Patent Publication Number: US-7596334-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2006-308212 filed Nov. 14, 2006. 
   BACKGROUND 
   1. Technical Field 
   The present invention relates to an image forming apparatus such as a photocopier and a printer. 
   2. Related Art 
   In an image forming apparatus such as a photocopier and a printer, there sometimes occurs a case, for example, in which a fusing unit is provided in which a recording member on which an image such as a toner image is held is heated and pressurized so as to fuse the image so held on to the recording material. 
   SUMMARY 
   According to an aspect of the invention, there is provided an image forming apparatus, including: a first fusing member provided rotatably; a second fusing member disposed rotatably in press contact with the first fusing member, so as to define, together with the first fusing member therebetween, a nip section through which a recording material which holds an image thereon is to be passed; and a changing unit that changes an angle formed by the nip section and a transport direction of the recording material which is passed through the nip section. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  illustrates a schematic diagram showing the configuration of an image forming apparatus to which the invention is applied; 
       FIG. 2  illustrates a side sectional view showing schematically the configuration of a fusing unit to which the invention is applied; 
       FIGS. 3A and 3B  illustrate plan views of the fusing unit; 
       FIGS. 4A and 4B  illustrate diagrams explaining a conventional fusing unit; 
       FIGS. 5A to 5D  illustrate explanatory diagrams illustrating a state of a fusing belt shown in  FIG. 4B ; 
       FIGS. 6A and 6B  illustrate diagrams explaining marks or wear generated on a fusing belt when the fusing unit is caused to rotate; and 
       FIGS. 7A and 7B  illustrate diagrams explaining marks or wear generated on the fusing belt when the fusing unit is caused to rotate substantially about a central portion of a fusing roller. 
   

   DETAILED DESCRIPTION 
   Hereinafter, an embodiment of the invention will be described by reference to the accompanying drawings. 
     FIG. 1  is a schematic drawing which shows the configuration of an image forming apparatus to which the invention is applied. This image forming apparatus is an intermediate transfer image forming apparatus which is generally referred to as a tandem type image forming apparatus. In this image forming apparatus, a plurality of image forming units  1 Y,  1 M,  1 C,  1 K in which toner images of respective color components are formed by means of an electrophotography method. In addition, primary transfer sections  10  are provided, respectively, in the image forming units  1 Y,  1 M,  1 C,  1 K, where toner images of the respective color components are sequentially transferred to an intermediate transfer belt  15  (a primary transfer). 
   Furthermore, in this image forming apparatus, a secondary transfer section  20  is provided where superposed toner images which are transferred on to the intermediate transfer belt  15  are transferred on to a sheet of paper P which is a recording material (a recording sheet of paper) altogether at one time (a secondary transfer). In addition, various types or rollers including a drive roller  31  are provided inside the intermediate transfer belt  15  which stretch the intermediate transfer belt  15  from the inside thereof and which are driven to rotate by a motor (not shown) which has a superior constant speed characteristic and the like. Furthermore, a fusing unit  60  is provided for fixing or fusing the secondarily transferred images on to the sheet of paper P. In addition, a control panel (not shown) which receives inputs from a user regarding the number of sheets on which images are formed, thickness and basic weight of sheets and types of sheets on which images are formed and a control unit  40  for controlling operations of constituent units (sections) of the image forming apparatus are provided. 
   In this embodiment, the following electrophotographic devices are sequentially provided in each of the image forming units  1 Y,  1 M,  1 C,  1 K. Firstly, a charge device  12  for charging a photoconductor drum  11 , which rotates in a direction indicated by an arrow A, is provided on the periphery of the photoconductor drum  11 . In addition, a laser exposure device  13  (in the figure, an exposure beam is denoted by reference character Bm) for writing a latent image on the photoconductor drum  11  is provided. Furthermore, a developing device  14  is provided which accommodates a color component toner and visualizes a latent image on the photoconductor drum  11  with the toner. In addition, a primary transfer roller  16  is provided which transfers the formed or visualized color component toner image to the intermediate transfer belt  15  at the primary transfer section  10 . Additionally, a drum cleaner  17  is provided which removes the toner which remains on the photoconductor drum  11 . 
   The primary transfer section  10  is configured to include the primary transfer roller  16  which is disposed in such a manner as to face oppositely the photoconductor drum  11  across the intermediate transfer belt  15 . 
   The secondary transfer section  20  is made up of a secondary transfer roller  22  which is disposed on a toner image holding surface side of the intermediate transfer belt  15  and a backup roller  25 . The secondary transfer roller  22  is disposed in press contact with the backup roller  25  across the intermediate transfer belt  15  or with the intermediate transfer belt  15  interposed therebetween. 
   The control unit  40  includes a CPU (Central Processing Unit) for controlling operations of the respective devices (the respective sections), a ROM (Read Only Memory) in which a program is recorded, a RAM (Random Access Memory) for temporarily storing various types of data and the like. In addition, the control unit  40  not only controls the operations of the respective devices (the respective sections) as has been described above but also counts sheets on which images are to be formed and determines types of sheets on which images are to be formed based on inputs made by the user through the control panel (not shown). 
   Next, a basic image creating process of the image forming apparatus according to the exemplary embodiment will be described. In the image forming apparatus according to the exemplary embodiment, image data is outputted from an image reading apparatus or the like, not shown. Then, a predetermined image processing is applied to the image data so outputted by an image processing apparatus, not shown, so as to be converted into color material gradation data of four colors such as Y (yellow), M (magenta), C (cyan), and K (black) to thereby be outputted to the laser exposure device  13 . 
