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The injection molding of thermoset composites gives certain advantages over compression molding techniques such as higher productivity rates, better part quality, and better adaptability to automation. The disadvantages of the injection molding process for thermoset composites include lower mechanical properties and fiber orientation effects. Recent developments in process and materials have significantly improved the mechanical properties and surface smoothness of injection molded thermoset composites. Even with these material improvements several molding problems occur in the injection molding production process. Two important production problems are cracks and porosity at the knitline areas and cracks during the handling of the hot part immediately after molding and other elevated temperature processes. This paper will illustrate the development of a unique mold configuration to simulate knitlines in a ribbed rectangular injection mold.

Sheet molding compound (SMC) has long been the composite material of choice for horizontal automotive body panels. The excellent surface, dimensional stability and stiffness have made it preferred for hoods, deck lids and roofs. Softer thermoplastic and thermosetting (RIM) materials have been considered for fenders and occasionally doors, i.e., vertical body panels. However recent work has shown some performance problems with these materials and automobile companies are examining the low profile SMC alternative. For fender applications increased flexibility over current horizontal body panel materials is desirable. Obtaining this flexibility can be readily achieved but usually at the expense of surface smoothness. This paper reports on work done to achieve a Class A flexible SMC system for compression molding. Markedly greater flexibility of molded parts has been achieved by new combinations of low profile additive and unsaturated polyester resin technology.

The automotive industry's need for thinner, smoother and tougher low profile sheet molding compound (SMC) composites for the evolving body panel parts of the 1980s has prompted the development of Union Carbide Low Profile Additives XLP-4514 and XLP-8521. Composites utilizing these additives have provided Class A surface characteristics at 80 mils thickness and above on developmental deck lid tools. Also improved surfaces on commercial parts such as a variety of models of grill opening panels, truck hoods, etc. have been observed. These studies were conducted using computer assisted experimental designs and results-analysis to ensure valid conclusions. XLP-4514 was engineered for use with modified rigid resins currently employed widely with Union Carbide Low Profile Additive LP-40A. XLP-8521 provides Class A surface with new, proprietary tougher resins as well as yielding composites with both superior surface and composite toughness.