Abstract:
A printing screen unit ( 110 ), comprising: a printing screen ( 112 ), comprising a sheet, at least sections of at least one pair of opposite edges of which include one of engagement apertures ( 124 ) or engagement projections; and a supporting frame ( 114 ) including at least one pair of interface units ( 116, 117 ) which are attached to the at least one pair of opposite edges of the printing screen ( 112 ), wherein the interface units ( 116, 117 ) each comprise at least one coupling element ( 130 ) which includes the other of engagement apertures or engagement projections ( 158 ) for inter-engaging with the one of engagement apertures ( 124 ) or engagement projections at a respective one of the opposite edges of the printing screen ( 112 ), and an interface member ( 126 ) which includes a coupling slot ( 142 ) for captively receiving the at least one coupling element ( 130 ) therein.

Description:
This application is a national phase of International Application No. PCT/GB2007/000392 filed Feb. 6, 2007 and published in the English language. 
     The present invention relates to printing screens, often alternatively referred to as stencils or masks, for use in the screen printing of workpieces, typically electronic substrates, such as circuit boards and components, frames therefor, and printing screen units. 
     BACKGROUND 
     Traditionally, printing screens have been mounted in large rectangular frames by a flexible, perforate sheet, typically a woven mesh of polypropylene or stainless steel strands, which acts to tension the printing screen. 
     Whilst the above-described mounting system has been well used, the mounting system does exhibit a number of drawbacks, notably in not allowing the printing screen to be detached from the frame without permanently damaging the perforate sheet, which requires the printing screens to be maintained in the mounted state, and thus requires a large storage facility for storing a large number of mounted printing screens. 
     More recently, as disclosed in WO-A-1997/003833, a mounting system has been developed which enables the printing screen to be demounted after use, and thus enables the use of a single frame with a large number of printing screens, thereby obviating the problem of the earlier mounting system regarding storage. In this mounting system, the printing screen, which comprises a thin, stiff metal sheet, typically of stainless steel, includes a plurality of fixing apertures at the opposite edges thereof by which the printing screen is mounted to a frame by mechanical tensioning mechanisms which engage the fixing apertures in the printing screen and act to tension the printing screen, with the tensioning force typically being provided by hydraulic, pneumatic or spring means. 
     Whilst the above-described mounting system has also been well used, this mounting system does suffer from a number of drawbacks. In particular, as the printing screens are very thin, typically having a thickness of from 0.1 to 0.25 mm, the edges thereof are razor sharp, which can lead to personal injury to a user if not handled with the utmost care. 
     Still more recently, as disclosed in WO-A-2003/093012 and illustrated in  FIGS. 1 to 3 , a mounting system has been developed which enables a printing screen to be demounted after use, and also, in utilizing an attachment frame, typically in the form of extrusions, shields the edges of the printing screen and thus prevents the possibility of personal injury to a user as could be caused by the sharp edges of a printing screen. 
     The above-described mounting system has been widely accepted. However, this mounting system does suffer from the drawback of requiring the edges of the printing screen to be cut and formed, typically by bending, to allow for the fitting of the attachment frame. Such cutting and forming adds both to the time and the cost in fabricating the resulting printing screen units. 
     SUMMARY OF THE INVENTION 
     The present invention provides a printing screen unit which at least partially overcomes the above-mentioned problems associated with existing printing screen units, and printing screens and frames for such printing screen units. 
     In one aspect the present invention provides a printing screen unit, comprising: a printing screen, comprising a sheet, at least sections of at least one pair of opposite edges of which include one of engagement apertures or engagement projections; and a supporting frame including at least one pair of interface units which are attached to the at least one pair of opposite edges of the printing screen, wherein the interface units each comprise at least one coupling element which includes the other of engagement apertures or engagement projections for inter-engaging with the one of engagement apertures or engagement projections at a respective one of the opposite edges of the printing screen, and an interface member which includes a coupling slot for captively receiving the at least one coupling element therein. 
     In one embodiment at least ones of the engagement apertures and engagement projections are circular in shape. 
     In one embodiment the at least one pair of opposite edges of the printing screen include engagement apertures and the at least one coupling element of each interface unit includes engagement projections for engagement with the engagement apertures in the respective edges of the printing screen. 
     In one embodiment at least sections of the at least one pair of opposite edges of the printing screen include a plurality of rows of one of engagement apertures or engagement projections which are disposed one inwardly of the other relative to the respective edge of the printing screen and the coupling elements of the interface units include a plurality of rows of the other of engagement apertures or engagement projections which correspond to the rows of the one of engagement apertures or engagement projections in the respective edges of the printing screen. 
