Patent Publication Number: US-8983316-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 are in contact with a fixing member (one of rotatable members). 
     At this time, the surface of the fixing member tends to be damaged by the lateral edge portion of the recording material. 
     When such 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. Therefore, it is desired to reduce the production of the edge flaw. 
     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. 
     However, with such a structure, the reciprocating operation may not be properly carried out due to the play or the like in the reciprocation mechanism when the moving direction of the fixing device switches. If this occurs, the fixing device remains at the same position with the result that the reduction of the edge flaw is insufficient. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a fixing device comprising: a fixing device including a pair of rotatable members which form a nip therebetween to fix a toner image on a sheet; a reciprocating mechanism, including a motor, configured to reciprocate the fixing device within a predetermined range by moving the fixing device in a longitudinal direction thereof for each passage of a predetermined number of sheets through the nip; and a controller configured to control the motor so that a drive time of the motor per the predetermined number of the sheets is longer in a first range including a point at which a moving direction of the fixing device reverses than in a second range in which the moving direction of the fixing device does not reverse. 
     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 schematic view illustrating an operation at the time when the moving direction of the reciprocating mechanism switches. 
         FIG. 11  is an enlarged view of the reciprocating mechanism illustrating a mounting play of a reciprocating cam. 
         FIG. 12  is a flow chart showing an example of a control flow for the fixing device. 
         FIG. 13  is a flow chart illustrating a relation between a detection state of a position sensor and a count of a pulse counter in a control flow for the fixing device. 
         FIG. 14  shows relations between a number of passages and a reciprocation displacement when a speed-up control of the reciprocation is carried out and not. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Referring to  FIG. 1  to  FIG. 14 , 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  ( 22 Y,  22 M,  22 C, and  22 K) is provided. 
     Photosensitive drum  20  is rotated in the direction indicated by the arrow in  FIG. 1 , 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 by the exposure device  22  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 reversed 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 (fixing device) of the fixing device  27  according to this embodiment will be described. As shown in  FIG. 2 , the heating 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 roller (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 composed 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 thereof 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 metal core  304   a , and an elastic layer  304   b  composed 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 metal core  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, extending 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 thermistor  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 thermistor  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 to be 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  releases 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 produce a 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 reciprocation 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 reciprocating operation 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 holes  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 carried by the reciprocation base plate  403  movably in the widthwise direction relative 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 the 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 reciprocating cam  430  as an inclination member, a reciprocating shaft  410  as an engageable member, and a motor M 2  as driving means (operating device). 
     The reciprocating 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. 
     Reciprocating 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 reciprocating cam  430 . That is, the reciprocating shaft  410  is inserted into the groove  430   d  of reciprocating 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 reciprocating cam  430  and the reciprocating 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 reciprocating 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 reciprocating cam  430  relative to the reciprocating shaft  410 , the engaging position between the reciprocating 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 reciprocating cam  430  reciprocates the reciprocating shaft  410  in the widthwise direction along the wave shape. With such a structure of this embodiment, the reciprocation moving operation of the heating unit  27 A is carried out. 
     The reciprocating 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 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 blocks 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. Therefore, as shown in  FIG. 9 , when the maximum size recording material is passed through the nip N, the heating unit  27 A is moved to the HP position, by which the center portion of the heat generation width and the center portion of the recording material of the maximum size can be aligned with each other. 
     In this embodiment, the relation between the sensor flag  440  and the position sensor  450  is set in such a manner, and therefore, the heat generation width of the heating belt  302  can be reduced. That is, when the center portion of the maximum size recording material and the center portion of the heat generation width are deviated from each other, it is necessary to make the heat generation width larger than the heating region of the maximum size recording material by the amount of the deviation in order to cover the maximum size recording material. On the other hand, by aligning the center portion of the maximum size recording material with the center portion of the heat generation width, the heat generation width may be the same as the heating region for the maximum size recording material, and therefore, there is no necessity for making the heat generation width large. 
     In this manner, in this embodiment, the reciprocating shaft  410  is engaged with the groove  430   d  formed in the reciprocating cam  430 , and the reciprocating cam  430  is rotated so that the 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 frequency of the reciprocation moving operations may be one for each sheet, of one for every 2, 3 sheets, 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. The predetermined numbers may include different numbers in mixture. For example, the numbers may be, one, two, one, two, or the like. 
