Patent Publication Number: US-9429887-B2

Title: Fixing device and image forming apparatus with air flow generator for cooling an internal space of a shaft to which a magnetic core is mounted

Description:
TECHNICAL FIELD 
     The present invention relates to a fixing device for applying a fixing process to a sheet and an image forming apparatus with the same. 
     BACKGROUND ART 
     An image forming apparatus such as a copier, a facsimile machine or a printer includes an image forming unit for forming an image on an image carrier (e.g. photoconductive drum), a transfer unit for transferring a toner image on the image carrier onto a sheet as an example of a recording medium and a fixing device for heating and fixing the toner image transferred onto the sheet to the sheet. 
     There is known a fixing device to which an electromagnetic induction heating (IH) method capable of quick heating and high efficiency heating is applied. In the electromagnetic induction heating method, an induction current is induced in a fixing roller and a fixing belt by a magnetic flux generated by the flow of a high-frequency current in an induction coil and Joule heating (induction heating) is caused in the fixing roller and the fixing belt. A toner image is fixed onto a sheet (recording medium) by this Joule heat. 
     A technology for suppressing excessive temperature increases of a fixing belt and a fixing roller in a fixing device of an electromagnetic induction heating type is, for example, known from Japanese Examined Patent Publication No. 2011-123409. A fixing device of Japanese Examined Patent Publication No. 2011-123409 includes a fixing belt configured to be induction-heated by a magnetic flux generated by a coil, a pressure roller configured such that a nip portion is formed between the fixing belt and the pressure roller, arch cores and side cores configured to form a magnetic path together with the fixing belt, a center core arranged between the arch cores and the fixing belt when viewed from the magnetic path and a magnetic shielding plate attached to the outer surface of the center core. The magnetic shielding plate comes to be located in the magnetic path with the rotation of the center core and suppresses excessive temperature increases of the fixing belt and the fixing roller in sheet non-passage areas by shielding or suppressing the magnetic flux in accordance with the sheet non-passage areas. Further, a technology for cooling an electromagnetic induction heating unit of a fixing device by cooling air is disclosed in Japanese Examined Patent Publication No. 2011-227445. 
     SUMMARY OF INVENTION 
     In the fixing device described in Japanese Examined Patent Publication No. 2011-123409, only parts of a peripheral surface of the center core are exposed on the upper surface of the electromagnetic induction heating unit through air gaps of the coil. Thus, there has been a problem that the center core cannot be sufficiently cooled when the coil is cooled by cooling air. 
     The present invention aims to efficiently cool a rotating core in a fixing device of an electromagnetic induction heating type and an image forming apparatus with the same. 
     A fixing device according to one aspect of the present invention includes a housing, a magnetic flux generation source, a first rotary body, a second rotary body, a first core, a second core, a shaft portion, a magnetic shielding body and an air flow generator. The magnetic flux generation source is fixed to the housing and generates a magnetic flux. The first rotary body is rotated in a predetermined direction and induction-heated by the magnetic flux. The second rotary body is rotated in a predetermined direction, and a nip portion through which a sheet carrying a toner image passes is formed between the first and second rotary bodies. The first core is made of a magnetic material and forms a magnetic path, along which the magnetic flux passes, together with the first rotary body. The second core is made of a magnetic material, arranged between the first core and the first rotary body when viewed from the magnetic path and rotatable in a predetermined direction. The shaft portion has a hollow cylindrical shape, holds the second core on a peripheral surface, includes an internal space extending along an axial direction inside and serves as a rotary shaft in the rotation of the second core. The magnetic shielding body is made of a nonmagnetic material and arranged on a peripheral surface of the second core or the shaft portion and shields or suppresses the magnetic flux by being located in the magnetic path with rotation about the shaft portion. The air flow generator generates an air flow flowing in the internal space from one end side toward the other end side in the axial direction. The magnetic flux generation source is fixed to the housing to surround the second core extending in the axial direction when viewed in a direction perpendicular to the axial direction. 
     An image forming apparatus according to another aspect of the present invention includes an image carrier, a transfer unit and the above fixing device. A toner image is formed on a surface of the image carrier. The transfer unit transfers the toner image onto a sheet. 
     According to the present invention, it is possible to efficiently cool a rotating core in a fixing device of an electromagnetic induction heating type and an image forming apparatus provided with the same. 
     An object, features and advantages of the present invention will become more apparent upon reading the following detailed description along with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view showing the internal structure of an image forming apparatus according to one embodiment of the present invention, 
         FIG. 2  is a sectional view showing a state where a magnetic shielding body is at a retracted position in a fixing device according to a first embodiment of the present invention, 
         FIG. 3  is a sectional view showing a state where the magnetic shielding body is at a shielding position in the fixing device according to the first embodiment of the present invention, 
         FIG. 4  is a perspective view of an electromagnetic induction heating unit of the fixing device according to the first embodiment of the present invention. 
