Patent Publication Number: US-2005128276-A1

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-416789, filed Dec. 15, 2003, 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 forming apparatus which discharges ink to a conveyed recording medium to thereby form an image.  
      2. Description of the Related Art  
      In general, an image forming apparatus to which a plurality of recording heads are fixed and which discharges four different colors (e.g., B: black, C: cyan, M: magenta, Y: yellow) of ink with respect to a conveyed recording medium (e.g., recording paper) to form a color image, a so-called ink jet type image forming apparatus has been known. A nozzle row of a plurality of nozzles arranged in one row is formed in this recording head in such a manner as to have a length which is not less than a width of the recording medium. Additionally, a plurality of recording heads having nozzle rows having a length shorter than the recording medium width are used, and connected to one another in such a manner as to have overlap portions on opposite sides in a direction (sub-scanning direction) crossing a conveying direction (main scanning direction) of the recording medium at right angles, and an image can be formed/finished by apparently one nozzle row.  
      For example, in Jpn. Pat. Appln. KOKAI Publication No. 2003-11377, an image forming apparatus is described on which four recording heads (referred to, e.g., as line heads) provided with one nozzle row not shorter than the width of the recording medium are mounted and which discharges different colors of ink. The nozzle row of this recording head is arranged along the sub-scanning direction. A constitution example is also described in which a plurality of recording heads are alternately arranged along the sub-scanning direction in such a manner as to have the overlap portions of the nozzle rows.  
      In these recording heads, an annular conveying belt constituting a conveying mechanism (belt platen) of the recording medium is extended between at least two rollers, and the recording heads are arranged in such a manner that nozzle surfaces (surfaces on which the nozzle rows are formed) of the recording heads are arranged facing the rotating conveying belt.  
      In the recording head of an ink jet system, ink sticks to the nozzle and solidifies, accordingly an ink discharge port is clogged, and therefore maintenance needs to be periodically performed. Therefore, a movable maintenance unit is disposed in the recording head. The maintenance unit comprises a protection unit which closely sticks to the nozzle surface of the recording head and occludes the surface from the outside to protect the recording head from the solidifying ink, sticking dust or the like when not used, that is, when any image is not formed.  
      In the above-described conventional image forming apparatus, an interval between the recording medium conveying surface and the nozzle (ink discharge port) in the belt platen is set to about 1 mm at the time of image formation. Therefore, at a maintenance or protection time, the recording head is raised, or the belt platen is lowered to expand this gap, and the maintenance unit is disposed facing the nozzle surface in the vicinity of the nozzle surface.  
      Then, as described in Jpn. Pat. Appln. KOKAI Publication No. 2003-11377, the maintenance units are alternately arranged facing the conveying surface of the recording medium between the recording heads in such a manner as to minimize a movement amount of the recording head or the maintenance unit.  
     BRIEF SUMMARY OF THE INVENTION  
      According to the present invention, there is provided an image forming apparatus in which a plurality of recording heads are arranged in the vicinity of one another, and a maintenance unit is disposed in the vicinity of these recording heads in accordance with the arrangement of the recording heads, so that color difference in color image formation is prevented, sufficient maintenance is realized, and an image is stably formed.  
      According to the present invention, there is provided an image forming apparatus comprising: image forming means of an ink jet system; medium holding/conveying means disposed facing the image forming means to support a recording medium; and maintenance means for maintaining an ink discharge surface of the image forming means, wherein the ink discharge surface of the image forming means and at least a part of the maintenance means are disposed facing the medium holding/conveying means, and the maintenance means is disposed on an upstream side of the image forming means in a conveying direction. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       FIG. 1  is a diagram showing a schematic constitution example of an image forming apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a diagram showing a constitution example of a belt platen unit in the first embodiment;  
       FIG. 3  is a diagram showing a schematic constitution example of a recording head;  
       FIG. 4  is a diagram schematically showing an ink channel in the image forming apparatus of the present invention;  
       FIG. 5  is a diagram showing an appearance constitution of a suction section and a cap section in a maintenance unit in the first embodiment;  
       FIG. 6  is a diagram showing a sectional constitution of an oil pan in the first embodiment;  
       FIG. 7  is a diagram showing an arrangement example of the recording head and maintenance unit in the first embodiment;  
       FIG. 8  is a diagram showing an inner constitution example in the image forming apparatus of the present invention;  
       FIG. 9  is a diagram showing one example of an image in which color difference is not generated;  
       FIG. 10  is a diagram showing a schematic constitution example of the image forming apparatus according to a second embodiment of the present invention;  
       FIG. 11  is a diagram showing an arrangement example of the recording head and maintenance unit in a conventional constitution;  
       FIG. 12  is a diagram showing one example of an image in which the color difference is generated; and  
       FIG. 13  is a diagram showing a constitution example of an image forming apparatus according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the present invention will be described hereinafter in detail with reference to the drawings.  