   The laser exposure device  13  emits an exposure beam Bm from, for example, a laser semiconductor to be shone on to the photoconductor drum  11  of each of the image forming units  1 Y,  1 ,  1 C,  1 K according to the color material gradation data inputted thereinto. A surface of each photoconductor drum  11  is charged by the charge device  12 , and thereafter, the charged surface is scan exposed by the laser exposure device  13  so as to form a latent image thereon. 
   The latent images so formed are then developed as toner images of the respective colors of Y, M, C, K by the respective developing devices  14  in the image forming units  1 Y,  1 M,  1 C,  1 K. 
   On the other hand, the intermediate transfer belt  15  is driven to circulate in the image forming apparatus at a predetermined speed in a direction indicated by an arrow B shown in  FIG. 1  by means of the various types of rollers including the drive roller  31 . The toner images formed on the photoconductor drum  11  are sequentially attracted to the intermediate transfer belt  15  in an electrostatic fashion at the primary transfer sections  10  where the respective photoconductor drums  11  come into abutment with the intermediate transfer belt  15 , whereby toner images are formed on the intermediate transfer belt  15  which are superposed one on another. After the images are formed on the intermediate transfer belt in the superposed fashion, the intermediate transfer belt  15  moves, so that the toner images are transported to the secondary transfer section  20 . At the secondary transfer section  20 , the secondary transfer roller  22  is pressed by the backup roller  25  via the intermediate transfer belt  15 . As this occurs, a sheet of paper P is transported to the secondary transfer section  20  in synchronism with the arrival of the intermediate transfer belt  15  holding the toner images thereon so as to be held between the intermediate transfer belt  15  and the secondary transfer roller  22 . 
   At the secondary transfer section  20 , a secondary transfer bias is applied between the secondary transfer roller  22  and the backup roller  25 , so as to form a secondary transfer electric field between the two members. Then, the images held on the intermediate transfer belt  15 , which have not yet been fixed or fused, are electrostatically transferred to the sheet P altogether at the secondary transfer section  20 . Thereafter, the sheet P on to which the toner images have electrostatically been transferred is transported by the secondary transfer roller  22  to a transport belt  55  which is provided on a downstream side of the secondary transfer roller  22  in a sheet transport direction. The transport belt  55  transports the sheet P to the fusing unit  60  at an optimum speed. In the fusing unit  60 , the sheet P which holds the transferred toner images is heated and pressurized so as to fuse the toner images on the sheet P. Then, the sheet P, on which the image fusing has been completed, is discharged to the outside of the image forming apparatus. 
   Next, the fusing unit  60  will be described. 
     FIG. 2  is a side sectional view which shows schematically the configuration of the fusing unit  60  to which the invention is applied. This fusing unit  60  is made up mainly of a fusing belt module  61 , an impression roller  62  which is disposed in press contact with the fusing belt module  61  and a support housing  63  as a support member which supports the fusing belt module  61  and the impression roller  62 . In addition, although the description was omitted in  FIG. 1 , a rotating unit  70  for rotating the whole fusing unit  60  via the support housing  63  is provided on a main body (not shown) side of the image forming apparatus. 
   The fusing belt module  61  includes a fusing belt  610  and a fusing roller  611  as a first fusing member. The fusing belt  610  is provided in such a manner as to rotate or move in circles, and the fusing roller  611  is provided rotatably and has a function to stretch the fusing belt  610 . 
   In addition, the fusing belt  610  has downstream of the fusing roller  611  in a rotational direction of the fusing belt  610  an external heating roller  613  for stretching the fusing belt  610  from the outside thereof and heating the fusing belt  610  from the outside thereof. 
   Furthermore, the fusing belt module  61  has downstream of an area where the fusing roller  611  and the impression roller  62  are in press contact with each other via the fusing belt  610  and in a position lying in the vicinity of the fusing roller  611  a release pad  64  for pressing the fusing belt  610  against the impression roller  62 . As a result, a nip section N where toner images are fused on a recording material is formed in the area where the fusing belt module  61  and the impression roller  62  are press contact with each other. 
   The fusing module  61  includes an idler roller  615  which stretches the fusing belt  610  downstream of the nip section N and a drive source (not shown) such as a motor for rotationally driving the fusing roller  611 . Furthermore, the fusing belt module  61  includes a heater  616   a  as a heat source for heating the fusing roller  611  in an interior of the fusing roller  611 . Moreover, the fusing belt module  61  includes in an interior of the external heat roller  613  a heater  616   c  as a heat source for heating the external heat roller  613 . In addition, the fusing belt module  61  includes a steering roller  612  which not only stretches the fusing belt  610  from the inside thereof but also adjusts the position of the fusing belt  610  in a direction which intersects the rotational direction of the fusing belt  610  (a direction which intersects substantially at right angles to the rotational direction of the fusing belt  610 , a width direction of the fusing belt  610 ). 
   Furthermore, the fusing belt module  61  includes a sensor (not shown) for detecting the position of the fusing belt  610  in the direction which intersects the rotational direction of the fusing belt  610 . In addition, the fusing belt module  61  includes in an interior of the steering roller  612  a heater  616   b  as a heat source for heating the steering roller  612 . Furthermore, the fusing belt module  61  includes a tensioner  65  for pushing the steering roller  612  towards the outside of the fusing belt  610  so as to impart a predetermined tension to the fusing belt  610 . In addition, the fusing belt module  61  includes a shifting unit  66  which shifts one end portion side of the steering roller  612  so as to impart an inclination or tilt to the steering roller  612 . 