     In one embodiment the rows of the one of engagement apertures or engagement projections in each edge of the printing screen are located in offset relation along a length of the respective edge of the printing screen and the rows of the other of engagement apertures or engagement projections on each coupling element are located in a counterpart offset relation. 
     In one embodiment the interface member includes a coupling section which comprises first and second body parts which together define the coupling slot. 
     In one embodiment the coupling slot comprises a support slot part which supports the at least one coupling element and includes an engagement surface at an inner edge thereof, against which the at least one coupling element is engaged when a tensioning force is applied to the interface member. 
     In one embodiment the coupling slot comprises a recessed slot part which includes at least one recess into which extend the distal ends of the engagement projections, such as to retain the printing screen captively thereon. 
     In one embodiment the supporting slot part and the recessed slot part are disposed in opposed relation. 
     In one embodiment the interface member includes an engagement section which, relative to the printing screen, is disposed outwardly of the coupling slot and provides for engagement to a tensioning mechanism. 
     In one embodiment the supporting frame includes one pair of interface units which are attached to one pair of opposite edges of the printing screen. 
     In another embodiment the printing screen has two pairs of opposite edges, at least sections of which include the one of engagement apertures or engagement projections, and the supporting frame comprises first and second pairs of interface units which are attached to the respective pairs of opposite edges of the printing screen. 
     In one embodiment the supporting frame further comprises corner pieces which are disposed adjacent ends of the interface units. 
     In one embodiment the corner pieces couple adjacent ends of the interface units. 
     In one embodiment the corner pieces are parts separate to the interface units. 
     In another embodiment the corner pieces are integral parts of at least ones of the interface units. 
     In one embodiment the interface members of one of the pairs of interface units are longer than the interface members of the other of the pairs of interface units, such that the ends of the interface members of the one of the pairs of interface units overlap the adjacent ends of the interface members of the other of the pairs of interface units. 
     In one embodiment the corner pieces are parts separate to the interface units and are attached to the ends of the interface members of the one of the pairs of interface units. 
     In another embodiment the corner pieces are defined by the ends of the interface members of the one of the pairs of interface units. 
     In one embodiment each coupling element comprises an elongate strip which includes the other of engagement apertures or engagement projections thereon. 
     In one embodiment at least ones of the interface units comprise a plurality of coupling elements. 
     In one embodiment the coupling elements are disposed in end-to-end relation along a length of the interface member. 
     In another embodiment the coupling elements are disposed in spaced relation along a length of the interface member. 
     In one embodiment the supporting frame provides for relative movement of the interface members on being tensioned by a tensioning mechanism. 
     In one embodiment the printing screen includes a pattern of printing apertures which define a pattern of deposits to be printed thereby. 
     In another embodiment the printing screen is a blank and includes no printing apertures therein. 
     In one embodiment the printing screen comprises a metal sheet. 
     In another embodiment the printing screen comprises a plastics sheet. 
     In another aspect the present invention provides a supporting frame for supporting a printing screen, comprising a sheet, at least sections of at least one pair of opposite edges of which include one of engagement apertures or engagement projections, the supporting frame comprising: at least one pair of interface units which are attachable to the at least one pair of opposite edges of the printing screen, wherein the interface units each comprise at least one coupling element which includes the other of engagement apertures or engagement projections for inter-engaging with the one of engagement apertures or engagement projections at a respective one of the opposite edges of the printing screen, and an interface member which includes a coupling slot for captively receiving the at least one coupling element therein. 
     In one embodiment at least ones of the engagement apertures and engagement projections are circular in shape. 
     In one embodiment the at least one pair of opposite edges of the printing screen include engagement apertures and the at least one coupling element of each interface unit includes engagement projections for engaging the engagement apertures in the edges of the printing screen. 
     In one embodiment at least sections of the at least one pair of opposite edges of the printing screen include a plurality of rows of one of engagement apertures or engagement projections which are disposed one inwardly of the other relative to the respective edge of the printing screen and the coupling elements of the interface units include a plurality of rows of the other of engagement apertures or engagement projections which correspond to the rows of the one of engagement apertures or engagement projections in the respective edges of the printing screen. 
     In one embodiment the rows of the one of engagement apertures or engagement projections in each edge of the printing screen are located in offset relation along a length of the respective edge of the printing screen and the rows of the other of engagement apertures or engagement projections on each coupling element are located in a counterpart offset relation. 
     In one embodiment the interface member includes a coupling section which comprises first and second body parts which together define the coupling slot. 