         FIG. 5  is an exploded perspective view showing a state in the electromagnetic induction heating unit of the fixing device according to the first embodiment of the present invention, 
         FIG. 6  is an enlarged perspective view showing one axial end side in the electromagnetic induction heating unit of the fixing device according to the first embodiment of the present invention, 
         FIG. 7  is an enlarged perspective view showing the other axial end side in the electromagnetic induction heating unit of the fixing device according to the first embodiment of the present invention, 
         FIG. 8  is an exploded perspective view of the electromagnetic induction heating unit of  FIG. 7 , 
         FIG. 9A  is a perspective view showing a drive transmission structure to a shaft portion of a second core of the fixing device according to the first embodiment of the present invention, 
         FIG. 9B  is a perspective view showing the drive transmission structure to the shaft portion of the second core of the fixing device according to the first embodiment of the present invention, 
         FIG. 10  is a sectional view of the second core of the fixing device according to the first embodiment of the present invention, 
         FIG. 11  is an exploded perspective view showing the drive transmission structure to the shaft portion of the second core of the fixing device according to the first embodiment of the present invention, 
         FIG. 12  is a side sectional view of the electromagnetic induction heating unit of the fixing device according to the first embodiment of the present invention, 
         FIG. 13A  is a perspective view showing a state in an electromagnetic induction heating unit of a fixing device according to a second embodiment of the present invention, 
         FIG. 13B  is a perspective view showing the state in the electromagnetic induction heating unit of the fixing device according to the second embodiment of the present invention, 
         FIG. 14  is an enlarged perspective view showing a peripheral surface of a second core of the fixing device according to the second embodiment of the present invention, 
         FIG. 15A  is a perspective view showing a state in an electromagnetic induction heating unit of a fixing device according to a third embodiment of the present invention, 
         FIG. 15B  is a perspective view showing the state in the electromagnetic induction heating unit of the fixing device according to the third embodiment of the present invention, 
         FIG. 16  is a perspective view of an electromagnetic induction heating unit of a fixing device according to a modification of the present invention, and 
         FIG. 17  is a side sectional view of the electromagnetic induction heating unit of the fixing device according to the modification of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described in detail using the drawings.  FIG. 1  is a sectional view showing the internal structure of an image forming apparatus  1  according to one embodiment of the present invention. The image forming apparatus  1  can be a printer, a copier, a facsimile machine, a complex machine having a combination of those functions, or the like for transferring and printing a toner image on a surface of a sheet as an example of a recording medium, for example, based on externally input image information. 
     The image forming apparatus  1  shown in  FIG. 1  is, for example, a tandem-type color printer. The image forming apparatus  1  includes an apparatus main body  2  in the form of a rectangular box for forming a color image on a sheet inside, and a discharge tray  3  to which a sheet printed with a color image is discharged is provided on an upper surface part of the apparatus main body  2 . In the apparatus main body  2 , a sheet cassette  5  for storing sheets is arranged in a lower part. Further, a stack tray  6  on which a manually fed sheet is placed is arranged on a right side surface of the apparatus main body  2  in  FIG. 1 . An image forming station  7  is provided in an upper part of the apparatus main body  2  and forms an image on a sheet based on image data such as characters and pictures transmitted from the outside of the apparatus. 
     A first conveyance path  9  for conveying a sheet fed from the sheet cassette  5  to the image forming station  7  is arranged at a position to the left of the image forming station  7  in  FIG. 1 . A second conveyance path  10  for guiding a sheet placed on the stack tray  6  to the first conveyance path is arranged at a position above the sheet cassette  5 . A pair of conveyor rollers  43  for conveying the sheet are arranged in each of the first and second conveyance paths  9  and  10 . Further, a fixing device  14  for applying a fixing process to a sheet having an image formed thereon in the image forming station  7  and a third conveyance path  11  for conveying the sheet, to which the fixing process was applied, to the discharge tray  3  are arranged in a left-upper part of the apparatus main body  2 . 
     The sheet cassette  5  is detachably mountable into the apparatus main body  2  and includes a storage  16 . At least two types of sheets having different sizes in a sheet feeding direction can be selectively stored in the storage  16 . The sheets stored in the storage  16  are fed to the first conveyance path  9  one by one by a feed roller  17  and a pair of separation rollers  18 . 
     The stack tray  6  is openable and closable relative to the apparatus main body  2  and sheets are placed on a manual feed surface  19  thereof. The sheets placed on the manual feed surface  19  are fed one by one by a pickup roller  20  and a pair of separation rollers  21 . 
     The first and second conveyance paths  9 ,  10  join at a position before a pair of registration rollers  22 . A sheet brought to the pair of registration rollers  22  temporarily waits here and is fed toward a secondary transfer unit  23  after a skew adjustment and a timing adjustment are performed. A full-color toner image on an intermediate transfer belt  40  is secondarily transferred to the fed sheet at the secondary transfer unit  23 . Thereafter, the sheet having the toner image fixed in the fixing device  14  is reversed in a fourth conveyance path  12  if necessary and a full-color toner image is secondarily transferred also to an opposite surface in the secondary transfer unit  23 . After the toner image on the opposite surface is fixed in the fixing device  14 , the sheet is discharged to the discharge tray  3  by a pair of discharge rollers  24  through the third conveyance path  11 . 
     The image forming station  7  includes four image forming units  26  to  29  for forming each of toner images of black (B), yellow (Y), cyan (C) and magenta (M) and an intermediate transfer unit  30  for combining and carrying toner images of the respective colors formed in the image forming units  26  to  29 . 
     Each image forming unit  26  to  29  includes a photoconductive drum  32  (image carrier), a charger  33  arranged to face a circumferential surface of the photoconductive drum  32 , a laser scanning unit  34  arranged downstream of the charger  33  in a rotating direction of the photoconductive drum  32  and configured to irradiate a laser beam to a specific position on the circumferential surface of the photoconductive drum  32 , a developing device  35  arranged downstream of a laser irradiation position from the laser scanning unit  34  in the rotating direction of the photoconductive drum  32  and arranged to face the circumferential surface of the photoconductive drum  32  and a cleaner  36  arranged downstream of the developing device  35  in the rotating direction of the photoconductive drum  32  and arranged to face the circumferential surface of the photoconductive drum  32 . 
