GB2329356A

GB2329356A - Contact lens manufacture

Info

Publication number: GB2329356A
Application number: GB9719867A
Authority: GB
Inventors: William R Mccall
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-09-19
Filing date: 1997-09-19
Publication date: 1999-03-24
Anticipated expiration: 2017-09-19
Also published as: GB9719867D0; GB2329356B

Abstract

A mould (1) for use in the manufacture of a lens from a material which undergoes a volume change on polymerisation. The mould comprises a male member (2) which has an arcuate portion (4) having a moulding surface 7, and a female member (3) having an arcuate portion (11) having a second moulding surface 14. The arcuate portion of at least one of the members is of varying thickness to allow the mould to accommodate for the volume change. When the mould is assembled the arcuate portions co-operate to define a moulding cavity (19) including the moulding surfaces. During moulding of a lens using the mould (1) the mould may be placed in a pressured atmosphere. A method of manufacturing a lens has also been described.

Description

CONTACT LENS MANUFACTURE This invention relates to contact lens manufacture and, in particular, a method of moulding a contact lens and a mould therefor.

Traditionally contact lenses have been manufactured by lathing techniques which basically involved casting lens material into a rod and cutting this into buttons, lenses are formed by lathing optical surfaces onto the button.

These lathing processes are expensive to operate not only in the time taken to produce a lens but also in the cost of the equipment needed to operate a line.

In an attempt to alleviate the problems of cost and time of manufacture associated with fully lathing lenses from buttons techniques have been developed in which buttons are moulded which incorporate one of the optical surfaces.

The second optical surface being lathed onto the button to form the lenses as required. This basic process whilst alleviating the problems to a degree does not fully overcome the problems, and as can be readily appreciated these methods of manufacture whilst giving lenses of good optical quality are still expensive to operate and somewhat time consuming.

Consequently over recent years there has been a drive in the industry to develop and bring on line methods of manufacture that involve the moulding of finished contact lenses or lenses in which subsequent down line operations are simple.

The first really serious attempt to do this was described by T Shepherd in US 4208364. However, it was quickly discovered that the method proposed and the mould for use therewith did not fully overcome all the problems of moulding high precision products such as contact lenses. In particular problems were found with regard to shrinkage of the monomer as it is polyimerised, which shrinkage could lead to bubbles forming in the moulded product due to inflexibility in the mould making it totally unacceptable for use as a contact lens.

GB 2185933 discloses a mould for, and a method of, moulding contact lenses in which the mould comprises a male and a female member with the male member having a weakened or diaphragmed section which is able to flex. Consequently during moulding of a lens as the monomer shrinks on polymerisation the weakened or diaphragmed section of the mould flexes and so accommodates the shrinkage of the lens material so preventing the formation of bubbles in the finished lens. It should also be noted that during the initial stages of the polymerisation operation whilst the monomer is still able to flow the mould allows the excess monomer material to be drawn back into the moulding cavity.

The mould proposed in this specification has problems in use in that due to the fact that in addition to the flexing the mould also allows for excess monomer material to be drawn or sucked back into the moulding cavity whilst the monomer is still able to flow. This means that the lenses made are likely to have rims of dross material meaning the lenses formed using this type of mould need to be edge finished.

US 4640489 discloses a mould for the manufacture of contact lenses or more specifically a tray of such moulds, wherein each mould includes a male member and a female member. The female member has a well defined lip which acts against an easily deformable male member so that as the lens material shrinks during the polymerisation the male member is drawn into the female member pivoting about the lip.

As with the earlier mentioned specification the lenses made using this mould design have problems with edge definition meaning that lenses formed using these moulds generally need to be edge finished.

The present invention is concerned with providing a mould for, and a method of, manufacturing contact lenses using the mould which enables the problems mentioned above to be overcome or at least alleviated.

In accordance with the present invention a mould for use in the manufacture of lenses from a polymerisable material which undergoes a volume change on polymerisation, comprises a male member which has an arcuate portion on which a moulding surface is defined and a female member having an arcuate portion on which a second moulding surface is defined, which moulding surfaces when the mould is in use co-operate with one another to define a moulding cavity, wherein the arcuate portion of at least one of the members in the region of the moulding cavity is of varying thickness.

As a consequence of varying the thickness of the mould member or members in the region of the moulding cavity as a polymer shrinks during the polymerisation the mould is able to deform inwardly in a controlled and predictable fashion to take up the change of volume. It should be noted that the degree of deflection of the mould members can be controlled by the variation of the thickness of each mould member with the thickest sections forming the optical zones (those sections which a wearer of for example a contact lens will actually look through) of the lens (defecting the least) and the tiniest sections forming the transitions and edge (deflecting the most) The way the mould will deform can be predicted and in view of the fact the shrinkage in the material being polymerised can also be determined, it is possible to design moulds in accordance with the present invention that will deform in such a fashion so as to provide a lens of known prescription at the end of the manufacturing process.

