Patent Publication Number: US-7899380-B2

Title: Image forming apparatus having fixing device and controller to execute a cleaning mode

Description:
This application is based on Japanese Patent Application Nos. 2007-040044 filed on Feb. 20, 2007, and 2007-281267 filed on Oct. 30, 2007, which are incorporated hereinto by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a technology for conducting cleaning of a fixing device for fixing a toner image onto the sheet in the image forming apparatus, such as a printer, a copier and a facsimile machine of an electro-photographic system. 
     An image forming apparatus of an electro-photographic system transfers a toner image onto a sheet and the toner image is fixed onto the sheet by heating with the fixing device. 
     On this image forming process, there is a case that a sheet jam is generated as a sheet gets stuck in the fixing device.  FIG. 9  illustrates the situation of occurrence of sheet jam in the fixing device.  FIG. 9  illustrates a schematic cross sectional view showing the sheet P, which generates the jam in the printer section in the image forming apparatus and in the fixing section of the printer. 
     As illustrated in  FIG. 9 , a printer section  40  includes an image forming section  41  and a fixing device  42 . The image forming section  41  includes photoreceptor drums  51 Y,  51 M,  51 C and  51 K and an intermediate transfer belt  43 , which is stretched around rollers. A secondary transfer member  45  conducts a secondary transfer of the toner image onto the sheet in the secondary transfer position D. The fixing device  42  includes a fixing member and a pressing member. The fixing member is configured by a fixing roller  421   a , a stretching roller  421   b  and a fixing belt  421  strained. A pressing roller  423  is utilized as a pressing member. 
     The image forming apparatus forms an electrostatic latent image on the photoreceptor drums  51 Y,  51 M,  51 C and  51 K by irradiating a laser beams thereon and forms a toner image by developing the electrostatic latent image using developer. Then, this toner image will be transferred (a primary transfer) onto the intermediate transfer belt  43 . The toner image on the intermediate transfer belt  43  arrives at the secondary transfer position D by a drive of the intermediate transfer belt  43 . The toner image arrived at the secondary transfer position D will be transferred (the secondary transfer) onto a sheet P. The toner image, which has been transferred onto the sheet P but has not been fixed yet, is transferred to the fixing device  42  and the toner image is fixed onto the sheet P with heat and pressure. After that, the sheet P, onto which the image has been formed, will be ejected from the apparatus. 
     On the image forming process described above, as illustrated in  FIG. 9 , there is a case that the sheet P is wound in the fixing device  42  and sheet jam occurs. In case when the sheet P is stopped by the sheet jam under a fixing process, there is a case that the toner, which has been fixed once onto the sheet, is melted by the heat of the fixing belt  421 , retransferred and fixed onto the fixing belt  421 . The residual toner remaining on the fixing belt  421  becomes toner dirt of the fixing device  42 . 
     This residual toner remains fixed onto the fixing belt  421 , and arrives at the nip portion of the fixing belt  421  and a pressing roller  423  by the rotation of the fixing belt  421 . Then the heat of the fixing device  42  melts the residual toner. The melted toner will be fixed onto the sheet, which is processed in the next operation and generates the toner dirt on the image. 
     In order to resolve the toner dirt, cleaning of the fixing device is necessary. However, the temperature of the fixing device is very high and there is a case that the cleaning by an operator is at risk. 
     Thus, an image forming apparatus having a fixing device with a cleaning mechanism, such as a cleaning blade, has been proposed. However, in case that the cleaning mechanism has been provided in the fixing device, the fixing belt is worn out and deteriorated due to the cleaning by the cleaning mechanism, and there is a possibility that belt durability is lost. Further, in case when the cleaning mechanism is provided in the image forming apparatus, the manufacturing process of the image forming apparatus becomes complicated and the cost of the image forming apparatus increases. 
     Thus, the cleaning of the fixing device needs to be safe without newly providing the cleaning mechanism. From this viewpoint, in the conventional image forming apparatus as illustrated in  FIG. 10 , a cleaning mode has been provided for cleaning a fixing device by sheet feeding a white sheet of paper. Here, the cleaning mode in the conventional image forming apparatus will be described by using  FIG. 10 . 
       FIG. 10  illustrates a schematic cross sectional view showing a situation where a sheet feeding is conducted by a cleaning mode executed in the printer in the conventional image forming apparatus. As illustrated in  FIG. 10 , a blank image is formed on the photoreceptive drums  51 Y,  51 M,  51 C and  51 K without forming a toner image, so that the no toner image is transferred to a sheet at the secondary transfer position D. Based on this arrangement, the sheet P will be conveyed to the fixing device  42  in a blank sheet state, onto where the toner image has not been transferred at the secondary transfer position D. 
     Next, the fixing device  42  executes fixing same as the normal image formation, against the sheet P having arrived to the nip portion. In this case, the residual toner adhered onto the fixing belt  421  is melted at the nip portion and retransferred and fixed onto the blank sheet P. As a result, the force to be adhered to the sheet P becomes larger than the force to be adhered onto the fixing belt  421 . Further, an exfoliation force due to the stiffness force of the sheet P is added to the force to be adhered to the sheet P and the toner exfoliates from the fixing belt  421 . Thus, as illustrated in  FIG. 10 , the cleaning of the toner dirt on the fixing belt  421  can be executed. This technology has been disclosed in Unexamined Japanese Patent Application Publication No. 2003-274151. 
     However, according to the sheet feed based on the execution of the cleaning mode in the image forming apparatus disclosed in Unexamined Japanese Patent Application Publication No. 2003-274151, as illustrated in  FIG. 9 , there is a case that the leading edge of the sheet P is adhered onto the fixing belt  421  due to the melted toner having a large adhesiveness and be wound in the fixing belt  421 . In this case, sheet jam due to the residual toner occurs. The situation of the sheet jam due to the residual toner will be explained by utilizing  FIG. 11 .  FIG. 11  illustrates a schematic cross sectional view wound by the printer section of the image forming apparatus and the fixing device of the printer. 
