Patent Publication Number: US-8523311-B2

Title: Capping mechanism and system for image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is a divisional of application Ser. No. 12/248,210, filed Oct. 9, 2008, which is based upon and claims the benefit of priority from Japanese Patent application No. 2007-276277, filed Oct. 24, 2007, each of which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention claims and discloses an image forming apparatus employing an inkjet system whereby an ink is discharged to a sheet (recording medium) in order to form an image on the sheet. 
     BACKGROUND OF THE INVENTION 
     In an image forming apparatus which forms an image on a sheet by discharging ink to the sheet with an inkjet head, a paper jam may occur during transportation of the sheet by a sheet transportation portion. In such an image forming apparatus, an ink discharge surface is provided on a lower surface of the inkjet head. The ink discharge surface has a large number of ink discharging nozzle openings for discharging the ink. The sheet transporting portion (i.e., a paper transport belt) is provided below the ink discharge surface at a small space interval. A user may insert his/her hand or the like in the vicinity of the inkjet head in order to rectify the paper jam occurring in the vicinity of the inkjet head. If a user&#39;s hand or the like touches the ink discharge surface of the inkjet head, it is likely to break meniscuses of the ink within the ink discharging nozzles which are open in the ink discharge surface. The meniscus as used herein is a surface on which the ink in the nozzles comes into contact with the open air. The state of the meniscus has an influence on the flying state of the ink. In the case where the meniscus is broken, a satisfactory discharge of the ink cannot be achieved and printing quality is deteriorated. Specifically, the hand or the like comes into contact with the nozzles of the inkjet head resulting in contaminants or the like entering the nozzles, or the ink within the nozzles soiling a hand or the like. In some cases, the meniscuses may be broken or at least damaged. 
     JP 2005-96187 relates to a conventionally proposed technology of covering the ink discharge surface with a sheet-like shutter in order to prevent, when paper jam occurs, a user from coming into direct contact with the ink discharge surface. For example, the direct contact can occur when a user inserts his/her hand in the vicinity of the inkjet head. With this technology, the user&#39;s hand will not be in contact with the ink discharge surface, thereby producing a high-quality image formation on a sheet. 
     However, in the image fowling apparatus described above, if the sheet remains attached to the ink discharge surface during paper jam, the sheet is caught in the shutter when the shutter moves to a position where the shutter faces the ink discharge surface. Therefore there is a likelihood that the ink discharge surface cannot be protected by the shutter. In such a scenario where the ink discharge surface is not protected, if the user touches the ink discharge surface with a hand, the hand may be stained with the ink. In addition, the meniscuses within the ink discharge nozzles may be broken and/or damaged and lead to the deterioration of printing quality. Moreover, there may be cases where the removal of the paper jam takes a long period of time. For instance, if the ink discharge surface is left unprotected for a period of time, i.e., 30 minutes or more under a state where the air is dry, there is a fear that the ink in the nozzles may dry and the nozzles will be clogged. Alternatively, there is also a fear that the ink in the nozzles will dry, which will cause increased viscosity of the ink and in turn cause defective discharge. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming apparatus capable of protecting an ink discharge surface even when a sheet is attached to the ink discharge surface. 
     An image forming apparatus according to one aspect of the present invention comprises: an image forming portion; a recording medium transporting portion; a moving mechanism; an ink discharge surface protecting mechanism; and a control unit. The image forming portion can include an inkjet head having an ink discharge surface for discharging ink. The image forming portion forms an image on a recording medium by discharging ink from the ink discharge surface. The recording medium transporting portion has a transport surface which is arranged below the ink discharge surface while facing the ink discharge surface and on which a recording medium is placed. The recording medium transporting portion transports the recording medium to a position facing the ink discharge surface. The moving mechanism moves the transport surface toward and away from the ink discharge surface. The ink discharge surface protecting mechanism includes a cap member and a driving device. The driving device moves the cap member. The control unit monitors a jam of the recording medium during transportation thereof. When the control unit determines that a jam of the recording medium occurs during the transportation of the recording medium, the control unit moves the transport surface away from the ink discharge surface to activate the ink discharge surface protecting mechanism. 
