Abstract:
An image recording apparatus comprises a transfer unit having a transfer surface for holding a recording medium and transferring the recording medium held on the transfer surface, a recording head opposed to the transfer unit and recording images on the recording medium transferred by the transfer unit, a suction-force changing unit drawing the recording medium, a support member providing at substantially the same height as the transfer surface of the transfer unit and supporting the suction-force changing unit, and a suction unit drawing the recording medium to a surface of the support member, via the suction-force changing unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-289131, filed Nov. 11, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image recording apparatus in which recording media, such as recording paper sheets, can be reliably supplied by suction to the transfer mechanism. 
     2. Description of the Related Art 
     Ink-jet printers have hither to known as image recording apparatuses. Any ink-jet printer has a recording head that has a plurality of nozzles, from which ink droplets are ejected, recording a high-quality image at high speed on a recording medium being transferred by a transfer mechanism. The ink-jet printer has a transfer belt as the transfer mechanism. The transfer belt has a plurality of suction holes. Any ink-jet printer that has a transfer belt comprises a suction unit in most cases. The suction unit draws air through each suction hole of the transfer belt. The suction unit has, for example, a suction fan. The transfer mechanism holds the recording medium on the transfer belt by action, as the suction unit draws air. 
     While the ink-jet printer is recording (or printing) an image, ink droplets are formed, or ink mist is generated. Therefore, the ink droplets or ink mist may stick to the transfer belt as the ink droplets or ink mist is drawn in the system wherein air is drawn to attract the recording medium to the transfer belt. The ink droplets or ink mist, if any on the transfer belt, may stick to the reverse side of the recording medium, possibly making the recording medium dirty. 
     An ink-jet printer of the type described above may use the technique of automatically closing those of the suction holes, which are not closed with a recording medium placed on the transfer belt, in order to prevent the medium from becoming dirty with ink droplets or ink mist, to prevent the suction force applied on the recording medium from decreasing due to an in-flow of unnecessary air, and to prevent power consumption of the suction fan. 
     For example, Jpn. Pat. Appln. KOKAI Publication No. 2003-159841 discloses the technique of automatically opening and closing the valves provided in suction holes. The suction plate has a plurality of suction holes. In each suction hole, a valve is provided, which can open and close in accordance with changes in the amount of air applied by a suction fan. In any suction hole made in that region of the suction plate, which does not contact a paper sheet, air flows in a large amount, closing the valve provided in the suction hole. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-35022 discloses the technique of opening and closing suction holes with spherical bodies. The drum has a plurality of suction holes. In each suction hole, a movable spherical body is provided. The paper sheet is attracted by suction to any suction hole that is covered with the paper sheet, because the spherical body does not plug the suction hole. In any suction hole that is not covered with the paper sheet, the spherical body plugs the hole, not allowing air to flow through the suction hole. 
     Jpn. Pat. Appln. KOKAI Publication No. 5-131692 discloses the technique of opening and closing suction holes by using valves. A platen has a plurality of suction holes. Each suction hole has a valve, which has a spring. Each valve automatically opens or closes in accordance with the amount of air flowing through the suction hole. Any suction hole that is not covered with a paper sheet is closed by the valve because air flows in a large amount in the suction hole. Any suction hole that is covered with a paper sheet is opened by the valve because air flows in a small amount in the suction hole. 
     BRIEF SUMMARY OF THE INVENTION 
     An image recording apparatus according to a first aspect of the present invention comprises a transfer unit having a transfer surface for holding a recording medium and configured to transfer the recording medium held on the transfer surface; a recording head opposed to the transfer unit and configured to record images on the recording medium transferred by the transfer unit; at least one suction-force changing unit configured to draw the recording medium; a support member provided at substantially the same height as the transfer surface of the transfer unit and supporting the suction-force changing unit; and a suction unit configured to draw the recording medium to a surface of the support member, via the suction-force changing unit, wherein: the suction-force changing unit includes an air passage unit that includes an opening area, and enables air to be drawn from the recording medium side to the suction unit side, and a valve that opens or closes the opening area of the air passage unit in accordance with a pressure difference between the recording medium side and the suction unit side; the valve closes the air passage unit, setting the opening area of the air passage unit to value B (0&lt;B), when the pressure in the suction unit side is lower than the pressure at the recording medium side, and opens the air passage unit, setting the opening area of the air passage unit to value A (B&lt;A), when the pressure at the recording medium side approaches the pressure in the suction unit side; and the suction-force changing unit draws air from the recording medium side to the suction unit side through the air passage unit having the opening area B, thereby opening the valve that closes the air passage unit because the pressure in the suction unit side is lower than the pressure at the recording medium side, thus lowering the pressure at the recording medium side toward the pressure in the suction unit side. 
