Abstract:
The case includes a middle part ( 3 ) and a back cover ( 5 ) delimiting a compartment ( 7 ) closed by a glass ( 10 ), made of a transparent material resistant to temperatures above 500° C., said glass being bonded or welded at joint zones ( 15 ) at least partially covering the shoulder ( 4 ) of the middle part ( 3 ). It is characterized in that the inner face ( 11 ) of the glass ( 10 ) includes an enamel deposition ( 20 ) formed on the zones of the joints ( 15 ) with the shoulder ( 4 ) of the middle part ( 3 ), either in a hollow ( 14 ), or on the surface of said inner face ( 11 ).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention concerns a sealed case closed by a bonded or welded glass, in an arrangement that enables the joint to be made invisible and/or a decorative effect to be obtained. The invention concerns more specifically the case of a timepiece wherein the glass, which forms the protective element for the subjacent displays, should not be detrimental to the overall aesthetic appearance, or conversely should participate in providing greater freedom as regards colours and hues.  
       BACKGROUND OF THE INVENTION  
       [0002]     When one wishes to assemble a glass on a case, in particular for edge-to-edge assembly, i.e. without any slot for the glass, and particularly when the case is made at least partially of ceramic material in order to obtain the broadest possible palette of colours, the usual technique consists in metallising the inner part of the glass in the zone where one wishes to form the joint, depositing a line of adhesive on the corresponding part of the case, then assembling the glass by pressure. The purpose of the metallising is primarily to conceal the line of adhesive, which may have, from the start, a more or less regular contour, making it visible to the naked eye. Metallising also enables the aging phenomena of the adhesive joint, such as shrinkage or yellowing, to be concealed. The method basically consists in depositing a primer layer, for example chromium, then by evaporating various metals (Cr, Ti, Pd, Au, . . . ) or alloys to obtain a large enough palette of shades that will all have a metallic hue. Metallising cannot provide matt non-metallic hues, or white, which has the drawback of limiting the choice of hues for the case or its external parts.  
         [0003]     This drawback is particularly important for cases including ceramic elements that can be manufactured in an infinite range of shades and hues but for which it is not always possible to find aesthetically appropriate metallising.  
       SUMMARY OF THE INVENTION  
       [0004]     It is therefore an object of the present invention to overcome the drawbacks of the aforecited prior art by providing a case on the aperture of which there is fixed a glass, whose joint with the shoulder of the aperture is not perceptible and which has the same hue as the case, or wherein, conversely, said joint provides an aesthetic appearance that can be altered.  
         [0005]     The invention therefore concerns a case including a middle part and a back cover delimiting a compartment formed by a glass made of a transparent material resistant to temperatures higher than 500° C., said glass being bonded or welded at joint zones at least partially covering the shoulder of the middle part, thus leaving at least one part of the middle part visible. The case is characterized in that the inner face of the glass includes an enamel deposition in the joint zones, enabling the glass to be assembled on the shoulder of the middle part by bonding, by applying a film or dots of adhesive, or by welding.  
         [0006]     The transparent material resistant to temperatures higher than 500° C. can be a mono or polycrystalline material, for example, quartz, spinelle or corundum, particularly sapphire. An amorphous material could also be used, such as a mineral glass provided that its softening point is higher than the temperature necessary for carrying out the enamel deposition.  
         [0007]     According to one aspect of the invention, it is possible to choose, within a large colour chart, an enamel hue that is perfectly suited to the hue of the middle part so as to make the hue of the joint zones indistinguishable from that of the middle part, which would not be possible with the prior metallising technique.  
         [0008]     According to another aspect of the invention, it is possible, conversely, via an appropriate choice of various enamel hues in the joint zones, and particularly when the latter are seporated, to have greater decorative freedom, as regards both hues and the shapes visible on the middle part.  
         [0009]     According to yet another aspect of the invention, deposition of an enamel layer on a glass made of a material of a close chemical nature allows very strong adhesion of said enamel layer, practically eliminating any risk of it being torn off after bonding or welding.  
