Numerical simulation for injection molding with a rapidly heated mold, Part II: Birefringence prediction

In the accompanying paper, Part I, the advantages of the rapid thermal response (RTR) molding process were investigated for thin-wall-mold filling by employing coupled analysis of flow and heat transfer. Besides the complete filling of the cavity, frozen-in molecular orientation is another major quality issue in thin wall molding. The frozen-in orientation causes residual stress and birefringence, and potential part distortion. The present work focuses on the prediction and visualization of birefringence in RTR-molded parts. To calculate birefringence, flow-induced residual stress is computed first and the stress-optical law is then applied. The simulation results show that the amount of molecular orientation, residual stress, and birefringence level considerably decrease in the RTR-molding process. The effect of the mold temperature on the level of birefringence was also studied and predicted birefringence patterns were compared with experimental results for a thin-walled rectangular strip. Both predicted and experimental patterns of birefringence are in agreement on the observation that the birefringence level diminishes significantly when the mold temperature is raised to above the glass transition temperature.