     The above and other objects, features, and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     In this text, the terms “comprising”, “comprise”, “comprises” and other forms of “comprise” can have the meaning ascribed to these terms in U.S. Patent Law and can mean “including”, “include”, “includes” and other forms of “include”. 
     The various features of novelty which characterize the invention are pointed out in particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying descriptive matter in which exemplary embodiments of the invention are illustrated in the accompanying drawings in which corresponding components are identified by the same reference numerals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description, given by way of example, but not intended to limit the invention solely to the specific embodiments described, may best be understood in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a view showing a schematic structure of an image forming apparatus according to one embodiment of the present invention; 
         FIG. 2  is an enlarged view of a transport surface according to one embodiment of the present invention; 
         FIG. 3  is a view showing a transport belt in a separate state according to another embodiment of the present invention; 
         FIG. 4  is a top view showing a structure of an ink discharge surface protecting mechanism according to yet another embodiment of the present invention; 
         FIG. 5  is a sectional view taken along the line A-A of  FIG. 4 , showing a cap member at a standby position according to the embodiment shown in  FIG. 4  of the present invention; 
         FIG. 6  is a sectional view taken along the line A-A of  FIG. 4 , showing the cap member at a protecting position according to the embodiment shown in  FIG. 4  of the present invention; 
         FIG. 7  is a control block diagram of the image forming apparatus according to still another embodiment of the present invention; 
         FIG. 8  is a flow chart showing a control flow in a normal image formation according to a further embodiment of the present invention; 
         FIG. 9  is a flow chart showing a control flow in an occurrence of a paper jam according to still a further embodiment of the present invention; and 
         FIG. 10A ,  FIG. 10B , and  FIG. 10C  are views showing an operation of the ink discharge surface protecting mechanism according to yet still a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Overall Structure 
     Reference will now be made in detail to various embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, and by no way limiting the present invention. In fact, it will be apparent to those skilled in the art that various modifications, combinations, additions, deletions and variations can be made in the present invention without departing from the scope or spirit of the present invention. For instance, features illustrated or described as part of one embodiment can be used in another embodiment to yield a still further embodiment. It is intended that the present invention covers such modifications, combinations, additions, deletions, applications and variations that come within the scope of the appended claims and their equivalents. 
     Hereinafter, description will be made of various embodiments of the present invention with reference to the accompanying drawings. A schematic structure of an image forming apparatus  1  according to one embodiment of the present invention is shown in  FIG. 1 . The image forming apparatus  1  is an inkjet printer that is connected to an external computer (not shown). The image forming apparatus  1  is an apparatus for performing an image formation based on image information transmitted from the external computer. The image forming apparatus  1  comprises an image forming portion  2 , a sheet accommodating portion  3 , a sheet transporting portion  4  serving as a recording medium transporting portion, a moving mechanism  7 , an ink discharge surface protecting mechanism  6  (See  FIGS. 4 to 6 ), a sheet holding mechanism  8  serving as a recording medium holding mechanism, a discharging portion  5 , and a control unit  9  (See  FIG. 7 ). 
     The image forming portion  2  is for forming an image based on image data or image information on a sheet as an example of recording medium. The image forming portion includes a plurality of inkjet heads  21 . Four of the inkjet heads  21  are arranged above a transport belt  42  of the sheet transporting portion  4  (to be described below) in a direction perpendicular to the sheet transporting direction. The inkjet heads  21  discharge ink on a sheet based on image data or image information. Note that each of the inkjet heads  21  according to one embodiment of the present invention as illustrated in the figures accommodates ink with different colors including an ink discharge surface  22  provided with a plurality of holes (ink discharging nozzles, not shown) for discharging the accommodated ink. The inkjet heads  21  are the so-called “line-type” inkjet heads which form an image by discharging the ink in a width substantially equal to a width perpendicular to the direction of the sheet being transported by the sheet transporting portion  4 . In  FIG. 1 , only one of the ink discharge surfaces is denoted by reference numeral  22  and additional reference numerals  22  are not shown in  FIG. 1 . The ink discharge surface  22  is provided to a lower surface in each of the inkjet heads  21  and has a long and thin rectangular shape in a front-to-back orientation. The image forming portion  2  causes the ink to be discharged from the ink discharge surface  22 . More specifically, the image forming portion  2  causes the ink to be discharged from the holes provided to the ink discharge surface  22 , thereby enabling the forming of an image on a sheet. Note that the “front-to-back orientation” mentioned above means an orientation perpendicular to the sheet of  FIG. 1 , and is synonymous with a longitudinal orientation of the ink discharge surface  22  or a longitudinal orientation of a cap member described below. Particularly, a near side in the direction perpendicular to the page of  FIG. 1  is referred to as “front”, and a far side is referred to as “back”. Further, in the following description, a “left-to-right orientation” means a left-to-right orientation of  FIG. 1  and is synonymous with a lateral direction of the ink discharge surface  22 . 
