JP2005234000A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device and an image forming apparatus that are inexpensive, as well as lightweight. <P>SOLUTION: The fixing device comprises a fixing member 22, fixed on a recording medium P by heating a toner image T on a recording medium P, a coil part 25 extended in the width direction so as to face the fixing member 22, and core parts 26 and 28 facing the coil part 25. The core parts 26 and 28 are formed of an electromagnetic steel plate. Furthermore, the fixing device is provided with an opening part, in which an opening length of the width direction is different on the peripheral face of the core part 28 and varies the heating range of the width direction of a fixing belt 22 according to the width of the recording medium P. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic method such as a copying machine, a printer, a facsimile, or a multifunction machine thereof, and a fixing device installed therein, and in particular, a fixing device using an electromagnetic induction heating method. And an image forming apparatus.

  2. Description of the Related Art Conventionally, a fixing device using an electromagnetic induction heating method for reducing energy consumption by reducing the start-up time of an image forming apparatus such as a copying machine or a printer is known (for example, Patent Documents). 1).

  In Patent Document 1 or the like, an electromagnetic induction heating type fixing device mainly includes a fixing belt as a fixing member stretched between a heating roller and a fixing roller, an induction heating unit facing the heating roller via the fixing belt, The pressure roller is opposed to the fixing roller via a fixing belt. The induction heating unit includes a coil unit extending in the width direction (a direction perpendicular to the conveyance direction of the recording medium), a core facing the coil unit, and the like. Inside the heating roller, a core (inner core) facing the coil portion via the fixing belt is also provided.

The fixing belt is heated at a position facing the induction heating unit. The heated fixing belt heats and fixes the toner image on the recording medium conveyed to the positions of the fixing roller and the pressure roller. Specifically, when a high-frequency alternating current is passed through the coil part, magnetic lines of force are formed between the core facing the coil part and the inner core in the heating roller, and an eddy current is generated on the surface of the heating roller. When an eddy current is generated in the heating roller, Joule heat is generated by the electric resistance of the heating roller itself. The fixing belt wound around the heating roller is heated by the Joule heat.
A fixing device using such an electromagnetic induction heating method is known to be capable of increasing the surface temperature (fixing temperature) of a fixing belt to a desired temperature and using a small amount of energy and a short start-up time. It has been.

JP 2002-82549 A

  The conventional fixing device described above has a problem that the cost of the device is high and the weight of the device is large.

This is because an expensive material such as ferrite is used as the material of the core portion of the fixing device. Usually, the core part made of ferrite is formed by sintering. For this reason, it has been difficult to reduce the weight of the core part by reducing the thickness.
Specifically, in the case of a roller-shaped inner core installed inside the heating roller, it was difficult to make a hollow structure due to manufacturing restrictions, and a solid structure with a large weight was obtained.

  SUMMARY An advantage of some aspects of the invention is to provide a fixing device and an image forming apparatus which are inexpensive and light in weight.

  According to a first aspect of the present invention, there is provided a fixing device that heats a toner image on a recording medium and fixes the toner image on the recording medium, and a width direction so as to face the fixing member. An extended coil part and a core part facing the coil part are provided, and the core part is made of an electromagnetic steel plate.

  According to a second aspect of the present invention, in the fixing device according to the first aspect, the fixing member is a fixing belt stretched between a heating roller and a fixing roller, and the heating roller includes: The fixing roller is disposed at a position facing the coil portion facing the outer peripheral surface of the fixing belt via the fixing belt, and the fixing roller is connected to the pressure roller that presses the recording medium to be conveyed via the fixing belt. The core portion is disposed at a position facing the coil portion, and is disposed at a position facing the coil portion via the fixing belt and inside the heating roller.

  A fixing device according to a third aspect of the present invention is the fixing device according to the first or second aspect, wherein the core portion is configured to be able to change a posture facing the coil portion, and the core The heating range in the width direction of the fixing member that is heated by electromagnetic induction when a current is passed through the coil portion by changing the posture of the portion is varied.

  A fixing device according to a fourth aspect of the present invention is the fixing device according to any one of the first to third aspects, wherein the core portion is a cylindrical body configured to be rotatable, and the cylindrical portion. The body is provided with a plurality of openings in which the length of the opening on the outer peripheral surface decreases or increases stepwise in the width direction.

