Fuser for preventing excessive increased temperature in paper passing region of a heat transferring part

A fuser includes an endless heat generating part including a conductive layer, an induced current generating part to heat the conductive layer by electromagnetic induction, and a magnetic shunt metal member that is located at a side opposite to the induced current generating part across the heat generating part, forms a first gap between the magnetic shunt metal member and the heat generating part in a first paper passing region of the heat generating part, and forms a second gap, which is different from the first gap in size, between the magnetic shunt metal member and the heat generating part in a second paper passing region different from the first paper passing region.

FIELD

Embodiments described herein relate generally to a fuser used in an image forming apparatus, and particularly to a fuser in which temperature of a heat generating part is uniformed.

BACKGROUND

As a fuser used in an image forming apparatus such as a copying machine or a printer, there is a fuser in which the heat capacity of a heat generating part is reduced, the energy is saved, and a quick temperature rise is achieved. The heat generating part having the small heat capacity is difficult to keep the surface temperature of the heat generating part uniformly in a direction perpendicular to the conveyance direction of a sheet.

In the heat generating part having the small heat capacity, heat transfer from the heat generating part to the sheet does not occur in a sheet non-passing region during fixation, and there is a fear that an abnormal increased temperature occurs. Because of the increased temperature of the sheet non-passing region, there is a fear that the image forming operation of the image forming apparatus must be placed in a stand-by state.

DETAILED DESCRIPTION

In general, according to one embodiment, a fuser includes an endless heat generating part including a conductive layer, an induced current generating part to heat the conductive layer by electromagnetic induction, and a magnetic shunt metal member that is located at a side opposite to the induced current generating part across the heat generating part, forms a first gap between the magnetic shunt metal member and the heat generating part in a first paper passing region of the heat generating part, and forms a second gap, which is different from the first gap in size, between the magnetic shunt metal member and the heat generating part in a second paper passing region different from the first paper passing region.

Hereinafter, embodiments will be described.

First Embodiment

FIG. 1is a schematic structural view showing a color MFP (Multi Functional Peripheral)1as a tandem-type image forming apparatus including a fuser of a first embodiment. The MFP1includes a printer section10as an image forming part, a paper feed part11, a paper discharge part12and a scanner13. The printer section10includes four sets of image forming stations16Y,16M,16C and16K for Y (yellow), M (magenta), C (cyan) and K (black) arranged in parallel along an intermediate transfer belt15. The image forming stations16Y,16M,16C and16K respectively include photoconductive drums17Y,17M,17C and17K.

The image forming stations16Y,16M,16C and16K respectively include chargers18Y,18M,18C and18K, developing devices20Y,20M,20C and20K, and photoreceptor cleaners21Y,21M,21C and21K around the photoconductive drums17Y,17M,17C and17K rotating in an arrow a direction. The printer section10includes a laser exposure device22constituting an image forming unit.

The laser exposure device22irradiates laser beams22Y,22M,22C and22K corresponding to the respective colors to the photoconductive drums17Y,17M,17C and17K. The laser exposure device22irradiates the laser beams and forms electrostatic latent images on the respective photoconductive drums17Y,17M,17C and17K.

The printer section10includes a backup roller27and a driven roller28to support the intermediate transfer belt15, and the intermediate transfer belt15runs in an arrow b direction. The printer section10includes primary transfer rollers23Y,23M,23C and23K at positions opposite to the photoconductive drums17Y,17M,17C and17K across the intermediate transfer belt15.

The primary transfer rollers23Y,23M,23C and23K primarily transfer and sequentially superimpose toner images formed on the photoconductive drums17Y,17M,17C and17K onto the intermediate transfer belt15. The photoreceptor cleaners21Y,21M,21C and21K respectively remove toners remaining on the photoconductive drums17Y,17M,17C and17K after the primary transfer.

The printer section10includes a secondary transfer roller31at a position opposite to the backup roller27across the intermediate transfer belt15. The secondary transfer roller31is driven by the intermediate transfer belt15and rotates in an arrow c direction. In the printer section10, a sheet P is taken out from the paper feed part11by a pickup roller34, and the sheet P is fed to the position of the secondary transfer roller31along a conveyance path36in synchronization with a timing when the toner images of the intermediate transfer belt15reach the position of the secondary transfer roller31. At the time of secondary transfer, the printer section10forms a transfer bias at a nip between the intermediate transfer belt15and the secondary transfer roller31, and collectively secondarily transfers the toner images of the intermediate transfer belt15onto the sheet P.

