GB2216065A

GB2216065A - Contact lens production

Info

Publication number: GB2216065A
Application number: GB8904140A
Authority: GB
Inventors: Geoffrey Harrison Galley
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-02-24
Filing date: 1989-02-23
Publication date: 1989-10-04
Also published as: GB8904140D0

Abstract

A two-part plastics material disposable mould for casting a contact lens preform from curable material comprises male 20 and female 22 mould members which seat together to define a mould cavity having an annular portion 31 which extends radially outwardly of the notional position of the circumferential edge of the eventual contact lens to permit the mould members to move relative to one another during curing of the curable material introduced into the mould cavity so as to accommodate shrinkage of the curing material without substantially affecting the optical quality of the moulded optical surfaces. A method of making a contact lens comprises utilizing such a mould to form a cured cast preform and then machining the preform to provide the desired circumferential edge of the lens. <IMAGE>

Description

CONTACT LENS PRODUCTION This invention relates to the manufacture of contact lenses, and in particular to disposable plastics material moulds for casting contact lens preforms from curable material and to methods of making contact lenses utilizing such moulds.

Contact lenses are traditionally manufactured by several processes including lathing, spin casting and cast moulding. Each of the above methods possesses advantages in terms of the cost of production of lenses or the variety of lens designs and materials which may be produced. Cast moulding offers significant advantages in respect of low cost of capital plant and low unit cost while being utilizable for a wide range of polymeric curable materials.

US patent No. 4208364 Shepherd and US patent No.

4209289 Newcomb, assigned to American Optical Corporation, describe currently utilized methods of cast moulding of contact lenses. In practice it has been found that these methods suffer relatively low production yields due to defects in or originating from the edge of the cast lens and, in the case of US patent No. 4209289 from numerous instances of poor optical quality.

US patent No. 4208364 Shepherd teaches the casting of a lens between two disposable plastics material mould members one of which is provided with a flexible deformable lip which facilitates relative movement of the mould members in order to compensate for the shrinkage of the curable material which occurs during its polymerisation.

That movement maintains contact between optical surfaces of the mould members and the curing material thus ensuring good optical quality in the eventual lens. However, the deformable lip against which the circumferential edge of the lens is formed, being of a disadvantageous cross-sectional form and very small dimensions, is difficult to create with the required high degree of precision on the mould members, which are themselves generally formed by an injection moulding process. The resulting lens edges frequently exhibit imperfections which may lead to the lens being judged unacceptable for use.

In addition, such imperfections often form sites for initiation of cracks which may propagate into the lens providing further cause for rejection of the lens on inspection.

Yet another limitation of the Shepherd method resides in its requirement for the presence of curable monomer on both sides of the deformable lip immediately prior to polymerisation. Exothermic heat produced during the polymerisation process limits the rate of heat loss from the lens edge as compared with the rate of heat loss from the remainder of the lens surfaces. This in turn can lead to differential hydration of the edges of hydrophilic lenses relative to the body of the lens which results in distortion or fluting of the lens edge during hydration. A further limitation of the Shepherd method results from variable deformation of the described flexible lip which in turn results in variation in the centre thickness of the cast lens.

As a result of the above described deficiencies, the production yield of lenses manufactured by the Shepherd method, being the number of acceptable lenses produced from a given number of cast mouldings, is found in practice to be generally of the order of 50% or less.

US patent No. 4209289 does not provide a means for the mould members to move towards each other during polymerisation other than by virtue of deformation of the surfaces of the mould members and hence is found in practice to result in frequent incidents of poor optical quality. The method further suffers from imperfection in the lens edge which is formed against the junction line between the two mould members. Misalignment and separation of mould members during polymerisation results in witness lines and flash rings at the lens edge which in turn result in defects similar to those described in respect of the Shepherd method.

It is an object of the present invention to provide a contact lens casting mould and a method of making contact lenses utilizing such moulds wherein the above described edge related problems are in effect transferred to a position distant from the edge of the lens and by so doing to permit the ultimately produced lens edge to be relatively free from the above described defects.

This in turn can permit the achievement of higher yields while maintaining consistently high optical quality in the eventual lens.

According to the present invention there is provided a plastics material mould for casting a contact lens preform from curable material, said mould comprising: a male mould member and a female mould member adapted to seat together to define a mould cavity and each presenting a curved surface for moulding a respective desired optical surface of a contact lens; wherein: said mould cavity defined by said seated mould members has an annular portion which extends radially outwardly of the notional position of the circumferential edge of the eventual contact lens to permit the mould members to move relative to one another during curing of said curable material introduced into the mould cavity to accommodate shrinkage of said curing material without substantially affecting the optical quality of said moulded optical surfaces.