     In this embodiment, the inclination angle of inclined surfaces  430   a ,  430   b  of cam  430  is selected such that the movement distance per one recording material is 0.15 mm in the range other than the moving direction switching range of the heating unit  27 A. The range of reciprocation control (reciprocation moving operation) is approx. 4-5 mm, for example. In other words, the heating unit  27 A movement is by an increment of 0.15 mm within the movement range of approx. 4-5 mm. In the case that the image is to be formed on each of the sides of the recording material, it is desired that the difference between a marginal blank range of the front side in the widthwise direction of the recording material and the marginal blank range of the back side is within a limitation. However, if the movement distance of the reciprocation of the heating unit  27 A by the reciprocating operation is too large, the different may not be within the limit. In order to make the different fall within the limit, the movement distance in one way of the reciprocation is preferably not more than 0.3 mm. On the other hand, if the movement distance is too small, the deviation between the center position of the sheet and the center position of the heat generation width may be kept for a long term, with the result of excessive temperature rise in the non-passing region. The temperature rise (temperature ripple) in the non-passing region is desirably within approx. 5 degree C. To make the temperature ripple not more than 5 degree C., the movement distance per recording material is preferably not less than 0.04 mm on the average. 
     The timing of the execution of the 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 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 a transfer defect. Therefore, in this embodiment, the timing of the execution of the reciprocating operation is selected as described above. 
     [Speed-Up Control at the End Portion of the Reciprocation] 
     In this manner, in this embodiment, the reciprocating shaft  410  is engaged with the groove  430   d  formed in the reciprocating cam  430 , and the reciprocating cam  430  is rotated so that the reciprocation moving operation of heating unit  27 A is effected. Therefore, the reciprocation moving operation may stagnate when the moving direction of the heating unit  27 A switches, due to a gap (play) between the reciprocating shaft  410  and the groove  430   d  of the reciprocating cam  430  or the like. Here, a predetermined range including the region where the moving direction of the heating unit  27 A switches is called reciprocation end portion. In this embodiment, the moving speed of the heating unit  27 A is increased at the reciprocation end portion (reciprocation end portion speed-up control). 
     Referring first to  FIGS. 10 and 11 , the stagnation of the reciprocating operation at the time when the moving direction of the heating unit  27 A switches will be described.  FIG. 10  schematically shows a movement locus of the reciprocating shaft  410  in the neighborhood of a reversing point (moving direction switching point) of the reciprocating operation, in the structure in which the reciprocating shaft  410  is engaged in the groove  430   d  of the reciprocating cam  430 . As shown in  FIG. 10 , there is a small gap between the outer peripheral surface of the reciprocating shaft  410  and the inclined surfaces  430   a ,  430   b  of the groove  430   d . In  FIG. 10 , the rotational moving direction of the reciprocating cam  430  is along an x-direction, and the moving direction of the heating unit  27 A is along a y-direction. 
     When the groove  430   d  moves to the left (−x direction) in  FIG. 10  by the rotation of the reciprocating cam  430 , the reciprocating shaft  410  is pushed by the inclined surface  430   a  of the groove  430   d  to move in the +y direction in  FIG. 10 . With the continuing rotation of the reciprocating cam  430 , it reaches the reversing point of the reciprocation, and the reciprocating shaft  410  is pushed by the inclined surface  430   b  of the groove  430   d . Therefore, at the reversing point of the reciprocation, the surface to which the reciprocating shaft  410  contacts changes between the inclined surface  430   a  and the inclined surface  430   b  (changing region). In such a region, the reciprocating shaft  410  is not pushed by either of the inclined surfaces  430   a ,  430   b , and therefore, the movement of the heating unit  27 A, more particularly, the displacement of the reciprocating shaft  410  in the y direction stagnates. In other words, in such a region, the reciprocating shaft  410  does not move even if the reciprocating cam  430  rotates, and therefore, the reciprocating operation of the heating unit  27 A does not occur. 
     Furthermore, as shown in  FIG. 11 , there exists play (gap g) which is necessary for the engagement, in a rotation center axis direction. As described hereinbefore, at the reversing point, the surface of the reciprocating cam  430  pushing the reciprocating shaft  410  switches. At this time, the direction of a component force (y) of a force received by the reciprocating cam  430  from the reciprocating shaft  410  (a reaction force of pushing the reciprocating shaft  410  by the inclined surface of the groove  430   d ) is reversed. That is, the surface of the reciprocating shaft  410  pushing the reciprocating cam  430  switches with the result of switching of the play killing direction. During the play killing motion, the reciprocating cam  430  per se is moved by the reaction to the reciprocating shaft  410 , and therefore, displacement of the reciprocating shaft  410  (y direction) stagnates. 
