JP2010002532A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve shortening of warming-up time in a fixing device where a fixing belt is subjected to induction heating. <P>SOLUTION: The fixing device includes a guide plate 156 guiding an endless fixing belt 155 in a circulating direction by coming into contact with the back of the fixing belt 155 on the inside of a circulating path of the fixing belt 155, and a magnetic flux generation part 170 arranged at a position opposed to the guide plate 156 across the fixing belt on the outside of the circulating path and generating magnetic flux. The fixing belt 155 includes a heating layer 155c generating heat by the magnetic flux, and a magnetic shunt alloy layer 155d that changes from a ferromagnetic state to a non-magnetic state when its temperature exceeds predetermined temperature. The guide plate 156 includes a low resistance conductive layer, and includes a thick part 156a larger in thickness than a center part at least at one of both ends in the circulating direction. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a fixing device and an image forming apparatus using the fixing device, and more particularly to a technique for suppressing heat generation of the guide plate in a fixing device having a guide plate that guides an induction-heated belt in its circumferential direction. .

In recent years, an image forming apparatus such as a printer has started to employ an electromagnetic induction heating type fixing device that can save energy compared to a fixing device using a halogen heater as a heat source (for example, Patent Document 1).
FIG. 10 is a cross-sectional view illustrating a configuration of an electromagnetic induction heating type fixing device 300 in Patent Document 1.

As shown in the figure, the fixing device 300 includes a fixing belt 301, a fixing roller 302, a pressure roller 303, a magnetic flux generator 304, a guide plate 305, and the like.
The fixing belt 301 is a cylindrical elastically deformable belt including an induction heat generating layer 301a and a magnetic shunt alloy layer 301b provided on the back surface thereof, and is driven to rotate in the direction of arrow P.
The magnetic shunt alloy layer 301b is a ferromagnetic material at a normal temperature, but has a characteristic of becoming non-magnetic when the Curie temperature is exceeded.

  The fixing roller 302 is disposed inside the circulation path of the fixing belt 301. The pressure roller 303 is disposed outside the circulation path of the fixing belt 301 and presses the fixing roller 302 via the fixing belt 301 to secure the fixing nip 310. The pressure roller 303 receives a driving force from a driving motor (not shown) and rotates in the arrow Q direction. When this driving force is transmitted to the fixing roller 302 and the fixing belt 301, the fixing roller 302 and the fixing belt 301 are driven to rotate.

The magnetic flux generation unit 304 is disposed outside the rotation path of the fixing belt 301 and at a position facing the pressure roller 303 with the fixing belt 301 interposed therebetween, and generates a magnetic flux for causing the induction heating layer 301 a of the fixing belt 301 to generate heat. .
The guide plate 305 is made of a non-magnetic material of a low-resistance conductive material, and is disposed at a position facing the magnetic flux generation unit 304 via the fixing belt 301 inside the circulation path of the fixing belt 301, and follows the curvature of the fixing belt 301. The belt is in contact with the back surface of the fixing belt 301 that is driven to rotate, and guides the fixing belt 301 in the rotating direction, thereby restricting the relative position between the fixing belt 301 and the magnetic flux generator 304.

  In such a configuration, when the magnetic flux is generated from the magnetic flux generator 304 while the fixing belt 301 is driven to rotate, the portion of the induction heating layer 301a in the fixing belt 301 facing the magnetic flux generator 304 mainly generates heat, This heat generating portion reaches the fixing nip 310, the temperature of the area of the fixing nip 310 is raised to a temperature suitable for fixing, and the toner image formed on the sheet S is heated and pressurized when passing through the fixing nip 310. Then, it is thermally fixed to the sheet S.

  At this time, in the central portion where the fixing belt 301 is in contact with the sheet S, the sheet S is deprived of heat and the temperature is lowered, but the portions on both ends where the sheet does not pass (hereinafter referred to as “non-sheet passing portion”). ), The temperature remains high without being deprived of heat. Therefore, when power is supplied to the magnetic flux generation unit 304 in order to adjust the central portion of the fixing belt 301 to the target temperature, the temperature of the non-sheet passing portion further increases. To rise.

