1. Field of the Invention
This invention relates to contact lenses and has particular reference to apparatus and method for casting the lenses to finished size, shape and desired edge configuration in a single operation.
2. Discussion of the Prior Art
In the manufacture of "soft" contact lenses, e.g. of cast silicon resin, edge finishing ordinarily consumes a very high percentage of the total labor required to make and inspect each lens. Edge finishing includes all operations involved in sizing a lens to desired diameter, contouring the edge to a comfortable shape and polishing or otherwise smoothing its surfaces. Prior art finishing equipment can be expected to cost more than half the total equipment cost (molds excepted) and occupy as much or more than half of the total manufacturing floor space with as much as three-quarters of the quantity yield losses for an entire lens manufacturing operation being attributed to the group of edge finishing steps. Additionally, the quality of contour and finish of the best of prior art edge finished lenses is usually considered only fair.
The disclosures of U.S. Pat. Nos. 2,990,664; 3,087,284; 3,160,039; 3,162,985; 3,369,329; 3,423,886; 3,458,959; 3,528,326; 3,835,590 and 3,948,007 are exemplary of lens edge finishing procedures and apparatuses.
Centrifugal casting technique, e.g. as in U.S. Pat. Nos. 3,408,429; 3,660,545 and 3,496,254 have been used with the objective, among others, of avoiding lens material shrinkage problems in the forming of thin edged lenses. The spin cast lens edge, however, becomes the edge of the posterior or concave surface of the lens. This surface is formed as a result of centrifugal forces, surface tension of the polymerizing lens material and other factors such as mold size and shape, volume, viscosity and volatility of the polymerizing mixture, condition of the mold surface, etc. The resulting surface accordingly becomes not only approximately parabolic in shape but very difficult to control and reproduce. There being no truly spherical optical zone and optimum optical quality is lacking. Also, post-polymerization edge finishing is usually required.
While lens edges requiring a minimum of finishing have been formed in systems which produce desirable lens surface profiles, e.g. as in U.S. Pat. Nos. 3,761,208; 3,915,609 and 4,017,238, parting lines between mold halves form the edges in locations which are subject to causing patient discomfort. For example, in the three-piece mold of U.S. Pat. No. 3,761,208 (FIGS. 1-6), two parting lines result at opposite sides of the cast lens edge. One would rest against the cornea and the other be engaged by the eyelid in use of the lens. These parting lines are very difficult and unduly costly to blend to the extent necessary for overcoming patient discomfort, especially in quantity production. On the other hand, the two-piece mold of U.S. Pat. No. 3,761,208 (FIG. 7) would inherently produce an edge flange requiring removal, e.g. by one or another of the aforesaid edge finishing procedures. It would be difficult for production quantities to obtain precise alignment of the mold halves in FIG. 7 of U.S. Pat. Nos. 3,761,208 so as to not produce a step at the parting line.
In the embodiments of U.S. Pat. Nos. 3,915,609 and 4,017,238, the sharp edge contact between a circular margin of one mold half and a spherically curved peripheral portion of the other mold half can minimize, if not eliminate, the need for most edge finishing operations. However, the mold parting lines may cause the cast lens edges to be located at positions and to extents tending to cause patient discomfort.
The foregoing comments illustrate the need in the present art for elimination of conventional costly and relatively ineffectual edge finishing operations and especially the need for improved lens edge design as well as apparatus and method all for accomplishing enhanced contact lens wearing comfort.
It is, accordingly, a principal object of the invention to provide a cast contact lens of improved edge design as well as apparatus and method of making the same.
A more specific object is to provide a contact lens which may be cast between separable mold halves having a single continuous mold parting line about the periphery of the lens casting cavity in a position relative to the cast lens edge which will lie between the eyelid and cornea and not rub on either when the lens is worn.
A corrollary objective is that of preventing occurrences of mold parting line blemishing or marking of cast contact lens edges at positions subject to engaging either the cornea or eyelid when the lenses are worn and further providing for modification of lens edge profile as needed to meet these particular requirements in various lens fitting situations.
The foregoing and other objects and advantages of the invention will become apparent from the following description.