Patent Publication Number: US-6671490-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, e.g., a copying machine or printer, which forms an image electrophotographically and, more particularly, to an image forming apparatus in which a toner-image-bearing transfer medium (to be referred to as a sheet hereinafter) after the fixing process can be correctly guided to a curl correcting means. 
     2. Description of the Related Art 
     According to some copying machine, a latent image formed on a photosensitive body through exposure (writing) in accordance with a document or image data is made visible by developing. The obtained toner image is transferred to the first surface (obverse surface) of a sheet by the operation of a transfer means, and is fixed by a heat roller type fixing unit. After that, the sheet is reversed by a reversing mechanism, and is fed to a transfer region again. Then, another toner image is transferred on the second surface (reverse surface) of the sheet. In this copying machine, conventionally, a curl (curling tendency) tends to occur in the sheet during the fixing process. Hence, after curling occurs, the sheet conveying performance degrades, and image transfer to the second surface of the sheet cannot be performed well. 
     In order to cope with these problems, a curl correcting means (curl correcting mechanism) may be added with which the sheet is drawn through a sponge roller pair by applying a force opposite to the direction of curl to the sheet after the fixing process, so that the curl is corrected. With this sponge roller method, the linear velocity of the sheet is difficult to manage. Simultaneously, the sheet cannot be regulated such that the leading end of the curled sheet is clamped well by the sponge roller pair. Thus, a satisfactory result cannot be obtained. 
     More specifically, for example, a sponge roller pair is provided at a position downstream of a fixing processing means and comparatively close to the fixing processing means. The fixing processing means has, as main constituent elements, an upper fixing roller incorporating a heat source, and a lower fixing roller which rotates in tight contact with the upper fixing roller. A guide member is provided at a position immediately before the sponge roller pair. 
     The guide member is arranged to guide the leading end of the sheet toward the nip position of the sponge roller pair. 
     It is, however, difficult to obtain an approach angle of the sheet for the nip of the sponge roller pair stably, and paper jamming and corner bent of the sheet occur often. 
     These inconveniences are apparently related to the arrangement itself of the curl correcting means and partly to the approach path of the sheet which extends from the fixing section to the curl correcting means. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus having such an arrangement that a sheet after the fixing process can be sent to a curl correcting means in a stable state. 
     In order to achieve the above object, according to the first aspect of the present invention, there is provided an image forming apparatus having an image carrier, image forming means for forming a toner image on the image carrier, transfer means for transferring the toner image formed on the image carrier onto a transfer medium, and fixing processing means including a pair of upper and lower fixing rollers for fixing the toner image onto the transfer medium, wherein the apparatus comprises curl correcting means, including a rotatable sponge roller and a fixed press member which presses the sponge roller while biting into part of an outer surface of the sponge roller, and provided downstream of the fixing processing means, and regulating means, provided immediately before the curl correcting means, so as to guide a leading end of the fixing-processed transfer medium to that position on the outer surface of the sponge roller which is tilted, within a range of 5° to 10° (both inclusive) from a reference line that connects a center of the sponge roller and a press start position of the press member with respect to the sponge roller, to a direction opposite to a rotating direction of the sponge roller. 
     According to the second aspect of the present invention, there is provided an image forming apparatus wherein the sponge roller according to the first aspect is rotated at a linear velocity larger than those of the pair of upper and lower fixing rollers, and the linear velocity of the sponge roller is 325 mm/s to 329 mm/s. 
     According to the third aspect of the present invention, there is provided an image forming apparatus wherein the sponge roller according to the second aspect has a rubber hardness of 2 degrees to 12 degrees (both inclusive) measured by Asker C measurement, and the press member bites into the sponge roller by a bite amount of 2.0 mm at a deepest portion. 
     According to the fourth aspect of the present invention, there is provided an image forming apparatus wherein the regulating means according to the first aspect comprises a first regulating member which has a regulating surface formed of an arcuate recessed surface formed on an approaching side of the fixing-processed transfer medium and a flat surface formed on an outlet side of the transfer medium continuously to the arcuate recessed surface, and a second regulating member which has a regulating surface formed of an arcuate projecting surface formed on an approaching side of the fixing-processed transfer medium and a flat surface formed on an outlet side of the transfer medium continuously to the arcuate projecting surface, and which is arranged to oppose the first regulating member. 
