Abstract:
A screen printing apparatus for screen printing onto a substrate comprises: a printing screen unit, with a printing screen and a frame for holding the printing screen in a substantially planar primary configuration; a support for the substrate; and a moveable print head for contacting and sweeping across a printing region of a first surface of the printing screen; an engagement structure mounted to contact a region of a surface of the printing screen which lies outside any part of the printing region which is drivable to move a portion of the printing screen in a direction orthogonal to the screen. Moving the screen away from the substrate improves separation between the printing screen and substrate. Moving the screen towards the substrate before printing can pre-tension the screen to reduce the tension which the print head has to overcome. Controlled movement of the screen towards and away from the substrate can reduce tension applied to the screen, and thus improve its working life.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a screen printing apparatus and a method for screen printing through a printing screen onto a substrate, typically an electronics substrate, such as a wafer for solar or fuel cells or a circuit board, using a print head, typically comprising a squeegee. 
       BACKGROUND OF THE INVENTION 
       [0002]    Various screen printing apparatus and methods already exist. However, the present inventor has recognized that these apparatus and methods suffer from a number of drawbacks. 
         [0003]    Firstly, there is no direct control over separation of the printing screen and the substrate, known as peel-off, and this peel-off occurs faster at the edges of the image area than in the center of the image area, which has been noted to result in uneven print quality across the area of the printed substrate. 
         [0004]    Secondly, the print head has to overcome the tension of the printing screen to apply pressure to the substrate, giving rise to an uneven print uniformity, because the resistance to travel of the print head is greater at the ends of the print head than in the center of the print head. 
         [0005]    Thirdly, the life of the printing screen is reduced, because in every print cycle, some of the printing screen must be moved by a distance corresponding to the required print gap between the printing screen and the substrate, which progressively causes a loss of tension in the printing screen, leading to variation in print quality over the life of the printing screen as the tension decreases, and ultimately premature failure of the printing screen. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an aim of the present invention to provide a screen printing apparatus and method which at least partially addresses one or more of the above-identified problems which have been recognized by the inventor. 
         [0007]    In accordance with the present invention, it has been discovered that this aim may be achieved by providing a mechanism for moving at least a portion of the printing screen relative to its normal plane. By moving the screen away from the substrate after printing, improved separation between the printing screen and substrate can be achieved. By moving the screen towards the substrate before printing, the screen can be pre-tensioned so that the tension which the print head has to overcome is reduced. Finally, by controlled movement of the screen towards and away from the substrate, it is possible to reduce tension applied to the screen, and thus improve its working life. 
         [0008]    In a preferred embodiment, the separation or peel-off of the printing screen from the substrate is controlled independently of the print speed and the print gap, thereby de-coupling the separation or peel-off from the print speed and the print gap, and so enabling optimisation of the peel-off in relation to the print speed. It is believed that peel-off has a greater effect on print quality than print speed, and, by independently controlling the peel-off, this aspect of the present invention enables the application of greater print speeds at the same separation or peel-off speed and yet maintains print quality, or enables the application of greater print speeds at a slower separation or peel-off speed and provides for improved print quality. 
         [0009]    In another preferred embodiment the printing screen is brought into contact with the substrate before the print stroke, which enables the print head to be applied with lower pressure, insofar as the print head does not have to overcome the tension of the printing screen, and this pressure is more evenly distributed over the substrate. The application of a lower pressure provides for improved print quality across the substrate, and also provides for the print head and the printing screen to have a longer life. 
         [0010]    In a further preferred embodiment the printing screen can be configured to have a neutral position from which the printing screen is moved before the print stroke in a first direction, typically downwards, by a distance less than the required print gap into contact with the substrate to be printed, and subsequently moved in a second direction opposite to the first direction, typically upwards, by a distance corresponding to the balance of the print gap, whereby the full print gap is established to enable separation and with a movement of the printing screen from the neutral position which is less than a distance corresponding to the required print gap. With this arrangement, the elastic stress on the printing screen is much reduced, so providing the printing screen with increased life. 