In another aspect the invention provides a method of making a contact lens comprising utilizing a mould as set forth in the immediately preceding paragraph to form a cured cast preform, and then machining the preform to provide the desired circumferential edge of the lens.

Figure 1 is a cross-section through a prior art contact lens mould and Figure 1A is an enlarged fragmentary view thereof; Figure 2 is a cross-section through another prior art contact lens mould and Figure 2A is an enlarged fragmentary view thereof; Figure 3 is a cross-section through a first embodiment of a mould according to the invention; Figure 4 is a cross-section through a lens preform produced by the mould of Figure 3; Figures 5 and 6 are cross-sectional views through a second embodiment of a mould according to the invention showing two steps in the production ' of a contact lens; Figure 7 is a cross-sectional view showing an alternative step in the production of a lens from a cast lens preform; Figures 8, 9 and 10 are partial cross-sectional views through third, fourth and fifth embodiments of a mould according to the invention; and Figure 11 is a cross-sectional view showing a step in the production of a lens from a lens preform cast in the mould of the third embodiment.

Figure 1 is a cross-section through a contact lens mould as illustrated in the above-mentioned prior art US patent No. 4208364 Shepherd, and Figure 1A is an enlarged fragmentary view thereof. The mould comprises a male mould member 10 and a female mould member 11, the male member incorporating an integrally formed deformable circumferential lip 12 against which the circumferential edge of a lens 13 is to be formed. Further details can be found in the Shepherd patent document and the practical disadvantages of that mould have been described above.

Figure 2 is a cross-section through a contact lens mould as illustrated in the above-mentioned prior art US patent No. 4209289. The mould comprises a male mould member 15 and a female mould member 16, the mould members seating together at a circumferential junction line 17 against which the circumferential edge of a lens 18 is to be formed. Further details can be found in US patent document No. 4209289 and the practical disadvantages of that mould have been described above.

Figure 3 is a cross-section through a two-part plastics material mould according to a first embodiment of the invention. The mould comprises a male mould member 20 having a convex curved surface 21 for moulding the concave optical surface of a contact lens, and a female mould member 22 having a concave curved surface 23 for moulding the convex optical surface of the lens. The mould members are advantageously produced by injection moulding of a thermo-moulding polymer such as polypropylene. The method of production is not, however, limited to this means and other methods and/or materials may be utilized to produce the mould members if so desired.

The male and female mould members 20,22 each include respective flat annular portions 24,25 which permit the mould members to seat together to define a mould cavity during the lens preform production process. In addition the female member 22 is shaped to provide a cavity or reservoir 26 in which excess monomeric material may collect during the assembly of the two mould members as later described. The reservoir may alternatively be incorporated in the male mould member.

The female mould member 22 is further provided with three or more spacing lugs 27. The spacing lugs may alternatively be incorporated in the male mould member.

The female mould member may advantageously be provided with three or more circumferentially spaced protrusions 28.

In a method of making a contact lens utilizing a mould according to the invention, a measured quantity of monomer mixture is introduced into the female mould member 22 and the male mould member 20 is applied to the female mould member by means of a jig or automated machine provided for this purpose and familiar to those skilled in the contact lens production art. The approaching male member encloses a volume of monomer between the curved optical surfaces of the mould members and excess material is ejected from the space between the optical surfaces for collection in the reservoir 26. The excursion of the male mould member towards the female member is terminated by the contact between the respective flat annular portions 23,25 of the male and female mould members and by contact between the spacing lugs 27 and the facing surface of the male mould member.On reaching its terminal position in relation to the female mould member the male mould member is retained in the terminal position by the action of the protrusions 28 provided on the female mould member in the manner shown in Figure 3.

After assembly of the male and female mould members in the manner described, the monomeric material contained between the mould members is cured, suitably by conventional means including for example heat polymerisation or ultraviolet polymerisation which may be used in conjunction with an ultra-violet transparent mould material The present method may be used in the described manner to produce a lens preform from any curable material which can be polymerised by known methods of polymerisation while contained within the mould members. Such polymeric materials may or may not be of a cross-linked variety and they may be of a hydrophilic or non-hydrophilic nature.

The above described casting process is used in accordance with the present invention to produce a shaped object as shown in Figure 4 and which is referred to herein as a lens preform. The preform consists of a central portion 32 which is in the form of a contact lens surrounded by an outer annular portion 33.