Consequently moulds to produce lenses of known prescription, taking into account the power and toricity of the lens, can be designed.

In a preferred arrangement of the present invention the male and female members of the mould are sized so that they are an interference fit with one another. The provision of the interference fit between the two members of the mould can be made as such an interference fit that the fit between the mould members acts as a seal for the moulding cavity during the moulding operation.

One of the problems commonly encountered in the moulding of contact lenses is to do with the finishing of the lenses and in particular the edges of the lenses where due to ineffective mating of the female and male mould members a rim of dross can be present. In order to remove this and provide a lens that has an acceptable finish and will not irritate the eye the lens would undergo a finishing or edge rubbing operation. With a mould made in accordance with the present invention the interference fit and shaping of the lens mould can be used to alleviate this problem.

Consequently a mould made in accordance with the present invention is shaped so that the area around the interference fit provides a tapered edge finish to the moulded lens.

Preferably the mould itself is formed by a moulding operation and most preferably is moulded from polypropylene.

The surfaces of the mould so formed may be treated to enhance the required properties, for example, by corona discharge so as to change the hydrophobic nature of the material and enable a pattern to be printed onto the lens surface for incorporation of that pattern in the moulded lens or to ease the removal of the lens from the mould once the moulding is complete.

In one arrangement of the present invention the mould is provided with means to retain the moulded lens preferentially on one of the mould members, for this any suitable means may be employed.

A mould made in accordance with the present invention is suited for use in the manufacture of contact lenses, and in particular, soft contact lenses. However it should be noted that this is not the only use to which moulds made in accordance with the present invention can be put.

In accordance with a second aspect of the present invention there is provided a method of manufacturing a lens from a material which undergoes a volume change during polymerisation which method comprises: selecting a male mould member and a female mould member with the correct characteristics to impart the desired shape to a lens moulded therewith; charging the female mould member with a measured amount of monomer material; bringing the male member and female member into alignment with each other to define a moulding cavity therebetween; placing the mould in a curing oven and pressurising the atmosphere in the oven; and curing the monomer in the moulding cavity whilst the mould is kept in the pressurised atmosphere.

Preferably a method in accordance with the present invention also includes: recovering the mould from the curing oven; and separating the mould members to enable removal of the lens.

In the case where a soft lens is being manufactured the lens which is removed from the mould will not be a finished lens suitable for use by a patient. As will be appreciated by those in the industry soft lens are made from a general class of materials which are known as hydrogels. The lens is initially formed from a material in what is termed its xerogel state. A state in which the material is rigid and fairly brittle in nature, and in order to get to the floppy flexible state we all associate with soft lenses this xerogel material needs to be hydrated. During this process hydrogen atoms in the polymer molecule are replaced with hydroxy groups, a bulkier group leading to expansion of the lens to form what can be termed as the finished lens.

Previously it has been proposed that the lens may be hydrated in the mould in order to speed up operations as well help with the release of the lens from the mould. This type of approach can be used with a mould in accordance with the present invention or alternatively the lens can be removed from the mould prior to the hydration step.

Preferably, the method also includes: hydrating the lens to form a finished lens.

The invention will now be illustrated by way of description of an example in accordance with the present invention and as shown in the following drawings, in which: Figure 1 shows a mould in accordance with the first aspect of the present invention; and Figure 2 shows an exploded view of the mould in the region of the mating between the two mould members.

Figure 1 of the drawings shows a mould 1 made in accordance with the present invention and suitable for use in the manufacture of lenses, in particular, contact lenses.

The mould 1 includes a male member 2 and a female member 3 both of which are moulded from polypropylene.

The male member 2 has an arcuate bowl like portion 4 and a circumferentially extending skirt portion 5. The arcuate bowl like portion 4 has an upper concave surface 6 and an lower convex surface 7 with a band 8 running around the periphery of the convex surface. The skirt portion 5 has an inner surface 9 and an outer surface 10, with the inner surface 9 running around the periphery of the band 8.

The band 8 is disposed perpendicularly to the inner surface 9 in this particular example.

The female mould member 3 comprises an arcuate bowl like portion 11, a circumferential skirt 12 and feet like portions 13. The arcuate bowl like portion 11 has an inner concave surface 14 and an outer convex surface 15 and the skirt 12 has an upper surface 16 extending around the periphery of the inner concave surface 14 and a lower surface 17. There is a sharp discontinuity between the inner concave surface 14 and the upper surface 16 which in effect defines a lip 18.

The mould members 2 and 3 are sized so that the respective bowl like portions 4 and 11 when correctly orientated with respect to one another form an interference fit. In this the band 8 on the male mould member 2 engages with the lip 18 on the female mould member. Further when the mould members 2 and 3 are orientated with respect to one another the lower convex surface 7 of the male mould member 2 and the inner concave surface 14 of the female mould member define a moulding cavity 19.

