow the pressurized gases

to blow their way out, a n d / o r sufficient opening movement to create a slight gap for trapped gases to escape effortlessly

To aid in controlling the thickness of molded parts a n d / o r support the pressure loads put on sections of a mold, lands in the mold are used Examples of lands are shown in Figure 14.2 Figure 14.4 shows the land locations used in a mold that supports the split-wedge in the mold

Figure 14,4 Example of land locations in a split-wedge mold

14 9 Compression molding 4 4 7

When plastics, particularly compounds, prepregs, and sheet materials that are filled with reinforcements such as glass fibers, the matting edges of the mold cavity require special treatment (Chapter 17) The target is to ensure proper and clean-cut edges of the parts The materials

of construction can overlap the edges prior to or during molding 3

Machine

The CM machines are usually referred to as compression presses They are primarily hydraulic or, in limited use, pneumatic Either of these systems can have the usual straight lockup system or toggle lockup system (Chapter 4) These presses may be either down-acting or up- acting The down-acting type is used for the fully automatic compression presses so that the lower mold half is at a fixed height to align with the material feeder and molded product stripper tables

Different actions in molds occur such as using ejector pins to remove molded parts from their cavities Side actions of molds may be required

to remove parts that have undercuts Other actions may be required such

as unscrewing threaded parts, including inserts, and so on (Chapter 17) The presses are available in all sizes to meet the many different require- ments for parts to be molded These differences include short to long curing cycle times, small to large parts requiring different pressures (clamp tonnages), and so on They range from less than a 1/2 to thousands of tons with platens 4 by 4 in to at least 10 by 20 ft The usual press has two platens and others have up to at least 30 platens that can simultaneously mold flat sheets or other products There are presses with shuttle molds and those that have a series of individual presses (3

to at least 25) that rotate providing the TS plastic to complete its curing cycle, permit ease of including inserts, etc Presses usually have their platens parallel to each other and there are those that open like clam shells referred to as book type Other processes reviewed in this book provide examples of these type presses to fabricate by their respective methods (Chapters 4, 12, 15, and 16)

In use arc stamping compression molding presses Plastic used can be

TS shcct molding compounds (SMCs) and stampablc rcinforccd thcrmoplastic sheet (STX) material (STX is a rcgistcrcd tradcnamc; Azdcl Inc., Shelby, NC) It is usually composcd of a glass fiber- thermoplastic RTP (Chaptcr 15)

4 4 8 Plastic Product Material and Process Selection Handbook

Plastic

; ; ~ ~ -

Different TS plastics are used such as phenolics, TS polyesters, DAPs, epoxies, ureas, melamines, and silicones, all with their own processing requirements and performance molded properties based on their compositions [Note there are TS and TP polyesters (Chapter 2)] Also used arc TPs TSs arc used primarily in CM and TPs in injection molding, extrusion, blow molding, etc In this review the emphasis is on TSs, which have different processing characteristics to TPs (Chapter 1)

Materials can be unreinforced or reinforced/filler compounds, sheet molding compounds (SMC), bulk molding compounds (BMC), prepregs, preforms or mats with liquid resins, etc With TSs, they cure via A-B-C stages that identify their heat cure cycle A-stage is uncured (in the form received from a material supplier), B-stage is partially cured with heat, and C-stage is fully cured A typical B-stage is TS molding compounds and preprcgs, which in turn arc processed to produce C-stage fully cured plastic material products in compression molds TSs when heated go through crosslinking chemical reactions to produce hard or rigid plastic product TPs during molding go through a melting stage when heated followed with a hardening stage when cooled (Chapter 1)

An example of very popular CM materials are bulk molding compounds (BMCs); in Europe they are called dough molding compounds (DMCs) They are formulated from different percentages of TS polyesters filled with glass fibers of lengths up to 1/2 in (13 mm) and fillers The BMCs flow easily and provide high strength (Chapter 15) Also popular as CM molding materials are the TS vinyls used for phonograph records, etc TP vinyls are crosslinked to turn them into

