Patent Publication Number: US-9891562-B2

Title: Image forming apparatus and conveyance control method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The entire disclosure of Japanese Patent Application No. 2016-051323, filed on Mar. 15, 2016, including description, claims, drawings and abstract are 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 a conveyance control method. 
     2. Description of Related Art 
     In general, an electrophotographic image forming apparatus (such as a printer, a copy machine, and a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on the surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner from a developing device to a photoconductor drum (image carrier) on which the electrostatic latent image is formed, whereby a toner image is formed. Further, the toner image is directly or indirectly transferred to a sheet, and then heat and pressure are applied to the sheet at a fixing nip to form a toner image on the sheet. 
       FIG. 1  illustrates a sheet conveyance state around a fixing nip. As illustrated in  FIG. 1 , fixing section  60  in the above-mentioned image forming apparatus includes fixing belt  61 , heating roller  62 , fixing roller  63  and pressure roller  64 . Fixing belt  61  is wound around heating roller  62  and fixing roller  63 . Pressure roller  64  forms a fixing nip by sandwiching fixing belt  61  between fixing roller  63  and pressure roller  64 . Fixing section  60  conveys sheet S while heating and pressing sheet S at the fixing nip. 
     SUMMARY OF THE INVENTION 
     Incidentally, for example, when sheet S is being sandwiched between the fixing nip and the conveyance section disposed on the upstream side of the fixing nip, jam or the like may occur in the image forming apparatus.  FIG. 1  illustrates a transfer section that includes secondary transfer roller  424 , intermediate transfer belt  421  and backup roller  423 B as the conveyance section. When jam or the like occurs in the image forming apparatus, the image formation operation, that is, the conveyance of sheet S is required to be stopped even during the image formation operation. In this case, when the timing of stopping the conveyance of sheet S in fixing section  60  is delayed, an end portion of sheet S passed through the fixing nip may be wound around fixing belt  61  in fixing section  60  (see the dashed line), and therefore it is desirable to immediately stop the conveyance of sheet S by fixing section  60 . 
     However, in the case where the image formation operation is stopped during the image formation operation, the conveyance of sheet S in the conveyance section may not be immediately stopped in some situation. For example, in the case of the transfer section, since stoppage is required to be performed after stopping the high-voltage power supply and the developing bias in the developing device, the conveyance operation is required to be continued for a predetermined time after the occurrence of jam. The reason for this is that the carrier in the developing device is attached to the photoconductor drum, and the photoconductor drum, the intermediate transfer belt and the like are damaged. 
     Therefore, even when the conveyance of sheet S in fixing section  60  is immediately stopped at the time of occurrence of jam or the like, the conveyance operation of the transfer section on the upstream side is continued for a predetermined time, and consequently the degree of upward slack of sheet S is undesirably increased at a position between the transfer nip and the fixing nip (see the broken line). In particular, in the case of sheet S having high rigidity such as sheet S having a large thickness and sheet S having a large basis weight, the shape of the slacked portion is held, and this portion may damage the surrounding members of intermediate transfer belt  421  and the like by making contact with the members and the like. 
     It is to be noted that Japanese Patent Application Laid-Open No. 2003-140488 discloses a configuration in which, when jam occurs, the sheet conveyance in the transfer section is stopped, and thereafter the sheet conveyance in the fixing section is stopped. When the sheet conveyance in the fixing section is stopped at such a timing, however, a certain period of time is required for stopping the fixing section, and consequently sheet winding around the fixing section may occur. 
     An image forming apparatus reflecting one aspect of the present invention includes: a fixing section configured to fix a toner image formed on a recording medium to the recording medium by conveying the recording medium while heating and pressing the recording medium at a fixing nip; a conveyance section configured to convey the recording medium toward the fixing nip while sandwiching the recording medium; and a control section configured to control the fixing section such that, at a timing after an image formation operation on the recording medium is stopped during the image formation operation, conveyance of the recording medium in the fixing section is stopped before the conveyance of the recording medium in the conveyance section is stopped, and that a timing of stopping the conveyance of the recording medium in the fixing section is changed in accordance with a type of the recording medium. 
     In a conveyance control method of an image forming apparatus reflecting one aspect of the present invention, the image forming apparatus includes: a fixing section configured to fix a toner image formed on a recording medium to the recording medium by conveying the recording medium while heating and pressing the recording medium at a fixing nip; and a conveyance section configured to convey the recording medium toward the fixing nip while sandwiching the recording medium, the method including: controlling the fixing section such that, at a timing after an image formation operation on the recording medium is stopped during the image formation operation, conveyance of the recording medium in the fixing section is stopped before the conveyance of the recording medium in the conveyance section is stopped, and that a timing of stopping the conveyance of the recording medium in the fixing section is changed in accordance with a type of the recording medium. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  illustrates a sheet conveyance state around a fixing nip; 
