Cylindritic structures of high-density polyethylene molded by multi-melt multi-injection molding

Abstract

The crystalline morphologies of high-density polyethylene (HDPE) molded by multi-melt multi-injection molding (MMMIM) and conventional injection molding (CIM) were studied by employing polarizing light microscopy (PLM) and scanning electronic microscopy (SEM). It was found that a special double skin-core structure was formed in MMMIM parts. Namely cylindritic structures appeared in both sub-skin layer and core layer, which were attributed to the strong shear flow introduced by the secondary melt penetration process. As the decrease in temperature and injection pressure of the second melt, the number of cylindritic structures in the core layer decreases and the cylindritic structures gradually develop into irregularly-arranged spherulites. Strong shear flow, as compared to CIM, is an important factor which dictates the formation of cylindritic structures in MMMIM. Meanwhile, on going from the interface to the core of MMMIM parts the distance between the centers of banded spherulites which consist of cylindritic structures and the radius of them increased gradually. This phenomenon was primarily caused by the coupling effect of the decreased shear intensity and the cooling rate of melts towards core layer. Based on the above observations, a modified model is proposed to interpret the mechanism of the formation of cylindritic structures during MMMIM process.