  The fixing device according to a fifth aspect of the present invention is the fixing device according to any one of the first to fourth aspects, wherein the core portion is a cylindrical body configured to be rotatable. The body is provided with an opening in which the length opening on the outer peripheral surface gradually decreases or gradually increases in the width direction.

  The fixing device according to a sixth aspect of the present invention is the fixing device according to any one of the first to fifth aspects, wherein the core portion is a cylindrical body configured to be freely rotatable. The body is provided with a blocking member whose length for blocking the magnetic flux is gradually reduced or increased in the width direction on the outer peripheral surface thereof.

  The fixing device according to a seventh aspect of the present invention is the fixing device according to any one of the first to sixth aspects, wherein the core portion is a cylindrical body configured to be freely rotatable. The body is provided with a blocking member whose length for blocking the magnetic flux is gradually decreased or increased in the width direction on the outer peripheral surface thereof.

  The fixing device according to an eighth aspect of the present invention is the fixing device according to any one of the fourth to seventh aspects, wherein the cylindrical body is formed by welding both ends of an electromagnetic steel sheet. The welded portion is located at a position not facing the coil portion when a current is passed through the coil portion.

  An image forming apparatus according to a ninth aspect of the present invention includes the fixing device according to any one of the first to eighth aspects.

  The present invention can provide a fixing device and an image forming apparatus that are reduced in cost and weight because the core portion is formed of a magnetic steel sheet.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

Embodiment 1 FIG.
A first embodiment of the present invention will be described in detail with reference to FIGS.
First, the configuration and operation of the entire image forming apparatus will be described with reference to FIG.
In FIG. 1, 1 is a main body of a laser printer as an image forming apparatus, 3 is an exposure unit that irradiates a photosensitive drum 18 with exposure light L based on image information, and 4 is detachably installed on the apparatus main body 1. A process cartridge as an image forming unit; 7, a transfer unit for transferring a toner image formed on the photosensitive drum 18 to a recording medium P; 10, a paper discharge tray on which an output image is placed; A paper feeding unit in which a recording medium P such as paper is stored, 13 is a registration roller that conveys the recording medium P to the transfer unit 7, and 15 is a recording medium P having a different size from the recording medium P of the paper feeding units 11 and 12. Reference numeral 20 denotes a manual paper feed unit used for transporting the unfixed image on the recording medium P.

With reference to FIG. 1, an operation during normal image formation in the image forming apparatus will be described.
First, exposure light L such as laser light based on image information is emitted from the exposure unit 3 (writing unit) toward the photosensitive drum 18 of the process cartridge 4. The photosensitive drum 18 rotates counterclockwise in the figure, and a toner image corresponding to image information is formed on the photosensitive drum 18 through a predetermined image forming process (charging process, exposure process, development process). Is done.
Thereafter, the toner image formed on the photosensitive drum 18 is transferred onto the recording medium P conveyed by the registration roller 13 in the transfer unit 7.

On the other hand, the recording medium P conveyed to the transfer unit 7 operates as follows.
First, one of the plurality of sheet feeding units 11 and 12 of the image forming apparatus main body 1 is automatically or manually selected (for example, the uppermost sheet feeding unit 11 is selected). ). The plurality of paper feeding units 11 and 12 store recording media P having different sizes and recording media P having the same size and different transport directions.

  Then, the uppermost sheet of the recording medium P stored in the paper feeding unit 11 is transported toward the position of the transport path K. Thereafter, the recording medium P passes through the conveyance path K and reaches the position of the registration roller 13. Then, the recording medium P that has reached the position of the registration roller 13 is conveyed toward the transfer unit 7 at the same timing in order to align with the toner image formed on the photosensitive drum 18.

After the transfer process, the recording medium P passes through the position of the transfer unit 7 and then reaches the fixing device 20 through the conveyance path. The recording medium P that has reached the fixing device 20 is fed between the fixing belt and the pressure roller, and the toner image is fixed by the heat received from the fixing belt and the pressure received from the pressure roller. The recording medium P on which the toner image has been fixed is sent from between the fixing belt and the pressure roller, and then is discharged from the image forming apparatus main body 1 as an output image and placed on the paper discharge tray 10.
Thus, a series of image forming processes is completed.