     Namely, normally, even when the melted toner, as illustrated in  FIG. 10 , is adhered onto both of the fixing belt  421  and the sheet P, based on the adhesive force onto the sheet P and the exfoliation force due to the stiffness of the sheet P itself, the toner exfoliates from the fixing belt  421  and the sheet P is conveyed into the conveyance direction (x direction in  FIG. 10 ). However, as illustrated in  FIG. 11 , in case when the melted residual toner is adhered onto the leading edge of the sheet P, which is wound to the fixing belt  421  through the residual toner causing sheet jam as illustrated in  FIG. 9 . 
     In case when sheet jam occurs in the sheet feed of the cleaning mode, the toner dirt of the fixing device is not resolved and the sheet feed needs to be executed again, which makes the cleaning mode complicated. Further, the efficiency of the sheet feed by the cleaning mode decreases. 
     With respect to the sheet jam in the sheet feed in a conventional cleaning mode, in order that the sheet is not wound on the fixing belt  421 , it is thought that a dedicated cleaning sheet be used to avoid the sheet from being wound. However, every time when executing a cleaning mode, the dedicated cleaning sheet has to be set, which makes the cleaning of the fixing device complicated. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invent is to safely and simply execute cleaning of a fixing device in the image forming apparatus without deterioration of the fixing device and increase in manufacturing cost of the image forming apparatus. 
     In order to solve the problems described above, in accordance with one aspect of the present invention, an image forming apparatus for forming an image onto a sheet, is provided with an image forming section for forming a toner image onto the sheet, a fixing device including a fixing member, a pressing member contacting the fixing member and releasing section for pressing and releasing the pressing member to or from the fixing member, and fixing the toner image onto the sheet by applying heat and pressure at a nip portion of the fixing member and the pressing member, a registration roller for feeding the sheet to a secondary transfer position in a downstream of a conveyance direction, and a controller for controlling an execution of a cleaning mode and the conveyance of the sheet P, in which the controller conveys the sheet from a sheet feed tray to the fixing device without forming the toner image onto the sheet and removes toner adhered onto the fixing member, wherein the controller controls the pressing member so as to release a contact to the fixing member before a leading edge of the sheet arrives to the nip portion, and press and contact the fixing member after the leading edge of the sheet has passed through the nip portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic structure of an image forming apparatus of an embodiment of the present invention. 
         FIGS. 2(   a )- 2 ( b ) illustrate schematic drawings of the fixing device of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIG. 3  illustrates a block diagram showing a schematic structure of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIG. 4  illustrates an execution selection screen of a cleaning mode of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIGS. 5(   a )- 5 ( b ) illustrate an execution control of a cleaning mode of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIG. 6  illustrates a schematic drawing of a fixing device of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIG. 7  illustrates a schematic diagram showing a sheet utilized for the sheet feed based on the execution of the cleaning mode. 
         FIG. 8  illustrates a flowchart showing a series of operations of the image forming apparatus in case when receiving an execution instruction of the cleaning mode of the image forming apparatus pertaining to the embodiment of the present invention. 
         FIG. 9  illustrates a schematic cross sectional view showing the sheet stuck in the fixing device of the printer section and the printer section in a conventional image forming apparatus. 
         FIG. 10  illustrates a schematic cross sectional view showing a situation of the sheet feed being executed based on the execution of the cleaning mode in the printer section in a conventional image forming apparatus. 
         FIG. 11  illustrates a schematic cross sectional view showing the sheet wound in the fixing device of the printer section and the printer section in the image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     [Structure] 
     Image forming apparatuses, such as, a printer, which is capable of outputting color images, and a color multifunction peripheral including a copier and a facsimile machine, will be described as embodiments of the present invention. 
     The structure of an image forming apparatus  10  will be described by referring to  FIGS. 1-3 .  FIG. 1  illustrates a schematic structure of the image forming apparatus  10  of an embodiment of the present invention.  FIGS. 2(   a )- 2 ( b ) illustrate schematic drawings of a fixing device  42  of the image forming apparatus  10  pertaining to the embodiment of the present invention.  FIG. 2(   a ) illustrates a pressure-contact state and  FIG. 2(   b ) illustrates a pressure-contact release state.  FIG. 3  illustrates a block diagram of a schematic structure of control of the image forming apparatus  10  pertaining to the embodiment of the present invention. 
     [Image Forming Apparatus] 
     As illustrated in  FIG. 1 , the image forming apparatus  10  includes an automatic document feeder  20 , a reading section  30  and a printer section  40 . The image forming apparatus is arranged to execute a sheet feed of a blank sheet of paper in the cleaning mode for cleaning residual toner adhered on the fixing device. In case when executing the cleaning mode, a controller  60  switches the fixing device  42  into a pressure-contact state. The structure of each section of the image forming apparatus  10  will be described hereinafter. 
     The automatic document feeder  20  conveys documents  2  stacked on the document tray  21  one by one to a reading section  30  and ejects the document after having read to a sheet ejection tray  27 . Further, in case when the documents  2  are a dual surface document, the documents  2 , of which one surface has been read, is reversed front and back by a reversing roller  26  and conveyed to the reading section  30  again. 
     The automatic document feeder  20  includes a sheet feed roller  22  for feeding a sheet from the top portion of the documents  2  placed on a document tray  21 , a conveyance roller  23  for conveying the document  2  onto the slit glass  31  located in the reading position and a guide roller  24  for conveying the document  2  along the conveyance roller  23 . Further, the automatic document feeder  20  includes a switching claw  25  for switching the moving direction of the document  2  passing through on the slit glass  31  and an ejection tray  27 , onto which a read document  2  is ejected. 
     The reading section  30  has a function for reading a color image of the document  2 . The reading  30  is arranged to optically read the document  2  by diving three colors of Red (R), Green (G) and Blue (B), and converts the read image data into four color density data of Yellow (Y), Magenta (M), Cyan (C) and Black (K) and respectively output the image data of the four colors. 
