JP2013037053A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus which adequately utilizes the high productivity thereof by effectively alleviating a temperature rise at a non-paper passing part in a short time caused by a successive heating of recording materials and resuming an image formation.SOLUTION: A pressure roller 2 contacts with a fixation belt 1 to form a heat nip for recording materials. An induction heating device 70 sets an induction heat area in the longitudinal direction of the fixation belt 1 in accordance with a size of a recording material in a conveyance width direction orthogonal to a conveyance direction of the recording material, and heats the fixation belt 1 inductively. When a toner image formation on an intermediate transfer belt 26 is interrupted, a control part 102 executes an alleviation mode for setting a heat area narrower than when the recording material is heated, in a state where the heating of the recording material is stopped, and continuing the induction heating of the fixation belt 1 by the induction heating device 70.

Description

  The present invention relates to an image forming apparatus capable of setting a heating area in a longitudinal direction in a heating nip between a first rotating body and a second rotating body according to the size of a recording material, and more specifically, a recording material passing area of a first rotating body. The present invention relates to a relaxation mode control that mitigates a local temperature rise that occurs outside.

  An image forming apparatus that forms a toner image and transfers it to a recording material, and then clamps and conveys the recording material onto which the toner image is transferred at a heating nip between the first rotating body and the second rotating body. Is widely used. An image heating apparatus using an induction heating apparatus as a heating source for the first rotating body (belt member or roller member) has also been put into practical use (Patent Documents 1 and 2).

  Since the induction heating device can concentrate heating on a part of the first rotating body having a relatively small mass, the temperature of the first rotating body rises quickly and is excellent in temperature control response. However, when the induction heating device is used, a region that locally becomes high temperature is formed outside the region that is cooled in contact with the recording material of the first rotator because it is not cooled by the recording material. . If the temperature rise is a so-called non-sheet-passing portion temperature and is exposed to an excessive non-sheet-passing portion temperature rise for a long time, the durability of the first rotating body may be reduced.

  In Patent Document 1, a plurality of magnetic cores are arranged in the longitudinal direction of the first rotating body (belt member), and the opposing spacing of the magnetic cores at both ends with respect to the first rotating body is changed, thereby preventing non-sheet passing. Temperature rise is avoided.

  In Patent Document 2, the size of the magnetic flux shielding plate that covers the end of the first rotating body (roller member) is switched, and the heating range at the outer position of the recording material is kept constant even when the size of the recording material changes. As a result, the temperature rise of the non-sheet passing portion is avoided. In addition, a temperature sensor is arranged at the end of the first rotating body to actually measure the temperature rise of the non-sheet passing portion, and the recording material is fed to the heating nip of the first rotating body and the second rotating body (number of sheets per minute or emergency Control).

JP 2001-194940 A JP 2006-120533 A

  If the feeding of the recording material is controlled so that the temperature rise of the non-sheet passing portion is kept below the upper limit design temperature of the first rotating body, the productivity of the image forming apparatus is limited by the temperature rise of the non-sheet passing portion. As a result, the original high productivity of the image forming apparatus cannot be exhibited. Therefore, a temperature sensor is arranged in the non-sheet passing portion to actually measure the non-sheet passing portion temperature rise, and when the non-sheet passing portion temperature rise of the first rotating body reaches the design upper limit value, the image formation is interrupted. Control to cool down the first rotating body was proposed. However, in this case, it takes time to return the heating nip to the original temperature state after cooling, and the total productivity tends to decrease.

  Further, there has been proposed a control in which when the temperature rise of the non-sheet passing portion of the fixing device reaches a design upper limit, image formation is interrupted and the non-sheet passing portion is forcibly cooled by a cooling fan. However, the position where the temperature rise in the non-sheet passing portion is a very narrow region and changes according to the size of the recording material, so that efficient cooling is difficult with a normal cooling fan with a wide blowing range.

  The present invention is an image in which the original high productivity can be fully utilized by effectively relieving the temperature rise of the non-sheet passing portion caused by continuous heating of the recording material in a short time and restarting image formation. An object is to provide a forming apparatus.

  An image forming apparatus according to the present invention includes an image forming unit that forms a toner image and transfers the toner image to a recording material, a first rotating body that contacts and heats the recording material to which the toner image is transferred, and the first rotating body. A second rotating body that abuts to form a heating nip of the recording material, and a heating area of induction heating in the longitudinal direction of the first rotating body according to the recording material length in the transport width direction perpendicular to the recording material transport direction And an induction heating device for induction heating the first rotating body. Then, in a state where the feeding of the recording material to the heating nip is stopped, the heating area is set narrower than the case where the recording material is fed to the heating nip, and the first heating by the induction heating device is performed. Control means capable of executing a relaxation mode for continuing induction heating of the rotating body is provided.

  In the image forming apparatus of the present invention, the relaxation mode control is executed by using the function of setting the heating area of the first rotating body by the induction heating apparatus, and the magnetic flux is limited to the area where the temperature increase of the non-sheet passing portion is generated. Stop heating. As a result, a large heat inflow amount difference is formed between the region where the temperature rise of the non-sheet-passing portion has occurred and the region outside the region, and the temperature difference between the two is quickly eliminated.

  Therefore, the image forming apparatus can be efficiently relieved in a short time by restarting the image formation by effectively reducing the temperature rise of the non-sheet passing portion caused by continuous heating of the recording material, without impairing the total productivity of the image forming apparatus. Can fully utilize the original high productivity.

1 is an explanatory diagram of a configuration of an image forming apparatus. FIG. 3 is an explanatory diagram of a configuration of a main part of the fixing device. FIG. 4 is a longitudinal sectional view of the fixing device as viewed from the secondary transfer unit side. FIG. 3 is an explanatory diagram of a layer configuration of the fixing belt 1. It is explanatory drawing of a movement of a magnetic body core. It is explanatory drawing of the moving mechanism of a magnetic body core. It is a perspective view of a fixing device. It is explanatory drawing of the temperature distribution of the longitudinal direction of a fixing belt when a heating area | region is narrow. It is explanatory drawing of the temperature distribution of the longitudinal direction of a fixing belt when a heating area | region is wide. It is explanatory drawing of the temperature distribution of the longitudinal direction of a fixing belt when a heating area | region is appropriate. 3 is a flowchart of relaxation mode control according to the first embodiment. It is explanatory drawing of the setting of the heating area | region during image formation. It is explanatory drawing of the setting of the heating area | region at the time of image formation interruption. It is explanatory drawing of the cancellation | release speed | rate of non-sheet passing part temperature rising in relaxation mode. 10 is a flowchart of relaxation mode control according to the fourth embodiment. It is explanatory drawing of the setting of the heating area | region at the time of image formation interruption. 12 is a flowchart of relaxation mode control according to the fifth embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. According to the present invention, in the configuration in which the heating area of the first rotating body is variable, as long as the heating area is narrowed and the induction heating is continued when the image formation is interrupted, a part or all of the configuration of the embodiment is replaced with that. Another embodiment in which the configuration is replaced can also be implemented.