   The fusing belt  610  is a flexible endless belt. In addition, this fusing belt  610  is formed into a three-layer construction and is made up of a base layer which is made from a polyimide or the like and which has a thickness of the order of 80 μm, an elastic layer which is made from a silicone rubber or the like and is laminated on a front surface side (an outer circumferential surface side) of the base layer and a surface layer (a release layer) which is formed from a fluorine-based resin and which has a thickness of the order of 30 μm, the surface layer being formed in such a manner as to cover the elastic layer. 
   On the other hand, the fusing roller  611  has no elastic layer which covers the fusing roller  611  and is a hard roller which is made up by coating a surface of a core metal (a core roller) of aluminum with a heat-resistant resin (a fluorine plastic). 
   The release pad  64  as a pressurizing member is a member which is formed into a thin plate-like shape of, for example, a SUS metal or resin and is also a member which is formed into an arc-like shape in section. The release pad  64  is provided downstream and in the vicinity of an area (a roller nip section N 1 ) where the impression roller  62  and the fusing roller  611  are brought into press contact with each other via the fusing belt  610 . In addition, the release pad  64  is provided on a rear surface side of the fusing belt  610  and is set in such a manner as to pressurize uniformly the impression roller  62  via the fusing belt  610  with a predetermined load over a predetermined width region on the impression roller  62 . As a result, a “release pad nip section N 2 ” is formed within the nip section N in such a manner as to be continued from the roller nip section N 1 . 
   The steering roller  612  is provided in such a manner as to be shifted obliquely upwards (refer to an arrow H in the figure) and obliquely downwards (refer to an arrow J in the figure) as viewed in the figure atone end portion thereof. Then, the steering roller  612  is provided in such a manner as to be tilted due to the one end portion side thereof being shifted in the oblique directions. 
   The shifting unit  66  changes the position (alignment) of the steering roller  612  by so shifting the one end portion of the steering roller  612  so as to impart an inclination or tilt to the steering roller  612 . The shifting unit  66  is made up of a drive source M 1  such as a motor and a transmission mechanism for transmitting driving force from the drive source M 1  to the steering roller  612 . Note that cams, rack and pinion or the like are raised as making up the transmission mechanism. 
   In addition, in this exemplary embodiment, the shifting unit  66  is provided on the main body side of the image forming apparatus. Although the shifting unit  66  can be provided on a fusing unit  60  side of the image forming apparatus, since, in the event that the shifting unit  66  is so provided, the shifting unit  66  is also replaced when the fusing unit  60  is replaced, and this increases the replacement cost of fusing units  60 . Then, in this exemplary embodiment, the configuration is adopted in which the shifting unit  66  is provided on the main body side of the image forming apparatus, so that even when fusing units  60  are replaced, the shifting unit  66  is allowed to remain on the main body side of the image forming apparatus. 
   The rotating unit  70  is made up of a drive source M 2  such a motor and a transmission mechanism (not shown) for transmitting driving force from the drive source M 2  to the support housing  63 . Note that cams, rack and pinion or the like are raised as making up the transmission mechanism. 
   On the other hand, the impression roller  62  as a second fusing member is made up of, as a base material, a cylindrical roller  621  of aluminum or the like. In addition, an elastic layer  622  which is made from a silicone rubber or the like and which has a thickness of the order of 10 mm and a release layer  623  which is made from a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) or the like and which has a thickness of the order of 100 μm are laminated on the base material sequentially in that order from the base material side, so as to form a soft roller. 
   In addition, the fusing unit  60  of the exemplary embodiment includes downstream of the nip section N a discharge sheet guide (not shown) for guiding a sheet which is discharged from the nip section N towards the outside of the fusing unit  60  and a posture correcting roller  614  for correcting the posture of the fusing belt  610  between the fusing roller  611  and the steering roller  612 . 
   Furthermore, the fusing unit  60  will be described in detail. 
     FIGS. 3A and 3B  are plan views of the fusing unit  60 . In this figure, the illustration of the release pad  64  and the posture correcting roller  614  is omitted. 
   As is shown in  FIG. 3A , the steering roller  612  includes a main body portion  612   a  which stretches the fusing belt  610  and a receiving portion  612   c  which supports the main body portion  612   a  at the one end portion side thereof and receives driving force from the shifting portion  66 . In addition, the steering roller  612  includes a support portion  612   b  which supports the main body portion  612   a  at the other end portion side of the main body portion  612   a  and which is supported by the support housing  63  of the fusing unit  60 . 
   In addition, the support housing  63  is provided in such a manner as to rotate about a predetermined rotating center and is configured to rotate about the rotating center by receiving driving force from the rotating unit  70 . This rotating center can be provided in an arbitrary position, and in this exemplary embodiment, a configuration is adopted in which the rotating center is disposed in the position of the end portion of the main body portion  612   a  of the steering roller  612 . Then, when the rotating unit  70  is rotated from this position by the control unit  40 , as is shown in  FIG. 3B , the fusing unit  60  rotates, and the fusing roller  611  and the impression roller  62  also rotate (swing) to thereby change their positions (or be displaced). To be more specific, the fusing roller  611  and the impression roller  62  are displaced in such a manner that the nip section N approaches a transport direction of sheets at one end thereof. In addition,  FIG. 3B  shows a state in which the support housing  63  is rotated counterclockwise through 5 degrees by the rotating unit  70 . 
