Patent Publication Number: US-7715774-B2

Title: Image forming apparatus which controls heating width of fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This patent application is based on Japanese patent application, No. 2006-050212 filed on Feb. 27, 2006 in the Japan Patent Office, the entire contents of which are incorporated by reference herein. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   Exemplary aspects of the present invention relate to an image forming apparatus, and more particularly to an image forming apparatus and the changing of the heating width of its fixing device. 
   2. Description of the Related Art 
   In general, image forming apparatus of recent years such as copiers, printers, facsimile machines, and multifunctional peripherals employ an electro-photographic method to form a toner image on an image carrier by a series of image forming processes such as charging, writing, developing, transferring, cleaning, and discharging. The toner image on the image carrier is transferred on a recording medium such as transfer paper and overhead projector films. The image forming apparatus includes a fixing device in which heat and pressure are applied by a fixing member and a pressure member to fix the toner image on the transfer medium. 
   Such image forming apparatuses are demanded to shorten a warm-up time when power is initially turned on. The fixing member of the fixing device increases the temperature thereof to a suitable level during warm-up so that the image forming apparatus can become operatable. 
   In attempting to shorten the warm-up time, fixing devices having a fixing member with a relatively low heat capacity have been developed and practically used. Specific examples thereof include a fixing device in which a nip is formed between a rotation member and a fixing film that has a relatively low heat capacity and faces the rotation member. Another example is a fixing device that employs a fixing belt as the fixing member, wherein and the fixing belt is tightly stretched by a plurality of rotation members. One of the plurality of rotation members has a relatively low heat conduction and forms a fixing nip with the a rotation member while another of the plurality of rotation members includes a heat source such as heaters. Recently, induction heating systems in which a fixing member having a relatively low heat capacity is directly heated have been used as the heat source as well as heaters. The fixing devices with the induction heating system have an advantage such that the fixing member having a relatively low heat capacity can increase the temperature thereof in a relatively short time period, resulting in shortening of the warm-up time of the fixing device. One example attempts to shorten the warm-up time by applying a relatively large amount of heat to the fixing belt in a short time period. The fixing device includes a contact member which presses the fixing belt so that the fixing belt and a heating roller having a heat source therein have a large contact length. Thereby, a relatively large amount of heat is applied to the fixing belt in a short time period. 
   According to another example of an attempt to reduce fixing errors when a variety of recording media having different widths are used, an increase in temperature at both ends of the fixing member in a width direction is controlled even when a plurality of recording media having small widths are successively fixed. In addition, the fixing device includes an induction heating unit including a shielding member for shielding and a core. The shielding member varies the heating area in a width direction of the fixing member by shielding a part in a width direction of the core when an electric current is applied to a coil of the induction heating unit. The core includes a protrusion member at the end in the width direction thereof. The protrusion member protrudes towards the coil of the induction heating. 
   Therefore, the related art fixing device attempts to reduce the increase in temperature of the fixing member at a non-sheet feeding area, by controlling the width of the heated portion of the fixing member when a recoding medium having a relatively small width is fed. 
   However, when the width of the recording medium is frequently switched for one printing operation, the control of the heating width is late due to the response speed of the mechanical operation or the response speed by the heat capacity of the fixing member. Thereby, the heating width is not properly controlled. When a related art heat fixing device with a low-speed and low-heat amount and a high-speed and high-heat amount is used, the temperature of a non-sheet feeding area cannot reach a risk range immediately because the control of the heating width is late. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the invention, an image forming device includes a fixing section, a sheet-width detector, a heating-width changing mechanism, a mixed feeding detector, and a controller. The fixing section includes at least a fixing member that fixes a toner image on a recording medium upon application of heat thereto. The sheet-width detector detects a width of the recording medium and outputs a sheet-width signal. The heat-width changing mechanism changes a heating width of the fixing member. The mixed feeding detector detects whether or not mixed feeding of a plurality of sheets of the recording medium having different widths is performed during a successive sheet feeding, and outputs a mixed feeding signal. The controller controls the heating-width changing mechanism to control the heating width based on the sheet-width signal and the mixed feeding signal. 