     The sheet accommodating portion  3  is for accommodating one or more sheets on which an image is formed, and arranged in a lower portion of the image forming apparatus  1 . The sheet accommodating portion  3  includes a sheet feeding roller  31  for feeding a sheet to the sheet transporting portion  4 . 
     The sheet transporting portion  4  transports a sheet fed from the sheet accommodating portion  3  to a position facing the image forming portion  2 . It then transports the sheet which has a surface on which an image is formed by the image forming portion  2  to the discharging portion  5 . The sheet transporting portion  4  includes but is not limited to a plurality of rollers  41  and a transport belt  42  for transporting a sheet. The transport belt  42  which is arranged below the inkjet heads  21  is an endless belt. Rollers  44  and  45  are arranged at the uppermost stream and the lowermost stream of the transport belt  42  for circulating the transport belt  42 . The transport belt  42  is supported by a belt supporting member  43  via the rollers  44  and  45 . The belt supporting member  43  has a case-like structure similar to a matchbox, and has a top surface serving as a guide plate  48  which is described below. Further, as shown in  FIG. 2 , the transport belt  42  is provided with a plurality of holes  46  passing therethrough so that air can pass therethrough in a vertical direction. In various embodiments according to the present invention the holes  46  have a diameter of about 4 mm, for example, and are uniformly provided on the transport belt  42  at intervals of about 20 mm. Note that only one of the holes is denoted by reference numeral  46  and additional reference numerals  46  are not shown in  FIG. 2 . 
     A top surface of the transport belt  42  is flat and faces the ink discharge surface  22  below the ink discharge surface  22  of the plurality of inkjet heads  21  to become a transport surface  47  on which a sheet is placed so as to face the ink discharge surface  22 . Note that in one exemplary embodiment, the distance between the top surface (transport surface  47 ) of the transport belt  42  and the ink discharge surface  22  is set to about 1 mm. Below the transport surface  47 , the guide plate  48  is provided along a transporting direction of a sheet transported on the transport surface  47 . As shown in  FIG. 2 , the guide plate  48  is provided with a plurality of slits  49  passing therethrough. Therefore, when the slits  49  overlap the holes  46  of the transport belt  42 , air can pass through the transport belt  42  and the guide plate  48 . Note that  FIG. 2  is a top view of the transport belt  42  in which the guide plate  48  is arranged overlapping under the transport belt  42  (transport surface  47 ). 
