Simulation of foaming in reaction injection molding.

Reaction injection molding in the polymerization and processing of low viscosity monomers in a cyclic polymer process operation. Blowing agents are often used with the reactive mixture to compensate for the shrinkage that occurs upon polymerization, thereby creating a structural foam part. Control of solid skin and foamed core thickness is critical to the ultimate mechanical properties of the molded part. It is well known that the exotherm of the chemical reaction is responsible for vaporization of the blowing agent but the mechanics of the process are not well understood. A numerical model has been developed to predict foaming behavior in reaction injection molding. It was applied to evaluate foaming in variable thickness cavities, against a metal insert, and in cast epoxy molding tools. Agreement with actual molded parts is good. The algorithm is useful for developing guidelines for mold design as well as arriving at process conditions to obtain desired skin/core thicknesses.