Next, the configuration / operation of the fixing device 20 installed in the image forming apparatus main body 1 will be described in detail with reference to FIG.
As shown in FIG. 2, the fixing device 20 mainly includes a fixing roller 21, a fixing belt 22, a heating roller 23, an induction heating unit 24, a pressure roller 30, a thermostat 37, an oil application roller 34, a guide plate 35, and a separation plate. 36 etc.

Here, the fixing roller 21 has an elastic layer made of silicone rubber or the like on its surface, and is driven to rotate counterclockwise in FIG. 2 by a driving unit (not shown).
The heating roller 23 is a cylindrical member made of a magnetic / conductive material such as SUS430 and rotates counterclockwise in the figure. Inside the heating roller 23, an internal core 28 (core portion) made of an electromagnetic steel plate is installed. The inner core 28 as a core portion is opposed to the coil portion 25 through the fixing belt 22. The rotation of the inner core 28 is performed separately from the rotation of the heating roller 23 (cylindrical member). The configuration and operation of the heating roller 23 will be described in detail with reference to FIG.

  A fixing belt 22 as a fixing member is stretched and supported by a heating roller 23 and a fixing roller 21. The fixing belt 22 is an endless belt having a multilayer structure including a base layer made of polyimide resin or the like, a heat generating layer made of silver, nickel, iron, or the like, a release layer (surface layer) made of a fluorine compound, or the like. The releasability for the toner T is secured by the release layer of the fixing belt 22.

The induction heating unit 24 includes a coil unit 25, a core 26, a coil guide 27, and the like.
Here, the coil unit 25 extends a litz wire in which thin wires are bundled in the width direction (in the direction perpendicular to the paper in FIG. 2) so as to cover a part of the fixing belt 22 wound around the heating roller 23. It is what. The coil guide 27 is made of a resin material having high heat resistance and holds the coil portion 25. The core 26 is made of an electromagnetic steel plate, and is provided with a center core 26a and a side core 26. The core 26 is installed so as to face the coil portion 25 extending in the width direction.

The “core portion” of the fixing device 20 refers to both opposing core portions that contribute to electromagnetic induction heating. Therefore, the core part of the fixing device 20 is the core 26 of the induction heating part 24 and the internal core 28 provided in the heating roller 23.
In the first embodiment, since the core 26 of the induction heating unit 24 and the internal core 28 provided in the heating roller 23 are formed of electromagnetic steel plates, the fixing device 20 is made inexpensive and lightweight. Can be

  The pressure roller 30 is formed by forming an elastic layer such as fluorine rubber or silicone rubber on the core metal, and is in pressure contact with the fixing roller 21 via the fixing belt 22. Then, the recording medium P is conveyed to a contact portion (a fixing nip portion) between the fixing belt 22 and the pressure roller 30.

A guide plate 35 for guiding the conveyance of the recording medium P is disposed on the entrance side of the contact portion between the fixing belt 22 and the pressure roller 30.
A separation plate 36 that guides the conveyance of the recording medium P and promotes the separation of the recording medium P from the fixing belt 22 is disposed on the exit side of the contact portion between the fixing belt 22 and the pressure roller 30. Yes.

  An oil application roller 34 is in contact with a part of the outer peripheral surface of the fixing belt 22. The oil application roller 34 supplies oil such as silicone oil onto the fixing belt 22. Thereby, the toner releasability on the fixing belt 22 is further ensured. The oil application roller 34 is in contact with a cleaning roller 33 that removes dirt on the surface.

A thermostat 37 is in contact with a part of the outer peripheral surface of the heating roller 23. Then, when the surface temperature on the heating roller 23 exceeds a predetermined temperature, the thermostat 37 cuts off the power supply to the induction heating unit 24. Thereby, overheating of the heating roller 23 by the induction heating unit 24 is limited.
Although not shown, a thermistor is in contact with the outer peripheral surface of the fixing belt 22. Then, the surface temperature (fixing temperature) on the fixing belt 22 is detected by the thermistor, and the heating amount by the induction heating unit 24 is adjusted.

