Method of manufacturing pressed lenses

DETAILED DESCRIPTION The present invention overcomes these problems by cooling the molded lens in a pressing mold but without pressure being applied.
Since the lens remains in the mold, no temperature difference is generated between the glass surface and the interior of the molded lens and shrinkage is prevented so that a pressed lens having a high surface accuracy can be obtained.
According to the present invention, a glass preform is inserted into a pressing mold in which an upper and lower mold are within a sleeve and at least the upper mold is made slidable.
A force is then applied to the upper mold to press the glass preform by using a pressing rod arranged separately from the upper mold at a temperature corresponding to a glass viscosity between 10.
sup.
8.
5 and 10.
sup.
10.
5 poise for a time from several seconds to several tens of seconds.
The force is then removed from the pressing rod and the thus obtained glass lens is cooled in the mold until the glass viscosity reaches a value not smaller than 10.
sup.
11.
5 poise.
In fact, the pressing rod may be withdrawn from the molds.
Thus, the present invention results from the discovery that high surface accuracy may be obtained even though the cooling step is not performed under pressure as long as pressing is initiated at a relatively high glass viscosity of 10.
sup.
8.
5 -10.
sup.
10.
5 poise and the entire glass body is uniformly cool by leaving the glass in the pressing mold.
In the prior art as described above, a pressing rod for applying a force to the mold is coupled, for example, with an upper mold, when the force is removed, and thereafter a temperature differential develops between upper and lower surfaces of the glass because the glass is in the lower mold.
Accordingly, if mold portions are separated at a glass viscosity of 10.
sup.
8.
5 -10.
sup.
10.
5 poise, high surface accuracy could not be obtained because shrinkage or the like might occur.
Therefore it has been necessary to continue pressing until the viscosity increases to a value not smaller than 10