         [0011]    In accordance with a first aspect of the present invention there is provided a screen printing apparatus for screen printing onto a substrate, comprising: 
         [0012]    a printing screen unit, the printing screen unit comprising a printing screen and a frame for holding the printing screen in a substantially planar primary configuration, the printing screen including a plurality of apertures in an image area and through which a print medium is printed onto a substrate in use; 
         [0013]    a support for supporting the substrate during printing; and 
         [0014]    a print head which is moveable in a print direction substantially parallel to the plane of the printing screen, to print print medium through the printing screen onto the substrate in use during a print stroke, the print head contacting and sweeping across a printing region of a first surface of the printing screen during the print stroke; 
         [0015]    an engagement structure mounted to contact in use a region of a surface of the printing screen which lies outside any part of the printing region and drivable to move at least a portion of the printing screen in a direction orthogonal to the plane of the screen. 
         [0016]    The region of the surface of the printing screen which is contacted necessarily lies outside the printing region so as not to interfere with the movement of the print head. The contacted region may comprise an area on the same side of the printing screen as the print region but arranged outwardly thereof, or alternatively on the opposite surface of the printing screen, in which case it need not be arranged outwardly but could advantageously be centrally located, e.g. directly opposed to the printing region. 
         [0017]    Advantageously, the apparatus may comprise at least one actuator to drive the engagement structure. 
         [0018]    The engagement structure may comprise an engagement member drivable to move the printing screen portion in a direction away from the support, to separate the printing screen from the substrate subsequent to the print stroke in use. The engagement member may in use contact a second surface of the printing screen, oppositely arranged to the first surface, and is drivable to push the printing screen portion away from the support. The engagement member may comprise a roller. 
         [0019]    The engagement member may be profiled to promote deflection of an intermediate region of the printing screen in relation to lateral edge regions thereof. 
         [0020]    In some embodiments, the engagement member is attached to the printing screen, and is drivable to pull the printing screen portion away from the substrate. 
         [0021]    The engagement structure may comprise an engagement member drivable to move the printing screen portion in a direction towards the substrate in use. The engagement member may in use contact the first surface of the printing screen, and is drivable to push the printing screen portion towards the substrate. Alternatively, the engagement member may be attached to the printing screen, and is drivable to pull the printing screen portion towards the substrate. 
         [0022]    The engagement member may be drivable towards the substrate into a secondary position in which the distance of the printing screen is displaced from that of the primary configuration by a distance which is less than a distance (d) corresponding to a required print gap. The engagement member may also be drivable away from the substrate into a tertiary position in which the distance of the printing screen is displaced from that of the primary configuration by a distance which is less than a distance (d) corresponding to a required print gap. 
         [0023]    The engagement structure may be drivable into a secondary configuration, and shaped so that, in the secondary configuration, the image area lies in a plane substantially parallel to the plane of the primary configuration. 
         [0024]    The engagement structure may comprise first and second engagement members disposed on opposing sides of the image area and outwardly thereof, each engagement member extending along a respective side so as to have a length at least equal to the dimension of the image area along that side. In this case, the apparatus may comprise at least one additional engagement member disposed between the first and second engagement members and outward of the image area. Alternatively, the engagement structure may comprise an engagement member including an inner frame which at least partially surrounds the image area. 
         [0025]    The or each engagement member may comprise a cam surface drivable in the print direction and the engagement structure comprises a cam element disposed to drive the cam surface orthogonally to the print direction. The cam element may be mounted to the print head. 
         [0026]    The engagement structure may extend in a direction parallel to the print direction. In this case, the engagement structure may comprise a plurality of engagement members, sequentially disposed to extend in the direction parallel to the print direction. The or each engagement member may be independently drivable. 
         [0027]    Alternatively, the engagement structure may comprise an engagement member which extends in the direction parallel to the print direction. The engagement member may be flexible. 
         [0028]    Each end of the engagement member may be independently drivable. In accordance with a second aspect of the present invention there is provided a method for screen printing onto a substrate using screen printing apparatus, the screen printing apparatus comprising: 
         [0029]    a printing screen unit comprising a printing screen and a frame for holding the printing screen in a substantially planar configuration, the printing screen including a plurality of apertures in an image area and through which a print medium is printed onto a substrate in use; 
         [0030]    a support for supporting the substrate during printing; and 
         [0031]    a print head which is moveable in a print direction substantially parallel to the is plane of the printing screen, 
         [0032]    the method comprising the steps of:
       i) performing a print stroke in which the print head contacts and sweeps across a printing region of a surface of the printing screen so as to print print medium through the printing screen onto the substrate, and   ii) moving at least a portion of the printing screen in a direction orthogonal to the plane of the screen, by engaging a region of a surface of the printing screen which lies outside any part of the printing region.       
