Patent Publication Number: US-9411283-B2

Title: Fixing device and control device

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a fixing device and a control device for an image forming apparatus such as a copying machine, a printer, a facsimile machine or a complex machine having a plurality of functions of those machines. In the image forming apparatus for forming an image through an electrophotographic type process, an image forming station forms a toner image, transfers the toner image onto a recording material (sheet) and fixes the toner image on the recording material by heating the recording material having the transferred toner image by a fixing device, 
     In such a fixing device, when the recording material is nipped by a nip, lateral edge portions (edges of widthwise ends) of recording material is in contact with a fixing member (rotatable member). At this time, the surface of the fixing member tends to be damaged by the lateral edge portion of recording material. 
     When such a damage by the edge of recording materials having a small width occurs, the resulting unsmoothness of the surface of the fixing member appears on a large width recording material subsequently processed. 
     In order to reduce the influence of the damage by the lateral edge, Japanese Laid-open Patent Application 2005-351939) proposes that an entirety fixing device (pair of rotatable members) is reciprocated in the widthwise direction of recording material. 
     Generally, in the fixing device, lengths of the fixing member and the heating source are determined so as to process the maximum width recording material. 
     If the proposal of Japanese Laid-open Patent Application 2005-351939 is employed for the fixing process on the recording material of the maximum width, the device may be upsized. 
     For example, in the case that the fixing process is executed on the maximum width recording material in the state that the fixing device (pair of rotatable members) is at one end of the reciprocable range thereof, the widthwise center portion of the fixing device (pair of rotatable members) is offset from the widthwise center portion of the recording material. Under the circumstances, it is required that the length of fixing device and the length of the heating source for heating the fixing member (one of rotatable members) are extended in the widthwise direction to properly carry out the fixing process even if the offset occurs. This would result in the upsizing of the device. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a fixing device comprising a fixing unit including a first rotatable member and a second rotatable member which form a nip to fix a toner image on a sheet, said fixing unit being capable of fixing the toner image on a maximum width sheet having a width for which said fixing unit is operable and a small width sheet having a width smaller than the maximum width; a reciprocating mechanism configured to reciprocate said fixing unit in a widthwise direction; and an operating device configured to operate said reciprocating mechanism, wherein said operating device operates said reciprocating mechanism for a fixing operation on the small width sheet, and said operating device does not operate said reciprocating mechanism for the fixing operation on the maximum width sheet. 
     These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following DESCRIPTION OF THE EMBODIMENTS of the present invention, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view of an image forming apparatus. 
         FIG. 2  is a schematic sectional view of a heating unit of a fixing device. 
         FIG. 3  is a schematic side view of a heating unit of the fixing device. 
         FIG. 4  is a schematic perspective view of a heating unit of the fixing device. 
         FIG. 5  is a schematic top plan view of the fixing device in which the recording material is passing the nip. 
         FIG. 6  is a schematic front view of the fixing device. 
         FIG. 7  is a schematic top plan view of the fixing device. 
         FIG. 8  is a view of a right-hand end portion of  FIG. 6 . 
         FIG. 9  is a schematic view illustrating a relation between a width of a heat generation and a maximum width size of the recording material. 
         FIG. 10  is a flow chart showing an example of a control flow for the fixing device. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1  to  FIG. 10 , an embodiment of the present invention will be described. Referring to  FIG. 1 , an image forming apparatus according to this embodiment will be described. 
     [Image Forming Apparatus] 
     The image forming apparatus  1  comprises a fixing device  27  as an image heating apparatus which fixes an unfixed image transferred onto a recording material (sheet) S such as paper by applying heat and pressure. In this embodiment, the image forming apparatus is of a full-color and intermediary transfer type, but the present invention is applicable to another type image forming apparatus comprising an image heating device. 
     The image forming apparatus  1  is tandem type in which image forming stations PY, PM, PC, PK for forming Y (yellow), M (magenta), C (cyan), K (black) toner images, respectively are provided. The image forming stations PY, PM, PC, PK are arranged along a rotational moving direction of an intermediary transfer belt  25  as an intermediary transfer member and carry out the toner image the processes for the respective colors in parallel. 
     The image forming stations have fundamentally the same structures, and therefore, the following description of the image forming stations applies commonly to them, although suffixes Y, M, C and K are added in the drawings and only when necessary. 
