Abstract:
A medium support member includes a table having a top surface; an overlay placed on the top surface of the table, the overlay having a top surface and a bottom surface and a plurality of through-holes; a spacer array provided between the top surface of the table and the bottom surface of the overlay to define a gap between the table and the overlay, and at least one vacuum passage communicating with said gap for creating an underpressure in the gap, wherein at least portions of the overlay and corresponding portions of the table are held together by magnetic attraction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a medium support member, especially for print media, and a method of forming a medium support member. 
         [0003]    2. Description of the Background Art 
         [0004]    In the field of printing, it is known to use medium support members for holding and flattening of print media during a print process in which the print media are scanned with a printhead. It is known to use a suction box as medium support member. Such a suction box usually has a perforated top surface, and the inner volume of the suction box is maintained at an underpressure by means of a vacuum pump. 
         [0005]    It is a disadvantage of this kind of medium supports that the suction box must have a very complex structure in order to be able to evenly distribute the underpressure from the suction device over the surface of the print medium. 
         [0006]    As the distance between the printhead and the print medium must be very well defined and preferably constant over the whole print area, the support surface should be very perfectly flat. 
         [0007]    US 2008/0055382 A1 discloses a medium support member comprising: 
         [0008]    a table having a top surface; 
         [0009]    an overlay placed on the top surface of the table, the overlay having a top surface and a bottom surface and a plurality of through-holes; 
         [0010]    a spacer array provided between the top surface of the table and the bottom surface of the overlay to define a gap between the table and the overlay, and 
         [0011]    at least one vacuum passage communicating with said gap for creating an underpressure in the gap. 
         [0012]    The spacers of the spacer array are formed by punching the overlay, which may be formed by a sheet metal, so that depressions or recesses are formed in the top surface of the overlay and projections corresponding to these recesses are formed in the bottom surface of the overlay. The spacers define a gap with a well-defined width between the overlay and the top surface of the table. Further, since the spacers are arranged in the form of separate islands, the hollow space surrounding the spacers forms a distribution manifold for evenly distributing the suction pressure over the through-holes in the overlay. 
         [0013]    When a sheet of a print medium is placed on the top surface of the overlay, the underpressure results in a force that will firmly draw the sheet against the overlay. Since the size of the print media may sometimes be smaller than the area of the medium support member, there may be a case where not all of the through-holes in the overlay are covered by the print medium sheet, but through-holes in a marginal area of the overlay are left open. In these areas, the overlay is held on the surface of the table only by gravity, assisted by a certain flow resistance which the through-holes will provide for the air that is being drawn in. The overlay, which may be relatively thin and flexible, should be safely prevented from forming warps which, in view of the very small distance between the printhead and the surface of the print medium, might lead to collisions between the printhead and upwardly bulged surface areas of the print medium or the support surface. 
         [0014]    Medium support members according to the preambles of claim  1  are disclosed in JP 2008 238674 A and US 2008/174652 A1. 
       SUMMARY OF THE INVENTION 
       [0015]    It is an object of the invention to provide a medium support member wherein the overlay is reliably secured on the top surface of the table. 
         [0016]    In the medium support member according to the invention, magnetic strips are secured to the overlay, and the the table is made of a magnetically attractable material, and in that the spacer array comprises spacers that are formed by the magnetic strips. 
         [0017]    Thus, the overlay will safely be attracted towards the surface of the table regardless of whether or not air is drawn-in through the through-holes, and the magnetic strips forming the spacers can be arranged such that they do not close-off the through-holes that are needed for applying underpressure to the media. 
         [0018]    In a preferred embodiment, it will be, at least among others, the marginal or outer peripheral portions of the overlay that are magnetically drawn against the table. Then, when the print media have a format that is smaller than the size of the support member, the area where underpressure is applied may be limited to the actual area of the print media, so as to avoid leakage of air through the open through-holes in the marginal portion of the overlay, thereby reducing the energy consumption of the vacuum pump. Nevertheless, the magnetic attraction will prevent the overlay from forming warps. 