     As described in the foregoing, the reciprocating operation of the reciprocating shaft  410  may stagnate at the fold-back point because of the play. The stagnation deteriorates the intended edge-flaw-suppressing effect of reciprocation at the end portions of the reciprocation range. 
     In this embodiment, in the predetermined range in which the contacted surface is switched and the play is being killed, the reciprocation end portion speed-up control is carried out. By the control of the controller  460 , the movement distance between the reciprocating cam  430  and the reciprocating shaft  410  per predetermined number of the recording materials is larger in the above-described predetermined range than the other range (central region of the reciprocation region). In this embodiment, the amount of rotation of the reciprocating cam  430  per one recording material is made larger. As described, when the predetermined number is plural, the relative movement distance per the number of the recording materials is made larger within a predetermined range. The above-described range is preferably the region in which the surface switching and the play killing is carried out, but may be smaller or larger than that. It will suffice if the predetermined range covers at least a part of the range in which the reciprocating operation stagnates. 
     Such a predetermined range is discriminated from the detection result of the above-described position sensor  450 . More particularly, the end portion of the reciprocating operation is detected using the sensor flag  440  and the position sensor  450 . This will be described. The controller  460  comprises a counter C for counting a position in the reciprocation. The counter C resets the count to zero, when the sensor flag  440  starts to block the light from the position sensor  450  or when the sensor flag  440  starts to let the light pass. The count increments by 1 by one forward or backward movement (in one direction) for one passing of one recording material through the nip N. 
     First, the heating unit  27 A moves in the direction from the front side to the rear side of the image forming apparatus, and the light from the position detecting sensor  450  is blocked (HP position) by the sensor flag  440 , and then the count of the counter C is reset to zero. During the time in which the heating unit  27 A is moving in the same direction, the sensor flag  440  keeps blocking the light from the position sensor  450 . 
     Subsequently, the predetermined number of the recording materials pass through the nip N, a free end of the sensor flag  440  passes by the position sensor  450  by way of the reversing point in the rear side, and the light receiving portion of the position sensor  450  starts to receive the light from the light emitting portion. At this time, the count is Nr. Here, the count of the counter C is reset to zero. With further passing of the recording material through the nip N, the sensor flag  440  continues to move in the state that the light of the position sensor  450  is not blocked. Thereafter, the sensor flag  440  again blocks the light from the position sensor  450  by way of the front side reversing point. The count of the counter C at this time is Nf. 
     The groove  430   d  of the reciprocating cam  430  displaces in a constant y direction by the rotation of the reciprocating cam  430 . That is, for one unidirectional movement of the reciprocating operation, the reciprocating cam  430  rotates through a predetermined amount relative to the reciprocating shaft  410 . The movement is determined by the number of the pulses of the motor M 2 , and one movement is carried out by feeding a predetermined number of pulses to the motor M 2 . 
     Therefore, the number of the movements of the heating unit  27 A in the forward stroke in the reciprocation is the same as that in the backward stroke, and the count at the fold-back point is as follows: Nr/2 for the rear side reversing point, and Nf/2 for the front side reversing point. Therefore, the controller  460  can discriminate the positions of the reciprocation end portion, and therefore, the above-described predetermined range) using the counter C. 
     In this embodiment, therefore, the end portion speed-up control is carried out using the count at the reversing point of the reciprocating operation. First, the comparison is made between the count at the reversing point and the current count. When the current count comes as close to the count at the reversing point as the reversing point count minus 3, the end portion speed-up control is started. 
     In this embodiment, the speed-up control is such that the normal moving operation is carried out twice for one recording material. In the range outside the above-described predetermined ranges, one unidirectional movement of reciprocation is effected for one recording material, but in the above-described predetermined range, two unidirectional movements of reciprocation are intermittently effected per recording material. In other words, in this embodiment, the number of the pulses supplied to the motor M 2  per one recording material is increased. As a result, the rotation amount of the reciprocating cam  430  per one recording material is larger in the above-described predetermined ranges than in the other range of the reciprocation. In this embodiment, the count is incremented by two by one recording material in the above-described predetermined range. 