     The moving mechanism  7  is a device for moving the transport surface  47  towards or away from the ink discharge surface  22  by moving the transport belt  42  up and down. The moving mechanism  7  which is arranged below the transport belt  42  has a pair of eccentric cams  71  and  72 . Of the pair of eccentric cams  71  and  72 , the eccentric cam  71  located on the left side of  FIG. 1  (hereinafter, referred to as “first eccentric cam  71 ”) is rotatably provided about a shaft portion  73  and is rotatably driven by a motor (not shown). The first eccentric cam  71  is provided with a plurality of bearings  74 . The bearings  74  partially protrude from an outer peripheral surface of the first eccentric cam  71 . The first eccentric cam  71  supports the belt supporting member  43  via the bearings  74 . Note that only one of the bearings is denoted by reference numeral  74  and additional reference numerals  74  are not shown in  FIG. 1 . Further, the size of the bearings  74 , the protruding amount of the bearings  74  from the surface of the first eccentric earn  71 , the arrangement of the bearings  74 , and the like are positioned such that the belt supporting member  43  is supported only by the bearings  74  when the transport belt  42  is moved in an up and down direction. The constitutions mentioned above are applied to the eccentric cam  72 . The eccentric cam  72  is located on the right side of  FIG. 1  (hereinafter, referred to as “second eccentric cam  72 ”) and has the same structure as that of the first eccentric cam  71  with a symmetric form with respect to the first eccentric cam  71  in the left-to-right orientation of  FIG. 1 .  FIG. 1  shows a state where the transport belt  42  is moved upward, and from this state, the pair of eccentric cams  71  and  72  rotates inward to move the transport belt  42  down (See  FIG. 3 ). When an image is formed on a sheet by the image forming portion  2 , the moving mechanism  7  moves the transport belt  42  upward to a position for forming the image shown in  FIG. 1  in order to provide a gap (of about 1 mm in this exemplary embodiment) suitable for printing between a sheet and the ink discharge surface  22  of the inkjet head  21 . Further, when the ink discharge surface  22  of the inkjet head  21  is protected with a cap member  60  (described later) in a shutdown state or when the disposal of a paper jam caused on the transport belt  42  is performed, the moving mechanism  7  moves the transport belt  42  down to a waiting position shown in  FIG. 3 . The effect is to enlarge the gap between the inkjet head  21  and the transport belt  42 . 
     The ink discharge surface protecting mechanism  6  includes but is not limited to a cap member  60 , a link mechanism  61 , and a driving device  62 , as shown in  FIGS. 4 to 6 . Note that  FIG. 4  shows a top view of the ink discharge surface protecting mechanism  6 , and  FIG. 5  and  FIG. 6  show sectional views taken along the line A-A of  FIG. 4 . 
     The cap member  60  is a member for protecting the ink discharge surface  22  by covering the ink discharge surface  22  of the inkjet head  21 . The cap member  60  includes but is not limited to a plate portion  63  and a side wall portion  64 . The plate portion  63  has a rectangular shape corresponding to a shape of the ink discharge surface  22 , which is made of a metal plate. The side wall portion  64  is uprightly arranged on a surface of the plate portion  63  facing the ink discharge surface  22 , that is, a top surface thereof. In other words, the side wall portion  64  is provided along an edge of the top surface of the plate portion  63 . The side wall portion  64  is formed of an elastic material such as rubber. The side wall portion  64  is provided over the entire periphery of the plate portion  63  so that the plate portion  63  is not brought into contact with the holes for ink discharge of the ink discharge surface  22 . Further, in the case where the ink discharge surface  22  needs to be protected, the side wall portion  64  abuts along an edge of a portion of the ink discharge surface, where the holes for ink discharge are provided. Note that the cap member  60  is constructed so as to shield space surrounded by the ink discharge surface  22  and an inner surface of the cap member  60  from the open air when the cap member  60  abuts on the ink discharge surface  22 . The cap member  60  is constructed as described above so that the ink in the holes ultimately deposited on the ink discharge surface  22  does not dry out, even over a period of time as the cap member  60  continues to protect the ink discharge surface  22 . The cap member  60  is provided so as to be moved by the link mechanism  61  mentioned below from a predetermined standby position through a position that is lower than the ink discharge surface  22 , and thereafter to a position in which the ink discharge surface  22  is covered. More specifically, as further described below in connection with the views shown in  FIGS. 5 and 6 , in this illustrative embodiment the cap member  60  is configured to be moved from a standby position where the cap member  60  is positioned at a time of image formation to a capping position where the cap member  60  covers the ink discharge surface  22  after being moved through a position lower than the capping position. Note that the standby position of the cap member  60  is a position above the transport belt  42 , constituting part of the sheet transporting portion  4  as shown in  FIG. 1 , and laterally adjacent to the inkjet head  21 . In this case, the position lower than the ink discharge surface  22  means a position existing in an ink discharging direction when viewed from the ink discharge surface  22 . 
     The link mechanism  61  can include a cap supporting member  65 , a side plate  66 , and a driving disk  67 . 