 
         [0035]    Step ii) may comprise moving the printing screen portion in a direction away from the substrate, to separate the printing screen from the substrate subsequent to performing the print stroke in step i). In this case, step ii) may comprise pushing the printing screen portion away from the substrate. Step ii) may further comprise deflecting an intermediate region of the printing screen in relation to lateral edge regions thereof. 
         [0036]    Alternatively, step ii) may comprise pulling the printing screen portion away from the substrate. 
         [0037]    Step ii) may comprise moving the printing screen portion in a direction towards the substrate, prior to performing the print stroke of step i). In this case, step ii) may comprise pushing the printing screen portion towards the substrate. Alternatively, step ii) may comprise pulling the printing screen portion towards the substrate. 
         [0038]    The method may comprise the step of moving the printing screen portion towards the substrate into a secondary position in which the distance of the printing screen is displaced from that of the primary configuration by a distance which is less than a distance (d) corresponding to a required print gap, prior to performing step i). In this case, the method may further comprise the step of moving the printing screen portion away from the substrate into a tertiary position in which the distance of the printing screen is displaced from that of the primary configuration by a distance which is less than a distance (d) corresponding to a required print gap, subsequent to performing step i). 
         [0039]    Step ii) may comprise moving the printing screen portion so that the image area lies in a plane substantially parallel to the plane of the primary configuration. 
         [0040]    Step ii) may comprise moving the printing screen portion by providing the engagement structure with a cam element drivable in the print direction and disposed to drive a cam surface member in engagement with the printing screen orthogonally to the print direction. In this case, the cam element may be mounted to the print head. 
         [0041]    Step ii) may comprise moving the printing screen portion independently of the print head. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    Preferred embodiments of the present invention will now be described hereinbelow by way of example only with reference to the accompanying drawings, in which: 
           [0043]      FIGS. 1( a ) and ( b )  illustrate a screen printing apparatus in accordance with a first embodiment of the present invention; 
           [0044]      FIG. 2  illustrates a screen printing apparatus in accordance with a second embodiment of the present invention; 
           [0045]      FIGS. 3( a ) and ( b )  illustrate a screen printing apparatus in accordance with a third embodiment of the present invention; 
           [0046]      FIG. 4  illustrates a screen printing apparatus in accordance with a third embodiment of the present invention; 
           [0047]      FIGS. 5( a ) to ( d )  illustrate a screen printing apparatus in accordance with a fifth embodiment of the present invention; 
           [0048]      FIG. 6  illustrates a screen printing apparatus in accordance with a sixth embodiment of the present invention; 
           [0049]      FIG. 7  illustrates a screen printing apparatus in accordance with a seventh embodiment of the present invention; 
           [0050]      FIG. 8  illustrates a screen printing apparatus in accordance with an eighth embodiment of the present invention; 
           [0051]      FIGS. 9( a ) to ( d )  illustrate a screen printing apparatus in accordance with a ninth embodiment of the present invention; and 
           [0052]      FIGS. 10( a ) and ( b )  illustrate a screen printing apparatus in accordance with a tenth embodiment of the present invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0053]      FIGS. 1( a ) and ( b )  illustrate a screen printing apparatus in accordance with a first embodiment of the present invention. 
         [0054]    The screen printing apparatus comprises a printing screen unit  101  through which a print medium is printed onto a substrate S, a support  103 , in this embodiment in the form of a platen, here disposed beneath the printing screen unit  101 , on which the substrate S is supported during printing, and a print head  105 , which is operative in a print stroke to print print medium onto the substrate S. 
         [0055]    The printing screen unit  101  comprises a frame  111 , in this embodiment of rectangular shape, which is pivotable about a pivot  115  at one edge of the frame  111 , and a printing screen  117  which is mounted under tension to the frame  111 . 
         [0056]    In this embodiment the printing screen unit  101  is pivoted about the pivot  115  by operation of an actuator (not illustrated), which acts to move the printing screen  117  away from the substrate S, in this embodiment upwardly in relation to the substrate S, thereby separating the printing screen  117  from the substrate S. 
         [0057]    The printing screen  117  includes a pattern of apertures  119  in an image area, which pattern defines the pattern of deposits to be printed on the substrate S. The image area is therefore effectively defined by the outer extremities of the pattern. 
         [0058]    In this embodiment the printing screen  117  comprises a mesh screen. 