     In one embodiment the coupling slot comprises a support slot part which supports the at least one coupling element and includes an engagement surface at an inner edge thereof, against which the at least one coupling element is engaged when a tensioning force is applied to the interface member. 
     In one embodiment the coupling slot comprises a recessed slot part which includes at least one recess into which extend the distal ends of the engagement projections, such as to retain the printing screen captively thereon. 
     In one embodiment the supporting slot part and the recessed slot part are disposed in opposed relation. 
     In one embodiment the interface member includes an engagement section which is disposed outwardly of the coupling slot and provides for engagement to a tensioning mechanism. 
     In one embodiment the supporting frame includes: one pair of interface units which are in use attached to one pair of opposite edges of the printing screen. 
     In one embodiment the printing screen has two pairs of opposite edges, at least sections of which include the one of engagement apertures or engagement projections, and comprises: first and second pairs of interface units which are in use attached to the respective pairs of opposite edges of the printing screen. 
     In one embodiment the supporting frame further comprises: corner pieces which are disposed adjacent ends of the interface units. 
     In one embodiment the corner pieces couple adjacent ends of the interface units. 
     In one embodiment the corner pieces are parts separate to the interface units. 
     In another embodiment the corner pieces are integral parts of at least ones of the interface units. 
     In one embodiment the interface members of one of the pairs of interface units are longer than the interface members of the other of the pairs of interface units, such that the ends of the interface members of the one of the pairs of interface units overlap the adjacent ends of the interface members of the other of the pairs of interface units. 
     In one embodiment the corner pieces are parts separate to the interface units and are attached to the ends of the interface members of the one of the pairs of interface units. 
     In another embodiment the corner pieces are defined by the ends of the interface members of the one of the pairs of interface units. 
     In one embodiment each coupling element comprises an elongate strip which includes the other of the engagement apertures or engagement projections thereon. 
     In one embodiment at least ones of the interface units comprise a plurality of coupling elements. 
     In one embodiment the coupling elements are disposed in end-to-end relation along a length of the interface member. 
     In another embodiment the coupling elements are disposed in spaced relation along a length of the interface member. 
     In one embodiment the supporting frame is configured to provide for relative movement of the interface members on being tensioned by a tensioning mechanism. 
     In a further aspect the present invention provides a printing screen, comprising a sheet, at least sections of at least one pair of opposite edges of which include one of engagement apertures or engagement projections. 
     In one embodiment at least ones of the engagement apertures or engagement projections are circular in shape. 
     In one embodiment the at least one pair of opposite edges of the printing screen include engagement apertures. 
     In one embodiment at least sections of the at least one pair of opposite edges of the printing screen include a plurality of rows of one of engagement apertures or engagement projections which are disposed one inwardly of the other relative to the respective edge of the printing screen. 
     In one embodiment the rows of the one of engagement apertures or engagement projections in each edge of the printing screen are located in offset relation along a length of the respective edge of the printing screen. 
     In one embodiment the printing screen has two pairs of opposite edges, at least sections of which include the one of engagement apertures or engagement projections. 
     In one embodiment the printing screen includes a pattern of printing apertures which define a pattern of deposits to be printed thereby. 
     In another embodiment the printing screen is a blank and includes no printing apertures therein. 
     In one embodiment the printing screen comprises a metal sheet. 
     In another embodiment the printing screen comprises a plastics sheet. 
     The present invention provides an improved printing screen unit which is compatible with existing support frame systems, with the interface members protecting edges of the printing screen from damage during handling and mounting, thereby increasing the life of the printing screen, and the sharp edges of the printing screen being enclosed, making the printing screen safer to handle. 
