a method and assembly for molding plastic optical lenses . the mold assembly comprises an annular retainer ring supporting a pair of generally disc shaped mold parts urged toward each other by a resilient force while separated by one or more gaskets , preferably including at least one soft gasket of uniform thickness between its inside and outside diameters which has the property of softening at elevated temperatures . when the cavity formed between the mold parts and bounded peripherally by the gaskets is filled with a resin formulation in the form of a liquid which then polymerizes when raised to an elevated curing temperature , shrinking and becoming hard and insoluble at room temperature , in the process the softening of the gasket permits the mold parts , under the action of the resilient force , to move towards each other to maintain contact with the formulation as it shrinks and hardens . a nest in which the assembly in the retainer ring is mounted has attached thereto a spring biased clamp arm for exerting the resilient force to urge the mold parts towards each other , the direction and location of such force being precisely determinable relative to the mounted retainer ring .