     The image forming station P includes a photosensitive drum  20  as an image bearing member on which a toner image is formed and carried. Around the photosensitive drum  20 , there are provided a charging device  21 , a developing device  23 , a primary transferring device  24  (unshown) and a cleaner. Above the image forming apparatus  1 , an exposure device  22  is provided. 
     Photosensitive drum  20  is rotated in the direction indicated by the arrow in the Figure, during which a surface of the photosensitive drum  20  is uniformly charged to a predetermined potential by the charging device  21 . Thereafter, the charged surface of the photosensitive drum  20  is exposed so that an electrostatic latent image is formed on the photosensitive drum  20 . The electrostatic latent image on the photosensitive drum  20  is developed with a developer by the developing device  23  into a visualized toner image. 
     The toner image formed by the developing device  20  is primary-transferred superposingly on an endless intermediary transfer belt  25  from the photosensitive drum  20  by a primary transferring device  24 . The toner images above intermediary transfer belt  25  are secondary-transferred all together onto the recording material S by a secondary transfer device  26 . The surface of the photosensitive drum  20  after the primary transfer and the surface of the intermediary transfer belt  25  after the secondary transfer are cleaned by the cleaner (unshown) to be prepared for the next image formation. 
     The recording material S is fed to a secondary transfer portion comprising a secondary transfer device  26  and the intermediary transfer belt  25 , by a feeding means such as a feeding roller, from a sheet feeding cassette  31 . After the secondary transfer, the recording material S carrying the toner image is fed to the fixing device  27 . The fixing device  27  heats and presses the unfixed toner image to melt and soften it, thus fixing it on the recording material S. The recording material S having the fixed toner image is discharged to a sheet discharge tray  28 . When an image is to be formed also on the back side of the recording material S, the recording material S is reverted by a recording material reversing path  29  and is refed to the secondary transfer portion along the duplex print feeding path  30 , where it receives the side on the back side. 
     As described in the foregoing, a series of image forming process operations including the charging, the exposure, the development, the transfer and the fixing is executed to form the image on the recording material S. If the image forming apparatus is a monochromatic image forming apparatus, only a black image forming station is provided. The structures and the order of the Y, M, C, K image forming stations are not limited to those described above. 
     [Fixing Device] 
     Referring to  FIG. 2  through  FIG. 5 , the fixing device  27  and a heating unit  27 A of the fixing device  27  according to this embodiment will be described. As shown in  FIG. 2 , the heating unit (fixing unit)  27 A comprises an endless heating belt (first rotatable member)  302  as a rotatable heating member, and a pressing roller (second rotatable member)  304  as a pressing rotatable member forming a nip N between an outer peripheral surface of the heating belt  302  and the heating belt  302 . The pressing roller  304  has a function also as a driving rotatable member for rotating the heating belt  302  as will be described hereinafter. Inside the heating belt  302 , there is provided a heater (ceramic heater)  300  as a heating mechanism. 
     The heater  300  comprises an elongated thin-plate-like ceramic substrate elongated in a perpendicular direction to the sheet of the drawing of  FIG. 1  (front and back direction), and a heat generating resistor layer provided on the surface of the substrate, as basic elements. Such a heater  300  is a low thermal capacity heater which is heated steeply by the electric power supply from a voltage source  309  to the heat generating resistor layer. 
     The heater  300  is fixed to a heater holder  301  as a supporting member. The heater holder  301  has a trough like shape having a substantially half-arc cross-section and is a heat insulation member of heat resistive resin material or the like elongated in the direction perpendicular to the sheet of the drawing of  FIG. 1 . The heater  300  is fitted into a groove portion formed in the lower surface of heater holder  301  along the length thereof and is fixed by a heat resistive adhesive, with the heater surface side facing downward. Designated by  303  is a stay provided inside of heater holder  301  to support the heater holder  301 . 
