JP4526019B2 - Fixing apparatus and image forming apparatus - Google Patents

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.

  Conventionally, in an image forming apparatus such as a copying machine or a printer, a fixing device using an electromagnetic induction heating method has been known for the purpose of saving energy by reducing the rise time of the device (see, for example, Patent Document 1). .)

  In Patent Document 1, etc., an electromagnetic induction heating type fixing device mainly includes a heating roller, a fixing auxiliary roller (fixing roller), a fixing belt (heat resistant belt) stretched between the heating roller and the fixing auxiliary roller, and a heating roller. And an induction heating unit facing the fixing belt via a fixing belt, a pressure roller facing the fixing auxiliary roller via the fixing belt, and the like. The induction heating unit includes a coil portion extending in the width direction (a direction perpendicular to the conveyance direction of the recording medium), a core (excitation coil core) facing the coil portion, and the like.

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 auxiliary fixing roller and the pressure roller. Specifically, when a high-frequency alternating current is passed through the coil portion, a magnetic field is formed around the coil portion, 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.
It is known that a fixing device using such an electromagnetic induction heating method can raise the surface temperature (fixing temperature) of a fixing belt to a desired temperature with a small start-up time with low energy consumption.

On the other hand, in Patent Document 2 and the like, a fixing device using an electromagnetic induction heating method is used for the purpose of preventing non-sheet passing portion temperature rise and end portion temperature sagging in a fixing member (fixing roller). A technique for providing a first magnetic core and a second magnetic core inside is disclosed.
Specifically, the first magnetic core is fixed to both ends in the width direction, and the second magnetic core is supported rotatably at the center in the axial direction. When passing a normal size recording medium, the central second magnetic core is separated from the exciting coil, and when passing a small size recording medium, the central second magnetic core is removed. The temperature distribution in the width direction of the fixing member is adjusted close to the excitation coil.

Japanese Patent Application Laid-Open No. H11-228561 is a fixing device using an electromagnetic induction heating method, and in order to prevent a non-sheet passing portion temperature rise in the fixing member (heating medium) A technique for providing a magnetic flux shielding plate that shields a magnetic flux that reaches the target is disclosed.
Specifically, the magnetic flux shielding plate has an arcuate curved surface covering the upper half of the induction coil and is configured to be rotatable. Then, by rotating the magnetic flux shielding plate according to the size of the recording medium, the magnetic flux in the non-sheet passing portion is shielded, and the non-sheet passing portion temperature rise is adjusted.

JP 2002-123106 A JP 2003-123961 A JP 2002-352948 A

  The conventional technology described above cannot efficiently suppress the temperature increase at both ends in the width direction of the fixing member, which occurs when a recording medium having a short width direction size is continuously fixed.

Details are as follows.
A general image forming apparatus is configured to form an image on several types of recording media having different sizes in the width direction. Here, the recording media having different sizes in the width direction include recording media of irregular sizes in addition to recording media of various regular sizes in the A and B rows of JIS dimensions. Even in the case of recording media of the same size (for example, A4 size), the transport direction is the short direction (the direction perpendicular to the longitudinal direction) when the longitudinal direction is the transport direction. In some cases, recording media having different sizes in the width direction are handled.

  When fixing such a recording medium having different width direction sizes with the fixing device, the temperature distribution in the width direction of the fixing belt varies depending on the width direction size of the recording medium, resulting in temperature unevenness. was there. For example, when fixing by passing a recording medium having a small size in the width direction, a lot of heat is taken away at the position of the fixing belt corresponding to the size in the width direction of the recording medium (the paper passing area). The fixing temperature is lower than at other positions (non-sheet passing area). Such a phenomenon becomes particularly prominent when a recording medium having a small size in the width direction is continuously fed.

  Therefore, when trying to control the fixing temperature in the entire width direction of the fixing belt with reference to the fixing temperature at the center portion in the width direction of the fixing belt, the fixing temperature at the center portion in the width direction of the fixing belt can be controlled to a desired temperature. The fixing temperature at both ends in the direction will increase. As described above, when a recording medium having a large width direction is fixed in a state where the fixing temperature at both ends in the width direction of the fixing belt is increased, a hot offset occurs on the recording medium corresponding to the temperature increase position. Further, when the fixing temperature at both ends in the width direction exceeds the heat resistance temperature of the fixing belt, it is also conceivable that the fixing belt is thermally damaged.

