Patent Abstract:
An image forming apparatus includes an image forming mechanism and an image fixing unit. The image forming mechanism forms a toner image on a recording sheet. The image fixing unit fixes the toner image onto the recording sheet. The image fixing unit includes a magnetic flux generator, a heat member, a magnetic flux adjuster, and a controlling member. The magnetic flux generator generates a magnetic flux. The heat member is heated inductively by the magnetic flux generated by the magnetic flux generator. The magnetic flux adjuster reduces the magnetic flux active on the heat member to form a heat reduction area in an outer circumferential surface of the heat member in a width direction thereof. The controlling member moves the magnetic flux adjuster to change the heat reduction area.

Full Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to an image fixing apparatus which uses an induction heater and is capable of stably controlling a fixing temperature. 
     2. Discussion of the Background 
     A background image forming apparatus such as a copy machine, a printer, a facsimile machine, and a multi-function machine capable of copying, printing, and faxing uses an electromagnetic induction type fixing mechanism to reduce a machine rise time for an energy savings. 
     One example of the electromagnetic induction type fixing mechanism includes a support roller, an auxiliary fixing roller, a fixing belt, a magnetic flux generator, and a pressure roller. The support roller serves as a heat roller, and the auxiliary fixing roller serves as a fixing roller. The fixing belt has a heat resistant property and is extended between the support roller and the auxiliary fixing roller. The magnetic flux generator faces the support roller via the fixing belt. The pressure roller faces the auxiliary fixing roller via the fixing belt. The magnetic flux generator includes a coil including a plurality of wire turns and a core such as an exciting coil core. The coil is wound around the core and is extended in a direction parallel to a surface of a recording sheet in conveyance and perpendicular to a conveyance direction of the recording sheet which is conveyed between the pressure roller and the auxiliary fixing roller. 
     The fixing belt is heated at a position facing the magnetic flux generator and applies heat to a toner image carried on a recording sheet which is transported to a nip formed between the auxiliary fixing roller and the pressure roller. More specifically, the coil receives an application of a high-frequency alternating current to generate a magnetic field around the coil. The magnetic field induces an eddy current near a surface of the support roller. This causes a generation of Joule heat due to an electrical resistance of the support roller itself. 
     The above-described electromagnetic induction type fixing mechanism is capable of increasing a fixing temperature of the fixing belt to a desired level in a relatively short time period and with a relatively small amount of energy. 
     However, the electromagnetic induction type fixing mechanism cannot assuredly suppress a temperature increase at longitudinal end sides of the fixing member, e.g., the fixing belt or roller, which may be overly heated, especially, when the image forming operation is consecutively performed on a narrower-sized recording sheet. 
     In general, an image forming apparatus is configured to handle various kinds of recording sheets specially in size for image forming: for example, standard A-series size such as A4, or irregular size as well. A recording sheet in A4 size, for example, is in a rectangular form and has a long side and a short side. Therefore, a surface area of the fixing belt facing the recording sheet can be changed by an orientation of image forming, depending on whether the recording sheet needs to be placed in landscape or portrait relative to the fixing belt. 
     Such a variation of width of the recording sheet causes the fixing belt to have an uneven temperature in the axis direction thereof. That is, during the fixing process, the recording sheet absorbs a certain amount of heat from the surface area of the fixing belt. This results in an uneven surface temperature of the fixing belt. Specifically, a sheet-contact area of the fixing belt which makes contact with the recording sheet has the temperature decreased and a non-sheet-contact areas around both end sides of the fixing belt which do not make contact with the recording sheet have higher temperatures. This problem occurs typically when the image forming is consecutively performed to a relatively small size recording sheet. 
     If the surface temperature of the fixing belt is adjusted to attempt to increase the lowered temperature of the sheet-contact area of the fixing belt, the lowered temperature of the sheet-contact area of the fixing belt can be adjusted to an appropriate level; however, at the same time, the temperature of the non-sheet-contact area are may exceedingly be increased. If the image forming operation is performed to a relatively large size recording sheet under this condition, a troublesome phenomenon referred to as a hot off-set may be caused at a surface area of the fixing belt where the fixing temperature is too high. That is, because of the exceedingly high temperature, a portion of toner included in the toner image carried on the recording sheet is melt on the recording sheet and is adhered to the fixing belt, not to the recording sheet. As a result, the toner image on the recording sheet loses a portion thereof. If the temperature is partly risen on the surface of the fixing belt in excess of a predetermined range of the fixing temperature, the fixing belt may cause a thermal breakdown. 
     In contrast, if the surface temperature of the fixing belt is adjusted to attempt to decrease the exceedingly risen temperature of the non-sheet-contact area of the fixing belt, the exceedingly risen temperature of the non-sheet-contact area of the fixing belt can be adjusted to an appropriate level; however, at the same time, the temperature of the sheet-contact area may exceedingly be decreased. If the image forming operation is performed under this condition, another troublesome phenomenon referred to as a cold off-set may be caused at a surface area of the fixing belt where the fixing temperature is too low. That is, because of the exceedingly low temperature, a portion of toner included in the toner image carried on the recording sheet is not melt on the recording sheet and is adhered to the fixing belt, not to the recording sheet. As a result, the toner image on the recording sheet loses a portion thereof. 
     One example technique attempts to solve the above-described problems by suppressing an increase of the fixing temperature at the non-sheet-contact area of the fixing roller. This technique provides a magnetic flux shield for shielding a part of the magnetic flux generated by the magnetic flux generator (e.g., an induction coil) disposed inside the fixing roller. More specifically, the magnetic flux generator is configured to change its position in accordance with a sheet-contact area of the fixing roller to change a range of area to shield accordingly so as to shield the magnetic flux applied to the fixing roller at the non-sheet-contact area of the fixing roller. Thereby, a temperature rise at the non-sheet-contact area of the fixing roller is suppressed. 
     SUMMARY OF THE INVENTION 
     This patent specification describes a novel image forming apparatus includes an image forming mechanism and an image fixing unit. The image forming mechanism is configured to form a toner image on a recording sheet. The image fixing unit is configured to fix the toner image onto the recording sheet. The image fixing unit includes a magnetic flux generator, a heat member, a magnetic flux adjuster, and a controlling member. The magnetic flux generator is configured to generate a magnetic flux. The heat member is configured to be heated inductively by the magnetic flux generated by the magnetic flux generator. The magnetic flux adjuster is configured to reduce the magnetic flux active on the heat member to form a heat reduction area in an outer circumferential surface of the heat member in a width direction thereof. The controlling member is configured to move the magnetic flux adjuster to change the heat reduction area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages 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 diagram of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an image fixing unit of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 3  is a schematic diagram of an interior of a support roller shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of an induction heater in relation to a fixing belt and a support roller; 
         FIG. 5  is a flowchart of an example procedure of a heat-reduction-area control operation for the image fixing unit of  FIG. 2 ; 
         FIGS. 6A-6C  are schematic diagrams for explaining relationships of a magnetic flux shield plate, a heating area, a heat reduction area, a center core, and a recording sheet in a width direction of the support roller; 
         FIG. 7  is a cross reference table representing a relationship between a print number and the heat reduction area and between a heating time and the heat reduction area; 
         FIG. 8  is a graph showing a relationship between a width position in a fixing surface of a fixing belt and a fixing temperature; 
         FIG. 9  is a flowchart of an example procedure of another heat-reduction-area control operation performed by the image forming apparatus of  FIG. 1 ; 
         FIG. 10  is a flowchart of an example procedure of another heat-reduction-area control operation performed by the image forming apparatus of  FIG. 1 ; 
         FIG. 11  is a graph showing a relationship between a print number and the fixing temperature when a magnetic flux shield plate is not installed; 
         FIG. 12  is a schematic diagram of an interior of another support roller for the image fixing unit shown in  FIG. 2 ; 
         FIGS. 13 and 14  are illustrations for explaining different magnetic flux shield plates; 
         FIG. 15  is a schematic diagram of another image fixing unit of the image forming apparatus shown in  FIG. 1 ; 
         FIG. 16  is a schematic diagram of a home position detector engaged with the support roller; 
         FIG. 17  is a schematic diagram of the home position detector seen in a direction indicated by an arrow; 
         FIG. 18  is a flowchart of an example procedure of a heat-reduction-are control operation performed by the image forming apparatus of  FIG. 1 ; 
         FIGS. 19A and 19B  are schematic diagrams for explaining a home position of the magnetic flux shield plate and its position for an image forming on a recording sheet in a B5T size; 
         FIG. 20  is a schematic diagram of another home position detector engaged with the support roller; 
         FIGS. 21A and 21B  are schematic diagrams showing relationships among the magnetic flux shield plate, the heating area, the heat reduction area, the center core, and the recording sheet in the width direction of the support roller; 
         FIGS. 22A and 22B  are illustrations schematically showing a distribution of the fixing temperature when the heating area is changed; 
         FIGS. 23 and 24  are schematic diagrams of an example procedure of another heat-reduction-area control operation performed by the image fixing unit of  FIG. 2 ; 
         FIG. 25  is a schematic diagram of another image fixing unit for the image forming apparatus shown in  FIG. 1 ; 
         FIGS. 26 and 27  are illustrations for explaining a structure of another support roller; 
         FIGS. 28A-28C  are illustrations for explaining variations of an outer circumferential surface length of an internal core when the internal core is rotated by different angles; and 
         FIG. 29  is a schematic diagram of another image fixing unit of the image forming apparatus shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred 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, particularly to  FIG. 1 , an image forming apparatus  1  according to an embodiment of the present invention is explained. The image forming apparatus  1  illustrated in  FIG. 1  is a laser printer as one example of the embodiment of the present invention. As shown in  FIG. 1 , the image forming apparatus  1  includes a control circuit unit  2 , an exposure unit  3 , a process cartridge  4 , an image transfer unit  7 , an output tray  10 , sheet cassettes  11  and  12 , a registration roller  13 , a manual input tray  15 , and an image fixing unit  20 . 
