JP2009150972A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable an electromagnetic induction heating type fixing device to perform stable fixing by suppressing excessive temperature rise in a paper non-passing area even when size of a conveyed recording sheet is changed, and to keep the device compact and inexpensive. <P>SOLUTION: When the recording sheet passes through a fixing roller, heat is taken away from the fixing roller 1 in a paper passing area, and temperature of the fixing roller 1 becomes high in the paper non-passing area. In the paper non-passing area where the temperature of the fixing roller 1 is high, temperature in main core parts 34a to 34d also rises by following up temperature rise of the fixing roller 1 and gets over Curie temperature. When the temperature of the main core parts gets over Curie temperature, magnetic permeability thereof is lowered rapidly and calorific power generated in a fixing belt 16 becomes low, so that the excessive temperature rise is suppressed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fixing device using an electromagnetic induction heating method, and an image forming apparatus including the fixing device.

Image forming apparatuses represented by laser printers, copiers, fax machines, multifunction machines, and the like include a fixing device, and after transferring a toner image to a recording sheet that is a recording medium (paper, OHP sheet, etc.), The recording sheet is heated and fixed by a fixing roller or the like.
In this fixing device, a method of heating the fixing roller using a halogen heater as a heat source has been conventionally used. However, in recent years, as the demand for downsizing and energy saving increases in image forming apparatuses, the fixing roller is heated by electromagnetic induction. A scheme has been proposed.

In particular, a fixing belt including a conductive heat generating layer is provided on the outer periphery of the fixing roller, a magnetic flux is generated by an exciting coil, and the magnetic material core such as a ferrite core guides the conductive heat generating layer of the fixing roller to generate heat. The type is adopted.
In this type of fixing device, the heat capacity of the conductive heat generating layer can be reduced, so the warm-up time can be shortened and a compact and high heat conversion efficiency can be obtained, but in the area where the recording sheet passes (paper passing area). Heat is deprived from the surface of the fixing roller to the recording sheet, and the temperature tends to be lower than that in the non-sheet passing area. Therefore, when the size of the recording sheet to be conveyed is small, particularly when continuously passing the sheet, if the sheet passing area of the fixing roller is maintained at the fixing temperature, the non-sheet passing area (both end areas) through which the recording sheet does not pass. There is a problem that the temperature rises excessively and the fixing roller and the excitation coil are damaged beyond the heat resistance limit.

On the other hand, Patent Document 1 adjusts the heat generation distribution of the fixing roller by providing an exciting coil and a magnetic core inside the fixing roller and using a material having a low Curie temperature for the magnetic core near the end of the roller. Techniques to do this are disclosed.
According to the technique of Patent Document 1, the heat of the heated portion of the fixing roller is scattered and conducted to the magnetic core, and when the magnetic core exceeds the Curie temperature, the magnetic field weakens and induction heating is suppressed. Thus, the magnetic field distribution when the fixing roller is induction-heated is automatically adjusted.

Therefore, when fixing a recording sheet having a size in which the end area using the core material having a relatively low Curie temperature is a non-sheet passing area, the temperature of the sheet passing area of the fixing roller is a temperature suitable for fixing. And an effect of suppressing excessive temperature rise in the non-sheet passing area can be obtained.
Patent Document 2 discloses a technique for adjusting a heat generation distribution of a fixing roller by providing a demagnetizing coil so as to overlap an end portion of an exciting coil and controlling the magnetic field distribution with this demagnetizing coil.
JP 2000-162912 A JP 2005-108603 A

  However, when the electromagnetic induction heating means is provided inside the fixing roller as in the fixing device described in Patent Document 1, heat is accumulated inside the fixing roller and the heat flows in the roller axial direction. Therefore, the temperature followability of the magnetic core with respect to the change in the surface temperature of the fixing roller is not good. Accordingly, in the non-sheet passing region of the fixing roller, the excessive temperature rise tends to occur before the heat of the heated portion is transmitted to the magnetic core and exceeds the Curie temperature.

  On the other hand, in the technique of Patent Document 2, an excessive temperature rise at both ends of the fixing roller can be suppressed by opening and closing a pair of degaussing coils and adjusting the magnetic field distribution. In addition to the exciting coil, the demagnetizing coil is installed in an overlapping manner, which leads to an increase in the size of the fixing device. Further, the size range of the recording sheet that can adjust the temperature of the sheet passing area of the fixing roller to an appropriate temperature and suppress the excessive temperature rise in the non-sheet passing area is limited to two types.

Here, in the technique of Patent Document 2, if several degaussing coils are arranged side by side and the opening and closing of the degaussing coil is controlled according to the recording sheet size, the temperature distribution of the fixing roller can be adjusted according to each recording sheet size. However, in this case, there is a problem that the apparatus is further increased in size and cost.
The present invention has been made in view of the above problems, and in an electromagnetic induction heating type fixing device, even if the size of a recording sheet to be fixed is changed, the device is kept compact and inexpensive, An object of the present invention is to reliably suppress an excessive temperature rise in a non-sheet passing region of a rotating body such as a fixing roller.

