Birefringence prediction of optical media

The flow and thermally induced birefringence of injection-compression molded optical media such as compact discs and digital video discs is predicted by applying a stress-optical rule to the flow and thermally induced stresses, which are estimated with a viscoelastic material model integrated into a non-isothermal compressible flow simulation. The resulting model considers flow and cooling induced molecular orientation, and the transient effect of thermal stress and pressure. Contrary to previous research for polystyrene, the validated results indicate that, for polycarbonate, the magnitude of the thermally induced birefringence is comparable to the flow induced birefringence. Simulation results of the flow and thermally induced in-plane birefringence for compact-disc-recordable moldings with an optical grade of polycarbonate compared well with experimental observations at different mold and melt temperatures. Both simulation and experiments indicate that mold and melt temperatures have a significant effect on the level of birefringence; increasing mold or melt temperature significantly reduces the birefringence. (C) 2004 Society of Plastics Engineers.