         [0010]     According to yet another aspect of the invention, the material used to form the middle part is a ceramic material, i.e. a material having the same palette of hues and shades as enamel.  
         [0011]     According to yet another aspect of the invention, the compartment of the case contains a timepiece movement and a dial to form a timepiece, such as a wristwatch.  
         [0012]     According to yet another aspect of the invention, the methods implemented to carry out the enamel depositions on the joint zones allow said zones to be precisely delimited, and layers of enamel of different hues to be deposited selectively, if desired. The enamel depositions can be carried out in hollows arranged in the edge of the glass, with levelling being achieved by subsequent polishing. The enamel depositions can also be carried out on top, by direct deposition or by chemical structuring, particularly when the edge of the glass is thin.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     Other features and advantages of the present invention will appear in the following description illustrated by a wristwatch in several embodiments given by way of non-limiting illustration with reference to the annexed drawings, in which:  
         [0014]      FIG. 1  shows a top view of a wristwatch;  
         [0015]      FIG. 2  shows a semi cross-section along the line II-II of  FIG. 1 ;  
         [0016]      FIGS. 2A, 2B  and  2 C show the various steps of a first manufacturing method;  
         [0017]      FIG. 3  shows a semi cross-section along the line III-III of  FIG. 1 ;  
         [0018]      FIG. 4  shows in semi cross-section an alternative assembly of the glass on the case;  
         [0019]      FIGS. 4A and 4   b  show the various steps of a second manufacturing method;  
         [0020]      FIGS. 5A  to  5 G show the various steps of a third manufacturing method, and  
         [0021]      FIG. 6  shows a top view of a wristwatch made in accordance with a variant of the manufacturing methods. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     In the following description the case of a wristwatch has been used as an example case.  
         [0023]     Referring first of all to  FIGS. 1, 2  and  2 A to  2 C, a first embodiment according to a first method will be described hereinafter.  
         [0024]     The wristwatch shown in  FIGS. 1 and 2  includes in a known manner a case  1  formed of a middle part  3 , closed in its lower part by a back cover  5 , and in its higher part by a glass  10  delimiting a sealed compartment  7 . Compartment  7  houses a watch movement  8  for displaying time data on a dial  9 . In the example shown, the display is an analogue display by means on hour hand  9 a, a minute hand  9 b and a second hand  9 c.  
         [0025]     As can be seen, glass  10  entirely covers shoulder  4  of middle part  3  to which it is bonded or welded at joint zones  15  formed by an enamel deposition  20  formed on inner face  11  of glass  10  at its external edge  12 , in this example covering all of shoulder  4  of middle part  3 . The hue visible to an observer through glass  10  at shoulder  4  of middle part  3  is thus the hue of enamel layer  20 . When the middle part is made of ceramic material, or has an external part made of ceramic material, it is thus very easy to find exactly the same hue of enamel such that the joint zone is not perceptible to an observer.  
         [0026]      FIGS. 2A  to  2 C show schematically the manufacturing steps according to a first method, more particularly suited to a glass  10  with quite a thick external edge  12 . The method consists in mechanically structuring the joint zone.  
         [0027]     In a first step ( FIG. 2A ), a hollow  14  is made in the external edge  12  of glass  10  by means of a diamond grinding-wheel over a depth comprised for example between 0.1 and 0.5 mm.  
         [0028]     In a second step ( FIG. 2B ), an enamel deposition  20  is carried out in one or several steps via known dipping, buffing, spraying or screen printing techniques, each step being followed by firing to obtain a slightly greater thickness than the desired final thickness.  
         [0029]     In a third step ( FIG. 2C ), the glass is reworked and polished by conventional means so as to eliminate any surplus or overspill of enamel and to machine the sides to achieve a well defined geometry, as regards both the enamel filling and external edge  12 .  
         [0030]     In a last step, shown in  FIG. 2 , glass  10  is applied to shoulder  4  of middle part  3  with the insertion of a film of adhesive (not shown). If assembly by welding is chosen, the joint zone is metallised beforehand.  
