Abstract:
An image forming apparatus includes: an image forming section; a fixing portion; a discharging portion; a sheet transport passage including a curved portion and connecting between the fixing portion and the discharging portion; a reverse transport passage formed by branching off the curved portion; a transport roller pair rotatable in normal and reverse directions; a fixed guide portion provided on an inner corner side of the curved portion; and a swingable guide portion supported to be swingable. The swingable guide portion is arranged selectively either in a first position where it protrudes farther into the sheet transport passage than a guide rib formed on a transport surface of the fixed guide portion or a second position where its protruding height from the transport surface is equal to a height of the guide rib or where the swingable guide portion is retracted farther toward the transport surface than the guide rib.

Description:
INCORPORATION BY REFERENCE 
       [0001]    This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2014-170303 filed on Aug. 25, 2014, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    The present disclosure relates to an image forming apparatus, such as a copier, a facsimile machine and a printer, that is provided with a double-sided printing function for printing on both sides of a sheet. 
         [0003]    Some known conventional image forming apparatuses have a double-sided printing function whereby, after an image is printed on one side of paper (a sheet), the paper is transported again to an image forming section with the printed side reversed so that an image is printed also on the other side of the paper on which no image has yet been printed. 
         [0004]    According to one method of reversing a printed side of paper, a reverse transport passage (double-side transport passage) is provided which branches off the transport passage used for regular printing. After a tail end of paper has passed through a branching portion, the transport direction is reversed; thus the paper then enters, starting with its tail end, the double-side transport passage (to perform a switchback) and has the printed side reversed. 
         [0005]    A known color printer is provided with a transport-in passage through which paper is received from a double-side transport passage, a reversal passage which is connected to the transport-in passage and in which the paper is reversed, and a transport-out passage through which the reversed paper is transported back toward a registration roller pair. In such color printers, a reversal switching claw is provided in a joining portion (branching portion) between the transport-in passage and the transport-out passage, and when the paper is transported in, the transport-in passage side of the reversal passage is opened, and when the paper is transported out, the transport-out passage side of the reversal passage is opened. In the joining portion, a sensor is provided to detect the head and tail ends of both paper which passes through the transport-in passage and paper which passes the transport-out passage. 
         [0006]    Moreover, sheet sorting mechanisms are known in which for the purpose of reducing switching motion noise in a branching portion, a branch claw provided in the branching portion of a transport passage and a transport roller driving portion ahead of the branching portion are connected to each other by a driving force transmission mechanism, a torque limiter is provided in a part of the driving force transmission mechanism, and a direction of the branch claw is switched through selection of the rotation direction of a transfer roller. 
       SUMMARY 
       [0007]    According to one aspect of the present disclosure, an image forming apparatus includes an image forming section, a fixing portion, a discharging portion, a sheet transport passage, a reverse transport passage, a transport roller pair, a fixed guide portion, and a swingable guide portion, and has a double-sided printing function whereby a sheet is, starting with its tail end, guided into the reverse transport passage by the reverse rotation of the transport roller pair and then the sheet is, with its obverse and reverse sides reversed, transported again to the image forming section. The image forming section forms an image on a sheet. The fixing portion fuses the image formed by the image forming section on the sheet. The discharging portion discharges the sheet having the image fused thereto by the fixing portion. The sheet transport passage includes a curved portion, and connects between the fixing portion and the discharging portion. The reverse transport passage is formed by branching off the curved portion of the sheet transport passage, and is connected to an upstream-side part of the image forming section. The transport roller pair is provided in the sheet transport passage so as to be rotatable in normal and reverse directions; it, on one hand, transports the sheet transported from the fixing portion by the normal rotation toward the discharging portion and, on the other hand, transports, by switching back, the sheet to the discharging portion by the reverse rotation toward the reverse transport passage. The fixed guide portion is provided on the inner corner side of the curved portion of the sheet transport passage off which the reverse transport passage branches. The swingable guide portion is supported so as to be swingable in a direction protruding out of or retracting into the fixed guide portion. The swingable guide portion is arranged selectively either in a first position where the swingable guide portion protrudes farther into the sheet transport passage than a guide rib formed on the transport surface of the fixed guide portion or in a second position where the protruding height of the swingable guide portion from the transport surface is equal to the height of the guide rib or where the swingable guide portion is retracted farther toward the transport surface than the guide rib. 