  Accordingly, the image heating apparatus heats the recording material to which the toner image has been transferred to fix the toner image on the recording material, and also heats the semi-fixed or fixed toner image to a desired image. It includes an image heating device that imparts surface properties. The first rotator and the second rotator are not limited to belt members, and may be roller members.

  The reason for interrupting image formation is not limited to adjustment of the developing device, but may be detection of overheating of the first rotating body, waiting for replenishment of recording material, size switching of recording material, download time of image data, and the like. In any case, it is possible to eliminate the temperature rise of the non-sheet passing portion by using a waiting time for other processing without providing a dedicated cooling time for the first rotating body.

  The image forming apparatus can be mounted with the image heating apparatus of the present invention without distinction between monochrome / full color, sheet-fed type / recording material conveying type / intermediate transfer type, toner image forming method, and transfer method. In the present embodiment, only main parts relating to toner image formation / transfer / fixing will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, in addition to necessary equipment, equipment, and a housing structure. The image forming apparatus can be used in various applications such as a multifunction peripheral.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. As shown in FIG. 1, the image forming apparatus E is a tandem intermediate transfer type full-color printer in which image forming portions PY, PC, PM, and PK of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 26. is there. The intermediate transfer belt 26 as an example of an image forming unit forms a toner image and transfers it to a recording material.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 21 (Y) and transferred to the intermediate transfer belt 26. In the image forming unit PC, a cyan toner image is formed on the photosensitive drum 21 (C) and transferred to the intermediate transfer belt 26. In the image forming portions PM and PK, a magenta toner image and a black toner image are formed on the photosensitive drums 21 (M) and 21 (K), respectively, and transferred to the intermediate transfer belt 26.

  The intermediate transfer belt 26 is composed of an endless resin belt, is stretched around a driving roller 27, a secondary transfer counter roller 28, and a tension roller 29, and is driven by the driving roller 27.

  The recording material P is taken out from the recording material cassette 31 one by one by the paper feed roller 32 and waits at the registration roller 33. The recording material P is fed to the secondary transfer portion T2 by the registration roller 33, and the toner image is transferred from the intermediate transfer belt 26. The recording material P to which the four color toner images have been transferred is conveyed to the fixing device A, and is heated and pressurized by the fixing device A to fix the toner image on the surface. Then, the recording material P is transferred to the external tray 37 through the discharge conveyance path 36. Discharged. It should be noted that when forming an image of only a single color (single color mode), a toner image is formed only for the target color, and is carried on the intermediate transfer belt 26 and transferred to a recording material.

  The image forming units PY, PC, PM, and PK are different from the toners used in the developing devices 23 (Y), 23 (C), 23 (M), and 23 (K) except for yellow, cyan, magenta, and black. The configuration is substantially the same. In the following, the image forming unit PY will be described, and redundant description regarding the image forming units PC, PM, and PK will be omitted.

  In the image forming unit PY, a charging roller 22, an exposure device 25, a developing device 23, a transfer roller 30, and a drum cleaning device 24 are arranged around the photosensitive drum 21. The charging roller 22 charges the surface of the photosensitive drum 21 to a uniform potential. The exposure device 25 scans the laser beam and writes an electrostatic image of the image on the photosensitive drum 21. The developing device 23 develops the electrostatic image and forms a toner image on the photosensitive drum 21. The transfer roller 30 is applied with a DC voltage to transfer the toner image on the photosensitive drum 21 to the intermediate transfer belt 26.

<Fixing device>
FIG. 2 is an explanatory diagram of a configuration of a main part of the fixing device. FIG. 3 is a longitudinal sectional view of the fixing device as seen from the secondary transfer portion side. FIG. 4 is an explanatory diagram of the layer configuration of the fixing belt 1. The fixing device A melts the toner (developer) of the toner image (unfixed image) transferred onto the conveyed recording material by heat and fuses it onto the recording material P.

  In the following description, the longitudinal direction of the fixing device or the fixing roller is a width direction perpendicular to the recording material conveyance direction in the recording material conveyance path surface. Further, the short direction of the fixing device or the fixing roller is a direction parallel to the conveyance direction of the recording material. The front surface of the fixing device is a surface of the fixing device as viewed from the recording material inlet side, and the rear surface is a surface of the fixing device as viewed from the recording material outlet side. The left and right sides of the fixing device are left or right when the fixing device is viewed from the front. The upstream side is the upstream side in the recording material conveyance direction, and the downstream side is the downstream side in the recording material conveyance direction.

  As shown in FIG. 2, the fixing belt 1, which is an example of a first rotating body, is heated in contact with the recording material onto which the toner image has been transferred. A pressure roller 2, which is an example of a second rotating body, contacts the fixing belt 1 to form a recording material heating nip. The induction heating device 70 induction-heats the fixing belt 1 by variably setting a heating area for induction heating in the longitudinal direction of the fixing belt 1.

  The fixing belt 1 is an endless belt member having a metal layer and an inner diameter of 30 mm. The pressure roller 2 is formed in a cylindrical shape having an outer diameter of 30 mm. The pressure roller 2 is in pressure contact with the outer surface of the fixing belt 1 supported on the inner surface by the pressure applying member 3, and between the fixing belt 1 and the recording material P. A fixing nip portion N is formed.

  The pressure roller 2 is provided with an elastic layer 2b of a silicone rubber layer having a thickness of about 5 mm on a cored bar 2a made of iron alloy having a diameter in the center in the longitudinal direction of 20 mm and a diameter at both ends of 19 mm. On the surface of the elastic layer 2b, a release layer 2c of a fluororesin layer (for example, PFA or PTFE) is provided with a thickness of 30 μm. The hardness at the center in the longitudinal direction of the pressure roller 2 is ASK-C70 ° C. The cored bar 2a is tapered because the pressure in the fixing nip N sandwiched between the fixing belt 1 and the pressure roller 2 is uniform over the longitudinal direction even if the pressure applying member 3 is bent when pressed. This is because it can be secured.

  Since the core 2a is tapered and the thickness of the elastic layer 2b is different between the center and both ends, the length of the fixing belt 1 and the fixing nip N of the pressure roller 2 in the conveying direction is determined by the fixing nip pressure. In 600N, it is about 9 mm at the longitudinal ends and about 8.5 mm at the center. As a result, the conveyance speed at both ends of the recording material P becomes faster than that at the central portion, so that there is an advantage that paper wrinkles are less likely to occur.

  The inner surface of the pressure applying member 3 is held by a metal stay 4, and the inner surface of the fixing belt 1 is supported by the outer surface of the pressure applying member 3. The pressure applying member 3 applies a pressing force to the pressure roller 2 through the fixing belt 1 to form a fixing nip portion N between the fixing belt 1 and the pressure roller 2. The pressure applying member 3 is a heat resistant resin. The pressure applying member 3 is provided with a magnetic flux shielding core 5 as a magnetic flux shielding member for preventing a temperature rise due to induction heating on the coil 6 side of the stay 4.