     An image recording apparatus according to a second aspect of the present invention comprises a transfer unit having a transfer surface for holding a recording medium and configured to transfer the recording medium held on the transfer surface; a recording head opposed to the transfer unit and configured to record images on the recording medium transferred by the transfer unit; at least one suction-force changing unit configured to draw the recording medium; a support member provided at substantially the same height as the transfer surface of the transfer unit and supporting the suction-force changing unit; and a suction unit configured to draw the recording medium to a surface of the support member, via the suction-force changing unit, wherein: the suction-force changing unit includes a plurality of air passage units which are provided between the recording medium side and the suction unit side and which serve to draw air toward the suction unit, and a valve that opens or closes only some of the air passage units; the valve closes some of the air passage units when the pressure in the suction unit side becomes lower than the pressure at the recording medium side; and a pressure difference between the recording medium side and the suction unit side decreases as air is drawn from the recording medium side toward the suction unit side, and the valve opens some of the air passage units, when air is drawn from the recording medium side toward the suction unit side through the other air passage units not closed by the suction-force changing unit, immediately after the suction unit is covered with the recording medium transferred. 
     An image recording apparatus according to a third aspect of the present invention comprises a transfer unit having a transfer surface for holding a recording medium and configured to transfer the recording medium held on the transfer surface; a recording head opposed to the transfer unit and configured to record images on the recording medium transferred by the transfer unit; at least one suction-force changing unit configured to draw the recording medium; a support member provided at substantially the same height as the transfer surface of the transfer unit and supporting the suction-force changing unit; a suction unit configured to draw the recording medium to a surface of the support member, via the suction-force changing unit; a first air passage unit provided in the suction-force changing unit and enabling air to flow between the recording medium side and the suction unit side; a valve configured to open and close the first air passage unit by virtue of a pressure difference between the recording medium side and the suction unit side; a second air passage unit provided in the suction-force changing unit and configured to draw air from the recording medium side toward the suction unit side and thereby decrease the pressure difference between the recording medium side and the suction unit side in order to open the first air passage unit, when the recording medium covers the suction unit in a state where the valve closes the first air passage unit. 
     An image recording apparatus according to a fourth aspect of the present invention comprises a transfer unit having a transfer surface for holding a recording medium and configured to transfer the recording medium held on the transfer surface; a recording head opposed to the transfer unit and configured to record images on the recording medium transferred by the transfer unit; at least one suction-force changing unit configured to draw the recording medium; a support member provided at substantially the same height as the transfer surface of the transfer unit and supporting the suction-force changing unit; a suction unit configured to draw the recording medium to a surface of the support member, via the suction-force changing unit; a first air passage unit provided in the suction-force changing unit and enabling air to flow between the recording medium side and the suction unit side; a valve configured to open and close the first air passage unit by virtue of a pressure difference between the recording medium and the suction unit; a second air passage unit provided in the suction-force changing unit and configured to draw air from the recording medium side to the suction unit side and thereby decrease the pressure difference between the recording medium side and the suction unit side in order to open the valve that closes the first air passage unit, when the recording medium covers the suction unit. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a side view showing the major components of a first embodiment of an image recording apparatus according to the present invention; 
         FIG. 2  is a top view showing the platen of the apparatus; 
         FIG. 3  is a sectional view showing the platen frame of the apparatus; 
         FIG. 4  is a sectional view showing one of the small chambers provided in the apparatus; 
         FIG. 5  is a perspective view showing one of the small chambers provided in the apparatus; 
         FIG. 6  is a block diagram of the control system provided in the apparatus; 
         FIG. 7A  is a diagram showing the state a valve assumes in the apparatus while no paper sheets are being transferred over depressions; 
         FIG. 7B  is a diagram showing the state the valve assumes in the apparatus when a paper sheet is transferred, covering the depressions; 
         FIG. 7C  is a diagram showing the depression covered with a paper sheet in the apparatus; 
         FIG. 8  is a perspective view showing one of the small chambers provided in a second embodiment of an image recording apparatus according to this invention; 
         FIG. 9  is a diagram showing two small chambers provided in the apparatus; 
         FIG. 10  is a sectional view showing one of the small chambers provided in the apparatus; 
         FIG. 11  is a diagram showing the configuration of a suction-hole selecting member used in the apparatus; 
         FIG. 12  is a block diagram of the controls system provided in the apparatus; 
         FIG. 13A  is a diagram of the suction-hole selecting member, showing the holes thereof aligned with those suction holes, which have the smallest open areas; 
         FIG. 13B  is a diagram of the suction-hole selecting member, showing the holes thereof aligned with those suction holes, which have the second smallest open areas; 
         FIG. 13C  is a diagram of the suction-hole selecting member, showing the holes thereof aligned with those suction holes, which have the largest open areas; 
         FIG. 14  is a diagram sowing the configuration of a modification of the second embodiment of the image recording apparatus according to this invention; 
         FIG. 15  is diagram showing the configuration of the suction-hole selecting member of the apparatus; 
         FIG. 16  is a diagram explaining how the open areas are varied in the apparatus; 
         FIG. 17  is a perspective view of one of the small chambers provided in a third embodiment of an image recording apparatus according to the present invention; 
         FIG. 18  is a diagram showing the configuration of a fourth embodiment of an image recording apparatus according to this invention; 
         FIG. 19  is a perspective view of one of small chambers provided in a fifth embodiment of an image recording apparatus according to this invention; 
         FIG. 20  is a diagram showing that the valves of the small chambers are opened in the apparatus; 
         FIG. 21  is a diagram showing that the valves of the small chambers are closed in the apparatus; 
         FIG. 22  is a diagram showing depressions of a modified shape, made in the upper surface of the platen frame provided in the apparatus; and 
         FIG. 23  is a diagram showing depressions of another modified shape, made in the upper surface of the platen frame provided in the apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment of this invention will be described with reference to the accompanying drawings. 
