Method of Molding Bifocal Contact Lenses

The use of plastic materials for making optical lenses has increased rapidly for the past ten years. This is due to the availability of better plastic materials and the physical advantages of the plastic resins for specific applications such as contact lenses. The technology for the production of high quality contact lenses has not kept pace with the material supply industry. It is important to advance both areas if the full potential is to be realized.

The current bifocal contact lenses are fabricated from plastic or glass, each lens is individually ground and polished to the required specifications. To achieve accurate reproduction of bifocals is most difficult and expensive. This new process makes possible exact reproductions and has many other distinct advantages which will become apparent from the following disclosure.

An object is to provide a process for making inexpensive bifocal contact lenses which may be made to identical specifications.

U.S. Pat. Nos. 4,239,712 Dec. 15, 1980 and 4,202,848 May 13, 1980 disclose lenses having a crescent shaped reading segment cut and polished on the front surface. In these patents, methods are disclosed wherein the reading segment is cut into the distance segment. U.S. Pat. Nos. 4,524,035, 3,440,306, 3,560,598 and 3,597,055 disclose methods of making fused crescent bifocals having two materials of different refractive index.

FIG. 1 shows the metal mold in section.

FIG. 2 shows the metal mold with the reading segment cut, in section.

FIG. 3 shows the metal mold with the reading and distant segments cut, in section.

FIG. 4 shows the mold with the resin negative mold in place, in section.

FIG. 5 shows the negative resin mold in the silicone sleeve and the bifocal lens monomer in place, in section.

FIG. 6 shows the resinous negative mold and the solid bifocal lens material removed from the silicone sleeve.

FIG. 7 shows the negative resinous mold and the bifocal lens with the concave surface cut, in section.

FIG. 8 shows the bifocal lens from the front.

The convex mold surface 2 FIGS. 1 and 2 is rough cut to a non-optical surface to remove surplus metal stock from the mold 1 FIGS. 1 and 2. The shorter convex radius reading segment 3 FIG. 2 is cut into the convex surface 2 FIG. 2 and polished to provide an optical surface having the required radius and the required thickness. The reading optical surface 3 FIG. 2 may have no prism with the optical center located at the geometric center or the optical center may be displaced from the geometric center. The location of the optical center will have the affect of changing the shape of the upper segment demarcation line. A further separation of the optical centers of the reading segment and the distant segment results in a longer radius of the upper segment demarcation line. The distant segment 2 FIG. 3 is cut and polished having the optical centers of the near and distant segments displaced one from the other to provide a difference of from 0.25 to 3.5 diopters of prism refractive power. A prism diopter is defined as an image displacement of one centimeter at a distance of one meter. The prism base or thicker edge 8 FIG. 7 is located at the lower lens edge at the midway point of the reading segment. The thin apex 9 FIG. 7 of the prism is located at the upper lens edge, 180.degree. removed from the prism base 8 FIG. 7. Smaller amounts of prism result in thinner lenses having a longer segment arc. Larger amounts of prism or greater differences in edge thickness result in a shorter segment arc. The optical distant convex surface 2 FIG. 3 is cut and polished by methods well known to the art. No special cover or protection is required for the convex reading segment 3 FIG. 3 as the polishing lap is well supported by the distant surface 2 FIG. 3 and no distortion of the near segment will occur while polishing the distant surface. The reading surface 3 FIG. 3 is well below the distant surface 2 FIG. 3 and therefore requires no cover or protection. A hard polishing lap is used therefore filling of the segment is not required. It is understood that the process may be reversed and the distant lens power made without prism or offset and the reading segment 3 FIG. 2 is cut with prism or the combination of prism in both distant or near segments may be employed to control the segment shape.

The metal mold 1 FIG. 3 is placed in a metal cylinder 10 FIG. 4. The convex mold 1 FIG. 4 having the crescent shaped reading segment 3 FIG. 4 surrounded by the distant mold segment 2 FIG. 4 is covered with a heated and softened resin material 4 FIG. 4. Pressure is applied either by compression molding or injection molding. The resin material is allowed to assume the shape of the mold surface 2 and 3 FIG. 4 and the inside of the cylinder 10 FIG. 4. The plastic heated resin is allowed to cool and become solid. The following resinous materials have been found useful: nylon, polypropylene, polyvinylchloride and polyethylene. The metal molds have been made from brass and stainless steel. The cool concave resinous mold 4 FIG. 4 is removed from the convex metal mold 1 FIG. 4.

The concave resinous mold 4 FIG. 5 is placed in a silicone sleeve 6 FIG. 5. The concave optical surfaces 2 FIG. 5 and 3 FIG. 5 are covered with a liquid lens monomer 5 FIG. 5. The liquid lens monomer 5 FIG. 5 is allowed to polymerize and form a transparent solid having the convex image of the concave optical surface 2 and 3 FIG. 5. The resinous concave mold 4 FIG. 6 and the solid lens material 5 FIG. 6 are removed from the silicone rubber sleeve 6 FIG. 5. A concave optical surface 7 FIG. 7 is cut and polished on the lens material 5 FIG. 7. The prism cut into surface 2 FIG. 7 provides a thin lens apex 9 FIG. 7 and a thick lens base 8 FIG. 7. Prism is necessary to provide ballast and position the reading segment 3 FIGS. 7 and 8 at the lower lens edge. The bifocal lens 2 FIG. 8 is removed from the resinous concave mold 4 FIG. 7. The crescent shaped segment 3 FIG. 8 has a positive refractive power.

For the production of soft hydrogel bifocal contact lenses the following monomer formulations have been found useful:

The hard xerogel lens 5 FIG. 7 is removed from the resinous mold 4 FIG. 7 and hydrated to form a soft hydrogel bifocal contact lens. The bifocal lenses are reproducible as many identical resinous concave molds 4 FIG. 4 can be made from the metal convex mold 1 FIG. 4.

A second convex resinous mold having the convex radius 7 FIG. 7 may be placed in the silicone rubber sleeve 6 FIG. 5 and provide the radius 7 FIG. 7 by molding. In this manner the lens is completely cast molded.

It is understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art.