   As a result of this, the nip section N, which is provided in such a manner as to intersect the transport direction of sheets which are transported by the transport belt  55  (refer to  FIG. 1 ) substantially at right angles (refer to  FIG. 3A ), is now disposed in a non-perpendicular state relative to the sheet transport direction. Looking at this from a different viewpoint, while in  FIG. 3A , an angle formed by an end portion of a sheet in a direction which intersects the sheet transport direction (the direction which intersects the sheet transport direction substantially at right angles, the width direction of the sheet) and an entrance of the nip section N is substantially a right angle, in  FIG. 3B , the angle formed in the way described above becomes an acute angle (an obtuse angle). In addition, in this embodiment, a changing unit for changing the angle formed by the nip section N and a sheet that is to be passed through the nip section N is configured by the control unit  40  (refer to  FIG. 1 ) and the rotating unit  70 . In addition in this exemplary embodiment, a swing unit for swinging at least one end side of the nip section N relative to the sheet transport direction is configured by the control unit  40  and the rotating unit  70 . 
   Hereinafter, in this specification, the state in which the nip section N is substantially at right angles to the sheet transport direction, as is shown in  FIG. 3A , is referred to as a “reference state,” whereas the state in which the nip section N is not at right angles to the sheet transport direction is referred to as a “non-reference state.” 
   Next, the operation of the fusing unit  60  will be described using  FIGS. 2 ,  3 A and  3 B. 
   When fusing is performed in the fusing unit  60 , firstly, a sheet of paper P on which toner images are held is introduced into the nip section N by the transport belt  55  (refer to  FIG. 1 ). As this occurs, receiving driving force from the drive source (not shown), the fusing roller  611  is rotating in a direction indicated by an arrow C. In addition, as the fusing roller  611  is rotating, following the fusing roller  611 , the fusing belt  610  is moving in circles or rotating in the direction indicated by the arrow C. When fusing is performed, the heaters  616   a  to  616   c  are fed by a power supply unit, not shown, whereby the fusing roller  611 , the steering roller  612  and the external heating roller  613  are controlled to be heated to a predetermined temperature. 
   In this state, the sheet P introduced into the nip section N is transported in a downstream direction by the fusing belt  610  and the impression roller  62  and passes sequentially through the roller nip section N 1  and the release pad nip section N 2 . Then, in the processes where the sheet P passes through the roller nip section N 1  and the release pad nip section N 2 , the sheet P is pressurized and heated by the fusing belt  610  and the impression roller  62 . As a result, the toner images are fused on the sheet P. Then, the fusing belt  610  which has passed through the release pad nip section N 2  continues to move along a side surface of the release pad  64 . 
   Then, the traveling direction of the fusing belt  610  is changed drastically so as to be bent towards the idler roller  615 . Because of this, the sheet P, which has been pressurized and heated at the nip section N, cannot follow the change in the traveling direction of the fusing belt  610  at a point in time at which the sheet P has emerged from the nip section N. As a result of this, the sheet P is separated from the fusing belt  610  by virtue of a so-called “nerve” of its own (self-stripping). Separation of curvature is performed stably in this way at an exit portion of the nip section N. The sheet P which is separated from the fusing belt  610  is, thereafter, discharged to the outside of the image forming apparatus by the discharge sheet guide (not shown) and a discharge sheet roll (not shown), ending the fusing process. 
   Incidentally, there sometimes occurs a case where the fusing belt  610 , fusing roller  611  and idler roller  615  are not molded properly. In addition, there sometimes occurs a case where the roller members such as the fusing roller  611 , idler roller  615 , external heating roller  613  and the like are not held in parallel. In these cases, there sometimes occurs a case where the fusing belt  610  is not transported accurately along the predetermined path and moves (walks) towards one of directions which intersect the traveling direction of the fusing belt  610 . Then, this moving distance is increased, wrinkles or the like are generated in the fusing belt  610  and sheets, leading to a problem that a stable fusing performance cannot be secured. Because of this, in this exemplary embodiment, in order to prevent the excessive movement of the fusing belt  610 , a process is adopted in which an end position of the fusing belt  610  is detected by a sensor (not shown), so that the steering roller  612  is tilted appropriately based on the result of such a detection. 
   For example, in  FIG. 3A , when the control unit  40  determines based on an output from the sensor (not shown) that the fusing belt  610  is being situated further downwards than the predetermined position as viewed in the figure (towards the receiving portion  612   c ), the control unit  40  causes the shifting unit  66  to shift the receiving portion  612   c  obliquely downwards (in the direction indicated by the arrow J in  FIG. 2 ) by a predetermined amount, so as to impart an inclination or tilt of a predetermined angle to the steering roller  612 . When such a tilt is imparted to the steering roller  612  the fusing belt  610  moves along the steering roller  612  so tilted. As a result, the fusing belt  610  moves upwards as viewed in the figure (in a direction in which the belt moves away from the receiving portion  612   c ). 
