Cavity compression molding

In cavity-compression molding the cold powder or mold material is placed under pressure in a heated mold. The process is also called hot pressing. Glass-fiber-reinforced, unsaturated polyester resins are processed by the heated-cavity-compression molding method. The vulcanization of rubber can also be carried out using this method. High-fidelity records are pressed out of the thermoplasts PVC or poly(ethylene-co-vinyl acetate), while cheaper records are injection molded. 

By contrast with ICM, a compression mold design is used where male plug fits into a female cavity rather than the usual flat surface parting line mold halves for IM (Fig. 8-27). The melt is injected into the cavity as a short shot thereby not filling the cavity. The melt in the cavity is literally stress-free it is literally poured into the cavity. Prior to receiving melt, the mold is slightly opened so that a closed cavity exists the male and female parts... 

For compression molding inside the disc-shaped cavity shown in Fig. 6.81, derive an expression for the flow field and pressure distribution inside the mold during filling for an arbitrary flow front location Rf. Assume a Newtonian fluid and the notation found in the figure. 

Figure 6.81 Schematic diagram of a disc-shaped compression mold cavity.

Figure 13.44 represents the various stages of the compression molding cycle from the point of view of the plunger force needed to close the mold at a constant rate. In the first region, t < the force increases rapidly as the preform is squeezed and heated. At tf, the polymer is presumably in the molten state and, as such, is forced to flow into the cavity and fill it. Filling terminates at tc, when compression of the polymer melt takes place, to compensate for the volume contraction that results from the polymerization reaction. The bulk of the chemical reaction occurs after tc. We now comment on each of the steps of the compression molding process. 

Compression molding method called coining is used for parts that are too complicated to be produced by machining. In coining, a sintered molding is heated to the melting point, then it is quickly pressed into a mold cavity and held under pressure until it solidifies.4... 

Transfer Molding. This preheats the molding powder in the upper part of the mold. Then a plunger pushes the fluid material down into the (lower) mold cavity and holds it there until cure is complete. This gives more fluid flow than compression molding, and avoids damage to delicate molds or molded parts. 

Either crystalline and amorphous types (Chapter 1) can be used with flow molding because the plastic is melted prior to forming. The forming temperature is usually lower than for injection molding or extrusion. Plastics need not be trimmed, as the composite is compression-molded to completely fill the mold cavity (Chapter 14). Very important, the flow molding process permits more complex parts to be formed than solid-state forming. The process cycle time is usually about 1 min. 

Since the plastic entering the cavities is melted it requires less force to 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. 

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... 

TS plastics in reinforced sheets and compounds are usually used. Also used are reinforced thermoplastic sheets and compounds. With TSs compression molding (CM) can use preheated material (dielectric heater, etc.) that is placed in a heated mold cavity. The mold is closed under pressure causing the material to flow and completely fill the cavity. Chemical crosslinking occurs solidifying the TS molding material. 

This process is a take-off from compression molding that uses solid material male and female matching mold halves. This unique process uses a precision-made, solid shaped heated cavity and a flexible plunger that is usually made of hard rubber or TS polyurethane. This two-part system can be mounted in a press, either hydraulic or air-actuated. Rather excellent product qualities are possible at fairly low production rates. The reinforcement can be positioned in the cavity and the liquid TS resin is poured on it. Also used are prepregs, BMC, and SMC. 

This process resembles the open molding process except it is closed like a two-part compression mold (Chapter 14). A measured amount of plastisol is poured or pumped into the closed mold cavity, similar to close molding except that a slight pressure of about 5 psi (34.5 kPa) is applied. The mold is heated to fuse the plastisol then cooled. Later the mold is opened and the product stripped out. This process can provide for accurate thickness control, filling very complex shaped parts, and so on. 

---