     The photoconductive drum  32  of each image forming unit  26  to  29  is rotated in a counterclockwise direction in  FIG. 1  by an unillustrated drive motor. The developing device  35  of each image forming unit  26  to  29  includes a developer container  51  storing two-component developer including black toner, yellow toner, cyan toner or magenta toner. 
     The intermediate transfer unit  30  includes a drive roller  38  arranged at a position near the image forming unit  26 , a driven roller  39  arranged at a position near the image forming unit  29 , a tension roller  42  arranged at a position between the drive roller  38  and the driven roller  39 , the intermediate transfer belt  40  mounted on the drive roller  38 , the driven roller  39  and the tension roller  42  and four transfer rollers  41  arranged to be pressable into contact with the photoconductive drums  32  of the respective image forming units  26  to  29  via the intermediate transfer belt  40 . 
     In the intermediate transfer unit  30 , a full-color toner image is formed by transferring toner images of the respective colors in a superimposed state onto the intermediate transfer belt  40  from the photoconductive drums  32  at the positions of the transfer rollers  41  of the respective image forming units  26  to  29 . 
     Conveyance paths  47  are provided upstream and downstream of the fixing device  14  in a sheet conveying direction. A sheet conveyed through the secondary transfer unit  23  is guided to the fixing device  14  through the upstream conveyance path  47 . The sheet, to which the fixing process was applied, is guided to the third conveyance path  11  through the downstream conveyance path  47 . 
     The third conveyance path  11  guides the sheet, to which the fixing process was applied in the fixing device  14 , to the discharge tray  3 . A pair of conveyor rollers  49  for conveying the sheet to the discharge tray  3  are arranged in the third conveyance path  11  and the pair of discharge rollers  24  are arranged at the exit of the third conveyance path  11 . 
     Next, a fixing device  14  according to a first embodiment of the present invention is described with reference to  FIGS. 2 to 11 .  FIG. 2  is a sectional view showing a state where a magnetic shielding body  60  to be described later is at a retracted position in the fixing device  14  according to the first embodiment of the present invention.  FIG. 3  is a sectional view showing a state where the magnetic shielding body  60  is at a shielding position in the fixing device  14 .  FIG. 4  is a perspective view of a coil unit  50  (electromagnetic induction heating unit) of the fixing device  14 .  FIG. 5  is an exploded perspective view showing a state in the coil unit  50 .  FIG. 6  is an enlarged perspective view showing one axial end side (front end side) in the coil unit  50 .  FIG. 7  is an enlarged perspective view showing the other axial end side (rear end side) in the coil unit  50 .  FIG. 8  is an exploded perspective view of the coil unit  50  of  FIG. 7 . In  FIG. 8 , a drive transmission member  593  and a driving unit  50 M to be described later are shifted backward as compared with  FIG. 7 .  FIGS. 9A and 9B  are perspective views showing a drive transmission structure to a center core shaft  59  of the fixing device  14 .  FIG. 10  is a sectional view of a second core  58  of the fixing device  14 .  FIG. 11  is an exploded perspective view showing the drive transmission structure to the center core shaft  59  of the fixing device  14 . Note that direction-indicating terms such as “upper” and “lower”, “front” and “back”, “left” and “right” used in the following description are merely for the purpose of clarifying the description and do not limit the principle of the fixing device at all. 
     The fixing device  14  applies a fixing process of fixing a toner image onto a sheet by heating and pressing the toner image transferred onto the sheet. The fixing device  14  includes a pressure roller  44  (second rotary body), a fixing belt  48  (first rotary body), a fixing roller  45 , a heat roller  46 , the coil unit  50  and a fan  65  ( FIG. 4 ) (air flow generator). These members are supported on an unillustrated frame which is a housing of the fixing device  14 . 
     The pressure roller  44  is a roller member rotatable counterclockwise in  FIG. 2  and includes a tubular core member made of, e.g. SUS, an elastic layer made of silicone rubber and laminated on the core member and a surface release layer made of PFA and laminated on the elastic layer. Note that a heat source such as a halogen heater may be arranged inside the pressure roller  44 . The elastic layer can be heated by the heat source. The pressure roller  44  is pressed against the fixing belt  48  by an unillustrated biasing member and a nip portion NP, through which a sheet carrying a toner image is passed, is formed between the fixing belt  48  and the pressure roller  44 . 
     The fixing belt  48  is an endless belt wound on the fixing roller  45  and the heat roller  46  to be described later and rotatable clockwise in  FIG. 2 . The fixing belt  48  has a widthwise dimension in a direction (front-back direction) perpendicular to a conveying direction T of the sheet passed through the nip portion NP. The fixing belt  48  includes a base member made of electroformed nickel and located on the side of the fixing roller  45 , an elastic layer made of silicone rubber and laminated on the base member on a side opposite to the fixing roller  45  and a surface release layer made of PFA, laminated on the elastic layer and located on the side of the pressure roller  44 . The fixing belt  48  is induction-heated by a magnetic flux generated by the coil unit  50  to be described later. 
