Patent Publication Number: US-2011069984-A1

Title: Fixing device, image forming apparatus and method for fixing image

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior U.S. Patent Application No. 61/244,727, filed on Sep. 22, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     This application is also based upon and claims the benefit of priority from Japanese Patent Application No. 2010-135829, filed on Jun. 15, 2010; and Japanese Patent Application No. 2010-152813, filed on Jul. 5, 2010, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Exemplary embodiments described herein relate to a fixing device loaded in an image forming apparatus to fix a toner image on a sheet. 
     BACKGROUND 
     An image forming apparatus such as a copy machine of an electro photographic system and a printer selectively exposes on a drum shaped photoconductor, for example, whose surface is charged uniformly, to form a latent image, and develops the latent image by toner. And the image forming apparatus transfers a toner image which is developed on the photoconductor onto a sheet, and fixes the image on the sheet by passing the sheet through a fixing nip formed by a fixing belt, a fixing roller and a pressing roller composing a fixing device. That is, the image forming apparatus fixes the image on the sheet by rotating the fixing roller and the pressing roller under a high nip pressure via the fixing belt which is heated up to a fixing temperature and by applying heat and pressure. 
     In the fixing device, in the state except fixing, the fixing roller and the pressing roller are rotated via the fixing belt in the state that the nip pressure is low so as to expand the life of the fixing belt, the fixing roller and the pressing roller. In such the fixing device, in case that the fixing roller and the pressing roller are rotated while keeping the nip pressure low, a flexure of the fixing belt may happen, and the flexure of the fixing belt may happen to make contact with a separator member positioning at the downstream side than the nip in the conveying direction of the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic construction view showing an image forming apparatus loaded with a fixing device in a first embodiment; 
         FIG. 2  is a front view of the fixing device in the first embodiment; 
         FIG. 3  is a schematic construction view of the fixing device in the first embodiment seen from above; 
         FIG. 4  is a view showing a gear sequence in the first embodiment; 
         FIG. 5  is a perspective view of the gear sequence in the first embodiment; 
         FIGS. 6A and 6B  are views, each showing a fixing roller and a pressing roller in the first embodiment; 
         FIG. 7  is a block diagram showing a controller of the fixing device in the first embodiment; 
         FIG. 8  is a flow chart showing an operation of the image forming apparatus in the first embodiment; 
         FIG. 9  is a view showing a gear sequence in a second embodiment; 
         FIG. 10  is a flow chart showing an operation of an image forming apparatus in the second embodiment; and 
         FIGS. 11A and 11B  are views, each showing a fixing roller and a pressing roller in a modification. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, there is provided a fixing device including: a fixing device includes a fixing roller having an elastic layer, a fixing belt wound around the fixing roller, a satellite roller to set up the fixing belt between the fixing roller and the satellite roller, a pressing roller to nip hold the fixing belt between the fixing roller and the pressing roller, a separator to separate a sheet at a position separate from the fixing belt, a pressure changing mechanism to change a nip pressure between the fixing roller and the pressing roller, and a driving device to rotate the fixing belt in a first direction when the nip pressure is a first nip pressure and to rotate the fixing belt in a second direction which is reverse to the first direction when the nip pressure is a second nip pressure which is lower than the first nip pressure. 
     Hereinafter, best embodiments of a fixing device, an image forming apparatus and a method for controlling an image fixing device will be described in detail with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a schematic construction view showing an internal construction example of an image forming apparatus  1  loaded with a fixing device  37  of a first embodiment. 
     The image forming apparatus  1  includes a scanner  2 , a sheet ejector  3  and an image forming portion  6 . The scanner  2  reads out an image on a document as a color image data (an image data with many values) or a monochrome image data (an image data with two values) by scanning the document face optically. The sheet ejector  3  houses a sheet P which was image formed and ejected from the chassis of the apparatus. 
     The image forming portion  6  forms an image on the sheet P based on the color image data or the monochrome image data. The image forming portion  6  has an image forming unit  11  including four image forming stations  11 Y,  11 M,  11 C and  11 K of yellow (Y), magenta (M), cyan (C) and black (K) arranged in parallel with along a lower side of an intermediate transfer belt  10 . 
     As each of the image forming stations  11 Y,  11 M,  11 C and  11 K has the same construction, accordingly, the construction and function will be described with respect to the image forming station  11 K for black use, and with respect to the other image forming stations, the detailed description will be omitted by giving corresponding references. 