Then, the non-sheet passing portion of the magnetic shunt alloy layer 301b included in the fixing belt 301 is heated to a temperature equal to or higher than the Curie temperature, turns from a ferromagnetic material to a non-magnetic material, and flows along the magnetic shunt alloy layer 301b until then. This magnetic flux penetrates through the magnetic shunt alloy layer 301b and enters the guide plate 305.
Since the guide plate 305 is made of a low resistance conductive material, the eddy current generated by the magnetic flux contributes to the generation of the magnetic flux in the direction that cancels out the entering magnetic flux rather than the heat generation. The magnetic flux density is reduced, and the temperature rise in the non-sheet passing portion of the fixing belt 301 is mitigated.

Thus, the fixing device 300 is excellent in thermal efficiency because the fixing belt 301 itself generates heat, and further, automatically by the interaction between the magnetic shunt alloy layer 301b and the guide plate 305 so that the non-sheet passing portion of the belt 301 does not overheat. The temperature can be controlled.
JP 2007-264421 A

However, although the guide plate 305 is made of a low-resistance conductive material, heat generation due to eddy current is inevitable, and since the thickness is as thin as about 0.5 mm, fixing of a small-sized sheet continues for a long time. Then, the amount of heat is accumulated and the temperature rises excessively.
There is a risk of deformation due to thermal strain.
In order to avoid this, it is conceivable to increase the heat capacity by increasing the thickness of the guide plate 305. However, if this is done, the amount of heat taken away from the fixing belt 301 in contact with the guide plate 305 increases, and there is a problem that the warm-up time becomes longer. .

  The present invention has been made in view of the above-described problems, and provides a fixing device capable of suppressing an excessive temperature rise of a guide plate without substantially increasing a warm-up time, and an image forming apparatus including the fixing device. It is an object.

  In order to achieve the above object, the fixing device according to the present embodiment is arranged on the inner side of the circulation path of the endless belt, and presses the first roller from the outer side of the circulation path of the belt by the second roller via the belt. Then, while securing a fixing nip between the belt surface and the second roller, the belt is circulated and heated by electromagnetic induction, and a sheet on which an unfixed image is formed is passed through the fixing nip. A fixing device for thermally fixing the unfixed image, wherein the belt extends in parallel with the axis of the first roller on the inner side of the belt circulation path, and contacts the back surface of the belt driven to rotate. A guide plate that guides the belt in the circulation direction, and a magnetic pole for heating the belt, disposed outside the circulation path of the belt at a position facing the guide plate with the belt interposed therebetween. The belt includes a heat generating layer that generates heat by the magnetic flux, and a magnetic shunt alloy layer that reversibly changes from ferromagnetic to non-magnetic when a predetermined temperature is exceeded, and the guide The plate includes a low-resistance conductive layer, and has a thick portion having a thickness larger than that of the central portion at at least one of the ends in the circumferential direction.

Since the end surface of the guide plate is easy for magnetic flux to enter, and at least one of the two ends has a thicker portion that is thicker than the central portion, the current density of this portion is reduced. Heat generation is suppressed.
Further, since the thickness other than the thick portion is thin, the increase in heat capacity is small and the warm-up time does not change much compared to the case where the thick portion is not provided.

The thick portion is preferably formed by bending an end portion of a plate body having a substantially uniform thickness by 180 degrees.
Thereby, a thick part can be formed easily and the production cost of a guide plate can be reduced.

