Abstract:
An image forming apparatus is provided that prevents paper from being scratched by a peeling claw and reducing the curl amount of the paper to be re-fed to a lower level. The rotational speed of a fixing and discharging roller is adequately kept higher than the rotational speed of a heating roller so as not to allow image stain to be produced in a paper due to scratching by a peeling claw in a fixing unit. Simultaneously, the rotational speed of reverse feeding rollers is set slightly higher than the rotational speed of the heating roller at the time when an automatic duplex unit performs paper re-feeding. As a result, “swelling” of the paper occurring in a reverse gate is eliminated by the reverse feeding roller which is rotated at a speed higher than the heating roller, thereby greatly reducing “curl amount” remaining in the paper due to the “swelling”.

Description:
This application claims priority from Japanese Patent Application 2005-65576, filed Mar. 9, 2005, which is incorporated herein by reference in its entirety. 
   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 such as a digital copying machine or a printer. 
   2. Description of the Related Art 
     FIG. 4  is a cross-sectional view showing a part of a fixing unit and an automatic duplex unit (ADU) of a conventional image forming apparatus. A printing paper PP onto which a toner image has been transferred from a photoconductor drum disposed in a toner image forming section is subjected to fixing processing performed by a heating roller  133  and a pressure roller  134  of the fixing unit  130  and fed to a fixing and discharging roller  135 . The fixing and discharging roller  135  then feeds the received printing paper PP to a reverse gate  136 . 
   In the case where a printing paper PP onto only one side of which a toner image has been transferred is reversed and discharged outside the machine, the reverse gate  136  feeds the printing paper PP received from the fixing and discharging roller  135  toward a paper re-feeding route  132   a.  In this case, the printing paper PP is fed through a feeding path that turns at an acute angle (in  FIG. 4 , turns at right angles) from the fixing and discharging roller  135  toward the paper re-feeding route  132   a.  The printing paper PP guided by the paper re-feeding route  132   a  is fed in a switchback manner by reverse feeding rollers  137   a  and  137   b  toward a printed paper discharge port  131 . 
   In the case where duplex printing is performed, when a toner image has been transferred onto only one side of the printing paper PP, the reverse gate  136  feeds the printing paper PP received from the fixing and discharging roller  135  toward the paper re-feeding route  132   a  as in the above case. The printing paper PP guided by the paper re-feeding route  132   a  is fed in a switchback manner by the reverse feeding rollers  137   a  and  137   b  toward an automatic duplex unit  170 . The automatic duplex unit  170  feeds the printing paper PP that has been fed thereto in a switchback manner toward the toner image forming section for paper re-feeding using reverse feeding rollers  137   c  and  137   d.    
   In the case where one-side printing or duplex printing for the printing paper PP has been completed as instructed and where the printing paper PP is to be discharged outside the machine without being reversed, the reverse gate  136  directly guides the printing paper PP that has been fed from the fixing and discharging roller  135  to the printed paper discharge port  131 . 
     FIG. 5  schematically shows the state where the printing paper fed from the fixing and discharging roller  135  toward the paper re-feeding route  132   a  in an easy-to-understand manner. In the conventional machine, a rotational speed V 3  of the reverse feeding roller  137  ( 137   a ,  137   b ,  137   c ,  137   d ) is set equal to a rotational speed V 1  of the heating roller  133  and pressure roller  134 . Only a rotational speed V 2  of the fixing/discharge roller  135  is set higher than the V 1  and V 3 . The reason that only the rotational speed V 2  is set higher is to prevent image stain from being produced when the printing paper is scratched by a peeling claw disposed in the fixing unit (refer to, for example, Jpn. Pat. Appln. Laid-Open Publication No. 6-156852 (pages 3 to 6, FIG. 1) and Jpn. Pat. Appln. Laid-Open Publication No. 7-267454 (pages 4 to 7, FIG. 1)). 
   However, in the conventional image forming apparatus, the leading end of the printing paper fed toward the reverse gate  136  hits against the reverse gate  136  to generate early “swelling” (refer to  FIG. 5 ). When the printing paper is passed through the reverse gate  136  and is further fed, this “swelling” moves to the rear side of the printing paper, with the result that the “swelling” remains at the rear side portion of the printing paper as “bending” or “curl”. In the case where the printing paper is reversed and re-fed to the toner image forming section, the “curl” stands at the leading end of the printing paper. In this case, when the printing paper has a bigger size and smaller thickness, the “curl” is likely to become greater. 
   Here, curl amount in an LD size printing paper was measured under the condition that V 1  and V 3  were set to 420 mm/sec, V 2  was set to 448 mm/sec or 429 mm/sec, and rotational speed V 3  of reverse feeding roller/rotational speed V 2  of fixing and discharging roller was changed by ±4%. As a result, the data as shown in the following Table 1 was obtained. In this case, V 2 H denotes the case where V 2  is 448 mm/sec, and V 2 L denotes the case where V 2  is 429 mm/sec. 
   
     
       
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               V3/V2 (%) 
               V2H (mm) 
               V2L (mm) 
             
             
                 
             
           
           
             
               −3.9 
               — 
               — 
             
             
               −3.7 
               — 
               16 
             
             
               −3.0 
               — 
               13 
             
             
               −2.2 
               38 
               11 
             
             
               −1.5 
               22 
               9 
             
             
               −0.7 
               18 
               11 
             
             
               +0.1 
               10 
               10 
             
             
               +1.0 
               8 
               10 
             
             
               +1.7 
               5 
               10 
             
             
               +4.0 
               5 
               9 
             
             
                 
             
           
        
       
     
   
     FIG. 6  is a graph showing the measurement result corresponding to Table 1. As can be seen from Table 1 and graph of  FIG. 6 , the smaller the value of rotational speed V 2  of the fixing and discharging roller, the smaller the curl amount becomes (when V 3 /V 2  (%) becomes a positive value, curl amount is reduced). That is, the decrease in the rotational speed of V 2  of the fixing and discharging roller reduces the curl amount, with the result that it is possible to keep the shape of the printed printing paper in an original state. On the other hand, the decrease in the rotational speed of V 2  of the fixing and discharging roller may allow the printing paper to be scratched by the peeling claw disposed in the fixing unit, producing image stain. As another means that reduces the curl amount, special equipment such as a dedicated de-curling unit or a heat pipe has been utilized. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the above problem, and an object thereof is to provide an image forming apparatus capable of adequately keeping the rotational speed of the fixing and discharging roller higher than the rotational speed of the heating roller so as not to allow image stain to be produced in the printing paper due to scratching by the peeling claw disposed in the fixing unit and, at the same time, capable of reducing the curl amount of the printing paper after paper re-feeding operation to a lower level without additionally providing special equipment. 
   To solve the above problem, according to an aspect of the present invention, there is provided an image forming apparatus comprising: a heating roller that fixes a toner image that has been transferred onto a sheet to the sheet; a fixing and discharging roller that feeds the sheet onto which the toner image has been fixed to a next process; a reverse feeding roller that feeds the sheet fed from the fixing and discharging roller to a reverse feeding path in a switchback manner; and a drive control section that controls drive of the heating roller, fixing and discharging roller, and reverse feeding roller such that a relation between the first speed V 1 , the second speed V 2 , and the third speed V 3  which are determined by the respective rollers satisfies V 1 &lt;V 3 &lt;V 2 . 
   With the above configuration, even when the sheet is fed from the fixing and discharging roller to the reverse feeding path and the feeding direction of the sheet is changed to the reverse feeding path, “swelling” produced in the reverse gate is eliminated by the reverse feeding roller which is rotated at a speed higher than the heating roller, thereby greatly reducing “curl amount” ultimately remaining in the printing paper due to the “swelling”. 
   Further, in the image forming apparatus having the above configuration, it is preferable that the drive control section set the second speed V 2  higher than the first speed V 1  by several percent and set the third speed V 3  higher than the first speed V 1  by 0.2 to 3.6%. 
   Further, in the image forming apparatus having the above configuration, it is more preferable that the drive control section set the second speed V 2  higher than the first speed V 1  by several percent and set the third speed V 3  higher than the first speed V 1  by 0.5 to 1.7%. 
   Further, in the image forming apparatus having the above configuration, it is preferable that the drive control section set the first speed V 1  to 420 mm/sec, the second speed V 2  to 448 mm/sec, and the third speed V 3  to 425 mm/sec. 
   As described above in detail, according to the present invention, even when the direction of the printing paper fed from the fixing and discharging roller to the reverse gate is changed by the reverse gate to the paper re-feeding path, the “swelling” to be produced in the reverse gate is eliminated by the reverse feeding roller which is rotated at a speed higher than the heating roller, thereby greatly reducing “curl amount” ultimately remaining in the printing paper due to the “swelling”. That is, it is possible to reduce the curl amount of the printing paper after paper re-feeding operation to a lower level without adding special equipment while adequately keeping the rotational speed of the fixing and discharging roller higher than the rotational speed of the heating roller so as not to allow image stain to be produced in the printing paper due to scratching by the peeling claw disposed in the fixing unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view showing an embodiment of an image forming apparatus according to the present invention; 
       FIG. 2  is a cross-sectional view showing, in an enlarged manner, a part of a fixing section and automatic duplex unit in the image forming apparatus of  FIG. 1 ; 
       FIG. 3  is a graph showing the curl amount of a printing paper obtained in printing operation of the image forming apparatus shown in  FIGS. 1 and 2  which involves paper re-feeding operation, in which a speed ratio between a heating roller and reverse feeding roller is changed in the range of ±4%; 
       FIG. 4  is a cross-sectional view showing a part of a fixing section and automatic duplex unit in a conventional image forming apparatus; 
       FIG. 5  is a schematic view help explain the description related to  FIG. 4  in an easy-to-understand manner; and 
       FIG. 6  is a graph showing a result obtained when duplex printing is performed for an LD size printing paper with rotational speed V 3  of reverse feeding roller/rotational speed V 2  of fixing and discharging roller changed in the range of ±4% and the curl amount after the printing operation is measured. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described below with reference to the accompanying drawings.  FIG. 1  is a cross-sectional view showing an embodiment of an image forming apparatus according to the present invention.  FIG. 2  is a cross-sectional view showing, in an enlarged manner, a part of a fixing section and automatic duplex unit in the image forming apparatus of  FIG. 1 .  FIG. 3  is a graph showing the curl amount obtained when a speed ratio between a heating roller and reverse feeding roller is changed in the image forming apparatus shown in  FIGS. 1 and 2 . 
   An image forming apparatus  100  of  FIG. 1  has a toner image forming section including a photoconductor drum  10 , a transfer belt  20  that transfers a toner image formed on the photoconductor drum  10  onto a printing paper, a fixing unit  30  that fixes the toner image on the printing paper onto which the toner image has been transferred and which has been fed by the transfer belt  20 , a CPU (corresponding to drive controller)  105 , and a MEMORY  106 . 
   The image forming apparatus  100  further has a paper feeding mechanism, a paper conveyance mechanism, and an automatic duplex unit  70  (for example, a stackless-type ADU  70 ). The paper feeding mechanism draws out a printing paper from a plurality of paper feed cassettes that store printing paper of various sizes and sequentially feeds the printing paper one sheet by one sheet to a paper feeding path  50 . The paper conveyance mechanism feeds the printing paper that has been fed from the paper feeding mechanism to an aligning position through the paper feeding path  50  at a paper feeding speed and, after aligning control, feeds the printing paper to a transfer nip between the photoconductor drum  10  and transfer belt  20  at a process speed. The automatic duplex unit  70  reverses the printing paper onto only one side of which a toner image has been transferred and re-feeds the printing paper to an aligning position in cooperation with an intermediate feeding roller  12 . 
   The paper feeding mechanism includes paper feed cassettes  61 ,  62 ,  63 ,  64 , pick-up rollers  41   c    42   c,    43   c,    44   c,  paper feeding rollers  41   a,    42   a,    43   a,    44   a,  and separating rollers  41   b,    42   b,    43   b,    44   b.  The paper conveyance mechanism includes first, second, third, and fourth feeding rollers  51 ,  52 ,  53 ,  54 , an intermediate feeding roller  12 , and a resist roller  11 . In this case, the intermediate feeding roller  12  is driven by a pulse motor and the CPU  105  controls the intermediate feeding roller  12  such that it can operate at three different speeds of paper feeding speed, process speed, and ADU speed (paper re-feeding speed), that is, the CPU  105  performs three-speed control. 
   In the present embodiment, the CPU  105  has a role of performing various processing (drive control for respective feeding rollers that perform sheet feeding) in the image forming apparatus and has another role of performing a program stored in the MEMORY  106  to realize various functions. The MEMORY  106  is constituted by, for example, a ROM or RAM and has a role of storing various information and programs utilized in the image forming apparatus. 
