Particle rearrangement and weldline formation during injection moulding of particulate composites

Optical microphotography, X-ray imaging and density analysis are used to examine the distribution of rigid spherical filler particles during the processing of thermoplastic particulate composites using injection moulding. Using simple rectangular cavities with circular inserts, parts are made with polystyrene containing between 30 and 50% by weight of glass beads ranging between 40 and 500 mu m in diameter and taken in narrow size distributions. The main feature of the observed microstructures is the formation of a skin-core structure in which the particulate phase forms a dense core at the centre plane of the cavity. A progressive increase in particle concentration with distance from the injection gate is also observed, as a result of a gradual accumulation of the particulate phase behind the advancing free surface. Flow past inserts result in the formation of two particle-enriched free surfaces which subsequently merge to form a particle enriched weldline. Under the conditions examined, it is observed that for the 425-500 mu m particles, the inhomogeneity occurs in all mixtures in which the particulate weight fraction is 25%.