  The present invention may be an image forming apparatus including the fixing device.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking as an example a case where it is applied to a tandem color digital printer (hereinafter simply referred to as “printer”).
(1) Overall Configuration of Printer FIG. 1 is a schematic cross-sectional view showing the overall configuration of the printer 1.
As shown in the figure, the printer 1 includes an image processing unit 3, a paper feeding unit 4, a fixing unit 5, and a control unit 60. The printer 1 is connected to a network (for example, a LAN) and is connected to an external terminal device (not connected). When a print job execution instruction is received from the figure, a toner image of each color of yellow, magenta, cyan, and black is formed based on the instruction, and a full color image formation is executed by multiple transfer of these toner images.

Hereinafter, the reproduction colors of yellow, magenta, cyan, and black are represented as Y, M, C, and K, and Y, M, C, and K are added as subscripts to the numbers of the components related to the reproduction colors.

The image processing unit 3 includes image forming units 3Y, 3M, 3C, and 3K, an optical unit 10, an intermediate transfer belt 11, and the like.

  The image forming unit 3Y includes a photosensitive drum 31Y, a charger 32Y, a developing unit 33Y, a primary transfer roller 34Y, and a cleaner 35Y for cleaning the photosensitive drum 31Y disposed around the photosensitive drum 31Y. A Y-color toner image is formed on the photosensitive drum 31Y. The other image forming units 3M to 3K also have the configuration of the chargers 32M to 32K as in the image forming unit 3Y except that the color of the toner is different. However, in order to prevent the drawings from becoming complicated, The sign of is not shown.

The intermediate transfer belt 11 is an endless belt, is stretched around a driving roller 12 and a driven roller 13, and is driven to rotate in the direction of arrow C.
The optical unit 10 includes a light emitting element such as a laser diode. The optical unit 10 emits laser light L for forming images of Y to K colors according to a drive signal from the control unit 60, and exposes and scans the photosensitive drums 31Y to 31K.

By this exposure scanning, electrostatic latent images are formed on the photosensitive drums 31Y to 31K charged by the chargers 32Y to 32K. The electrostatic latent images are developed by developing units 33Y to 33K, and Y to K color toner images are superimposed on the same positions on the intermediate transfer belt 11 and primarily transferred onto the photosensitive drums 31Y to 31K. It is executed at different timings.
The toner images of the respective colors are sequentially transferred onto the intermediate transfer belt 11 by the electrostatic force acting on the primary transfer rollers 34Y to 34K to form a full-color toner image, and further move toward the secondary transfer position 46.

  On the other hand, the paper feed unit 4 includes a paper feed cassette 41 that accommodates the paper S, a feed roller 42 that feeds the paper S in the paper feed cassette 41 one by one onto the transport path 43, and a secondary feed of the fed paper S. A timing roller pair 44 and the like are provided for timing to send the toner to the transfer position 46, and the paper S is fed from the paper supply unit 4 to the secondary transfer position in accordance with the movement timing of the toner image on the intermediate transfer belt 11. The toner images on the intermediate transfer belt 11 are secondarily transferred onto the sheet S all at once by the action of electrostatic force by the secondary transfer roller 45.

The sheet S that has passed the secondary transfer position 46 is further conveyed to the fixing unit 5, and the toner image (unfixed image) on the sheet S is fixed on the sheet S by heating and pressurization in the fixing unit 5. The paper is discharged onto a discharge tray 72 via a discharge roller pair 71.
(2) Configuration of Fixing Unit FIG. 2 is a partial cross-sectional perspective view showing the configuration of the fixing unit 5, and FIGS. 3A and 3B are cross-sectional views of the main part.

As shown in FIG. 2, the fixing unit 5 includes a fixing roller 150, a fixing belt 155, a guide plate 156, a pressure roller 160, and a magnetic flux generation unit 170.
As shown in FIG. 3A, the fixing roller 150 is formed by covering the circumference of a long and cylindrical cored bar 152 with an elastic body layer 153, and inside the circulation path (circulation traveling path) of the fixing belt 155. Arranged.

The cored bar 152 is a cylindrical body having an outer diameter of about 20 mm made of, for example, aluminum, iron, stainless steel, or the like.
For example, the elastic layer 153 has a thickness of about 10 mm, and the outer diameter of the fixing roller 150 is about 40 mm.
The material is a foamed elastic body such as silicone rubber or fluororubber, and a material having high heat resistance and heat insulation is desired.

