GB2191144A

GB2191144A - Casting lenses

Info

Publication number: GB2191144A
Application number: GB08712924A
Authority: GB
Inventors: Albert Henry Morland; Timothy John Warren
Original assignee: CooperVision Optics Ltd
Current assignee: CooperVision Optics Ltd
Priority date: 1985-03-19
Filing date: 1987-06-02
Publication date: 1987-12-09
Also published as: GB8712924D0; GB2191144B

Abstract

A process for casting lenses using male and female mould halves, comprises placing a polymerisable liquid composition into the female mould half, introducing a male mould into the female mould along a substantially vertical axis until the male mould touches the surface of the liquid composition and then allowing the male mould to fall under its own weight into the female mould half and curing or allowing the polymerisable composition to cure. The moulds are formed from a thermoplastic material and are maintained under a predetermined closing pressure by spot welding their flanges together.

Description

SPECIFICATION Process and apparatus for casting lenses This invention relates to the manufacture of lenses, especially contact lenses, by a casting procedure.

Although casting potentially provides one of the cheapest methods of manufacture of contact lenses, its adoption by manufacturers has been restricted because of difficulties in producing lenses of consistent quality. Typical problems are development of bubbles or voids in the cured polymer of the lenses or surface or edge distortions, all of which lead to rejection of a high proportion of the moulded lenses at the inspection stage.

T.H. Shepherd in U.K. patent No. 1,575,694 proposed the use of polypropylene moulds in which the edge ofthe cast lens is defined by an integral, flexible rim, usually on the male mould half.

According to the process described by Shepherd, lenses are cast by filling a suitable polymerisable liquid composition intothefemale mould halfand pressing the male mould halfintothefemalemould cavity until excess polymerisable liquid is displaced.

The filled mould is maintained at a controlled temperature until the composition has polymerised to a clear solid. During polymerisation, a monomer mixture will undergo shrinkage, which depends on the natureofthe monomer mixture but has been estimated to lie normally between 10 and 20volume %. Cast lenses are frequently observed to contain bubbles or voids and Shepherd attributed these to the failure to allowforthe shrinkage arising on polymerisation. His flexible rim was therefore intended to deflect inwardly during polymerisation and inthiswaytocompensateforthevolume shrinkage. However, formation of bubbles and voids has not been eliminated in the process described in the Shepherd patent, particularly in the case of lenses cast from hydroxyethyl methacrylate (HEMA).

Rawlings in U.S. Patent No. 4,469,646 attributes theformation of bubbles and voids in the operation ofthe Shepherd process to the manner in which the two mould halves are brought together. While it is certainly important to control the manner in which the mould is closed, the Rawlings procedure will not ensure that bubbles and voids are prevented and that cast lenses of consistently good quality are produced.

Nevertheless, the manner in which the mould is filled and closed is of some importance. We have found that ifthe mould halves are brought together too quickly, bubbles are invariably produced,while closure at too slow a speed can cause formation of voids. In addition, we have determined that the mould halves should come together properly aligned to the centre-line of the moulds.

According to one aspect of the present invention there is provided a process for casting lenses using male andfemale mould halves formed from a plastics material in which a polymerisable liquid composition is placed in the female mould half, said process comprises inserting the male mould into the female mould until the male mouldtouchesthe surface ofthe liquid composition, allowing the male mould half to fall under its own weight intothe female mould half and then pressing the mould halves together and maintaining the resultant closing pressure until the composition has polymerised.

The invention also includes a substantially horizontal supportforthefemale mould, a holderfor releasably holding the male mould, means for movingtheholdertowardsand into the female mould along a substantially vertical path passing through the axes of the moulds and meansfor detecting contact between the male mould and the surface of the liquid composition and causing said holderto release the male mould.

In the mould closing procedure of the present invention,the male mould continually moves towards the female mould from the moment closure is initiated. This is in contrast to the procedure described by Rawlings in which the mould closure is arrested and preferably reversed when the male mould contacts the surface of the liquid monomer.

The speed of approach prior to contacting the liquid monomer surface is immaterial. Ontheinstantof contact with the liquid surface, the male mould is released and falls into the female mould. On being wetted with the monomer, surface tension assists the drawingtogetherofthetwo mouldswhich align themselves correctly during this stage.