TS vinyls (Chapter 1)

Very soft flowing TSs are required for molding around very delicate inserts Large quantities of electronic components such as resistors, capacitors, diodes, transistors, integrated circuits, etc arc encapsulated with such soft-flowing TS compounds Principally used are epoxies by compression molding (and transfer molding) Silicone molding com- pounds are used occasionally where higher environmental temperatures are required of the encapsulated part that can be exposed up to 500F (260C) or more TS polyester compounds, that are less expensive than epoxies, or silicones that are more expensive, arc also selected when their requirements suffice (Chapter 2)

In the use of preform and mat-reinforced molding, the plastic may be added either before or after the reinforcement is positioned in the cavity The preform can be a spray-up of chopped glass fibers deposited

14 Compression molding 449

on a shaped screen with a minimum of plastic binder (about 1 to 5wt%

of resin compatible with the molding resin) Different techniques are used to provide desirable surfaces (Chapter 15)

Depending on the type of material used, and the size and thickness of the product, different temperatures, pressure, and time schedules are used Temperatures range from about 200 to 350F (93 to 177C) Pressurewise, the range is from about 1,000 to 2,000 psi (6.9 to 13.8 MPa) Time cycles can range from less than 1 minute to many minutes The process called matched-die molding, generally identifies CM operating at the lower pressures

Polytetrafl uoroethylene

Processing the usual thermoplastics (PE, PP, PVC, PS, etc.) sets up no special technique However certain TPs such as polytetrafluoro- ethylenes (PTFEs) require special techniques because they do not have the usual easy melt flow CM is used to fabricate cylindrical billet, molding, or sheet shapes of PTFE This type of CM is also called isostatic compression molding As an example electric-grade tapes are sldved from billets with a wall thickness of 75 to 100 mm The granular plastic goes through the three stages of preforming, sintering (heating) and cooling

The large quantity of plastic and the length of time needed to produce

a shape requires careful attention to factors that affect fabrication such

as the handling and storage of the plastic High temperature storage of granular PTFE can lead to compaction during handling Plastic should

be conditioned at temperatures of 21 to 25C (70 to 77F) before molding

to reduce clumping and ease handling Dew point conditions should be avoided to prevent moisture from condensing on the cold powder that will expand during sintering and crack the molding Molding below 20C (68F) should be avoided because PTFE will undergo a 1% volumetric change at a transition temperature of 19C (66F)

Preforms molded below 20C can crack during sintering Sintcring is the process of holding the fusible pressed powder part (PTFE, nylon, etc.)

at a temperature just below its melting point for a specific time period Powdered particles are fused (sintered) together but the mass as a whole does not melt PTFE is an excellent thermal insulator that is about 2,000 times less than copper The most common way of delivering heat to the preform is by circulation of hot air A large volume of air has to be recirculated because of its low thermal capacity

4 5 0 Plastic Product Material and Process Selection Handbook

After being removed from the mold it is heated to a higher temperature

to completely fuse the sintered material resulting in property increases (tensile strength, etc.), ductility, and usually density Good temperature control is critical to achieving uniform and reproducible part dimensions and properties Sintering of the preform takes place in an oven where massive volumes of heated air are circulated Initial heating

of the preform leads to thermal expansion of the part After PTFE melts, relaxation of the residual stresses occur where additional recovery takes place and the part expands The remaining air begins to diffuse out of the preform after heating starts The adjacent molten particles begin to coalesce slowly; usually hours are required because of the massive size of PTFE molecules Fusion of the particles is followed by elimination of the voids, where almost no air is left

Cooling at a controlled rate after sintering takes place ensures proper crystallization and annealing of the plastic Properties of PTFE (similar

to other semicrystalline polymers) are controlled by the crystalline phase content of the part (Chapter 1) To remove residual stresses in the plastic, annealing is used, which takes place after a period of time The final crystallinity of the part depends on the annealing temperature