     Furthermore, the attachment of a printing screen to interface members further simplifies the manufacture of the printing screen unit as a whole, in obviating the need for an adhesive bond. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a plan view of a prior art printing screen unit; 
         FIG. 2  illustrates an exploded perspective view from above of one corner region (region A in  FIG. 1 ) of the printing screen unit of  FIG. 1 ; 
         FIG. 3  illustrates a vertical sectional view (along section I-I in  FIG. 1 ) through an edge region of the printing screen unit of  FIG. 1 ; 
         FIG. 4  illustrates a plan view of a printing screen unit in accordance with a first embodiment of the present invention; 
         FIG. 5  illustrates an exploded perspective view from below of one corner region (region B in  FIG. 4 ) of the printing screen unit of  FIG. 4 ; 
         FIG. 6  illustrates a vertical sectional view (section II-II in  FIG. 4 ) through an edge region of the printing screen unit of  FIG. 4 ; 
         FIG. 7  illustrates a plan view of a printing screen unit in accordance with a second embodiment of the present invention; 
         FIG. 8  illustrates a vertical sectional view (along section III-III in  FIG. 7 ) of the printing screen unit of  FIG. 7 ; 
         FIG. 9  illustrates a plan view of a printing screen unit in accordance with a third embodiment of the present invention; 
         FIG. 10  illustrates a vertical sectional view (along section IV-IV in  FIG. 9 ) of the printing screen unit of  FIG. 9 ; 
         FIG. 11  illustrates a vertical sectional view (along section IV-IV in  FIG. 9 ) of one modification of the printing screen unit of  FIG. 9 ; 
         FIG. 12  illustrates a plan view of a printing screen unit in accordance with a fourth embodiment of the present invention; 
         FIG. 13  illustrates a vertical sectional view (along section V-V in  FIG. 12 ) of the printing screen unit of  FIG. 12 ; and 
         FIG. 14  illustrates an exploded perspective view from above of one corner region (region A in  FIG. 1 ) of one modification of the printing screen unit of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 3  illustrate a prior art printing screen unit  10 , as disclosed in WO-A-2003/093012. 
     The printing screen unit  10  comprises a printing screen  12 , in this embodiment of rectangular shape, and a supporting frame  14  for supporting the printing screen  12 , which comprises first and second pairs of interface members  16 ,  16 ,  17 ,  17  which are attached to the respective pairs of opposite edges of the printing screen  12  and first to fourth corner pieces  18   a - d  which couple the respective ones of the interface members  16 ,  16 ,  17 ,  17 . 
     The printing screen  12  includes a pattern of printing apertures  20  which define the pattern of deposits to be printed. The pattern of printing apertures  20  are typically formed by laser cutting or etching. The printing screen  12  typically comprises a sheet of a metal, such as stainless steel, or a plastic. 
     The edges of the printing screen  12  to which tensioning mechanisms are attached each comprise a plurality of spaced projections  22  along the length thereof which define a castellated structure, and are folded to define attachment elements  24 . The edges of the printing screen  12  are each folded along a line intersecting the projections  22 , such that the projections  22  define a hook element and the attachment elements  24 , as defined thereby, enclose an acute angle with the main body of the printing screen  12 . 
     The interface members  16 ,  16 ,  17 ,  17  each include an attachment section  26  which defines an attachment slot  32  which extends along the length thereof and in which a respective one of the attachment elements  24  of the printing screen  12  is located. 
     The interface members  16 ,  16 ,  17 ,  17  each further include an engagement section  40  which, relative to the printing screen  12 , is disposed outwardly of the attachment section  26  and provides for engagement to a tensioning mechanism. 
     The engagement section  40  includes an inwardly-facing engagement surface  42  which extends along the length of the respective interface member  16 ,  16 ,  17 ,  17  and defines a hook arrangement which provides for captive engagement with a tensioning mechanism. 
     The interface members  16 ,  16 ,  17 ,  17  each further include a recess  44 , in this embodiment of substantially rectangular section, which extends to the distal ends thereof for receiving the tongue elements  46 ,  48  of respective corner pieces  18   a - d , as will be described in more detail hereinbelow. 
     The corner members  18   a - d  each include first and second tongue elements  46 ,  48 , which extend in orthogonal directions, for fitting in respective ones of the recesses  44  in the distal ends of the adjacent interface members  16 ,  16 ,  17 ,  17 . 
     In this embodiment the tongue elements  46 ,  48  of the corner members  18   a - d  are configured such as to be a tight fit in the recesses  44  in the distal ends of interface members  16 ,  16 ,  17 ,  17 , but such as to allow for movement of the tongue elements  46 ,  48  in the respective recesses  44  in the direction of a tension as applied to the printing screen  12 . With this configuration, the interface members  16 ,  16 ,  17 ,  17  present a substantially rigid frame when handled, but allow for relative movement of respective opposite pairs of the interface members  16 ,  16 ,  17 ,  17  on being tensioned by tensioning mechanisms. Also, in allowing for relative movement of the interface members  16 ,  16 ,  17 ,  17 , design tolerances can be accommodated in order to provide for uniform tensioning of the printing screen  12 . 
       FIGS. 4 to 6  illustrate a printing screen unit  110  in accordance with a first embodiment of the present invention. 