     According to the third aspect of the present invention, there is provided an image forming apparatus further having, downstream of the curl correcting means, a reversal delivery/re-feeding mechanism for reversely delivering and re-feeding the transfer medium which is curl-corrected through the regulating means and the curl correcting means according to the first aspect, wherein the regulating means further comprises a convey path switching member which can direct the transfer medium toward the reversal delivery/re-feeding mechanism. 
     As is apparent from the above aspects, according to the present invention, the arrangement of the regulating member which is located at a position where it is easily thermally affected by heat associated to fixing is improved, and sheet feeding to the curl correcting means can be performed stably. Thus, the curling tendency of the sheet is corrected reliably, and accordingly image formation on the second surface of the sheet can be performed well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing the overall arrangement of a digital copying machine as an example of an image forming apparatus; 
     FIG. 2 is an enlarged view of the arrangement of the main part of a fixing unit in order to explain it; 
     FIG. 3 is an enlarged schematic view showing the arrangement of a curl correcting mechanism and its vicinity; and 
     FIG. 4 is a schematic plan view of the curl correcting mechanism including a second regulating member, lower guide member, and fixing delivery rollers. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described with reference to the accompanying drawings. 
     As shown in FIG. 1, an image forming apparatus of this embodiment is exemplified by a digital copying machine having an automatic document feeder  1 , image reading unit  2 , image forming section  3 , sheet storing section  4 , sheet feeding section  5 , reversal delivery/re-feeding section  6 , and reversal convey section  8 . 
     The automatic document feeder  1  feeds out document sheets one by one to convey them to an image reading position and delivers the document sheets after image reading to a predetermined position. 
     The automatic document feeder  1  has a document table  11  on which a document is to be placed, a document separating means  12  for separating the document placed on the document table  11 , a document conveying means  13  including a plurality of rollers which convey the document separated by the document separating means  12 , a document delivery means  14  for delivering the document conveyed by the document conveying means  13 , a document delivery table  15  on which the document delivered by the document delivery means  14  is to be placed, and a document reversing means  16  comprised of a reversing roller pair for turning over the document in the double-sided copy mode. 
     A plurality of document sheets (not shown) placed on the document table  11  are separated one by one by the document separating means  12 , and are conveyed by the document conveying means  13  toward an image reading position. 
     The document reading position is located below the document conveying means  13 . At this position, the image of the document is read through a slit  21  of the image reading unit  2 . 
     The document from which the image has been read is delivered onto the document delivery table  15  by the document delivery means  14 . 
     When reading images on the two sides of the document, the document from which the image on one side has been read is guided to the document reversing means  16 . When the trailing end of the document is clamped by the reversing roller pair constituting the document reversing means  16 , the reversing roller pair is rotated in the reverse direction to turn over the document. Then, the document is conveyed by the document conveying means  13  again, and the image on the other side (second surface) can be read at the document reading position. 
     This process is repeated a number of times corresponding to the number of the document sheets placed on the document table  11 . 
     The automatic document feeder  1  can be raised. When the automatic document feeder  1  is raised upright to open up the space above a platen glass plate  22 , a document can be placed directly on the platen glass plate  22  and be copied. 
     The image reading unit  2  serves to read the image of the document to obtain image data. The image reading unit  2  has a first mirror unit  23  formed by integrating a lamp  231  which illuminates the document through the slit  21  and a first mirror  232  which reflects light from the document, a second mirror unit  24  formed by integrating a second mirror  241  which reflects light from the first mirror  232  and a third mirror  242 , an image forming lens  25  which causes the light reflected by the second mirror unit  24  to form an image on a CCD (to be described later) serving as an imaging element, and a linear CCD  26  which obtains image data by photoelectrically converting the optical image formed by the image forming lens  25 . 
     The image data is subjected to an appropriate image process, and is then stored once in a memory (not shown). 
     When the document which is being fed by the automatic document feeder  1  is to be read by the image reading unit  2 , the first and second mirror units  23  and  24  are fixed at positions shown in FIG.  1 . 
     When the image of the document directly placed on the platen glass plate  22  is to be read, the image is read by moving the first and second mirror units  23  and  24  along the platen glass plate  22  while maintaining their optical path lengths. 
     The image forming section  3  is an image forming region where a toner image formed by using an electrophotographing process is transferred onto a sheet. The image forming section  3  has a photosensitive drum  31  having a photoconductive photosensitive layer serving as an image carrier on its surface, a charging unit  32  for uniformly charging the surface of the photosensitive drum  31 , a laser write system  33  serving as an exposure means which is operated based on the image data after image processing and exposes the photosensitive drum  31  to form an electrostatic charge latent image, a developing unit  34  for reversely developing the electrostatic charge latent image formed on the photosensitive drum  31  to form a toner image, a transfer electrode  35  for transferring the visible toner image onto the sheet, a discharging unit  36  for performing AC corona discharge from the reverse surface of the sheet, on which the toner image has been transferred, thus promoting separation of the sheet from the photosensitive drum  31 , a cleaning means  37  for cleaning the photosensitive drum  31  after the transfer step, and the like. 
     Reference numeral  38  denotes a convey belt for conveying the separated sheet toward a heat roller type fixing unit  9 ; and  63 , delivery rollers. 
     In the above arrangement, if an image is formed by transferring a toner image onto the sheet, the constituent members, ranging from the image carrier  31  to the transfer electrode  35 , make up the image forming means. 
     The fixing unit  9  has, as a main element, a fixing processing means comprised of an upper fixing roller  900  which incorporates a heating source H and rotates about it independently, and a lower fixing roller  903  which rotates in tight contact with the upper fixing roller  900 , and integrally has fixing delivery rollers  61 , a switching means/first regulating means (to be referred to as switching means or first regulating means depending on the situation hereinafter, and will be referred to as the switching means in the description of FIG.  1  and as the first regulating means in the description of FIG. 2)  62 , having the function of a switching means (switching member) which selectively switches the sheet convey path and the function of a regulating member which regulates the behavior of the sheet under conveyance, a curl correcting means, and the like. 
     The curl correcting means is arranged at a position before convey rollers  600  provided downstream of the fixing delivery rollers  61 . The curl correcting means is omitted in FIG. 1 due to the limited space, and will be described with reference to FIG. 2 together with a second regulating member  65  according to the present invention which is arranged to oppose the first regulating member  62 , and the like. 
     Reference numeral C in FIG. 1 denotes a cleaning web provided in contact with the surface of the upper fixing roller  900 . The cleaning web C is taken up little by little by a take-up core shown on the left side of FIG. 1 at an appropriate time interval during the operation of the apparatus. 
     Image formation with the above arrangement is achieved in the following manner. The photosensitive drum  31  which is rotated by an appropriate driving means in a direction indicated by an arrow is sequentially charged by the charging unit  32 . After that, the laser write system  33  performs dot exposure to form an electrostatic charge latent image corresponding to the document image on the photosensitive drum  31 . The developing unit  34  develops the electrostatic charge latent image into a toner image. Then, the toner image is transferred, through the operation of the transfer electrode  35 , onto a sheet which is fed when registration rollers  56 , serving as the second sheet feed means, start rotation. 
     Actually, after the sheet arrives at the registration rollers  56 , a process of forming the toner image on the photosensitive drum  31  is started synchronously when the sheet is fed upon start of rotation of the registration rollers  56 . 
     For this purpose, the distance from the exposure portion to the transfer electrode  35  and that from the registration rollers  56  to the transfer electrode  35  are set equal so that the toner image and the sheet overlap at the transfer region where the transfer electrode  35  exists. Also, the linear velocities of the photosensitive drum  31 , the registration rollers  56 , and pre-transfer rollers  57  are set equal. 
     The toner-image-transferred sheet is separated from the photosensitive drum  31  by the operation of the discharging unit  36 , is heated and pressed by the fixing unit  9 , and is discharged outside the copying machine. 
     The photosensitive drum  31  that has passed through the transfer region further continues rotation. The residual toner on the photosensitive drum  31  is accordingly removed by the cleaning means  37 , to prepare for next image formation. 
     Returning to the description of the arrangement, in the sheet storing section  4 , sheet feed trays  400 ,  410 , and  420  which store sheets P in a stacked state are arranged in the vertical direction (the sheets P are shown only in the sheet feed tray  400 ). The respective sheet feed trays have exclusive sheet feed rollers  401 ,  411 , and  421  and pairs of separation rollers  403 ,  413 , and  423  serving as double-feed preventive separating means such that they can be withdrawn integrally. 
     The sheet feeding section  5  has convey roller pairs (to be also referred to as convey rollers hereinafter) R 1 , R 2 , R 3 , R 4 , R 5 , and R 6  as convey means which convey the sheets P from the corresponding sheet feed trays  400 ,  410 , and  420  to the image forming section  3 . 
     The convey rollers R 1  to R 3  are preferably formed as pre-registration rollers integrally with the sheet feed trays  400 ,  410 , and  420 , and are integrally formed with them in this embodiment. 
     Reference symbols PS denote photosensors. For example, a photosensor PS has a function of detecting whether or not a sheet P fed from the sheet feed tray  400  by the sheet feed roller  401  has reached the convey roller pair R 1  formed downstream of the separation rollers  403 . This photosensor PS is disposed at a position immediately before the convey rollers R 1 . 
     Reference numeral  55  denotes convey rollers provided downstream of the convey rollers R 4  in the convey direction of the sheet P. The convey rollers  55  are formed at a merge portion of a convey path for a sheet fed again through the reversal convey section  8  and a convey path for a sheet fed from, e.g., the sheet feed tray  400 . 
     Reference numeral  56  denotes the registration rollers serving as the second sheet feed means; and  57 , the pre-transfer rollers. 
     The reversal delivery/re-feeding section  6  is a region where a sheet after transfer and fixing is reversely delivered and the sheet P is fed again in accordance with the double-sided image formation mode. The reversal delivery/re-feeding section  6  has the switching means (already described)  62  which switches the convey paths when the sheet P delivered by the fixing delivery rollers  61  is to be directly delivered outside the copying machine, when the sheet is to be turned over and then delivered, and when the sheet P is to be fed again toward the registration rollers  56  so that an image is formed on the reverse surface (second surface) of the sheet P. 
     When the sheet P on which an image has been formed is to be delivered directly, i.e., with its image-transferred surface facing up, the switching means  62  is held at the position indicated by an alternate long and short dashed line in FIG.  1 . When the sheet P on which an image has been formed is to be turned over and delivered, the switching means  62  is held at the position indicated by a solid line in FIG.  1 . The sheet P conveyed by the fixing delivery rollers  61  is fed to a mid position (portion) of the convey path of the reversal delivery/re-feeding section  6  provided with the rollers  600  and rollers  610 . The operation of the roller group is stopped simultaneously when the trailing end of the sheet P reaches a position before the convey rollers  600 . After that, the convey rollers  600  are rotated in the opposite direction to that described above. As a result, the sheet P is passed on the left side of the switching means  62  and is delivered to a delivery tray  64  outside the copying machine through the delivery rollers  63 . 
     In the double-sided image formation mode for forming an image on the second surface of the sheet successively to the first surface, the switching means  62  is held at the position indicated by the solid line in FIG.  1 . The sheet P conveyed by the fixing delivery rollers  61  is fed to the reversal convey section  8  through the respective convey rollers  600  and  610  of the reversal delivery/re-feeding section  6  which are driven by a delivery motor. After the sheet P is turned over at the reversal convey section  8 , it is fed out toward the registration rollers  56 . 
     The process that takes place while a transfer image is obtained on the second surface of the sheet P is the same as that described above. After the fixing process; the sheet P is delivered in either manner described above. 
     The reversal convey section  8  has a convey roller pair  800  which is controlled to drive so that it feeds the sheet P conveyed by the convey rollers  610  further to the right, is stopped rotating when it clamps the trailing end of the sheet P, and is then rotated in the reverse direction. The sheet P is conveyed along a convey path which extends from a mid point of conveyance to the left by the reverse rotation of the convey roller pair  800 , forms an arc upward, and then extends to the right. 
     Reference numerals  803 ,  805 ,  807 ,  809 , and  811  as well as  800  denote a plurality of convey roller pairs (to be merely referred to as convey rollers hereinafter) formed along the convey path of the reversal convey section  8 . 
     FIG. 2 is an enlarged descriptive view showing the arrangement of the main part of the fixing unit. The right side in FIG. 2 corresponds to the inlet of the sheet P which is conveyed by the convey belt  38  after the transfer process. In FIG. 2, the same members (means) as those that are already described are denoted by the same reference numerals, and a repetitive description will be omitted unless it is necessary. 
     In FIG. 2, reference numerals  905  and  907  denote inlet side upper and lower guides, respectively. 
     For example, the upper fixing roller  900  described above is obtained by forming a PFA layer on a hollow metal core made of aluminum and coating the surface of the PFA layer with fluoride. The lower fixing roller  903  is obtained by forming a silicone rubber layer on an iron metal core. 
     Fixing temperature control can be performed by using a known technique that detects the surface temperature of the upper fixing roller  900  with a temperature sensor. 
     Position-fixed upper and lower guide members  910  and  920  oppose each other at the outlet of the two fixing rollers  900  and  903 . 
     Reference numeral  913  denotes an upper separation pawl  913 . The upper separation pawl  913  is controlled by a coil spring and stopper (neither is shown) so that its distal end comes close to or into light contact with part of the outer surface of the upper fixing roller  900 . 
     A plurality of upper separation pawls  913  are arranged at appropriate intervals in the longitudinal direction of the upper fixing roller  900 . 
     With the above arrangement, even when the sheet P after the fixing process attaches to the upper fixing roller  900  and moves upward, it is separated at its leading end from the outer surface of the upper fixing roller  900  by the upper separation pawls  913 , and is sent to the left while being regulated by the lower bellies (guide portions) of the upper separation pawls  913  and a tilted guide surface  911  of the upper guide member  910 , so that it is eventually sent out while being supported by the guide portion of the lower guide member  920 . 
     The lower guide member  920  has a position-fixed first member  921  and a vertically movable second member  925 . 
     More specifically, the first member  921  rises as it extends downstream (to the left in FIG. 2) from an upper portion (also called an upstream edge)  922  of its vertical wall on the lower fixing roller  903  side, continues to a substantially horizontal guide surface, and terminates at its end (also called a downstream edge)  923 . 
     In other words, the first member  921  is formed such that the height of the downstream edge  923  is higher than that of the upstream edge  922  when seen along the moving direction of the sheet P. 
     This aims at obtaining an angle with which water vapor is prevented as much as possible from attaching to the tilted guide surface, which starts from the upstream edge  922  of the first member  921 , and forming water droplets to stay on the tilted guide surface. In this embodiment, the angle formed by the tilted guide surface and the horizontal line is 20°. 
     The first member  921  is formed of a metal sheet, more particularly, an iron sheet, has a plurality of steps and depressions formed by drawing in the tilted guide surface (guide portion of the tilted surface), and is subjected to surface treatment by nickel plating. 
     Reference numeral  700  denotes steps;  710 , depressions; and  720 , rises. 
     Reference numeral  925  denotes the second member. The second member  925  has a gutter-like cross section, is entirely located closer to the lower fixing roller  903  than the first member  921 , and is integrated with the first member  921  by coil springs  750  provided with respect to the first member  921 . 
     The second member  925  can move perpendicularly (vertically) to the first member  921 . 
     The narrow upper side of the second member  925  has a tilted surface that rises downstream from an upper portion (also called an upstream edge)  926  of its wall close to the lower fixing roller  903  and terminates at its end (also called a downstream edge)  927 . 
     The angle formed by the upper side of the second member  925  and the horizontal line is about 30° in this embodiment. 
     As is apparent from FIG. 2, when the second member  925  is at the upward position (fixed position), the height of its downstream edge  927  is higher than the upstream edge  922  of the first member  921  that opposes the downstream edge  927 . Accordingly, a gap S is formed between the downstream and upstream edges  927  and  922 . 
     The gap S has a function of causing the water droplets dropping from the guide surface of the first member  921  to flow onto the second member  925 . 
     The water droplets flowing onto the second member  925  are collected in an appropriate bottle or the like. 
     The height of the upstream edge  926  of the second member  925  is lower than the upstream edge  922  of the first member  921 , in other words, is on the extension line of the tilted guide surface of the first member  921 . 
     As described above, regarding the guide surfaces of the first and second members  921  and  925 , their downstream edges ( 923  and  927 ) are higher than their upstream edges ( 922  and  926 ). In the lower guide member  920 , the downstream edge  923  is higher than the upstream edge  926 . 
     Reference numeral  929  denotes a lower separation pawl. The lower separation pawl  929  is supported by the first and second members  921  and  925  such that its distal end comes close to or into light contact with part of the outer surface of the lower fixing roller  903 . 
     More specifically, the bottom of the lower separation pawl  929  is pushed up by the second member  925  in accordance with the function of the coil springs  750 , which lie between a locking member (two projecting piece members formed at a distance from each other such that they project by utilizing the upper thin portion of the separation pawl  929  while their portions close to that base portion of the separation pawl  929  which is thicker than its upper portion do not project; denoted by  724  in FIG. 4) formed on the first member  921  and the gutter-like bottom side of the second member  925  and are arranged between the first and second members  921  and  925 , so that the lower separation pawl  929  maintains a predetermined posture through locking of part of its base portion and the locking members  724 . 
     The lower separation pawl  929  includes a plurality of separation pawls which are arranged at appropriate intervals in the longitudinal direction of the lower fixing roller  903 . 
     In this embodiment, the top portion (upper side) of each lower separation pawl  929  forms a tilted surface which has a size almost covering the entire region occupied by the upper side of the second member  925  in the sheet convey direction, is high, and rises downstream. 
     Reference numeral  61  denotes the fixing delivery rollers; and  62 , the switching means/first regulating member (to be referred to as the first regulating member in the description of FIG. 2, as described above). The first regulating member  62  is located at such a position in FIG. 2 where it directs the sheet P after the fixing process toward a portion for reversal delivery or re-feeding. 
     In other words, the sheet P after the fixing process is guided by the upper and lower guide members  910  and  920 , is conveyed through the fixing delivery rollers  61  having a nip at almost the same height as those of the upper and lower guide members  910  and  920 , is guided downward obliquely to the left (downward obliquely forward in the convey direction) to draw an arc while being regulated by one regulating surface (the same meaning as the guide surface)  620  which regulates the sheet P on the first regulating member  62  and a regulating surface  650  of the second regulating member  65  opposing the first regulating member  62 , and approaches a predetermined position of a curl correcting means  68  provided below (downstream of) the two regulating members  62  and  65  at a predetermined angle. 
     The regulating surface  620  of the first regulating member  62  has a length that opposes the lower fixing delivery roller  61  as well. 
     The sheet convey path formed by the regulating surfaces of the first and second regulating members  62  and  65  has a wide inlet  660  and narrows toward its outlet  663 . 
     This aims at easy acceptance of a sheet P which is slightly curled during the fixing process and at determining the sheet sending direction in a predetermined direction gradually. 
     The outlet end of the second regulating member  65  has a length projecting from the outlet end of the first regulating member  62 , but the present invention is not limited to this. 
     The first and second regulating members  62  and  65  are both molded products made of PBT (polybutylene terephthalate). 
     In this embodiment, the second regulating member  65  which often comes into contact with the sheet P is formed of a plurality of divisional portions, preferably three or more divisional portions (which are called regulating components), that are arranged in the widthwise direction (in the obverse-to-reverse direction in FIG.  2 ). The gap formed by the regulating surface  620  of the first regulating member  62  and the regulating surface  650  of the second regulating member  65  is larger at its center than at its ends when seen in the widthwise direction. 
     The two regulating members  62  and  65  have sectional shapes each like half of a hanging bell, as shown in FIG. 2, which are substantially of the same size. 
     Reference numeral  68  denotes the curl correcting means having a rotatable sponge roller  680  and a press member  690  formed of a fixed round stainless steel (SUS) rod. Part of the outer surface of the press member  690  bites into the outer circumference of the sponge roller  680 . 