   On the other hand, when the above state, that is, the state in which the receiving portion  612   c  is situated downwards is maintained, the fusing belt  610  moves gradually upwards as viewed in the figure. Then, when the control unit  40  determines based on an output from the sensor (not shown) that the fusing belt  610  is has moved to be situated further upwards than the predetermined position as viewed in the figure, the control unit  40  causes the shifting unit  66  to shift the receiving portion  612   c  of the steering roller  612  obliquely upwards (in the direction indicated by the arrow H in  FIG. 2 ) by a predetermined amount, whereby the moving direction of the fusing belt  610  is reversed, and the fusing belt  610  moves gradually downwards as viewed in the figure. The, when the fusing belt  610  has moved to be situated further downwards than the predetermined position as viewed in the figure, the control unit  40  causes the shifting unit  66  to move the receiving portion  612   c  of the steering roller  612  obliquely downwards by the predetermined amount, so as to change again the tilt angle of the steering roller  612 . Namely, in this embodiment, the angle of the steering roller  612  is changed by swinging the receiving portion  612   c  in the oblique directions repeatedly, so that the fusing belt  610  is allowed to move (walk) in the directions which intersect the rotational direction of the fusing belt  610  within a predetermined range. 
   Incidentally, with a conventional fusing unit, there is caused a problem that an edge portion (an end portion) or edge portions of a sheet (hereinafter, referred to as a “sheet edge portion”) is brought into press contact with fusing members, whereby a mark or wear is caused in the fusing members, and an image defect (failure) is eventually caused in a resulting image fused by the fusing members. This will be described below. 
     FIGS. 4A and 4B  are drawings which describes a conventional fusing unit. As is shown in  FIG. 4B , the fusing unit includes a fusing belt  710  which is provided in such a manner as to move in circles or rotate, a cylindrical fusing roller  711  which stretches the fusing belt  710  and an impression roller  72  which is disposed in such a manner as to be press contact with the fusing roller  711  via the fusing belt  710 . In addition, the fusing belt  710  is formed into an endless shape. Additionally, a nip section L is formed between the fusing belt  710  and the impression roller  72  for fusing toner images on a sheet of paper on thereto. The image forming apparatus also includes a plurality of tension rollers (not shown) which stretch the fusing belt  710 , a drive motor (not shown) for rotating the fusing roller  711 , a heat source (not shown) for heating the fusing roller  711  and the like. 
   In the fusing apparatus shown in the figure, a sheet passes through the nip section L in such a state that toner images are held on a side thereof which faces the fusing belt  710 , whereby the toner images are fused on to the sheet. Furthermore, the fusing apparatus also includes a roller-like member (not shown) which is similar to the steering roller  612  shown in  FIGS. 2 ,  3 A and  3 B a shifting unit (not shown) for shifting one end of the roller-like member, whereby the fusing belt  710  is configured to be shifted in directions which intersect a rotational direction of the fusing belt  710  by tilting the roller-like member by the shifting unit. 
   On the other hand,  FIG. 4A  is an enlarged view of part of  FIG. 4B . This figure illustrates a state occurring in the vicinity of the sheet edge portions in such a state that the sheet P is situated in the nip section L. When the sheet P is inserted into the nip section L, the sheet edge portions are brought into strong contact with the fusing belt  710 . In addition, the sheet edge portions are also brought into strong contact with the impression roll  72 . As a result, a mark or wear is caused in the fusing belt  710  and the impression roller  72 . 
   In addition, as is shown in  FIG. 4A , when the sheet P is inserted into the nip section L, the fusing members are deformed due to the existence of the sheet P, and the traveling (rotating) speed of the fusing member is caused to differ depending upon positions thereon. In this fusing apparatus, since the fusing roller  711  is formed into the cylindrical shape, the fusing belt  710  is configured to have a constant rotational radius at the nip section L to thereby travel along the circumference of the fusing roller  711  at a constant speed. When the sheet is passed through the nip section L, however, as is shown in the figure, the fusing belt  710  is caused to be recessed at a portion where the fusing belt  710  is brought into contact with the sheet P, whereas the fusing belt  710  is left in press contact with the impression roller  72  at a portion where the fusing belt  710  is not in contact with the sheet P. Because of this, there is caused a difference in outside diameter in the fusing belt  710 , and a speed at the portion on an outer circumferential surface of the fusing belt  710  where the belt is in contact with the sheet P becomes slower than a speed at the portion where the fusing belt  710  is not in contact with the sheet P. Then, in the event that the difference in speed in a width direction of the fusing belt  710  in this way, the fusing belt  710  and the sheet P come to slide on each other, whereby there may occur a case where wear is generated in the fusing belt  710 . 
   Furthermore, the fusing unit is configured to be shifted in the directions which intersect the rotational direction of the fusing belt  710  by tilting the roller-like member as has been described before. As a result, as is shown in  FIG. 4B , a mark or wear is generated at both edge portions of the fusing belt  710  within a moving width (a waking width) over which the fusing belt  710  moves (in the figure, indicated as a “mark/wear generating portion”). 
   Then, when the mark or wear is generated in the fusing belt  710  and the impression roller  72  in the way described above, there is a fear that the lives of the fusing belt  710  and the impression roller  72  may be reduced. In addition, when the mark or wear is generated in the fusing belt  710 , since the fusing belt  710  is brought into contact with the toner images on the sheet, there is a fear that an image defect such as irregularity in gloss of the toner images that are fused may be generated. In particular, when the fusing belt  710  is configured to move in the directions which intersect the rotational direction of the fusing belt  710  as has been described above, there is a fear that a gloss irregularity in the form of a strip may be generated in the toner images that are fused. 
   In addition, although the fusing belt  710  is used in the fusing unit, there is also known a fusing unit adopting a so-called roll-nip method in which no fusing belt  710  is used. 