         [0008]    Further features and advantages of the present disclosure will become apparent from the description of embodiments given below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a side sectional view showing an internal structure of an image forming apparatus  100  according to one embodiment of the present disclosure; 
           [0010]      FIG. 2  is a partly enlarged view of and around a curved portion  20  in  FIG. 1 ; 
           [0011]      FIG. 3  is an exterior perspective view of a fixing portion  15 ; 
           [0012]      FIG. 4  is an enlarged view of and around first transport guides  30   a  and  30   b  and a second transport guide  31   b , with the first transport guides  30  arranged in a first position; 
           [0013]      FIG. 5  is a side sectional view of first transport guides  30  located in a first position; 
           [0014]      FIG. 6  is a side sectional view of first transport guides  30  located in a second position; and 
           [0015]      FIG. 7  is an enlarged view of and around first transport guides  30   a  and  30   b  and a second transport guide  31   b , with the first transport guides  30  arranged in a second position. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.  FIG. 1  is a side sectional view showing an internal structure of an image forming apparatus  100  according to one embodiment of the present disclosure. In the image forming apparatus (for example, a monochrome printer)  100 , there is arranged an image forming section P that forms a monochrome image through the processes of electrostatic charging, exposure to light, image development, and image transfer. In the image forming section P, there are arranged, along the rotation direction (clockwise direction in  FIG. 1 ) of a photosensitive drum  5 , a charging unit  4 , an exposure unit (such as a laser scanning unit)  7 , a developing unit  8 , a transfer roller  14 , a cleaning device  19 , and a static eliminator (unillustrated). 
         [0017]    When an image forming operation is performed, the photosensitive drum  5  rotating in the clockwise direction is electrostatically charged uniformly by the charging unit  4 , an electrostatic latent image is formed on the photosensitive drum  5  by a laser beam from the exposure unit  7  based on document image data, and developer (hereinafter referred to as toner) is attached to the electrostatic latent image by the developing unit  8  to form a toner image. 
         [0018]    The toner is fed to the developing unit  8  from a toner container  9 . The image data is transmitted from a personal computer (unillustrated) or the like. Moreover, the static eliminator (unillustrated) which removes residual electric charge on the surface of the photosensitive drum  5  is arranged on the downstream side of the cleaning device  19 . 
         [0019]    Toward the photosensitive drum  5  on which the toner image has been formed as described above, paper is transported through a paper transport passage  12  and a registration roller pair  13  from a paper feed cassette  10  or a manual paper feed device  11 . Then, the toner image formed on the surface of the photosensitive drum  5  by the transfer roller  14  (an image transferring portion) is transferred to the paper. The paper to which the toner image has been transferred is separated from the photosensitive drum  5  and is transported to a fusing device  15 , where the toner image is fused. The paper which has passed through the fusing device  15  is transported to an upper part of the apparatus through a paper transport passage  16 . When an image is formed only on one side of paper (during single-sided printing), the paper is discharged onto a discharge tray  18  by a discharge roller pair  17 . 
         [0020]    On the other hand, when images are formed on both sides of paper (during double-sided printing), after the tail end of the paper has passed through a curved portion  20  of the paper transport passage  16 , the transport direction is reversed. Thus, the paper is sorted to a reverse transport passage  21  which branches off the curved portion  20 , and is transported again to the registration roller pair  13  with the image side reversed. Then, the next toner image formed on the photosensitive drum  5  is transferred to the side of the paper on which an image has not yet been formed by the transfer roller  14 . The paper to which the toner image has been transferred is transported to the fusing device  15 , where the toner image is fused, and is then discharged onto the discharge tray  18  by the discharge roller pair  17 . A procedure for transporting paper during both-sided printing will be described in detail later. 
         [0021]      FIG. 2  is a partly enlarged view of and around the curved portion  20  in  FIG. 1 , and  FIG. 3  is an exterior perspective view of the fusing device  15 . As shown in  FIG. 2 , on the downstream side of the fusing device  15  with respect to the paper transport direction (the solid arrow in  FIG. 2 ), the reverse transport passage  21  branches off the curved portion  20  of the paper transport passage  16 . In the paper transport passage  16 , transport roller pairs  23  and  25  are arranged between the fusing device  15  and the discharge roller pair  17 . The paper which has passed through the fusing device  15  is transported via the transport roller pairs  23  and  25  to the discharge roller pair  17 . 