The fixing device 20 configured as described above operates as follows.
As the fixing roller 21 is driven to rotate, the fixing belt 22 rotates in the direction of the arrow in FIG. 2, the cylindrical member of the heating roller 23 also rotates counterclockwise, and the pressure roller 30 also rotates in the direction of the arrow. The fixing belt 22 is heated at a position facing the induction heating unit 24. Specifically, the magnetic field lines are alternately switched between the core 26 and the inner core 28 by flowing a high-frequency alternating current through the coil portion 25. At this time, an eddy current is generated on the surface of the heating roller 23 and Joule heat is generated by the electric resistance of the heating roller 23 itself. The fixing belt 22 wound around the heating roller 23 is heated by the Joule heat.

Thereafter, the surface of the fixing belt 22 that generates heat by the induction heating unit 24 reaches a contact portion with the pressure roller 30. Then, the toner image T on the conveyed recording medium P is heated and melted.
Specifically, the recording medium P carrying the toner image T through the image forming process described above is fed between the fixing belt 22 and the pressure roller 30 while being guided by the guide plate 35 (indicated by an arrow Y10). It is movement in the transport direction.) The toner image T is fixed to the recording medium P by the heat received from the fixing belt 22 and the pressure received from the pressure roller 30, and the recording medium P is sent from between the fixing belt 22 and the pressure roller 30.

3 to 5, the configuration and operation of the heating roller 23 and the inner core 28, which are characteristic in the first embodiment, will be described in detail.
FIG. 3 is a front view of the heating roller 23 installed in the fixing device 20 of FIG. 2 as viewed in the width direction from the induction heating unit 24 side. FIG. 4 is a perspective view showing the inner core 28 installed in the heating roller 23.

As shown in FIG. 3, a cylindrical inner core 28 is rotatably installed in the cylindrical member of the heating roller 23.
Specifically, referring to FIG. 4, the inner core 28 as a core portion is formed by bending a rectangular electromagnetic steel sheet having a thickness of about 1 mm and then welding both ends thereof into a cylindrical body (see FIG. 4). 28b in 4 is a welding part.). Here, the electromagnetic steel sheet is made by adding silicon (silicone) to inexpensive iron among materials (ferromagnetic materials) that are easy to pass magnetic lines of force, or by adjusting the crystal orientation and the width of magnetic domains. , Improved magnetic properties.

3 and 4, the outer peripheral surface of the inner core 28 is provided with a plurality of openings 28a at both ends in the width direction. The plurality of openings 28a are formed so that the length of the openings increases (or decreases) stepwise in the width direction.
The plurality of openings 28a are formed by punching at a plate-like stage before bending that finishes the electromagnetic steel sheet into a cylindrical body. Thereafter, as shown in FIG. 4, flanges having shaft portions are press-fitted into both ends in the width direction of the inner core 28 finished into a cylindrical body. The cylindrical inner core 28 having a flange is rotatably installed in the heating roller 23 (in the state shown in FIG. 3).

  Here, the plurality of openings 28 a provided in the inner core 28 are provided to vary the heating range in the width direction of the fixing belt 22 heated by the induction heating unit 24. That is, when the inner core 28 facing the coil portion 25 of the induction heating unit 24 has the opening 28a, the density of the magnetic flux formed between the core 26 of the induction heating unit 24 is within the range of the opening 28a. Decrease drastically. Accordingly, the range of the fixing belt 22 corresponding to the range of the opening 28a is hardly heated.

Specifically, the inner core 28 in the state of FIG. 3 is rotated by a predetermined angle in the circumferential direction so that the longest opening 28 a of the inner coil 28 is opposed to the coil portion 25. At this time, the fixing belt 22 corresponding to the longest opening width is not heated, and only the outside of the region (the region of the center width L2) is the heating range of the fixing belt 22.
This state is suitable when the recording medium P having the width L2 is continuously fixed. Specifically, when fixing the recording medium P having the minimum width (for example, 148 mm) handled by the image forming apparatus, the posture facing the coil portion 25 of the inner core 28 is fixed to the above-described state, as shown in FIG. Perform the fixing process described.

  At this time, the fixing temperature distribution in the width direction on the fixing belt 22 is as shown by a solid line R2 in FIG. That is, in the range exceeding the width L2 of the fixing belt 22, the temperature does not increase, so that thermal damage to the fixing belt 22 can be suppressed.