     The printing screen unit  110  comprises a printing screen  112 , in this embodiment of rectangular shape, and a supporting frame  114  for supporting the printing screen  112 , which comprises first and second pairs of interface units  116 ,  116 ,  117 ,  117  which are attached to the respective pairs of opposite edges of the printing screen  112  and first to fourth corner pieces  118   a - d  which couple the respective ones of the interface units  116 ,  116 ,  117 ,  117 . 
     The printing screen  112  includes a pattern of printing apertures  120  which define the pattern of deposits to be printed. The pattern of printing apertures  120  can be formed, for example, by laser cutting or etching, as is well known in the art. In preferred embodiments the printing screen  112  comprises a sheet of a metal, such as stainless steel, or a plastic. 
     The edges of the printing screen  112  to which tensioning mechanisms are attached each comprise a plurality of engagement apertures  124   a  and  124   b  (herein also collectively referenced as  124 ) along the length thereof which define an apertured engagement structure, which is engaged by at least one coupling element  130  of a respective one of the interface units  116 ,  116 ,  117 ,  117 , such as to couple the respective edge of the printing screen  112  to the respective one of the interface units  116 ,  116 ,  117 ,  117 , as will be described in more detail hereinbelow. 
     In this embodiment the engagement apertures  124  are circular in shape. The engagement apertures  124  could, however, have any shape, for, example, square, triangular, teardrop or elongate. 
     In this embodiment the edges of the printing screen  112  each include first and second rows of engagement apertures  124   a, b , which are disposed one inwardly of the other and are located in offset relation along the length of the respective edge of the printing screen  112 . 
     In this embodiment the engagement apertures  124  are formed by etching (chemical milling). In other embodiments the engagement apertures  124  could be formed using a press tool or laser. In further embodiments the engagement apertures  124  could be formed by electro-discharge machining (EDM) or water jet milling, which fabrication techniques would allow several printing screens  112  to be fabricated simultaneously. In still another embodiment the printing screen  112  could be formed by electroforming, and the engagement apertures  124  would be formed when forming the printing screen  112 . 
     In one embodiment the engagement apertures  124  can be formed simultaneously with the pattern of printing apertures  120 . 
     The interface units  116 ,  116 ,  117 ,  117  each comprise an interface member  126  which provides for engagement to a tensioning mechanism, in this embodiment an elongate member which extends along the length of the respective edge of the printing screen  112 , and at least one coupling element  130  which couples the interface member  126  to the respective edge of the printing screen  112 . 
     The interface member  126  includes a coupling section  136  which comprises a first, lower body part  138  and a second, upper body part  140 , which together define a coupling slot  142  which extends along the length thereof and in which the at least one coupling element  130  and a respective one of the edges of the printing screen  112  are located, such that the respective edge of the printing screen  112  is held captive in the coupling section  136  by the at least one coupling element  130 . 
     In this embodiment the coupling slot  142  comprises a first, support slot part  144 , here including a flat supporting section, which supports the at least one coupling element  130  and includes an engagement surface  146  at an inner edge thereof, against which the at least one coupling element  130  is engaged when a tensioning force is applied to the interface member  126 . 
     In this embodiment the coupling slot  142  further comprises a second slot part  148  which includes at least one recess, here as defined by first and second parallel slots  150   a, b , which are in registration with the first and second rows of engagement apertures  124   a, b  in each of the edges of the printing screen  112 . 
     The interface member  126  further includes an engagement section  152  which, relative to the printing screen  112 , is disposed outwardly of the coupling section  136  and provides for engagement to a tensioning mechanism. 
     The engagement section  152  includes an inwardly-facing engagement surface  154 , in this embodiment defined by a continuous surface, which extends along the length of the interface member  126 . The engagement surface  154  is outwardly inclined from the free edge thereof relative to the printing screen  112 , such as to define a hook arrangement which provides for captive engagement with a tensioning mechanism. 
     As will be appreciated, the engagement section  152 , in being defined by a continuous surface, allows for fabrication by a wide variety of manufacturing techniques, such as machining, routing and extrusion. 
     The interface member  126  further includes recesses  156 , in this embodiment of substantially rectangular section, at the distal ends thereof for receiving the tongue elements  166 ,  170  of respective corner pieces  118   a - d , as will be described in more detail hereinbelow. 
     In this embodiment the coupling elements  130  each comprise an elongate body  157 , here in the form of an elongate bar or strip of rectangular section, which is configured to fit in the support slot part  144  of the coupling slot  142  in a respective one of the interface members  126 , and a plurality of engagement projections  158   a  and  158   b  (herein also collectively referenced as  158 ), here in the form of projecting studs, along the length thereof, which define a projecting engagement structure and are configured to engage the counterpart engagement apertures  124  in a respective one of the edges of the printing screen  112 , as described hereinabove. 