         [0059]    In this embodiment the print head  105  comprises a squeegee  121  which acts to force print medium into the apertures  119  in the printing screen  117  when the print head  105  is moved in a first, printing direction X, and a flood bar  123  which acts to flood the surface of the printing screen  117  with print medium when the print head  105  is moved in a second, flood direction Y, which is opposite to the printing direction X. 
         [0060]    The screen printing apparatus further comprises an engagement structure including an engagement member  131  which is driven by an actuator (not illustrated), in this embodiment disposed within the support  103 , and which acts to engage a central region of the under surface of the printing screen  117 , which is intermediate the lateral edges of the printing screen  117 , at a location upstream of the print apertures  119  in the printing direction X, and move the printing screen  117  away from the substrate S, in this embodiment upwardly in relation to the substrate S, thereby separating a central section of the printing screen  117  from the substrate S. 
         [0061]    With this configuration, the engagement member  131  can be controlled independently of pivoting of the print screen unit  101 , so as to provide that the central band of the printing screen  117  separates at the same speed as the lateral edges of the printing screen  117 . 
         [0062]    In this embodiment the engagement member  131  can be actuated simultaneously with or prior to pivoting of the printing screen unit  101 . 
         [0063]    In this embodiment the engagement member  131  is controlled independently of the print head  105 , whereby the rate of separation or peel-off is controlled separately of the speed of the print head  105  and the print gap, and so enabling optimization of the print quality. 
         [0064]    In this embodiment separation of the printing screen  117  from the substrate S is initiated during the print stroke of the print head  105 . 
         [0065]    In an alternative embodiment, separation of the printing screen  117  from the substrate S could be initiated subsequent to the print stroke of the print head  105 . 
         [0066]    In this embodiment the engagement member  131  comprises an elongate bar which extends across a width of the printing screen  117 , here across substantially a center third of the width of the printing screen  117 . 
         [0067]    In one embodiment the engagement member  131  extends across a width of the printing screen  117  corresponding to the image area as defined by the pattern of apertures  119  in the printing screen  117 . 
         [0068]    In this embodiment the engagement member  131  extends across a width of the printing screen  117  from about a center third to about a center two-thirds of the width of the printing screen  117 . 
         [0069]      FIG. 2  illustrates a screen printing apparatus in accordance with a second embodiment of the present invention. 
         [0070]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the first-described 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. 
         [0071]    The screen printing apparatus of this embodiment differs from that of the first-described embodiment in that the frame  111  has a fixed position during the print stroke and is not pivotable, and in that the engagement member  131  acts to deflect the printing screen  117 , in this embodiment upwardly, in relation to the frame  111  and upstream of the image area in relation to the printing direction X, so as to separate or peel-off the printing screen  117  from the substrate S. 
         [0072]    In this embodiment the engagement member  131  is provided by a roller on the support  103 , which can be operative to transfer substrates S. 
         [0073]    Operation is otherwise the same as for the first-described embodiment. 
         [0074]      FIGS. 3( a ) and ( b )  illustrate a screen printing apparatus in accordance with a third embodiment of the present invention. 
         [0075]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the first-described 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. 
         [0076]    The screen printing apparatus of this embodiment differs from that of the first-described embodiment in that the frame  111  has a fixed position during the print stroke and is not pivotable, in that the engagement member  131  is profiled to promote deflection of a central band or region of the printing screen  117 , in this embodiment upwardly, in relation to the frame  111 , so as to separate or peel-off the printing screen  117  from the substrate S. 
         [0077]    In this embodiment the engagement member  131  has a profiled upper surface  132 , here an outwardly-projecting arcuate or curved surface which extends across a width of the printing screen  117 . 
         [0078]    With this arcuate or curved profile, on raising the engagement member  131  in relation to the printing screen  117 , which is located flush or sub-flush to the printing screen  117  during the printing operation, separation of the central band or region of the printing screen  117  is promoted, so as to ensure a substantially uniform separation of the printing screen  117  from the substrate S across the width of the printing screen  117 . Without this operation of the engagement member  131 , the lateral edge regions of the printing screen  117  would separate ahead of the central region of the printing screen  117 . 
         [0079]    In one alternative embodiment the screen printing apparatus could comprise a pair of engagement members  131  which are disposed in spaced relation to opposite sides of the image area. 
         [0080]    In one embodiment the engagement members  131  could be raised simultaneously or sequentially. 
         [0081]    Operation is otherwise the same as for the first-described embodiment. 
         [0082]      FIG. 4  illustrates a screen printing apparatus in accordance with a fourth embodiment of the present invention. 
