Patent Publication Number: US-6669189-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and more particularly to an image forming apparatus of the type operable in a duplex print mode and capable of stacking prints face down in order of page. 
     2. Description of the Background Art 
     An electrophotographic copier, printer, facsimile apparatus or similar image forming apparatus of the type described usually includes a first path and a second path arranged at the sheet discharge side of an image forming section  102 . The first path conveys a sheet carrying an image thereon toward a sheet outlet section while the second path conveys it in a duplex print mode or to reverse the sheet and then discharge it. A path selector is located at the position where the first and second paths part from each other. The path selector selectively steers a sheet coming out of the image forming section to the first path or the second path. 
     The second path merges into a switchback path that reverses the sheet by switching it back. A reverse roller is positioned on the switchback pack and movable into and out of contact with the sheet (up-and-down direction). In the duplex print mode, the refeed path again conveys the sheet switched back by the switchback path toward the image forming section. The reverse discharge path discharges the above sheet to a sheet outlet section face down in order of page. The refeed path is positioned below the switchback path. A path selector is located at a position where the refeed path and reverse discharge path part from each other. This path selector selectively steers the sheet switched back by the switchback path to the refeed path or the reverse discharge path. 
     To reverse the sheet carrying an image on one side thereof and then discharge it, the path selector again delivers the sheet to the second path. In this case, the path selector is so positioned as to steer the sheet driven out of the switchback path to the reverse discharge path. As a result, the sheet is driven out to the sheet outlet section face down via the reverse discharge path. 
     The conventional image forming apparatus described above has undesirably great height because of the switch back path and refeed path arranged one above the other. 
     Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 6-236086 and Japanese Patent No. 2,941,021. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an image forming apparatus capable of enhancing the productivity of image formation in a repeat print mode by reducing the interval between consecutive sheets at an image forming section, and obviating the need for an exclusive switchback path for a duplex print mode. 
     An image forming apparatus of the present invention includes a sheet tray loaded with a stack of sheets. A separating and feeding device feeds one sheet from the sheet tray while separating it from the other sheets. An image forming section forms an image on the sheet fed by the separating and feeding device. A conveyance path conveys the sheet from the sheet tray to the image forming section. A switchback path is connected to the intermediate portion of the conveyance path for receiving the sheet being conveyed along the conveyance path. A reversing device is selectively switchable to a first position for switching back the preceding sheet introduced into the switchback path to thereby feed it to the conveyance path or a second position for allowing the preceding sheet being fed from the switchback path and the following sheet to be introduced into the switchback path after the preceding sheet to at least partly overlap each other. A refeed path is connected to the sheet outlet side of the image forming section and part of the conveyance path upstream of the switchback path. A conveying device conveys the sheet driven into the refeed path to the conveyance path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a view showing a conventional image forming apparatus; 
     FIG. 2 is a view showing an image forming apparatus embodying the present invention; 
     FIG. 3 is an enlarged front view of a mechanism arranged along a conveyance path in the illustrative embodiment; 
     FIG. 4 is an isometric view of an inlet roller and a path selector; 
     FIG. 5A shows reversing means included in the illustrative embodiment in an open position; 
     FIG. 5B shows the reversing means in a closed position; 
     FIG. 6 is an isometric view showing a roller and another path selector also included in the illustrative embodiment; 
     FIGS. 7 and 8 are views showing how consecutive sheets are sequentially fed to an image forming section included in the illustrative embodiment; 
     FIG. 9 is a view showing how a sheet is fed in a duplex print mode; 
     FIG. 10 shows the order of sheet feed and the interval between sheets, as seen at the junction of the conveyance path and a refeed path in a two-sheet interleaf, duplex print mode; 
     FIG. 11 is a view how a sheet is fed in a reverse discharge mode; 
     FIG. 12 is a diagram showing the feed of sheets in a repeat print mode; 
     FIG. 13 is a view showing the feed of sheets in the reverse discharge mode unique to an alternative embodiment of the present invention; 
     FIGS. 14 and 15 are views demonstrating sheet feed in the reverse discharge mode particular to the embodiment shown in FIG. 13; 
     FIG. 16 is a view showing a switchback mechanism representative another alternative embodiment of the present invention; and 
     FIG. 17 is a view showing a switchback mechanism representative of a further alternative embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To better understand the present invention, brief reference will be made to a conventional image forming apparatus, shown in FIG.  1 . The image forming apparatus to be described is of the type operable in a duplex print mode and capable of discharging prints face down in order of page. As shown, the image forming apparatus includes an image forming section  102  including a photoconductive drum  100  and a developing device  101 . The image forming section  102  prints an image on a sheet. 
     A first path  104  and a second path  105  are arranged at the sheet discharge side of the image forming section  102 . The first path  104  conveys the sheet carrying an image thereon toward a sheet outlet section  103  while the second path  105  conveys it in the duplex print mode or to reverse the sheet and then discharge it. A path selector  106  is located at the position where the first path  104  and second path  105  part from each other. The path selector  106  selectively steers the sheet coming out of the image forming section  102  to the first path  104  or the second path  105 . 
     The second path  105  merges into a switchback path  107  that reverses the sheet by switching it back. A reverse roller  108  is positioned on the switchback pack  107  and movable into and out of contact with the sheet (up-and-down direction). 
