Role of gas cooling time on crystalline morphology and mechanical property of the HDPE parts prepared by gas-assisted injection molding

In this study, the hierarchical crystalline structures of high-density polyethylene (HDPE) samples molded by gas-assisted injection molding (GAIM) with different gas cooling times were characterized via scanning electron microscopy, two-dimensional wide-angle X-ray scattering, tensile testing techniques, and differential scanning calorimetry, respectively. It was found that the shish-kebab, the oriented lamellae, and common spherulite structures orderly distributed from the skin region to gas channel region of samples. More importantly, the wider area with highly oriented structure (shish-kebab) was obtained in the samples with longer gas cooling time, in that the longer gas cooling time tends to increase the cooling rate of polymer melt, and then much more stretched chains are retained. Although lower crystallinity, the higher degree of orientation, and much more shish-kebab structures lead to significant reinforcement from 28 to 785 MPa of the samples with gas cooling time of 0.5 s to 32 and 879 MPa of the samples with gas cooling time of 20 s for tensile strength and modulus, respectively. Finally, combined the HDPE molecular parameter with characteristic of the GAIM temperature field and flow field, the formation and stability of crystalline morphology in different regions of sample were discussed.