     The cap supporting member  65  supports an end of the cap member  60  in the front-to-back orientation thereof and has a shape extending in a vertical direction (See  FIG. 5  and  FIG. 6 ). A lower end of the cap supporting member  65  is connected to the cap member  60 . The cap member  60  is perpendicularly attached to the cap supporting member  65 . An upper end of the cap supporting member  65  is provided with a shaft member  68 , which has a column-like shape extending in the front-to-back orientation. In addition, the shaft member  68  is provided so as to protrude from the cap supporting member  65  to an opposite side of the cap member  60 . 
     The side plate  66  is a guide member for guiding the cap member  60 . The side plate  66  is a plate-like member arranged in the front (near side in the direction perpendicular to the page of  FIG. 1 ) of the cup member  60  and the inkjet head  21 . The side plate  66  is provided with a slit  69  for guiding the cap member  60 . The slit  69  as shown in  FIG. 5  can include a horizontal portion  69   a  extending in the left-to-right orientation, a first vertical portion  69   b  connected to an end of the horizontal portion  69   a  and is also vertically extending, and a second vertical portion  69   c  connected to another end of the horizontal portion  69   a  and vertically extending. Note that the side plate  66  is a member constituting part of a housing frame of the image forming apparatus  1 . 
     The driving disk  67  is a driving force transmitting member for transmitting a driving force transmitted from the driving device  62  to the cap member  60  so that the cap member  60  is moved along the slit  69  of the side plate  66 . The driving disk  67  is rotatably provided. A rotation center of the driving disk  67  is located between the first vertical portion  69   b  and the second vertical portion  69   c  of the slit  69  of the side plate  66 . Besides, the driving disk  67  is provided with a slit  67   a  extending from the rotation center toward a radial direction. The slit  67   a  of the driving disk  67  moves along a trace such that the slit  67   a  and the slit  69  of the side plate  66  are overlapped by the rotation of the driving disk  67 . The above-mentioned shaft member  68  of the cap member  60  passes through the slit  69  of the side plate  66  and the slit  67   a  of the driving disk  67 , and furthermore, can move along the respective slits  67   a  and  69 . Note that, as shown in  FIG. 5 , in the case where the slit  67   a  of the driving disk  67  overlaps with the first vertical portion  69   b  of the slit  69  of the side plate  66 , and the shaft member  68  is located at the first vertical portion  69   b , the cap member  60  is located at the standby position which is laterally adjacent to the inkjet head  21  and is higher than the ink discharge surface  22 . Moreover, as shown in  FIG. 6 , in the case where the slit  67   a  overlaps with the second vertical portion  69   c  of the slit  69  of the side plate  66  and the shaft member  68  is located at the second vertical portion  69   c , the cap member  60  is located at a protecting position in which the ink discharge surface  22  is covered. In the case where the slit  67   a  of the driving disk  67  overlaps with the horizontal portion  69   a  of the slit  69  of the side plate  66  and the shaft member  68  is located at the horizontal portion  69   a , the cap member  60  is located at a position lower than the ink discharge surface  22  (see a cap member  60  indicated by the chain double-dashed line of  FIG. 6 ). Accordingly, as depicted by  FIGS. 5 and 6 , the cap member  60  is configured to be moved from a standby position (e.g., as shown in  FIG. 5 ) where the cap member  60  is positioned at a time of image formation to a capping position where the cap member  60  covers the ink discharge surface (e.g., as shown by the solid-line cap member  60  in  FIG. 6 ) after being moved through a position lower than the capping position (e.g., as shown by the chain double-dashed line cap member  60  in  FIG. 6 ). Only a front end side of the ink discharge surface protecting mechanism  6  is shown in  FIG. 4 , but a back end side thereof also has the same structure. Note that, in place of the driving device  62  described later, a bearing (not shown) for rotatably supporting a rotating shaft X 1  fixed to the driving disk  67  is arranged. 
