Use of core retraction to achieve low density foams in microcellular injection molded polypropylene parts

Core retraction was used with the conventional microcellular injection molding (MIM) process to foam thick polypropylene (PP) parts with high density reductions of 30% and 55%. The cavity volume was changed by changing the retraction distance, which resulted in varying density reductions. The lowest densities were achieved with a core retraction-Aided microcellular injection molding (CRMIM) process, the results of which could not have been achieved by the conventional MIM process alone. The effects of delay time and weight reduction on the microstructure of the core and skin layer were investigated. It was shown that the CR-MIM process yielded a better microstructure and a higher tensile modulus than the conventional MIM process. Use of core retraction also yielded more consistent densities and tensile properties at different distances from the gate location.