Patent Publication Number: US-2011050778-A1

Title: Ink jet recording apparatus and sheet feeding method of ink jet recording apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Provisional Application No. 61/238,753 filed on Sep. 1, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an ink jet recording apparatus which ejects ink or the like through an ink jet head to form an image on a medium. 
     BACKGROUND 
     Conventionally, an ink jet recording apparatus which forms an image using a head for ink ejection has the function of cleaning a nozzle surface in order to prevent non-ejection of ink from the head. 
     Further, there was used a type of ink jet recording apparatus that is provided with an image forming unit including a plurality of heads on a continuous feeding passage. In such an ink jet recording apparatus, when non-ejection of ink from any head is detected during image formation, an image forming operation of the entire apparatus should be stopped in order to clean a nozzle surface. In addition, in such an ink jet recording apparatus, the stop time of the image forming operation is lengthened as the number of cleanings of the nozzle surface increases. Thus, such an ink jet recording apparatus has the problem of lowering user convenience. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary lateral section view illustrating an ink jet recording apparatus according to an embodiment. 
         FIG. 2  is an exemplary lateral section view illustrating an ink jet recording apparatus according to an embodiment. 
         FIG. 3  is an exemplary lateral section view illustrating an ink jet recording apparatus according to an embodiment. 
         FIG. 4  is an exemplary diagram illustrating detection of non-ejection of ink according to an embodiment. 
         FIG. 5  is an exemplary block diagram illustrating a control system of an ink jet recording apparatus according to an embodiment. 
         FIG. 6  is an exemplary flowchart illustrating a single-side image formation in an ink jet recording apparatus according to an embodiment. 
         FIG. 7  is an exemplary flowchart illustrating a double-side image formation in an ink jet recording apparatus according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an ink jet recording apparatus includes a first ink jet head, a second ink jet head, a main feeding passage, a first roundabout feeding passage, and a controller. The first ink jet head forms an image on a medium. The second ink jet head forms an image on the medium. The main feeding passage feeds the medium so that the first ink jet head and the second ink jet head form an image on the medium. The first roundabout feeding passage feeds the medium without allowing the medium to face the first ink jet head. The controller controls so as to feed the medium through the first roundabout feeding passage if the first ink jet head is unusable. 
     Hereinafter, an embodiment will be described with reference to the accompanying drawings.  FIG. 1  is a lateral section view illustrating an ink jet recording apparatus  1  according to an embodiment. The ink jet recording apparatus  1  includes, inside a casing  10 , a first cassette  20 , a second cassette  30 , a first image forming unit  40 , and a second image forming unit  50 . The first cassette  20  contains a bundle P including a plurality of sheets of mediums (papers). The second cassette  30  stores a bundle P′ including a plurality of sheets of mediums different in size from the medium contained in the first cassette  20 . The first cassette  20  is disposed under the second cassette  30 . 
     The first image forming unit  40  feeds the medium and forms an image. The second image forming unit  50  performs the same operation. The first image forming unit  40  is disposed above the second cassette  30 . The second image forming unit  50  is disposed above the first image forming unit  40 . Further, the ink jet recording apparatus  1  includes therein a first roundabout feeding passage  60  and a second roundabout feeding passage  70 . In the casing  10  is installed a discharge port  80  which discharges the medium. A main feeding passage  100  is formed in a feeding path ranging from a medium supplying position of the first cassette  20  to the discharge port  80  via the first image forming unit  40  and the second image forming unit  50 . 
     Next, a configuration of the first image forming unit  40  will be described. The first image forming unit  40  includes a feeding unit  401 , a head mounting unit  402  and a maintenance unit  403 . The feeding unit  401  includes a feeding belt  401   a , a driving roller  401   b , a driven roller  401   c  and a negative pressure chamber  401   d . The feeding belt  401   a  extends over the driving roller  401   b  and the driven roller  401   c  in an endless manner. The feeding belt  401   a  operates depending upon the rotation of the driving roller  401   b . In a surface of the feeding belt  401   a  are formed holes at a predetermined interval. In order to allow the medium to be held onto the feeding belt  401   a , the negative pressure chamber  401   d  connected with a fan  90  is installed inside the feeding belt  401   a.    