   An image forming apparatus includes an image bearing member, a transfer device and the fixing device. The image bearing member bears a toner image thereon. The transfer device transfers the toner image onto a recording medium. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the exemplary aspects of the invention and many of the attendant advantage thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
       FIG. 1  is a schematic cross section illustrating an image forming apparatus according to an exemplary embodiment of the present invention; 
       FIG. 2  is a schematic cross section illustrating a fixing device for use in the image forming apparatus of  FIG. 1 ; 
       FIGS. 3A and 3B  are schematic front elevations illustrating a heating roller for use in the fixing device of  FIG. 2  when observed from the induction heating unit side; 
       FIG. 4  is a schematic diagram illustrating a driving system for driving the heating roller of the fixing device of  FIG. 2 ; 
       FIG. 5  is a schematic block diagram illustrating a control system for controlling the fixing device in the image forming apparatus of  FIG. 1 ; 
       FIG. 6  is a flowchart illustrating an example procedure for controlling the fixing device of  FIG. 2 ; 
       FIG. 7  is a flowchart illustrating another example procedure for controlling the fixing device of  FIG. 2 ; 
       FIG. 8  is a schematic flowchart illustrating yet another example procedure for controlling the fixing device of  FIG. 2 ; and 
       FIG. 9  is a schematic cross section illustrating another fixing device according to an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
   In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
   Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, an image forming apparatus  1  according to an exemplary embodiment of the present invention is described. 
   Referring to  FIG. 1 , the image forming apparatus  1  includes an exposure unit  3 , a fixing device  20 , a transfer unit  7 , a pair of registration rollers  13 , a plurality of sheet feeding units such as a first sheet feeding unit  11  and a second sheet feeding unit  12 , an ejection tray  10 , and a process cartridge  4 . The process cartridge includes a photoconductor drum  18 . The exposure unit  3  emits light L, for example, a laser beam, based on image information. The fixing device  20  fixes a toner image transferred onto a recording medium P. The transfer unit  7  transfers the toner image on the photoconductor drum onto the recording medium P. The registration rollers  13  convey and register the recording medium P. The first sheet feeding unit  11  stores the recording medium P. The second sheet feeding unit  12  stores the recording medium P. The ejection tray  10  is a tray on which the recording medium is ejected. The process cartridge  4  includes the photoconductor  18  and at least one of image forming elements such as charging devices, cleaning devices, and development devices. The photoconductor drum  18  forms an electrostatic latent image thereon by the light L emitted from the exposure unit  3 . 
   According to the exemplary embodiment of the present invention, the exposure unit  3  emits the light L towards the photoconductor drum  18  of the process cartridge  4 . The photoconductor drum  18  rotates counterclockwise, and forms the toner image thereon by a series of image forming processes such as charging, exposing, and developing. The toner image includes the image information. The transfer unit  7  transfers the toner image from the photoconductor drum  18  onto the recording medium P which is conveyed by the registration rollers  13 . 
   When the recording medium P is conveyed to the transfer unit  7 , one of the plurality of sheet feeding units included in the image forming apparatus  1  is automatically or manually selected. 
   Each of the plurality of sheet feeding units, for example, the sheet feeding units  11  and  12 , stores certain size sheets of the recording medium P. In this way, the plurality of sheet feeding units stores different size sheets of the recording medium P. 
   An uppermost sheet of the recording medium P in the first sheet feeding unit  11  is conveyed towards a conveyance path K, and reaches the position of the registration roller  13 . The registration rollers  13  register the recording medium P at a desired time and convey the recording medium P to the transfer unit  7  so that a toner image on the photoconductor  18  is transferred onto a predetermining portion of the recording medium P. 
   When the transfer unit  7  transfers the toner image from the photoconductor  18  onto the recording medium P, the recording medium P is conveyed to the fixing device  20  that includes a fixing belt  22  (shown in  FIG. 2 ) and a pressure roller  30  (shown in  FIG. 2 ). The recording medium P is fed into a nip between the fixing belt  22  and the pressure roller  30  so that the toner image on the recording medium P is fixed by heat and pressure applied by the fixing belt  22  and the pressure roller  30 . When the toner image on the recording medium is fixed, the recording medium P is fed out from the nip between the fixing belt  22  and the pressure roller  30 . The recording medium P with a fixed toner image is ejected from the image forming apparatus  1  as an output image, and is stacked on the ejection tray  10 . Thus, the series of image forming processes are completed. 
   Referring to  FIG. 2 , the fixing device  20  included in the image forming apparatus  1  of  FIG. 1  according to the exemplary embodiment of the present invention is illustrated in a schematic cross section. The fixing device  20  includes a fixing roller  21 , the fixing belt  22 , a heating roller  23 , an induction heating unit  24 , the pressure roller  30 , a thermostat  37 , an oil applying roller  34 , a guide plate  35 , and a separation plate  36 . The heating roller  23  includes an internal core  23   a  and shielding members  23   b  therein. The induction heating unit  24  includes a coil  25 , a core  26 , a side core  27 , a center core  28 , and a coil guide  29 . The oil applying roller  34  includes a cleaning roller  33  in a vicinity thereof. 