The pressure roller 160 is formed by laminating a release layer 163 around a cylindrical cored bar 161 with an elastic body layer 162 interposed therebetween, and is disposed outside the circumference path of the fixing belt 155, and from the outside of the fixing belt 155. A fixing nip 155n having a predetermined width in the circumferential direction is formed between the surface of the fixing belt 155 by pressing the fixing roller 150 through the fixing belt 155.
The core metal 161 is made of, for example, aluminum, the elastic body layer 162 is made of silicon sponge rubber or the like, and the release layer 163 is made of PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer) or PTFE (polytetrafluoroethylene). Fluoroethylene) coat and the like. The outer diameter of the pressure roller 160 is about 35 mm.

  The fixing roller 150 and the pressure roller 160 are rotatably supported at both ends in the axial direction of the core bars 152 and 161 on a bearing portion of a frame (not shown), and the pressure roller 160 is supplied from a drive motor (not shown). Is driven to rotate in the direction of arrow B. As the pressure roller 160 rotates, the fixing belt 155 and the fixing roller 150 are driven to rotate in the direction of arrow A.

The fixing belt 155 is a cylindrical belt. As shown in FIG. 3C, the release layer 155a, the elastic layer 155b, the heat generation layer 155c, and the magnetic shunt alloy layer 155d are separated in this order. The mold layer 155a is laminated so as to be on the outer peripheral surface side.
As the fixing belt 155, a belt that can stand independently and hold a cylindrical shape is used. The belt width direction (corresponding to the rotation axis direction of the fixing roller 150) of the fixing belt 155 is longer than the width direction length of the maximum size sheet.

The release layer 155a is a cylindrical body made of PFA or the like, and its thickness is determined as appropriate from the range of 30 μm or more and 40 μm or less based on experience.
The elastic body layer 155b is made of silicone rubber having a thickness of about 200 μm.
In addition to this, fluorine rubber or the like may be used.
The heat generating layer 155c is made of nickel or the like having a thickness of about 40 μm, and generates heat by the magnetic flux generated from the magnetic flux generator 170.

The magnetic shunt alloy layer 155d is made of, for example, an alloy of nickel and iron, and has a characteristic that, for example, it has a thickness of about 30 μm and is ferromagnetic at a normal temperature but becomes non-magnetic when the Curie temperature is exceeded. This Curie temperature can be adjusted by the mixing ratio of nickel and iron. In this embodiment, the Curie temperature is set to a temperature approximately 20 ° C. higher than the temperature suitable for fixing (target temperature).
In addition, an alloy such as nickel, iron, and chromium may be used.

Returning to FIG. 2, the magnetic flux generator 170 has a coil bobbin 171, a hem core 172, an exciting coil 173, a core 174, and a cover 175, and is outside the circulation path of the fixing belt 155. With respect to the position facing the pressure roller 160 with the gap therebetween as a reference, it is arranged along the width direction of the fixing belt 155 slightly upstream in the circumferential direction from here.
The exciting coil 173 generates a magnetic flux for heating the heat generating layer 155 c included in the fixing belt 155, and is wound around the coil bobbin 171.

  The alternating magnetic flux generated from the exciting coil 173 is guided to the fixing belt 155 by the core 174 and the bottom core 172, and mainly faces the magnetic flux generating portion 170 of the heat generating layer 155c (see FIG. 3C) of the fixing belt 155. The eddy current is generated in this portion to generate heat in the heat generating layer 155c itself, and the fixing belt 155 is heated. As the fixing belt 155 is heated, the temperature of the pressure roller 160 in contact with the fixing belt 155 is also increased.