In order to achieve the above object, a fixing nip is formed by arranging a pair of rotating bodies close to each other, and at least one of the pair of rotating bodies is heated by electromagnetic induction heating means, and an image to be fixed is formed. A fixing device that heat-fixes the recorded sheet through the fixing nip portion, wherein the electromagnetic induction heating means is an excitation arranged in the longitudinal direction along the circumferential surface of at least one of the rotating bodies. It has a coil and a magnetic core arranged in the direction perpendicular to the recording sheet conveyance direction along the exciting coil, and corresponds to the non-sheet passing area when the recording sheet of the minimum size passes through the magnetic core. The Curie temperature of the portion to be heated is Tc, the temperature of the magnetic core when the rotating body heated by the electromagnetic induction heating means reaches a predetermined fixing temperature, Tf, and the heated rotating body reaches its heat resistant temperature. When magnetism When the temperature of the core and Th
Tf ≦ Tc <Th
The relationship is established.

  In the fixing device of the present invention, when the exciting coil is energized, a magnetic flux passing through each magnetic core and the rotating body is generated, and the rotating body generates heat due to an eddy current generated along with the magnetic flux change. Here, when the rotating body heated by the electromagnetic induction heating means has reached a predetermined fixing temperature, the magnetic body core has a Curie temperature Tc at least in a portion corresponding to a region other than the paper passing portion of the recording sheet of the minimum size. Since the temperature of the magnetic core is set to be equal to or higher than Tf and the temperature of the magnetic core when the heated rotating body reaches its heat resistant temperature is set to a range lower than Th, The temperature of the magnetic core at the position corresponding to the non-sheet-passing region exceeds the Curie temperature until the temperature of the non-sheet-passing region reaches the fixing temperature and reaches the heat-resistant temperature, and the magnetic permeability is noticeable. descend. As a result, the magnetic flux density formed in the rotating body with the drive of the exciting coil is reduced, and the amount of induction heat generated in the rotating body is also reduced, so that excessive temperature rise can be suppressed.

  As described above, in the fixing device of the present invention, the electromagnetic induction heating means is arranged along the fixing roller on the outer side, and the Curie temperature of the magnetic core corresponding to the non-sheet passing region is set as described above. Since partial overheating can be suppressed, it is not necessary to provide a plurality of sizes of demagnetizing coils as in the prior art, and the manufacturing cost can be reduced and the apparatus can be made compact.

Further, in the present invention, the electromagnetic induction heating means is disposed in a direction perpendicular to the recording sheet conveyance direction along the excitation coil disposed in the longitudinal direction along the circumferential surface of at least one rotating body and the excitation coil. Since it is composed of the arranged magnetic cores, it is suitable for improving the temperature tracking and responsiveness of the magnetic cores.
Here, if the magnetic body core is arranged in an arch shape along the outer peripheral surface of the rotating body, heat transfer between the rotating body and the magnetic body core is improved, and temperature followability and responsiveness of the magnetic body core are improved. .

  Further, if the magnetic core has a configuration in which a plurality of core portions are arranged, heat transfer in the width direction of the recording sheet is reduced in the magnetic core, so that temperature followability of the magnetic core is improved.

Embodiments of a fixing device and an image forming apparatus including the fixing device according to the present invention will be described below with reference to the drawings.
(1) Overall Configuration of Image Forming Apparatus FIG. 7 is a schematic sectional view showing the overall configuration of the image forming apparatus 1000 according to the present embodiment.

As shown in the figure, the image forming apparatus 1000 includes an image forming unit 1500, a paper feed / conveying unit 1515, a fixing device 1520, a control unit 100, and the like.
The image forming unit 1500 includes an intermediate transfer belt 1502 that is stretched around a driving roller 1505 and a driven roller 1504 and is driven to rotate in the direction of arrow A, and yellow (Y) and magenta arranged below the intermediate transfer belt 1502. The image forming unit includes four image forming units 1506Y, 1506M, 1506C, and 1506K corresponding to (M), cyan (C), and black (K), and print head portions 1509Y to 1509K.

Each of the image forming units 1506Y to 1506K includes photosensitive drums 1507Y to 1507K, and around the photosensitive drums 1507Y to 1507K, chargers 1508Y to 1508K and developing units 1510Y to 1510K are sequentially arranged along the rotation direction. In addition, primary transfer rollers 1511Y to 1511K and the like are disposed.
A secondary transfer roller 1503 is pressed against a portion of the intermediate transfer belt 1502 supported by the driving roller 1505, and a nip portion between the secondary transfer roller 1503 and the intermediate transfer belt 1502 becomes a secondary transfer region 1530. Yes.

A paper feed cassette 1517 is detachably disposed below the image forming unit 1500.
The sheet feeding cassette 1517 can store a recording sheet P of one size selected from a plurality of sizes (P1, P2, P3, P4), and is stored by a sensor (not shown). The size of can be detected.
The fixing device 1520 includes a fixing roller 1 and a pressure roller 2, and a pressure contact portion between the fixing roller 1 and the pressure roller 2 is a fixing nip region 1531.

  In such a configuration of the image forming apparatus 1000, when the control unit 100 receives a print job from an external terminal connected via a LAN or the like, the image signal in the received print job is changed to yellow, cyan, magenta, and black. A color-converted digital image signal is created, and based on the digital image signal, the print head units 1509Y to 1509K of the image forming units 1506Y, 1506M, 1506C, and 1506K emit light to perform exposure scanning. Thereby, electrostatic latent images for the respective colors are formed on the surfaces of the respective photosensitive drums 1507Y to 1507K.