       FIG. 1  shows a schematic constitution example of an image forming apparatus according to a first embodiment of the present invention. The example will be described. In the following embodiment, it is assumed that a conveying direction of a recording medium is an X-axis direction or a main scanning direction, and a direction crossing the conveying direction is a Y-axis direction or a sub-scanning direction.  
      This image forming apparatus is an image forming apparatus of a so-called ink jet system, in which a plurality of recording heads are fixed, and discharge four different colors (e.g., B: black, C: cyan, M: magenta, Y: yellow) of ink to thereby form a color image onto a conveyed recording medium such as recording paper.  
      This image forming apparatus roughly comprises: a medium supplying/discharging mechanism which supplies or discharges a recording medium; a conveying mechanism which conveys the recording medium; an image forming mechanism which discharges ink onto the recording medium to thereby form an image; an ink supplying/storing mechanism which supplies or stores the ink; and a maintenance mechanism which maintains the recording heads.  
      First, the medium supplying/discharging mechanism comprises: a recording medium supply base  1  on which a recording medium  2  is to be charged; a supply roller  3  which supplies/feeds the recording medium  2 ; a separation roller  4  which prevents superimposition of the recording mediums  2 ; a resist roller pair  5  which corrects skewing of the recording medium  2 ; a discharge roller pair  10  which discharges the recording medium  2 ; and a discharge tray  11  which stores the discharged recording medium.  
      In this constitution, the supply roller  3  is disposed in a take-out port of the recording medium supply base  1  onto which the recording medium  2  is charged, and abuts on the uppermost surface of the recording medium  2  to take out and supply/feed the recording medium  2 . The separation roller  4  abuts on the back surface of the taken recording medium  2  to rotate the recording medium  2  in a returning direction toward the recording medium supply base  1  via a torque limiter (not shown) in such a manner as to prevent the taken recording mediums  2  from being superimposed on each other. The resist roller pair  5  brings a tip of the recording medium  2  into contact with the stopped roller pair, and corrects the skewing utilizing looseness generated at this time, while feeding the recording medium  2  on the side of the conveying mechanism.  
      Next, the conveying mechanism mainly comprises a belt platen unit  7  and a platen vertical movement mechanism (not shown). In this belt platen unit  7 , as shown in  FIG. 2 , a case formed of a box-shaped sheet metal is sealed by a platen  25  which is formed of an aluminum plate having a thickness of about 5 mm and in which a large number of through holes are made from above the case to thereby constitute a suction chamber  24 . A plurality of suction fans  26  for setting pressure in the suction chamber to be negative are attached to a bottom part in the suction chamber  24 . Discharge ports  24   a ,  24   b  for discharging exhaust air from the suction fans  26  are disposed in one side surface of the case.  
      Moreover, a driving roller  21 , a driven roller  23 , and a tension roller  22  are disposed on an upstream side, on a downstream side, and under the conveying direction of the suction chamber  24 , and a belt  27  is rotatably attached in such a manner as to surround the rollers. The belt  27  has a structure which passes air, for example, a structure in which a large number of suction holes are formed. While tension is imparted by the tension roller  22 , the belt is rotated by the driving roller  21  in a contact state with respect to the surface of the platen  25 . By this structure, the recording medium is adsorbed on the side of the platen  25  together with the belt  27  by the suction fans  26 , and can be conveyed.  
      As shown in  FIG. 1 , the image forming mechanism is constituted of four head units  9  having a plurality of ink jet system recording heads, arranged facing the belt platen unit  7  (belt conveying surface). These recording heads discharge four different colors of ink for each head unit  9 .  
      Moreover, in the maintenance mechanism, a maintenance unit  8  is disposed facing the belt conveying surface in the same manner as in the head unit  9 . Therefore, the maintenance unit is disposed on the upstream side of the conveying direction, and the head unit  9  is disposed on the downstream side thereof.  
      The ink supplying/storing mechanism mainly comprises: a plurality of bottles  12  in which four colors of ink are charged; sub-tanks  13  constituting tanks for supplying the ink to the recording heads from the bottles  12 ; a suction pump  15  which sucks unnecessary ink; and a waste liquid bottle  16  which collects and stores the unnecessary ink.  
      The bottles  12  are arranged as bottles  12 K to  12 Y described later in which the four colors of ink are charged in a highest position in the ink channel in an upper part of an image forming apparatus main body. The bottles  12  are connected to the sub-tanks  13  via replenishment paths (not shown). The ink which has overflowed the sub-tank  13  is stored in the waste liquid bottle  16  by the suction pump  15 .  
