Reduction of cycle time during press molding of glass lenses

Glass press molding of optical lenses is a time-consuming process due to the time to heat and cool the glass material. Cycle times can range from 15 to 30 minutes with over 80% of the cycle time consumed in heating and cooling. The heating process is limited by the heater capacity, while the cooling process is limited by the maximum cooling rate required to achieve the desired minimum residual stress or birefringence. Typically, glass press molding machines utilize radiant heating elements; therefore, the heating power is limited by the exposure of the mold and glass to the heating elements. Also, temperature gradients in the mirror are difficult to control by radiant heating. This paper presents a new glass press molding machine design that utilizes two heating element types with several independently controlled zones, allowing for higher heating capacity and better control of thermal gradients. The cooling rate during gradual cooling is also controlled to a finer tolerance through nitrogen gas cooling combined with the independently controlled heating zones. Glass lenses pressed with the new process exhibit a shorter cycle time and reduced birefringence.