       FIG. 1  shows the major components of an image recording apparatus  1  according to the invention. The recording medium is, for example, a paper sheet  10 . The direction in which the paper sheet  10  is transferred is Y-axis direction. In the side of the paper sheet  10 , on which an image will be formed, the direction orthogonal to the Y-axis direction is X axis. The direction orthogonal to X axis and Y axis is Z axis. 
     The image recording apparatus  1  comprises a sheet feeding unit  2 , a platen unit  3 , an image recording unit  4 , a medium-floating detecting unit  5 , and a recording-medium ejecting unit  6 . 
     The sheet feeding unit  2  comprises a sheet tray  7 , a pickup roller  8 , and a pair of registration rollers  9 . The sheet tray  7  can hold sheets  10  that are used as recording media. The pickup roller  8  is a sheet extracting mechanism. The pickup roller  8  is supported by the main frame of the apparatus  1  and can rotate. The pickup roller  8  extracts sheets  10 , one by one, from the sheet tray  7 . 
     The registration rollers  9  have two rollers. The registration rollers  9  are supported by the main frame of the apparatus  1  and can rotate. The registration rollers  9  correct the position of any paper sheet  10  extracted by the pickup roller  8  if the paper sheet  10  is in, for example, a slanting position. That is, the rollers  9  align the paper sheet  10 , coming in a slanting position, with the transfer direction (Y-axis direction). The registration rollers  9  feed the paper sheet  10  to the platen unit  3  at the time the image recording unit  4  forms an image. 
     The platen unit  3  transfers the sheet  10  fed from the sheet feeding unit  2 . The platen unit  3  has a transfer belt  11 , a drive roller  12   a , a plurality of driven rollers  12   b , an upper driven roller  13 , and a platen frame  14 . The platen frame  14  has a platen suction unit  15 . 
     The transfer belt  11  is a belt shaped endless. The transfer belt  11  has a plurality of small through holes, which are made in the entire surface of the belt  11 . The transfer belt  11  is wrapped around, between a drive roller  12   a  and the driven rollers  12   b.    
     The transfer belt  11 , the drive rollers  12   a , and the driven rollers  12   b  constitute a belt conveyor. The belt conveyor transfers the paper sheet  10  in the Y-axis direction. 
     At the upstream of the platen unit  3 , the upper driven roller  13  is provided. The upper driven roller  13  prevents the paper sheet  10  from floating. The upper driven roller  13  is arranged at the end of the transfer belt  11 , which is upstream with respect to the direction in which the paper sheet  10  is transferred. 
     The platen suction unit  15  has a fan  53 . The fan  53  generates an air stream that flows from the upper part of the platen unit  3  toward the lower part thereof. By virtue of the air stream, the paper sheet  10  is drawn to the platen unit  3 . 
     The image recording unit  4  ejects ink to the paper sheet  10 . The image recording unit  4  is arranged above the transfer belt  11 . The image recording unit  4  has a recording head  16  and a carriage  17 . 
     The recording head  16  comprises ink-jet recording heads  16   k ,  16   c ,  16   m  and  16   y , which are associated with, for example, black (K), cyan (C), magenta (M) and yellow (Y), respectively. The ink-jet recording heads  16   k ,  16   c ,  16   m  and  16   y  eject black (K) ink, cyan (C) ink, magenta (M) ink and yellow (Y) ink, respectively. The ink-jet recording heads  16   k ,  16   c ,  16   m  and  16   y  are arranged, at almost regular intervals, in the direction (Y-axis direction) in which the paper sheet  10  is transferred. 
     The ink-jet recording heads  16   k ,  16   c ,  16   m  and  16   y  have a width (in X-axis direction) equal to or greater than the maximum width the paper sheet  10  used in this apparatus  1  may have. If the largest paper sheet on which the apparatus  1  can record images is, for example, the A3-size sheet, the recording heads  16   k ,  16   c ,  16   m  and  16   y  have a width (in X-axis direction) equal to or larger than the width of the A3-size sheet. 
     At upstream of the recording heads  16   k ,  16   c ,  16   m  and  16   y , with respect to the sheet transfer direction, second sheet guides  18  are arranged. Each of the second sheet guides  18  is a roller. Each second sheet guide  18  can rotate, supported at both ends by the main frame of the apparatus. 
