Patent Publication Number: US-2010128098-A1

Title: Inkjet printer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from prior Japanese Application No. 2008-303017, filed Nov. 27, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inkjet printer that is capable of easily performing maintenance on an ink path. 
     2. Description of the Related Art 
     Conventionally, an inkjet printer that records an image on a recording medium by jetting ink from a recording head is known. As systems for supplying ink to the recording head in such the inkjet printer, there are two types of systems, i.e., a circulation system and a non-circulation system. 
     For example, Japanese Patent Application Publication No. 2001-219580 discloses an ink path using the circulation system. In order to supply ink to a recording head, the ink path in Japanese Patent Application Publication No. 2001-219580 comprises a first ink container provided higher than the recording head, a second ink container provided lower than the recording head, and a pump. 
     Ink is supplied to the recording head by means of the difference in elevation between the first and second ink containers. The ink collected in the second ink container is sent to the first ink container using the pump. In this way, the ink path in Japanese Patent Application Publication No. 2001-219580 circulates ink. 
     As another example, Japanese Patent Application Publication No. 2008-162175 discloses an ink path using the non-circulation system. The ink path in Japanese Patent Application Publication No. 2008-162175 comprises a sub tank for temporarily pooling ink supplied from an ink bag as a cartridge. The ink in the sub-tank is supplied to the recording head. 
     The ink paths in Japanese Patent Application Publication No. 2001-219580 and Japanese Patent Application Publication No. 2008-162175 described above are accommodated in the same housing as the one which accommodates a recording unit including the recording head and accommodates a transportation unit for transporting the recording medium. 
     SUMMARY OF THE INVENTION 
     The inkjet printer according to the present invention comprises a transportation unit for transporting a recording medium, a recording unit including a recording head that jets ink to the recording medium transported by the transportation unit, an ink path for supplying ink to the recording head, a first housing accommodating the transportation unit and the recording unit, and a second housing attached to the first housing and accommodating a portion of the ink path. The second housing is attached to the first housing in such a way that it can be moved to a first position and to a second position that is different from the first position. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  are diagrams showing a schematic of an inkjet printer of a first embodiment of the present invention, with illustration of an ink path being omitted;  FIG. 1A  shows the configuration when an image is recorded and  FIG. 1B  shows the configuration when an image is not recorded. 
         FIG. 2  is a diagram showing a first and a second housing that configure a body frame of the inkjet printer of the first embodiment. 
         FIG. 3  is a diagram showing the configuration of a transportation unit of the inkjet printer of the first embodiment. 
         FIG. 4  is a perspective view in which the configuration of a recording unit of the inkjet printer of the first embodiment is viewed from the under side. 
         FIG. 5  is a diagram showing an ink path of the inkjet printer of the first embodiment and showing the relationship between the first and a second housing and the ink path. 
         FIG. 6  is a diagram in which the relationship between the first housing, the second housing, and the ink path of the inkjet printer of the first embodiment is viewed from the top side. 
         FIGS. 7A and 7B  are diagrams showing exemplary modification  1  of the form for attaching the second housing to the first housing of the inkjet printer of the first embodiment. 
         FIG. 8A  is a diagram showing exemplary modification  2  of the form for attaching the second housing to the first housing of the inkjet printer of the first embodiment. 
         FIG. 8B  is a perspective view showing only the second housing of exemplary modification  2  of the first embodiment. 
         FIG. 9  is a diagram showing an ink path of an inkjet printer of a second embodiment of the present invention and showing the relationship between a first and a second housing and the ink path. 
         FIG. 10  is a diagram showing an ink path of an inkjet printer of a third embodiment and showing the relationship between a first and a second housing and the ink path. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  are diagrams showing a schematic of an inkjet printer of a first embodiment of the present invention, with illustration of an ink path being omitted.;  FIG. 1A  shows the configuration when an image is recorded and  FIG. 1B  shows the configuration when an image is not recorded. 
       FIG. 2  is a diagram showing a first and a second housing that configure a body frame of the inkjet printer of the first embodiment. 
       FIG. 3  is a diagram showing the configuration of a transportation unit of the inkjet printer of the first embodiment. 
       FIG. 4  is a perspective view in which the configuration of a recording unit of the inkjet printer of the first embodiment is viewed from the under side. 