  As shown in FIG. 3, the stay 4 is made of iron because it needs rigidity to apply pressure to the pressure contact portion between the fixing belt 1 and the pressure roller 2. The stay 4 is close to the exciting coil 6 particularly at both ends. In order to prevent the stay 4 from generating heat, the stay 4 has a magnetic flux shielding core on the upper surface of the stay 4 over the longitudinal direction in order to shield the magnetic field generated by the exciting coil 6. 5 is arranged.

  The fixing flange 10 is a right and left restricting member that restricts the longitudinal movement and the circumferential shape of the fixing belt 1. The stay pressurizing spring 9b is contracted between the spring receiving portion 4a of the stay 4 that is inserted into the fixing flange 10 and the spring receiving member 9a on the apparatus chassis side. Is acting. As a result, the lower surface of the pressure applying member 3 and the upper surface of the pressure roller 2 are pressed across the fixing belt 1 to form a fixing nip portion N of the recording material image.

  Since the rotating fixing belt 1 has a base layer made of a metal, as a means for restricting the shift in the width direction even in the rotating state, the fixing simply by receiving the end of the fixing belt 1 is performed. It is sufficient to provide the flange 10. Thereby, there is an advantage that the configuration of the fixing device A can be simplified.

  As shown in FIG. 4, the fixing belt 1 has a nickel base layer (metal layer) 1 a having a thickness of 40 μm manufactured by an electroforming method. For the base layer 1a, iron alloy, copper, silver or the like can be appropriately selected in addition to nickel. Moreover, the structure of laminating | stacking these metals on a resin base layer may be sufficient. The thickness of the metal layer 1a may be adjusted according to the frequency of a high-frequency current flowing through an exciting coil, which will be described later, and the permeability / conductivity of the metal layer, and may be set between about 5 and 200 μm.

  An elastic layer 1b of a heat resistant silicone rubber layer is provided on the outer periphery of the base layer 1a. The thickness of the elastic layer 1b is preferably set within a range of 100 to 1000 μm. Here, the thickness of the elastic layer 1b is 300 μm in consideration of reducing the heat capacity of the fixing belt 1 to shorten the warm-up time and obtaining a suitable fixed image when fixing a color image. The silicone rubber of the elastic layer 1b has a JIS-A hardness of 20 degrees and a thermal conductivity of 0.8 W / mK. On the outer periphery of the elastic layer 1b, a release layer 1c of a fluororesin layer (for example, PFA or PTFE) is provided with a thickness of 30 μm. On the inner surface side of the base layer 1a, in order to reduce the sliding friction between the inner surface of the fixing belt and the central thermistor (TH1: FIG. 2), a slipping layer 1d made of a resin layer such as a fluororesin or polyimide is provided at 10 to 50 μm. . The sliding layer 1d was provided with 20 μm of polyimide.

<Induction heating device>
As shown in FIG. 2, the induction heating device 70 is a heating source for induction heating the fixing belt 1. The induction heating device 70 is disposed on the upper surface side of the outer peripheral surface of the fixing belt 1 so as to face the fixing belt 1 with a predetermined gap (gap).

  The exciting coil 6 is formed by using, for example, a litz wire as an electric wire, which is horizontally long and shaped like a ship bottom, and is opposed to the peripheral surface and part of the side surface of the fixing belt 1. The exciting coil 6 has an inner diameter in the longitudinal direction of 352 mm and an outer diameter of 392 mm.

  The magnetic core 7 a covers the exciting coil 6 so that the magnetic field generated by the exciting coil 6 does not substantially leak to other than the metal layer (conductive layer) of the fixing belt 1. The magnetic core 7 a serves to efficiently guide the alternating magnetic flux generated from the exciting coil 6 to the fixing belt 1. It is used to increase the efficiency of the magnetic circuit of AC magnetic flux and to shield magnetic flux that avoids induction heating of the peripheral members by leaking magnetic flux to the surroundings. As the material of the magnetic core 7a, a material having high magnetic permeability and low residual magnetic flux density such as ferrite may be used.

  The mold member 7c supports the exciting coil 6 and the magnetic core 7a with an electrically insulating resin. The fixing belt 1 and the exciting coil 6 are kept electrically insulated by a 0.5 mm mold. The distance between the fixing belt 1 and the exciting coil 6 is constant at 1.5 mm (the distance between the mold surface and the fixing belt surface is 1.0 mm).

  In the rotating state of the fixing belt 1, a high frequency current of 20 to 50 kHz is applied from the power supply device (excitation circuit) 101 to the excitation coil 6 of the induction heating device 70, and the magnetic field generated by the excitation coil 6 causes the fixing belt 1 to The metal layer (conductive layer) generates induction heat.

  The central thermistor TH1 is a temperature sensor (temperature detection element), and is disposed in contact with the position of the center portion in the width direction of the fixing belt 1. Since the central thermistor TH1 is attached to the pressure applying member 3 via an elastic support member, even if a positional variation such as a wave of the contact surface of the fixing belt 1 occurs, it is good to follow this. The contact state is maintained.

  The central thermistor TH1 detects the temperature of the inner side surface of the fixing belt 1 at approximately the center of the conveyance paper area of the recording material, and the detected temperature information is fed back to the control unit 102.

  The control unit 102 controls the electric power input from the power supply device 101 to the exciting coil 6 so that the detected temperature input from the central thermistor TH1 is maintained at a predetermined target temperature (fixing temperature). When the detected temperature of the fixing belt 1 rises to a predetermined temperature, the control unit 102 cuts off the energization to the excitation coil 6. The control unit 102 changes the frequency of the high-frequency current based on the detected value of the central thermistor TH1 so that the detected temperature of the fixing belt 1 is constant at 180 ° C., which is the target temperature of the fixing belt 1, Temperature control is performed by controlling the power input to the exciting coil 6. The exciting coil 6 of the induction heating device 70 connected to the power supply device 101 is controlled by the control unit 102, and the fixing belt 1 is heated to a predetermined fixing temperature.

  As described above, a high frequency current of 20 to 50 kHz is applied to the exciting coil 6, and the metal layer 1a of the fixing belt 1 generates induction heat. In the temperature adjustment, the power input to the exciting coil 6 is controlled by changing the frequency of the high-frequency current based on the detection value of the central thermistor TH1 so as to be constant at the target temperature of 180 ° C. of the fixing belt 1.

  Since the induction heating device 70 including the exciting coil 6 is disposed outside the fixing belt 1 that is at a high temperature, the temperature of the exciting coil 6 is not easily increased. Further, the loss due to Joule heat generation can be reduced even when a high frequency current is passed without increasing the electrical resistance. Further, the arrangement of the exciting coil 6 on the outside contributes to a reduction in the diameter (lower heat capacity) of the fixing belt 1, and it can be said that it is excellent in energy saving.

  Since the warm-up time of the fixing device of this embodiment has a very low heat capacity, for example, when 1200 W is input to the exciting coil 6, it can reach the target temperature of 165 ° C. in about 15 seconds. Since the heating operation during standby is unnecessary, it is possible to keep the power consumption very low.