         FIG. 2  schematically illustrates a general configuration of an image forming apparatus according to the present embodiment; 
         FIG. 3  illustrates a principal part of a control system of the image forming apparatus according to the present embodiment; 
         FIG. 4  is a timing chart showing a stopping state of an image formation operation, a driving state of a fixing section and a driving state of an intermediate transfer belt; 
         FIG. 5  is a timing chart showing a sandwiching state of a sheet at a secondary transfer nip, a stopping state of an image formation operation, a driving state of a fixing section and a driving state of an intermediate transfer belt; 
         FIG. 6  is a flowchart of an exemplary sheet conveyance operation of the image forming apparatus according to the present embodiment; 
         FIG. 7  illustrates a sheet conveyance state around a fixing nip according to modification 1; and 
         FIG. 8  illustrates a sheet conveyance state around a fixing nip according to modification 2. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples. 
     In the following, the present embodiment is described in detail with reference to the drawings.  FIG. 2  schematically illustrates a general configuration of image forming apparatus  1  according to the present embodiment.  FIG. 3  illustrates a principal part of a control system of image forming apparatus  1  according to the present embodiment. 
     Image forming apparatus  1  illustrated in  FIGS. 2 and 3  is a color image forming apparatus of an intermediate transfer system using electrophotographic process technology. Specifically, image forming apparatus  1  transfers color toner images of C (cyan), M (magenta), Y (yellow), and K (black) formed on a photoconductor onto an intermediate transfer body (primary-transfer), superposes the toner images of the four colors on the intermediate transfer body, and then transfers the images onto a sheet (secondary transfer), thereby forming an image. Sheet S corresponds to the “recording medium” of the embodiment of the present invention. 
     A longitudinal tandem system is adopted for image forming apparatus  1 . In the longitudinal tandem system, respective photoconductor drums  413  corresponding to the four colors of YMCK are placed in series in the travelling direction (vertical direction) of intermediate transfer belt  421 , and the toner images of the four colors are sequentially transferred to intermediate transfer belt  421  in one cycle. 
     Image forming apparatus  1  includes image reading section  10 , operation display section  20 , image processing section  30 , image forming section  40 , sheet conveyance section  50 , fixing section  60  and control section  100 . 
     Control section  100  includes central processing unit (CPU)  101 , read only memory (ROM)  102 , random access memory (RAM)  103  and the like. CPU  101  reads a program suited to processing contents out of ROM  102 , develops the program in RAM  103 , and integrally controls an operation of each block of image forming apparatus  1  in cooperation with the developed program. At this time, CPU  101  refers to various data stored in storage section  72 . Storage section  72  is composed of, for example, a non-volatile semiconductor memory (so-called flash memory) or a hard disk drive. 
     Control section  100  transmits and receives various data to and from an external apparatus (for example, a personal computer) connected to a communication network such as a local area network (LAN) or a wide area network (WAN), through communication section  71 . Control section  100  receives, for example, image data (input image data) transmitted from the external apparatus, and performs control to form an image on sheet S on the basis of the image data. Communication section  71  is composed of, for example, a communication control card such as a LAN card. 
     Image reading section  10  includes auto document feeder (ADF)  11 , document image scanning device  12  (scanner), and the like. 
     Auto document feeder  11  causes a conveyance mechanism to feed document D placed on a document tray, and sends out document D to document image scanner  12 . Auto document feeder  11  enables images (even both sides thereof) of a large number of documents D placed on the document tray to be successively read at once. 
     Document image scanner  12  optically scans a document fed from auto document feeder  11  to its contact glass or a document placed on its contact glass, and brings light reflected from the document into an image on the light receiving surface of charge coupled device (CCD) sensor  12   a , to thereby read the document image. Image reading section  10  generates input image data on the basis of a reading result provided by document image scanner  12 . Image processing section  30  performs predetermined image processing on the input image data. 
     Operation display section  20  includes, for example, a liquid crystal display (LCD) provided with a touch panel, and functions as display section  21  and operation section  22 . Display section  21  displays various operation screens, image conditions, operating statuses of functions, and the like in accordance with display control signals received from control section  100 . Operation section  22  includes various operation keys such as numeric keys and a start key, receives various input operations performed by a user, and outputs operation signals to control section  100 . 
