Effect of rapid heating cycle injection mold temperature on crystal structures, morphology of polypropylene and surface quality of plastic parts

Abstract

The present study mainly investigated the effect of mold cavity surface temperature in the filling process on the crystallization status of polypropylene (PP) in different thickness layers of injected plastic parts, and revealed the inner relationship between the surface quality of plastic parts, crystallinity and residual stress under rapid heating cycle molding (RHCM) process conditions. The morphology of PP was studied by using a transmission-polarized optical microscope (POM) and a scanning electron microscope (SEM). The crystal structures of PP on different thickness layers of plastic parts were examined by using a wide-angle X-ray diffraction (WAXD). The surface quality of plastic parts was characterized by using a glossiness tester. With the increase of mold cavity surface temperature, the thickness of frozen layer of plastic parts is drastically reduced, and the crystallinity of PP in the external surface first increases, then decreases, and finally tends to a constant, but the crystallization in the sub-surface layer first increases, then decreases, and finally increases again. When the temperature of mold cavity surface is close to the glass transition temperature of PP, the crystallinity in the external surface reaches the maximum and the crystallization status in the sub-surface and core layers is close to each other. Meanwhile, the surface glossiness of plastic parts also achieves its best. In addition, the high crystallinity and low residual stress in the external surface are both favorable conditions for obtaining a high surface glossiness of plastic parts.