  In order to solve such a problem, in the above-mentioned Patent Document 2 and the like, the second magnetic core that rotates with the fixed first magnetic core in the magnetic core provided in the fixing member (fixing roller) It is divided into and. The position of the second magnetic core is varied depending on whether the recording medium is a normal size or a small size.

  However, the technique of Patent Document 2 and the like corresponds to recording media of two sizes (normal size and small size), and does not correspond to recording media having three or more different sizes. That is, when a recording medium having an intermediate size between the normal size and the small size is continuously passed, a temperature rise occurs in the intermediate size non-passing region of the fixing member.

  On the other hand, in order to cope with recording media having three or more different sizes, a method of increasing the number of magnetic cores that rotate independently can be considered. However, in that case, since a plurality of prepared magnetic cores must be individually moved and controlled, it is conceivable that the configuration and control of the apparatus become complicated and the cost increases. Further, since a gap is generated between the divided magnetic core and the magnetic core, there is a possibility that the heating on the fixing member corresponding to the position becomes insufficient.

  On the other hand, in Patent Document 3 and the like described above, the magnetic flux shielding plate having an arcuate curved surface is rotated according to the size of the recording medium to shield the magnetic flux in the non-sheet passing portion. The effect of suppressing the temperature rise in the non-sheet passing area can be expected for each recording medium. That is, even for an intermediate size recording medium, the temperature increase in the non-sheet passing area is highly likely to be suppressed by adjusting the rotation angle of the magnetic flux shielding plate to optimize the magnetic flux shielding area.

  However, since the magnetic flux shielding plate is provided inside the fixing member in the technique disclosed in Patent Document 3, the size of the core and the induction coil provided inside the fixing member together with the magnetic flux shielding plate is limited. It is necessary to install the core and the induction coil apart from the inner peripheral surface of the fixing member at least for the thickness of the magnetic flux shielding plate. For this reason, energy loss for heating the fixing member by electromagnetic induction occurs.

  On the other hand, a measure of directly attaching a magnetic flux shielding member that shields the magnetic flux to the core portion facing the fixing member is also conceivable. However, even in that case, the core portion cannot be brought close to the fixing member by the thickness of the adhered magnetic flux shielding member. Further, when the accuracy of sticking the magnetic flux shielding member to the core portion is poor, the effect of suppressing the temperature rise in the non-sheet passing region cannot be sufficiently obtained. Furthermore, it is conceivable that when the atmospheric temperature becomes high, the sticking force of the magnetic flux shielding member is reduced, causing a problem of peeling from the core portion.

  The present invention has been made to solve the above-described problems. Even when a recording medium having a short width is continuously fixed, the temperature rise at both ends in the width direction of the fixing member is relatively simple. It is an object of the present invention to provide a fixing device and an image forming apparatus that can be efficiently suppressed by the configuration.

According to a first aspect of the present invention, there is provided a fixing device that heats a toner image to fix the toner image on a recording medium, and fixes the fixing belt stretched between a heating roller and a fixing auxiliary roller; via the fixing belt roller extending in the width direction so as to face Rutotomoni, a coil portion provided with the circumferential center core in a central position of at a position opposed through the fixing belt to the coil unit The cylinder is disposed inside the heating roller and is configured to be rotatable and so that the length in the width direction of the outer peripheral surface of the coil portion facing the center core gradually increases or decreases with rotation. the outer peripheral surface of the inner core portion of the outer peripheral surface of the body comprises an inner core of the columnar which is cutaway, and facing the center core of the coil portion in accordance with the rotation of the inner core The length in the width direction gradually increasing or by decreasing, and adjusts the heating range of the width direction of the fixing belt is heated by electromagnetic induction by the coil portion.

The fixing device according to a second aspect of the present invention is the fixing device according to the first aspect, wherein the fixing auxiliary roller is configured to pass the fixing belt against the pressure roller that presses the conveyed recording medium. It is arrange | positioned in the position which opposes.