     In this embodiment the engagement projections  158  are circular in shape, and have a shape and configuration such as to be a close fit in the counterpart engagement apertures  124  in the printing screen  112 . The engagement projections  158  could, however, have any shape, for, example, square, triangular, teardrop or elongate, in the same manner as the engagement apertures  124  in the printing screen  112 . 
     In this embodiment the length of at least ones of the engagement projections  158  is such that the distal ends of those engagement projections  158  extend into the at least one recess  150  in the second slot part  148  of the coupling slot  142  of the respective interface member  126  when the at least one coupling element  130  is fitted in the first slot part  144  of the coupling slot  142  of the respective interface member  126 . 
     In this embodiment the coupling elements  130  each include first and second rows of engagement projections  158   a, b , which are disposed one inwardly of the other and are located in offset relation along the length of the coupling element  130 . 
     In this embodiment the interface units  116 ,  116 ,  117 ,  117  each include a single coupling element  130  which extends along the length of the respective interface member  126 , or at least substantially the entire length thereof. 
     In an alternative embodiment the interface units  116 ,  116 ,  117 ,  117  could each include a plurality of coupling elements  130  which together extend along the length of the respective interface member  126 , or at least substantially the entire length thereof. In one embodiment the ends of the adjacent coupling elements  130  could be disposed in juxtaposed, end-to-end relation. In another embodiment the ends of the adjacent coupling elements  130  could be spaced one from the other. With these configurations, coupling elements  130  of a stock set of lengths can be combined to accommodate interface members  126  of different length. 
     In this embodiment the corner pieces  118   a - d  each comprise first and second coupling members  159 ,  160 , which lock together to define the respective corner pieces  118   a - d.    
     In this embodiment the first coupling member  159  comprises a body element  162  which includes a slot  164 , here of the same rectangular section as the recesses  156  in the distal ends of the interface members  126 , which extends in a first direction therethrough, and a tongue element  166  for fitting in a respective one of the recesses  156  in the distal ends of the interface members  126  which extends from the body element  162  in a second direction orthogonal to the first direction of the slot  164  therein. 
     In this embodiment the second coupling member  160  comprises a body element  168 , and a tongue element  170  for fitting in an adjacent one of the recesses  156  in the distal ends of the interface members  126  which extends from the body element  168 . The body element  168  of the second coupling member  160  is configured to be a tight, snap-fit in the slot  164  in the body element  162  of the first coupling member  159  such that, when inserted therein, the tongue elements  166 ,  170  of the coupling members  159 ,  160  are locked in a coupling configuration. In one embodiment the coupling members  159 ,  160  could be bonded together. 
     With this configuration, the coupling members  159 ,  160  allow for the in situ assembly of the last of the corner pieces  118   a - d  to be fitted, in enabling the tongue elements  166 ,  170  of the respective pair of the coupling members  159 ,  160  to be inserted into respective recesses  156  in the distal ends of the interface members  126  in assembling the last corner piece  118   a - d . It will be appreciated that three of the corner pieces  118   a - d  can be pre-assembled and need not be assembled in situ. Indeed, in one embodiment three of the corner pieces  118   a - d  can be fabricated as an integral unit, typically as a moulding or casting. 
     In this embodiment the tongue elements  166 ,  170  of the coupling members  159 ,  160  are configured such as to be a tight fit in the recesses  156  in the distal ends of the interface members  126 , but such as to allow for movement of the tongue elements  166 ,  170  in the respective recesses  156  in the direction of a tension as applied to the printing screen  112 . With this configuration, the interface members  126  present a substantially rigid frame when handled, but allow for relative movement of respective opposite pairs of the interface members  126  on being tensioned by tensioning mechanisms. In allowing for relative movement of the interface members  126 , design tolerances can be accommodated in order to provide for uniform tensioning of the printing screen  112 . 