   The fixing roller  21  has an elastic layer on a surface thereof. The elastic layer may be silicon rubber, for example. The fixing roller  21  is rotationally driven by a driving unit (not shown) in a counterclockwise direction in  FIG. 2 . The heating roller  23  serving as a heating member is a non-magnetic cylindrical member, and rotates counterclockwise in  FIG. 2 . The heating roller  23  includes a non-magnetic material such as SUS304 (a stainless steel defined by Japanese Industrial Standard). The internal core  23   a  includes a ferromagnetic material such as ferrite, and the shielding members  23   b  includes a low magnetic permeability material such as copper. The internal core  23   a  is opposed to the coil  25  with the fixing belt  22  therebetween. 
   The shielding members  23   b  are configured to shield both ends of the internal core  23   a  in the width direction. The internal core  23   a  and the shielding members  23   b  are configured to rotate integrally. The internal core  23   a  and the shielding members  23   b  are rotated independently of the heating roller  23 . 
   The fixing belt  22  serving as a fixing member is tightly stretched by the heating roller  23  and the fixing roller  21 . The fixing belt  22  is a multilayer endless belt. For example, the endless belt includes a base layer including a polyimide resin, a heat generation layer including a material such as silver, nickel, and iron, and a releasing layer serving as an outmost layer including a fluorine compound. The releasing layer of the fixing belt  22  allows a toner image T on the fixing belt  22  to be released from the fixing belt  22 . 
   The coil  25  included in the induction heating unit  24  is a litz wire that has a bundle of fine lines. The litz wire of the coil  25  extends in the width direction of the core (i.e., in the vertical direction perpendicular to the surface of a paper sheet on which  FIG. 2  is printed) so as to cover a part of the fixing belt  22  winding around the heating roller  23 . The coil guide  29  of the induction heating unit  24  includes a heat-resistant resin material, and holds the coil  25 , the core  26 , the side core  27 , and center core  28 . Each of the core  26 , the side core  27 , and center core  28  includes a high magnetic permeability material such as ferrite. The core  26  is opposed to the coil  25 . The side core  27  is disposed at an end of the coil  25 . The center core  28  is disposed at a center of the coil  25 . 
   The fixing device  20  includes a core unit. This core unit includes the core  26 , the side core  27 , the center core  28 , and the internal core  23   a , and contributes to electromagnetic induction heating. Since the heating roller  23  includes the internal core  23   a  therein, a proper magnetic field can be formed between the core  26  and the internal core  23   a . Thereby, the heating roller  23  and the fixing belt  22  can be heated efficiently. 
   The pressure roller  30  includes a metal core on which there is an elastic layer made of a material such as a fluorine rubber or a silicon rubber. This pressure roller  30  is pressure-contacted with the fixing roller  21  with the fixing belt  22  therebetween. The recording medium P is conveyed to a contact area (referred to as a fixing nip) formed between the fixing belt  22  and the pressure roller  30 . 
   The guide plate  35  guides the recording medium P. This guide plate  35  is disposed at an inlet side of the fixing nip formed between the fixing belt  22  and the pressure roller  30 . The separation plate  36  is disposed at an outlet side of the fixing nip. This separation plate  36  guides the recording medium P while separating the recording medium P from the fixing belt  22 . 
   The oil applying roller  34  contacts a portion of an outer circumference surface of the fixing belt  22 . The oil applying roller  34  applies an oil such as silicon oils, on the fixing belt  22  to impart toner releasability to the toner image T on the fixing belt  22 . The oil applying roller  34  contacts the cleaning roller  33  that cleans the surface of the oil applying roller  34 . 
   The thermostat  37  contacts an outer circumference surface of the heating roller  23 . The thermostat  37  detects the temperature of the heating roller  23 . When the temperature of the heating roller  23  is higher than the predetermined temperature, the thermostat  37  shuts off the power to the induction heating unit  24 . A thermistor (not shown) serving as a temperature detection mechanism is disposed on the fixing belt  22 . The thermistor directly detects the temperature (referred to as fixing temperature) of the surface of the fixing belt  22  to control the fixing temperature. The fixing belt  22  may employ a thermopile as the temperature detection mechanism that detects the temperature of fixing belt  22  without being contacted therewith, although any suitable temperature detection mechanism may be used. 
   The fixing device  20  operates as follows. The fixing belt  22  is rotated in a direction, indicated by an arrow shown in  FIG. 2 , by the fixing roller  21  while the heating roller  23  rotates counterclockwise, and the pressure roller  30  rotates in a direction indicated by an arrow. The surface of the fixing belt  22  is heated at a position substantially opposed to the induction heating unit  24 . When the coil  25  is applied with a high frequency alternating current, magnetic lines of force are formed between the core  26  and the internal core  23   a  which alternate direction. In this case, an eddy current is formed on a surface of the heating roller  23 , thereby, generating Joule heat therein due to the electric resistance thereof. The fixing belt  22  which is wrapped around the heating roller  23  is heated by the Joule heat. 