       FIG. 5  is a diagram showing an ink path of the inkjet printer of the first embodiment and showing the relationship between the first and a second housing and the ink path. 
       FIG. 6  is a diagram in which the relationship between the first housing, the second housing, and the ink path of the inkjet printer of the first embodiment is viewed from the top side. 
     In the following description, a transportation direction of a recording medium is referred to as an X-axis direction and a direction orthogonal to the transportation direction is referred to as a Y-axis direction or the width direction of the recording medium. A direction orthogonal to the X-axis direction and the Y-axis direction is referred to as a Z-axis direction or a vertical direction. 
     As shown in  FIGS. 1A and 1B , an inkjet printer  1  includes a body frame  2 . As shown in  FIG. 2 , the body frame  2  comprises a first housing  3  ( 3   a,    3   b,    3   c  and  3   d ) and a second housing  4 . 
     The first housing  3  comprises at least side face frames  3   a  and  3   b,  a tie bar  3   c  (see  FIG. 6 ), and a bottom face  3   d.    
     The second housing  4  is configured in such a way that, as indicated by arrow a, it can rotate about a rotation axis  5  relative to the first housing  3  so as to be positioned at a first position indicated by a solid line and at a second position indicated by a dashed line. In the present embodiment, the second housing  4  is attached to the side face frame  3   a  of the first housing  3 . An opening  7  through which a tube from a recording unit  6  can pass is formed on the side face frame  3   a  (a detailed descriptions of this will be given later). 
     A medium supplying unit  8 , a movement unit  9 , a transportation unit  10 , a recording unit  6 , and a medium ejection unit  11  are placed in the first housing  3 . 
     The medium supplying unit  8  includes a feed tray  12  and a pickup roller  13 . The feed tray  12  and the pickup roller  13  are supported by the side face frames  3   a  and  3   b.    
     A plurality of recording medium  14  composed of, for example, paper or the like are placed in the feed tray  12 . The recording medium  14  placed on the feed tray  12  are supplied one by one to the transportation unit  10  by the pickup roller  13 , supplying from the one placed on the top. 
     The transportation unit  10  is accommodated in the first housing  3 . As shown in  FIGS. 1 and 3 , the transportation unit  10  includes a resist roller pair  15 , a platen  16 , a plurality of rollers  17 , a transportation belt  18 , a fan  19 , and a transportation frame  20  ( 20   a  and  20   b ). 
     The resist roller pair  15  clears (i.e., corrects) inclinations of the recording medium  14  sent by the pickup roller  13 . The resist roller pair  15  then transports the recording medium  14  to the transportation belt  18  at a proper timing for image recording. 
     The resist roller pair  15  is supported by the side face frames  3   a  and  3   b.    
     As shown in  FIG. 3 , the transportation frames  20   a  and  20   b  are placed at both ends of the side that is orthogonal direction to the running direction of the transportation belt  18 . In other words, the transportation frames  20   a  and  20   b  are placed at the upper part and the lower part in  FIG. 3 . 
     The transportation frames  20   a  and  20   b  support the plurality of rollers  17  in such a way that these rollers can rotate. 
     The transportation belt  18  is formed of endless belt having a large number of suction holes  21 . The transportation belt  18  held under tension by the plurality of rollers  17 . 
     The transportation frames  20   a  and  20   b  also support the platen  16 , which was processed to have a predetermined flatness, and the fan  19  placed below the platen  16 , such that the recording medium  14  does not meander. 
     On the platen  16 , a plurality of grooves  22 , which extend in the recording medium transportation direction, are formed in the region facing the transportation belt  18 . A suction hole  23  in communication with the fan  19  is formed on the grooves  22 . 
     In this way, the suction hole  23  of the platen  16  and the suction hole  21  of the transportation belt  18  serve to adsorb the recording medium  14  on the transportation belt  18  by means of the suction force of the fan  19 . 
     The transportation belt  18  moves via a drive motor attached to at least one of the plurality of rollers  17  being driven. The transportation belt  18  then adsorbs and holds the recording medium  14  sent by the resist roller pair  15  he transportation belt  18  transport the recording medium  14  downstream of the transportation direction at a predetermined rate. 
