Injection molding with thermosets

In recent years, injection molding with thermosets has been perfected using the gas assist more commonly used when injection molding thermoplastic parts that are fairly thick in cross section (say, 1 to 10... 

Thermoplastic RPs (RTPs), even with their relatively lower properties when compared to thermoset RPs (RTSs), consume about 55wt% of all RP products. Practically all of the RTPs are injection molded with very fast cycles using short glass fiber producing highly automated and high performance products. 

Recendy, storm doors have been constmcted of advanced thermoplastic composites. Stampable, glass-mat reinforced polypropylene sheet is used to create a high strength outer skin. These compression molded skins are welded together using a friction or ultrasonic process then injection molded with a polyurethane foam core to produce an insulated structure. New technology for window frames incorporate the pultrusion of frame channels to produce a thermoset composite channel that can be filled with fiber glass for further insulation enhancement (12,31,33,34,48,49,54—56,60—67). 

The impression space into which the molding compound is injected-known as cavity-is located within the mold. The material solidifies by cooling within the cavity, thus forming the molding. With thermosets/elastomers, the article is produced by chemical crosshnking reaction. 

Injection Molding. Both thermosetting and thermoplastic polymers can be used in the injectionmolding process, though the practical requirements are very different for the two types of polymers. In injection molding, a cavity in the shape of the desired object is filled with molten thermoplastic or a thermosetting prepolymer resin. When the sohd polymer has formed, the object is removed from the mold and... 

Orifice flow ii (test and mold) (Rg. 2.2 ). As shown, the mold has a pot in the lower half and a plunger affixed to the top half. The pot block is designed with a sprue hole in the bottom of the cavity, which feeds the molding compound into a runner that comes out one side of the mold. The pot is charged with 90 g of compound, and the mold is closed under 1000 Ib/in with a mold temperature of 300°F (148°C). The compound exiting from the runner can he weighed once the flow has ceased. If the time of flow from the start has been timed from start to finish, the rate of flow can he calculated as X g/s. This is a very useful tool for use in transfer or injection molding of thermosets. 

In much the same manner that vacuum-assisted venting is used with transfer molding, it may be utilized equally successfully in injection molding of thermosets and of thermoplastics. 

Liquid-Injection Molding. In Hquid-injection mol ding (LIM), monomers and oligomers are injected into a mold cavity where a rapid polymerization takes place to produce a thermoset article. Advantages of these processes are low cost, low pressure requirement, and flexibiHty in mold configuration. Conventional systems, such as isocyanate with polyol, release Htfle or no volatiles. The generation of substantial volatiles in the mold is obviously undesirable and has represented a significant obstacle to the development of a phenoHc-based LIM system. A phenoHc LIM system based on an... 

Newer resins include polysulfone, polyethersulfone, polyetherimide, and polyetherketone. Some of these newer materials are high temperature thermoplastic, not thermoset, resins. They are being promoted for the design of injection-molded printed circuit boards in three-dimensional shapes for functional appHcations as an alternative to standard flat printed circuit boards. Only semiadditive or fully additive processing can be used with these devices. 

Phenolic phenol formaldehydes (PFs) are the low-cost workhorse of the electrical industry (particularly in the past) low creep, excellent dimensional stability, good chemical resistance, good weatherability. Molded black or brown opaque handles for cookware are familiar applications. Also used as a caramel colored impregnating plastics for wood or cloth laminates, and (with reinforcement) for brake linings and many under-the-hood automotive electricals. There are different grades of phenolics that range from very low cost (with low performances) to high cost (with superior performances). The first of the thermosets to be injection-molded (1909). 

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