         [0031]      FIG. 3  shows in cross-section a variant wherein the enamel deposition is only carried out above one part of shoulder  4  of middle part  3 , preferably in the form of a strip close to the edge of glass  10  in order to guarantee sealing. The parts not covered in enamel can be left as they are so that the hue of the middle part appears via transparency through the glass. These parts could, conversely, be subjected to metallising  17  for aesthetic purposes, after having formed the enamelled zones.  
         [0032]     Referring now to  FIGS. 4, 4A  and  4 B, a second embodiment will be described in accordance with a second method more particularly suited to a glass  10  whose external edge  12  is thin and in which a hollow cannot be machined. The addition of enamel  20  in joint zone  15  on inner face  11  of glass  10  can be carried out by a direct structured deposition, as shown in  FIGS. 4A and 4B . The deposition of the enamel powder can be carried out by conventional pad or screen printing techniques, in several steps with firing after each step, until the desired thickness is obtained, comprised for example between 0.1 and 0.5 mm. The glass is assembled on the middle part as previously by applying a film of adhesive on shoulder  4  of middle part  3 , or by welding.  
         [0033]     When the glass used is made of sapphire, it is also possible to carry out structuration by chemical means via photolithographic means of the type used for manufacturing MEMS on a glass substrate. The method, schematised in  FIGS. 5A  to  5 G basically consists in the following steps:  
         [0034]     depositing over the entire inner surface  11  of glass  10 , for example by screen printing, a layer of enamel  19  in one or several steps as previously indicated until a thickness, comprised for example between 0.1 and 0.5 mm, is obtained. This step may be followed by polishing or mechanical lapping to ensure a constant thickness ( FIG. 5A );  
         [0035]     depositing over the entire surface of enamel  19  a chromium and gold mask  21  via known vapour deposition or sputtering techniques ( FIG. 5B ), over a thickness of approximately 100 to 500 nm;  
         [0036]     depositing a uniform layer of photoresist  22  over the entire surface of mask  21  ( FIG. 5C );  
         [0037]     structuring mask  21  by UV radiation and removing the non-polymerised photoresist ( FIG. 5D );  
         [0038]     etching the enamel with HF or BHF acid ( FIG. 5E ), ensuring that the sapphire glass is not, or only very slightly etched;  
         [0039]     cleaning off the residual mask  21  above the zones including enamel deposition  20  provided for the joint zones ( FIG. 5F ); and  
         [0040]     if necessary after cleaning off mask  21 , stabilising the enamel deposition by reheating in a furnace ( FIG. 5G ).  
         [0041]      FIGS. 5A  to  5 G illustrate the example of a flat glass, but it is clear that the same method could be applied to a convex glass.  
         [0042]     This method has the advantage of giving great freedom as regards the shapes of the joint zones. Before the first step of the method, it is for example possible, as shown in  FIG. 6 , to carry out a partial metallisation  18  inside the part of glass  10  that has to be positioned above shoulder  4  of middle part  3 . Structuring then allows enamel deposition  20  to be carried out in the non-metallised parts.  
         [0043]     According a variant, by using transluced-enamal, the deposition can be carried out on the entire shoulder leaving the metalised parts to appear by transparency.  
         [0044]     In the example shown, the metallised parts form two threads giving an impression of continuity with the external edges of metal wristband strands  2   a ,  2   b , and the hue of the enamelled parts is in harmony with the ceramic wristband strands.  
         [0045]     The examples that have just been given could be subject to numerous variants available to those skilled in the art without departing from the scope of the invention. It is for example possible to alter the aesthetic appearance of the wristwatch at will by using several enamel hues for the joint zones. In the entire preceding description, the invention was illustrated by a glass mounted without a glass snap. It is clear that the technique according to the invention could also apply to a glass mounted in a conventional manner on a joint favouring the decorative appearance.  
         [0046]     Likewise, the invention is not limited to a timepiece: it could for example find application in the manufacture of the cover of a jewellery box.