     The control circuit unit  2  includes a CPU (central processing unit)  2   a , a ROM (read only memory)  2   b , and a RAM (random access memory)  2   c . The process cartridge  4  includes a photosensitive drum  18 . The sheet cassettes  11  and  12  include sheet size detectors  11   a  and  12   a , respectively. The manual input tray  15  includes a sheet size detector  15   a.    
     The control circuit unit  2  controls the entire operations of the image forming apparatus  1 . Specifically, the CPU  2   a  controls the entire operations of the image forming apparatus  1  in accordance with programs including an image forming program stored in the ROM  2   b  by utilizing memories and counters formed in the RAM  2   c . The memories and counters are configured to store various kinds of information including temperature values, count values, recording sheet sizes, a print number in a print job, and so forth. 
     The exposure unit  3  irradiates an exposure light beam L modulated according to image information to a surface of the photosensitive drum  18 . The process cartridge  4  serves as an image forming engine and is configured to be a single exchangeable unit. The photosensitive drum  18  is configured to rotate anticlockwise in the drawing. The image transfer unit  7  configured to transfer a toner image formed on the surface of the photosensitive drum  18  onto a recording sheet P. The output tray  10  is configured to receive and store the recording sheets P after the image forming operations. Each of the sheet cassettes  11  and  12  is configured to store a plurality of recording sheets P. The sheet size detector  11   a  of the sheet cassette  11  is configured to detect a sheet size of the recording sheet stored in the sheet cassette  11 , and the sheet size detector  12   a  of the sheet cassette  12  is configured to detect a sheet size of the recording sheet stored in the sheet cassette  12 . The registration roller  13  is configured to configured to transport the recording sheet P to the image transfer unit  7 . The manual input tray  15  is configured to insert manually a recording sheet. The sheet size detector  15   a  of the manual input tray  15  is configured to detect a sheet size of the recording sheet stored in the manual input tray  15 . The image fixing unit  20  is configured to fix a not-fixed toner image formed on the recording sheet P. 
     Each of the sheet size detectors  11   a ,  12   a , and  15   a  includes a photosensor configured to detect a position of sheet fence (not shown). The sheet fence is provided inside each of the sheet cassettes  11  and  12  and the manual input tray  15  and is configured to support the stored recording sheet P horizontally in the width direction of the recording sheet P. 
     In  FIG. 1 , a reference  1   a  is a sheet thickness detector configured to detect a thickness of the recording sheet P. Reference  1   b  and  1   c  are a transfer speed detector configured to detect a transfer speed of the recording sheet P. Reference  1   d  is an environment detector configured to detect environment conditions such as an environment temperature, humid, etc., around the image forming apparatus  1 . The sheet thickness detector  1   a  may be used as a sheet kind detector configured to detect a sheet kind of the recording sheet P. 
     With reference to  FIG. 1 , example operations of the image forming apparatus  1  are explained. The exposure unit  3  starts to irradiate the exposure light beam L modulated according to image information to the surface of the photosensitive drum  18  of the process cartridge  4 . The photosensitive drum  18  is rotated in an anticlockwise direction and is subjected to an electrophotographic image forming process including charging, exposing, developing processes, and so forth, thereby forming a toner image on the surface thereof. During this image forming process, the recording sheet P is transported towards the image transfer unit  7  by the registration roller  13 . Then, the toner image formed on the surface of the photosensitive drum  18  and the recording sheet P being moved in synchronism with each other meet at the image transfer unit  7 . Thereby, the toner image is transferred onto the recording sheet P by the image transfer unit  7 . 
     Apart from the above-described operations, to start the image forming process, one of the sheet cassettes  11  and  12  and the manual input tray  15  is selected automatically or manually. The sheet cassettes  11  and  12  are typically used to store the recording sheets P of different size or of same size but in different orientation, and the manual input tray  15  is typically used in occasions using a special recording sheet such as an OHP (overhead projector) sheet, for example. 
     In this discussion, it is assumed that the sheet cassette  11  is selected. An uppermost sheet of the plurality of recording sheets P stored in the sheet cassette  11  is transported towards a transportation passage K. The recording sheet P transported is subsequently transferred to the position of the registration roller  13  through the transportation passage K. The registration roller  13  once stops the recording sheet P and restarts to transfer the recording sheet P in synchronism with the movement of the photosensitive drum  18  so that the toner image and the recording sheet accurately meet at a transfer position of the image transfer unit  7 . 
     After passing through the image transfer unit  7 , the recording sheet P is further transferred towards the image fixing unit  20  through the transportation passage K. Then, the recording sheet P is caused to enter the image fixing unit  20  in which the recording sheet P is pressed and heated between a fixing belt and a pressure roller which are included in the image fixing unit  20 . Thus, the toner image on the recording sheet P is melt and fixed in the image fixing unit  20 . The recording sheet P having the fixed toner image thereon is driven off from the image fixing unit  20  and is ejected onto the output tray  10  from the image forming apparatus  1 . In this way, the series of the image forming operation is executed. 
     With reference to  FIG. 2 , an example structure and operation of the image fixing unit  20  is explained. As illustrated in  FIG. 2 , the image fixing unit  20  includes an auxiliary fixing roller  21 , a fixing belt  22 , a support roller  23 , an induction heater  24 , a pressure roller  30 , a cleaning roller  33 , an oil-coated roller  34 , a guide plate  35 , a separation plate  36 , a thermopile  37 , a thermistor  38 , and a thermostat  39 . 
     The auxiliary fixing roller  21  includes a surface layer which is an elastic layer including a silicone rubber or the like and is configured to be driven by a driving unit (not shown) to rotate in an anticlockwise direction in the drawing. 
     The support roller  23  may be referred to as a heat roller. This support roller  23  includes a non-magnetic material such as a stainless steel (e.g., SUS304), for example, and is configured to have a cylindrical shape driven to rotate in a counterclockwise direction in the drawing. As illustrated in  FIG. 2 , the support roller  23  internally includes an internal core  28  and a magnetic flux shield plate  29 , both of which are held for rotation in the support roller  23 . The internal core  28  includes a ferromagnetic material such as a ferrite, for example. The magnetic flux shield plate  29  covers a part of the surface of the internal core  28 . The internal core  28  adjacently faces the induction heater  24  via the fixing belt  22  and the support roller  23 . A driving mechanism for the support roller  23  and for the internal core  28  and the magnetic flux shield plate  29  are separately provided. 
     As illustrated in  FIG. 2 , the fixing belt  22  is held and extended between the auxiliary fixing roller  21  and the support roller  23 . This fixing belt  22  is configured to be an endless belt of a multi-layered structure including a base material, a heat layer, an elastic layer, and a release layer. 
     The base material of the fixing belt  22  includes a heat-resisting resin material such as a polyimide resin, a polyamide-imide resin, a PEEK (polyether ether ketone) resin, a PES (polyether sulfone) resin, a PPS (polyphenylene sulfide) resin, a fluorocarbon resin and the like. The heat layer includes any one of materials such as nickel, stainless steel, iron, copper, cobalt, chrome, aluminum, gold, platinum, silver, tin, and palladium, or an alloy of at least two metals from among these metals. The elastic layer includes any one of materials such as a silicone rubber, a fluoro-silicone rubber, or the like. The release layer includes any one of fluorocarbon resins such as a PTFE (polytetrafluoroethylene) resin, a polytetrafluoroethylene perfluoroalkyl vinyl ether copolymer, i.e., a FEP (fluorinated ethylene propylene resin), or an amalgamation of these resins. 
     In this example of the fixing belt  22 , the base material and the heat layer together form a composite layer, that is, three of the heat layer are formed with space in the base material. On such a composite layer, the elastic layer and the release layer are formed in this order. 
     As illustrated in  FIG. 2 , the induction heater  24  includes a coil  25 , a core  26 , and a coil guide  27 . The coil guide  27  has a curbed shape in accordance with a round portion of the fixing belt  22  supported by the support roller  23 . The coil  25  includes a litz wire formed by binding a plurality of thin wires. This litz wire is wound and is extended along the coil guide  27  and in a direction perpendicular to the surface of the drawing so as to cover an external circumferential surface of the fixing belt  22  supported by the support roller  23 . The coil guide  27  includes a resin material having a relatively high heat-resisting property, and is configured to hold the coil  25 . This coil guide  27  also serves as a frame of the induction heater  24 . The core  26  includes a ferromagnetic material such as a ferrite having a relative permeability of about 2500 and is provided with a center core  26   a  and a side core  26   b . The core  26  has a cubed shape in accordance with the coil guide  27  and is disposed in a way so as to closely face the coil  25 . The center core  26   a  is disposed at an approximately circumferential-middle position of the coil  25  where a density of magnetic flux generated around and by the coil  25  reaches its peak value. The coil  25  is connected to a high-frequency power source (not shown) and receives an application of an alternating current having a frequency in the range of from approximately 10 kHz to approximately 1 MHz from the high-frequency power source. 