Referring again to Figure 3 it will be seen that the mould cavity defined by the seated mould members has a central portion 30, for casting the eventual lens, and an annular portion 31 which extends radially outwardly of the notional position 35 of the circumferential edge of the eventual contact lens. This structure permits the mould members 20,22 to move relative to one another during curing of the curable material in the mould subsequent to filling and assembly of the mould members to accommodate shrinkage of the curing material without substantially affecting the optical quality of the moulded optical surfaces.

A spherical surface, in particular the curved optical surfaces 21,23 is less readily deformed than a surface in which angles are present between adjacent areas of the surface. It will be seen from Figure 3 that the angle which is present between the flat surfaces of the annular portion 31 of the mould cavity (and thus of the lens preform) and the central curved portion 30 of the mould cavity (and thus of the lens preform) provides for a circumferential hinging effect which facilitates the small amount of movement necessary to compensate for shrinkage of the lens polymer during curing.

After removal of the cured preform from the mould formed by the male and female members 20,22, the annular portion 33 may be utilized to accurately position the preform in a special purpose lathing machine, which may be constructed by those skilled in the contact lens production art, on which the annular portion 33 is separated from the central portion 32 of the preform by a machining process in which a diamond cutting tool may advantageously be used. The circumferential edge of the eventual lens is thus produced by a machining process and does not result directly from the moulding process. The machined lens edge may, if desired, be polished on a conventional lens edge polishing machine or by other means, but the relatively small surface area of the machined portion of the lens edge will in general render polishing of the lens edge unnecessary.

The annular portion 33 provides protection for the periphery of the central portion 32 of the preform at which location the lens edge will ultimately be formed by the machining process. Thus damage to the edge of the lens cannot occur during de-moulding of the lens or subsequent handling prior to the point at which the eventual edge is produced by the machining process.

The edge may be of any desirable cross-sectional form which may be conveniently produced utilizing the special purpose lathing machine. The preform may be loaded onto the lathing machine by hand or the lathing machine may advantageously be fitted with an automated feeding system which automatically loads and unloads the lens preforms on the lathing machine. The annular portion 33 which is removed from each lens preform during the edge machining process may be adantageously ejected from the lathing machine by mechanical means or by other means such as an air jet. The lathing machine may advantageously be fitted with a swarf extraction system so as to remove swarf and fragments produced during the edge machining process.

It is recognized that the edge is produced by a secondary operation, adding marginally to the cost of producing the eventual lens, but the resulting improvement in yield will more than offset the marginal cost increase by substantially increasing the number of acceptable lenses produced.

After removal of the annular portion 33 from the.

lens preform the resulting lens may be processed in a conventional manner to achieve an appropriately verified and packaged lens.

In other embodiments the lens preform may be loaded onto the special purpose lathing machine while still contained between the male and female mould members 20,22.

In this case features of the male and/or female mould member are used to position the preform on the special purpose lathing machine. The annular portion 33 may now be removed together with portions of the female and/or male mould members by the machining process. This is illustrated in Figures 5 to 7 which will now be described.

Figure 5 is cross-sectional view through a second embodiment of a mould. Constructional features similar to the mould of Figure 3 are identified by the same reference numerals. The mould cavity, and thus the lens preform moulded therein, is shaped to provide first and second concentric sections 40,41. The first, inner section 40 extends at a small angle outwardly from the cylindrical, and the second, outer section 41 extends in a substantially radial plane. The sections 40,41 are thus at an angle to one another to create a circumferential hinge zone 42 for the same reason as explained in detail in relation to Figures 3 and 4. In addition, the male mould member 20 is provided with a ring portion 43 of reduced mould wall thickness adjacent the hinge zone 42 to facilitate the pivotal movement to accommodate shrinkage during curing.

After curing the lens preform, still contained between the mould members 20,22 is loaded into a lathing machine (not shown) having a mandrel portion shown schematically at 44 which accurately locates the mould/preform assembly by seating against an upstanding inclined peripheral surface 45 of the female mould member, as shown in Figure 5. A cutting tool, shown schematically at 46, is then advanced towards the . female mould member and cuts successively through the female mould member, the preform and then partially into the male mould member 20. The cutting position of the tool 46 when about to enter the male member 20 is shown in chain lines 47 in Figure 5.

Referring now to Figure 6 the tool 46 has been retracted after completion of its cutting operation. The severed central portion of the female mould member 22 is removed, as shown, and the lens 32 having a machined circumferential edge 48 is then removed, leaving the male mould member 20 with a small portion machined away at 49 together with the annular portions of the lens preform and the female mould member. The edge 38 may then be polished if desired as described above.

