Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to obtaining an optical lens from a polymerizable material, in particular, although not exclusively, an ophthalmic lens. 
     2. Description of the Related Art 
     It is known that it is possible to obtain a lens of this kind with a mold formed of two shells disposed at a distance from and facing each other and an annular seal cooperating with the periphery of the shells to define a molding cavity. The faces of the shells facing toward the cavity are generally concave and convex, respectively, and have a shape corresponding to the anterior face and the posterior face, respectively, of the lens to be obtained, in order for the anterior and posterior faces to have the required shape upon removal from the mold or after machining to finish them. 
     Diverse methods of obtaining an optical lens with a mold of this kind are known already, and in particular a method in which the first step is to place the molding shells on the seal, a spring clamp for retaining the shells is then fitted, the cavity is then filled manually using an injection nozzle that is inserted between the seal and one of the shells thanks to deformation of the seal and slight movement of the shell in question against the action of the clamp, filling continuing until overflowing occurs, in order to expel excess air, and the filled mold held in the assembled state by the clamp is then transported to a water bath in which the material introduced into the mold is polymerized by heat. 
     To enable the mold to accompany shrinkage of the material that it contains during polymerization, the seal is adapted to be compressed axially by the effect of the movement of the shells toward each other caused by the shrinkage. 
     There have already been proposed, in particular in U.S. Pat. No. 3,136,000 and in international application WO 00/30836, molds of the kind mentioned above in which the seal has respectively one lateral hole and several lateral holes, the polymerizable material being introduced into the cavity of the mold by means of a nozzle fitted into the filling hole in the manner of a syringe, the air initially present in the molding cavity escaping respectively via a passage separate from the nozzle and via at least one vent hole separate from the filling hole. After introduction of the polymerizable material into the mold, the single hole and the multiple holes are respectively plugged by a flap that returns to a position in front of the single hole during the removal of the nozzle and by plugs that are pushed into the multiple holes from the outside. 
     There has also been proposed, in particular in U.S. Pat. No. 4,251,474, for situations in which there is no hole in the seal, to dispose the mold horizontally, to introduce the filling nozzle between the seal and the upper shell and then, once the mold has been filled and the nozzle removed, to move the upper shell downward by means of an external mechanical force while bracing the lower shell, so that the excess portion of the material introduced into the mold gets past the lip of the seal cooperating with the upper shell and overflows out of the mold. When the upper shell is released, the seal tends to return to its original configuration, but because air is not able to penetrate into the molding cavity, each molding shell is held in contact with the seal in the manner of a sucker, so that there is no need to use the spring clamp referred to above, which could anyway provide only a modest spring force compared to the forces that may be produced mechanically. 
     SUMMARY OF THE INVENTION 
     The invention aims to improve the conditions of use of molds of the kind referred to above. 
     To this end it proposes a method of obtaining an optical lens from a polymerizable material, which method uses a mold that is formed of two facing and spaced shells and an annular seal comprising means for cooperating with the periphery of said shells to define a molding cavity, said seal being adapted to be compressed elastically when said shells are moved toward each other, which method comprises a step of filling said molding cavity with said polymerizable material, a step of polymerizing the material filling the molding cavity, a step of applying an external mechanical force to move said shells toward each other, and a step of releasing said force, and which method is characterized in that a filling hole is provided in said annular seal, away from said means for cooperating with the periphery of the shells, in that during said filling stage said material is introduced into the cavity through said filling hole, and in that said material is introduced after said step of applying a mechanical external force to move said shells toward each other and before said step of releasing said force. 
     Applying the external mechanical force for moving the shells toward each other before filling the mold, rather than after filling the mold, combined with filling through a hole in the seal away from the means with which the latter is provided to cooperate with the periphery of the shells, enables the mold to be filled without any leakage from the molding cavity between the shells and the seal. 
     This absence of leaks is particularly advantageous, not only because of the saving in polymerizable material resulting from the absence of wastage as a result of overflow around the shells, but also, and more importantly, because, it avoids the operations following removal from the mold that were previously associated with the overflow of material at the location of the means for cooperating with the periphery of the shells, which in practice comprise a lip with a groove on the outside; this avoids the need to work on the lens after its removal from the mold to eliminate flash and facilitates cleaning the seal before recycling. 
