Electromagnetic induction heating fixing apparatus and image forming apparatus having the same

A fixing apparatus includes: a magnetic flux generating source which generates a magnetic flux; an endless belt which inductively generates heat by the magnetic flux while rotating in a prescribed direction; a rotating body which rotates in a prescribed direction and, together with the belt, forms a nip section through which a recording medium carrying a toner image passes; a core which is made of a magnetic material, and directs the magnetic flux to the belt; a heat value adjustment member for adjusting an amount of heat generated in the belt; and a gripping piece which is a non-rotating member, is disposed in a position corresponding to the nip section, and contacts an inner surface of the belt to rotatably grip the belt against the rotating body. The belt is wrapped between the heat value adjustment member and the gripping piece.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a fixing apparatus which heats a toner image that has been transferred to a recording medium and thereby fixes the toner image onto the recording medium, and to an image forming apparatus using such a fixing apparatus.

2. Description of the Related Art

The basic constituent elements of an image forming apparatus such as a copying machine, a facsimile machine or a printer include: an image forming section which forms a toner image on an image carrying body (for example, a photosensitive drum); a transfer unit which transfers the toner image on the image carrying body to a sheet, which is one example of a recording medium; and a fixing apparatus which heats and fixes the toner image transferred to the sheet, onto the sheet.

A growing number of fixing apparatuses employ a belt system in which the heat capacity can be set to a low value with a view to shortening the warm-up time at apparatus start-up and reducing energy consumption, etc. Furthermore, attention has been drawn to electromagnetic induction heating (IH) systems, which are capable of very fast heating and high-efficiency heating. An electromagnetic induction heating system excites an induction current in a heating roller or fixing belt, by means of a magnetic flux generated by passing a high-frequency current through an induction coil, and uses the resistance of the actual heating roller or fixing roller itself to produce Joule heat in the heating roller or fixing belt (by induction heating). By means of this Joule heat, the toner image is fixed to a sheet (recording medium) in a nip section between the fixing roller (or fixing belt) and a pressurization roller. A fixing apparatus which combines an electromagnetic induction heating system and a belt system has been developed as a product.

A conventional fixing apparatus which combines an electromagnetic induction heating system and a belt system includes: a fixing belt; a pressurization roller which, together with the fixing belt, forms a nip section through which a sheet carrying a toner image is passed; a fixing roller and a heating roller about which the fixing belt is wrapped; and a coil unit, disposed in a position opposing the heating roller, which lets the fixing belt generate heat by induction heating.

The coil unit includes a plurality of cores which form magnetic paths along which the magnetic flux generated by the coil passes, and a magnetic shielding plate is installed on a center core of these cores. The position of the magnetic shielding plate is switched between a shielding position where the plate is disposed in the magnetic path and shields the magnetic flux, and a withdrawn position where the plate is withdrawn from the magnetic path and does not shield the magnetic flux, in accordance with the amount of rotation of the center core. By appropriately switching the position of the magnetic shielding plate between the shielding position and the withdrawn position in accordance with the size of the sheet which is passed through the nip section, overheating of the fixing belt outside the paper passage region, where the sheet is not in contact with the fixing belt, is suppressed.

However, although a conventional fixing apparatus is able to suppress overheating outside the paper passage region of the fixing belt, it is difficult to shorten the warm-up time of the fixing belt. Since the fixing roller and the heating roller which are wrapped about the fixing belt have a high heat capacity, then a large amount of heat is transferred from the fixing belt that generates heat by induction heating, to the fixing roller and the heating roller. If the amount of heat transfer is large, then the time required until the fixing belt is sufficiently heated increases.

SUMMARY OF THE INVENTION

The object of the present disclosure is to provide a function for suppressing overheating of a fixing belt, while shortening a belt warm-up time.

The fixing apparatus relating to one aspect of the present disclosure which achieves this object is a fixing apparatus including: a magnetic flux generating source which generates a magnetic flux; an endless belt which inductively generates heat by the magnetic flux while rotating in a prescribed direction; a rotating body which rotates in a prescribed direction and, together with the belt, forms a nip section through which a recording medium carrying a toner image passes; a core which is made of a magnetic material, and directs the magnetic flux to the belt; a heat value adjustment member for adjusting an amount of heat generated in the belt; and a gripping piece which is a non-rotating member, is disposed in a position corresponding to the nip section, and contacts an inner surface of the belt to rotatably grip the belt against the rotating body; wherein the belt is wrapped between the heat value adjustment member and the gripping piece.

Furthermore, an image forming apparatus relating to a further aspect of the present disclosure includes: an image forming unit which forms a toner image; a transfer unit which transfers the toner image formed by the image forming unit onto a recording medium; and a fixing apparatus which fixes the toner image onto the recording medium; the fixing apparatus having the composition described above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present disclosure will be described in detail with reference to the drawings.FIG. 1is a schematic cross-sectional drawing showing the composition of an image forming apparatus1according to one embodiment. The image forming apparatus1may be implemented as a printer, a copying machine, a facsimile machine, or a multi-functional peripheral combining the functions of these, which performs printing by transferring a toner image onto a surface of a paper sheet T, which is one example of a recording medium, on the basis of image information input from an external source, for example.