     A refeed path  109  and a reverse discharge path  110  are arranged downstream of the switchback path  107  in the direction of sheet conveyance. In the duplex print mode, the refeed path  109  again conveys the sheet switched back by the switchback path  107  toward the image forming section  102 . The reverse discharge path  110  reverses the above sheet and then discharges it to the sheet outlet section  103  face down in order of page. The refeed path  109  is positioned below the switchback path  107 . A path selector  111  is located at a position where the refeed path  109  and reverse discharge path  110  part from each other. The path selector  111  selectively steers the sheet switched back by the switchback path  107  to the refeed path  109  or the reverse discharge path  110 . 
     In a duplex print mode, the path selector  106  is so positioned as to steer a sheet carrying an image one side thereof toward the second path  105 . In this condition, the sheet is conveyed to the switchback path  107  via the second path  105 . The roller  108  switches back the sheet out of the switchback path  107 . At this instant, the path selector  111  is so positioned as to steer the sheet coming cut of the switchback path  107  to the refeed path  109 , so that the sheet is again conveyed to the image forming section  102 . After an image has been formed on the other side of the sheet, the sheet is driven out to the sheet outlet section  103  via the first path  104 . 
     To reverse the sheet carrying an image on one side thereof and then discharge it, the path selector  106  again delivers the sheet to the second path  105 . In this case, the path selector  111  is so positioned as to steer the sheet driven out of the switchback path  107  by the roller  108  to the reverse discharge path  110 . As a result, the sheet is driven out to the sheet outlet section  103  face down via the reverse discharge path  110 . 
     The conventional image forming apparatus described above has great height because of the switch back path  107  and refeed path  109  arranged one above the other, as stated earlier. 
     Referring to FIGS. 2 through 12, an image forming apparatus embodying the present invention will be described. As shown in FIG. 2, the image forming apparatus includes a sheet tray  1  loaded with a stack of sheets. An image forming section  2  forms an image on one side of a sheet fed from the sheet tray  1  or the other side of a sheet carrying an image on one side thereof and again fed thereto. A sheet outlet section  3  drives the sheet coming out of the image forming section  2  out of the apparatus. A plurality of paths are arranged inside of the apparatus for conveying a sheet between the sheet tray  1 , the image forming section  2 , and the sheet outlet section  3 . 
     Specifically, a conveyance path  4  extends from the sheet tray  1  to the image forming section  2  and branches into a switchback path  6  at a point  5 . The switchback path  6  switches back the sheet being conveyed along the conveyance path  4 . More specifically, the sheet conveyed along the conveyance path  4  is steered into the switchback path  6  via the point  5  and then again returned to the path  4  by reversing means, which will be described later specifically. 
     A roller pair  7  is positioned upstream of the point  5  of the path  4  in the direction of sheet conveyance and rotated by a motor not shown. An inlet roller  8 , an inlet sensor  9  and a path selector  10  are located around the point  5 . A motor, not shown, causes the inlet roller  8  to rotate for introducing the sheet into the switchback path  6 . The inlet sensor  9  is positioned upstream of the inlet roller  8  in the direction of sheet conveyance for sensing the leading edge of the sheet. The path selector  10  steers the sheet from the conveyance path  4  to the switchback path  6  or steers it from the path  6  to the path  4 . In the illustrative embodiment, the sheet sensor  9  is implemented as a reflection type optical sensor made up of a light emitting device and a light-sensitive device, although not shown specifically. 
     An intermediate roller pair  11  is positioned downstream of the point  5  in the direction of sheet conveyance. A motor, not shown, drives the roller pair  11  for conveying the sheet coming out of the switchback path  6  toward the image forming section  2 . A registration sensor  12  and a registration roller pair  13  are positioned downstream of the roller pair  11 . The registration roller pair  13  starts conveying the sheet sensed by the registration sensor  12  to the image forming section  2  in synchronism with the operation of the section  12 . The registration sensor  12  is also a reflection type optical sensor. 
     A reversing device, or the previously mentioned reversing means,  14  and a reversal sensor  15  are positioned on the switchback path  6 . The reversing device  14  selectively takes a closed or feed position or an open position. In the closed position, the reversing device  4  again feeds the sheet from the switchback path  6  to the conveyance path  4 . In the open position, the reversing device  14  allows the sheet being again fed to the conveyance path  4  and the subsequent sheet being introduced into the switchback path  6  to at least partly overlap each other. The reversal sensor  15  is responsive to the leading edge of the sheet being introduced into the switchback path  6  and is also implemented by a reflection type optical sensor. 
     FIGS. 5A and 5B show the reversing device  14  specifically. As shown, the reversing device  14  is made up of a reverse roller  16  and a driven roller  17 . A stepping motor, not shown, causes the reverse roller  16  to intermittently rotate. The driven roller  17  contacts the reverse roller  16  with the intermediary of the switchback path  6  and is driven by the reverse roller  16 . Part of the circumference of the reverse roller  16  is removed, forming a flat face  16   a . As shown in FIG. 5A, when the flat face  16   a  faces the driven roller  17 , the former and latter form a gap therebetween. 
     As shown in FIG. 5, the reverse roller  16  is rotated in a direction indicated by an arrow in contact with the driven roller  17 , causing the driven roller  17  to rotate. In this condition, the reverse roller  16  and driven roller  17  convey a sheet toward the conveyance path  4 . In the condition shown in FIG. 5, a sheet is introduced into the switchback path  6  via the gap between the reverse roller  16  and the driven roller  17 . It should be noted that a sheet moves at a higher speed when fed from the conveyance path  4  to the switchback path  6  than when fed from the latter to the former. 