     Image processing section  30  includes a circuit that performs a digital image process suited to initial settings or user settings on the input image data, and the like. For example, image processing section  30  performs tone correction on the basis of tone correction data (tone correction table), under the control of control section  100 . In addition to the tone correction, image processing section  30  also performs various correction processes such as color correction and shading correction as well as a compression process, on the input image data. Image forming section  40  is controlled on the basis of the image data that has been subjected to these processes. 
     Image forming section  40  includes: image forming units  41 Y,  41 M,  41 C, and  41 K that form images of colored toners of a Y component, an M component, a C component, and a K component on the basis of the input image data; intermediate transfer unit  42 ; and the like. 
     Image forming units  41 Y,  41 M,  41 C, and  41 K for the Y component, the M component, the C component, and the K component have similar configurations. For ease of illustration and description, common elements are denoted by the same reference signs. Only when elements need to be discriminated from one another, Y, M, C, or K is added to their reference signs. In  FIG. 2 , reference signs are given to only the elements of image forming unit  41 Y for the Y component, and reference signs are omitted for the elements of other image forming units  41 M,  41 C, and  41 K. 
     Image forming unit  41  includes exposing device  411 , developing device  412 , photoconductor drum  413 , charging device  414 , drum cleaning device  415  and the like. 
     Photoconductor drum  413  is a negative-charging type organic photoconductor (OPC) having photoconductivity in which an undercoat layer (UCL), a charge generation layer (CGL), and charge transport layer (CTL) are sequentially stacked on a peripheral surface of a conductive cylindrical body made of aluminum (aluminum raw pipe), for example. 
     Charging device  414  causes corona discharge to evenly negatively charge the surface of photoconductor drum  413  having photoconductivity. 
     Exposure device  411  is composed of, for example, a semiconductor laser, and configured to irradiate photoconductor drum  413  with laser light corresponding to the image of each color component. The positive charge is generated in the charge generation layer of photoconductor drum  413  and is transported to the surface of the charge transport layer, whereby the surface charge (negative charge) of photoconductor drum  413  is neutralized. An electrostatic latent image of each color component is formed on the surface of photoconductor drum  413  by the potential difference from its surroundings. 
     Developing device  412  is a developing device of a two-component reverse type, and attaches toners of respective color components to the surface of photoconductor drums  413 , and visualizes the electrostatic latent image to form a toner image. Developing device  412  forms a toner image on the surface of photoconductor drum  413  by supplying the toner included in the developer to photoconductor drum  413 . 
     Drum cleaning device  415  includes a drum cleaning blade that is brought into sliding contact with the surface of photoconductor drum  413 , and removes residual toner that remains on the surface of photoconductor drum  413  after the primary transfer. 
     Intermediate transfer unit  42  includes intermediate transfer belt  421 , primary transfer roller  422 , a plurality of support rollers  423 , secondary transfer roller  424 , belt cleaning device  426  and the like. Intermediate transfer unit  42  corresponds to the “conveyance section” of the embodiment of the present invention. 
     Intermediate transfer belt  421  is composed of an endless belt, and is stretched around the plurality of support rollers  423  in a loop form. At least one of the plurality of support rollers  423  is composed of a driving roller, and the others are each composed of a driven roller. When driving roller rotates, intermediate transfer belt  421  travels in direction A at a constant speed. Intermediate transfer belt  421  has conductivity and elasticity, and is driven into rotation with a control signal from control section  100 . 
     Primary transfer rollers  422  are disposed on the inner periphery side of intermediate transfer belt  421  to face photoconductor drums  413  of respective color components. Primary transfer rollers  422  are brought into pressure contact with photoconductor drums  413  with intermediate transfer belt  421  therebetween, whereby a primary transfer nip for transferring a toner image from photoconductor drums  413  to intermediate transfer belt  421  is formed. 
     Secondary transfer roller  424  is disposed to face backup roller  423 B disposed on the downstream side in the belt travelling direction relative to driving roller  423 A, at a position on the outer peripheral surface side of intermediate transfer belt  421 . Secondary transfer roller  424  is brought into pressure contact with backup roller  423 B with intermediate transfer belt  421  therebetween, whereby a secondary transfer nip for transferring a toner image from intermediate transfer belt  421  to sheet S is formed. The secondary transfer nip corresponds to the “transfer nip” of the embodiment of the present invention. 
     Belt cleaning device  426  removes transfer residual toner which remains on the surface of intermediate transfer belt  421  after a secondary transfer. 
     When intermediate transfer belt  421  passes through the primary transfer nip, the toner images on photoconductor drums  413  are sequentially primary-transferred to intermediate transfer belt  421 . To be more specific, a primary transfer bias is applied to primary transfer rollers  422 , and an electric charge of the polarity opposite to the polarity of the toner is applied to the rear side, that is, a side of intermediate transfer belt  421  that makes contact with primary transfer rollers  422  whereby the toner image is electrostatically transferred to intermediate transfer belt  421 . 
