GB2503037A

GB2503037A - Mould and method for forming a Contact Lens

Info

Publication number: GB2503037A
Application number: GB1210695.1A
Authority: GB
Inventors: Paul David Stidworthy; James Parton; Armin Ebrahimpour
Original assignee: DEALTFORCE Ltd
Current assignee: DEALTFORCE LIMITED
Priority date: 2012-06-15
Filing date: 2012-06-15
Publication date: 2013-12-18
Also published as: WO2013186573A1; GB201210695D0

Abstract

A kit for forming a contact lens and a method of using the kit; the kit comprises a male mould portion 10 and a female mould portion 5, the male and female mould portions each having a contact surface for, in use, contacting the surface of a moulded contact lens 15, and wherein the polarity of the contact surfaces of the male and female mould portions differ. Preferably, the polarity of the male mould portion is greater than the female mould portion which may be substantially non-polar. The contact surfaces of the mould portions may comprise polypropylene, the contact surface of the male portion may comprise a maleic anhydride graft copolymer. The method comprises forming a contact lens precursor in the mould cavity defined between the mould portions and separating the male and female portions whereby the differing polarities cause the lens precursor to be preferentially retained on either the male or female mould portion. The exposed surface of the contact lens precursor may then be lathed before being released from the remaining mould portion. The use of a polar polymer to retain a contact lens precursor on a surface of a support during a lens finishing and shaping process is further claimed.

Description

Mould and method for forming a Contact Lens The present invention relates to a kit for forming a contact lens and to a method of using the kit. In particular, the invention relates to the use of dissimilar mould surfaces to avoid a number of problems associated with the prior art.

In general, soft contact lenses are manufactured by one of two main processes.

According to the first conventional method, lenses are made by a cast moulding process where the contact lens material is pressed between two thermoplastic moulds which are formed by an injection moulding process. This is normally a high volume, highly automated, high throughput process that requires a large amount of investment. Lenses manufactured by this process can be made very economically and are manufactured for a few pence. However, the economics of the process require large volumes of the same prescription lenses to be produced.

According to the second conventional method, a contact lens is lathe-cut from a cylindrical disc of the contact lens forming material. This is a much slower and more expensive manufacturing process for producing lenses and is used normally for making bespoke speciality contact lenses. Lenses manufactured by this route can cost several pounds to manufacture but the process provides the only economically feasible route to preparing low volume prescriptions.

Accordingly, it is desirable to provide an improved method for forming a lens and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.

In a first aspect, the present invention provides a kit for moulding a contact lens, the kit comprising a male mould portion and a female mould portion, the male and female mould portions each having a contact surface for, in use, contacting the surface of a moulded contact lens, and wherein the polarity of the contact surfaces of the male and female mould portions differ.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to an intermediate or hybrid' manufacturing process that combines aspects of the two conventional approaches. In this manufacturing process a generic "blank" lens, or lens precursor, is manufactured by cast moulding.

In particular, the process involves moulding the finished back surface that goes on the eye. This means that only of a relatively small number of stock keeping units are required to be held to account for the eye shape and dimensions generally required.

It is also possible, although not essential, to mould the finished back surface to provide correction for astigmatism. This would require a larger number of stock keeping units, but this would still be relatively small in comparison with conventional methods. In a second step, the precursor lens is then machined or "lathe-cut" to achieve the desired refractive correction on the front surface of the lens. The lathe-cutting allows for millions of permutations in the final lens prescription.

This hybrid process reduces the manufacturing cost over the fully lathed contact lens by approximately 50%. While the process does not offer the total flexibility of back surfaces of the fully lathed product, it allows for a flexible process for intermediate volumes of lenses. Indeed, by offering all 3 manufacturing processes, the correct procedure can be selected depending upon the volumes required. In all 3 of these processes a dry lens is produced which is then hydrated in a subsequent process to produce the final soft lens. Water content of the final lens can vary typically from 20% to 80% depending upon the material.

The present inventors have discovered that by having mould contact surfaces with different polarities it is possible to selectively retain a conventional contact lens on one surface of the mould when a mould is opened after moulding. Moreover, it is then possible to retain the lens on the mould portion during lathe treatment without requiring additional retention means.