The electrostatic latent images are developed by the developing units 1510Y to 1510K to become toner images of the respective colors, and are sequentially primary transferred and superimposed on the intermediate transfer belt 1502 by the action of the primary transfer rollers 1511Y to 1511K. .
On the other hand, the recording sheets P stacked and accommodated in the paper feed cassette 1517 are sent one by one from the uppermost one to the transport path 1540 by the paper feed roller 1518. The timing roller 1570 conveys the recording sheet P to the secondary transfer area 1530 at a predetermined timing.

In the secondary transfer region 1530, the superimposed toner images of each color are collectively transferred onto the conveyed recording sheet P by the action of the secondary transfer roller 1503.
The recording sheet P on which the toner image is secondarily transferred reaches the fixing nip region 1531, and the toner image is fixed on the recording sheet P by the fixing device 1520. The recording sheet P on which the toner image is fixed is discharged to a paper discharge tray 1513 via a paper discharge roller 1514.

FIG. 8 is a diagram illustrating the configuration of the control unit 100 that controls the operation of each unit in the image forming apparatus. As shown in the figure, the control unit 100 includes a CPU 101, an image signal processing unit 102, a RAM 103, a ROM 104, and the like.
When print job data is received from an external terminal (personal computer) via a communication interface (not shown), the image signal processing unit 102 performs smoothing processing, edge enhancement processing, etc. on the R, G, B image data. And a color conversion process to generate a digital image signal for each development color of YMCK.

The ROM 104 stores a control program necessary for an image forming operation, a temperature control program for the fixing roller 1 in the fixing device 1520, and the like.
The RAM 105 serves as a work area when the above programs are executed.
The CPU 101 reads out a program necessary for control from the ROM 104, and comprehensively controls the operations of the image signal processing unit 105, the intermediate transfer unit 1500, the sheet feeding / conveying unit 1515, the fixing device 1520, and the like, thereby performing a smooth image forming operation. To realize.

Further, based on the detection signal from the temperature sensor 61, the CPU 101 controls the output of the high-frequency inverter 110 that drives the exciting coil 31, as will be described below, and controls the temperature of the sheet passing area of the fixing roller 1 during fixing. Maintain a predetermined target temperature (fixing temperature).
(2) Configuration of Fixing Device Next, the configuration of the fixing device 1520 will be specifically described.

FIG. 1 is a perspective view showing the main part of the fixing device 1520, which is partially cut away to show the internal structure. FIG. 2 is a schematic view of the fixing device 1520 when viewed from the direction B in FIG.
The fixing device 1520 is provided with a recording sheet separation claw 8 and the like in addition to the fixing roller 1 and pressure roller 2 for heating and pressing the recording sheet P, and the electromagnetic induction heating device 3 for induction heating the fixing roller 1. .

The fixing roller 1 and the pressure roller 2 are arranged in parallel, and both ends of each shaft are rotatably supported by a bearing member (not shown). The pressure roller 2 is urged toward the fixing roller 1 by a pressure mechanism (not shown) using a spring or the like, and presses the surface of the fixing roller 1 with a predetermined pressure to form a fixing nip region 1531. .
The pressure roller 2 is rotationally driven at a predetermined peripheral speed in a clockwise direction indicated by an arrow by a drive mechanism (not shown). The fixing roller 1 rotates following the rotation of the pressure roller 2.

The fixing roller 1 has a five-layer structure including a core metal 11 as a support layer, a heat insulating layer 12, an electromagnetic induction heat generating layer 13, an elastic layer 14, and a release layer 15 in order from the inside to the outside, and the electromagnetic at the outer peripheral portion. The induction heating layer 13, the elastic layer 14, and the release layer 15 constitute a fixing belt 16.
The fixing belt 16 is a cylindrical body whose shaft extends in the sheet passing width direction (the axial direction of the fixing roller 1) and corresponds to a rotating body that is electromagnetically heated in the present invention. The fixing roller 1 has an ASKER-C hardness of 30 to 90 degrees, for example.

As long as the material of the core 11 can secure the strength as a support layer, it is possible to use a heat-resistant molded pipe such as iron or PPS (polyphenylene sulfide), but it prevents the core metal from generating heat. Therefore, it is desirable to use a non-magnetic or low resistance material that is less affected by electromagnetic induction heating. As a specific example of the core metal 11, an aluminum one having a diameter of 32 mm can be given.
The heat insulation layer 12 is for heat insulation holding of the electromagnetic induction heat generating layer 13, and a sponge material (heat insulation structure) made of a rubber material or a resin material having heat resistance elasticity is used. As a result, the electromagnetic induction heat generating layer 13 is insulated and held, and the pressure induction nip width is increased by allowing the electromagnetic induction heat generating layer 13 to be bent, and the roller hardness is decreased to improve the paper discharge performance and the recording sheet separation performance. Fulfill.

In particular, it is desirable to use a silicon sponge material for the heat insulating layer 12, in which case the thickness is set to 2 to 10 mm, particularly 3 to 7 mm, and the hardness is in the range of 20 to 60 degrees with an Asker rubber hardness meter, particularly 30 to 50. It is desirable to set the degree range. The heat insulating layer 12 may have a two-layer structure of a silicon sponge rubber material and a sponge body.
The electromagnetic induction heat generating layer 13 is an endless belt, and is made of a material having a relatively high magnetic permeability μ and an appropriate resistivity ρ so that the temperature can be easily raised by heat generation by electromagnetic induction. A specific example is a nickel electroformed belt having a thickness of 10 to 100 μm, preferably 20 to 50 μm. Other examples of the material for the electromagnetic induction heating layer 13 include magnetic materials (magnetic metal) such as magnetic stainless steel and permalloy. Moreover, not only a magnetic material but a non-magnetic material may be used, for example, a conductive non-magnetic material such as a metal may be formed in a thin film.