According to a third aspect of the present invention, there is provided a fixing device that heats a toner image to fix the toner image on a recording medium, and contacts the pressure roller that presses the conveyed recording medium; A coil portion that extends in the width direction so as to face the outer peripheral surface of the fixing roller and that has a center core at a central position in the circumferential direction, and a position that faces the coil portion via the fixing roller. The cylindrical body is disposed inside the fixing roller and is configured to be rotatable and so that the length in the width direction of the outer peripheral surface of the coil portion facing the center core gradually increases or decreases with rotation. A columnar inner core in which a part of the outer peripheral surface is cut off, and in the width direction of the outer peripheral surface of the inner core facing the center core of the coil portion as the inner core rotates Incremental or by decreasing of, and adjusts the heating range of the width direction of the fixing roller is heated by electromagnetic induction by the coil portion.

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 width direction of the inner core is in accordance with the size of the recording medium in the width direction. The heating range is adjusted by gradually increasing or decreasing the length.

A 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 inner core is made of ferrite.

An image forming apparatus according to a sixth aspect of the invention includes the fixing device according to any one of the first to fifth aspects.

  In the present invention, a core portion configured to be able to vary a facing range facing the coil portion is provided, and the heating range in the width direction of the fixing member is adjusted by varying the facing range. Thereby, even when a recording medium having a short width is continuously fixed, a temperature increase at both ends in the width direction of the fixing member is efficiently suppressed with a relatively simple configuration. Can be provided.

  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.
The 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 paper feeding units 11, 12, and 15 of the image forming apparatus main body 1 is automatically or manually selected (for example, the uppermost paper feeding unit 11 is selected). To do.) Each of the plurality of paper feeding units 11 and 12 stores a recording medium P having a different size and a recording medium 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 auxiliary 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. It consists of a plate 36 and the like.

Here, the fixing auxiliary roller 21 has an elastic layer such as silicone rubber formed on the surface thereof, and is driven to rotate counterclockwise in FIG. 2 by a driving unit (not shown).
The heating roller 23 is a cylindrical body made of a nonmagnetic material such as SUS304 and rotates counterclockwise in the figure. An internal core 28 (core portion) made of a ferromagnetic material such as ferrite is installed inside the heating roller 23. The inner core 28 as a core portion is opposed to the coil portion 25 through the fixing belt 22. The rotational drive of the inner core 28 is performed separately from the rotational drive of the heating roller 23. The configuration and operation of the heating roller 23 and the inner core 28 will be described in detail later.

A fixing belt 22 as a fixing member is stretched and supported by a heating roller 23 and a fixing auxiliary roller 21. The fixing belt 22 is a multi-layered endless belt in which a heat generation layer, an elastic layer, and a release layer are formed on a base material.
The base material of the fixing belt 22 is made of a heat-resistant resin material. For example, polyimide, polyamideimide, PEEK (polyetheretherketone), PES (polyethersulfone), PPS (polyphenylene sulfide), fluorine resin, or the like can be used. . As the heat generating layer, nickel, stainless steel, iron, copper, cobalt, chromium, aluminum, gold, platinum, silver, tin, palladium, an alloy made of a plurality of these metals, or the like can be used. As the elastic layer, silicone rubber, fluorosilicone rubber, or the like can be used. As the release layer, fluorine resins such as tetrafluoroethylene resin (PTFE) and tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer (FEP), or a mixture of these resins can be used.

The induction heating unit 24 includes a coil unit 25, a core 26, a coil guide 27, and the like.
Here, the coil portion 25 is wound around a litz wire in which fine wires are bundled so as to cover the outer peripheral surface of the fixing belt 22 wound around the support roller 23 and is in the width direction (in the direction perpendicular to the paper surface of FIG. 2). It is extended. The coil guide 27 is made of a highly heat-resistant resin material or the like, and holds the coil part 25 and functions as a frame of the induction heating part 24. The core 26 is made of a ferromagnetic material such as ferrite having a relative permeability of about 2500, and is provided with a center core 26a and a side core 26b. The core 26 is installed so as to face the coil portion 25 extending in the width direction. The center core 26 a is at a substantially central position in the circumferential direction of the coil portion 25. The coil unit 25 is connected to a high frequency power supply unit (not shown), and an alternating current of 10 k to 1 MHz is applied from the high frequency power supply unit.

  The pressure roller 30 is formed by forming an elastic layer such as fluorine rubber or silicone rubber on a cylindrical member made of aluminum, copper, stainless steel or the like. The elastic layer of the pressure roller 30 has a thickness of 1 to 5 mm and an Asker hardness of 20 to 50 degrees. The pressure roller 30 is in contact with the fixing auxiliary 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 conveyance of the recording medium P and promotes 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.

