Abstract:
The invention relates to a lens blank coming from a mold having a sheet insert placed therein prior to injection, the blank including peripheral ejector marks, the marks lying outside the insert.

Description:
[0001]     The present invention relates to a lens blank fabricated by injecting a thermoplastic material into a mold that is prefitted with an insert (generally a sheet of polarizing or photochromic or colored material that has been cut to shape and preformed). The invention also relates to a method of fabricating such a blank.  
       BACKGROUND OF THE INVENTION  
       [0002]     This type of component is known. It is used in particular for fabricating correcting lenses for ophthalmic eyeglasses or comfort lenses (for sunglasses, protective glasses, . . . ).  
         [0003]     Several fabrication techniques are known for this kind of component. By way of example, one of them is illustrated by patent document US 2004/0217495. In that document, a mold is used comprising two parts that open relative to each other, one of the parts defining a complex cavity in which a kind of spot face is provided for containing an insert in sheet form, such as a portion of polarized film, the spot face being defined by a plurality of mold portions that are adjustable relative to one another.  
         [0004]     The mold as disclosed therein would appear to consume a large quantity of injected material, much of which will constitute scrap when making a lens from the molded blank.  
         [0005]     In addition, the blank from that mold is not suitable for automatic handling, nor even for building up stacks since the surface presenting the polarized film projects from the blank as a whole, and is thus exposed to external aggression that might scratch the film.  
         [0006]     Industrializing the method disclosed in that document would appear to require equipment that is complicated, demanding incessant and accurate adjustments, thereby unfavorably influencing the cost price of the fabricated blank.  
       OBJECT OF THE INVENTION  
       [0007]     The present invention proposes a novel design for such a lens blank that makes it possible to implement a fabrication method that is much more simple and that serves to produce a component that is competitively priced.  
       SUMMARY OF THE INVENTION  
       [0008]     To this end, in a first aspect, the invention provides a lens blank coming from a mold having a sheet insert placed therein prior to injection, the blank including peripheral ejector marks, said marks lying outside the insert. This characteristic is the signature of the invention. As explained below, no portion of the insert, which is an expensive ingredient, is used for those portions of a blank that are designed to be destroyed while fabricating a lens for eyeglasses or for an eyepiece.  
         [0009]     In preferred manner, each mark is carried by a tab having extra thickness on the same side as the insert, and a recess in its other side, the recess forming an element for centering the spacer of an adjacent blank, which is superposed thereon. By means of these tabs, it is possible to stack the blanks on one another without the surface of an insert coming into contact with the blanks immediately below it in the stack, said surface then being protected from the external environment.  
         [0010]     In a second aspect, the invention provides a method of fabricating the above-specified blank by injection molding, using a mold that opens into two parts, one of the parts being provided with ejectors that are movable perpendicularly to the closure plane of the mold, which method consists, starting from the open mold, in: 
        placing the ejectors in the ejection position;     engaging the sheet insert between the ejectors and retaining it between them by elastic wedging;     closing the mold while retracting the ejectors into the corresponding part;     injecting a thermoplastic material into the mold;     opening the mold after the thermoplastic material has hardened;     placing the ejectors back in the ejection position; and     extracting the composite blank as obtained in this way.        
 
         [0018]     Naturally, in fabrication tooling, the opening mold will have a plurality of cavities, each being provided with at least three ejectors so as to be able to hold the insert stationary within each of the cavities by wedging.  
         [0019]     Other characteristics and advantages of the invention appear from the description given below of an embodiment thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Reference is made to the accompanying drawings, in which:  
         [0021]      FIG. 1  is a diagrammatic plan view of a lens blank in accordance with the invention;  
         [0022]      FIG. 2  is a section on one or other of the lines II-II of  FIG. 1 , through a stack of blanks in accordance with the invention; and  
         [0023]      FIGS. 3, 4 , and  5  show three stages in the method of making such blanks. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     With reference to  FIGS. 1 and 2 , it can be seen that a lens blank in accordance with the invention is composite, comprising a film  1  of polarizing material and a base  2  of transparent resin that may optionally be colored, or possibly even include photochromic pigments.  