   The surface of fixing belt  22  heated by the induction heating unit  24  reaches the nip between the fixing belt  22  and the pressure roller  30 , and the toner image T on the recording medium P is heated and fused. For example, the recording medium P on which the toner image T is formed by the series of image forming processes is guided by the guide plate  35 , and is fed into the nip between the fixing belt  22  and the pressure roller  30  as indicated by an arrow Y 10  in  FIG. 2 . The toner image T on the recording medium P is fixed by the heat and pressure applied by the fixing belt  22  and the pressure roller  30 , and is fed out from the nip between the fixing belt  22  and the pressure roller  30 . 
   Referring to  FIG. 3A  and  FIG. 3B , the heating roller  23  for use in the fixing device  20  is illustrated in schematic front elevations when observed from the induction heating unit  24 . The heating roller  23  is a cylindrical member, and includes the internal core  23   a  and the shielding members  23   b  therein. The internal core  23   a  is a column having a width L 1  and a diameter D 1 . The internal core  23   a  and the shielding members  23   b  are rotatably provided in the heating roller  23 . 
   The shielding members  23   b  are integrally disposed at both ends of the internal core  23   a  in the width direction. The shielding members  23  have such a shape as to increase or decrease the area of the shielded circumference surface of the internal core  23   a  toward the end of the internal core  23   a . By rotating, the internal core  23   a  with the shielding members  23   b,  the shielding area in the width direction of the internal core  23   a  opposed to the coil  25  in the induction heating unit  24  can be varied. 
   Referring to  FIG. 4 , the heating roller  23  of the fixing device  20  includes a stepping motor  23   e  that is configured to drive the heating roller  23   b . As shown in  FIG. 4 , a shaft  23   c  is connected to the internal core  23   a , and can be rotated by the stepping motor  23   e  through gears  23   d . Therefore, the area of the shielded portion of the internal core  23   a  opposed to the coil  25  can be varied. The stepping motor  23   e  is a driving system, and is separated from another driving system (not shown) configured to drive elements such as the fixing roller  21 , the fixing belt  22 , and the heating roller  23 . 
   For example, when the internal core  23   a  and the shielding member  23   b  in the state of  FIG. 3A  are rotated by 90 degrees in the circumference direction thereof, the internal core  23   a  and the shielding member  23   b  achieve the state of FIG.  3 B. In this state, the area of the shielded portion of the internal core  23   a  opposed to the induction heating unit  24  is increased by the shielding members  23   b . The magnetic lines of force to be formed between the induction heating unit  24  and the core  26  are shielded by the shielded portion. Thereby, the portion of the fixing belt  22  corresponding to the shielded portion is hardly heated. However, another portion of the fixing belt  22 , for example, the portion having a width L 2  in  FIG. 3A , can be heated. 
   This state in which the heated portion of the fixing belt  22  has the width L 2  is suitable for successively feeding toner images on a plurality of sheets of the recording media P with the width L 2 . When the recording medium P with a minimum width (for example, 148 mm) is fixed, the position of the internal core  23   a  and the shielding member  23   b  is preferably fixed in the state of  FIG. 3B  so as to fix the toner image on the recording media P. 
   When the internal core  23   a  and the shielding member  23   b  in the state of  FIG. 3B  are further rotated by 180 degree in the circumference direction thereof, the portion of the internal core  23   a  opposed to the induction heating unit  24  is not shielded by the shielding members  23   b . In this case, the magnetic lines of force are formed between the opened internal core  23   a  and the core  26  in the induction heating unit  24 , and thereby the entire surface of the fixing belt  22  in the width direction thereof (i.e., the portion with width L 1  in  FIG. 3A ) is heated. 
   This state in which the heated portion of the fixing belt  22  has the width L 1  is suitable for successively feeding the toner images on a plurality of sheets of the recording media P with the width L 1 . When the recording medium P with a maximum width (for example, 297 mm) is fixed, the position of the internal core  23   a  and the shielding member  23   b  is rotated by 180 degree from the state of  FIG. 3B  and is preferably fixed so as to fix the toner image on the recording media P. 
   When sheets of the recording medium P that have a width larger than L 2  and smaller than L 1  is fixed, the internal core  23   a  and the shielding members  23   b  are rotated to a proper angle depending on the width of recording medium P so that the heating range of the fixing belt  22  is suitable for the width of the recording medium P. Thereby, the temperature distribution in the width direction of the fixing belt  22  is adjusted so as to be suitable for the sheets width range of the recording medium P, resulting in a production of fixed images having good fixing property. In this case, the temperature of the potions of the fixing belt  22  outside the width range of the recording medium P hardly increases, resulting in a reduction of risk of damaging the fixing belt  22 . 
