Patent Publication Number: US-8992006-B2

Title: Image forming apparatus configured for bi-directional feeding of medium

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is based on and claims the benefit of priority under 35 U.S.C §119 of Japanese Patent Application Nos. 2012-204855 filed Sep. 18, 2012 and 2013-116539 filed Jun. 1, 2013, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus. 
     2. Description of the Related Art 
     As an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, a multi-functional peripheral (MFP) thereof or the like, more specifically, as an image forming apparatus employing a liquid-discharge recording method using a recording head ejecting ink droplets or the like, an inkjet recording apparatus and the like have been popular. 
     As an image forming apparatus in related art, there has been known an apparatus in which the recording heads are arranged in a manner that ink droplets discharged are ejected in the horizontal direction. The feeding unit to feed the recording sheet from the sheet supplying unit in the vertical (upward) direction, so that an image is formed by the recording heads on the recording sheet feeding the vertical (upward) direction. 
     The recording sheet on which the image is formed is discharged to the sheet discharge unit disposed on the upper side of the apparatus main body. In double-sided printing, the discharged recording sheet is returned on the upstream side of the feeding unit, so that the recording sheet is further fed in the vertical (upward) direction and an image is formed on the opposite side where the previous image has been formed. The double-side printed recording sheet is discharged to the sheet discharge unit (see, for example, Japanese Laid-open Patent Publication No. 2012-106055). 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an image forming apparatus include an image forming unit forming an image on a recording medium; and a feeding unit feeding the recording medium facing the image forming unit in a first feeding direction relative to the image forming unit and a second feeding direction opposite to the first feeding direction. 
     Further, when the recording medium is supplied from a first side of the feeding unit, while the feeding unit feeds the recording medium in the first feeding direction, the image forming unit forms an image on the recording medium, and the feeding unit further feeds the recording medium to a second side of the feeding unit, the second side being opposite to the first side relative to the feeding unit, and when the recording medium is supplied from the second side of the feeding unit, while the feeding unit feeds the recording medium in the second feeding direction, the image forming unit forms an image on the recording medium, and the feeding unit further feeds the recording medium to the first side of the feeding unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features, and advantages of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is an external perspective view of an image forming apparatus according to a first embodiment; 
         FIG. 2  is a side view of the image forming apparatus when viewed from the X direction of  FIG. 1 ; 
         FIG. 3  is a side view of a feeding mechanism part of the image forming apparatus; 
         FIGS. 4A and 4B  are side views of a feeding mechanism part according to a second embodiment; 
         FIG. 5  is a side view similar to  FIG. 2  according to a third embodiment; 
         FIG. 6  is a flowchart illustrating a printing operation in a manual single-sheet printing according to the third embodiment; 
         FIG. 7  is a side view similar to  FIG. 2  according to a fourth embodiment; 
         FIG. 8  is a top view of the image forming apparatus when viewed from the Y direction of  FIG. 7 ; 
         FIG. 9  is a side view of an image forming apparatus according to a fifth embodiment; 
         FIG. 10  is a side view of an image forming apparatus according to a sixth embodiment; 
         FIG. 11  is a side view of an image forming apparatus according to a seventh embodiment; 
         FIGS. 12A and 12B  are drawing illustrating the image forming apparatuses that are installed in different manners; 
         FIG. 13  is an external perspective view of an image forming apparatus according to an eighth embodiment; 
         FIG. 14  is a side view of the image forming apparatus according to the eighth embodiment; 
         FIG. 15  is an external perspective view of an image forming apparatus according to a ninth embodiment; 
         FIG. 16  is a side view of an image forming apparatus according to the ninth embodiment; 
         FIG. 17  is a side view of an image forming apparatus according to a tenth embodiment; 
         FIG. 18  is a top view of the image forming apparatus according to an eleventh embodiment; 
         FIG. 19  illustrates an example nozzle arrangement of the recording heads according to the eleventh embodiment; and 
         FIGS. 20A and 20B  are drawings illustrating images formed by the image forming units according to the eleventh embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the image forming apparatus in the related art, the feeding direction of the recording medium on which an image is formed in the image forming process is always one direction. Namely, the image is formed while the recording medium is fed from the sheet supplying side to the sheet discharging side. 
     Due to this limitation, the sheet supplying position and the sheet discharging position are fixed; thereby limiting the degree of freedom on how to layout the apparatus. 
     With such a configuration, it may be difficult to freely install the apparatus to fit the installation space. Further, it may be difficult to add a device for adding an expanded function to meet the user&#39;s request. 
     The present invention is made in light of the problems and may provide an image forming apparatus whose sheet supplying side and sheet discharging side are functionally replaceable, so that a degree of freedom in installation manners and additional function setting may be enhanced. 
     In the following, embodiments of the present invention are described with the accompanying drawings. 
     First, a first embodiment of the present invention is described with reference to  FIGS. 1 through 3 .  FIG. 1  is an external perspective view of an image forming apparatus.  FIG. 2  is a side view of the image forming apparatus when viewed from the X direction of  FIG. 1 .  FIG. 3  is a side view of a feeding mechanism part of the image forming apparatus. The view of  FIG. 3  is rotated by approximately 45 degrees relative to the view in  FIG. 2 . 