     The heating belt  302  is made of a heat resistive film, for example, and is loosely fitted around the heater holder  301  including the heater  300 . The heating belt  302  has a composite layer in order to improve a quick start property by reducing the thermal capacity as follows. The belt comprises a base layer of metal such as SUS or Ni, having a film thickness of not more than 100 μm, preferably 20-50 μm. The outer peripheral surface thereof is coated with a heat resistive rubber such as silicone rubber or fluorine-containing rubber, or an elastic layer of a foam member of silicone rubber. The outer peripheral surface thereof is further coated with PTFE, PFA or the like layer having a thickness of approx. 5-50 μm. An inner surface of the base layer is provided with a protection layer of PI (polyimide) or the like having a thickness of several μm to reduce a sliding friction between the heater  300  and the metal layer of the heating belt  302 . 
     The pressing roller  304  comprises a core metal  304   a , and an elastic layer  304   b  of heat resistive rubber such as silicone rubber or fluorine-containing rubber or a foam member of silicone rubber, and the opposite end portions of the core metal  304   a  are rotatably supported by side plates  400 ,  401 . As shown in  FIG. 2 , above the top side of the pressing roller  304 , the heater  300 , the heater holder  301 , the heating belt  302  and an assembly of the stay  303  are provided extended in parallel with the pressing roller  304  with the heater  300  side facing downward. The stay  303  is urged toward the pressing roller  304  by a variable pressure mechanism  500  which will be described hereinafter. By this, the lower surface ( FIG. 2 ) of the heater  300  is press-contacted toward the outer peripheral surface of pressing roller  304  through the heating belt  302  against the elastic of the elastic layer  304   b  to form a nip N having a predetermined width. 
     A temperature of the heating belt  302  is monitored by a thermister  307  as a temperature detecting means outputting a detection signal to a controller (CPU)  308  of the control device. The controller  308  adjusts a current applied to the heater  300  by the voltage source  309  on the basis of the signal of the thermister  307 , so that the heating belt  302  keeps a predetermined target temperature during the fixing operation. 
     In the state that the temperature of the heating belt  302  is controlled, the recording material carrying the toner image is fed into the nip N, and the unfixed toner image is heated and pressed so that the toner image is fixed on the recording material. The recording material after the fixing is separated from the heating belt  302 , and is discharged from the nip N along a separation guide  306  provided downstream of the nip N in the feeding direction. The separation guide  306  is disposed spaced from the heating belt  302  so that the recording material discharged from the nip N is not wrapped around the heating belt  302  and so that the heating belt  302  is not damaged. Such a separation guide  306  is engaged with a part of a flange  305  which will be described hereinafter, and is fixed by an urging means such as a spring. 
     The flange  305  is supported by the side plates  400  and  401  constituting a frame (case) of the heating unit  27 A as shown in  FIGS. 3 and 4 , and is movable toward and away from the pressing roller  304 . The flange  305  is provided with a regulating member for supporting opposite end portions (rotation axial direction of the heating belt  302 ) of stay  303  and the heater holder  301  and for regulating a configuration in the circumferential direction and a movement in the longitudinal direction of the heating belt  302 . 
     The heating belt  302  supported by such a flange  305  is urged toward the pressing roller  304  by the variable pressure mechanism  500  shown in  FIGS. 3 and 4 . The variable pressure mechanism  500  is provided at each of the opposite ends of the heating belt  302 , and comprises a pressing cam  501 , a pressing member rotational shaft  502 , a pressing cam rotational shaft  504 , a pressing member  505 , a pressing adjusting screw  506 , pressing supporting plate  507  and an urging spring  508 . 
     The pressing member  505  and the pressing supporting plate  507  are supported by the side plates  400 ,  401  through the pressing member rotational shaft  502 , and the pressing member  505  can move rotatably relative to the pressing supporting plate  507 . The pressing supporting plate  507  is fixed to the side plates  400 ,  401 . To the pressing supporting plate  507 , the pressing adjusting screw  506  is fastened, and by rotating the pressing adjusting screw  506 , a seat of the pressing adjusting screw  506  contracts the spring of the urging spring  508  to increase the spring load applied to the pressing member  505 . The pressing member  505  is rotatably supported relative to the pressing supporting plate  507  as described above, and therefore, the compressive force of the urging spring  508  produces a moment about the pressing member rotational shaft  502 . 
     The pressing member  505  is contacted to the flange  305 . Therefore, the moment produced in the pressing member  505  pushes the flange  305  toward the pressing roller  304  to form the above-described nip N between the pressing roller  304  and the heating belt  302 . 