     The pressure roller  30  includes a cylindrical member which includes an aluminum, a copper, or a stainless steel. The cylindrical member is coated with an elastic layer including a fluorocarbon rubber, a silicone rubber, or the like. Such elastic layer of the pressure roller  30  has a thickness of from approximately 1 mm to approximately 5 mm and an Asker hardness of from approximately 20 degrees to approximately 50 degrees. The pressure roller  30  contacts the fixing belt  22  supported by the auxiliary fixing roller  21  with an application of a pressure to the fixing belt  22  so that a fixing nip area is formed between the pressure roller  30  and the fixing belt  22 . The fixing nip area is an area into which the recording sheet P is transported in a direction Y to receive the image fixing operation. 
     The guide plate  35  is disposed around an entrance of the fixing nip area and is configured to guide the recording sheet P towards the fixing nip area. The separation plate  36  is disposed around an exit of the fixing nip area and is configured to guide the recording sheet P and also to help separation of the recording sheet P from the fixing belt  22 . 
     The oil coating roller  34  is arranged in contact with the fixing belt  22  which applies oil such as a silicone oil to a surface of the fixing belt  22 . With such an application of oil to the fixing belt  22 , releasing a toner image T from the fixing belt  22  can be made with reliability. 
     The cleaning roller  33  contacts the oil coating roller  34  to remove contamination from the surface of the oil coating roller  34 . 
     The thermopile  37  is a non-contact type temperature detector and is disposed at a position to face an approximately middle portion of the fixing belt  22  widthwise. This position is out of an area for adjustment of the fixing belt  22 , which is explained afterwards. 
     The thermistor  38  is a contact type temperature detector and is disposed at a position to contact a circumferential edge surface of the fixing belt  22 . This position is within the area for the adjustment of the fixing belt  22 . 
     The above-explained thermopile  37  and the thermistor  38  detect surface temperatures of the fixing belt  22 , that is, the fixing temperature of the fixing belt  22 . Based on the detected fixing temperature, the induction heater  24  which includes an inverter power source circuit which is a high-frequency power source adjusts its output using this inverter power source circuit. Thus, the fixing temperature on the surface of the fixing belt  22  is held at a constant level. In addition, based on the detected temperatures by the thermopile  37  and the thermistor  38 , the magnetic flux acting around lateral edges of the support roller  23  is adjusted, which is explained afterwards. 
     The thus-structured image fixing unit  20  performs the fixing operation in a way as described below. As illustrated in  FIG. 2 , when the auxiliary fixing roller  21  is driven to rotate, the fixing belt  22  is driven to rotate in a direction indicated by an arrow, the support roller  23  rotates counterclockwise, and the pressure roller  30  rotates in a direction indicated by an arrow. The fixing belt  22  is heated at a position facing the induction heater  24 . More specifically, the induction heater  24  is configured to alternately switch directions of generated magnetic lines of force between the core  26  and the core  28  by an application of an alternating current with a high frequency to the coil  25 . At this moment, an eddy current is generated in a surface of the support roller  23  and in the heat layer of the fixing belt  22 . Consequently, a Joule heat is generated due to electrical resistances of the support roller  23  and the heat layer of the fixing belt  22 . Accordingly, the fixing belt  22  is heated by heat of the heat layer thereof and by heat from the support roller  23 . As such, the support roller  23  serves as a heating member and the fixing belt  22  serves as a heating member on one hand and also a member to be heated on the other hand. 
     The surface of the fixing belt  22  heated by the induction heater  24  is then caused to pass by the thermistor  38  and to reach a position to contact the pressure roller  30  so as to heat the toner image T held on the recording sheet P transported thereto. 
     More specifically, the recording sheet P carrying the toner image T through the above-described image forming process is guided in the direction Y by the guide plate  35  and is caused to enter the fixing nip area formed between the fixing belt  22  and the pressure roller  30 . Accordingly, the toner image T is fixed on the recording sheet P by heat from the fixing belt  22  and by pressure from the pressure roller  30 , and the recording sheet P having the fixed toner image T is ejected from the fixing nip area between the fixing belt  22  and the pressure roller  30 . 
     After passing by the pressure roller  30 , the heated surface of the fixing belt  22  is then caused to pass sequential by the oil coating roller  34  and the thermopile  37  and returns to the position where it is initially heated. 
     The fixing process in the image forming operation is executed by continuously repeating such series of operations as described above. 
     With reference to  FIG. 3 , an example structure and operations of the support roller  23  are explained.  FIG. 3  illustrates the support roller  23  in cross section seen from the induction heater  24 . As illustrated in  FIG. 3 , the internal core  28  and the magnetic flux shield plate  29  are arranged for rotation inside the support roller  23 . 
     The internal core  28  which is of a cylindrical shape and is ferromagnetic has lateral edge sides covered by the magnetic flux shield plate  29  of diamagnet such as a copper or the like. The magnetic flux shield plate  29  includes a slanted side  29   a  at each of lateral edge sides thereof. With the slanted side  29   a , an area for shutting a circumferential surface of the internal core  28  is gradually decreased or increased from an edge of the internal core  28 . Thereby, it becomes possible to vary a magnetic flux shield area formed in a lateral direction of the internal core  28 , which faces the coil  25  of the induction heater  24 , by driving the internal core  28  and the magnetic flux shield plate  29  to rotate. 
     More specifically, with reference to  FIG. 4 , a normal peak magnetic flux is generated along dashed-imaginary-lines in  FIG. 4  when the magnetic flux shield plate  29  does not intervene the magnetic flux between the center core  26   a  of the core  26  and the internal core  28 . However, when the magnetic flux shield plate  29  intervenes, such normal peak magnetic flux is accordingly reduced. Thus, the heating efficiency is reduced in a surface area of the support roller  23  intervened by the magnetic flux shield plate  29  as the magnetic flux reduces. The surface area of the support roller  23  in which the heating efficiency is varied in response to the change of the magnetic flux shield area is referred to a heat reduction area. 
     The heat reduction area formed in the lateral direction of the support roller  23  by the intervention of the magnetic flux shield plate  29  can be adjusted by changing an attitude of the magnetic flux shield plate  29  relative to the core  25 . More specifically, the heat reduction can be made at the both sides of the support roller  23  within a length range of from 0 to (L 1 −L 2 )/2 by turning the magnetic flux shield plate  29  together with the internal core  28 , as illustrated in  FIG. 3 . In this way, the magnetic flux shield plate  29  functions as a magnetic flux adjusting member to vary the magnetic flux shield area for the magnetic flux acting on the support roller  23  or the fixing belt  22  in the width direction, which ultimately changes the heat reduction area of the support roller  23  or the fixing belt  22 . 
     The internal core  28  and the magnetic flux shield plate  29  are driven with a driving mechanism (not shown) such as a stepping motor connected to a shaft of the internal core  28 . This driving mechanism may be independent from a driving mechanism for driving the auxiliary fixing roller  21 , the fixing belt  22 , and the support roller  23 . 
     To be more specific, the internal core  28  and the magnetic flux shield plate  29  are turned by a specific angle along in a circumferential direction of the support roller  23  so that the greatest area of the magnetic flux shield plate  29  faces the center core  26   a . At this time, the heat reduction area is adjusted to its maximum and, as a result, an area of L 2  which is out of the heat reduction area is a main heating area of the fixing belt  22 . This condition may be suitable for the image forming operation handling the recording sheet P with a lateral size of L 2 . 
     When the internal core  28  and the magnetic flux shield plate  29  are further turned by another specific angle along in the circumferential direction of the support roller  23  so that the greatest area of the magnetic flux shield plate  29  does not face the center core  26   a . At this time, the heat reduction area is adjusted to its minimum, that is, zero and, as a result, an entire area of L 1  is a main heating area of the fixing belt  22 . 
     The thus-structured image fixing unit  20  is capable of performing the image forming operations consecutively with a plurality of recording sheets P by turning the attitude of the magnetic flux shield plate  29  to change the heat reduction area. 
     Referring to  FIGS. 5-8 , an example procedure of an heat-reduction-area control operation for the image fixing unit  20  is explained. In a flowchart of  FIG. 5 , when the image forming apparatus  1  is energized in Step S 2 , a home position search is performed for the magnetic flux shield plate  29  in Step S 3 . That is, the magnetic flux shield plate  29  is driven to turn to its home position.  FIG. 6A  demonstrates a condition in that the magnetic flux shield plate  29  is at its home position where the magnetic flux shield plate  29  does not intervene and no heat reduction area is formed. In  FIG. 6A , M represents a heating area, B5T represents the recording sheet P of B5 size in a landscape orientation, that is, the short side of the recording sheet P being set perpendicular to the transportation direction of the recording sheet P. Accordingly, under the condition of  FIG. 6A , the magnetic flux is fully activated across an entire width of the heating area M. 
