Abstract:
A doctor blade support ( 2 ) for a machine for printing paste by silk screen printing, includes:
       a doctor blade ( 6 ) of substantially square cross-section which is to come into contact with the meshes of a silk screen printing screen in order to apply the paste to an element to be printed;   a first ( 12 ) and a second ( 14 ) clamping jaws suitable for clamping the doctor blade ( 6 );   at least two elements ( 20 ) for tightening the first clamping jaw ( 12 ) towards the second clamping jaw ( 14 ) in order to clamp or withdraw the doctor blade ( 6 );   characterized in that each tightening element ( 20 ) includes a resilient element ( 40 ) applying a force between the two clamping jaws ( 12, 14 ) in order to move them towards each other, and elements for regulating the deformation of the resilient element ( 40 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a doctor blade support used in silk screen printing machines. 
     In particular, the invention relates to a doctor blade support intended to print a silver- or aluminium-based conductor paste on a support, a substrate or a silicon wafer. 
     2. Description of the Related Art 
     Such printing machines are used in the field of micro-electronics to produce printed circuits and in the field of the manufacture of solar wafers for the production of solar panels. 
     Conductor pastes must be printed regularly at a constant thickness in order not to impair the electrical characteristics of the circuits or solar panels so formed. 
     In addition, the pattern to be printed must be reproduced with precision. 
     As a strong pressure is necessary to transfer the conductor paste from the screen to the element to be printed, the doctor blade support must have a high degree of rigidity. 
     In order to produce such printing, a doctor blade composed of synthetic elastomeric material is secured in a metal support. 
     The doctor blade has a rectangular parallelepipedal shape of substantially square cross-section. It is generally obtained by cutting a strip from a sheet of elastomeric material. This cutting operation brings about variations in the dimensioning of the square cross-section greater than one tenth of a millimeter. 
     For printing, the doctor blade support is displaced in accordance with a translational movement above a screen for silk screen printing placed above the element to be printed. A pressing force is exerted on the doctor blade during its displacement in order to force the conductor paste to pass through the open meshes of the screen in order to deposit a pattern to be printed on the element to be printed. 
     In order to print conductor pastes in a uniform manner, and with a high degree of precision, in particular the U.S. Pat. No. 3,482,819 has proposed a doctor blade support carrying a doctor blade of synthetic elastomeric material having a square cross-section, one of the corners of which forms the surface in contact with the screen. This doctor blade is clamped between two metal clamping jaws reproducing the shape of the square cross-section of the doctor blade. Only a small portion of the corner of the doctor blade projects relative to the two clamping jaws. Generally, the doctor blade is clamped in the clamping jaws in such a manner that one of the diagonals of the square cross-section of the doctor blade is in a vertical plane, so that the angle formed between the surface of the screen and one of the faces of the doctor blade is equal to 45°. 
     The document U.S. Pat. No. 4,122,771 proposes a variant of that doctor blade support in which the angle formed between the surface of the screen and one of the faces of the doctor blade is equal to 30°. 
     In the documents U.S. Pat. No. 3,482,819 and U.S. Pat. No. 4,122,771, the clamping jaws of the doctor blade supports are secured to each other by several screws aligned in the longitudinal direction of the clamping jaws. 
     The clamping jaws of the doctor blade supports may also be secured to each other by a single tightening handle as described, for example, in the document U.S. Pat. No. 5,458,060. 
     The doctor blade obtained does not always have a completely uniform cross-section along the entire parallelepiped, so that, after the tightening of the tightening handle described in the document U.S. Pat. No. 4,122,771, or even of the screws of the doctor blade supports described in the documents U.S. Pat. No. 3,482,819 and U.S. Pat. No. 4,122,771, the fine edge of the corner of the doctor blade is not completely flat and straight. It has protuberances at the sites where the area of the cross-section of the doctor blade is largest. For, at those sites, the force exerted by tightening has the effect of causing the fine edge of the corner of the doctor blade to bulge by compression of the elastomeric material. Of course, in order to avoid such a phenomenon, all that is required is to tighten to a slightly lesser extent at those sites. 