         [0022]    As shown in  FIG. 3 , a housing  15   a  of the fusing device  15  is provided with the transport roller pair  23  and first transport guides (swingable guides)  30 . On the transport roller pair  23 , four rollers fixed on a shaft are arranged at four positions of the housing  15   a  in its longitudinal direction (the paper width direction). The first transport guides  30  are arranged near the transport roller pair  23 , on its downstream side with respect to the paper transport direction, at four positions of the housing  15   a  in its longitudinal direction. In  FIG. 3 , only one of the rollers composing the transport roller pair  23  is illustrated. 
         [0023]    As shown in  FIG. 2 , the side surface, at one side (the front left side in  FIG. 3 ), of the housing  15   a  forms a part of the transport surface of the curved portion  20  and the reverse transport passage  21 . Specifically, the side surface above the transport roller pair  23  forms an inner corner-side transport surface  20   a  (second transport guides  31   a  to  31   e ) of the curved portion  20 , and the side surface below the transport roller pair  23  forms a transport surface  21   a  of the reverse transport passage  21 . 
         [0024]      FIG. 4  is an enlarged view of and around the first transport guides  30   a  and  30   b  and the second transport guide  31   b , and  FIG. 5  is a side sectional view of the first transport guides  30   a  to  30   d . The first transport guides  30  are arranged at four positions corresponding to both end parts of each of a large size (A3 and B4 size) and a small size (A4 and B5 portrait) in the paper width direction so as to protrude into the paper transport passage  16 . One of the first transport guides  30  is composed of three guide claws arrayed in the paper width direction. As shown in  FIG. 5 , the claws are supported on the housing  15   a  so as to be swingable about a pivot O as a center. The first transport guides  30   c  and  30   d  and the second transport guide  31   d  have a structure symmetrical with that shown in  FIG. 4 , and therefore no overlapping description will be repeated. 
         [0025]    The centers of gravity G of the first transport guides  30   a  to  30   d  are located farther outward of the housing  15   a  (leftward in  FIG. 5 ) than the swingable fulcrum O is. Thus, the first transport guides  30   a  to  30   d  are acted on by a moment in the direction indicated by arrow A, and are ordinarily supported in an upright state as shown in  FIG. 4  with the bottom end parts of the first transport guides  30   a  to  30   d  abutting on a support portion  33 . 
         [0026]    Moreover, the second transport guides  31   a  to  31   e  (fixed guides) are provided outward of and between the first transport guides  30   a  and  30   d . The second transport guides  31   a  to  31   e  are each formed integrally with a transport rib  32   a  which extends in the paper transport direction (the up/down direction in  FIG. 4 ). Moreover, the second transport guides  31   b  and  31   d  in the second position as counted from opposite sides with respect to the paper width direction are each formed integrally with a guide rib  32   b  with a larger protruding height than the transport rib  32   a . The first transport guides  30  are, in an upright state as shown in  FIG. 5 , arranged at a position protruding farther into the curved portion  20  (hereinafter referred to as the first position) than the guide rib  32   b.    
         [0027]    The arrangement of the first transport guides  30   a  to  30   d  and the second transport guides  31   a  to  31   e  will be described in more detail. As shown in  FIG. 3 , the first transport guides  30   a  to  30   d  and the second transport guides  31   a  to  31   e  are arranged alternately in the paper width direction. Specifically, both end parts of A3 size paper in the width direction face the outermost second transport guides  31   a  and  31   e  respectively. Both end parts of B4 size paper in the width direction face the middle one of the three guide claws of the outer first transport guides  30   a  and  30   d  respectively. Both end parts of A5 portrait size paper in the width direction face the inner first transport guides  30   b  and  30   c  respectively. 
         [0028]    Moreover, of the ribs formed on the second transport guides  31   b  and  31   d  in the second position as counted from opposite sides in  FIG. 3 , the outer ribs are the guide ribs  32   b  and are formed at positions that both end parts of A4 portrait size paper in the width direction face. Both end parts of B5 portrait size paper in the width direction face the transport ribs  32   a  formed inward of and next to the guide ribs  32   b  in the paper width direction on the second transport guides  31   b  and  31   d.    
         [0029]    Now, how the first transport guides  30   a  to  30   d  and the second transport guides  31   a  to  31   e  function when A4 portrait size paper passes through the curved portion  20  will be described. First, a description will be given of a case where plain paper is subjected to single-sided printing. In this case, plain paper to which a toner image formed on the photosensitive drum  5  (see  FIG. 1 ) has been transferred by the transfer roller  14  (see  FIG. 1 ) has the toner image fused in the fusing device  15 . 