     The image forming station  11 K includes a photoconductor drum  12 K. Around the photoconductor drum  12 K, a charger  13 K, a developing device  14 K and a photoconductor cleaning device  16 K are arranged along a rotation direction shown by an arrow m. Exposing light is irradiated by a laser exposing device  17  on a drum surface of the photoconductor drum  12 K at a position between the charger  13 K and the developing device  14 K. When the exposing light is irradiated, an electrostatic latent image is formed on the rotating photoconductor drum  12 K. 
     The developing device  14 K has a two-component black (K) developer composed of toner and carrier, for example. The developing device  14 K feeds black toner to the electrostatic latent image on the photoconductor drum  12 K. 
     The intermediate transfer belt  10  is set up by a backup roller  21 , a driven roller  20  and a first to a third tension roller  22  to  24 . The intermediate transfer belt  10  faces to and contacts with the photoconductor drums  12 Y,  12 M,  12 C and  12 K. At the positions of the intermediate transfer belt  10  facing the photoconductor drums,  12 Y,  12 M,  12 C and  12 K, primary transfer rollers  18 Y,  18 M,  18 C and  18 K are provided so as to primarily transfer the images formed on the photoconductor drums  12 Y,  12 M,  12 C and  12 K which are the toner images to the intermediate transfer belt  10 , respectively. Each of the toner images of each color formed on each of the photoconductors is sequentially transferred on the intermediate transfer belt  10  by each of the primary transfer rollers to become a color image. 
     At a secondary transfer portion of the intermediate transfer belt  10 , a secondary transfer roller  27  is arranged. At the secondary transfer portion, a secondary transfer bias is applied to the backup roller  21 . Below the laser exposing device  17 , a sheet cassette  4  is provided to feed the sheet P to the secondary transfer roller  27 . At the right side of the image forming apparatus  1 , a manual sheet feeder  31  to feed the sheet P manually is provided. 
     At the positions between the sheet cassette  4  and the secondary transfer roller  27 , a pickup roller  4   a , a separation roller  28   a , a conveying roller  28   b  and an aligning roller  36  are provided so as to convey the sheet P from the sheet cassette  4 . At the positions between a manual sheet feed tray  31   a  of the manual sheet feeder  31  and the aligning roller  36 , a manual sheet pickup roller  31   b  and a manual sheet separation roller  31   c  are provided so as to convey the sheet P on the manual sheet feed tray  31   a.    
     While the sheet P is sandwiched and conveyed between the intermediate transfer belt  10  and the secondary transfer roller  27 , the toner image on the intermediate transfer belt  10  is secondarily transferred on the sheet P. After the secondary transfer is finished, the intermediate transfer belt  10  is cleaned by a belt cleaner  10   a . Along the running direction of the sheet p, a fixing device  37  is provided at the downstream of the secondary transfer roller  27 . The sheet p fed from the sheet cassette  4  or the manual sheet feeder  31  is conveyed along a conveying path  34  to the fixing device  37  through the aligning roller  36  and the secondary transfer roller  27 . 
     At the downstream of the fixing device  37 , a gate  33  is provided, which sorts the sheet P in a direction for an ejection roller  41  or in a direction for a re-conveying unit  32 . The sheet P guided to the ejection roller is ejected to the sheet ejector  3 . The sheet guided to the re-conveying unit  32  is guided again in a direction for the secondary transfer roller  27 . 
       FIG. 2  is a schematic construction view of the fixing device  37  seen from the front, and  FIG. 3  is a schematic construction view of the fixing device  37  seen from above. The fixing device  37  has an endless fixing belt  42  wound around a fixing roller  38  and a satellite roller  141  and a pressing roller  43 . The fixing roller  38  is formed by coating a foamed rubber (sponge) layer  38   b  with a thickness of 8.5 mm as an elastic layer around a cored bar  38  with a thickness of 2 mm, for example. The outer diameter of the fixing roller  38  is 48.5 mm, for example. The outer diameter of the satellite roller  141  is 17 mm, for example. The satellite roller  141  is formed by coating on the surface of an aluminum metallic pipe, for example. Material for the metallic pipe may be iron, copper and stainless, and so on. In addition, in place of the metallic pipe, a heat pipe with further higher heat conductivity may be used. The satellite roller  141  is harder and is less deformable compared with the elastic layer of the fixing roller  38 . A spring  44  biases the satellite roller  141  in a back away direction from the fixing roller  38 . 
