SURFACE-ROUGHNESS AND IMPACT STRENGTH OF INJECTION-MOLDED POLYSTYRENE

The formation of regular surface waves in injection moulded polystyren e was investigated as a function of the moulding conditions and the mo lecular weight distribution of the polymer. The injection velocity and the position of measurement relative to the gate of the mould have th e greatest effect on the average roughness measured with a Talysurf ma chine. The packing pressure has only a minor effect in reducing the ro ughness. It is proposed that the waves occur if there is time for surf ace buckling in the fountain flow at the melt front. The waves are sho wn to have negligible effect on the unnotched impact strength, which i s dominated by molecular orientation, so the waves merely detract from the surface appearance of the moulding.