Figure 7 illustrates schematically an alternative step for machining the lens preform produced by the method illustrated in Figure 5. Instead of machining through the mould and preform assembly as described in relation to Figure 6, the preform is first removed from the mould members and is then loaded onto the lathing machine.

The annular portion 33 of the preform is gripped by peripheral gripper means shown schematically at 50 and the circumferential edge 51 of the lens is machined by a cutting tool advancing through the gripped preform on a line shown at 52.

Figures 8, 9 and 10 are partial cross-sectional views through third, fourth and fifth embodiments of the mould. Constructional features similar to the moulds of Figures 3 and 5 are again identified by the same reference numerals. In these embodiments at least one of the disposable mould members 20,22 is provided with an annular projection along the edge of the central portion of the mould cavity. In Figure 8 the female mould member 22 has a projection 60, in Figure 9 the male mould member has a projection 61, and in Figure 10 the mould members each have opposing projections 62,63.

The annular projections 60 to 63 are each substantially rigid, i.e. effectively non-deformable, and each provide a moulding surface 64 which partially defines the circumferential edge of the eventual contact lens. The annular projections are not intended to engage with the surface of the opposing mould member so as to form a complete edge and thus they are not required to deform during movement of the mould members towards each other during polymerisation.This movement is accommodated by the mould cavity, and hence the lens preform, having an annular portion 33 extending radially outwardly of the notional position of the circumferential edge of the eventual contact lens to permit the mould members to move relative to one another during curing of the curable -material introduced into the mould cavity to accommodate shrinkage of the curing material as described in detail in relation to Figure 3. The outer periphery of the mould members is not shown in Figures 8 to 10 but may be similar to the outer portions as shown in Figures 3 and 5.

Since the projection or projections 60 to 63 in these embodiments are not required to be deformable in order to permit said movement of the mould members during curing, they may be of such cross-sectional form as will facilitate accurate moulding by the injection moulding process which is used to produce the mould members, and moreover the dimensions of the projections 60 to 63 may be such as to enhance their accurate moulding.

As can be seen from Figures 8, 9 and 10, the projections 60 to 63 produce an annular groove in one or both surfaces of the lens preform, the cross-sectional form of which corresponds with the cross-sectional form of the projection forming such groove. The preform material remaining between the projection and the surface of the opposing mould member or, in the case of projections on both surfaces, the preform material remaining between the projections themselves, constitutes a flash ring of a thickness which is a fraction of the thickness of the edge of the lens so that the flash ring may be readily sheared from the edge of the lens by removal of the prefor#m from the mould member using a small force as may be applied with a suction holder or by means of an appropriate flexing of the mould member.

The remnants of the flash ring which remain attached to the edge of the lens at which may be observed on microscopic examination of the edge may be removed by a conventional polishing process or a machining process or by an appropriate burnishing process.

Where the projection 60 is located on the female mould member 22 then the flash ring will occur between the projection and the surface of the male mould member, i.e. at the posterior surface of the lens. When the projection 61 is located on the male mould member 20, then the flash ring will occur at the anterior surface of the lens. When projections 62,63 are located in aligned positions on both male and female mould members 20,22 then the flash ring will occur between the two projections which may be advantageously positioned so as to locate the flash ring at about the middle of the lens edge.

The formation of the edge of the lens in the above described manner removes the tendency for the edge to fracture or chip as described in relation to the prior art Shepherd patent, while at the same time providing a desirable cross-sectional edge shape which needs only minimal polishing in order to remove the remnants of the flash ring.

In another embodiment, illustrated schematically in Figure 11, a device is provided for removal of the flash ring by a machining process utilizing the groove 70 produced in the anterior surface of the lens, by projection 60, as a guide for the positioning of a machining tool 71 which may advantageously be a diamond tool.

The device is provided with a rotating spindle 72, a means (not shown) for loading the mould assembly onto the spindle, which may advantageously be an automated loading system, and a sensing finger 73 which, after the removal of a portion of the female mould member 22 covering the anterior surface of the preform, travels across the preform surface from the outer edge towards the centre until it engages in the groove 70 or detects the position of the groove 70 in the preform surface.

The machining tool 71 may be attached directly to the sensing finger or alternatively the motion of the tool may be linked to the sensing finger by electronic, electromechanical or other appropriate means 74 such that the tool 71 will engage with the lens edge at the precise point of contact of the flash ring so as to remove the flash ring while providing a continuous surface on the circumferential edge of the lens.

In a further embodiment (not shown) the relevant mould member may be provided with two concentric annular projections so as to provide two concentric grooves in the preform surface. In this case the sensing finger may be used to sense the position of the outer groove in order to accurately position the machining tool in the inner groove which bears a precise location relative to the outer groove.