A sensor for detecting the surface temperature of the center portion in the width direction of the fixing belt 155 is separately provided (not shown), and the controller 60 determines that the temperature of the fixing belt 155 is based on the detection signal of the sensor. The power supply to the exciting coil 173 is controlled so as to be maintained at (about 180 ° C.).
When the sheet S passes through the fixing nip 155n while the fixing nip 155n is maintained at the target temperature, an unfixed toner image on the sheet S is heated and pressed to be thermally fixed on the sheet S ( (See FIG. 2).

The guide plate 156 is a long plate made of a non-magnetic low-resistance conductive material and arranged in parallel with the axis of the fixing roller 150. The guide plate 156 is in contact with the back surface of the fixing belt 155, and the fixing belt 155 155 is guided around the belt.
Specifically, the low-resistance conductive material is copper, and aluminum or the like can also be used.

The guide plate 156 is supported at both ends in the longitudinal direction by a frame (not shown).
(3) the guide plate shaped guide plate 156, as shown in FIG. 3 (b), a total length at L ₀ in the belt rotation direction, at both ends in the circumferential direction, a large meat thickness than each central portion a plate thickness portions 156a and 156b are provided, the outer peripheral surface thereof is formed by a curve such that the radius of curvature R _1.

Here, the total length _{L 0} is has a 35 mm, also, the radius of curvature _{R 1} has a 20 mm.
The lengths L ₁ and L ₂ in the thick portions 156a and 156b are each 1 mm, and the thicknesses t ₁ and t ₂ in the thick portions 156a and 156b are both 1 .5mm.
On the other hand, the thickness _{t 0} of the central portion is 0.5 mm.

Note that the radius of curvature R ₁ of the guide plate 156 is substantially equal to the radius of curvature of the back surface of the portion of the fixing belt 155 that faces the magnetic flux generator 170 when stationary.
When the magnetic shunt alloy layer 155d is not higher than the Curie temperature and is a ferromagnetic material (hereinafter referred to as “before the magnetic shunt effect is activated”), the magnetic flux from the magnetic flux generator 170 is applied to the fixing belt 155. It penetrates through the heat generating layer 155c, enters the magnetic shunt alloy layer 155d from the approximate center in the belt circumferential direction, branches to both sides thereof, and travels the path leading to the skirt core 172 at the closest position.

At this time, the heat generating layer 155c is induction-heated by the eddy current generated by the magnetic flux passing therethrough.
In the magnetic shunt alloy layer 155d, the temperature in the non-sheet passing portion becomes equal to or higher than the Curie temperature, and the portion changes from ferromagnetic to non-magnetic (hereinafter referred to as “after the magnetic shunt effect is activated”), the magnetic flux generating portion 170. The magnetic flux generated in the above step penetrates the heat generating layer 155c and the magnetic shunt alloy layer 155d and further enters the guide plate 156.

At this time, since the guide plate 156 generates a magnetic flux in the direction opposite to the direction of the magnetic flux that enters, the magnetic flux density in the guide plate 156 and its periphery is lowered, and as a result, overheating in the heat generating layer 155c is suppressed.
Thus, when a large number of small sized sheets are continuously printed, the temperature of the non-sheet passing portion P (see FIG. 2) of the fixing belt 155 of the fixing belt 155 is set to the Curie temperature (of the fixing belt 155). Control target temperature + 20 ° C.) is not significantly exceeded, and it is prevented that the fixing belt 155 reaches a high temperature causing damage.

Note that the Curie temperature to be set is not limited to the above temperature, and depends on the configuration of the fixing unit 5 so that the temperature of the sheet passing portion maintains a predetermined fixing temperature and the non-sheet passing portion does not overheat. It is set as appropriate depending on the experiment.
(4) Effect by providing a thick portion at the end portion of the guide plate 156 As a result of intensive studies, the inventors have found that the thickness of the guide plate 156 is larger than the central portion at both ends in the belt rotation direction. It has been found that by providing the portions 156a and 156b, the temperature rise is mitigated as compared with a conventional guide plate having no thick portion.