      The respective constituting components will be described hereinafter in detail.  
      First, a constitution of the head unit  9  will be described with reference to  FIG. 3 .  
      In recording heads  31  to  36 , about 636 nozzles (not shown) are arranged in one nozzle row or two nozzle rows at a density of 300 dpi (interval or 84.6 μm) in a direction (sub-scanning direction) crossing a medium conveying direction (main scanning direction) at right angles. A surface on which this nozzle row is disposed is hereinafter referred to as a nozzle surface.  
      These recording heads  31  to  36  are alternately arranged along the sub-scanning direction. The recording heads are arranged in the medium conveying direction in such a manner that opposite ends of the nozzle row are superimposed (additionally, one end of the recording head  31  or  36  is superimposed). A discharge timing at which the ink is discharged for each recording head is, needless to say, adjustable. By this arrangement, apparently the recording heads  31  to  36  form the image as if the recording heads had one nozzle row having a nozzle pitch of 300 dpi and a nozzle number of about 3816. Therefore, even if the nozzles are arranged in a plurality of rows with respect to the medium conveying direction, one straight line can be formed in the sub-scanning direction (lateral direction) on the recording medium  2  by adjustment of the discharge timing.  
      As shown in  FIG. 3 , these recording heads  31  to  36  are connected to an ink distributor  9   a  via tubes  9   b . The ink is replenished to this ink distributor  9   a  from the sub-tank  13  via a tube  9   c . This ink distributor  9   a  is planar, and has an ink channel therein, and an upper part of the ink distributor has a tilt gradient. This is because bubbles generated inside are collected in a highest position, and entrance of the bubbles into the nozzles is prevented. The recording heads  31  to  36  are connected to a control substrate via individual wirings (not shown).  
      Moreover, the nozzle surfaces in which the nozzles of the respective recording heads  31  to  36  are formed are arranged facing the belt  27  in such a manner as to obtain an interval of about 1 mm from the belt surface. Correctly, a distance from the recording medium adsorbed and conveyed by the belt  27  is set to about 1 mm. These recording heads  31  to  36  and the ink distributor  9   a  which supplies the ink to these heads constitute the head unit  9  ( 9 K,  9 C,  9 M,  9 Y). The head unit  9  is divided into units of the respective colors. Here, black ink is charged in a head unit  9 K, cyan ink is similarly charged in a head unit  9 C, magenta ink is charged in a head unit  9 M, and yellow ink is charged in a head unit  9 Y.  
       FIG. 4  schematically shows an ink channel in the image forming apparatus of the present invention. Next, the ink channel will be described.  
      In this apparatus, the respective colors of ink are charged in four bottles  12 K to  12 Y, and the bottles are attached to highest positions in the ink channel. Black will be described in accordance with an example of one system of the ink. The replenishment path is connected to a sub-tank  13 K from the bottle  12 K via an electromagnetic valve  13 Ka. A tube for releasing the inside to the atmosphere is extended from a top part of the sub-tank  13 K, and the sub-tank is connected to an overflow tank  18  via an atmosphere release magnetic valve  13 Kc. As shown in  FIG. 1 , a pressurizing pump  14  is disposed in the apparatus, and connected to the sub-tank  13 K via the electromagnetic valve  13 Kc.  
      Moreover, a liquid suction port is disposed in the sub-tank  13 K in such a manner as to pump up accumulated black ink from a bottom part, and connected to the ink distributor  9   a  of the head unit  9 K by a replenishment tube  93 . A liquid level detection sensor (not shown) is disposed in the sub-tank  13 . When a liquid level drops, the electromagnetic valve  13 Ka is opened, and the ink is replenished to the sub-tank  13 K from the bottle  12 K by an ink weight. A cap section  42  and a suction section  41  are prepared for each of head units  9 K to  9 Y. The overflow tank  18  for receiving the ink which has overflowed the sub-tank  13 , an ink pan  40 , and the suction pump  15  are connected to the waste liquid bottle  16 , and the finally unnecessary ink is stored in the waste liquid bottle  16 .  
      Next, the maintenance unit will be described with reference to  FIGS. 5 and 6 . Here,  FIG. 5  shows an appearance constitution of the suction section and cap section in the maintenance unit, and  FIG. 6  is a diagram showing a sectional constitution of an oil pan.  