Further, the electromagnetic induction heat generating layer 13 may be made of a material in which heat generating particles are dispersed in a resin, or a resin coated with a heat generating material may be used. By using a resin-based material for the electromagnetic induction heat generating layer 13, the separation of the recording sheet P from the fixing roller 1 can be improved.
The elastic layer 14 has a role of improving the adhesion between the recording sheet and the surface of the fixing roller, and is formed of a rubber material or a resin material having heat resistance and elasticity. Specific examples of the material include heat-resistant elastomers such as silicon rubber and fluorine rubber that can withstand use at a fixing temperature. Various fillers may be mixed in the elastic layer 14 in order to impart thermal conductivity, reinforcement, or the like. As the thermally conductive particles, diamond, silver, copper, aluminum, marble, glass, or the like may be used, but in practice, silica, alumina, magnesium oxide, boron nitride, and beryllium oxide are preferably used.

The thickness of the elastic layer 14 is preferably in the range of 10 to 800 μm, particularly preferably in the range of 100 to 300 μm. This is because if the thickness of the elastic layer is less than 10 μm, it becomes difficult to obtain elasticity in the thickness direction, while if it exceeds 800 μm, the heat generated in the heat generating layer is difficult to reach the outer peripheral surface of the fixing film. This is because there is a tendency to get worse.
When silicon rubber is used for the elastic layer, the JIS hardness is 1 to 80 degrees, preferably 5 to 30 degrees. Within this JIS hardness range, it is possible to prevent poor toner fixability while preventing a decrease in strength and poor adhesion of the elastic layer.

As the silicone rubber, one-component, two-component or three-component or more silicone rubber, LTV type, RTV type or HTV type silicone rubber, condensation type or addition type silicone rubber can be used. Here, the elastic layer 14 is a silicon rubber layer having a JIS hardness of 10 degrees and a thickness of 200 μm.
The release layer 15 is the outermost layer and plays a role of improving the release property of the surface of the fixing roller 1.

  The release layer 15 is a material that can withstand use at a fixing temperature and has toner releasability, specifically, a fluororesin such as silicon rubber, fluororubber, PFA, PTFE, FEP, PFEP, or the like. It is preferable to use a mixture. 5-100 micrometers is preferable and, as for the thickness of a mold release layer, 10-50 micrometers is more preferable. Further, in order to improve the interlayer adhesive force between the elastic layer 14 and the release layer 15, an adhesion treatment with a primer or the like may be performed. In addition, a conductive material, an abrasion resistant material, and a good heat conductive material can be added to the release layer 15 as a filler, if necessary.

  Similar to the fixing roller 1, the pressure roller 2 is a roller in which a heat insulating layer 22 and a release layer 25 that enhances the surface releasability are provided on the outer periphery of the cored bar 21. The material of the cored bar 21 is only required to be strong. For example, a pipe molded with a heat-resistant material such as iron or PPS (polyphenylene sulfide) can be used, but in order to prevent heat generation by electromagnetic induction. It is desirable to use a nonmagnetic material.

As a specific example of the pressure roller 2, the core metal 21 is aluminum having a diameter of 27 mm, the heat insulating layer 22 is silicon sponge rubber having a thickness of 3 to 10 mm, PTFE, PFA, or the like, and the release layer 25 is fluorine having a thickness of 10 to 50 μm. It is formed with a release layer made of a resin.
The pressure roller 2 is pressed against the fixing roller 1 with a load of 300 to 500N. In this case, the nip width is about 5 to 15 mm, but the nip width is changed by changing the load.

The thickness of the silicon sponge rubber layer in the heat insulating layer 22 can be appropriately changed according to the use conditions. The heat insulating layer 22 may have a two-layer structure of silicon rubber and silicon sponge.
(3) Configuration of Electromagnetic Induction Heating Device 3 The electromagnetic induction heating device 3 in the fixing device 1520 includes an exciting coil 31, a magnetic core 32, and a core heater 37 for heating a part of the magnetic bodies 32, as shown in FIG. And a coil bobbin 33 for holding the exciting coil in a wound state, and provided outside the outer peripheral surface of the fixing roller 1.

FIG. 3 is a perspective view showing main parts of the exciting coil 31 and the magnetic core 32.
The exciting coil 31 has a structure in which a conducting wire is wound in an oval shape along the longitudinal direction of the fixing roller 1 as shown in FIGS. 1 and 3, and has a arched cross section as shown in FIG. 2. And the magnetic body core 32 is provided so that the outer surface of this exciting coil 31 may be covered.
The exciting coil 31 is supplied with high frequency power of 10 to 100 [KHz] and 100 to 2000 [W] from the high frequency inverter 110 (FIG. 8). It is preferable to use a material coated with a heat-resistant resin in consideration of insulation during heating.

As shown in FIG. 2, the magnetic core 32 includes a main core 34, an end core 35, and a skirt core 36. The magnetic core 32 increases the efficiency of the magnetic circuit and shields magnetism from leaking outside.
In the present invention, the temperature of the fixing roller 1 is automatically controlled by utilizing the fact that the magnetic permeability is lowered when the temperature of the magnetic core 32 exceeds the Curie temperature. It is set within a predetermined range.