  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 output in the induction heating unit 24 including the inverter circuit is adjusted. Thus, the fixing temperature on the fixing belt 22 is kept constant.

The fixing device 20 configured as described above operates as follows.
By the rotation driving of the auxiliary fixing roller 21, the fixing belt 22 rotates in the direction of the arrow in FIG. 2, 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 the heat generating layer of the fixing belt 22, and Joule heat is generated by the electric resistance of the heating roller 23 and the heat generating layer. Thus, the fixing belt 22 is heated by the heat received from the heat roller 23 that has generated heat and the heat generated by its own heat generation layer.

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 the arrow Y). 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 6, the configuration and operation of the internal core 28 that are characteristic in the first embodiment will be described in detail.
FIG. 3 is a 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, and shows the inside of the heating roller 23. FIG. 4 is a perspective view showing the inner core 28 installed in the heating roller 23.

Referring to FIGS. 3 and 4, an inner core 28 is rotatably installed in a cylindrical heating roller 23.
Specifically, referring to FIG. 4, the inner core 28 as a core portion is a columnar member in which a part of the outer peripheral surface of the cylindrical body is cut off, and shaft portions 28 b are inserted at both ends thereof. . That is, the core portion 28 includes a non-notched portion 28a1 (a central portion in the width direction) in which the outer peripheral surface is not cut off and a cutout portion 28a2 (both ends in the width direction) in which the outer peripheral surface is cut off. ) And. The notch 28a2 is formed so as to cut out an arc curved surface at both ends of the cylindrical body.

  Referring to FIG. 3, shaft portions 28 b provided at both ends of the inner core 28 are inserted into bearings (not shown) provided at both end surfaces of the heating roller 23, and are inserted into the frame of the fixing device 20. It is supported rotatably. One shaft portion 28b is connected to a stepping motor (not shown). With such a configuration, the inner core 28 can be rotated by a desired angle. The rotational drive system of the inner core 28 has a configuration independent of the rotational drive system of the heating roller 23.

With such a configuration, when the inner core 28 rotates, the length in the width direction (opposed range) of the outer peripheral surface of the inner core 28 facing the coil portion 25 is gradually increased or decreased.
Here, the facing range facing the coil portion 25 is, in particular, the length in the width direction of the outer peripheral surface of the inner core 28 facing the center core 26a at the center position of the coil portion 25.

  The opposed range that is varied with the rotation of the inner core 28 is for varying the heating range in the width direction of the fixing belt 22 heated by the induction heating unit 24. That is, according to the width direction length of the outer peripheral surface of the inner core 28 facing the coil portion 25 (particularly, the position of the center core 26a) of the induction heating unit 24, the space between the induction heating unit 24 and the core 26 is increased. The density of the magnetic flux formed on the surface increases or decreases. When the width direction length of the outer peripheral surface of the opposed inner core 28 is large, the heating range of the fixing belt 22 is also increased, and when the width direction length is small, the heating range of the fixing belt 22 is also decreased.

  5A to 5C are cross-sectional views showing a state in which the internal core 28 provided in the heating roller 23 is rotated (cross-sectional views taken along AA, BB, and CC in FIG. 3). It is. The broken line arrows in FIGS. 5A to 5C indicate the position of the center core 26a.

FIG. 5A shows a state in which the longest facing range (the width L1 in FIG. 3) of the inner core 28 is opposed to the coil portion 25. At this time, the fixing belt 22 corresponding to the longest outer peripheral surface length (L1) is heated.
The state shown in FIG. 5A 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 (for example, 297 mm) handled by the image forming apparatus, the length of the outer peripheral surface facing the coil portion 25 of the inner core 28 is fixed to the above state. The fixing process described in FIG. 2 is performed.

FIG. 5 (B) shows a state in which the opposed range (in the middle of the width L1 and the width L2 in FIG. 3) of the inner core 28 is opposed to the coil portion 25. At this time, the fixing belt 22 corresponding to the intermediate outer peripheral surface length is heated.
The state shown in FIG. 5B is suitable for the case where the medium width recording medium P is continuously fixed. Specifically, when fixing a recording medium P having an intermediate width (for example, 210 mm) handled by the image forming apparatus, the outer peripheral surface length facing the coil portion 25 of the inner core 28 is fixed to the above-described state. The fixing process described in FIG. 2 is performed.