      The maintenance unit  8  comprises the box-shaped ink pan  40  divided into a plurality of chambers  40   a  to  40   d  by walls in such a manner as to prevent the overflow of the ink in the apparatus, and maintenance sections disposed in the respective chambers  40   a  to  40   d  to maintain or protect the nozzle surfaces of the recording heads  31  to  36 . In the present embodiment, since the maintenance unit is assigned to each recording head, the chambers are disposed as many as the recording heads. Needless to say, the present invention is not limited to this embodiment. When one maintenance unit takes charge of the maintenance of a plurality of recording heads, the corresponding number of the chambers are disposed.  
      In each of the chambers  40   a  to  40   d , an annular guide portion  41   c  is disposed in a direction crossing the medium conveying direction at right angles, and the guide portion  41   c  is fitted and attached in such a manner as to be slidable in a direction of an arrow  41   d  (sub-scanning direction).  
      These suction sections  41  face the nozzle surfaces of the recording heads  31  to  36 , and suction ports  41   b  for sucking surplus ink which has dropped from each nozzle are disposed in the suction sections. The suction pump  15  which applies a pressure (negative pressure) for suction from the suction ports  41   b  is connected to a convex pipe port  41   a  disposed in the side surface of the suction section  41  by a tube.  
      This suction section  41  is moved along the guide portion  41   c  in the sub-scanning direction by the driving mechanism (not shown). Moreover, the suction section is rotatable in an arrow  41   e  direction, and moved in the arrow  41   d  direction while a triangular guide portion  41   f  on the upper surface is pressed against an edge of each of the recording heads  31  to  36 . By this movement, the suction ports  41   b  of the suction section  41  are moved along the nozzle row, facing the nozzles, and the suction is possible in this state. It is to be noted that the suction ports  41   b  are prevented from being brought into contact with the nozzle surface by the triangular guide portion  41   f.    
      Moreover, the cap section  42  is vertically movably disposed facing under the nozzle surfaces of the recording heads  31  to  36 . The cap section  42  is formed, for example, of an elastic member such as rubber or resin, and a groove  42   a  which is a concave portion is formed only in a portion facing the nozzles of each of the recording heads  31  to  36 . A hole is made in a bottom part of the groove  42   a , and connected to a pipe  42   b , and the ink accumulated in the groove  42   a  is discharged to the outside. This cap section  42  is controlled in such a manner as to be attachable/detachable with respect to the recording heads  31  to  36  by an attaching/detaching mechanism (not shown). Furthermore, the cap section  42  is urged in such a manner as to be pressed by a spring  42   c , when abutting on the recording heads  31  to  36 . It is to be noted that the suction section  41  moves among the recording heads  31  to  36  in a state in which the cap section  42  is detached from the recording heads  31  to  36  to thereby suck the surplus ink sticking to the nozzle surfaces.  
      Moreover, in the present embodiment, it has been described that the suction section  41  is a section separate from the cap section  42  in the maintenance unit, but the present invention is not limited to this embodiment. For example, the suction section  41  may be disposed in the cap section  42  to thereby constitute a maintenance device of such a type that the ink is sucked from the nozzle, when the cap section abuts on the nozzle section.  
      As shown in  FIG. 1 , the maintenance unit  8  of the present embodiment is disposed facing the belt  27  on the upstream side of the medium conveying direction of the head units  9 K to  9 Y. Since the respective head units  9 K to  9 Y are arranged adjacent to one another, as shown in  FIG. 7 , a distance L 5  is formed. On the other hand, as shown in  FIG. 11 , the head unit  9  ( 9 K to  9 Y) has heretofore had a distance L 2 , and the distance between the recording heads in the present embodiment is shorter.  
      Moreover, in the conventional constitution shown in  FIG. 11 , an auxiliary roller  6  abuts on the roller  23  via a belt by its own weight. There has heretofore been a distance L 3  from the auxiliary roller  6  to the first head unit  9 K, but there is a distance L 6  in the present embodiment, and the distance largely lengthens.  
      As to this distance, in the conventional constitution shown in  FIG. 11 , the recording medium needs to be brought into close contact with the belt immediately after the tip of the recording medium rides on the belt. Additionally, at this time, most of suction holes disposed in the belt are not closed. This constitution requires a strong suction force also in a top portion of the belt on the upstream side in order to prevent the recording medium abutting on the belt from being shifted or floated. A large number of suction fans are required, and the suction fan is also disposed on the upstream side.  
      On the other hand, in the present embodiment, as shown in  FIG. 7 , the maintenance unit is disposed on the upstream side of the head units  9 K to  9 Y. Therefore, the recording medium passes under the maintenance unit  8 , and reaches the first head unit  9 K. The lower surface of the maintenance unit can be disposed in a position apart from the head with respect to the belt  27 . Concretely, the lower surface is disposed apart from the belt by 5 mm.  