     Each second sheet guide  18  prevents the paper sheet  10  from floating, and guides the paper sheet  10 , making the same travel below the recording heads  16   k ,  16   c ,  16   m  and  16   y.    
     The medium-floating detecting unit  5  has a first sheet guide  19 , a sheet-floating detecting plate  21 , and a sensor  22 . Only one first sheet guide  19  is provided. The first sheet guide  19  is arranged, more downstream than all recording heads  16   k ,  16   c ,  16   m  and  16   y , with respect to the sheet transfer direction. The first sheet guide  19  is a roller that suppresses the floating of the paper sheet  10 . The first sheet guide  19  can rotate, supported at both ends by the main frame of the apparatus  1 . 
     The sheet-floating detecting plate  21  is located more downstream than the first sheet guide  19 , with respect to the transfer direction of the paper sheet. The sheet-floating detecting plate  21  detects that the paper sheet  10  being transferred has reached a level higher than a prescribed level, in order to prevent the paper sheet  10  from contacting the recording heads  16   k ,  16   c ,  16   m  and  16   y . The sheet-floating detecting plate  21  is pivotally supported by a shat  23 . The sensor  22  is arranged in the vicinity of the upper edge of the sheet-floating detecting plate  21 . When the paper sheet  10  abuts on the sheet-floating detecting plate  21 , the sensor  22  detects the motion of the sheet-floating detecting plate  21 . 
     The sheet-floating detecting plate  21  is arranged at such a position that a predetermined gap is provided between its lower end and the transfer surface of the transfer belt  11 . 
     The recording-medium ejecting unit  6  is arranged more downstream, in the sheet transfer direction, than the position where the platen unit  3  and the image recording unit  4  are arranged. The recording-medium ejecting unit  6  ejects the paper sheet  10  having an image printed by the image recording unit  4 , out of the main body of the apparatus  1 . The recording-medium ejecting unit  6  has a pair of ejection-transfer rollers  24 , a pair of ejection-port rollers  25 , and an ejection tray  26 . 
     The ejection-transfer rollers  24  transfer the paper sheet  10  transferred from the platen unit  3 , toward the ejection-port rollers  25 . The ejection-port rollers  25  eject the paper sheet  10  transferred from the ejection-transfer rollers  24 , onto the ejection tray  26 . 
       FIG. 2  is a top view showing the platen unit  3 . For the purpose of illustrating the configuration of the platen unit  3 , the right half of the transfer belt  11  is shown in section taken along the Y-axis direction. 
     A roller drive motor  27  is connected to the shaft  12 - 1  of the drive rollers  12   a.    
     A plurality of suction holes  28  are made in the entire surface of the transfer belt  11 , forming a uniform pattern. Below the transfer belt  11 , the platen frame  14  is provided. The platen frame  14  supports the transfer belt  11  in a horizontal position. The platen frame  14  supports the drive rollers  12   a  and the driven rollers  12   b , allowing them to rotate, and further holds the platen suction unit  15  shown in  FIG. 1 . 
     The platen suction unit  15  comprises a fan  53  configured to attract the paper sheet  10 , by suction, to the transfer belt  11 . 
     The platen frame  14  has an XY surface as a belt transfer surface, which extends in the X-axis direction and Y-axis direction. As shown in  FIG. 2 , a plurality of depressions  29  are made in the XY surface of the platen frame  14 . Each depression  29  has a suction hole  31  made almost at the center. 
       FIG. 3  is a sectional view of the platen frame  14 .  FIG. 4  is a sectional view showing the small chamber  40  of each unit. The upper surface (i.e., belt transfer surface) of the platen frame  14  has the depressions  29 . Each depression  29  has a suction port  31  in the bottom. Below the depression  29 , small chambers  40  are provided, each in association with one suction hole  31 . The suction hole  31  connects the depression  28  and the small chamber  40 . 
       FIG. 5  is a perspective view showing one of the small chambers  40 . In the bottom of the small chamber  40 , a large suction hole  32  and a small suction hole  36  are made, the former having a larger opening area than the latter. The large suction hole  32  and the small suction hole  36  communicate with the interior of the small chamber  40  and the platen suction unit  15 , respectively. The large suction hole  32  and the small suction hole  36  have a rectangular shape in this embodiment. They can be circular, nevertheless. Further, the large suction hole  32  and the small suction hole  36  can be made at any other positions, so far as they are made in the bottom of the small chamber  40 . 
     A valve  33  is made in the bottom of each small chamber  40 . The valve  33  can rotate around an axle  35 . The valve  33  can open and close the large suction hole  32 . 
     The valve  33  is formed integrally with a weight  34  that is used as a balancer. The weight  34  is a weight that facilitates the opening and closing of the valve  33 . When no negative pressure is generated by the platen suction unit  15 , the valve  33  rotates in, for example, the clockwise direction ( FIG. 4 ), around the axle  35  by virtue of the weight of the weight  34 , thereby opening the suction hole  32 . When a negative pressure is generated by the platen suction unit  15 , the valve  33  is drawn to the suction hole  32  side, because of the pressure difference between the small chamber  40  and the platen suction unit  15 . As a result, the valve  33  rotates in the counterclockwise direction ( FIG. 4 ) around the axle  35 , closing the suction hole  32 . To open the valve  33 , a spring may be used in place of the weight  34 . 
     The small suction hole  36  has an opening area far smaller than that of the large suction hole  32 . The small suction hole  36  is made in the bottom of the small chamber  40 , at the position where it is not closed by the valve  33 . Thus, the small suction hole  36  communicates with the small chamber  40  and platen suction unit  15 , at all times. 
     The large suction hole  32  and the small suction hole  36  constitute an opening part, which is made in the bottom of the small chamber  40 . Therefore, as the large suction hole  32  is opened or closed, the opening area of the opening part provided in the bottom of the small chamber  40  varies. That is, the bottom of the small chamber  40  has opening area A when the valve  33  opens the large suction hole  32 , opening area B when the valve  33  closes the large suction hole  32 . Hence, the opening area can have three values 0, B and A, which have the relation of 0&lt;B&lt;A. That is, the suction hole  32  and the suction hole  36  have such diameters as define the relation of 0&lt;B&lt;A. 
     In other words, the opening part has a large suction hole  32  and a small suction hole  36 . The valve  33  is provided to open and close the large suction hole  32 . While the valve  33  keeps opening the large suction hole  32 , the total opening area is A, i.e., the sum of the opening areas of the suction hole  32  and suction hole  36  is A. While the valve  33  is closing the large suction hole  32 , the total opening area is B, i.e., the sum of the opening area of the small suction hole  36  only. The size of the valve  33 , the opening areas of the suction holes  32  and  36  are have been determined to achieve the above-mentioned relation of 0&lt;B&lt;A. 
       FIG. 6  is a block diagram showing the configuration of the control system provided in the apparatus  1 . The system has a control unit  50 . The control unit  50  comprises, for example, a CPU, a RAM and a ROM. The control unit  50  controls the sequence of image recording in the apparatus  1 . To the control unit  50 , a console panel  51 , the recording heads  16   k ,  16   c ,  16   m  and  16   y , and the platen unit  3  are connected. The control unit  50  controls the recording heads  16   k ,  16   c ,  16   m  and  16   y  and the platen unit  3 , as the sequence of image recording proceeds. The console panel  51  has various buttons. The buttons may be operated by the user to designate the material, number, printing mode and the like about the paper sheets  10  used to record images. 
     The control unit  50  is connected via a network to a host apparatus. The host apparatus supplies commands to the control unit  50 . The commands designate the number, printing mode and the like about the paper sheets  10 . 
     The image recording that the apparatus  1  configured as described above performs will be explained in brief. 
     First, the power switch is turned on. Next, the console panel  51  or the host apparatus is operated, inputting the data representing the material, number and the like of paper sheets  10  to use in image recording. The control unit  50  starts a printing process, recording characters, images, and the like. That is, the control unit  50  controls the recording heads  16  and the platen unit  3  as the sequence of image recording proceeds. 
     The pickup roller  8  extracts the paper sheets  10 , one by one, from the sheet tray  7 , first extracting the uppermost paper sheet. It extracts one sheet  10  every time it rotates once. The pickup roller  8  feeds each paper sheet  10  extracted from the sheet tray  7 , to the registration rollers  9  that is arranged in pair. The registration rollers  9  temporarily stop rotating, and corrects the position of the paper sheet  10  if the paper sheet  10  assumes, for example, a slanting position with respect to the main scanning direction (i.e., X direction). The registration rollers  9  wait until the time the paper sheet  10  should be transferred forwards. When this time comes, the registration rollers  9  starts transferring the paper sheet  10  toward the platen unit  3 . 
     When the paper sheet  10  fed to the platen unit  3  reaches that end of the platen unit  3 , which is upstream with respect to the sheet transfer direction, the paper sheet  10  is guided by the upper driven roller  13  to the first sheet guide  19 . The paper sheet  10  travels below the first sheet guide  19  and sheet-floating detecting plate  21 , while being prevented from floating by the first sheet guide  19 , and is then placed onto the transfer belt  11  and transferred forwards. The paper sheet  10  reaches the second sheet guide  18  located immediately before the first recording head  16   k . Next, the paper sheet  10  travels below the recording heads  16   k ,  16   c ,  16   m  and  16   y , one after another. While the paper sheet  10  is so traveling, characters and an image are recorded on the paper sheet  10 . The paper sheet  10  is then transferred to the recording-medium ejecting unit  6 . 
     How each valve  33  operates during the image recording process described above will be explained with reference to  FIG. 7A  to  FIG. 7C . 
