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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While the PET bottles are blow-molded and heat-set in the hot mold, the “Mirror-like” aluminum mold surface becomes rough by transferring a substance from PET bottles. The outer surface of bottles becomes also rough because of copying the rough mold surface. After all, the appearance of bottles gets partially hazy. It results in the loss of the commercial value of the bottles. This phenomenon was analyzed by using the experimental device. The phenomenon was successfully reproduced by the heat-set of the biaxially oriented PET sheets with the experimental device of the aluminum block. As the number of the heat-set times increases, the deposit accumulates. The substance was determined to be a cyclic trimer (Cy 3) by the use of liquid chromatography. The mechanism of the adherent phenomena could be elucidated by a series of experiments using the monomer; BHET (Bishydroxyethyl terephthalate) and the oligomer; Cy 3. BHET was adhered to the aluminum block in the liquid phase at the early stage. Cy 3 was then adhered to the aluminum block with the existence of BHET as an adhesive. BHET evaporates or transforms into the polymer under the environment of the higher temperature. The polymer is then deformed to Cy 3, which is stable even exposed to the environment of the higher temperature. Therefore only Cy 3 is substantially observed on the hot aluminum block after the long period of time.

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