     As shown in  FIG. 1 , an image forming apparatus  1  includes an apparatus main body  2  and a printing mechanism part  11  that is disposed in the apparatus main body  2 . The printing mechanism part  11  performs an image forming process. The apparatus main body  2  includes an outer cover (housing member)  3 . 
     On the upper surface of the outer cover (housing member)  3 , an upper opening (first sheet supplying and discharging opening)  4  is formed which is a first medium opening serving as a sheet supplying opening and a sheet discharging opening through which a sheet (i.e., a medium to be recorded) is supplied and discharged. 
     Similarly, on a side surface of the outer cover (housing member)  3 , a side opening (first sheet supplying and discharging opening)  5  is formed which is a second medium opening serving as a sheet supplying opening and a sheet discharging opening through which a sheet is supplied and discharged. 
     As described above, the upper opening  4  and the side opening  5  are formed on the upper surface and the side surface, respectively, of the apparatus main body  2 , and no other opening for supplying or discharging the sheet is formed on any other surfaces of the outer cover (housing member)  3 . 
     Due to this configuration, the image forming apparatus  1  may be installed in a manner that a surface other than the upper surface and the side surface is attached to a (vertical) wall. Even in this manner, an image may be formed by the image forming apparatus  1 . Further, the image forming apparatus  1  may be installed in a space-saving manner. 
     Next, as shown in  FIG. 2 , the printing mechanism part  11  includes an image forming section  21  that serves as an image forming unit that forms an image on the medium to be recorded (“recording medium”). The printing mechanism part  11  further includes a feeding mechanism part  22  that serves as a feeding unit that feeds the recording medium in a first feeding direction facing the image forming section  21  and in a second feeding direction opposite to the first feeding direction facing the image forming section  21 . 
     The image forming section  21  includes a carriage  33  that is movably supported in the main-scanning direction (i.e. in the direction vertical to the surface of  FIG. 2 ) by a stay  31 . The carriage  33  includes a recording head  34  that includes a liquid discharging head for discharging link droplets in various colors such as, for example, yellow (Y), magenta (M), cyan (C), and black (K) colors. Further, as the image forming unit, a recording head other the liquid discharging head may be used. 
     Here, the recording head  34  is mounted on the carriage  33  in a manner that the recording head  34  is tilted at a predetermined angle (approximately 45 degrees in this example) relative to the horizontal plane so that the liquid droplets discharged from the recording head  34  are directed in an obliquely downward direction. 
     The feeding mechanism part  22  includes a platen member  41  that guides the feeding of a sheet  10  (see,  FIG. 3  and described below) facing the recording head  34 . The feeding mechanism part  22  further includes a first feeding rotary body pair  42  on the upper opening  4  side and a second feeding rotary body pair  45  on the side opening  5  side. The first feeding rotary body pair  42  includes an upper feeding roller  43  and a spur  44 . The second feeding rotary body pair  45  includes a side feeding roller  46  and a spur  47 . 
     Further, a guide member  51  is arranged between the upper opening  4  and the first feeding rotary body pair  42  so as to guide the sheet  10  therebetween. Also, a guide member  52  is arranged between the side opening  5  and the second feeding rotary body pair  45  so as to guide the sheet  10  therebetween. 
     In this case, the feeding path (feeding route)  50  of the sheet  10  is depicted in a dashed-dotted line. 
     Hereinafter, a feeding direction in which the sheet  10  is fed from the upper opening  4  to the side opening  5  is referred to as a “first feeding direction A”, and a feeding direction in which the sheet  10  is fed from the side opening  5  to the upper opening  4  is referred to as a “second feeding direction B”. Accordingly, the first feeding rotary body pair  42  is disposed on the upstream side of the recording head  34  in the first feeding direction A, and the second feeding rotary body pair  45  is disposed on the upstream side of the recording head  34  in the second feeding direction B. 
     Further, as shown in  FIG. 3  as well, in the direction orthogonal to a nozzle surface  34   a  of the recording head  34  (i.e., in the liquid droplet discharging direction), the upper feeding roller  43  of the first feeding rotary body pair  42  is arranged in a manner that a part of the periphery of the upper feeding roller  43  of the first feeding rotary body pair  42  protrudes beyond the surface level (surface position) of the platen member  41  (the surface position is depicted in the dashed-two dotted line “S” in  FIG. 3 ) on the nozzle surface  34   a  of the recording head  34 . 
     Further, the spur  44  is arranged relative to the periphery of the upper feeding roller  43  in a manner that when the sheet  10  supplied from the upper opening  4  is fed in the first feeding direction A, the sheet  10  is pushed onto the surface platen member  41 . 
     Similarly, in the direction orthogonal to a nozzle surface  34   a  of the recording head  34  (the liquid droplet discharging direction), the side feeding roller  46  of the second feeding rotary body pair  45  is arranged in a manner that a part of the periphery of the side feeding roller  46  of the second feeding rotary body pair  45  protrudes beyond the surface level (surface position) of the platen member  41  on the nozzle surface  34   a  of the recording head  34 . 
     Further, the spur  47  is arranged relative to the periphery of the side feeding roller  46  in a manner that when the sheet  10  supplied from the side opening  5  is fed in the second feeding direction B, the sheet  10  is pushed onto the surface platen member  41 . 
