Orientation development in the injection molding of amorphous polymers

Frozen-in orientation in the injection-molding of amorphous polymers has been considered in terms of flow- and cooling-induced birefringence. In particular, measurements of the frozen-in orientation distribution in polystyrene (PS) molded strips and circular runners have been performed. Three birefringence components, Δn, n22 − n33, and n11 − n33, have been measured for strips, and two components, Δn and nrr − nθθ, for runners. The effects of various processing conditions, of strip thickness, and of runner diameter on orientation development have been analyzed and compared with those predicted by our previously developed viscoelastic theory. In addition to injection-molding experiments, free and constrained quenching experiments for PS and poly(methyl methacrylate) (PMMA) strips have been carried out and the gapwise distribution of cooling-induced (thermal) birefringence has been measured. Relaxation of thermal birefringence following quenching has been observed for PMMA. The effects of flow- and cooling-induced orientation on various components of birefringence in molded parts have been elucidated and limitations on the applicability of the stress-optical law to the injection-molding of amorphous polymers have been discussed