     In practice, the tightening is adjusted in accordance with the straightness of the fine edge of the corner of the doctor blade by placing the doctor blade on a surface plate before it is mounted in the machine. Such a regulation of the tightening may involve major time losses, the duration of this regulation being in accordance with the tolerances in the cross-section of the doctor blade. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a doctor blade support which receives doctor blades of not always uniform cross-section and which does not require such tedious regulation and which always enables a conductor paste to be printed regularly, precisely and at a constant thickness on the element to be printed. 
     To that end, the invention relates to a doctor blade support for a silk screen printing machine, comprising:
         a doctor blade of substantially square cross-section which is to come into contact with the meshes of a silk screen printing screen in order to apply the paste to an element to be printed;   a first and a second clamping jaws suitable for clamping the doctor blade;   at least two means for tightening the first clamping jaw towards the second clamping jaw in order to clamp or withdraw the doctor blade;       

     characterised in that each tightening means comprises a resilient element applying a force between the two clamping jaws in order to move them towards each other, and means for regulating the deformation of the resilient element. 
     According to particular embodiments, the doctor blade support comprises one or more of the following features, taken in isolation or in combination:
         the resilient elements of each tightening means have the same stiffness constant, so that the force exerted by the tightening means on the first and the second clamping jaws is constant for the same deformation of the resilient elements of each tightening means;   the resilient means are compression springs;   the length of all of the compression springs is equal in order to apply a constant force to the doctor blade along the entire length thereof;   the regulating means comprise a template;   each tightening means comprises:
           a through-opening formed in the first clamping jaw;   an at least partially threaded rod suitable for being secured to the second clamping jaw, the rod being moveable in the opening of the first clamping jaw; and   a nut screwed onto the threaded portion of the rod in order to move the second clamping jaw towards the first clamping jaw;   
           the nut is a self-locking nut;   the doctor blade has a substantially square cross-section, the first and the second clamping jaw comprising V-shaped grooves which are suitable for each receiving a portion of the doctor blade so that the diagonal of the square cross-section of the doctor blade extends substantially parallel with the major faces of the clamping jaws, the fine edge of the corner of the doctor blade coming into contact with the meshes of the screen in order to apply the conductor paste to the element to be printed;   the doctor blade support comprises means for loosening the clamping jaws which are separate from the means for tightening the clamping jaws,   the loosening means comprise a rod screwed into a threaded orifice of the first clamping jaw, the end of the rod being suitable for coming into abutment with a surface of the second clamping jaw in order to move the second clamping jaw away from the first clamping jaw, during the screwing of the rod into the threaded orifice of the first clamping jaw;   at least one portion of each of the edges of the mutually opposing faces of the first and the second clamping jaw is bevelled in order to enable the clamping jaws of the doctor blade support to move apart.       

     The invention relates also to a tool for loosening two clamping jaws of a doctor blade support, characterised in that the tool comprises:
         a profile suitable for carrying the doctor blade support;   at least one upright secured to the profile and extending perpendicularly to the major face of the profile;   at least one press-screw which has a pointed end and which is screwed into an opening in the upright and the purpose of which screw is to be inserted between the bevelled edge portions of the first and the second clamping jaw in order to move them away from each other.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood on reading the following description which is given purely by way of example with reference to the appended drawings, in which: 
         FIG. 1  is a perspective view of a doctor blade support secured to a carrier block thereof; 
         FIG. 2  is a perspective view of a doctor blade support according to a first embodiment of the invention; 
         FIG. 3  is a view, sectioned in accordance with the plane III.III, of the doctor blade support illustrated in  FIG. 2 ; 
         FIG. 4  is a view, sectioned in accordance with the plane IV.IV, of the doctor blade support illustrated in  FIG. 2 ; 
         FIG. 5  is a perspective view of a doctor blade support according to a second embodiment of the invention; 
         FIG. 6  is a perspective view of a tool for loosening the clamping jaws of the doctor blade support according to the second embodiment of the invention; 
         FIG. 7  is a top view of the loosening tool illustrated in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , the doctor blade support  2  according to the invention is secured to a carrier block  4  capable of being displaced vertically in order to press a doctor blade  6  against the meshwork  8  of a silk screen printing screen  10  in order to apply a conductor paste to a wafer or substrate generally composed of silicon. 
