The effects of melt flow behavior and adjacent weld line formation behind the obstacle on the flow induced birefringence in the weld line region of the parts were investigated.Polystyrene parts with weld lines were injection-molded under various process conditions by means of a specially designed plate cavity with an obstacle.The photo-elasticity method was used to determine the gapwise-averaged birefringence along the flow direction of injection-molded parts.The results showed that melt temperature and packing pressure had a significant impact on the birefringence distribution of the molded parts.Birefringence pattern around the weld line region just behind the obstacle was apparently different from other regions of the part

which was attributed to the local flow behavior and molecular orientation distribution after the melt flow fronts met.The maximum value of gapwise-averaged birefringence along the weld line occurred at a small distance from the obstacle.Heat shrinkage test results of different locations at the adjacent weld line region were also consistent with the above results.