   This roll-nip method adopting fusing unit is made up of a pair of roller-like members, and no belt-like member such as the fusing belt  710  exists. Therefore, it is natural that the moving (walking) of the fusing belt  710  does not occur. As a result, sheets pass through substantially the same place on the roller-like members. Because of this, marks or wear that is generated on the roller-like members by sheet edge portions of the sheets tends to be formed in such a manner as to be collected to a specific location and deeply when compared to a case where a belt-like member is used in such a manner as to be allowed to move (walk). Because of this, irregularities come to be generated more conspicuously in a fused image, and the lives of the roller-like members are also reduced. 
   In addition, there sometimes occurs a case where a surface layer (a release layer) made from a fluorine-based resin is formed on the surface of the fusing belt  710 . When a fluorine-based resin is used as a surface material for the fusing belt  710 , there is provided an advantage that since the relevant resin provides good releasability, toner images become easy to be released without using oil. However, the fluorine-based resin is less elastic than the silicone rubber or the like which has been conventionally used as a release material, and there sometimes occurs a case where a recess or mark is generated on the surface of the fusing belt  710  by a single passage of a sheet of thick paper between the fusing belt  710  and the impression roller  72 . When printing (fusing), in particular, a document or picture or the like which has a high image density on to a sheet of paper, the mark appears as a linear defect in the resulting image. 
   Here, the mark/wear generating portion describe by use of  FIG. 4B  will be described in greater detail. 
     FIGS. 5A to 5B  show explanatory diagrams which illustrate the state of the fusing belt  710  shown in  FIG. 4B . When a sheet is inserted into the nip section L in the fusing unit shown in  FIG. 4B , the fusing belt  710  and the sheet edge portions come into contact with each other as has been described above. 
     FIG. 5A  illustrates a state in which the fusing belt  710  is situated substantially in an axially central portion of the fusing roller  711  (refer to  FIG. 4B ). In the state shown in the figure, when the sheet is passing through the nip section L (refer to  FIGS. 4A and 4B ), the sheet edge portions come into contact with both the end portions of the fusing belt  710 . Note that contact portions of the fusing belt  710  where the fusing belt  710  is brought into contact with the sheet edge portions are indicated by broken lines. 
     FIG. 5B  illustrates a state in which the fusing belt  710  moves (walks) towards one end of the fusing roller  711 . When the sheet passes through the nip section L in the state shown in the same figure, the sheet edge portions come into contact with the fusing belt  710  in positions which deviate towards the other end of the fusing roller  711 . 
     FIG. 5C  illustrates a state in which the fusing belt  710  moves (walks) towards the other end of the fusing roller  711 . When the sheet passes through the nip section L in the state shown in the same figure, the sheet edge portions come into contact with the fusing belt  710  in positions which deviate towards the one end of the fusing roller  711 . 
   As a result, as is shown in  FIG. 5D , a plurality of contact portions are formed where the fusing belt  710  comes into contact with the sheet edge portions, and a state results in which the plurality of contact portions are aligned parallel to each other. Then, in these contact portions, marks or wear is generated on the surfaces of the fusing belt  710  as has been described above. 
   When a fusing process is performed on other sheets in this state, since marks or wear is aligned parallel to each other in the contact portions, the aligned marks or wear appears as an image line on a fused image which is easy to be recognized by (or is standing out so as to be clearly visible to) the user or the like. In addition, as this occurs, since marks are collected to the same location to some extent, there is also caused a problem that the life of the fusing belt is reduced. 
   Then, there are some techniques for suppressing the generation of the problems attributed to the marks or wear. However, it is difficult to suppress the generation of the problems effectively even with those techniques. For example, there is proposed a technique in which a release agent such as oil is coated on a member corresponding to the fusing belt so as to protect the surface thereof. In this technique, the release agent so coated is transferred to a sheet of paper, and this leads to a problem that a defect in the form of a streak of oil tends to be easily generated on an image which results after fusing. 
   In addition, there is also proposed a technique in which a fusing unit and a transport unit for transporting sheets to the fusing unit are disposed askew to each other in advance so that sheets are inserted askew relative to the fusing unit so as to widen the width of wear on a fusing member which is caused by sheet edge portions. In the case of this technique, however, since sheets are inserted askew into the fusing units at all times, there is caused a problem that when fusing is performed on thin sheets, wrinkles or the like are easily generated thereon. 
   Furthermore, similar to the configurations shown in  FIGS. 4A to 5D , there is proposed a technique in which a fusing member is moved in directions which intersect the transport direction of sheets that are transported substantially at right angles so that the width of wear caused on the fusing member by sheet edge portions is widened so as to extend the life of the fusing member. In this configuration, however, as has been described in  FIGS. 5A to 5D , since marks are formed in such a manner as to be aligned substantially parallel to the transport direction of sheets, the marks appear in the form of an image line which is easily recognized by the user or the like on an image which results after fusing. 
   Then, in the embodiment of the invention, as is shown in  FIGS. 1 to 3B , the configuration is adopted in which the fusing unit  60  is rotated by the rotating unit  70  so as to disperse marks caused on the fusing belt  610 . Hereinafter, marks will be described which are generated on the fusing belt  610  when the fusing unit  60  is rotated by the rotating unit  70 . 
     FIGS. 6A and 6B  show diagrams which explain marks which are generated on the fusing belt  610  when the fusing unit  60  is rotated by the rotating unit  70 . 