     The doctor blade  6  has a rectangular parallelepipedal shape of substantially square cross-section. 
     According to a first embodiment of the invention illustrated in  FIGS. 2 to 4 , the doctor blade support  2  is formed by a first  12  and a second  14  clamping jaw, and a doctor blade head  16  articulated to the first clamping jaw  12  by means of a hub  18 . The doctor blade head  16  is secured to the carrier block  4 . 
     The clamping jaws  12  and  14  have a generally rectangular parallelepipedal shape. They extend in a plane X-Z in  FIG. 2 . They are connected to each other by several regulatable tightening means  20 . 
     The clamping jaws  12  and  14  are suitable for clamping the doctor blade  6 . 
     A lug  22  projects in a central portion of the upper section of the first clamping jaw  12 . It is provided with a hole  24  through which the hub  18  extends in order to enable the clamping jaws  12 ,  14  to pivot relative to the doctor blade head  16 . 
     A groove  26  is formed in the faces  28 ,  30  of the mutually opposing clamping jaws. This groove  26  extends along the lower edge of the faces  28 ,  30  of the clamping jaws. 
     Each groove  26  is in the shape of a V inclined by an angle of 90° and having an opening of 90°. The V shape is open in the direction towards the opposing face of the other clamping jaw. The upper branch of this V shape has a slightly smaller length than the length of one side of the square cross-section of the doctor blade  6 . 
     The grooves  26  are suitable for each receiving a portion of the doctor blade  6 , so that the diagonal of the square cross-section of the doctor blade  6  extends parallel with the major faces of the clamping jaws  12 ,  14 . Thus, the fine edge of the corner  31  of the doctor blade  6  comes into contact with the meshes  8  of the screen in order to apply the conductor paste to the element to be printed. 
     The area of the square cross-section of the doctor blade  6  is slightly larger than the area of the opening formed by the grooves  26  which are to receive the doctor blade. 
     The first  12  and the second  14  clamping jaws comprise in their upper portion several, for example six, holes  32  and  34 , respectively, having common axes. 
     The holes  32  are tapped. The holes  34  are smooth and have a slightly larger diameter than the diameter of the screws  36  guided in the holes  34 . 
     Screws  36  are screwed into the openings  32 . 
     A self-locking nut  38  is screwed to the end of each screw  36 . The screws  36  and the nuts  38  form regulatable means  20  for tightening the first clamping jaw  12  against the second clamping jaw  14 . 
     Six compression springs  40  are mounted between the first clamping jaw  12  and each nut  38  of each tightening means  20 . 
     The spring  40  is suitable for transmitting the tightening force exerted by the nut  38  and by the head of the screw  36  in order to move the clamping jaws  12 ,  14  towards each other. The set of six springs  40  has the same stiffness constant k so that the length of each spring  40  is representative of the tightening force exerted on the doctor blade  6  by the nut  38  in contact with this spring. 
     In order to apply a constant tightening force to the doctor blade  6  by the various tightening means  20 , the compression of the six springs  40  must be identical. This compression can be measured by means of a template or a distance piece arranged between the nut  38  and the first clamping jaw  12 . 
     Thus, the springs  40  form a means of checking the force exerted by the tightening means  20  on the doctor blade  6  at the site where the tightening means is located. This force is practically constant, even if there is a variation in the thickness of the doctor blade at the site where the tightening means is located, since the force exerted on the doctor blade  6  is proportional to the length of the compressed spring. For example, a variation of one tenth of a millimeter in the length of the spring for a length of spring of 10 mm brings about a variation of less than 1% in the force exerted on the doctor blade. This variation is negligible because it cannot cause a deformation of the doctor blade. 
     Countersinking  42 , shown in  FIG. 3 , is produced around the hole  34  in the first clamping jaw  12  in order to receive a portion of the spring  40 . 