         [0019]    The spacers provided in the peripheral portions of the overlay may be configured as seals for sealing the distribution manifold formed in the gap between the top surface of the table and the bottom surface of the overlay. 
         [0020]    The spacers that are distributed over the central part of the overlay may also be formed by magnetic strips. When all spacers are formed by magnetic strips that have exactly the same thickness, the width of the gap between the table and the manifold will be defined with high precision. 
         [0021]    When the spacers, i.e. magnetic strips, are detachably secured at the bottom surface of the overlay, the configuration of the spacer array may easily be modified and may thus be adapted to varying sizes of the print media. 
         [0022]    Since it is not necessary to punch the overlay and to form recesses in the top surface thereof, in order to provide the spacers at the bottom surface, the top surface of the overlay will be an almost continuous flat surface that is perforated only by the through-holes which may have relatively small diameters so as to prevent the portions of the print media covering these through-holes from being bent. 
         [0023]    According to the invention, a method of forming a medium support member of the type that has been described above comprises a step of providing a jig formed with cavities adapted to the contours of the magnetic strips and located in predetermined relative positions, placing magnetic strips in the cavities of the jig, and superposing the overlay and the jig with the magnetic strips contained therein so as to mount the magnetic strips at the predetermined positions on the bottom surface of the overlay. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only and are thus not limitative of the invention, and wherein: 
           [0025]      FIG. 1  is a schematic view showing a medium support member according to an embodiment of the present invention; 
           [0026]      FIG. 2  is a cross-sectional view (not to scale) of the support member showing in  FIG. 1 ; 
           [0027]      FIG. 3  is a bottom view of one half of an overlay of the support member shown in  FIGS. 1 and 2 ; and 
           [0028]      FIG. 4  is a perspective view of a jag that is used in a method according to the invention for forming the medium support member. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]      FIG. 1  shows a schematic view of a printing system comprising a medium support member  10  according to an embodiment of the present invention. The support member  10  comprises a table  12  and an overlay  14  positioned on top of the table  12 . 
         [0030]    The overlay  14  is perforated by a plurality of through-holes  16  that are regularly distributed over the top surface of the overlay  14 . The through-holes  16  connect the top surface of the overlay  14  to the bottom surface thereof (not visible in  FIG. 1 ), where a distribution manifold  18  ( FIG. 2 ) is formed to which an underpressure as applied by means of a vacuum pump  20  (acting as a suction device) and a suction duct  22 . 
         [0031]    The underpressure causes ambient air to be drawn-in through the through-holes  16 . As a consequence, when a sheet of a print medium  24  is supported on the support member  10 , the sheet will be firmly sucked against the top surface of the overlay  14 . In this way, the medium  24  will be kept stationary and flat. A carriage (not shown) comprising one or more printheads  26  is controlled to move across the print medium  24  to scan the same with high speed in a main scanning direction (fast direction) X and a sub-scanning direction (slow direction) Y, while ink droplets are jetted out from the printhead  26  and onto the medium  24  to form an image thereon. In this embodiment, the printhead  26  ejects droplets of UV-curable ink, but it will be clear for the skilled person that other types of marking material such as solvent ink, water based inks or hot melt inks may be used instead. 
         [0032]    In a modified embodiment, the printhead  26  may be moved only in the main scanning direction X to print a swath of the image while the medium  24  is held stationary, and the medium  24  is intermittently advanced in the sub-scanning direction Y so as to print successive swathes. 
         [0033]    The dimensions of the overlay  14  and the table  12  may be 3 by 4 meters, for example, and the overlay  12  may be foamed by an aluminum sheet having a thickness of not more than 100 to 150 μm, for example. The through-holes  16  are arranged in rows and columns with a row-to-row distance of 20 mm and may have a diameter of 1.5 mm. 
         [0034]    In the cross-sectional view shown in  FIG. 2 , the dimensions in thickness direction of the overlay  14  have been exaggerated. A spacer array  28  is provided on the bottom surface of the overlay  14 , so that a gap with a predetermined height of, e.g., 1.1 mm is formed between the bottom surface of the overlay  14  and the top surface of the table  12 . 