Contact between the male mould and the liquid can be sensed, e.g. by directing an infra-red beam along the axes of the moulds, and the male mould released at this instant.

In order to produce lenses of consistent quality, it has been found that it is also important to maintain a predetermined pressure on the moulds during the entire period from closing the mould until polymerisation ofthe monomer composition is complete. In the past this has involved careful control ofthe lens closing and polymerisation steps by maintaining thefilled moulds in a special jig which is loaded with a predetermined weight. This has been an expensive and laborious procedure.

According to another aspect of the present invention there is provided a process for casting lenses in which a polymerisable liquid monomer composition is filled into a mould comprising male and female mould halves and held in the mould until the monomer composition has polymerised to a solid condition, wherein after introduction ofthe liquid monomer composition the mould is closed under a predetermined load and the mould halves bonded together (preferably by welding) while undersaid load. This avoids the need to maintain the mould underafixed load provided byaweightor spring pressure. Instead, the welds ensure thatthe mould halves are locked together underthe correct degree of compressive load.

Conveniently, the mould halves are bonded together by welding in the region of the peripheries of the mould halves.

The mould halves are preferably formed from a thermoplastic polymer, preferably a polyolefin, such as polypropylene, which is readilywelded by contact with a heated metal tool. Itis unnecessary to weld the mould halves together by a continuous weld line.

Indeed, it is preferable to spot weld the rim portions ofthe mould halves together at spaced locations around the rims of the mould halves.

Preferablythe mould filling and closing procedure of this invention is combined with the method of bonding the moulds together under load just described.

One illustrativeform of the present invention will now be described in conjunction with the accompanying drawings in which: Figure 1 is a side elevation ofthe mould filling and closing apparatus.

Figure 2 is a partial side elevation of the mould after closure showing the step of bonding the two mould halves together by welding.

Figure 3 is a scrap view in elevation showing details ofthe welding tool and the deflection ofthe flange ofthe male (upper mould half) during welding.

Figure4is a cross-sectional view taken on the line X-Xin Figure 2.

Figures5, Sand 7are diagrammatic views ofthe mould illustrating steps in the filling and closing.

Figure 8 is an enlarged view of the mould in the Figure 8is an enlarged view of the mould in the fully closed and filled condition of Figure 7.

Referring to the drawings and particularly to Figurel,theapparatuscomprisesatablefor supporting the female mould half 2. Figure 1 shows a malemould3alreadyreceived bythefemalemould from a closure head 4 mounted above table 1.

Closure head 4 is arrangedto move along axisA-A towards and away from table 1, e.g. by means ofan hydraulic ram (not shown) and is provided with vacuum holders 5 (in fact3 holders are spaced equallyaroundaxisA-A)forholdinga male mould half. Vacuum is supplied to holders 5 along annular passage 6 between a tube 6a and a coaxial innertube 7, andthrough a port8. Afilling tube (notshown) is mounted adjacent to the closure head and arranged to enterthefemale mould half and introduce a measured amount of liquid polymerisable composition intothefemale mould using a metering pump.Asuitable pump is described in our concurrent patent application entitled "Metering Pump" No. 86 06325.Thefilling tube is then moved away and the head 4 moves down towards the female mould alongthe IineA-Acarrying the male mould held on vacuum holders 5.

Figure 5 indicates the movement of the male mould into the female while the former is supported onthevacuum holders.

It is important to detect the moment when the tip of the male touches the surface ofthe liquid monomer as illustrated in Figure 6. This is achieved by directing an infra-redbeamthroughtheinnertube7 along the IineA-A,throughthe mould halves and onto a fibre optic sensor mounted beneath the table 1 at9.Thesensorgivesasharpresponseatthe moment when the male mould touches the liquid surface and this sharp discontinuity in the response ofthe sensor is conveniently used as a signal to cause the vacuum supplied to holding tubes 5 to be shutoffandtherebycausethe male mould to fall into the female mould.It is believed thatthe reason for this sharp response is that the pool of liquid monomer in the female mould half acts as a positive power lens and focusses radiation from the infra-red source onto the sensor. When the male halftouches the surface of the liquid, this 'lens' is destroyed so that there is a sharp reduction in the infra-red radiation falling on the sensor. This movement is probably effected by a combination of gravity and surface tension and during this movement the male mould is guided by the mating surfaces on the mould halves so that it entersthefemale mould correctly aligned to the proper axis.

