Plastic casting molds

[0002] This claims the benefit of Provisional application No. 60/151,669 filed Aug. 31, 1999.

[0003] The invention relates to a process for the preparation of mouldings, especially optical lenses and in particular contact lenses, to a corresponding device for the preparation of mouldings, and to the mouldings that have been prepared or are obtainable by the process or using the device, especially optical lenses and in particular contact lenses, in accordance with the preamble of each independent patent claim.

[0004] Contact lenses, which are to be manufactured economically in large unit numbers, are preferably manufactured by the so-called mould or full-mould process. In these processes, the lenses are manufactured into their final shape between two moulds, so that there is no need to subsequently finish the surfaces of the lenses, nor to finish the edges. Such moulds consist of a female mould half and a male mould half, the cavity being formed between the two mould halves defining the shape of the moulding. Mould processes are described for example in PCT application no. WO/87104390 or in European patent application EP-A-0 367 513.

[0005] In these known mould processes, the geometry of the contact lenses to be manufactured is defined by the mould cavity. The edge of the contact lens is likewise formed by the mould which normally consists of two mould halves. The geometry of the edge is defined by the contour of the two mould halves in the area in which they touch one another.

[0006] To prepare a contact lens, first of all a certain amount of a flowable starting material is placed in the female mould half. Afterwards, the mould is closed by placing the male mould half thereon. Normally, a surplus of starting material is used, so that, when the mould is closed, the excess amount is expelled into an overflow area outwardly adjacent to the mould cavity. The subsequent polymerisation or crosslinking of the starting material takes place by radiation with UV light, or by heat action, or by another non-thermal method. Both the starting material in the mould cavity and the excess material in the overflow area are thereby hardened. In order to obtain error-free separation of the contact lens from the excess material, a good seal or expulsion of the excess material must be achieved in the contact zone of the two mould halves. Only in this way can error-free contact lens edges be obtained.

[0007] The contact lenses produced in this manner are moulded parts having little mechanical stability and a water content of more than 60% by weight. After manufacture, the lens is inspected, then packed and subjected to heat sterilisation at 121° C. in an autoclave.

[0008] The materials used for these moulds are preferably plastics, e.g. polypropylene. The moulds are produced by injection moulding and are only used once. This is because, among other things, the moulds are partially contaminated by the surplus material, are damaged when the contact lens is separated or are irreversibly deformed in partial areas when the mould is closed. In particular, because of the quality requirements of the contact lenses edges, the moulds are only used once, since a certain amount of deformation of the moulds at the area of their edge cannot be excluded with certainty.

[0009] In U.S. Pat. No. 5,508,317, a new contact lens material is described, which represents an important improvement in the chemistry of polymerisable starting materials for the manufacture of contact lenses. The patent discloses a water-soluble composition of a prepolymer, which is filled into the mould cavity and then crosslinked photochemically. Since the prepolymer has several crosslinkable groups, the crosslinking is characterised by its high quality, so that a finished lens of optical quality can be produced within a few seconds, without the necessity for subsequent extraction or reworking steps. Owing to the improved chemistry of the starting material as illustrated in the patent, contact lenses can be produced at considerably lower cost, so that in this way it is possible to produce disposable lenses that are used only once.

[0010] EP-A-0 637 490 describes a process by means of which a further improvement may be obtained in the preparation process of contact lenses with the prepolymer described in U.S. Pat. No. 5,508,317. Here, the material is filled into a mould comprising two halves, whereby the two mould halves do not touch, but a thin circular gap is located between them. The gap is linked to the mould cavity, so that surplus lens material can flow away into the gap. Crosslinking of the prepolymer takes place by radiation especially with UV light, whereby radiation is restricted to the mould cavity by a chromium mask. In this way, only the material in the mould cavity is crosslinked, so that there is high reproducibility of the edges of the lens without closing the two mould halves.

[0011] In this process, instead of the polypropylene moulds that may be used only once, reusable quartz/glass moulds are used. Because of the water-soluble basic chemistry, after a lens has been produced, the uncrosslinked prepolymer and other residues can be removed from the moulds rapidly and effectively and dried in the air. In addition, quartz is notable for its good UV permeability and is very hard and refractory.

