2007 Volume 35 Issue 5 Pages 273-281
The striped patterns of moldings formed parallel to the material flow direction in the vicinity of injection gate was investigated for polypropylene (PP)/ethylene-propylene rubber (EPR)/ethylene-butylene rubber (EBR)/talc blends from the viewpoint of morphologies and rheological properties. The trace of material flow generated at the packing stage of injection molding process was observed on the cross section cut in the direction perpendicular to the stripes. The trace is circular in shape, and the area of the trace shows a maximum at a position just under stripes. The morphologies of the blends at the trace were compared with those in the other region of the moldings. It is found that the morphology at the trace appears co-continuous, but the interfaces between two phases is smeared, differing from usual co-continuous morphologies with distinct interfaces. The emergence of the smeared interfaces indicates that the morphology in the trace originates in part from the large deformation of rubber particulates. A large scale of material flow enlarging the deformation of particulates occurs especially in the trace just under stripes. Namely, the force generated by the flow deforms the materials in the thickness direction of molding and the strain gives rise to the stripes on the surface. This mechanism allows that the stripes would not be observed on the surfaces of moldings if the stress relaxation rate is faster than that of crystallization of PP in the blends in the packing and solidification stages. In fact, the moldings of the blends with the shorter longest relaxation time have the excellent surface appearances without any stripes.