The image forming apparatus1shown inFIG. 1is a tandem type color printer. The image forming apparatus1includes a square box-shaped apparatus main body2, inside which a color image is formed on a sheet T. An output tray3for receiving a sheet T on which a color image has been printed is provided in the upper surface section of the apparatus main body2. A paper supply cassette5which accommodates sheets T is provided in the lower portion inside the apparatus main body2. Furthermore, a stacking tray6for supplying sheets T manually is provided in the right side face of the apparatus main body2as seen inFIG. 1. An image forming section7is provided in the upper portion of the apparatus main body2and the image forming section7forms an image on a sheet T on the basis of image data, such as text characters, pictures, and the like, transmitted from an external source.

A first conveyance path9for conveying a sheet T fed from the paper supply cassette5to the image forming section7is provided in a left position of the image forming section7inFIG. 1. A second conveyance path10for guiding a sheet T loaded on the stacking tray6, to the first conveyance path9, is provided in a position above the paper supply cassette5. Pairs of conveyance rollers43for conveying a sheet T are provided respectively in the first conveyance path9and the second conveyance path10. Furthermore, a fixing apparatus14which applies a fixing process to a sheet T on which a toner image has been formed by the image forming section7, and a third conveyance path11for conveying a sheet T which has undergone a fixing process, to the output tray3, are provided in the upper left portion of the interior of the apparatus main body2.

The paper supply cassette5can be inserted into and removed from the apparatus main body2, and has an accommodating unit16. The accommodating unit16is capable of selectively accommodating at least two types of sheets T having different sizes in the paper supply direction. Sheets T accommodated in the accommodating unit16are fed to the first conveyance path9, one sheet at a time, by a paper supply roller17and a separating roller pair18.

The stacking tray6can be opened and closed with respect to the apparatus main body2, and sheets T are disposed on a manual feed surface19of this stacking tray6. Sheets T loaded on the manual feed surface19are fed to the second conveyance path10, one sheet at a time, by a pick-up roller20and a separating roller pair21.

The first conveyance path9and the second conveyance path10converge before a resist roller pair22. A sheet T which has been conveyed to the resist roller pair22waits provisionally in a state of abutting against the resist roller pair22, and after skew adjustment and timing adjustment, is fed toward a secondary transfer unit23(transfer unit). In the secondary transfer unit23, a full-color toner image on an intermediate transfer belt40is secondarily transferred onto the sheet T which has been fed in this way. Thereupon, the sheet T on which the toner image has been fixed by the fixing apparatus14is inverted in a fourth conveyance path12, if necessary, and a full-color toner image is also secondarily transferred onto the opposite surface of the sheet T in the secondary transfer unit23. After the toner image on the opposite surface has been fixed by the fixing apparatus14, the sheet T passes along the third conveyance path11and is output to the output tray3by an output roller pair24.

The image forming section7includes four image forming units26to29which form respective toner images of black (Bk), yellow (Y), cyan (C) and magenta (M), and an intermediate transfer unit30which carries, in mutually superimposed fashion, the toner images of the respective colors formed by the image forming units26to29.

The image forming units26to29each include: a photosensitive drum32(image carrying body); a charger33which is disposed so as to oppose the circumferential surface of the photosensitive drum32; a laser scanning unit34which irradiates a laser beam onto a specific position on the circumferential surface of the photosensitive drum32on the downstream side of the charger33in terms of the direction of rotation of the photosensitive drum32; a developing apparatus35which is disposed so as to oppose the circumferential surface of the photosensitive drum32on the downstream side of the laser beam irradiation position from the laser scanning unit34in terms of the direction of rotation of the photosensitive drum32; and a cleaner36which is disposed so as to oppose the circumferential surface of the photosensitive drum32on the downstream side of the developing apparatus35in terms of the direction of rotation of the photosensitive drum32.

The photosensitive drums32of the image forming units26to29rotate in the counter-clockwise direction in the drawings, by means of a drive motor which is not illustrated. The developing apparatuses35of the respective image forming unit26to29each include a development vessel51which accommodates a two-component developer, respectively containing black toner, yellow toner, cyan toner and magenta toner.

The intermediate transfer unit30includes: a drive roller38which is disposed in a position in the vicinity of the image forming unit26; a driven roller39which is disposed in a position in the vicinity of the image forming unit29; a tension roller42which is disposed in a position between the drive roller and the driven roller39; an intermediate transfer belt40which is disposed about the drive roller38, the driven roller39and the tension roller42; and four transfer rollers41which are disposed so as to be able to press against the photosensitive drums32of the respective image forming units26to29, via the intermediate transfer belt40.

In the intermediate transfer unit30, toner images of respective colors are transferred in a mutually superimposed state from the photosensitive drums32, onto the intermediate transfer belt40, at the positions of the transfer rollers41of the image forming units26to29, thereby forming a full-color toner image.

Viewed in terms of the sheet conveyance direction, a conveyance path72is provided on the upstream side and the downstream side of the fixing apparatus14. A sheet T which is conveyed via the secondary transfer unit23passes along the upstream-side conveyance path72and is guided to the fixing apparatus14. A sheet T which has undergone a fixing process passes along the downstream-side conveyance path72and is guided to the third conveyance path11.

The third conveyance path11guides a sheet T which has undergone a fixing process in the fixing apparatus14, to the output tray3. A conveyance roller pair71for conveying the sheet T to the output tray3is provided in the third conveyance path11, and furthermore the output roller pair24described above is provided at the outlet of the third conveyance path11.

First Embodiment

Next, a fixing apparatus14relating to a first embodiment is described with reference toFIG. 2.FIG. 2is a vertical cross-sectional diagram of a fixing apparatus14. The fixing apparatus14carries out a fixing process for fixing a toner image to a sheet T, by applying heat and pressure to the toner image which has been transferred to the sheet T. The fixing apparatus includes a pressurization roller44(rotating body), a fixing belt45(endless belt), a gripping piece49, a heat value adjustment member46, and a coil unit50.