     Thereafter, when sheet S passes through the secondary transfer nip, the toner image on intermediate transfer belt  421  is secondary-transferred to sheet S. To be more specific, a secondary transfer bias is applied to backup roller  423 B, and an electric charge of the polarity identical to the polarity of the toner is applied to the front side, that is, a side of sheet S that makes contact with intermediate transfer belt  421  whereby the toner image is electrostatically transferred to sheet S. 
     Fixing section  60  includes upper fixing section  60 A having a fixing side member disposed on a fixing surface side, that is, a side of the surface on which a toner image is formed, of sheet S, lower fixing section  60 B having a rear side supporting member disposed on the rear surface side, that is, a side of the surface opposite to the fixing surface, of sheet S, and the like. The back side supporting member is brought into pressure contact with the fixing side member, whereby a fixing nip for conveying sheet S in a tightly sandwiching manner is formed. 
     At the fixing nip, fixing section  60  applies heat and pressure to sheet S on which a toner image has been secondary-transferred to fix the toner image on sheet S. 
     Upper side fixing section  60 A includes endless fixing belt  61 , heating roller  62  and fixing roller  63 , which serve as a fixing side member. Fixing belt  61  is installed in a stretched state between heating roller  62  and fixing roller  63 . 
     Heating roller  62  incorporates a heating source (halogen heater) and applies heat to fixing belt  61 . The heating source applies heat to heating roller  62 , and as a result, fixing belt  61  is heated. 
     Fixing roller  63  is rotated clockwise by control section  100 . When fixing roller  63  rotates, fixing belt  61  and heating roller  62  rotate in the clockwise direction to follow the rotation of fixing roller  63 . 
     Lower fixing section  60 B includes pressure roller  64  that is the rear side supporting member. Together with fixing belt  61 , pressure roller  64  forms a fixing nip for conveying sheet S in a sandwiching manner. Pressure roller  64  is driven into rotation in the counterclockwise direction by control section  100 . 
     In fixing section  60  of the present embodiment, the conveyance of sheet S is stopped when the operations of fixing roller  63  and pressure roller  64 , that is, the rotation operations, are stopped under the control of control section  100 . Fixing roller  63  and pressure roller  64  correspond to the “pair of rotation member” of the embodiment of the present invention. 
     Sheet conveyance section  50  includes sheet feeding section  51 , sheet ejection section  52 , conveyance path section  53  and the like. Three sheet feed tray units  51   a  to  51   c  included in sheet feeding section  51  store sheets S (standard sheets, special sheets) discriminated on the basis of the basis weight, the size, and the like, for each type set in advance. 
     Conveyance path section  53  includes a plurality of pairs of conveyance rollers such as a pair of registration rollers  53   a  and the like. Sheets S stored in sheet tray units  51   a  to  51   c  are output one by one from the uppermost, and conveyed to image forming section  40  by conveyance path section  53 . At this time, the registration roller section in which the pair of registration rollers  53   a  are arranged corrects skew of sheet S fed thereto, and the conveyance timing is adjusted. Then, in image forming section  40 , the toner image on intermediate transfer belt  421  is secondary-transferred to one side of sheet S at one time, and a fixing process is performed in fixing section  60 . Sheet S on which an image has been formed is ejected out of the image forming apparatus by sheet ejection section  52  including sheet ejection rollers  52   a.    
     In addition, in the present embodiment, jam detection section  73  that detects the occurrence of jam in image forming apparatus  1  is provided as illustrated in  FIG. 3 . Jam detection section  73  is, for example, sensors which are not illustrated in the drawing and are disposed on the upstream side and on the downstream side of the secondary transfer nip in the conveyance direction. Jam detection section  73  detects passage of an end of sheet S to detect jam of sheet S. Specifically, jam detection section  73  determines that jam of sheet S is caused at secondary transfer nip when, after a certain period has elapsed after passage of an end of sheet S is detected by the sensor on the upstream side, the sensor on the downstream side does not detect passage of an end of sheet S. When jam of sheet S is caused, jam detection section  73  outputs jam information relating to the jam to control section  100 . 
     When acquiring jam information, control section  100  performs an operation of stopping the image formation operation during the image formation operation, or more specifically, an operation of stopping the conveyance of sheet S which is being conveyed. In addition to the acquisition of jam information, when failure of a component or the like occurs in image forming apparatus  1 , control section  100  acquires failure information relating to the failure and performs an operation of stopping the image formation operation during the image formation operation. 
     Incidentally, in the case where the image formation operation is stopped during the image formation operation, when the timing of stopping the conveyance of sheet S in fixing section  60  is delayed, sheet S is wound around fixing belt  61 . In view of this, in the case where the image formation operation is stopped during the image formation operation, it is desirable to stop the conveyance of sheet S by fixing section  60  as soon as possible. In addition, in intermediate transfer unit  42 , the stoppage is required to be performed after the high-voltage power supply and the developing bias in developing device  412  are stopped, and therefore the conveyance operation is required to be continued for a predetermined time after the occurrence of jam. 
