There are many methods of molding plastic products. Injection molding is used widely because it can manufacture an intricately shaped product in a short period of time. However, one problem with injection-molded products is that they are apt to warp. Although warpage prediction by warp analysis provides qualitative agreement with actual warping, quantitative prediction accuracy is still insufficient. An investigation into the cause of that problem revealed that the thermal expansion coefficient of injection molded products was characterized by anisotropy and a large distribution in the layers along their thickness. However, it was found that the prediction method was not capable of analyzing that anisotropy and distribution with sufficient accuracy. Based on that result, it was assumed that prediction of the anisotropy and distribution of the thermal expansion coefficient was important for improving warp analysis accuracy, and a method of predicting that anisotropy and distribution was investigated.
In this study, injection molding was used to form a plate made of Polystyrene (PS), an amorphous material, and the residual stress generated by the anisotropy and distribution of the thermal expansion coefficient was examined. It was found that flow and thermal factors were dominant in inducing residual stress in the surface layer and in the inner layer of the PS plate, respectively. The molecular orientation ratio of each layer was also measured. The surface layer where the flow factor was dominant had a high molecular orientation ratio and the inner layer where the thermal factor was dominant had a small molecular orientation ratio. The thickness of the plate was then sliced into six layers and measurements were made of the anisotropy and distribution of the thermal expansion coefficient in order to examine the relationship with the molecular orientation ratio. The results indicated that the thermal expansion coefficient had a wide distribution in each layer. Moreover, a correlation was observed between the thermal expansion coefficient and the molecular orientation. It was found that the anisotropy and distribution of the thermal expansion coefficient can be predicted from the molecular orientation.
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