     Abutting units  25  ( 25   a,    25   b,    25   c  and  25   d ) on which gap adjustment members  24  ( 24   a,    24   b,    24   c  and  24   d ) abut for adjusting the space between the recoding unit  6  and the transportation unit  10  are formed on the platen  16 . 
     The movement unit  9  comprises a driving source  9   a  and a wire  9   b.  One end of the wire  9   b  is fixed on a shaft (not shown) extending from the driving source  9   a.  The other end of the wire  9   b  is fixed on the platen  16  via the gap adjustment members  24 . In the present embodiment, four wires are used as the wires  9   b  and the other ends above of the four wires  9   b  are respectively fixed on the abutting units  25  ( 25   a,    25   b,    25   c  and  25   c ), which are the four corners of the platen  16 . 
     By driving the driving source  9   a,  the movement unit  9  rotates the shaft (not shown) to reel or unreel the wires  9   b.  In this way, the transportation unit  10  moves to an image recording position shown in  FIG. 1A  and to an evacuation position (i.e., non image-recording position) shown in  FIG. 1B . 
     In the present embodiment, the transportation unit  10  is moved by the wires  9   b.  However, the configuration is not limited to this. The movement unit  9  can have any configuration as long as the transportation unit  10  can move to the image recording position and the evacuation position. 
     The recording unit  6  is accommodated in the first housing  3 . The recording unit  6  is placed facing the transportation unit  10 . As shown in  FIGS. 1 and 4 , the recording unit  6  includes head units  26 ,  27 ,  28  and  29 , a head holder  31 , supporting units  32 ,  33  and  34 , and the gap adjustment members  24  ( 24   a  to  24   d ) described above. 
     The head units  26 ,  27 ,  28  and  29  respectively jet black (K) ink, cyan (C) ink, magenta (M) ink, and yellow (Y) ink to the recording medium  14  transported by the transportation unit  10 . 
     The head units  26 ,  27 ,  28  and  29  extend in the direction orthogonal to the transportation direction of the recording medium  14  and have a width that is as great as or greater than a recording region on the recording medium  14 . In this way, the head units  26 ,  27 ,  28  and  29  configure a line head. 
     As an example, as shown in  FIG. 5 , the head unit  26  in the present embodiment comprises an ink distributor  35  and four recording heads  36 - 1  to  36 - 4 . The ink distributor  35  is provided to temporarily pool ink to be distributed to each of the recording heads  36 - 1  to  36 - 4 . 
     The head units  26 ,  27 ,  28  and  29  are fixed and held by the head holder  31 . The head units  27 ,  28  and  29  also have configurations similar to that of the head unit  26 . 
     Two supporting units  32  and  33  are provided on the feeding-side face of the head holder  31  and the supporting unit  34  is provided on the ejecting-side face of the head holder  31 . 
     The head holder  31  is supported on the first housing  3  by three supporting units  32 ,  33  and  34 . In detail, the supporting units  32  and  33  are supported by the side face frames  3   a  and  3   b  as shown in  FIG. 2 . The supporting unit  34  is, as shown in  FIG. 1 , supported by the tie bar  3   c  that serves as a bridge between the side face frames  3   a  and  3   b.    
     The gap adjustment members  24  ( 24   a  to  24   d ) are provided on a face of the head holder  31 , which is opposed to the transportation unit  10 . The gap adjustment mechanisms  24  ( 24   a  to  24   d ) are provided at positions that are opposed to the abutting units  25   a,    25   b,    25   c  and  25   d  formed on the platen  16 . 
     When the transportation unit  10  is lifted upward using the wires  9   b  described above, the gap adjustment members  24   a,    24   b,    24   c  and  24   d  abut on the abutting units  25   a,    25   b,    25   c  and  25   d  formed on the platen  16 . 
     This determines the positions of the head holder  31  and the platen  16  in the height direction (Z direction). In other words, the space between the head units  26 ,  27 ,  28  and  29  and the transportation belt  18  is defined in accordance with a predetermined gap amount. 
     The medium ejection unit  11  includes an ejection roller pair  37  and an ejection tray  38 . The ejection tray  38  and the ejection roller pair  37  are supported by the side face frames  3   a  and  3   b.  The recording medium  14  on which an image is recorded by the recording unit  6  is ejected into the ejection tray  38  by the ejection roller pair  37 . 