     Then, in Step S 4 , the inverter power source circuit of the image fixing unit  20  is energized so that the induction heater  24  is caused to start heating. Then, after reloading the power to the image fixing unit  20  in Step S 5 , a determination is performed in Step S 6  as to whether the image forming operation is commanded. 
     When the image forming operation is determined as not being commanded in Step S 6 , the determination is repeated via a predetermined standby time period in Step S 7 . 
     When the image forming operation is determined as being commanded in Step S 6 , the image forming apparatus  1  selects a recording sheet P from among the sheet cassettes  11  and  12  and the manual input tray  15 , in Step S 8 . In this process, the recording sheet P in a suitable size for the commanded image forming operation is detected by the sheet size detector  11   a ,  12   a , or  15   a , for example. According to this selection of the recording sheet P in suitable size for the image forming operation, a non-sheet-passing area is defined in the surfaces of the support roller  23  and the fixing belt  22 , at which the temperature may excessively be increased. The selection of the recording sheet P may also be executed based on any input command entered by an operator. In this example operation, the size of the recording sheet P selected is B5 which is stored in the sheet cassette  11 , for example, and which is relatively small and has a relatively small width in parallel to the width of the support roller  23 , as illustrated in  FIGS. 6A-6C . 
     Then, in Step S 9 , the magnetic flux shield plate  29  is caused to turn in accordance with the size information of the recording sheet P selected. In this case, as illustrated in  FIG. 6B , a heat reduction area N is grown to an extent within the non-sheet-passing area and the heating area M is narrowed instead. More specifically, the heating area M has a coverage wider than the recording sheet P by a degree of X 2 , as illustrated in  FIG. 6B . This arrangement is made because the temperatures at the non-sheet-passing areas of the support roller  23  and the fixing belt  22  may not increase immediately after the heating operation and because a temperature around the boarder between the non-sheet-passing area and a sheet-passing area may excessively be reduced if the magnetic flux is reduced across the entire width of the non-sheet-passing area. 
     Then, in Step S 10 , the fixing process is started in a consecutive manner for the plurality of the recording sheet P. At this time, a heating time and an image forming number are counted with counters formed in the RAM  2   c  of the image forming apparatus  1 . The heating time is an accumulated time that the high-frequency power source applies power to the induction heater  24 . The image forming number is an accumulated number of printed sheets through the image forming operations. 
     Then, in Step S 11 , the position of the magnetic flux shield plate  29  is adjusted so as to grow the heat reduction area N and instead to shorten the heating area M at an occurrence of one of events that the heating time reaches a predetermined count value counted by one of the counters and the image forming number reaches another predetermined count value counted by another one of the counters. 
     Specifically, the magnetic flux shield plate  29  initially set at the position indicated in  FIG. 6B  is controlled so that the heat reduction area N is stepwise widen according to an increase of the count value. Upon an excess of the predetermined count value, the heat reduction area N is wider than the non-sheet-passing area and the heating area M is shorter than the sheet-passing area. As illustrated in  FIG. 6C , the heat reduction area N is wider than the non-sheet-passing area by an extent of X 3 . 
     The relationship between the count values and the heat reduction area N is summarized into a cross reference table, as shown in  FIG. 7 , which is stored in the image forming apparatus  1 . As shown in the cross reference table of  FIG. 7 , the magnetic flux shield plate  29  is controlled with an increase of the image forming number or the heating time so that the heat reduction area N is stepwise grown wider. 
     The above-described arrangement of  FIG. 7  is made because transmission of heat gradually occurs from the heating area M to the heat reduction area N which is not directly heated as the heating time and the image forming number increase after the consecutive image forming operations begin. If the heat reduction area N is fixed during the consecutive image forming operations, an overheated area may be generated in the heat reduction area N and close to the heating area M. 
     In this example, as described above, the magnetic flux shield plate  29  is controlled with an increase of the image forming number or the heating time so that the heat reduction area N is stepwise grown wider. Therefore, the heat reduction area N is protected from generating an overheated area due to a transmission of heat from the heating area M. 
     After a completion of the consecutive image forming operations in Step S 12 , the magnetic flux shield plate  29  is returned to its home position in Step S 13 . Then, in Step S 14 , the inverter power source circuit is turned off so that the induction heater  24  is caused to stop heating. Then, the process ends. 
       FIG. 8  demonstrates a temperature distribution of the fixing belt  22  in the width direction. In  FIG. 8 , a horizontal axis represents longitudinal positions in the width direction of the fixing belt  22 , expressed as a distance in millimeter from the width center of the fixing belt  22 , and the vertical axis represents a surface temperature of the fixing belt  22 , that is, the fixing temperature. Further, curbed lines R 1  and R 2  represent temperature distributions when the consecutive image forming operations are performed with the recording sheet P having the width L 1  and when the consecutive image forming operations are performed with the recording sheet P having the width L 2 , respectively. 
     It is possible to maintain the temperature distribution of the fixing belt  22  over time during the consecutive image forming operations in a way as shown in  FIG. 8  by adjusting, finely over time, the heat reduction area N according to the attitude of the magnetic flux shield plate  29 . Thereby, the fixing belt  22  can be free from being overheated at its surface area beyond a width of the recording sheet P and therefore it can be free from a thermal breakdown. 
     As described above, the image fixing unit  20  of the image forming apparatus  1  controls the heat reduction area N in which the magnetic flux acting on the fixing belt  22  and the support roller  23  is reduced, during the consecutive image forming operations. Thereby, it becomes possible to suppress the temperature rises with reliability at the both sides of the fixing belt  22  and the support roller  23 . 
     In this example, both of the fixing belt  22  having the heat layer and the support roller  23  are used as a heating member. Alternatively, it is possible to use one of the fixing belt  22  and the support roller  23  as a heating member. In such a case, the effect of suppression generated in the image forming apparatus  1 , as described above, may be achieved in a similar manner by optimizing the heat reduction area N according to the attitude of the magnetic flux shield plate  29  during the consecutive image forming operations. 
     In addition, the image forming apparatus  1  may be provided with a halogen heater inside the pressure roller  30 . Furthermore, an additional thermistor and oil coating roller may be provided in contact with a circumferential surface of the pressure roller  30 . In these cases, the effect of suppression generated in the image forming apparatus  1 , as described above, may be achieved in a similar manner. 
     The image forming apparatus  1  is an example embodiment in a form of a black and white image forming machine; however, it is possible to apply the present invention to a color image forming machine with the effect of suppression generated in the image forming apparatus  1 , as described above. 
     Referring to  FIG. 9 , another example procedure of the shield-area control operation is explained. In this example, the magnetic flux shield plate  29  is driven based on a temperature detected by the thermopile  37 , instead of using the counters to count the count values. The flowchart of  FIG. 9  applies Steps S 2 -S 10  of  FIG. 5  to its introduction stage and Steps S 12 -S 14  of  FIG. 5  to its ending stage, and replaces Step S 11  of  FIG. 5  with new Steps S 21 -S 26 . Therefore, the discussion below avoids repetition of Steps S 2 -S 10  and Steps S 12 -S 14  of  FIG. 5 , but focuses on new Steps S 21 -S 26 . 
     After the start of the consecutive image forming operations in Step S 10 , the temperature of the fixing belt  22  is detected by the thermopile  37  in Step S 21 . The thermopile  37  is arranged at a position to face an approximate width center area of the fixing belt  22 . This approximate width center area is out of the heat reduction area N even when the heat reduction area N is changed by the adjustment, thereby making it possible to detect a temperature variation of the fixing belt N at an area out of the heat reduction area N. 
     Then, in Step S 22 , a determination is made as to whether a temperature T detected by the thermopile  37  is equal to or lower than a predetermined temperature D. When the temperature T detected by the thermopile  37  is determined in Step S 22  as being equal to or lower than the predetermined temperature D, the magnetic flux shield plate  29  is driven in Step S 23  so as to shorten the width of the heat reduction area N having a width adjusted in Step S 9 . Accordingly, the heat of the heating area M is transferred to the shield area N so that a temperature reduction at edges of the sheet-passing area is suppressed while temperature rises at the non-sheet-passing areas are suppressed. 
     Then, in Step S 26 , a determination is performed as to whether an image forming job commanded is completed. When the image forming job commanded is determined in Step S 26  as not being completed, the processes after Step S 21  are repeated. When the image forming job commanded is determined in Step S 26  as being completed, the processes of Steps S 12 -S 14  are performed and the procedure ends. 
     When the temperature T detected by the thermopile  37  is determined in Step S 22  as not being equal to or lower than the predetermined temperature D, another determination is made in Step S 24  as to whether the temperature T is equal to or greater than a predetermined temperature E which is greater than the predetermined temperature D. When the temperature T is determined in Step S 24  as being equal to or greater than the predetermined temperature E, the magnetic flux shield plate  29  is driven in Step S 25  so as to lengthen the width of the heat reduction area N having the width adjusted in Step S 9 . Accordingly, a heat transfer rate from the heating area M to the heat reduction area N is made smaller so that temperature reductions at the non-sheet-passing areas are suppressed. 
     Then, in Step S 26 , a determination is performed as to whether an image forming job commanded is completed. Also, when the temperature T is determined in Step S 24  as not being equal to or greater than the predetermined temperature E, the procedure goes to Step S 26 . When the image forming job commanded is determined in Step S 26  as not being completed, the processes after Step S 21  are repeated. When the image forming job commanded is determined in Step S 26  as being completed, the processes of Steps S 12 -S 14  are performed and the procedure ends. 