     Referring again to FIG. 2, a pickup roller  18  pays out the sheets from the sheet tray one by one while a reverse roller  19  separates one sheet being paid out from the other sheets. The pickup roller  18  and reverse roller  19  constitute a separating and feeding device  20 . 
     The image forming section  2  includes a photoconductive drum  21 . A charger  22  uniformly charges the surface of the drum  21 . A digital, optical writing unit or means  23  optically writes a latent image on the charged surface of the drum  21 . A developing unit  24  develops the latent image with toner to thereby form a corresponding toner image. An image transferring device transfers the toner image from the drum  21  to the sheet. A drum cleaner  26  removes toner left on the drum  21  after the image transfer. A fixing device  27  fixes the toner image on the sheet. The image forming section  2  executes a digital, electrophotographic image forming process. 
     As shown in FIG. 3, the path selector  10  located at the point  5  has a generally triangular contour including a first guide surface  10   a , a second guide surface  10   b , and a tip  10   c  with an acute angle between the first and second guide surfaces  10   b . The first guide surface  10   a  guides the sheet being fed into the switchback path  6  while the second guide surface  10   b  guides the sheet being fed out of the switchback path  6 . The tip  10   c  races the switchback path  6 . 
     As shown in FIG. 4, a plurality of path selectors  10  each having the contour shown in FIG. 3 are mounted on a shaft  28 , which adjoins and extends in parallel to a shaft  8   a  supporting the inlet roller  8 . The inlet roller  8  is also implemented as a plurality of rollers  8 , as illustrated. The shaft  28  is rotatably supported by bearings not shown. An arm  29  is connected at one end to one end of the shaft  28 . A spring  30  is anchored at one end to the other end of the arm  29 . A stop  31  restricts the rotation of the arm  29  being constantly biased downward by the spring  30 . 
     The shaft  28  is rotatable to selectively move the path selectors  10  to a first position indicated by a solid line in FIG. 3 or a second position indicated by a phantom line in FIG.  3 . In the first position, the sheet introduced into the switchback path  6  angularly moves the path selectors  10  due to its own elasticity against the bias of the spring  30  and passes through the gap between the inlet rollers  8  and the first guide surfaces  10   a  of the path selectors  10 . In the second position, no sheets are present between the inlet rollers  8  and the first guides  10   a  of the path selectors  10 ; the arm  29  abuts against the stop  31  due to the bias of the spring  30  with the tips  10   c  of the path selectors  10  being positioned radially inward of the circumferences of the inlet rollers  8 . Let the inlet rollers  8  and path selectors  10  be respectively represented by a single inlet roller  8  and a single selector  10  for simplicity hereinafter. 
     As shown in FIG. 2, a direct discharge path  32  extends between the sheet discharging side of the image forming section  2  and the sheet outlet section  3 . A refeed path  33  extends between the sheet discharging side of the image forming section  2  and part of the conveyance path  4  upstream of the point  5  in the direction of sheet conveyance. A path selector  34  is located at a point where the direct discharge path  32  and refeed path  33  part from each other at the sheet discharging side of the image forming section  2 . The path selector  34  selects either one of the direct discharge path  32  and refeed path  33 . 
     The refeed path includes a vertical portion  33   a  extending downward and a horizontal portion  33   b  extending from the lower end of the vertical portion  33   a  in substantially the horizontal direction The end of the horizontal portion  33   b  remote from the vertical portion  33   a  merges into the conveyance path  4 . 
     A reverse discharge path  35  branches from the refeed path  33  at the bent portion between the vertical portion  33   a  and the horizontal portion  33   b . The reverse discharge path  35  is connected to the sheet outlet section  3 . 
     A refeed roller  36  is positioned on the horizontal portion  33   b  of the refeed path  33 . The refeed roller  36  plays the role of conveying means and reverse discharging means at the same time. Specifically, the refeed roller  36  is selectively rotatable in the forward direction for feeding the sheet out of the refeed path  33  to the conveyance path  4  in the duplex print mode (duplex-print feed state) or rotatable in the reverse direction for switching back the sheet toward the reverse discharge path  35  (reverse discharge state). More specifically, the refeed roller  36  is implemented by a plurality of roller pairs  37  each having a reversible drive roller and a driven roller movable into and out of contact with the drive roller, as illustrated. The drive rollers of the roller pairs  37  each can be driven independently of the others. The refeed roller  36  is capable of nipping the sheet on the horizontal path  33   b  to thereby interrupt conveyance and again driving it toward the conveyance path  4  at a suitable timing in response to, e.g., the output of a sheet sensor not shown. 
     A roller  38  and a path selector  39  are located at a position where the refeed path  33  merges into the reverse discharge path  33 . The roller  38  is rotated to convey the sheet from the vertical portion  33   a  to the horizontal portion  33   b  of the refeed path  33 . As shown in FIG. 6, the path selector  39  is also implemented as a plurality of path selectors  39  mounted on a shaft  40 , which adjoins and extends in parallel to a shaft  38   a  supporting the roller  38 . The roller  38  is also implemented as a plurality of rollers  38 . Each path selector  39  has a generally triangular contour including a first guide surface  39   a  for guiding the sheet being conveyed from the vertical portion  33   a  to the horizontal portion  33   b , a second guide surface  39   b  for guiding the sheet being switched back toward the reverse discharge path  35 , and a tip  39   c  with an acute angle between the two guide surfaces  39   a  and  39   b . An arm  41  is connected at one end to one end of the shaft  40 . A spring  42  is anchored at one end to the other end of the arm  41 . A stop  43  restricts the movement of the arm  41  being constantly biased downward by the spring  42 . 
