GB2040213A

GB2040213A - Method and apparatus for making plastics lenses

Info

Publication number: GB2040213A
Application number: GB7943824A
Authority: GB
Original assignee: American Optical Corp
Current assignee: American Optical Corp
Priority date: 1979-01-15
Filing date: 1979-12-20
Publication date: 1980-08-28
Also published as: FR2446161B1; DE3001253A1; CA1143518A; FR2446161A1; GB2040213B

Abstract

The apparatus comprises a set 10 of disposable casting moulds for casting cured plastics lenses, which comprises a strip 12 comprising debossed mould recesses (16) having the dimensions and contours required for the desired lenses. The strip is formed of a material which maintains its dimensional stability at the curing temperature of the cast plastics material 22 used to make the lenses and has sufficient strength that the strip can serve as a workpiece holder during subsequent machining and/or polishing of the cured plastics lens. The plastics material is cast and cured in the mould recesses, the latter being covered by a separate cover sheet 14 having depressions (20) fitting snugly in recesses (16). The plastics material may be cured by immersion of the filled and covered mould recesses in a bath of water 24 maintained at curing temperature. The process may be continuous mould recesses being filled and closed successively. <IMAGE>

Description

SPECIFICATION Method and apparatus for making plastics lenses The present invention is concerned with the manufacture of plastics lenses by casting, and a set of disposable casting moulds for use in such a casting method.

Plastics ophthalmic lenses are traditionally individually cast between circumferentially gasketed mould halves, at least one of which is painstakingly ground and polished to the final shape and finish required of a surface of a lens to be cast. (See, for example, U.S. Patents 2,964,501; 3,821,333; 3,902,693 and 3,946,982).

The production of such moulded halves and the inventory and maintenance thereof are expensive and assembly with gasketing and separate filling and curing for lens casting are tedious. U.S. Patents 3,806,079; 3,871,610 and 3,871,803 exemplify prior art schemes for higher volume multiple casting. The latter, however, have similar disadvantages of high equipment costs, maintenance and expensive inventorying, and are generally unsuitable for automatic production of lenses.

We have now developed apparatus for producing cured plastics lenses which enables high quality lenses to be mass-produced with low capital cost.

According to one aspect of the invention, there is provided a set of disposable casting moulds for casting cured plastics lenses, which comprises a strip comprising a plurality of debossed mould recesses having the dimensions and contours required for the desired lenses, the strip being formed of a material which maintains its dimensional stability at the curing temperature of the cast plastics material and having sufficient mechanical strength so that the strip can serve as a workpiece holder during subsequent machining and/or polishing of the cured plastics lens.

According to another aspect of the invention, there is provided a method of producing lenses, which comprises providing a set of disposable casting moulds as just defined, and casting and curing a curable plastics material in the mould recesses thereof. The curing is preferably effected while the strip is in heat-exchange relationship with a heattransfer fluid, such as water or air, for example, at the curing temperature required.

The term "lens" as it is used herein is intended to include any one or all forms of the article whether unfinished, semitinished (one side only) or finished (both convex and concave surfaces).

The final lens surface shapes are preferably generated by machining with the lenses supported in the moulding recesses, the base of each recess being machined away prior to machining the relevant lens; the lens may then be conventionally optically polished. Material of the strip may be retained and used to avoid lens blocking in polishing orfinal surfacing operations and therefore dissolved, peeled or otherwise removed from the cured lens.

The method according to the invention is readily adaptable to continuous process technology and eliminates the need for mould inventories and affords the opportunity to machine precise lens surface curvatures thereby relieving the cast lens produced of mould surface impurities, dye streaking and/or other such blemishing.

In a preferred embodiment of the invention, the strip is provided with a cover sheet of the same material as the strip, the cover sheet being adapted to cover the mould recesses in the strip. The cover sheet preferably has plurality of depressions formed therein, these depressions being shaped and arranged such that each depression fits snugly in a corresponding mould recess in the strip.

In order that the invention may be more fully understood, preferred embodiments thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is an illustration, in perspective, of a preferred form of a two-part lens casting mould having its two parts separated for clarity of illustration; Figure 2 illustrates use of a lens casting mould according to the invention; Figures 3 and 4 are diagrammatic illustrations of lens surfacing operations which may be performed subsequent to lens casting and curing with the mould of Figures 1 and 2; and Figures 4 and 5 diagrammatically illustrate additional lens surfacing operations which may be performed upon the cast lenses.

Referring to Figure 1, lens casting mould 10 comprises a main strip 12 and a cover 14 each in the form of a debossed strip of sheet plastics or metal.

Strip 12 is provided with debossed casting recesses 16 each having a diametral size, depth and curvature of bottom 18 which corresponds to the shape and size desired of lenses to be cast therein. Depressions 20 in cover 14 are so contoured that each fits snugly into a corresponding casting recess 16 when the mould 10 is closed as illustrated in Figure 2, thus sealing out air which would normally be detrimental to the curing of casting material in recesses 16.

Mould 10 may comprise multiple parallel rows of recesses 16 and depressions 20 either in staggered or aligned juxtaposition. The single row embodiment shown in figure 1 is presented merely for ease and simplification of illustration.