Moreover, the lengths L ₁ and L ₂ in the belt circumferential direction of the thick portions 156a and 156b are as short as 1 mm, and the increase in volume is extremely small.
There is almost no increase in the heat capacity of the guide plate 156, and the warm-up time is not extended.
<Simulation results>
In order to verify the above effect, the temperature of the guide plate 156 was simulated by a computer before and after the activation of the magnetic shunt effect using the finite element method, and the following results were obtained.
<Specifications of test product>
FIG. 4 shows the specifications of the example products 1, 2, and 3 according to the present invention in which the simulation was performed and the guide plate 156 according to the conventional product.

As shown in the figure, the example products 1, 2, and 3 are obtained by setting the thicknesses of the thick portions provided at both ends in the belt circumferential direction to 1.0 mm, 1.5 mm, and 2.0 mm, respectively. It is. The thickness of the central part other than both end parts is unified to 0.5 mm.
For each example product, the length of the thick portion in the belt circumferential direction is changed to 1 mm, 3 mm, and 5 mm, and the simulation is performed.

The conventional product is uniform with a thickness of 0.5 mm and has no thick part.
In this simulation, in order to match the energization conditions to the excitation coil 173 of the actual machine, an alternating current of 40000 Hz is energized to the excitation coil 173 and 7.69 A is energized before the magnetizing effect is activated, and 9.65 A is energized after the magnetizing effect is activated. The condition was

In this embodiment, the value obtained by dividing the difference between the calorific value of the conventional product and each of the implemented products 1, 2 and 3 by the calorific value of the conventional product in% is called the “heating rate” and is adjusted. The rate of change in heat generation before and after the magnetic effect is activated is expressed as Has1 (n) and Has2 (n), respectively. It should be noted that n = 1, 2, 3 means the number of the example product.
The heat generation increase / decrease rate Has1 (n) before the magnetic shunt effect is activated is expressed by the following equation 1.
Has1 (n) = (HJ1 (n)-HJ1 (0)) / HJ1 (0) (Formula 1)

HJ1 (n): Calorific value of the non-sheet-passing part before the magnetic shunt effect is activated in the example product (n)
HJ1 (0): Heat value of the non-sheet passing part before the magnetic shunt effect of the conventional product is activated

Further, the heat generation increase / decrease rate Has2 (n) after activation of the magnetic shunt effect is expressed by the following equation 2.
Has2 (n) = (HJ2 (n)-HJ2 (0)) / HJ2 (0) (Formula 2)

HJ2 (n): Heat value of the non-sheet-passing part after the magnetic shunt effect of the example product (n) is activated
HJ2 (0): Heat generation amount in non-sheet passing portion after activation of magnetic shunting effect of conventional product FIG. 5 is a diagram showing a heat generation increase / decrease rate Has1 of each example product and the conventional product before triggering the magnetic shunting effect. .

From the figure, it can be seen that, before the magnetic shunt effect is activated, each of the products of Examples 1 to 3 has a lower calorific value than the conventional product.
FIG. 6 is a diagram showing a heat generation increase / decrease rate Has2 after activation of the magnetic shunt effect between each of the examples and the conventional product.

In addition, Example product 2 and Example product 3 overlap, and it looks like one graph.
From the figure, after the magnetic shunt effect is activated, the thickness of the thick part is 1.5 mm and 2 mm, and the thickness of the thick part is 1.5 mm and the thickness of the example product 2. It can be seen that the calorific value of Example Product 3 is reduced.
Since there is no difference in the simulation results between the example product 2 and the example product 3, even if the thickness is made larger than 1.5 mm, it is considered that a further heat generation amount reduction effect cannot be expected.

On the other hand, in order to maintain the warm-up time, it is desirable that the heat capacity, that is, the volume (mass) of the guide plate 156 be as small as possible.
From the above, among the test products, the guide plate 156 of the example product 2 in which the thickness in the thick portion is 1.5 mm and the length is 1 mm is the most excellent.
Thus, the inventor considered as follows why the heat generation amount of the guide plate 156 is reduced by providing the thick portions at both ends of the guide plate 156 in the belt-circulating direction.