         [0083]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the first-described 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. 
         [0084]    The screen printing apparatus of this embodiment differs from that of the first-described embodiment in that the frame  111  has a fixed position during the print stroke and is not pivotable, in that the engagement member  131  acts to deflect the printing screen  117 , in this embodiment upwardly, in relation to the frame  111  so as to separate or peel-off the printing screen  117  from the substrate S, and in that the engagement member  131  is provided to a lateral side of the image area as defined by the printing apertures  119 . In one alternative embodiment the screen printing apparatus could comprise a pair of engagement members  131  which are disposed in spaced relation to opposite sides of the image area. 
         [0085]    In one embodiment the engagement members  131  could be raised simultaneously or sequentially. 
         [0086]    Operation is otherwise the same as for the first-described embodiment. 
         [0087]      FIGS. 5( a ) to ( d )  illustrate a screen printing apparatus in accordance with a fifth embodiment of the present invention. 
         [0088]    The screen printing apparatus comprises a printing screen unit  201  through which a print medium is printed onto a substrate S, a support  203 , in this embodiment in the form of a platen, on which the substrate S is supported during printing, in this embodiment beneath the printing screen unit  201 , and a print head  205 , which is operative in a print stroke to print print medium onto the substrate S. 
         [0089]    The printing screen unit  201  comprises a first, outer frame  211 , in this embodiment of rectangular shape, a second, inner frame  213 , in this embodiment of rectangular shape, which is located within the outer frame  211 , and a printing screen  217  which is mounted under tension to the outer and inner frames  211 ,  213 . 
         [0090]    In this embodiment the inner frame  213  is adhesively bonded to the printing screen  217 . 
         [0091]    In an alternative embodiment the inner frame  213  could be otherwise mechanically fixed to the printing screen  217 , such as by clamping, for example, vacuum or magnetic clamping. 
         [0092]    The printing screen  217  includes a pattern of apertures  219  which are located within the inner frame  213  and define the image area and the pattern of deposits to be printed on the substrate S. 
         [0093]    In this embodiment the printing screen  217  comprises a mesh screen. 
         [0094]    In this embodiment the inner frame  213  comprises a first pair of elongate engagement members  220  which are arranged in spaced relation to opposite ends in the printing direction X, here in parallel relation, and a second pair of elongate engagement members  221  which interconnect the respective ends of the first pair of elongate engagement members  220 . 
         [0095]    In this embodiment the first pair of elongate engagement members  220  each include an engagement feature  223 , here in the form of a lateral groove in an outer face thereof, which are engaged by respective ones of first and second actuators  231 ,  233 , as will be described in more detail hereinbelow. 
         [0096]    In this embodiment the elongate engagement members  220 ,  221  comprise rigid members. 
         [0097]    In an alternative embodiment the first pair of elongate engagement members  220  could comprise rigid members and the second pair of elongate engagement members  221  could comprise flexible members, with the rate and shape of the separation or peel-off being determined in accordance with the flexibility of the lateral elongate engagement members  221 . In one embodiment the lateral elongate engagement members  221  can be configured so as to have non-uniform flexibility along the length thereof. 
         [0098]    In this embodiment the print head  205  comprises a squeegee  227  which acts to force print medium into the apertures  219  in the printing screen  217  when the print head  205  is moved in the printing direction X. 
         [0099]    The screen printing apparatus further comprises first and second actuators  231 ,  233  which are coupled to respective ones of the first pair of elongate engagement members  220 , in this embodiment to the respective engagement features  223  thereof, and are operable independently of one another, whereby the rate of separation of the printing screen  217  and the substrate S can be controlled separately of the speed of the print head  205  and the print gap, so enabling optimisation of the print quality. 
         [0100]    Operation of the screen printing apparatus will now be described with reference to  FIGS. 5( b ) to ( d ) . 
         [0101]    In a first step, as illustrated in  FIG. 5( b ) , the inner frame  213  is lowered from a neutral position, as defined by the natural tension of the printing screen  217 , in which the printing screen  217  is substantially parallel, to a lowered, printing position, in which the printing screen  217  within the inner fame  213  is spaced from the neutral position by a distance d corresponding to the required print gap. 
         [0102]    Then, as illustrated in  FIG. 5( c ) , the print head  205  is displaced to print print medium through the printing apertures  219  in the printing screen  217  onto the substrate S, and the first actuator  231  is operated to move, in this embodiment raise, the elongate engagement member  220  which is coupled thereto, being upstream of the image area in relation to the printing direction X, in this embodiment to return the printing screen  217  at the elongate engagement member  220  to the neutral position, moving the upstream elongate engagement member  220  through the distance d. 
         [0103]    In this embodiment separation or peel-off of the printing screen  217  from the substrate S by operation of the first actuator  231  is initiated during the print stroke of the print head  205 . 
         [0104]    In an alternative embodiment separation or peel-off of the printing screen  217  from the substrate S could be initiated subsequent to the print stroke of the print head  205 . 
         [0105]    Next, as illustrated in  FIG. 5( d ) , the second actuator  233  is operated to move, in this embodiment raise, the elongate engagement member  220  which is coupled thereto, being downstream of the image area in relation to the printing direction X, in this embodiment to return the printing screen  217  at the elongate engagement member  220  to the neutral position, moving the downstream elongate engagement member  220  through the distance d. 
         [0106]    In this embodiment the first and second actuators  231 ,  233  each comprise first and second actuator elements  235  which are coupled to respective ends of the elongate engagement members  220 . 
         [0107]      FIG. 6  illustrates a screen printing apparatus in accordance with a sixth embodiment of the present invention. 
         [0108]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the fifth-described embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being identified by like reference signs. 
         [0109]    The screen printing apparatus of this embodiment differs from that of the fifth-described embodiment in that the lateral elongate engagement members  221  each comprise a plurality of segments  221   a - h , and in further comprising a plurality of actuators  241   a - h  which are coupled to the respective segments  221   a - h  of the elongate engagement members  221 , such that the actuators  241   a - h  can be selectively controlled to control the advancing separation or peel-off of the printing screen  217  from the substrate S. 
         [0110]    In one alternative embodiment the lateral elongate engagement members  221  could be flexible members, with the actuators  241   a - h  being coupled to adjacent sections thereof, in a similar manner to the respective segments  221   a - h  of the elongate engagement members  221 . 
         [0111]    Otherwise, operation is the same as for the fifth-described embodiment. 
         [0112]      FIG. 7  illustrates a screen printing apparatus in accordance with a seventh embodiment of the present invention. 
         [0113]    The screen printing apparatus of this embodiment is very similar to the screen printing apparatus of the fifth-described 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. 
         [0114]    The screen printing apparatus of this embodiment differs from that of the fifth-described embodiment in that the inner frame  213  comprises instead only the first pair of elongate engagement members  220 , with the second pair of elongate engagement members  221  being omitted. 
         [0115]    Operation is the same as for the fifth-described embodiment. 
         [0116]      FIG. 8  illustrates a screen printing apparatus in accordance with an eighth embodiment of the present invention. 
         [0117]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the fifth-described 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. 
         [0118]    The screen printing apparatus of this embodiment differs from that of the fifth-described embodiment in omitting the inner frame  213  and instead comprising first and second elongate clamping engagement members  251 ,  253  which are located in spaced relation outwardly of the image area as defined by the printing apertures  219 , and operable in the manner of the first pair of elongate engagement members  220  in the fourth-described embodiment to provide for movement of the printing screen  219  therebetween in a first direction from the neutral position of the printing screen  217 , in this embodiment lowering, by the distance d corresponding to the print gap, and in a second direction, opposite to the first direction, in this embodiment raising, by the distance d corresponding to the print gap to return the printing screen  217  to the neutral position, with the first and second clamping engagement members  251 ,  253  operated sequentially to control separation or peel-off of the printing screen  217  in relation to the substrate S. 
         [0119]    In this embodiment the clamping engagement members  251 ,  253  are provided by vacuum bellows, which can be controlled to set the height of the printing screen  217  thereat by the extent of the vacuum, which acts against the tension in the printing screen  217 . 
         [0120]    In one embodiment the clamping engagement members  251 ,  253  are segmented along the length thereof, such that the pressure at the individual engagement segments is controllable independently, whereby progressive control of the separation or peel-off can be achieved along the printing direction X. 
         [0121]    The screen printing apparatus of this embodiment further differs from the fifth-described embodiment in that the clamping engagement members  251 ,  253  are disposed to lateral sides of the image area in the printing direction X. In an alternative embodiment the clamping engagement members  251 ,  253  could be disposed to opposite ends of the image area in the printing direction X. 
         [0122]      FIGS. 9( a ) to ( d )  illustrate a screen printing apparatus in accordance with a ninth embodiment of the present invention. 
         [0123]    The screen printing apparatus comprises a printing screen unit  301  through which a print medium is printed onto a substrate S, a support  303 , in this embodiment in the form of a platen, on which the substrate S is supported during printing, in this embodiment beneath the printing screen unit  301 , and a print head  305 , which is operative in a print stroke to print print medium onto the substrate S. 
         [0124]    The printing screen unit  301  comprises a first, outer frame  311 , in this embodiment of rectangular shape, a second, inner frame  313 , in this embodiment of rectangular shape, which is located within the outer frame  311 , and a printing screen  317  which is mounted under tension to the outer and inner frames  311 ,  313 . 
         [0125]    In this embodiment the inner frame  313  is attached to a surface, here an upper surface, of the printing screen  317 , which is opposite to the support  303 . 
         [0126]    In this embodiment the inner frame  313  is adhesively bonded to the printing screen  317 . 
         [0127]    In an alternative embodiment the inner frame  313  could be otherwise mechanically fixed to the printing screen  317 , such as by clamping, for example, vacuum or magnetic clamping. 
         [0128]    The printing screen  317  includes a pattern of apertures  319  which are located within the inner frame  313  and define the image area and the pattern of deposits to be printed on the substrate S. 
         [0129]    In this embodiment the printing screen  317  comprises a mesh screen. 
         [0130]    In this embodiment the inner frame  313  comprises a first pair of elongate engagement members  320  which are arranged in spaced relation to lateral sides of the image area in the printing direction X, here in parallel relation, and a second pair of elongate engagement members  321  which are arranged in spaced relation to opposite ends of the image area in the printing direction X and interconnect the respective ends of the first pair of elongate engagement members  320 . 
         [0131]    In this embodiment the elongate engagement members  320 ,  321  comprise rigid members. 
         [0132]    In an alternative embodiment the first pair of elongate engagement members  320  could comprise flexible members, with the rate and shape of the separation or peel-off being determined in accordance with the flexibility of the lateral elongate members  320 . In one embodiment the lateral elongate engagement members  320  can be configured so as to have non-uniform flexibility along the length thereof. 
         [0133]    In one alternative embodiment the second pair of elongate engagement members  321  could be omitted. 
         [0134]    In this embodiment the print head  305  comprises a squeegee  327  which acts to force print medium into the apertures  319  in the printing screen  317  when the print head  305  is moved in the printing direction X. 
         [0135]    The screen printing apparatus further comprises cam element  341  which engages the first pair of lateral elongate engagement members  320  to displace the printing screen  317  from the neutral position thereof, in this embodiment downwardly, whereby the rate of separation or peel-off of the printing screen  317  from the substrate S can be controlled separately of the speed of the print head  305  and the print gap, so enabling optimization of the print quality. 
         [0136]    In this embodiment the cam element  341  is coupled to the print head  305  so as to be moved in the printing direction X together with the print head  305 . 
         [0137]    In an alternative embodiment the cam element  341  could be coupled to a separate actuator which moves the cam element  341  in the printing direction X. 
         [0138]    In this embodiment the cam element  341  comprises a pair of cam members  343  which engage respective ones of the lateral elongate engagement members  320 . 
         [0139]    In this embodiment the cam members  343  each have a cam surface  345  which bears on a cam surface of a respective one of the lateral elongate engagement members  320 , and, with movement of the cam element  341  in the printing direction X, the cam members  343  are rotated, such that a length of the cam surface  345  bears on a length of the respective lateral elongate engagement member  320 . 
         [0140]    In this embodiment the cam surface  345  is an arcuate surface. 
         [0141]    In this embodiment the cam members  343  are interconnected, here by a shaft  347 , such as to be moved in unison. 
         [0142]    Operation of the screen printing apparatus will now be described with reference to  FIGS. 9( b ) to ( d ) . 
         [0143]    In a first step, as illustrated in  FIG. 9( b ) , the cam element  341  is lowered in relation to the printing screen  317 , and the cam surfaces  345  of the cam members  343  engage respective ones of the lateral elongate engagement members  320  at one, the upstream end thereof in the printing direction X, whereby the inner frame  313  is lowered at the one, upstream edge thereof from a neutral position, as defined by the natural tension of the printing screen  317 , by a distance d corresponding to the required print gap. 