     According to preferred features, a plug is provided for plugging each hole in said seal and said filling step comprises, after introducing said material and before said step of releasing said force, a step of plugging each of said holes with one of said plugs. 
     Accordingly, when the force is released, there is a suction effect at the level of each shell whereby the mold remains in the assembled configuration of its own accord. 
     Said filling hole is preferably the only hole in said annular seal. 
     The operation of filling the mold is therefore particularly simple and convenient, since after the operation of introducing the material through the filling hole, a single operation is sufficient to fit a plug. 
     Said filling step is preferably effected with said mold placed vertically with said filling hole situated at the top of said seal. 
     The operation of introducing the polymerizable material into the mold may therefore be effected under excellent conditions, in particular where the evacuation of the air initially contained in the mold is concerned: the outlet of the filling hole being situated at the top of the cavity, it is certain that the air initially contained in the mold has been entirely evacuated when the filling hole itself begins to fill. 
     According to other features preferred for reasons of simplicity and convenience of implementation:
         a boss is provided around said filling hole and projects relative to the lateral surface of the seal; and/or   said filling hole is oriented in a radial direction; and/or   said filling hole is halfway or substantially halfway between said means adapted to cooperate with a first shell and with the second shell, respectively; and/or   said step of applying a mechanical external force to move said shells toward each other is adapted to move said shells toward each other until a predetermined force is reached; and optionally   said force to move said shells toward each other is produced by an actuator connected to a control center; and/or   during said step of applying a force to move said shells toward each other, said force is applied to said shells by way of respective sleeves having a diameter slightly less than that of said shells.       

     According to other features preferred for reasons of simplicity and convenience of implementation:
         a unit is provided for receiving said mold and said step of applying a force to move said shells toward each other and said step of releasing said force are effected by means of said receiving unit; and optionally   centering means and angular positioning means are provided in said unit for receiving the mold; and optionally   said centering means comprise two fingers oriented longitudinally and adapted to cooperate with the lateral surface of said seal and said angular positioning means comprise a fork adapted to cooperate with a boss projecting relative to the lateral surface of said seal; and/or   for effecting said filling step, there are provided a station for introducing said polymerizable material into the cavity of said mold and a station for plugging said filling hole, said introduction station and said plugging station being disposed side by side, and in that said unit for receiving the mold is movable between a first position in which it is vertically aligned with the plugging station and a second position in which it is vertically aligned with the introduction station.       

     According to other features preferred for the same reasons:
         said step of introducing the polymerizable material is effected by means of a station that comprises a nozzle for introducing said material and a nozzle for aspirating surplus material; and optionally   said nozzle for introducing said material is inclined; and/or   the distal end of said nozzle for introducing said material is below the distal end of said aspiration nozzle.       

     According to other features preferred for the same reasons:
         a plug is provided for plugging said filling hole and comprises a blind hole and a step of plugging said filling hole with said plug is effected at a plugging station comprising a canula adapted to be engaged in said blind hole of said plug; and optionally   said canula is connected to a vacuum system to hold said plug in place on said canula and said canula is vented to the atmosphere to release said plug; and/or   said plug is pressed into said filling hole by driving said plugging station.       

     According to other features preferred for the same reasons:
         there is provided a relatively narrow lug projecting relative to the lateral surface of said seal and having a transverse end surface incorporating a predetermined location relative to the remainder of the seal; and optionally   for effecting said filling step, there are provided a station for introducing said polymerizable material into the cavity of said mold and a station for plugging said filling hole, said introduction station and said plugging station being disposed side by side, a unit is provided for receiving said mold in a predetermined position and said receiving unit is mobile between a first position in which it is vertically aligned with the plugging station and a second position in which it is vertically aligned with the introduction station; and optionally   at least one optical cell is provided for detecting the position of the mold relative to said filling station or said plugging station and said unit for receiving the mold is placed in vertical alignment with said filling station or in vertical alignment with said plugging station in conjunction with said optical cell; and optionally   respective optical cells are provided for said filling station and said plugging station.       

     A second aspect of the invention provides the annular seal as explained hereinabove and a third aspect of the invention provides a device comprising means for implementing the method explained above. 
