Apparatus and Method for Making Cast Ophthalmic Lenses

1. Field of the Invention

Ophthalmic lens casting with particular reference to method and apparatus for making casting mold pieces.

2. Description of Prior Art

Methods of producing the traditional glass casting molds used in the manufacture of ledged multifocal lenses involve, in each case, either machining the mold from one piece of glass or fusing one or more sections of glass together or onto a preformed casting surface of a major mold component. U.S. Pat. Nos. 3,109,696; 3,460,928; and 3,555,126 are exemplary. These methods have serious drawbacks, i.e. in addition to the tediousness and high cost of fabrication, ledged glass molds suffer from fracture limitations and other mechanical shortcomings not expected of plastic molds.

Plastic casting molds produced from a single master die offer the advantage of accurate configurational duplication and economical replacement as needed.

Heretofore, however, master dies required for producing plastic multifocal casting molds were costly, tedious and difficult to manufacture. The problems and limitations experienced in making glass casting molds were similarly encountered. For example, considerable difficulty was found in producing acceptable multifocal ledge cornering and finish. Moreover, an entire line of dies offering a reasonable choice of lens distance viewing and reading portion power combinations has required an excessive number of master dies, e.g. 72 master dies were needed for supplying six distance viewing surface curvatures with twelve reading portion combinations, trifocal combinations excluded. Also, traditional mold or die making practices do not permit bringing multifocal ledge height to zero at the mold center.

In view of the above, this invention has an objective of facilitating the manufacture of cast ophthalmic lenses, more particularly by improvement in master molding dies from which molded plastic multifocal lens casting pieces may be made.

Another object is to provide a novel master die construction and method of making the same wherewith the present tediousness and costliness of die manufacture and inventory are overcome.

Another object is to provide means and method for making molded multifocal casting pieces and lenses cast therefrom of designs heretofore considered impractical, if not impossible, to produce, e.g. straight-line one-piece multifocals with zero ledge thickness at center may be made according to the invention.

Other objects and advantages of the invention will become more readily apparent from the following description.

The foregoing is accomplished according to the present invention by provision of a split-insert technique for making a master die component from which molded plastic multifocal lens casting mold pieces may be conveniently, economically and accurately reproduced as needed.

The master die molding surface is made up of juxtapositioned die components each precision surfaced to a particular curvature and finish corresponding to that of a multifocal surface desired to be finally cast by plastic casting pieces molded from the die.

With proper selection, juxtapositioning and securing together of die components having preselected molding surfaces and shapes, plastic casting pieces are molded against the assembled die components. With judicious selection of die components, great numbers of multifocal surface combinations can be provided with minimal die component inventory and expenditure. Also, with the advantage of mechanical positioning of die components, multifocal lens ledge height may be readily controlled and brought to zero at ultimate lens center, if desired.

Details of the invention will become more readily apparent from the following description when taken in conjunction with the accompanying drawings.

FIG. 1 is an illustration, in cross-section, of a lens casting cell incorporating a casting mold piece formed according to principles of the present invention;

FIG. 2 is an illustration, also in cross-section, of molding apparatus incorporating master die means for producing lens casting pieces according to the invention;

FIG. 3 is a face view of the master die means taken from line 3--3 in FIG. 2 looking in the direction of the arrows;

FIG. 4 is an illustration, in cross-section, of an alternative form of master die adaptable to the molding apparatus of FIG. 2;

FIG. 5 is a face view of the die illustrated in FIG. 4;

FIG. 6 is an illustration in cross-section of still another form of master die; and

FIG. 7 is a face view of the die of FIG. 6.

Illustrated in the drawings is a multifocal lens casting cell 10 (FIG. 1) which comprises the usual resilient gasket 12 supporting mold pieces 14 and 16 between which a casting precursor 18 is injected for casting and curing to a lens shape having opposite sides 20 and 22 corresponding to facing sides 24 and 26 of pieces 14 and 16 respectively.