A part which is annealed below the crystallization temperature range [<300C (<572F)] will only undergo stress relief Annealing at a temperature in the crystallization range [300 to 325C (572 to 612F)] results in higher crystallinity The result is higher specific gravity and opacity in addition to stress relief

Processing

Processing conditions such as temperature, pressure, and molding cycle differ for the different plastics There exists a wide range of flow characteristics with the different plastics and also within a specific plastic when they have different compositions These molding compounds are mixtures of constituents, usually of different size and shape The compounds themselves present the greatest number of variables that must be understood and properly applied The processing conditions with TSs as well as TPs ultimately effect mechanical, chemical, electrical, aesthetic, and other properties

Many TS compounds are heated to about 300 to 400F (149 to 204C) for optimum cure; but can operate as high as 1200F (650C) Over heating any materials could degrade their performance or could cause them to solidify rapidly before the mold cavity is completely filled

14 9 Compression molding 4 5 1

Preheating is often used to reduce the molding cycle It can aid in providing even heat through the material and can cause a more rapid rise in heat than occurs in the mold cavity A warm surface plate, infrared lamps, hot-air oven, or screw/barrel preheatcr can accomplish preheating The best and quickest method is high-frequency (dielectric) heating

Preheating is usually carried out at 150 to 300F (66 to 149C) followed

by quick transfer to a mold cavity The actual heat depends on the material, the heater capability, and the speed of transfer Circular prcforms are normally used with dielectric heaters so they can be rotated to obtain uniform heating Pills of compressed compound are used to produce preforms to facilitate handling, reduce the bulk factor

in the cavity, and control the uniformity of charges for mold loading Preforms can also be the shape of the mold cavity

Compared to other processes, particularly injection molding (IM) for shaping plastics, CM is fairly labor-intensive even if it is automated However it requires lower capital investment Molding cycles for CM arc generally longer than for IM If the material used is preheated or preplasticizcd before it is placed in a mold cavity, molding cycles may bc comparable to IM When CM flash formation in the molds occur, their viscosity during the melting action resembles that of water Techniques can be used to significantly reduce flash by modifying the mold design

To aid in reducing cycle time there are molded parts that can finish their cycle in a fixture After a molded product is removed from the cavity it is still hot and the material is not fully rigid Any internal stresses in the material may therefore cause the shape of the product to change while it is cooling Where close tolerances are required and especially where products have thin sections, dimensional accuracy can

be met by placing the hot, molded product on a fixture near the press that will hold it until it has cooled

To improve properties such as mechanical, thermal, and dimensions of certain molded TSs, also certain TPs, they arc exposed to a postcure The part is literally baked in an oven Experience or a material supplier's recommended times and temperature profiles required to enhance properties arc used Baiting also improves creep resistance and reduc- tion in stresses This postcuring is also used with certain TPs after IM

or extrusion to improve their performance

Postcuring heat is usually below the actual molding heat It is usually performed in a multistage heat cycle The reinforcement system of the compound will dictate heating cycles Products molded from com- pounds using organic reinforcements arc postcured at lower heats than

452 Plastic Product Material and Process Selection Handbook

those using glass and mineral reinforcements Products of uneven thick- ness will exhibit uneven shrinkage This shrinkage effect is included in the mold design

eater

There are many heat choices available and a wide choice of temperature controls as there is with other fabricating processes They range from simple mechanical thermostats to solid state units with P l D control, to microprocessors that are proportional, programmable, and self-tuning (Chapter 3 )

Electrical heating, through the use of heater coils, strips, or cartridges,

is the most popular method Higher temperatures for faster cycles are easily obtained Recognize that electric mold heating is only as fast as the wattage put into it It is a cleaner system than steam that was used many decades ago

Steam heat provided the fastest recovery time of any system because of the oversize source available in the boiler room It offered a uniform mold temperature, as do all liquid systems, but is limited to about 350F (177C) maximum Steam heat is also messy and requires good main- tenance, or rusty pipes and leaks become all too common Steam controls and the accompanying valves are expensive and many are not dependable