     It will be noted that the seal referred to above may be used under conditions other than those explained above and in particular to mold materials that are not subject to any shrinkage, in which case the capacity for axial deformation of the seal is not indispensable. 
     It will also be noted that it is beneficial for the material of the plug to be the same as the material of the rest of the seal, in particular for reasons connected with recycling, regardless of the material, and/or with the operating conditions, and that the same remark applies to detecting the position of the mold by means of the relatively narrow lug projecting from the external surface of the mold and at least one optical cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The explanation of the invention continues next with the description of one embodiment of the invention given hereinafter by way of nonlimiting example and with reference to the appended drawings. In the drawings: 
         FIG. 1  is an exploded perspective view of a mold for obtaining an optical lens in accordance with the invention; 
         FIG. 2  is a partial sectional view in perspective of the seal of the mold, without its plug, the section plane containing the axis of the seal and the axis of the filling hole; 
         FIG. 3  is a view similar to  FIG. 2  but with the plug fitted into the filler hole; 
         FIG. 4  is a plan view of the mold, without the plug, in position on the support fingers and the angular positioning fork of the receiving unit of the mold provided in the device shown diagrammatically in  FIG. 6 ; 
         FIG. 5  is an elevation view in section taken along the line V-V in  FIG. 4 ; 
         FIG. 6  is a diagrammatic elevation view of a device for obtaining an optical lens using the mold shown in  FIGS. 1 to 5 ; 
         FIG. 7  is a diagrammatic elevation view in section taken along the same line as in  FIGS. 2 and 3  of the mold in the assembled configuration without the plug of the seal; 
         FIG. 8  is a similar view of the mold in place in the receiving unit (the support fingers and the fork are not shown), this unit exerting on the mold an external mechanical force for moving the shells toward each other; 
         FIGS. 9 to 13  are similar partial views of the mold showing successive steps of the filling operation; and 
         FIG. 14  is a similar view of the filled mold which is self-supporting by virtue of the suction effect when it is extracted from the receiving unit of the device shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The mold  1  of which an exploded perspective view is shown in  FIG. 1  comprises two shells  2  and  3  and an annular seal  4  provided with a removable plug  5 . 
     The shells  2  and  3  are conventionally made from relatively thick glass with one concave face, one convex face and a circular contour edge. The active face  6  of the shell  2 , situated on the side facing the seal  4 , is concave and has a conformation corresponding to the anterior face of the lens to be obtained. The active face  7  of the shell  3 , also situated on the side facing the seal  4 , is convex and corresponds to the posterior face of the lens to be obtained. 
     The seal  4  is molded from a synthetic elastomer material. It has a globally annular conformation with an external belt  8  to the inside of which is attached a bead  9  that is narrower than the belt  8  and has a dovetail-shaped cross section whose narrower side is that by which the bead is attached to the belt  8 . 
     Thus there exists on either side of the bead  9  a recess adapted to receive one of the shells  2  and  3 , the peripheral surface of this recess being the internal surface  10  of the belt  8 , whose diameter corresponds to that of the shells  2  and  3 , while the shoulder surface of the recess is the flank surface  11  of the bead  9 , which is inclined outward and toward the center of the seal. Each edge  12  at the junction between a surface  11  and the inside peripheral surface  13  of the bead  9  thus forms the edge of a lip  14  adapted to cooperate sealing fashion with one of the shells  2  and  3 , as seen in  FIGS. 7 and 8 . 
     The external surface  15  of the seal  4  has a slight relief on either side of a central edge  16  that corresponds to the plane of the seal of the mold used to fabricate the seal  4 . 
     As seen in  FIGS. 4 to 6 , there projects from the lateral surface  15  a relatively narrow lug  17  that extends to only one side of the edge  16 , the lug  17  having a transverse end surface  18  in the same plane as the edge  16 . 
     The angular offset between the lug  17  and the hole  19  that is plugged by the plug  5  is such that, when the hole  19  is at the top of the seal  4 , as shown in the drawings, the lug  17  is visible above the seal  14 . 
     This is used to position the mold  1  and thus the hole  19  in the axial direction, as explained later. 
     The hole  19  is oriented in a radial direction and is centered in the plane that contains the edge  16 , half way or substantially half way between the two opposite recesses each delimited by the surfaces  10  and  11  of the seal  4  that cooperate with the shells  2  and  3 . 