     The reading section  30  includes an exposure and scanning section  35  configured by a light source  33  and a mirror  34 , a CCD (Charge Coupled Device)  36  of a color system for outputting electric signals for each color corresponding to the light intensity after receiving the reflected lights from the document  2 , a collective lens  37  for collecting the reflected lights from the document to the CCD  36  and a mirror  38  for guiding the reflected lights from the mirror  34  to the CCD  36 . 
     The exposure and scanning section  35  reads the document  2  conveyed by the conveyance roller  23  by moving the document  2  to the read position beneath the slit glass  31  and stopping it, in case when reading the document  2  fed by the automatic document feeder  20 . The exposure and scanning section  35  reads a document in a still state placed on the platen glass  32  by scanning the exposure and scanning section  35  from left to right along the under surface of the platen glass  32  when reading the document  2  placed on the platen glass  32 . 
     A printer section  40  includes an image forming section  41 , the fixing device  42 , a sheet feed section  70  for sheet feeding a sheet and a sheet conveyance section  80  for conveying the fed sheet. The image forming section  41  includes an intermediate transfer belt  43 , a plurality of image forming units  50 Y,  50 M,  50 C and  50 K for forming respective mono-color toner images of Y, M, C and BK on the intermediate transfer belt  43 , a primary transfer member, a cleaning section  44  and a secondary transfer section  45 . 
     The image forming unit  50 Y forms a Yellow (Y) color toner image on the intermediate transfer belt  43 . The image forming unit  50 M forms a Magenta (M) color toner image on the intermediate transfer belt  43 . The image forming unit  50 C forms a Cyan (C) color toner image on the intermediate transfer belt  43 . The image forming unit  50 K forms a Black (K) color toner image on the intermediate transfer belt  43 . 
     The image forming unit  50 Y includes a cylindrical shaped photoreceptor drum  51 Y, onto which an electrostatic latent image is formed, a charging unit  52 Y disposed around the photoreceptor drum  51 Y, a developing unit  53 Y and a cleaning unit  54 Y. The image forming unit  50 Y further includes a laser unit  55 Y structured by a laser diode, a polygon mirror, various lenses and a mirror. 
     The photoreceptor drum  51 Y rotates in an arrow A direction by being driven by a drive section. The charging unit  52 Y evenly charges the photoreceptor drum  51 Y. The laser unit  55 Y irradiates laser beams, which is turned on and off corresponding to the Yellow colored image data, onto the photoreceptor drum  51 Y to form the electrostatic latent image thereon. The developing unit  53 Y forms a toner image by developing electrostatic latent using Yellow colored toner. This toner image is transferred onto the intermediate transfer belt  43  at the primary transfer position. The cleaning unit  54 Y removes and collects the residual toner remaining on the surface of the photoreceptive drum  51 Y by using a blade after the transfer. 
     The image forming units  50 M,  50 C and  50 K have the same structure as the image forming unit  50 Y except that the laser beams are to be turned on and off by the corresponding image data of respective colors. Symbols M, C and K are given to the corresponding structural elements instead of the symbol Y even though the number portions are the same. 
     The intermediate transfer belt  43  is supported so as to be capable of rotating around a plurality of rollers. The intermediate transfer rollers  43  rotate in an arrow B direction by being driven by a drive section. On the rotating process, the toner image of each color formed by the image forming units  50 Y,  50 M,  50 C and  50 K is formed onto the intermediate transfer belt  43  in the order of (Y), (M), (C) and (K). This color toner image is transferred onto the sheet from the intermediate transfer belt  43  (secondary transfer) at once in the secondary transfer member  45  provided in lower edge of a circuit path of the intermediate transfer belt  43 . After the secondary transfer, the cleaning section  44  removes the toner remaining on the intermediate transfer belt  43 . 
     The fixing device  42  includes a fixing belt  421 , which is a fixing member, a fixing roller  421   a  and a stretching roller  421   b  for stretching the fixing belt  421  and a pressing roller  423 , which is a pressing member. Further, the fixing device  42  sandwiches the sheet P, onto which a toner image is transferred, with the fixing roller  421   a  and the pressing roller  423  via the fixing belt  421  and fixes the toner onto the sheet P by applying heat and pressure. The sheet P ejected from the fixing device  42  is ejected outside the apparatus or conveyed to a reversing path  80   b . The fixing device  42  will be described in detail later by referring to  FIGS. 2(   a )- 2 ( b ). 
     Sheet feed section  70  includes a plurality of sheet feed trays  71  for storing different sheet sizes and sheet types. In the downstream side of each sheet feed tray  71 , a first sheet feed roller  72  for feeding the sheet stored on the top of the sheet feed tray  71  to a conveyance section  80  is provided. Further, in downstream side of each sheet feed tray  71 , a sheet feed tray  73  for detecting the fed sheet is provided. 
     The conveyance section  80  includes a normal path  80   a  for sheet feeding the sheets conveyed from the sheet feed tray  71  through the secondary transfer member  45  and the fixing device  42  and ejecting the sheets outside the apparatus, and a reverse path  80   b  for reversing the sheets passed through the fixing device  42  and merging the sheets into the normal path  80   a  in the upstream of the secondary transfer member  45 . The respective paths  80   a  and  80   b  include a plurality of conveyance rollers  81  with a distance, which is shorter than the size in the conveyance direction of the minimum sheet size. 
     A leading edge detecting sensor  82  for detecting the leading edge of the sheet is provided in the upstream of the secondary transfer member  45  in the conveyance section  80 . Further, a registration roller  83  is provided in the upstream side. Further, in the upstream side of a registration roller  83 , a second sheet feed sensor  84  for detecting the arrival of the sheet is provided. 