     Due the configuration described above, it is possible that the sheet  10  supplied from the upper opening  4  is fed in the first feeding direction A while the sheet  10  is followed on the surface platen member  41  by the feeding mechanism part  22 , so that an image is formed on the sheet  10  by discharging liquid droplets from the recording head  34  of the image forming section  21  and the sheet  10  on which the image is formed is discharged from the side opening  5 . 
     Further, it is also possible that the sheet  10  supplied from the side opening  5  is fed in the second feeding direction B while the sheet  10  is followed on the surface platen member  41  by the feeding mechanism part  22 , so that an image is formed on the sheet  10  by discharging liquid droplets from the recording head  34  of the image forming section  21  and the sheet  10  on which the image is formed is discharged from the upper opening  4 . 
     Namely, when the recording medium is supplied from a first side of the feeding unit, the recording medium is fed in the first feeding direction A, an image is formed on the recording medium by the image forming unit, and the recording medium on which the image is formed is fed to a second side, opposite to the first side, of the feeding unit. On the other hand, when the recording medium is supplied from the second side of the feeding unit, the recording medium is fed in the second feeding direction B, an image is formed on the recording medium by the image forming unit, and the recording medium on which the image is formed is fed to the first side of the feeding unit. 
     By doing this, a sheet supplying side and a sheet discharging side may be changed (interchanged with each other). 
     Further, as described above, the external form of the apparatus main body  2  is rectangular (cuboid). Due to the external form, a surface other than the upper and side surfaces where the upper opening  4  and the side opening  5 , respectively, are formed may be in close contact with a wall or the like. For example, the image forming apparatus  1  (apparatus main body  2 ) may be installed in a space next to a space where books are on the corner of a desk. 
     Otherwise, for example, the image forming apparatus  1  (apparatus main body  2 ) may be installed in a manner that the rear surface side (i.e., the side surface opposite to the other side surface where the side opening  5  is formed) is in close contact with the wall surface of the installation site (space). 
     Further, in the printing mechanism part  11 , the number of rollers for feeding the sheet  10  is reduced so that the flatness of sheet  10  in the position facing the recording head  34  is maintained by the platen member  41 . By doing this, the structure of the printing mechanism part  11  is simplified. 
     In this embodiment, it is described that the recording head  34  is tilted at a predetermined angle (approximately 45 degrees in this example) relative to the horizontal plane. However, it should be noted that the angle of the recording head  34  is not limited to the approximately 45 degrees schematically shown in  FIG. 2 . Namely, the recording head  34  may be arranged so that the liquid droplet discharging direction corresponds to any direction including the vertical direction and the horizontal direction. 
     However, in order for the feeding load in the direction from the upper opening  4  (i.e., the first feeding direction A) to be similar to the feeding load in the direction from the side opening  5  (i.e., the second feeding direction B) and that the difference in recording quality between the first feeding direction A and the second feeding direction B is reduced, it is preferable that the angle of the recording head  34  relative to the horizontal plane be set as shown in  FIG. 2  Specifically, it is preferable that the above angle be within a range from 30 degrees to 60 degrees. 
     When the recording head  34  is disposed in a manner that the tilted angle of the recording head  34  is closer to 45 degrees, each of the flexion angles of the sheet  10  from the upper opening  4  to the recording position (i.e., position at the platen member  41 ) and the flexion angles of the sheet  10  from the side opening  5  to the recording position (position at the platen member  41 ) may be reduced. 
     By doing this, the overall sheet feeding load may be reduced, thereby enabling highly-accurate feeding even when a nip part is formed by using a combination of a spur and a roller each having relatively lower feeding power and reducing the number of rollers for feeding the sheet  10 . To that end, it is preferable that the tilted angle of the recording head  34  be in a range from 40 degrees to 50 degrees. 
     Further, in each of the cases where the sheet  10  is fed from the upper opening  4  and the side opening  5 , the first and the second feeding rotary body pairs  42  and  45  are arranged so as to press the sheet  10  onto the surface of the platen member  41  to maintain the flatness of sheet  10  in the position facing the recording head  34 . 
     Due to this, even when the printing is performed while any of the header part and the end part of the sheet  10  in the feeding direction is on the platen member  41 , it may become possible to prevent the sheet  10  from separating from the surface of the platen member  41 , thereby enabling forming an image in high equality. 
     Next, a second embodiment is described with reference to  FIGS. 4A and 4B .  FIGS. 4A and 4B  are side views of a feeding mechanism part according to the second embodiment. In  FIGS. 4A and 4B , the direction of the surface of the platen member  41  is substantially parallel to the horizontal direction (i.e., rotated by approximately by 45 degrees relative to the direction of the surface of the platen member  41  in  FIG. 2 ). 
     Here, a rotary body member  61 , that includes a spur  44  and a roller  48 , is disposed facing the upper feeding roller  43  of the first feeding rotary body pair  42 . The rotary body member  61  includes a rotation axis  62 . The spur  44  and the roller  48  are connected to the rotation axis  62  by the arm members  63   a  and  63   b , respectively, so that the spur  44  and the roller  48  are rotatably supported by the rotation axis  62 . The angle between the arm members  63   a  and  63   b  is predetermined. 
     Similarly, a rotary body member  64 , that includes a spur  47  and a roller  49 , is disposed facing the side feeding roller  46  of the second feeding rotary body pair  45 . The rotary body member  64  includes a rotation axis  65 . The spur  47  and the roller  49  are connected to the rotation axis  65  by the arm members  66   a  and  66   b , respectively, so that the spur  47  and the roller  49  are rotatably supported by the rotation axis  65 . The angle between the arm members  66   a  and  66   b  is predetermined. 