     In fitting the supporting frame  114  to the printing screen  112 , the interface units  116 ,  116 ,  117 ,  117  are fitted to the respective edges of the printing screen  112 , and the ends of the interface units  116 ,  116 ,  117 ,  117  are coupled by the corner pieces  118   a - d . In fitting an interface unit  116 ,  116 ,  117 ,  117  to the printing screen  112 , the at least one coupling element  130  of the interface unit  116 ,  116 ,  117 ,  117  is fitted to one edge of the printing screen  112  by inserting the engagement projections  157  of the at least one coupling element  130  into the counterpart engagement apertures  124  in the respective edge of the printing screen  112 , and then sliding the at least one coupling element  130  and the edge of the printing screen  112  into the coupling slot  142  of the respective interface member  126 . With the at least one coupling element  130  and the edge of the printing screen  112  so inserted into the interface member  126 , the edge of the printing screen  112  is then held captive to the interface member  126 . This procedure is then repeated in fitting the other interface units  116 ,  116 ,  117 ,  117  to the other edges of the printing screen  112 . With the interface units  116 ,  116 ,  117 ,  117  so fitted, the corner pieces  118   a - d  are fitted such as to couple the respective ends of the interface members  126  and thereby form the supporting frame  114 . 
       FIGS. 7 and 8  illustrate a printing screen unit in accordance with a second embodiment of the present invention. 
     The printing screen unit  110  of this embodiment is quite similar to the printing screen unit  110  of the above-described first embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being designated by like reference signs. 
     The printing screen unit  110  of this embodiment differs from the printing screen unit  110  of the above-described first embodiment in the configuration of the interface units  116 ,  116 ,  117 ,  117  and the corner pieces  118   a - d.    
     In this embodiment one of the opposite pairs of the interface units  116 ,  116  are longer than the other of the opposite pairs of interface units  117 ,  117 , such as at least partially to overlap the adjacent ends of the opposite pair of interface units  117 ,  117 , and the corner pieces  118   a - d  are defined by stop elements which are disposed at the ends of the one pair of interface units  116 ,  116 . In this embodiment the one pair of interface units  116 ,  116  have a length which is substantially the same as the respective edges of the printing screen  112 , but, as illustrated in  FIG. 8 , the interface members  126  of the one pair of interface units  116 ,  116  are marginally longer than the at least one coupling elements  130  thereof and the respective edges of the printing screen  112 , in order to accommodate stretching of the printing screen  112  when tensioned. 
     With this configuration, the one pair of interface units  116 ,  116  act to trap the other pair of interface units  117 ,  117 , and prevent the same from sliding from the respective edges of the printing screen  112 , and the one pair of interface units  116 ,  116  are prevented from sliding from the printing screen  112  by the corner pieces  118   a - d.    
     In this embodiment the corner pieces  118   a - d  comprise end caps which are attached to the respective ends of the one pair of interface units  116 ,  116 . In preferred embodiments the corner pieces  118   a - d  can be formed of a metal or a synthetic material. 
     In this embodiment the corner pieces  118   a - d  are fixedly coupled to the ends of the interface units  116 ,  116 , here by bonding. In alternative embodiments the corner pieces  118   a - d  could be fixed by pins or threaded screws. 
     In an alternative embodiment the corner pieces  118   a - d  could comprise end caps which are attached to the respective ends of the other pair of interface units  117 ,  117 . 
       FIGS. 9 and 10  illustrate a printing screen unit in accordance with a third embodiment of the present invention. 
     The printing screen unit  110  of this embodiment is quite similar to the printing screen unit  110  of the above-described first embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being designated by like reference signs. 
     The printing screen unit  110  of this embodiment differs from the printing screen unit  110  of the above-described first embodiment in the configuration of the interface units  116 ,  116 ,  117 ,  117  and the corner pieces  118   a - d.    
     In this embodiment one of the opposite pairs of the interface units  116 ,  116  are longer than the other of the opposite pairs of interface units  117 ,  117 , such as at least partially to overlap the adjacent ends of the opposite pair of interface units  117 ,  117 , and the corner pieces  118   a - d  are defined by stop elements which are disposed at the ends of the one pair of interface units  116 ,  116 . In this embodiment the one pair of interface units  116 ,  116 , that is, the interface member  126  and the at least one coupling element  130  thereof, have a length which is longer than the respective edges of the printing screen  112 . As illustrated in  FIG. 10 , the at least one coupling elements  130  are marginally shorter than the interface members  126  of the one pair of interface units  116 ,  116 , in order to accommodate stretching of the printing screen  112  when tensioned. 
     With this configuration, the one pair of interface units  116 ,  116  act to trap the other pair of interface units  117 ,  117 , and prevent the same from sliding from the respective edges of the printing screen  112 , and the one pair of interface units  116 ,  116  are prevented from sliding from the printing screen  112  by the corner pieces  118   a - d.    
     In this embodiment the corner pieces  118   a - d  comprise end caps which are attached to the respective ends of the one pair of interface units  116 ,  116 . In preferred embodiments the corner pieces  118   a - d  can be formed of a metal or a synthetic material. 