     The head mounting unit  402  includes an ink jet head  402 P, an ink jet head  402 C, an ink jet head  402 M, an ink jet head  402 Y and an ink jet head  402 K, in order from the upstream side of a feeding direction of the medium. The head mounting unit  402  is disposed above the medium feeding unit  401 . The ink jet head  402 P ejects a preprocessing solution. The ink jet head  402 C ejects a cyan toner. The ink jet head  402 M ejects a magenta toner. The ink jet head  402 Y ejects a yellow toner. The ink jet head  402 K ejects a black toner. The head mounting unit  402  is also referred to as a first ink jet head. 
     In each of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K, nozzles which eject ink droplets are arranged along a widthwise direction of the medium with a predetermined resolution. In each of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K, the nozzles are arranged in a direction perpendicular to the feeding direction of the medium by the feeding belt  401   a.    
     Each of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K ejects ink droplets of each color in synchronization with the feeding operation of the medium by feeding belt  401   a . Thus, the ink jet heads  402 P,  4020 ,  402 M,  402 Y and  402 K form a color image in a desired position of a surface of the medium. 
     The maintenance unit  403  performs maintenance for each nozzle surface whenever the image forming operation through the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K terminates. Further, if a nozzle surface of any one of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K is in a state where the nozzle surface cannot eject ink, even though a printing request is made, the maintenance unit  403  maintains the nozzle surface. 
     Next, a configuration of the second image forming unit  50  will be described. The second image forming unit  50  includes a medium feeding unit  501 , a head mounting unit  502  and a maintenance unit  503 . The head mounting unit  502  is also referred to as a second ink jet head. The feeding unit  501  includes a feeding belt  501   a , a driving roller  501   b , a driven roller  501   c  and a negative pressure chamber  501   d . The feeding belt  501   a , the driving roller  501   b , the driven roller  501   c  and the negative pressure chamber  501   d  each have the same configuration as the feeding belt  401   a , the driving roller  401   b , the driven roller  401   c  and the negative pressure chamber  401   d  as described above, and thus detailed description thereof will be omitted. 
     The head mounting unit  502  includes an ink jet head  502 P, an ink jet head  502 C, an ink jet head  502 M, an ink jet head  502 Y and an ink jet head  502 K, in the order from the upstream side of the feeding direction of the medium. The ink jet head  502 P, the ink jet head  502 C, the ink jet head  502 M, the ink jet head  502 Y and the ink jet head  502 K each have the same configuration as the ink jet head  402 P, the ink jet head  402 C, the ink jet head  402 M, the ink jet head  402 Y and the ink jet head  402 K as described above, and thus detailed description thereof will be omitted. The maintenance unit  503  has the same configuration as the above-described maintenance unit  403 , and thus detailed description thereof will be omitted. 
     In this respect, the operation of the first image forming unit  40  and the second image forming unit  50 , and mainly, the maintenance operation for the nozzle surface is disclosed in a copending, commonly owned application entitled “IMAGE FORMING APPARATUS AND EJECTION LIQUID CIRCULATING METHOD” having application Ser. No. 12/707,810, which is incorporated herein by reference. 
     Next, the feeding path of the medium in the ink jet recording medium  1  will be described. Here, it is assumed that the first image forming unit  40  and the second image forming unit  50  are normal (usable) and images are to be formed on the both surfaces of the medium. A first medium supplying roller  101  extracts the medium corresponding to a selected size from the top of the bundle P contained in the first cassette  20  sheet by sheet and feeds the medium through a pair of feeding rollers  102 , a pair of feeding rollers  103  and a pair of first register rollers  104  in the order. Similarly, a second medium supplying roller  105  extracts the medium corresponding to a selected size from the bundle P′ contained in the second cassette  30  sheet by sheet and feeds the medium through a pair of first feeding rollers  106 , the pair of feeding rollers  103  and the pair of first register rollers  104  in that order. The pair of first register rollers  104  supplies the medium to the first image forming unit  40 . The feeding belt  401   a  feeds the medium from the upstream side on which the ink jet head  4029  is installed towards the downstream side on which the ink jet head  402 K is installed. That is, the feeding belt  401   a  serves as the feeding path of the medium. The first image forming unit  40  forms a full color image on an upper surface of the medium. 
     A pair of feeding rollers  107  feeds the medium image-formed on one surface thereof in the first image forming unit  40  through a pair of feeding rollers  108  and a pair of second register rollers  109  in the order. The pair of second register rollers  109  supplies the medium to the second image forming unit  50 . Here, the pair of feeding rollers  107 , the pair of feeding rollers  108  and the pair of second register rollers  109  supply the medium to the feeding belt  501   a  so that a surface of the medium reverse to the surface image-formed in the first image forming unit  40  becomes an upper surface. The feeding belt  501   a  feeds the medium from the upstream side on which the ink jet head  502 P is installed towards the downstream side on which the ink jet head  502 K is installed. That is, the feeding belt  501   a  serves as the feeding path of the medium. The second image forming unit  50  forms a full color image on the upper surface (surface reverse to the surface image-formed in the first image forming unit  40 ) of the medium. A pair of feeding rollers  110  discharges the medium image-formed in the second image forming unit  50  to the outside of the casing  10  through the discharge port  80 . In this respect, when the image is to be formed only on one surface of the medium, the ink jet recording apparatus  1  can form the image on the medium through any one of the first image forming unit  40  and the second image forming unit  50 . 
     Next, the first roundabout feeding passage  60  will be described. One end of the first roundabout feeding passage  60  is connected with the main feeding passage  100  which feeds the medium to the first image forming unit  40 , in a downstream position in the feeding direction of the pair of feeding rollers  103  which feeds the medium extracted from the second cassette  30  disposed on the most upstream side of the feeding direction. The first flapper  111  is installed around the one end of the first roundabout feeding passage  60 . The other end of the first roundabout feeding passage  60  is connected with the main feeding passage  100  which feeds the medium output from the first image forming unit  40 , in a downstream position of the feeding direction of the first image forming unit  40 . The first roundabout feeding passage  60  is installed in parallel under the first image forming unit  40 . Next, the second roundabout feeding passage  70  will be described. One end of the second roundabout feeding passage  70  is connected with the main feeding passage  100  which feeds the medium to the second image forming unit  50 , in a downstream position of the feeding direction of the other end of the first roundabout feeding passage  60 . The second flapper  112  is installed around the one end of the second roundabout feeding passage  70 . The other end of the second roundabout feeding passage  70  is connected with the main feeding passage  100  which feeds the medium output from the second image forming unit  50 , in a downstream position of the feeding direction of the second image forming unit  50 . The second roundabout feeding passage  70  is installed in parallel under the second image forming unit  50 . 
     Next, a case where the second image forming unit  50  forms an image on one surface of the medium when the first image forming unit  40  is under maintenance (unusable) will be described.  FIG. 2  illustrates a medium feeding path of the medium in this case. The first flapper  111  is switched so that the medium is fed through any one of the pair of first register rollers  104  or the first roundabout feeding passage  60 . The medium supplied to the first roundabout feeding passage  60  is supplied to the pair of feeding rollers  108  without being image-formed in the first image forming unit  40 . The first roundabout feeding passage  60  feeds the medium from the side on which the first flapper  111  is installed toward the side on which the pair of feeding rollers  108  is installed, for example, using a pair of feeding rollers  601  and a pair of feeding rollers  602 . Accordingly, the pair of feeding rollers  103  feeds the medium to the first roundabout feeding passage  60  by the first flapper  111 , not to the pair of first resist roller  104 . The pair of feeding rollers  601  and the pair of feeding rollers  602  feed the medium, which is not image-formed in the first image forming unit  40 , to the pair of feeding rollers  108 . 
     Further, the second flapper  112  is switched so that the medium is fed to any one of the second image forming unit  50  and the second roundabout feeding passage  70 . The medium supplied to the second roundabout feeding passage  70  is supplied to the pair of feeding rollers  110  without being image-formed in the second image forming unit  50 . The second roundabout feeding passage  70  feeds the medium from the side on which the second flapper  112  is installed toward the side on which the pair of feeding rollers  110  is installed, for example, using a pair of feeding rollers  701  and a pair of feeding rollers  702 . Accordingly, the pair of second register rollers  109  supplies the medium to the second image forming unit  50  by the second flapper  112 . The second image forming unit  50  forms a full color image on the upper surface of the medium. The pair of feeding rollers  110  discharges the medium image-formed on one surface thereof in the second image forming unit  50  to the outside of the casing  10  through the discharge port  80 . 
     Further, when the second image forming unit  50  is to form an image on the both surfaces of the medium, the medium is fed through a path as will be described below. The pair of feeding rollers (switchback unit)  110  feeds (switches back) the medium image-formed on one surface thereof in the second image forming unit  50 , as described above, in the direction opposite to the discharging direction. That is, the pair of feeding rollers  110  feeds the medium to the second roundabout feeding passage  70 . The pair of feeding rollers  702  and the pair of feeding rollers  701  supply the medium to the pair of second register rollers  109 . The pair of second register rollers  109  supplies the medium to the second image forming unit  50  once again. The upper surface of the medium on the feeding belt  501   a  is a surface reverse to the surface previously image-formed in the second image forming unit  50 . That is, the second image forming unit  50  can independently form images on the both surfaces of the medium by the second roundabout feeding passage  70 . Then, the pair of feeding rollers  110  discharges the medium image-formed on the both surfaces thereof in the second image forming unit  50  to the outside of the casing  10  through the discharge port  80 . 
     Next, a case where the first image forming unit  40  forms an image on one surface of the medium when the second image forming unit  50  is under maintenance will be described.  FIG. 3  illustrates a medium feeding path in this case. 
     The pair of feeding rollers  103  feeds the medium to the pair of first register rollers  104  by the first flapper  111 . The first image forming unit  40  forms a full color image on the upper surface of the medium. The pair of feeding rollers  107  feeds the medium image-formed on one surface thereof in the first image forming unit  40  to the pair of feeding rollers  108 . The pair of feeding rollers  108  feeds the medium through the second roundabout feeding passage  70  by the second flapper  112 , not to the second image forming unit  50 . The pair of feeding rollers  701  and the pair of feeding rollers  702  feed the medium image-formed in the first image forming unit  40  to the pair of feeding rollers  110 . The pair of feeding rollers  110  discharges the medium image-formed on one surface thereof in the first image forming unit  40  to the outside of the casing  10  through the discharge port  80 . 
     Further, when the first image forming unit  40  is to form images on the both surfaces of the medium, the medium is fed through a path which will be described below. The pair of feeding rollers (switchback unit)  108  feeds (switches back) the medium image-formed on one surface thereof in the first image forming unit  40 , as described above, in the direction opposite to the discharging direction. That is, the pair of feeding rollers  108  feeds the medium image-formed in the first image forming unit  40  to the first roundabout feeding passage  60 . The pair of feeding rollers  602  and the pair of feeding rollers  601  supply the medium to the pair of first register rollers  104 . The pair of first register rollers  104  supplies the medium to the first image forming unit  40  once again. The upper surface of the medium on the feeding belt  401   a  is a surface reverse to the surface previously image-formed in the first image forming unit  40 . That is, the first image forming unit  40  can independently form images on the both surfaces of the medium by the first roundabout feeding passage  60 . Then, the medium image-formed on the both surfaces thereof in the first image forming unit  40  is discharged to the outside of the casing  10  through the discharge port  80 , via the second roundabout feeding passage  70 . 
       FIG. 4  illustrates a detection mechanism which detects non-ejection of ink from the nozzle surface of the ink jet head  402 P. The head mounting unit  402  includes a light source  901  and an optical sensor  902 . The light source  901  and the optical sensor  902  are installed under the ink jet head  402 P so as to coincide with an arrangement direction of the nozzles. The optical sensor  902  detects non-ejection of ink from the nozzle surface according to a detected level of light emitted from the light source  901 . The light source  901  and the optical sensor  902  detect the presence or absence of non-ejection for every nozzle of the ink jet head  402 P through pre-ejection during, before or after image formation. Further, the detection mechanism may be a mechanism which detects a disconnection of an ejection heater of the ink jet head  402 P, or may be a mechanism which electrically detects the ink ejection on the side facing the nozzle surface. 
     If the optical sensor  902  detects non-ejection of ink from any one of the nozzles of the ink jet head  402 P, the maintenance unit  403  moves to a position facing the ink jet head  402 P and maintains the nozzle surface. Hereinbefore, the description was made for the ink jet head  402 P, but can be similarly applied to the ink jet heads  402 C,  402 M,  402 Y and  402 K. The head mounting unit  402  is independently provided with the detection mechanism including the light source  901  and the optical sensor  902  for every ink jet head. Further, the second image forming unit  50  has the same detection mechanism as described above. 
       FIG. 5  is a block diagram illustrating a control system of the ink jet recording apparatus  1  according to the embodiment. The ink jet recording apparatus  1  includes a main controller  800 , a CPU  801 , a memory  802 , a medium supplying unit control circuit  803 , a motor  804 , a motor  805 , a main feeding passage control circuit  811 , a motor  812 , a motor  813 , a first roundabout feeding passage control circuit  814 , a motor  815 , a motor  816 , a second roundabout feeding passage control circuit  817 , a motor  818 , a motor  819 , a first flapper control circuit  820 , a solenoid  821 , a second flapper control circuit  822 , a solenoid  823 , a first image forming unit controller a  830 , a second image forming unit controller a  831 , a first image forming unit controller b  832 , a second image forming unit controller b  833 , an I/F  840 , a sensor control circuit  850 , an holding unit control circuit  860 , and a motor  861 . 
     The main controller  800  is a microcomputer including the CPU  801  which is a processor and the memory  802 . The CPU  801  performs a variety of operations according to programs stored in the memory  802 . The CPU  801  outputs control signals to the respective elements. 
     The medium supplying unit control circuit  803  controls the driving of a plurality of motors including the motors  804  and  805 . The motor  804  drives the pair of first register rollers  104 . The motor  805  drives the pair of second register rollers  109 . The main feeding passage control circuit  811  controls the driving of a plurality of motors including the motors  812  and  813 . The motor  812  drives the pair of feeding rollers  108 . The pair of feeding rollers  108  can feed the medium in a bi-directional manner under the control of the main feeding passage control circuit  811 . The motor  813  drives the pair of feeding rollers  110 . The pair of feeding rollers  110  can feed the medium in a bi-directional manner under the control of the main feeding passage control circuit  811 . 
     The first roundabout feeding passage control circuit  814  controls the driving of a plurality of motors including the motors  815  and  816 . The motor  815  drives the pair of feeding rollers  601 . The motor  816  drives the pair of feeding rollers  602 . The pair of feeding rollers  601  and the pair of feeding rollers  602  can feed the medium in a bi-directional manner under the control of the first roundabout feeding passage control circuit  814 . The second roundabout feeding passage control circuit  817  controls the driving of a plurality of motors including the motors  818  and  819 . The motor  818  drives the pair of feeding rollers  701 . The motor  819  drives the pair of feeding rollers  702 . The pair of feeding rollers  701  and the pair of feeding rollers  702  can feed the medium in a bi-directional manner under the control of the second roundabout feeding passage control circuit  817 . The first flapper control circuit  820  controls the driving of the solenoid  821 . The solenoid  821  drives the first flapper  111 . The second flapper control circuit  822  controls the driving of the solenoid  823 . The solenoid  823  drives the second flapper  112 . 
     The first image forming unit controller a  830  controls the operations of the feeding unit  401  and the head mounting unit  402  of the first image forming unit  40 . The second image forming unit controller a  831  controls the operations of the feeding unit  501  and the head mounting unit  502  of the second image forming unit  50 . The first image forming unit controller b  832  controls the operation of the maintenance unit  403  of the first image forming unit  40 . The first image forming unit controller b  832  controls the maintenance unit  403  to move to the position facing the nozzle surface of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K mounted on the head mounting unit  402  and to maintain the nozzle surface. The second image forming unit controller b  833  controls the operation of the maintenance unit  503  of the second image forming unit  50 . The second image forming unit controller b  833  similarly controls the maintenance unit  503  to move to the position facing the nozzle surface of the ink jet heads  502 P,  502 C,  502 M,  502 Y and  502 K mounted on the head mounting unit  502  and to maintain the nozzle surface. 
     The I/F  840  is connected with an external computer (not shown). The I/F  840  receives input of a printing request and an image signal transmitted from the external computer to the ink jet recording apparatus  1 . The sensor control circuit  850  controls the operations of the light source  901  and the optical sensor  902 . The sensor control circuit  850  outputs the detection result of the optical sensor  902  to the main controller  800 . The holding unit control circuit  860  controls the driving of the motor  861 . The motor  861  drives the fan  90 . 
     Next, the operation of the ink jet recording apparatus  1  when the image is formed on one surface of the medium will be described.  FIG. 6  is a flowchart illustrating the operation of the ink jet recording apparatus  1  in this case. Firstly, the main controller  800  detects whether there is non-ejection of ink from each ink jet head mounted on the head mounting unit  402  and the head mounting unit  502 , through the light source  901  and the optical sensor  902  (Act  101 ). If the main controller  800  detects non-ejection of ink in the head mounting unit  402  or the head mounting unit  502  (YES in Act  101 ), the corresponding image forming unit controller drives the corresponding maintenance unit (Act  102 ). Here, if the corresponding image forming unit controller is the first image forming unit controller b  832 , the maintenance unit  403  is driven to start the maintenance for the nozzle surface of the ink jet heads  402 P,  402 C,  402 M,  402 Y and  402 K mounted on the head mounting unit  402 . If the corresponding image forming unit controller is the second image forming unit controller b  833 , the second image forming unit controller b  833  similarly drives the maintenance unit  503  to start the maintenance for the nozzle surface of the ink jet heads  502 P,  502 C,  502 M,  502 Y and  502 K mounted on the head mounting unit  502 . 
     The main controller  800  determines whether the first image forming unit  40  is under maintenance under the condition that a printing request is received (Act  103 ). If the first image forming unit  40  is not under maintenance (NO in Act  103 ), the main controller  800  gives an instruction to the first flapper control circuit  820  so that the medium is fed to the first image forming unit  40  using the first flapper  111  (Act  104 ). The main controller  800  gives an instruction to the first image forming unit controller a  830  so that an image is formed on the upper surface of the medium using the feeding unit  401  and the head mounting unit  402  of the first image forming unit  40  (Act  105 ). 
     The main controller  800  gives an instruction to the second flapper control circuit  822  so that the medium image-formed in the first image forming unit  40  is fed to the second roundabout feeding passage  70  using the second flapper  112  (Act  106 ). The main controller  800  gives an instruction to the second roundabout feeding passage control circuit  817  so that the medium is fed to the pair of feeding rollers  110  through the second roundabout feeding passage  70  (Act  107 ). The main controller  800  gives an instruction to the main feeding passage control circuit  811  so that the medium is discharged to the outside of the casing  10  through the discharge port  80 . Further, if all of the first image forming unit  40  and the second image forming unit  50  are not under maintenance, the main controller  800  may give an instruction so that an image is formed on one surface of the medium in the second image forming unit  50 . In this case, the main controller  800  may perform control so that the medium is fed through the roundabout feeding passage making a pair with the image forming unit which does not perform the image-formation. Alternatively, the main controller  800  may give an instruction to the main feeding passage control circuit  811  so that the medium is fed to the second image forming unit  50 , not to the roundabout feeding passage. 
     If the main controller  800  determines that the first image forming unit  40  is under maintenance (YES in Act  103 ), the main controller  800  determines whether the second image forming unit  50  is under maintenance (Act  108 ). If it is determined that the second image forming unit  50  is not under maintenance (NO in Act  108 ), the main controller  800  gives an instruction to the first flapper control circuit  820  so that the medium is fed to the first roundabout feeding passage  60  using the first flapper  111  (Act  109 ). The main controller  800  gives an instruction to the first roundabout feeding passage control circuit  814  so that the medium is fed to the pair of feeding rollers  108  through the first roundabout feeding passage  60  using the pair of feeding rollers  601  and the pair of feeding rollers  602  (Act  110 ). In this way, it is possible to feed the medium to the downstream side of the head mounting unit  402  in the feeding direction without allowing the medium to face the head mounting unit  402 . 
     The main controller  800  gives an instruction to the second flapper control circuit  822  so that the medium is fed to the second image forming unit  50  using the second flapper  112  (Act  111 ). The main controller  800  gives an instruction to the second image forming unit controller  831  so that an image is formed on the upper surface of the medium using the feeding unit  501  and the head mounting unit  502  of the second image forming unit  50  (Act  112 ). The main controller  800  gives an instruction to the main feeding passage control circuit  811  so that the medium image-formed on one surface thereof in the second image forming unit  50  is discharged to the outside of the casing  10  through the discharge port  80 , using the pair of feeding rollers  110 . 
     If the main controller  800  determines that the second image forming unit  50  is under maintenance (YES in Act  108 ), the main controller  800  waits until the maintenance for any one of the first image forming unit  40  and the second image forming unit  50  ends. 
     Next, the operation of the ink jet recording apparatus  1  when images are formed on the both surfaces of the medium will be described.  FIG. 7  is a flowchart illustrating the operation of the ink jet recording apparatus  1  in this case. Acts  201  to  203  are the same as Acts  101  to  103 , and thus description thereof will be omitted. Further, Acts  205  to  207  are the same as Acts  104  to  106 , and thus description thereof will be omitted. After finishing Act  207 , the main controller  800  gives an instruction to the second image forming unit controller a  831  so that an image is formed on the upper surface of the medium using the feeding unit  501  and the head mounting unit  502  of the second image forming unit  50  (Act  208 ). 
     If the main controller  800  determines that the second image forming unit  50  is under maintenance (YES in Act  204 ), the main controller  800  gives an instruction to the first flapper control circuit  820  so that the medium is fed to the first image forming unit  40  using the first flapper  111  (Act  209 ). The main controller  800  gives an instruction to the first image forming unit controller a  830  so that an image is formed on the upper surface (front surface) of the medium using the feeding unit  401  and the head mounting unit  402  of the first image forming unit  40  (Act  210 ). The main controller  800  controls the main feeding passage control circuit  811  so that the medium image-formed in the first image forming unit  40  is fed to the first roundabout feeding passage  60  using the pair of feeding rollers  108 . The main controller  800  gives an instruction to the first roundabout feeding passage control circuit  814  so that the medium is fed to the pair of first register rollers  104  through the first roundabout feeding passage  60  using the pair of feeding rollers  601  and the pair of feeding rollers  602 . Then, the main controller  800  gives an instruction to the medium supplying unit control circuit  803  so that the medium is supplied to the first image forming unit  40  once again using the pair of first register rollers  104  (Act  211 ). That is, in the operation of Act  211 , a reverse turn of the medium is instructed. 
     The main controller  800  controls the first image forming unit controller a  830  so that an image is formed on the upper surface (rear surface) of the medium using the feeding unit  401  and the head mounting unit  402  of the first image forming unit  40  (Act  212 ). The main controller  800  controls the second flapper control circuit  822  so that the medium image-formed on the both surfaces thereof in the first image forming unit  40  is fed to the second roundabout feeding passage  70  using the second flapper  112  (Act  213 ). The main controller  800  controls the second roundabout feeding passage control circuit  817  so that the medium is fed to the pair of feeding rollers  110  through the second roundabout feeding passage  70  using the pair of feeding rollers  701  and the pair of feeding rollers  702  (Act  214 ). The main controller  800  controls the main feeding passage control circuit  811  so that the medium image-formed on the both surfaces thereof in the first image forming unit  40  is discharged to the outside of the casing  10  through the discharge port  80  using the pair of feeding rollers  110 . 
     If the main controller  800  determines that the first image forming unit  40  is under maintenance (YES in Act  203 ), the main controller  800  determines whether the second image forming unit  50  is under maintenance (Act  215 ). If the main controller  800  determines that the second image forming unit  50  is not under maintenance (NO in Act  215 ), the main controller  800  gives an instruction to the first flapper control circuit  820  so that the medium is fed to the first roundabout feeding passage  60  using the first flapper  111  (Act  216 ). The main controller  800  gives an instruction to the first roundabout feeding passage control circuit  814  so that the medium is fed to the pair of feeding rollers  108  through the first roundabout feeding passage  60  using the pair of feeding rollers  601  and the pair of feeding rollers  602  (Act  217 ). 
     The main controller  800  gives an instruction to the second flapper control circuit  822  so that the medium is fed to the second image forming unit  50  using the second flapper  112  (Act  218 ). The main controller  800  gives an instruction to the second image forming unit controller a  831  so that an image is formed on the upper surface (front surface) of the medium using the feeding unit  501  and the head mounting unit  502  of the second image forming unit  50  (Act  219 ). The main controller  800  gives an instruction to the main feeding passage control circuit  811  so that the medium image-formed in the second image forming unit  50  is fed to the second roundabout feeding passage  70  using the pair of feeding rollers  110 . The main controller  800  gives an instruction to the second roundabout feeding passage control circuit  817  so that the medium is fed to the pair of second register rollers  109  through the second roundabout feeding passage  70  using the pair of feeding rollers  701  and the pair of feeding rollers  702  (Act  220 ). That is, in the operation of Act  220 , a reverse turn of the medium occurs. The main controller  800  gives an instruction to the second image forming unit controller a  831  so that an image is formed on the upper surface (rear surface) of the medium using the feeding unit  501  and the head mounting unit  502  of the second image forming unit  50  (Act  221 ). The main feeding passage control circuit  811  performs control so that the medium image-formed on the both surfaces thereof in the second image forming unit  50  is discharged to the outside of the casing  10  through the discharge port  80  using the pair of feeding rollers  110 . If the main controller  800  determines that the second image forming unit  50  is under maintenance (YES in Act  215 ), the main controller  800  waits until the maintenance for any one of the first image forming unit  40  and the second image forming unit  50  ends. 
     As described above, in the embodiment, the first roundabout feeding passage  60  is used when the first image forming unit  40  is under maintenance, but the first roundabout feeding passage  60  can be also used when the first image forming unit  40  is unusable due to malfunction or other causes. This is the same case with the second image forming unit  50 . Further, the first roundabout feeding passage  60  is installed under the first image forming unit  40 , but the position is not limited thereto. The first image forming unit  40  and the second image forming unit  50  may be disposed with the same height in the horizontal direction. The first roundabout feeding passage  60  has only to be installed so that the medium can be fed to the pair of feeding rollers  108  in order not to form images in the first image forming unit  40 . For example, the first roundabout feeding passage  60  may be installed above the first image forming unit  40 . The above modification of the first roundabout feeding passage  60  may be similarly applied to second roundabout feeding passage  70 . 
     Further, as shown in  FIG. 1 , the downstream side of the first image forming unit  40  and the upstream side of the second image forming unit  50  are nearly the same positions in the widthwise direction. In addition, the upstream side of the first image forming unit  40  and the side on which the medium is discharged from the first cassette  20  and the second cassette  30  are nearly the same positions in the widthwise direction. Accordingly, the medium is fed inside the ink jet recording apparatus  1  in an “S” (zigzag) shape. Thus, the ink jet recording apparatus  1  can occupy a small installation area. Further, the first image forming unit  40  and the second image forming unit  50  are arranged in the vertical direction, but may be arranged in the horizontal direction. 
     According to the embodiment, even though non-ejection is detected in any one of ink jet head during printing, the printing can be continuously performed through the image forming unit which does not include the ink jet head. Thus, the ink jet recording apparatus  1  can secure an image quality through image formation without stopping the printing operation. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.