     Around the hole  19  the seal  4  has a boss  20  projecting relative to the surface  15 . 
     Between its two outlets, onto the external surface of the boss  20  and onto the internal surface  13  of the bead  9 , respectively, the hole  19  has three sections  21 ,  22  and  23  with different diameters. 
     The section  21 , which is the narrowest, extends between a first end, at the surface  13 , and a second end at which it is joined to a first end of the section  22 , which has a larger diameter than the section  21 ; a slight chamfer is formed at the junction between the sections  21  and  22 , the latter extending as far as a second end at which it is joined to the section  23 , which has a greater diameter, the section  23  extending as far as the upper surface of the boss  20 . 
     Thus the sections  21 ,  22  and  23  of the hole  19  have a generally stepped arrangement. 
     The plug  5  is made from the same material as the seal  4 . It has a head  24  and a body  25 . The diameter of the head  24  corresponds to the diameter of the section  23 , the diameter of the body  25  corresponds to the diameter of the section  21 , and the length of the body  25  corresponds to the sum of the lengths of the section  21  and the section  22 . Accordingly, when the plug  5  is fitted into the hole  19 , as shown in  FIG. 3  in particular, the free end of the body  25  is flush with the surface  13  and the transverse surface of the head  24  flanking the body  25  bears on the shoulder surface of the section  23 . 
     The fit between the section  21  and the body  25  is moderately tight so that when the body  25  is in place in the section  21  the latter is plugged and sealed. Similarly, the fit between the head  24  and the section  23  is moderately tight, so that fitting of the head  24  into the section  23  plugs and seals the latter. 
     In the example shown, the diameter of the section  21  is of the order of 7.8 mm, that of the section  22  is of the order of 16.6 mm, and that of the section  23  is of the order of 17.8 mm, the length of the section  21  is of the order of 3 mm, that of the section  22  is of the order 3.7 mm, and that of the section  23  is of the order of 2.4 mm. 
     It will be noted when the plug  5  is in place in the hole  19 , there exists within the section  22 , around the body  25 , a sealed chamber  26  plugged on one side by the cooperation between the body  25  and the section  21  and on the other side by the cooperation between the head  24  and the section  23 . 
     Given the dimensions specified above, it should also be noted that the volume of the chamber  26  is greater than the volume of the section  21 . 
     As shown in  FIGS. 10 to 13 , which, like  FIGS. 6 to 9 , are highly diagrammatic, unlike  FIGS. 1 to 5 , the result of this is that if the molding cavity delimited by the shells  2  and  3  fitted into the seal  4  is filled until the polymerizable material reaches the junction between the sections  21  and  22  and the plug  5  is then fitted, the material contained in the section  21  may be accommodated in the chamber  26  and retained therein in a sealed manner when the plug  5  has been pushed all the way in, which prevents overflow of polymerizable material out of the mold. 
     To obtain an optical lens using the mold  1 , the shells  2  and  3  are fitted into the seal  4 , as shown in  FIG. 7  in particular, after which the mold  1  obtained in this way is placed in the filling device  30  shown in  FIG. 6 , to be more precise in a receiving unit  31  of the mold of the device. 
     To receive the mold  1 , the unit  31  comprises two fingers  32  and a fork  33 , all of which are oriented longitudinally. 
     As seen in  FIGS. 4 and 5  in particular, the mold  1  is disposed vertically in the unit  31  with the filling hole at the top of the seal  4 , the fingers  32  and the fork  33  being at angular positions substantially corresponding to 4 o&#39;clock, 8 o&#39;clock and 12 o&#39;clock on a clock face. The seal  4  simply rests on the fingers  32 , which center the seal  4  with respect to the unit  31 , while the boss  20  is engaged in the fork  33 , which positions the seal  4  angularly relative to the unit  31  and in particular ensures that the hole  19  is at the top of the seal  4 . 
     On each side of the location for the mold  1  the unit  31  comprises a respective sleeve  34  and  35 , seen in more detail in  FIG. 8  (in which the fingers  32  and the fork  33  are not shown). 