  In the fixing belt 1, the pressure roller 2 is rotationally driven by a motor M <b> 1 controlled by the control unit 102, so that the conveyance speed of the recording material P conveyed from the secondary transfer unit T <b> 2 in FIG. And is driven to rotate at substantially the same peripheral speed. In the fixing device A, the surface rotation speed of the fixing belt 1 is 300 mm / sec, and 80 sheets per minute for A4 size lateral feed full-color images, and 58 sheets per minute for A4 size longitudinal feed. It can be fixed continuously.

  The recording material P having the unfixed toner image T is guided by the guide member 7 with the toner image carrying surface side facing the fixing belt 1 and is pressed by the fixing belt 1 and the pressure roller 2. It is introduced into the fixing nip N. The recording material P is in close contact with the outer peripheral surface of the fixing belt 1 at the fixing nip portion N, and is nipped and conveyed along the fixing nip portion N together with the fixing belt 1.

  The unfixed toner image T is fixed on the surface of the recording material P under the pressure of the fixing nip N while the heat of the fixing belt 1 is applied. Since the surface of the fixing belt 1 is deformed at the exit portion of the fixing nip portion N of the recording material P that has passed through the fixing nip portion N, the recording material P self-separates from the outer peripheral surface of the fixing belt 1 and goes out of the fixing device A. Be transported.

  By the way, in general, an image heating device reheats an image fixed on a recording material in addition to a fixing device that melts the toner of a toner image transferred onto the recording material by heat and fuses it onto the recording material. There is also a gloss applying device that improves the glossiness of an image.

  In order to enable high-speed temperature increase of the image heating apparatus, a fixing roller having a small diameter, a heater heating from the inside of a resin belt, an induction heating of a thin metal rotating body, and the like have been proposed. Also from the viewpoint of material cost and energy efficiency, it is desired to employ a thin rotating body to reduce the heat capacity and to heat with an induction heating device having good heating efficiency.

  However, when a thin rotating body is used, the cross-sectional area of the cross section perpendicular to the axis is extremely small, so the heat transfer rate in the axial direction is not good. This tendency becomes more conspicuous as the wall becomes thinner, and is even lower for materials such as resins with low thermal conductivity.

This is apparent from Fourier's law that the heat quantity Q transmitted per unit time when the thermal conductivity is λ, the temperature difference between two points is θ1-θ2, and the length is L is expressed by the following equation. is there.
Q = λ · f (θ1-θ2) / L

  If a thin rotating body is used and the recording material of a small size is continuously fed in a state where the heat transfer rate in the axial direction is not good, the temperature of the non-sheet passing area of the rotating body rises higher than the passing area. This causes a problem of so-called non-sheet passing portion temperature rise in which temperature unevenness occurs in the longitudinal direction of the rotating body.

  When a non-sheet passing part temperature rise occurs, if a large size recording material is heated immediately after a small size recording material is continuously heat-treated, heating unevenness will occur on the recording material, and paper wrinkles and fixing unevenness will occur. Cause. When the temperature of the non-sheet-passing portion is significantly increased, the life of the peripheral member made of a resin material may be reduced.

  The temperature rise in the non-sheet passing area increases as the heat capacity of the recording material being conveyed increases and the number of prints per unit time increases, so in a high-productivity copier, a thin rotating body and induction heating with good heating efficiency An image heating apparatus combined with the apparatus could not be adopted. In a copier with high productivity, in many cases, the non-sheet-passing portion temperature rise is avoided by dividing a halogen lamp heater or a resistance heating element and heating an area corresponding to the recording material size.

  Incidentally, some image heating apparatuses in which a thin rotating body and an induction heating apparatus with good heating efficiency are combined can set the heating area in the longitudinal direction of the rotating body according to the recording material size. However, if a plurality of induction heating devices are provided or divided, the control circuit becomes complicated and the cost increases accordingly. In the case of a thin rotating body, there is a problem that the temperature distribution becomes discontinuous near the boundary between the divided heating regions, and the rotating body cannot satisfy the required fixing performance.

  Therefore, in the fixing device A, the magnetic core 7a that can variably set the magnetic flux guided from the induction heating device to the first rotation body in an area of every 10 mm is disposed between the first rotation body and the induction heating device. . By moving the magnetic core 7a by the number corresponding to the size of the recording material, the magnetic flux reaching from the induction heating device is reduced except in the heating area where heating is necessary, and the heat generation of the fixing belt 1 itself is suppressed. As a result, the heating area is controlled, and the temperature distribution of the fixing belt 1 to be heated can be precisely controlled.

<Movement mechanism of magnetic core>
FIG. 5 is an explanatory view of the movement of the magnetic core. FIG. 6 is an explanatory diagram of a magnetic core moving mechanism. FIG. 7 is a perspective view of the fixing device.

  As shown in FIG. 2, the exciting coil 6, which is an example of an exciting coil member, generates a magnetic flux that enters the fixing belt 1. The power supply device 101 controls power supply to the exciting coil 6 so as to keep the temperature of the fixing belt 1 within a predetermined temperature range necessary for fixing the toner image.

  The plurality of magnetic cores 7a are arranged in the longitudinal direction of the fixing belt 1 to guide the magnetic flux generated by the exciting coil 6 to the fixing belt 1 in each region. The core moving mechanism 71 moves the plurality of magnetic cores 7a in the direction in which they are in contact with or separated from the fixing belt 1. The controller 102 sets the heating region by bringing the number of magnetic cores 7a corresponding to the length of the recording material in the width direction closer to the fixing belt 1 than the other magnetic cores 7a.

  As shown in FIG. 3, the magnetic core 7 a is divided in the longitudinal direction of the fixing belt 1, and the width in one longitudinal direction is 10 mm, and they are arranged with an interval of 1.0 mm. In the paper passing portion, by narrowing the gap between the exciting coil 6 and the magnetic core 7a, the density of magnetic flux passing through the fixing belt 1 is increased, and the heat generation amount of the fixing belt 1 is increased. On the other hand, in the non-sheet passing portion, the gap between the exciting coil 6 and the magnetic core 7a is widened to reduce the magnetic flux density passing through the fixing belt 1 and to reduce the heat generation amount of the fixing belt 1.

  As shown in FIG. 5A, in the paper passing area, the interval between the exciting coil 6 and the magnetic core 7a is 0.5 mm. As shown in FIG. 5B, in the non-sheet passing region, the interval between the exciting coil 6 and the magnetic core 7a is separated to 10 mm.

  As shown in FIG. 6, the magnetic core 7 a is held by the magnetic core holder 77 and is accommodated in the housing 76. The magnetic core holder 77 is movable in a direction in which the gap between the magnetic core 7a and the exciting coil 6 is changed. The link member 75 is assembled so as to be rotatable around the rotation shaft 78, and the long hole portion at the end is connected to the magnetic core holder 77. When the link member 75 rotates about the rotation shaft 78 in the Q1 direction, the magnetic core holder 77 and the magnetic core 7a move in the P1 direction. When the link member 75 rotates in the Q2 direction, the magnetic core holder 77 and the magnetic core 7a move in the P2 direction. The link member 75 is urged in the direction rotating in the Q1 direction by the excitation coil spring 74, but the rotation of the link member 75 in the Q1 direction is restricted by the restriction member 73.