     In order to release the pressure, the pressing cam  501  eccentric by a predetermined amount is rotated to push the pressing member  505  up. The pressure is released by rotating the pressing cam  501  until the pressing member  505  and the flange  305  becomes non-contacted relative to each other. The pressing cam  501  is rotated by a motor M 1  as a driving source. The pressing cams  501  are provided at the opposite sides of the fixing belt  302  and are fixed to the opposite end portions of the pressing cam rotational shaft  504  with the same phase, so that they are rotated with the same phase by the motor M 1 . By this, the variable pressure mechanisms  500  at the opposite sides of heating belt  302  can be actuated to switch between the pressing and releasing states to the pressing roller  304 . The normal pressure is 300 N, for example. 
     When the image forming operation starts, the variable pressure mechanisms  500  press-contact the heating belt  302  to the pressing roller  304  to form the nip N. On the other hand, when the image forming operation is finished, the variable pressure mechanisms  500  releasing the heating belt  302  from the pressing roller  304 , and the released state is kept. 
       FIG. 5  shows the fixing device during the image forming operation. During the image forming operation, the nip N is formed between the heating belt  302  and the pressing roller  304  by the variable pressure mechanisms  500 , and the fixing step (fixing process) is completed by passing the recording material through the nip N. The edges of the recording material have small burrs produced by cutting, and the burrs flaw surface of the heating belt  302  during the fixing step at the position corresponding to the edges of the recording material, and the flaws may appear on the prints. 
     When the recording materials of the same size are continuously processed, a temperature difference occurs between the recording material passing portion of the surface of the heating belt  302  and the non-passing portion of the surface of the heating belt  302 . Because the heat of heating belt  302  is consumed for the toner fixing in the passing portion, but it is not consumed in the non-passing portion. By the temperature difference, a surface speed of the heating belt  302  is higher in the non-passing portion region than in the passing portion region with the result of slippage in the lateral end portions of the recording material. Therefore, the surface of the heating belt  302  results in having fine unsmoothness (fine pits and projections, damage by the lateral edges or edge flaw). 
     [Reciprocating Mechanism] 
     In this embodiment, in order to reduce such edge flaws, the base plate which is a supporting portion for the heating unit  27 A is reciprocated in the longitudinal direction (widthwise direction of the recording material or direction perpendicular to the feeding direction of recording material). Referring to  FIG. 6  through  FIG. 8 , a reciprocating mechanism for reciprocation controlling will be described. 
     As shown in  FIGS. 6 and 7 , the heating unit  27 A of the fixing device  27  includes a frame  400 A having the front side plate  400 , the rear side plate  401  and a bottom plate  402 . Thus, the heating belt  302  and the pressing roller  304  including the assembly such as the heater  300  are supported by the frame  400 A. In this embodiment, the front side and the rear side are based on the installed state of the image forming apparatus, and the front side is the side where the user operates the image forming apparatus, and the rear side is the opposite side. 
     At each of four corners of the bottom plate of the frame  400 A, a roller  420  is rotatably provided using a bearing  421 , and the surface of the roller  420  is slightly projected downwardly beyond the bottom plate  402 . In addition, the bottom plate  402  is provided with two elongated holes  405  extending in the widthwise direction (longitudinal direction, left-right direction of  FIG. 6  through  FIG. 8 ) as an engaged portion, the elongated hole  405  being spaced from each other and being provided at a sheet discharging side. 
     The frame  400 A of such a heating unit  27 A is a part of the fixing device  27 , and is support by the base plate  403  movably in the widthwise direction relative to the main assembly of the image forming apparatus. More particularly, by the rollers  420  provided in the bottom plate  402  rolls on the base plate  403 , the frame  400 A and the heating unit  27 A can move in the widthwise direction relative to the base plate  403 . In this manner, the bottom plate  402  is supported by the rollers  420  on the base plate  403 , and therefore, the rollers  420  rotate at the time of reciprocation in which the sliding resistance is minimized. 
     The reciprocation base plate  403  is provided with two shafts  404  as an engaging portion in the sheet discharging side so as to engage with the elongated holes  405  of the bottom plate  402 , respectively. Therefore, the frame  400 A is guided in the widthwise direction by the engagement between the shaft  404  and the elongated hole  405 . A movement distance in the widthwise direction is regulated by a length of the elongated hole  405  measured in the widthwise direction. 
