Tiger-Striped Flow Mark of Polypropylene Alloys

We investigated the relationship between rheological properties and tiger-striped flow marks in injection moldings of polypropylene alloys prepared under a wide range of molding conditions, taking into consideration the visibility of flow marks. The three polypropylene alloys used in this study were composed of ethylene propylene block copolymer, ethylene butene rubber, and talc. By using a reduced shear rate in which the shear rate of the flow front in the mold was normalized by the relaxation time of the polypropylene alloys, it was possible to perform flow mark evaluations that considered the influences of molding conditions such as injection speed, molding temperature, and mold temperature. In all three polypropylene alloys, the flow mark evaluations improved as the reduced shear rate increased in the high reduced shear rate region. On the other hand, the flow marks were generated in the low reduced shear rate region, but they gradually became less noticeable and the flow mark evaluations improved as the reduced shear rate decreased from about 50. Design guidelines for polypropylene alloys recommend shortening their relaxation time in order to eliminate flow marks in the low reduced shear region. These material design guidelines are markedly different from those for eliminating flow marks in the high reduced shear region.