As defined herein, a male mould portion is the portion of a mould cavity that has a convex surface upon which the contact lens is formed. As will be appreciated, contact lenses are generally shaped to form a portion of the surface of a sphere. As defined herein, a female mould portion is the complementary portion of a mould cavity that has a concave surface within which the contact lens is formed.

In use, the male and female mould portions are held so as to define a cavity therebetween within which a contact lens precursor may be formed, preferably by cast moulding. Cast moulding is quick and efficient.

The contact surfaces as defined herein are the surface upon or against which the contact lens blank is formed. Thus, the male and female mould portions each have a contact surface which together forms an inner surface of the mould cavity.

According to the present invention, the polarity of the contact surfaces of the male and female mould portions differ. That is, the contact surface of the male mould has a different polarity to that of the contact surface of the female mould. In one embodiment, one contact surface may be substantially non-polar, while the other surface is polar. In another embodiment, both surfaces may be polar, but the polarity of one surface may be more polar than the other.

Preferably the polarity of the contact surface of the male mould portion is greater than the polarity of the contact surface of the female mould portion. This can be used to allow the male portion of the mould, which contacts the generic eye-shaped surface on the lens precursor, to preferentially retain the contact lens when the mould is opened.

Preferably the contact surface of the female mould portion is substantially non-polar and/or the contact surface of the male mould portion is substantially polar.

Measurement of polarity is well known in the art. Polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole-dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in electronegalivily between atoms in a compound and the asymmetry of the compound's structure. For example, a molecule of water is polar because of the unequal sharing of its electrons between oxygen and hydrogen in which the former has larger electronegativity than the latter, resulting in a "bent" structure, whereas methane is considered nonpolar because the carbon shares the electrons with the hydrogen atoms almost uniformly. In general, polarity can be best determined by comparing the polarity of two materials and, indeed, orders of polarity are commonly cited.

A suitable contact surface for the male and female mould portions is polypropylene.

Preferably the contact surface of the male mould portion and/or the contact surface of the female mould portion comprises polypropylene. The use of a polypropylene contact surface allows for the ready injection moulding formation of the desired mould pieces. Moreover, the polypropylene can be supplemented by co-polymers to finely tune the desired polarity of one or both contact surfaces.

Preferably the contact surface of the male mould portion comprises a maleic anhydride graft co-polymer. It has been found that the use of maleic anhydride is particularly suitable as an additive for the formation of a polar mould surface.

Preferably the contact surface of the male mould portion comprises from 0.05 to 5wt% maleic anhydride, more preferably from 0.1 to 2wt% and most preferably from 0.5 to lwt%. The material of the contact surface will preferably be the same material as the entire mould portion for ease of manufacture. Preferably the contact surface of the female mould portion does not comprise maleic anhydride and is substantially non-polar. This permits the selective retention of a conventional contact lens material on the male portion of the mould.

The use of maleic anhydride as the active ingredient in a polypropylene mould promotes adhesion between the polypropylene surface and the dry contact lens material preform. By controlling the level of maleic anhydride the adhesion can be controlled and different levels can be selected for different contact lens materials.

According to a second aspect, the present invention provides a method for making a contact lens using the kit described herein, the method comprising: forming a contact lens precursor in a mould cavity defined between the contact surfaces of the male and female mould portions; separating the male and female mould portions, whereby the differing polarities of the contact surfaces of the male and female mould portions cause the lens precursor to be preferentially retained on either the male or the female mould portion; lathing an exposed surface of the contact lens precursor to form a contact lens and then releasing the contact lens from the remaining mould portion.

The contact lens precursor is formed by moulding, preferably cast moulding. Cast moulding techniques and the formulations suitable for forming contact lenses in this way are well known in the art. Cast moulding and the techniques disclosed herein are suitable, for example, for both hydrogels and silicone hydrogels.