     Further, in this embodiment, when the sheet  10  is fed in the first feeding direction A, as shown in  FIG. 4A , the roller  48  of the rotary body member  61  is in contact with the periphery of the upper feeding roller  43  of the first feeding rotary body pair  42  and the spur  47  is in contact with the periphery of the side feeding roller  46  of the second feeding rotary body pair  45 . 
     Similarly, when the sheet  10  is fed in the second feeding direction B, as shown in  FIG. 4B , the roller  49  of the rotary body member  64  is in contact with the periphery of the side feeding roller  46  of the second feeding rotary body pair  45  and the spur  44  is in contact with the periphery of the upper feeding roller  43  of the first feeding rotary body pair  42 . 
     By doing this, before an image is formed, a feeding force may be applied to the sheet  10  by the roller pair, and a feeding force applied by the roller pair is greater than the feeding force applied by the combination of the roller and the spur. Further, after the image is formed, by the spur in contact with thee surface of the sheet  10  on which the image is formed, it may become possible to prevent the degradation of the image which may occur due to friction to the formed image on the sheet  10 . 
     Next, a third embodiment is described with reference to  FIGS. 5 and 6 .  FIG. 5  is a side view according to the third embodiment similar to  FIG. 2 . 
     In this embodiment, the configuration according to this embodiment differs from that in the first embodiment in that sheet sensors  71  and  72  are provided. The sheet sensor  71  detects the sheet  10  near the upper opening  4  as a sheet detecting unit (detecting unit). The sheet sensor  72  detects the sheet  10  near the side opening  5  as the sheet detecting unit. 
     The sheet sensors  71  and  72  may be, for example, photo sensors. However, any other sensor may be used as the sheet sensors  71  and  72 . 
     Basically, the first feeding direction A or the second feeding direction B is selected by a user input. However, the user may confirm the selected direction based on a signal from the sheet sensor  71  or  72 . Namely, although the user input indicates that the first feeding direction A is selected, if the sheet sensor  72  first detects the sheet set, an error message is displayed and the sheet  10  is not fed. This also applies to the case where although the user input indicates that the second feeding direction B is selected, if the sheet sensor  71  first detects the sheet set. 
     In another example, the detection signals from the sheet sensors  71  and  72  is not used for checking the sheet feeding direction, but may be used for determining the sheet feeding direction. The specific process of determining the sheet feeding direction is described with a flowchart of  FIG. 6  in which a printing operation in a manual single-sheet printing. 
     As shown in  FIG. 6 , a user instructs printing (step S 61 ). A control section (not shown) of the apparatus main body  2  causes an operation section (not shown) to display a message (sign) indicating that printing is ready (step S 62 ). Then, the user sets (inserts) one sheet  10  through the upper opening  4  or the side opening  5  (step S 64 ), and presses a print OK button on the operation section of the apparatus main body  2  (step S 65 ). 
     In this case, the position of the sheet in the width direction may be adjusted by side fence adjustment (not shown) (step S 63 ). 
     Here, the control section of the apparatus main body  2  checks the detection signals from the sheet sensors  71  and  72  and determine whether the sheet  10  is set at the upper opening  4  or the side opening  5  to determine the feeding direction of the sheet  10  (i.e., the driving direction of the feeding mechanism part  22 ) (step S 66 ). 
     Then, while the sheet  10  is fed in the determined feeding direction by the feeding mechanism part  22 , the recording head  34  is driven to perform a printing process of forming an image on the sheet  10  (step S 67 ). In this case, the sheet  10  is intermittently fed and the carriage  33  is moved in the main-scanning direction to discharge liquid droplets from the recording head  34 . By repeating the operation, the image is formed on the sheet  10 . 
     By feeding the sheet  10  while the image is formed, the sheet  10  is further fed in the feeding direction and discharged from the side opening  5  or the upper opening  4  (step S 68 ). 
     By doing this, when manual printing is performed, even when a user may arbitrarily select either the upper opening  4  or the side opening  5  to set the sheet  10  for printing, the apparatus main body  2  may accept the setting by selectively changing the sheet supplying opening and the sheet discharging opening to print the sheet  10  based on the user&#39;s selection. 
     Further, by using the sheet sensors  71  and  72 , paper jamming may be detected. Further, by measuring the distance of the sheet  10  passing through the two sheet sensors  71  and  72 , the slipping of the sheet (magnification error) may be corrected. Further, when a sheet supplying unit where plural sheets can be stacked is additionally attached, the rear end part of the sheet is detected, and based on the detected information, the next sheet is fed, so that continuous sheet feeding may be performed. 
     In the above printing operation flow, the step in checking the pressing of the print OF button may be omitted, or the print OK button may be directly removed. In this case, the sheet sensor  71  or  72  may detect that a sheet is inserted by a user, then the feeding mechanism part  22  is driven to be rotated in the corresponding direction, or after the preparation of printing is completed, the feeding mechanism part  22  is rotated. Further, to facilitate the sheet setting, a mechanism of drawing the sheet that is manually set (e.g., a drawing roller) may be added. 