     The paper sheet  10  laid on the transfer belt  11  travels over the depressions  29  made in the upper surface of the platen frame  14 . 
       FIG. 7A  shows the state that the paper sheet  10  does not reach a position over a depression  29  yet. At this time, the suction hole  31  made in the depression  29  is open. An air stream flows from the small chamber  40  (a recording medium side) toward the platen suction unit  15  side(a suction unit side). This is due to the difference between the pressure in the small chamber  40  and the pressure in the platen suction unit  15 . The air stream rotates the valve  33  around the axle  35  in the counterclockwise direction. Hence, the valve  33  closes the large suction hole  32 . 
     At this time, the small suction hole  36  is not closed. Nonetheless, air flows in a small amount through the small suction hole  36  into the platen suction unit  15  side. Furthermore, the small chamber  40  communicates with the atmosphere through the large suction hole  31 . Therefore, the difference of the pressure in the small chamber  40  (the recording medium side) and the pressure in the platen suction unit  15  (the suction unit side) changes little. The valve  33  therefore keeps closing the suction hole  32 . 
       FIG. 7B  shows the state that immediately after the paper sheet  10  has been transferred, covering the depressions  29 . Each suction hole  31  is closed by the paper sheet  10 , whereby the small chamber  40  isolated from the atmosphere. By contrast, the small suction hole  36  remains open, and an air stream flows into the platen suction unit  15  side through the small suction hole  36 . Consequently, the pressure in the small chamber  40  falls fast. 
     As a result, the difference between the pressure in the small chamber  40  and the pressure in the platen suction unit  15  side gradually decrease. The moment deriving from the weight of the weight  34  becomes larger than the force (moment) that pushes down the valve  33  to close the large suction hole  32 . The valve  33  therefore rotates in the clockwise direction, opening the large suction hole  32  as shown in  FIG. 7C . 
     When the large suction hole  32  is opened, the pressure in the small chamber  40  becomes equal to the pressure in the platen unit  15  side. As a result, a difference is made between the pressure outside the platen unit  14  and the pressure in the small chamber  40 . This pressure difference draws the paper sheet  10  to the transfer belt  11  on the upper side of the platen frame  14 . 
     In the first embodiment described above, while no paper sheets  10  remain on the platen frame  14 , each valve  33  automatically closes the associated large suction hold  32 , preventing a flow of unnecessary air. If the paper sheet  10  covers the suction hole  31  made in the depression  29 , air will flow from the small chamber  40  through the small suction hole  36 , fast lowering the pressure in the small chamber  40 . Therefore, the valve  33  rotates in the clockwise direction, opening the large suction hole  32  as shown in  FIG. 7C . The paper sheet  10  is thereby drawn to the upper surface of the platen frame  14 , more precisely onto the transfer belt  11 . 
     In the present embodiment, the small suction hole  36  made in the bottom of each small chamber  40  serves to open and close the valve  33 . No dedicated mechanisms are required to open and close the valve  33 . This can reduce the manufacturing cost. 
     A second embodiment of this invention will be described with reference to the accompanying drawings. Note that the image recording apparatus  1  according to the second embodiment is identical in overall configuration to the apparatus shown in  FIG. 1  and  FIG. 2 . 
       FIG. 8  is a perspective view showing one of the small chambers provided in the image recording apparatus  1 . In this apparatus  1 , the opening size of the small suction holes  36  varies. The bottom of each small chamber  40  has, for example, three small suction holes  36 - 1 ,  36 - 2  and  36 - 3 , not one small suction hole  36  as in the first embodiment. The small suction holes  36 - 1 ,  36 - 2  and  36 - 3  have different opening areas. More specifically, the suction hole  36 - 1  is the smallest, the suction hole  36 - 2  is the next smallest, and the suction hole  36 - 3  is the largest of the three. The small suction holes  36 - 1 ,  36 - 2  and  36 - 3  are arranged in, for example, the X-axis direction that is orthogonal to the sheet transfer direction. 
       FIG. 9  is a diagram showing, for example, two of the small chambers  40  as viewed from above. The small suction holes  36 - 1 ,  36 - 2  and  36 - 3  of each small chamber  40  are arranged on a line extending in the X-axis direction. Since more than two small chambers  40  are arranged in the X-axis direction, the small suction holes  36 - 1 ,  36 - 2  and  36 - 3  of each small chamber  40  are arranged on a line extending in the X-axis direction. 
       FIG. 10  is a sectional view that shows one of the small chambers  40 . On the bottom of the small chamber  40 , a suction-hole selecting member  38  is provided. The suction-hole selecting member  38  contacts the bottom of the small chamber  40  and can slide in the X-axis direction. The suction-hole selecting member  38  can slide over all small suction holes  36 - 1 ,  36 - 2  and  36 - 3  of the small chamber  40 . The suction-hole selecting member  38  is shaped like, for example, a plate as is illustrated in  FIG. 11 . 
