Study of the Effects of Mold Coating on the Birefringence of Injection‐Molded Optical‐Quality Polycarbonate

ABSTRACT This study investigates the impact of mold surface coatings on the birefringence of injection‐molded optical‐quality polycarbonate lenses. Birefringence, caused by residual stresses during molding, affects optical clarity and performance. To explore how mold coatings influence these stresses, experiments were conducted using nickel‐plated and non‐plated aluminum molds for two different lens designs. Photoelastic stress analysis revealed that nickel plating significantly reduced residual stress and birefringence by altering the heat transfer at the mold/polymer interface, delaying skin layer formation, and allowing more time for polymer relaxation. Injection pressure monitoring and numerical simulations confirmed that changes in the thermal contact resistance introduced by the plating led to reduced flow resistance and stress development. The findings highlight the importance of mold surface properties in optimizing optical injection molding, offering insights for improved lens manufacturing with enhanced optical performance.