[0012] When using a prefunctionalised PVA (polyvinyl alcohol) polymer as lens material, moreover, important material properties of quartz casting moulds are the excellent replication of the surface geometry, as well as the transparency of the material, so that it is possible to provide visual control of the lens in the mould.

[0013] However, moulds made from quartz or glass are very expensive to produce, so that owing to the high costs, the moulds ought to have quite long service life in order to ensure that the process is economical. Therefore, for economic reasons, only a limited number of variants can be realised, for example in respect of the dioptre number.

[0014] In order to solve this problem, the intended objective is to replace at least one of the two mould halves, especially the female mould half, with a mould half consisting of plastic, and thus to employ one mould half made of plastic in combination with one mould half made of quartz or glass.

[0015] In the plastic moulds, especially polypropylene moulds, which were previously known in the prior art, there was however the problem that the lenses produced with these moulds had a slippery surface. This is because the slipperiness is caused by oxygen, which leads to an inhibition of the crosslinking reaction at the surface of the lens, this becoming apparent-macroscopically in a “slipperiness” of the lens. It is assumed that oxygen, which is already present at the surface of the casting mould or migrates to the surface of the mould during the polymerisation process, inhibits polymerisation of the lens material at the lens surface. Thus, the interface between the mould and the lens material appears to be crucial.

[0016] In order to avoid this problem, it is known from EP-A-0 687 550 that the plastic mould halves can be degassed and then transported and filled in a nitrogen atmosphere. This however requires a technical process that is very complex and in addition is very cost-intensive.

[0017] WO-A-96/11782 describes a process operating under an inert gas atmosphere and/or one using moulds in which the oxygen contained therein has been drawn off completely by means of treatment with a vacuum or with an inert gas, thus producing “non-slippery” lenses. It also describes that the oxygen in the material from which the lenses are produced is completely drawn off prior to lens manufacture. However, the cost of these additional process steps is very high and the process steps are very time-consuming.

[0018] It is the aim of the present invention to further develop a process and a device of the generic kind and to improve them in such a way that it is possible to use mould halves of plastic in conjunction with mould halves of quartz or glass, without the above-mentioned difficulties and problems arising. In particular, the plastic mould halves should be reusable, and any burr or web formation on the finished contact lenses should, be avoided, so that the rejection rate of the contact lenses is very low. In addition, optical monitoring of the lens through the casting mould should be possible.

[0019] The invention solves the problem by means of the features indicated in claim 1 and claim 11. As far as further essential developments of the process according to the invention and of the device according to the invention are concerned reference is made to the dependent claims.

[0020] Through the choice of polymers for the production of plastic mould halves, which have very low oxygen permeability, the oxygen is prevented to a great extent from reaching the cavity during polymerisation, and thus prevented from making contact with the starting material for the lenses. The idea behind this is that the oxygen which impairs polymerisation or crosslinking of the lens surface originates from the casting mould with which the lens material comes into contact. It could be shown that when using polymers for the mould, which have only slight oxygen permeability, lenses having no surface slipperiness can be produced. It is thus possible to manufacture lenses in an atmospheric environment whilst simultaneously using plastic mould halves. Since it is not necessary to produce the lenses in a nitrogen atmosphere, production costs may be substantially reduced.

[0021] Further aspects and advantages of the process according to the invention and of the device according to the invention will be seen from the description that follows.

[0022] A device is designed for the manufacture of contact lenses from a liquid starting material which may be polymerised or crosslinked by UV radiation. It comprises a mould and an energy source, here a UV light source, as well as means for directing the energy provided by the energy source to the mould in the form of an essentially parallel beam. Of course, the energy source and means can also be combined to form a single unit.

[0023] The mould consists of two mould halves, each having a curved mould surface which together define a mould cavity, which in turn determines the shape of the contact lens to be manufactured. The mould surface of the upper mould half is convex and determines the rear and basic face of the contact lens with the connected edge area; this mould half is normally called the father mould half. Conversely, the mould surface of the other mould half, which is correspondingly called the mother mould half, is concave and determines the front face of the contact lens to be manufactured, likewise with the connected edge area.