     The sleeve  34  is fixed directly to the frame  36  of the unit  31  whereas a hydraulic actuator  37  is provided between the sleeve  35  and the frame  36  whose body is fixed to the frame  36  and whose piston rod is fixed to the sleeve  35 . The chambers of the actuator  37  are connected by respective pipes  38  and  39  to a control center  40  which conventionally comprises a hydraulic pump and distribution and regulation means. 
     The unit  31  has a configuration for fitting and removing the mold  1  in which the piston rod  37  is retracted, i.e. a position in which the sleeve  35  is offset to the right relative to the position shown in  FIG. 8 . 
     With the unit  31  in this configuration the mold  1  is put into position by sliding it along the fingers  32  until the shell  2  comes into contact with the sleeve  34  and engaging the boss  20  in the fork  33 . 
     At the control center  40 , the operator then starts the automatic control means provided therein to deploy the piston rod  37  until the pressure in the pressurized chamber of the actuator, in this example the chamber on the right as seen in  FIG. 8 , reaches a predetermined pressure threshold, the control means provided in the control center  40  then holding the piston rod  37  in this position. 
     During the deployment of the piston rod  37 , the sleeve  35  comes into contact with the shell  3 , after which further movement of the piston rod applies a force moving the shells  2  and  3  toward each other, the effect of which is that the seal  4  is compressed elastically in the axial direction, in particular at the location of the lips  14 , as may be seen by comparing  FIGS. 7 and 8 . 
     This compression of the seal  4  by an external mechanical force provides an excellent seal between the seal  4  and the shells  2  and  3 . 
     In the example shown, the synthetic elastomer from which the seal  4  is made has, as measured by the test method NF T 46-002, a modulus of elasticity at 100% of the order of 0.8 to 1.3 MPa, a yield strength of the order of 3 to 7 MPa, and an elongation at the yield point of the order of 500 to 750%; its SHORE A (30 s) hardness, using the NF ISO 868 test method, is of the order of 30 to 49; the diameter of the actuator  37  is 40 mm and the predetermined pressure threshold at which the control means in the control center  40  deploy the piston rod  37  is of the order of 0.3 MPa. 
     The external mechanical force applied by the actuator  37  to the mold  1  via the sleeves  34  and  35  is therefore relatively high, of the order of 37.7 daN. 
     It will be noted that the contact surface between the sleeve  34  and the shell  2  is opposite the contact surface between the annular lip  14  and the shell  2  and likewise the contact surface between the sleeve  35  and the shell  3  is opposite the contact surface between the annular lip  14  and the shell  3 , which minimizes the risk of deformation of the shells  2  and  3 . 
     The structure and operation of the device  30  are described in more detail next. 
     The device  30  comprises, in addition to the unit  31 , a station  41  for introducing polymerizable liquid material and a plugging station  42 . 
     Each of the stations  41  and  42  is mobile in the vertical direction and adapted to be driven upward and downward, as shown by the double-headed arrows  43  and  44 , whereas the receiving unit  31  is mobile in the horizontal direction and is adapted to be driven toward the right and toward the left, as shown by the double-headed arrow  45 . 
     At each of the stations  41  and  42  there is a respective optical cell  46  and  47  for detecting the position of the mold  1  in the axial direction, each of the cells  46  and  47  comprising an emitter and a receiver of light beams such as the incident beam and reflected beams  47 ′ shown in part in  FIG. 5 . The incident beam  47 ′ is oriented vertically and when the mold  1  is under the cell emitting the beam, the latter is at the level of the surface  15 , and always at a distance from the latter less than the thickness of the lug  17 . When the mold is in a position such that the lug  17  is vertically in line with the incident beam  47 ′, the beam is reflected by the lug  17  and the cell that emitted the beam receives and detects a reflected beam. On the other hand, when the mold  1  is in a position in which the lug  17  is not vertically aligned with the incident beam  47 ′, no reflection of the beam occurs at the lug  17 , and there is therefore no reflected beam. 
     It is therefore possible, based on the information as to whether the respective cells  46  and  47  receive a reflected beam or not, to position the mold  1  very accurately with the surface  18  of the lug  17  that is aligned with the emitted beam, i.e. the surface  18  situated in the plane in which the hole  19  is centered, in a position in which the hole is centered with respect to the polymerizable liquid material introduction station  41  and the plugging station  42 , respectively. 