     As described above, in this example, the shield area N having an effect of reducing the magnetic flux active on the fixing belt  22  and the support roller  23  is changed in accordance with the temperature variations detected around the width center of the fixing belt  22 , during the consecutive image forming operations. Thereby, a temperature rise at width edges of both fixing belt  22  and support roller  23  is suppressed with reliability. 
     As described above, in this example, the temperature of the fixing belt  22  which serves as a heating member is directly detected and, based on the detected temperature, the heat reduction area N is varied. As an alternative, a temperature of the support roller  23  which also serves as a heating member may directly be detected in order to be used for a control of the heat reduction area N. 
     In a case the fixing belt includes no heat layer, that is, the fixing belt is not a heating member but a member to be heated, it is also possible to detect the temperature of the fixing belt and to use the detected temperature for a control of the heat reduction area N. In this case, it is understood that the temperature of a heating member is indirectly detected via the fixing belt. 
     Referring to  FIGS. 10 and 11 , another example procedure of the heat-reduction-area control operation is explained. In this example, the magnetic flux shield plate  29  is driven based on a temperature detected by the thermistor  38  at width edge portions of the fixing belt  22 , not at the width center of the fixing belt  22 . The flowchart of  FIG. 10  applies Steps S 2 -S 10  of  FIG. 5  to its introduction stage and Steps S 26  of  FIG. 9  and S 12 -S 14  of  FIG. 5  to its ending stage, and replaces Step S 11  of  FIG. 5  with new Steps S 31 -S 33 . Therefore, the discussion below avoids repetition of Steps S 2 -S 10  and Steps S 26  and S 12 -S 14 , but focuses on new Steps S 31 -S 35 . 
     After the start of the consecutive image forming operations in Step S 10 , the temperature of the fixing belt  22  is detected by the thermistor  38  in Step S 31 . The thermistor  38  is arranged at a position in contact with a width edge area of the fixing belt  22 . This width edge area is within the heat reduction area N even when the heat reduction area N is changed by the adjustment, thereby making it possible to detect a temperature variation of the fixing belt N at an area within the heat reduction area N. 
     Then, in Step S 32 , a determination is made as to whether a temperature T detected by the thermistor  38  is equal to or greater than a predetermined temperature F. When the temperature T detected by the thermistor  38  is determined in Step S 32  as being equal to or greater than the predetermined temperature F, the magnetic flux shield plate  29  is driven in Step S 33  so as to widen the width of the heat reduction area N having a width adjusted in Step S 9 . Accordingly, a heat transfer rate from the heating area M to the heat reduction area N is made smaller so that temperature reductions at the non-sheet-passing areas are suppressed. 
     Then, in Step S 26 , a determination is performed as to whether an image forming job commanded is completed. When the image forming job commanded is determined in Step S 26  as not being completed, the processes after Step S 31  are repeated. When the image forming job commanded is determined in Step S 26  as being completed, the processes of Steps S 12 -S 14  are performed and the procedure ends. 
     When the temperature T detected by the thermistor  38  is determined in Step S 32  as not being equal to or greater than the predetermined temperature F, another determination is made in Step S 34  as to whether the temperature T is equal to or smaller than a predetermined temperature G which is smaller than the predetermined temperature F. When the temperature T is determined in Step S 34  as being equal to or smaller than the predetermined temperature G, the magnetic flux shield plate  29  is driven in Step S 35  so as to shorten the width of the heat reduction area N having the width adjusted in Step S 9 . Accordingly, the heat of the heating area M is transferred to the heat reduction area N so that a temperature reduction at edges of the sheet-passing area is suppressed while temperature rises at the non-sheet-passing areas are suppressed. 
     Then, in Step S 26 , a determination is performed as to whether an image forming job commanded is completed. Also, when the temperature T is determined in Step S 34  as not being equal to or smaller than the predetermined temperature G, the procedure goes to Step S 26 . When the image forming job commanded is determined in Step S 26  as not being completed, the processes after Step S 31  are repeated. When the image forming job commanded is determined in Step S 26  as being completed, the processes of Steps S 12 -S 14  are performed and the procedure ends. 
       FIG. 11  is a graph showing a relationship between a print number by a job of consecutive image forming operations as a horizontal axis and the fixing temperature as a vertical axis, in a case when the magnetic flux shield plate  29  is not installed. In  FIG. 11 , a curbed line S 1  represents variations of the fixing temperature over time in the sheet-passing area, that is, the width middle area of the fixing belt  22 . Also, a curbed line S 2  represents variations of the fixing temperature over time in the non-sheet-passing area, that is, the width side areas of the fixing belt  22 . As illustrated in  FIG. 11 , the fixing temperature in the sheet-passing area, indicated by the curbed line S 1 , is relatively low during a time the heating is started and is then soon stabilized. On the other hand, the fixing temperature in the non-sheet-passing area, indicated by the curbed line S 2 , is relatively low during a time the heating is started and is not stabilized even afterwards. The present example effectively suppresses such a faulty phenomenon before it grows. That is, the present example can stabilize the fixing temperature at the width side areas of the fixing belt  22  so as to suppress an excessive temperature rise by changing the heat reduction area N based on the temperature variations at the width side areas of the fixing belt  22 , at which the fixing temperature is not stable. 
     As described above, in this example, the magnetic flux shield area having an effect of reducing the magnetic flux active on the fixing belt  22  and the support roller  23  is changed in accordance with the temperature variations detected around the width edge area of the fixing belt  22 , during the consecutive image forming operations. Thereby, a temperature rise at width edges of both fixing belt  22  and support roller  23  is suppressed with reliability. 
     Referring to  FIG. 12 , another example magnetic flux shield plate  129  for the support roller  23  of the image fixing unit  20  is explained.  FIG. 12  illustrates the support roller  23  in a manner similar to  FIG. 3 , except for the magnetic flux shield plate  129 . The magnetic flux shield plate  129  includes a plurality of copper members having widths different from each other. The magnetic flux shield plate  129  are adhered to a circumferential surface of the internal core  28 . The plurality of copper members of the magnetic flux shield plate  129  are arranged so that an area for shutting a circumferential surface of the internal core  28  is gradually decreased or increased from an edge of the internal core  28 . Thereby, it becomes possible to vary the magnetic flux shield area in a lateral direction of the internal core  28 , which faces the coil  25  of the induction heater  24 , by driving the internal core  28  and the magnetic flux shield plate  129  to rotate. 
     As explained above, the image fixing unit  20  having the magnetic flux shield plate  129  of  FIG. 12  can change the magnetic flux shield area to reduce or increase the magnetic flux active on the fixing belt  22  and the support roller  23  during the consecutive image forming operations. Thereby, the image fixing unit  20  having the magnetic flux shield plate  129  of  FIG. 12  is capable of suppressing with reliability a temperature rise at the width sides of each of the fixing belt  22  and the support roller  23 . Therefore, the image fixing unit  20  having the magnetic flux shield plate  129  of  FIG. 12  can achieve the effects performed by the previously described embodiments in a similar manner. 
     Referring to  FIG. 13 , another example magnetic flux shield plate  229  for the support roller  23  of the image fixing unit  20  is explained.  FIG. 13  illustrates the magnetic flux shield plate  229  which includes a stepwise slant side  229   a  at each of lateral edge sides thereof. With the stepwise slant side  229   a , an area for shutting a circumferential surface of the internal core  28  is gradually decreased or increased from an edge of the internal core  28 . 
     As is in the previously explained examples, this example can also drive the magnetic flux shield plate  229  to precisely control the magnetic flux shield area by which the magnetic flux in the width direction of the fixing belt  22  can be changed in accordance with the heating time or the temperature of the fixing belt  22 . 
     As explained above, the image fixing unit  20  having the magnetic flux shield plate  229  of  FIG. 13  can change the magnetic flux shield area to reduce or increase the magnetic flux active on the fixing belt  22  and the support roller  23  during the consecutive image forming operations. Thereby, the image fixing unit  20  having the magnetic flux shield plate  229  of  FIG. 13  is capable of suppressing with reliability a temperature rise at the width sides of each of the fixing belt  22  and the support roller  23 . Therefore, the image fixing unit  20  having the magnetic flux shield plate  229  of  FIG. 13  can achieve the effects performed by the previously described embodiments in a similar manner. 
     Further, referring to  FIG. 14 , another example magnetic flux shield plate  329  for the support roller  23  of the image fixing unit  20  is explained.  FIG. 14  illustrates the magnetic flux shield plate  329  which includes a plurality of copper members having widths different from each other and tapered side edges, as illustrated in FIG.  14 . The magnetic flux shield plate  329  are adhered to a circumferential surface of the internal core  28 . The plurality of copper members of the magnetic flux shield plate  329  are arranged so that an area for shutting a circumferential surface of the internal core  28  is gradually decreased or increased from an edge of the internal core  28 . Thereby, it becomes possible to vary the magnetic flux shield area in a lateral direction of the internal core  28 , which faces the coil  25  of the induction heater  24 , by driving the internal core  28  and the magnetic flux shield plate  329  to rotate. 
