This paper investigates the process of precision compression-molding of plastics lenses with near-zero residual birefringence in the molded lenses. First, plastics preforms are injection molded by a conventional injection molding machine by net-shape molding technology. The preforms are then re-heated by infrared heaters and compressed under hydrostatic pressure conditions for form-finishing and stress-relieving. Once the preforms are heated above glass transition temperature, the moldings would release the frozen-in residual-stresses and retain the form accuracy via low compression pressure employed for shrinkage compensation. The compression molding process exhibits advantages of low process costs, high production efficiency, and good dimensional accuracy with zero-birefringence. First, experiments incorporated with Taguchi method are conducted for parameter optimization in residual stresses and form accuracy. The preliminary results show that plastic lenses optimized for form accuracy could have p-v values smaller than 0.35 micron plus RMS surface roughness less than 60 run. More importantly, the plastic lenses are molded with little birefringence throughout the process.
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