     The fixing roller  45  is a roller member arranged inside the fixing belt  48  in parallel to the pressure roller  44  and rotatable in a clockwise direction which is the same as the rotating direction of the fixing belt  48 . The fixing roller  45  includes a tubular core member made of, e.g. SUS, an elastic layer made of silicone rubber and laminated on the core member and a surface release layer made of PFA, laminated on the elastic layer and held in contact with the fixing belt  48 . The fixing roller  45  rotates while being held in contact with the inner surface of the fixing belt  48  at a position corresponding to the nip portion NP. This causes the fixing belt  48  to rotate while being sandwiched between the fixing roller  45  and the pressure roller  44  at the nip portion NP. 
     The heat roller  46  is a roller member arranged inside the fixing belt  48  in parallel to the fixing roller  45  at a position opposite to the pressure roller  44  across the fixing roller  45  and rotatable clockwise in  FIG. 2 . The heat roller  46  includes a tubular core member made of a magnetic metal such as SUS and a surface release layer made of PFA and laminated on the core member. Note that a thermistor  62  is arranged in the heat roller  46 . The thermistor  62  detects a surface temperature of the heat roller  46 . The detected surface temperature is transmitted to an unillustrated controller, which controls an alternating current bias power supply V based on that surface temperature and adjusts the density of a magnetic flux generated by the coil  52  to be described later. 
     Each of the pressure roller  44 , the fixing roller  45  and the heat roller  46  has an axial dimension set larger than a maximum width of sheets to be passed through the nip portion NP and the widthwise dimension of the fixing belt  48  is set larger than the maximum width of sheets. 
     The coil unit  50  induction-heats the fixing belt  48  at a position where the fixing belt  48  is wound around the heat roller  46 . The coil unit  50  is arranged above the heat roller  46 . With reference to  FIGS. 4 and 5 , the coil unit  50  includes a supporting unit  50 L (housing), a shield  50 H (magnetic shield), the driving unit  50 M, the coil  52  (magnetic flux generation source), a bobbin  53 , an arch core unit  50 S, a plurality of pairs of arch cores  54 , a pair of side cores  56 , the center core  58 , the center core shaft  59  and the magnetic shielding body  60 . 
     The supporting unit  50 L supports each member of the coil unit  50 . As shown in  FIG. 5 , the supporting unit  50 L has a substantially rectangular shape having a predetermined width in a lateral direction and extending in the front-back direction. The supporting unit  50 L constitutes a part of the frame of the aforementioned fixing device  14 . 
     The shield  50 H ( FIG. 4 ) is mounted on an upper surface part of the supporting unit  50 L. The shield  50 H is formed by bending a plate member and has a chevron shape. The shield  50 H covers the center core  58  and the coil  52  to be described later from above and prevents magnetic or electrical noise from entering the coil unit  50  from the outside of the coil unit  50 . The shield  50 H includes an exhaust duct  50 H 1 . The exhaust duct  50 H 1  is a rectangular duct formed on a rear end part of a right wall surface of the shield  50 H. The exhaust duct  50 H 1  has a function of exhausting an air flow discharged from exhaust ports  593 H of a drive transmission member  593  to be described later to the outside of the coil unit  50 . 
     The driving unit  50 M ( FIG. 5 ) is arranged on a rear end part of the supporting unit  50 L. The driving unit  50 M is a unit for generating a rotational drive force for rotating the center core  58  to be described later. 
     The bobbin  53  ( FIG. 2 ) is arranged on a lower surface part of the supporting unit  50 L. The bobbin  53  is a member for holding the coil  52  and extends along an axial direction of the heat roller  46  (i.e. width direction of the fixing belt  48 ). The bobbin  53  is a member formed into a semi-cylindrical shape to extend along the fixing belt  48  at a position opposite to the fixing roller  45 . The bobbin  53  extends along an arc beyond the widthwise dimension of the fixing belt  48  and over a substantially half of a circumferential length of the fixing belt  48 . The bobbin  53  is at a predetermined distance from the fixing belt  48 . A material of the bobbin  53  is a heat resistant resin (e.g. PPS, PET, LCP). 
     The coil  52  is arranged in a state wound along an extending direction of the bobbin  53  on the hollow semi-cylindrical bobbin  53 . A winding area of the coil  52  is set to have a dimension larger than the widthwise dimension of the fixing belt  48 . Further, with reference to  FIG. 5 , the coil  52  is fixed to the bobbin  53  of the supporting unit  50 L to surround the center core  58  extending in the axial direction when viewed in a direction perpendicular to the axial direction of the heat roller  46  (from above). The coil  52  is fixed to the bobbin  53 , for example, by a silicone-based adhesive. Further, the alternating current bias power supply V ( FIG. 2 ) is connected to the coil  52  and the coil  52  generates a magnetic flux when an alternating current bias is applied thereto. 
     The arch core unit  50 S ( FIG. 5 ) is mounted on the supporting unit  50 L from above. At this time, the arch core unit  50 S is arranged to cover the coil  52  and the center core  58  from above. Further, the aforementioned shield  50 H is arranged above the arch core unit  50 S. The arch core unit  50 S supports a plurality of arch cores  54 . 
     The arch cores  54  and the side cores  56  are made of a magnetic material such as ferrite and form a magnetic path, along which the magnetic flux generated by the coil  52  passes, together with the fixing belt  48 . The arch cores  54  are paired to surround the coil  52  from opposite sides with the center core  58  as a center. The arch cores  54  are formed into an arch shape with the center core  58  interposed while being paired. A plurality of arch cores  54  are arranged at intervals along the axial direction of the center core  58 . The plurality of arch cores  54  are arranged also in an area beyond the winding area of the coil  52 . 