     Next, a fourth embodiment is described with reference to  FIGS. 7 and 8 .  FIG. 7  is a side view according to a fourth embodiment similar to  FIG. 2 .  FIG. 8  is a top view of the image forming apparatus when viewed from the Y direction of  FIG. 7 . Here, in  FIG. 7 , a side of the exterior of the apparatus main body  2  is assumed to be transparent to view the elements inside the apparatus main body  2 . 
     In this embodiment, each of the upper feeding roller  43  and the side feeding roller  46  of the feeding mechanism part  22  is rotatably supported by a left side plate  80 A and a right side plate BOB. 
     Further, the feeding mechanism part  22  includes a motor  81 , a motor gear  82  that rotates by the motor  81 , an upper feeding roller gear  83  that is attached to an axle part  43   a  of the upper feeding roller  43  and is in engagement with the motor gear  82 , and a side feeding roller gear  84  that is attached to an axle part  46   a  of the side feeding roller  43  and is in engagement with the motor gear  82 . 
     The feeding mechanism part  22  further includes an upper joint gear  85  that is in engagement with the upper feeding roller gear  83  and a side joint gear  86  that is in engagement with the side feeding roller gear  84 . Those joint gears  85  and  86  are provided to transmit the feeding driving force to an extension unit that can be attached to the upper opening  4  or the side opening  5  as described below. 
     Due to the structure of the driving mechanism, the upper feeding roller  43  and the side feeding roller  46  rotate in the same direction by rotating the motor  81 . 
     Further, in this configuration described above, the motor  81  is disposed at the center part and a single reducing motor gear  82  fixed to the motor shaft is used to drive the rotation of the upper feeding roller  43  and the side feeding roller  46 . Due to this configuration, the stiffness property of the gear and delay caused by a delaying element (e.g., backlash) may be improved because many gears are not used in this configuration. 
     By doing this, it becomes possible to commonly control the precise feedback of the feeding rollers and improve the stiffness property of the gear and delay caused by a delaying element (e.g., backlash) because not many gears are used. 
     Further, in the apparatus main body  2 , to attach the extension unit to the apparatus main body  2 , a position determination section  91 A and a fastening section  92 A are formed on the left side plate  80 A and a position determination section  91 B and a fastening section  92 B are formed on the right side plate  80 B. Those position determination sections  91 A and  91 B and the fastening section  92 A and  92 B are used to as a unit (means) to externally connect the feeding path (feeding route)  50  of the apparatus main body  2 . 
     Here, the position determination sections  91 A and  91 B and the fastening section  92 A and  92 B have a hole shape so that when the extension unit has a convex shaped part for position determining and a click (snap fit) for fastening (fixing), the position of the extension unit may be fixed relative to the apparatus main body  2  and the extension unit may be jointed to the apparatus main body  2 . 
     Further, the position determination sections  91 A and  91 B may have a convex shape and the fastening section  92 A and  92 B may have a click (snap fit). By doing this, when the extension unit has hole-shaped parts, the extension unit may also be jointed to the apparatus main body  2 . 
     By having the same configuration on the upper opening  4  side, the extension unit may also be jointed to the apparatus main body  2  on the upper side as well. 
     Next, a fifth embodiment is described with reference to  FIG. 9 .  FIG. 9  is a side view of an image forming apparatus according to the fifth embodiment. 
     In this embodiment, a side sheet supplying unit  101  is attached on the side opening  5  side of the apparatus main body  2 . 
     The side sheet supplying unit  101  includes a sheet supplying cassette  102  to stack the sheets  10 , a sheet feeding roller  103  that separates and supplies the sheet  10  one by one, a side unit joint gear  104  that is to be in engagement with the side joint gear  86  in an engagement state, and a sheet supplying gear  105  that is fixed to the axle part of the sheet feeding roller  103  and is to be in engagement with the side unit joint gear  104 . 
     By doing this, the driven force may be transmitted from the side joint gear  86  on the apparatus main body  2  side to the side sheet supplying unit  101  so as to feed the sheet  10 . 
     Further, by connecting the side sheet supplying unit  101  that can stack a plurality of sheets  10  to the apparatus main body  2 , continuous printing may be performed. 
     Next, a sixth embodiment is described with reference to  FIG. 10 .  FIG. 10  is a side view of an image forming apparatus according to the sixth embodiment. 
     In this embodiment, in addition to the configuration in the fifth embodiment, an upper sheet discharging unit  111  is attached on the upper opening  4  side of the apparatus main body  2 . 
     The upper sheet discharging unit  111  includes a sheet feeding tray  112  that stocks discharged sheets  10 , a roller  113  and a spur  114 , a sheet discharging roller  115  and spur  116 , and a sheet discharging roller  117  and a spur  118  that feed the sheet  10  discharged from the upper opening  4  of the apparatus main body  2  to the sheet feeding tray  112 , an upper unit joint gear  119  that is in engagement with the upper joint gear  85  in an engagement state, a sheet discharging gear  120  provided on the axle part of the sheet discharging roller  115 , and a relay gear  121  that is in engagement with the upper unit joint gear  119  and the sheet discharging gear  120 . 
     By having the above configuration, a driving force from the upper joint gear  85  on the apparatus main body  2  side is transmitted to the upper sheet discharging unit  111 , so that the sheets  10  can be discharged to the sheet feeding tray  112 . 
     Accordingly, a large number of the sheets  10  after continuous printing may be stacked. 
