Patent Publication Number: US-8109600-B2

Title: Inkjet recording apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2007-223482, which was filed on Aug. 30, 2007, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet recording apparatus which ejects ink droplets to record an image on a recording medium. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Publication No. 69176/2006 (Tokukai 2006-69176) discloses an inkjet printer including a conveyance mechanism having a drum which rotates to convey a sheet carried on the outer circumferential surface thereof; a plurality of inkjet heads each having an ejection surface, which are aligned in a conveyance direction of the sheet so that the ejection surfaces of the each inkjet heads faces the outer circumferential surface of the drum; and a maintenance unit for performing maintenance of the ejection surface. The maintenance unit has a tray for receiving ink ejected from the inkjet heads. The length of the tray in the axial direction of the drum is the same as that of the drum. The inkjet heads are moveable between a printing position and a non-printing position. The printing position is a position in which the ejection surfaces of the inkjet heads are disposed close to the outer circumferential surface of the drum. The non-printing position is a position in which the ejection surfaces of the inkjet heads are disposed farther apart from the outer circumferential surface of the drum, compared to the printing position. On the other hand, the maintenance unit is moveable in the axial direction of the drum between the retracted position and a maintenance position. The retracted position is a position in which the maintenance unit does not face the ejection surfaces, where as the maintenance position is a position in which the maintenance unit faces the ejection surfaces. During a printing operation, the maintenance unit is in the retracted position and the inkjet heads in the printing position. Further, during a maintenance operation, the inkjet heads move to the non-printing position, after which the maintenance unit moves from the retracted position to the maintenance position. 
     SUMMARY OF THE INVENTION 
     The tray of the above mentioned inkjet printer for receiving the ink however is positioned outside the drum in the axial direction of the drum. The inkjet printer therefore is large in the axial direction of the drum. 
     In view of the above problem the present invention was made, and it is an object of the present invention to provide an inkjet recording apparatus whose size in the axial direction of a drum thereof is small. 
     An inkjet recording apparatus of the present invention includes: a drum, an inkjet head, a cap, a head-moving mechanism, and a cap-moving mechanism. The drum has a cylindrical outer circumferential surface, and rotates about a rotation axis thereof to convey a recording medium retained on the outer circumferential surface. The inkjet head has an ejection surface which extends in the axial direction and faces the outer circumferential surface, the ejection surface having a plurality of nozzles for ejecting ink droplets. The cap faces the outer circumferential surface. The head-moving mechanism moves the inkjet heads to any one of positions including a recording position, a wiping position, and a capping position, wherein the recording position is a position where inkjet head is disposed at a time of forming an image on a recording medium on the outer circumferential surface by ejecting ink droplets from the nozzles to the recording medium, the wiping position is a position in which the ejection surface of the inkjet head is farther distanced from the outer circumferential surface than in the recording position, and the capping position is a position in which the ejection surface of the inkjet head is farther distanced from the outer circumferential surface than in the recording position. The cap-moving mechanism moves the cap to an open position or an opposing position in the circumferential direction of the drum along the outer circumferential surface, wherein the open position is a position where the cap does not face the ejection surface, and the opposing position is a position where the cap faces the ejection surface and covers the inkjet head while the inkjet head is in the capping position. 
     With the present invention, the cap facing the outer circumferential surface of the drum moves in the circumferential direction along the outer circumferential surface of the drum. The cap therefore does not protrude from the position of the drum in the axial direction, whether the cap is in the open position or in the opposing position. Therefore, downsizing of the inkjet recording apparatus in the axial direction of the drum is possible. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other and further objects, features and advantages of the invention will appear more fully from the following description taken in connection with the accompanying drawings in which: 
         FIG. 1  is a side view illustrating an embodiment of an inkjet printer according to the present invention. 
         FIG. 2  is a cross sectional view taken along the line II-II of  FIG. 1 . 
         FIG. 3  is a development view of a cap unit of  FIG. 1  developed on a plane. 
         FIG. 4  is a side view illustrating an operation of the capping unit of the inkjet printer illustrated in  FIG. 1 . 
         FIG. 5  is a plane view of the inkjet printer illustrated in  FIG. 1 . 
