Method of Predicting the Distribution of Material Properties during Injection Molding (IV)

Abstract

Warpage of injection-molded products can be largely attributed to two principal factors. One is a flow factor related to the anisotropy or distribution of the coefficient of linear expansion as a result of the molecular orientation produced by the resin flow. The other is a thermal factor related to shrinkage differences stemming from different rates of cooling in the molded product. In an actual molding process, the mold temperature is asymmetrical due to the mold and product geometries. Shrinkage differences between the low and high temperature sides are especially apt to cause warpage. It is also thought that Warpage may occur due to the effect of the asymmetrical mold temperature on the resin flow. However, there do not appear to be any reports in the literature of studies concerning the effects of the asymmetry of the mold temperature on resin flow characteristics.In the present research, visualization experiments were conducted to determine the effects of the asymmetry of the mold temperature on the resin flow and its relationship to the molecular orientation ratio. The results made the following points clear.(1) Under conditions of an asymmetrical mold temperature, the solid layer grows more slowly on the high temperature side than on the low temperature side and the mainstream of the resin flow shifts to the high temperature side.(2) Shear stress induced by the distribution of the resin flow velocity in the thickness direction has the effect of increasing the molecular orientation ratio on the high temperature side, with the result that the distribution of the thermal expansion is affected.