Microstructural and mechanical analysis of the skin/shell/core gradient structure of glass fibre- and talc-reinforced injection moulded iPP

Injection moulding of filled polypropylene often results in a distinct skin–shell–core morphology, leading to heterogeneous mechanical and thermal properties across the part thickness. Characterizing the morphology of each structural region offers great potential for understanding structure–property relationships. However, this characterization presents significant challenges due to the limited thickness of each region, the gradual transitions between them, and the difficulty in isolating and characterizing their distinct morphological features. To achieve layer-resolved morphological and mechanical insight, a custom slicing method was applied to separate the skin, shell, and core regions into sections with thicknesses ranging from 100 to 200 μ m. Each region was then characterized using ashing, differential scanning calorimetry (DSC), tensile testing, and optical microscopy. In this study, isotactic polypropylene filled with 30 wt.% glass fibre (PP-GF) and with 30 wt.% talc (PP-Talc) was analysed to investigate the structural and property variations within the injection moulded cross section. The shell region was identified as a key morphological feature, bridging the highly sheared and rapidly cooled skin and the more isotropic core. Notably, the skin regions of both PP-GF and PP-Talc generally exhibited the lowest mean crystallinity (29.87% and 31.91%) and lowest filler contents, 11.3% and 14.2%, respectively. Flow simulation were performed to identify the source of the heterogeneity. Simulation results revealed steep gradients in shear rate and cooling rate across the part thickness, with maximum values occurring at the skin and decreasing toward the core. These gradients directly influence filler distribution, orientation, and matrix crystallinity. The study highlights the roles of the different regions in the overall mechanical behaviour and demonstrates the importance of correlating morphology with processing conditions for improved injection moulding design and analysis.