The step of separating the male and female mould portions simply involves separating the two mould portions. This can be achieved manually or automatically using a machine. Due to the differing polarities of the contact surfaces of the male and female mould portions, the lens precursor is preferentially retained on either the male or the female mould portion. Preferably the lens precursor is retained on the male portion as it is easier to lathe the "male" side of the lens precursor than the "female".

The step of lathing is well known in the art. The lathe is used on the exposed surface of the contact lens precursor to form a contact lens. That is, the lathe cuts away material from the lens surface to achieve the desired prescription.

Preferably the contact lens is released from the mould portion by hydrating the contact lens. This causes the lens to swell and increases the ease of removing the lens. In other embodiments the lens can be physically removed or separated from the supporting mould by force.

According to a third aspect, the present invention provides for the use of a polar polymer to retain a contact lens precursor on a surface of a support during a lens finishing or shaping process. The standard moulding systems used in the prior art use the same materials for both surfaces of the mould with no appreciation that the use of a polar polymer can allow for the selective retention described herein.

The invention will be described in relation to the following non-limiting figures, in which: * Figure 1 a shows a closed assembly of male and female mould halves.

* Figure lb show an exploded view of a lens between male and female mould halves.

* Figure 2 shows the male mould mounted on a mandrel ready to go into the lathe.

* Figure 3 shows a cross-section of a lens preform on a male mould during lathing.

* Figure 4a shows a finished lens still mounted on the male mould following the lathing.

* Figure 4b shows a more detailed view of a male mould on a mandrel.

The method of the present invention will now be described in more detail in relation to the following non-limiting embodiment.

Figure 1 a shows a two part assembly 1 of a male mould portion 10 and a female mould portion 5. The female mould portion 5 has a substantially cylindrical side wall which, in the assembly 1, overlaps a cylindrical base of the male mould portion 10.

The female mould portion 5 has a top layer that forms the upper concave surface of a cavity (not shown) between the male mould portion 10 and the female mould portion 5. The male mould portion 10 has a top layer that forms the lower convex surface of a cavity (not shown) between the male mould portion 10 and the female mould portion 5. Handle portions formed on the male mould portion 10 and female mould portion 5 allow for the transfer of the portions and to allow the portions to be held secure relative to each other.

In use, the cavity between the male mould portion 10 and the female mould portion 5 is filled with a solution for forming a contact lens 15. The technique for forming the lens 15 is cast moulding.

Once the lens has been cast, the assembly 1 is transferred to the lathe (not shown).

The assembly is kept closed until just before lathe processing in order to avoid damage to the lens 15.

Before lathing, the lens 15 is exposed by the separation of the male mould portion 10 from the female mould portion 5 as shown in Figure 2. The contact surface of the male mould portion 10 which forms the lower convex surface of the cavity (not shown) is formed of a polar material, such as a polypropylene and maleic acid graft copolymer in a 50:50 weight ratio. The maleic graft copolymer comprises 2.5wt% maleic acid. The contact surface of the female mould portion 5 which forms the upper concave surface of the cavity (not shown) is formed of a non-polar material, such as a polypropylene. Due to the differences in the polarity, the lens is preferentially retained on the male mould portion 10. The opening of the assembly 1 exposes excess lens material 25 around the fringe of the precursor lens 15.

In some embodiments, the male mould 10 and female mould 5 may fit with sufficient tolerance to allow the edge of the precursor lens 15 to be moulded without excess peripheral material. However, this is not essential, and embodiments are considered in which excess material around the precursor lens 15 results from moulding and is subsequently removed by the lathe.

The male mould portion 10 is mounted on a mandrel 20 for supporting the portion 10 during the subsequent lathing operation. As shown in Figure 3, during lathing, the excess lens material 25 is removed and the desired prescription is obtained by lathing the surface of the lens 15 to transform it from its precursor state to a final prescription lens.

After lathing, the male mould 10 is removed from the mandrel 20 and may be reused. The lens 15 is removed from the male mould 10 by the addition of water and then mechanically lifting the lens from the mould 10.

In the hybrid process a preform is manufactured by the cast moulded procedure.