     The shaft  40  is rotatable to selectively move the path selectors  39  to a first position or a second position. In the first position, the sheet advancing from the vertical portion  33   a  to the horizontal portion  33   b  angularly moves the path selectors  39  due to its own elasticity against the bias of the spring  42  and passes through the gap between the rollers  38  and the first guide surfaces  39   a  of the path selectors  39 . In the second position, no sheets are present between the rollers  38  and the first guides surface  39   a  of the path selectors  39 , as shown in FIGS. 2 and 6; the arm  41  abuts against the stop  43  due to the bias of the spring  42  with the tips  39   c  being positioned radially inward of the circumferences of the rollers  38 . In the second position, the tips  39  prevent the sheet switched back from entering the vertical path  33   a  while guiding it toward the reverse discharge path  35 . Again, let the rollers  38  and path selectors  39  be respectively represented by a single inlet roller  38  and a single selector  39  for simplicity hereinafter. 
     The sheet being switched back from the horizontal portion  33   b  to the reverse discharge path  35  and the sheet being transferred from the vertical portion  33   a  to the horizontal portion  33   b  can pass each other. More specifically, the horizontal portion  33   b  has a height great enough to allow two sheets to pass each other. In addition, assume that the refeed roller  36 , i.e., roller pairs  37  are held in the reverse discharge state for feeding the preceding sheet into the reverse discharge path  35 . Then, as soon as the following sheet enters the horizontal portion  33   b , the driven rollers of the roller pairs  37  are released from the drive rollers to allow the sheet into the horizontal portion  33   b.    
     The sheet is conveyed along the refeed path  33  at a speed higher than the image forming process speed of the image forming section  2 . For example, while the process speed of the image forming section  2  is 330 mm/sec, the sheet is conveyed along the refeed path  33  at a speed of 560 mm/sec. This is also true with the conveyance of the sheet along the reverse discharge path  35 . 
     In the configuration described above, the separating and feeding device  20  feeds one sheet from the sheet tray  1  to the conveyance path  4  while separating it from the other sheets. As soon as the inlet sensor  9  located at the point  5  senses the leading edge of the sheet, the inlet roller  8  is caused to start rotating. Further, when the leading edge of the sheet abuts against the first guide surface  10   a  of the path selector  10 , which is held in the second position, the sheet raises the path selector  10  to the first position due to its own elasticity. The sheet then advances to the switchback path  6  via the gap between the first guide surface  10   a  and the inlet roller  8 . At this instant, the reversing device  14  is held in the open position shown in FIG.  5 A. 
     As the trailing edge of the sheet moves away from the gap between the first guide surface  10   a  and the inlet roller  8 , the path selector  10  automatically returns to the second position due to the bias of the spring  30 . In the second position, the tip  10   c  of the path selector  10  surely prevents, when the sheet is driven out of the switchback path  6 , the leading edge of the sheet from entering between the inlet roller  8  and the first guide surface  10   a . This guarantees smooth feed of the sheet from the switchback path  6  toward the image forming section  2 . 
     To feed the sheet out of the switchback path  6 , the reverse roller  16  of the reversing device  14  is rotated counterclockwise, as viewed in FIG. 5B, causing the driven roller  17  to rotate. The reverse roller  16  and driven roller  17  therefore convey the sheet out of the switchback path  6  by nipping it. The intermediate roller pair  11  nips and conveys the leading edge of the sheet coming out of the switchback path  6 . At this time, the reversing device  14  is brought to the condition shown in FIG. 5A, releasing the sheet. In the position shown in FIG. 5A, the reversing device  14  allows the following sheet into the switchback path  6 . 
     The intermediate roller pair  11  conveys the sheet until the leading edge of the sheet abuts against the registration roller pair  13 . The registration roller pair  13  starts rotating at a preselected timing to convey the sheet to the image forming section  2 . 
     Reference will be made to FIGS. 7 and 8 for describing the flow of consecutive sheets to occur in a repeat print mode As shown, as soon as the preceding sheet a is driven out of the switchback path  6  toward the conveyance path  4 , the following sheet b is conveyed toward the switchback path  6 . More specifically, the reversing device  15  first conveys the sheet a and then releases it, as shown in FIG.  5 A. Subsequently, the intermediate roller pair  11  conveys the sheet a toward the image forming section  2 . On the other hand, the roller pair  7  and inlet roller  8  sequentially convey the following sheet b, so that the sheet b enters the switchback path  6  via the reversing device  14  held in the position shown in FIG.  5 A. At this instant, the trailing edge portion of the preceding sheet a and that of the following sheet b (leading edge when switched back) momentarily overlap each other. 
     After the preceding sheet a has been fully fed out of the switchback path  6 , the following sheet b is fed out of the switchback path  6  at the time when the trailing edge of the sheet a and the leading edge of the sheet are spaced by an adequate short distance. This successfully enhances the productivity of image formation. 
     FIG. 12 is a diagram demonstrating the conveyance of the consecutive sheets a and b. As shown, the roller pair  7  and inlet roller  8  convey each of the sheets a and b from the sheet tray  1  to the stop position on the switchback path  6  at a speed of Va. Subsequently, the reverse roller  16  and intermediate roller pair  11  convey the sheet from the above stop position to the image forming section  2  at a speed of Vb equal to the image forming speed. The speed Va is selected to be higher than the speed Vb. 