  In a state where the link member 75 is pushed in by the restriction member 73, the link member 75 rotates in the Q2 direction against the exciting coil spring 74. At this time, the magnetic core holder 77 moves in the direction of the arrow P <b> 2 and the magnetic core 7 a is approaching the exciting coil 6.

  When the pressing by the restricting member 73 is released, the link member 75 is urged by the exciting coil spring 74, rotates in the Q1 direction, abuts against the frame 79, and stops. As a result, the magnetic core holder 77 moves in the direction of the arrow P1 and the magnetic core 7a moves away from the excitation coil 6.

  As shown in FIG. 7, the regulating member 73 is connected to the central pinion gear 80, and can be moved in the width direction (Y1, Y2 direction) perpendicular to the recording material conveyance direction by the rotational movement of the pinion gear 80. ing. When the regulating member 73 moves in the Y1 direction, the pushing by the regulating member 73 is released in order from the link member 75 on the end side, and the magnetic core 7a moves away from the exciting coil 6 in order from the end side to the center side. . In FIG. 7, the four magnetic core holders 77 from the end side are released from being pushed by the restricting member 73, and the gap between the magnetic core 7 a and the excitation coil 6 is widened.

  The control unit 102 controls the core moving mechanism 71 to release the pressing by the regulating member 73 for the number of magnetic core holders 77 determined according to the recording material conveyance width direction. As a result, the gap between the magnetic core 7a located outside the recording material and the exciting coil 6 is enlarged to prevent the temperature rise of the non-sheet passing portion. In order to correspond to various recording material sizes such as postcard size, A5, B4, A3, A3 Nobi size, the position of the regulating member 73 is made different depending on the size of the recording material, and a heating area corresponding to the size of each recording material is set. Thus, the temperature rise of the non-sheet passing portion is suppressed.

<Temperature distribution of fixing belt>
FIG. 8 is an explanatory diagram of the temperature distribution in the longitudinal direction of the fixing belt when the heating region is narrow. FIG. 9 is an explanatory diagram of the temperature distribution in the longitudinal direction of the fixing belt when the heating region is wide. FIG. 10 is an explanatory diagram of the temperature distribution in the longitudinal direction of the fixing belt when the heating region is appropriate.

In each drawing, the broken line is 30 seconds after the fixing device is started and before the start of image formation. The solid line is the end of 500 continuous image formations. The recording material is AOSEDARE (registered trademark) 105 g / m ² A4 size plain paper. In a 15 ° C. environment, image formation and fixing were performed with a productivity of 80 ppm and a process speed of 320 mm / sec. The temperature control temperature of the fixing belt was set to 180 ° C.

  As shown in FIG. 8, when the heating area is set to the same length in the transport width direction for an A4 size recording material, the temperature decreases at the end of the recording material before and after the image formation of 500 sheets. Was observed, and fixing failure occurred. A so-called cold offset occurred in which the toner adheres to the fixing belt 1 and is taken away.

  As shown in FIG. 9, when an extra heating area of 20 mm for two magnetic cores 7a is set outside the A4 size recording material, fixing failure does not occur, but the recording material after forming 500 sheets of images. The non-sheet passing part temperature rise occurred in the outer area of the paper and exceeded the design temperature.

  As shown in FIG. 10, when an extra heating area of 10 mm for one magnetic core 7a is set outside the A4 size recording material, no fixing failure occurs, and the recording material after forming 500 images. Even if a temperature increase in the non-sheet passing portion occurred in the outer region of the paper, the design temperature was not exceeded.

  By the way, as described above, the image heating apparatus that heats a thin rotator with an induction heating apparatus has a problem of so-called non-sheet passing portion temperature rise in which the rotator locally increases the temperature outside the recording material. is there.

  In the case of an image with a small margin, it is necessary to ensure a sufficient temperature to fix the toner at both ends of the recording material in the conveyance width direction. Therefore, as shown in FIG. 10, the fixing belt 1 is heated by injecting an induction heating magnetic flux to the outside of the recording material.

  However, if image formation is continued in this state, the amount of heat taken away by the recording material in the non-sheet passing portion is smaller than that in the non-sheet passing portion, so that heat gradually accumulates and a large non-sheet passing portion temperature rise is formed. End up. The temperature increase in the non-sheet passing portion appears more conspicuously in the fixing belt 1 using a low heat capacity film material. Since the endurance life of the fixing belt 1 is reduced by the non-sheet passing portion temperature rise, it is necessary to reduce the non-sheet passing portion temperature rise.

  Therefore, in the following embodiments, in a configuration in which the amount of heat generation can be partially adjusted, the heat generation width is narrower than usual during idle rotation operation in which paper passing is interrupted or when productivity is reduced. Move the magnetic core 7a. As a result, the temperature rise of the non-sheet passing portion is suppressed / reduced and the durability life of the fixing belt 1 is extended.

<Example 1>
FIG. 11 is a flowchart of the relaxation mode control according to the first embodiment. FIG. 12 is an explanatory diagram of setting of the heating area during image formation. FIG. 13 is an explanatory diagram of the setting of the heating area when image formation is interrupted. FIG. 14 is an explanatory diagram of the elimination speed of the non-sheet passing portion temperature increase in the relaxation mode.

  As shown in FIG. 2, the fixing belt 1, which is an example of a first rotating body, is heated in contact with the recording material on which the toner image is transferred. A pressure roller 2, which is an example of a second rotating body, contacts the fixing belt 1 to form a recording material heating nip. The induction heating device 70 induction-heats the fixing belt 1 by setting a heating area for induction heating in the longitudinal direction of the fixing belt 1 according to the recording material length in the conveyance width direction perpendicular to the conveyance direction of the recording material. The control unit 102, which is an example of a control unit, performs induction heating by setting a heating area narrower than when the recording material is fed to the heating nip in a state where feeding of the recording material to the heating nip is stopped. A relaxation mode in which the induction heating of the fixing belt 1 by the device 70 is continued can be executed. The control unit 102 executes the relaxation mode when the toner image formation on the intermediate transfer belt 26 is interrupted.

  As shown in FIG. 1, in the case of normal image formation, the control unit 102 interrupts image formation and adjusts the developing device every 500 image formations. In addition, when a high duty image with a large amount of toner used is continuously formed, the toner density and toner charge amount in the developing device 23 fluctuate greatly, so that the developing device is frequently adjusted.

  In the adjustment of the developing device, the control unit 102 forms a color patch toner image on the photosensitive drum 21 under a predetermined image forming condition in the image forming unit PY and transfers it to the intermediate transfer belt 26. The color patch toner image on the intermediate transfer belt 26 is irradiated with infrared light by the optical sensor 105 to detect reflected light.

