Analysis of fibre orientation effects on injection moulded components

Fibre orientation, and thus the mechanical behaviour of short-fibre-reinforced thermoplastics (SFRTPs), depends greatly on the flow conditions in the mould. In order to design with SFRTPs, a thorough understanding of the processing conditions is required, together with the influence that the fibre orientation distribution (FOD) has upon the resulting mechanical properties. This work investigates the influence of primary injection moulding process parameters on the final properties of a simple planar injection-moulded component through design of experiments (DOE) and analysis of variance (ANOVA) techniques. Four factors are seen to have greatest effect on the mechanical properties: cavity thickness, packing pressure, packing time, and melt temperature. A systematic procedure is then employed to vary the levels of each factor, and the FOD and mechanical properties are studied. The final orientation is complex and is shown to vary throughout the part and depend heavily on the values selected for each parameter. Experimentally measured orientation results are then compared against those predicted by commercially available software for this planar geometry component. It is found that the predicted orientations were significantly over-predicted (34 per cent on average), from a minimum of 15 per cent for a 2 mm mould cavity up to 40 per cent for the larger 5 mm cavity thickness. This discrepancy in turn leads to an over-prediction, of approximately 50–60 per cent, between predicted and measured stiffness.