     The suction-hole selecting member  38  has selection holes  39 . The selection holes  39  are positioned not to cover one of the small suction holes  36 - 1 ,  36 - 2  and  36 - 3 , and to cover the remaining two small suction holes. In other words, as the suction-hole selecting member  38  slides, each selection hole  39  comes into alignment with one of the small suction holes  36 - 1 ,  36 - 2  and  36 - 3 , thereby selecting one of the small suction holes  36 - 1 ,  36 - 2  and  36 - 3 , and closing the other two small section holes. 
     The intervals at which the selection holes  39  are arranged is equal to the distance between the small suction holes  36 - 1  of the small chambers  40 , to the distance between the small suction holes  36 - 2  thereof, and to the distance between the small suction holes  36 - 3  thereof. The suction-hole selecting member  38  has two selection holes  39 , which are associated with the small chambers  40  shown in  FIG. 9 . In the actual situation, however, the suction-hole selecting member  38  has as many selection holes  39  as the small chambers  40  arranged in the X-axis direction as illustrated in  FIG. 2 . 
       FIG. 12  is a block diagram of the controls system provided in the apparatus  1  according to the second embodiment. The system has a control unit  50 . To the control unit  50 , a selection-hole selecting member drive unit  52  is connected. The selection-hole selecting member drive unit  52  slides the suction-hole selecting member  38  in the X-axis direction. 
     The control unit  50  drives and controls the selection-hole selecting member drive unit  52 , which slides the suction-hole selecting member  38  in the X-axis direction. The control unit  50  slides the suction-hole selecting member  38  in the X-axis direction, in accordance with the speed with which the platen unit  3  transfers the paper sheet  10 . One of the small suction holes  36 - 1 ,  36 - 2  and  36 - 3  is thereby selected. The opening area defined by the large suction hole  32  and one of the small suction holes  36 - 1 ,  36 - 2  and  36 - 3  is thereby changed. 
     To be more specific, if the platen unit  3  transfers the paper sheet  10  at high speed, the control unit  50  slides the member  38  to increase the opening area. That is, the control unit  50  selects, for example, the small suction hole  36 - 3 . Conversely, the platen unit  3  may transfer the paper sheet  10  at low speed. In this case, the control unit  50  slides the member  38  to decrease the opening area. Thus, the control unit  50  selects, for example, the small suction hole  36 - 1 . 
       FIG. 13A  shows the suction-hole selecting member  38  so positioned that the selection holes  39  are aligned with the small suction holes  36 - 1  of the small chambers  40 . Therefore, only each small suction hole  36 - 1 , which has the smallest opening area of the three suction holes  36 - 1  to  36 - 3 , is opened, and the two other suction holes  36 - 2  and  36 - 3  are closed. 
       FIG. 13B  shows the suction-hole selecting member  38  so positioned that the selection holes  39  are aligned with the suction holes  36 - 2  of the small chambers  40 . Thus, only each small suction hole  36 - 2 , which has the second smallest opening area, is opened, and the two other suction holes  36 - 1  and  36 - 3  are closed. 
       FIG. 13C  shows the suction-hole selecting member  38  so positioned that the selection holes  39  are aligned with the suction holes  36 - 3  of the small chambers  40 . In this case, only each small suction hole  36 - 3 , which has the largest smallest opening area, is opened, and the two other suction holes  36 - 1  and  36 - 2  are closed. 
     In the second embodiment, three suction holes  36 - 1  to  36 - 3  having different opening areas are made in the bottom of each small chamber  40 . The suction-hole selecting member  38  that has selection holes  39  is provided to slide on the bottoms of the small chambers  40 . The control unit  50  controls the sliding of the suction-hole selecting member  38 , in accordance with the speed with which the paper sheet  10  is transferred by the platen unit  3 . The suction-hole selecting member  38  therefore selects one of the small suction holes  36 - 1  to  36 - 3 . The opening area defined by the large suction hole  32  and one of the small suction holes  36 - 1  to  36 - 3  is thereby changed. 
     The user may change the recording mode of the image recording apparatus  1 , changing the transfer speed of the paper sheet  10 . In this case, the control unit  50  controls the sliding of the suction-hole selecting member  38 . One of the small suction holes  36 - 1  to  36 - 3  is thereby selected. 
     The case where the largest of the small suction holes  36 - 1  to  36 - 3  is selected will be explained first. As mentioned above,  FIG. 7B  shows the state that immediately after the paper sheet  10  has been transferred, covering the depressions  29 . At this time, more air flows through the suction hole  36 - 3  than through the suction hole  36 - 2 . The pressure in the small chamber  40  therefore falls relatively fast. Hence, the valve  33  is opened fast, and the paper sheet  10  is fast drawn onto the transfer belt  11 . 
     If the suction hole  36 - 3 , which has the largest opening area, is selected, the paper sheet  10  being transferred can be efficiently prevented from floating at its distal part. Therefore, the selection of the suction hole  36 - 3  is advisable in the case where the sheet transfer speed is high. 