     In this embodiment the corner pieces  118   a - d  are fixedly coupled to the ends of the interface units  116 ,  116 , here by bonding. In alternative embodiments the corner pieces  118   a - d  could be fixed by pins or threaded screws. 
     In an alternative embodiment the corner pieces  118   a - d  could comprise end caps which are attached to the respective ends of the other pair of interface units  117 ,  117 . 
     In one modification, as illustrated in  FIG. 11 , the at least one coupling elements  130  of the one pair of interface units  116 ,  116  are formed so as to be shorter than the interface members  126  thereof, in this embodiment having substantially the same length as the respective edges of the printing screen  112 , and the corner pieces  118   a - d  each include a tongue element  172  which is a push fit in the ends of the coupling slots  142  in the interface members  126  of the one pair of interface units  116 ,  116 , such as to provide for frictional retention of the corner pieces  118   a - d . In this embodiment the tongue element  172  of each of the corner pieces  118   a - d  has substantially the same length as the extended length at each of the ends of the interface members  126  of the one pair of interface units  116 ,  116 , but are marginally shorter, in order to accommodate stretching of the printing screen  112  when tensioned. 
       FIGS. 12 and 13  illustrate a printing screen unit in accordance with a fourth embodiment of the present invention. 
     The printing screen unit  110  of this embodiment is quite similar to the printing screen unit  110  of the above-described first embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being designated by like reference signs. 
     The printing screen unit  110  of this embodiment differs from the printing screen unit  110  of the above-described first embodiment in the configuration of the interface units  116 ,  116 ,  117 ,  117  and the corner pieces  118   a - d.    
     In this embodiment one of the opposite pairs of the interface units  116 ,  116  are longer than the other of the opposite pairs of interface units  117 ,  117 , such as at least partially to overlap the adjacent ends of the opposite pair of interface units  117 ,  117 , and the corner pieces  118   a - d  are defined by the respective ends of the one pair of interface units  116 ,  116 . 
     In this embodiment the interface members  126  of the one pair of interface units  116 ,  116  have a length which is longer than the respective edges of the printing screen  112  and the at least one coupling elements  130  of the one pair of interface units  116 ,  116  are formed so as to be shorter than the interface members  126  thereof, in this embodiment having substantially the same length as the respective edges of the printing screen  112 , and the extended length at each of the ends of the interface members  126  of the one pair of interface units  116 ,  116  defines the corner pieces  118   a - d.    
     In this embodiment the one pair of interface units  116 ,  116  each include retaining elements  174  which are disposed to the respective ends of the interface members  126  thereof and act to retain the interface members  126  to the respective edges of the printing screen  112 . In this embodiment the retaining elements  174  are located at positions marginally beyond the ends of the printing screen  112 , such as to accommodate stretching of the printing screen  112  when tensioned. In this embodiment the retaining elements  174  comprise pins, but in other embodiments could comprise screws. 
     With this configuration, the one pair of interface units  116 ,  116  act to trap the other pair of interface units  117 ,  117 , and prevent the same from sliding from the respective edges of the printing screen  112 , and the one pair of interface units  116 ,  116  are prevented from sliding from the printing screen  112  by the retaining elements  174  thereof. 
     Finally, it will be understood that the present invention has been described in its preferred embodiments and can be modified in many different ways without departing from the scope of the invention as defined by the appended claims. 
     For example, in one modification, the printing screen  112  could include any arrangement of engagement apertures  124  along each of the edges thereof and the coupling elements  130  could each include any counterpart arrangement of engagement projections  158 , with the interface member  126  including one or more counterpart recesses  150 . In such alternative embodiments the printing screen  112  could include any number of rows of engagement apertures  124  along each of the edges thereof, for example, a single row or three or more rows, and the coupling elements  130  could each include any counterpart number of rows of engagement projections  158 , with the interface member  126  including one or more counterpart recesses  150 . In other embodiments the engagement apertures  124  in the printing screen  112  and the engagement projections  158  of the coupling elements  130  can be arranged in other than rows. 
     In another modification, as illustrated in  FIG. 14 , the printing screen  112  could include the engagement projections  158  instead of the embodied engagement apertures  124  and the coupling elements  130  of the interface units  116 ,  116 ,  117 ,  117  could include the engagement apertures  124  instead of the embodied engagement projections  158 . In another embodiment the printing screen  112  could include a combination of the engagement apertures  124  and the engagement projections  158  and the coupling elements  130  of the interface units  116 ,  116 ,  117 ,  117  could include the counterpart combination of the engagement apertures  124  and the engagement projections  158 .