     The driving device  62  shown in  FIG. 4  can be a stepping motor for rotating the driving disk  67 . The rotating shaft X 1  of the driving device  62  is fixed to the center of the driving disk  67 . The driving device  62  moves the shaft member  68  by rotating the driving disk  67 . The movement of the shaft member  68  can cause the cap member  60  to move. The driving device  62  is fixed to a driving device supporting plate  59  constituting a part of the housing frame of the image forming apparatus  1 . 
     The sheet holding mechanism  8  is for holding a sheet on the transport surface  47 . The sheet holding mechanism  8  includes a plurality of blower fans  81  and a motor (not shown) for rotatably driving blower fans  81 , as shown in  FIG. 1 . With the movement of the blower fans  81 , an air flow passing through the transport surface  47  and the guide plate  48 , that is, a downward air flow passing from an upper side of the transport surface  47  toward a lower side of the guide plate  48 , is generated. With this air flow, the sheet holding mechanism  8  can suck the sheet via the transport surface  47  so that the sheet can be held on the transport surface  47 . Note that only one of the blower fans is denoted by reference numeral  81  and additional reference numerals  81  are not shown in  FIG. 1 . 
     The discharging portion  5  is a portion for discharging a sheet on which an image has been formed by the image forming portion  2 . The discharging portion  5  is arranged at an upper portion of the image forming apparatus  1 . The discharging portion  5  includes but is not limited to at least one roller and preferably a plurality of rollers  51 . The sheet on which an image has been formed by the image forming portion  2  is transported by the transport belt  42  and the rollers  51  and is discharged outside the apparatus from an outlet  52 . 
     As shown in  FIG. 7 , the control unit  9  performs various processing including but not limited to image formation by controlling the image forming portion  2 , the sheet transporting portion  4 , the discharging portion  5 , the moving mechanism  7 , the sheet holding mechanism  8 , the ink discharge surface protecting mechanism  6 , and the like, according to inputs from an operating unit  11  which is operated by a user and a communicating unit  12  for communicating with an external computer (not shown). Furthermore, the control unit  9  is connected to a first sheet detecting sensor  29  which serves as a first recording medium detecting member and a second sheet detecting sensor  30  which serves as a second recording medium detecting member which are described below. The detecting sensors monitor whether or not a sheet can cause a paper jam during the time when the sheet is transported by sheet transporting portion  4 . 
     Control of Image Forming Apparatus 
     Hereinafter, the control of the image forming apparatus  1  performed by the control unit  9  is described. First, a control flow at a normal image formation is described with reference to  FIG. 8 . 
     In the first step S 1 , the transport belt  42  is moved. In this case, the moving mechanism  7  operates to move the transport belt  42 , which stands by at the waiting position and away from the ink discharge surface  22 , to the position for forming the image. This position is where the transport belt  42  comes close to the ink discharge surface  22  (See  FIG. 1 ). 
     In the second step S 2 , the sheet holding mechanism  8  operates. Note that other embodiments of the present invention allow the second step S 2  and the first step S 1  to be inversely performed or simultaneously performed. 
     After operation of the sheet holding mechanism  8 , the sheet transporting portion  4  operates to start transporting a sheet in step S 3 . 
     In the fourth step S 4 , the ink is discharged from the ink discharge surface  22  to form an image on the sheet. Ink discharge starts after a specific time has elapsed after the leading end of the sheet is detected by the first sheet detecting sensor  29 . The first sheet detecting sensor  29  is typically installed upstream of the inkjet head  21  in the uppermost stream of a path where the sheet is transported. The specific starting time is set longer for the inkjet heads  21  located more downstream. Therefore the discharge of the ink is sequentially started from the inkjet heads  21  located upstream of a sheet transporting direction to form an image on the sheet which is transported below the inkjet heads  21 . Then, in the fifth step S 5 , the sheet on which the image has been formed is discharged outside the apparatus. 
     After the image formation onto the sheet is completed, the sheet transporting portion  4  is stopped in the sixth step S 6  and the sheet holding mechanism  8  is stopped in the seventh step S 7 . 
     In the eighth step S 8 , the transport belt  42  is moved. In this step, the moving mechanism  7  operates to move the transport belt  42  from the position for forming an image where the transport belt  42  comes close to the ink discharge surface  22  to the waiting position (See  FIG. 3 ) where the transport belt  42  is away from the ink discharge surface  22 . 
