Effect of the skin-core morphology on the mechanical properties of injection-moulded parts

Abstract

The presented study deals with the effects of different processing parameters during injection moulding on the morphological structure through the thickness of the injection-moulded samples and, consequently, on their mechanical properties. In this work, tensile bars of an isotactic polypropylene were injected under different conditions such as the flow rate, the melt temperature and the mould temperature. The morphological structure of the samples was investigated with polarized light microscopy using thin cross-sections cut perpendicularly to the flow direction. The fountain flow in the mould cavity influenced the crystallization kinetics and the presence of three distinct crystalline zones was observed; namely, the highly oriented non-spherulitic skin, the shear-nucleated spherulitic intermediate layer and the inner core composed of spherulites with a low orientation. The results showed that the flow rate has the highest influence on the thickness of the oriented skin layer. The mechanical properties of the tensile samples demonstrated that the larger thickness of the two outer skins provides the higher tensile strength. The same effect was also confirmed with a microhardness test where the skin layer was harder than the inner spherulitic core.