As the material, a material having high magnetic permeability and low loss, for example, permalloy (Fe—Ni alloy) is used. In the case of an alloy such as permalloy, the Curie temperature varies depending on the alloy composition. Therefore, the Curie temperature can be set by selecting an alloy composition having an appropriate Curie temperature as the material of the magnetic core 32.
For example, in the case of Fe—Ni alloy, when the atomic percentage (at%) is Fe 70% and Ni 30%, a Curie temperature of about 100 ° C. is obtained. In the Fe—Ni—Cr alloy, Ni 31%, Fe 61%, In the case of 8% Cr, a Curie temperature of about 70 ° C. is obtained. And this Curie temperature changes according to Ni content.

In addition to this, a Cu—Ni alloy or a Mn—Pt alloy can also be used as the material of the magnetic core 32. In this case, a desired Curie temperature can be obtained by changing the composition ratio.
An alloy such as permalloy may have a laminated structure because eddy current loss in the core due to high frequency increases. If magnetic circuit parts such as the excitation coil 31 and the magnetic core 32 are made of a resin material in which magnetic powder is dispersed, the magnetic permeability is relatively low, but the shape can be freely set.

  As shown in FIGS. 3 and 4A, the main core 34 includes a plurality of (here, twelve) main core portions 34a to 34e, which extend over the entire axial direction (paper passing width direction) of the fixing roller 1. Has been placed. Each of the main core portions 34a to 34e extends in an arch shape (see FIG. 2) along a plane perpendicular to the axis of the fixing roller 1 so as to cover the outside of the exciting coil 31, and the length thereof is, for example, 100 mm. It is.

By arranging the main core 34 in this way, when the exciting coil 31 is energized, it passes through the main core 34 and a part of the electromagnetic induction heat generating layer 13 of the fixing roller 1 along a plane perpendicular to the axis of the fixing roller 1. A loop of magnetic flux (broken line Φ in FIG. 2) is formed.
Here, since the magnetic flux Φ changes with the high-frequency current supplied to the exciting coil 31, an eddy current flows in the electromagnetic induction heat generating layer 13 and the layer itself generates Joule heat. The fixing belt 16 is heated by the heat generated by the electromagnetic induction heat generating layer 13.

A projecting portion is provided at the center of the main core 34 to form an E-shaped cross section, and the projecting portion is fitted into the hole of the exciting coil 31 toward the fixing roller 1. If they are close, the heat generation efficiency by electromagnetic induction can be increased.
The hem core 36 has a quadrangular cross section and is continuously arranged so as to connect the end portions of the main core portions 34 a to 34 e over the entire axial length of the fixing roller 1.

The end core 35 has a rectangular cross section as shown in FIG. 2 and a length of 5 to 10 mm. The end core 35 is disposed at positions substantially corresponding to both axial ends of the fixing roller 1.
(4) Temperature control of fixing roller 1 The temperature of the fixing roller 1 during the image forming operation is controlled as follows.
(A) Output Control of Excitation Coil 31 During the image forming operation, as a temperature control of the fixing roller 1, the control unit 100 performs output control of the excitation coil 31 to set the temperature of the sheet passing area on the surface of the fixing roller 1 to the fixing temperature. Maintain Tr.

For temperature control, as shown in FIG. 4B, a temperature sensor 61 is provided on the outer peripheral surface of the fixing roller 1, and the temperature detection signal is input to the control unit 100. The temperature sensor 61 is disposed at a position close to the center of the fixing roller 1 so that the temperature of the sheet passing area can be detected under the assumption that the recording sheet passes through the center area of the fixing roller 1.
The temperature sensor 61 may be a contact type or a non-contact type with respect to the surface of the fixing roller 1, and an infrared sensor (trade name NC sensor), a thermistor, or the like can be used.

The control unit 100 uses a high frequency inverter so that the temperature detection signal (surface temperature in the central region of the fixing roller 1) input from the temperature sensor 61 becomes the fixing temperature Tr (for example, 170 ° C.) set in advance as the operating temperature. To control the amount of power supplied to the exciting coil 31.
Specifically, the high-frequency inverter is provided with a main capacitor that forms an exciting coil and an LC oscillation circuit, an IGBT that turns the LC oscillation circuit on and off, and the control unit monitors a detected temperature signal from the temperature sensor 61. Meanwhile, the IGBT is ON / OFF controlled so that the detected temperature signal approaches the fixing temperature Tr.

By controlling the output from the high frequency inverter to the exciting coil 31 in this way, the surface temperature of the central region of the fixing belt 16 is maintained near the fixing temperature Tr.
Even when the recording sheet P passes through the outer surface of the fixing roller 1, heat is deprived from the fixing belt 16 to the recording sheet P. The amount of deprived heat is immediately transferred to the fixing belt 16 by the electromagnetic induction heating device 3. Since the replenishment is performed, the temperature of the central region of the fixing belt 16 is maintained at the fixing temperature Tr.

