Patent Publication Number: US-2007102835-A1

Title: Automated in-line moulding/heating process and apparatus for preparing contact lenses

Description:
FIELD OF THE INVENTION  
      The present invention relates to an in-line process for preparing contact lenses. More particularly, the present invention relates to an in-line process continuous process for making contact lenses and in particular, silicone containing hydrogel lenses.  
     BACKGROUND OF THE INVENTION  
      Processes for making contact lenses and in particular silicone hydrogel lenses are well known in the art. Reference is made to U.S. Pat. No. 5,260,000, the disclosure of which is incorporated herein by reference, for a description of a typical process for producing such lenses.  
      Briefly, the current practice involves the preparation of a mixture of a silicone-containing monomer, a hydrophilic monomer and an organic diluent. This mixture is cast into a mold and cured or polymerized to obtain a shaped lens article. The lens article, still retained in the mold is placed in an oven and maintained at an elevated temperature for a time sufficient to drive off the organic diluent that may be entrapped within the polymerized lens matrix. After cooling, the article is removed from the mold and subjected to further treatments to produce the contact lens.  
      Current manufacturing processes use a batch oven process for heating to remove the organic diluent. Such batch heating requires dedicated and mostly manual material handling techniques to load and unload the heating oven. Batch trays also are dedicated for parts handling and orientation within the heating oven. In a typical batch operation, trays containing a plurality of the molds are stacked in a forced air oven and are kept in the oven for up to three hours at 60° C. in order to remove the organic diluent.  
      Accordingly, to date, the production of silicone hydrogel contact lenses has been labor intensive first to manually load a plurality of molds onto a dedicated tray, then to manually load a plurality of individual trays into the heating oven and then to manually unload the trays from the oven.  
      Accordingly, it is an object of the present invention to provide a method of producing silicone hydrogel lenses that reduces the amount of manual handling as opposed to the current batch method.  
      Another object is to provide a method that provides a first-in-first out product flow.  
      A further object is to provide a method that provides for the continuous movement of molds or batches of molds through a heating/cooling chamber.  
     BRIEF SUMMARY OF THE INVENTION  
      In accordance with the present invention, the molds containing the polymerized silicone hydrogel are arranged in trays placed on a moving conveyor. The conveyor moves along a path of travel that extends into and through a heating chamber. The speed of the conveyor, the size of the chamber and the temperature control are all selected to provide a residence time and temperature conditions within the camber sufficient to drive off the organic diluent from the polymerized silicone hydrogel. The path of the conveyer subsequently carries the trays through a cooling zone and then through a treating zone where lenses are removed from the molds for further treatment.  
      The heating chamber can be horizontally oriented with the cooling zone located downstream along a horizontal path of travel. In an alternative arrangement the heating chamber is vertically oriented so heating occurs during ascent through the heating chamber and cooling occurs on the subsequent descent.  
      In one embodiment, a plurality of individual molds each containing a silicone hydrogel article are placed in trays which are then placed on the conveyor for travel through the heating and cooling chambers. An alternative is to pass individual molds continuously through the heating and cooling chambers.  
      Accordingly, the present invention may be characterized in one aspect thereof by a method for producing a shaped ophthalmic biomedical article blank such as a silicone hydrogel contact lens or the like comprising:  
      a) casting a curable liquid formulation including an organic diluent into a mold;  
      b) curing the liquid formulation within the mold to form the article blank;  
      c) conveying the article blank and the mold along a path of travel extending through a heating zone, the conveying allowing the article blank and the mold to remain in the heating zone for a time sufficient to drive off the organic diluent and produce a substantially diluent-free article blank within the mold; and  
      d) moving the article blank and the mold from the heating zone and along the path of travel through a subsequent treating zone to further treat the substantially diluent-free article blank within the mold.  