[0024] The mould cavity is not completely and tightly closed, in a first embodiment, is open around its peripheral edge which defines the edge of the contact lens to be manufactured, and is linked to a relatively narrow annular gap. The annular gap is limited or formed by a flat mould wall on each of the father mould half and the mother mould half. In order to prevent complete closure of the mould, spacers, for example in the form of several bolts, are provided on the mother mould, and these interact with a collar or flange of the father mould and keep the two mould halves at such a distance apart that the said annular gap results. The spacers may also be of adjustable or spring-action formation. In this way, the two mould halves can be moved towards one another during the crosslinking process to balance out leakage, by adjusting the spacers or against a spring action. Of course, the mould can, be opened and closed in the usual manner, for example by means of a closure unit. Adjustment of the gap between the two mould halves to balance out leakage, may also be effected e.g. using this external closure unit.

[0025] It is also conceivable that, instead of the continuous annular gap and the spacers, a series of segmentous gaps may be provided, the intermediate areas between the individual segment gaps taking over the function of the spacers. Of course, other configurations of mould halves are also conceivable.

[0026] On the mould wall in the area of the annular gap, there is a mask which is impermeable to the energy form employed, here this is UV light, (or a mask which at least has poor permeability compared with the permeability of the mould), and this mask extends right to the mould cavity, and with the exception of the same, screens all the other parts, hollow spaces or areas of the mould that are in contact with or may come into contact with the liquid, uncrosslinked, possibly excess material, from the radiated energy. Partial areas of the lens edge are therefore formed not by a limitation of the material by mould walls, but by a spatial limitation of the radiation or other forms of energy triggering polymerisation or crosslinking.

[0027] In the case of UV light, the mask may be preferably a chromium layer, that can be produced by processes known e.g. from photography or UV-lithography. The mask does not necessary have to be fixed; it may also be, for example, removable or exchangeable.

[0028] Other configurations are however also possible. For example, in a second embodiment a casting mould similarly consists of two mould halves, each of which has a curved surface, which together define a mould cavity. The mould surface of the upper mould half is convex and determines the rear and basic face of the contact lens; this mould half is normally called the father mould half. Conversely, the mould surface of the other mould half is concave and is called the mother mould half. It defines the front face of the contact lens to be manufactured. The edge region of the contact lens is defined by a limiting area. After filling the lens material into the mother mould half the mould is closed by placing the male mould half excess amount is expelled into an overflow area outwardly adjacent to the mould cavity. The subsequent polymerisation or crosslinking of the starting material takes place by radiation with UV light, or by heat action, or by another non-thermal method. Both the starting material in the mould cavity and the excess material in the overflow area are thereby hardened. The geometry of the edge of the contact lens is defined by the contour of the two mould halves in the area in which they touch one another. In order to obtain error-free separation of the contact lens from the excess material, a good seal or expulsion of the excess material must be achieved in the contact zone of the two mould halves. Only in this way can error-free contact lens edges be obtained.

[0029] In general, the oxygen permeability coefficient P of a polymer may be described by the following formula:

[0030] P=D×S,

[0031] with

[0032] P: oxygen permeability of the polymer

[0033] D: oxygen diffusion constant of the polymer

[0034] S: oxygen solubility in the polymer

[0035] The permeability coefficient P is a measure of the degree of oxygen which acts on the lens material during the hardening process and thus effects an inhibition of the polymerisation reaction. The lower the oxygen permeability coefficient, the less oxygen that reaches the surface of the mould cavity and the lower the inhibition of polymerisation at the surface of the lens.

[0036] Since, in the embodiments shown here, the UV radiation only comes from one side, from the top, in fact only the father mould half needs to be permeable to UV light. If radiation comes from below through the mother mould half, of course the same applies in reverse. According to an especially suitable and advantageous arrangement of the invention, the mould half which is exposed to UV light is made of quartz, while the other mould half is made of a polymer. However, within the scope of the invention, both mould halves may also be made of a polymer, and it is also conceivable that the impacting of the crosslinkable material located in the mould cavity with energy that effects crosslinking, may also be effected not only from one side, but also from two sides. In this case, it must be ensured that both mould halves are permeable to UV light.