Mould 10 may be of any length considered appropriate for a "continuous" lens casing operation or used in relatively short sections each having two, four, six or other numbers of casting recesses 16.

Strip 12 and cover 14may be formed, for example, of a polyamide (or nylon) or a polyester or other synthetic polymer having similar characteristics, e.g.

a commercially available synthetic polymer sold under the trade name Trogamid. Alternatively, metals such as aluminium, or alloys thereof, or other similar metallic compositions may be used.

Referring to Figure 2, casting recesses 16 are filled with a catalyzed liquid monomer material 22, e.g.

allyl diglycol carbonate which is sold under the trade name CR-39, the catalyst being, for example, a peroxide.

The cover 14 is then fitted to the strip 12 with the depressions 20 fitted into recesses 16, which forces a marginal outward flow of excess casting monomer and tight sealing of each recess 16 of strip 12. The filling and covering of recesses 16 maytake place successively along the length of strip 12 in a continuous process, or sections of strip 12 and cover 14 may be individually respectively filled and covered.

In addition to the tight sealing of recesses 16 with depressions 20 of cover 14, the cover may be cemented, crimped, heat sealed or otherwise prevented from accidental displacement.

The filled and covered mould 10 is subjected to resin curing temperatures, e.g. by immersion in a liquid heat transfer fluid such as water 24 (Figure 2).

This may be accomplished on a continuous basis or by separately immersing relatively short sections of the covered mould into the heat transfer fluid.

Various time-temperature cycles may be employed for curing (polymerising) the lens casting resin, for example, as disclosed in the abovementioned U.S. Patents (see, in particular, lines 26-35, column 3 of U.S. Patent 3,902,693).

Following the curing of the monomer material 22, the resulting lenses 22a (Figures 3-6) may be surface finished as follows: With mould 10 inverted (Figures 3 and 4), and the convex bottom 18 of a recess 16 presented to a preformed plunge cutting tool 26, or vice versa, bottom 18 is removed by tool 26. Continued cutting into lens 22a forms a surface shape corresponding to that of the tool cutting face 28. This removes surface blemishes resulting from contamination with mould material, roughness or other irregularities and further removes any less than completely polymerised lens surface material (the bulk material of the lens 22a is inevitably more completely polymerised than the surface material). Any superficial contamination of the cast lens is therefore not deleterious.

Having so machined the convex surface of lens 22a (Figure 3) the surface 30 may be optically polished in conventional fashion, e.g. with a polishing tool 32 (Figure 4).

Second surface finishing, i.e. complete surface finishing, of lenses 22a may be accomplished with plunge cutting tool 34 (Figure 5) or sweep generating tool 36 (Figure 6). The former produces spherical surface curvatures only while the latter allows the production of compound (toric) surface curvatures.

Mould 10 may, in either case, be retained as support for the lens 22a thereby avoiding the need to use lens block for second surface finishing, this being an advantageous feature of the present invention.

Various modifications of the embodiment illustrated may be employed. For example, recesses 16 of mould 10 may be compression moulded to the precise surface 18 curvatures and finish required of convex surface of lenses 22a. By such means, volume production lenses may be accomplished without the need for the first surface finishing operations shown in Figures 3 and 4.

Claims

1. A set of disposable casting moulds for casting cured plastic lenses, which comprises a strip comprising a plurality of debossed mould recesses having the dimensions and contours required for the desired lenses, the strip being formed of a material which maintains its dimensional stability at the curing temperature of the cast plastics material and having sufficient mechanical strength so that the strip can serve as a workpiece holder during subsequent machining and/or polishing of the cured plastics lens.

2. A set according to claim 1, in which said material is plastics.

3. A set according to claim 1, in which said material is metallic.

4. A set according to any of claims 1 to 3, which comprises a cover sheet of said material, said cover sheet being adapted to cover the mould recesses in the strip.

5. A set according to claim 4, in which the cover sheet has a plurality of depressions formed therein, said depressions being shaped and arranged such that each depression fits snugly in a corresponding mould recess in the strip.

6. A set of disposable casting moulds for casting cured plastics lenses, substantially as herein described with reference to Figure 1 of the accompanying drawings.

7. A method of producing lenses, which comprises providing a set of disposable casting mqulds according to any of claims 1 to 3, and casting and curing a curable plastics material in the mould recesses thereof.

8. A method according to claim 7, in which the curable plastics material is cured while the strip is in heat exchange relationship with a heat-transferfluid for a predetermined time.

9. A method according to claim 7 or 8, in which the open ends of the mould recesses are covered during curing of the plastics material.

10. A method according to claim 7 or 8, in which the open ends are covered with a cover sheet as defined in claim 5, each depression in the cover sheet being fitted snugly into a corresponding mould recess in the strip.

11. A method according to any of claims 7 to 10, further comprising machining and polishing at least one surface of each resulting lens to a desired curvature and optical finish.

12. A method according to claim 11, in which the machining is effected with the lenses supporting in the moulding recesses, the base of each recess being machined away prior to machining the respective lens.

13. A method of producing lenses, substantially as herein described with reference to Figures 2 to 4 of the accompanying drawings.