      That is, even if the recording medium slightly floats up, the medium can pass under the maintenance unit  8 , and any paper is not jammed. Thereafter, the half of the suction holes made in the belt  27  are closed by the recording medium until the recording medium reaches the head unit  9 K. An internal pressure of the chamber is maintained to be high, and even the tip portion of the recording medium completely closely adheres to the belt  27 . Therefore, the number of the suction fans can be reduced as compared with the conventional constitution.  
      The image formation in the image forming apparatus constituted in this manner will be described.  
      First, the recording medium  2  supplied from the recording medium supply base  1  is supplied/fed by the supply roller  3 , separated into one sheet by the separation roller  4 , and abuts on the resist roller pair  5 . The sheet is supplied again from the resist roller pair  5  at a taken timing, and the tip thereof is nipped and conveyed between the auxiliary roller  6  and the belt  27 . At this time, a conveying speed of the resist roller pair is set to be lower than that of a medium conveying speed by the supply roller  3  by 1%. Furthermore, the conveying speed by the belt  27  is set to be lower than that of the resist roller pair  5  by 1%. The medium tip which has ridden on the belt  27  is adsorbed by the belt  27  by a suction force (negative pressure) generated by the suction fan  26 .  
      The medium tip held between the belt  27  and the auxiliary roller  6  is not sufficiently adsorbed by the belt  27 , but is securely adsorbed as the medium advances toward the head unit  9 K. Immediately after the sheet tip is held between the auxiliary roller  6  and the belt  27 , the sheet moves at the conveying speed of the resist roller pair  5 . However, as the sheet is securely adsorbed by the belt  27 , the sheet is conveyed at a speed equal to the conveying speed of the belt  27 .  
      In the present embodiment, the head units  9 K to  9 Y are arranged adjacent to one another, and the maintenance unit  8  is disposed facing the belt platen unit on the upstream side of the head unit. Accordingly, while the sheet tip passes under the maintenance unit  8 , the conveying speed is stabilized in accordance with the speed of the belt  27 . Since the sheet securely closely adheres to the belt  27 , the float-up is eliminated, and the contact with the head unit  9 K is prevented.  
      The recording medium  2  nipped between the auxiliary roller  6  and the belt  27  is adsorbed by the belt  27  by the negative pressure in a suction chamber, and passes under the head unit  9 . In this case, a change of the position of the recording medium  2  is read by a line sensor (CCD) disposed immediately after the auxiliary roller  6 , and the ink discharge from the nozzle is controlled. Concretely, the timing at which the medium tip passes under each head unit  9  is calculated, and controlled in such a manner as to match a discharge start timing. A change of the position of a side-surface end of the recording medium is read, a medium position in a lower part of each head unit  9  is predicted, and a mask process is performed in such a manner as to prevent the ink from being discharged to a portion other than the recording medium.  
      Moreover, after reaching the portion under the head unit  9 K, the conveyed recording medium  2  successively takes the timing in  9 C,  9 M,  9 Y to thereby form an image. It is to be noted that as to a reference of this timing, an encoder (not shown) is disposed in the belt platen unit, and a pulse signal generated in accordance with the movement amount of the belt is used. Since the conveying speed of the recording medium  2  is constant by this conveyance, the color difference of the image formed in each color can be minimized.  
      Thereafter, the recording medium  2  on which the image has been formed is discharged by the discharge roller pair  10 , and stored in the discharge tray  11 .  
      The conveying direction sometimes slightly differs in the above-described conveyance of the recording medium  2 . This is a phenomenon referred to as so-called skewing, in which a length direction of the recording medium  2  does not match the conveying direction, and is oblique. That is, the medium is not constantly conveyed in a direction crossing the sub-scanning direction of the head unit  9  at right angles. The medium is conveyed in a slightly different direction by eccentricity of the head unit  9  or a conveying direction error with respect to the belt  27 .  
      The color difference by the skewing will be described.  
       FIG. 9  shows a state in which any color difference is not generated, and  FIG. 12  shows a state in which the color difference is generated by the skewing. That is, in comparison of the conventional distance L 2  between the head units  9 K and  9 Y shown in  FIG. 11  with the distance L 5  between the head units  9 K and  9 Y of the present embodiment, the distance L 5  in the present embodiment is shorter than the distance L 2 . Therefore, as apparently seen from comparison of  FIG. 9  with  FIG. 12 , in the present embodiment, the color difference is minimized with respect to an equal skewing amount of the recording medium.  
      Next, a sensor (line sensor) which detects a tip portion of the recording medium will be described.  