FIG. 5C shows a state where the shortest facing range (the width L2 in FIG. 3) of the inner core 28 is opposed to the coil portion 25. At this time, the fixing belt 22 corresponding to the shortest outer peripheral surface length (L2) is heated.
The state shown in FIG. 5C 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 outer peripheral surface length facing the coil portion 25 of the inner core 28 is fixed to the above-described state. The fixing process described in FIG. 2 is performed.

  Note that the information relating to the size (size in the width direction) of the recording medium P fed from the paper feeding units 11, 12, and 15 before the fixing process is performed (for example, the detection information of the size detection sensor). ), The step of adjusting the rotation angle of the stepping motor that rotationally drives the internal core 28 is performed.

FIG. 6 is a graph showing the temperature distribution in the width direction on the fixing belt 22.
In FIG. 6, the horizontal axis indicates the position in the width direction of the fixing belt 22, 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 facing range in the inner core 28 is maximized and the heating range of the fixing belt 22 is maximized (in the state of FIG. 5A). A solid line R2 indicates a temperature distribution when the facing range in the inner core 28 is minimized and the heating range of the fixing belt 22 is minimized (the state in FIG. 5C).

  As shown in FIG. 6, the outer peripheral surface length (opposing range) of the inner core 28 in the heating roller 23 is changed according to the width of the recording medium P to be passed, so that the sheet passing width of the recording medium P is reached. If the temperature exceeds the range, the temperature of the fixing belt 22 does not increase. Thereby, the thermal damage of the fixing belt 22 is suppressed. Such an effect is also obtained when various recording media having a size between the maximum size L1 and the minimum size L2 are passed though illustration in FIG. 6 is omitted.

  As described above, the heating range of the fixing belt 22 varies when the outer peripheral surface length of the inner core 28 is varied. The size of the outer peripheral surface of the inner core 28 that faces the coil portion 25 substantially in parallel. This is because the density of the magnetic flux formed between the core 26 and the inner core 28 fluctuates according to the above. That is, in the state where the notch portion 28a2 faces the coil portion 25, the magnetic flux density at that position becomes low, and the heating efficiency of the fixing belt 22 becomes low.

  As described above, in the first embodiment, the fixing belt is provided with the inner core 28 configured to change the facing range facing the coil portion 25 in the heating roller 23 and changing the facing range. The heating range in the width direction of 22 is adjusted. Thereby, even when the recording medium P having a short width (including the medium-sized recording medium) is continuously fixed, the temperature increase at both ends in the width direction of the fixing belt 22 can be reliably suppressed.

  In particular, in the first embodiment, the heating range of the fixing belt 22 is adjusted by adjusting the circumferential posture of the inner core 28 as a columnar member having the notch 28a2 without using a magnetic flux shielding member or the like. is doing. Therefore, the fixing device is relatively inexpensive and inexpensive. Further, since the inner core 28 can be brought close to the inner peripheral surface of the heating roller 23, energy loss due to electromagnetic induction heating is reduced, and an efficient fixing device is obtained. Further, since the shape of the inner core 28 described above can gradually increase or decrease the facing range with respect to the coil portion 25, a fixing device that suppresses the temperature rise in the non-sheet passing area with respect to the recording medium P of various sizes. Become.

  In the first embodiment, the fixing device 20 is configured so that the pressure roller 30 contacts the fixing auxiliary roller 21 via the fixing belt 22. In contrast, the fixing device 20 is configured such that the pressure roller 30 contacts the surface of the fixing belt 22 from the heating roller 23 (or the fixing auxiliary roller 21) to the fixing auxiliary roller 21 (or the heating roller 23). You can also. Also for such a fixing device, the same effect as in the first embodiment can be obtained by providing the inner core 28 that can change the facing range with respect to the coil portion 25.

Embodiment 2. FIG.
A second embodiment of the present invention will be described in detail with reference to FIG.
FIG. 7 is a cross-sectional view showing the fixing device according to the second embodiment. The fixing device of the second embodiment is different from that of the first embodiment in which the fixing roller 31 is used as the fixing member, and the fixing belt is used as the fixing member.