Details are as follows.
The image forming apparatus according to the first embodiment is configured to be able to form images on several types of recording media P having different widths. Here, the recording media P having different widths include recording media of irregular sizes in addition to recording media of various regular sizes in the A and B rows of JIS dimensions. Further, even in the case of the recording medium P having the same size (for example, A4 size), when the longitudinal direction is the transport direction, the short direction (the direction perpendicular to the longitudinal direction) is the transport direction. In this case, recording media having different widths are handled.

  When such a recording medium P having a different width is fixed by the fixing device 20, the heat distribution in the width direction of the fixing belt 22 varies according to the width of the recording medium P (the length in the width direction). As a result, temperature unevenness occurs. For example, when the recording medium P having a small width is passed and fixed, a large amount of heat is taken away at the position of the fixing belt 22 corresponding to the width of the recording medium P (the sheet passing position). The fixing temperature is lower than the position (the non-sheet passing position). Such a phenomenon becomes particularly remarkable when the recording medium P having a small width is continuously fed.

  Therefore, if it is attempted to control the fixing temperature in the entire width direction of the fixing belt 22 based on the fixing temperature at the center in the width direction of the fixing belt 22, the fixing temperature in the center in the width direction of the fixing belt 22 can be controlled to a desired temperature. The fixing temperature at both ends in the width direction will increase. As described above, when the recording medium P having a large width is fixed in a state where the fixing temperature at both ends in the width direction of the fixing belt 22 is increased, a hot offset is generated on the recording medium P corresponding to the temperature increasing position. Further, when the fixing temperature at both ends in the width direction exceeds the heat resistance temperature of the fixing belt 22, the fixing belt 22 is thermally damaged and the life is shortened.

  In the fixing device 20 according to the first embodiment, the heating range in the width direction of the fixing belt 22 is varied in accordance with the width of the recording medium P. Therefore, the temperature rise at both ends in the width direction of the fixing belt 22 is suppressed. be able to.

  FIG. 5 is a graph showing the temperature distribution in the width direction on the fixing belt 22. In FIG. 5, the horizontal axis indicates the position of the fixing belt 22 in the width direction, and the vertical axis indicates the temperature of the fixing belt 22 (fixing temperature). Here, 0 in the width direction position on the horizontal axis indicates the center position in the width direction of the fixing belt 22. A solid line R1 indicates a temperature distribution when the opening range of the opening 28a in the inner core 28 is minimized and the heating range of the fixing belt 22 is maximized, and a solid line R2 is fixed with the opening range of the opening 28a in the inner core 28 being maximized. The temperature distribution when the heating range of the belt 22 is minimized is shown.

The inner core 28 in the state of FIG. 3 is further rotated by a predetermined angle in the circumferential direction so that none of the openings 28 a of the inner coil 28 is opposed to the coil portion 25. At this time, the maximum range of the fixing belt 22 (the entire range of the width L1) is the heating range.
This state is suitable when the recording medium P having the width L1 is continuously fixed. Specifically, when fixing the recording medium P having the maximum width (e.g., 297 mm) handled by the image forming apparatus, the posture of the inner core 28 facing the coil portion 25 is fixed to the above-described state, as shown in FIG. Perform the fixing process described.

  When the recording medium P having a width smaller than L1 and larger than L2 is fixed (image formation), the inner core 28 is rotated by a predetermined angle according to the width of the recording medium P, and the heating range of the fixing belt 22 is changed. Adjust so that becomes the width. As a result, in the range exceeding the width of the recording medium P on the fixing belt 22, the temperature does not increase, so that thermal damage to the fixing belt 22 can be suppressed.

  In the first embodiment, the heating range on the fixing belt 22 is varied by forming the opening 28a on the outer peripheral surface of the cylindrical inner core 28 made of an electromagnetic steel plate. Therefore, the heating range on the fixing belt 22 can be varied with a relatively simple configuration without dividing the core portion into a plurality of portions in the width direction or providing a magnetic flux shielding member on the peripheral surface of the core portion. . Furthermore, since the opening 28a does not protrude from the outer peripheral surface of the inner core 28, the outer diameter of the inner core 28 can be brought close to the inner peripheral surface of the heating roller 23 without restriction. Thereby, the strength of the magnetic field formed between the core 26 and the inner core 28 is increased, and the heating efficiency for the fixing belt 22 is improved.