     The center core  58  is arranged between upper end parts of the paired arch cores  54 . An upper part of the center core  58  is covered by a center core protecting portion  58 H ( FIG. 5 ) of the arch core unit  50 S. On the other hand, the side cores  56  extending along the extending direction of the bobbin  53  are coupled to lower end parts of the arch cores  54 . The arch cores  54  and the side cores  56  are supported by the above arch core unit  50 S made of a heat resistant resin (e.g. PPS, PET, LCP). In this embodiment, the arch cores  54  and the side cores  56  constitute a first core. 
     The center core  58  is a tubular core made of a magnetic material such as ferrite and arranged at a side opposite to the heat roller  46  across the bobbin  53  in  FIG. 2  and extends in parallel to the heat roller  46 . The center core  58  is facing an area of the bobbin  53  where the coil  52  is absent in a circumferential direction ( FIG. 2 ). The center core shaft  59  is inserted through the center core  58  and the center core  58  rotates clockwise or counterclockwise with the rotation of the center core shaft  59 . 
     The center core shaft  59  is formed of a hollow cylindrical member and holds the center core  58  on a peripheral surface. The center core shaft  59  includes an internal space AF ( FIG. 10 ) extending along the axial direction in a hollow interior. The center core shaft  59  serves as a rotary shaft in the rotation of the center core  58 . The center core shaft  59  is formed of a thin pipe (cylindrical tube member) made of nonmagnetic SUS and rotated by a motor M to be described later. 
     The magnetic flux generated by the coil  52  passes along the magnetic path formed among the fixing belt  48 , the side cores  56 , the arch cores  54  and the center core  58 . The center core  58  is located between the arch cores  54  and the fixing belt  48  when viewed from the magnetic path and the magnetic shielding body  60  is mounted on the outer peripheral surface of the center core  58 . 
     The magnetic shielding body  60  is a thin plate member made of a nonmagnetic material with good electrical conductivity, e.g. oxygen-free copper. The magnetic shielding body  60  is arranged on the peripheral surface of the center core  58 . The magnetic shielding body  60  is rotatable between the shielding position and the retracted position with the rotation of the center core  58 . The magnetic shielding body  60  is located in the magnetic path formed by the fixing belt  48 , the side cores  56 , the arch cores  54  and the center core  58 , i.e. faces the fixing belt  48 , and shields or suppresses the magnetic flux when being at the shielding position ( FIG. 3 ). On the other hand, the magnetic shielding body  60  is retracted from the magnetic path, i.e. moves to a position not facing the fixing belt  48 , and neither shields nor suppresses the magnetic flux when being at the retracted position ( FIG. 2 ). 
     With reference to  FIG. 5 , the center core  58  is actually divided into a plurality of pieces in the axial direction and composed of a first center core  581  (central core portion) and a pair of second center cores  582  (outer core portions). The first center core  581  is arranged around the center core shaft  59  over the entire circumference in an axial central part of the center core shaft  59 . The first center core  581  forms a magnetic path in accordance with a first sheet width of sheets which pass through the nip portion NP ( FIG. 2 ). The second center cores  582  are adjacent to the first center core  581  at axially outer sides and arranged around the center core shaft  59  while being partially exposed in the circumferential direction ( FIG. 10 ). The second center cores  582  form a magnetic path in accordance with a second sheet width larger than the first sheet width together with the first center core  581 . 
     Note that the first center core  581  and the second center cores  582  are formed by adjacently arranging a plurality of cylindrical tube members mounted on the peripheral surface of the center core shaft  59  in the axial direction as shown in  FIG. 5 . Note that the first center core  581  and the second center cores  582  may also be formed by mounting a plurality of arched members on the peripheral surface of the center core shaft  59 . 
     The magnetic shielding body  60  is arranged in the same areas as the second center cores  582  at least in the axial direction. As shown in  FIG. 10 , the magnetic shielding body  60  is arranged to cover areas different from areas where the second center cores  582  are exposed in the circumferential direction. In other words, the areas where the second center cores  582  are exposed to a radially outer side are set by covering parts of the peripheral surfaces of the second center cores  582  by the magnetic shielding body  60 . 
     The division of the center core  58  and the magnetic shielding body  60  is determined according to the size of a sheet to be passed through the nip portion NP. Specifically, an axial dimension from a front end side of the first center core  582  to a rear end side thereof is determined to correspond to sheets having the first sheet width size (e.g. A4 sheets longitudinal). On the other hand, an axial dimension from the second center core  582  on a front end side to the second center core  582  on a rear end side is determined to correspond to sheets having the second sheet width size larger than the first sheet width (e.g. A3 sheets). Note that the center core  58  and the magnetic shielding body  60  may be divided in a plurality of patterns in the circumferential direction and the axial direction in accordance with further different sheet widths. 
     Note that the plurality of arch cores  54  are arranged along the axial direction of the center core  58  as described above and an arrangement density of the pairs of arch cores  54  is determined according to a magnetic flux density (magnetic field intensity) distribution of the coil  52 . By appropriately arranging the arch cores  54  and the side cores  56 , the magnetic flux density distribution (temperature difference) in the axial direction of the heat roller  46  (width direction of the fixing belt  48 ) is uniformized. 
     The fan  65  ( FIG. 4 ) is a fan mounted on the unillustrated frame of the fixing device  14 . The fan  65  generates an air flow flowing from the front side toward the rear side in the internal space AF of the center core shaft  59  by being rotated by the unillustrated controller. 
     Further, the coil unit  50  includes an introducing portion ( FIG. 6 ) and the drive transmission member  593  (transmitting portion). Further, with reference to  FIG. 11 , the driving unit  50 M includes the motor M and a transmission gear MG. 