     The sheet fed from the sheet feed tray  71  hits the registration roller  83  and pauses. Then the conveyance is resumed thereafter. The sheet, onto which an image has been formed, is ejected outside the image forming apparatus  10 . 
     [Fixing Device] 
     Next, the schematic structure of the fixing device  42  pertaining to an embodiment of the present invention will be described by using  FIGS. 2(   a )- 2 ( b ). 
     The fixing roller  421   a  in the fixing device  42  is disposed against the pressing roller  423 . 
     The fixing roller  421   a  includes a heat source  422  structured by two halogen lamps inside, and functions as a heating section on the fixing process. Further, the both ends of a pressing roller  423 , which is a pressing member, are supported by the concave section  424   a  of a first arm  424  and the pressing roller  423  can pressure contact or be released from the fixing belt  421  by swinging the first arm  424  around the swing shaft  424   b  (C 1  and C 2  directions in  FIG. 2(   a )). 
     One end of the swing shaft  426   a  is pivotally supported by the second arm  426  in the center section of the first arm  424 . The second arm  426  swings around the swing shaft  426   a  by being pushed by a cam  427 . Further, a pressing spring  425  is arranged between the edge sections, which is opposite side of the swing shaft  424   b  of the first arm  424  against the swing shaft  424   b  and the edge section, which is opposite side of the of the second arm  426  against the swing shaft in the second arm  426 . Meanwhile, the cam  427  is arranged to be driven by a motor (not shown). 
     The pressing method of the fixing device  42  is as follows. Namely, in case when the second arm  426  is rotated in the sheet conveyance path direction (in the C 1  direction (counterclockwise) in  FIG. 2(   b )) by being pressed by the cam  427 , the pressing spring  425  moves in the sheet conveyance path side. In accordance with this movement, also the first arm  424  rotates counterclockwise and firstly, the pressing roller  423  contacts the fixing roller  421   a . Further, in case when the cam  427  continues to rotate and further presses the second arm  426 , the nip portion of the pressing roller  423  and the fixing roller  421   a  is pressed while being deformed by the pressing force of the pressing spring  425 . Meanwhile, the sheet, onto which a toner image has been transferred but not fixed yet (not shown), moves from right to left in  FIGS. 2(   a ) and  2 ( b ) and passes through the nip portion of the fixing roller  421   a  and the pressing roller  423 . Then, a toner image is melted and fixed onto the sheet. 
     Meanwhile, the releasing method of the pressing of the fixing device  42  is as follows. The state shown in the  FIG. 2(   a ) can be shifted to the state shown in the  FIG. 2(   b ) by swinging the second arm  426  to the opposite direction (C 2  direction in  FIG. 2(   a )) of the sheet conveyance path side by the cam  427 . Based on this operation, the pressing spring  425  is extended and the edge section of the first arm  424  is lowered from the position “X” shown in a dotted line to the position “Y” apart from the position “X” by distance d, which is shown in a solid line. The pressing roller  423  held by the first arm  424  is separated from the fixing roller  421   a.    
     [Control] 
     Next, the controller and the schematic structure of each section of the image forming apparatus  10  pertaining to the embodiment in  FIG. 3  will be described. 
     The image forming apparatus  10  includes the controller  60  shown in  FIG. 3 , a memory section  92 , I/F (interface)  91 , a display and operation section  93  and a counting section  94 . 
     The interface  91  receives the document data transmitted from an external terminal U via network  100  and stores the document data to the memory section  92 . With respect to the network  100 , for example, LAN (Local Area Network) is listed. 
     The controller  60  is configured by a CPU (Central Processing Unit), a ROM (Read Only Memory) and RAM (Random Access Memory). In the memory section  92 , a control program has been stored in advance. The CPU extends the control program on the RAM when necessary and executes the control. Practically, there are conducted a reading control, a transmission and reception control, an image processing control, a counting control, a sheet feed control, a display control, a cleaning mode execution control and a sheet feed control. Each process and control will be described later. The memory section  92  stores image data and display image data to be displayed on the display section  93   b.    
     The memory section  92  stores the image data received from the interface  91 . Further, the memory section  92  stores the document data read from the reading section  30 . The controller  60  image processes the stored document data, which will be stored in the memory section  92 . 
     A display operation section  93  is configured by an operation section  93   a  and a display section  93   b . With regard to the operation section  93   a , for example, a touch panel, which is a display and an input device, is used and has roles of a display function and an input function. The operation section  93   a  includes an operation device, such as button. The operation section  93   a  and display section  93   b  may be combined. 
     The counting section  94  has a counting function and transmits the counting signal of the controller to each control of the controller  60  (a sheet feed control, an execution control of a cleaning mode, a sheet feed control and a counting process), which will be described later, to the controller  60 . This counting signal denote a signal used for an execution control of the cleaning mode, which will be described later, and the time measurement of the time used for the sheet feed control. 
     Here, each process and the each control of the controller  60  in the image forming apparatus  10  of the embodiment will be described hereinafter. 
     [Control of each Section] 
     The controller  60  executes the control of automatic document feeder  20  and the reading section  30 , the transmission and reception of the document data, and a compression process and an extension process of the document data stored in the memory section  92 . 
     [Sheet Feed Control] 
     The sheet feed control by the controller  60  controls the sheet conveyance start timing for conveying the sheet from the registration roller  83  to the secondary transfer member  45  in case of image formation. This sheet conveyance timing is adjusted based on the detection of the sheet P 1  by a second sheet feed sensor  84 , the conveyance speed of the sheet P 1  by the registration roller  83  and the distance to the fixing device  42  from the registration roller  83 . 
     [Display Control] 
     The display control by the controller  60  displays the display screen for the operator to set the items of the jobs related to the document or other items on the display section  93   b . The items of job denotes, for example, items of the sheet related to the output image data (magnification, reduction ratio, a sheet direction, an output sheet, a sheet feed tray), an output order, an output method and a post process. Other items denote, for example, the execution of the cleaning mode for cleaning the fixing device  42  and the selection of the execution. 