      A sensor  19  for detecting the conveyed sheet tip is disposed in the vicinity of the head unit  9 K on the upstream side. The sensor  19  is a line sensor which is capable of detecting the sheet tip over the width of the belt  27  and which optically detects the medium tip in a non-contact state, such as a CCD sensor. The medium tip supplied again by the resist roller pair  5  is captured by the sensor  19 , and the discharge by each of the head units  9 K to  9 Y is controlled at a predetermined timing detected by the encoder or the like based on the signal.  
      In the present embodiment, the maintenance unit  8  is disposed on the upstream side of the head units  9 K to  9 Y, and the sensor  19  detects the recording medium  2  conveyed at a speed equal to that of the belt  27  in a state in which the recording medium  2  sufficiently adheres to the belt  27 . Therefore, any scattering does not occur in a recording start position in the head units  9 K to  9 Y, and the color difference in each color can be reduced.  
      The sensor  19  is disposed facing the belt  27  on the lower surface of the maintenance unit  8 . This position is preferably disposed in the vicinity of the head unit  9 , and is preferably on the downstream side in the lower surface of the maintenance unit facing the belt. Alternatively, the sensor may be disposed facing the belt  27  between the maintenance unit  8  and the head unit  9 K.  
      Next, powder dust (e.g., paper powder) generated in a case where the recording medium is paper (recording sheet) will be described with reference to  FIG. 7 .  
      When the paper is used as the recording medium  2  in the conventional apparatus, the paper powder is generated by friction at a cutting time, by the supply roller  3 , or by separation roller  4 . The paper powder sticks to the recording sheet, and moves, but leaves the recording sheet because of a rapid posture change in abutting on the resist roller pair  5  to form a loop (broken line shown in  FIG. 11 ), and flies in the atmosphere.  
      Furthermore, a loop (broken line shown in  FIG. 7 ) is formed between the auxiliary roller  6  and the resist roller pair  5  by a difference between the conveying speed by the belt  27  and that of the resist roller pair  5  until a recording sheet rear end leaves the resist roller pair  5 . Moreover, the paper powder sticking to the recording sheet flies in the atmosphere also because of a rapid posture change at the moment at which the sheet leaves the resist roller pair  5 .  
      The flying paper powder moves downstream together with the recording sheet by air viscosity on the recording sheet surface, and reaches the head unit  9 . Here, when the powder sticks to the nozzle of the head unit  9 , discharge defects are generated, and cause nozzle clogging.  
      In the present embodiment, the head units  9 K to  9 Y of the respective colors are arranged adjacent to one another together in the conveying direction. Moreover, the maintenance unit  8  is disposed on the upstream side of the head unit. Accordingly, as to the distance from the auxiliary roller  6  or the resist roller pair  5  to the head unit  9 , the distances L 1 , L 3  shown in the prior art of  FIG. 11  can be expanded to the distances L 4 , L 6  shown in the present embodiment of  FIG. 7 .  
      Therefore, a probability increases that the paper powder generated on the upstream side from the auxiliary roller  6  is sucked via the hole in the belt by the function of the suction fan  26  until the powder reaches the head unit  9 K, the adhering to the head unit  9  is prevented, and further the generation of the discharge defects can be prevented.  
      Next, a recovery operation in a case where the dust sticks to the nozzle surface of the head unit  9  or the nozzle is clogged will be described.  
      To perform the recovery operation, the ink which has dropped from the nozzle needs to be prevented from being attached to the belt  27  or the platen  25 . As shown in  FIG. 1  or  8 , a vertical movement mechanism (not shown) is disposed in the belt platen unit  7 , and the belt platen unit  7  is lowered and retracted from a facing position with respect to the head unit  9  at a recovery operation time.  
      A space is generated by this retraction, and the maintenance unit  8  once moves down from the head unit  9 , and moves to the position facing the nozzle surface on the downstream side of the medium conveying direction (main scanning direction). Next, the unit rises to a predetermined height in the facing position. A mechanism in which the belt platen unit  7  vertically moves may be utilized as a movement mechanism in which the cap section  42  contacts/leaves the head unit  9 . For example, the maintenance unit  8  is disposed on the belt platen unit  7 , and the belt platen unit  7  is raised. Accordingly, the cap section  42  is raised to abut on the head unit  9 .  
      Moreover, when the height for raising the maintenance unit  8  is changed, the position where the triangular guide portion  41   f  of the suction section  41  abuts on the head unit  9 , and the position where the cap section  42  abuts can be selectively used.  