     In addition to the above arrangement, the sponge roller  680  is driven to rotate at a linear velocity larger than those of the upper fixing rollers ( 900  and  903 ) so as to draw the approaching sheet P, thus correcting the curl. 
     Referring to the schematic enlarged view of FIG. 3 which shows the arrangement of the curl correcting means  68  and its vicinity, the regulating surface  620  of the first regulating member  62  is formed of an arcuate recessed surface formed on the sheet approaching side, and a flat surface formed on the outlet side continuously to the arcuate recessed surface. A position  681  where an extension line L 1  (indicated by an alternate long and two short dashed line) of this flat surface hits the outer surface of the sponge roller  680  corresponds to the position of one end of a region where it is appropriate for the leading end of the sheet being sent to abut against the sponge roller  680 . 
     More specifically, this indicates that position  681  on the outer surface of the sponge roller  680  which is tilted to the right (opposite to the rotating direction of the sponge roller  680 ) at an angle θ 1  (5°) from a reference line K that connects a center C of the sponge roller  680  and a press start position (the upstream end which is in contact with the press member  690  in the rotating direction of the sponge roller  680 ; corresponding to the contact end of the sheet-approaching-side sponge roller  680  and the press member  690 )  691  of the sponge roller  680  and press member  690 . 
     The regulating surface  650  of the second regulating member  65  is formed of an arcuate projecting surface formed on the sheet approaching side, and a flat surface formed on the outlet side continuously to the arcuate projecting surface. A position  683  where an extension line L 2  (indicated by an alternate long and two short dashed line) of this flat surface hits the outer surface of the sponge roller  680  corresponds to the position of the other end of a region where it is appropriate for the leading end of the sheet being sent to abut against the sponge roller  680 . 
     More specifically, this indicates that position  683  on the outer surface of the sponge roller  680  which is tilted to the right (opposite to the rotating direction of the sponge roller  680 ) at an angle θ 2  (10°) from the reference line K. 
     In other words, it is desirable that the leading end of the sheet P is not directed toward the press start position  691  but is regulated so as to abut against the outer surface of the sponge roller  680  within an angular range of 5° to 10° (both inclusive) from the reference line K. 
     If this angle is less than 5°, the leading end of the sheet P tends to easily hit the press member  690 . Then, it is difficult to send the sheet P to the nip starting from the press start position  691 . Paper jamming, sheet waving, or corner bent tends to occur easily. 
     If this angle exceeds 10°, the two corners at the leading end of the sheet may be bent, or paper jamming occurs easily. 
     These inconveniences are observed often particularly with a sheet having two sides which are curled by the fixing process or the like. 
     In this embodiment, the sponge roller  680  had a rubber hardness of 2 degrees to 12 degrees (according to the Asker C measurement). The bite amount (the amount of squeeze of the sponge roller  680 , which indicates an amount from the regular outer circumference to the push-in position) of the press member  690  into the sponge roller  680  was 2.0 mm at the deepest portion. The sponge roller  680  had a diameter of 20.3 mm. The press member  690  had a diameter of 8 mm. The fixing rollers  900  and  903  had linear velocities of 320 mm/s. The sponge roller  680  had a linear velocity of 320 mm/s to 329 mm/s. The fixing rollers  900  and  903  had fixing control temperatures of 175° C. to 205° C. 
     The softer the rubber, the more easily the sponge roller  680  squeezes, and the larger the drawing effect. The rubber hardness within the above range is suitable because it allows easy handling of the sponge roller  680  and easy management of the linear velocities. 
     FIG. 4 is a schematic plan view including the second regulating member  65 , lower guide member  920 , and fixing delivery rollers  61 , and is employed particularly for describing the arrangement of the second regulating member  65 . 
     In FIG. 4, the same members (means) as those that are already described are denoted by the same reference numerals. 
     Reference numerals  651 ,  652 ,  653 , and  654  denote divisional regulating components for making up the second regulating member  65 , which are fixed in an array in the widthwise direction (left-to-right direction in FIG. 4) on a support  657 . 