     As is in the previously explained examples, this example can also drive the magnetic flux shield plate  329  to precisely control the magnetic flux shield area by which the magnetic flux in the width direction of the fixing belt  22  can be changed in accordance with the heating time or the temperature of the fixing belt  22 . 
     As explained above, the image fixing unit  20  having the magnetic flux shield plate  329  of  FIG. 14  can change the magnetic flux shield area to reduce or increase the magnetic flux active on the fixing belt  22  and the support roller  23  during the consecutive image forming operations. Thereby, the image fixing unit  20  having the magnetic flux shield plate  329  of  FIG. 14  is capable of suppressing with reliability a temperature rise at the width sides of each of the fixing belt  22  and the support roller  23 . Therefore, the image fixing unit  20  having the magnetic flux shield plate  329  of  FIG. 14  can achieve the effects performed by the previously described embodiments in a similar manner. 
     Referring to  FIG. 15 , another example image fixing unit  420  is explained.  FIG. 15  illustrates the image fixing unit  420  which has a structure similar to the image fixing unit  20  of  FIG. 2 , except for a fixing roller  423  which combines the functions of the fixing belt  22  and the support roller  23  of  FIG. 2 . That is, the fixing roller  423  of  FIG. 15  serves as a fixing member as well as a heating member. 
     The fixing roller  423  includes a heat layer  423   a , an elastic layer (not shown), and a release layer. The elastic layer mainly includes a silicone rubber, and the release layer mainly includes a fluorine compound. The fixing roller  423  has a shape of hollow circular cylinder in which the internal core  28  and the magnetic flux shield plate  29  are held for rotation. 
     The induction heater  24  includes the coil  25 , the core  26 , and the coil guide  27 , as described in the previous example of  FIG. 2 . The coil  25  is configured to receive an application of an alternating current having a frequency in the range of from approximately 10 kHz to approximately 1 MHz. As a result, magnetic lines of force are generated between the core  26  and the core  28  and the fixing roller  423  is consequently heated by the action of an electromagnetic induction. The thus-heated fixing roller applies heat to the toner image carried on the recording sheet P transferred thereto in the direction Y. Thereby, the toner image is melt and fixed on the recording sheet P while passing through the gap between the fixing roller  423  and the pressure roller  30 . 
     As described above, this example changes the magnetic flux shield area by which the magnetic flux in the width direction of the fixing roller  423  can be changed in accordance with the heating time or the temperature of the fixing roller  423  during the consecutive image forming operations. Thereby, a temperature rise of the fixing roller  423  in the width direction can be suppressed with reliability. 
     Referring to  FIG. 16 , an example detector for the home position of the support roller  23  is explained. As illustrated in  FIG. 16 , the internal core  28  of the support roller  23  illustrated in  FIG. 3  has a shaft  28   a  to which a disc  41  is provided. The internal core  28  and the shaft  28   a  are engaged with each other, and the disc  41  is rotated together with the core  28  and the magnetic flux shield plate  29  when the shaft  28   a  of the internal core  28  is driven to rotate. As illustrated in  FIG. 17 , the disc  41  has a half circle shape and is arranged to be linked with the position of the magnetic flux shield plate  29 . In other words, the position of the magnetic flux shield plate  29  can be recognized by detecting the attitude of the half round disc  41 . To detect the attitude of the disc  41 , a transmissive photosensor  42  is provided in the vicinity of the disc  41 . The transmissive photosensor  42  includes a light emitting element such as a laser diode and a light sensitive element such as a photodiode, and is configured to detect the disc  41  when a radial edge of the half round the disc  41  is driven to move in either of a clockwise or counterclockwise direction and passes a position  42   a  between the light emitting element and the light sensitive element. By detecting the position of the disc  41  in this way, the position of the magnetic flux shield plate  29  which is engaged with the disc  41  is determined. For example, as illustrated in  FIG. 17 , when the internal core  28  is rotated clockwise so that the detection status of the disc  41  by the transmissive photosensor  42  is changed from a status of “being not detected” to a status of “being detected” when the radial edge of the half round the disc  41  passes the position  42   a . At this moment, the magnetic flux shield plate  29  is recognized at a position, as illustrated in  FIG. 17 . This position is referred to as a home position of the magnetic flux shield plate  29 . 
     With this example structure described above, the magnetic flux shield plate  29  is initially returned to the home position and is then subjected to the heat-reduction-area control operation in accordance with the size of the recording sheet P. 
     Referring to  FIGS. 18 and 19A  and  19 B, an example procedure of the shield area control operation performed by the image fixing unit  20  is explained.  FIG. 18  is a flowchart of an example procedure of the heat-reduction-area control operation according to an embodiment of the present invention.  FIG. 19A  demonstrates a condition in that the magnetic flux shield plate  29  is at its home position where the magnetic flux shield plate  29  does not intervene and no heat reduction area N of the magnetic flux is formed.  FIG. 19B  shows a condition in that the magnetic flux shield plate  29  is moved to a position where the magnetic flux shield plate  29  intervenes the magnetic flux in an area outside the recording sheet P, i.e., the non-sheet-passing area. In this case, the magnetic flux shield area N for the magnetic flux is formed around the non-sheet-passing area. 
     When the image forming apparatus  1  is energized, the image fixing unit  20  starts the heat-reduction-area control operation in which the magnetic flux shield plate  29  is initially needed to return to its home position. In Step S 42  of  FIG. 18 , the magnetic flux shield plate  29  is driven to rotate together with the internal core  28  and the disc  41 . Then, the transmissive photosensor  42  detects the radial edge of the disc  41 , in Step S 43 . By this detection, it is determined that the magnetic flux shield plate  29  is at the home position. At the home position, the magnetic flux shield plate  29  is away from the center core  26   a  by a distant Y along the circumferential surface of the core  26  in the circumferential direction of the core  26 , as illustrated in  FIG. 19A , so that no magnetic flux shield area is formed and the entire width of the internal core  28  is exposed to the magnetic flux. In other words, at this time, the heat reduction area N of the support roller  23  is null and the heating area M is applied to the entire width of the support roller  23 . 
     Then, the magnetic flux shield plate  29  is stopped in Step S 44 , and the home position of the magnetic flux shield plate  29  is determined in Step S 45 . Subsequently, the inverter power source circuit, i.e., the high-frequency power source is energized and accordingly heating by the induction heater  24  is started, in Step S 46 . 
     Then, the sheet size detector  11   a , for example, detects the size of the recording sheet P in accordance with an image forming command entered by an operator, in Step S 47 . Based on the sheet size detected by the sheet size detector  11   a , for example, an initial control position of the magnetic flux shield plate  29  is determined, in Step S 48 . Then, in Step S 49 , the magnetic flux shield plate  29  is turned to the initial control position. 
     More specifically, when the sheet size of the recording sheet P detected by the sheet size detector  11   a , for example, is B5T (i.e., B5 landscape), the magnetic flux shield plate  29  is driven to turn from the home position, as illustrated in  FIG. 19A , to the initial control position, as illustrated in  FIG. 19B . Thus, the heat reduction area N is approximately equal to the non-sheet-passing area, that is, outside the recording sheet P of B5T size. In addition, the heating area M is approximately equal to the sheet-passing area, that is, within the width of the recording sheet P of B5T size. 
     At each time a series of fixing operations is performed, the processes of Steps S 47 -S 49  are repeated, and the procedure of the image forming job ends. 
     In this example, the position of the magnetic flux shield plate  29  is adjusted so that the heat reduction area N and the heating area M are in accordance with the non-sheet-passing area and the sheet-passing area, respectively, as illustrated in  FIGS. 19A and 19B . However, it is preferable to adjust the position of the magnetic flux shield plate  29  in accordance with the distribution of temperature of the fixing belt  22  or the support roller  23  in the width direction, as illustrated in  FIGS. 6A-6C . 
     With the structure of the support roller  23  with the disc  41  and the transmissive photosensor  42 , the magnetic flux shield plate  29  is initially moved to the home position and is then adjusted in accordance with the size of the recording sheet P, thereby improving variation accuracy of the heat reduction area N. As a result, the distribution of temperature with respect to the fixing belt  22  is constantly maintained in a shape, as illustrated in  FIG. 8 . Therefore, the temperature rise of the fixing belt  22  is suppressed in the heat reduction area N and the fixing belt  22  would not cause a thermal damage. 
     As described above, in this example, the image forming apparatus  1  controls the magnetic flux shield plate  29  based on the width information of the recording sheet P and the position of the magnetic flux shield plate  29 . Thereby, the heat reduction N is accurately adjusted and the temperature rise of the fixing belt  22  and the support roller  23  is suppressed in the width direction with reliability. 
     This example uses the fixing belt  22  including the heat layer and the support roller  23  as heat members. As an alternative, not both but one of the fixing belt  22  and the support roller  23  may be used as a heat member. Even with such a structure, the fixing procedure can be performed in a similar manner with a similar effect. 
     Further, in this example, the pressure roller  30  may be provided internally with a halogen heater. Also, it is possible to provide a thermistor and an oil coating roller at positions in contact with the outer circumferential surface of the pressure roller  30 . 
     Furthermore, the image forming apparatus  1  is, as described above, a black and white image forming machine; however, the present invention can easily be applied to a color image forming apparatus. 
