Back melt flow in injection–compression molding: Effect on part thickness distribution

Abstract

The flexibility of injection–compression molding (ICM) was demonstrated to be adequate for replication of precise polymeric parts in this work. Under the optimal processing conditions, the rectangular polystyrene (PS) plates with the standard deviation of 0.15% for the part thickness were molded by ICM. Experimental analyses suggested that the part thickness uniformity was significantly affected by the melt temperature and compression-related processing factors (including the compression force, compression stroke, and compression speed), but was much less affected by the injection rate. Furthermore, with the generalized Hele–Shaw flow simulation, the mechanism dominating the part thickness distribution was investigated. The simulated results gave an insight into the back melt flow within the cavity during the compensation stage. It was revealed that the melts flowed back from the cavity to the gate when the compression was completed. Such back flow, which was mainly controlled by the melt temperature and compression-related factors, could obviously redistribute the melts within the cavity, and more importantly distribute the final part thickness. As an increased amount of melts within the cavity was transferred via the back flow, the area near the gate received more compensation for the part shrinkage, whereas the area near the end of the cavity exhibited greater part shrinkage correspondingly.