Molecular orientation induced by cooling stresses. Birefringence in polycarbonate: III. Constrained quench and injection molding

Abstract

AbstractResidual stress and birefringence distributions are determined in polycarbonate samples obtained by quenching in a specially designed apparatus and by injection molding. The molecular orientation is distinguished from the thermally and pressure‐induced residual stresses. The birefringence in the quenched samples is found to be positive and almost constant, independent of the quench temperature, but varying strongly with initial quench temperature between 150 and 180°C. The residual stress level, as determined by layer removal and sectioning, is very low. The birefringence distribution is mainly due to a tensile equibiaxial orientation induced by transient cooling stresses built up above Tg. The samples which are injection‐molded with a high injection speed and without packing pressure display the same birefringence distribution as the quenched samples, apart from a local maximum beneath the surface due to the shear flow during filling. Apart from the flow during filling and packing, the frozen‐in molecular orientation in injection‐molded samples is also induced by transient thermal stresses present during vitrification. The birefringence from thermally induced orientation was found to be of comparable magnitude to that from flow‐induced orientation. For a correct prediction of molecular orientation the thermal strains above Tg must therefore be included in simulation programs. Because of the low level of thermal stresses, the application of a packing pressure will lead to tensile stresses at the surface in general. © 1994 John Wiley & Sons, Inc.