     A reciprocating mechanism  470  controls the reciprocal moving operation. Referring to  FIG. 8 , the reciprocating mechanism  470  will be described. The reciprocating mechanism  470  is disposed at the side plate  401  side in the rear side of the fixing device  27 . More specifically, the reciprocating mechanism  470  includes a cam  430  as an inclination member, a shaft  410  as an engageable member, and a motor M 2  as driving means (operating device). 
     The cam  430  is provided on one of the heating unit  27 A and the supporting portion, more particularly on the base plate  403  which is a supporting portion in this embodiment, and is provided with a pair of inclined surfaces  430   a ,  430   b  inclined relative to the widthwise direction. The cam  430  has a substantially cylindrical shape and is integral with the gear  430   c  to which a rotational force is applied from the motor M 2 , and it is provided with V-like grooves  430   d  as seen from a diametrically outside over the entire cylindrical outer peripheral surface. Opposite side surfaces of the groove  430   d  constitute the inclined surfaces  430   a ,  430   b , respectively. The inclined surfaces  430   a ,  430   b  extend in parallel with each other, and are waved at regular intervals when they are expanded. 
     The shaft  410  is provided on the other of the heating unit  27 A and the supporting portion, more particularly on the side plate  401  of the heating unit  27 A in this embodiment, and is engaged with the inclined surfaces  430   a ,  430   b  of the cam  430 . That is, the shaft  410  is inserted into the groove  430   d  of cam  430 , and the outer peripheral surface of the shaft  410  is contacted to at least one of the inclined surfaces  430   a ,  430   d.    
     The motor M 2  causes a relative movement between the cam  430  and the shaft  410  to reciprocate the heating unit  27 A through the engagement between the shaft  410  and the inclined surfaces  430   a ,  430   b . In this embodiment, the motor M 2  is a pulse motor, and is driven in accordance with a pulse number fed from the controller (CPU)  460  of the control device so as to rotate the cam  430  through an amount (angle) corresponding to the pulse number. The controller  460  may be common with the above-described controller  308  for controlling the electric power supply to the heater  300 . 
     By the relative rotation of the cam  430  relative to the shaft  410 , the engaging position between the shaft  410  and the inclined surfaces  430   a ,  430   b  changes. Since the inclined surfaces  430   a ,  430   b  are inclined relative to the widthwise direction as described above, the changing of the engaging position moves the shaft  410 , and therefore the heating unit  27 A fixed to the shaft  410 , in the widthwise direction. Here, the heating unit  27 A is movable only in the direction along the elongated hole  405  of the bottom plate  402  as described hereinbefore, and therefore, the heating unit  27 A defined by broken lines in  FIG. 8  moves only in the widthwise direction. 
     In addition, the pair of inclined surfaces  430   a ,  430   b  is in the form of a wave continuously extending in the circumferential direction as described above, and therefore, the rotation of the cam  430  reciprocates the shaft  410  in the widthwise direction along the wave shape. With such a structure of this embodiment, the reciprocation control (reciprocation moving operation) of the heating unit  27 A is carried out. 
     The cam  430  as the inclination member may be provided on the heating unit  27 A side, and the shaft  410  as the engageable member may be provided on the base plate  403  side (supporting portion). 
     In this manner, in this embodiment, the shaft  410  is engaged with the groove  430   d  formed in the cam  430 , and the cam  430  is rotated so that the reciprocation control (reciprocation moving operation) of heating unit  27 A is effected. It is unnecessary to employ an urging means such as spring to urge the cam  430  to the cam surface, and therefore, the required torque can be reduced. By this, the driving structure can be downsized, and therefore, the space required by the reciprocating mechanism can be reduced. 
     Such a reciprocation control (reciprocation moving operation) is carried out for each recording material. That is, the controller  460  moves the heating unit  27 A through a predetermined amount for each passage of the recording material through the nip N. In this embodiment, the heating unit  27 A is moved during the recording material passing through the nip N after the trailing edge of recording material depart the secondary transfer portion. The movement distance is preferably approx. 0.1-0.2 mm per sheet. 