     When the current count reaches the reversing point count plus 2, the speed-up control is stopped to effect the normal reciprocation control. That is, one movement is effected per one recording material. With such control, the stagnation time at the reciprocation end portion can be reduced as compared with the case of normal movement. For example, it is assumed that the stagnation region can be passed by passing of 6 recording materials with the normal movement control. Using the above-described end portion speed-up control, the stagnation region can be passed by passing of 3 recording materials. It is possible to modify the speed-up control such that the stagnation region can be passed by one recording material. 
     Referring to  FIG. 12 , such an example of control of this embodiment will be described. First, the main switch of the device is actuated, and the start-up of the fixing device  27  begins (S 1 ), and the motor M 1  for the pressing roller  304  starts (S 2 ). Then, the electric power supply to the heater  300  for the heating belt  302  is started (S 3 ). During the start-up operation for the fixing device, a reciprocation profile is acquired (S 4 ). 
     The acquisition of the reciprocation profile is effected by detecting the position of end portion of the reciprocation by one reciprocation of the heating unit  27 A by rotating the reciprocating cam  430 . That is, the counts at reversing points of the reciprocation are acquired. The acquisition of the reciprocation profile is fundamentally carried out when the device is renewed such as when the fixing device is exchanged, but in this embodiment, it is carried out also upon the actuation of the main switch. Once the reciprocation profile is acquired, a step acquisition step may be omitted until the renewal of the device. By carrying out the acquisition of the reciprocation profile, the reciprocation end portion speed-up control can be carried out from the initial stage of start-up of the fixing device. The counter is operated at all times during the reciprocating operation since the counts at the reversing points are always renewed. 
     After completion of the start-up of the fixing device and the acquisition of the reciprocation profile (S 5 ), the discrimination is made as to whether or not a job (JOB) is produced (S 6 ). If not, the rotation of the motor M 2  for the pressing roller  304  is stopped (S 8 ), while keeping the electric power supply to the heater  300  ON, and waits for the production of the job (S 7 ). 
     If a job is produced in step S 6 , the motor M 2  of the pressing roller  304  is rotated (S 9 ), and a heater  300  is supplied with the electric power (S 10 ). When the temperature of the heating belt  302  reaches a predetermined target temperature (S 11 ), the job is executed (S 12 ). Then, the discrimination is made as to whether or not the current count of the counter C indicates the reciprocation end portion (S 13 ). If so, the reciprocation end portion speed-up control is executed (S 14 ). Thereafter, the discrimination is made as to whether or not the job is completed (S 15 ), and if not, the operation returns to step S 13 . If the discrimination at step S 13  does not indicate the reciprocation end portion, the normal movement stroke is carried out (S 16 ), and the operation goes to step S 15 . If the discrimination as step S 15  indicates the completion of the job, the electric power supply to the heater  300  is rendered OFF (S 17 ), and the rotation of the motor M 1  for the pressing roller  304  is stopped (S 18 ). 
     Referring to  FIG. 13 , a flow of control based on the count of counter C will be described. When the main switch of the device is actuated, the acquisition of the reciprocation profile is started, and the count operation of the counter C is started (S 21 ). At this time, the discrimination is made as to whether or not the position sensor  450  changes from the light blocking state to the light passing state (S 22 ). If so, it is discriminated that the heating unit  27 A is moving from the rear side to the front side of the device. The count α of the counter C is incremented by one per one recording material (S 23 ). Here, the count is Nf. Then, the discrimination is made as to whether or not the position sensor  450  changes from the light passing state to the light blocking state (S 24 ). 
     If the discrimination at the step S 24  is affirmative (Y), the value Nf can be determined (S 25 ). Thereafter, the operation returns to step S 22 . If the result of discrimination at step S 24  is negative (N), the discrimination is made as to whether or not the count α, is not less than {(Nf/2)−3}, and not more than {(Nf/2)+2} (S 26 ). The result of discrimination at step S 26  is affirmative (Y), that is, the heating unit  27 A is in the predetermined range in which the moving direction reverses, the count α of the counter C is incremented by two because of the above-described two actuations of the reciprocating operation (S 27 ). That is, the reciprocation end portion speed-up control is carried out. If the result of discrimination at step S 26  is negative (N), the operation returns to step S 23 . 
     On the other hand, if the result of discrimination at step S 22  is negative (N), that is, the position sensor  450  changes from the light passing state to the light blocking state, it is discriminated that the heating unit  27 A is moving from the front side to the rear side of the device. The count α of the counter C is incremented by one per one recording material (S 28 ). Here, the count is Nr. Then, the discrimination is made as to whether or not the state of the position sensor  450  changes from the light blocking state to the light passing state (S 29 ). 