[0037] According to the invention, the materials that may be considered for the casting moulds are a number of polymers, which however in respect of their oxygen permeability must be below the value $5\times {10}^{-11}\frac{\left(\mathrm{ml}{O}_2\mathrm{at}\mathrm{STP}\right)\left(\mathrm{mm}\right)}{\left({\mathrm{cm}}^2\right)\left(s\right)\left(\mathrm{torr}\right)}$

[0038] Table 1 lists the oxygen permeability P of various polymers, whereby the relative values are taken from the book: “Polymer Handbook”, and have been recalculated into absolute values corresponding to the value given for polypropylene in Table 1 on page 61 of the book “Polymer permeability, 1. Polymers and polymerization”, from Elsevier Applied Science Publishers LTD 1985, Reprinted 1986. The unit of oxygen permeability is: $\frac{\left(\mathrm{ml}{O}_2\mathrm{at}\mathrm{STP}\right)\left(\mathrm{mm}\right)}{\left({\mathrm{cm}}^2\right)\left(s\right)\left(\mathrm{torr}\right)}$

[0039] According to the invention, at least one of the two mould halves and consists of a polymer that has an oxygen permeability lower than

[0040] The table shows that the permeability coefficient for polyacrylonitrile (PA) and polymethyl-methacrylate (PMMA) is 10 to 1000 times lower than the values for polypropylene (PP) and polystyrene (PS). It has been shown that the lenses that were produced with plastic moulds made of PP and PS in a normal ambient atmosphere had a considerably higher degree of slipperiness than lenses produced with moulds made of PA or PMMA.

[0041] According to a further concept of the invention, it is advantageous to select a material that has a high absorption coefficient for UV light, or to provide materials with UV absorbers, in order to attain the desired UV absorption. The UV stability of the casting moulds and thus their service life may thereby be decisively improved.

[0042] Further studies were carried out with CP-75 UVA and CP-71 UVA (both PMMA with a UV absorber), PMMA GS-222 from the company Rohm, Goldflex (GF), Deirin (DR), Luran 368R (styrene/acrylonitrile copolymer, SAN), Terlux KR-2812 (methylmethacrylate/acrylonitrile/-butadiene/styrene polymer, MABS), Barex 210 (methacrylate/acrylonitrile polymer, MAN), Topas from the company Ticono, as well as CR-39 (chemically crosslinked polymer consisting of allyl diglycol carbonate monomer), which were similarly notable for their low oxygen permeability coefficient. It is demonstrated that very good results are obtained as regards shape and UV stability of the moulds, as well as good lens quality, especially using materials CP-75, CP-71, CR-39, PMMA GS-22 and Barex.

[0043] To crosslink the starting material, UV light with a wavelength on λ>280 nm is preferably used. By using longer wavelength UV light, the demands placed on the UV stability of the polymers are substantially lowered.

[0044] It has been shown that when choosing a polymer for the mother mould half, which has high UV absorption, the quality of the edge of a lens produced in a device described above can be improved, compared with a device in which both mould halves consist of quartz/glass.

[0045] In the first embodiment described above, radiation with UV light to crosslink the polymer takes place through the male casting mould. If the female mould half consists of a material that does not absorb UV light, reflections of the UV light can occur. The reflected UV light now crosslinks the lens material below the mask of the male casting mould, whereupon a non-sharp contact lens edge is produced, since the material around the actual edge is crosslinked by reflected UV light. If, on the other hand, UV-absorbing materials are used for the female mould half, these problems do not arise.

[0046] The processes which may be considered for the production of plastic moulds may be a number of techniques, for example injection moulding, lathing and polishing. These technologies are established, so that the production process is relatively simply to carry out and they do not require particularly great resources.

[0047] In addition, almost all of the above-listed plastics enable optical control of the lenses taking place through the casting mould, since they are transparent and also do not become hazy during long-term exposure to UV light.