Structural hierarchy developed in co-injection molded polystyrene/polypropylene parts

In this paper, the structural gradients developed in co-injection molded polypropylene/polystyrene parts were investigated as a function of process history and injection sequence. For this purpose, a series of parts with PP skin/PS core, PS skin/PP core, and PP skin/PP core were prepared under selected mold temperatures and injection speeds and detailed structural analyses were performed using a series of structural analysis techniques. The optical microscopy, microbeam-WAXS and birefringence techniques revealed that crystalline orientation levels in PP are high when it is injected first to form the outer (skin) layer of the part. On going from the mold surface to the polystyrene surface the orientation level decreases slightly but remains high even at the PS/PP interface that is located at the interior of the part. This result is mainly caused by the additional deformation that the PP experiences as a result of secondary shearing by the polystyrene injection that occurs a short time delay after the primary injection. When PP is injected as a core layer, the orientation levels in PP were found to be low and mainly concentrated near the PP/PS interface. This low level of orientation is as a result of slower cooling that causes the relaxation of the chain orientation developed during the flow prior to the crystallization. In all polystyrene skin samples, the birefringence between the two skins was found to be the highest at the intermediate distances. When the injection speed of the core polypropylene is increased, this peak position shifts towards the mold surface as a result of shear heating. The use of low injection speeds in the core layer injection was found to distribute this layer more uniformly along the flow path. This is attributed to the attainment of increased levels of viscosity for the first injected layers during the course of injection of core layers at slower speeds. (C) 1999 Elsevier Science Ltd. All rights reserved.