(B) Temperature self-control of the fixing belt 16 by changing the magnetic permeability of the magnetic core When the fixing belt 16 is induction-heated during the fixing operation, the main core 34 receives radiant heat and conduction heat from the fixing belt 16. In addition, heat is radiated to a colder external environment while being influenced by the self-heating of the exciting coil 31.
Here, when the temperature rises more than necessary in the non-sheet passing region of the fixing belt 16, the temperature of the main core 34 becomes equal to or higher than the Curie temperature, and the magnetic permeability of the main core 34 decreases (i) Each of the main cores 34 is configured to change the temperature well following the temperature change of the area facing the main core 34 on the surface of the fixing belt 16, and (ii) in an area that may become a non-sheet passing area. The Curie temperature of the existing main core portions 34a to 34d is set to be lower than the fixing temperature.

Each is described in detail below.
(I) Temperature Followability of Main Core 34 FIG. 5 is an example of a characteristic diagram showing the relationship between the temperature of the fixing belt, the temperature of the exciting coil, and the temperature of the main core. This characteristic diagram shows the temperature of the fixing belt 16, the exciting coil 31, and the main core 34 in a steady state by using an experimental fixing device, setting the fixing belt 16 to several temperatures of 170 ° C. to 240 ° C. It is what. This temperature measurement was performed in the central region, but not only in the central region but also in other regions, when in a steady state, it is considered that the temperature relationship is similar to that shown in FIG.

FIG. 6 is a diagram created based on FIG. 5, and when the temperature of the fixing belt 16 is 230 ° C., 220 ° C., 210 ° C., 190 ° C., and 170 ° C. in a steady state, the exciting coil 31 and the main core 34 are illustrated. Represents the temperature taken by.
As can be seen from FIG. 5, the temperature of the exciting coil 31 is lower than the temperature of the fixing belt 16 and the temperature of the main core 34 is lower, and the exciting coil 31 to the exciting coil 31 and the main core 34 in the steady state. Further, it can be seen that a steady heat flow is generated toward the external environment.

As described above, in the present embodiment, the main core 34 exists outside the outer peripheral surface of the fixing belt 16, and a low-temperature external environment exists outside the main core 34, so that the heat flow is substantially perpendicular to the axis of the fixing roller 1. Along the plane.
Therefore, the temperature followability when the temperature of the opposing main core 34 changes following the temperature change in each region of the fixing belt 16 is compared with the case where the exciting coil and the magnetic core are provided inside the conventional fixing roller. It will be much better.

Next, the response when the temperature of the opposing main core 34 changes following the temperature change in each region of the fixing belt 16 will be considered.
If the temperature changes in a certain area of the fixing belt 16, the temperature of the main core portions 34a to 34e in the area also changes and approaches the steady state after the temperature change.
For example, if there is a region of the fixing belt 16 where the temperature rises as D → C → B → A, the amount of heat flowing to the main core portions 34a to 34e in that region increases, and the main core portions 34a to 34e. The temperature rises as follows: d → c → b → a.

On the other hand, when the temperature of the fixing belt 16 is lowered as A → B → C → D, the amount of heat flowing to the main core portions 34a to 34e in the region is reduced, and the temperature of the main core portions 34a to 34e is changed from a → b →. It descends as c → d.
Here, a time delay occurs when the temperature of the main core portions 34a to 34e changes following the temperature change of the fixing belt 16, and the smaller the time delay, the better the response.

  If the responsiveness is not good, it takes time until the temperature of the main core 34 rises even if the temperature of the fixing belt 16 rises in the non-sheet-passing region. In this embodiment, as described above, the heat flow from the fixing belt 16 to the main core 34 is along a plane perpendicular to the axis of the fixing roller 1 in the present embodiment. There is little heat flow to.

In addition, since the main core 34 is formed in an arch shape in cross section as described above to increase the facing area between the fixing belt 16 and the main core portions 34a to 34e, each region portion of the fixing belt 16 and the main core portion are arranged. Heat transfer is good between 34a-34e.
Therefore, the response when the temperature of the main core 34 changes following the temperature change of the fixing belt 16 is also good.

Further, in the present embodiment, the plurality of main core portions 34a to 34e are arranged in the axial direction of the fixing roller 1 and the main core portions 34a to 34e are spaced from each other, so that it is difficult to conduct heat. The main core portions 34a to 34e contribute to improving the response when the temperature follows the temperature.
In order to further improve the temperature followability and response of the main core 34, it is effective to use a material with good thermal conductivity for the main core 34 or to reduce the heat capacity.

In order to further improve the heat transfer performance, the distance between the fixing belt 16 and the main core 34 is set to be small so that heat transfer is improved between each region portion of the fixing belt 16 and the main core portions 34a to 34e. In addition, it is effective to use a material having good thermal conductivity for the main core 34.
In addition, providing a protruding portion at the center of the main core 34 as described above also contributes to improving heat transfer.

In order to reduce the heat capacity of the main core 34, it is desirable to reduce the volume by setting the thickness of the main core 34 small within a range in which the function of guiding the original magnetic flux of the main core 34 can be ensured.
It is also desirable to use a material with a small specific heat as the material of the main core 34.
(Ii) Curie temperature of the main core 34 As shown in FIG. 4B, the minimum size of the recording sheet P that can be used in the image forming apparatus 1000 is P4 (sheet passing width D), and the center of the fixing roller 1 (in the sheet passing width direction). Assuming that the non-sheet passing area is mirror-symmetrical with respect to the center of the fixing roller 1, the main core portions 34 a to 34 e are mainly divided by four at both ends. The areas of the core portions 34a to 34d can be non-sheet passing areas.