The pressurization roller44is a roller member capable of rotating in the counter-clockwise direction inFIG. 2, and is constituted by a tubular stainless steel core member47, a silicone rubber elastic layer48which is laminated onto the core member47, and a PFA surface separating layer (not illustrated), which is laminated onto the elastic layer48. A heat source, such as a halogen heater, may be arranged inside the core member47. The elastic layer48can be heated by this heat source.

The fixing belt45is an endless belt which is wrapped about the gripping piece49and the heat value adjustment member46, and is capable of rotating in the clockwise direction inFIG. 2. The pressurization roller44is pressed towards the fixing belt45by a biasing member (not illustrated), and a nip section NP through which a sheet T carrying a toner image passes is formed between the pressurization roller44and the fixing belt45. The fixing belt45has a width dimension in a direction perpendicular to the conveyance direction of the sheet T passing through the nip section NP.

The fixing belt45includes an electroplated nickel base member which faces the interior of the fixing belt45, a silicone rubber elastic layer which is laminated onto the base member, and a PFA surface separating layer, which is layered on the elastic layer. The thickness of the base member is 30 to 50 μm, for example, and the thickness of the elastic member is 200 to 500 μm, for example. The thickness of the surface separating layer is approximately 30 μm, for example.

The gripping piece49is disposed inside the fixing belt45and contacts the inner surface of the fixing belt45(in other words, the base member), at a position corresponding to the nip section NP, thereby gripping the fixing belt45in rotatable fashion, against the pressurization roller44. The gripping piece is a flat plate-shaped member which extends along the nip section NP in parallel with the pressurization roller44, and has a width direction dimension extending along the conveyance direction of the sheet T. A portion of the circumferential surface of the pressurization roller44deforms elastically in a flat shape due to being pressed against the flat plate-shaped gripping piece49, and a portion of the fixing belt45also deforms to a flat planar shape following the flat plate-shaped gripping piece49. By this means, a straight line-shaped nip section NP having a prescribed length in the conveyance direction of the sheet T is formed. The width direction dimension of the gripping piece49in a longitudinal cross-section is set in such a manner that the nip section NP has a sufficient nip width in the conveyance direction of the sheet T, as well as so as to enable the gripping piece to exert sufficient gripping force against the fixing belt45. Furthermore, the gripping piece49is a member which is in a fixed non-rotating position, in contrast to the roller member.

The gripping piece49serves to rotatably support the fixing belt45in a state of contact with the base member of the fixing belt45, and therefore the material of the gripping piece49is selected from materials which give suitable rigidity to the gripping piece49with respect to the fixing belt45. Furthermore, surface treatment may also be applied to the surface of the gripping piece49in order to lower the friction between the gripping piece49and the fixing belt45.

The gripping piece49need only be a member capable of exerting sufficient gripping force with respect to the fixing belt45, and therefore may be formed to small dimensions. Consequently, the heat capacity of the gripping piece49can be reduced. The gripping piece49is not limited to a flat plate shape, provided that it is capable of ensuring a sufficient nip width and exerting sufficient gripping force with respect to the fixing belt45.

The heat value adjustment member46is a member which is disposed inside the fixing belt45, and which adjusts the amount of heat generated in the fixing belt45by adjusting the amount of magnetic flux guided from the coil unit50to the fixing belt45. The heat value adjustment member46is also used as a member about which the fixing belt45is wrapped. In the first embodiment, the heat value adjustment member46includes a base member51, a magnetic shielding plate52, and a coating layer53.

The base member51is a member having a cylindrical shape which is made of a magnetic material, such as iron or stainless steel, and extends in parallel with the pressurization roller44and is disposed in a position opposing the pressurization roller44via the nip section NP and the gripping piece49. The base member51is made of a magnetic material, and therefore is able to generate heat by the magnetic flux from the coil unit50. The base member51is a thin member having a thickness of 0.3 to 1.0 mm, for example.

The base member51is composed so as to be rotatable in a prescribed direction. The heat value adjustment member46sets the position of the fixing belt45with respect to the coil unit50by contacting the inner surface of the fixing belt45from the opposite direction to the direction in which the gripping piece49applies a gripping force to the fixing belt45. The fixing belt has intrinsic rigidity, and therefore the fixing belt45is supported by the heat value adjustment member46in a position opposing the center core60of the coil unit50, which is described below, and in a position in the vicinity thereof. Therefore, the fixing belt45does not contact the outer circumferential surface of the base member51(coating layer53) except for at the position where the base member51opposes the center core60and a position in the vicinity thereof.

The fixing belt45which is wrapped about the heat value adjustment member46and the gripping piece49is driven to rotate in the clockwise direction due to the pressurization roller44rotating in the counter-clockwise direction inFIG. 2by means of a drive source, which is not illustrated.