  The control unit 102 adjusts the developing condition (developing sleeve DC voltage) of the developing device 23 based on the detection result of the color patch toner image, forms the color patch toner image again, and repeats the same control. By executing this in parallel in the image forming units PY, PC, PM, and PK, a downtime of 5 to 20 seconds occurs. In this way, toner density adjustment and the like in development are performed.

  In the first exemplary embodiment, by using the downtime, the fixing device A executes the relaxation mode control to eliminate the temperature increase in the non-sheet passing portion that has occurred through the 500 image formation from the previous time. .

  As shown in FIG. 11 with reference to FIG. 2, the control unit 102 starts image formation (S100). At this time, as shown to (a) of FIG. 12, the heating area | region which added the length for 2 pieces of the magnetic body core 7a to the conveyance width direction length of A4 size is set.

  The control unit 102 executes the relaxation mode when the time from the start of image formation to the interruption of toner image formation is a predetermined time or more, but does not execute it when the time is less than the predetermined time. When image formation is interrupted due to the adjustment of the developing device (23) (S101), the control unit 102 determines whether or not the elapsed time from the start of image formation exceeds a predetermined time (S102). The predetermined time is set as shown in Table 1 according to the environmental temperature and the weight (basis weight) per unit area of the recording material.

  In Example 1, since the temperature of the non-sheet passing portion is not measured, the temperature distribution is estimated by setting the predetermined time in this way. The predetermined time is desirably set to 20 seconds or more. When the predetermined time is too short, the temperature rise at the end is not so much, and there is no need to move the magnetic core 7a. As described above, if the heating region is narrowed in a situation where the temperature rise of the non-sheet passing portion is not so much generated, there is a possibility that the fixing of the recording material may occur due to temperature sag at both ends of the recording material in the conveyance width direction.

  As shown in FIG. 12B, if a predetermined time has elapsed from the start of image formation to the occurrence of adjustment processing, as shown by the solid line, a temperature increase in the non-sheet passing portion occurs in the fixing belt 1. Yes. If it is immediately after the start of image formation, the non-sheet passing portion temperature rise has not occurred as shown by the broken line. Therefore, when the temperature distribution becomes a solid line over a predetermined time, the temperature rise in the non-sheet passing portion is reduced by narrowing the heating region.

  When a predetermined time has elapsed from the start of image formation (YES in S102), the control unit 102 moves the outer magnetic cores 7a away from the exciting coil 6 as shown in FIG. The heating area is reduced to the length of the recording material in the conveyance width direction (S107).

  In this state, the control unit 102 adjusts the power supplied to the exciting coil 6 so that the detected temperature of the central thermistor TH1 is 180 ± 5 ° C. (S103). By removing the region where the non-sheet-passing portion temperature rise has occurred from the heating region and continuing the heating of the fixing belt 1, the temperature difference between the regions where the non-sheet-passing portion temperature rise has not occurred is rapidly increased. Thus, the non-sheet passing portion temperature rise is quickly resolved.

  As indicated by a solid line in FIG. 13B, a considerable temperature rise of the non-sheet passing portion has occurred in the fixing belt 1 before the magnetic core 7a is moved. However, during the down sequence for adjusting the developing device 23, as shown in FIG. 13A, the magnetic cores 7a located at both ends are moved away from the exciting coil 6 (S107). As a result, the heating area of the fixing belt 1 is narrowed and the temperature rise of the non-sheet passing portion is reduced (S103), and the durability of the fixing belt 1 is improved.

  When a predetermined time has elapsed after the relaxation mode is started, the control unit 102 returns the heating area to the state of heating the recording material and continues induction heating of the fixing belt 1 by the induction heating device 70. When the adjustment time of the developing device 23 is long, the time during which the heating region of the fixing belt 1 is set narrow may be too long. At this time, an unnecessary temperature drop proceeds in the inner region of the recording material, which causes a fixing failure in subsequent image formation. For this reason, the control unit 102 brings the outer magnetic cores 7a close to the exciting coil 6 and returns to the original state in about 10 seconds after the heating region is set narrow (S105). When the adjustment time of the developing device 23 is 10 seconds or less, the heating area is returned to the original state simultaneously with the restart of image formation after the adjustment is completed. Thereafter, the control unit 102 resumes image formation (S106).

  As shown in FIG. 14, according to Example 1 with the movement of the magnetic core 7 a, the heating area is narrowed, so that the heating area is widened more than twice without the movement of the magnetic core 7 a. The temperature rise at the non-sheet passing portion is quickly eliminated.

  According to Example 1 with the movement of the magnetic core 7a, the temperature increase of the non-sheet passing portion can be rapidly reduced to about 30 ° C. in 10 seconds, and therefore the non-sheet passing at the time when the image formation is restarted. As a result, the temperature rise in the area is reduced, and the durability performance of the fixing belt 1 is improved.

Next, using Osedale (registered trademark) 105 g / m ² A4 size plain paper as the recording material P, continuous image formation was performed in a 15 ° C. environment under the conditions of a productivity of 80 ppm and a process speed of 320 mm / ses. The temperature was adjusted so that the temperature detected by the central thermistor TH1 was maintained at 180 ° C. Example 1 with movement of the magnetic core 7a and no movement of the magnetic core 7a under the condition that the adjustment process of the developing device 23 enters a down sequence (passing of paper) for 10 seconds every 500 sheets passed. The durability of the fixing belt 1 was compared with the comparative example.

  As shown in Table 2, in Example 1 in which the heating region is narrowed during the down sequence by the magnetic core 7a, the durability life of the fixing belt 1 is longer than in the comparative example in which the heating region is not narrowed during the down sequence.

  By reducing the heating area during the down sequence by moving the magnetic core 7a, the non-sheet passing portion temperature rise can be reduced, and the non-sheet passing portion temperature rise during continuous image formation can be reduced by about 10 ° C. on average. did it. Thereby, in terms of the number of formed images, an improvement in durability life of 50000 sheets or more was observed.

<Example 2>
In Example 1, since the temperature of the non-sheet passing portion was not measured, as shown in Table 1, the temperature increase of the non-sheet passing portion was estimated by the duration after the start of image formation. However, as described in Patent Document 2, the temperature of the non-sheet passing portion may be measured to directly determine the occurrence state of the non-sheet passing portion temperature rise.

  As shown in FIG. 10, the end thermistor TH <b> 2 that is an example of the first detection unit can detect the local temperature rise of the fixing belt 1. The control unit 102 executes the relaxation mode when the local temperature rise detected by the end thermistor TH2 exceeds the predetermined range, but does not execute it when it does not exceed the predetermined range. An end thermistor TH3, which is an example of the second detection means, can detect the temperature of the fixing belt 1 at a position corresponding to the end of the recording material in the conveyance width direction. When the temperature of the fixing belt 1 detected by the end thermistor TH3 reaches a predetermined lower limit temperature necessary for fixing the toner image, the control unit 102 restores the heating area to the state at the time of heating the recording material to induce the induction heating device. The induction heating of the fixing belt 1 by 70 is continued.