         FIG. 6  and  FIG. 7  are side views of the inkjet printer illustrated in  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes, with reference to  FIG. 1 , an embodiment of inkjet printer according to the present invention. An inkjet printer  1  which is an inkjet recording apparatus is a color inkjet printer, and includes: a sheet conveyance mechanism  20 , four inkjet heads  2 , a head-moving mechanism  51 , a cap unit  61  including four caps  62 , four wipers  66  (see  FIG. 5 ), a cap moving mechanism, a wiper moving mechanism, and a control device  10 . The sheet conveyance mechanism  20  includes a drum  21  having a cylindrical outer circumferential surface  21   a . The control device  10  serves as a control means which controls operations of the above mentioned members. Note that illustration of the wipers  66  are omitted in  FIG. 1  as a matter of convenience. In the lower part of the inkjet printer  1  is provided a sheet-feeding tray  11 . On the top surface of the casing is provided a sheet receiving tray  12 . Further, in the inkjet printer  1 , a sheet conveyance path is formed. This sheet conveyance path extends from the sheet-feeding tray  11  to the lower end of the drum  21 , and from the lower end of the drum  21  to the sheet receiving tray  12  via the upper end of the drum  21  along the outer circumferential surface  21  of the drum  21 . 
     Each of the four inkjet heads  2  has ink passages having nozzles for ejecting ink droplets, and is formed in a parallelepiped shape elongated in the axial direction of the drum  21 . From each of these inkjet heads  2  is ejected ink of one of the following four colors: magenta, yellow, cyan, and black. These four inkjet heads  2  are aligned in the circumferential direction of the drum  21  along the outer circumferential surface thereof. In short, the inkjet printer  1  is a line printer. 
     The lower surface of each of the inkjet head  2  serves as an ejection surface  2   a  having thereon nozzle openings. This ejection surface  2   a  of the inkjet head  2  faces the outer circumferential surface  21   a.    
     The sheet-feeding tray  11  has a tray main body  13  opened in the upper direction. In the tray main body  13  is stored a stack of sheets P. Inside the tray main body  13  are arranged a plate  14  which supports the sheets P from the bottom, and two springs  15  which urges the plate  14  upward. The plate  14  is formed in a plane shape which is substantially the same as the tray main body  13 , and is disposed in such a manner that the plate  14  is able to move upward or downward within the tray main body  13 . 
     Further, the inkjet printer  1  includes a pickup roller  31  and a pickup roller moving mechanism  32  which rotates the pickup roller  31 . The pickup roller  31 , while being rotated, contacts a sheet P at the top of the stack of sheets P so as to feed out the sheet P to the drum  21 . 
     The pickup roller moving mechanism  32  includes a turning arm  33 , a shaft  34 , and a not-illustrated drive motor. A lower end of the turning arm  33  rotatably supports the pickup roller  31 , and an upper end of the same is fixed to the shaft  34 . The drive motor rotates the shaft  34  to rotate the turning arm  33  about the shaft  34 . With this pickup roller moving mechanism  32 , the turning arm  33  is moved so that the pickup roller  31  is positioned in one of the following two positions: a contact position where the pickup roller  31  contacts the uppermost sheet P in the tray main body  13 ; and a separation position where the pickup roller  31  is apart from the uppermost sheet P in the tray main body  13 . While the pickup roller  31  is in the separation position, the sheet-feeding tray  11  is easily attached or detached. Note that aforementioned springs  15  are structured in such a manner that a friction within a predetermined range is generated between the pickup roller  31  and the uppermost sheet P, irrespective of the number of sheets P stored in the tray main body  13 . This prevents a problem in feeding out a sheet P by the pickup roller  31 . 
     Further, as a part of a sheet conveyance path, the inkjet printer  1  includes an introduction path  41  through which a sheet P sent out from the sheet-feeding tray  11  by the pickup roller  31  is lead to the outer circumferential surface  21   a  of the drum  21 , which surface adsorbs and retains thereon the sheet P. The introduction path  41  is a substantially straight path which extends from the sheet-feeding tray  11  to the lower end of the drum  21 . 
     The drum  21  is further detailed with reference to  FIG. 2 . The drum  21  has a pipe-like shape and the axial direction thereof conforms with a direction perpendicular to the surface of the  FIG. 1 . The both ends of the drum  21  is closed. The centers of the both closed ends of the drum  21  are connected with each other via a tubular shaft  27  extending in the axial direction and penetrating the both ends. The tubular shaft  27  rotates with the drum  21 . 
     The outer circumferential surface  21   a  is formed on a circumferential wall of the drum  21 . This circumferential wall has through holes  22  communicating an internal space  23  of the drum  21  with the outside of the drum. These through holes  22  are all uniformly formed. Further, the circumferential wall of the tubular shaft  27  in the internal space  23  has through holes  27   a  communicating the inside of the tubular shaft  27  with the internal space  23 . These through holes  27   a  are uniformly formed. The left end of the tubular shaft  27  is sealed, and the right end of the same is connected to an air-suction device  28  having a fan  28   a . Driving of the air-suction device  28  aspirates the air in the internal space  23  into the tubular shaft  27  via the through holes  27   a , and from the tubular shaft  27  to the outside the drum  21 . Thus, an air flow from the outside of the drum  21  to the internal space  23  via the through holes  22  is created. This enables the outer circumferential surface  21   a  of the drum  21  to adsorb thereon a sheet P. 
     On the left end of the tubular shaft  27  is supported a pulley  24  which rotates along with the tubular shaft  27 . Further, a belt  26  is looped around the pulley  24  and a pulley  29  attached to the rotational shaft of a conveyance motor  25 . The conveyance motor  25  rotates the tubular shaft  27  and the drum  21  counterclockwise in  FIG. 1 , via the belt  26  and the pulleys  24  and  29 , thereby conveying a sheet adsorbed and retained on the outer circumferential surface  21   a  in a conveyance direction A indicated by the arrow; that is, in the circumferential direction of the drum  21 , along the outer circumferential surface  21   a  of the drum  21 . 
     In the present embodiment, the conveyance motor  25 , belt  26 , tubular shaft  27 , pulleys  24  and  29 , or the like form a drum drive device. This drum drive device, drum  21 , and the air-suction device  28  form the sheet-conveyance mechanism  20 . Driving the air-suction device adsorbs and retains on the outer circumferential surface  21   a  a sheet P having conveyed through the introduction path  41  from the sheet-feeding tray  11 . Then, as the drum drive device drives the drum  21  to rotate the same in the counter clockwise direction, the sheet P is conveyed in the conveyance direction A. Then, the inkjet heads  2  eject ink of different colors to the sheet P conveyed on the drum  21 , while the respective ejection surfaces  2   a  of the inkjet heads  2  face the sheet P. Thus, an intended colored image is printed on the sheet P. 
     Towards the downstream of the drum  21  in the conveyance direction A is provided a separator  45 . The separator  45  separates a sheet P from the outer circumferential surface  21   a  of the drum  21  retaining the sheet P so as to feed the sheet P to an output path  43  which is also a part of the sheet conveyance path. 
     The head-moving mechanism  51  moves the four inkjet heads  2  simultaneously so that relative positions of the four inkjet heads do not change. The head-moving mechanism  51  has a guide rail  52 , and a head frame  53  supporting the four heads. The guide rail  52  retains the four inkjet heads  2  so that the inkjet heads  2  are positioned symmetrically with respect to the center of these heads  2  along the circumferential direction of the drum  21 . Further, the guide rail  52  supports the head frame  53  so as to enable the head  53  to slide along the radial direction. A line extended in the sliding direction of the head frame  53  is perpendicular to the outer circumferential surface  21   a . In other words, the center of the head frame  53  in the circumferential direction slides along the guide rail  52  to move along the line which extends through the center of the guide rail and which is perpendicular to the outer circumferential surface  21   a  of the drum  21 . 
     The control device  10  controls a not-illustrated actuator to move the head frame  53  along the guide rail  52 , thereby determining the positions of the four inkjet heads  2 , which are fixed on the head frame  53 , relative to the outer circumferential surface  21   a . Specifically, the head-moving mechanism  51  positions the four inkjet heads  2  so that the inkjet heads  2  are in any one of the following positions: a printing position (see  FIG. 1 ), a capping position (See  FIG. 4 ), a wiping position (See  FIG. 6 ), and a retracted position (See  FIG. 7 ). The printing position is a position where the inkjet heads  2  are disposed at a time of performing printing to a sheet P by ejecting ink droplets from the ejection surfaces  2   a . The capping position is a position in which each ejection surface  2   a  and one of the caps  62  facing the ejection surface  2   a  form a sealed space having the ejection surface  2   a  as an inner wall surface. The wiping position is a position in which each ejection surface  2   a  contacts a portion nearby the leading end of a wiper  66  (see  FIG. 5 ) facing the ejection surface  2   a . The retracted position is a position in which each ejection surface  2   a  is separated from the leading end of the wiper  66  facing the ejection surface  2   a.    
     The distance between the outer circumferential surface  21   a  of the drum  21  and the ejection surface of each of the four inkjet heads  2  is hereinafter referred to as a separation distance. The separation distance of each of the inkjet heads  2  in the above mentioned positions is expressed as follows: the separation distance of the inkjet head  2  in the printing position&lt;the separation distance of the inkjet head  2  in the capping position&lt;the separation distance of the inkjet head  2  in the wiping position&lt;the separation distance of the inkjet head  2  in the retracted position. Each of the four inkjet heads  2  is disposed so that, while the inkjet head  2  is in the printing position, the normal to the center of the ejection surface  2   a  is perpendicular to the outer circumferential surface  21   a . Note however that the normal to the center of the ejection surface  2   a  of each of the inkjet heads  2  does not conform with the moving direction of the inkjet head  2 . As such, the normal to the center of the ejection surface  2   a  is not perpendicular to the outer circumferential surface  21   a  while the inkjet head  2  is in a position other than the printing position. 
     Next, a capping unit  61  is described with reference to  FIG. 3  and  FIG. 4 . The cap unit  61  is used when performing a maintenance of the ejection surfaces  2   a  of the inkjet heads  2 , and includes: four caps  62  aligned in the conveyance direction A of the sheet P, a cap tray  64  supporting the caps  62 , a waste liquid tray  71 , and a waste liquid tank  72 . 
     Each of the caps  62  is made of an elastic material such as an ink-resistance rubber or resin. The caps  62  each has an annular projection having a substantially rectangular plane shape, which projection defines a recessed portion whose bottom surface is the cap tray  64 . The recessed portion defined by the annular projection extends in the axial direction of the drum  21  in a plane view. The size of the recessed portion is smaller than the ejection surface  2   a , but is sufficient for including the entire region of the ejection surface  2   a  where the nozzle openings are provided. 
     The cap tray  64  is formed substantially in the shape of sector, and includes a curved part and a plane part connected to the curved part. The curved part faces the outer circumferential surface  21   a  of the drum  21 , and is curved along the outer circumferential surface  21   a  of the drum  21 . The plane part faces an end surface of the drum  21 , and is capable of swinging about the rotational axis of the drum  21  so as to move the curved part in the circumferential direction of the outer circumferential surface  21   a  of the drum  21 . The cap tray  64  is swung by a not-illustrated maintenance motor controlled by the control device  10 . Note that lines hidden by the cap tray  64  in  FIG. 1  are solid lines, though these lines should be dotted-lines. The outer surface of the curved part of the cap tray  64  holds the four caps  62 , in such a manner that the caps  62  are positioned on the surface in the conveyance direction A of the sheet P. Note that the caps  62  correspond to the inkjet heads  2  on one-to-one basis. 
     The control device  10  controls the angular orientation of the cap tray  64  so as to move the four caps  62  between the open position (standby position) illustrated in  FIG. 1  and the opposing position illustrated in  FIG. 4 , in the circumferential direction along the outer circumferential surface  21   a  of the drum  21 . The open position is a position which is upstream from the four inkjet heads  2  in the conveyance direction A, and is a position where the cap tray  64  faces none of the ejection surfaces  2   a . In other words, the standby position is a position where the cap tray  64  does not face an area including the four ejection surfaces  2   a . The opposing position is a position where the cap tray  64  faces all the ejection surfaces  2   a . While the caps  62  are in the opposing position, each of the caps  62  faces the corresponding ejection surface  2   a  of the corresponding inkjet head  2 . Further, while the caps  62  are in the open position, the openings of the recessed portions of all the caps  62  are faced downward. Thus, waste ink stored in the caps  62  from the caps  62  due to its own weight. In the present embodiment, the cap tray  64  and the maintenance motor or the like form a cap-moving mechanism. 
     While the caps  62  are in the opposing position, the inkjet heads  2  move from the retracted position or the wiping position to the capping position, and the ejection surface  2   a  of each of the inkjet heads  2  closely touches the annular projection of corresponding one of the caps  62 , thus creating a sealed space having the ejection surface  2   a  as an inner wall surface. This restrains drying and thickening of the ink inside the nozzles. Further, through a purging operation for discharging thickened ink inside the nozzles, the ink ejected from the ejection surface  2   a  is received inside the cap  62  while avoiding dispersion of ink droplets outside the cap  62 . 
     The waste liquid tray  71  is positioned so as to be located below the caps  62  while the caps  62  are in the open position. This waste liquid tray  71  leads, to the waste liquid tank  72 , the waste ink discharged from the caps  62  in the open position. The waste liquid tank  72  is positioned below the waste liquid tray  71 , and stores therein the waste ink having lead by the waste liquid tray  71 . 
     Next, the wiper-moving mechanism  65  is further detailed with reference to  FIG. 5  to  FIG. 7 . Note that the ejection surfaces  2   a  of the inkjet heads  2  in the wiping position are indicated by dotted lines in  FIG. 5 . As illustrated in the figures, the wiper-moving mechanism  65  includes: a wiper tray  67  supporting the four wipers  66  aligned in the conveyance direction of the sheet P; a pair of wipe-guides  68  which are parallel to each other; and a not-illustrated actuator. 
     The pair of wipe-guides  68  extend parallel to the rotational axis of the drum  21 . The both of the wipe-guides  68  are distanced from the outer circumferential surface  21  by the same distance. One of the wipe-guides  68  is slightly upstream from the separator  45 , and the other wipe-guide  68  is slightly upstream from one of the inkjet heads  2  disposed uppermost stream relative to the conveyance direction A. These wipe-guides  68  respectively support both ends of the wiper tray  67  in the conveyance direction A so that the wiper tray  67  made of a thin and hard plate having a small width (length in the axial direction) is able to slide in the axial direction of the drum. The wiper tray  67  is inflected at three positions so as to form a shape that extends along the outer circumferential surface  21   a , consequently forming outer planes respectively facing the four ejection surfaces  2   a  on the wiper tray  67 . 
     The wiper tray  67  is moved in the axial direction of the drum by the actuator controlled by the control device  10 . The wiper tray  67  is moveable between two positions (wiping start position and wiping end position) sandwiching the ejection surfaces  2   a.    
     The wiper tray  67  preferably does not protrude from the drum  21  in the axial direction. However, an increase in the size of the printer  1  in the axial direction is prevented, as long as the length of the wiper tray  67  in the axial direction is sufficiently small. In a structure where the wiper tray  67  protrude from the drum  21  in the axial direction, it is preferable that the wiper tray  67  be moveable within a range from a position between the air-suction device  28  and the drum  21  to a position between the drum  21  and the pulley  24  or the motor  25 , as illustrated in  FIG. 5 . 
     Each of the four wipers  66  is a blade for wiping the ejection surface  2   a , and is made of an elastic material such as an ink-resistant rubber or resin. The wipers  66  have the same size, and are all thinner than the wiper tray  67 . Further, each of the wipers  66  is fixed perpendicularly to the outer plane of the wiper tray  67 , and extends in the width direction of the ejection surface  2   a . The leading end of each of the wipers  66  is in a rectangular shape, and is parallel to the ejection surface  2   a  of the inkjet head  2 . The height of each of the wiper  66  is slightly larger than the separation distance of the inkjet head  2  in the wiping position. Thus, when the wiper  66  moves in the axial direction to face the ejection surface  2   a , the wiper  66  is able to wipe the ejection surface  2   a.    
     The wiper tray  67  is entirely slanted downwards so that an angle between the outer planes and the horizontal plane increases towards the right of  FIG. 6 . Thus, among the four wipers  66 , a wiper  66  on the left of  FIG. 6  is positioned higher than a wiper  66  on the right of the figure. Further, the lower end (right end of  FIG. 6 ) of the wiper tray  67  is positioned above the waste liquid tray  71 . With this, ink adhered to the wiper tray  67  is more likely to flow down into the waste liquid tray  71 . 
     In the present embodiment, the wipers  66  move from the wiping start position (e.g. the position shown in  FIG. 5 ) in the axial direction, after the inkjet heads  2  move to the wiping position shown in  FIG. 8 . At this point, each of the wipers  66  moves in the axial direction of the drum  21  while contacting the corresponding ejection surface  2   a , thus wiping the ejection surface  2   a  and shaping the meniscus. Ink adhered to the wipers  66  then flows down along the wiper tray  57 , and then flows into the waste liquid tank  72  via the waste liquid tray  71  located below the wiper tray  67 . The ink is stored in the waste liquid tank  72 . The wiping of the ejection surface  2   a  is completed when the wipers  66  reach the wiping end position which is on the opposite side of the wiping start position across the ejection surfaces  2   a  in the axial direction. After that, the inkjet heads  2  move to the retracted position shown in  FIG. 7 , and the wipers  66  move back from the wiping end position to the wiping start position. At this point, the inkjet heads  2  are in the retracted position, the wipers  66  do not contact the ejection surfaces  2   a , and the meniscus formed on each of the nozzle openings therefore are not damaged. This series of operations for wiping the ejection surfaces  2   a  is hereinafter referred to as wiping operation. 
     Next, the following describes an operation which takes place at a time of performing printing to the sheet P. It is supposed that the inkjet heads  2  are brought to the printing position prior to the printing. First, a sheet P is fed to the drum  21  from the sheet-feeding tray  11 . In this step, the uppermost sheet P among a stack of sheets P stored in the sheet-feeding tray  11  contacts the pickup roller  31 . The pickup roller  31  rotates counter clockwise while being in contact with the sheet P, thereby feeding the sheet P to the introduction path  41 . The sheet P fed out is then lead through the introduction path  41 , and arrives at the drum  21 . 
     At the same time, the drive mechanism drives the drum  21  to rotate the drum  21  in the conveyance direction A, and the air-suction device  28  is driven to adsorb and retain the sheet P on the outer circumferential surface  21   a  of the drum  21 . The sheet P retained on the outer circumferential surface  21   a  of the drum  21  is then conveyed in the conveyance direction A with the rotation of the drum  21 . 
     When the sheet P reaches a position so as to face the ejection surface  2   a , the ejection surfaces  2   a  of the four inkjet heads  2  eject ink from their nozzles to form an image on the sheet P. Then, the sheet P on which the image is formed is separated from the outer circumferential surface  21   a  of the drum  21  by the separator  45 , and is fed out to the output path  43 . The sheet P fed to the output path  43  is conveyed towards the sheet receiving tray  12 , and is received by the sheet receiving tray  12 . 
     Next, the following details a maintenance operation of the inkjet heads  2 . The maintenance operation of the inkjet heads  2  includes a capping operation, a purge operation, and the wiping operation. The capping operation is an operation in which each of the caps  62  creates a sealed off space having the corresponding ejection surface  2   a  as an inner wall. The purge operation is an operation in which thickened ink inside the nozzles opened on the ejection surfaces is discharged into the caps  62 . Further, in the wiping operation, meniscus formed on each of the nozzle openings is shaped, as hereinabove mentioned. The maintenance operation is performed at a time of powering on the inkjet printer  1 , or immediately before the printing to a sheet P. 
       FIG. 1  illustrates an ordinary state where printing to the sheet P is possible. During this state, the inkjet heads  2  are in the printing position, the caps  62  are in the open position, and the wipers  66  are in the wiping start position. When the maintenance operation starts during this state illustrated in  FIG. 1 , the inkjet heads  2  first move from the printing position to the retracted position. Then, the caps  62  move from the open position to the opposing position. Thus, each of the caps  62  faces corresponding one of the ejection surfaces  2   a . Then, the inkjet heads  2  move from the retracted position to the capping position and each of the ejection surfaces  2   a  is closely attached to the annular projection of the corresponding cap  62 , thereby creating a sealed space having the ejection surface as an inner wall. The capping operation is then ended. When the capping operation ends, the purging operation for discharging the thickened ink inside each nozzle is performed. The purging operation is performed by driving a not-illustrated pump connected to the ink supply path of the inkjet head  2 . At this point the ink droplets ejected from the ejection surface  2   a  is received in the cap  62 . 
     When the purge operation is ended, the inkjet heads  2  move from the capping position to the wiping position illustrated in  FIG. 6 . Then the caps  62  move to the open position. Then, waste ink in the caps  62  in the open position is discharged therefrom to the waste liquid tray  71 . The discharged waste ink is then lead to the waste liquid tank  72  through the waste liquid tray  71 . During this state the wipers  66  move from the wiping start position to the wiping end position so as to wipe the ejection surfaces  2   a . At this point, the four wipers  66  each wipes different one of the ejection surfaces  2   a . Thus, meniscus to be formed at each of the nozzle openings is shaped. Further, the ink adhered to the wiper  66  flows down along the wiper tray  67 , and flows into the waste liquid tray  71  located below the wiper tray  71 . Thus, the ink is stored in the waste liquid tank  72 . 
     After that, the inkjet heads  2  move to the retracted position as illustrated in  FIG. 7 . Further, the wipers  66  move to the wiping start position and the wiping operation is completed. Thus, the maintenance operation of the inkjet heads  2  is completed. 
     Each of the caps  62  forms a sealed off space having the ejection surface  2   a  as an inner wall, except for occasions of performing printing and maintenance operation. This restrains drying and thickening of the ink inside the nozzles. 
     According to the present embodiment as hereinabove described, each of the caps  62  facing the circumferential surface  21   a  of the drum  21  moves in the circumferential direction along the outer circumferential surface  21   a  of the drum  21 . The cap  62  therefore does not protrude from the drum  21  in the axial direction whether it is in the open position or in the opposing position. Thus, downsizing of the printer  1  in the axial direction is possible. 
     Further, the wiper  68  wipes the corresponding ejection surface  2   a  in the axial direction of the drum  21 . This allows the wiper  66  to evenly wipe the ejection surface  2   a , while preventing the ejection surface  2   a  from being contaminated by the different ink. 
     Further, in the capping operation, each of the caps  62  form a sealed space having the corresponding one of the ejection surfaces  2   a  as an inner wall, and each of the wipers  66  wipes different one of the ejection surfaces  2   a . This prevents the contamination by the different ink without fail. 
     Further, ink adhered to each of the wipers  66  flows down along the wiper tray  67 , flows into the waste liquid tank  72  via the waste liquid tray  71  located below the wiper tray  67 , and is stored in the waste liquid tank  72 . The waste ink therefore is efficiently collected. 
     Further, the four inkjet heads  2  move simultaneously, and the relative positions thereof do not change. Therefore, the structure of the head-moving mechanism  51  is made simple. 
     In addition, the inkjet heads  2  are moveable to the retracted position by means of the head-moving mechanism  51 . Thus, when no wiping operation is needed, the wipers  66  are able to move while avoiding contacting the ejection surfaces  2   a  when the wiping operation is not needed. This restrains damage to meniscus formed on each nozzle opening. 
     The above-mentioned embodiment deals with a case where the inkjet printer  1  has four inkjet heads  2 . However, the number of the inkjet heads  2  may be any given number. For example, the number of the inkjet heads  2  may be one to three or, five or more. 
     Further, the above embodiment allows the inkjet heads  2  to be moved to the retracted position. However, a structure which does not move the inkjet heads  2  to the retracted position is also possible. In such a case, the inkjet heads  2  are moved to the wiping position, instead of the retracted position. Note, however, that the ejection surfaces  2   a  are wiped even when the wiping operation is not necessary. 
     Further, the above embodiment deals with a case where the caps  62  receive the ejected ink droplets in the purging operation. However, while keeping these caps in the same structure, it is possible to provide a purge area capable of retaining ink to a predetermined position of the outer circumferential surface  21  of the drum  21 , and to eject ink droplets to the purge area during the purging operation. This allows omission of the process of forming with the cap  62  a sealed space having the ejection surface  2   a  as an inner-wall surface, in the purging operation. As a result, the maintenance operation is promptly completed. 
     Further, the above-mentioned embodiment deals with a case where the wipers  66  do not move in the circumferential direction of the drum  21 . It is however possible to adopt a structure in which the wipers  66  move in the circumferential direction along the outer circumferential surface  21   a  of the drum  21  so that the wiper tray  67  faces the waste liquid tray  71  and the drum  21  in the radial direction of the drum  21 . With this structure, the ink adhered to the wiper tray  67  more efficiently flows down into the waste liquid tray  71 . Note that the structure may be further adapted so that the wiping operation is performed while the wipers  66  move in the circumferential direction. 
     Further, the above embodiment deals with a case where a sheet P is held on the outer circumferential surface of the drum  21  by means of the air-suction device. However, the outer circumferential surface of the drum  21  may posses adhesiveness. This simplifies the structure of the device. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.