     FIG. 6A  illustrates a state of the fusing roller  611  which results when the fusing unit  60  is viewed from a direction indicated by an arrow K in  FIG. 2 . Note that in this figure, the other members than the fusing roller  611  and the rotating unit  70  are omitted from illustration. In addition,  FIG. 6B  shows contact portions where the fusing belt  610  comes into contact with sheet edge portions. 
   In this exemplary embodiment, as is shown in  FIG. 6A , the rotating center which constitutes a center about which the support housing  63  (refer to  FIGS. 3A and 3B ) rotates when it so does is provided at the one end portion of the fusing roller  611 . In addition, in this exemplary embodiment, the support housing  63  is provided so as to rotate clockwise and counterclockwise about the rotating center. Then, the support housing  63  is rotated about the rotating center by the rotating unit  70 . In addition,  FIG. 6A  illustrates three modes including a state in which the fusing roller  611  (the nip section N (refer to  FIGS. 3A and 3B )) is disposed in the direction which intersects the transport direction of sheets that are transported (refer to  FIGS. 3A and 3B ) substantially at right angles (a reference state), a state in which the fusing roller  610  is rotated clockwise through 5 degrees about the rotating center (a non-reference state), and a state in which the fusing roller  610  is rotated counterclockwise through 5 degrees about the rotating center (a non-reference state). 
   Then, when sheets are passed through the nip section N in these three modes, respectively, sheet edge portions come into contact with the fusing belt  610  in different positions. In addition, in this embodiment, as has been described above, the steering roller  612  (refer to  FIG. 2 ) is provided, and the fusing belt  610  moves in the directions which intersect the rotating direction of the fusing belt  610 . As a result, as is shown in  FIG. 6B , a plurality of contact portions are formed where the fusing belt  610  comes into contact with the sheet edge portions. In addition, since the nip section N is disposed so as to be put in the reference state and the non-reference states, these contact portions come to intersect each other, resulting in a state where the contact portions are dispersed further than the contact portions shown in  FIG. 5 . 
   In the examples shown in  FIGS. 4A to 5D , since the fusing belt  710  is configured to simply be moved in the directions which intersect the rotating direction of the fusing belt  710  substantially at right angles, the contact portions where the fusing belt  70  comes into contact with the sheet edge portions are formed aligned parallel to each other as has been described above. As a result, as has been described above, when fusing is performed on other sheets of paper, the problem is caused that the image line which is easily recognized by the user of the like is easily generated. On the other hand, in the exemplary embodiment of the invention, since the fusing unit  60  is configured to be rotated, the contact portions can be disposed in the non-parallel state, and hence, marks or wear that is generated in the contact portions can be situated in a dispersed fashion. 
   The rotating center can be disposed at other locations on the fusing unit  60 . Next, an example will be described in which the rotating center is provided substantially in a central portion of the fusing roller  611 . 
     FIGS. 7A and 7B  show diagrams which explain marks or the like which are generated on the fusing belt  610  when the fusing unit  60  is rotated about substantially a central portion of the fusing roller  611 . 
   In the exemplary embodiment, as is shown in  FIG. 7A , the rotating center about which the support housing  63  (refer to  FIGS. 3A and 3B ) rotates when it so does is provided in an axially central portion of the fusing roller  611  which lies on the axis of the fusing roller  611 . In addition, in this exemplary embodiment, the support housing  63  is provided so as to rotate clockwise and counterclockwise about the rotating center. Then, the support housing  63  is rotated about the rotating center by the rotating unit  70 . 
     FIG. 7A  illustrates three modes including a state in which the fusing roller  611  (the nip section N (refer to  FIGS. 3A and 3B )) is disposed in the direction which intersects the transport direction of sheets that are transported (refer to  FIGS. 3A and 3B ) substantially at right angles (a reference state), a state in which the fusing roller  610  is rotated (swung) clockwise about the rotating center (a non-reference state), and a state in which the fusing roller  610  is rotated (swung) counterclockwise about the rotating center (a non-reference state). In addition, looking at this from a different viewpoint,  FIG. 7A  illustrates a state in which the nip section N (refer to  FIGS. 3A and 3B ) is disposed in the direction which intersects the transport direction of sheets that are transported (refer to  FIGS. 3A and 3B ) substantially at right angles (a reference state) and a state in which one end side of the nip section N is moved towards or away from the transport direction of sheets, and the other end side thereof is moved away from or towards the transport direction of sheets (a non-reference state). 
   Then, when sheets are passed through the nip section N in these three modes, respectively, sheet edge portions come into contact with the fusing belt  610  in different positions. In addition, in this exemplary embodiment, as has been described above, the steering roller  612  (refer to  FIG. 2 ) is provided, and the fusing belt  610  moves in the directions which intersect the rotating direction of the fusing belt  610 . As a result, as is shown in  FIG. 7B , a plurality of contact portions are formed where the fusing belt  610  comes into contact with the sheet edge portions. In addition, since the nip section N is disposed so as to be put in the reference state and the non-reference state, these contact portions come to intersect each other, resulting in a state where the contact portions are not disposed parallel to each other but are disposed to intersect each other. 
   Thus, while the example has been described in which the fusing unit  60  is rotated about the rotating center, the degree of dispersion of the contact portions comes to differ depending on the position of the rotating center. For example, in the example shown in  FIGS. 6A and 6B , since the rotating center is disposed in the position of an end portion of the fusing roller  610  which is a position different from the axially central portion of the fusing roller  610  or a position which deviates from the axially central portion of the fusing roller  610 , the contact portions are generated over a relatively wide range. On the other hand, as is shown in  FIGS. 7A and 7B , when the rotating center is disposed in the axially central portion of the fusing roller  611 , the range where the contact portions are generated is narrowed when compared with the case shown in FIGS.  6 A and  6 B. 
