The influence of molecular weight and mould temperature on the skin-core morphology in injection-moulded polypropylene parts containing weld lines

Abstract

The sandwich structure of injection-moulded polypropylene parts with and without weld lines has been investigated using optical microscopy. X-ray scattering, and differential scanning calorimetry. It was found that the skin-layer thickness is strongly dependent on whether the mould is injected through one gate or through two gates. Samples containing a weld line show a much lower skin-layer thickness than samples without a weld line. This difference, however, depends strongly on the molecular weight of the polypropylene. The skin-layer thickness also varies along the flow path and decreases with increasing mould temperature. While most of the polypropylene crystallizes in the crystallographic α-modification, in the textured skin layer, some β-modification occurs. The β-crystallinity depends on the molecular weight but disappears in the weld line. X-ray investigations exhibited a bimodal crystal orientation in the skin layer, one with thec-axis oriented parallel to the flow direction, the second one with thea ^*-axis oriented parallel to the flow direction. The core exhibited only a weakc-axis orientation. In the skin, the metastable α₁-modification was found.