Effect of processing conditions on shrinkage and warpage and morphology of injection moulded parts using microcellular injection moulding

This research investigated the effects of processing conditions on the shrinkage and warpage (SW) behaviour of a box shaped, polypropylene part using conventional and microcellular injection moulding. Two sets of 2^6–1 fractional factorial design of experiments (DOE) were employed to perform the experiments and proper statistical theory was used to analyse the data. After the injection moulding process reached a steady state, moulded samples were collected and measured using an optical coordinate measuring machine (OCMM), which had been evaluated using a proper repeatability and reproducibility (RR) measurement study. By analysing the statistically significant main and two factor interactions, the results show that the supercritical fluid (SCF) content (nitrogen in this case, in terms of SCF dosage time) and the injection speed affect the SW of microcellular injection moulded parts most significantly, whereas hold pressure and hold time have the most significant effect on the SW of conventional injection moulded parts. Investigation of the cell morphology indicated that fine and dense cells are not critical for achieving excellent results regarding SW. Also, the present study quantitatively showed that, within the processing range studied, a reduction in the SW could be achieved using the microcellular injection moulding process.