  In Example 2, the end thermistor TH2 is provided 5 mm outside the A4 size sheet passing width, and the surface temperature of the fixing belt 1 is detected in a non-contact state. Further, an end thermistor TH3 is provided 5 mm inside the A4 size sheet passing width to detect the surface temperature of the fixing belt 1 in a non-contact state. Further, as shown in FIG. 2, a central thermistor TH1 is disposed inside the fixing belt 1 at the center of the fixing belt 1 to measure the temperature control temperature.

  The control unit 102 periodically captures the output of the non-contact type end portion thermistor TH2, and accurately grasps the temperature rise of the non-sheet passing portion of the fixing belt 1. The control unit 102 reduces the temperature rise of the non-sheet passing portion of the fixing belt 1 by narrowing the heat generation width when entering the down sequence by the adjustment processing of the developing device 23.

  The control unit 102 compares the non-sheet passing portion temperature rise detected by the end thermistor TH2 with the design temperature at the time of entering the down sequence accompanying the adjustment of the developing device 23. If the controller 102 exceeds the design temperature, the controller 102 reduces the non-sheet-passing portion temperature rise to the temperature control temperature, as shown in FIG. Prevent magnetic flux from entering the generated area.

  Then, immediately before the temperature detected by the end thermistor TH2 falls to the upper limit value of the temperature control temperature, the magnetic core 7a is returned to the state shown in FIG. 10 to resume image formation. However, when the temperature of the fixing region detected by the end thermistor TH3 falls below the lower limit value of the temperature adjustment temperature, priority is given to returning the magnetic core 7a to the temperature adjustment temperature by returning it to the state shown in FIG. .

  In the second embodiment, although there is a cause of cost increase of the end thermistors TH2 and TH3, more precise temperature adjustment control can be performed than the time estimation control in the first embodiment.

<Example 3>
In the first embodiment, the movable magnetic core 7 a is used as means for adjusting the magnetic flux density distribution in the longitudinal direction in which the magnetic flux generated by the induction heating device 70 enters the fixing belt 1. However, as shown in Patent Document 2, a configuration is adopted in which a movable magnetic flux shielding plate is arranged between the fixing belt 1 and the induction heating device 70 to adjust the magnetic flux density distribution in the longitudinal direction incident on the fixing belt 1. May be.

  In the third exemplary embodiment, the fixing device A movably disposes a magnetic flux shielding plate that shields part of the magnetic flux reaching the fixing belt 1 from the induction heating device 70 between the fixing belt 1 and the induction heating device 70. A displacement mechanism for moving the magnetic flux shielding plate in the longitudinal direction of the fixing belt 1 is provided, and an arbitrary heating region is set in the fixing belt 1 by controlling the displacement mechanism.

  In Example 3, by providing a magnetic flux shielding plate and moving it, the magnetic flux reaching from the induction heating device 70 is shielded except for the necessary part, and the heating area is controlled by suppressing the heat generation itself, The temperature distribution of the temperature rise at the non-sheet passing portion of the fixing belt 1 is controlled.

<Example 4>
FIG. 15 is a flowchart of the relaxation mode control according to the fourth embodiment. FIG. 16 is an explanatory diagram of setting of the heating area when image formation is interrupted. In Example 1, when the toner image formation was interrupted, the temperature increase of the non-sheet passing portion was eliminated or reduced using the interruption time. On the other hand, in Example 4, the toner image formation is actively interrupted before the size of the recording material to be heated becomes large, and the recording material is generated by continuous heating of the recording material of a small size until then. In addition, the temperature rise at the non-sheet passing part is eliminated.

  As shown in FIG. 15 with reference to FIG. 2, the control unit 102 interrupts the formation of the toner image on the intermediate transfer belt 26 before the length of the heated recording material in the transport width direction becomes large, thereby reducing the mode. Can be executed. The control unit 102 assumes that an image forming job for a large-size recording material has occurred (S202) while a small-size recording material is being passed (S201). If a predetermined time has passed since the start of image formation of a small-sized recording material (YES in S203), the control unit 102 narrows the heating area of the fixing belt 1 (S206).

  As shown in FIG. 10, when the A4 size recording materials are passed, the fixing belt 1 is locally heated on the outside of the recording material, and the non-sheet passing portion is heated. If the heating area is enlarged by the size difference from the A3 nobi size in order to start image formation of the A3 nobi size in this state, the non-sheet passing portion temperature rise is taken into the recording material.

  As shown in FIG. 16B, the local temperature rise of the fixing belt 1 generated by the passage of the A4 size recording material does not drop moderately, and the A3 nobi size (13 inch) recording material is passed as it is. When paper is used, a difference in gloss of the output image occurs between the inside and the outside of the locally heated portion. There may be a case where a hot offset image defect occurs in which the toner is carried away to the fixing belt 1 in a locally heated portion. For this reason, normally, in order to lower the temperature of the local temperature rise, it shifts to a wait time, resulting in a long wait for the user.

  Therefore, in the fourth embodiment, in order to solve these problems, when an image forming job having a larger size is generated after the A4 size recording material is passed, once the A4 size recording material is passed, the processing is temporarily performed as shown in FIG. As shown in (a), the heating area of the fixing belt 1 is narrowed. The temperature of the temperature rising portion is rapidly decreased by locally stopping the heating of the local temperature rising portion.

  As in the first embodiment, the control unit 102 narrows the heating region if the paper passing time exceeds the predetermined time shown in Table 2 (YES in S203) (S206). After that, the time for maintaining the state in which the heating area is narrowed is set to 10 seconds as in Example 1 in order to prevent the fixing belt 1 from lowering the temperature at the end of the conveyance width of the recording material to cause fixing failure.

  The controller 102 moves the magnetic core 7a after 10 seconds to obtain a heat generation width suitable for a 13 inch size recording material (S204). Then, image formation for a 13 inch size recording material is started (S205).

  According to the relaxation mode of Example 4, the time taken for the wait time and the time until the temperature of the subsequent 13-inch heating region to be stabilized could be shortened to 20 seconds from what normally took 30 seconds.

<Example 5>
FIG. 17 is a flowchart of the relaxation mode control according to the fifth embodiment. In the first embodiment, the control of the relaxation mode in the embodiment using the thin fixing belt has been described. However, if the relaxation mode is similarly applied to the embodiment using the thin fixing roller, the same effect is obtained. In Example 5, an embodiment using a thin fixing roller for further high-speed productivity will be described. In addition, the same code | symbol is attached | subjected to the structure which has the same function as Example 1, and the overlapping description is abbreviate | omitted.

  As shown in FIG. 2, in Example 5, the fixing belt 1 is replaced with a fixing roller 1R. The fixing roller 1R is formed by forming an elastic layer of silicone rubber having a thickness of 400 μm on the outside of an iron core bar having an inner diameter of 40 mm and a thickness of 1 mm, and covering the outer periphery of the elastic layer with a release layer of fluororesin (FRP). The The temperature control of the fixing roller 1R is performed by adjusting the output of the induction heating device 70 so that the temperature detected by the central thermistor TH1 disposed on the inner surface of the fixing roller 1R is maintained within a predetermined temperature control temperature range. .