     There is a selection of the execution of the cleaning mode after the occurrence of sheet jam as an example of the selection of the execution of the cleaning mode. The controller  60  detects an malfunction operation of the conveyance roller  81  and the fixing device  42  (for example, the status where the conveyance process to the normal path  80   a  and the reverse path  80   b  by the conveyance roller  81  cannot be executed) in case when sheet jam occurs in the fixing device  42  and determines whether sheet jam occurs based on this detection. In this case the controller  60 , for example, reads out the data of the display screen for displaying a resolution method for resolving the sheet jam from the memory section  92  and display it on the display section  93   b . In case when the operator has resolved the sheet jam in response to the display screen displayed on the display screen, the controller  60  detects sheet jam clearing. 
     In case when the controller  60  detects the sheet jam clearing, the controller  60  reads out the display data of the selection screen for selecting the execution of a cleaning mode from the memory section  92 . This selection screen will be described by referring to  FIG. 4 . 
       FIG. 4  illustrates a selection screen of a cleaning mode. The operator can select an execution of the cleaning mode or non-execution, which will be described later, through the cleaning mode selection screen by using the operation section  93   a.    
     Namely, in case when the operator selects the execution key of the selection screen illustrated in  FIG. 4 , the controller  60  executes the cleaning mode. Meanwhile, in case when a non-execution key N is selected, the controller  60  does not execute a cleaning mode. The selection in the cleaning mode selection screen is not limited to the timing after the controller  60  has detected the sheet jam clearing. The controller  60  may be configured so that the operator can execute the selection at an arbitrary timing by using the display operation section  93 . 
     [Execution Control of Cleaning Mode] 
     In the execution control of the cleaning mode by the controller  60 , the cleaning of the toner dirt adhered on the fixing device  42  is executed by conducting the sheet feed of a blank sheet in response to the request of the execution of the cleaning mode on the selection screen. This execution control of the cleaning mode will be explained by using  FIGS. 5(   a )- 5 ( b ).  FIG. 5(   a ) illustrates the situation where the sheet P 1  has arrived in front of the nip portion of the fixing device  42  under the execution control of the cleaning mode. Further,  FIG. 5(   b ) illustrates the situation where the cleaning of the fixing device  42  is executed under the execution control of the cleaning mode. 
     The controller  60 , as the execution control of the cleaning mode, controls the sheet conveyance timing, by which the sheet P 1  arrives at the fixing device  42  under the control of registration roller  83 . The sheet conveyance timing of sheet P 1  is determined based on the conveyance speed of the sheet P 1 , the distance L 1  (refer to  FIGS. 5(   a )- 5 ( b )) and the detection of the sheet P 1  by the sensor. 
     The controller  60  measures the conveyance timing of sheet P 1  and conveys the sheet P 1  to the secondary transfer member  45 . Under the execution control of the cleaning mode, the transfer of a toner image onto the sheet P 1  arrived at the secondary transfer member  45  is not conducted. Namely, in case of image formation, a toner image is formed onto the intermediate transfer belt  43  by the image formation units  50 Y,  50 M,  50 C and  50 K and the toner image is transferred onto the sheet P 1  passing through the secondary transfer member  45 . In case when conducting the execution of the cleaning mode, the controller  60  controls image formation units  50 Y,  50 M,  50 C and  50 K not to form the toner image. Thus, the toner image is not transferred onto the intermediate transfer belt  43 , further the toner image is not transferred onto the sheet P 1 , even the sheet P 1  passes through the secondary transfer section  45  and the sheet P 1  is conveyed to the fixing device  42  in a blank sheet condition. 
     Further, under the execution control of the cleaning mode by the controller  60 , as illustrated in  FIG. 5(   a ), prior to the arrival of the sheet P 1  to the fixing device  42 , the controller  60  controls the pressing roller  423  of the fixing device  42  so as to be away from the fixing roller  421   a  to release the pressing state in the fixing device  42 . 
     Namely, the controller  60  releases the pressure of the fixing device  42  when a first time period has passed from the time when the registration roller  83  started the conveyance of the sheet P 1 . 
     This first time period corresponds to the a time period, for which the sheet P 1  needs to reach in front of the nip portion of in the fixing device  42  from the registration roller  83 . This first time period can be obtained from the distance L 1  from the registration roller  83  to the place, which is in front of the nip portion (refer to  FIGS. 5(   a )- 5 ( b )), and the conveyance speed of the registration roller  83 , which has been stored in the memory section  92  in advance. As described above, since the controller  60  releases the pressure of the fixing device  42  prior to the arrival of the sheet P 1  to the nip portion of the sheet P 1  and the leading edge of the sheet P 1  passes in the released state, it is prevented that the leading edge of the sheet P 1  is wound around the fixing device  42  by the residual toner, and further the occurrence of sheet jam under the execution of the cleaning mode can be prevented. Thus, even though the melted residual toner and the conveyed sheet P 1  arrive at the nip portion at the same time, since the fixing device  42  has been released ( FIG. 5(   b )) and at least the leading edge of the sheet passing through the fixing device  42  is able to pass without being pressed. Accordingly, the effects of the stiffness of the sheet P 1  prevent the melted toner from adhering onto the sheet P 1 . Further, the effects of the stiffness of the sheet P 1  prevent the situation where the sheet P 1  is wound from occurring. Thus, the occurrence of the sheet jam in case when cleaning the toner dirt on the fixing belt  421  can be avoided. 
     Further, in the execution control of the cleaning mode, the controller  60  releases the pressure of the fixing device  42  prior to the arrival of the sheet P 1  to the nip portion of the fixing device  42  and controls the fixing device  42  to resume in a pressured state after the leading edge of the sheet P 1  has passed through the nip portion. 
     Namely, the controller  60  controls the fixing device  42  so as to resume to be in the pressured state after a second time period has passed, which is a period from the time when the pressure of the fixing device  42  has been released to the time when the leading edge of the sheet P 1  has passed through the nip portion. 