  As described above, the inner core 28 in the heating roller 23 changes the posture in the rotation direction (the posture facing the coil portion 25) according to the width of the recording medium P to be passed. . Here, the welded portion 28b of the inner core 28 is set to be in a position not facing the coil portion 25 during electromagnetic induction. That is, when fixing any recording medium P from the maximum width L1 to the minimum width L2, when the coil portion 25 is energized and the fixing belt 22 is heated, the welded portion 28b of the inner core 28 is coiled. The position of the welded portion 28b is set so as not to face the portion 25. Thereby, inconvenience that magnetic lines of force are formed between the inner core 28 and the core 26 in a state where the welded portion 28b of the inner core 28 faces the coil portion 25 is prevented. That is, the magnetic field lines formed between the welded portion 28b and the core 26 become non-uniform, and the problem that the temperature unevenness in the width direction occurs on the fixing belt 22 is suppressed.

As described above, in the first embodiment, since the core portions 26 and 28 of the fixing device 20 are formed of electromagnetic steel plates, the device can be made cheaper and lighter. In particular, when the inner core 28 is formed of a magnetic steel sheet, the inner core 28 can have a hollow structure, so that the degree of weight reduction is large.
In addition, a plurality of openings 28 a having different opening widths are provided in the inner core 28, so that the heating range in the width direction of the fixing belt 22 is variable. Thereby, even when the recording medium P having a short width is continuously fixed, the temperature rise at both ends in the width direction of the fixing belt 22 can be suppressed.

  In the first embodiment, the heating range in the width direction of the fixing belt 22 is varied by providing the inner core 28 with a plurality of openings 28a. On the other hand, it can also be set as the structure which provides the interruption | blocking member which interrupts | blocks a magnetic flux in the position corresponding to several opening part 28a, without providing the several opening part 28a in the inner core 28, respectively. As the blocking member, a material having a low magnetic permeability can be used. For example, copper having a thickness of about 0.5 mm can be used. Even in such a case, substantially the same effect as the first embodiment can be obtained.

Embodiment 2. FIG.
6 and 7, the second embodiment of the present invention will be described in detail.
FIG. 6 is a front view of the heating roller 23 installed in the fixing device 20 according to the second embodiment when viewed in the width direction from the induction heating unit 24 side, and corresponds to FIG. 3 of the first embodiment. is there. FIG. 7 is a perspective view showing the inner core 28 installed in the heating roller 23 of FIG. 6 and corresponds to FIG. 4 of the first embodiment.
In the second embodiment, the shape of the inner core 28 installed in the heating roller 23 is different from that in the first embodiment.

As shown in FIG. 6, a substantially cylindrical inner core 28 is rotatably installed in the cylindrical member of the heating roller 23.
Specifically, referring to FIG. 7, an inner core 28 as a core portion is formed by welding both ends of a rectangular electromagnetic steel sheet into a cylindrical body (28b in FIG. 7 is a welded portion).

Referring to FIGS. 6 and 7, openings 28 a are provided on the outer peripheral surface of the inner core 28 at both ends in the width direction. Unlike the first embodiment, the opening 28a of the second embodiment is formed such that the length of the opening gradually increases (or gradually decreases) in the width direction.
Here, the opening 28 a provided in the inner core 28 is provided to vary the heating range in the width direction of the fixing belt 22 heated by the induction heating unit 24, as in the first embodiment. It is.

  Also in the second embodiment, as in the first embodiment, the inner core 28 in the heating roller 23 is rotated by a predetermined angle and fixed according to the width of the recording medium P on which image formation is performed. As a result, the opening width of the opening 28 a facing the coil portion 25 is adjusted, and the heating range of the fixing belt 22 is adjusted to a range corresponding to the width of the recording medium P. Then, temperature rise in the range exceeding the width of the recording medium P on the fixing belt 22 is prevented, and thermal damage to the fixing belt 22 is suppressed.

As described above, in the second embodiment, as in the first embodiment, since the core portions 26 and 28 of the fixing device 20 are formed of electromagnetic steel plates, the cost of the device is reduced and the weight is reduced. be able to.
Further, an opening 28 a whose opening width gradually increases is provided in the inner core 28, and the heating range in the width direction of the fixing belt 22 is varied. Thereby, even when the recording medium P having a short width is continuously fixed, the temperature rise at both ends in the width direction of the fixing belt 22 can be suppressed.