     For example, position fixing and position regulation can be achieved by forming holes in the support  657 , forming a projection on one end of each of the components  651  to  654 , fitting the holes and projections, adjusting, and fixing the support  657  and components  651  to  654  to each other with machine screws. 
     The adjacent regulating components are arranged at an appropriate distance. The lowest ends of the regulating surfaces of the regulating components  651  and  654  at the left and right ends are located on a reference line  655  indicated by an alternate long and two short dashed line. The lowest ends of the regulating surfaces of the regulating components  652  and  653  at the center are located at positions retreated from the reference line  655 . 
     In other words, the regulating surfaces of the regulating components  651  and  654  at the ends are located close to the regulating surface  620  of the first regulating member  62  shown in FIG. 2, while the regulating surfaces of the regulating components  652  and  654  at the center are located far from the regulating surface  620  of the first regulating member  62 , so that the convey path is large in the widthwise direction at the center. 
     The second regulating member  65  is divided as described above, so that when it is formed of one resin member, its warp based in thermal expansion occurring in the widthwise direction is decreased. The positions of the regulating components  652  and  653  at the center are set far from the positions of the regulating members at the two ends because thermal expansion occurs also in a direction perpendicular to the widthwise direction and because the influence of thermal expansion appears largely particularly at the center. Hence, the inconveniences in sheet conveyance can be suppressed. 
     The distances of retreat of the regulating components  652  and  653  from the reference line  655  can be determined appropriately, and are about 0.5 mm in this embodiment. 
     The fixing delivery rollers  61  form round slices which are fixed on a shaft  605 . 
     The first member  921  which constitutes the lower guide member  920  is formed of a metal sheet, and has the plurality of steps  700 , plurality of depressions  710 , and plurality of rises  720 , formed by drawing, in the longitudinal direction. 
     The “steps” refer to depressions formed in a region extending from a downstream position in a tilted guide surface  928  toward an upstream side and including the upstream edge  922  (this means that the bottom surface includes as far as the upstream edge). 
     The “depressions” are recesses (depressions) formed in the region of the tilted guide surface  928 . The “rises” are projections formed in this region, with reference to the tilted guide surface before processing, on the opposite side to the side where the steps  700  and depressions  710  are formed, and are located within the region of the tilted guide surface  928 . 
     The steps  700  are formed at positions phase-shifted from the lower separation pawls  929 . The depressions  710  are formed at positions with almost the same phase as those of the lower separation pawls  929 . The rises  720  are formed on the left and right of the steps  700  and depressions  710 . 
     Reference numerals  724  denote the locking members. The locking members  724  serve to support the lower separation pawls  929  in cooperation with the second member  925 , and are formed of part of the first member  921 . Reference numerals  750  denote the coil springs. The coil springs  750  are elements that integrate the first and second members  921  and  925  to be movable relative to each other. These elements are mentioned in the description of FIG.  2 . 
     The process in the arrangement of FIG. 2 will be briefly described by taking the arrangements of FIGS. 3 and 4 into consideration. 
     The sheet P after the transfer process is conveyed while it is regulated by the inlet side guides  905  and  907 , and is heated and pressed by the upper and lower fixing rollers  900  and  903 . Then, toner powder which forms an image on the sheet P is melted, and is fixed to the sheet P. 
     The leading end of the sheet P after the fixing process is fed out by the operation of the lower or upper separation pawls  929  or  913 , or the like, and is regulated by the first and second regulating members  62  and  65 , to abut against a predetermined region on the outer surface of the sponge roller  680  which forms the curl correcting means. 
     After this, the sheet P is conveyed by the rotation of the sponge roller  680 , is sent to the nip with respect to the press member  690 , is subjected to a drawing operation (curl correcting operation), and is fed out toward a predetermined portion of the reversal delivery/re-feeding section  6  (see FIG.  1 ). 
     In this embodiment, the switching means for the sheet convey path serves as the first regulating member as well. However, a switching means and a first regulating member can apparently be formed of separate members. The hardness and size (diameter) of the sponge roller, the amount of squeeze of the sponge roller, the size (diameter) of the press member, the linear velocities of the fixing rollers and sponge roller, and the like can also be determined appropriately.