     As described above, by making the extension unit attachable, when the side sheet supplying unit  101  and the upper sheet discharging unit  111  are attached to the apparatus main body  2  in a user&#39;s house, the apparatus (i.e., the apparatus main body  2  with attached side sheet supplying unit  101  and upper sheet discharging unit  111 ) can be used as a printer that can print many sheets by continuous printing). In addition, the user can bring out only the apparatus main body  2  so as to use the apparatus (i.e., apparatus main body  2  alone) as a mobile printer for manual single sheet printing. 
     In the configuration of the mobile printer without any attached sheet feeding tray and sheet discharging tray, if the sheet  10  is inserted into the side opening  5 , the sheet  10  discharged from the upper opening  4  cannot be hold. Therefore, for convenience purposes, opposite to the case where the extension units are attached to the apparatus main body  2 , it is preferable that the supplied sheet  10  be inserted into the upper opening  4 . 
     Next, a seventh embodiment is described with reference to  FIG. 11 .  FIG. 11  is a side view of an image forming apparatus according to the seventh embodiment. 
     In this embodiment, as another example where an extension unit is attached (connected), an upper double-sided unit  131  (for double-sided printing) is attached (connected) to the upper opening  4  side of the apparatus main body  2 . 
     As shown in  FIG. 11 , the upper double-sided unit  131  includes an inversion path  132  to invert the sheet  10  fed from the upper opening  4 , a roller  133  and a spur  134 , a roller  135  and a spur  136 , a roller  137  and a roller  138 , a roller  139  and a roller  140 , and a roller  141  and a roller  142  to feed the sheet  10 . 
     By having the above configuration, it becomes possible that the sheet  10  fed from the upper opening  4  on the apparatus main body  2  and having one-side surface on which an image is formed is inverted in the upper double-sided unit  131 , so that the inverted sheet  10  is supplied through the upper opening  4  again. Then, the inverted sheet  10  is fed in the opposite direction, so that an image is formed on the other side surface of the sheet  10 , and the sheet  10  is discharged from the side opening  5 . 
     In a double-sided unit for an image forming apparatus employing a one direction recording method of the related art, the sheet discharged from the printing section is required to be returned back to the sheet supplying side again by bypassing the printing section (i.e., in the case of  FIG. 11 , the sheet discharged from the upper opening  4  is required to be returned back to the side opening  5  by bypassing the apparatus main body  2 ). By doing this, however, the distance of the feeding path becomes longer and the size of the apparatus may be increased. 
     On the other hand, in this embodiment, it is possible to change the sheet feeding direction. Therefore, it is necessary to simply return the sheet  10  back to the upper opening  4  which is the same opening as that from which the sheet is discharged. As a result, it may become possible to attach (install) the upper double-sided unit  131  without substantially increasing the installation area. 
     As described in the fifth through the seventh embodiments, by simply attaching various extension units to the apparatus main body  2  that is a core elements of the image forming apparatus of the present invention, it may become possible for a user to use the apparatus in various applications. 
     Next, modified examples of the installation modes of the image forming apparatus of the present invention are described with reference to  FIGS. 12A and 12B .  FIGS. 12A and 12B  are drawing illustrating the image forming apparatus installed in different manners. 
     As illustrated in the above embodiments, the image forming apparatus  1  according to embodiments may be used in an installation mode (e.g., longitudinally installed) where an operation section  90  and the upper opening  4  of the apparatus main body  2  are positioned on the upper side and the side opening  5  is positioned on the lateral side relative to an installation plane  200 . 
     In this case, on the opposite side of the operation section  90  relative to the recording head  34 , four-color ink cartridges  91 Y,  91 M,  91 C, and  91 K supplying the inks to the recording head  34  are provided mounted. The ink cartridges may be inserted, for example, in the direction orthogonal to the sheet plane of  FIG. 12A  (i.e., in the side direction when the operation section  90  is viewed in front) to be set to the image forming apparatus  1 . 
     Further, as shown in  FIG. 12B , the image forming apparatus  1  according to embodiments may be used in another installation mode (e.g., laterally installed) where the side opening  5  of the apparatus main body  2  is positioned on the upper side and the operation section  90  and the upper opening  4  are positioned on the lateral side relative to the installation plane  200 . 
     Ink discharging performance may greatly differ depending on whether the ink droplets are discharged in the vertical direction or a horizontal direction. However, in the embodiments, the recording head is inclined by approximately 45 degrees relative to the installation plane  200 . By doing this, it becomes possible that the ink discharging performance when longitudinally installed is substantially the same as the ink discharging performance when laterally installed. 
     Further, in the image forming apparatus  1  according to embodiments, the function on the sheet supplying side and the function on the sheet discharging side may be replaced. Therefore, for example, in the installation (use) mode of  FIG. 12A , it is possible that the upper opening  4  is used as the sheet supplying side to supply the sheet  10 , so that the sheet  10  is fed in the first feeding direction A and an image is formed on the sheet  10 . Similarly, in the installation (use) mode of  FIG. 12B , it is possible that the side opening  5  is used as the sheet supplying side to supply the sheet  10 , so that the sheet  10  is fed in the second feeding direction B and an image is formed on the sheet  10 . 
     Namely, in these modified examples, regardless of the installation (use) modes, in the image forming apparatus  1  according to embodiments, the upper opening  4  may be used as a media opening through which a recording medium is supplied and the side opening  5  may be used as an sheet discharge opening. 