         [0025]     The film  1  is cut out so as to present two rectilinear edges  3  and  4  and two curved end edges  5  and  6 . Outside the edges  3  and  4 , the blank has ejection marks  7 ,  8 ,  9 , and  10  where the ejectors have acted to extract the blank from the cavity of the fabrication mold, as explained in detail below.  
         [0026]     In the particular example shown in the figures, these ejection marks are in fact cylindrical tabs defined by a bottom surface  11  and a top surface  12  parallel thereto, with the bottom surface  11  being provided with extra thickness  13 , while the top surface  12  presents a recess  14 . The distance between the bottom of the recess  14  and the top of the extra thickness  13  is such that when two blanks are superposed, as shown in  FIG. 2 , there remains clearance  15  between the polarizing film  1  of an upper blank and the base  2  of the lower blank on which the upper blank is placed. The extra thickness  13  thus constitutes a spacer that penetrates into the inside of the recess  14  and that also constitutes means for centering two superposed blanks relative to each other.  
         [0027]     The mold  20  for making the blank of the invention is shown diagrammatically in  FIGS. 3, 4 , and  5 , comprising two parts  21  and  21  that are movable relative to each other between an open position as shown in  FIG. 3  and a closed position as shown in  FIG. 4 . When the mold is closed, it defines a join plane  23  and a cavity  24 .  
         [0028]     The bottom part  21  presents a bottom surface  25  of the cavity that is generally concave, while the top part  22  presents a top cavity surface  26  that is generally concave. The bottom part  21  is provided with four ejectors passing through the bottom surface  25 . Two of the ejectors  27  and  28  are shown in the figures.  
         [0029]     The method of the invention consists initially in moving the part  22  away, while the mold is open, so as to provide free access to the cavity in the bottom part  21 . The ejectors  27  and  28  are in their positions where they project above the surface  25  so as to constitute pillars between which it is possible to place a polarizing film  1  that has already been preshaped to have a surface that is substantially identical to one or other of the surfaces  25  or  26 , but of transverse dimension, i.e. the distance at rest between the longitudinal edges  3  and  4  of the polarizing film  1 , that is slightly greater than the distance between two ejectors such as  27  and  28 . It will thus be understood that prior to coming into contact with the surface  25 , the polarizing film can be held stationary by being wedged elastically (braced) between the ejectors. Three ejectors could suffice.  
         [0030]     With the film  1  put into place in this way, using any suitable robotic arm or mechanism, the mold is closed by bringing the mold part  22  over the part  21  and by causing their surfaces  23  to coincide so as to constitute the join plane of the mold. Simultaneously, the ejectors  27 ,  28  are retracted below the surface  25  as shown in  FIG. 4 . It can be seen that the top surfaces of the ejectors  27 ,  28  possess setbacks, while the bottom surface  26  of the top part  22  of the mold presents a substantially circular portion facing each ejector and presenting a projection. The cavity  24  is thus shaped in such a manner that in register with the ejectors  27 ,  28  it is possible to obtain the shapes of the tabs  7 ,  8 ,  9 , and  10  as described above.  
         [0031]     Thereafter the thermoplastic material for constituting the base  2  of the blank is injected. This injection takes place substantially parallel to the join plane  23  and in a direction perpendicular to the plane of FIGS.  3  to  5 , i.e. in the longitudinal direction of the insert  1 . The shape of the cavity  24  is such that the curvilinear edges  5  and  6  of the insert fit closely to the corresponding edges of the cavity  24 . In the example shown, the cavity  24  has a curvilinear edge identical to the edge  6  and a rectilinear edge tangential to the edge  5 . Under such conditions, the injected stream of thermoplastic material leads to practically no variation in the position of the insert  1  even though the ejectors  27  and  28  are retracted.  
         [0032]     Once the material has hardened, the mold is opened and the ejectors  27  and  28  are extended, as shown in  FIG. 5 . It is then possible to take charge of the blank of the invention by any appropriate robotic means.  
         [0033]     It will be understood that the design of the lens blank of the invention and its method of fabrication make it possible to simplify the shape of the mold cavity and thus the design of the mold, to save on polarizing material by ensuring that this material does not extend over the ejectors, but without that requiring films of polarizing material to be cut out in a complex manner, and finally, the invention makes it easier to store blanks made by the fabrication method of the invention, in particular by protecting the sensitive outside face of the polarizing film  1  from any aggression from the environment.