     The introducing portion  590  is mounted on a front end part (one axial end side) of the center core shaft  59  and introduces an air flow into the internal space AF. The introducing portion  590  includes a front bearing  591  (bearing member) and an introducing tube  592  (introducing tube). The front bearing  591  is made of a hollow cylindrical heat resistant material. The front bearing  591  rotatably supports the front end part of the center core shaft  59 . With reference to  FIG. 6 , the front bearing  591  is mounted in a U-shaped front supporting portion  50 L 1  formed on the front side of the supporting unit  50 L. An end part of the center core shaft  59  slightly projecting from the front end part of the center core  58  (magnetic shielding body  60 ) is inserted into the front bearing  591 . As a result, the center core shaft  59  is rotatably supported on the front bearing  591  in a state where the front bearing  591  is fixed. Thus, even if the center core shaft  59  and the front bearing  591  rub against each other with the rotation of the center core shaft  59 , the deformation and breakage of the front bearing  591  are prevented since the front bearing  591  is made of the heat resistant material. Note that a cylindrical interior of the front bearing  591  communicates with the internal space AF. 
     The introducing tube  592  is mounted on a small-diameter portion  591 A of the front bearing  591  and introduces the air flow toward the internal space AF via the front bearing  591 . A front end side of the introducing tube  592  communicates with the fan  65 . Since the introducing tube  592  is formed of a deformable elastic tube, a later-described introduction path for the air flow is easily arranged. 
     The drive transmission member  593  is coupled to the motor M via the transmission gear MG and mounted on a rear end side (other axial end side) of the center core shaft  59 . The drive transmission member  593  transmits a rotational drive force of the motor M to the center core shaft  59 . The drive transmission member  593  is formed of a substantially hollow cylindrical member and has a communication space  593 S ( FIG. 9B ) inside. When the drive transmission member  593  is mounted on the center core shaft  59 , the communication space  593 S communicates with the internal space AF. Note that the communication space  593 S extends in the axial direction from a front end part of the drive transmission member  593  to an area facing exhaust ports  593 H to be describe later from a radially inner side. 
     With reference to  FIGS. 9A and 9B , an end part of the center core shaft  59  slightly projecting from the rear end part of the center core shaft  58  (magnetic shielding body  60 ) is inserted into the drive transmission member  593 . At this time, a pair of projections  593 T formed on a front end part of an inner peripheral part of the drive transmission member  593  are engaged with a pair of cut portions  59 T formed on the rear end part of the center core shaft  59 . As a result, the center core shaft  59  and the drive transmission member  593  are made integrally rotatable. Note that the drive transmission member  593  is rotatably supported in a substantially U-shaped rear supporting portion  50 L 2  ( FIG. 8 ) formed on a rear end part of the supporting unit  50 L. Further, the drive transmission member  593  includes a gear portion  593 G ( FIG. 11 ) and the exhaust ports  593 H. 
     The gear portion  593 G is a gear fixed to a rear end part of the drive transmission member  593 . The gear portion  593 G is engaged with the transmission gear MG to be described later. 
     The exhaust port  593 H is a long and narrow opening communicating with the communication space  593 S and open on the outer peripheral surface of the drive transmission member  593 . A plurality of exhaust ports  593 H are arranged in the circumferential direction as shown in  FIG. 11 . The respective exhaust ports  593 H are arranged while being displaced in the axial direction. Thus, the air flow ejected from each exhaust port  593 H can cool members around the drive transmission member  593  in a range wide in the axial direction. 
     With reference to  FIG. 7 , the exhaust ports  593 H rotate about the center core shaft  59  above an end adjacent area  52 A of the coil  52 . Here, the end adjacent area  52 A is an area of the coil  52  adjacent to the axial rear end part of the center core  58  in the axial direction and a part where a coil wire of the coil  52  extends in the lateral direction. 
     The motor M ( FIG. 11 ) is a motor for generating a rotational drive force for rotating the center core  58  (center core shaft  59 ). The motor M includes an output shaft MH. The transmission gear MG is a gear rotatably supported in the driving unit  50 M. The transmission gear MG is coupled to the output shaft MH of the motor M and transmits the rotational drive force to the drive transmission member  593 . The transmission gear MG is engaged with the gear portion  593 G of the drive transmission member  593 . 
     When an image forming operation is started in the image forming apparatus  1 , the center core  58  is rotated according to the sheet width of a sheet to be used. As a result, the center core  58  is arranged in the magnetic path in accordance with a sheet passage area and the magnetic shielding body  60  is arranged in sheet non-passage areas. The fixing belt  48  is induction-heated by the magnetic flux passing along the magnetic path formed by the fixing belt  48 , the arch cores  54 , the side cores  56  and the center core  58 . As a result, a toner image is fixed to the sheet passing through the nip portion. Further, unnecessary temperature increases of the fixing belt  48  and the coil unit  50  in the sheet non-passage areas are prevented. Particularly, when the magnetic shielding body  60  enters the magnetic path with the rotation of the center core  58 , the magnetic flux axially outwardly of the first center core  581  is shielded or suppressed. Accordingly, when a sheet having the first sheet width passes through the nip portion NP, temperature increases of the coil  52  and the center core  58  in the sheet non-passage areas are suppressed. On the other hand, with the rotation of the center core  58 , the second center cores  582  enter the magnetic path, thereby forming the magnetic path in accordance with the second sheet width. As a result, the fixing process can be applied to sheets having different widths while suppressing an excessive temperature increase of each member of the fixing device  14 . 
