Patent Publication Number: US-6220845-B1

Title: Mold clamping and precure of a polymerizable hydrogel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 08/825,281, filed Mar. 27, 1997, now U.S. Pat. No. 5,981,618, which is a division of application Ser. No. 08/636,507, filed Apr. 26, 1996, now abandoned, which is a continuation-in-part of application Ser. No. 08/257,792, filed Jun. 10, 1994, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an apparatus and method for molding soft contact lenses, and particularly an apparatus and method for precuring a polymerizable monomer or monomer mixture under pressure to form a soft contact lens. 
     2. Discussion of the Prior Art 
     U.S. Pat. No. 4,495,313 to Larsen, now assigned to the assignee of the present application, discloses the polymerization of a monomer to form a soft contact lens by means of ultraviolet light. In addition, this reference also teaches that an ultraviolet light catalyst can be added to the polymerization system to effect an essentially complete polymerization. 
     U.S. Pat. No. 5,039,459 to Kindt-Larsen et al. entitled “Method of Forming Shaped Hydrogel Articles Including Contact Lenses” teaches the polymerization of soft contact lenses wherein the monomer/diluent mixture in a transparent polystyrene mold is exposed on one side to 1.7 jewels/centimeter squared of ultraviolet radiation for ten minutes at 55° C. (TL 09 lamps with peak radiation of 350 nm). 
     In addition to these two references, U.S. Pat. Nos. 4,565,348; 4,680,336; 4,640,489 and 5.080,839 all disclose the polymerization of a monomer with ultraviolet light to form a soft contact lens. The entire disclosure of each of these references are hereby incorporated by reference into this patent application. 
     SUMMARY OF THE INVENTION 
     The present invention is an apparatus and a method for precuring a polymerizable monomer mixture to form a soft contact lens in a mold with avoids decentration of the resultant lens, enables a more uniform cure for the lens during the cure step, and which reduces “puddling” or cavitation of the lens in the mold during cure. 
     In accordance with the present invention, a polymerizable monomer or monomer mixture is deposited in a lens mold having first concave and second convex mold halves. The mold halves are formed of polysytrene or other material transparent to actinic or visible radiation. The mold halves are transported from the deposition and mold assembly station to the precure station of the present invention, and clamped together under predetermined pressure for a predetermined period of time which may be done in a low oxygen environment. The second or convex mold half may be slightly thinner than the first or concave mold half to enable mold compliance during cure as the monomer is polymerized. The clamping pressure aligns flanges formed on the first and second mold halves to ensure that the flanges are parallel and that the respective curves of the molds are aligned. The clamping pressure also seats the second convex mold half against an annular edge formed on the first mold half to essentially sever any excess monomer from the monomer contained within the mold. 
     After a predetermined clamping period, the hydrogel is exposed to actinic or visible radiation, such as an UV light source, to partially cure the hydrogel to a gel state. After a second predetermined period of exposure under clamping pressure, the clamping action and the UV radiation may be removed, and the partially precured hydrogel lens is transported in the mold to a curing station for complete polymerization and cure. Alternatively, a complete cure may be effected in the apparatus. 
     It is an object of the present invention to provide a novel means for precuring a polymerizable monomer or monomer mixture to form a soft contact lens. The means includes a transport for intermittently transporting a plurality of contact lens molds to the precure station in a low oxygen environment. The contact lens molds include first and second mold halves with a polymerizable monomer or monomer mixture therebetween. The means also include a plurality of means for clamping the first mold half to the second mold half for a predetermined period of time in a low oxygen environment. The polymerizable monomer or monomer mixture is then exposed to actinic radiation for a predetermined period of time while it is under pressure to partially or completely cure the monomer or monomer mixture. 
     The clamping pressure may be applied by an annular air cylinder, spring driven annular cylinder, or physical weights which allows actinic radiation to pass through the annulus of the cylinder and through one or more of the mold halves and into the monomer or monomer mixture. The means may include a means for controlling the duration and intensity of the clamping pressure, and the duration and intensity of the actinic radiation. It has been found that under certain conditions involving a pre-cure or partial cure of the monomer, that the surface energy attraction between the clamping means and the mold half may result in premature separation of one or more sets of mold halves from the carrier pallet when the clamping means is retracted at the end of the pre-cure step. 
     The exact cause of this phenomena is unknown, but the mold halves and monomer are very light in weight, and the attraction may involve surface energy, static electricity, or migration of minute amounts of oils from the mold halves to the clamping means over time. When the precure apparatus cycles, the attracted set of mold halves is stacked on another set in a different pallet, eventually leading to rupture of the mold halves and contamination of the production equipment with partially cured monomer. While the incidence of such premature separation is low, the cost of shutting down a line operating in a controlled atmosphere for cleaning is very high, and thus to be avoided. 