     By doing this, it becomes possible to freely select an appropriate installation (use) mode (i.e., the longitudinal installation or the lateral installation) depending on, for example, a shape of the installation space. More specifically, for example, when the image forming apparatus  1  is installed on a desk, the image forming apparatus  1  may be longitudinally installed to reduce the occupation space, and when a shelf is placed on the desk, the image forming apparatus  1  may be laterally installed due to the height limitation. In any of the installation modes, the sheet  10  may be supplied from the upper side. 
     Next, an eighth embodiment is described with reference to  FIGS. 13 and 14 .  FIG. 13  is an external perspective view of an image forming apparatus according to the eighth embodiment, and  FIG. 14  is a side view of the image forming apparatus according to the eighth embodiment. 
     In this embodiment, as shown in  FIGS. 13 and 14 , it is assumed that the image forming apparatus according to above embodiment corresponds to each of image forming units  1 A and  1 B. Further, those image forming units  1 A and  1 B are connected in a manner that the side opening  5  of the image forming unit  1 A is in contact with the side opening  5  of the image forming unit  1 B. 
     In this case, to ensure that the position of the side opening  5  of the image forming unit  1 A fits the position of the side opening  5  of the image forming unit  1 B so that the sheet  10  (recording medium) can smoothly be fed from one side opening  5  to another, the image forming units  1 A and  1 B may be connected with each other by using, for example, position determining bosses and corresponding joint screws. 
     In the image forming apparatus, the sheet  10  is supplied and discharged using the upper openings  4  of the image forming units  1 A and  1 B. Further, the image forming apparatus may be installed in a manner that a surface of the image forming apparatus other than the surface where the upper openings  4  are formed is in close contact with a wall surface and used. 
     According to this embodiment, since plural image forming units are connected to each other, it becomes possible to use the recording heads  34  two or more times. Namely, it becomes possible to virtually increase the number of nozzles to be used for the same surface of the recording medium by ensuring the respective printing positions between the image forming units. By doing this, it may become possible to provide an image forming apparatus having higher productivity than before and stable image forming with an inexpensive configuration. 
     In the case of this embodiment, two recording heads  34  are used. Therefore, the number of the nozzles may be twice as many as the number of nozzles when a single recording head  34  is used, and the size of the printing area on the recording medium per unit time may be doubled. As a result, it may become possible to double the print productivity while the installation space is saved. 
     Next, a ninth embodiment is described with reference to  FIGS. 15 and 16 .  FIG. 15  is an external perspective view of an image forming apparatus according to the ninth embodiment.  FIG. 16  is a side view of the image forming apparatus according to the ninth embodiment. 
     In this embodiment, three image forming units  1 A,  1 B, and  1 C are connected to configure an image forming apparatus. 
     Here, the image forming units  1 A and  1 B are connected to each other in a manner that the side opening  5  of the image forming unit  1 A is in contact with (fits) the side opening  5  of the image forming unit  1 B. Further, the image forming units  1 A and  1 C are connected to each other in a manner that the upper opening  4  of the image forming unit  1 A is in contact with (fits) the upper opening  4  of the image forming unit  1 C. 
     In this case, to ensure that the position of the side opening  5  of the image forming unit  1 A fits the position of the side opening  5  of the image forming unit  1 B and the position of the upper opening  4  of the image forming unit  1 A fits the position of the upper opening  4  of the image forming unit  1 C so that the sheet  10  (recording medium) can smoothly be fed from one side opening  5  to another and from one upper opening  4  to another, the image forming units  1 A and  1 B and the image forming units  1 A and  1 C may be connected with each other by using, for example, position determining bosses and corresponding joint screws. 
     In the image forming apparatus, the sheet  10  is supplied and discharged using the upper opening  4  of the image forming unit  1 B and the side opening  5  of the image forming unit  1 C. Further, the image forming apparatus may be installed in a manner that a surface of the image forming apparatus other than the surfaces where the upper opening  4  of the image forming unit  1 B and the side opening  5  of the image forming unit  1 C are formed is in close contact with a wall surface and used. 
     In the case of this embodiment, three recording heads  34  are used. Therefore, the number of the nozzles may be triple as many as the number of nozzles when a single recording head  34  is used, and the size of the printing area on the recording medium per unit time may be tripled. As a result, it may become possible to triple the print productivity while the installation space is saved. 
     Next, a tenth embodiment of the present invention is described with reference to  FIG. 17 .  FIG. 17  is a side view of an image forming apparatus according to the tenth embodiment. 
     In this embodiment, as shown in  FIG. 17 , it is assumed that the image forming apparatus according to above embodiment corresponds to each of image forming units  1 D and  1 E. Further, those image forming units  1 D and  1 E are connected in a manner that the upper opening  4  of the image forming unit  10  is in contact with the upper opening  4  of the image forming unit  1 E. 
     In this case, to ensure that the position of the upper opening  4  of the image forming unit  1 D fits the position of the upper opening  4  of the image forming unit  1 E so that the sheet  10  (recording medium) can smoothly be fed from one upper opening  4  to another, the image forming units  1 D and  1 E may be connected with each other by using, for example, position determining bosses and corresponding joint screws. 
     In the image forming apparatus, the sheet  10  is supplied and discharged using the side opening  5  of the image forming units  1 D and  1 E. 
     Here, the image forming unit  1 D and the image forming unit it are arranged (connected) with each other in a positional relationship that images can be formed on different (both-sided) surfaces of the recording medium. 
     Therefore, for example, the recording head  34  of the image forming unit it forms an image on the rear surface of the recording medium supplied from the side opening  5  of the image forming unit  10 , then the recording medium is fed to the image forming unit  1 E on the lower side. 
     Then, the recording head  34  of the image forming unit  1 D forms an image on the surface of the recording medium, and the recording medium is discharged from the side opening  5  of the image forming unit  1 D. 
     As described above, double-sided printing may be performed without adding a mechanism (unit) to invert the recording medium. 
     Further, in this case, plural recording heads are used to perform double-sided printing on a recording sheet. Therefore, the productivity in the double-sided printing may be improved. 
     Next, an eleventh embodiment is described with reference to  FIG. 18 .  FIG. 18  is a top view of an image forming apparatus according to the eleventh embodiment. This configuration corresponds to the configuration according to the eighth embodiment. 
     In this embodiment, as shown in  FIG. 18 , the home position of the carriage  33  of the image forming unit  1 A is disposed on the opposite side of the home position where the carriage  33  of the image forming unit  18  is disposed in the main scanning direction (back and forth moving direction). The printing is performed by moving the carriage  33  of the image forming unit  1 A in the printing direction PA and moving the carriage  33  of the image forming unit  1 B in the printing direction PB. 
     By doing this, it may become possible to prevent the occurrence of color differences that may occur when bi-directional printing is performed using one recording head to increase the printing speed. 
     Namely, for example, as shown in  FIG. 19 , the recording head  34  includes nozzle lines  34 Y,  34 M,  34 C, and  34 K discharging Yellow, Magenta, Cyan, and Black color ink droplets respectively and arranged in a manner that the direction of the nozzle lines are orthogonal to the main scanning direction. 
     In this case, the recording head  34  is used in both back and forth directions. The order of superimposing colors in the back direction is different from the order of superimposing colors in the forth direction. Due to this difference, color difference may occur. 
     To resolve the problem, in this embodiment, the printing direction PA in which the image forming unit  1 A performs printing and the printing direction PB in which the image forming unit  1 B performs printing are opposite to each other. 
     Further, for example, the recording head  34  of the image forming unit  1 A is used. In this case, in the movement in the forward direction, the carriage  33  is used to perform multicolor printing by superimposing colors in the order of C ink color and M ink color. Next, in the movement in the backward direction, the carriage  33  is used to perform multicolor printing by superimposing colors in the order of M ink color and C ink color. By doing this, as shown in  FIG. 20A , a synthetic color of light blue having a color difference is formed. 
     Next, the recording head  34  of the image forming unit  1 B is used to perform multicolor printing by superimposing colors in the order of C ink color and M ink color in the forward direction on the part where the recording head  34  of the image forming unit  1 A is used to perform multicolor printing in the backward direction before. 
     Also, the recording head  34  of the image forming unit  1 B is used to perform multicolor printing by superimposing colors in the order of M ink color and C ink color in the backward direction on the part where the recording head  34  of the image forming unit  1 A is used to perform multicolor printing in the forward direction before. 
     By doing this, as shown in  FIG. 20B , a synthetic color of light blue having regular (formal) density is formed. 
     Therefore, the color difference caused by performing the bi-directional printing including forward and backward printings using a single recording head may be cancelled and prevented. As a result, it may become possible to form a high-quality image without color difference and without reducing the productivity by bi-directional printing with an inexpensive simple configuration. 
     Further, in the present description, the “sheet” is not limited to a sheet whose material is paper but also any materials including, but not limited to, an OHP, cloth, glass, and substrate, and materials to which ink droplets, other liquid, image forming agent or the like. The “sheet” further includes, but not limited to, materials called a medium to be recorded, a recording medium, a recording paper, a recording sheet and the like. Further, herein, the terms “image forming”, “recording”, “copying”, “imaging”, and “printing” have the same meaning. 
     Further, herein the term “image forming apparatus” refers to an apparatus that performs image forming by applying liquid or image forming agent to a medium such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, ceramics or the like. Further, the term “image forming” refers to not only a process of applying a meaningful image of characters, figures and the like to a medium but also a process of applying a meaningless image of a pattern or the like to a medium (a process of simply applying liquid droplets or image forming agent to a medium). 
     Further, herein, unless otherwise limited, the term “ink” is not limited to a material which may called “ink” but is used as a collective name of the liquids including a liquid called a recording liquid, a fixing processing liquid, a liquid and the like and all the liquids that may be used to perform image processing. Also, the term “ink” further includes, for example, a DNA sample, a resist, a pattern material, and resin. 
     Further, herein, the term “image” is not limited to a two-dimensional image, but includes an image applied to a three-dimensionally formed material and a figure formed by performing three-dimensional modeling on a solid material or the like. 
     Further, herein, the term “horizontal direction” as the liquid discharging direction refers to a direction other than the vertical direction (i.e., the direction that is inclined upward or downward relative to the horizontal line). 
     Further, herein, the term “vertical direction” as the sheet feeding direction refers to a direction other than the direction in the horizontal line (i.e., the direction that is inclined upward or downward relative to the vertical line). 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.