The magnetic shielding plate52is a thin plate-shaped member made of a non-magnetic material having high conductivity, such as copper or aluminum, and is installed on the outer circumferential surface of the base member51. The thickness of the magnetic shielding plate52is 0.3 to 1.0 mm, for example. The position of the magnetic shielding plate52is switched between a shielding position and a restricted shielding position, in accordance with rotation of the base member51.FIG. 2shows a state where the magnetic shielding plate52is positioned in the restricted shielding position andFIG. 3shows a state where the magnetic shielding plate52is positioned in the shielding position. When positioned in the shielding position, the magnetic shielding plate52is moved to a position near the coil unit50, and in particular, a position in the vicinity of the center core60, and thereby shields or suppresses the magnetic flux. On the other hand, when positioned in the restricted shielding position, the magnetic shielding plate52is in a position which is distant from the center core60. Therefore, the shielding of the magnetic flux is weakened. The magnetic shielding plate52cancels out the magnetic flux which seeks to pass through the fixing belt45, by generating a reverse magnetic flux when the magnetic flux from the center core60passes through the fixing belt45.

FIG. 4is a perspective diagram of the heat value adjustment member46. As shown inFIG. 4, a magnetic shielding plate52is installed on the outer circumferential surface of the base member51in the respective end portions in the axial direction. The pair of magnetic shielding plates52have laterally symmetrical shapes, and each magnetic shielding plate52is designed in such a manner that the circumferential dimension thereof in the circumferential direction of the base member51gradually becomes smaller in the inward axial direction of the base member51. More specifically, the magnetic shielding plates each include: a large shielding portion52alocated in the endmost portion of the axial direction of the base member51; a medium shielding portion52blocated in the inward axial direction of the base member51from the large shielding portion52a; and a small shielding portion52clocated in the inward axial direction of the base member51from the medium shielding portion52b. The large shielding portion52a, the medium shielding portion52band the small shielding portion52care formed in an integrated fashion.

The large shielding portions52a, the medium shielding portions52band the small shielding portions52ccorrespond respectively to the width dimensions of sheets T which pass through the nip section NP (the sizes of the sheets T in the direction perpendicular to the conveyance direction of the sheet T in the nip section NP). The distance between the pair of small shielding portions52ccorresponds to a sheet T1having a minimum width dimension (for example, an A5 sheet). The distance between the pair of medium shielding portions52bcorresponds to a sheet T2having a medium width dimension (for example, an A4 sheet (portrait)). The distance between the pair of large shielding portions52acorresponds to a sheet T3having a maximum width dimension (for example, an A3 sheet). Moreover, the respective circumferential dimensions of the large shielding portions52a, the medium shielding portions52band the small shielding portions52cin the circumferential direction of the base member51are set to dimensions which enable shielding of the magnetic flux directed to the fixing belt45by the center core60when the magnetic shielding plates52are positioned in the shielding position.

The base member51of the heat value adjustment member46also includes flanges56which close off the respective end portions in the axial direction, and rotating shaft members55which pass through the flanges56. By means of the rotating shaft member55rotating in a prescribed direction by means of a drive source (not illustrated), the position of the magnetic shielding plates52is switched between the shielding position and the restricted shielding position.

The coating layer53is formed over substantially the whole of the surface of the heat value adjustment member46which contacts the inner surface of the fixing belt45, in other words, the surface of the base member51and the surfaces of the magnetic shielding plates52. The coating layer53is made of fluorine resin and reduces the friction between the surface of the heat value adjustment member46and the inner surface of the fixing belt45. InFIG. 2andFIG. 3, the thickness of the coating layer53is depicted in exaggerated fashion.

The coil unit50serves to let the base member51of the fixing belt45generate heat by induction heating and includes a coil54(magnetic flux generating source), arch cores58, a pair of side cores59, and a center core60.

The coil54is a winding which is disposed so as to oppose the outer circumferential surface of the fixing belt45at a position opposite to the pressurization roller44with respect to the fixing belt45, and this winding has a linear portion following the width direction of the fixing belt45. The coil54is supported by a bobbin (not illustrated) in a state where the coil is separated by a prescribed distance from the fixing belt45. The wiring region of the coil54is set to a size which exceeds the width dimension of the fixing belt45. Furthermore, the coil54is connected to an AC bias power source V, and when an AC bias is applied to the coil54, the coil54generates a magnetic flux.

The arch cores58, the pair of side cores59and the center core60are ferrite cores which create a magnetic path along which the magnetic flux generated by the coil54passes. The arch cores58have an arch shape which extends through a range exceeding the winding region of the coil54. The arch cores58are held by a core holder made of heat-resistant resin (for example, PPS, PET, LCP), which is not illustrated.

The arch cores58have a pair of free ends58awhich are disposed on either side of the coil54and which extend in the direction of extension of coil54, and each of the pair of side cores59is connected to the corresponding free ends58a. The side cores59are also held by a core holder made of a heat-resistant resin, which is not illustrated.

The center core60is a core which is installed on the arch cores58so as to be disposed between the arch cores58and the fixing belt45from the viewpoint of the magnetic path. The center core60extends in the direction of extension of an arrangement area of arch cores58and opposes the fixing belt45in the region where the coil54is not present. The center core60guides the magnetic flux passing through the arch core58to the fixing belt45. The center core60and the magnetic shielding plates52of the heat value adjustment member46are situated in closest mutual proximity when the magnetic shielding plates52are in the shielding position.

Next, the fixing operation by the fixing apparatus14having the composition described above will be explained. When an AC bias is applied to the coil54from the AC bias power source V, the coil54generates a magnetic flux. The magnetic flux passes along a magnetic path formed between the fixing belt45, the side cores59, the arch cores58and the center core60. When the magnetic flux passes through the fixing belt45, an induction current is generated. When the induction current is passed through the fixing belt45, Joule heat is generated by the intrinsic resistance of the fixing belt45itself, in other words, induction heating occurs in the fixing belt45. The whole of the fixing belt45inductively generates heat as the belt rotates. Furthermore, the base member51of the heat value adjustment member46generates heat by the passage of magnetic flux.