  Here, if the combination of the normal use environment and the recording material is used, the temperature of the fixing roller 1R does not fall below the fixable temperature by design. However, when a thick recording material having a large heat capacity is continuously heat-treated in an ultra-low temperature environment of 5 ° C. or lower, the supplied power is not in time, and the temperature of the fixing roller 1R falls below the fixable temperature (YES in S302). If the temperature of the fixing roller 1R falls below the fixable temperature, a fixing failure such as cold offset may occur as described above. Sequence processing is executed.

  In the fifth embodiment, by using the down-sequence waiting time for recovering the temperature of the fixing roller 1R, the relaxation mode is executed, so that the temperature increase of the non-sheet passing portion that has occurred in the image formation up to that point can be increased. Let go. By narrowing the heating area in the down sequence that shifts when the temperature of the fixing roller 1R falls below the fixable temperature, the temperature rise of the non-sheet passing portion that has occurred in the same manner as the fixing belt 1 shown in FIG. 10 is reduced. The

  As shown in FIG. 17 with reference to FIG. 2, the control unit 102 starts to raise the temperature of the fixing roller 1R for the print job (S300). Then, the sheet feeding is started when the fixing roller 1R exceeds a predetermined sheet feeding start temperature (S301). In the image forming job, a sheet of A4 size is passed at a process speed of 400 mm / sec with a productivity of 100 sheets per minute (100 ppm), and the sheet feeding start temperature of the fixing roller 1R is set to 190 ° C. Suppose there is.

  If the inner surface temperature of the fixing roller 1R detected by the central thermistor TH1 falls below 170 ° C., which is the lower limit of the fixable temperature, during sheet passing (YES in S302), a down sequence is entered (S303). In the down sequence, similarly to the first embodiment, the determination is made based on the elapsed time in Table 1 (YES in S304), and the heating region is set narrow by moving the magnetic core 7a (S310).

  Thereafter, when the temperature of the fixing roller 1R recovers to the fixable temperature (YES in S305), the control unit 102 returns the heating area to the original state (S306) and restarts image formation (NO in S307). When the image forming job is completed (YES in S307), the control unit 102 stops the fixing device A (S308).

  As described in the first exemplary embodiment, when the heating area is narrowed to eliminate the temperature increase in the non-sheet passing portion during the normal printing operation, a temperature drop occurs at the recording material end portion position of the fixing roller 1R, thereby causing a cold offset. Fixing failure occurs. However, during the down sequence, since the heating of the recording material is interrupted, the amount of heat taken away by the recording material is reduced, so that the temperature decrease at the recording material end portion of the fixing roller 1R is not so much seen. It is possible to effectively reduce the temperature rise of the non-sheet passing portion by narrowing the area.

  As described in the first embodiment, if the predetermined time in Table 1 has elapsed from the start of image formation, the non-sheet passing portion temperature rise has occurred, so the relaxation mode is executed and the magnetic core 7a. The non-sheet-passing portion temperature rise is eliminated by moving and narrowing the heating area. However, if the predetermined time has not been exceeded, the non-sheet passing portion temperature rise has not occurred, so the relaxation mode is not executed.

  As described in the first exemplary embodiment, if the heating area is narrowed for a long time during the down sequence, the temperature sag in the sheet passing area gradually progresses, so the temperature of the fixing roller 1R does not recover to the fixable temperature. However, when 10 seconds elapse, the heating area is returned to the original state.

  According to the relaxation mode of the fifth embodiment, the temperature of the non-sheet-passing portion generated during normal image formation is quickly eliminated, so that the temperature of the non-sheet-passing portion can be kept low on average. Durability life is improved.

DESCRIPTION OF SYMBOLS 1 Fixing belt, 2 Pressure roller, 3 Pressure application member, 4 Stay 5 Magnetic shielding core, 6 Excitation coil, 7a Magnetic body core 8 Mold member, 9a Spring receiving member, 9b Stay pressurization spring 10 Fixing flange, 11 Outer magnetism Body core, 12 support side plate 21 photosensitive drum, 23 developing device, 26 intermediate transfer belt 31, recording material cassette, 33 registration roller 70 induction heating device, 71 core moving mechanism, 101 power supply device 102 control unit, 105 optical sensors PY, PM , PC, PK Image forming unit TH1 Central thermistor, TH2, TH3 End thermistor

Claims (8)

An image forming portion that forms a toner image and transfers it to a recording material, a first rotating body that contacts and heats the recording material to which the toner image is transferred, and a heating nip of the recording material that contacts the first rotating body A first rotating body, and a heating area of induction heating in the longitudinal direction of the first rotating body is set according to the recording material length in the transport width direction perpendicular to the transport direction of the recording material. In an image forming apparatus comprising an induction heating device for induction heating a body,
In the state where feeding of the recording material to the heating nip is stopped, the heating region is set narrower than the case where the recording material is fed to the heating nip, and the first rotating body by the induction heating device is set. An image forming apparatus comprising: a control unit capable of executing a relaxation mode for continuing induction heating.   The image forming apparatus according to claim 1, wherein the control unit is capable of executing the relaxation mode when toner image formation in the image forming unit is interrupted.   The control unit can execute the relaxation mode by interrupting the formation of a toner image in the image forming unit before the length of the heated recording material in the conveyance width direction is increased. The image forming apparatus according to 1 or 2.   The control unit executes the relaxation mode when the time from the start of image formation to the interruption of toner image formation is a predetermined time or more, but does not execute when the time is less than the predetermined time. The image forming apparatus according to 2 or 3.   The control means returns the heating area to a state during heating of the recording material after a predetermined time has elapsed after the relaxation mode is started, and continues induction heating of the first rotating body by the induction heating device. The image forming apparatus according to claim 4. Comprising first detection means capable of detecting the local temperature rise of the first rotating body;
The control means executes the relaxation mode when the local temperature rise detected by the first detection means exceeds a predetermined range, but does not execute when the local temperature rise does not exceed the predetermined range. The image forming apparatus according to claim 2. A second detection means capable of detecting the temperature of the first rotating body at a position corresponding to an end of the recording material in the conveyance width direction;
When the temperature of the first rotating body detected by the second detection means reaches a predetermined lower limit temperature necessary for fixing the toner image, the control means returns the heating area to the state when the recording material is heated. The image forming apparatus according to claim 6, wherein the induction heating of the first rotating body by the induction heating device is continued. The induction heating device includes an exciting coil member that generates a magnetic flux incident on the first rotating body, and the exciting coil member that maintains a temperature of the first rotating body within a predetermined temperature range necessary for fixing a toner image. And a plurality of magnetic cores arranged in the longitudinal direction of the first rotating body and guiding the magnetic flux generated by the exciting coil member to the first rotating body in each region And a core moving mechanism for moving the plurality of magnetic cores in a direction of moving toward and away from the first rotating body,
The core moving mechanism sets the heating region by bringing the number of magnetic cores corresponding to the length of the recording material in the width direction closer to the first rotating body than the other magnetic cores. The image forming apparatus according to any one of claims 1 to 7.

Images