The Relation between the Layered Structure and the Resin Behavior in Injection Molding Process

In injection molding process, the molten resin flows speedy into the thin cavity and this causes high shear stress condition. In addition, the resin is rapidly cooled and solidified from the surface of the molded even if the molten resin flows in the cavity. It was reported that these phenomena result in the layered structure of the molded.In this paper, the cross-sectional polarizing microphotograph of injection molded plates were analyzed for various molding condition so as to examine the effect of molding condition on the layered structure. In order to clarify the generating mechanism of layered structure, the observed layered structures were correlated with the solidified layer thickening process during injection molding and time dependence of maximum shear stress, which were obtained by the injection molding simulation. As a result, it was shown that the maximum shear stress occurred between the solid and liquid layers drastically changes when the thickening rate of the solidified layer changes discontinuously, and that the depth of the boundaries among the layers is in accordance with the depth at where the maximum shear stress changes drastically. These two facts enable us to conclude that the generation of the layered structure is dominated both by the changes of solidified layer thickening and by the maximum shear stress between the solid and liquid layers.