Shear-induced crystallization morphology and mechanical property of high density polyethylene in micro-injection molding

The advances of the polymer melt flow-induced crystallization behaviour and its influence on mechanical properties of high density polyethylene (HDPE) in micron injection (MI) were studied in the present paper. Analysis of mechanical performance, including yield stress and elongation at break, for samples adopted from different regions in a molded plaque showed that a higher injection speed, a higher mold temperature and a longer cooling time could effectively enhance the yield stress but negatively promoted the ductility. Then, the mechanisms of such variation of mechanical performance and the factors affecting it were investigated by means of differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and polarized light microscopy (PLM). The super high shear rate during cavity feeding in MI molding not only induced a typical three-layered structure but also developed a highly oriented fibrously morphological structure in the skin layer. However, such fully oriented morphology was much negative in the interlayer and even could not be observed in the core layer. The results from SEM and PLM observations indicated that the orientation morphology varied significantly through the plaque's cross-section and thickness of the each layer changed with the process parameters and geometric position, and finally led to variation of the mechanical performance.