      The recovery operation is performed at a height at which the triangular guide portion  41   f  of the suction section  41  abuts on the head unit  9  to bring the suction ports  41   b  close to and facing the nozzle surface. The electromagnetic valves  13 Ka,  13 Kb are closed, and the electromagnetic valve  13 Kc is opened in order to transmit the pressure of the pressurizing pump  14  to the sub-tank. The ink is pushed out of the sub-tank by this operation, and reaches the ink distributor  9   a . The ink is pushed out of the nozzle of the head unit  9 , and recovery is performed.  
      As a result, the dust which has adhered to the nozzle and its vicinity is pushed away. Since the surplus ink adheres to the nozzle portion or the nozzle surface, the suction section  41  is moved from end to end of the nozzle surface of the head unit  9  while sucking the ink. The ink sucked by the suction section  41  or the ink dropped inside the ink pan  40  is collected in the waste liquid bottle  16 .  
      Next, a capping operation with respect to the recording heads by the cap section  42  will be described.  
      This capping operation is different from the recovery operation in that the cap section  42  is closely attached to the head unit  9  by an elastic force by the spring  42   c . When the maintenance unit  8  is raised, the cap section  42  can be closely attached to the head unit  9  by an urging force of the spring  42   c . This is because the cap section is formed of a rubber or the like having elasticity.  
      Additionally, since the groove  42   a  is disposed in the cap section facing the nozzle, the nozzle does not directly contact the cap section  42 . This capping operation occludes the nozzle from the outside air, and prevents the ink from being evaporated and solidified in the nozzle. The dust from the outside is also prevented from being attached.  
      Furthermore, when the sheet is jammed in a platen portion, the belt platen unit  7  lowers, and the jammed recording medium can be removed. When the hand is inserted between the head unit  9  and the belt  27 , the nozzle of the head unit  9  can be prevented from being damaged because of operator&#39;s carelessness. This is because the maintenance unit  8  moves to a position facing the head unit  9  to achieve a capped state.  
      According to the first embodiment described above, the following function/effect can be obtained.  
      (1) The head units (or the recording heads) of the respective colors are arranged adjacent to one another in the conveying direction, and these head units are arranged together in the short distance in the medium conveying direction. Therefore, the distance between the recording heads is shorter as compared with the conventional constitution, and the color difference can be minimized with respect to a predetermined skewing amount of the recording medium.  
      (2) The maintenance unit is disposed on the upstream side of the head unit of each color, and the units are arranged in such a manner as to face the conveying surface of the conveying belt. Accordingly, the recording medium can closely adhere to the belt until reaching the first head unit (recording head). The sheet jam can be prevented.  
      (3) The maintenance unit is disposed on the upstream side of the head unit of each color, and the units are arranged in such a manner as to face the conveying surface of the conveying belt. Accordingly, the half of the suction holes in the belt are closed until the recording medium reaches the first head unit (recording head). The adsorption is possible by a weaker suction force as compared with the conventional constitution, and the suction fans can be reduced.  
      (4) The maintenance unit is disposed on the upstream side of the head unit of each color, and the units are arranged in such a manner as to face the conveying surface of the conveying belt. Accordingly, the recording medium can closely adhere to the belt until reaching the first head unit (recording head). The recording medium sufficiently follows the conveying speed of the belt, and reaches each head unit. Therefore, speed fluctuation of the recording medium is little from when the image formation starts, and recording timing shift (color difference) in the conveying direction can be minimized.  
      (5) The maintenance unit is disposed on the upstream side of the head unit of each color, the units are arranged in such a manner as to face the conveying surface of the conveying belt, and the sensor for detecting the tip of the recording medium is disposed opposite to the conveying path on the conveying-direction downstream side of the lower surface of the maintenance unit or between the maintenance unit and the head unit. Accordingly, after the medium speed sufficiently follows the belt speed, the sensor detects the medium tip. Therefore, the positional shift of the image by the head unit (recording head) of each color can be minimized.  
      (6) The maintenance unit is disposed on the upstream side of the head unit of each color, and the units are disposed in such a manner as to face the conveying surface of the conveying belt. Accordingly, a distance from the auxiliary roller to the first recording head  9 K lengthens, and the paper powder generated on the upstream side from the auxiliary roller is sucked into the chamber, and does not easily reach the head unit. As a result, non-discharge by the attached paper powder can be prevented.  
      Next, a second embodiment of the present invention will be described with reference to a schematic constitution example of  FIG. 10 .  
      In the present embodiment, a paper powder capturing section  20  is disposed on the lower surface of a maintenance unit  8 .  
      This paper powder capturing section  20  comprises: a brush-shaped roller  20   a  which contacts a recording medium (recording sheet) surface to float paper powder adhering to the recording sheet surface; a duct suction port  20   b  disposed in the vicinity of the roller  20   a  to suck the paper powder; and a suction fan  20   d  connected to the duct suction port  20   b  via a tube  20   c  to suck the paper powder, and sucks air including the paper powder in the atmosphere from the duct suction port  20   b.    