     The flow of a sheet to occur in the duplex print mode will be described with reference to FIG.  9 . As shown, a sheet carrying an image on one side thereof (one-sided sheet hereinafter) is conveyed from the image forming section  2  to the refeed path  33 . The refeed roller  36  conveys the one-sided sheet straight to the path  4 . Subsequently, the one-sided sheet, like a sheet fed from the sheet tray  1 , is conveyed to the switchback path  6  and then switched back toward the image forming section  2  via the conveyance path  4 . The one-sided sheet has therefore been reversed when reaching the image forming section  2 . The image forming section  2  forms an image on the other side or reverse side of the one-sided sheet, producing a two-sided or duplex print. 
     As stated above, the illustrative embodiment switches back a sheet in the duplex print mode by using the switchback path  6  that is originally directed toward high productivity, thereby obviating the need for an exclusive path for the duplex print mode. The image forming apparatus is therefore reduced in height despite the presence of the switchback path  6 . 
     The illustrative embodiment executes so-called interleaf sheet feed, i.e., interleaves a new sheet and a one-sided sheet in the duplex print mode. More specifically, in the duplex print mode, a plurality of (e.g. two or three) new sheets are continuously fed from the sheet tray  1  to the image forming section  2 . The image forming section  2  prints images on one side of the consecutive sheets in preselected order of page (e.g. the first and third pages in the case of two sheets or the first, third and fifth pages in the case of three sheets). After the resulting first one-side sheet has been positioned in the horizontal portion  33   b  of the refeed path  33 , the refeed roller  36  refeeds the one-sided sheet to the conveyance path  4  at a preselected timing. Thereafter, the one-sided sheets and new sheets paid out from the sheet tray  1  are alternately fed to the path  4 . Two-sided sheets, or duplex prints, are sequentially driven out to the sheet outlet section  3  via the direct discharge path  32 . 
     The sheet is conveyed along the refeed path  33  at a speed higher than the process speed of the image forming section  2 , as stated earlier. Therefore, in the interleaf, duplex print mode, the sheet being conveyed along the refeed path  33  can be rapidly brought to the junction of the paths  33  and  4 . This successfully reduces the interval between the sheet fed from the sheet tray  1  and the one-sided sheet fed from the refeed path  33  to the path  4  and thereby enhances the productivity of image formation. 
     FIG. 10 shows the order of sheets being conveyed and the interval between the sheets, as seen at the junction of the paths  4  and  33 , on the assumption that two sheets are continuously fed from the sheet tray  1  in the interleave, duplex print mode. Numbers attached to the sheets indicate the order of feed from the sheet tray  1  while the word “front” attached to the numbers refers to a one-sided sheet. Further, T indicates a time interval between the time when the trailing edge of a sheet fed from the sheet tray  1  (e.g. sheet ( 3 )) moves away from the junction of the paths  4  and  33  and the time when the leading edge of the sheet ( 3 ) “front” to again reach the above junction via the image forming section  2  and path  33 . 
     In the two-sheet interleaf moved, the time interval T is expressed as: 
     
       
           T =(2 D+ 3 X )/ V   Eq. (1)  
       
     
     where D denotes the length of the sheet, X denotes the distance between sheets measured at the junction of the paths  4  and  33 , and V denotes a sheet speed also measured at the junction. It is to be noted that in the two-sheet interleaf mode, two sheets pass through the junction of the paths  4  and  33  during the period of time T. 
     Generally, in an n-sheet interleaf mode, the period of time T is expressed as: 
     
       
           T=[ 2( n− 1) D +(2 n −1) X]/V   Eq. (2)  
       
     
     In this connection, in a three-sheet interleaf mode, four sheets pass through the junction of the paths  4  and  33  during the period of time T. 
     When the period of time T increases, the distance X between sheets, of course, increases and lowers productivity. The illustrative embodiment enhances productivity by reducing the interval between sheets in the interleaf, duplex print mode, as will be described hereinafter. First, the sheet speed V included in the Eq. (1) or (2) may be increased to reduce the interval between sheets. This can be done if a sheet is conveyed along the refeed path  33  at a speed higher than the process speed of the image forming section  2 . Specifically, a control unit, not shown, included in the image forming apparatus stores a data table listing distances X and time intervals T in relation to sheet sizes and the number of sheets to be fed first. The control unit finds an adequate distance X and an adequate time interval T out of the data table and substitutes them for the Eq. (1) or (2) to thereby determine an adequate sheet speed V. 
     Second, a period of time necessary for a sheet to move from the switchback path  6  to the registration roller pair  13  may be reduced. More specifically, the duration of a stop of a sheet on the switchback path  6  maybe reduced to increase the sheet conveying speed from the path  6  to the registration roller pair  13 . 
     As for the sheet speed V or the duration of a stop, assume that a period of time necessary for a sheet to move from the image forming section  2  to the junction of the paths  4  and  33  is fixed. Then, the control unit subtracts the fixed period of time from the time interval T and then calculates a duration of a stop or a sheet speed V that allows a sheet to move from the switchback path  6  to the registration roller  13  within the remaining period of time. 
     FIG. 11 shows how a one-sided sheet is driven out to the sheet outlet section  3  after being reversed. As shown, a one-sided sheet is conveyed along the refeed path  33  to the horizontal portion  33   b . Subsequently, the refeed roller  36  switches back the sheet to the sheet outlet section  3  via the reverse discharge path  35 . As a result, such sheets are sequentially stacked on the sheet outlet section  3  face down in order of page. 