     If the discrimination at the step S 29  is affirmative (Y), the value Nr can be determined (S 30 ). Thereafter, the operation returns to step S 22 . If the result of discrimination at step S 29  is negative (N), the discrimination is made as to whether or not the count α, is not less than {(Nr/2)−3}, and not more than {(Nr/2)+2} (S 31 ). If the result of discrimination at step S 31  is affirmative (Y), that is, the heating unit  27 A is in the predetermined range in which the moving direction reverses, the count α of the counter C is incremented by two because of the above-described two actuations of the reciprocating operation (S 32 ). That is, the reciprocation end portion speed-up control is carried out. If the result of discrimination at step S 31  is negative (N), the operation returns to step S 28 . 
     Thus, in above-described flow of the control, either of the Nr or the Nf is incremented, depending on whether the state of the position sensor changes from the light blocking state to the light passing state or it changes from the light passing state to the light blocking state. In addition, the increment is either one or two, depending on whether or not the device is in the reciprocation end portion. The above-described flow chart is an example, and the discrimination criteria and the executing operations are not inevitable. 
     In this embodiment, as described above, the heating unit  27 A is moved (a part of reciprocating operation) for each passage of a predetermined number of the recording materials through the nip. Therefore, the 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 deleterious effects of an edge flaw. 
     In addition, in this embodiment, the relative movement is caused between the reciprocating cam  430  and the reciprocating shaft  410  to move the heating unit  27 A by the engagement between the reciprocating shaft  410  and the pair of inclined surfaces  430   a ,  430   b  of the reciprocating cam  430  to effect the reciprocation movement in a long term. 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 permitting the use of a reciprocating mechanism extending over a small space. 
     In addition, in the predetermined range where the moving direction of the heating unit  27 A switches (reciprocation), the relative movement distance between the reciprocating cam  430  and the reciprocating shaft  410  per one recording material is made larger than in the other region. That is, the reciprocation end portion speed-up control is executed. Therefore, the stagnation of the operation of the heating unit  27 A can be reduced where the moving direction switches. As a result, the production of the edge flaw can be reduced in the end portions of the reciprocation. 
       FIG. 14  shows the results of experiments to check the effects of this embodiment. The experiments determined the relation between the number of passages of the recording material and the reciprocation displacement (y direction displacement) when the reciprocation end portion speed-up control of this embodiment is carried out and when it is not carried out. In  FIG. 14 , the solid line represents the case in which the reciprocation end portion speed-up control is carried out, and the broken line represents the case in which the reciprocation end portion speed-up control is not carried out. The left part of this graph indicated by the broken line double head arrow at the bottom represents the range in which one actuation of the reciprocation control is performed in the case of the reciprocation end portion speed-up control. The part of this graph indicated by the solid line double head arrow at the bottom represents the range in which two actuations of the reciprocation control are performed in the case in of the reciprocation end portion speed-up control. As indicated at the top of this graph, the left half shows the case in which the reciprocation end portion speed-up control is executed, and the right half shows the case in which it is not carried out. 
     As will be understood from  FIG. 14 , the motion (y direction) stagnates in the neighborhood of the reversing point of the reciprocating motion without the reciprocation end portion speed-up control (right half), but the stagnation is reduced when the reciprocation end portion speed-up control is carried out. 
     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. 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 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 has 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. In this case, the fixing device is reciprocated by rotating the motor in forward and backward directions. It will suffice if the reciprocation movement can be carried out. Using a screw mechanism, the stagnation phenomenon-occurs at the point where the moving direction switches, due to the backlash, 
     In the foregoing embodiments, two actuations of the reciprocating operation are carried out at the end portion, but the number may be three or more, as long as the number is larger than in the normal range. 
     The reciprocation end portion speed-up control may be carried out by another method. For example, the relative moving speed between the inclination member and the engageable member per one recording material is increased in the predetermined reciprocation end portion range. Specifically, the frequency of the motor for the reciprocating operation is increased in the predetermined end ranges. The speed in the end ranges may be twice of thrice. In the foregoing embodiments, the counts for the reciprocation end portion speed-up control are the counts at the reversing points minus 3 and plus 2, but the value may be changed taking into account the actual play between the inclined surface of the cam groove and the shaft, and the mounting play of the reciprocating cam per se. 
     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. 195672/2012 filed Sep. 6, 2012, which is hereby incorporated by reference.