     On the other hand, if the suction hole  36 - 1 , which has the smallest opening area, is selected, air flows through this suction hole  36 - 1  in a relatively small amount. The airflow therefore less influences the ejection of ink droplets. In addition, the operating ability of the negative-pressure generating device, such as platen unit  15 , can be lowered. Since it takes longer to draw the paper sheet  10 , the selection of the suction hole  36 - 1  is desirable in the case where the sheet transfer speed is low. 
     Hence, it is desired that the suction hole  36 - 3  having the largest opening area be selected if the paper sheet  10  is transferred at high speed, and the suction hole  36 - 1  having the smallest opening area be selected if the paper sheet  10  is transferred at low speed. 
     The second embodiment may be modified as will be described below. 
     In the second embodiment described above, a plurality of suction holes  36 - 1  to  36 - 3  are made in the platen frame  14  for, each small chamber  40 . One suction-hole selecting member  38  has selection holes  39 . The second embodiment may be modified such that selection holes  39  are made in the platen frame  14  and a plurality of suction holes  36 - 1  to  36 - 3  are made in the suction-hole selecting member  38 . 
     Alternatively, the second embodiment described above may be modified as follows.  FIG. 14  is a diagram showing two of the small chambers  40  as viewed from above. One rectangular hole  36 - 10  is made in the bottom of each small chamber  40 . The rectangular hole  36 - 10  is elongated in, for example, the Y-axis direction. On the bottom of the small chamber  40 , such a suction-hole selecting member  38  as shown in  FIG. 15 , used as a closing member, is provided and able to slide in the X-axis direction. The suction-hole selecting member  38  has opening adjusting holes  39 - 1 . As seen from  FIG. 16 , the opening adjusting holes  39 - 1  can change the opening area of the rectangular holes  36 - 10  as the suction-hole selecting member  38  is slid in the X-axis direction. 
     A third embodiment of this invention will be described. 
       FIG. 17  is a perspective view showing one of the small chambers that are provided in the third embodiment of an image recording apparatus according to this invention. A suction hole  36  is made in the center part of a valve  33 . The suction hole  36  may be made in any other part of the valve  33 , not necessarily in the center part. In this embodiment, only the suction hole  32  need be made in the bottom of each small chamber  40 . The area the chamber  40  occupies can therefore be smaller than otherwise. This makes it possible to arrange as many suction holes as possible in the platen frame  14 . 
     A fourth embodiment of this invention will be described. 
       FIG. 18  is a sectional view showing the configuration of one of the small chambers  40 . On the rim of the suction hole  32 , except that part of the rim, at which an axle  35  is positioned, a porous member  41  is provided. The porous member  41  can be made of, for example, foamed urethane. The porous member  41  performs the function of tiny suction holes. The use of the porous member  41  can reduce the sound generated when the valve  33  is closed. The sound the image recording apparatus makes as it operates can be thereby decreased. 
     A fifth embodiment of this invention will be described. 
       FIG. 19  is a perspective view of one of small chambers  40  used in the fifth embodiment.  FIG. 20  shows the state in which the valve of each small chamber  40  is open. The valve structure comprises a valve  33  and a suction hole  32 . The suction hole  32  is made in the bottom of the small chamber  40 . A projecting suction hole  32   a  is provided and is continuous to the suction hole  32 . The projecting suction hole  32   a  remains open if the valve  33  closes the suction hole  32 . That is, only the projecting suction hole  32   a  is open even if the suction hole  32  is closed. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 
     In some cases it may not be advisable to make many depressions  29  in the upper surface of the platen frame  14 , in view of manufacturing cost. If so, less depression  29  should better be made in the platen plate  14 .  FIG. 22  shows an exemplary configuration, wherein the number of depressions  29  is decreased. A plurality of depressions  29  are made in the upper surface of the platen frame  14 , each being shaped like a band and extending in the sheet transfer direction (i.e., Y-axis direction). In each depression  29 , one or more suction holes  31  are made. 
     The configuration described above is appropriate if the paper sheet  10  is long in its transfer direction and if its width varies, like a rolled paper sheet. Since one depression  29  need not be provided for each suction hole  31 , the manufacturing cost can be lowered. The configuration shown in  FIG. 22  has a plurality of suction holes  31 . Nonetheless, only one suction hole  31  may be provided. 
       FIG. 23  shows a configuration in which the number of depression is reduced. Depressions  29  made in the upper surface of the platen frame  14  are arranged in rows extending in a direction (X-axis direction) that is orthogonal to the sheet transfer direction. One or more suction holes  31  are made in each depression  29 . This configuration is appropriate if the paper sheets  10  have a constant width that is almost equal to the width of the depressions  29 . In this case, too, one depression  29  need not be made for each suction hole  31 , to lower the manufacturing cost. In  FIG. 23 , a plurality of suction holes  31  are made. Nonetheless, only one suction hole  31  may be made.