     The polymerizable liquid material introduction station  41  comprises two nozzles projecting downward, respectively a nozzle  48  for introducing the polymerizable material into the cavity of the mold  1  and a nozzle  49  for aspirating surplus material in the hole  19 . The nozzle  49  is in the transverse plane containing the beam emitted by the cell  46  and is centered with respect to the unit  31 . 
     The plugging station  42  comprises a canula  50  for receiving the plug  5  projecting downward, situated in the transverse plane that contains the beam emitted by the cell  47  and centered with respect to the unit  31 . 
     To cooperate with the canula  50 , the plug  5  comprises a blind hole  51  that is open at the end adjoining the head  24  and closed at the end adjoining the body  25 , the hole  51  having a diameter corresponding to the external diameter of the canula  50 , which is connected to a vacuum system and surrounded, at the end opposite its free end, by an abutment  52  forming a shoulder around the canula  50 . 
     When the unit  31  is in its configuration for fitting and removing the mold  1  its axial position is such that the mold is vertically aligned with the plugging station  42 , as shown in  FIG. 6 . 
     The operator places the mold  1  by hand onto the fingers  32  and the fork  33 , in the manner previously explained, and, for example using his other hand, threads the plug onto the canula  50 , the latter penetrating into the blind hole  51  until the face of the head  24  opposite the body  25  comes up against the abutment  52 , the plug being retained by aspiration by virtue of the fact that the canula  50  is at this time connected to a vacuum system. 
     As previously explained, using the control center  40 , the operator commands the deployment of the piston rod  37  until an external force of predetermined intensity is exerted that moves the shells  2  and  3  toward each other. 
     The unit  31  is then driven toward the polymerizable liquid material introduction station  41 , i.e. toward the left in  FIG. 6 , the drive means being controlled in conjunction with the cell  46  so that the mold  1  is positioned with the nozzle  49  centered relative to the hole  19 , in the manner explained above. 
     The polymerizable liquid material introduction station  41  is then driven downward until the distal ends of the nozzles  48  and  49  are inside the hole  19 , as shown in  FIG. 9 . 
     In the example shown, the distal end of the polymerizable material introduction nozzle  48  and the distal end of the surplus material aspiration nozzle  49  are in the section  22  of the hole  19  and in any event above and facing the section  21 , the distal end of the nozzle  49  being above that of the nozzle  48 . 
     Although the nozzle  49  is vertical, the nozzle  48  is slightly inclined so that the jet of polymerizable material that this nozzle emits is oriented obliquely to the section  21 , the jet in question passing across this section and then encountering the shell  3 , along which the polymerizable material flows as the cavity of the mold  1  is filled. 
     It will be noted that, in practice, to enable the jet of material to cross the section  21  in the manner just indicated the nozzle  48  is less inclined than in the highly diagrammatic  FIG. 9 . 
     The fact that the material introduced into the molding cavity runs along the wall of the shell  3  helps to produce a homogeneous lens and in particular avoids the formation of bubbles. 
     Introduction of the polymerizable material continues until the section  21  and a portion of the section  22  are filled with polymerizable material. 
     Given that the section  21  is at the top of the molding cavity, when this section is itself filled with polymerizable material it is certain that no air remains in the molding cavity. 
     The nozzle  49  then aspirates the overflowing polymerizable material. Its distal end is positioned relative to the top of the section  21  so that when aspiration ceases the level of polymerizable material is level with the top of the section  21 , as shown in  FIG. 10 . The height difference between the distal end of the nozzle  49  and the top of the section  21  corresponds to the height of the column of polymerizable liquid material that remains between the time at which the polymerizable material level falls below that of the distal end of the nozzle  49  and the time at which aspiration stops. 
     It will be noted that the surface of the polymerizable liquid material is shown perfectly horizontal in the diagrams of  FIGS. 10 and 11 , whereas in practice this surface is domed. 
     Once introduction of the polymerizable material has finished, i.e. when aspiration of the material by the nozzle  49  ceases, the station  41  is driven upward to return to its position as shown in  FIG. 6 , after which the receiving unit  31  of the mold  1  is driven horizontally toward the plugging station  42  until the hole  19  is positioned by the cell  47  and the lug  17 , in the manner previously explained, in a position in which the canula  50 , fitted with the plug  5 , is centered relative to the hole  19 . 