      By the paper powder capturing section  20 , the paper powder generated from the recording sheet existing upstream or the remaining paper powder adhering to the recording sheet can be sucked and removed by the suction fan in the belt platen before reaching the head unit  9 .  
      In the present embodiment, the brush-shaped roller  20   a  has been used in order to remove the paper powder adhering to the recording sheet. However, when the atmospheric air is only sucked via the suction port  20   b  without disposing the roller  20   a , the effect is obtained.  
      Moreover, in the arrangement of constituting components in the present embodiment, the paper powder capturing section  20  (roller  20   a ) has been disposed under the maintenance unit  8 , but may be disposed between the auxiliary roller  6  and the head unit  9 . The maintenance unit  8  may be disposed on the downstream side of the auxiliary roller  6 , and the capturing section may be disposed between the maintenance unit  8  and the head unit  9  downstream in order. Alternatively, the capturing section may be disposed between the auxiliary roller  6  and the maintenance unit  8 .  
      When the capturing section is disposed on the downstream side of the maintenance unit  8  as in the present embodiment, a space for disposing the paper powder capturing section  20  (roller  20   a ) can be secured without enlarging a conveying surface size of the belt platen unit  7 . The paper powder is sucked via the suction hole in the belt, and additionally dust floating in the vicinity of the recording medium is positively sucked. Accordingly, the nozzles can be prevented from being clogged. Moreover, when a brush roller contacts the sheet surface to remove the paper powder, the electrostatically adhering paper powder can be removed.  
      According to the second embodiment constituted as described above, the following function/effect can be obtained.  
      (1) The maintenance unit is disposed on the upstream side of the head unit of each color, the units are disposed in such a manner as to face the conveying surface of the conveying belt, and the paper powder capturing section is disposed opposite to the conveying path on the maintenance unit lower surface or between the maintenance unit and the head unit. Accordingly, the paper powder generated on the upstream side from the auxiliary roller is sucked by the paper powder capturing section, and does not easily reach the head unit (recording head). As a result, non-discharge by the attached paper powder can be reduced.  
      (2) Since the paper powder capturing section or the sheet tip detection sensor is disposed on the maintenance unit lower surface, the section or the sensor can be disposed facing the conveying surface without increasing the conveying length of the belt platen unit.  
      Next, a third embodiment will be described with reference to a constitution example of  FIG. 13 .  
      In the above-described first and second embodiments, a cut sheet has been described as a recording medium, but the present invention is not limited to the sheet. As shown in  FIG. 13 , a rolled/wound continuous sheet may be used in a recording apparatus. It is to be noted that a head unit  9  and a maintenance unit  8  have constitutions equivalent to those of the first embodiment. Here, a characteristic constitution will be described, other constituting components are denoted with the same reference numerals, and description thereof is omitted.  
      The image forming apparatus comprises a chamber  24  disposed facing the head unit  9  and the maintenance unit  8  and having a planar platen  25  attached on a facing side, and the belt  27  is omitted. Similarly as shown in  FIG. 1 , a large number of suction holes are made in the surface of the platen  25 , and a negative pressure is generated by suction fans  26  disposed inside the chamber  24 .  
      A continuous sheet  67  is conveyed to a roller  66  on a take-up side from a roller  65  on a supply/feed side via the upper surface of the platen  25  by a pair of conveying rollers  61 ,  62 . The surfaces of the conveying roller  61  and a guide roller  63  are set at a height substantially equal to that of the platen upper surface, and the continuous sheet  67  adsorbed by the platen is conveyed.  
      By this constitution, a function/effect equivalent to that of the first embodiment can be obtained.  
      As described above, in the first and second embodiments, the belt conveying unit  7  is used as means disposed facing the head unit  9  to hold/convey the recording medium, but the present invention is not limited to this conveying mechanism. The means facing the head unit to support the recording medium may be a planar platen as in the third embodiment. Roller-shaped support means may be used as long as posture of the recording medium is determined while supporting the recording medium opposite to the head unit  9 . The conveying means is not limited to the belt, and a pair of conveying rollers may be disposed on the upstream or downstream side of the support means.  
      According to the present invention, there can be provided an image forming apparatus in which a plurality of recording heads are arranged in the vicinity of one another, a maintenance unit is disposed in the vicinity of the recording heads in accordance with the arrangement of the recording heads, color difference in forming a color image accompanying the skewing of the recording medium is prevented, sufficient maintenance is realized, and the image is stably formed.