         [0035]    As can be seen in a bottom plan view in  FIG. 3 , the spacers of the array  28  are arranged in the form of separated islands, so that the hollow part of the gap not filled by the spacers forms a contiguous distribution manifold  18  capable of evenly applying the underpressure to all of the through-holes  16 . As shown in  FIG. 2 , the manifold  18  is connected to the vacuum duct  22  ( FIG. 1 ) via internal suction passages  30  of the table  12 . 
         [0036]      FIG. 3  shows only one half of the bottom surface of the overlay  14 , the rest of the overlay being symmetric thereto relative to a symmetry axis A. The locations of the mouths of the suction passages  30  have been indicated in the phantom lines. 
         [0037]    The individual spacers forming the spacer array  28  are formed by magnetic strips  32 - 36  that are bonded to the bottom surface of the overlay  14  by means of an adhesive. Some of these strips, designated as  32  and  34 , extend along the periphery of the overlay  14  as a kind of frame structure. An innermost frame formed by the strips  34  has the function of a seal that limits and seals the manifold  18 . Other, shorter, strips  36  are arranged inside the manifold  18  in parallel lines separated by aisles  38 . Thus, the underpressure applied via the suction passages  30  is distributed over the entire surface of the manifold  18  while the overlay  14  is supported in both, the edge portion and the interior portion by the various strips of the spacer array  28 . In the example shown, the strips  36  extend in the main scanning direction X. They are arranged in the intervals between every second pair of rows of the through-holes  16 , so that, on the one hand, the through-holes  16  are not covered by the strips and, on the other hand, the overlay  14  is particularly supported against the suction force acting in the vicinity of the through-holes  16 . Thus, the strips  36  have mutual spacings of 40 mm. 
         [0038]    The marginal portions of the overlay  14  which are not perforated by the through-holes  16  are safely attached to the table  12  by means of the frames formed by the magnetic strips  32  and  34 , so that the thin overlay  14  is held in a perfectly flat condition even in the marginal areas where no suction pressure is present or where the suction pressure is reduced because the through-holes  16  are not closed-off by the medium  24 . 
         [0039]    When print media  24  of a different format are to be used, the spacer array  28  may be modified so as to limit and concentrate the suction pressure to the area that is actually covered by the print medium. In this way, it is possible to reduce the number of through-holes  16  that are not covered by the print medium  24  and thereby to reduce the amount of leakage of air through these through-holes. 
         [0040]    A method of forming the medium support member  10  with a spacer array adapted to a specific format of the print media will now be explained by reference to  FIG. 4 . 
         [0041]    As is shown in  FIG. 4 , jig  40  is used for positioning the magnetic strips  16 ,  32  in the correct positions. The jig  40  is formed by a rigid plate  42  forming a mold  44  with cavities  32 ′,  36 ′ into which the magnetic strips  36 ,  32  may be inserted. The cavities  36 ′,  32 ′ are adapted to the contours of the respective strips and define the target positions thereof. The depth of the cavities is slightly smaller than the thickness of the strips  36 ,  32  so that the strips will slightly project out of the cavities, as has been shown for the strip  36  in  FIG. 4 . The surfaces of the strips  36 ,  32  that form the top surfaces in  FIG. 4  are coated with an adhesive  46 . When all strips have been inserted in their respective cavities, the overlay  14  is placed onto the jig  40  and its bottom face is pressed against the projecting strips  16 ,  32 , so that these strips are bonded to the overlay  14 . Positioning pins  48  formed on the plate  42  will engage into corresponding positioning holes (not shown) that may be formed in the overlay so as to assure that the strips forming the spacer array  28  will be positioned relative to the pattern of the through-holes  16  with high precision. 
         [0042]    Preferably, the butting edges of the strips  32  forming the outer frame of the spacer array will be sealed by means of a curable sealing liquid or the like so as to form an air-tight seal. 
         [0043]    Finally the overlay with the strips  32 ,  36  adhering thereto will be lifted off from the jig  40  and will be placed on the top surface of the table  12 . The table  12  is formed of a magnetically attractable steel so that the overlay with the spacer array will be held in place by magnetic attraction between the strips  32 ,  36  and the table  12 .