The Effects of Processing Conditions on Residual Stresses and Birefringence in Injection Molded Articles
Tai Hun Kwon, Jae Gu Lee, Young Bok Lee, Tai Hun Kwon
POSTECH
Korea(South)
Keywords: injection molding, birefringence, residual stress
In the present study, extensive experimental and numerical studies were carried out in order to investigate the effects of processing conditions on residual stress and birefringence in injection molded articles. Residual stresses were measured by the layer removal method and appropriate fitting scheme and birefringence was measured over thickness direction by using a polarizing microscope. In general, the resulted residual stress is tensile in the core region, becomes compressive in the shell region near the surface, and tends to become tensile at the surface. Birefringence distribution shows that there are two peaks that consist of an outer peak due to high shear filling flow and subsequent rapid cooling near the surface during the filling stage and an inner peak developed by an additional flow during the packing stage. The effects of various processing conditions on the birefringence and residual stresses were extensively studied in a center-gated disk and a flat plate: the processing conditions in this investigation include melt temperature, mold temperature, packing pressure, packing time and injection speed. Finally, experimental results obtained from various processing conditions were compared with the corresponding numerical analysis results which are obtained from a numerical analysis system based on Leonov model, free volume theory, stress optical law, linear viscoelasticity and photoviscoelasticity.