     In the image recording operations performed by the inkjet printer  1  that is configured as described above, under the state shown in  FIG. 1A , the recording medium  14  stored in the feed tray  12  are taken out one by one by the pickup roller  13  and are sent to the resist roller pair  15 . 
     The resist roller pair  15  corrects the inclination of the recording medium  14  and transports it to the transportation belt  18  at the proper timing for image recording. 
     The recording medium  14  transported to the transportation belt  18  is adsorbed and held on the transportation belt  18  by the fan  19 , such that it is transported in accordance with the movement of the transportation belt  18 . 
     In this way, the recording medium  14  sequentially arrive at positions below the head units  26 ,  27 ,  28  and  29 . Then, the head units  26 ,  27 ,  28  and  29  jet ink and record information, such as characters or images, on the recording medium  14 . The recording medium  14  on which images are recorded is ejected into the ejection tray  38  by the ejection roller pair  37 . 
     Next, an ink path of the present embodiment will be described using  FIGS. 5 and 6 . 
       FIG. 5  shows the ink path for K-color ink only. The ink paths for the other colors are similar to the one for K-color. 
     As shown in  FIG. 5 , the ink path for supplying ink to the head unit  26  comprises a sub tank  40 K accommodated in the second housing  4 , and a tube  41 K that connects the sub tank  41 K to the head unit  26  accommodated in the first housing  3 . 
     A liquid surface detector  42  and an air releasing valve  43  are provided at the sub tank  40 K. The liquid surface detector  42  monitors the amount of the ink in the sub tank  40 K such that the amount is a predetermined amount. Specifically, the liquid surface detector  42  performs monitoring such that the amount of the ink in the sub tank  40 K is an ink liquid surface  45 . 
     When the liquid surface detector  42  senses that the amount of the ink in the sub tank  40 K is equal to or smaller than a predetermined amount, ink is supplied from an ink cartridge (not shown) through a tube  44 K. The ink cartridge may be accommodated in either of the first housing  3  and the second housing  4 . 
     The air releasing valve  43  is provided to let air in and out of the sub tank  40 K. The sub tank  40 K configured as described above is placed in the second housing  4  in such a way that it is positioned lower than the head unit  26  in the direction of a gravitational force. 
     In more detail, the sub tank  40 K is placed in the second housing  4  in such a way that the ink liquid surface  45  of the sub tank  40 K is lower than a nozzle face  46  of the recording heads  36 - 1  to  36 - 4 , by height H in the direction of the gravitational force. 
     In this way, when the air releasing valve  43  is opened, a predetermined negative pressure is applied to each of the recording heads  36 - 1  to  36 - 4  and proper meniscuses are formed. In other words, when an image is recorded, the air releasing valve  43  is opened to record the image. A purging process for each of the recording heads  36 - 1  to  36 - 4  can be performed by closing the air releasing valve  43  to apply pressure into the sub tank  40 K. 
     One end of the tube  41 K is soaked in ink in the sub tank  40 K. The other end of the tube  41 K is connected to the ink distributor  35  of the head unit  26  accommodated in the first housing  3 . 
     In the present embodiment, the tube  41 K is connected to the ink distributor  35 . However, it may also be directly connected to each of the recording heads  36 - 1  to  36 - 4  without the ink distributor  35  being provided. 
     As shown in  FIG. 6 , the sub tanks  40 K,  40 C,  40 M and  40 Y corresponding to the head units  26  to  29  are accommodated in the second housing  4 . 
     The head units  26  to  29  and the sub tanks  40 K to  40 M are connected via the respective tubes  41 K,  41 C,  41 M and  41 Y that pass through the opening  7  formed on the side face frame  3   a.    
     When recording is performed, the second housing  4  is located at the first position indicated by a solid line. When maintenance or inspection of the ink path is performed, the second housing  4  can be rotated (opened and closed) about the rotation axis  5  from the first position to the second position indicated by dashed lines. 
     The second position is upright relative to the side face frame  3   a.    
     In this way, the second housing  4  is attached to the first housing  3  in such a way that it can be moved to the first position and to the second position that is different from the first position. Accordingly, the handling of the sub tanks  40 K,  40 C,  40 M and  40 Y accommodated in the second housing  4  is simplified. In other words, the maintenance or the inspection of the ink path can be easily performed. 
     When the maintenance or the inspection is terminated, the second housing  4  positioned at the second position is returned to the first position again. 
     As described above, in the present embodiment, in addition to the first housing  3  accommodating at least the transportation unit  13  and the recording unit  6 , the second housing  4  is provided for accommodating the sub tanks  40 K to  40 Y that configure a portion of the ink path. 
     Relative to the first housing  3 , the second housing  4  is rotated (opened and closed) from the first position to the second position, and the maintenance or the inspection of the ink path is performed when the second housing  4  is at the second position. In this way, the operations are riot needed to be performed in the first housing  3  whose inside is jammed (i.e., the space is narrow) because the transportation unit  13  and the recording unit  6  are accommodated. 
     Exemplary Modification 1 of the First Embodiment 
       FIGS. 7A and 7B  are diagrams showing exemplary modification 1 of the first embodiment. In the first embodiment, the second housing  4  is rotated (opened and closed) relative to the first housing  3 . Exemplary modification 1 is different in the sense that the second housing  4  is attached in such a way that it can slide relative to the first housing  3 . 
     Specifically, the second housing  4  slides in the vertical direction (see  FIG. 7A ) or in the transportation direction (see  FIG. 7B ) so as to shuttle between the first position indicated by a solid line and the second position indicated by a dashed line. 
     At the second position, at least a portion of the second housing  4  is not in contact with the first housing  3  such that the sub tanks in the second housing  4  can be easily accessed. Other than this, the configuration of exemplary modification 1 is similar to those in the first embodiment described above. 
     Exemplary Modification 2 of the First Embodiment 
       FIG. 8A  is a diagram showing exemplary modification 2 of the first embodiment.  FIG. 8B  is a perspective view showing only the second housing  4  of exemplary modification 2 of the first embodiment. 
     In the first embodiment, the second housing  4  is rotated (opened and closed) relative to the first housing  3 . Exemplary modification 2 is different in the sense that the second housing  4  is attached to the first housing  3  in such a way that it can be detached from the first housing  3 . In  FIG. 8A , the first and second positions of the second housing  4  are indicated by dashed lines and solid lines, respectively. 
     The tubes connected to the head units  26  to  29  are connected to first joints  47 K to  47 Y shown in  FIG. 8A . The tubes connected to the sub tanks  40 K to  40 Y are connected to second joints  48 K to  48 Y shown in  FIG. 8B . 
     When the second housing  4  is positioned at the first position, the second joints  48 K to  48 Y are connected to the first joints  47 K to  47 Y. 
     Each of the first joints  47 K to  47 Y and each of the second joints  48 K to  48 Y is configured such that ink does not leak when the second housing  4  is removed from the first housing  3 . Other than this, the configuration of exemplary modification 2 is similar to those in the first embodiment described above. 
     Second Embodiment 
       FIG. 9  is a diagram showing an ink path configuration of an inkjet printer of the second embodiment. In the present embodiment, in comparison with the ink path of the first embodiment using a non-circulatory system, an ink path is configured to use a circulatory system in order to eliminate bubbles in the ink path. 
     A head unit  26  of the present embodiment is provided with an ink collector  50  for temporarily collecting ink that was not jetted from recording heads  36 - 1  to  36 - 4 .  FIG. 9  shows the ink path for K-color ink only. The configurations of those for the other colors are similar to that of the ink path for K-color ink. 
     In the ink path of the present embodiment, a sub tank  40 K placed lower than the recording heads  36 - 1  to  36 - 4  in the direction of the gravitational force, a tube  41 K connecting the sub tank  40 K to an ink distributor  35 , a tube  51 K connecting the sub tank  40 K to the ink collector  50 , and a pump  52  placed on the tube  51 K configure an ink circulation unit. 
     In the present embodiment, the sub tank  40 K and the pump  52  are placed in a second housing  4 . 
     As is the case with the first embodiment described above, a liquid surface detector  42  and an air releasing valve  43  are provided in the sub tank  40 K. 
     One end of the tube  41 K is soaked in ink in the sub tank  40 K. The other end of the tube  41 K is connected to the ink distributor  35  of the head unit  26  accommodated in a first housing  3 . In the present embodiment, the tube  41 K is connected to the ink distributor  35 . However, it may also be directly connected to each of the recording heads  36 - 1  to  36 - 4  without the ink distributor  35  being provided. 
     One end of the tube  51 K is soaked in ink in the sub tank  40 K. The other end of the tube  51 K is connected to the ink collector  50  of the head unit  26  accommodated in the first housing  3 . 
     In the present embodiment, the tube  51 K is connected to the ink collector  50 . However, it may also be directly connected to each of the recording heads  36 - 1  to  36 - 4  without the ink collector  50  being provided. 
     When an attempt is made to remove bubbles from the ink path, the pump  52  is driven and sends the ink in the sub tank  40 K to the ink collector  50 . 
     In the ink circulation unit, the sub tank  40 K is placed such that an ink liquid surface  45  is lower than a nozzle face  46  of the recording heads  36 - 1  to  36 - 4  by height H. 
     When an image is recorded, the air releasing valve  43  is opened so as to put the sub tank  40 K in an air release state and the pump  52  is disabled. In this way, a predetermined negative pressure is applied to the recording heads  36 - 1  to  36 - 4  so as to form meniscuses, as is the case with the first embodiment. 
     When an attempt is made to remove bubbles from the ink path, the pump  52  is driven while the sub tank  40 K is maintained in the air release state. When the pump  52  is driven, ink is circulated in the order of the sub tank  40 K→the pump  52 →the ink collector  50 →each of the recording heads→the ink distributor  35 →the sub tank  40 K. This enables bubbles to be removed from the ink path. 
     When the amount of the ink in the sub tank  40 K is equal to or smaller than a predetermined amount, ink is supplied to the sub tank  40 K from a cartridge (not shown) through a tube  44 K. 
     The attachment relationship between the first housing  3  and the second housing  4  of the present embodiment is similar to those in  FIGS. 2 ,  7 A and  7 B or  FIGS. 8A and 8B  indicated for the first embodiment and the exemplary modification of the first embodiment described above. 
     In the present embodiment, the sub tank  40 K and the pump  52  are placed in the second housing  4 . However, it is also possible for one of the sub tank  40 K and the pump  52  to be placed in the second housing  4  and for the other to be placed in the first housing  3 . 
     Third Embodiment 
       FIG. 10  is a diagram showing an ink path configuration of an inkjet printer of the third embodiment. In the present embodiment, in comparison with the ink path of the first embodiment using a non-circulatory system, an ink path is configured to use a circulatory system. 
     In the present embodiment, an image can be recorded while circulating ink. A head unit  26  of the present embodiment is provided with an ink collector  50  for temporarily collecting ink that was not jetted from recording heads  36 - 1  to  36 - 4 .  FIG. 10  shows the ink path for K-color ink only. The configurations of those for the other colors are similar to that of the ink path for K-color ink. 
     In the ink path of the present embodiment, a sub tank  40 K placed lower than the recording heads  36 - 1  to  36 - 4  in the direction of the gravitational force, an upper sub tank  53 K placed higher than that of the recording heads  36 - 1  to  36 - 4  in the direction of the gravitational force, a tube  49 K connecting the ink collector  50  to the sub tank  40 K, a tube  54 K connecting the upper sub tank  53 K to the sub tank  40 K, a pump  52  placed on the tube  54 K, and a tube  55 K connecting the upper sub tank  53 K to an ink distributor  35  configure an ink circulation unit. 
     In the present embodiment, the sub tank  40 K, the upper sub tank  53 K, and the pump  52  are placed in a second housing  4 . 
     As is the case with the first embodiment described above, a liquid surface detector  42  and an air releasing valve  43  are provided in the sub tank  40 K. 
     The upper sub tank  53 K is placed in a second housing  4 . A liquid surface detector  57  and an air releasing valve  58  are provided in the upper sub tank  53 K. The liquid surface detector  57  monitors the amount of ink in the upper sub tank  53 K such that this amount is a predetermined amount. Specifically, the liquid surface detector  57  performs monitoring such that the amount of ink in the upper sub tank  53 K is an ink liquid surface  56 . The air releasing valve  58  is provided to let air in and out of the upper sub tank  53 K. 
     One end of the tube  49 K is soaked in ink in the sub tank  40 K. The other end of the tube  49 K is connected to the ink collector  50  of the head unit  26  accommodated in a first housing  3 . In the present embodiment, the tube  49 K is connected to the ink collector  50 . However, it may also be directly connected to each of the recording heads  36 - 1  to  36 - 4  without the ink collector  50  being provided. 
     One end of the tube  54 K is soaked in ink in the sub tank  40 K. The other end of the tube  54 K is soaked in ink in the upper sub tank  53 K. 
     One end of the tube  55 K is soaked in ink in the upper sub tank  53 K. The other end of the tube  55 K is connected to the ink distributor  35  of the head unit  26  accommodated in the first housing  3 . 
     In the present embodiment, the tube  55 K is connected to the ink distributor  35 . However, it may also be directly connected to each of the recording heads  36 - 1  to  36 - 4  without the ink distributor  35  being provided. 
     When the ink in the upper sub tank  53 K is reduced, the pump  52  is driven and supplies ink from the sub tank  40 K to the upper sub tank  53 K. 
     In this ink circulation unit, the upper sub tank  53 K is placed such that the height of the ink liquid surface  56  is greater than the height of a nozzle face  46  of the recording heads  36 - 1  to  36 - 4 , by height H 1 . 
     The sub tank  40 K is placed such that the height of the ink liquid surface  45  is less than the height of the nozzle face  46  of the recording heads  36 - 1  to  36 - 4 , by height H 2 . 
     Heights H 1  and H 2  above are set such that a predetermined negative pressure is applied to the recording heads  36 - 1  to  36 - 4 . In this way, moderate meniscuses are formed on the recording heads  36 - 1  to  36 - 4 . 
     When ink is circulated while an image is being recorded, the air releasing valve  58  and an air releasing valve  43  are opened. Since the air releasing valve  58  is opened, the internal pressure of the upper sub tank  53 K is equal to an ambient pressure. The internal pressure of the sub tank  40 K is also equal to an ambient pressure since the air releasing valve  43  is opened. 
     Therefore, because of the difference in elevation between the upper sub tank  53 K and the sub tank  40 K, ink runs down by means of its own weight. Then, when the amount of the ink in the upper sub tank  53 K is equal to or smaller than a predetermined amount, the pump  52  is driven and ink from the sub tank  40 K is supplied to the upper sub tank  53 K. In other words, during ink circulation, ink is circulated in the order of the sub tank  40 K→the pump  52 →the sub tank  53 K→the ink distributor  35 , each of the recording heads→the ink collector  50 →the sub tank  40 K. 
     When the amount of the ink in the sub tank  40 K is equal to or smaller than a predetermined amount, ink is supplied from a cartridge (not shown) through the tube  44 K to the sub tank  40 K. 
     As described above, heights H 1  and H 2  are set such that a predetermined negative pressure is applied to the recording heads  36 - 1  to  36 - 4 . Therefore, even while ink is circulated, meniscuses are formed on the recording heads  36 - 1  to  36 - 4  and a recording operation (i.e., ink jetting operation) can be performed. 
     During a non-recording period (i.e., non ink-circulation period) in which an image is not recorded, the air releasing valve  58  is closed. Here, ink does not drip off since a predetermined negative pressure is applied to the recording heads because of the difference in elevation (i.e., height H 2 ) between the ink liquid surface  45  in the sub tank  40 K and the nozzle face  46  of the recording heads  36 - 1  to  36 - 4 . 
     The attachment relationship between the first housing  3  and the second housing  4  of the present embodiment is similar to those in  FIGS. 2 ,  7 A and  7 B or  FIGS. 8A and 8B  indicated for the first embodiment and the exemplary modification of the first embodiment described above. 
     In the present embodiment, the sub tank  40 K, the upper sub tank  53 K, and the pump  52  are placed in the second housing  4 . However, it is also possible for at least one of the sub tank  40 K, the upper sub tank  53 K, and the pump  52  to be placed in the second housing  4  and for the others to be placed in the first housing  3 . 
     In the first, second and third embodiments described above, a line-type inkjet printer employing a line head is described. However, needless to say, it is also possible to apply these embodiments to a serial-type inkjet printer that performs recording while moving a recording head in the width direction of a recording medium. 
     In the implementation phase of the present invention, various changes can be made to the extent that the gist of the invention is not changed.