Unusual Tiger-Striped Flow Mark Pattern in Injection Moldings for Polypropylene/Talc Compounds II

We investigated the thickness of the skin layer just below the surface of the glossy part and cloudy part of tiger-striped flow marks that appeared in injection moldings of polypropylene (PP)/talc compounds, and we examined the correlation between flow marks with the same-phase (SP)-type pattern and “flow front slip” inferred in our previous paper. The cycle of gloss change on the surface of injection moldings of PP/talc materials showing one SP-type and two different-phase (DP)-type flow marks was the same as the cycle of skin layer thickness change, regardless of the flow mark pattern. In both the patterns, the thickness of the skin layer was minimal in the glossy area and maximal in the cloudy area. Furthermore, the minimum and maximum values of the skin layer thickness in the SP type were smaller compared with the DP type. Regardless of the unstable flow mechanisms of the flow front, the glossy part and the cloudy part were generated due to the time lag when the molten resin contacted the mold, after which the flow marks were generated. The SP-type flow marks that appear in the PP/talc compounds used in this study can be attributed to flow front slip.