Empirical Investigation of the Relationship Between Processing Parameters and Crystalline Morphology in Injection-Moulded Polypropylene Components Produced in Nigerian Automotive Assembly Plants

Crystalline morphology developed during injection moulding of polypropylene components significantly influences their mechanical and dimensional performance, yet the systematic relationship between moulding process parameters and resulting crystalline structure has not been empirically established for production conditions in Nigerian automotive assembly plants. This study investigates the effects of melt temperature, mould temperature, injection speed, and holding pressure on the crystalline morphology and mechanical properties of injection-moulded polypropylene components at an automotive assembly facility in Lagos State, South West Nigeria. A full factorial experimental design with centre points was employed at two levels of each process variable. Wide-angle X-ray diffraction characterised crystalline phase composition and degree of crystallinity, while polarised light microscopy revealed spherulite size and morphology at varying depths through component cross-sections. Tensile strength, notched Izod impact strength, and flexural modulus were correlated with morphological measurements using multivariate regression analysis. Results demonstrate that mould temperature exerted the strongest influence on spherulite size, with higher mould temperatures producing larger spherulites of 35 to 68 micrometres diameter at the component core compared to 8 to 15 micrometres at the rapidly cooled surface. Elevated mould temperature increased core-layer crystallinity from 52 to 64 percent. Impact strength was negatively correlated with spherulite size, while flexural modulus increased with overall crystallinity. Optimal mechanical performance was achieved at intermediate mould temperature of 40 degrees Celsius with high holding pressure of 70 MPa. The study provides empirically validated processing parameter guidelines for polypropylene injection moulding in Nigerian automotive manufacturing, contributing to quality specification development. Keywords: polypropylene, injection moulding, crystalline morphology, mechanical properties, automotive