Advances in Thermoset Injection Molding Part IV: Knitline and Elevated Temperature Properties in Thermoset Composites

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. By studying mechanical properties in these knitline areas, a method to evaluate process and material variables has been developed. By measuring mechanical properties at elevated temperatures, the strength of the composite during high temperature processes can be simulated. This provides another method to evaluate material variables and their effect in a simulated production situation.