     When the above one-sided sheet is about to reach the horizontal portion  33   b , the path selector  39  is held at the second position. The sheet therefore moves the path selector  39  to the first position due to its own elasticity and then enters the horizontal portion  33   b . As soon as  6  the trailing edge of the sheet moves away from the path selector  39 , the path selector  39  automatically restores the second position due to the bias of the spring  42 . Therefore, when the sheet is switched back toward the reverse discharge path  35 , the path selector  39  surely guides the sheet to the reverse discharge path  35  by preventing it from entering the vertical portion  33   a  and jamming the portion  33   a . Further, the spring  42  is simpler than a solenoid or similar electronic actuator. In addition, the movement of the path selector  39  effected by the spring  42  is sure and adaptive to high-speed sheet feed. 
     The sheet switched back to the reverse discharge path  35  and a sheet advancing toward the horizontal portion  33   b  via the vertical portion  33   a  can pass each other, as stated earlier. Therefore, just after the leading edge of the preceding sheet moved away from the path selector  39  into the reverse discharge path  35  has been nipped by a roller pair located on the path  35 , the refeed roller  36  can be switched to its open position so as to allow the following sheet into the horizontal portion  36 . It follows that smooth reverse discharge is achievable despite the short distance between one-sided sheets. 
     Further, the one-sided sheet is conveyed along the reverse discharge path  35  at a speed higher than the process speed of the image forming section  2 , as also stated earlier. It is therefore possible to increase the distance between consecutive sheets sequentially conveyed along the reverse discharge path  35  to the sheet outlet section  3 . This facilitates punching, stapling or similar finishing that may be executed with the sheets stacked on the sheet outlet section  3 . 
     Referring to FIGS. 13 through 15, an alternative embodiment of the present invention will be described. In the alternative embodiment, as well as in the other alternative embodiments to be described later, structural elements identical with the structural elements of the previous embodiment are designated by identical reference numerals and will not be described specifically in order to avoid redundancy. 
     As shown in FIGS. 13 through 15, first reversible rollers  50  and  51 , a second reversible roller  52  and a reverse roller  53  are arranged in the horizontal portion  33   b  of the refeed path  33 . The first reversible rollers  50  and  51  constitute conveying means, reverse discharging means and refeed rollers at the same time. The second reversible roller  52  constitutes conveying means and reverse discharging means at the same time. The reverse roller  53  also plays the role of reverse discharging means. A trailing edge sensor  54  is positioned in the vertical portion  33   a  of the refeed path  33  in order to sense the trailing edge of a sheet. Further, the roller  38  and path selector  39  are positioned at the junction of the refeed path  33  and reverse discharging path  35 . 
     The reversible rollers  50  and  51  each are implemented as a drive roller connected to a stepping motor, DC servo motor or similar reversible motor, not shown, and a driven roller constantly held in contact with the drive roller. The rollers  50  and  51  each are selectively rotatable in the forward direction for conveying a sheet from the horizontal portion  33   b  to the conveyance path  4  or in the reverse direction for switching it back to the reverse discharge path  35 . 
     As shown in FIG. 14, the horizontal portion  33   b  includes a passing range X downstream of the junction of the refeed path  33  and reverse discharge path  35 . A preceding sheet A entering reverse discharge path  35  and a following sheet B entering the horizontal portion  33   b  pass each other in the range X. The passing range X varies in accordance with the size and speed of the sheets A and B and varies every moment in accordance with the positions of the sheets A and B being conveyed. The passing range X shown in FIG. 14 is the maximum range. 
     The first reversible rollers  50  and  51  are located outside of the passing range X. This obviates an occurrence that the following sheet B reaches the rollers  50  and  51  when the rollers  50  and  51  are conveying the preceding sheet A toward the reverse discharge path  35 . More specifically, when the following sheet B reaches the rollers  50  and  51 , the preceding sheet A has already been released from the rollers  50  and  51 . The rollers SO and  51  can therefore rotate in the forward direction to convey the following sheet B entered the horizontal portion  33   b  to a preselected position downstream of the rollers  50  and  51 . The rollers  50  and  51  therefore do not have to be switched to the open position even when the two sheets A and B pass each other. That is, the rollers  50  and  51  should only be switched in the direction of rotation. 
     The second reversible roller  52  is identical in basic structure with the first reversible rollers  50  and  51 . The difference is that a driven roller forming part of the roller  52  is movable away from a drive roller forming the other part of the roller  52 . This is effected by a solenoid not shown. Further, the roller  52  lies or does not lie in the passing range, depending on the size of the sheet to be driven into the reverse discharge path  35 . When the preceding sheet A and following sheet B are to pass each other at the position of the roller  52 , the roller  52  is brought to its open position so as not to obstruct the pass. 
     The reverse roller  53  adjoins the junction of the horizontal portion  33   b  of the refeed path  33  and reverse discharge path  35 . The reverse roller  53  also has a drive roller and a driven roller and so rotates as to drive a sheet toward the reverse discharge path  35 . A solenoid, not shown, selectively brings the driven roller into or out of contact with the drive roller. When the reverse roller  53  rotates to drive the sheet toward the reverse discharge path  35 , the roller  53  and the other rollers  50 ,  51  and  52  rotate at the same peripheral speed in synchronism with each other. This prevents the sheet being conveyed toward the reverse discharge path  35  from being pulled or slackened between the rollers  50 ,  51  and  52  and the roller  53 . 
     The trailing edge sensor  54  adjoins the junction of the refeed path  33  and reverse discharge path  35  at a position upstream of the junction. The trailing edge sensor  54  senses the trailing edge of a sheet being conveyed along the refeed path  33 . The direction of rotation of the rollers  50 ,  51  and  52  are switched from forward to reverse in accordance with the output of the sensor  54 . Also, the rollers  53  and  52  are brought to the closed position in accordance with the output of the sensor  54 . 