   According to the exemplary embodiment of the present invention, the heating range in the width direction of the fixing belt  22  is varied by employing the shielding members  23   b  shielding a portion of the internal core  23   a . In addition, the internal core  23   a  has a large diameter portion  23   a   2  at the end thereof, and therefore a distance between the coil  25  and the ends of internal core  23   a  becomes shorter than that at the a middle portion in the width direction by having large diameter portion  23   a   2  at the both ends in the width direction of the internal core  23   a . Therefore, when toner images on a plurality of sheets of the recording media P with a small width are successively fixed, the likelihood of an increase in the temperature at both ends in the width direction of the fixing belt  22  can be reduced, and the likelihood of poor fixing can be decreased even when the recording media P having various widths are used. 
   Referring to  FIG. 5 , a control system for controlling the fixing device  20  in the image forming apparatus  1  of  FIG. 1  is illustrated in a schematic block diagram. As shown in  FIG. 5 , the image forming apparatus  1  of the exemplary embodiment of the present invention includes a mixed sheet feeding detector  101 , a sheet-width detector  102 , a controller  103 , and the stepping motor  23   e . The mixed sheet feeding detector  101  detects mixed feeding of a plurality of sheets of the recording medium P with different widths during a successive sheet feeding, and outputs a mixed feeding signal. The sheet-width detector  102  detects widths of sheets of the recording medium P, and outputs a sheet-width signal. 
   According to one embodiment of the invention, the sheet width detector  102  is implemented in software and may be part of the image forming apparatus. Take for example a situation where the image forming apparatus is a printer connected to one or more computing devices. In such a case, a user selects or specifies a size of recording sheets used within a printing job, and commands or information provided to selector indicate the size of the recording sheet. According to one embodiment, the recording sheet is specified through the use of a printer driver within a computer which sends a printing job. According to another embodiment, the user can select the recording size using an operation panel on a copier or MFP (multi-function printer). Further, a facsimile machine or any other image forming apparatus may use different size sheets and information on the size of the recording sheets can be detected or determined, as desired. In addition to the sheet width the detector  102  operating based on a detection of printer commands, printer control language, or by other means in which the size of the recording sheets are specified, the sheet width detector alternatively can be implemented using sensors, such as optical, or mechanical sensors, which detect the physical size of the paper. The sheet width detector indicates the size of the recording sheet to the controller  103  and/or the mixed feeding detector  101 . When a user or a plurality of users select or specify different sizes of recording sheets within printing jobs, the sheet width detector  102  detects the width of the selected or specified recording sheet and the mixed feeding detector  101 , through the use of a controller, programmed controller, or other controlling device, detects that the widths of the recording sheets are changed. For example, the system may determine, detect, or otherwise realize that a recording sheet of size A4 is being conveyed in a vertical direction (portrait), and the next selected sheet is still A4 size but is being conveyed in a horizontal (landscape) direction. In such a case, the mixed feeding detector  101  detects the change of the sheet width. This mixed feeding detector  101  may also determine or detect that the width of the sheet is changed in a plurality of times, for example five times. While the mixed feeding detector  101  and the sheet width detector  102  are illustrated as separate elements in  FIGS. 4 and 5 , in actual implementation, these devices can be implemented together, and implemented within the controller  103 , if desired. 
   The controller  103  controls the heating width of the fixing member based on the mixed feeding signal and/or the sheet-width signal. The controller  103  is a micro-computer, for example, and controls driving of the stepping motor  23   e.    
   The operation of the image forming apparatus  1  of the exemplary embodiment of the present invention will be explained below referring to the flowchart of  FIG. 6 . According to this example procedure, it is determined during the successive sheet feeding whether or not the fed sheets of the recording medium P have different widths (i.e., whether or not mixed sheet feeding is performed). When mixed feeding is not performed, the width of sheets of the recording medium P is detected, and the heating width is controlled each time. When mixed feeding is performed, the heating width is not changed by a heating-width changing mechanism until an appropriate controlling timing. 
   According to the example procedure of  FIG. 6 , the mixed sheet feeding detector  101  determines whether or not mixed feeding is performed (Step SA 1 ). When mixed feeding is not performed, the mixed sheet feeding detector  101  does not output a mixed feeding signal, and flow proceeds to step SA 2  in which the sheet-width detector  102  detects the width of recording medium P and outputs the sheet-width signal. Next, in step SA 3 , the controller  103  drives the stepping motor  23   e  according to the sheet-width signal output from the sheet-width detector  102 , and changes the position of the shielding members  23   b  to control the heating width which changes the heating width (Step SA 4 ). 