     In this embodiment, the controller  60  controls the fixing device  42  to resume to be in the pressured state after the time period has passed, which is obtained by adding the first and the second time periods, from the starting time when starting the conveyance of the sheet P 1  from the registration roller  83 . However, it also may be possible that the fixing device  42  resumes to be in the pressured state after the second time period has passed from the starting time when the controller  60  releases the pressure of the fixing device  42 . 
     The second time period denotes a time period obtained by diving the distance L 2  ( FIGS. 5(   a ) and  5 ( b )), which is a distance from in front of the nip portion (the position where the controller  60  releases the pressure of the fixing device  42 ) to the position where the leading edge of the sheet P 1  has passed the nip portion, by the conveyance speed of the sheet P 1 . The position where the leading edge of the sheet P 1  has passed is preferably, for example, the position where the leading edge of the sheet P 1  is not less than 10 mm away from the nip portion. With this distance, the leading edge of the sheet P 1  will not be wound and it becomes possible for the sheet to conduct cleaning in a wider portion. 
     As describes above, by resuming the fixing device  42  to the pressured state, the sheet P 1  passing through the nip portion of the fixing device  42  passes through the nip portion under the pressured state and the residual toner adhered onto the fixing belt  421  moves onto the sheet P 1 . As a result, the residual toner is removed from the fixing belt  421 . Thus, the toner dirt of the fixing device  42  can be resolved. 
     Further, controller  60  preferably controls the switching of the pressure and the release based on the thickness of the sheet P 1  used for the cleaning mode. For example, the controller  60  determines the thickness of the sheet P 1  based on the sheet information inputted by the operator, for example, the nominal weight. 
     The controller  60  determines whether the thickness of the sheet P 1  is within a prescribed thickness and releases the pressure only when the thickness is not more than the prescribed thickness. In case when the thickness is more than the prescribed thickness, it is preferred not to conduct the switch. This prescribed thickness denotes a thickness of normal sheet. For example, the prescribed thickness is a thickness when the nominal weight of the sheet is 200 g/m 2 . 
     In case of the sheet having the nominal weight is more than 200 g/m 2 , even though the release of the pressure of the fixing device  42  is not executed, the leading edge of the sheet P 1  reaches to the nip portion of the fixing device  42  and the residual toner adheres the leading edge of the sheet. Since the stiffness of the sheet is strong, the leasing edge of the sheet will not be wound. Thus, sheet jam does not occur in the cleaning mode. 
     (Sheet Refeed Control) 
     When the entire fixing belt  421  cannot be cleaned in response to the length of the sheet P 1 , the controller  60  executes a sheet re-feed control, which cleans by the first sheet P 1  and the following sheet P 2 . 
     The controller  60  in the sheet re-feed control obtains the range of the fixing belt  421 , which has not been cleaned in the sheet feeding of the first sheet P 1 , and executes the following process in order to re-execute a cleaning mode on non-cleaned area. 
     A length of the circumference of the fixing belt  421 , which is stored in a memory section  92  in advance, and a length L 3  of the conveyance direction drawn from the size of the sheet P 1  used in the cleaning mode (refer to  FIG. 6 ) are compared. As a result, when the length L 3  of the sheet P 1  is less than a length L 4  of the fixing belt  421 , the sheet re-feed control is executed by the following sheet P 2 . The length of the circumference, for example, is the length of an area A 1  and area A 2  combined in  FIG. 6 . 
     When the length of the sheet P 1  is less than the length L 4  of the fixing belt  421 , the controller  60  executes the sheet re-feed control. In case when the sheet re-feed control of the cleaning mode re-executed, a control of the sheet conveyance start timing of a following sheet P 2  from the registration roller  83  is executed. 
     The sheet conveyance start timing denotes a timing to start the conveyance of the sheet P 2  from the registration roller  83  in order to have a leading edge of the sheet P 2  passing through the nip portion at the same time with a section, which the cleaning of the fixing belt  421  should start, reaching the nip portion. The sheet conveyance start timing of the sheet re-feed control in the controller  60  will be explained using  FIG. 6 .  FIG. 6  illustrates a schematic drawing of the fixing belt  421 , the pressuring roller  423  and the sheet P 1  in the fixing device  42 .  FIG. 7  illustrates a plan view showing the sheet P 1 . 
     When the length L 3  of the first sheet P 1  (refer to  FIG. 7 ) is less than a length of the fixing belt  421 , the fixing belt  421 , which cleaning is executed by the first sheet P 1 , is divided into a cleaned area A 1  and non-cleaned area A 2 . Here, a sheet feeding by the cleaning mode of a following sheet P 2  will clean the non-cleaned area A 2  by the following sheet P 2 . Therefore, in case when the leading edge of the following sheet P 2  reaches the nip portion, the sheet conveyance timing conduct a contact at a boundary section between the cleaned area A 1  and the non-cleaned area A 2  of the fixing belt  421 . Further, it is preferred to have a contact at the cleaned area A 1  side (Z direction side in  FIG. 6 ) from the border portion, and to have a part of the cleaning area of the sheet P 1  and P 2  overlap. When a cleaning is executed by the following sheet P 2 , it is possible to avoid having a gap in the border portion of a following edge of the cleaned area Al and the leading edge of the non-cleaned area A 2 , and prevent an omission of the cleaning by overlapping parts of the cleaning area of the sheet P 1  and P 2 . 
     Namely, the sheet conveyance start timing for the following sheet P 2  is set as follows. 
     The controller  60  divides the length L 3  of the sheet conveyance direction of the sheet P 1  by the sheet conveyance speed of the registration roller  83  and stores memory of the third time period needed for the sheet P 1  to pass through the nip portion to the memory section in advance. 