       FIG. 4  is a timing chart showing a stopping state of an image formation operation, a driving state of fixing section  60  and a driving state of intermediate transfer belt  421 . In  FIG. 4 , “ON” of “stoppage of image formation operation” means that the image formation operation is in a stopping state, and “OFF” means that the image formation operation is in an operation state. In addition, in  FIG. 4 , “ON” of “driving of fixing section” means that fixing section  60  is in an operation state, and “OFF” means that fixing section  60  is in a stopping state. In addition, in  FIG. 4 , “ON” of “driving of intermediate transfer belt” means that the intermediate transfer belt  421  is in an operation state, and “OFF” means that intermediate transfer belt  421  is in a stopping state. 
     In consideration of winding of sheet S around fixing belt  61  and continuation of the conveyance operation of intermediate transfer unit  42 , as illustrated in  FIG. 4 , control section  100  stops the conveyance of sheet S in fixing section  60  at a timing (for example, time T 1 ) between the timing when the image formation operation is stopped during the image formation operation (time T 0 ), and the timing when the conveyance of sheet S in intermediate transfer unit  42  is stopped (time T 2 ). With such an operation, sheet S is sandwiched at both of the secondary transfer nip and the fixing nip, and therefore upward slack of sheet S is caused. When the degree of the slack becomes excessive, sheet S may make contact with surrounding members such as intermediate transfer belt  421 , and may damage the surrounding members at the time of unjamming, for example. 
     In view of this, control section  100  performs an operation for changing the timing of stopping the conveyance of sheet S in fixing section  60  (which is hereinafter referred to as “stop timing”) in accordance with the type of sheet S. The type of sheet S is the basis weight and the thickness of sheet S. Sheet S having a large basis weight and sheet S having a large thickness have high rigidity, and therefore tend to easily damage the surrounding members when the degree of the slack is excessive and the shape of the slacked sheet is held. In view of this, control section  100  delays the stop timing of fixing section  60  as the basis weight of sheet S increases. In addition, control section  100  delays the stop timing of fixing section  60  as the thickness of sheet S increases. In the example illustrated in  FIG. 4 , for example, in the case of sheet S having a large basis weight greater than that of sheet S whose stop timing of fixing section  60  is set at time T 1 , the stop timing of fixing section  60  is set at time T 12  which is later than time T 1 . In this manner, the degree of the slack of sheet S between the secondary transfer nip and the fixing nip can be prevented from becoming excessive, and in turn, the surrounding members can be prevented from being damaged. In addition, in the case of sheet S having high rigidity, even when the stop timing of fixing section  60  is delayed to a certain degree, the sheet does not easily stick to fixing belt  61  for its rigidity, and therefore the winding around fixing belt  61  is not easily caused. 
     In addition, since sheet S having a small basis weight and sheet S having a small thickness have low rigidity, such sheets S stick to fixing belt  61  of fixing section  60 , and are easily wound around fixing belt  61  of fixing section  60  when the stop timing of fixing section  60  is delayed. In view of this, control section  100  advances the stop timing of fixing section  60  as the basis weight of sheet S decreases. In addition, control section  100  advances the stop timing of fixing section  60  as the thickness of sheet S decreases. In the example illustrated in  FIG. 4 , for example, in the case of sheet S having a basis weight smaller than that of sheet S whose stop timing of fixing section  60  is set at time T 1 , the stop timing of fixing section  60  is set at time T 11  which is earlier than time T 1 . In this manner, in the case where the image formation operation is stopped during the image formation operation, winding of sheet S around fixing belt  61  can be suppressed. In addition, in the case of sheet S having low rigidity, even when the stop timing of fixing section  60  is advanced and the slack of sheet S increased, the sheet does not easily damage the surrounding members for its low rigidity, and therefore, it is possible to suppress damaging of the surrounding members even when the stop timing is advanced. 
     In addition, control section  100  determines the stop timing of fixing section  60  in accordance with the type of sheet S by referring to Table 1 and in Table 2 in storage section  72  and the like, for example. Table 1 shows stop timings of corresponding basis weights of sheet S, and Table 2 shows stop timings of corresponding thicknesses of sheet S. In addition, in Table 1 and Table 2, the distance between the secondary transfer nip and the fixing nip is set to 169 mm, and the conveyance speed of sheet S is set to 460 mm/sec. In addition, in Table 1 and Table 2, the stop timing is based on the case where the period from stoppage of the image formation operation until completion of stoppage of intermediate transfer unit  42  is 500 msec. In addition, the stop timing of fixing section  60  is the period from stoppage of the image formation operation until an end of sheet S advances halfway around the fixing belt, when it is assumed that that no surrounding member is provided after the fixing nip. For example, the stop timing of fixing section  60  is set as the time for advancing halfway around fixing belt  61  by 150 mm in the case where fixing belt  61  has a diameter of 100 mm and a circumference of 314 mm. In Table 1 and Table 2, the stop timing of fixing section  60  is defined by the time elapsed from the timing (time T 0 , in  FIG. 4  and  FIG. 5 ) at which the image formation operation is stopped during the image formation operation. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Basis weight(b/m2) 
                 Stop timing 
               