In the printer section10, the toner images are fixed to the sheet P by a fusing unit32as a fuser, and the sheet P is discharged to the paper discharge part12by a paper discharge roller pair33.

The image forming apparatus is not limited to a tandem type, and the number of developing devices is not limited. The image forming apparatus may directly transfer a toner image from a photoreceptor to a recording medium.

Next, the fusing unit32will be described in detail. As shown inFIG. 2andFIG. 3, the fusing unit32includes a fusing belt60as a heat generating part, a press roller61as a pressure part, an induced current generating coil (hereinafter abbreviated to an IH coil)70as an induced current generating part, a nip forming member74, a heat equalizing plate78including a magnetic shunt metal layer78aas a magnetic shunt metal member, and a non-contact thermopile infrared temperature sensor67. The fusing unit32includes a peeling plate64as a peeling member at a discharge side of the sheet P with respect to a nip63on the periphery of the fusing belt60.

The fusing belt60includes a multi-layer structure. For example, as shown inFIG. 4, the fusing belt60includes a release layer60bhaving a thickness of 30 μm and made of fluorine resin such as, for example, PFA resin, on a surface of a heat generating layer60ahaving a thickness of, for example, 40 μm and made of nickel (Ni). A structure of the fusing belt is not limited. The fusing belt has only to include the heat generating layer, and an elastic layer may be disposed between the heat generating layer and the release layer. The thickness of the fusing belt is not limited. The heat generating layer may be made of nonmagnetic metal such as stainless, aluminum (Al), copper (Cu), silver (Ag) or composite material of stainless and aluminum. Flanges62support both sides of the fusing belt60. The fusing belt60, together with the flanges62, is driven by the press roller61or drive independently.

The nip formation member74is formed of, for example, heat resistant silicone sponge or silicone rubber, and includes a release layer of, for example, fluorine resin on a surface. A stay75supports the nip formation member74, and fixes the nip formation member74in the inside of the fusing belt60.

The press roller61includes, for example, a heat resistant silicone sponge or silicone rubber layer around a core metal, and includes a release layer made of fluorine resin such as, for example, PFA resin on a surface. A press roller frame80to support the press roller61rotates around a fulcrum80awith respect to a fusing belt frame90to support the fusing belt60. The press roller61includes a pressure changing mechanism87to adjust the pressing force of the press roller61to the nip formation member74. The pressure changing mechanism87includes a cam81, a bearing82and a pressure spring85. The pressure spring85presses the press roller61in an arrow r direction.

At the time of use of the fusing unit32, a cam surface83bclose to a rotation center81acontacts the bearing82, and the cam81of the pressure changing mechanism87presses the press roller61to the nip formation member74at a high pressure by the pressure spring85. If the fusing unit32is not used, in the cam81, a cam surface83aremote from the rotation center81acontacts the bearing82. The press roller frame80rotates in an arrow t direction, reduces the pressure of the press roller61to the nip formation member74, and prevents permanent deformation of the press roller61.

The press roller frame80fixes and supports the peeling plate64. At the time of peeling, the tip of the peeling plate64approaches the fusing belt60along the nip formation member74squashed by the high pressure of the press roller61and certainly peels the sheet P. If the fusing unit32is not used, the press roller61reduces the pressure to the nip formation member74, and the nip formation member74which deformed by pressure is restored. When the nip formation member74is restored, the peeling plate64rotates in the arrow t direction by the press roller frame80and separates from the nip formation member74. When the nip formation member74is restored, the tip of the peeling plate64does not contact the fusing belt60.

As shown inFIG. 5, the IH coil70includes a coil71and a magnetic core72to intensify the magnetic field of the coil71. The magnetic core72includes an upstream core72aat an upstream end along a rotation direction of an arrow u direction of the fusing belt60, and includes a downstream core72bat a downstream end along the rotation direction of the arrow u direction of the fusing belt60. A magnetic flux generation region (heating region) by excitation of the IH coil70in the rotation direction of the fusing belt60is determined by the upstream core72aand the downstream core72b. With respect to the magnetic flux generating region of the IH coil70, the upstream core72adetermines a magnetic flux generation upstream end77aand the downstream core72bdetermines a magnetic flux generation downstream end77b.

As the coil71, for example, a litz wire is used in which plural copper rods coated with heat resistant polyamide-imide as insulating material are bundled. When a high frequency current is applied to the coil71to generate a magnetic flux, an eddy current is generated in the heat generating layer60aof the fusing belt60. Joule heat is generated by the eddy current and the resistance value of the heat generating layer60a, and the surface of the fusing belt60is heated over the whole length in the longitudinal direction.

In order to enable quick temperature rise, the heat capacity of the heat generating layer60aof the fusing belt60is made low and the thickness thereof is made thin. The thickness of the heat generating layer60aof the fusing belt60is thinner than a skin depth at a frequency applied to the IH coil70. As shown inFIG. 6, the magnetic flux of the IH coil70is induced in the heat generating layer60aand forms a first magnetic path73a. Further, the magnetic flux passes trough the thin heat generating layer60a, is induced in the heat equalizing plate78arranged inside the fusing belt60, and forms a second magnetic path73b.

The heat equalizing plate78is formed into an arc shape along the inner peripheral surface of the fusing belt60while gaps t1and t2are formed between the heat equalizing plate and the inner peripheral surface of the fusing belt60. Both ends of the heat equalizing plate78are supported by the flanges62, and are fixed inside the fusing belt60. The function of the heat equalizing plate78is changed at the Curie temperature at which the magnetic shunt metal layer78achanges from a ferromagnetic material to a paramagnetic material. If the temperature of the magnetic shunt metal layer78adoes not reach the Curie temperature, the heat equalizing plate induces the magnetic flux from the IH coil70and generates heat, and further accelerates the quick temperature rise of the fusing belt60. If the temperature of the magnetic shunt metal layer78areaches the Curie temperature, the heat equalizing plate78reduces the magnetic flux from the IH coil70, and prevents abnormal heat generation of the fusing belt60. For example, if the heat equalizing plate78is made of Fe—Ni alloy (Permalloy), the heat equalizing plate78has a reversible property, and returns to the ferromagnetic state if the temperature is reduced.

As shown inFIG. 7, the heat equalizing plate78includes, for example, release layers78bhaving a thickness of 0.03 mm on both surfaces of a magnetic shunt metal layer78ahaving a thickness of 0.15 mm. The magnetic shunt metal layer78ais formed of, for example, Fe—Ni alloy (Permalloy) having a Curie temperature of 200° C. The magnetic shunt metal layer78ais not limited to the Fe—Ni alloy. The magnetic shunt metal layer78amay be made of any material as long as the Curie temperature at which the material changes from a ferromagnetic material to a paramagnetic material is higher than the fusing temperature of toner and not higher than the upper temperature limit of the fusing belt60, for example, about 200° C.

As the release layer78b, a material having a low friction coefficient and high heat resistance, for example, PFA resin is used. Since the friction coefficient of the release layer78bis low, even if the heat equalizing plate78contacts the inner peripheral surface of the fusing belt60, the occurrence of drive load on the fusing belt60is prevented. The heat equalizing plate78supports a thermostat92on a side opposite to a side facing the fusing belt60. The release layer78bkeeps the gap between the magnetic shunt metal layer78aand the thermostat92. The thermostat92detects abnormal heat generation of the fusing unit32, and cuts off power supply to the IH coil70. The thickness of the heat equalizing plate78is not limited.

The heat equalizing plate78formed into the arc shape along the inner peripheral surface of the fusing belt60has, for example, an arc shape whose center is a rotation center66of the fusing belt60. For example, a first angle between a line connecting the rotation center66of the fusing belt60and the magnetic flux generation upstream end77aof the IH coil70and a line connecting the rotation center and the magnetic flux generation downstream end77bof the IH coil70is made an angle α (magnetic flux generation angle of the IH coil70at the rotation center66). A second angle between a line connecting the rotation center66of the fusing belt60and an upstream side end79aof the heat equalizing plate78in the rotation direction of the fusing belt60and a line connecting the rotation center66and a downstream side end79bof the heat equalizing plate78is made an angle β (center angle of the arc-shaped heat equalizing plate78). The center angle β of the arc shape is made larger than the angle α as the magnetic flux generation angle of the IH coil70, so that the heat equalizing plate78prevents to leak the magnetic flux of the IH coil70passing through the fusing belt60to the surrounding of the heat equalizing plate78.

In the longitudinal direction of the fusing belt60perpendicular to the rotation direction of the fusing belt60, the size of the gap between the heat equalizing plate78and the fusing belt60varies. As shown inFIG. 4, in the longitudinal direction of the fusing belt60, in a center region (A) of the fusing belt60as a first paper passing region, the gap between the heat equalizing plate78and the fusing belt60is set to a first gap t2. In the longitudinal direction of the fusing belt60, in a side region (B) as a second paper passing region, the gap between the heat equalizing plate78and the fusing belt60is set to a second gap t1narrower than the first gap t2.

For example, if the fusing belt60fixes the sheet P of JIS standard “A4” vertical size (297 mm) at the maximum, the center region (A) is made, for example, JIS standard “A4” horizontal size (210 mm). The second gap t1is set to, for example, t1≦1.5×t2. The second gap t1is preferably, for example, 2 mm or less. In the image forming apparatus, a sheet is not necessarily conveyed while aligned to the center. For example, if a sheet is conveyed while aligned to the end, in the longitudinal direction of the fusing belt, the rear side of the image forming apparatus is made a base point, and a region where a small size sheet passes is set to a first region, and the remaining region on the front side may be set to a second paper passing region.

The non-contact thermopile infrared temperature sensor67detects the temperature of the fusing belt60, and inputs the detection result to a body control part100to control the MFP1. The body control part100controls an IH control part100ato control application of high frequency current to the IH coil70and a drive control part100bto control pressure adjustment or rotation driving of the press roller61.

If printing starts, the drive control part100bcontrols rotation of the cam81of the fusing unit32, and causes the cam surface83bclose to the rotation center81aof the cam81to contact the bearing82. The press roller frame80rotates in the arrow r direction by the spring force of the pressure spring85. The press roller61presses the nip formation member74at high pressure. The peeling plate64supported by the press roller frame80rotates in the arrow r direction, and its tip approaches the fusing belt60. The drive control part100brotates the press roller61in an arrow q direction, and the fusing belt60is rotated or independently rotated in an arrow u direction.

The IH control part100aexcites the coil71. The IH control part100afeedback controls the IH coil70from the detection result of the infrared temperature sensor67, and keeps the fusing belt60at fusing temperature. The magnetic flux of the coil71generates the eddy current in the heat generating layer60aof the fusing belt60and heats the fusing belt60. Further, the magnetic flux of the coil71passing through the heat generating layer60agenerates the eddy current in the magnetic shunt metal layer78aof the heat equalizing plate78, and heats the heat equalizing plate78.

At the time of heating start of the fusing belt60, the heat of the heat equalizing plate78is conducted to the fusing belt60through the gap, and accelerates the quick temperature rise of the fusing belt60. The sheet P on which a toner image is formed comes in close contact with the fusing belt60while passing through the nip63, and the toner image is fixed. The peeling plate64peels the sheet P, which passed through the nip63, from the fusing belt60.

If the width of the sheet P is equal to the whole length of the fusing belt60in the longitudinal direction, the whole length of the fusing belt60in the longitudinal direction contacts the sheet P during fixation. During fixation, the temperature of the fusing belt60is almost uniformly reduced over the whole length in the longitudinal direction, and there is no fear that a specific region abnormally generates heat.

If the sheet P has a small size, if the fusing operation is continued, although the temperature of the paper passing region of the sheet P is reduced in the longitudinal direction of the fusing belt60, the temperature of the sheet non-passing region gradually increases. For example, if the sheets P having “A4” lateral size (210 mm) width are continuously fixed, in the center region (A) of the fusing belt60which becomes the paper passing region, the temperature is absorbed by the passage of the sheet P. However, in the side region (B) of the fusing belt60which becomes the paper non-passing region, the temperature gradually increases. If the temperature in the side region (B) increases, and the temperature of the magnetic shunt metal layer78aof the heat equalizing plate78reaches the Curie temperature, the magnetic flux from the IH coil70is quickly decreased in the side region (B). In the side region (B), the fusing belt60and the magnetic shunt metal layer78astop self heat generation, and abnormal heat generation in the side region (B) of the fusing belt60is prevented.

In the side region (B) of the fusing belt60, the gap between the heat equalizing plate78and the fusing belt60is the second gap t1, and the heat equalizing plate78is close to the fusing belt60. Accordingly, thermal conductivity from the fusing belt60to the heat equalizing plate78is high in the side region (B). If the temperature in the side region (B) of the fusing belt60increases while the small size sheets P are continuously fixed, heat of the fusing belt60in the side region (B) is quickly conducted to the heat equalizing plate78close to the fusing belt60. Increased temperature in the side region (B) of the fusing belt60immediately increases the temperature of the magnetic shunt metal layer78a.

The heat equalizing plate78is close to the fusing belt60, so that the timing when the magnetic shunt metal layer78ain the side region (B) of the fusing belt60reaches the Curie temperature is quickened. The self heat generation in the side region (B) as the paper non-passing region is stopped at the early timing, and the abnormal heat generation in the side region (B) is efficiently prevented. If the side region (B) of the fusing belt60abnormally generates heat, the print operation must be waited until the temperature of the side region (B) is reduced. The timing when the magnetic shunt metal layer78areaches the Curie temperature is quickened, and the occurrence of the wait mode of the fusing unit32is prevented.

In the center region (A) of the fusing belt60, the gap between the heat equalizing plate78and the fusing belt60is the first gap t2, and the heat equalizing plate78is somewhat separate from the fusing belt60. Thus, as compared with the side region (B), the thermal conductivity from the fusing belt60to the heat equalizing plate78in the center region (A) is reduced. The timing when the temperature of the magnetic shunt metal layer78ain the center region (A) of the fusing belt60increases by the heat conduction from the fusing belt60is delayed, and it is prevented that the temperature of the magnetic shunt metal layer78ain the center region (A) reaches the Curie temperature during fixation. The abrupt reduction in temperature reducing of the center region (A) of the fusing belt60due to the decrease of the magnetic flux is prevented, and the center region (A) as the paper passing region of the fusing belt60is kept at the fusing temperature.

If printing is ended, the drive control part100brotates and controls the cam81of the fusing unit32, and causes the cam surface83aremote from the rotation center81aof the cam81to contact the bearing82. The press roller frame80rotates in the arrow t direction against the spring force of the pressure spring85. The press roller61reduces the pressure to the nip formation member74. The nip formation member74which deformed by pressure is restored, and the peeling plate64moves in the arrow t direction by the rotation of the press roller frame80and separates from the fusing belt60.

There is a case where during printing, for example, the fusing belt60or the heat equalizing plate78is heated, and the fusing unit32abnormally generates heat. If the fusing unit32abnormally generates heat, the thermostat92is turned off, power supply from a power supply circuit93to the IH coil70is cut off, and the abnormal heat generation of the fusing unit32is stopped.

According to the first embodiment, the gap between the heat equalizing plate78including the magnetic shunt metal layer78aand the fusing belt60is made such that the second gap t1in the side region (B) is narrower than the first gap t2in the center region (A). If the small size sheets P are continuously fixed, the temperature increases in the side region (B) of the fusing belt60is quickly heat-conducted to the magnetic shunt metal layer78ain the side region (B), and the timing when the magnetic shunt metal layer78ain the side region (B) reaches the Curie temperature is quickened. The magnetic shunt metal layer78ain the side region (B) reaches the Curie temperature, and the self heat generation of the fusing belt60and the magnetic shunt metal layer78ain the side region (B) is stopped, to prevent abnormal heat generation of the fusing belt60and the fusing unit32. The occurrence of the wait mode of the fusing unit32, which is caused if the side region (B) of the fusing belt60abnormally generates heat, is prevented, and the performance of the MFP1for printing different sizes of paper is improved.

In the center region (A) of the fusing belt60, heat conduction from the fusing belt60to the magnetic shunt metal layer78ais reduced, and the timing when the magnetic shunt metal layer78ain the center region (A) reaches the Curie temperature by the heat conduction from the fusing belt60is delayed. It is prevented that the center region (A) reaches the Curie temperature during fixation, the abrupt reduction in temperature in the center region (A) of the fusing belt60is prevented, and the performance of the MFP1is improved.

Second Embodiment

Next, a second embodiment will be described. In the second embodiment, slits are formed in a magnetic shunt metal member. In the second embodiment, the same component as the component described in the first embodiment is denoted by the same reference numeral and its detailed description is omitted.

As shown inFIG. 8, a heat equalizing plate110including a magnetic shunt metal layer110aof the second embodiment includes slits111at specified intervals throughout the entire area. The slits111are formed by, for example, press-working the heat equalizing plate110. If the heat equalizing plate110does not include the slits111, as indicated by a dotted line inFIG. 8, the heat equalizing plate110generates a large eddy current112throughout the whole area of the heat equalizing plate110by magnetic flux from an IH coil70. Thus, if the heat equalizing plate110does not include the slits111, there is a fear that the whole area of the heat equalizing plate110reaches the Curie temperature by self heat generation caused by the large eddy current112. If the whole area of the heat equalizing plate110reaches the Curie temperature, there is a fear that in the longitudinal direction of a fusing belt60, the temperature of a region where fixation is being performed is also abruptly lowered, and fusing can not be performed.

If the heat equalizing plate110is provided with the slits111, as shown by a solid line inFIG. 8, small eddy currents113are generated between the slits111in the heat equalizing plate110by the magnetic flux from the IH coil70. Since the eddy current generated in the heat equalizing plate110is small irrespective of the magnetic flux from the IH coil70, self heat generation of the heat equalizing plate110by the eddy current is small, and it is prevented that the temperature of the heat equalizing plate110reaches the Curie temperature by the self heat generation. Further, since the self heat generation of the heat equalizing plate110is small, the increased temperature of the inside of the fusing belt60is prevented.

Since the self heat generation of the heat equalizing plate110is suppressed to be low, the increased temperature due to the heat conduction from the fusing belt60is more reflected on the heat equalizing plate110. Similarly to the first embodiment, in the longitudinal direction of the fusing belt60, a gap between the heat equalizing plate110and the fusing belt60in a center region (A) is set to a first gap t2, and a gap between the heat equalizing plate110and the fusing belt60in a side region (B) is set to a second gap t1narrower than the first gap t2. Accordingly, in the side region (B) close to the fusing belt60, the increased temperature of the fusing belt60is quickly conducted to the heat equalizing plate110. If small size sheets P are continuously fixed and the temperature in the side region (B) of the fusing belt60increases, the heat of the fusing belt60in the side region (B) is quickly reflected on the increased temperature of the magnetic shunt metal layer110a. The magnetic shunt metal layer110ain the side region (B) of the fusing belt60reaches the Curie temperature at an early timing, and the prevention of the increased temperature of the fusing belt60is advanced.

In the center region (A) of the fusing belt60, since the heat equalizing plate110is somewhat separate from the fusing belt60, thermal conductivity from the fusing belt60to the heat equalizing plate110is reduced. The timing when the temperature of the magnetic shunt metal layer110ain the center region (A) of the fusing belt60is increased by the heat conduction from the fusing belt60is delayed, and it is prevented that the magnetic shunt metal layer110ain the center region (A) reaches the Curie temperature during fixation. Abrupt reduction in temperature of the center region (A) of the fusing belt60is prevented, and the center region (A) of the fusing belt60is kept at fusing temperature.

According to the second embodiment, similarly to the first embodiment, if small size sheets P are continuously fixed, the increased temperature in the side region (B) of the fusing belt60is quickly conducted to the magnetic shunt metal layer110a, and the timing when the magnetic shunt metal layer110ain the side region (B) reaches the Curie temperature is quickened. If the magnetic shunt metal layer110ain the side region (B) reaches the Curie temperature, the self heat generation of the fusing belt60and the magnetic shunt metal layer110ain the side region (B) is stopped, and the abnormal heat generation of the fusing belt60and the fusing unit32is prevented. The occurrence of the wait mode of the fusing unit32is prevented and the performance of the MFP1is improved. In the center region (A) of the fusing belt60, the heat conduction from the fusing belt60to the magnetic shunt metal layer110ais reduced to delay the timing when the magnetic shunt metal layer110ain the center region (A) reaches the Curie temperature by the heat conduction from the fusing belt60. It is prevented that the center region (A) reaches the Curie temperature during fixation, abrupt reduction in temperature of the center region (A) of the fusing belt60is prevented, and the performance of the MFP1is improved.

According to the second embodiment, the heat equalizing plate110is provided with the slits. The eddy current113generated in the heat equalizing plate110is reduced irrespective of the magnetic flux from the IH coil70. Accordingly, the self heat generation of the heat equalizing plate110by the eddy current is suppressed, and it is certainly prevented that the whole area of the heat equalizing plate110reaches the Curie temperature by the self heat generation, the reduction in temperature of the fusing region of the fusing belt60during fixation is prevented certainly, and the performance of the MFP1is improved.

Third Embodiment

Next, a third embodiment will be described. In the third embodiment, in a longitudinal direction of a heat generating part, magnetic shunt metal members different in Curie temperature are used in a center region and a side region. In the third embodiment, the same component as the component described in the first embodiment is denoted by the same reference numeral and its detailed description is omitted.

As shown inFIG. 9, in the third embodiment, a heat equalizing plate120is divided into a center heat equalizing plate121and side heat equalizing plates122and123. The center heat equalizing plate121includes a magnetic shunt metal layer121amade of MS 220 (made by Neomax Material Co., Ltd.) which is a magnetic shunt metal member whose Curie temperature is 220° C. The side heat equalizing plates122and123respectively include magnetic shunt metal layers122aand123amade of MS 190 (made by Neomax Material Co., Ltd.) which is a magnetic shunt metal member whose Curie temperature is 190° C.

Accordingly, in the heat equalizing plate120, if the center heat equalizing plate121in the center region (A) reaches 220° C., the magnetic flux from the IH coil70is abruptly reduced. If the side heat equalizing plates122and123in the side regions (B) reach 190° C., the magnetic flux from the IH coil70is abruptly reduced.

Similarly to the first embodiment, in the longitudinal direction of the fusing belt60, a gap between the center heat equalizing plate121and the fusing belt60in the center region (A) is set to a first gap t2, and a gap between the side heat equalizing plate122,123and the fusing belt60in the side region (B) is set to a second gap t1narrower than the first gap t2. In the side region (B) close to the fusing belt60, increased temperature of the fusing belt60is quickly conducted to the heat equalizing plate110.

If small size sheets P are continuously fixed and the temperature in the side regions (B) of the fusing belt60increases, the heat of the fusing belt60in the side regions (B) is quickly conducted to the magnetic shunt metal layers122aand123a. Further, since the magnetic shunt metal layers122aand123ain the side regions (B) of the fusing belt60reach the Curie temperature at 190° C., abnormal heat generation of the fusing belt60is prevented while the temperature in the side regions (B) of the fusing belt60is relatively low.

In the center region (A) of the fusing belt60, the center heat equalizing plate121is somewhat separate from the fusing belt60, and the timing when the magnetic shunt metal layer121aof the center heat equalizing plate121reaches the Curie temperature by the heat conduction is delayed. Further, since the Curie temperature of the magnetic shunt metal member121ain the center region (A) of the fusing belt60is as high as 220° C., even if the temperature in the center region (A) of the fusing belt60slightly increases and the temperature of the magnetic shunt metal layer121aof the center heat equalizing plate121increases it is prevented that the magnetic shunt metal layer121areaches the Curie temperature. Even if the temperature in the center region (A) of the fusing belt60slightly increases, abrupt reduction in temperature in the center region (A) of the fusing belt60is prevented, and the fusing region of the fusing belt60is kept at fusing temperature.

According to the third embodiment, when small size sheets P are continuously fixed, the magnetic shunt metal layers122aand123aare quickly heated to the Curie temperature while the temperature in the side region (B) is relatively low, magnetic permeabilities of the fusing belt60and the magnetic shunt metal layers122aand123ain the side region (B) are decreased, and the abnormal heat generation of the fusing belt60is prevented. On the other hand, in the center region (A) of the fusing belt60, even if the temperature slightly increases, the magnetic shunt metal layer121adoes not reach the Curie temperature. Even if the temperature in the center region (A) of the fusing belt60slightly increases, desired fusing temperature is obtained, and the performance of the MFP1is improved.

According to at least one of the embodiments, even if small size sheets are continuously fixed, the increased temperature of the heat generating part in the paper non-passing region is quickly conducted to the magnetic shunt metal member. The timing when the magnetic shunt metal member in the paper non-passing region reaches the Curie temperature is quickened, the abnormal heat generation of the heat generating part is prevented, and the performance of the image forming apparatus is improved. In the paper passing region, heat conduction from the heat generating part to the magnetic shunt metal layer is reduced. The timing when the magnetic shunt metal layer in the paper passing region reaches the Curie temperature is delayed, it is prevented that the paper passing region reaches the Curie temperature during fixation, and the performance of the image forming apparatus is improved.