     Further, the controller  60  divides the length L 4  of the fixing belt  421  by a rotation speed of the fixing belt  421  and stores memory of the fourth time period needed for the fixing belt  421  to make a full turn to the memory section in advance. 
     The controller  60  obtains the fifth time period by adding the fourth time period to the third time period stored in memory section in advance. The sheet conveyance start timing, which starts the sheet conveyance of the following sheet P 2  from the registration roller, is considered to be after the fifth time period has passed based on the time of starting the sheet conveyance of the first sheet P 1  from the registration roller. 
     Further, also in the sheet re-feed control, the controller  60  executes the switching control of pressuring and the releasing of the fixing device  42 . The switching controller is as is mentioned above, therefore, the explanation will be omitted. 
     By controlling in this way, the cleaning of the outer circumference section of the fixing belt  421  is executed. Therefore, a complicated operation, where an operator determines the existence of the omission of the cleaning of the fixing device  42 , can be avoided. 
     Further, when the non-cleaning area is determined to be present in the outer circumference section of the fixing belt  421  after the first cleaning mode has been executed, the controller  60  executes the sheet re-feed control. Thus, even when the cleaning of the residual toner of the fixing device  42  is not completed by the first sheet, the sheet feeding is executed by repeating the cleaning mode with the second sheet. Therefore, the omission of the cleaning of the fixing device by the sheet feeding is avoided, and the sheet feeding is efficiently executed by the execution of the cleaning mode. 
     Next, a flow of the execution of the cleaning mode in the image forming apparatus  10  of the embodiment of the present invention is explained by using a flow chart illustrated in  FIG. 8 . 
     The controller  60  determines the sheet jam clearing after the occurrence of the sheet jam in the fixing device  42  (S 01 ). When the sheet jam clearing is detected (S 01 : YES), the controller  60  displays a selection screen for the execution of the cleaning mode (refer to  FIG. 4 ) on an operation section  93   a  (S 02 ). In the selection screen, the determination is made whether the execution has been selected by an operator or not (S 03 ). When the execution is selected (S 03 : YES), the controller  60  starts controlling the cleaning mode (S 04 ). Meanwhile, when the execution was not selected by the operator (S 03 : NO), the flow is terminated, and the cleaning mode will not be executed. 
     The controller  60  starts controlling of the cleaning mode in STEP  04  (S 04 ). 
     Firstly, the controller  60  determines if it has reached the sheet conveyance start timing (S 05 ). 
     When it is determined not to have reached the sheet conveyance start timing (S 05 : NO), the controller  60  repeats the determination of STEP S 05 . When it is determined to have reached the sheet conveyance start timing (S 05 : YES), the registration roller  83  is controlled and the sheet conveyance of the sheet P 1  is started (S 06 ). Further, a timer is started based on the sheet conveyance starting timing. 
     Next, the controller  60  determines whether a thickness of the sheet P 1  used to execute the cleaning mode is less than a prescribed thickness or not (S 08 ). The determination of the prescribed thickness is made based on, for example, the data of the information of a sheet, such as, the size of the sheet P 1  and a nominal weight of the sheet, inputted by an operator. For example, the determination is made based on whether the nominal weight of the sheet P 1  is less than 200 g/m 2 . 
     When the sheet P 1  is determined to be not less than the prescribed thickness (S 08 : NO), the controller  60  sheet feeds the sheet P under the pressured state without switching the pressure and release of the fixing device  42 , and terminates the flow. 
     When the sheet P 1  is determined to be less than the prescribed thickness (S 08 : YES), the controller  60  determines whether the time of the timer has become the first time period or not (S 09 ). The first time period denotes the time from the point of the start of the sheet conveyance of the sheet P 1  from the registration roller  83  up to the point of the leading edge of the sheet P 1  to reach before the nip portion of the fixing section  42 , and is the release timing for releasing the pressure. When the first time period has determined not to have passed (S 09 : NO), the controller  60  repeats the determination until the first time period passes. 
     When it is determined that the first time period has passed (S 9 : YES), and a release timing of the pressure of the fixing device  42  has arrived, the controller  60  puts the cam  427  of the fixing device  42  in drive and releases the pressure of the fixing device  42  after a pressuring roller  423  is separated from a fixing roller  421   a  via the second arm  426  and the first arm  424  (S 10 ). 
     Next, the controller  60  determines whether the time, which the first time period and the second time period has combined, has passed (S 11 ). The second time period denotes the time from the releasing point of the pressure of the fixing device  42  up till a point of the first sheet reaching the nip portion, which the sheet P 1  passes through. The pressuring point is from a point of the release of the pressure to a point of the distance L 2  after the sheet P 1  passes through the nip portion. Since a timer is counting a time from the point of the start of the sheet conveyance, a time, which the first time period and the second time period has combined, becomes the timing for pressuring. When time, which the first time period and the second time period have been combined, is determined to have not passed (S 11 : NO), the controller  60  repeats the determination. When time, which the first time period and the second time period have been combined, is determined to have passed (S 11 : YES), the controller  60  puts the cam  427  of the fixing device  42  to drive, and the pressuring roller  423  is pressured onto the fixing roller  421   a  again via the second arm  426  and the first arm  424  (S 12 ). After the pressuring roller has been pressured, the cleaning mode is executed while the sheet P 1  is in the state of being closely attached to the fixing belt  421 . 
