Relationships between Cooling Conditions in Mold and Three-Dimensional Birefringences of Molded Thin Discs

The relationships between the cooling conditions in the mold and three-dimensional distribution of birefringences in molded thin polycarbonate resin discs have been studied in order to analyze the mechanism of the growth on the birefringence in the injection molded specimen. (1) The birefringence (n1-n2) in the plane of the molded disc is not affected by the mold temperature and cooling time. But the birefringence (n1-n3) in the cross section of the molded disc decreases with increasing the mold temperature and it increased with increasing the cooling time. (2) The molding shrinkage factor decreases with increasing the cooling time. The residual strain caused by the force preventing the resin shrinkage in the mold. Thus, the birefringence (n1-n3) increases with increasing cooling time. (3) The reason why (n1-n3) decreases with increasing the mold temperature is that the tensile stress received from the mold is small during the cooling stage. (4) The above phenomena are interpreted by means of a mechanical model which consists of the thermal expansion element and a four-parameter model.