     Further, in this embodiment, the unillustrated controller rotates the fan  65  ( FIG. 4 ), whereby the air flow flows in the internal space AF ( FIG. 10 ) of the center core shaft  59 . As a result, the center core  58  and the magnetic shielding body  60  are cooled via a wall surface of the center core shaft  59 . Accordingly, a temperature increase of the center core  58  due to the heat generation of the center core  58  itself is prevented and a temperature increase of the coil  52  arranged around the center core  58  due to radiation heat from the fixing belt  48  is suppressed. This prevents insulation breakdown due to the melting of a coating of the coil wire (Litz wire) of the coil  52  and the breakage of the coil  52  associated with a temperature increase. Further, the breakage of the alternating current bias power supply V ( FIG. 2 ) due to an abnormal change of an inductance value of the coil  52  is prevented. As just described, in this embodiment, a cooling mechanism for cooling the center core  58  and the coil  52  is arranged in a compact manner utilizing the interior of the center core shaft  59  of the center core  58 . 
     Further, according to the above configuration, the air flow is introduced into the front end side (one end side) of the internal space AF of the center core shaft  59  by the introducing portion  590  ( FIG. 6 ). Further, the air flow is exhausted from the exhaust ports  593 H of the drive transmission member  593  arranged on the rear end side (other end side) of the center core shaft  59 . Thus, the flow of air in one direction can be formed in the internal space AF and the center core  58  can be stably cooled. Further, as the exhaust ports  593 H rotate about the center core shaft  59 , the air flow coming out from the exhaust ports  593 H is blown to the end adjacent area  52 A ( FIG. 7 ) of the coil  52 . As a result, parts of the coil  52  axially outwardly of the center core  58  can be effectively cooled. 
     Furthermore, in this embodiment, with reference to  FIG. 11 , in the case where a sheet having the first sheet width passes through the nip portion NP ( FIG. 3 ), the second center cores  582  are retracted upwardly from the magnetic path and the magnetic shielding body  60  is arranged in the magnetic path when the center core  58  is rotated about the center core shaft  59 . Specifically, the center core  58  of  FIG. 11  is rotated by 180° about the center core shaft  59 . At this time, one exhaust port  593 H is arranged to face the end adjacent area  52 A of the coil  52 . The air flow ejected downwardly from the above exhaust port  593 H (arrow DF of  FIG. 11 , actually downward facing) is exhausted from the exhaust duct  50 H 1  ( FIGS. 4 and 12 ) after cooling the coil wire around the end adjacent area  52 A. Thus, parts of the center core shaft  59 , the center core  58  and the coil  52  corresponding to the sheet non-passage areas can be effectively cooled. Thus, even if heat is transferred to the center core  58  and the coil  52  from the sheet non-passage areas of the fixing belt  48  heated by the previously performed fixing operation, temperature increases of the center core  58  and the coil  52  can be suppressed. Further, since the exhaust duct  50 H 1  is formed on the same rear end side as the exhaust ports  593 H, the air flow ejected from the exhaust ports  593 H can be quickly exhausted from the fixing device  14 . 
     Next, a coil unit  50 P according to a second embodiment of the present invention is described with reference to  FIGS. 13A, 13B and 14 . The coil unit  50 P is mounted in the fixing device  14  similarly to the coil unit  50  according to the previous embodiment.  FIGS. 13A and 13B  are perspective views showing a state in the coil unit  50 P.  FIG. 14  is an enlarged perspective view showing a peripheral surface of a center core  58 P of the coil unit  50 P. This embodiment differs from the coil unit  50  according to the previous first embodiment in that the center core  58 P and a magnetic shielding body  60 P include hole portions ( 58 HP,  59 HP,  60 HP). Thus, the description is centered on this point of difference and common points are not described. Further, in  FIGS. 13A, 13B and 14 , members having structures and functions common to the coil unit  50  of the previous first embodiment are denoted by reference signs obtained by adding P in the ends of the same reference signs as in the first embodiment. 
     With reference to  FIGS. 13A, 13B and 14 , the hole portions  58 HP and  60 HP are formed on the peripheral surfaces of the center core  58 P and the magnetic shielding body  60 . These hole portions communicate with an internal space of a center core shaft (not shown) and are open on the peripheral surface of the center core  58 P or the magnetic shielding body  60 P through a wall surface of the center core shaft between front and rear end parts of the center core shaft. Further, as shown in  FIGS. 13A and 13B , a plurality of hole portions  58 HP and  60 HP are arranged at intervals in the circumferential and axial directions. Specifically, a plurality of hole portions  58 HP are formed on peripheral surfaces of a first center core  581 P and second center cores  582 P of the center core  58 P. Note that the hole portion  58 HP is formed of a cut portion formed on one adjacent end edge of the first center core  581 P or the second center core  582 P formed of a cylindrical tube member ( FIG. 14 ). Thus, the hole portions can be formed utilizing clearances between adjacent first and second center cores  581 P and  582 P and an air flow can be caused to flow out. Note that the hole portions  59 HP ( FIG. 14 ) formed on the center core shaft are facing the respective hole portions  58 HP and  60 HP from a radially inner side. 