     A reverse discharge roller  55  is positioned on the reverse discharge path  35  and rotated to discharge a sheet along the path  35 . The reverse discharge roller  55  is positioned such that before the leading edge of the following sheet B being conveyed along the refeed path  33  reaches the reverse roller  53 , the leading edge of the preceding sheet A reaches the roller  55 . In this condition, when the reverse roller  53  is switched to the closed position for allowing the two sheets A and B to pass each other, the roller  55  can nip the leading edge of the sheet A to thereby smoothly discharge the sheet A. Further, the roller  55  and rollers  50 ,  51  and  52  drive a sheet toward the reverse discharge path  35  at a speed higher than the speed at which a sheet is introduced into the refeed path  33 . This is achievable by, e.g., assigning a particular motor speed to each of forward rotation and reverse rotation. 
     FIG. 13 shows a condition in which the preceding sheet A has started moving out of the horizontal portion  33   a  toward the reverse discharge path  35  while the following sheet B is being conveyed toward the horizontal portion  33   b  via the vertical portion  33   b . At this instant, the rollers  50 ,  51  and  52 , as well as the reverse roller  53 , all are rotated in the reverse direction, conveying the sheet A toward the reverse discharge path  35 . 
     FIG. 14 shows a condition in which the preceding sheet A and following sheet B are passing each other at the horizontal portion  33   b . At this time, the roller  52  and reverse roller  53  lying in the passing range X are switched to the open position and therefore do not obstruct the following sheet B. The preceding sheet A has its leading edge nipped by the reverse discharge roller  55  and can therefore be smoothly conveyed toward the sheet outlet section  3  even when released from the rollers  50  through  53 . 
     FIG. 15 shows a condition in which the entire preceding sheet A has entered the reverse discharge path  35  while the following sheet B is being conveyed along the horizontal portion  33   b  toward the path  4 . The rollers  50  through  52  are rotated in the forward direction while the reverse roller  53  is held in the open position. When the trailing edge sensor  54  senses the trailing edge of the following sheet B arrived at the preselected position in the horizontal portion  33   b , the condition shown in FIG. 13 is again set up. As a result, the rollers  50  through  52  and reverse roller  53  start switching back the sheet B toward the reverse discharge path  35 . It follows that the sheet B does not become free during the switching of the direction at all and is therefore prevented from skewing or shifting in the widthwise direction. 
     If the sheet being switched back toward the reverse discharge path  3 S is relatively long, then the roller  52  and reverse roller  53  are omissible; the rollers  50  and  51  can switch back the sheet alone. 
     FIG. 16 shows another alternative embodiment of the present invention. As shown, a roller  60 , a first and a second roller  61  and  62  facing the roller  60 , a pad or gripping means  63  and a sensor  64  are arranged around the junction of the refeed path  33  and reverse discharge path  35 . The circumference of the roller  60  partly faces the refeed path  33  and reverse discharge path  35 . A motor, not shown, rotates the roller  60  in the direction in which a sheet being conveyed along the refeed path  33  advances and the direction in which a sheet being conveyed along the reverse discharge path  35  advances. The roller  60  includes a cylindrical roller body  60   a  and a number of lugs  60   b  extending radially outward from the circumference of the roller body  60   a . The lugs  60   b  are formed of an elastic material. 
     The first roller  61  contacts and is driven by the roller  60  to drive a sheet being conveyed along the refeed path  33 . At the position where the first roller  61  contacts the roller  60 , the lugs  60   b  of the roller  60  yield and become substantially flush with the circumference of the roller body  60   a . The second roller  62  also contacts and is driven by the roller  60  to drive a sheet being conveyed along the revere discharge path  35 . At the position where the first roller  62  contacts the roller  60 , the lugs  60   b  of the roller  60  yield and become substantially flush with the circumference of the roller body  60   a.    
     The sensor  64  is positioned upstream of the first roller  61  in the direction of sheet conveyance and senses the leading edge and trailing edge of a sheet. A solenoid or similar actuator, not shown, selectively moves the pad  63  to a gripping position for gripping and stopping a sheet on the refeed path  33  or a releasing position for releasing it. 
     In operation, a sheet conveyed along the refeed path  33  is conveyed by the roller  61  and first roller, which are rotating in directions indicated by arrows in FIG.  16 . Just after the sensor  64  has sensed the trailing edge of the sheet, but before the trailing edge moves away from the roller  60  and first roller  61 , the pad  63  is moved to the gripping position to grip the sheet for thereby stopping the movement of the sheet. Even after the pad  63  has gripped the sheet, the roller  60  is continuously rotated to convey the trailing edge of the sheet along the refeed path  33 . As a result, the sheet bends between the pad  63  and the roller  60  little by little. When the trailing edge of the sheet moves away from the roller  60  and first roller  61 , the lugs  60   b  of the roller  60  retain the trailing edge of the sheet and convey it toward the reverse discharge path  35  in accordance with the rotation of the roller  60 . AS soon as the trailing edge of the sheet arrives at a preselected position adjoining the reverse discharge path  35 , the bent sheet bounces upward due to its own restoring force. Consequently, the trailing edge of the sheet is released from the lugs  60   b  and enters the nip between the roller  60  and the second roller  62 . 