It should also be noted that the mould members are shaped such that when they are in correct orientation with respect to one another the relationship between the mould members forms a tapered zone in the moulding cavity 19 which forms the edge of the contact lens.

Both the mould members 2 and 3 in the region of the arcuate bowl like portions 4 and 11 are provided with walls of varying thickness with the upper concave surface 6 and the outer convex surface 15 having discontinuous surfaces which have distinct sections of a flat aspect.

In the moulding of a contact lens using the mould 1 as described above a female mould member 3 and a male mould member 2 are selected which will impart to the finished lens the desired optical characteristics. The characteristics imparted to the lens by the male member are most commonly those characteristics which control the fit of the lens to a wearer's eye, i.e. shaping the inner curve of the lens to match that of the wearer's eyeball, whilst those characteristics imparted to the lens by the female mould member are those which adjust the power of the lens.

Once the appropriate mould members have been selected a charge of monomer can be deposited into the arcuate bowl like portion 11 of the female mould member 3 and the male member 2 is correctly orientated with respect to the female mould member 3.

Once this has been done the mould members are placed in a curing oven (not shown) and the oven is pressurized.

The mould is now heated to effect the curing of the monomer material to form the polymerised lens.

During the curing operation the mould and the monomer are heated, to the appropriate temperature. It should be remembered that the physical characteristics of the material of the mould, polypropylene, will alter with temperature and in particular as the mould is heated the suppleness and deformability of the material will increase. Once the monomer material begins to undergo polymerisation the material begins to shrink in volume and as this proceeds a negative pressure will be exerted on the mould from within the moulding cavity 19. This will cause the mould members 2,3 to deform inwardly in the region of the moulding cavity 19.

Due to the fact that the walls of the mould members 2,3 in the region of the arcuate bowl like portions 4,11 have their thicknesses varied so that the moulding cavity 19 will vary in shape in a predictable fashion. As the polymerisation proceeds the shape of the moulding cavity 19 alters to the desired shape for the lens. Once the polymerisation is complete the monomer in the moulding is solid and in a fixed shape.

The moulds are now allowed to cool, and this may be controlled where the polymer formed during the curing stage is brittle in nature so as to avoid faults forming in the lens which would render the lens unacceptable for use.

When the mould 1 has cooled sufficiently the mould is opened carefully and the lens removed. The lens may be removed directly or it may be soaked off in succeeding operations.

In the case where the lens is a soft contact lens the lens formed during the moulding operation is not the finished lens it is more of a precursor which needs to undergo subsequent treatment to turn it into a lens suitable to be worn. The precursor is formed from a hydrogel material in its xerogel state and to make it acceptable for use the lens has to be hydrated. This operation has to be carried out carefully as the lens can be easily cracked if the hydration is carried out to quickly, as during hydration the lens material generally expands.

Once the lens has been hydrated the lens is inspected and the optical characteristics checked and verified.

Assuming that the lens is of acceptable quality it is now packaged and prepared for sale.

Claims

1. A mould for use in the manufacture of lenses from a polymerisable material which undergoes a change of volume on polymerisation, which mould comprises a male member having an arcuate portion on which a moulding surface is defined, and a female member having an arcuate portion on which a second moulding surface is defined, which arcuate portions when the mould is assembled co-operate with one another to define a moulding cavity including said moulding surfaces wherein the arcuate portion of at least one of the mould members has a region of varying thickness.

2. A mould as claimed in claim 1, wherein the male and female mould members are an interference fit with one another

3. A mould as claimed in claim 2, wherein the interference fit provides a seal for the moulding cavity during moulding.

4. A mould as claimed in claims 1, 2 or 3, wherein the male and female mould members are shaped to provide a tapered edge finish to the moulded lens.

5. A mould as claimed in any one of the preceding claims, wherein the male and female mould members formed by a moulding process.

6. A mould as claimed in claim 5, wherein the male and female mould members are moulded from polypropylene.

7. A mould as claimed in any one of the preceding claims, wherein one of the mould members is provided with means to retain the moulded lens.

8. A method of manufacturing a lens from a material which undergoes a change of volume during polymerisation, which method comprises: selecting a male mould member and a female mould member with the correct characteristics to impart the desired shape to a lens moulded therewith; charging the female mould member with an amount of monomer material; bringing the male member and female member into alignment with each other to define a moulding cavity therebetween in an assembled mould; placing the mould in a curing oven; pressurising the atmosphere in the oven; and curing the monomer in the mould whilst the mould is kept in the pressurised atmosphere.

9. A method as claimed in claim 8, wherein the method also includes recovering the mould from the curing oven; and separating the mould members to enable removal of the lens.

10. A method as claimed in either claim 8 or claim 9, wherein the method also includes hydrating the lens to form a finished lens.

11. A mould substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings.

12. A method of manufacturing a lens substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings.