Patent Publication Number: US-7914215-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2005-375590, filed on Dec. 27, 2005, the entire subject matter of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Aspects of the present invention relate to an image forming apparatus applicable to an electrophotographic image forming apparatus such as a laser printer or a copying machine. 
     BACKGROUND 
     An image forming apparatus usually prints an image such as letters or the like on a recording face of a recording sheet while conveying the recording sheet on a conveying path set in the image forming apparatus (or a casing thereof). If the recording sheet passes through a curved conveying path, the recording sheet may have a bending tendency (or a curl). 
     Therefore, the bending tendency is offset and eliminated usually by curving the recording sheet in a direction opposite to the bending tendency. As specific means for curving the recording sheet in a direction opposite to the bending tendency, there is known an uncurling mechanism, which includes a conveying roller and a pair of follower rollers arranged along a circumference direction of the conveying roller and confronting the conveying roller. 
     In this uncurling mechanism, the pair of follower rollers winds the recording sheet on the conveying rollers so that the recording sheet is curved in the direction opposite to the bending tendency thereby to offset and remove the bending tendency. 
     However, different recording sheets have different warping rigidity and different bending tendency in accordance with their kinds. Thus, the bending tendency of all kinds of the recording sheets may not be properly cleared if they are forcibly curved under a common condition. 
     JP-A-6-183628 discloses a configuration in which a pair of follower rollers are integrated by a support plate or the like, and the support plate is moved in parallel with the conveying direction of the recording sheet. The Support plate is moved such that one of the pair of follower rollers is brought into contact with the recording sheet on the conveying roller in one case, and the one of the follower rollers is separated from the recording sheet in another case. Thus, various kinds of recording sheets can be dealt with. 
     JP-A-8-175733 discloses a configuration in which the follower roller on the upstream side in the conveying direction is moved in parallel with a direction perpendicular to the conveying direction such that the follower roller is brought into and out of contact with the recording sheet on the conveying roller, so that various kinds of recording sheets can be dealt with. 
     SUMMARY 
     In the configuration disclosed in JP-A-6-183628, however, the support plate is moved in parallel with the conveying direction of the recording sheet, thereby to switch the cases, in which one of the pair of follower rollers is brought into and out of contact with the conveying rollers of the recording sheet. As a result, the distance between the rotation center of the other follower roller and the rotation center of the conveying roller varies as the one follower roller is brought into and out of contact with the conveying roller. 
     When the distance between the rotation center of the follower rollers and the rotation center of the conveying roller varies, the contacting facial pressure between the follower rollers and the conveying roller changes so that the recording sheet cannot be stably conveyed. 
     If the contacting facial pressure becomes excessively high, a roller scar may be formed to damage the recording sheets when the recording sheet is clamped between the conveying roller and the follower rollers. If the contacting facial pressure becomes excessively low, on the contrary, the recording sheet cannot be sufficiently clamped so that it cannot be stably conveyed. 
     In the configuration disclosed in JP-A-8-175733, on the other hand, since the follower roller on the upstream side of the conveying direction is switchably moved to be brought into and out of contact with the recording sheet on the conveying roller, the recording sheet cannot be stably conveyed for the reasons to be described later. 
     Aspects of the present invention provide an image forming apparatus which can remove the bending tendency reliably for various kinds of recording sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing the exterior of a laser printer according to an aspect of the invention; 
         FIG. 2  is a sectional side view of the laser printer; 
         FIG. 3  is an enlarged view of a discharge chute and intermediate conveying rollers; 
         FIG. 4  is a sectional view taken of  FIG. 3  from a plane being normal to an axial direction of the intermediate conveying rollers; 
         FIG. 5  is another sectional view taken of  FIG. 3  from a plane being normal to the axial direction of the intermediate conveying rollers; 
         FIG. 6  is a perspective view of the follower rollers; 
         FIG. 7  is a perspective view taken from a rear side of  FIG. 6 ; 
         FIG. 8  is another perspective view of the follower rollers; and 
         FIG. 9  is another perspective view taken from the rear side of  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
     General Overview 
     According to a first aspect of the invention, there is provided an image forming apparatus comprising: an image forming unit which forms an image on a recording sheet; a casing which houses the image forming unit and holds the image forming unit; a conveying roller which applies a conveying force to the recording sheet by contacting with the recording sheet; a first roller which is fixed to a position confronting the conveying roller, the first roller pushing the recording sheet toward the conveying roller and being rotatable; a second roller which is arranged at a position confront the conveying roller and downstream side of the first roller in a conveying direction, the second roller being displaceable; and a roller displacing mechanism which displaces the second roller in such a manner as to change a distance between the second roller and the conveying roller. 
     According to a second aspect of the invention, the image forming apparatus further comprises a first guide unit which is disposed in an upstream side of a contact portion between the first roller and the second roller in the conveying direction and guides the recording sheet to the contact portion. 
     According to a third aspect of the invention, the image forming apparatus further comprises a second guide unit which has a guide portion continuing from the upstream side of the conveying roller in the conveying direction to the downstream side of the conveying roller in the conveying direction and guides the recording sheet. 
     According to a fourth aspect of the invention, if the second roller is spaced from the conveying roller, the recording sheet discharged from the conveying roller is conveyed to the downstream side being in contact with the second guide unit but without colliding with the second roller. 
     According to a fifth aspect of the invention, the image forming apparatus further comprises a third guide unit which guides the recording sheet discharged from the conveying roller to the downstream side without colliding with the second roller if the second rollers are the most spaced from the conveying rollers. 
     According to a sixth aspect of the invention, the first roller and the second roller are connected by a roller holder, and the second roller is rocked and displaced on a rotation center of the first roller. 
     According to a seventh aspect of the invention, the roller displacing mechanism includes a cam plate which extends substantially in parallel with the rotation axis direction of the conveying roller and is movable in parallel with the same direction, and the cam plate includes a grip portion which operates the cam plate at a longitudinal end portion thereof. 
     According to an eighth aspect of the invention, the image forming unit includes a fixing unit which heats a toner transferred to the recording sheet and fixes the toner on the recording sheet, and the conveying roller is arranged in a downstream side of the fixing unit in the conveying direction. 
     According to a ninth aspect of the invention, the image forming apparatus further comprises a discharge roller which discharges the recording sheet having ended an image formation to a discharge tray in a downstream side of the conveying roller in the conveying direction. 
     According to a tenth aspect of the invention, The image forming apparatus further comprises a guide member which is disposed in the downstream side of the fixing unit in the conveying direction and an upstream side of the discharge rollers in the conveying direction and guides the recording sheet while curving the same, wherein the conveying roller is arranged to curve the recording sheet in a direction opposite to the curvature by the guide member. 
     &lt;Illustrative Aspects&gt; 
     With reference to the accompanying drawings, aspects of the invention will be described, in which an image forming apparatus is applied to an apparatus such as a laser printer. 
     1. Exterior Structure of Laser Printer 
       FIG. 1  is a perspective view showing the exterior of a laser printer  1  according to the aspect of the invention. 
     The laser printer  1  has a substantially box-shaped (or stereoscopic) casing  3 , in which a later-described image forming unit  10  or the like is housed. A discharge tray  5  is provided on an upper face side of the casing  3 . Recording sheets discharged from the casing  3  after printing are stacked on the discharge tray  5 . In this aspect, sheets such as paper sheets or OHP sheets may be used as the recording sheets. 
     The discharge tray  5  is configured to have such a slope face  5   a  which increases in slope the farther the distance from an upper face of the casing  3  the closer to a rear side. A discharge port  7 , from which the printed recording sheet is discharged, is provided on a rear end side of the slope face  5   a.    
     2. Internal Structure of Laser Printer 
       FIG. 2  is a sectional side view of the laser printer  1 . The image forming unit  10  form images on the recording sheets. A feeder unit  20  constitutes a portion of a conveying unit for feeding the recording sheets to the image forming unit  10 . 
     A discharge chute  30  constitutes a guide member for causing the recording sheet having formed with the image at the image forming unit  10  to make a U-turn of about 180 degrees in the conveying direction and to turn upside-down so as to guide the recording sheet into the discharge port  7  formed above a fixer unit  90 . 
     The intermediate conveying rollers  40  for conveying the recording sheet to the discharge port  7  are disposed at the intermediate portion of the conveying path  31 , which is formed by the discharge chute  30 . A turning roller  32  for guiding the recording sheets which is discharged from the fixing unit  90  (or a heating roller  91 ), to turn about 90 degrees in the conveying direction upward, is disposed on an exit side of the fixing unit  90 . Discharge rollers  33 , which discharge the conveyed recording sheet to the discharge tray  5 , are disposed at the most downstream portion of the conveying path  31 . 
     2.1. Feeder Unit 
     The feeder unit  20  includes a sheet feed tray  21 , a sheet feed roller  22 , a separating roller  23  and a separating pad  24 . The sheet feed tray  2  is housed in the lowermost portion of the casing  3 . The sheet feed roller  22  is disposed above the front end portion of the sheet feed tray  21  for conveying the recording sheets to the image forming unit  10 . The separating roller  23  and the separating pad  24  separate one by one the recording sheets to be conveyed to the sheet feed roller  22 . The recording sheets, which are stacked on the sheet feed tray  21 , are conveyed, after being U-turned on the front side in the casing  3 , to the image forming unit  10  arranged substantially at the center portion in the casing  3 . 
     In the recording sheet conveying path from the sheet feed tray  21  to the image forming unit  10 , a paper dust removing roller  25  is provided on the outer side of the top portion of the substantially U-shaped turn. The paper dust removing roller  25  removes the paper dust or the like which has stuck to the image forming face (or the printing face) of the recording sheet. An opposed roller  26  for pushing the conveyed recording sheet to the paper dust removing roller  25  is provided on the inner side of the top portion. 
     At the entrance of the image forming unit  10  of the conveying path from the sheet feed tray  21  to the image forming unit  10 , moreover, there are arranged a pair of resister rollers  27  for applying a conveying resistance to the recording sheet thereby to arrange the conveyed state of the recording sheet. 
     2.2. Image Forming Unit 
     The image forming unit  10  includes a scanner unit  70 , a process cartridge  80  and a fixing unit  90 . 
     2.2.1. Scanner Unit 
     The scanner unit  70  is disposed in the upper portion of the casing  3  and forms an electrostatic latent image on the surface of a photosensitive drum  81 . Specifically, the scanner unit  70  includes a laser light source, a polygon mirror, an fθ lens and reflecting mirrors. 
     The laser beam, which is emitted from a laser light source and based on the image data, is deflected by the polygon mirror and passed through the fθ lens. The optical path is then folded back and bent downward by the reflecting mirrors so that the surface of the photosensitive drum  8  is irradiated with the laser beam to form the electrostatic latent image. 
     2.2.2. Process Cartridge 
     The process cartridge  80  is removably arranged in the casing  3  on the lower side of the scanner unit  70 . This process cartridge  80  includes the photosensitive drum  81 , a charger  82 , a transfer roller  83  and a developer cartridge  84 . 
     The photosensitive drum  81  acts as an image carrying unit for carrying the image to be transferred to the recording sheet. The photosensitive drum  81  includes a cylindrical drum body  81   a  having its outermost layer made of a positively chargeable photosensitive layer such as polycarbonate, and a drum shaft  81   b  extending axially and longitudinally in the drum body  81   a  for supporting the drum body  81   a  rotatably. 
     The charger  82  acts as a charging unit for charging the surface of the photosensitive drum  81 . The charger  82  is so arranged obliquely above the rear side of the photosensitive drum  81  to confront the photosensitive drum  81  across a predetermined spacing so that it does not contact the photosensitive drum  81 . The charger  82  may be a scorotron type charger for charging the surface of the photosensitive drum  81  substantially uniformly with a positive charge by corona discharge. 
     The transfer roller  83  is arranged to confront the photosensitive drum  81  and to rotate in association with the photosensitive drum  81 . This transfer roller  83  acts as a transfer unit for transferring the toner having stuck to the surface of the photosensitive drum  81  to the printing surface of the recording sheet. The transfer roller  83  transfers the toner by causing the charge (i.e., the negative charge), which is opposite to the charge at the photosensitive drum  81 , to act on the recording sheet from the side opposite to that of the printing face. 
     The discharge chute  30  includes a toner housing chamber  84   a  housing the toner, a toner feed roller  84   b  for feeding the toner to the photosensitive drum  81 , and a developing roller  84   c.    
     The toner, which is housed in the toner housing chamber  84   a , is fed toward the developing roller  81   c  by the rotation of the toner feed roller  84   b . The toner fed toward the developing roller  84   c  is carried on the surface of the developing roller  84   c . After regulated to a predetermined constant (or uniform) thickness, the toner is fed to the surface of the photosensitive drum  81  exposed by the scanner unit  70 . 
     2.2.3. Fixing Unit 
     The fixing unit  90  is arranged on the down stream side of the photosensitive drum  81  in the conveying direction of the recording sheet. The fixing unit  90  fixes the toner, which is transferred to the recording sheet, by heating and melting the toner. Specifically, the fixing unit  90  includes a heating roller  91  and a pressure roller  92 . The heating roller  91  is arranged on the printing face side of the recording sheet for applying the conveying force, while heating the toner, to the recording sheet. The pressure roller  92  is arranged on the opposite side of the heating roller  91  across the recording sheet and pushes the recording sheet toward the heating roller  91 . 
     Incidentally, the heating roller  9  is driven by a unit such as a motor (not shown), and the pressure roller  92  follows and rotates while receiving the rotating force from the heating roller  91  through the recording sheet contacting with the heating roller  91 . 
     2.2.4. Actions of Image Forming Unit 
     The drum  8  is uniformly and positively charged by the charger  82  as it rotates, and is then exposed to the laser beam which is irradiated from and scanned at a high speed by the scanner unit  70 . As a result, the electrostatic latent image corresponding to the image to be formed on the recording sheet is formed on the surface of the photosensitive drum  81 . 
     Next, the toner, which is carried on the developing roller  84   c  and positively charged, is fed to the electrostatic latent image formed on the surface of the photosensitive drum  81  when brought to face and contact with the photosensitive drum  81  by the rotation of the developing roller  84   c . Namely, the roller is fed to such an exposed portion in the uniformly and positively charged surface of the photosensitive drum  8  which has been exposed to the laser beam to lower its potential. As a result, the roller image by the reversal phenomenon is carried on the surface of the photosensitive drum  81 . 
     The toner image, which is carried on the surface of the photosensitive drum  81 , is transferred to the recording sheet by the transfer bias applied to the transfer roller  83 . The recording sheet having the toner image transferred is conveyed to and heated by the fixing unit  90  go that the toner transferred as the roller image is fixed on the recording sheet. Thereby, the image formation is completed. 
     2.3. Discharge Chute  30  and Intermediate Conveying Rollers  40   
     2.3.1. Structures of Discharge Chute  30  and Intermediate Conveying Rollers  40   
       FIG. 3  is an enlarged view of the discharge chute  30  and the intermediate conveying rollers  40 .  FIG. 4  and  FIG. 5  are sectional views taken along a plane in  FIG. 3 , plane being normal to the axial direction of the intermediate conveying rollers  40 .  FIG. 6  and  FIG. 8  are perspective views of follower rollers  41  and  42  and so on.  FIG. 7  is a perspective view taken from the rear side of  FIG. 6 .  FIG. 9  is a perspective view taken from the rear side of  FIG. 8 . 
     The discharge chute  30  constitutes a guide wall for turning the recording sheet, discharged from the fixing unit  90  (or the heating roller  91 ), as shown in  FIG. 2 , upward by about 180 degrees thereby to guide the recording sheet to the discharge rollers  33 . On the upstream side of the turning roller  32  in the conveying direction, there is arranged a peeling blade  34  for peeling the recording sheet, heated at the heating roller  91 , from the heating roller  91 . 
     The intermediate conveying rollers  40 , which are disposed at the intermediate portion of the conveying path  31 , come into contact with the discharged recording sheet from the fixing unit  90 , thereby to apply the conveying force to the recording sheet. At the position confronting the intermediate conveying rollers  40  across the recording sheet to be conveyed, there is arranged the pair of follower rollers  41  and  42 , which is pushed to the side of the intermediate conveying rollers  40  by urging unit (coil springs, as shown in  FIG. 4 ). 
     As a result, the pair of follower rollers  41  and  42  pushes the conveyed recording sheet to the side of the intermediate conveying rollers  40 , and rotates while following the conveyance of the recording sheet. The follower rollers on the upstream side in the conveying direction will be called the first follower rollers  41 , and the follower rollers on the downstream side in the conveying direction will be called the second follower rollers  42 . 
     At least an outer circumference of the intermediate transfer roller  40  is made of an elastically deformable elastic material such as rubber. The pair of follower rollers  41  and  42  is made of a hard material such as a resin. 
     The first follower rollers  41  and the second follower rollers  42  are connected by roller holders  14  such that they are rotatably supported by the roller holders  44  through roller shafts  41   a  and  42   a . The pair of follower rollers  41  and  42 , which is connected by the roller holders  44  to form the pairs, is juxtaposed in pairs in the widthwise direction (as will be called the sheet width direction) of the recording sheet to be conveyed, as shown in  FIG. 6  and  FIG. 8 . 
     The sheet width direction is a direction perpendicular to both the conveying direction of the recording sheet and the thickness direction of the recording sheet. The sheet width direction is aligned with the rotation axis direction of the intermediate conveying rollers  40 . 
     The individual roller holders  44  are fixed to a holder stay  45  extending in the sheet width direction. The holder stay  45  is assembled with the casing  3  such that the holder stay  45  rotatable at a position corresponding to the rotation centers of the first follower rollers  41 . 
     In this aspect, arm portions  45   a  and  45   b , which extend in parallel with the roller holders  44 , are provided on the two end sides of the longitudinal direction in the holder stay  45 . The arm portion  45   a  on one side has a pin hole  45   c , into which a pin (not shown) formed on the casing  3  is rotatably inserted. The arm portion  45   b  on the other side has a pin  45   d , which is rotatably inserted into a pin hole (not shown) formed in the casing  3 . 
     When viewed in the sheet width direction, the rotation centers of the pin hole  45   c  and the pin  45   d  are substantially aligned with the rotation center of the first follower rollers  41 , as shown in  FIG. 6  and  FIG. 8 . Therefore, as the holder stay  45  rotates (or rocks), the rotation center of the first follower rollers  41  do not move, whereas the rotation center of the second follower rollers  42  rotate (or rock) on the rotation center of the first follower rollers  41 . 
     In other words, the center distance d 1  between the first follower roller  41  and the intermediate conveying roller  40  is constant at all times irrespective of the state of the holder stay  45 , whereas the center distance d 2  between the second follower roller  42  and the intermediate conveying roller  40  changes with the state of the holder stay  45 , as shown in  FIG. 4  and  FIG. 5 . 
     As shown in  FIG. 7  and  FIG. 9 , the holder stay  45  has cam faces  45   e  protruding in a substantially trapezoidal shape. On the other hand, a cam plate  46  is assembled to the casing  3  such that the cam plate  46  is movable in parallel with the aforementioned rotation axis direction. The cam plate  16  extends in the rotation axis direction (or the sheet width direction) of the intermediate conveying roller  40  and has cam faces  46   b  come into contact with the cam faces  45   e.    
     As shown in  FIG. 7  and  FIG. 9 , a grip portion  46   a , which is manually operated by a user to move the cam plate  46  in the A-direction or in the opposite direction, is provided on one end side in the longitudinal direction of the cam plate  46 . When the cam plate  46  is brought relative to the holder stay  45  to a position shown in  FIG. 7 , in which the A-direction side end portion of the cam plate  46  is positioned on the side of the A-direction with respect to the A-direction side end portion of the holder stay  45 , the cam faces  45   e  of the holder stay  45  are lifted by the cam faces  46   a  of the cam plate  46 . 
     As a result, the holder stay  45  takes the state, in which both the pair of follower rollers  41  and  42  contacts with the intermediate conveying roller  40 , as shown in  FIG. 4 . On the other hand, when the cam plate  16  is brought to a position shown in  FIG. 9  with respect to the holder stay  45 , that is, a position, in which the A-direction side end portion of the cam plate  46  is located on the opposite side in the A-direction with respect to the A-direction side end portion of the holder stay  45 , the recessed portions of the cam faces  46   b  and the protruded portions of the cam faces  45   e  confront with each other. As a result, the holder stay  45  falls by its own weight so that only the first follower rollers  41  contact with the intermediate conveying rollers  40 , as shown in  FIG. 5 , whereas the second follower rollers  42  are spaced from the intermediate conveying rollers  40 . 
     As described above, in this aspect, the holder stay  45  and the cam plate  16  constitute a roller displacing mechanism for changing the distance between the second follower rollers  42  and the intermediate conveying rollers  40 . 
     The first follower rollers  4  are so arranged that the upstream side of the recording sheet in the conveying direction is directed rearward with respect to the contact points P 1  between the intermediate conveying rollers  10  and the first follower rollers  41 , as shown in  FIG. 4 . The second follower rollers  42  are so arranged that the downstream side of the recording sheet in the conveying direction is directed backward with respect to the contact points P 2  between the intermediate conveying rollers  40  and the second follower rollers  42  while the second follower roller  42  and the intermediate conveying roller  40  are contacting with each other. 
     Herein the word “rearward” means a direction in which the recording sheet is conveyed from the contact portion between the heating roller  91  and the recording sheet. In this aspect, the rearward side coincides with the rearward of the laser printer  1 . 
     In the conveying path  31 , the conveying path between the turning rollers  32  and the intermediate conveying roller  40  has an enlarged space  31   a . The enlarged space  31   a  is enlarged rearward from the contact points P 1  between the intermediate conveying rollers  40  and the first follower rollers  41 , as shown in  FIG. 2 . The enlarged space  31   a  increases in width toward the rearward direction and toward the upstream (the downward in the drawing) of the conveying direction. 
     As shown in  FIG. 3 , the discharge chute  30  is provided with first guide ribs  35  and second guide ribs  36 . The first guide ribs  35  and second guide ribs  36  protrude toward the recording sheet (or the conveying path  31 ) to be conveyed so that leading end sides of the first guide ribs  35  and second guide ribs  36  come into contact with the recording sheet and guide the recording sheet. 
     The first guide ribs  35  constitute a first guide unit, which is disposed on the upstream side in the conveying direction of the contact portion P 1  between the intermediate conveying rollers  40  and the first follower rollers  4  and guides the conveyed recording sheet to the contact points P 1  (refer to  FIG. 4 ). The second guide ribs  36  have guide portions  36   a  continuing from the upstream side of the intermediate conveying rollers  40  in the conveying direction to the downstream side of the intermediate conveying rollers  40  in the conveying direction. The second guide ribs  36  constitute a second guide unit for guiding the recording sheet. 
     The guide portions  36   a  are portions correspond to the leading ends of the second guide ribs  36  and contact the recording sheet to be conveyed so as to guide the conveying direction of the recording sheet. Likewise, the first guide ribs  35  have guide portions  35   a  which are the leading ends of the first guide ribs  35 . These leading ends contact the recording sheet to be conveyed, to guide the conveying direction of the recording sheet. The guide portions  35   a  of the first guide ribs  35  end near the contact portion P 1 . 
     The protrusion amount of the second guide ribs  36  are set such that, when the second follower rollers  42  are spaced from the intermediate conveying rollers  40  as shown in  FIG. 5 , the recording sheet discharged from the intermediate conveying rollers  40  is brought into contact with the guide portions  36   a  of the second guide ribs  36  and is conveyed to the downstream side without any collision against the second follower rollers  42 . 
     That is, the second guide ribs  36  are configured such that, when the second drive rollers  42  are spaced from the intermediate conveying rollers  40 , the second follower rollers  42  are positioned closer to the image forming unit  10  than the guide portions  36   a  of the second guide ribs  36  when viewed in the direction parallel to the axial direction of the hinge pins  42   b.    
     2.3.2. Conveying of Recording Sheet in Discharge Chute and Intermediate Conveying Rollers 
     In the state where the first follower rollers  41  and the second follower rollers  42  contact with the intermediate conveying rollers  40  (refer to  FIG. 4 ), the discharged recording sheet from the fixing unit  90  is guided by the first guide ribs  35  and clamped between the first follower rollers  4  and the intermediate conveying rollers  40 . 
     Then, the discharged recording sheet from the first follower rollers  41  is guided by the second guide ribs  37  to the contact portion P 2  between the second follower rollers  42  and the intermediate conveying rollers  40 , and is fed out to the discharge rollers  33  while being clamped between the second follower rollers  42  and the intermediate conveying rollers  40 . 
     At this time, the recording sheet is forcibly curled on the intermediate conveying rollers  40 , as shown in  FIG. 4 . As a result, the recording sheet having acquired a bending tendency to bulge rearward is so forcibly curved as to bulge oppositely (or to the front side) thereby to offset the bending tendency (or the curl) having occurred at or downstream of the fixing unit  90 . 
     When the cam plate  46  is operated such that the second follower rollers  42  leave the intermediate conveying rollers  40 , the recording sheet, which is guided by the first guide ribs  35  and clamped between the first follower rollers  41  and the intermediate conveying rollers  40  as shown in  FIG. 5 , is guided by the second guide ribs  36  and is fed to the discharge rollers  33  without being forcibly curled on the intermediate conveying rollers  40 . 
     At this time, the first follower rollers  4  are so arranged that the upstream side of the recording sheet in the conveying direction is directed rearward with respect to the contact points P 1  between the intermediate conveying rollers  40  and the first follower rollers  41 . As a result, the recording sheet is curved oppositely (or to the front side) of the bending tendency although the degree of curvature is smaller than that of the case (refer to  FIG. 4 ), in which the first follower rollers  41  and the second follower rollers  42  are made to contact with the intermediate conveying rollers  40  thereby to curl the bending tendency of the recording sheet forcibly on the intermediate conveying rollers  40  (as shown in  FIG. 4 ). Therefore, the recording sheet is sufficiently eliminated, if it is so thin and likely to cause the bending tendency. 
     3. Operations of Laser Printer 
     As has been described hereinbefore, the second follower rollers  42  arranged on the downstream side in the conveying direction are displaced while the distance between the rotation centers of the first follower rollers  41  and the rotation centers of the intermediate conveying rollers  40  arranged on the upstream side in the conveying direction being constant. 
     As a result, the contacting facial pressure between the first follower rollers  4  and the intermediate conveying rollers  40  can be held substantially constant. Accordingly, the recording sheet can be sufficiently clamped while preventing the recording sheet from being damaged. 
     In order to eliminate the bending tendency, it is necessary to curve the recording sheet oppositely to the bending tendency by winding the recording sheet on the intermediate conveying rollers  40 . Accordingly, a high conveying resistance occurs in the curved portion (or the conveying path) from the first follower rollers  41  on the upstream side in the conveying direction to the second follower rollers  42  on the downstream side in the conveying direction. 
     A conveying force is directed toward the intermediate conveying rollers  40  and applied by the heating roller  91  to the recording sheets, which is conveyed toward the intermediate conveying rollers  40 . If the recording sheet is not clamped by the first follower rollers  41  on the upstream side in the conveying direction, the recording sheet is buckled and deformed at the curved portion acting as the fixed point. 
     If the recording sheet is buckled and deformed at the curved portion as the fixed point, the leading end side of the sheet in the conveying direction may possibly be directed to a direction different from the conveying direction along the curved portion. As a result, the sheet jam may occur at the curved portion. 
     On the other hand, if the first follower rollers  41  on the upstream side in the conveying direction contact with the intermediate conveying rollers  40 , the recording sheet is buckled and deformed at the contact points between the first follower rollers  41  and the intermediate conveying rollers  40  on the upstream side in the conveying direction as the fixed point. However, the recording sheet is clamped at the contact between the first follower rollers  4  and the intermediate conveying rollers  40  on the upstream side in the conveying direction, and the distance between the curved portions and the contact points is short. Thus, the recording sheet is hardly buckled and deformed between the curved portions and the contact points. 
     Therefore, if the recording sheet is always clamped by the first follower rollers  41  on the upstream side in the conveying direction, the recording sheet can be conveyed more stably than that of the configuration disclosed in JP-A-8-175733, in which the recording sheet is always clamped by the follower rollers on the downstream side in the conveying direction. 
     Accordingly it is possible to provide a laser printer, which can remove the bending tendency reliably for recording sheets of various kinds. 
     In this aspect, the first guide ribs  35  for guiding the recording sheet to the contact portions P 1  between the first follower rollers  41  and the intermediate conveying rollers  40  are provided. As a result, the recording sheet, which is conveyed to the intermediate conveying rollers  40 , can be reliably clamped by the first follower rollers  41  and the intermediate conveying rollers  40 . Accordingly, the recording sheet can be conveyed stably. 
     In this aspect, the guide portions  36   a , which continue from the upstream side in the conveying direction of the intermediate conveying rollers  40  to the downstream side in the conveying direction of the intermediate conveying rollers  10 , is provided. As a result, the recording sheet can be guided at its leading end side in the conveying direction along the curved portions. Accordingly, the recording sheet can be conveyed stably. 