   Although there is a possibility that the marks or wear generated in the contact portions appears as the image defect as has been described above, the wider the range where the contact portions are generated, the wider marks or wear generated tends to be dispersed, and hence, the image defect becomes difficult to be recognized by the extent that the marks or wear is so dispersed. Because of this, the rotating center is desirably disposed in the position which deviates from the axially central portion of the fusing roller  611  as is shown in  FIGS. 6A and 6B  from the view point that the image defect is made difficult to be recognized by the user or the like. Note that when attempting to cause the rotating center to deviate from the axially central portion of the fusing roller  611 , the rotating center can be disposed at the end portion of the fusing roller  611  as is shown in  FIGS. 6A and 6B , or the rotating center can be disposed at the end portion of the steering roller  612  as is shown in  FIGS. 3A and 3B . Namely, when the rotating center is disposed in the position deviating from the axially central portion of the fusing roller  611 , the position where the rotating center is disposed is not limited to positions on the fusing roller  611 , and hence, the rotating center can be disposed in any other position than those on the fusing roller  611 . 
   Incidentally, the timing or the like at which the fusing unit  60  is rotated so as to put the nip section N in the reference state or the non-reference state is set by the control unit  40  based on kinds of sheets such as basic weight, thickness and the like or the number of sheets on which fusing is performed. In addition, the angle or the like through which the fusing unit  60  is rotated is also set by the control unit  40  based on kinds of sheets or the number of sheets. Hereinafter, the timing at which the nip section N is put in the reference state or the non-reference state and the rotational angle through which the fusing unit  60  is rotated will be described in detail. 
   For example, the shift of the nip section N from the reference state to the non-reference state can be performed when the basic weight of sheets on which fusing is performed is large. This is because the thicker a sheet or the larger the basic weight thereof, the more easily marks or wear is generated on the fusing belt  610  in general and in the event that fusing is performed without rotating the fusing unit  60 , marks or wear is generated concentrically at a specific location on the fusing belt  610 . Whether a sheet is thick or the basic weight is large is determined by the control unit  40  based on an input made by the user through the control panel (not shown). 
   Specifically speaking, the control unit  40  receives information given by the user through the control panel (not shown) as the receiving section and determines whether a sheet on which fusing is performed is thick or the basic weight thereof is large. Then, when determining that the sheet on which fusing is performed is thick or the basic weight thereof is large, the control unit  40  causes the fusing unit  60  to rotate so as to put the nip section N, which is being in the reference state, in the non-reference state. 
   Note that with the nip section N left in the non-reference state, when fusing is performed on a sheet which is thin or whose basic weight is small, wrinkles tend to be generated on the sheet. Because of this, when fusing is performed on a sheet which is thin or whose basic weight is small, the nip section N is desirably put back in the reference state. Namely, with a sheet which is thin or whose basic weight is small, the nip section N is desirably put in the reference state, whereas with a sheet which is thick or whose basic weight is large, the nip section N is desirably put in the non-reference state. In addition, in this exemplary embodiment, while the example is described in which whether or not the sheet is thick is determined based on information inputted through the control panel, there can be adopted a configuration in which a sensor is provided for detecting the thickness of a sheet that is supplied for fusing, so that whether or not the sheet is thick can be determined based on an output from the sensor so provided. In this case, the control unit  40  (refer to  FIG. 1 ) functions as the receiving section. 
   In addition, the rotation of the fusing unit  60  can be performed based on the number of sheets on which fusing is performed. For example, when receiving an instruction to produce a plurality of printed sheets from the user via the control panel, the control unit  40  causes the fusing unit  60  to rotate so as to put the nip section N, which is being in the reference state, in the non-reference state after a predetermined number of sheets have been subjected to fusing. In addition, the control unit  40  can cause the fusing unit  60  to rotate so as to dispose the nip section N in such a manner as to increase the angle relative to the reference state after a predetermined number of sheets have been subjected to fusing. Furthermore, the control unit  40  can be configured to grasp a total number of sheets which are to be subjected to fusing so as to cause the fusing unit  60  to rotate when a predetermined number of sheets have been subjected to fusing. 
   In the exemplary embodiment described above, while the nip section N is put in the non-reference state by causing the fusing unit  60  to rotate, the nip section N can also be put in the non-reference state by causing a member or section provided on the main body side of the image forming apparatus such as the transport belt  55  (refer to  FIG. 1 ). When causing the member or section provided on the main body side of the image forming apparatus to rotate, however, since there is a fear that the image forming apparatus is enlarged in size or wrinkling or jamming (wedging) is generated in the transporting process of sheets, the fusing unit  60  is preferably caused to rotate. In addition, in the embodiment, while the configuration is described in which the fusing belt  610  is used, the configuration of the embodiment can, of course, be applied, as well, to a fusing unit which adopts the so-called roll-nip method in which a pair of roller-like members are used in place of a belt-like member corresponding to the fusing belt  610 . In this case, the pair of roller-like members correspond to the first fusing member and the second fusing member, respectively. 
   The foregoing description of the exemplary embodiments of the present invention has been provided for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments are chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various exemplary embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.