Effect of preform shape on formability and internal stress for glass molded concave meniscus lens

In this study, finite element analyses are conducted for the glass molding press of a large-diameter concave meniscus lens. As press molding may commence in a non-uniform temperature field, thermal-structure two-way coupling simulations are applied to the molding process to understand the sequential changes in the lens shape and temperature distribution. Three types of glass preform, namely, ball model, approximate curvature model, and near net model, are prepared to investigate the effects of preform shape on formability and internal stress. Preforms with a small clearance (shape difference) from the mold shape require less time for complete transfer; however, high internal stresses occur during cooling. The internal stress generated during cooling is thermal stress, caused by the internal temperature difference; it can be reduced by expanding the contact regions between the lens surfaces and molds. Moreover, to improve formability and reduce internal stress when forming using an approximate curvature model, the preform shape and heat input temperature conditions to the upper and lower molds are optimized. In both cases, although the internal stress is slightly higher during the molding process, the entire lens surface is in contact with the mold during cooling, thus resulting in a significant decrease in internal stress.