The frequency of the alternating magnetic flux generated in the exciting coil 173 is as high as 40000 Hz, and the eddy current generated in the guide plate 156 is also high in frequency, so that the current tends to concentrate on the surface of the end portion of the guide plate 156 due to the skin effect. It seems to be.
By providing a thick part at the end where current is concentrated in this way, it is presumed that the current density at the time of eddy current generation is reduced and induction heating is suppressed.
Further, as the circumferential lengths L ₁ and L ₂ of the effective thick portion for preventing the temperature rise of the guide plate 156 are smaller, the temperature reduction effect is greater.

These specific dimensions can be determined by those skilled in the art based on the above disclosure, based on the model to be applied (high speed machine or low speed machine) and other design contents.

As described above, according to the present embodiment, the guide plate 156 has a simple configuration in which both end portions in the belt circumferential direction are thickened, and the guide is particularly provided after the magnetic shunt effect is activated while maintaining the warm-up time. The temperature rise of the plate 156 can be suppressed.

<Modification>
As described above, the present invention has been described based on various embodiments. However, the content of the present invention is not limited to the above embodiments, and for example, the following modifications can be considered. .
(1) In the above embodiment, the thickness of the thick portions 156a and 156b in the guide plate 156 is 1.5 mm and the length is 1 mm. However, the present invention is not limited to this, and the guide plate It suffices to have a thick portion having a shape (thickness and length) that allows the rate of increase / decrease in heat generation later to be smaller than that of the conventional product.
(2) In the above embodiment, the guide plate 156 is provided with thick portions at both ends in the belt circumferential direction. However, the present invention is not limited to this. For example, as shown in FIG. Alternatively, the guide plate 256 may be provided with a thick portion 256a at one of both end portions in the belt circumferential direction.

As described above, even when the thick portion 256a is provided at one of the both end portions, the temperature rise of the guide plate 256 after activation of the magnetic shunt effect as compared with the case where the thick portion is not provided at all. The effect which suppresses is produced.
In the case where the thick portion 256a is provided on the upstream side in the fixing belt circumferential direction among the both end portions, as shown in FIG. 8, the back surface of the fixing belt (not shown) runs over the edge of the thick portion 256a. Since the upstream end portion has a larger friction than the downstream end portion of the guide plate 256, the R portion having the curvature R ₂ is formed on the outer peripheral side edge portion of the thick portion 256a so as to reduce this friction. 256b may be provided.

The value of the curvature R ₂ are, the degree of reduction of the frictional force, in consideration of the influence of the heating rate of change Has2 after magnetic shunt effect triggering by the provision of the R portion, it is desirable to determine appropriately.
(3) In the above embodiment, the thickness portions 156a and 156b are formed by locally changing the thickness at both ends in the belt circumferential direction in the guide plate 156. As shown in FIG. 9, the guide plate 356 may be formed by providing thick portions 356 a and 356 b by bending both ends of a substantially uniform plate having a thickness of t ₀ by 180 degrees.

In that case, the thick portions 356a and 356b has a thickness of twice the _{t 0.}
At this time, it is desirable that a gap is not generated between the faces facing each other by bending.
By doing in this way, a thick part can be formed easily and the production cost of a guide plate can be reduced.
(4) In the above embodiment, the overall length L ₀ of the belt rotation direction guide plate 156 was 35 mm, not intended to limit the overall length of the belt rotation direction of the guide plate 156 to a predetermined value, the design conditions What is necessary is just to determine suitably.
(5) In the above embodiment, the guide plate 156 is made of a nonmagnetic low-resistance conductive material. However, the guide plate 156 is not limited to this. For example, the guide plate 156 has a multilayer structure, and one of a plurality of layers. One may be a low-resistance conductive layer.