As shown in FIG. 7, the fixing device 20 according to the second embodiment mainly includes a fixing roller 31 (fixing member), a pressure roller 30, an induction heating unit 24, and the like.
The fixing roller 31 includes a heat generating layer 31a, an elastic layer made of silicone rubber, a release layer made of a fluorine compound, and the like. The inside of the fixing roller 31 has a hollow structure, and the inner core 28 is rotatably installed. The inner core 28 provided in the fixing roller 31 is a columnar member formed by cutting off a part of the outer peripheral surface of the columnar body as in the first embodiment.

  The induction heating unit 24 includes a coil unit 25, a core 26, a coil guide 27, and the like, as in the first embodiment. Then, when an alternating current of 10 k to 1 MHz is supplied to the coil portion 25, a magnetic line of force is formed between the core 26 and the inner core 28, and the fixing roller 31 is heated by electromagnetic induction. In this way, the heated fixing roller 31 heats and melts the toner image on the recording medium P conveyed from the direction of the arrow and fixes the toner image on the recording medium P.

  Also in the second embodiment, as in the first embodiment, the length of the outer peripheral surface of the inner core 28 facing the coil portion 25 is variable in accordance with the size in the width direction of the recording medium P to be passed. Is done. As a result, the heating range in the width direction of the fixing roller 31 substantially matches the sheet passing area of the recording medium P to be passed.

  As described above, in the second embodiment, the fixing roller 31 is provided with the inner core 28 configured so that the facing range facing the coil portion 25 can be changed, and the facing range is changed and fixed. The heating range in the width direction of the roller 31 is adjusted. Thereby, even when the recording medium P having a short width (including the medium-sized recording medium) is continuously fixed, the temperature rise at both ends in the width direction of the fixing roller 31 can be reliably suppressed.

In the present application, 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 each of the above embodiments, the heating range of the fixing member is adjusted by configuring the inner core 28 as the core portion so that the facing range can be varied. On the other hand, it is possible to adjust the heating range of the fixing member by configuring the core 26 as the core portion so that the facing range with respect to the coil portion 25 can be varied.

  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 view showing the inside of 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. It is sectional drawing which shows the state which rotated the internal core provided in the heating roller of FIG. 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 fixing device in Embodiment 2 of this invention.

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 auxiliary 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), 28a1 non-notched part, 28a2 notched part,
28b Shaft portion, 30 pressure roller,
31 Fixing roller (fixing member), P recording medium.

Claims (6)

Fixing the toner image on the recording medium by heating the toner image, and a fixing belt stretched between a heating roller and a fixing auxiliary roller;
A coil unit provided with the center core to the fixing belt through the extending in the width direction so as to face Rutotomoni, the circumferential center position to the heating roller,
The outer peripheral surface of the coil portion that faces the center core of the coil portion at a position facing the coil portion via the fixing belt and being disposed inside the heating roller and configured to be rotatable. A columnar inner core in which a part of the outer peripheral surface of the cylindrical body is cut off so that the length in the width direction of the cylindrical body gradually increases or decreases,
With
Said width direction of the outer peripheral surface of the inner core that faces the center core of the coil portion in accordance with the rotation of the inner core length is gradually increased or gradually decreased, the fixing belt heated by electromagnetic induction by the coil unit A fixing device that adjusts a heating range in a width direction. The fixing device according to claim 1, wherein the fixing auxiliary roller is disposed at a position facing the pressure roller that presses the conveyed recording medium via the fixing belt. A fixing roller that heats the toner image and fixes the toner image to the recording medium, and abuts against a pressure roller that pressurizes the recording medium conveyed;
A coil portion extending in the width direction so as to face the outer peripheral surface of the fixing roller, and having a center core at a central position in the circumferential direction;
The outer peripheral surface of the coil portion facing the center core of the coil portion at a position facing the coil portion via the fixing roller and being disposed inside the fixing roller and configured to be rotatable. A columnar inner core in which a part of the outer peripheral surface of the cylindrical body is cut off so that the length in the width direction of the cylindrical body gradually increases or decreases,
With
As the inner core rotates, the length of the outer peripheral surface of the inner core facing the center core of the coil portion is gradually increased or decreased, and heated by electromagnetic induction by the coil portion. A fixing device that adjusts a heating range in a width direction. 4. The heating range is adjusted by gradually increasing or decreasing the length of the inner core in the width direction according to the width direction size of the recording medium. 5. The fixing device described. The fixing device according to claim 1, wherein the inner core is made of ferrite. An image forming apparatus comprising the fixing device according to claim 1.

Images