     The fixing belt  42  comprises by sequentially laminating a solid rubber layer with a thickness of 200 μm of silicone rubber and a release agent layer with a thickness of 30 μm of a PTFE (Polytetrafluoroethylene) tube on a nickel metallic conductive layer with a thickness of 40 μm, for example, which is a metallic layer. The fixing belt  42  has an outer diameter of 60 mm, when formed in a cylindrical shape. The fixing belt  42  is set up around the fixing roller  38  and the satellite roller  141  by the spring  44 . 
     The fixing device  37  includes a belt heating portion which will be separately described at the outer circumference of the fixing belt  42 , and the fixing belt  42  is heated to a fixing temperature by the belt heating portion. 
     A first temperature sensor  53   a  to detect a temperature of a center portion of the fixing belt  42  which is heated as described above and a second temperature sensor  53   b  to detect a side portion of the fixing belt  42  are arranged around the fixing belt  42 . As the first temperature sensor  53   a  and the second temperature sensor  53   b , a thermopile sensor to detect infra-red ray without contacting is used, for example. Using the first temperature sensor  53   a  and the second temperature sensor  53   b , the temperature distribution of the fixing belt  42  in the width direction is controlled. The number of the temperature sensors to detect the temperatures of the fixing belt  42  is not limited to two. By providing the three temperature sensors, and by measuring the temperatures at the center portion and both the side portions of the fixing belt  42 , the temperature distribution in the width direction may be controlled. 
     The pressing roller  43  has a rubber layer  43   b  around a cored bar  43   a , for example. The outer diameter of the pressing roller  43  is 50 mm, for example. As the rubber layer, silicone rubber or fluorine-containing rubber is used. The pressing roller  43  has a roller temperature sensor  47  at the circumference. The pressing roller  43  pressing contacts to the fixing roller  38  and the fixing belt  42 . By pressure contacting the pressing roller  43 , a nip portion  50  is formed between the fixing belt  42  and the pressing roller  43 . To plan further speeding up FCOT (First Copy Output Time) is possible by providing a heating device such as an electromagnetic induction heater or a halogen lamp. A pressure of the pressing roller  43  against the fixing roller  38  which is a nip pressure can be adjusted. 
     The pressing roller  43  rotates in a direction of an arrow t by a drive motor  51  which is a driving portion. Hereinafter, a rotation in the direction of the arrow t is determined as a forward rotation, and a rotation in a direction of an arrow s which is a direction reverse to the direction of the arrow t is determined as a reverse direction. The fixing roller  38 , the satellite roller  141  and the fixing belt  42  rotate in a direction of an arrow v while driven by the pressing roller  43 . The driving speed of the drive motor  51  can be adjusted. 
     The fixing device  37  fixes the toner image on the sheet P passing through the nip portion  50  between the fixing belt  42  and the pressing roller  43  from the upstream side to the downstream side in a direction of an arrow w. With respect to the sheet P adhered to the fixing belt  42  and the pressing roller  43  by the toner melted by heat and the nip pressure, a separator  52   a  to separate the sheet P from the fixing belt  42  and a separator  52   b  to separate the sheet P from the pressing roller  43  are provided at the downstream side of the nip portion  50 . A width between the fixing belt  42  and the separator  52   a  is about 0.3 mm to 0.5 mm. 
     The fixing device  37  has a coil  56  and a magnetic body  57  which are a belt heating portion. A first coil  56   a  makes a center portion including a center in the width direction of the fixing belt  42  generate heat. A second coil  56   b  and a third coil  56   c  make both side portions not including the center in the width direction of the fixing belt  42  generate heat. The second coil  56   b  and the third coil  56   c  are connected in series, and are driven under the same control. The first coil  56   a , the second coil  56   b  and the third coil  56   c  are driven by being changed selectively. In any coils, an output can be adjusted at 200 W to 1500 W, for example. 
     Each of the coils  56   a  to  56   c  includes a conducting wire  58  wound around each of magnetic cores  57   a  to  57   c . As the conducting wire  58 , a litz wire is used which is formed by binding up sixteen cupper wire rods each with a diameter of 0.5 mm coated with heat-resisting polyamide-imide. When high-frequency current is flown through the conducting wire  58 , each of the coils  56   a  to  56   c  generates magnetic flux. By the magnetic flux, Joule heat generates by the resistance value of the metallic conductive layer of the fixing belt  42 , and the fixing belt  42  generates heat instantaneously. The high-frequency current to be flown through each of the coils  56   a  to  56   c  is in a range of frequency of 20 to 100 kHz, for example. 
       FIG. 4  is a view showing a gear sequence  100  to control a rotation direction of the pressing roller  43  and the nip pressure, and  FIG. 5  is a view showing a perspective view of the gear sequence  100 . The gear sequence  100  is provided at a side face of the fixing device  37 . 
     The gear sequence  100  as a driving device has a plurality of gears, a motor shaft  51   a  and an electromagnetic clutch gear  101 . The drive motor  51  whose rotation direction is controlled by a CPU  72  described later drives the motor shaft  51   a . The motor shaft  51   a  rotates in a direction p shown in  FIG. 5  and a reverse direction of the direction p. The motor shaft  51   a  rotates forwardly in the direction p. The motor shaft  51   a  rotates reversely in the reverse direction of the direction p. The motor shaft  51   a  rotates adjacent gears. 
     A gear  100   a  rotates a cam  81  and a shutter  83 . A gear  100   b  rotates the pressing roller  43 . 
     The electromagnetic clutch gear  101  is a clutch to switch over whether to transmit or to cut the rotation of the motor shaft  51   a  to the cam  81 . The electromagnetic clutch gear  101  is a two-stage gear composed of an upper stage and a lower stage, and the operation changes by whether or not to flow the current. The lower stage gear connects so as to rotate gears in a direction of an arrow j, and the upper stage gear connects so as to follow with the rotation of gears in a direction of an arrow k. When current is flown, the upper stage along with the lower stage of the two-stage gear rotate, and rotate gears in the direction of the arrow k, which are ranging to the gear  100   a . When the current is cut off, the gear of the lower stage runs idle and the gear of the upper stage rotates. 
     At the time of the forward rotation, the motor shaft  51   a  rotates forwardly the electromagnetic clutch gear  101  and the gears in the direction of the arrow j. When the gear  100   b  rotates forwardly, the pressing roller  43  rotates forwardly in the direction of the arrow t in  FIG. 2 . With respect to the fixing belt  42  which is rotated by the forwardly rotating pressing roller  43 , a face facing the separator  52   a  goes slack. 
     At the time of reverse rotation, the motor shaft  51   a  rotates reversely the electromagnetic clutch gear  101  and the gears in the direction of the arrow j. When the gear  100   b  rotates reversely, the pressing roller  43  rotates reversely. With respect to the fixing belt  42  which is rotated reversely by the rotating pressing roller  43 , a face facing the separator  52   a  goes tight. 
     A concrete example of a mechanism to adjust a pressing power of the pressing roller  43  against the fixing roller  38  will be described.  FIG. 6A  is a view showing a state where the nip pressure is high, and  FIG. 6B  is a view showing a state where the nip pressure is low. 
     The fixing device  37  includes, as a pressure changing mechanism, a pressing roller frame  80 , the cam  81 , a bearing  82 , the shutter  83 , a sensor  84 , a pressure spring  85 , a fixing roller frame  90 , for example. 
     The fixing roller frame  90  supports rotatably the fixing roller  38 . The fixing roller frame  90  supports a supporting point  80   a . The fixing roller frame  90  supports the sensor  84 . The pressing roller frame  80  rotates regarding the supporting point  80   a  as the center against the fixing roller frame  90 . A pressing roller shaft  143   a  is supported by the pressing roller frame  80 . When the pressing roller frame  80  rotates regarding the supporting point  80   a  as the center, the pressing roller  43  along with the pressing roller shaft  143   a  rotate against the fixing roller frame  90 . 
     The cam  81  is of an ellipse shape, and supported by the shaft eccentrically and is rotatable. As the cam  81  is eccentric, there are a cam face distant from the rotation center and a cam face near the rotation center. When the cam face near the rotation center of the cam  81  contacts with the bearing  82 , the nip pressure is high. When the cam  81  rotates so that the cam face distant from the rotation center of the cam  81  makes contact with the bearing  82 , the pressing roller frame  80  rotates in a direction to separate from the fixing roller  38 , and the nip pressure lowers. If the nip pressure lowers, as the foamed rubber layer  38   b  which is crushed by the high nip pressure is restored, the fixing belt  42  approaches the separator  52   a.    
     The bearing  82  constantly contacts with the cam face of the cam  81  to operate as a follower, and moves by the rotation of the cam  81 . 
     The shutter  83  is fan-like as shown in  FIG. 6 , and is coaxial with the cam  81 . A tip portion of the cam  83  projects in a direction of the cam face near the rotation center of the cam  81 . The tip portion of the cam  83  may project in a direction of the cam face distant from the rotation center of the cam  81 . 
     The sensor  84  has a detecting concave portion, for example, so that the shutter  83  overpasses the sensor  84 , and a light emitting element faces a light receiving element across the detecting concave portion. When the shutter  83  does not shield the detecting concave portion of the sensor  84  as shown in  FIG. 6A , the nip pressure is high. When the shutter  83  shields the detecting concave portion of the sensor  84  as shown in  FIG. 6B , the nip pressure is low. The CPU  72  discriminates from an output of the sensor  84  whether or not the shutter  83  shields the detecting concave portion of the sensor  84 . The pressing spring  85  biases the pressing roller frame  80  toward the fixing roller frame  90  (in a direction of an arrow u). 
     In the case of moving from a state where the nip pressure is high to a state where the nip pressure is low, current is flown in the electromagnetic clutch gear  101 . The lower stage along with the upper stage of the electromagnetic clutch gear  101  rotate. When the gear  100  rotates by 180°, the cam  81  fixed coaxially rotates by 180°. When the shutter  83  comes to the detecting concave portion of the sensor  84  and shields the sensor  84 , to flow the current in the electromagnetic clutch gear  101  is stopped, and the came  81  stops in the state that the cam face distant from the rotation center of the cam  81  makes contact with the bearing  82 . Along with the rotation of the cam  81 , the pressing roller frame  80  rotates regarding the supporting point  80   a  as the center in a direction to separate from the fixing roller  38 , and becomes the state where the nip pressure is low. 
     On the other hand, in the case of moving from a state where the nip pressure is low to a state where the nip pressure is high, current is flown in the electromagnetic clutch gear  101  in the same manner as moving to the state where the nip pressure is low. When the gear  100  rotates by 180°, the cam  81  fixed coaxially rotates by 180°. When the shutter  83  separates from the detecting concave portion of the sensor  84  and no one shields the detecting concave portion, to flow the current in the electromagnetic clutch gear  101  is stopped, and the came  81  stops in the state that the cam face near the rotation center of the cam  81  makes contact with the bearing  82 . Along with the rotation of the cam  81 , the pressing roller frame  80  rotates regarding the supporting point  80   a  as the center in a direction to approach the fixing roller  38 , and becomes the state where the nip pressure is high. 
     In the first embodiment, the pressing roller  43  rotates reversely in the direction of the arrow s in the state where the nip pressure is low. By expanding a distance between the shafts of the fixing roller  38  and the pressing roller  43  compared with in the fixing mode, in the state where an area of the nip portion  50  formed by the fixing belt  42  and the pressing roller  43  decreases, the fixing belt  42  and the pressing roller  43  rotate in a direction reverse to the rotating direction in the fixing mode. To start transiting from the state where the nip pressure is high to the state where the nip pressure is low and to start rotating reversely the pressing roller  43  may be concurrent. 
     The distance between the shafts of the fixing roller  38  and the pressing roller  43  is determined as a distance to connect the rotation center of the fixing roller  38  and the rotation center of the pressing roller  43 , for example. Assuming that the diameter of the fixing roller  38  is 30 mm φ, and the diameter of the pressing roller is 30 mm φ, the distance between the shafts becomes 29 mm in the state where the nip pressure is low, and becomes 27 to 28 mm in the state where the nip pressure is high, for example. 
       FIG. 7  shows a controller  60  of the fixing device  37 . The controller  60  has an inverter circuit  61  to control power supplied to each of the coils  56   a  to  56   c . The controller  60  has a rectifier circuit  63  to supply a DC current obtained by smoothing the current from a commercial AC source  62  to the inverter circuit  61 . A transformer  64  is arranged at a former stage of the rectifier circuit  63 , and an overall consumption power can be detected via an input detector  64   a.    
     The controller  60  has the CPU  72  to control the inverter circuit  61  in accordance with the detection results of the first temperature sensor  53   a  and the second temperature sensor  53   b . The CPU  72  controls the drive motor  51  in accordance with the detection results of the first temperature sensor  53   a  and the roller temperature sensor  47 . The power detected by the input detector  64   a  is fed back to the CPU  72 . 
     The CPU  72  realizes various functions by executing the program stored in a memory  72   a  as a controller, and executes various processing of the image forming apparatus  1  including the fixing device  37  and so on. 
     The memory  72   a  is a RAM (Random Access Memory), a ROM (Read Only Memory), a DRAM (Dynamic Random Access Memory), a SRAM (Static Random Access Memory), a VRAM (Video RAM) and so on, for example, and has a function to store the various information and programs utilized in the image forming apparatus  1 . The memory  72   a  stores a table to set the temperature and speed of the fixing device  37 . 
     In the inverter circuit  61 , a first capacitor  66   a  for resonance use is connected in parallel with the first coil  56   a  to form a first resonance circuit  67   a , and a second capacitor  66   b  for resonance use is connected in parallel with the second coil  56   b  and the third coil  56   c  to form a second resonance circuit  67   b . In the inverter circuit  61 , a first switching element  68   a  connects to the first resonance circuit  67   a , and a second switching element  68   b  connects to the second resonance circuit  67   b.    
     A first drive circuit  70   a  and a second drive circuit  70   b  respectively connect to control terminals of the first switching element  68   a  and the second switching element  68   b . The first and second drive circuits  70   a ,  70   b  connect to the CPU  72  via first and second control circuits  71   a ,  71   b , respectively. The first control circuit  71   a  and the second control circuit  71   b  are instructed by the CPU  72 , and control ON times of the first and second switching elements  68   a ,  68   b  via the first drive circuit  70   a  and the second drive circuit  70   b , respectively. By controlling the ON times of the first switching element  68   a  and second switching element  68   b , the frequency is changed variably in a range of 20 to 100 kHz to change an output value. 
     A control process of the fixing device  37  will be described. After the power source is switched ON and the image forming apparatus  1  is started, the control process of the fixing device  37  is started. The control process of the fixing device  37  has a warming up mode, a ready mode, a fixing mode, a preheat mode, a sleep mode, a preheat return mode, and a sleep return mode. 
     In the warming up mode, after starting the image forming apparatus  1  by making the power source ON, the fixing device  37  becomes at fixing temperatures. An fixing temperature of the fixing belt  42  is 160° C., for example. An fixing temperature of the pressing roller  43  is 80° C., for example. The nip pressure in the warming up mode is high. In the warming up mode, the drive motor  51  rotates forwardly the pressing roller  43  at 270 mm/sec. 
     In the ready mode, the fixing belt  42  and the pressing roller  43  are held at the fixing temperatures. The nip pressure in the ready mode is lower than the nip pressure in the warming up mode. In the ready mode, the drive motor  51  rotates reversely the pressing roller  43  at 90 mm/sec. 
     In the fixing mode, the fixing belt  42  and the pressing roller  43  are held at the fixing temperatures. The nip pressure in the fixing mode is high in such an extent to fix sufficiently at the fixing temperatures. The nip pressure in the fixing mode is higher than the nip pressure in the ready mode. The nip pressure in the fixing mode may be made equal to the nip pressure in the warming up mode. In the fixing mode, the drive motor  51  rotates forwardly the pressing roller  43  at 270 mm/sec. 
     In the preheat mode, the fixing belt  42  is held at a preheat temperature. The preheat temperature of the fixing belt  42  is lower than the fixing temperature of the fixing belt  42 . The preheat temperature is 80° C., for example. The image forming apparatus  1  changes to the preheat mode, when a certain time (three minutes, for example) elapses in the ready mode constantly. The nip pressure in the preheat mode is lower than the nip pressure in the warming up mode. In the preheat mode, the drive motor  51  rotates reversely the pressing roller  43  at 90 mm/sec. 
     In the sleep mode, as the fixing belt  42  is not heated, the temperature of the fixing belt  42  is lower than the preheat temperature. The image forming apparatus  1  changes to the sleep mode, when a certain time (27 minutes, for example) elapses in the preheat mode. The nip pressure in the sleep mode is lower than the nip pressure in the warming up mode. The nip pressure in the sleep mode may be made equal to the nip pressure in the preheat mode. In sleep mode, the drive motor  51  does not rotate the pressing roller  43 . 
     In the preheat return mode, the image forming apparatus  1  becomes in the ready mode from the preheat mode. In the preheat return mode, the fixing belt  42  becomes at the fixing temperature from the preheat temperature. The nip pressure in the preheat return mode is higher than the nip pressure in the ready mode. The nip pressure in the preheat return mode may be made equal to the nip pressure in the fixing mode. In the preheat return mode, the drive motor  51  rotates forwardly the pressing roller  43  at 270 mm/sec. 
     In the sleep return mode, the image forming apparatus  1  becomes in the ready mode from the sleep mode. In the sleep return mode, the fixing belt  42  becomes at the fixing temperature from a temperature lower than the preheat temperature. The nip pressure in the sleep return mode is higher than the nip pressure in the ready mode. The nip pressure in the sleep return mode may be made equal to the nip pressure in the fixing mode. In the sleep return mode, the drive motor  51  rotates forwardly the pressing roller  43  at 270 mm/sec. 
       FIG. 8  is a flow chart showing an operation of the image forming apparatus  1  to transit from the fixing mode to the ready mode after image forming. 
     The image forming apparatus  1  rotates forwardly the pressing roller  43  ( 402 ). 
     The image forming apparatus  1  confirms current mode. The image forming apparatus  1  detects the fixing mode by the shutter  83  not shielding the sensor  84 . The nip pressure is in the high state then ( 404 ). 
     If the shutter  83  shields the sensor  84 , the cam  81  and the shutter  83  are rotated by flowing current in the electromagnetic clutch gear  101  till the shutter  83  does not shield the sensor  84  so as to make the nip pressure in the high state, while rotating the pressing roller  43  forwardly ( 406 ). 
     The image forming apparatus  1  rotates the pressing roller  43  forwardly and confirms that the nip pressure is in the high state in the fixing mode ( 408 ), and performs image forming. The image forming apparatus  1  takes out the sheet P from the sheet cassette  4  or the manual sheet feeder  31 . The image forming apparatus  1  conveys the sheet P from the aligning roller  36  to the secondary transfer roller  27  along the conveying path  34 . The image forming apparatus  1  conveys the sheet P on which the toner image is transferred to by the secondary transfer roller  27  to the fixing device  37 . The image forming apparatus  1  in the fixing mode rotates forwardly the pressing roller  43  in the direction of the arrow t. The fixing roller  38  rotates driven by the pressing roller  43 . The nip portion  50  formed by the fixing roller  38  and the pressing roller  43  fixes the toner image on the sheet P by applying heat and pressure. The image forming apparatus  1  finishes image forming by ejecting the sheet P on which the toner image is fixed to the sheet ejector  3  ( 410 ). 
     The image forming apparatus  1  rotates the pressing roller  43  reversely after finishing image forming ( 412 ). 
     The image forming apparatus  1  confirms that the shutter  83  shields the sensor  84  so as to confirm that the nip pressure is in the low state ( 414 ). 
     If the shutter  83  does not shield the sensor  84 , the cam  81  and the shutter  83  are rotated by flowing current in the electromagnetic clutch gear  101  till the shutter  83  shields the sensor  84  so as to make the nip pressure in the low state, while rotating the pressing roller  43  reversely ( 416 ). 
     The image forming apparatus  1  becomes in the ready mode if the shutter  83  shields the sensor  84  while rotating the pressing roller  43  reversely ( 418 ). 
     When the pressing roller  43  rotates reversely, as the flexure of the fixing belt  42  is difficult to happen at the face facing the separator  52   a , even if the nip pressure lowers and the foamed rubber layer  38   b  is restored, the fixing belt  42  becomes difficult to contact, as the orbit is stable even though approaching the separator  52   a . When the pressing roller  43  rotates forwardly, as the face of the fixing belt  42  facing the first temperature sensor  53   a  and the second temperature sensor  53   b  goes tight, the first temperature sensor  53   a  and the second temperature sensor  53   b  can detect the temperature of the fixing belt  42  more accurately. In the mode to raise the temperature rapidly such as the warming up mode, the preheat return mode and the sleep return mode, the pressing roller  43  is rotated forwardly and the temperature is detected accurately to avoid overheating. 
     To make the rotation speed of the pressing roller  43  lower when the pressing roller  43  rotates reversely than when the pressing roller  43  rotates forwardly can suppress the damage of the fixing belt  42  even if the fixing belt  42  makes contact with the separator  52   a.    
     In place of moving an absolute position of the pressing roller  43  so as to change the nip pressure, an absolute position of the fixing roller  38  may be moved, and both the absolute positions may be changed. 
     The rotation speed of the pressing roller  43  according to the temperature difference between the fixing belt  42  and the pressing roller  43  in the return mode is not limited. The rotation speed of the pressing roller  43  may be changed according to the property of the fixing device  37 , such as a branch point of the changing temperature difference of the fixing belt  42 , and the number of variations of the rotation speeds of the fixing belt  42 . 
     Second Embodiment 
       FIG. 9  is a view showing a gear sequence  100  to control a rotation direction of the pressing roller  43  and the nip pressure in a second embodiment. The gear sequence  100  includes a plurality of gears, the motor shaft  51   a , the electromagnetic clutch gear  101  and a sub-electromagnetic clutch gear  102 . The sub-electromagnetic clutch gear  102  is a clutch to switch over whether to transmit or to cut the rotation of the motor shaft  51   a  to the pressing roller  43 . The sub-electromagnetic clutch gear  102  is a two-stage gear composed of an upper stage and a lower stage, and the operation changes by whether or not to flow the current. The upper stage gear connects so as to follow with the rotation of gears in the direction of the arrow j. When current is flown, the upper stage along with the lower stage of the two-stage gear rotate, and rotate gears in the direction of the arrow j, which are ranging to the gear  100   b . When the current is cut off, the gear of the upper stage runs idle and the gear of the lower stage rotates. 
     A flow of the forward rotation and the reverse rotation of the pressing roller  43  will be described.  FIG. 10  is a flow chart showing an operation of the image forming apparatus  1  to transit from the fixing mode to the ready mode after image forming in the second embodiment. 
     The image forming apparatus  1  stops the drive motor  51  ( 452 ). 
     The image forming apparatus  1  confirms current mode. The image forming apparatus  1  detects the fixing mode by the shutter  83  not shielding the sensor  84 . The nip pressure is in the high state then ( 454 ). If the shutter  83  does not shield the sensor  84 , the image forming apparatus  1  cuts off the electromagnetic clutch gear  101  and connects the sub-electromagnetic clutch gear  102  ( 456 ). If the shutter  83  shields the sensor  84 , the image forming apparatus  1  connects the electromagnetic clutch gear  101  and cuts off the sub-electromagnetic clutch gear  102  so as to make the nip pressure in the high state ( 458 ). 
     The image forming apparatus  1  rotates the drive motor  51  forwardly ( 460 ). 
     The image forming apparatus  1  confirms current mode. If the electromagnetic clutch gear  101  is connected and the sub-electromagnetic clutch gear  102  is cut off, the nip pressure becomes in the high state by the rotation of the cam  81  and the pressing roller  43  does not rotate. The image forming apparatus  1  confirms that the shutter  83  does not shield the sensor  84  so as to confirm that the nip pressure is in the high state ( 462 ). If the shutter  83  does not shield the sensor  84 , the image forming apparatus  1  cuts off the electromagnetic clutch gear  101  and connects the sub-electromagnetic clutch gear  102  ( 466 ). If the electromagnetic clutch gear  101  is cut off and the sub-electromagnetic clutch gear  102  is connected, the cam  81  does not rotate, but the pressing roller  43  rotates. 
     The image forming apparatus  1  rotates the pressing roller  43  forwardly and confirms that the nip pressure is in the high state in the fixing mode ( 468 ), and performs image forming ( 470 ). 
     The image forming apparatus  1  stops the drive motor  51  when image forming is finished ( 472 ). 
     The image forming apparatus  1  confirms that the shutter  83  shields the sensor  84  so as to confirm that the nip pressure is in the low state ( 474 ). If the shutter  83  shields the sensor  84 , the image forming apparatus  1  cuts off the electromagnetic clutch gear  101  and connects the sub-electromagnetic clutch gear  102  ( 476 ). If the shutter  83  does not shield the sensor  84 , the image forming apparatus  1  connects the electromagnetic clutch gear  101  and cuts off the sub-electromagnetic clutch gear  102  so as to make the nip pressure in the low state ( 478 ). 
     The image forming apparatus  1  rotates the drive motor  51  reversely ( 480 ). 
     The image forming apparatus  1  confirms that the shutter  83  shields the sensor  84  so as to confirm that the nip pressure is in the low state ( 482 ). If the electromagnetic clutch gear  101  is connected and the sub-electromagnetic clutch gear  102  is cut off, the cam  81  rotates, the nip pressure becomes in the low state and the pressing roller  43  does not rotate. If the shutter  83  becomes to shield the sensor  84 , the image forming apparatus  1  cuts off the electromagnetic clutch gear  101  and connects the sub-electromagnetic clutch gear  102  ( 484 ). 
     The image forming apparatus  1  rotates the pressing roller  43  reversely, and becomes to the ready mode ( 486 ). 
     In the second embodiment, the position of the fixing belt  42  moves against the separator  52  and is easy to contact the separator  52 , and at the time of changing the nip pressure, the pressing roller  43  and the fixing belt  42  are not rotated. 
     Whether the nip pressure is in the high state or in the low state may be different from the operation of  FIG. 6 . That is, the nip pressure may be in the high state, when the shutter  83  shields the sensor  84  as shown in  FIG. 11A , and the nip pressure may be in the low state when the shutter  83  does not shield the sensor  84  as shown in  FIG. 11B . 
     The fixing belt  42  and the pressing roller  43  may contact with each other in the state that the nip pressure is low, and may be separated so as not to connect with each other. 
     While certain embodiments have been described, those embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and devices described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and devices described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.