         [0144]    Then, as illustrated in  FIG. 9( c ) , the print head  305  is moved in the printing direction X to print print medium through the printing apertures  319  in the printing screen  317  onto the substrate S, and the cam element  341  is moved simultaneously in the printing direction X to cause the inner frame  313  to be lowered from the neutral position forwardly of the print head  305  by a distance d corresponding to the required print gap, whereby the image area of the printing screen  317  is in in contact with the substrate S during application of the squeegee  327 . 
         [0145]    As illustrated in  FIG. 9( d ) , at the end of the print stroke, the cam surfaces  345  of the cam members  343  engage respective ones of the lateral elongate engagement members  320  at the other, downstream end thereof in the printing direction X, whereby the inner frame  313  is lowered at the other, downstream edge thereof from the neutral position by the distance d corresponding to the required print gap, and the one, upstream edge of the inner frame  313  is returned substantially to the neutral position under the action of the tension in the printing screen  317 . 
         [0146]    Following the print stroke, the cam element  341  is raised in relation to the printing screen  317 , and the other, downstream edge of the inner frame  313  is returned substantially to the neutral position under the action of the tension in the printing screen  317 . 
         [0147]    In one alternative embodiment the cam element  341  could be provided as a roller or slide which engages with and is moved along a length of the lateral elongate engagement members  320 , causing the inner frame  313  to be deflected downwardly as the roller or slide passes thereover. 
         [0148]    In another alternative embodiment the cam element  341  could be provided as one of a track or follower which engages with counterpart one of a track or follower on the lateral elongate engagement members  320 , causing the inner frame  313  to be deflected downwardly as the track or follower passes along a length of the lateral elongate engagement members  320 . 
         [0149]      FIGS. 10( a ) to ( c )  illustrate a screen printing apparatus in accordance with a tenth embodiment of the present invention. 
         [0150]    The screen printing apparatus of this embodiment is similar to the screen printing apparatus of the fifth-described embodiment, and thus, in order to avoid unnecessary duplication of description, only the differences will be described in detail, with like parts being identified by like reference signs. 
         [0151]    The screen printing apparatus of this embodiment differs from that of the fifth-described embodiment in the mode of operation. 
         [0152]    In this embodiment the actuators  231 ,  233  are controlled such that the printing screen  217  is moved in any one direction through a distance which is less than a distance d corresponding to the required print gap, in this embodiment by a distance d/2 which is not greater than half of the required print gap. 
         [0153]    First, print medium is applied, e.g. flooded, to the printing screen  217  while it is in a neutral position, as defined by the natural tension of the printing screen  217 . 
         [0154]    Next, as illustrated in  FIG. 10( a ) , the actuators  231 ,  233  are controlled to move the printing screen  217  in a first direction away from the neutral position (primary configuration) of the printing screen  217 , as defined by the natural tension of the printing screen  217  held by frame  211 , in this embodiment downwardly, into a secondary configuration in which a portion of the printing screen  217  has been displaced by a distance d/2, such that the image area of the printing screen  217  is brought into contact with the substrate S. 
         [0155]    Then, then print head  205  is moved in a print stroke to print print medium through the apertures  219  in the printing screen  217  and onto the substrate S. Subsequently, as illustrated in  FIG. 10( b ) , the actuators  231 ,  233  are controlled to move the printing screen  217  in a second direction, opposite to the first direction, towards and through the neutral position of the printing screen  217 , in this embodiment upwardly, by a distance d, into a tertiary configuration such that the image area of the printing screen  217  is moved a distance d/2 to the opposite side of the neutral position of the printing screen  217 . 
         [0156]    With this mode of operation, the printing screen  217  is moved from the neutral position by a distance which corresponds to a half of the required print gap, which advantageously provides that the printing screen  217  is put under significantly less stress as compared to the mode of operation in which the printing screen is moved from the neutral position in a single direction by a distance which corresponds to the required print gap. 
         [0157]    In alternative embodiments the actuators  231 ,  233  could be controlled such that the printing screen  217  is moved from the neutral position in either direction by a distance kd (where k&lt;1), which is less than the print gap. 
         [0158]    In another alternative embodiment the actuators  231 ,  233  could be controlled such that the printing screen  217  is moved from the neutral position in a first direction by a distance kd and in the second opposite direction by a distance (1−k)d/k (where k is less than 1 and d is the print gap). 
         [0159]    It should be understood that this concept of moving the frame away from the plane of the printing screen by an amount&lt;d can be achieved in various ways, in particular by using apparatus similar to that shown in any of  FIGS. 1-8 . 
         [0160]    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.