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Birefringence in injection-compression molding of amorphous polymers: Simulation and experiment



The influence of the processing variables on the residual birefringence was analyzed for polystyrene and polycarbonate disks obtained by injection-compression molding under various processing conditions. The processing variables studied were melt and mold temperatures, compression stroke, and switchover time. The modeling of flow-induced residual stresses and birefringence of amorphous polymers in injection-compression molded center-gated disks was carried out using a numerical scheme based on a hybrid finite element/finite difference/control volume method. A nonlinear viscoelastic constitutive equation and stress-optical rule were used to model frozen-in flow stresses in moldings. The filling, compression, packing, and cooling stages were considered. Thermally-induced residual birefringence was calculated using the linear viscoelastic and photoviscoelastic constitutive equations combined with the first-order rate equation for volume relaxation and the master curves for the Young's relaxation modulus and strain-optical coefficient functions. The residual birefringence in injection-compression moldings was measured. The effects of various processing conditions on the measured and simulated birefringence distribution Δn and average transverse birefringence <nrrnθθ> were elucidated. Comparison of the birefringence in disks manufactured by the injection molding and injection-compression molding was made. The predicted and measured birefringence is found to be in fair agreement. POLYM. ENG. SCI., 2013. © 2013 Society of Plastics Engineers