Thehead4followsthedownward movementof the male mould until it rests on the upperflangevia bearing pins 11. As in the case of the tube 5 there are conveniently 3 bearing pins 11 equispaced around the head 4. The weight of the head 4 provides a predetermined closing pressure applied to the mould halves thus ensuring that the cavity defined by the two mould halves is completely filled with monomer. The situation afterthe mould halves have been pressed together is illustrated in Figures 7 and 8.

As can be seen from Figures 2 and 3, the load applied to the flange 10 through the pins 11 causes deformation of portions of the flange of the male mould downwardly into contact with the corresponding portion of the flange of the female mould 2. At this point, hot wire welding probes 12 are lowered on carriage 13 to weld together the contacting edge portions of the flanges of the male and female mould halves at three or more spaced locations around the mould. The probes 12 are then retracted but the closing pressure is maintained by holding the metal block in place for a few seconds to ensure that the welds are set. Carriage 13 is then retracted and the mould is then moved to a thermostatically controlled environment (e.g. an air circulating oven or water bath) until polymerisation is complete.After the monomer has polymerised to a solid lens, the mould is opened by cutting through the spot welds, opening the mould and removing the lens.

Referring to Figure 8, this shows a mould comprising a female mould half 2 and a male mould half 3 inserted therein in the fully closed condition.

The mould halves are made from a stiff thermoplastic polymer such as polypropylene. Each mould half comprises a flange portion 24 and 25 and an integrally moulded hollow body portion 26 and 27. As indicated in the drawing the body portions 26 and 27 have a slight inward taper. In the base areas 28 and 29 of the body portions, the surfaces have a smooth surface corresponding to highly polished surfaces ofthe master metal moulds of the injection machine on which they are produced.

The curvature of surface 28 determines the base curve ofthe lens and the curvature ofthe surface 29 determines the power of the lens. The lens cavity is defined by the space bounded by the surfaces 28 and 29 and a peripheral skirt 30 which is integral with male mould half 3.

It will be appreciated that this invention is not limited to the use of electrically heated welding probes and that other methods of heating can be employed to effect the temporary welding of the two mould halves together. It is even possible to use means otherthan welding to form the bond between the two mould halves, e.g. hot melt adhesive, although spot welding has practical advantages.

It has been found that by virtue ofthe accurate alignmentofthe mould halves achieved by the process of this invention, the profile ofthe edge of the cast lenses conforms closely to the desired form.

As a consequence, lenses can be produced which require little or no edge polishing.

Claims

CLAIMS 1. Aprocessforcasting lenses in which a polymerisable liquid monomer composition is placed in a mould comprising male and female mould halves and held in the mould until the composition has polymerised to a solid condition, wherein after introduction ofthe liquid composition, the mould is closed under a predetermined load and the two mould halves bondedtogetherwhile under said load. 2. A process according to claim 1 in which the mould halves areformed from athermoplastic polymer and the mould halves are bonded together bywelding. Amendments to the claims have been filed and have the following effect: (b) Newortextually amended claims have beenfiled asfollows: CLAIMS

1. A process for casting lenses in which a polymerisable liquid monomer composition is placed in a mould comprising an individual mould half and a female mould half and held in the mould until the composition has polymerised to a solid condition,wherein after introduction ofthe liquid composition, the mould is closed undera predetermined load and the two mould halves bonded together while under said load.

2. A process according to claim 1 in which the mould halves are formed from a thermoplastic polymer and the mould halves are bonded together by welding.

3. A process according to claim 2 in which the mould halves each have a flange or rim portion and mould halves are welded together at spaced locations around the flange or rim portions.

4. A process according to claim 3 in which the flange or rim portions ofthe male mould half are deflected downwardly at said spaced locations while under said load.

5. A process according to anyone of claims 2 to 4 in which the thermoplastic polymer is a polyolefin.

6. A process according to any one of claims 2 to 5 whereinthefilled mould is placed in a thermostatically controlled environment until polymerisation is complete, the mould opened by cutting throughthewelds and the lens removed.

7. A cast lens whenever produced by the process claimed in any one of the preceding claims.