     The frequency of the reciprocation controls (reciprocation moving operations) may be one for each sheet, of one for every 2, 3 or another plurality of sheets. The heating unit  27 A is moved at every predetermined number of sheets passing the nip N. The predetermined number of sheets may be constant, or may be variable depending on the kind, the size of recording material, the number of the processed sheets or the like. 
     In this embodiment, the inclination angle of inclined surfaces  430   a ,  430   b  of cam  430  are selected such that the movement distance per one recording martial is 0.15 mm in the range other than the moving direction switching range. The range of reciprocation control (reciprocation moving operation) is approx. 4-5 mm, for example. In other words, the heating unit  27 A movement by increment of 0.15 mm within the movement range of approx. 4-5 mm. 
     The timing of the execution of the reciprocation control (reciprocation moving operation) is in the period in which no recording material is in the nip N, that is, so-called sheet interval, but in this embodiment, the timing is selected as described above. More particularly, it is after the trailing edge of recording material departs the secondary transfer portion, before the leading end reaches the nip N and during the period in which the recording material is nipped only by the nip N. This is because by the reciprocation control (reciprocation moving operation) during a sheet interval may result in the reduction of the productivity. In addition, if the heating unit  27 A carries out the reciprocating operation while the recording material is nipped by the secondary transfer portion and the nip N of the heating unit  27 A, the nip N deviates the recording material in the widthwise direction with the result of transfer defect. Therefore, in this embodiment, the timing of the execution of the reciprocating operation is selected as described above. 
     In addition, in this embodiment, there is provided a position sensor  450  as a position detecting means for detecting a position of the heating unit  27 A with respect to the widthwise direction. The position sensor  450  is fixed on the base plate  403  and includes a light emitting portion and a light receiving portion for receiving the light emitted by the light emitting portion, the light emitting portion and the light receiving portion being disposed opposed to each other. In addition, a sensor flag  440  is provided on the rear side plate  401  of heating unit  27 A. The sensor flag  440  enters between the light emitting portion and the light receiving portion of the position sensor  450  to block the light from the light emitting portion, by which the position sensor  450  detects a predetermined position of heating unit  27 A with respect to the widthwise direction. The detection signal is fed to the controller  460 , and the controller  460  controls the motor M 2  on the basis of the signal. 
     In this embodiment, a home position (HP position) is the position at which the sensor flag  440  just block the light of position sensor  450  by the movement of the heating unit  27 A from a position not blocking the light of the position sensor  450 . In position HP, a widthwise center portion of recording material entering the nip N and a widthwise center portion of the heat generation width of heating belt  302  (widthwise center portion of heating region) are substantially aligned with each other. 
     The edge flaw of the surface of the heating belt  302  produced by a recording material of a size may appear on the image on the recording material having a size larger than that. If, on the other hand, the subsequent has a larger size, the edge flaw does not appear thereon. For this reason, of above-described reciprocation control (reciprocation moving operation) is unnecessary for the recording material having a maximum width usable with the image forming apparatus. Correspondingly, the device can be downsized. 
     If after the fixing process of the recording material having the maximum width, the next recording material has a width within width range having caused the edge flaw produced by the maximum width sheet does not appear on the next sheet. 
     However, when the fixing process is carried out for the recording material having the maximum width size in a state that the heating unit  27 A is at an end portion of the reciprocation range, the widthwise center portion of a heating region of a heating belt  302  is not aligned with the widthwise center portion of recording material. Here, when the heating region of the heating belt  302  is set corresponding to the width of the recording material having the maximum width, the maximum width recording material may be partly outside the heat generation width if the widthwise position of the heating unit  27 A is deviated by the reciprocation control (reciprocation moving operation), as shown in  FIG. 9 . As a result, the fixing property may not be assured for an image at the widthwise end portion of recording material. In addition, in the case that the fixing process is carried out continuously on the maximum width recording materials, the maximum width recording material may be partly outside the fixable heat generation width, similarly. 