     Next, how to use a control flow to remedy a paper jam is described with reference to  FIG. 9 . 
     Initially the occurrence of the paper jam is detected in the eleventh step S 11 . Step S 11  is performed during the image formation operation in the fourth step  54 . The sheet transporting portion  4  is stopped in the twelfth step S 12 . Note that the occurrence of the paper jam is detected as follows. In this exemplary case in accordance with the present invention, the second sheet detecting sensor  30  is arranged immediately upstream of the rollers  51  which are arranged downstream of the transport belt  42 . That is, rollers  51  are located immediately downstream of the inkjet heads  21  which are located in the lowermost stream. In other words, in the case where a specific time (time A) has elapsed after the leading end of the sheet is detected by the first sheet detecting sensor  29 , and further when the leading end of the sheet cannot be detected by the second sheet detecting sensor  30 , the control unit  9  determines that the paper jam has occurred in the transporting path in the vicinity of the transport belt  42 . Alternatively, in the case where a specific time (time B) has elapsed after the leading end of the sheet is detected by the first sheet detecting sensor  29 , and further when a trailing end of the sheet cannot be detected by the second sheet detecting sensor  30 , it can be determined that the leading end of the sheet has passed through a position opposite to the second sheet detecting sensor  30 . It can then also be determined that the paper jam has occurred on the transporting path before the trailing end of the sheet reaches the position opposite to the second sheet detecting sensor  30 . Accordingly, the control unit  9  determines that the paper jam is being caused on the transporting path in the vicinity of the transport belt  42 . Note that in various other embodiments the two determinations described above for the paper jam may be combined. 
     In the thirteenth step S 13 , the transport belt  42  is moved. In this case, the moving mechanism  7  operates to move the transport belt  42  from the position for forming an image where the transport belt  42  comes close to the ink discharge surface  22  to the waiting position where the transport belt  42  is away from the ink discharge surface  22 . After the occurrence of the paper jam is detected and while the transport belt  42  is being moved, the sheet holding mechanism  8  is in an operating state. In this state an attraction force acting on the sheet for holding the sheet on the transport surface  47  is maintained. For that reason, in the case where the sheet is attached to the ink discharge surface  22 , the sheet can be separated from the ink discharge surface  22  by the attraction force due to the sheet holding mechanism  8  and a moving force of the transport belt  42 . Alternatively, the sheet which caused the paper jam can be positioned or moved so that it is difficult for the sheet to be attached to the ink discharge surface  22 . 
     When the transport belt  42  is moved in the thirteenth step S 13  and the distance between the ink discharge surface  22  and the transport surface  47  is expanded, then the ink discharge surface protecting mechanism  6  operates in the fourteenth step S 14 . In this case, the cap member  60  is moved from the standby position (See  FIG. 10A ) to the protecting position (See  FIG. 10C ), whereby the ink discharge surface  22  is protected by the cap member  60 . During this transition, the cap member  60  passes through a lower position than the ink discharge surface  22  when moving from the standby position to the protecting position. Therefore, in the case where a sheet  100  cannot be taken off from the ink discharge surface  22  even by the operation of the sheet holding mechanism  8 , the sheet  100  is separated from the ink discharge surface  22  by a force generated when the cap member  60  is intended to move to the lower position than the ink discharge surface  22  (See  FIG. 10B ). In particular, the cap member  60  comes close to the ink discharge surface  22  while horizontally moving in the lower position than the ink discharge surface  22 . This movement causes the sheet  100  attached to the ink discharge surface  22  to be reliably separated. Therefore, even in the case where the sheet  100  cannot be separated from the ink discharge surface  22  in the thirteenth step S 13 , as described above, the sheet  100  can be reliably separated from the ink discharge surface  22  in the fourteenth step S 14 . Further, after the sheet  100  is separated from the ink discharge surface  22 , the ink discharge surface  22  can be protected with the cap member  60  (See  FIG. 10C ). When a user removes the sheet  100  that caused the paper jam, it is possible to prevent the hand or the like of the user from unintentionally coming in contact with the ink discharge surface  22 . As a result, defective images are prevented from being generated in a subsequent image formation. 
