Patent Document

This application is a divisional of Ser. No. 09/095,703, filed on Jun. 9, 1998, now U.S. Pat. No. 5,968,422, which claims the benefit of U.S. Provisional Application No. 60/058,096 filed on Jun. 30, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method of injection molding contact lens molds for cast molding contact lenses having a rotationally asymmetric lens surface, and apparatus for carrying out the method. 
     One method in practice for making contact lenses is cast molding. Cast molding of contact lenses involves depositing a curable mixture of polymerizable monomers in a mold cavity formed by two mold sections, curing the monomer mixture, and disassembling the mold assembly and removing the lens. Other processing steps, for example, hydration in the case of hydrogel lenses, may also be employed. One mold section forms the anterior lens surface (anterior mold section), and the other mold section forms the posterior lens surface (posterior mold section). Prior to the cast molding of the contact lens, each of the mold sections is formed by injection molding a resin in the cavity of an injection molding apparatus. Mounted in the injection molding apparatus are tools for forming the optical surfaces on the mold sections. Whereas the mold sections are typically used only once for casting a lens, the injection molding tools are used to make hundreds of molds. 
     Several known cast molding methods have the potential to mold a finished contact lens, for example, U.S. Pat. No. 5,271,875 (Appleton et al.). Since these methods avoid time-consuming and labor-intensive operations such as lathing, the methods have been found to offer the potential to reduce production time and cost for the manufacture of spherical contact lenses. 
     However, various problems have been encountered in employing cast molding technology for manufacturing other types of contact lenses, especially contact lenses that have at least one rotationally asymmetric surface. As one example, toric contact lenses (i.e., contact lenses having a toric optical zone that are used to correct refractive abnormalities of the eye associated pith astigmatism) have at least one surface that is not rotationally symmetric. The problems encountered may be due to several factors. First, the toric optical zone is not spherical. Second, toric contact lenses include some type of ballast (such as prism ballast or slab-off zones) to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism; in order to provide such ballast, the edge thickness of the lens is not uniform about the entire circumference of the lens. As another example of such lenses, many mulitfocal designs are not rotationally symmetric. 
     Applicant found that, in forming contact lens molds for molding lenses having a rotationally asymmetric lens surface, problems were encountered in consistently obtaining contact lens molds having the same geometries. Such inconsistencies in the contact lens mold geometries translated to inconsistencies in cast molding contact lenses in the molds. The present invention solves this problem. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved method for injection molding a contact lens mold having a rotationally asymmetric molding surface. The method comprises: providing a first molding tool including a convex molding surface, and a second molding tool including a concave molding surface, wherein one of said convex or concave molding surfaces has an optical quality finish and is rotationally asymmetric, said one molding surface for forming an optical surface on the contact lens mold, and the other of said convex or concave molding surface is rotationally asymmetric; positioning the molding tools in opposed relationship to form a space therebetween, such that the respective molding surfaces are spaced substantially uniformly across their surfaces; and injecting a plastic resin into a space formed between the molding surfaces. 
     The method is especially useful for injection molding contact lens molds having a mold cavity defining surface for forming a toric contact lens surface molded thereagainst, and more particularly, for contact lens molds having a mold cavity defining surface shaped to provide ballast to a contact lens surface molded thereagainst. 
     The invention further includes an assembly for carrying out the method, and contact lens molds formed by the method. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional view of a representative toric contact lens. 
     FIG. 2 is a schematic exploded view of a representative mold assembly. 
     FIG. 3 is a schematic cross-sectional view of the mold assembly of FIG. 2 assembled for cast molding a contact lens. 
     FIG. 4 is a schematic cross-sectional view of tooling for injection molding an anterior mold section of the assembly shown in FIGS.  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 schematically illustrates a representative contact lens having a rotationally asymmetric surface. For this described preferred embodiment, contact lens  1  is a toric contact lens, although the invention is applicable to other contact lenses having at least one rotationally asymmetric surface. As used herein, the term “rotationally asymmetric surface” denotes a surface that is not a second-order surface of revolution, such as a torus section. 
     Central zone  11  of posterior surface  3  is toric, i.e., this zone has a toric surface that provides the desired cylindrical correction. Posterior surface  3  may optionally include at least one peripheral curve  12  surrounding the central toric zone  11 . For the described embodiment, central zone  21  of anterior surface  4  is spherical, and the spherical curve is matched with central zone  11  to provide the desired spherical correction to the lens. Anterior surface  4  includes at least one peripheral curve  22  surrounding central zone  21 . Lens  1  is provided with ballast so that the lens maintains a desired rotational orientation on the eye. For the described embodiment, schematically shown in FIG. 1, peripheral section  24  has a different thickness than an opposed peripheral section  25  of the lens periphery due primarily to the ballast in surface  4 ; thus, anterior surface  4  is not rotationally symmetric. Other ballast types for inhibiting rotation of the contact lens on the eye are known in the art, and the invention is applicable for such other ballast types that require a rotationally asymmetric surface. It is further noted that for toric lens designs, the centerpoint of central zone  21  is not necessarily aligned with the center of lens  1 , thereby further contributing to surface  4  not being rotationally symmetric. 
