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 440 Plastic Product Material and Process Selection Handbook Table 14.1 Example of applications for compression molded thermoset plastics Material Performance Application i i Phenol-formaldehyde General-purpose Durable, lowcost Small housings Electrical grade High dielectric strength Circuit breakers Heat resistant Low heat distortion Stove knobs Impact resistant Strong Appliance handles Urea formaldehyde Color stable Kitchen appliances Melamine formaldehyde Hard surface Plastic dinnerware Alkyd Arc resistant Electrical switchgear Polyester Arc resistant Electrical switchgear Diallyl phthalate High dielectric strength Multipin connectors Epoxy Soft flowing Encapsulating electronic components Silicone Heat resistant Encapsulating electronic 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 l'r~e~ wl~, MPa kJt C~Pu lIP p=i M]~ k~i Heat Tensile Impact Distortion Di~-'~ric Strength S~rength at l.SMPa St~ngth MPa ksi J/m ft. Ibf/fl C |: kV/cm kVfin, f~lycJt~r SMC Comp~h~ 25-~ ~ F]larnen~ 3(~&2 sLms,.~ winding epoxy P~ttr~ ~on, ~ ~as~ mat- ~olye~rer IOt~i71) 15-25 5.'~-|20.L~-|.~ ItO [~,Xl |6 2H ~) t2() |01) 211) |~4-~1 11-|7 1.6-2~ 120-2|0 I1¢ ~! 55-1,40 !~0 2]0 t5 3~ 6.2-]4 ~Lg ~O 7(I 280 IO.~ 170 2t0 9-IX 210-640 48-]44 175-~)5 3.~, 400 ~) I~() ~K)-4(Xl 8-20 L'~)-I,I~) ~Xcv-264 20~-2/,/J 4fiO ~O 120-1~') 3(KN~} 25-30 ;30-I,0~) 120-240 175-205 350-400 120-240 3iO-41~ 4.'~ 7n 2X ~2 4.~-9.O 69(~-1~150 i(X),-270 550 I,7(X) ~3-~_~)l~)-32(X) 4~) 72~ |75-20S 350-4(~ 120-[60 3(X) 4(X) 210-480 30-~I) 2~ 4! 4,0.6.0 6~-1,0~0 I00-150 410-1,05,0 60-150 400-3200 540-7"20 205-260 400-500 80-160 2{X) ~O t o 3 o ~t 3 o et t.o 4~ 4~ t 442 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, and/or 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 lb/yr 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. 444 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 [:igure 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~ 3 m, o -,I =I o =,- 4~ 4~ 446 Plastic Product Material and Process Selection Handbook escape. They will, in some cases, filter through the flash and/or 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, and/or sufficient opening movement to create a slight gap for trapped gases to escape effortlessly. To aid in controlling the thickness of molded parts and/or 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 447 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). 448 Plastic Product Material and Process Selection Handbook Plastic ; i; ;.~7~ - . 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 [...]... 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 4 5 6 Plastic Product Material and Process Selection Handbook RPs, also called plastic composites or composites, are tailor-made materials... than composite materials The dividing line is not sharp and differences of opinion do exist Regardless the name composite literally identifies thousands of different combinations with very few that include the use of plastics In using the term composites when plastics are involved the more appropriate term is plastic composites or just RP 460 Plastic Product Material and Process Selection Handbook Many... 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... reinforcement of structural plastics continues to increase Only with nonwoven fiber sheet 462 Plastic Product Material and Process Selection Handbook structures can the full potential of fiber strength be realized 427 Great advances have been made developing new fibers and resins, in new chemical finishes given to the fibers, in methods of bonding the fiber to the resin, and in mechanical processing methods... 454 Plastic Product Material and Process Selection Handbook Figure 14. 5 Schematicof 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... postcured at lower heats than 452 Plastic Product Material and Process Selection Handbook those using glass and mineral reinforcements Products of uneven thickness 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... ply may be very close, and the fiber content may be very high The higher fiber content made from individual plies tends to make it stiffer and stronger than the mat construction 468 Plastic Product Material and Process Selection Handbook -.-.-.-.: r - ~ :: v.- v There is a relationship between the way the glass is arranged and the amount of glass... heterogeneous mass that 4 7 0 Plastic Product Material and Process Selection Handbook has uniform properties For design purposes, many heterogeneous materials are treated as homogeneous (uniform) This is because a reasonably small sample of material cut from anywhere in the body has the same properties as the body An unfilled (unreinforced) TP is an example of this type of material One must be aware that... successful lay-ups depend on the operators' skills to innovate In England the National Physical Laboratory (NPL) is preparing a prepreg standard qualification plan (SQP) that will provide a Good Practice Guide (GPG) 467 472 Plastic Product Material and ProcessSelection Handbook , Sheet Molding Compound SMC is a ready-to-mold material representing a special form of a Bstage prepreg From the original... 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 . 4~ t 442 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. into the desired shapes 440 Plastic Product Material and Process Selection Handbook Table 14. 1 Example of applications for compression molded thermoset plastics Material Performance Application. composcd of a glass fiber- thermoplastic RTP (Chaptcr 15). 448 Plastic Product Material and Process Selection Handbook Plastic ; i; ;.~7~ - . Different TS plastics are used such as phenolics,