     Just after the trailing edge of the sheet has entered the nip between the roller  60  and the second roller  62 , the pad  63  is moved to the releasing position. As a result, the sheet is conveyed by the roller  60  and second roller  62  to the reverse discharge path  35 . 
     As stated above, the illustrative embodiment does not locate a path selector or similar hard member around the junction of the refeed path  33  and reverse discharge path  35 . This protects a sheet and therefore an image carried thereon from damage ascribable to friction otherwise acting between the sheet and such a path selector. Further, at the time of switchback, the lugs  60   b  convey the leading edge of a sheet (trailing edge before switchback) without causing it to shift in the oblique direction, thereby preventing the sheet from skewing. 
     FIG. 17 shows a further alternative embodiment of the present invention. As shown, this embodiment is identical with the embodiment described with reference to FIG. 16 except that a reversible roller  70  is substituted for the pad  63  as alternative gripping and stopping means. In the event of reverse discharge, the roller  70  is rotated in a direction a for conveying a sheet coming in through the refeed path  33 . On the elapse of a preselected period of time since the sensor  64  has sensed the trailing edge of the sheet, e.g., when the trailing edge moves away from the roller  60  and first roller  61 , the roller  70  is caused to rotate in a direction b for again conveying the sheet toward the roller  60 . Consequently, the lugs  60   b  retain the leading edge of the sheet (trailing edge before switchback) and convey it toward the reverse discharge path  35 . This is followed by the conveyance described with reference to FIG.  16 . 
     The reversible roller  70  substituted for the pad  63 , FIG. 16, makes it needless for a sheet to bend between the roller  60  and the roller  70  and therefore frees the sheet from curling. 
     If desired, at the time when the direction of rotation of the roller  70  is switched, the conveying speed of the roller  70  conveying a sheet on the refeed path  33  may be reduced below the conveying speed of the roller  60  and then stopped and switched in the direction of rotation. In this configuration, when the roller  70  stops rotating while gripping the sheet, the sheet can gently bend between the rollers  60  and  70 . At the time of reverse discharge, the edge of the sheet is prevented from parting from the roller  60 , so that a switchback time is reduced. 
     In summary, it will be seen that the present invention provides an image forming apparatus having various unprecedented advantages, as enumerated below. 
     (1) When a preceding sheet is switched back from a switchback path toward a conveyance path, the preceding sheet and the following sheet being introduced into the switchback path at least partly overlap each other. Therefore, by controlling the speed of the sheet being conveyed to the switchback path and the speed and timing of the sheet being driven out of the same path, it is possible to accurately maintain a short distance between sheets to be fed to an image forming section. This can be done without regard to irregularity in the timing of sheet feed from a sheet tray or in the speed of conveyance to the switchback path. Further, in a duplex print mode, the switchback path can be used as a path for switching back a one-sided sheet and again feeding it toward an image forming section. This makes an exclusive path for the duplex print mode needless. The apparatus is therefore reduced in thickness despite the presence of the switchback path 
     (2) When the one-sided sheet is driven into a refeed path, reverse discharging means switches back the sheet and conveys it into the reverse discharge path. The sheet can therefore be driven out to a sheet outlet section face down in order of page. 
     (3) A refeed roller for switching the direction of sheet feed plays the role of conveying means and reverse discharging means at the same time, simplifying the structure of the apparatus. At the time of reverse discharge, the refeed roller rotating in the forward direction nips the sheet introduced into the refeed path and then rotates in the reverse direction to thereby drive the sheet toward the reverse discharge path. The sheet therefore does not become free during conveyance to the reverse discharge path and is therefore prevented from skewing. 
     (4) The sheet being switched back to the reverse discharge path and the following sheet coming in through the refeed path can pass each other. Therefore, the following sheet can enter the refeed path when the trailing edge of the preceding sheet entering the reverse discharge path is still positioned in the refeed path. The preceding sheet can therefore smoothly reversed and discharged even when the distance between sheets being conveyed along the refeed path is short. 
     (5) When the refeed roller is rotating in the reverse direction to nip and convey the preceding sheet to the reverse discharge path, the following sheet coming in through the refeed path is prevented from reaching the refeed roller. Therefore, even when the preceding sheet and following sheet pass each other, the refeed roller does not have to be opened, but should only be switched in the direction of rotation. 
     (6) A simple biasing member suffices to switch the position of a path selector that deals with a sheet to enter the refeed path or the reverse discharge path. This not only obviates the need for a solenoid or similar electronic actuator, but also realizes sure, high-speed switching. 
     (7) The sheet being switched back toward the reverse discharge path does not contact a path selector or similar member and is therefore free from damage ascribable to rubbing, preventing image quality from being degraded. Further, lugs retain the leading edge of the sheet and convey the leading edge to the reverse discharge path side without causing it to shift in the oblique direction. The sheet is therefore from skew when being conveyed along the reverse discharge path. 
     (8) A pad is movable to a gripping position in order to grip the sheet coming in through the refeed path and stop it. This is also true with a roller that can stop rotating. 
     (9) The sheet can be driven out of the refeed path to the conveyance path at an adequate timing, implementing interleaf sheet feed. 
     (10) In an Interleaf, duplex print mode, the sheet being conveyed along the refeed path can be rapidly brought to the junction of the refeed path and conveyance path. This successfully reduces the distance between the sheet being fed from the sheet tray and the sheet being driven out of the refeed path to the conveyance path, thereby enhancing the productivity of image formation. 
     (11) Punching, stapling or similar finishing is easy to execute with a stack of sheets because the distance between sheets sequentially discharged to the sheet outlet section can be increased. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.