     Next, the controller  60  determines whether there is a non-cleaning area present on the fixing belt  421  or not (S 13 ). When the length L 3  of the first sheet P 1  utilized in the execution of the cleaning mode is less the length L 4  of the fixing device, it is determined that the non-cleaning area is present (S 13 : YES). When the non-cleaning area is determined to be present (S 13 : YES), the controller  60  restarts the controlling of the cleaning mode by the second sheet P 2  (S 14 ). The controller  60  calculates the fifth time period, which is the sheet conveyance start timing of the following sheet P 2 , by adding the fourth time period needed to circuit around the fixing belt  421  and the third time period, needed to convey the length L 3  of the sheet conveyance direction of the first sheet P 1  based on the sheet conveyance start timing of the first sheet P 1  from the registration roller  83  (S 15 ). The timing is a combination of a timing that a following edge B of an area A 1  cleaned by the first sheet P 1  reaches to the nip portion again and a timing that a following sheet P 2  reaching the nip portion. Next, the determination of whether the timer has counted up the fifth time period or not (S 16 ). When the timer has not counted up the fifth time period, the determination of S 16  is repeated (S 16 : NO). When the timer has counted up the fifth time period (S 16 : YES), the timer is cleared (S 17 ), and returns to the STEP  06 . The same controlling flow mentioned above is executed hereinafter. 
     A function and an effect of an image forming apparatus, which is related to the embodiment explained above, will be explained. 
     The image forming apparatus  10  related to the embodiment of the present invention is structured to release the pressure of the fixing device  42  before the sheet P 1  and P 2  reaches the nip portion of the fixing device  42  by the controller  60  in response to the execution order of the cleaning mode from the operator. 
     Accordingly, when the sheet feeding is executed by the cleaning mode in the image forming apparatus  10 , the sheet reaches the nip portion of the fixing device  42  after the sheet conveyance from the registration roller  83 . Even when the residual toner left on the fixing belt  421  and a leading edge F 1  (refer to  FIG. 7 ) of the sheet P 1  conveyed reaches the nip portion at the same time, the pressure of the fixing section  42  has been released, thus, only a leading edge section of the sheet passes through the fixing device  42  without pressure. And, it prevents the residual toner strongly attached to the leading edge of the sheet P 1  to be taken into the fixing belt  421 , and the sheet jam can be avoided when a cleaning of the toner dirt on the fixing belt  421  is executed. 
     Further, when the operator is not in need of cleaning the toner dirt on the fixing device  42 , and since the temperature inside of the image forming apparatus  10  is high, the risk of the operator cleaning the inside can be prevented. Further, there is no need to provide a cleaning mechanism around the fixing device  42 . The increase of the production cost is not involved, and a wear out and a deterioration of a belt by contact and slide of the cleaning mechanism can be prevented. Also, space inside the image forming apparatus may be conserved. 
     When the second time period, which the leading edge of the sheet P 1  and P 2  passes through the nip portion of the fixing device  42  corresponding to the controlling of the cleaning mode by the controller  60 , the image forming apparatus  10  relating to the embodiment tries to put back the fixing device  42  under pressure by pressuring the pressuring roller  423  of the fixing device  42  contacted against the fixing roller  421   a.    
     Therefore, the pressure of the fixing device  42  is released only at the time when the leading edge of the sheet P 1  and P 2  passes through. Since the fixing device  42  will be put under the pressure again after the leading edge passes through, the cleaning of toner dirt on the fixing belt  421  can be executed without the occurrence of a sheet jam. 
     Next, a modified example of the image forming apparatus of the present invention will be described hereinafter. 
     In the image forming apparatus  10  pertaining to the embodiment described above, the release timing of the pressure of the fixing device  42  is controlled based on the conveyance times of the sheets P 1  and P 2  from the registration roller  83 . However, the present invention is not limited to this embodiment. 
     For example, the controller  60  controls the fixing device  42  so that the pressure of the fixing device  42  may be released at the timing when detecting the leading edge of the sheet P 1  by providing a leading edge detection sensor  82  between the registration roller  83  and the fixing device  42 . By releasing the pressure of the fixing device  42  at the timing described above, sine the leading edge of the sheet passes through the fixing device  42  without being pressed, it becomes possible to prevent the sheet to be wound around the fixing device  42  and sheet jam in case of the cleaning mode can be dissolved. 
     Further, in the image forming apparatus  10  pertaining to the embodiment, the conveyance period of time of sheets P 1  and P 2  control the timing of allowing the fixing device  42  to resume the pressing state. However, the present invention is not limited to this embodiment. 
     For example, the resume timing to the pressing state may be controlled based on the detection of the arrival of the leading edges of sheets P 1  and P 2  by providing the sensor for detecting that the leading edges of sheets P 1  and P 2 . By resuming the pressing state as described above, the leading edges of sheets P 1  and P 2  can be surely conveyed to the downstream, the winding around the fixing section  42  can be prevented and the sheet jam when feeding a sheet in the cleaning mode can be dissolved. 
     Further, in the fixing device  42  pertaining to the embodiment, a fixing belt  421  and fixing rollers  421   a  and  421   b  configure the fixing member. However, a heat roller may configure the fixing member. 
     In the image forming apparatus  10  pertaining to the embodiment of the present invention, the controller  60  is configured to calculate the cleaning area cleaned by the fist sheet P 1  based on the length determined by the size of the sheet P 1  when the controller  60  executes cleaning by utilizing the second sheet P 2 . However, the present invention is not limited to this embodiment. 
     For example, by providing the sensor for detecting the residual toner in the circumference of the fixing device  42 , the controller  60  may check the cleaning are by the sheet P 1  and the area, which has not cleaned, and control the conveyance timing of the second sheet P 2 . 
     In this embodiment, the controller is configured to release the pressure of the fixing device before the leading edge reaches to the nip portion of the fixing member and presses the fixing member after the leading edge has passed through the nip portion, in case when the controller executes the cleaning mode of the fixing member. Thus, it becomes possible to prevent the leading edge of the sheet, which passes through the fixing member when executing the cleaning mode, to adhere to the fixing member and to be wound around the fixing member in case of the sheet feed in the cleaning mode. Further sheet jam associated with the sheet feed in the cleaning mode can also be prevented. Further, the melted residual toner surely adheres to the sheet and the toner dirt of the fixing member can be removed by pressing the fixing member after the leading edger of the sheet has passed through the nip portion. As a result, the cleaning of the fixing member in the image forming apparatus can be safely and easily executed without the deterioration of the fixing member and increase of the manufacturing cost.