     It is therefore an object of the present invention to minimize the surface energy attraction between the mold halves and the clamping means, and the opportunity for premature separation of the mold halves from the carrier pallet. 
     It is another object of the present invention to provide a clamping means which will provide radially uniform pressure to clamp the mold halves together. 
     Another object of the present invention is to provide a novel method of precuring a polymerizable monomer or monomer mixture to form a soft contact lens. The method includes a step of depositing a polymerizable monomer or monomer mixture in a contact lens mold having first concave and second concave mold halves with the monomer therebetween. The mold halves are then clamped together with a predetermined pressure and then exposed to radiation to partially polymerize the monomer to a precured gel-like state. The clamping pressure is then relieved and the lens is then cured with additional radiation or a combination of heat and radiation. 
     In the preferred embodiment the radiation source may be actinic, electron beam or radioactive source, but preferably is an ultraviolet lamp which irradiates the monomer at 2-4 mW/cm 2  for 5-60 seconds, but preferably 20-40 seconds. Radiation may also be from a high intensity UV source that is pulsed or cycled. 
     The clamping step aligns the mold halves, causes excess monomer to be removed from the mold cavity, and seats a compliant second mold half against a first mold half for the precure step. 
     It is an object of the present invention to precure a polymerizable monomer or monomer mixture to a gel-like state wherein polymerization has been initiated throughout the monomer, and utilize the shrinkage of the monomer and the compliance of the mold halves to seal and hold the mold halves together for the remainder of the cure period. This substantially avoids decentration defects which can occur if the back curve mold half is tipped or rotated with respect to the front curve mold half prior to cure. 
     It is another object of the present invention to precure the hydrogel in a low oxygen environment to prevent undesired absorption of oxygen by the monomer. 
     It is another object of the present invention to provide an adjustable means for varying the clamping pressure during the precure step. 
     It is another object of the present invention to minimize the surface energy attraction between the clamping means and the mold halves while simultaneously providing a radially uniform clamping pressure during the clamping step. 
     It is another object of the present invention to provide a reciprocating means for positioning the actinic light source at an exposure position, and then retracting the light source to a second position for transport of the lens molds. 
     It is further an object of the present invention to provide an alternate reciprocating means for positioning the lens molds against the light source for exposure, and retracting the molds to a second position for transport. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing objects and advantages of the present invention for precuring a polymerizable monomer or monomer mixture may be more readily understood by one skilled in the art with reference to the following detailed description of the two preferred embodiments, taken in conjunction with the accompanying figures wherein like elements are designated by identical reference numerals throughout the several views. 
     FIG. 1 is a partially cut away elevation view of one of the embodiments for precuring a polymerizable monomer or monomer mixture to form a soft contact lens. 
     FIG. 2 is an end elevation view of the apparatus illustrated in FIG.  1 . 
     FIG. 3 is a diagrammatic and schematic illustration of one embodiment of the invention. 
     FIG. 4 is a plan view of a carrier used to transport a plurality of contact lens molds having a polymerizable monomer or monomer mixture therebetween to and from the precure station. 
     FIG.  4 ( a ) is a diagrammatic and cross-sectional view of a pair of mold halves in the pallet carrier of FIG.  4 . 
     FIG.  5 ( a ) is a diagrammatic illustration of one embodiment of the present invention which uses an air driven clamp for clamping the mold halves together. 
     FIG.  5 ( b ) is a diagrammatic illustration of a second embodiment of the present invention which uses a spring driven clamp for clamping the mold halves together. 
     FIG.  5 ( c ) is an elevation view of one of the clamping members used in the second embodiment illustrated in FIG.  5 ( b ) which illustrates one clamp construction that minimizes surface energy attraction. 
     FIG.  5 ( d ) is a top elevation view of the clamping member illustrated in FIG.  5 ( c ). 
     FIG. 6 is a plan view of a reciprocating portion of the apparatus for precuring a polymerizable monomer or monomer mixture to form a contact lens. 
     FIG. 7 is an elevational view of the apparatus illustrated in FIG.  6 . 
     FIG. 8 is an end elevational view of the apparatus illustrated in FIG.  6 . 
     FIG. 9 is an elevational end view of a second embodiment of the present invention used to precure a polymerizable monomer or monomer mixture to form a soft contact lens. 
     FIG. 10 is an elevational side view of the apparatus illustrated in FIG.  9 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is useful in a process for forming soft contact lenses from a polymerizable monomer or monomer mixture. The soft contact lenses are formed in a mold assembly having a first concave and a second convex mold half. As illustrate in FIG. 4 a l, the mold halves are formed of polystyrene transparent to visible and ultraviolet light, with a central curved section defining a concave surface  31 , a convex surface  33  and circular circumferential edge  31 ( c ), and integral with said edge an annular essentially uniplanar flange  31 ( a ). At least a part of the concave surface  31  and the convex surface  33  have the dimensions of the front or back curves, respectively of a contact lens to be produced in the mold assembly, and are smooth so that the surface of the contact lens formed by polymerization of said polymerizable composition in contact with the surface is optically acceptable. The mold is thin enough to transmit heat therethrough rapidly and has rigidity sufficient to withstand prying forces applied to separate the mold halves during demolding step which occurs after the cure step in the manufacturing process. 
     The present invention is directed to an additional precure step introduced between the assembly of the mold with a polymerizable monomer or monomer mixture, and the cure step. The precure step partially cures the polymerizable monomer or monomer mixture to a viscous gel and initiates polymerization throughout the mixture. During this precure step the parallel alignment of the mold halves are “frozen” by the viscous gel like nature of the partially polymerized monomer gel which prevents decentration and enables unattended and unweighted cure during the remainder of the cure period. The addition of this precure step has substantially reduced the number of defective lenses resulting from traditional methods of manufacture. 
     The complimentary pair of first  31  and second  33  mold halves which define the shape of the final desired lens are used to direct mold the monomer mixture wherein the mixture is dissolved in a nonaqueous water displacable solvent. After the filling or dosing step, in which the front concave mold half  31  is substantially filled with a polymerization mixture  32 , the concave front mold half  31  is covered with a base mold half  33  under a vacuum to ensure that no air bubbles are trapped between the mold halves. The base mold half is then brought to rest on the circumferential edge  31 ( c ) of the concave front mold half to ensure that the resultant lenses are properly aligned and without distortion. 
     The first and second mold halves are then clamped together. This clamping step may be done following assembly of the mold halves, during precure, or both. The clamping step displaces any surplus monomer from the mold area and properly aligns the mold halves by alignment of the mold flanges. When the clamping step occurs in the precure process, the mold halves are clamped under pressure, the monomer or monomer mixture is then exposed to actinic light, preferably from a UV lamp, while the mold halves are clamped. Typically the mold halves are clamped for approximately 40 seconds with 30 seconds of actinic radiation. At the completion of the precure step, the monomer or monomer mixture has formed a partially polymerized gel, with polymerization initiated throughout the mixture. 
     Following the novel precure step, the monomer/diluent mixture is then cured in a UV oven whereby polymerization of the monomer(s) is completed. This irradiation with actinic, visible or ultraviolet radiation produces a polymer/diluent mixture in the shape of the final desired hydrogel lens. After the polymerization process is completed, the two halves of the mold are separated in a demolding step typically leaving the contact lens in the first or front curve mold half, from which it is subsequently removed. The front and base curve mold halves are used for a single molding and then discarded or disposed of. After the demolding step, the solvent is displaced with water to produce a hydrated lens, which when fully hydrated and buffered, will be of the final shape and size which, in most cases, is nominally 10% larger than the original molded polymer/diluent article. 
     The present invention is therefore a novel precure step, and includes two separate embodiments of an apparatus for performing the same. The precure step is inserted in the process immediately after the polymerizable composition is placed in the front curve mold half, and the mold halves assembled. 
     The compositions to which this precure step may be directed include copolymers based on 2-hydroxyethyl methacrylate (“HEMA”) and one or more comonomers such as 2-hydroxyethyl acrylate, methyl acrylate, methyl methacrylate, vinyl pyrrolidone, N-vinyl acrylamide, hydroxypropyl methacrylate, isobutyl methacrylate, styrene, ethoxyethyl methacrylate, methoxy triethyleneglycol methacylate, glycidyl methacrylate, diacetone acrylamide, vinyl acetate, acrylamide, hydroxytrimethylene acrylate, methoxyethyl methacrylate, acrylic acid, methacryl acid, glyceryl methacrylate, and dimethylamino ethyl acrylate. 
     Preferred polymerizable compositions are disclosed in U.S. Pat. No. 4,495,313 to Larsen, U.S. Pat. No. 5,039,459 to Larsen et al. and U.S. Pat. No. 4,680,336 to Larsen et al., the disclosures of which are hereby incorporated herein by reference. Such compositions comprise anhydrous mixtures of a polymerizable hydrophilic hydroxy ester of acrylic acid or methacrylic acid and a polyhydric alcohol, and a water displaceable ester of boric acid and a polyhydroxyl compound having preferably at least 3 hydroxyl groups. Polymerization of such compositions, followed by displacement of the boric acid ester with water, yields a hydrophilic contact lens. The mold assembly of the present invention described herein may be used to make hydrophobic or rigid contact lenses, but the manufacture of hydrophilic lenses is preferred. 
     The polymerizable compositions preferably contain a small amount of a cross-linking agent, usually from 0.05 to 2% and most frequently from 0.05 to 1.0%, of a diester or triester. Examples of representative cross linking agents include: ethylene glycol diacrylate, ethylene glycol dimethacrylate, 1,2-butylene dimethacrylate, 1,3-butylene dimethacrylate, 1,4-butylene dimethacrylate, propylene glycol diacrylate, propylene glycol dimethacrylate, diethylglycol dimethacrylate, dipropylene glycol dimethacrylate, diethylene glycol diacrylate, dipropylene glycol diacrylate, glycerine trimethacrylate, trimethylol propane triacrylate, trimethylol propane trimethacrylate, and the like. Typical cross-linking agents usually, but not necessarily, have at least two ethylenically unsaturated double bonds. 
     The polymerizable compositions generally also include a catalyst, usually from about 0.05 to 1% of a free radical catalyst. Typical examples of such catalysts include lauroyl peroxide, benzoyl peroxide, isopropyl percarbonate, azobisisobutyronitrile and known redox systems such as the ammonium persulfate-sodium metabisulfite combination and the like. Irradiation by ultraviolet light, electron beam or a radioactive source may also be employed to catalyze the polymerization reaction, optionally with the addition of a polymerization initiator. Representative initiators include camphorquinone, ethyl-4-(N,N-dimethylamino)benzoate, and 4-(2-hydroxyethoxy)phenyl-2-hydroxyl-2-propyl ketone. 
     Polymerization of the polymerizable composition in the mold assembly is preferably carried out by exposing the composition to polymerization initiating conditions. The preferred technique is to include in the composition initiators which work upon expose to ultraviolet or visible radiation; and exposing the composition to ultraviolet or visible radiation of an intensity and duration effective to initiate polymerization and to allow it to proceed. For this reason, the mold halves are preferably transparent to ultraviolet or visible radiation. After the precure step, the monomer is again exposed to ultraviolet or visible radiation in a cure step in which the polymerization is permitted to proceed to completion. The required duration of the remainder of the reaction can readily be ascertained experimentally for any polymerizable composition. 
     After the polymerizable composition has polymerized, the mold assembly is disassembled to permit further processing of the polymerized hydrogel into a contact lens (such processing including e.g. washing and hydrating, and packaging of the lens). Preferably, the flanges of the front and back curve mold halves are gripped and pulled away from each other, either in directly opposite directions or through an angle in a prying sort of motion. Advantageously, the back curve is first heated moderately to facilitate separation of the polymerized article from the back curve mold half surfaces. 
     THE PRECURE APPARATUS 
     FIG.  1  and FIG. 2 represent a side elevation view and an end elevation view, respectively, of an apparatus for practicing the present invention. As illustrated in FIGS. 1 and 2, a support frame  11  having legs  12  provides support for the device at an elevation of an infeed conveyor  33 . As shown in FIG. 3, the precure apparatus receives a plurality of pallets, one of which is illustrated in FIG. 4, having a plurality of contact lens molds therein, from the infeed conveyor  13 . As seen in FIG. 3, the infeed conveyor  13  delivers the pallets  30  and molds  31 ,  33  to a low oxygen environment, which environment is accomplished by pressurizing an enclosure  14  with nitrogen gas. Prior to polymerization, the monomer is susceptible to absorption of oxygen which results in degradation of the resultant lens. The enclosure  14  may be pivoted about pivot point  15  by lifting on handle member  16 . A gas strut assist device  17  will hold the enclosure  14  in an open position for servicing of the apparatus. 
     As illustrated in FIG. 3, the conveyor  13  delivers pallets  30  containing a plurality of molds  31 ,  33  to an accumulating section generally indicated  18  which gathers a plurality of pallets for the precure step. In the embodiment illustrated in FIGS. 1-2 and  6 - 8 , twelve pallets of the type illustrated in FIG. 4 are accumulated for a total of 96 molds in each batch operation. In the embodiment illustrated in FIGS. 9 and 10 a total of 12 pallets having 96 contact lens molds thereon are batched for each precure operation. Accumulator  18  thus enables the precure apparatus of the present invention to batch process a plurality of molds for an extended period of time of 30 to 60 seconds while continuously receiving new pallets from the production line at the rate of 1 every 6 to 12 seconds. 
     The precure assembly  19  is partially visible in the breakaway portion of FIG. 1, and is further described with respect to FIGS. 6-8. It is raised and lowered into engagement with pallets containing contact lens molds by virtue of a pneumatic cylinder  20  which raises and lowers an intermediate support beam  21  and reciprocating shaft members  22  which are journaled for reciprocating support in member  23  as will be hereinafter subsequently described in greater detail. After the precure operation, the pallets with contact lens molds therein are discharged through a nitrogen ventilation airlock mechanism  24  (illustrated in FIG. 1) for subsequent cure by heat and cycled actinic radiation. 