     According to such a configuration, a part of the air flow flowing in the internal space of the center core shaft can be caused to flow out to outer sides of the center core  58 P and the magnetic shielding body  60 P from the hole portions  58 HP and  60 HP. The air flow flowing out cools a coil (not shown) arranged to surround the center core  58 P. Thus, an effect of cooling the center core  58 P and the coil can be increased in a wide range. Particularly, the plurality of hole portions  58 HP and  60 HP are arranged at intervals in the circumferential and axial directions. Thus, the effect of cooling the center core  58 P and the coil can be further increased. 
     Next, a coil unit  50 Q according to a third embodiment of the present invention is described with reference to  FIGS. 15A and 15B . The coil unit  50 Q is mounted in the fixing device  14  similarly to the coil unit  50  according to the previous embodiment.  FIGS. 15A and 15B  are perspective views showing a state in the coil unit  50 Q. This embodiment differs from the coil unit  50 P according to the previous second embodiment in the arrangement of hole portions  58 HQ. Thus, the description is centered on this point of difference and common points are not described. Further, in  FIGS. 15A and 15B , members having structures and functions common to the coil unit  50  of the previous first embodiment are denoted by reference signs obtained by adding Q in the ends of the same reference signs as in the first embodiment. 
     With reference to  FIGS. 15A and 15B , the hole portions  58 HQ and  60 HQ are formed on peripheral surfaces of second center cores  582 Q of a center core  58 Q and a magnetic shielding body  60 . On the other hand, no hole portion is formed on a peripheral surface of a first center core  581 Q. In other words, the hole portions  58 HQ and  60 HQ are arranged in areas axially outwardly of the first center core  581 Q. An area corresponding to the first center core  581 Q serves as a sheet passage area when a sheet passes through the nip portion NP of the fixing device  14  regardless of a sheet width. Thus, a fixed amount of heat is constantly transferred from a belt surface of the fixing belt  48  facing the first center core  581 Q. On the other hand, the areas axially outwardly of the first center core  581 Q may serve as sheet non-passage areas and heat from the belt surface of the fixing belt  48  is less likely to be consumed. Thus, the center core  58 Q and a coil  52 Q tend to be warmed by the fixing belt  48 . Thus, by arranging the hole portions at axially outer sides of the first center core  581 Q as described, the second center cores  582 Q, the magnetic shielding body  60 Q and, further, the surrounding coil  52 Q can be effectively cooled. 
     The fixing device  14  and the image forming apparatus  1  provided with the same according to the embodiments of the present embodiment have been described above. According to these inventions, a temperature increase of the center core  58 ,  58 P,  58 Q is prevented and a temperature increase of the coil (magnetic flux generation source) arranged around each center core is suppressed. As a result, the process of fixing the toner to the sheet is stably realized. Note that the present invention is not limited to these embodiments and, for example, the following modifications can be adopted. 
     (1) Although the air flow is introduced from the introducing tube  592  arranged on the front end side of the center core shaft  59 , discharged from the drive transmission member  593  arranged on the rear end part and exhausted from the exhaust duct  50 H 1  formed on the rear side of the shield  50  in the above embodiments, the present invention is not limited to this.  FIG. 16  is a perspective view of a coil unit  50 R according to a modification of the present invention and  FIG. 17  is a sectional view of the coil unit  50 R. Note that, in  FIGS. 16 and 17 , members having structures and functions common to the coil unit  50  of the previous first embodiment are denoted by reference signs obtained by adding R in the ends of the same reference signs as in the first embodiment. 
     With reference to  FIG. 16 , the coil unit  50 R includes an exhaust duct  50 H 2  open on a shield  50 HR. The exhaust duct  50 H 2  is open on a front end part of the shield  50 HR. Thus, as shown in  FIG. 17 , an air flow discharged from an exhaust port  593 HR after flowing in an internal space AF of a center core shaft  59 R is exhausted to the outside of the coil unit  50 R from the exhaust duct  50 H 2  after being guided forward along an inner wall of the shield  50 HR. At this time, a peripheral surface of a center core  58 R and an unillustrated coil are effectively cooled by the air flow flowing forward from the exhaust port  593 HR. 
     (2) Although the hole portions are arranged axially outwardly of the first center core  581 Q in the above third embodiment, the present invention is not limited to this. As in the second embodiment, the hole portions may be formed on the first center core  581 P, the second center cores  582 P and the magnetic shielding body  60 P and the number of the hole portions arranged axially outwardly of the first center core  581 P may be set larger than the number of the hole portions arranged on the first center core  581 P. Even in this case, parts of the center core  58 P and the coil  52 P corresponding to the sheet non-passage areas can be effectively cooled when a sheet having the first sheet width passes through the nip portion NP. 
     (3) Further, although the above first embodiment is described using the fixing belt  48  as the first rotary body, the present invention is not limited to this. The first rotary body to be induction-heated by the coil unit  50  may be a rotary roller body. 
     (4) Further, although the air flow is ejected from the exhaust ports  593 H open on the drive transmission member  593  in the above first embodiment, the present invention is not limited to this. Similar openings may be formed on the front bearing  591  on the front end side of the center core shaft  59  or on the introducing tube  592 . In this case, an end adjacent area of the coil  51  located on the front side of the center core shaft  59  can be effectively cooled. 
     (5) Further, although the air flow flowing into the internal space AF of the center core shaft  59  is generated by the fan  65  in the above first embodiment, the present invention is not limited to this. A fan may be arranged near the exhaust duct  50 H 1  of the shield  50 H as another air flow generator. Further, another air flow generator may be arranged at another position. 
     (6) Further, although the magnetic shielding body  60  is arranged on the peripheral surface of the center core  58  in the above embodiments, it may be directly arranged on the peripheral surface of the center core shaft  59 .