H o t oil heat offers the benefits of higher temperatures from a liquid system It results in probably the most uniform mold temperatures primarily because the fluid is being constantly circulated Recovery time, however, is limited to the total heat capacity designed into the circulating unit

High pressure water systems are also available that heat by continuously circulating hot water The advantage is less corrosion than steam since the oxygen is not replenished in the closed circuit Also, temperatures are more uniform than steam because, like hot oil, it is a dynamic system These systems are expensive and costly to maintain

Gas flames have been used on rotary presses Gas also has been used with some exotic materials requiring very high temperatures [over 2000F (1,093C)]

Automation

A variety of feeders have been designed to move the molding material into the mold, and special strippers built to remove the molded parts All of these have the common goal of faster, more efficient, and

14 9 Compression molding 4,63

automatic production The feeders include feeding cold powder through tubes from overhead hoppers, reciprocating, feed boards, etc., and the feeding of preheated, partially or fully plasticated material from infrared heated hoppers, RF heating units, or screw plasticators

Automatic strippers have included many combinations of air blow-off, metal combs, and catch trays or chutes Programmable robots are used for this type of work These sophisticated units are also used to add inserts before loading the mold

Recently all of the temperature, pressure, and time controls have been replaced with a single microprocessor-based controller A number of these are available and they allow for complex pressure and temperature curves to be programmed with multiple soaking levels and variables that can be chosen Built-in memories recall previous programs and cassettes can store them on the shelf Interfaces can connect with a central host computer for data collection or actual machine setup and supervision The result is more flexible, more exacting, and easier to control modern molding equipment (Chapter 3)

Transfer Molding

Shaw of Pennsylvania developed this plastic transfer molding process during the 1930s It is a method of compression molding, principally thermoset plastics Heat and pressure in a transfer chamber (pot) first soften plastic After the heating cycle it is forced by a ram at high pressure through suitable sprues, runners, a n d / o r gates into a closed mold to produce the molded part or parts using one or normally two or more cavities (Figure 14.5) Usually dielectrically preheated circular preforms are fed into the pot Plastic remaining in the transfer chamber after mold filling is called a cull Unless there is slight excess in this chamber, one cannot be sure that the cavity(s) was completely full

Since the plastic entering the cavities is melted it requires less force t o fill the cavities than compression molding With conventional CM there

is more force in the cavities as the solid plastic is melted The result is that more intricate parts can be molded as well as encasing intricate devices such as electronic

Compression-Injection Molding

Usually called injection-compression molding (ICM) Details are in Chapter 4 The essential difference when compared to IM lies in the manner in which the thermal contraction in the mold cavity that occurs during cooling (shrinkage) is compensated With conventional injection

454 Plastic Product Material and Process Selection Handbook

Figure 14.5 Schematic of transfer molding

molding, the reduction in material volume in the cavity due to thermal contraction is compensated basically by forcing in more melt during thc pressure-holding phase By contrast with CIM, a compression mold design is used where a male plug fits into a female cavity rather than the usual fiat surface parting line mold halves for IM

Hydrostatic Compression Molding

Hydrostatic molding is a suitable alternative to compression molding techniques for the production of plastics that do not have the usual melt flow behavior, such as previously reviewed in the Plastic section for polytetrafluoroethylene

REINFORCED PLASTIC

Overview

Industry continues to go through a major evolution in reinforced plastic (R P) structural and semi-structural materials RP has been developed to produce an exceptionally strong and corrosive material The RP products normally contain from 10 to 40wt% of plastic, although in some cases plastic content may go as high as 60% or more (Figures 15.1 and 15.2)

F i g u r e I 5~ 1 Effect of matrix content on strength (F) or elastic moduli (E) of reinforced plastics

Figure 15~2 Properties vs amount of reinforcement