It being understood that the cell 10 components may be variously clamped together for avoiding leakage of precursor 18 prior to and during its curing, the assembly is placed and held in a mildly heated over (e.g. from 40.degree. C. to 65.degree. C.) for the period of time needed to at least partially cure (solidify) precursor 18, e.g. 10-20 hours.

When finally or partially cured, precursor 18 as cast lens L is "picked," i.e. removed from cell 10 by separation of its parts 12, 14 and 16. If required, final curing of lens L may be accomplished independently of cell 10 in a suitably heated oven.

The lens L material may be allyl diglycol carbonate, sold under the trademark CR-39 or another of the commonly used ophthalmic lens resins, examples of which may be found in U.S. Pat. No. 3,136,000 together with examples of typical curing times and temperatures. U.S. Pat. No. 3,821,333 may also be referred to for details of a typical gasket 12 construction and procedure for casting ophthalmic lenses.

Casting piece 14 of cell 10 may be conventionally formed of glass, metal or plastic and produces a side of lens L which is eventually finished by grinding and polishing. The crux of the present invention, however, lies in the construction of casting piece 16 and others similar thereto which are molded of plastic (e.g. nylon) and are particularly designed for ledged multifocal lens surface replication.

To this end, casting piece 16 is molded (e.g. by injection procedure) against a master die 28 (FIG. 2) which is illustrated in a position of use within a split injection molding head 30. A backing insert 32 and supporting sleeves 34, 36 complete the casting cavity 38 when head 30 is closed, as illustrated.

Sections 40 and 42 of head 30 are separable along line 3--3 for removal of articles formed in cavity 38 and changing or maintenance of die 28 and insert 32.

With the exception of master die 28, molding head 30 having injection inlet 44 is of conventional design requiring no further description.

Die 28, however, is uniquely designed to provide a positively curved (convex) molding surface 46 against which a negatively curved (concave) side of a lens casting piece 16 may be molded in cavity 38 so as to, in turn, form a positively curved side 22 of a cast lens L (FIG. 1). The shape of side 22 of lens L will correspond to the shape of surface 46 of die 28.

More particularly, die 28 is designed to facilitate the casting of lens surfaces having plural viewing fields, i.e. areas of different surface curvature, with a ledged separation therebetween of clean, sharp and uninterrupted edge contour. Such surfaces are referred to in the art as multifocal surfaces and may comprise the combination of two adjoining fields of different curvature (bifocal); three adjoining fields of different curvature (trifocal) as well as various sizes, arrangements and shapes of each as will become apparent from the following detailed description of FIGS. 2-7.

In FIGS. 2 and 3, master die 28, with two die pieces 48 and 50, is used for molding a bifocal surface on side 26 of casting piece 16. The bifocal surface has a ledged dividing line produced by step 52 extending across surface 46.

Step 52 is sharply cornered by separate machining, surface finishing and final juxtapositioning of die pieces 48 and 50.

Die piece 50, having a molding surface of shorter radius of curvature than the adjoining molding surface of die piece 48, produces step 52. Step 52 may be set to desired ledge height, including zero or near zero at its midpoint, by relative sliding adjustment of die pieces 48, 50 along their line of adjoinment 54 and selective shimming against base 56 (FIG. 1).

Die pieces 48 and 50 of die 28 are bolted together and to base 56 (e.g. as illustrated in FIG. 1) with shims 58 and 60 therebetween. Shims 58 and 60 may each comprise a single piece of conventional shim stock of appropriate thickness or layered stock making up the desired thickness.

Base 56 of die 28 is secured to section 40 of molding head 30 with bolt 62 and insert 32 is secured to section 42 with bolt 64 thereby completing the injection molding system of head 30 which, when closed as shown in FIG. 2, is adapted to receive a molding precursor through inlet 44.

Once the molding of casting piece 16 is accomplished, separation of sections 40 and 42 allow piece 16 to be removed for use in casting cell 10.

Either or both of die pieces 48 and 50 may be replaced by others having differently curved molding faces for producing various combinations of ledged multifocal casting surfaces on casting pieces 16 molded thereagainst.

In all cases of combinations of die pieces 48 and 50, their molding faces and adjoining sides along line 54 are provided with a finish corresponding to or superior to that expected of ophthalmic lenses ultimately cast from casting pieces 16. In cases where it may be desired to provide cast ophthalmic lenses with ledges having a textured finish for reflection-reduction purposes, the corresponding texture may be provided upon the surface of die piece 48 which adjoins die piece 50. Also, it should be noted that the line 54 of adjoinment of die pieces 48 and 50 is set at an angle .theta. of tilt (e.g. 5.degree.) from the axis x of mold 28 to provide step 52 with a draft for facilitation of removal of a molded casting piece 16 and avoidance of edge damage to the ledge in casting piece 16. Draft angles of more or less than 5.degree., including zero draft, may be used.

The whole die 28 may also be replaced by one of different design, e.g. die 28a of FIGS. 4 and 5 having three bolted-together die pieces 66, 68 and 70 which produce a molding face 46a corresponding to that desired of the front surface of a ledged trifocal lens to be cast from a plastic casting piece molded against face 46a. Each of die pieces 66, 68 and 70 have a different molding surface curvature, e.g. surface 66 being of longest radius, surface 70 of shortest radius and surface 68 having an intermediate radius of curvature.

The bolted-together die pieces 66, 68 and 70 are further bolted to base 56a and separately shimmed with pieces 71, 72 and 74 according to desired height of steps 76 and 78. It should be understood that one or both of steps 76 and 78 may be brought to zero or near zero at their respective midpoints, if desired.

Another embodiment of master die which may replace either of the above dies 28 and 28a is illustrated in FIGS. 6 and 7. This die 28b is used to produce casting pieces 16 from which ophthalmic lenses having the configuration of molding face 46b can be produced. This form of ophthalmic lens is well known in the art and is commonly referred to as a "flat top" trifocal.

Die 28b with base 56b comprises a major die piece 80 having a bore 82 into which the assembly of juxtapositioned die pieces 84 and 86 is fitted and anchored by screws 88. Bore 82 may be produced by conventional electro-forming process, for example.

Shims 90 and 92 are used to establish desired height of steps 94 and 96 and a draft angle for steps 94 and 96 is provided.

It should be understood that a single insert may be substituted for the assembly of die pieces 84 and 86 in cases where a "flat top" bifocal configuration is preferred for die 28b or another die of similar construction.

It is preferred, but not necessary, that the above die pieces 48, 50, 66, 68, 70, 80, 84, 86 and others referred to be formed of stainless steel.

The molding of lens casting pieces 16 with dies 28, 28a, 28b or variations thereof is preferably accomplished by injection of a suitable plastic material. An appropriate commercially available material is sold under the trademark Trogamid.

From the above, it can be seen that the ledge requirements of cast multifocal lenses are met according to the invention with minimum cost and complication. Provision is made for satisfying all combinations of multifocal surface curvatures with a minimum number of master die components, mechanical relative positioning of master die components permits ready selection of ultimate multifocal ledge height and uniquely permits production of zero ledge height at lens center if desired. Furthermore, accurate, rapid and economical casting piece replication is made possible, i.e. lens production demands for new and replacement casting pieces can be satisfied without the heretofore undue delay time of individual mold piece machining and hand-working. Additionally, casting piece inventory can be greatly reduced, damaged or otherwise spent casting pieces can be economically disposed of and replaced and the heretofore costly reconditioning and/or other maintenance of casting pieces is avoided.

The terms "lens" and "lenses" used herein are intended to include all of the various forms of ophthalmic optics whether with only one or both sides finally finished and/or of final edge shape or uncut (not edged), i.e. components known in the art as "lens blanks" are included.

It should be understood that various modifications and adaptations of the precise forms of the invention here shown and described may be made without departure from the scope of the invention. The above illustrations are, accordingly, not intended to be interpreted as restrictive of the inventions beyond that necessitated by the following claims.