         [0030]    The plain paper which has passed through the fusing device  15  is transported upward (the solid arrow in  FIG. 2 ) through the paper transport passage  16  by the transport roller pair  23 . Here, the plain paper passes through the curved portion  20  while making contact with the first transport guides  30   b  and  30   c . As described previously, the first transport guides  30   b  and  30   c  are swingable about the pivot O as a center. Thus, as shown in  FIG. 7 , the first transport guides  30   b  and  30   c  rotate in the clockwise direction (direction indicated by arrow A′) against the moment (see  FIG. 5 ) acting on the first transport guides  30   b  and  30   c  resulting from the plain paper making contact with them. As a result, the first transport guides  30   b  and  30   c  are brought into a state inclined toward the transport surface  20   a , and this helps reduce the transport load applied when the plain paper is guided upward. Then, the plain paper is discharged via the transport roller pair  25  and the discharge roller pair  17  onto the discharge tray  18 , and thus single-sided printing is completed. After the passage of the plain paper, the first transport guides  30   b  and  30   c  return to the first position. 
         [0031]    Next, in a case where plain paper is subjected to double-sided printing, the plain paper which has passed through the fusing device  15  is transported upward (the solid arrow in  FIG. 2 ) through the paper transport passage  16  by the transport roller pairs  23  and  25  and the discharge roller pair  17 . Then, at the time that the tail end of the plain paper has just passed through the curved portion  20  (with the tail end of the plain paper overlapping the first transport guides  30   b  and  30   c ) the transport roller pair  25  and discharge roller pair  17  are rotated in the opposite direction. This reverses the transport direction of the plain paper, and the plain paper, now starting with its tail end, passes downward through the curved portion  20  again. Then, the first transport guides  30   b  and  30   c  return to the first position by swinging, as if pushing back the plain paper, in the direction indicated by arrow A in  FIG. 5 . Thus, the tail end of the plain paper is guided to the reverse transport passage  21  along the first transport guides  30  (the broken-line arrow in  FIG. 2 ). 
         [0032]    The plain paper which has passed through the reverse transport passage  21  is transported again to the registration roller pair  13  (see  FIG. 1 ) with the image side reversed. Then, the next tonner image formed on the photosensitive drum  5  is transferred to the side of the plain paper on which an image has not yet been formed by the transfer roller  14 . The plain paper to which the toner image has been transferred is transported again to the fusing device  15 , where the toner image is fused, and is then discharged onto the discharge tray  18  by the transport roller pairs  23  and  25  and the discharge roller pair  17 , and thus double-sided printing is completed. 
         [0033]    As described previously, during double-sided printing on plain paper, with the first transport guides  30   b  and  30   c  swingably provided in the curved portion  20 , it is possible to guide the plain paper to the reverse transport passage  21  smoothly without applying a transport load to the plain paper. 
         [0034]    Now, a description will be given of a case where thick paper is subjected to single-sided printing. The thick paper which has passed through the fusing device  15  is transported upward (the solid arrow in  FIG. 2 ) through the paper transport passage  16  by the transport roller pair  23 . Then, as shown in  FIG. 6 , the first transport guides  30   b  and  30   c  are rotated in the clockwise direction (direction indicated by arrow A′) against the moment (see  FIG. 5 ) acting on themselves. 
         [0035]    As a result, as with the plain paper, as shown in  FIG. 7 , the first transport guides  30   b  and  30   c  are brought into a state inclined toward the transport surface  20   a . As shown in  FIG. 6 , when in the inclined state, the first transport guides  30   b  and  30   c  are arranged in a position where their maximum protrusion height relative to the transport surface  20   a  is the same as the height of the guide rib  32   b  (hereinafter referred to as the second position). Specifically, at least upstream-side (upper in  FIG. 6 ) parts of the first transport guides  30   b  and  30   c  are retracted from the guide rib  32   b  toward the transport surface  20   a  with respect to the paper transport direction from the paper transport passage  16  through the curved portion  20  to the reverse transport passage  21 . 
         [0036]    Then, as with the plain paper, the thick paper is discharged via the transport roller pair  25  and the discharge roller pair  17  onto the discharge tray  18 , and thus single-sided printing is completed. After the passage of the thick paper, the first transport guides  30   b  and  30   c  return to the first position by their weight. 