This is a semi finished dry lens that has the back surface already formed by the cast moulding process but is typically thicker than a conventional dry moulded contact lens to allow a proportion of material to be removed from the front surface by the lathing process. In this case however the preform is not removed from the male and female injection moulded shells as would be the case in the normal cast moulding process. Instead the closed shells are used to protect the preform from atmospheric effects or mechanical damage as it is transferred to the lathing process. Here the female mould is removed, leaving the preform on the male mould. This can then be presented to the lathe where the final refractive surface can be cut into the preform giving the final dry lens. This finished dry lens can then either be hydrated away from the male mould shell or mechanically removed and subsequently hydrated.

One of the key factors however is to ensure that the semi-finished preform stays on the male mould during separation of the male and female moulds but also in the lathing process when the work-piece is subjected to lateral forces to cut away material from the surface. If the lens is not well adhered it will fly off during the lathing process. This adhesion is normally achieved by the design of the preform and the male mould giving a mechanical interlock rather than a surface adhesion.

A preferred feature of the mandrel 15 is shown in Figure 4b. As can be seen in this figure, an indentation is formed in the mandrel 15 into which the handle of the male mould 10 is inserted when the male mould 10 is mounted on the mandrel 15.

Insertion of the handle into the indentation prevents relative rotation between the mandrel 15 and the male mould 10.

The inventor has discovered that it is possible to modify the moulding material of the male mould with an adhesion promoter that causes the semi-finished preform to stay on the male mould during removal of the female mould and during the lathing process. By carefully selecting the level of adhesion promoter, the finished lens can subsequently be simply removed from the male mould by mechanical means if required. The preferred adhesion promoter selected in this process is maleic anhydride.

In one embodiment the male mould currently consists of a 50% blend by weight of a standard injection moulding grade of homopolymer polypropylene, manufactured by Total grade PPH11O12 and 50% of a polypropylene modified with maleic anhydride (ExxonMobil Exxelor P01020). This contains between 0.5 and 1.0% grafted maleic anhydride. This is for silicone hydrogel contact lens material.

For traditional hydrogel contact lens materials (for instance methafilcon) the amount of Exxelor can be dramatically reduced to 10% and still see the adhesion to the male mould.

Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.

Claims

Claims: 1. A kit for moulding a contact lens, the kit comprising a male mould portion and a female mould portion, the male and female mould portions each having a contact surface for, in use, contacting the surface of a moulded contact lens, and wherein the polarity of the contact surfaces of the male and female mould portions differ.
2. A kit according to claim 1, wherein the polarity of the contact surface of the male mould portion is greater than the polarity of the contact surface of the female mould portion.
3. A kit according to claim 1 or claim 2, wherein the contact surface of the female mould portion is substantially non-polar and/or the contact surface of the male mould portion is substantially polar.
4. A kit according to any of the preceding claims, wherein the contact surface of the male mould portion and/or the contact surface of the female mould portion comprises polypropylene.
5. A kit according to any of the preceding claims, wherein the contact surface of the male mould portion comprises a maleic anhydride graft co-polymer.
6. A kit according to claim 5, wherein the contact surface of the male mould portion comprises from 0.05 to Swt% maleic anhydride.
7. A method for making a contact lens using the kit of claim 1, the method comprising: forming a contact lens precursor in a mould cavity defined between the contact surfaces of the male and female mould portions; separating the male and female mould portions, whereby the differing polarities of the contact surfaces of the male and female mould portions cause the lens precursor to be preferentially retained on either the male or the female mould portion; lathing an exposed surface of the contact lens precursor to form a contact lens and then releasing the contact lens from the remaining mould portion.
8. A method according to claim 7, wherein the contact lens precursor is formed by moulding.
9. A method according to claim 7 or claim 8, wherein releasing the contact lens from the remaining mould portion is achieved by hydrating the contact lens.
10. A method according to any of claims 7 to 9, wherein the lens precursor is retained on the male mould portion.
11. A male mould portion for use in the method according to claim 10, wherein the contact surface of the male mould portion comprises a maleic anhydride graft co-polymer.
12. The use of a polar polymer to retain a contact lens precursor on a surface of a support during a lens finishing or shaping process.