If desired the mounting of the mould assembly on the spindle of the machining device may be reversed and the machining process may be executed from the posterior surface of the lens.

By the above means it is possible to form an edge of desirable cross-reference section which requires no further treatment after removal of the flash ring by the machining method described.

Claims

CLAIMS:

1. A plastics material mould for casting a contact lens preform from curable material, said mould comprising: a male mould member and a female mould member adapted to seat together to define a mould cavity and each presenting a curved surface for moulding a respective desired optical surface of a contact lens; characterized in that: said mould cavity defined by said seated mould members has an annular portion which extends radially outwardly of the notional position of the circumferential edge of the eventual contact lens to permit the mould members to move relative to one another during curing of said curable material introduced into the mould cavity to accommodate shrinkage of said curing material without substantially affecting the optical quality of said moulded optical surfaces.

2. A mould according to claim 1 characterized in that said mould members are adapted to create a circumferential hinge zone, in the area of said annular portion, about which one or both said mould members can move pivotally during said curing.

3. A mould according to claim 2 characterized in that said annular portion of said mould cavity extends at an angle to the edge of the central portion between the curved surfaces to create said circumferential hinge zone at said edge.

4. A mould according to claim 2 characterized in that said annular portion of said mould cavity includes first and second concentric sections at an angle to one another to create said circumferential hinge zone.

5. A mould according to claim A: characterized in that said first, inner concentric section extends at -a small angle outwardly from the cylindrical, and said second, outer concentric section extends in a substantially radial plane.

6. A mould according to any one of claims 2 to 5 characterized in that .at least one of said mould members is provided with a ring portion of reduced mould wall thickness adjacent said hinge zone to facilitate said pivotal movement.

7. A mould according to claim 1 characterized in that said annular portion of said mould cavity extends outwardly with a generally similar continuing curvature to that of the central portion between the curved surfaces.

8. A mould according to any one of claims 1 to 7 characterized in that said mould members are each provided with seating structure to adapt them to seat together to define said mould cavity, said seating structure being annular and extending radially outwardly of said annular portion of the mould cavity.

9. A mould according to any one of claims 1 to 8 characterized in that said annular portion of said mould cavity includes a reservoir for collection of excess curable material during closure of the filled mould.

10. A mould according to any one of claims 1 to 9 characterized in that at least one of said mould members is provided with an annular projection along the edge of the central portion of the mould cavity, said annular projection being substantially rigid and having a moulding surface partially defining the circumferential edge of the eventual contact lens.

11. A method of making a contact lens characterized by utilizing a mould according to any one of claims 1 to 10 to form a cured cast preform and then machining the preform to provide the desired circumferential edge of the lens.

12. A method according to claim 11 characterized in that said machining step is carried out while the preform remains within the mould, said machining comprising moving a cutting tool through one said mould member, through the preform to provide said desired edge, and then partially into the other said mould member.

13. A method according to claim 11 characterized by removing said preform from said mould, locating said preform on a cutting machine by means of the annular portion of the preform cast in said annular portion of the mould cavity, and then carrying out said machining step.

14. A method of making a contact lens characterized by utilizing a mould according to claim 10 to form a cured cast preform, removing at least the central portion of one of said mould members to expose the annular groove cast in said preform by said annular projection, and moving a cutting tool into said groove and through said preform to complete formation of the desired circumferential edge of the lens.

15. A method according to claim 14 characterized by the step of utilizing a sensing finger to sense the position of said groove, and moving said cutting tool into the groove in response to said sensing by said sensing finger.

16. A method according to claim 15 characterized by utilizing a said mould provided with inner and outer concentric said annular projections whereby the cured cast preform has inner and outer concentric said annular grooves, the sensing finger sensing the outer said groove and the cutting tool moving into the inner said groove in response to said sensing by said sensing finger.

17. A method of making a contact lens characterized by utilizing a mould according to claim 10 wherein said annular projection or projections extend substantially completely, but not wholly, across the mould cavity so that the moulding surface or surfaces thereof substantially completely, but not wholly, define the circumferential edge of the eventual contact lens, removing said preform from said mould, and finishing the circumferential edge of the lens.

18. A plastics material mould for casting a contact lens preform from curable material substantially as described herein with reference to Figures 3 to 11 of the accompanying drawings.

19. A method according to claim 11 and substantially as described herein with reference to Figures 3 to 11 of the accompanying drawings.

20. A contact lens preform when made in a mould according to any one of claims 1 to 10 and 18.

21. A contact lens when made by a method according to any one of claims 11 to 17 and 19.