Before causing the fixing belt45, and the like, to generate heat, the base member51of the heat value adjustment member46is rotated appropriately by rotational force applied to the rotational shaft member55, in accordance with the width dimension of the sheet T. By this means, the large shielding portions52ato the small shielding portions52cof the magnetic shielding plate52are switched between a shielding position where they are situated in the magnetic path and shield or suppress the magnetic flux, and a restricted shielding position where they are withdrawn from the magnetic path and shielding of the magnetic flux is weakened.

For example, in the case of a sheet T1having a minimum width dimension which is passing through the nip section NP, the base member51of the heat value adjustment member46is rotated in such a manner that all of the large shielding portions52ato the small shielding portions52cof the magnetic shielding plate assume a shielding position. Consequently, only the paper passage region of the fixing belt45which is in contact with the sheet T1in the nip section NP generates heat by induction heating without restriction of generating heat, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T1in the nip section NP, induction heating is restricted.

Moreover, in the case of a sheet T2having a medium width dimension which is passing through the nip section NP, the base member51is rotated in such a manner that the small shielding portions52cof the heat value adjustment member46assume a restricted shielding position, whereas the medium shielding portions52band the large shielding portions52aassume a shielding position. Consequently, only the paper passage region of the fixing belt45which is in contact with the sheet T2in the nip section NP generates heat by induction heating without restriction of generating heat, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T2in the nip section NP, induction heating is restricted.

Furthermore, in the case of a sheet T3having a maximum width dimension which is passing through the nip section NP, the base member51is rotated in such a manner that the small shielding portions52cand the medium shielding portions52bof the heat value adjustment member46assume a restricted shielding position, whereas the large shielding portions52aassume a shielding position. Consequently, only the paper passage region of the fixing belt45which is in contact with the sheet T3in the nip section NP generates heat by induction heating without restriction of generating heat, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T3in the nip section NP, induction heating is restricted.

In this way, by suitably rotating the heat value adjustment member46in such a manner that induction heating is restricted in the region outside the paper passage region on the fixing belt45, the small shielding portions52cto the large shielding portions52aare switched between a shielding position and a restricted shielding position. By this means, the amount of magnetic flux directed to the fixing belt45, in other words, the amount of heat generated in the fixing belt45is adjusted, and overheating of the region outside the paper passage region of the fixing belt45is suppressed. The positional relationship between the small shielding portions52cto the large shielding portions52ais set in such a manner that the small shielding portions52cto the large shielding portions52acan suitably be positioned at a shielding position or a restricted shielding position, in accordance with the sheets T1to T3.

When any one of the sheets T1, T2or T3enters into the nip section NP following the conveyance direction of sheet T, the toner image on any one of the sheet T1, T2or T3receives heat from the fixing belt45while being gripped between the fixing belt45and the pressurization roller44. By this means, the toner image is fixed onto the sheet.

A thermistor (not illustrated) is provided in a position in the vicinity of the fixing belt45. The thermistor detects the surface temperature of the fixing belt45. The surface temperature thus detected is sent to a control unit, which is not illustrated. The control unit controls the AC bias power source V on the basis of the surface temperature of the fixing belt45, and adjusts the density of the magnetic flux generated by the coil54.

According to the fixing apparatus14relating to the first embodiment described above, the fixing belt45is wrapped between a gripping piece49and a heat value adjustment member46. The gripping piece49is a non-rotating member which rotatably grips the fixing belt45against the pressurization roller44, at a position corresponding to the nip section NP, and it has a small heat capacity. The heat value adjustment member46is a member which, together with the gripping piece49, rotatably supports the fixing belt45while applying tension to the fixing belt45. Consequently, it is possible to reduce the amount of heat transferred to the other members from the fixing belt45which generates heat by using induction heating, compared to a conventional composition having a fixing belt which rotates by being wrapped about two roller members having a large heat capacity (for example, a fixing roller and a heating roller). Accordingly, it is possible to shorten the warm-up time of the fixing belt45. Furthermore, the heat value adjustment member46about which the fixing belt45is wrapped is a member which is used in order to adjust the amount of the magnetic flux directed to the fixing belt45, and therefore the fixing apparatus14has a function for suppressing overheating of the fixing belt45.

Moreover, a coating layer53is formed on the surface of the heat value adjustment member46, and therefore the slidability of the fixing belt45with respect to the heat value adjustment member46is improved. By this means, the fixing belt45can rotates smoothly.

Moreover, since the heat value adjustment member46is constituted by a thin base member51and thin plate-shaped magnetic shielding plates52, then the amount of heat transferred to the heat value adjustment member46from the fixing belt45generating heat by induction heating is small.

Furthermore, since the base member51of the heat value adjustment member46has a cylindrical shape, it is possible to rotate the magnetic shielding plates52in a 360° angular range. Therefore, the magnetic shielding plates52of the heat value adjustment member46can be switched easily between a shielding position and a restricted shielding position.

Second Embodiment

Next, a fixing apparatus140relating to a second embodiment is described with reference toFIG. 5.FIG. 5is a vertical cross-sectional diagram of a fixing apparatus140. Similarly to the fixing apparatus14according to the first embodiment, the fixing apparatus140includes a pressurization roller44, a fixing belt45, a gripping piece49, a heat value adjustment member61, and a coil unit50. In the fixing apparatus140according to the second embodiment, only the composition of the heat value adjustment member61differs from that of the fixing apparatus14according to the first embodiment, and therefore description of the other members is omitted here.

In the fixing apparatus140according to the second embodiment, the heat value adjustment member61adjusts the amount of heat generated in the fixing belt45by generating heat itself. The heat value adjustment member61includes a base member62and a heat generating plate63.

The base member62is a member having a cylindrical shape which is made of a non-magnetic resin material having high heat resistance, such as LCP, and extends in parallel with the pressurization roller44and is disposed in a position opposing the pressurization roller44via the nip section NP and the gripping piece49. The heat value adjustment member61is composed so as to be rotatable in a prescribed direction. The base member62is a thin member having a thickness of 2 mm, for example.

The heat value adjustment member61sets the position of the fixing belt45with respect to the coil unit50by contacting the inner surface of the fixing belt45from the opposite direction to the direction in which the gripping piece49applies a gripping force to the fixing belt45. The fixing belt45has intrinsic rigidity, and therefore the fixing belt45is supported by the heat value adjustment member61in a position opposing the center core60of the coil unit50and in a position in the vicinity thereof. Therefore, the fixing belt45does not contact the outer circumferential surface of the heat value adjustment member61except for at the position where the base member62opposes the center core60and a position in the vicinity thereof.

The heat generating plate63is a thin plate-shaped member made of a magnetic material, such as iron or stainless steel, and is installed on the outer circumferential surface of the base member62. The heat generating plate63has a property that the plate generates heat when a magnetic flux generated by the coil54is passed therethrough. The thickness of the heat generating plate63is 0.3 to 1.0 mm, for example. The heat generating plate63is attached to the base member62by, for example, embedding the heat generating plate63following the outer circumferential surface of the base member62. A coating layer (not illustrated) made of fluorine resin, for example, is formed on the surface of the heat value adjustment member61, in other words, on substantially the whole surface of the heat generating plate63and the whole surface of the base member62.

The position of the heat generating plate63is switched between a heat generating position and a restricted heat generating position, in accordance with rotation of the heat value adjustment member61.FIG. 5shows a state where the heat generating plate63is positioned in a heat generating position andFIG. 6shows a state where the heat generating plate63is positioned in a restricted heat generating position. When positioned in the heat generating position, the heat generating plate63is at a position close to the center core60, and generates heat due to the passage of the magnetic flux. On the other hand, when positioned in the restricted heat generating position, the heat generating plate63is at a position which is distant from the center core60. Therefore, magnetic flux does not readily pass through the heat generating plate63, and the generation of heat in the heat generating plate63is suppressed.

FIG. 7is a perspective diagram of the heat value adjustment member61. As shown inFIG. 7, the heat generating plate63extends from one end portion to another end portion in the axial direction of the base member62. The heat generating plate63has a circumferential dimension extending in the circumferential direction of the base member62, and this circumferential dimension is set to become larger in the inward axial direction of the base member62. More specifically, the heat generating plate63includes: a middle heat generating portion63awhich is disposed in the middle part of the axial direction of the base member62, a pair of first heat generating portions63bwhich are disposed respectively to the outside of the middle heat generating portion63ain the axial direction of the base member62, a pair of second heat generating portions63cwhich are disposed respectively to the outside of the pair of first heat generating portions63bin the axial direction of the base member62, and a pair of third heat generating portions63dwhich are disposed to the outside of the pair of second heat generating portions63cin the axial direction of the base member62, and more specifically, in the endmost portions of the base member62in the axial direction thereof. The middle heat generating portion63ahas the largest circumferential dimension, and the third heat generating portions63dhave the smallest circumferential dimension. The middle heat generating portion63a, the first heat generating portions63b, the second heat generating portions63cand the third heat generating portions63dare formed in an integrated fashion.

The middle heat generating portion63a, the first heat generating portions63b, the second heat generating portions63cand the third heat generating portions63dare arranged in accordance with the width dimensions of the sheets T which pass through the nip section NP. The middle heat generating portion63acorresponds to a sheet T1having a minimum width dimension (for example, an A5 sheet). The pair of first heat generating portions63bcorrespond to a sheet T2having a medium width dimension (for example, an A4 sheet (portrait)). The pair of second heat generating portions63ccorrespond to a sheet T3having a maximum width dimension (for example, an A3 sheet). The pair of third heat generating portions63dare set to exceed the width dimension of the sheet T3. Furthermore, the respective circumferential dimensions of the middle heat generating portion63a, the first heat generating portions63b, the second heat generating portions63cand the third heat generating portions63dare set to dimensions which enable the heat generating plate63to receive the magnetic flux passing through the fixing belt45when the heat generating plate63is disposed in the heat generating position.

The base member62of the heat value adjustment member also includes flanges65which close off the respective end portions in the axial direction, and rotating shaft members64which pass through the flanges65. By means of the rotating shaft member64rotating in a prescribed direction by mean of a drive source which is not illustrated, the position of the magnetic shielding plate63is switched between a heat generating position and a restricted heat generating position.

Next, a fixing operation by the fixing apparatus140relating to the second embodiment will be described. When an AC bias is applied to the coil54from the AC bias power source V, the coil54generates a magnetic flux. The magnetic flux passes along a magnetic path formed by the fixing belt45, the side cores59, the arch cores58and the center core60. When the magnetic flux passes through the fixing belt45, an induction current is generated. When the induction current is passed through the fixing belt45, Joule heat is generated by the intrinsic resistance of the fixing belt45itself, in other words, induction heating occurs in the fixing belt45. The whole of the fixing belt45inductively generates heat as the belt rotates. Furthermore, the heat generating plate63of the heat value adjustment member61generates heat by the magnetic flux which passes through the fixing belt45.

In this case, the heat value adjustment member61is rotated appropriately by rotating the rotating shaft member64in accordance with the width dimension of the sheet T, and the positions of the middle heat generating portion63aand the first heat generating portions63bto the third heat generating portions63dof the heat generating plate63are switched between a heat generating position and a restricted heat generating position.

For example, in the case of a sheet T1having a minimum width dimension which is passing through the nip section NP, the heat value adjustment member61is rotated by the rotating shaft member64in such a manner that the middle heat generating portion63aof the heat generating plate63is positioned in a heat generating position, whereas the first heat generating plates63bto the third heat generating plates63dare positioned in a restricted heat generating position. Consequently, generating heat is promoted in the paper passage region of the fixing belt45which is in contact with the sheet T1in the nip section NP, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T1in the nip section NP, generating heat is suppressed.

Furthermore, in the case of a sheet T2having a medium width dimension which is passing through the nip section NP, the heat value adjustment member61is rotated by the rotating shaft member64in such a manner that the middle heat generating portion63aand the pair of first heat generating portions63bare positioned in a heat generating position, whereas the pair of second heat generating plates63cand the pair of third heat generating plates63dare positioned in a restricted heat generating position. Consequently, generating heat is promoted in the paper passage region of the fixing belt45which is in contact with the sheet T2in the nip section NP, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T2in the nip section NP, generating heat is suppressed.

Moreover, in the case of a sheet T3having a maximum width dimension which is passing through the nip section NP, the heat value adjustment member61is rotated by the rotating shaft member64in such a manner that the middle heat generating portion63a, the pair of first heat generating portions63band the pair of second heat generating portions63care positioned in a heat generating position, whereas the pair of third heat generating plates63dare positioned in a restricted heat generating position. Consequently, generating heat is promoted in the paper passage region of the fixing belt45which is in contact with the sheet T3in the nip section NP, whereas in the region outside the paper passage region of the fixing belt45which is not in contact with the sheet T3in the nip section NP, generating heat is suppressed. The third heat generating portions63dare never positioned in a heat generating position with respect to the sheets T1to T3of any of the sizes, and therefore they do not have to be formed on the base member62.

In this way, by suitably rotating the heat value adjustment member61so as to switch the positions of the middle heat generating portion63aand the first heat generating portions63bto third heat generating portions63d, between the heat generating position and the restricted heat generating position, generating heat is promoted in the paper passage region of the fixing belt45, whereas overheating is suppressed in the region outside the paper passage region of the fixing belt45. The positional relationships between the middle heat generating portion63a, the first heat generating portions63b, the second heat generating portions63cand the third heat generating portions63dare set so as to enable the middle heat generating portion63a, the first heat generating portions63b, the second heat generating portions63cand the third heat generating portions63dto be positioned appropriately in a heat generating position or a restricted heat generating position in accordance with the sheets T1to T3.

When any one of the sheets T1, T2or T3enters into the nip section NP following the conveyance direction of the sheet T, the toner image on any one of the sheet T1, T2or T3receives heat from the fixing belt45while being gripped between the fixing belt45and the pressurization roller44. By this means, the toner image is fixed onto the sheet.

In the fixing apparatus140relating to the second embodiment described above, a fixing belt45is wrapped between a gripping piece49and the heat value adjustment member61, similarly to the fixing apparatus14relating to the first embodiment. Consequently, it is possible to reduce the amount of heat transferred from the fixing belt45which generates heat by using induction heating, compared to a conventional composition having a fixing belt which rotates by being wrapped about two roller members having a large heat capacity (for example, a fixing roller and a heating roller). Accordingly, it is possible to shorten the warm-up time of the fixing belt45. Furthermore, since the heat value adjustment member61about which the fixing belt45is wrapped adjusts the amount of heat generated in the fixing belt45, then it is possible to add a function of suppressing overheating of the fixing belt45, to the fixing apparatus140.

Furthermore, since the heat value adjustment member61is constituted by a thin base member62and thin plate-shaped heat generating plates63, then the amount of heat transferred to the heat value adjustment member61from the fixing belt45generating heat by using induction heating is small. Moreover, if the heat generating plate63is disposed in the heat generating position, then the aforementioned effect is further enhanced, and in the case of a composition where the temperature of the heat generating plate63becomes higher than the temperature of the fixing belt45, heat is transferred from the heat generating plate63to the fixing belt45and it is possible to heat the fixing belt45even more efficiently.

Furthermore, since the base member62of the heat value adjustment member61has a cylindrical shape, it is possible to rotate the heat generating plate63in a 360° angular range. By this means, the position of the heat generating plate63can be switched readily between a heat generating position and a restricted heat generating position.

Third Embodiment

Next, a third embodiment of the disclosure is described with reference toFIG. 8andFIG. 9.FIG. 8is a longitudinal cross-sectional diagram of the fixing apparatus150relating to a third embodiment, and shows a state where a magnetic shielding plate52is positioned in a restricting shielding position.FIG. 9is a longitudinal cross-sectional diagram of the fixing apparatus150relating to a third embodiment, and shows a state where a magnetic shielding plate52is positioned in a shielding position.

Similarly to the fixing apparatus14according to the first embodiment, the fixing apparatus150according to the third embodiment includes a pressurization roller44, a fixing belt45, a gripping piece49, a heat value adjustment member46, and a coil unit50. In the fixing apparatus150according to the third embodiment, only the composition of the heat value adjustment member46differs from that of the fixing apparatus14according to the first embodiment, and therefore description of the other members is omitted here.

In the fixing apparatus150according to the third embodiment, the heat value adjustment member46which is used in the fixing apparatus14in the first embodiment has a semi-cylindrical shape, rather than a cylindrical shape. More specifically, the base member51aof the heat value adjustment member46is formed with a semi-cylindrical shape, rather than a cylindrical shape. A magnetic shielding plate52and a coating layer53are provided on the base member51ahaving the semi-cylindrical shape.

By rotating the heat value adjustment member46in a prescribed direction, the magnetic shielding plate52is switched between a shielding position where the magnetic shielding plate52is positioned so as to oppose the center core60, and a restricted shielding position where the magnetic shielding plate52is positioned in a position distant from the center core60. As stated previously, when the magnetic shielding plate52is positioned in the shielding position, the magnetic flux directed from the center core60to the fixing belt45is shielded or suppressed, whereas when the magnetic shielding plate52is positioned in the restricted shielding position, the shielding of the magnetic flux is weakened. In the third embodiment, the central angle of the coil54which covers the fixing belt45is set to approximately 100° to 120°, the central angle of the base member51ais set to approximately 180° to 200°, and the central angle of the magnetic shielding plate52is set to approximately 60° to 90°. As mentioned above, the position of the magnetic shielding plate52is switched appropriately between a shielding position and a restricted shielding position in accordance with the sheets T1to T3which pass through the nip section NP.

According to the fixing apparatus150relating to the third embodiment, the heat value adjustment member46is formed in a semi-cylindrical shape, and therefore it is possible to further reduce the heat capacity of the heat value adjustment member46compared to a composition where the heat value adjustment member is formed in a cylindrical shape. The amount of heat transferred from the fixing belt45which generates heat by using induction heating to the heat value adjustment member46is reduced accordingly. Consequently, it is possible further to shorten the warm-up time of the fixing belt45.

Fourth Embodiment

Next, a fixing apparatus160relating to a fourth embodiment is described with reference toFIG. 10andFIG. 11.FIG. 10is a longitudinal cross-sectional diagram of the fixing apparatus160relating to a fourth embodiment, and shows a state where a heat generating plate63is positioned in a heat generating position.FIG. 11is a longitudinal cross-sectional diagram of a fixing apparatus160, and shows a state where the heat generating plate63is positioned in a restricted heat generating position.

Similarly to the fixing apparatus140according to the second embodiment, the fixing apparatus160according to the fourth embodiment includes a pressurization roller44, a fixing belt45, a gripping piece49, a heat value adjustment member61, and a coil unit50. In the fixing apparatus160according to the fourth embodiment, only the composition of the heat value adjustment member61differs from that of the fixing apparatus140according to the second embodiment, and therefore description of the other members is omitted here.

In the fixing apparatus160according to the fourth embodiment, the heat value adjustment member61which is used in the fixing apparatus140in the second embodiment has a semi-cylindrical shape, rather than a cylindrical shape. More specifically, the base member62aof the heat value adjustment member61is formed with a semi-cylindrical shape, rather than a cylindrical shape. A heat generating plate63and a coating layer (not illustrated) are provided on the base member62ahaving a semi-cylindrical shape. The structure of the heat generating plate63is as shown inFIG. 7.

By rotating the heat value adjustment member61in a prescribed direction, the heat generating plate63is switched between a heat generating position where the heat generating plate63is opposes the major part of the center core60and the coil54, and generates heat due to the passage of magnetic flux, and a restricted heat generating position where the heat generating plate63is away from the center core60and generation of heat is suppressed. InFIG. 10, each one of the middle heat generating portion63aand the first heat generating portions63bto the third heat generating portions63dhave been moved to positions opposing the major part of the center core60and the coil54, and hence are positioned in the heat generating position. InFIG. 11, only the middle heat generating portion63ahas been moved to a position opposing the center core60and hence is in a heat generating position, whereas the first to third heat generating portions63bto63dhave been moved to a position distant from the center core60and hence are positioned in a restricted heat generating position.

In the fourth embodiment, the central angle of the coil54which covers the fixing belt45is set to approximately 100° to 120°, the central angle of the middle heat generating portion63aof the heat generating plate63is set to approximately 180° to 200°, and the central angle of the third heat generating portions63dof the magnetic shielding plate63is set to approximately 100° to 120°. As mentioned above, the position of the heat generating plate63is switched appropriately between a heat generating position and a restricted heat generating position in accordance with the sheets T1to T3which pass through the nip section NP.

According to the fixing apparatus160relating to the fourth embodiment, the base member62aof the heat value adjustment member61is formed in a semi-cylindrical shape, and therefore it is possible to reduce the heat capacity of the heat value adjustment member61in comparison with a composition where the base member62is formed in a cylindrical shape. The amount of heat transferred from the fixing belt45which generates heat by using induction heating to the heat value adjustment member61is reduced accordingly. Consequently, it is possible further to shorten the warm-up time of the fixing belt45.

According to the fixing apparatus and the image forming apparatus relating to the present disclosure which were described above, it is possible to shorten the warm-up time of a belt, while providing a function of suppressing overheating of the belt.

This application is based on Japanese Patent application No. 2010-199166 filed in Japan Patent Office on Sep. 6, 2010, the contents of which are hereby incorporated by reference.