     As a further alternative, it is possible to use a reflection type photosensor instead of the transmissive photosensor  42 . In using the transmissive photosensor, an absence of the disc  41  is determined when the light sensitive element detects the light emitted by the light emitting element. However, in using the reflection type photosensor, a presence of the disc  41  is determined when the light sensitive element detects a reflected light of the light emitted by the light emitting element. 
     Referring to  FIG. 20 , another example detector for detecting the home position with respect to the support roller  23  is explained. As illustrated in  FIG. 20 , the support roller  23  is provided with a disc  41   a  which includes a first section  41   b , a second section  41   c , and a third section  41   c . The support roller  23  is also provided with a transmissive photosensor  42   a  which includes light sensitive elements  42   b ,  42   c , and  42   d , each of which is paired with a light emitting element (not shown). 
     The first, second, and third sections  41   b ,  41   c , and  41   d  have fan-like shapes with different radiuses and are arranged one another. These sections correspond to the variations of the heat reduction area N. For example, the first section  41   b  corresponds to the heat reduction area N for a sheet size of A3T, that is, a A3-size recording sheet in landscape orientation. Similarly, the second section  41   c  corresponds to the heat reduction area N for a sheet size of A4T, that is, a A4-size recording sheet in landscape orientation, and the third section  41   d  corresponds to the heat reduction area N for a sheet size of A5T, that is, a A5-size in landscape orientation. 
     The disc  41   a  is turned in a manner similar to the disc  41  of  FIG. 17 , when the internal core  28  is driven to rotate together with the magnetic flux shield plate  29 . The light sensitive elements  42   b ,  42   c , and  42   d  are arranged at positions corresponding to the first, second, and third sections  41   b ,  41   c , and  41   d  so that, when the disc  41   a  is turned, the first section  41   b  is detected by the light sensitive element  42   b , the second section  41   c  is detected by the light sensitive element  42   c , and the third section  41   d  is detected by the light sensitive element  42   d.    
     When the disc  41   a  is turned by a degree so that the photosensor  42   a  only detects the first section  41   b , the heat reduction area N corresponds to the recording sheet of A3T. Similarly, the heat reduction area N corresponds to the recording sheet of A4T when the photosensor  42   a  detects the first and second sections  41   b  and  41   c . Further, the heat reduction area N corresponds to the recording sheet of A5T when the photosensor  42   a  detects the first, second, and third sections  41   b ,  41   c , and  41   d . In this way, the photosensor  42   a  directly detects the attitude of the magnetic flux shield plate  29 . 
     In this example, the detectors for the home position of the magnetic flux shield plate  29  using the photosensor such as the transmissive photosensors  42  and  42   a  or the like is applied to the image fixing unit employing the support roller shown in  FIG. 3 . However, such a home position detector can also be applied to the image fixing units employing variations of the support rollers shown in  FIG. 12 , for example. Further, the home position detector can be applied to the cases that employ the variations of the magnetic flux shield plate shown in  FIGS. 13 and 14 , for example. Further, the home position detector can also be applied to the image fixing unit shown in  FIG. 15 , for example. 
     Referring to  FIGS. 21A and 21B , an example procedure of another heat-reduction-area control operation for the image fixing unit  20  is explained.  FIG. 21A  demonstrates a case in which the recording sheet P in a B5T size is used and  FIG. 21B  demonstrates a case in which the recording sheet P in a A4T size. In this example, the magnetic flux shield plate  29  is rotated so that the heating area M is made as included in the sheet-passing area which is equivalent to the width L. 
     In a case of the recording sheet P of B5T having the width L 2 , the magnetic flux shield plate  29  is rotated to shield a part of the center core  26   a  so as to change the heat reduction area N to a heat reduction area N 2  on each side of the support roller  23 , entering into the width L 2  of B5T by a marginal distance. Accordingly, the heating area M is changed to a heating area M 2  which is narrower than the width L 2 , as illustrated in  FIG. 21A . The above marginal distance is expressed as (L 2 −M 2 )/2. 
     Subsequently, the inverter power source circuit of the image fixing unit  20  is energized so that the induction heater  24  is caused to start heating. The time of energizing the inverter power source circuit is not limited to it and can be executed before starting the rotation of the magnetic flux shield plate  29 , for example. 
     In a case of the recording sheet P of B4T having the width L 1 , the magnetic flux shield plate  29  is rotated to shield a part of the center core  26   a  so as to change the heat reduction area N to a heat reduction area N 3  on each side of the support roller  23 , entering into the width L 1  of B4T by a marginal distance. Accordingly, the heating area M is changed to a heating area M 3  which is narrower than the width L 2 , as illustrated in  FIG. 21A . The above marginal distance is expressed as (L 1 −M 3 )/2. 
     Subsequently, the inverter power source circuit of the image fixing unit  20  is energized so that the induction heater  24  is caused to start heating. 
     As described above, this example drives the magnetic flux shield plate  29  so that the heating area M is made as included in the sheet-passing area which is equivalent to the width L. Therefore, a leveling of the temperature distribution can be performed with consideration of thermal transmission from the heating area M to the heat reduction area N, as shown in comparative illustrations of  FIGS. 22A and 22B , wherein L is the width of the recording sheet P, T is the temperature, and M is the heating area. 
     Furthermore, since this example drives the magnetic flux shield plate  29  so that the heating area M is made as included in the sheet-passing area which is equivalent to the width L, the distribution of temperature with respect to the fixing belt  22  is constantly maintained in a shape, as illustrated in  FIG. 8 . Therefore, the temperature rise of the fixing belt  22  is suppressed in the heat reduction area N and the fixing belt  22  would not cause a thermal damage. 
     This example uses the fixing belt  22  including the heat layer and the support roller  23  as heat members. As an alternative, not both but one of the fixing belt  22  and the support roller  23  may be used as a heat member. Even with such a structure, the fixing procedure can be performed in a similar manner with a similar effect. 
     Further, in this example, the pressure roller  30  may be provided internally with a halogen heater. Also, it is possible to provide a thermistor and an oil coating roller at positions in contact with the outer circumferential surface of the pressure roller  30 . 
     Furthermore, the image forming apparatus  1  is, as described above, a black and white image forming machine; however, the present invention can easily be applied to a color image forming apparatus. 
     Still further, this example procedure of the heat-reduction-area control operation can also be applied to the image fixing units employing variations of the support rollers shown in  FIG. 12 , for example. Further, the example procedure of the heat-reduction-area control operation can be applied to the cases that employ the variations of the magnetic flux shield plate shown in  FIGS. 13 and 14 , for example. Further, the example procedure of the heat-reduction-area control operation can also be applied to the image fixing unit shown in  FIG. 15 , for example. 
     Referring to  FIGS. 23 and 24 , an example procedure of another heat-reduction-area control operation for the image fixing unit  20  is explained. This image forming unit  20  includes the magnetic flux shield plate  229  of  FIG. 13  for the support roller  23 . As explained above, the magnetic flux shield plate  229  of  FIG. 13  includes the stepwise slant side  229   a  at each of lateral edge sides thereof. With the stepwise slant side  229   a , an area for shutting a circumferential surface of the internal core  28  is stepwise decreased or increased from an edge of the internal core  28 . 
     As illustrated in  FIG. 23 , the stepwise slant side  229   a  of the magnetic flux shield plate  229  has seven steps prepared for different sizes of the recording sheet P: A6, B6, A5, B5, A4, B4, and A3, for example. Therefore, in this example, the heating area M can be changed in seven steps. For example, the illustration of  FIG. 23  demonstrates a condition of the magnetic flux shield plate  229  in a case of the recording sheet P of A5, in which the magnetic flux shield plate  229  is appropriately positioned relative to the center core  26   a  for the recording sheet P of A5. Under this condition, the heating area M is substantially equivalent to the width L of the recording sheet P, that is, the width of A5. In this example, the magnetic flux shield plate  229  is rotated so that the heat reduction area N faces the non-sheet-passing area and the heating area M faces the sheet-passing area which is equivalent to the width L. 
     In this way, the image fixing unit  20  using the magnetic flux shield plate  229  can handle the recording sheets P in various sheet sizes such as A6, B6, A5, B5, A4, B4, and A3, for example. 
     As illustrated in  FIG. 23 , the stepwise slant side  229   a  is a leading side when the magnetic flux shield plate  229  is rotated. Therefore, as demonstrated in  FIG. 24 , when the magnetic flux shield plate  229  is positioned with a slight positional error in the sheet transportation direction relative to the center core  26   a  for the recording sheet P of A5, the positional error is extended only for a distance G, in the width direction, which is relatively small. That is, when the magnetic flux shield plate  229  is moved inaccurately by an erroneous distance (e.g., the distance G), such an erroneous distance is not caused across the magnetic flux shield plate  220  but is restricted within a relatively small range. 
     As described above, since, in this example, the leading side, that is, the stepwise slant side  229   a  of the magnetic flux shield plate  229  has a plurality of steps, the distribution of temperature with respect to the fixing belt  22  can constantly be maintained in a shape, as illustrated in  FIG. 8 , even when the magnetic flux shield plate  229  is moved with a slight error. Therefore, the temperature rise of the fixing belt  22  is suppressed in the heat reduction area N and the fixing belt  22  would not cause a thermal damage. 
