Compression molding similar processes

PGA, is another biodegradable polymer with applicability in bionanocomposites [372] and, like PVA and PVAc, PGA is readily soluble in water and can be processed by extrusion, injection, and compression molding similarly to other thermoplastics. Murugan et al. [373] produced bionanocomposites of PGA and clay by polymerizing glycolic acid under vacuum in the presence and absence of nanoday which act as a catalyst to the condensation polymerization of PGA. They found that addition of clay improved flame retardancy. 

Rigid caul plates are typically constructed of thick metal or composite materials. Thick caul plates are used on very complex part applications or cocured parts where dimensional control is critical. Many rigid caul plates result in a matched die configuration similar to compression or resin transfer molding. Parts processed in this manner are extremely challenging because resin pressure is much more dependant on tool accuracy and the difference in thermal expansion between the tool and the part. Tool accuracy is critical to ensure no pinch points are encountered that would inhibit a tool from forming to the net shape of the part. 

Granular PTFE resins are most frequently processed by compression molding using a technique similar to that common in powder metallurgy and by ram extrusion. Each of these processes requires a specific type of granular resins. 

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. 

Metal powder processing techniques in which a preform is molded and sintered are used to process PTFE. Compression molding may also be used to fabricate PTFE parts. Its dispersions are applied by similar techniques to other coatings. Paste extrusion in which PTFE is blended with a hydrocarbon, prior to molding a preform, is used to continuously fabricate PTFE into tubes, tapes, and wire insulation. The hydrocarbon is vaporized before the parts are sintered. PVF is dispersed in a polar latent solvent such as dimethyl acetamide and is melt-extruded as a plastisol, followed by solvent removal by drying. 

Poly([ethyl acrylate]-g-plvalolactone) was found to be easily processable on conventional rubber working equipment. It was easily processable on a two-roll mill, had excellent calendering properties, could be compression molded at 225-230°C, and could be Injection molded at 225°C. Extrusion was more difficult requiring high temperatures (250°C) and slow extrusion rates. Physical properties of the graft copolymers were similar to those of the parent elastomeric polyacrylates that had been compounded with carbon block and chemically crossllnked. 

This process is an economical press molding method for manufacturing an intermediate number of parts, such as 200 to 2,000. It uses low-pressure, room-temperature curing resins, and inexpensive molds. Cold press is similar to compression molding except that the resin curing action occurs via its own exothermic heat of reaction (after resins are mixed with catalyst, etc.). Pressures are moderate, usually 20 to 50 psi. Thus molds can be made of relatively inexpensive metals, plaster, or reinforced plastics. The edges need not be trimmed. Ribs, bosses, and other fairly complex shapes are not easily produced. Two good mold surfaces are obtained. 

The Prolong process is similar to the previously described DURASUL except that an electron beam irradiation dose of 65 kGy is used for Prolong and continuous compression-molded bars are processed as with Longevity. Components are machined from processed Prolong maferial, enclosed in gas-permeable packaging, and sterilized by gas plasma (Figure 15.6). 

---