JP2790262B

A method for press-forming a glass preform includes the steps of applying positioning pressure to an upper mold as the temperature of the upper mold and a lower mold increase to set values, maintaining a first temperature difference between the upper and lower molds of at least 10 DEG C. and increasing the temperature of the glass preform to a value above its glass transition temperature. Additional steps include cooling the upper and lower molds at a rate of 30 DEG to 100 DEG C. per minute while maintaining a temperature difference between the upper and lower molds at at least the first temperature difference, applying pressure to the lower mold, and opening the upper and lower molds to release the formed optical element after the temperatures of the upper and lower molds decrease to below a set value and a temperature difference between the upper and lower molds becomes greater than the first temperature difference.