Influences of processing conditions on gradient structures in injection-molded polyethylene caps and their property correlations

In this study, the influences of processing conditions, that is, injection speed and holding pressure, on the gradient crystal structural formation in the injection-molded cap specimens were investigated. Skin-, shear- and core-layers across the specimen thickness were investigated in detail by using small-angle x-ray scattering, wide-angle x-ray diffraction, and atomic force microscopy techniques. The results showed that high holding pressure caused high density and large weight of the cap specimens. This strongly contributed to high displacement resistant on applied pressure (doming resistant), while the lamellar thickness (L-c) of the skin layer also played a role. On the contrary, low holding pressure promoted the formation of the random-oriented lamellae in the core layer, which contributed to high environmental cracking resistance (ESCR). Moreover, the lower injection speed caused the thicker L-c of the skin layer, and this improved the ESCR of the specimen. The results allowed us to propose its failure mechanism.