The birefringence problem in optical disk substrates: A modeling approach



An analytical model has been constructed to predict the birefringence level in an injuection-molded disk with direct application to optical disk substrates. The model incorporates simplified analyses for the flow in a center-gated cavity during the filling stage, the heat transfer during the cooling stage, and the stress relaxation following the cessation of flow at the end of the filling stage of the injection-molding cycle. The stress relaxation process is analyzed with the integral constitutive equation of Wagner and the residual stresses are converted to retardation via the stress-optical law. Predictions of normal retardation profiles are in line with experimental data and the general effects of melt temperature, mold temperature, and injection speed are closely captured by the model. By contrast, the observed transverse retardation is not in accord with the predicted response. This discrepancy is attributed to the fact that the transverse retardation is induced by cooling stresses and thus cannot be explained in terms of a flow stress mechanism employed in the present analysis.