     A representative mold assembly  25  for the method of this invention is shown in FIGS. 2 and 3. The mold assembly includes posterior mold  30  having a posterior mold cavity defining surface  31  (which forms the posterior surface of the molded lens), and anterior mold  40  having an anterior mold cavity defining surface  41  (which forms the anterior surface of the molded lens). Each of the mold sections is injection molded from a plastic resin, such as polypropylene or polystyrene, in an injection molding apparatus, as described in more detail below. When the mold sections are assembled, a mold cavity  32  is formed between the two defining surfaces that corresponds to the desired shape of the contact lens molded therein. Accordingly, for the described embodiment, posterior mold cavity defining surface  31  has a toric central zone (for forming the toric posterior surface of the toric contact lens) having a cylindrical axis, and anterior mold cavity defining surface  41  has a configuration that will provide ballast to a lens molded in mold cavity  32 . Of course, surfaces  31 ,  41  may also include curves for forming desired peripheral curves on the lens, and the central zones of surfaces  31 ,  41  may be designed to provide a desired spherical correction to the molded toric lens. 
     As mentioned above, the posterior and anterior mold sections are injection molded from a plastic resin in an injection molding apparatus. FIG. 4 illustrates schematically an injection mold arrangement for the injection molding of anterior mold section  40 . As seen in the Figures, anterior mold section  40  includes surface  42  opposed to anterior mold cavity defining surface  41 , surfaces  41  and  42  defining segment  43  therebetween of mold section  40 . Tools  51 , 52  are mounted in the injection molding apparatus. Tool  51  has an optical quality finish on its molding surface  53  since tool  51  is used to form mold anterior cavity defining surface  41 . (As used herein, the term “optical quality finish” denotes a molding surface that is sufficiently smooth for ultimately forming the optical surface of a contact lens, e.g., the produced contact lens is suitable for placement in the eye without the need to machine or polish the formed lens surface.) Tool  52 , used to form opposed surface  42 , does not need to have an optical quality finish on its molding surface  54  since opposed surface  42  of contact lens mold  40  does not contact the polymerizable lens mixture in casting contact lenses, i.e., opposed surface  42  does not form part of mold cavity  32 . 
     According to conventional methods of injection molding such a contact lens mold, the shape of opposed surface  42  was not considered particularly critical. Therefore, tool molding surface  54  would generally have a shape that was easy to machine in order to avoid unnecessary labor and expense in forming the molding surface on tool  52 , i.e., this tool molding surface would be formed of rotationally symmetric curves especially spherical curves. 
     However, applicant found that, in forming contact lens molds for molding lenses having a rotationally asymmetric lens surface, a problem of inconsistent molding of contact lens molds was encountered. More specifically, it was discovered that when surface  53  of tool  52  was made with a rotationally symmetric molding surface as in conventional methods, it was difficult to obtain contact lens molds having consistent geometries, which translated to inconsistencies in the casting of lenses in the contact lens molds. It is believed that there was still sufficient mismatch between the shapes of surfaces  41  and  42 , especially in the region of the molding surfaces that provide ballast, that uneven resin flow occurred in injection molding the contact lens mold, thus causing the inconsistency in the injection molding process. 
     The present invention solved this problem by providing molding tool  52  with a molding surface  54  that is rotationally asymmetric, such that when the two molding tools  51 , 52  are positioned in opposed relationship, molding surfaces  53 , 54  are spaced substantially uniformly across their surfaces. Preferably, tools  51  and  52  are locked into these positions with respect to one another. It is believed that this uniformity in the space formed between the molding surfaces  53 , 54  results in more uniform flow of resin during injection molding, and thereby provides more consistency in the injection molding of the contact lens mold sections. Preferably, surface  54  has curves approximating each of the curves on surface  53 . Molding surfaces  53 ,  54  should be shaped so that the thickness of section  43  varies no more than 0.2 mm, more preferably no more than 0.15 mm, and especially no more than 0.1 mm, across its profile. 
     (It is noted that due to the scale of FIG. 4, the various curves of surfaces  41  and  42  are not visibly illustrated; similarly, the various curves of surface  31  is not illustrated in FIG.  3 . However, for the described embodiment, it is evident that surface  31  would be shaped to provide contact lens surface  21 , and surfaces  41 , 42  of the tools would be shaped accordingly. As discussed above, for the described embodiment, lens  1  does not include a uniform peripheral thickness due primarily to inclusion of a ballast.) 
     Tools  51 , 52  are typically made from brass, stainless steel or nickel or some combination thereof, and the desired molding surface is formed on the tools according to generally methods known in the art, such as lathe cutting. Alternately, if the tool surface has a shape that is difficult to lathe cut, other methods are generally available in the art, such as electrodischarge machining. After forming the desired surface, surface  53  of tool  51  is polished to achieve precision surface quality so that no surface imperfections are transferred to the mold section being injection molded therefrom. Surface  54  of tool  52  does not require such degree of polishing, since it is not used to form an optical surface, and therefore, the molding surface  54  of tool  52  does not need to correspond exactly to surface  53 . As shown schematically in FIG. 4, the end of tool  52  opposite surface  54  is designed to mount the tool in insert  55 , surrounded by ejector sleeve  56 , and tool  51  is surrounded by sleeve  57 . This assembly is mounted in blocks  58 , 59 , with a gate  60  provided for introducing resin As would be apparent to one skilled in the art, the exact design or configuration to accommodate the molding tools will depend on the injection molding apparatus. 
     Although certain preferred embodiments have been described, it is understood that the invention is not limited thereto and modifications and variations would be evident to a person of ordinary skill in the art. As one example, the invention is applicable to toric contact lenses having other ballast means than that illustrated for the described embodiment, and for other types of contact lenses having at least one rotationally asymmetric surface. As another example, the invention is not limited to injection molding of anterior mold sections, but is also applicable to injection molding of posterior mold sections that have a rotationally asymmetric mold cavity defining surface. As yet another example, the invention is applicable to contact lens mold types other than those illustrated in FIGS. 2 to  4 , and the various injection molding set-ups therefor.

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