     In view of the above, in this embodiment, when the width of the recording material is larger than a predetermined value, the reciprocation control (reciprocation moving operation) is not executed, and the heating unit  27 A is placed at a predetermined position with respect to the widthwise direction. On the other hand, when the width of the recording material is not more than the predetermined value, the reciprocation control (reciprocation moving operation is carried out. In heating unit  27 A is moved every predetermined number of sheet processings of the recording materials through the nip N. In order to accomplish this, the controller  460  comprises an information acquiring portion  480  for acquiring the information of the width of the fed recording material. In this embodiment, the information acquiring portion  480  acquires the width from the information set by the user for the sheet size on an operation panel  490 . The information of the width of the recording material may be acquired from the detecting means provided in the apparatus may be acquired from the information of the recording material inputted from an external information terminal. 
     For example, when the user designates A4 size on the operating portion  490 , the information acquiring portion  480  acquires the width of the A4 size recording material. The controller  460  controls the reciprocation control (reciprocation moving operation) of the heating unit  27 A on the basis of the width information acquired by the information acquiring portion  480 . More particularly, if the width is not more than a predetermined value, the reciprocation control (reciprocation moving operation) is carried out, otherwise, the reciprocation control the reciprocation moving operation) is not carried out, and place the heating unit  27 A in the predetermined position. 
     It is not inevitable that the controller  460  compares the acquired width with a predetermined length. For example, in an alternative structure, a corresponding relation between the size of recording material and whether to effect the reciprocation control (reciprocation moving operation) is stored beforehand in a table (memory), and the determination is made by the table. 
     Here, the predetermined width may be properly selected depending on the device and/or a usage thereof, but in this embodiment, the width of the maximum width recording material is larger than the predetermined width, and the other recording materials have the width not more than the predetermined width. The maximum width recording material is the one usable with the device. In the case that the maximum width recording material is hardly used, or in the case that the image quality on the maximum width recording material is not concerned, the width less than the maximum width may be included in the range larger than the predetermined width. 
     The predetermined position of heating unit  27 A is the above-described HP position in this embodiment. That is, the predetermined position is such a position that the widthwise center portion of the recording material fed to the nip is aligned with the widthwise center portion of heating region (fixable heat generation width) of the heating belt  302 . The predetermined position may be properly selected depending on the device and/or a usage thereof, and it may be a position where the widthwise center portion of recording material fed to the nip is not aligned with the widthwise center portion of the heating region of the heating belt  302 . For example, it may be deviated by several mm. In addition, in the case that the size larger than the predetermined includes a plurality of sizes of the recording materials, the predetermined position may be different depending on the sizes. 
     In summary, in this embodiment, when the width information acquired by the information acquiring portion  480  is smaller than the maximum width capable of passing through the nip N, the heating unit  27 A is moved for each of the predetermined number of recording materials passing the nip N by the above-described reciprocation control (reciprocation moving operation). On the other hand, when the width information acquired by the information acquiring portion  480  is the maximum width, the reciprocation control (reciprocation moving operation) is not carried out, and the heating unit  27 A is placed in the HP position in the widthwise direction. More specifically, when the heating unit  27 A is deviated from the HP position by the past reciprocating operations or the like, the heating unit  27 A is moved to the HP position before the maximum width recording material is fed to the nip N. When the heating unit  27 A is at the HP position after the passing of the immediately prior recording material, the heating unit  27 A is kept at the HP position without movement. 
     In the case of this embodiment, while the recording materials having the predetermined width larger than a predetermined width as acquired by the information acquiring portion  80  are continuously fed into the nip N, the heating unit  27 A is not moved from the predetermined position when the recording material passes the nip N. In this embodiment, while the maximum width recording materials are continuously fed, the reciprocation control (reciprocation moving operation) is not carried out, but the heating unit  27 A is placed in the HP position. 
     In this manner, when a job (fixing process) is carries out for the maximum width recording materials, the widthwise center portion of recording material can be aligned with the widthwise center portion of heat generating region, if the heating unit is moved to the HP position or the heating unit is kept at the HP position. 
     Referring to  FIG. 10 , such an example of control of this embodiment will be described. When the job starts (S 1 ), the motor M 1  of the pressing roller  304  is rotated (S 2 ), and the heater  300  is supplied with electric power (S 3 ). Then, the controller  460  discriminates whether or not the width of the recording material acquired by the information acquiring portion  480  is the maximum width (S 4 ). If it is a maximum width recording material, the reciprocation control (reciprocation moving operation) is not executed, and the heating unit  27 A is moved to or kept at the HP position (S 5 ). On the other hand, if it is not a maximum width recording material (S 4 ), the reciprocation control (reciprocation moving operation) of the heating unit  27 A is executed (S 6 ). Thereafter, when the temperature of the heating belt  302  reaches the predetermined level (target temperature) (S 7 ), the fixing operation is started (S 8 ). 