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       FIG. 1  is a schematic representation of the steps in the method of the present invention wherein the heating and cooling zones are horizontally oriented;  
       FIG. 2  is a schematic view showing another apparatus for implementing the method of the present invention wherein the heating and cooling zones are vertically oriented;  
       FIG. 3  is a schematic view of one embodiment of a vertically oriented heating chamber for practicing the method of the present invention;  
       FIG. 4  is a plan view of a portion of the Figure;  
       FIG. 5  is a view taken along lines  5 - 5  of  FIG. 4 ; and  
       FIG. 6  is a view taken along lines  6 - 6  of  FIG. 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to the drawings,  FIG. 1  shows the steps in the method of the present invention. As a first step a composition is prepared for casting an ophthalmic biomedical article wherein the composition contains an organic diluent.  
      In the context of the present invention the article is a silicone hydrogel lens. A composition and method for making a silicone hydrogel lens are known in the art and reference is made to the composition and method as described in U.S. Pat. No. 5,260,000, the disclosure of which is incorporated herein by reference. Briefly, and as stated in U.S. Pat. No. 5,260,000, a monomeric mixture is prepared comprising a silicone-containing monomer, a hydrophilic monomer and an organic diluent.  
      This mixture  10  as shown in  FIG. 1  is then charged into a mold  12  having the desired shape of the article to be formed, in this case a silicone hydrogel lens blank  14 . The mixture charged into the mold is cured as shown at  16 . Curing preferably involves exposure of the mixture in the mold to ultraviolet light to initiate the polymerization of the monomers in the mixture.  
      After cure is complete, the lens blanks still in the mold  12  is heated to drive off the organic diluent. In the embodiment of the invention as shown in  FIG. 1 , this involves placing a plurality of the molds into a holding tray  18 . In this respect upwards of 150 or more of the molds, each containing a single lens blank, are placed into each tray. The tray  18  then is placed onto a conveyor  20  that carries the tray along a horizontal path of travel first through a heating zone  22  and then a cooling zone  24 . While the Figure shows trays  18  being arranged in a single file on the conveyor, it should be appreciated that the conveyor can be wide enough to accommodate two or more trays side-by-side to increase the capacity and the production rate.  
      Progress of the tray  18  is continuous and other trays  18  are added to the conveyor as the mixture in additional individual molds  12  go through a curing step. The speed of the conveyor along its horizontal path of travel together is sufficient to allow each tray to remain in the zone  22  for a time sufficient to drive off the organic diluent from the cast lenses. In this respect, the size of the heating zone  22  and the temperature/air flow parameters within the heating zone are selected so these parameters together with the linear speed of the conveyor allow for the appropriate residence time.  
      After passage through the heating zone  22 , the conveyor carries the trays  18  through the cooling zone  24 . Upon passage thought the cooling zone, the trays are removed from the conveyor and emptied. Individual molds  12  then are treated to effect the removal of the cast article  14 .  
      In the embodiment of  FIG. 2  the conveyor is arranged to travel through a vertical path of travel. In this respect  FIG. 2  shows that a tray  18  containing a plurality of molds (not shown) enters the heating zone  22  at a low level and is placed on a shelf  26 . The ends of the shelf are attached to an endless drive chain or belt  28  that move the shelves vertically through the heating zone. At an upper level of the heating zone the endless belts start on a downward path that extends through a cooling zone  24 .  
      Suitable mechanisms for maintaining the trays horizontal during the transition from upward to downward travel (and from downward to upward travel) are known in the art. Trays  18 , upon reaching the bottom or end of the cooling zone are removed from the shelves are then transferred to a horizontal path for conveying to a station where the lenses are separated from the molds.  
       FIGS. 3-6  illustrate a still further vertically oriented embodiment of the invention.  FIG. 3  shows a heated chamber  30 . Within the heated chamber are two vertically oriented cylindrical shells. The shells are concentric and include an outer shell  32  and an inner shell  34 . The heating chamber includes various components, not shown, including heating elements, fans for convection heating, insulation to retain heat, filters for incoming air and provisions to exhaust waste air. It should be understood that the appropriate temperature controls also are provided.  
      Also associated with the heating chamber  30  are load/unload systems for moving molds to be heated into the heating chamber through a bottom located inlet  31  and out of the chamber through a top located outlet  33 . The load/unload systems are schematically represented in  FIG. 3  by an inlet conveyor  36  and an outlet conveyor  38  respectively.  
      The outer and inner shells  32 ,  34  are vertically oriented and one of the shells rotates about a vertical axis  40  relative to the other. For purposes of description the apparatus will be described as if the inner shell  34  rotates and the outer shell  32  is fixed.  
      Fixed to the outer shell  32  and extending inward from the inner surface of the outer shell is a helical rail  42 . As best seen in  FIGS. 4 and 6 , the rail  42  spans a portion of the space  44  between the inner and outer shells but does not extend to the inner shell. The rail spirals upwards along the outer shell with the helical pitch or spacing  46  between adjacent portions of the rail being constant ( FIG. 5 ).  
      Provided on the outer surface of the inner shell  34  are vertically oriented ribs  48 . These ribs are at equally spaced intervals around the periphery of the inner shell. As noted hereinabove, the helical rail  42  does not extend to the inner shell. Accordingly there is a space between the rail and the surface of the ribs so the ribs pass close to, but do not contact the rail.  
      In the context of one embodiment, the relationship between the diameter of a mold  12  and the space  44  between the outer and inner shells  32 ,  34  is critical. In this respect, and as best seen in  FIGS. 4 and 6 , the diameter of each mold  12  is less than the width of the distance across the space  44  but greater than the width of the helical rail so a mold will over hang the edge of the rail. The diameter of each mold also is greater than the distance between the outer shell  32  and the crown of the ribs  48 .  
      In operation, a first of the molds  12  that contains the cast article for heating is loaded through the bottom located inlet  31  and onto the lowermost portion of the helical rail  42 . As the inner cylindrical shell  34  rotates in a continuous or stepwise fashion, one of the vertical ribs  48  engages the mold on the rail and pushes the mold along the helical rail. Movement of the mold is constrained by the inner surface of the outer shell  32  so the mold is forced to move up the helical rail.  
      Now, when a second mold is loaded through the inlet  31  and onto the helical rail, the rotation or step wise indexing of the inner shell  34  brings a following rib  48  into engagement with the second mold and moves it along the helical rail. In this fashion molds are loaded sequentially onto the helical rail and are moved progressively, either continuously or step wise, upwardly along the helical rail and through the heating zone represented by the heating chamber  30 .  
      The angle of inclination of the helical rail  42 , the pitch  46  of the rail and the timing of the rotation of the inner shell  34  determine the overall time the molds  12  take to travel through the heating chamber. As with the previous embodiments, this time, considered the residence time of the molds within the heating chamber  30 , is sufficient to drive off the organic diluent component of the lens forming mixture. Upon leaving through the outlet  33  the heated molds are conveyed through a cooling zone after which the lens is separated from the mold for further operations.  
      In a typical batch operation of the prior art, two hours of production may involve the handling and processing of upwards of 1500 individual molds. With an apparatus as shown in  FIGS. 3-6 , the production volume becomes a function of mold diameter, mold height, the diameter of the stationary outer shell  32 , and the helical pitch of the rail  42 . Given current mold dimensions, the same two hours of batch operation can be accommodated in a chamber that has a mold center-to-mold center of 24 inches and that is 20 inches tall.  
      Accordingly, the method and apparatus of the present invention provides an improved method and apparatus for heating the silicone hydrogel lenses to drive off the organic diluent. The invention further provides an inventory management system that is a true first-in-first-out system as opposed to a batch operation and that has a larger through put than the conventional batch operation.  
      Accordingly, it should be appreciated that the present invention accomplishes its intended objects by providing a method of producing silicone hydrogel lenses that reduces the amount of manual handling as opposed to the current batch method. The present invention further provides a method and apparatus that allows for a first-in-first out product flow and that provides for the continuous movement of molds or batches of molds through a heating /cooling chamber.