     This example uses the fixing belt  22  including the heat layer and the support roller  23  as heat members. As an alternative, not both but one of the fixing belt  22  and the support roller  23  may be used as a heat member. Even with such a structure, the fixing procedure can be performed in a similar manner with a similar effect. 
     Further, in this example, the pressure roller  30  may be provided internally with a halogen heater. Also, it is possible to provide a thermistor and an oil coating roller at positions in contact with the outer circumferential surface of the pressure roller  30 . 
     Furthermore, the image forming apparatus  1  is, as described above, a black and white image forming machine; however, the present invention can easily be applied to a color image forming apparatus. 
     Still further, this example procedure of the heat-reduction-area control operation can also be applied to the image fixing units employing variations of the support rollers shown in  FIG. 12 , for example. Further, the example procedure of the heat-reduction-area control operation can be applied to the cases that employ the variations of the magnetic flux shield plate shown in  FIGS. 13 and 14 , for example. Further, the example procedure of the heat-reduction-area control operation can also be applied to the image fixing unit shown in  FIG. 15 , for example. 
     In this example, the magnetic flux shield plate  229  is adjusted to change the heat reduction area N and the heating area M based on the detection result by the sheet detector  11   a ,  12   a , or  15   a . However, as an alternative, it is possible to adjust the magnetic flux shield plate  229  in accordance with the detection result by the sheet thickness detector  1   a . This arrangement is particularly effective for a case in which heating efficiencies of the fixing belt  22  and the support roller  23  are susceptible to the change of a thickness of the recording sheet P. With such an arrangement, a temperature rise at both sides of the fixing belt  22  and the support roller  23  in the width direction can be suppressed with reliability, regardless of variations of the thickness of the recording sheet P. 
     When heating efficiencies of the fixing belt  22  and the support roller  23  are susceptible to the change of a thickness of the recording sheet P, the sheet thickness detector  1   a  is used to detect a sheet kind of the recording sheet P, and the magnetic flux shield plate  229  is adjusted in accordance with the detection result by the sheet thickness detector  1   a . With such an arrangement, a temperature rise at both sides of the fixing belt  22  and the support roller  23  in the width direction can be suppressed with reliability, regardless of variations of the kind of the recording sheet P. 
     As another alternative to the detection result by the sheet detector  11   a ,  12   a , or  15   a , it is possible to adjust the magnetic flux shield plate  229  in accordance with the detection result by the transfer speed detectors  1   b  and  1   c . This arrangement is particularly effective for a case in which the image forming apparatus is capable of changing the sheet transfer speed and in which heating efficiencies of the fixing belt  22  and the support roller  23  are susceptible to the change of the sheet transfer speed. With such an arrangement, a temperature rise at both sides of the fixing belt  22  and the support roller  23  in the width direction can be suppressed with reliability, regardless of variations of the sheet transfer speed of the recording sheet P. 
     As another alternative to the detection result by the sheet detector  11   a ,  12   a , or  15   a , it is possible to adjust the magnetic flux shield plate  229  in accordance with the detection result by the environment detector  1   d . This arrangement is particularly effective for a case in which heating efficiencies of the fixing belt  22  and the support roller  23  are susceptible to the change of environmental factors such as a temperature and humid, for example. With such an arrangement, a temperature rise at both sides of the fixing belt  22  and the support roller  23  in the width direction can be suppressed with reliability, regardless of variations of the environmental factors such as a temperature and humid, for example. 
     Referring to  FIG. 25 , another example image fixing unit  520  is explained.  FIG. 25  illustrates the image fixing unit  520  which has a structure similar to the image fixing unit  20  of  FIG. 2 , except for a support roller  523  and a thermostat  537 . The support roller  523  includes an internal core  528  having no magnetic flux shield plate. The thermostat  537  is arranged in contact with an outer circumferential surface of the support roller  523 . 
     As described above, the thermistor  38  arranged in contact with the outer circumferential surface of the fixing belt  22  is configured to regularly detect the fixing temperature from the surface of the fixing belt  22 . The inverter power source circuit is activated based on the detection result from the thermistor  38  so as to adjust its output. As a result, the fixing belt  22  maintains the fixing temperature at a constant level. However, as described above, the thermostat  537  arranged in contact with the support roller  523  detects an event in that the surface temperature of the support roller  523  exceeds a predetermined temperature. When detecting such an excess temperature, the thermostat  537  shuts off the power to the induction heater  24 . Thereby, the induction heater  24  is restricted to apply heat to the support roller  23 . 
     As illustrated in  FIG. 26 , the internal core  528  of the support roller  523  employed by the image fixing unit  520  has sides both canted off and includes a main body  528   a , canted surfaces  528   b , and a shaft  528   c . The canted surfaces  528   b  of the internal core  528  are more clearly shown in  FIG. 27 . The thus-structured support roller  523  of  FIG. 26  is similar to the support roller  23  of  FIG. 3 , except for these crosswise cuttings. 
     The internal core  528  structured in this way has in its width direction an outer circumferential surface length which faces the coil  25 . This outer circumferential surface length of the internal core  528  facing the coil  528  is gradually increased or decreased by a rotary movement of the internal core  528  itself. 
     Since the internal core  528  is configured to be driven to rotate by an arbitrary angle in a manner similar to the internal core  28 , it is possible to change the heating area M and the heat reduction area, as is performed by the support roller  23 , by rotating the internal core  528  to cause the canted surfaces  528   c  to face the center core  26   a  with a desired angle. 
     More specifically, seeing from one of the two canted surfaces  528   c , an area of the canted surface  528   c  facing the center core  26   a  can be changed by a rotary movement of the internal core  528 . Therefore, a change of the area of the canted surface  528   c  corresponds to a variation of the heating area M and the heat reduction area N shown in  FIG. 6B , for example. That is, an amount of the magnetic flux generated between the core  26  and the internal core  528  is increased or decreased in accordance with the outer circumferential length of the internal core  528  facing the coil  25 . When the outer circumferential surface length of the internal core  528  facing the coil  25  is relatively long, the heating area M is relatively long and the heat reduction area N is relatively short. Similarly, when the outer circumferential surface length of the internal core  528  facing the coil  25  is relatively long, the heating area M is relatively short and the heat reduction area N is relatively short.  FIGS. 28A-28C  show example conditions when the outer circumferential surface length of the internal core  528  facing the coil  25  is extended to its maximum length, a middle length, and its minimum length. In each of  FIGS. 28A-28C , an arrow with a dotted line indicates a direction in which the magnetic flux is applied. 
       FIG. 28A  shows a cross-sectional view of the support roller  523  seen in lines A-A, B-B, and C-C of  FIG. 26 , when the internal core  528  is rotated so that the outer circumferential surface length of the internal core  528  facing the coil  25  is extended to its maximum length, i.e., the width L 1 . 
     Similarly,  FIG. 28B  shows a cross-sectional view of the support roller  523  seen in lines A-A, B-B, and C-C of  FIG. 26 , when the internal core  528  is rotated so that the outer circumferential surface length of the internal core  528  facing the coil  25  is extended to a middle length between the width L 1  and the width L 2 . 
     Similarly,  FIG. 28C  shows a cross-sectional view of the support roller  523  seen in lines A-A, B-B, and C-C of  FIG. 26 , when the internal core  528  is rotated so that the outer circumferential surface length of the internal core  528  facing the coil  25  is extended to its minimum length, i.e., the width L 2 . 
     In this way, the image fixing unit  520  of the image forming apparatus  1  is provided with the internal core  528  which has the canted surfaces  528   c . Rotation of the canted surfaces  528   c  makes it possible to control the magnetic flux acting on the fixing belt  22  and the support roller  23  so as to change the heating area M and the heat reduction area N. Thereby, the image fixing unit  520  can suppress the temperature rises with reliability at the both sides of the fixing belt  22  and the support roller  23 . 
     Referring to  FIG. 29 , another example image fixing unit  620  is explained.  FIG. 29  illustrates the image fixing unit  620  which has a structure similar to the image fixing unit  20  of  FIG. 2 , except for a support roller  623 . As illustrated in  FIG. 29 , the support roller  523  includes a heat layer  523   a  and is arranged in contact directly with the pressure roller  30  to catch the recording sheet P transported in the direction Y. Therefore, in this structure, the image fixing unit  620  does not need the fixing belt. Such a support roller  623  may be referred to as a heat roller or a fixing roller. 
     In this structure, the image fixing unit  620  employs the internal core  528  of  FIG. 26 , which has the canted surfaces  528   c . Therefore, rotation of the canted surfaces  528   c  makes it possible to control the magnetic flux acting on the fixing belt  22  and the support roller  23  so as to change the heating area M and the heat reduction area N, in a similar manner as is performed by the image fixing unit  520 . Thereby, the image fixing unit  620  can suppress the temperature rises with reliability at the both sides of the support roller  623 . 
     The above-described embodiments are illustrative, and numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative and exemplary embodiments herein may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. 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. 
     This patent specification is based on Japanese patent applications, No. 2004-255114 filed on Sep. 2, 2004, No. 2004-259590 filed on Sep. 7, 2004, No. 2004-260717 filed on Sep. 8, 2004, No. 2004-264165 filed on Sep. 10, 2004, and No. 2004-213244 filed on Jul. 21, 2004, in the Japan Patent Office, the entire contents of each of which are incorporated by reference herein.

Technology Classification (CPC): 6