     In the control shown in  FIG. 10 , the rotation of the motor M 1  for the pressing roller  304  and the electric power supply to the heater  300  are started earlier than the reciprocation control (reciprocation moving operation) because the heating of the heating belt  302  to the target temperature requires longer time than the reciprocation control (reciprocation moving operation the. The order of such controls is determined so as to minimize the required time to the first copy FCOT (first copy time), and therefore, the order may be different depending on the structure of fixing device. 
     In this embodiment, when the width of the recording material is not more than the predetermined width (less than the maximum width), the reciprocation control (reciprocation moving operation) of the heating unit  27 A is executed after each predetermined number (one in this embodiment) of sheets passing the nip. Therefore, lateral edges of the recording materials do not pass the same portions of the nip N, and the surface of the heating belt  302  is protected from the edge flaw. 
     On the other hand, when the width of the recording material is larger than the predetermined width (maximum width in this embodiment), the widthwise position of the heating unit  27 A is placed at the predetermined position (HP position in this embodiment). Therefore, the width (dimension measured in the widthwise direction of the heating region) of the heating belt  302  is not required to be too long. That is, since the center portion of maximum width recording material and the center portion of the heating region of the heating belt  302  are aligned with each other, the heating region may be determined to meet the maximum width. As a result, the lengths of the heating belt  302  and the heater  300  measured in the widthwise direction is not required to be longer than necessary, despite the structure with which the reciprocation control (reciprocation moving operation) of the heating unit  27 A is possible, and therefore, the upsizing of the device can be avoided. 
     In addition, in this case, by the feeding of the maximum width recording materials through the nip N, the edge flaws may results on the surface of the heating belt  302 , but the edge flaws are produced at the positions corresponding to the end portions of maximum width recording material. Therefore, the produced edge flaws are outside, with respect to widthwise direction, the next recording material, are not transferred onto said next recording material. 
     When this embodiment, the relative movement is caused between the cam  430  and the shaft  410 , and the heating unit  27 A is reciprocated through the engagement between the shaft  410  and the pair of the inclined surfaces  430   a ,  430   b  of the cam  430 . Therefore, no spring or the like is required in order to move the heating unit  27 A. Therefore, the motor is not required to drive the cam against an urging force of the spring, and the torque required for the rotation of the cam  430  may be relatively small, thus accomplishing the reciprocating mechanism with the small space. 
     According to this embodiment, as described in the foregoing, the edge flaw of heating belt  302  is reduced, and therefore, the image quality and the lifetime can be improved, without upsizing the device. 
     Other Embodiments 
     The present invention is not limited to the foregoing embodiment. In the above-described embodiment, the fixing device is an on-demand type fixing device using a film-like heating belt as the rotatable heating member. The rotatable heating member may be a roller or belt. The heating mechanism in the foregoing embodiment is a ceramic heater, but it may be a halogen heater, or an induction heating mechanism using an excitation coil (IH). 
     The positional relation between the sensor flag and the position sensor may be the opposite. More particularly, the position sensor may be provided on the reciprocation movement side, and the sensor flag is provided on the non-reciprocation side. The means for detecting the position of the heating unit with respect to the widthwise direction may be the combination of the sensor flag and the position sensor, or may use an encoder. For example, an encoder is provided on the rotation shaft of the motor, and the rotation amount of the encoder is counted, and the home position can be made detected, by which the position of the heating unit from the home position can be detected. It will suffice if the position of the heating unit with respect to the widthwise direction can be detected. 
     The reciprocating mechanism have used the cam and the shaft in the foregoing, but another structure is usable. For example, the inclination member may be a screw shaft having an outer peripheral surface male screw, and the engageable member may be a nut screwed on the screw shaft. It will suffice if the reciprocation movement can be carried out. 
     While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purpose of the improvements or the scope of the following claims. 
     This application claims priority from Japanese Patent Application No. 195664/2012 filed Sep. 6, 2012, which is hereby incorporated by reference.