Abstract:
A two piece contact lens mold having a pair of blunt, continuous mating surfaces extending around the periphery of the mold cavity is disclosed. Parameters for mold materials and important dimensions are also disclosed. The mold can produce finished contact lenses which do not require further machining.

Description:
BACKGROUND OF THE INVENTION 
     Prior Art 
     U.S. Pat. No. 4,121,896 issued Oct. 24, 1978 to Shepherd. The mold disclosed has a flexible lip that permits the mold halves to move closer together during curing to compensate for lens material shrinkage. 
     U.S. Pat. No. 4,113,224 issued Sept. 12, 1978 to Clark. This patent discloses a plurality of passages connecting the mold cavity with the riser. The structure compensates for shrinkage by keeping the material in the riser fluid longer than that in the mold cavity. Machining is essential to obtaining a finished lens. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION AND DRAWINGS 
     FIG. 1 is a side view of the assembled mold in crosssection; and 
     FIG. 2 is an enlargement of the mold junction area. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, the female mold half is formed by frustroconical support 1 having flange 2 extending from the large end. Concave molding surface 3 has continuous flat mating surface 4 extending radially outwardly from periphery 5 of surface 3. Outwardly tapering wall 6 extends from outer edge 7 of mating surface 4 to the narrow end 8 of frustroconical support 1. 
     The male mold half is formed by frustroconical side 9 with flange 10 extending outwardly from the large end and convex molding surface 11 extending across the small end of side 9. Recess 12 forms a groove extending around the periphery of surface 11 to define the edge shape of a lens cast in cavity 13. Continuous flat mating surface 14 extends radially outwardly from recess 12 to side 9. 
     The radii of surfaces 3 and 11 are selected to provide a chosen power and base curve in the lens product well known to those skilled in the contact lens art. The surfaces have a common axis 15 when the mold halves are assembled. 
     Referring to FIG. 2, registration of the mold halves is obtained by an interference fit between edge 7, at the juncture of mating surface 4 with wall 6, and edge 16, at the juncture of mating surface 15 and side 9. The amount of interference must be such that the shape of neither recess 12 nor surface 11 is distorted when mating surface 4 is in contact with mating surface 14. Riser 17 is formed by a difference in taper angles for side 9 and wall 6 and accomodates an excess of lens material necessary to ensure completely filling cavity 13. 
     Since soft contact lens materials generally shrink 7% to over 20% by volume during curing, it is important that the mold be carefully designed to avoid prerelease or distortion during curing. Critical parameters, which must be considered in addition to the mold material being inert to the contact lens precursor, include softening temperature of the mold material, curing temperature of the contact lens material, mold thickness in the areas of the cavity, and flexibility of the mold material. In addition, size of the lens may require consideration and adjustment of the mold dimensions. 
     Polypropylene is a preferred mold material for lenses that typically are 0.3 to 0.7 inch in diameter, have a thickness in the range of 0.003 to 0.04 inch, and that shrink 10% to 20% by volume. A mating surface dimension in the order of 0.004 to 0.020 inch has been found suitable for lenses having the above parameters and a dimension of about 0.01 inch is preferred. This dimension is sufficient for the mating surfaces to accurately control lens thickness, and avoid flash at the mold joint when a weight of about two to three pounds per mold is placed on top of the male mold member. The mold thickness in the vicinity of the cavity, i.e., behind the concave and convex molding surfaces, can range from 0.015 to 0.045 with thickness of about 0.035 being preferred. Examples of suitable contact lens materials include those disclosed in U.S. Pat. Nos. 3,700,761 and 4,036,814.