     After the transport belt  42  is located at the waiting position by the moving mechanism  7 , the sheet holding mechanism  8  is stopped in the fifteenth step S 15  for safety purposes in order to facilitate the sheet removal from the transport belt  42  by the user. 
     In the image forming apparatus  1 , the sheet can be easily separated from the ink discharge surface  22  even when the sheet is attached to the ink discharge surface  22 . Further, after the sheet is separated from the ink discharge surface  22 , the ink discharge surface  22  can be covered with the cap member  60  for protection. 
     Other Embodiments In Accordance of the Present Invention 
     In another embodiment of the present invention, when the occurrence of the paper jam is detected in the eleventh step S 11 , the operation of the sheet holding mechanism  8  may be caused to be stopped in order to separate the sheet from the ink discharge surface  22  only, by the movement of the cap member  60  in the fourteenth step S 14 . 
     In the above-mentioned embodiments, the air flow headed from a sheet side of the transport surface  47  to an opposed side thereof is generated to suck the sheet, whereby the force for holding the sheet on the transport surface  47  is generated. However, in yet another embodiment of the present invention, a different mechanism from the above, for example, may be used employing an electrostatic attraction force for holding the sheet on the transport surface  47 . 
     In the above-mentioned various embodiments, after the transport surface  47  is separated from the ink discharge surface  22  (thirteenth step S 13 ), the cap member  60  is moved to the protecting position (fourteenth step S 14 ). However, the transport surface  47  is not moved to the waiting position in the thirteenth step S 13 , but in various other embodiments of the present invention the transport surface  47  may be slightly moved so as to ensure a gap into which the cap member  60  can be inserted. The cap member  60  may also be moved to the protecting position, and then the transport surface  47  may be moved again to the waiting position while a state in which the sheet holding mechanism  8  is in operation. Accordingly, the sheet attached to the ink discharge surface  22  can be more reliably separated from the ink discharge surface  22 . 
     In the above-mentioned embodiments, a color printer is exemplified as the image forming apparatus  1 . However, the present invention is not limited thereto, and may be applied to a multi-function peripheral having functions such as a copying machine, a facsimile machine, or a device only having a copy function. Further, the present invention is not limited to a device which can form a color image, and may be applied to an image processing apparatus capable of forming only a monochrome image. 
     In the above-mentioned embodiments, the moving mechanism  7  and the ink discharge surface protecting mechanism  6  are operated by the control unit  9 . In other embodiments of the present invention, a front door (not shown) may be provided on the front surface side of the image fowling apparatus  1 . Preferably the front door can be locked while the moving mechanism  7  and the ink discharge surface protecting mechanism  6  operate and can be opened by the user to check the inside thereof for the disposal of the paper jam or the like while the moving mechanism  7  and the ink discharge surface protecting mechanism  6  don&#39;t operate. With this, it is possible to prevent the hand or the like of the user from being caught in the moving mechanism  7  and the ink discharge surface protecting mechanism  6  under operation when the user abruptly opens the front door to insert the hand into the image forming apparatus  1 . Further, it is possible to prevent the user&#39;s hand from coming into direct contact with the ink discharge surface  22  of the inkjet head  21  which is not protected by the cap member  60 . 
     According to the above-mentioned embodiments of the present invention, after the operations of the moving mechanism  7  and the ink discharge surface protecting mechanism  6  by the control unit  9  are completed, a display indicating that the paper jam has occurred may be made on a display portion (not shown) in the image forming apparatus  1 . Alternatively in other embodiments of the present invention, after the operations of the moving mechanism  7  and the ink discharge surface protecting mechanism  6  by the control unit  9  are completed, a display indicating that the paper jam has occurred may be made on a display screen of a connected computer that ordered printing for the sheet which has caused the paper jam. Then, after the operations of the moving mechanism  7  and the ink discharge surface protecting mechanism  6  are completed, the user is informed of the occurrence of the paper jam. Therefore, the user can safely perform the disposal of the paper jam, and further, the user can be prevented from touching by the hand the ink discharge surface  22  of the inkjet head  21  which is not protected by the cap member  60 .