冷却速度制御による精密射出成形法の開発 第3報　粘弾性モデルによる円筒形状部品の収縮メカニズムの解明

抄録

In this study, we applied our precision injection molding method, which enhances molding precision by controlling cooling rate, to cylindrical parts and reviewed the shrinkage mechanisms of PPS, PBT, and POM. Using a viscoelastic model, we calculated elastic strain after demolding, taking into account the generation and relaxation of thermal stress during the cooling process. We then added thermal strain after demolding to this to analyze the total shrinkage strain, and investigated the changes in the shrinkage strain in respect to the initial mold temperature (T_M). The results showed that for PPS and PBT, whose glass transition temperature (T_g) is higher than room temperature, their shrinkage characteristics were found to change around T_g as the borderline. When T_M was higher than T_g, the shrinkage strain increased gradually in normal cooling. The shrinkage strain at this time was dependent on the thermal strain in the case of PPS, and dependent on both the elastic and thermal strain in the case of PBT. However, with PBT, as strain relaxed gradually, the elastic strain decreased with longer cooling time, resulting in reduced shrinkage strain. On the other hand, when T_M was below T_g, both resin materials showed a balanced relation between the thermal and elastic strain because the elastic strain increased with greater control of the thermal strain, resulting in a more or less uniform shrinkage strain. The same results were observed when the molded product was rapidly cooled. In the case of POM, whose T_g is lower than room temperature, no balanced relation was seen between the thermal and elastic strain. When T_M was higher, the thermal strain was dominant, resulting in a gradual increase in the shrinkage strain during normal cooling. These results show that the shrinkage characteristics obtained in experiments were reproduced well by the analysis, and that the shrinkage mechanisms of the various resins can be explained by the correlation between thermal and elastic strain.