In order to enhance the accuracy of precision injection molding, the authors proposed the "precision injection molding by cooling rate control" which enhances accuracy by controlling the shrinkage of molded products, and are studying the mechanism for realizing high precision. To date, we have investigated the shrinkage characteristics of cylindrical products, while limiting the scope of our investigations to only shrinkage along the radius. In this study, we extended the scope of investigations to two-dimensional deformations and applied the proposed molding method to U-shaped products. The width W₁ at the base and width W₂ at the opening of the U-shape were measured, and the effects of the initial mold temperature (T_M) and cooling rate on deformations at the side walls of PPS products were investigated. It was found that W₁ is easily affected by the effects of in-plane shrinkage of the top board, and more or less the same characteristics as deformation characteristics along the radius in cylindrical products were seen. Rapid cooling was confirmed to be effective for controlling deformation when T_M is higher than the glass transition temperature (T_g). In addition, the deformation characteristics of W₂, which directly reflect the tilting amount of the side walls, also changed around T_g like W₁. When T_M was higher than T_g, the side walls were found to tilt inwards in normal cooling, and this could be reduced by uniform rapid cooling. However, in non-uniform rapid cooling, when T_M was below T_g, no differences between the cooling methods were seen, however, deformation was found to be most controlled around T_g. When this led to low temperature, the tilting amount of the side walls increased. The same characteristics were seen for PBT and POM. These results also demonstrate that these characteristics can be explained by the generation of thermal stress depending on cooling rate and progress of stress relaxation.

View full abstract