     The first  12  and the second  14  clamping jaws also comprise two tapped through-holes  44  in which are mounted two threaded rods  46  equipped at one of their ends with a wheel  48  and at the other end with an abutment element  50 . 
     A pin  51  is secured to the end of each threaded rod  46  near the abutment element  50 . 
     When a user causes the wheel  48  to pivot on itself, the abutment element  50  applies a force against the second clamping jaw  14  in order to move it away from the first clamping jaw  12  and thus, for example, to enable the doctor blade  6  to be changed. 
     In the embodiment illustrated in  FIGS. 2 and 4 , the pin  51  is accommodated in a recess  52  formed in the face  28  of the second clamping jaw. The pin  51  and the recess  52  have a slightly larger diameter than the diameter of the hole  44  and of the rod  46 , so that the loosening means  53  cannot be withdrawn from the first clamping jaw  12  and lost. 
     The threaded rod  46  provided with the wheel  48  and with the abutment element  50  constitutes a means  53  for loosening the clamping jaws and moving them away from each other. This loosening means  53  has no influence on the two clamping jaws  12 ,  14  at the moment when they are tightened. 
     Advantageously, the loosening means  53  is separate from the tightening means  20 . 
     In a variant, the hole  32  is not tapped, and the spring  40  is arranged between the screw head and the second clamping jaw  14 , the screw  36  being secured to the first clamping jaw  12 . 
     In another variant, the screw  36  is replaced by a threaded rod secured to the second clamping jaw  14 . 
     According to a third variant, the compression spring  40  is replaced by a leaf spring, a Belleville washer or a spring in the shape of a gendarme&#39;s two-pointed hat. 
     The doctor blade support  54  according to the second embodiment of the invention is illustrated in  FIG. 5 . The elements of the second embodiment of the doctor blade support that are identical to the elements of the first embodiment bear the same references and will not be described a second time. 
     The doctor blade support  54  comprises the same elements as the doctor blade support  2  according to the first embodiment except that the means  53  for loosening the clamping jaws is constituted by a separate tool, and that each of the edges  55 ,  56  of the mutually opposing faces  28 ,  30  of the first  12  and the second  14  clamping jaw is bevelled in order to enable the clamping jaws  12 ,  14  to move apart with the aid of a loosening tool  57  independent of the doctor blade support  54 . 
     The bevelled edges  55 ,  56  are located between the faces  28 ,  30  and the lateral sections of the clamping jaws  12 ,  14 . 
     In a variant, only a portion of the edges  55 ,  56  is bevelled. 
     The loosening tool  57  is used solely when the doctor blade  6  has to be changed. 
     Referring to  FIGS. 6 and 7 , the tool  57  for loosening the doctor blade support  54  comprises a support profile  58  on which two uprights  60 ,  62  are secured in such a manner as to be separated from each other by a distance corresponding to the length of the doctor blade support  54 . 
     The uprights  60 ,  62  extend in a direction perpendicular to the principal plane of the support profile  58 . 
     A press-screw  64  having a pointed end is screwed into a threaded opening  66  in the upright  60 . The screw  64  is to be inserted between the bevelled edges  55 ,  56  of the first  12  and the second  14  clamping jaws in order to move them away from each other. 
     The invention relates also to a method for tightening the doctor blade support according to the first and second embodiment of the invention. The method comprises the following steps:
         mounting the doctor blade  6  between the first  12  and the second  14  clamping jaws;   securing the threaded rod  36  to the second clamping jaw  14  for each tightening means  20 ;   screwing a nut  38  onto the end of each threaded rod  36  for each tightening means  20 ;   checking that the distance D between the nut  38  and the first clamping jaw  12  is identical for all of the tightening means  20 .       

     Advantageously, the fine edge of the corner of the doctor blade mounted in the doctor blade support according to the invention is planar and rectilinear, even when the doctor blade has a cross-section of non-uniform size along its entire length. 
     Also advantageously, the doctor blade support according to the invention is readily loosened either with the loosening means or with the independent loosening tool.