   In contrast, when the mixed sheet feeding detector  101  detects that mixed feeding is performed, the mixed sheet feeding detector  101  outputs a mixed feeding signal (Yes in Step SA 1 ) and flow proceeds to Step SA 5 . In step SA 5 , the controller  103  determines whether or not it is the appropriate controlling timing for changing the heating width. As an example, when it is determined that five or more subsequent recording sheets have a different width, then it would be appropriate to modify the heating width. However, if all sheets being fed were A4 landscape pages, and one A4 portrait page was recorded on, then that may be considered not to be an appropriate situation to modify the heating width. The “appropriate timing” can be based on a sheet count of changed sheets, and according to one embodiment, utilizes five sheets of different width from the previous sheets which were recorded upon, but any number of sheets may be utilized in step SA 5 , for example 1, 2, 3, 4, 5, . . . 10, etc. When the controller determines that it is the appropriate controlling timing for changing the heating width, the controller  103  controls the heating width (Step SA 6 ). When the controller determines that it is not the timing for changing the heating width, the heating width is not changed (Step SA 7 ). Series of image forming processes are conducted while this example procedure is executed for each sheet of the recording medium P. 
   When the width of the recording medium P is frequently switched during printing, it may not be feasible or appropriate to change the heating width for each sheet having a different width, and therefore the controller  103  sets an appropriate controlling timing. Thereby, the image forming apparatus  1  can control the heating width more appropriately, resulting in reduction of occurrence of increasing the temperature of a non-sheet feeding portion of the fixing belt. 
   Referring to the flowchart of  FIG. 7 , another example procedure for controlling the fixing device  20  of  FIG. 2  is explained. The example procedure shown in the flowchart of  FIG. 7  is similar to the example procedure of  FIG. 6 . For example, the processes or functions of steps SB 1 , SB 2 , SB 3 , SB 4 , SB 6 , and SB 7  of  FIG. 7  are similar to those of steps SA 1 , SA 2 , SA 3 , SA 4 , SA 6 , and SA 7  of  FIG. 6 , respectively. As shown in  FIG. 7 , the controller  103  determines whether or not a predetermined number of sheets of the recording medium P are fed (Step SB 5 ). When the predetermined number of sheets of the recording media P are fed, the controller  103  controls the heating width (Step SB 6 ). When the predetermined number of sheets of the recording media P are not fed (for example, the number of sheets of the recording media fed is smaller than the predetermined number), the heating width is not changed (Step SB 7 ). When mixed feeding is not performed, steps SB 1 , Steps SB 2 , SB 3 , and SB 4  of  FIG. 7  that are similar to steps SA 2 , SA 3 , and SA 4  of  FIG. 6 , respectively, are executed. 
   According to this example procedure, the controller  103  determines whether or not it is the appropriate timing for changing the heating width based on the number of the fed sheets of the recording medium P, which is a specific number, for example, five. Thereby, the image forming apparatus  1  can facilitate the heating width control process. 
   In another example procedure of the control timing of  FIG. 8 , the heating width is controlled based on the average width of the fed sheets of the recording medium P when mixed feeding is performed. The example procedure shown in the flowchart of  FIG. 8  is similar to the example procedure of  FIG. 6 . For example, the steps or processes of steps SC 1 , SC 2 , SC 3 , and SC 4  of  FIG. 8  are similar to those of steps SA 1 , SA 2 , SA 3 , and SA 4  of  FIG. 6 , respectively. When a predetermined number of sheets of the recording medium P are fed (Yes in Step SC 5 ), the controller  103  calculates the average width of the fed sheets measured by the sheet-width detector  102  (Step SC 6 ), and controls the heating width (Step SC 7 ) based on the calculated average width. When mixed feeding is not performed, steps SC 2 , SC 3 , and SC 4  of  FIG. 7  that are similar to steps SA 2 , SA 3 , and SA 4  of  FIG. 6 , respectively, are executed. When a predetermined number of sheets of the recording medium P are not fed (No in Step SC 5 ), the controller  103  determines it is not the appropriate timing for changing the heating width (Step SC 8 ). According to this example procedure, the controller  103  controls the sheet width based on the calculated average width. Thus, the image forming apparatus  1  controls the appropriate heating width on the basis of the average width of the fed sheets, and thereby occurrence of increasing the temperature in the non-sheet feeding area of the fixing belt and shortage of the heat amount applied to the end portions of the recording medium P can be prevented. 
   The calculated average width determined in step SA 6  may be classified as categories. By using categories, the heating width may be controlled based on the calculated average width. For example, when the calculated average width is 130 mm, the control determines that the A5 size sheet being conveyed in a vertical direction (portrait) is to be printed so as to control the heating width. The following table may be used to correlate the paper width with the paper type: 
   