               
                   
               
             
            
               
                   
                 176 or smaller 
                  0 msec 
               
               
                   
                 177-256 
                 300 msec 
               
               
                   
                 257 or greater 
                 450 msec 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Thickness (μm) 
                 Stop timing 
               
               
                   
               
             
            
               
                   
                 190 or smaller 
                  0 msec 
               
               
                   
                 191-280 
                 300 msec 
               
               
                   
                 281 or greater 
                 450 msec 
               
               
                   
               
            
           
         
       
     
     It is to be noted that “176 or smaller” in Table 1 and “190 or smaller” in Table 2 represent the basis weight or the thickness of a thin sheet whose risk of winding around the fixing belt is high. In addition, “177 to 256” in Table 1 and “191 to 280” in Table 2 represent the basis weight or the thickness of a plain sheet whose risk of winding around the fixing belt is relatively low, and whose risk of damaging the surrounding members when the slack is increased is relatively low. In addition, “257 or greater” in Table 1 and “281 or greater” in Table 2 represent the basis weight or the thickness of a thick sheet whose risk of damaging the surrounding members when the slack is increased is high. 
     In addition, the size of sheet S may be used as the type of sheet S. The reason for this is that, when the length of sheet S in the conveyance direction is large, the slack between the secondary transfer nip and the fixing nip may possibly be increased by its length, and consequently the stop timing of fixing section  60  is required to be changed in accordance with the length. In this case, preferably, control section  100  performs an operation of delaying the stop timing of fixing section  60  as the length of sheet S in the conveyance direction increases. 
     In addition, the brand of sheet S may be used as the type of sheet S in combination with the above-mentioned type of sheet S. The reason for this is that, even with the same basis weight, the quality of sheet S can be slightly different from each other depending on the brand of sheet S, and in turn, the stop timing of fixing section  60  can be different from each other, and consequently, the stop timing of fixing section  60  is required to be changed in accordance with the difference in brand. That is, even when sheet S has the same basis weight and thickness, the rigidity (hardness) can be different depending on the quality of sheet S, and the stop timing of fixing section  60  is changed in accordance with the brand of sheet S. 
     In addition, for example, when the sandwiching of sheet S at the secondary transfer nip is released, sheet S does not slacken between the secondary transfer nip and the fixing nip. In view of this, to suppress the winding of sheet S around fixing belt  61 , it is required to stop the conveyance in fixing section  60  at an earlier timing.  FIG. 5  is a timing chart showing a sheet sandwiching state at the secondary transfer nip, a stopping state of an image formation operation, a driving state of fixing section  60  and a driving state of intermediate transfer belt  421 . In  FIG. 5 , “ON” of “sheet sandwiching at the secondary transfer nip” means that sheet S is sandwiched at the secondary transfer nip, and “OFF” means that sheet S is not sandwiched at the secondary transfer nip. 
     As illustrated in  FIG. 5 , when the sandwiching of sheet S at the secondary transfer nip is released before the conveyance of sheet S in fixing section  60  is stopped, control section  100  stops the conveyance of sheet S in fixing section  60  at the timing when the sandwiching of sheet S at the secondary transfer nip is released. In the example illustrated in  FIG. 5 , in the case where the stop timing of fixing section  60  is originally set at time T 1 , when the sandwiching of sheet S at the secondary transfer nip is released at time T 13  which is earlier than time T 1 , the stop timing of fixing section  60  is set to time T 13 . In this manner, winding of sheet S around fixing belt  61  can be further suppressed. 
     It is to be noted that the timing when the sandwiching of sheet S at the secondary transfer nip is released may be calculated based on the feeding timing of sheet S and the distance from the secondary transfer nip in intermediate transfer unit  42 , or may be calculated based on the feeding timing of sheet S and a predetermined time period until the sheet S is conveyed to the secondary transfer nip from the feeding position. 
     Now an exemplary sheet conveyance operation in image forming apparatus  1  having the above-mentioned configuration is described.  FIG. 6  is a flowchart of an exemplary sheet conveyance operation of image forming apparatus  1  according to the present embodiment. The process in  FIG. 6  is executed when control section  100  receives a request of performing a printing job. It is to be noted that, in  FIG. 6 , the stop timing of fixing section  60  is determined based only on the basis weight of sheet S. 