As a layer other than the low-resistance conductive layer, for example, a surface that contacts the fixing belt 155 may be coated with PTFE or the like to provide a low friction layer that reduces friction with the fixing belt 155.
In this case, a thick portion thicker than the central portion of the low-resistance conductive layer may be provided on at least one of both end portions in the belt circumferential direction of the low-resistance conductive layer.
(6) Although the tandem color printer has been described in the above embodiment, the present invention is not limited to this, and may be, for example, a monochrome printer, and has additional functions such as a copying machine and a fax machine. In short, the present invention may be applied to all image forming apparatuses including a fixing device that uses a fixing belt and a guide plate that guides the fixing belt in a circumferential direction.

  The present invention can be widely applied to a fixing device using a fixing belt and a guide plate that guides the fixing belt in a circumferential direction, and an image forming apparatus using the fixing device.

1 is a schematic cross-sectional view of a tandem color digital printer according to an embodiment of the present invention. FIG. 3 is a partial cross-sectional perspective view of a fixing unit according to an embodiment of the present invention. (A) And (b) is a cross section which shows the structure of the principal part of the fixing part which concerns on embodiment of this invention, (c) is a fragmentary sectional view of a fixing belt. It is a figure which shows the specification of the test article which performed simulation. It is a figure which shows the simulation result before the magnetic shunt effect is activated. It is a figure which shows the simulation result after a magnetic shunt effect is activated. It is a figure which shows the modification of the guide plate which concerns on embodiment of this invention. It is an enlarged view which shows the modification of the guide plate which concerns on embodiment of this invention. It is a figure which shows the modification of the guide plate which concerns on embodiment of this invention. It is sectional drawing of the conventional fixing part.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer 3 Image process part 3Y, 3M, 3C, 3K Image creation part 4 Paper feed part 5 Fixing part 10 Optical part 11 Intermediate transfer belt 12 Drive roller 13 Driven roller 31 Photoreceptor drum 32 Charger 33 Developer 34 Primary transfer roller 35 Cleaner 41 Paper Feed Cassette 42 Feeding Roller 43 Conveying Path 44 Timing Roller Pair 45 Secondary Transfer Roller 46 Secondary Transfer Position 60 Control Unit 71 Discharge Roller Pair 72 Discharge Tray 150 Fixing Roller 152 Core Bar 153 Elastic Body Layer 155 Fixing Belt 155a Release layer 155b Elastic layer 155c Heat generation layer 155d Magnetic shunt alloy layer 155n Fixing nip 156 Guide plate 156a, 156b Thick part 160 Pressure roller 161 Core metal 162 Elastic body layer 163 Release layer 170 Magnetic flux generation part 171 Coil bobbin 172 Bottom Core 17 Excitation coil 174 core 175 cover

Claims (3)

Arranged inside the endless belt circumference path, the first roller is pressed from the outside of the belt circumference path by the second roller through the belt, and the fixing nip is formed between the belt surface and the second roller. A fixing device that heats the unfixed image by passing the sheet on which the unfixed image is formed through the fixing nip by heating by electromagnetic induction while circling the belt,
A guide plate that extends in parallel with the axis of the first roller on the inner side of the belt rotation path, contacts the back surface of the belt driven to rotate, and guides the belt in the rotation direction;
A magnetic flux generation unit that is disposed at a position facing the guide plate across the belt, and generates a magnetic flux for heating the belt, on the outer periphery of the belt.
The belt has a heat generating layer that generates heat by the magnetic flux, and a magnetic shunt alloy layer that reversibly changes from ferromagnetic to non-magnetic when a predetermined temperature is exceeded,
The fixing device according to claim 1, wherein the guide plate includes a low-resistance conductive layer, and has a thick portion having a thickness larger than that of a central portion at at least one end portion of both ends in the circumferential direction.   The fixing device according to claim 1, wherein the thick portion is formed by bending an end portion of a plate body having a substantially uniform thickness by 180 degrees.   An image forming apparatus comprising the fixing device according to claim 1.

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