     The present invention is particularly adapted to cooperate with a plurality of pallets as illustrated in FIGS. 4 and 4 a  which have a plurality of cavities for receiving a plurality of contact lens molds. As illustrated in FIG. 4 a , the contact lenses of the present invention are formed by placing an amount of polymerizable composition, generally on the order of about 60 μl, in the first or concave mold half  31 . The desired amount depends on the dimensions (i.e., the diameter and thickness) of the desired lens, taking into account the generation of by-products upon polymerization and exchange water for those by-products and diluent, if any, following polymerization. 
     Then a second or convex mold half  33  is placed onto the polymerizable composition  32  with the first and second mold halves aligned so that their axes of rotation are collinear and the respective flanges  31 ( a ),  33 ( a ) are parallel. The mold halves  31  are carried in an annular recess  30 ( a ) which receives and supports the annular flange  31 ( a ) of the first or concave mold half. The pallet  30  also has a plurality of recesses  30 ( b ) for receiving the concave portion of the mold. The pallet also carries a plurality of oriented recesses  30 ( c ) which receive a triangular tab portion  31 ( c ) of the base mold half to provide a predefined angular position. The second or convex mold half  33  also includes a triangular tab  33 ( c ) which overlies tab  31 ( c ) to provide a collinear axis of rotation with respect to the two mold halves. The pallet  30  illustrated in FIG. 4 also contains a unique bar code number  35  for use in pallet tracking and quality control procedures. 
     FIG. 3 is a diagrammatic representation of a portion of the apparatus more fully illustrated in FIGS. 6-9 and FIGS. 9-10 which is particularly suited to an overview of the present invention. As illustrated in FIG. 3, the apparatus includes a conveyor system  13  for transporting a plurality of pallets  30  into and away from the precure station. Batch mode forks  36 ( a ),  36 ( b ) are used to gather and move a plurality of pallets into the precure apparatus. 
     The apparatus  19  includes multiple vertical reciprocal movements, a first one of which is in response to movement from air cylinders  20 ( a ) and reciprocating beam  21 ( a ). As the precure apparatus  19  is lowered in the direction illustrated by arrow A, a plurality of annular clamping means  41  will engage the upper annular flange  33 ( a ) of each of the mold halves contained within pallets  30 . A plurality of annular clamping means  40  are mounted on and travel with a reciprocating platform  41  of the apparatus, and are resiliently mounted therein for a second reciprocal movement along the direction of arrow B illustrated in FIG.  3 . 
     As illustrated in FIGS.  3  and  5 ( a )-( d ), the clamping means  40  are biased within frame  41  by springs  42  (illustrated diagrammatically) which may be the air spring  42 ( a ) illustrated in FIG. 5 a  or a helical spring  42  as illustrated in FIG. 5 b . As the apparatus is lowered, the clamping means will engage and clamp the first and second mold halves together with the force determined by the spring means  42 . When air springs are used, the force will be determined by the amount of pressure provided to the air cylinder  42 ( a ). While clamping means  40  have been illustrated as four members in FIG. 3 for illustrative purposes, it is understood that in the embodiments illustrated in FIGS. 6-8 and the embodiment illustrated in FIGS. 9-10 that there are 96 individual clamping means, with an individual clamping means for each of the mold halves. 
     Positioned above the clamping apparatus are a plurality of actinic light sources  44  which may be UV lamps. The lamps are separated from the clamping area by Pyrex glass  100  which separates the precure apparatus into two separate heat zones. This enables cooling of the actinic lamps  44 , while maintaining the clamping zone at an optimal cure temperature. The glass  100  also protects the lamps from monomer emissions which collect on the glass  100  rather than the lamps  44 . After the clamping means has engaged the mold halves to clamp them together, a shutter mechanism  45  is opened by air cylinder  46  to enable the actinic light source  44  to initiate polymerization of the polymerizable composition in each of the mold halves  33 . Shutter  45  has a plurality of openings  43  defined therein and is reciprocal along the x axis as indicated by arrow C in FIG. 3 in order to open and close the exposure passage ways  47 . 
     The operation of the precure apparatus  19  is set by control circuit  10  which controls the duration of the clamping period by the length of time air cylinder  20 ( a ) is activated to its reciprocal down position. Control circuit  10  also controls the amount of radiation received by the molds controlling the duration of the exposure period through operation of shutter  45  and air cylinder  46 . The intensity may also be manually adjusted by raising or lowering the lamps  44  with respect to molds  31 ,  33 . Optionally, the polymerization radiation may be generated remotely and routed to the clamped mold halves and polymerizable material via a fiber optic system, with control system  10  providing control of exposure time and energy level. 