   The above-described fixing unit  30  receives the printing paper PP onto which a toner image has been transferred from the transfer belt  20  and uses a heating roller  33  and pressure roller  34  to fix the toner image on the printing paper PP. The printing paper PP that has been subjected to the fixing processing is fed to a fixing and discharging roller  35 , by which the printing paper PP is fed toward a reverse gate  36 . In the case where one-side printing or duplex printing for the printing paper PP has been completed as instructed or where the one-side printing paper whose other side will not be subjected to printing has been reversed, the reverse gate  36  feeds the received printing paper PP to a printed paper discharge port  31  in the direction same as the direction that the printing paper PP has been fed thereto since there is no paper re-feeding instruction issued from the controller. 
   In the case different from the above, that is, in the case where the reverse gate  36  receives an instruction of duplex printing or reverse processing and where only one-side printing has been completed or reverse processing has not been completed, there is a paper re-feeding instruction from the controller, so that the reverse gate  36  feeds the printing paper PP received from the fixing and discharging roller  35  in the direction different from the direction that the printing paper PP has been fed (in the example of  FIGS. 1 and 2 , the reverse gate  36  feeds the printing paper PP in the direction substantially perpendicular to the direction that the printing paper PP has been fed), that is, toward a paper re-feeding route  32   a.  The automatic duplex unit  70  re-feeds the printing paper PP that has been fed to the paper re-feeding route  32   a  toward the resist roller  11  and toner image forming section through paper re-feeding routes  32   a,    32   b,    32   c.  In this case, the printing paper PP that has been fed to the paper re-feeding route  32   a  is fed by reverse feeding rollers  37   a,    37   b,    37   c,    37   d,  and the like disposed along the paper re-feeding route. Note that the reverse feeding rollers  37   a  and  37   b  correspond to the reverse feeding roller recited in the claim. 
   The heating roller  33  has a role of fixing the toner image transferred onto a sheet to the sheet, the fixing and discharging roller  35  has a role of feeding the sheet onto which the toner image has been fixed, to a next process, and the reverse feeding roller  37  has a role of feeding the sheet that has been fed from the fixing and discharging roller  35  to (at least) the reverse feeding path in order for the sheet to be fed in a switchback manner. 
   At the paper re-feeding time, in the case of conventional image forming apparatus, the leading end of the printing paper PP is hit against the reverse gate  36  to generate early “swelling”. With proceeding of the feeding operation of the printing paper by the reverse feeding roller, this “swelling” moves to the rear side of the printing paper, with the result that the “swelling” remains at the rear side portion of the printing paper. The “swelling” at the rear side of the printing paper becomes “swelling” at the front side of the printing paper at the time when the printing paper PP is reversed and re-fed to the toner image forming section. In this case, when the printing paper PP was discharged from the printed printing paper discharge port  31  after completion of the duplex printing or reverse processing, the “swelling” remained largely as so-called “curl” (in particular, in the case of an LD size printing paper). However, the present inventor succeeded in reducing the “curl” to a very small level in the image forming apparatus  100  shown in  FIGS. 1 and 2 . 
   In the image forming apparatus  100 , the following experiment was made to reduce the “curl”. That is, in order to prevent the printing paper from being scratched by the peeling claw provided in the fixing unit  30 , the rotational speed of the fixing and discharging roller  35  was set higher by several percent than the rotational speed of the heating roller  33  and pressure roller  34  as is conventionally done. On the other hand, the rotational speed of the reverse feeding roller  37  was changed in the range of ±4% relative to the rotational speed of the heating roller  33 . To be more precise, the rotational speed of the heating roller  33  and pressure roller  34  was set to 420 mm/sec and the rotational speed of the fixing and discharging roller  35  was set to 448 mm/sec. In the above setting, maximum values (mm) of the “curl amount” were measured while the rotational speed of the reverse feeding roller is changed. The following Table 2 shows the result of the measurement. Here, LD, A4-R, A4 size printing papers were used. 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
                 
               Max value in 
                 
             
             
                 
               MAX value in 
               A4-R size 
               Max value in 
             
             
               V3/V1 (%) 
               LD size paper 
               paper 
               A4 size paper 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               −3.9 
                 
                 
                 
             
             
               −3.7 
               — 
               16 
               15 
             
             
               −3.0 
               — 
               13 
               16 
             
             
               −2.2 
               38 
               11 
               12 
             
             
               −1.5 
               22 
               9 
               12 
             
             
               −0.7 
               14 
               11 
               12 
             
             
               +0.1 
               11 
               10 
               10 
             
             
               +0.5 
               4 
               — 
               — 
             
             
               +0.9 
               3 
               — 
               — 
             
             
               +1.7 
               5 
               10 
               10 
             
             
               +2.5 
               5 
               — 
               — 
             
             
               +3.3 
               4 
               — 
               — 
             
             
               +4.0 
               — 
               9 
               8 
             
             
                 
             
           
        
       
     
   
     FIG. 3  is a graph showing the result corresponding to Table 2. As can be seen from Table 2 and graph of  FIG. 3 , the “curl amount” is reduced to a lower level when V 3 /V 1  (%) falls within the range of from +0.5% to +3.3%. In particular, the “curl amount” of the LD size printing paper is reduced to a very low level when V 3 /V 1  (%) is 0.9%. Additionally, duplex printing was performed with the rotational speed of the heating roller  33  and pressure roller  34  set to 420 mm/sec, the rotational speed of the fixing and discharging roller  35  set to 448 mm/sec, and the rotational speed of the reverse feeding roller  37  set to 425 mm/sec. As a result, a highly favorable result was obtained with regard to the “curl amount” (3 mm). 
   In the present embodiment, when the heating roller  33  feeds a sheet at a feeding speed of 420 mm/sec, the drive control section (CPU  105 ) performs drive control in such a manner to make the sheet feeding speed determined by the reverse feeding roller  37  higher than the feeding speed determined by the heating roller  33  by about 1 to 15 mm/sec (that is, the feeding speed V 3  determined by the reverse feeding roller is made higher than the sheet feeding speed V 1  by 0.1 to 4.0% of the V 1 ). In order to further reduce the occurrence of the curl of the sheet, it is preferable that the third speed V 3  be made higher then the first speed V 1  by 0.2 to 3.6% of the V 1 . As a matter of course, in order to further reduce the curl amount in the sheet, it is possible for the drive control section to make the second speed V 2  higher than the first speed V 1  by several percent and the third speed V 3  higher than the first speed V 1  by 0.5 to 1.7% of the V 1  (refer to  FIG. 3 ). 
   As described above, in the image forming apparatus according to the embodiment of the present invention, the drive control section (CPU  105 ) controls drive of the heating roller, fixing and discharging roller, and reverse feeding roller such that a relation between the first speed V 1 , the second speed V 2 , and the third speed V 3  which are determined by the respective rollers satisfies V 1 &lt;V 3 &lt;V 2 . 
   This application claims priority from Japanese Patent Application 2005-65576, filed Mar. 9, 2005, which is incorporated herein by reference in its entirety.