     The station  42  is then driven downward, so that the plug  5  progressively penetrates into the hole  19 , as shown in  FIGS. 11 to 13 . 
     When the body  25  of the plug penetrates into the section  21 , the polymerizable liquid material that was contained in this section progressively passes into the chamber  26  in which, as explained above, it is trapped in a sealed manner because of the plugging of the section  21  by the body  25  of the plug and the plugging of the section  23  by the head  24 . 
     It will be noted that expelling the polymerizable liquid material that was contained in the section  21  toward the chamber  26 , i.e. away from the molding cavity, makes certain that no air is introduced into the molding cavity on the occasion of fitting the plug. 
     While the body  25  is being pressed into the section  21 , the small section of the polymerizable liquid material between the body  25  and the perimeter of the section  21  causes a certain pressure rise in the molding cavity, but this dynamic pressure rise does not cause any leaks between the seal  4  and the shells  2  and  3  because of the external force applied by the sleeves  34  and  35 , which external force has been maintained throughout the filling step, as shown by arrows in  FIGS. 9 to 13 . 
     Once the fitting of the plug  5  into the hole  19  has finished, i.e. once the station  42  is at the end of its downward travel, the connection between the canula  50  and the vacuum system is broken and the canula  50  is vented to the atmosphere, or even subjected to a slightly increased pressure, which releases the plug  5  from the station  42 , which is then returned to its uppermost position shown in  FIG. 6 . 
     Using the control center  40 , the operator then causes the hydraulic fluid to flow in the pipes  38  and  39  in the direction that retracts the piston rod  37  into the body of the actuator, to retract the sleeve  35 , i.e. to move it toward the right in  FIG. 8 . 
     The releasing of the external force moving the shells  2  and  3  toward each other that occurs at the start of the retraction of the sleeve  35  has no effect on the positions of the shells  2  and  3  relative to the seal  4 , each of the shells  2  and  3  being held in place by a suction effect. 
     The mold  1 , filled with polymerizable material, therefore remains in the assembled configuration of its own accord, as shown in  FIG. 14 . The operator then removes the filled mold  1  from the unit  31 , or more generally from the device  30 , and it is then transported to a water bath in which it remains for the time required to polymerize the material in the molding cavity. 
     Once polymerization has been completed, the lens is removed from the mold by removing the seal  4  and then the shells  2  and  3 . 
     It will be noted that the lack of leaks between the shells  2  and  3  and the lips  14  means that there is no polymerized material between the surfaces  10  and  11  of the seal  4  and no intervention is required on the lens after removal from the mold to eliminate flash, and this also facilitates cleaning the seal to eliminate polymerized material before recycling. 
     In fact, the only portion of the seal  4  in which polymerized material remains is the chamber  26 , which is a very localized area that is relatively easy to deal with. 
     In a variant, not shown, only the cell  46  is provided: the position of the mold  1  in vertical alignment with the station  42  before the plugging operation is obtained by moving the unit  31  from the position in which the mold  1  is vertically aligned with the station  41  by the exact distance between the stations  41  and  42 , using a stepper motor. 
     In this variant, the mold  1  is placed on the unit  31  in a position between the positions in which the unit  31  is respectively vertically aligned with the station  41  and vertically aligned with the station  42 : this provides the operator with more space for threading the plug  5  onto the canula  50 . 
     In the example shown, the cells  46  and  47  are above the station  41  and the station  42 , respectively, but they may be located at any other location from where the lug  17  is visible. 
     In other variants, not shown, the filling hole comprises only the sections  21  and  22 , the head of the plug bearing directly on the outside lateral surface of the seal; there is no hole in the seal away from the means for cooperating with the molding shells, such as the shoulder delimited by the surfaces  10  and  11 , the double-acting hydraulic actuator  37  is replaced by some other type of actuator, for example a single-acting pneumatic actuator, and/or, more generally, a type of mechanism other than that shown in  FIG. 8  is provided for producing an external mechanical force for moving the shells toward each other, this mechanism possibly being hand operated, like a vice or a chuck. 
     Numerous variants are also possible in respect of the dimensions and the mechanical characteristics of the components shown. 
     More generally, it is pointed out that the invention is not limited to the examples described and shown.

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