         [0037]    On the other hand, in a case where thick paper is subjected to double-sided printing, the thick paper which has passed through the fusing device  15  is transported upward through the paper transport passage  16  by the transport roller pairs  23  and  25  and the discharge roller pair  17  (the solid arrow in  FIG. 2 ). Then, at the time that the tail end of the thick paper has just passed through the curved portion  20 , the transport roller pair  25  and discharge roller pair  17  are rotated in the opposite direction. This reverses the transport direction of the thick paper, and the thick paper, now starting with its tail end, passes downward through the curved portion  20  again. 
         [0038]    Here, the thick paper has been transported upward first, and has not yet completely passed through the first transport guides  30   b  and  30   c ; it thus has its tail end overlapping the first transport guides  30   b  and  30   c . During double-sided printing on plain paper, which is pliable, the first transport guides  30   b  and  30   c  return to the first position as if pushing back the paper; however, with thick paper, which is less pliable than plain paper, the first transport guides  30   b  and  30   c  cannot return to the first position. Thus, the first transport guides  30   b  and  30   c  remain in the second position, where they cannot serve to guide the thick paper to the reverse transport passage  21 . 
         [0039]    As a solution, the second transport guides  31   b  is provided with the guide rib  32   b  such that the guide rib  32   b  protrudes farther from the transport surface  20   a  than the first transport guides  30   b  and  30   c  in the second position. This allows thick paper to pass through the curved portion  20  along the guide rib  32   b  and be guided to the reverse transport passage  21 . Thereafter, the thick paper that has, as with plain paper, a toner image transferred and fused to the side on which an image has not yet been formed is discharged onto the discharge tray  18  by the transport roller pairs  23  and  25  and the discharge roller pair  17 , and thus double-sided printing is completed. 
         [0040]    In this embodiment, the second transport guides  31   b  and  31   d  are each provided with the guide rib  32   b , and the first transport guides  30   b  and  30   c  are swingable between the first position protruding farther into the curved portion  20  than the guide rib  32   b  and the second position retracted farther from the curved portion  20  than the second guide  32   b . With this configuration, during single- and double-sided printing on plain paper and during single-sided printing on thick paper, it is possible to guide the paper along the first transport guides  30   b  and  30   c  arranged in the first position to a predetermined transport passage smoothly without applying a transport load to the paper. 
         [0041]    On the other hand, during double-sided printing on thick paper, instead of being guided by the first transport guides  30   b  and  30   c  which are inclined by the passage of the thick paper into the second position, the thick paper is guided smoothly to the reverse transport passage  21  along the guide rib  32   b  protruding into the curved portion  20 . Thus, it is possible to prevent a jam resulting from the thick paper that has entered the curved portion  20  starting with its tail end being guided to the transport roller pair  23  (the paper transport passage  16 ). 
         [0042]    Here, as shown in  FIG. 6 , the first transport guides  30   b  and  30   c  are configured such that their maximum protrusion height H is the same as the height of the guide rib  32   b . This, however, is not meant as any limitation; the maximum protrusion height H of the first transport guides  30   b  and  30   c  may be less than the height of the guide rib  32   b.    
         [0043]    The embodiments described above are in no way meant to limit the present disclosure, which thus allows for many modifications and variations within the spirit of the present disclosure. For example, although in the above described embodiment, the first transport guides  30   a  to  30   d  are provided at four positions in the paper width direction, and the guide ribs  32   b  are provided at two positions in the paper width direction, the number and arrangement of the first transport guides  30  and the guide ribs  32   b  may be changed as necessary according to width of the paper used or the like. 
         [0044]    Also, the magnitude of the moment that acts on the first transport guides  30   a  to  30   d  can be adjusted by adjusting the weight of the first transport guides  30   a  to  30   d  and the positional relationship between the swing pivot O and the center of gravity G according to the thickness and pliability of the paper used. 
         [0045]    Needless to say, the present disclosure is applicable, not only to monochrome printers like the one shown in  FIG. 1 , but also to other types of image forming apparatus provided with a reverse transport passage for double-sided printing, such as color printers, monochrome and color copiers, digital multifunction peripherals, and facsimile machines. 
         [0046]    The present disclosure is applicable to image forming apparatuses provided with a double-sided printing function for printing on both sides of a recording medium. Based on the present disclosure, it is possible to provide an image forming apparatus with a simple configuration that can guide paper, irrespective of its type, smoothly to a reverse transport passage and that can prevent a jam during double-sided printing.