     
       
         
             
             
          
             
                 
                 
             
             
                 
               Size (mm) 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
                 
               &lt;100 
               101 &lt; 125 
               126 &lt; 150 
               151 &lt; 175 
               176 &lt; 200 
               201 &lt; 225 
               225&lt; 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
          
             
               Paper Type 
               Postcard 
               Letter- 
               A5 
               A5 
               A4 
               A4 
               . . . 
             
             
                 
                 
               sized 
               sized 
               sized 
               sized 
               sized 
             
             
                 
                 
               (V) 
               (V) 
               (H) 
               (V) 
               (H) 
             
             
                 
             
          
         
       
     
   
   In the exemplary embodiments described above, the appropriate timing for changing the heating width in mixed feeding is preferably set to be longer than the time needed for driving the shielding members  23   b  to the target positions by the stepping motor  23   e . By using this method, the shielding plate can reach the target position when needed, and the control of heating width can be secured. 
   In the exemplary embodiments described above, the fixing device  20  uses electromagnetic induction heating. However, any other known heater can be used as the heating source. 
   Referring to  FIG. 9 , another fixing device  20  is illustrated in a schematic cross section. This fixing device  20  includes a heating roller  40 , the fixing roller  21 , the fixing belt  22 , the pressure roller  30 , a cleaning roller  33   b,  an oil applying roller  34   b , the guide plate  35 , the separation plate  36 , and the thermostat  37 . The heating roller  40  includes a plurality of heaters therein that are separated in the axis direction of the heating roller. A description of elements in  FIG. 9  which have already been described with respect to  FIG. 2  and the other figures is omitted. According to this fixing device  20  of  FIG. 9 , the control system properly activates one or more of the plural heaters so as to change the heating width. The control system conducts the control processes as shown in  FIG. 6 ,  FIG. 7 , and  FIG. 8 . 
   The present invention includes and can also be implemented according to any of the teachings of JP 2005-258383, which is incorporated herein by reference. 
   Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein.