In this study, we applied our new process "precision injection molding by cooling rate control" to cylindrical products and investigated the shrinkage mechanism of various resins. With this method, the molded product is rapidly cooled, after which the parts requiring accuracy are left inside the mold and cooled locally to control shrinkage. We particularly focused on the four factors, initial mold temperature, cooling method, cooling time, and time waited for rapid cooling to investigate their effects on shrinkage characteristics. For resins whose glass transition temperature (T_g) was below room temperature, shrinkage control effects could be obtained from this molding method when the initial mold temperature was above room temperature. On the other hand, for resins whose T_g was above room temperature, shrinkage control effects could be obtained only when the initial mold temperature was above T_g. When below T_g, our new method was not better than normal molding methods. Moreover, when the initial mold temperature rose by more than about 70°C above T_g, the temperature differed according to the method used for cooling the molded product. Shrinkage could be controlled better when the molded product was rapidly cooled from outside the mold than inside the mold. With PA 66 and PBT, shrinkage could be controlled by extending the cooling time. This may be because the thermal stress occurring during cooling decreases with time. Furthermore, we found that extending the starting time of rapid cooling further improves the crystallinity of PP. These results clarified that our new method promotes crystallinity at moderate cooling, and controls shrinkage by rapidly cooling the molded product to improve accuracy.

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Structures of polymer/paint interfaces were investigated for injection and compression molded polystyrene (PS), poly(styrene-co-acrylonitrile) (SAN) and poly(methyl methacrylate) (PMMA) specimens. For the interfaces of injection molded specimens, a peculiar "zigzag" structure formation of several tens μm in size was observed similar to the ABS system reported before. In contrast, the interface structure was not observed for these compression molded specimens and the injection molded specimens annealed above the glass transition temperature. These facts indicate that this peculiar interface structure is a universal phenomenon relating to the residual stress and molecular orientation at the surface of the injection molded specimens.

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