Study of wave like flow marks of PP composites in injection molding

Abstract

Injection molding is one of the most popular processing techniques for polymers and occupies the dominant position in the mass production of plastic products. Recently, resins with enhanced optical and physical properties have been developed; consequently, the injection molding process is becoming more important in the manufacturing of products such as camera lenses, compact disk, automotive parts including bumper and consol body. In most application, exterior surface quality is crucial and any defect on the molded surface may become a major problem. The wavelike flow mark phenomenon is one of the surface defects that can occur during the injection stage of the injection molding process. It is characterized by a surface with periodic hills and valleys running perpendicular to the direction of the movement of the flow front [1]. In many cases the flow marks are asynchronous or out of phase on opposite surfaces of the part, with a cloud part on one surface of the part located across from a luster part on the opposite surface. Surface defects known as wavelike flow marks in injection molding are encountered with a variety of thermoplastic materials, including neat polymer, filled polymers, and polymer blend [2-4]. These defects have been reported at resin flow velocities ranging from 1 to 200 mm/s with wavelengths from 1000 to 10 μm and at a depth between 150 and 0.01 μm [5-7]. In this study, the flow mark on injection molded polymer composites was investigated. The materials used were PP/rubber composites and PP/talc composites, which are widely used for automobile part. PP/rubber and PP/talc composites were fabricated and the mechanism for appearance of a flow mark was clarified through observation of the structure of the injection molded specimens.