The temperature of each main core portion 34a to 34e changes following the temperature change in the region of the fixing belt 16 facing the main core portion. During the fixing operation, the temperature in the central region of the fixing belt 16 is the fixing temperature. It is maintained at Tr.
The temperature of the main core portion 34e in the central region is lower than the fixing temperature Tr by a temperature difference α0 (Tr−α0) (see FIG. 6).

This temperature difference α0 is a characteristic value determined by the structure of the fixing roller 1, the excitation coil 31, the magnetic core 32, etc., and corresponds in the example of FIG. 6 when the fixing temperature Tr is the temperature at point D (170 ° C.). The temperature of the main core portion 34e is about 70 ° C. at the point d, and the temperature difference α 0 is about 100 ° C.
Therefore, if the Curie temperature of the main core portions 34a to 34e is set to be higher than the above temperature (Tr−α0), the temperature of the main core portions 34a to 34e is increased until the temperature of the fixing belt 16 is raised to the fixing temperature Tr. Since the temperature does not exceed, the entire fixing roller 1 can be quickly raised to the fixing temperature Tr.

On the other hand, the upper limit of the Curie temperature range of the main core portions 34a to 34d may be set so that the fixing belt 16 does not exceed the heat resistant temperature when the temperature of the main core portions 34a to 34d reaches the Curie temperature. .
For example, when the responsiveness of the main core 34 following the temperature is very good, the temperature of the main core 34 corresponding to the heat resistance temperature (about 220 ° C.) of the fixing belt 16 and the excitation coil 31 (particularly shown in FIG. 6). Although it has been confirmed by experiments that it is about 110 ° C.)) can be considered the upper limit of the Curie temperature range.

Of course, when the temperature responsiveness of the main core 34 is not so good, a slightly lower temperature is set to the upper limit of the Curie temperature range in consideration of the time delay from the temperature rise of the fixing belt 16 to the temperature rise of the main core 34. It does not matter.
For example, in the fixing device having the characteristics shown in FIG. 6, in consideration of responsiveness, β ° C. (for example, 210 ° C. lower by 10 ° C.) lower than the heat-resistant temperature (about 220 ° C.) is set as the upper limit temperature of the fixing belt. Since the corresponding temperature (210 ° C.−α 1) of the main core 34 is a point b (about 100 ° C.) lower than the point B by a temperature difference α 1, this temperature may be considered as the upper limit of the Curie temperature range.

The value of β can be easily obtained by experiments based on the temperature rise rate of the fixing roller 1 and the responsiveness of the temperature rise of the main core 34.
As described above, if the Curie temperature of the main core portions 34a to 34d is set within the range obtained based on the fixing temperature Tr and the heat resistance temperature of the fixing belt 16 and the excitation coil 31, the fixing roller 1 is fixed to the fixing temperature Tr ( The temperature can be rapidly raised to 170 ° C., and the heat-resistant temperature is not greatly exceeded.

In the characteristics shown in FIG. 6, the values of the temperature differences α0 and α1 between the fixing belt 16 and the main core 34 change with changes in the structure of the fixing roller 1, the excitation coil 31, the magnetic core 32, and the like. To do.
For example, when the magnetic core 32 is in contact with the excitation coil 31, the temperature of the excitation coil 31 is substantially equal to the temperature of the magnetic core 32, and the temperature differences α0 and α1 are also reduced. As described above, when the temperature differences α0 and α1 are small, the upper limit of the Curie temperature range of the main core portions 34a to 34d corresponding to the belt upper limit temperature is also high and is about 200 ° C.

Therefore, the range in which the Curie temperature of the main core portions 34a to 34d is to be set can be determined by mounting it in a specific device and repeating the experiment.
For the main core portion 34e existing in the central region, the same magnetic material as that of the main core portions 34a to 34d may be used. However, this region is always a paper passing region, and an excessive temperature rise of the fixing belt 16 does not occur. Therefore, a magnetic material having a higher Curie temperature may be used.

When the same magnetic material is used, the manufacture of the main core 34 becomes easy. If a magnetic material having a high Curie temperature is used, the main core portion 34e does not exceed the Curie temperature and the magnetic permeability remains high. For the 16 central region, the fixing belt 16 can be heated to the fixing temperature Tr more reliably.
Further, the target for setting the Curie temperature as described above may be only the main core 34, but may also be set similarly for the end core 35 and the hem core 36.

When the position through which the recording sheet P passes is not the center of the fixing roller 1 but the main core portion that can be a non-sheet passing region is changed, the Curie temperature of the target main core portion may be set similarly. .
As described above, by ensuring the temperature followability of the main core 34 and setting the Curie temperature within the Curie temperature range, the temperature of the fixing belt 16 is adjusted as follows.

Until the temperature detected by the temperature sensor 61 reaches near the fixing temperature Tr, the magnetic core 32 is kept at a lower temperature than the Curie temperature and its magnetic permeability remains high. A high-density magnetic flux flows in the entire area in the sheet passing width direction of the fixing roller 1 and the temperature is rapidly increased.
After the temperature detected by the temperature sensor 61 reaches the fixing temperature Tr, the central area where the main core portion 34e of the fixing belt 16 is provided is maintained near the fixing temperature Tr.

Here, when the recording sheet P passes through the fixing roller 1, heat is removed from the fixing belt 16 in the sheet passing area, and a temperature distribution is formed in which the temperature of the fixing belt 16 increases in the non-sheet passing area.
In the fixing device of the present embodiment, since the Curie temperature is set within the above Curie temperature range (about 70 ° C. to 100 ° C.), the main core portions 34a to 34d follow the temperature rise of the fixing belt 16. The temperature also rises and exceeds the Curie temperature in the region where the temperature of the fixing belt 16 is high.

When the temperature of the main core 34 exceeds the Curie temperature, the magnetic permeability rapidly decreases. When the temperature exceeds the Curie temperature by 10 ° C. to 20 ° C., the magnetic permeability decreases to near 1. Along with this, the amount of heat generated in the electromagnetic induction heat generating layer 13 is also reduced (in general, the amount of heat generated by the electromagnetic induction is proportional to the 1/2 power of the magnetic permeability).
Therefore, in the region where the temperature of the main core 34 exceeds the Curie temperature, the magnetic flux density formed by the exciting coil 31 is reduced, and the amount of heat generated by electromagnetic induction is also reduced.

Thereby, a suitable excessive temperature rise suppression effect is obtained in the non-sheet passing region.
On the other hand, in the region where the temperature of the main core 34 is not more than the Curie temperature including the central region where the main core portion 34e of the fixing belt 16 is provided, the magnetic flux density formed by the exciting coil 31 does not decrease, and electromagnetic induction Since the heat generation amount in the heat generating layer 13 is also large, the fixing temperature Tr is stably maintained.

As described above, in the fixing device of the present embodiment, the overheating of the fixing belt 16 is suppressed by self-control so that the induction heat generation amount is lowered in the region where the surface temperature of the fixing belt 16 is high, and other than that. In the central region, a sufficient amount of heat is obtained by the fixing belt 16. This effect can be obtained in the same manner even when the sheet passing width of the recording sheet P changes.
Therefore, even if the size of the paper passing P changes, the excessive temperature rise in the non-paper passing area can be reduced while performing stable fixing.

Further, since it is not necessary to provide a degaussing coil or the like for adjusting the induction heat generation amount of the fixing belt 16, the apparatus cost can be reduced and the apparatus can be kept compact.
<Modification>
(1) In the fixing device 1520, the setting of the fixing temperature Tr may be changed.
When the fixing temperature setting can be changed in this way, the lower limit of the Curie temperature of the portion in the non-sheet passing region of the main core 34 is set based on the highest temperature among the fixing temperatures that can be set.

  (2) Further, the image forming apparatus to which the fixing device according to the above embodiment is applied may be a monochrome printer as well as a color printer, and a copier, a multifunction machine, A FAX apparatus may be used.

  The present invention is suitable as a fixing device of an image forming apparatus typified by a laser printer, a copying machine, a FAX, a multifunction machine, or the like.

FIG. 3 is a perspective view in which the fixing device according to the embodiment is partially cut away. It is the schematic which looked at the said fixing device from the B direction of FIG. It is a perspective view which shows the principal part of an exciting coil and a magnetic body core. (A) (b) is a figure explaining the mechanism which controls the temperature of the fixing roller concerning Embodiment. FIG. 6 is an example of a characteristic diagram illustrating a relationship between a fixing belt temperature, an exciting coil temperature, and a main core temperature. This shows the temperature that the exciting coil and the main core take when the temperature of the fixing belt is steady at each temperature. 1 is a diagram illustrating an overall configuration of an image forming apparatus according to an embodiment. FIG. 3 is a diagram illustrating a configuration of a control unit in the image forming apparatus according to the present embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fixing roller 2 Pressure roller 3 Electromagnetic induction heating means 13 Electromagnetic induction heating layer 14 Elastic layer 15 Release layer 16 Fixing belt 31 Excitation coil 32 Magnetic body core 33 Coil bobbin 34 Main cores 34a-34d Main core part 35 End core 36 Hem core 61 Temperature sensor 100 Control unit

Claims (5)

A fixing nip portion is formed by arranging a pair of rotating bodies close to each other, and at least one rotating body of the pair of rotating bodies is heated by electromagnetic induction heating means, and the recording sheet on which an image to be fixed is formed is fixed. A fixing device that heats and fixes paper through a nip part,
The electromagnetic induction heating means includes
An exciting coil disposed in the longitudinal direction along the circumferential surface of the at least one rotating body;
A magnetic core disposed along the excitation coil in a direction perpendicular to the conveyance direction of the recording sheet,
The Curie temperature of the magnetic core corresponding to the non-sheet passing area when the minimum size recording sheet is passed is Tc, and the rotating body heated by the electromagnetic induction heating means has reached a predetermined fixing temperature. When the temperature of the magnetic core is Tf and the temperature of the magnetic core when the heated rotating body reaches its heat resistant temperature is Th
Tf ≦ Tc <Th
A fixing device characterized in that the relationship is established. The fixing according to claim 1, wherein the magnetic core is formed in an arch shape so as to cover a peripheral surface of the rotating body in a cross section orthogonal to a longitudinal direction of the rotating body to be heated. apparatus. The magnetic core is
The fixing device according to claim 1, wherein the plurality of core portions are arranged in a longitudinal direction of the rotating body. The magnetic core is
The fixing device according to claim 1, wherein the fixing device is made of permalloy.   An image forming apparatus comprising the fixing device according to claim 1.

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