     As illustrated in  FIG. 6 , control section  100  determines whether the image formation operation is stopped during the image formation operation (step S 101 ). When it is determined that the image formation operation is not stopped (step S 101 , NO), the process is advanced to step S 110 . On the other hand, when it is determined that the image formation operation is stopped (step S 101 , YES), control section  100  determines whether sheet S is sandwiched at the secondary transfer nip (step S 102 ). 
     When it is determined that sheet S is not sandwiched at the secondary transfer nip (step S 102 , NO), the process is advanced to step S 107 . On the other hand, when it is determined that sheet S is sandwiched at the secondary transfer nip (step S 102 , YES), control section  100  determines whether the basis weight of sheet S is greater than a first basis weight (for example, 257 g/m 2 ) (step S 103 ). 
     When it is determined that the basis weight of sheet S is greater than the first basis weight (step S 103 , YES), control section  100  stops fixing section  60  at a first stopping time (step S 104 ). The first stopping time is set to “450 msec” which is a stop timing corresponding to a basis weight of “257 or greater” in Table 1, for example. On the other hand, when the basis weight of sheet S is equal to or smaller than the first basis weight (step S 103 , NO), control section  100  determines whether the basis weight of sheet S is not smaller than a second basis weight (for example, 176 g/m 2 ) (step S 105 ). 
     When it is determined that the basis weight of sheet S is not smaller than the second basis weight (step S 105 , YES), control section  100  stops fixing section  60  at a second stopping time (step S 106 ). The second stopping time is set to “300 msec” which is a stop timing corresponding to a basis weight of “177 to 256” in Table 1, for example. On the other hand, when the basis weight of sheet S is smaller than the second basis weight (step S 105 , NO), control section  100  stops fixing section  60  at a third stopping time (step S 107 ). The third stopping time is set to “0 msec” which is a stop timing corresponding to a basis weight of “176 or smaller” in Table 3, for example. 
     After step S 104 , step S 106  and step S 107 , control section  100  stops intermediate transfer belt  421  (step S 108 ). Next, control section  100  determines whether the conveyance of sheet S is resumed (step S 109 ). When it is determined that the conveyance of sheet S is not resumed (step S 109 , NO), the process of step S 109  is repeated. On the other hand, when it is determined that the conveyance of sheet S is resumed (step S 109 , YES), control section  100  determines whether the conveyance of sheet S is completed (step S 110 ). 
     When the conveyance of sheet S is not completed (step S 110 , NO), the process is returned to step S 101 . When the conveyance of sheet S is completed (step S 110 , YES), control section  100  terminates this process. 
     In image forming apparatus  1  having the above-mentioned configuration, the stop timing of fixing section  60  is changed in accordance with the type of sheet S, and thus wounding of sheet S around fixing belt  61  can be suppressed while suppressing the damaging of the surrounding members due to excessive slack of sheet S between the fixing nip and the secondary transfer nip. 
     In addition, since the stop timing of fixing section  60  is delayed as the rigidity of sheet S increases, the degree of the slack of sheet S between the secondary transfer nip and the fixing nip can be prevented from becoming excessive, and in turn, damaging of the surrounding members can be suppressed. 
     In addition, since the stop timing of fixing section  60  is advanced as the rigidity of sheet S decreases, wounding of sheet S around fixing belt  61  can be suppressed. Thus, in the case where fixing section  60  is driven with sheet S being wound around fixing belt  61 , a situation where unjamming cannot be easily performed can be suppressed, and damaging of the surface of fixing belt  61  can be suppressed. 
     In addition, when the sandwiching of sheet S at the secondary transfer nip is released, the stop timing of fixing section  60  is set to the timing when the sandwiching of sheet S at the secondary transfer nip is released, and thus winding of sheet S around fixing belt  61  can be further suppressed. 
     While Table 1 and Table 2 are set from the view point of preventing damaging of the surrounding members and winding of sheet S due to slack of sheet S in the above-mentioned embodiment, the present invention is not limited to this. For example, as shown in Table 3 and Table 4, the table may be set in consideration of only damaging of the surrounding members due to slack of sheet S. Table 3 shows stop timings of corresponding basis weights of sheet S, and Table 4 shows stop timings of corresponding thicknesses of sheet S. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Basis weight(b/m 2 ) 
                 Stop timing 
               