     In this aspect, when the second follower rollers  42  are spaced from the intermediate conveying rollers  40 , the discharged recording sheet from the intermediate conveying rollers  40  is conveyed in contact with the second guide ribs  36  to the downstream side without any collision with the second follower rollers  42 . When the second follower rollers  42  are spaced from the intermediate conveying rollers  40 , therefore, the leading end side of the recording sheet in the conveying direction can be prevented from colliding with the second follower rollers  42  and from being turned in a direction different from the conveying direction. Accordingly, the recording sheet can be conveyed stably. 
     In this aspect, the first follower rollers  4  and the second follower rollers  42  are connected by the roller holders  44 . Therefore, even if the displacing actions of the second follower rollers  42  are repeated over a long period of time, the relative positional relations between the first follower rollers  41  and the second follower rollers  42  can be prevented in advance from being largely changed. Accordingly, the recording sheet can be stably conveyed over a long period of time. 
     Moreover, the second follower rollers  42  are rocked and displaced on the rotation centers of the first follower rollers  41  so that the movable area of the second follower rollers  42  can be made smaller than that of the case, in which the second follower rollers  42  are moved in parallel. Therefore, the space for mounting the second follower rollers  42  can be secured while suppressing the size increase of the laser printer. 
     If the configuration is such that the second follower rollers  42  are displaced by rotating the cam plate  46 , it is necessary to provide a handle for turning the cam plate  46 . Thus, the turning area of the handle has to be retained. 
     On the contrary, this aspect is configured such that the second follower rollers  42  are displaced by moving the cam plate  46  substantially in parallel with the direction of the roller shafts of the intermediate conveying rollers  40 . It is, therefore, unnecessary to secure the turning area of the handle. Thus, it is possible to reduce the size of the mechanism (or the roller displacing mechanism) for displacing the second follower rollers  42 . 
     The recording sheet is heated in the fixing unit  90  and water contained therein evaporates. Thus, the bending tendency is liable to occur especially at the fixing unit  90 . Therefore, the bending tendency can be effectively removed, if the intermediate conveying rollers  40  and the pair of follower rollers  41  and  42  having the uncurling function are arranged on the downstream side of the fixing unit  90  in the conveying direction. 
     The discharge chute  30 , which guides the recording sheet while curving the same, is disposed on the downstream side of the fixing unit  90  in the conveying direction and on the upstream side of the discharge rollers  33  in the conveying direction. The intermediate conveying rollers  40  are arranged to curve the recording sheet in the direction opposite to that by the discharge chute  30 . 
     With this configuration, the curvature of the recording sheet by the discharge chute  30  and the curvature of the recording sheet by the intermediate conveying rollers  40  are directionally opposite so that the transfer resistance is increased. Thus, the recording sheet may be caught when it is conveyed by the intermediate conveying rollers  40 . 
     However, in this aspect, the distance between the first follower rollers  4  on the upstream side in the conveying direction and the intermediate conveying rollers  40  is not changed, as described hereinbefore. Accordingly, it is possible to suppress the sheet jam effectively. 
     Incidentally, the image forming unit  10  corresponds to an image forming unit. The intermediate conveying rollers  40  correspond to a conveying roller. The first follower rollers  4  correspond to a first roller. The second follower rollers  42  correspond to a second roller. The holder stay  45  and the cam plate  46  constitute a roller displacing mechanism. The discharge chute  30  corresponds to a guide member. 
     Moreover, the first guide ribs  35  correspond to a first guide unit. The second guide ribs  36  correspond to a second guide unit and a third guide unit. The roller holders  44  correspond to a roller holder. The fixing unit  90  corresponds to a fixing unit. 
     (Other Aspects) 
     In the aforementioned aspect, the second guide ribs  36  act as the third guide unit and the second guide unit. However, the invention should not be limited thereto but may be additionally provided with guide ribs corresponding to the third guide unit. 
     In the aforementioned aspects the second guide ribs  36  are rocked and displaced. However, the invention should not be limited thereto. For example, the second guide ribs  36  may be moved in parallel. 
     Alternatively, either the first guide ribs  35  or the second guide ribs  36  may be eliminated. 
     In the aforementioned aspect, the pair of follower rollers  41  and  42  is connected by the roller holders  44 , but the invention should not be limited thereto. 
     In the aforementioned aspect, the holder stay  45  and the cam plate  46  constitute a roller displacing mechanism. However, the invention should not be limited thereto. For example, the second follower rollers  42  may be displaced by turning the cam plate.