Numerical prediction of residual stresses and birefringence in injection/compression molded center-gated disk. Part II: Effects of processing conditions

Abstract

The accompanying paper, Part L has presented the physical modeling and basic numerical analysis results of the entire injection molding process, in particular with regard to both flow-induced and thermally-induced residual stress and birefringence in an injection molded center-gated disk. The present paper, Part II, investigates the effects of various processing conditions of injection/compression molding process on the residual stress and birefringence. The birefringence is significantly affected by injection melt temperature, packing pressure and packing time. However, the thermally-induced birefringence in the core region is insignificantly affected by most of the processing conditions. On the other hand, packing pressure, packing time and mold wall temperature affect the thermally-induced residual stress rather significantly in the shell layer, but insignificantly in the core region. The residual stress in the shell layer is usually compressive, but could be tensile if the packing time is long, packing pressure is large, and the mold temperature is low. The lateral constraint type turns out to play an important role in determining the residual stress in the shell layer. Injection/compression molding has been found to reduce flow-induced birefringence in comparison with the conventional injection molding process. In particular, mold closing velocity and initial opening thickness for the compression stage of injection/compression molding have significant effects on the flow-induced birefringence, but not on the thermal residual stress and the thermally-induced bireffingence.