               
                   
               
             
            
               
                   
                 256 or smaller 
                  0 msec 
               
               
                   
                 257 or greater 
                 450 msec 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                 Thickness (μm) 
                 Stop timing 
               
               
                   
               
             
            
               
                   
                 280 or smaller 
                  0 msec 
               
               
                   
                 281 or greater 
                 450 msec 
               
               
                   
               
            
           
         
       
     
     It is to be noted that, also with the stop timing of fixing section  60  in Table 3 and Table 4, the stop timing of fixing section  60  is set to 0 msec in the case of the basis weight (“256 or smaller” in Table 3) and the thickness (“280 or smaller” in Table 4) corresponding to a thin sheet and a plain sheet which can cause winding, and thus the winding of sheet S around fixing belt  61  is not easily caused. 
     In addition, while the stop timing of fixing section  60  is changed in accordance with only the type of sheet S in the above-mentioned embodiment, the present invention is not limited to this. For example, control section  100  may control the stop timing of fixing section  60  in accordance with the information of the image formed at an end portion of sheet S. An example of the image information is the toner amount of a toner image. To be more specific, preferably, control section  100  advances the stop timing of fixing section  60  as the amount of the toner formed at an end portion of sheet S increases. When the amount of the toner formed at an end portion of sheet S is large, sticking to fixing belt  61  easily occurs, and therefore fixing section  60  is required to be stopped at an earlier timing. In this manner, with the above-mentioned operation, wounding of sheet S around fixing belt  61  can be suppressed. 
     In addition, the stop timing of fixing section  60  may be changed in accordance with the conveyance state of sheet S. For example, as illustrated in  FIG. 7 , the stop timing of fixing section  60  may be changed in accordance with a detection result of passage detection section  74  that detects passage of conveyed sheet S. 
     In this configuration, passage detection section  74  is disposed on the downstream side relative to fixing section  60  in the conveyance direction of sheet S such that whether sheet S is conveyed without being wound around fixing belt  61  can be detected. Passage detection section  74  is configured to be able to move between a broken line position located on the conveyance path of sheet S and a solid line position located outside the conveyance path. Passage detection section  74  is located at the broken line position when sheet S has not yet reached the position of passage detection section  74 . When turned down by sheet S reaching that position and moved to the solid line position, passage detection section  74  detects passage of sheet S, and outputs the detection result to control section  100 . 
     In the case where the image formation operation is stopped during the image formation operation of sheet S, when sheet S is sandwiched at secondary transfer nip and sheet S is detected by passage detection section  74 , control section  100  delays the timing of stopping the conveyance of fixing section  60  in comparison with the case where sheet S is not detected by passage detection section  74 . In the case where sheet S is detected by passage detection section  74  located at a position on the downstream side relative to fixing section  60 , it is possible to confirm that sheet S is conveyed without being wound around fixing belt  61 . Therefore, in this case, the slack amount of sheet S can be reduced by delaying the stop timing of fixing section  60 . 
     In addition, in the case where the image formation operation of sheet S is stopped during the image formation operation, when no sheet S is detected by passage detection section  74  for a predetermined sheet feeding time after sheet S is fed, control section  100  advances the timing of stopping the conveyance of fixing section  60  in comparison with the case where sheet S is detected by passage detection section  74 . The predetermined sheet feeding time is set to a time which is calculated from the conveyance distance between the position to which sheet S is fed and the position of passage detection section  74 , and from the conveyance speed of sheet S, for example. When sheet S does not reach the position of passage detection section  74  within the predetermined sheet feeding time, it is recognized that sheet S is wound around fixing belt  61 . Therefore, in this case, the winding of sheet S around fixing belt  61  can be suppressed by advancing the stop timing of fixing section  60 . 
     In addition, as illustrated in  FIG. 8 , the stop timing of fixing section  60  may be changed in accordance with the slack amount of sheet S. In this configuration, first slack detection section  75  and second slack detection section  76  are provided between the secondary transfer nip and the fixing nip in the conveyance direction. 
     First slack detection section  75  is disposed on the conveyance path such that occurrence of slack of sheet S can be detected. First slack detection section  75  has a configuration similar to that of passage detection section  74  in  FIG. 7 . When first slack detection section  75  is located at the solid line position, control section  100  determines that slack of sheet S is not caused, or, that the slack amount is small. When first slack detection section  75  is located at the broken line position, control section  100  determines that the slack amount of sheet S is at a predetermined value or greater. 
     Second slack detection section  76  is located above the conveyance path, and includes contact section  76 A capable of making contact with sheet S having a predetermined slack amount or greater. Second slack detection section  76  is movable in the upper and lower direction, and detects the slack amount of sheet S when contact section  76 A makes contact with sheet S. Contact section  76 A is set at the position illustrated with the chain double-dashed line in an initial state where no slack of sheet S is detected, for example. When sheet S makes contact with contact section  76 A at the position of the chain double-dashed line, control section  100  determines that the slack amount of sheet S is increased. 
     Next, the operation of control section  100  having the configuration illustrated in  FIG. 8  is described. Control section  100  changes the conveyance speed of sheet S in fixing section  60  based on a detection result detected by at least one of first slack detection section  75  and second slack detection section  76 . For example, when the slack amount of sheet S is increased before the stop timing which is set based on the type of sheet S, the slack amount may be undesirably increased until the set stop timing is reached. Therefore, in this case, it is possible to prevent the slack amount of sheet S from being undesirably increased by increasing the conveyance speed of fixing section  60 . 
     In the case where the image formation operation of sheet S is stopped during the image formation operation, control section  100  resumes the conveyance of sheet S in fixing section  60  when at least one of first slack detection section  75  and second slack detection section  76  detects slack of sheet S of a predetermined value or greater after the conveyance of sheet S in fixing section  60  is stopped, that is, when the slack amount of sheet S has a first slack amount. After the conveyance of sheet S is resumed, control section  100  continues the conveyance of sheet S in fixing section  60  until the slack amount of sheet S becomes a value equal to or smaller than the second slack amount that is smaller than the first slack amount. In this manner, when the slack amount is increased more than anticipated, it is possible to set the position to a position where the slack amount of sheet S is reduced, by resuming the conveyance of sheet S. 
     The reference for determining the first slack amount of first slack detection section  75  may be set to the broken line position in  FIG. 8 , and the reference for determining the first slack amount of second slack detection section  76  may be set to the solid line position in  FIG. 8 . The reference for determining the second slack amount of first slack detection section  75  may be set to the solid line position in  FIG. 8 , and the reference for determining the second slack amount of the second slack amount of second slack detection section  76  may be set to the chain double-dashed line position in  FIG. 8 . 
     It is to be noted that, as the configuration of  FIG. 8 , it is also possible to adopt a configuration in which only one of first slack detection section  75  and second slack detection section  76  is provided. 
     In addition, while the conveyance of sheet S in fixing section  60  is stopped by stopping fixing section  60  in the above-mentioned embodiment, the present invention is not limited to this. For example, the conveyance of sheet S may be stopped by releasing the fixing nip in fixing section  60 . In this manner, sheet S at the fixing nip is released, and consequently slack of sheet S between the fixing nip and the secondary transfer nip is not easily caused. Thus, it is possible to advance the stop timing of fixing section  60  to suppress winding of sheet S around fixing belt  61 . It should be noted that, in the case where the fixing nip is released after slack of sheet S is caused due to the difference in conveyance speed between fixing section  60  and the transfer section or the like, the shape of the slacked sheet S may possibly not be reset, and therefore, it is preferable to perform another operation for resuming the conveyance of sheet S in fixing section  60 . 
     In addition, while intermediate transfer unit  42  is the conveyance section in the above-mentioned embodiment, the present invention is not limited to this. For example, the conveyance section may be a conveyance roller pair or the like. 
     The embodiments disclosed herein are merely exemplifications and should not be considered as limitative. While the invention made by the present inventor has been specifically described based on the preferred embodiments, it is not intended to limit the present invention to the above-mentioned preferred embodiments but the present invention may be further modified within the scope and spirit of the invention defined by the appended claims. 
     The present invention is applicable to an image forming system composed of a plurality of units including an image forming apparatus. The units include, for example, a post-processing apparatus, an external apparatus such as a control apparatus connected with a network, and the like. 
     Although embodiments of the present invention has been described and illustrated Specifically, it is clearly understood that the same is by way of illustrated and example only and is not to be taken by way of limitation, the scope of the present invention is interpreted by terms of the appended claims.