  In the second embodiment, the opening 28 a is provided in the inner core 28 to change the heating range in the width direction of the fixing belt 22. On the other hand, without providing the opening part 28a in the internal core 28, it can also be set as the structure which provides the interruption | blocking member of the shape equivalent to it in the position corresponding to the opening part 28a. Even in such a case, substantially the same effect as in the first embodiment can be obtained.

  It should be noted that the present invention is not limited to the above-described embodiments, and within the scope of the technical idea of the present invention, the above-described embodiments can be modified as appropriate in addition to those suggested in the above-described embodiments. Is clear. Further, the number, position, shape, and the like of the constituent members are not limited to the above-described embodiments, and the number, position, shape, and the like that are suitable for implementing the present invention can be used.

1 is an overall configuration diagram illustrating an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view showing a fixing device installed in the image forming apparatus of FIG. 1. FIG. 3 is a cross-sectional view showing a heating roller installed in the fixing device of FIG. 2. FIG. 4 is a perspective view showing an inner core provided in the heating roller of FIG. 3. 3 is a graph showing a temperature distribution in a width direction on a fixing belt in the fixing device of FIG. 2. It is sectional drawing which shows the heating roller installed in the fixing device in Embodiment 2 of this invention. It is a perspective view which shows the internal core installed in the heating roller of FIG.

Explanation of symbols

1 image forming apparatus main body (apparatus main body), 3 exposure unit, 4 image forming unit,
7 Transfer section, 10 Paper discharge tray, 11, 12 Paper feed section,
20 fixing device, 21 fixing roller, 22 fixing belt (fixing member),
23 heating roller, 24 induction heating unit, 25 coil unit,
26 core, 26a center core, 26b side core,
28 inner core (core part), 28a opening, 28b welded part,
30 pressure roller, 33 cleaning roller, 34 oil application roller,
35 guide plate, 36 separation plate, 37 thermostat, P recording medium.

Claims (9)

A fixing member that heats the toner image on the recording medium and fixes the toner image on the recording medium;
A coil portion extending in the width direction so as to face the fixing member;
A core portion facing the coil portion,
The fixing device, wherein the core portion is made of an electromagnetic steel plate. The fixing member is a fixing belt stretched between a heating roller and a fixing roller,
The heating roller is disposed at a position facing the coil portion facing the outer peripheral surface of the fixing belt via the fixing belt,
The fixing roller is disposed at a position facing the pressure roller that presses the conveyed recording medium via the fixing belt,
The fixing device according to claim 1, wherein the core portion is disposed inside the heating roller at a position facing the coil portion via the fixing belt. The core part is configured to be able to vary the posture facing the coil part,
3. The heating range in the width direction of the fixing member that is heated by electromagnetic induction when a current is passed through the coil portion by changing the posture of the core portion. The fixing device according to 1. The core portion is a cylindrical body configured to be rotatable,
4. The fixing according to claim 1, wherein the cylindrical body includes a plurality of openings in which a length of an opening on an outer peripheral surface thereof is gradually reduced or increased in a width direction. 5. apparatus. The core portion is a cylindrical body configured to be rotatable,
5. The fixing device according to claim 1, wherein the cylindrical body includes an opening whose length that opens on the outer peripheral surface thereof gradually decreases or increases in the width direction. 6. The core portion is a cylindrical body configured to be rotatable,
6. The fixing according to claim 1, wherein the cylindrical body includes a blocking member whose length for blocking the magnetic flux gradually decreases or increases in the width direction on an outer peripheral surface thereof. apparatus. The core portion is a cylindrical body configured to be rotatable,
The fixing device according to claim 1, wherein the cylindrical body includes a blocking member whose length for blocking the magnetic flux is gradually decreased or gradually increased in the width direction on an outer peripheral surface thereof. The cylindrical body is formed by welding both ends of a magnetic steel sheet,
The fixing device according to any one of claims 4 to 7, wherein the welded portion of the cylindrical body is in a position that does not face the coil portion when a current is passed through the coil portion. An image forming apparatus comprising the fixing device according to claim 1.

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