Flow and thermally induced birefringence in gas-assisted tubular injection moldings: Simulation and experiment

Various components of birefringence in polystyrene (PS) and polycarbonate (PC) tubular moldings, obtained by gas-assisted injection molding (GAIM), are measured and simulated considering both flow- and thermally induced stresses. The flow- and thermally induced stresses are calculated using nonlinear and linear viscoelastic theories, respectively, based on the flow and temperature histories that the polymer experiences during the GAIM cycle. Flow-induced birefringence components are calculated from flow stresses using the stress optical rule, while thermally induced birefringence components are calculated from thermal stresses using a photoviscoelastic model. The simulated total residual birefringence takes into account the contributions of both flow- and thermally induced birefringence generated during the melt injection, gas injection, and cooling stages of GAIM. The simulated results are in better agreement with the measured birefringence in PS moldings than in PC moldings. Considering both thermal and flow birefringence, simulations provides a better description of the experimental results indicating that, in GAIM moldings, the birefringence near the mold was caused mainly by flow stresses while that located towards the polymer/gas interface was caused mainly by thermal stresses