     The amount of force applied by clamping means  40  may be varied from approximately 0.5 Kgf to 2.0 Kgf/per lens and is applied to keep the flange  33 ( a ) of the second convex mold half parallel to the flange  31 ( a ) of the first concave mold half for the duration of the exposure. The clamping weight is applied for 10 to 60 seconds, but typically for a period of 40 seconds by control means  10 . After approximately 0-20 seconds of weight, preferably 10 seconds, actinic radiation from UV lamps  44  is applied to the assembled mold and the polymerizable monomer. Typically, the intensity of the UV light source is 2-4 mW/cm 2 , and this intensity of light is applied for 10 to 50 seconds, but in the preferred embodiment, is applied for 30 seconds. 
     In the preferred embodiment, the UV lamp source are tubular low pressure mercury vapor fluorescent lamps which emit a long wave ultraviolet radiation having a highly concentrated radiation between 320 and 390 nm. In one embodiment, by way of example, the lamps are manufactured by Philips as model ‘TL’ 29 D 16/09 N having a nominal wattage of 14 watts and an average radiation of 1.8 watts in the desired spectrum. 
     The mold halves are first clamped together for a predetermined period of time, prior to exposure, in order to allow equilibrium to develop between the monomer and the mold cavity, and to allow any excess monomer to be extruded out of the mold cavity into the space between flanges  31 ( a ) and  32 ( a ) where it forms a ring of excess monomer  33 ( a ), which is generally referred to as a “HEMA” ring when hydroxyethylmethacrylate monomer is used. The first or concave mold cavity includes a sharp annular edge  31 ( c ) to cleanly contact the convex portion of mold half  33  and thereby separate the contact lens  32  from the HEMA ring  32 ( a ). The pre-exposure clamping period allows for any excess monomer to migrate from the mold cavity to the HEMA ring, enables the second mold cavity to seat cleanly on parting edge  31 ( c ), and allows an equilibrium to develop between the mold halves and the monomer. 
     Although the mold halves may be held together until the monomer(s) are completely cured, in the preferred embodiment actinic radiation on the order of 2-4 mW/cm 2  is applied for approximately 30 seconds. Different intensities and exposure times could be used, including pulsed and cycled high intensity UV on the order of 10 to 150 mW/cm 2  with exposure times running from 5 to 60 seconds. In a pulsed or cycled exposure, the actinic radiation may be cycled on for 3 to 10 seconds and then off for 3 to 10 seconds with a total of 1 to 10 cycles of radiation. At the end of the radiation period, the shutter  45  is closed by reciprocating it to the right as illustrated in FIG.  3  and the weight is removed by energizing cylinder  20 ( a ) to lift the precure assembly  19  upwardly by means of push rods  22 ( a ). As the assembly  19  is lifted, the clamping means  40  will be lifted clear of the molds and pallets to enable them to be transported out of the precure means by means of conveyor  13 ( a ). During the precure time, the temperature in the system may be varied from ambient to 50° C. 
     At the conclusion of the precure process, the monomer has gone through initiation and some degree of polymerization. The resultant product is in a gel state with some areas of the product that have the least thickness, i.e., the edge, having a higher degree of polymerization than the body. As the monomer polymerizes it forms a seal along the parting edge  31 ( c ), which together with the inherent shrinkage of the monomer from polymerization, forms a secure mold package that may be cured in atmospheric conditions. 
     The clamping pressure aligns the mold flanges to provide substantially parallel alignment of the flanges, and the seal maintains them in a parallel position which results in improved optical characteristics near the theoretical maximum of the plastic mold halves. Further, the clamping, and resultant seal from the partial cure results in highly repeatable values for centration and an improved “clean” edge at the circumference of the lens. 
     FIGS. 5 a  and  5   b  are schematic illustrations of two alternate means for clamping the mold halves together. In FIG. 5 a  an air spring is used to displace the clamping means  40  downwardly with respect to the movement of reciprocating platform  41 . In the embodiment illustrated in FIG. 5 b , a resilient coil spring  42  is used to drive the clamping means  40  downwardly with respect to the movement of reciprocating platform  41 . The clamping means  40  illustrated in FIGS.  5 ( c ) and  5 ( d ) is particularly adapted for use with the embodiment illustrated in FIG.  5 ( b ), but with minor modifications could be used in the embodiment of FIG.  5 ( a ). 
     As illustrated in FIG. 5 a , reciprocating platform  41  includes a plurality of annular cylinders  40 , each of which has an annular flange  51  formed thereon, which reciprocates within a cylinder defined by spacer plate  52  to form an air spring or air cylinder  42 ( a ). The cylinder is vented to atmosphere by vents, one of which is illustrated at  53  in FIG. 5 a  to enable air pressure in the air cylinder  42 ( a ) to drive the flange  51  and clamping means  40  downwardly against a stop plate  55  when the air cylinder  42 ( a ) is pressurized. The carriage  50  also includes an upper stop member  56  which defines the upper limit of travel by annular flange  51 , and serves as reciprocal bearing support for the clamping means  40 . The upper stop member  56  also defines a plurality of air plenums, one of which is schematically illustrated at  71 , which provide air pressure for the air cylinder  42 ( a ). 
     Air pressure is supplied to plenum  71  by solenoid air valve  72  which is activated by control means  10 . As indicated previously, the air pressure supplied to air cylinder  42 ( a ) is variable to generate a clamping force of 0.5 Kgf to 2.0 Kgf on the annular flange  33 ( a ). As the reciprocating platform  41  is lowered over the pallet  30  and the contact lens molds, the annular clamping means  40  and the plurality of engagement members formed thereon engage the annular flange  33 ( a ) formed on the second or convex mold half to urge it downwardly and clamp it into engagement with mold half  31 . The movement of reciprocating platform  41  is defined by the movement of the entire assembly  19  as driven by air cylinder  20 . 
     As illustrated in FIGS. 5 b-d , the annular clamping cylinder  40  also includes a rectangular flange  51  which is engaged by a resilient coil spring  42  and driven downwardly, as illustrated in FIG. 5 b , against stop plate  55 . The other end of coil spring  42  is mounted within upper stop plate  56 ( a ). Spacer plate  52  and the upper stop plate  56  provide support and guidance for the vertical reciprocation of clamping members  40 . 
     The clamping member  40 , as illustrated in FIGS.  5 ( c ) and ( d ) is formed of stainless steel, and optionally the mass of the clamping members may be increased to enable the clamping members to clamp by weight alone, without air or spring assist. However, the combination of the individual reciprocal clamping means  40  and locally driven resilient biasing means enable precise control of the amount of clamping pressure exerted on the plastic flanges  33 ( a ) of the upper mold regardless of the amount of force needed to move the mass of the entire precure assembly  19 . Formed on the engaging annulus of clamping number  40  are a plurality of engagement members  40 ( a )-( d ) which minimize the surface area of engagement between the clamping member and the mold halves. These engagement members are integrally formed from the annular cylinder as downwardly extending square tooth members, by cutting away portions of the annular member. In the embodiment illustrated in FIG.  5 ( b ),  5 ( c ) and  5 ( d ), the flange member  51  is rectangular, with mounting holes  51 ( a ), ( b ) which are used to mount the clamping member to coil spring member  42 . When formed for use with the embodiment illustrated in FIG.  5 ( a ), flange member  51  would be annular, and member  40  would be uniform in diameter through out its length. 
     The engagement members  40 ( a )-( d ) are approximately 3.5 mm in thickness, and 4 mm in length, which has been found sufficient to ensure evenly distributed pressure around the knife edge  31 ( c ) (FIG.  4 ( a ) while minimizing the area of engagement which might otherwise be susceptible to surface energy attraction or oil migration from the mold halves. While the preferred configuration for the engagement members is illustrated in FIG.  5 ( c ), any configuration of engagement members  40 ( a )-( d ) would work, provided the clamping pressure is distributed in a radially uniform manner about the circular knife edge  31 ( c ). Non-uniform distribution of the clamping pressure may result in tipping of the upper or back curve mold half with respect to the lower or front curve mold half 
     As noted earlier, while the incidence of attraction between the clamping member  40  and the clamped set of mold halves is very low, and a very small fraction of 1%, the cost in down time associated with changing atmospheres, cleaning the contaminated area, recharging the inert atmosphere and restarting the line is very high. 
     FIGS. 6-8 are top side and end elevation views of one embodiment of the apparatus for performing the present invention. As illustrated in FIGS. 6-8, the precure assembly  19  is mounted on reciprocating support shafts  22  which are journaled for reciprocating motion within support members  23  which are fixably mounted to a frame. Each of the support members  23  include upper and lower bearings or journals  23 ( a ),  23 ( b ) to provide for precision guidance of the reciprocating shafts  22 . Shafts  22  are driven by intermediate support frame  21  which is in turn driven by air cylinder  20  as previously described. 
     The precure assembly  19  is reciprocated vertically with respect to the upper most surface of the conveyor  13  illustrated schematically in FIGS. 7 and 8. The precure assembly  19  is supported from an upper support frame  60  from which various operating components are suspended by intermediate support members at each corner generally illustrated at  61 ,  62  in FIG. 7 an  62 ,  63  in FIG.  8 . These members suspend the reciprocating platform  41 , into which the individual clamping means  40  are fitted for a secondary reciprocating along the same axis. A separate subframe  65  is mounted on platform  41  and provides support for the ultraviolet lamps  44  and the ballast members  66  which provide the high voltage for the UV lamps, and the starters  67  which are used to initiate fluorescence. In one embodiment, by way of example, the ballast members may be BTP 30C05S, as manufactured by Philips and the starters may be model S2, as manufactured by Philips. The intermediate support frame  65  may be lifted free from the reciprocating platform  41  by means of a plurality of handles  67  which are mounted via straps  68  to the intermediate support frame  65 . Pyrex glass member  100  separates the lamps  44  from the clamping area to protect the lamps from monomer emissions, and to enable the lamps to be cooled, while keeping the precure zone at a temperature of ambient to 50° C. to facilitate polymerization. An oxygen detector  69  is also provided to monitor the low oxygen environment maintained within the protective cabinet  14  and an alarm is initiated if the oxygen content rises to an unacceptable level. Shutter  45  is positioned above the reciprocating platform  41  between the UV lamps  44  and the clamping means  40  and is reciprocated between its open and closed position by virtue of air cylinder  46 . 
     The embodiment illustrated in FIGS. 6-8 utilizes the air cylinder or air springs depicted schematically in FIG. 5 a  with an internal common plenum formed in the reciprocating platform  41  by means of passageways in stop plate  56 . Thus, each of the 96 clamping members  40  are reciprocated by their respective air cylinders, which are fed from common plenums, one of which is illustrated in FIG. 5 a  as plenum  71  formed in the upper stop plate  56 . 
     FIGS. 9 and 10 depict a second preferred embodiment for practicing the present invention. As illustrated in FIGS. 6-8, the first preferred embodiment reciprocated the UV lamps and clamping members into and out of engagement with the mold halves and pallets carried by conveyor means  13 . In the embodiment illustrated in FIGS. 9 and 10, the UV lamps are stationary, and the pallets are lifted from the conveyor into engagement with the clamping means for the precure period. 
     The clamping means utilized by the embodiment illustrated in FIGS. 9 and 10 utilizes the clamping means previously described with respect to FIG. 5 b . In this embodiment, a plurality of clamping means  40 ( a ) are mounted above a roller conveyor illustrated in side view of FIG. 10 by rollers  80 . A plurality of lifting standards  81  are positioned between groups of rollers  80  on centers approximate the width of the pallets  30 . In FIG. 10, a first row of pallets  30  is depicted resting on rollers  80  with adjoining edges of each of the pallets aligned along the top of the lifting standards  81 . 
     The pallets  30  are aligned in position by means of stop means  83  which is lifted by air cylinder  82  during the loading of the precure apparatus. During loading of the device, the stop means  83  is reciprocated upwardly, and the requisite number of pallets  30  are advanced into the precure apparatus. When 6 pallets in each row have been advanced, a second stop means  84  is lifted by air cylinder  85  to define a limit on x axis travel as illustrated in FIG. 10. A separate air cylinder  87  is used in cooperation with stop means  83  to align the adjoining edges of the pallets  30  above the centers of the lifting standards  81 . After the pallets have been aligned, the lifting standards  81  are reciprocated upwardly by means of intermediate support frame  88  and a pair of pneumatic motors  90  and  91 . 
     The pallets are reciprocated upwardly to the position illustrated at  30 ( a ) in FIG. 10, in which position they engage the clamping member  40 ( a ) as previously described with respect to FIG. 5 b . Each of the clamping members  40 ( a ) also include a separate independent and resilient spring  42  for driving clamping member  40 ( a ) and the upper mold half against the lower mold half during the precure period. 
     After the pallets and mold halves have been raised by air cylinders  90 ,  91 , and the first and second mold halves clamped together by means of clamping means  40 ( a ), a reciprocating shutter  45 ( a ) is shifted as illustrated in FIG. 3 to align a plurality of openings therein with the central openings formed in the clamping means  40 ( a ) and thereby enable exposure of the monomer in the mold halves by means of actinic light sources  44 ( a ). The upper precure assembly  19 ( a ) is fixably supported by means of support beams  92  and  93 . High voltage ballast means  66 ( a ) and starter  67 ( a ) also initiate and maintain fluorescence within the UV light sources  44 ( a ). The reciprocal subassembly of the embodiment illustrated in FIGS. 9 and 10 is mounted on an intermediate support frame  94  which may be raised and lowered by means of pneumatic cylinder  95  to provide for cleaning and servicing of the apparatus, and in particular, the clearing of the individual clamping means  40 . Pyrex glass member  101  protects the lamps  44 ( a ) from monomer emissions, and may be removed for clearing. Glass member  101  also divides the apparatus into two temperature zones, enabling cooling of lamps  44 ( a ). The intermediate support frame  94  is guided and aligned with the exposure assembly  19   a  by means of support rods  95 ,  96  and reciprocal bushing members  97 ,  98 . 
     The clamping period and the amount of exposure to radiation are controlled by control means  10  in the manner previously described with respect to FIGS. 3 and 5. 
     Following the precure of the monomer in mold halves  31 ,  33  the pallets  30  are reciprocated downwardly to the position illustrated in FIG.  10  and advanced by conveyor rollers  80  to a subsequent conveyor (not shown) which transports the pallets to the final cure apparatus. 
     While the invention has been particularly shown and described with respect to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention, which should be limited only by the scope of the appended claims.