JP5481439B2 - Fixing apparatus and image forming apparatus having the same - Google Patents

Description

  The present invention relates to a fixing device used for a copying machine, a printer, a facsimile, a composite machine thereof, and an image forming apparatus including the same.

  Conventionally, in a fixing device, a recording medium carrying an unfixed toner image is fed into a nip portion between a heating member and a pressure member, and at the same time the toner on the recording medium is melted by the heating action of the heating member, The toner image is fixed to the recording medium by applying a pressure action from the pressure member to the melted toner image.

  In recent years, as a heating member, so that image formation can be started immediately from the power-on state of the image forming apparatus, and image forming operations can be executed at any time in the standby state of the image forming apparatus. Some use an endless fixing belt. Since the fixing belt is made of a thin-walled material having good thermal conductivity and has a small heat capacity, it can warm up in a short time. Furthermore, the non-tensionable fixing belt can reduce the area in contact with other members, reduce heat transfer to the other members, and perform more efficient warm-up.

  An example of a fixing device that is rotationally driven without stretching a fixing belt is disclosed in Patent Document 1. The fixing device of Patent Document 1 includes a fixing belt having an endless peripheral surface, a pressure roller that is in contact with the outer peripheral surface of the fixing belt and rotationally driven, and an inner peripheral surface of the fixing belt that is in contact with the fixing belt. A pressing member sandwiched between the pressing roller and an electromagnetic induction heating unit that heats the fixing belt. Further, a guide member that is slidably in contact with both side edges of the inner peripheral surface of the fixing belt, a driving roller that is rotationally driven by a motor, etc., and the driving roller and the outer peripheral surface of both side edges of the fixing belt are wound around. And a driving belt for transmitting the rotational driving force of the driving roller to the fixing belt. With this configuration, even if the frictional force between the pressing member and the inner peripheral surface of the fixing belt acts on the fixing belt, the driving belt transmits the rotational driving force from the driving roller to the fixing belt, so that the fixing belt is driven. It rotates reliably based on the peripheral speed of the roller.

JP 2005-266376 A (paragraphs [0033] to [0037], FIGS. 1 and 2)

  Since the fixing belt has a relatively small rigidity, when the driving belt is driven on the side edge of the fixing belt as in Patent Document 1, the stress is concentrated on the portion of the fixing belt where the driving belt is applied. There is a problem that the fixing belt is damaged by long-term use.

  SUMMARY An advantage of some aspects of the invention is that it provides a fixing device in which a fixing belt maintains durability and always rotates smoothly, and an image forming apparatus including the fixing device. And

  In order to achieve the above object, the first invention is an endless fixing belt that is rotatably supported, a heating unit that heats the fixing belt, and a rotational drive in a state of being pressed against the outer peripheral surface of the fixing belt. And a nip portion for fixing the toner image by heating and pressurizing an unfixed toner image formed on the recording medium between the pressure belt and the pressure roller for driving and rotating the fixing belt. And a pressing member that is disposed on an inner peripheral surface of the fixing belt and presses the recording medium and the fixing belt between the pressing roller and the fixing belt, and the fixing belt has an N pole in the rotation direction. A first magnetized portion in which S poles are alternately magnetized is provided, and is arranged opposite to the first magnetized portion in the nip portion, and N poles and S poles are alternately magnetized. A second magnetized portion is further provided, and the second magnetized portion is Tsu is characterized by being configured such that the opposite polarities of the magnetic poles relative to the first magnetized portion of the magnetic pole passing through the up part.

  According to a second aspect of the present invention, in the above fixing device, the second magnetized portion is alternately provided with N and S poles in a circumferential direction of the pressure roller.

  According to a third aspect of the present invention, in the above fixing device, the pressing member includes an electromagnet disposed away from the fixing belt, and the electromagnet passes through the nip portion. It is characterized in that the direction of current can be switched so that the magnetic poles have the same polarity as the magnetic poles.

  According to a fourth aspect of the present invention, in the above-described fixing device, an on / off signal is provided in response to the passage of the first magnetized portion as the fixing belt rotates and is disposed in the vicinity of the fixing belt. The magnetic pole of the first magnetized portion passing through the nip portion is detected based on the output reed switch and the on / off signal of the reed switch, and the magnetic pole of the electromagnet has the same polarity as the detected magnetic pole And a controller that switches the direction of the current so that

  According to a fifth aspect of the present invention, in the above fixing device, the second magnetized portion is an electromagnet disposed on the pressing member, and the electromagnet is the first magnetized portion passing through the nip portion. It is characterized in that the direction of current can be switched so that the magnetic pole has a polarity opposite to that of the magnetic pole.

  According to a sixth aspect of the present invention, in the above-described fixing device, an on / off signal is provided in response to the passage of the first magnetized portion as the fixing belt rotates and is disposed in the vicinity of the fixing belt. Based on the reed switch to be output and the reed switch on / off signal, the magnetic pole of the first magnetized portion passing through the nip portion is detected, and the magnetic pole of the electromagnet is opposite in polarity to the detected magnetic pole. And a controller that switches the direction of the current so that

  According to a seventh aspect of the present invention, in the above-described fixing device, an on / off signal is provided according to the passage of the first magnetized portion that is disposed in the vicinity of the fixing belt and that is caused by the rotation of the fixing belt. A reed switch for output, and a control for detecting the presence or absence of rotation of the fixing belt based on an on / off signal of the reed switch and switching on and off the power supply of the heating unit based on the presence or absence of rotation of the fixing belt And a section.

  According to an eighth aspect of the present invention, in the fixing device, the heating unit includes an induction heating unit that generates heat from the eddy current generated in the conductive layer of the fixing belt.

  According to a ninth aspect of the invention, there is provided an image forming apparatus including the fixing device having the above-described configuration.

  According to the first invention, when the recording medium to be fixed is sandwiched at the nip portion and conveyed by the rotation of the fixing belt and the pressure roller, the second magnetized portion of the fixing belt is the first portion of the fixing belt. It is comprised so that it may become a magnetic pole of a reverse polarity with respect to the magnetic pole of a magnetization part. As a result, a magnetic force attracted by the first magnetized portion and the second magnetized portion is generated in the nip portion, and this attracting magnetic force acts to promote the rotational force of the fixing belt. Normally, a fixing belt having relatively low rigidity is subjected to a load due to the pressure contact of the nip portion, and the fixing belt may be damaged if used for a long period of time. However, the attractive magnetic force between the first magnetized portion and the second magnetized portion may be damaged. Thus, since the fixing belt rotates smoothly, the load applied to the fixing belt can be reduced and the durability can be improved.

1 is a diagram illustrating a schematic configuration of an image forming apparatus including a fixing device according to a first embodiment of the present invention. Sectional drawing which shows schematic structure of the fixing device which concerns on 1st Embodiment. FIG. 3 is a schematic cross-sectional view showing the periphery of the nip portion of the fixing device according to the first embodiment. Schematic sectional view showing the periphery of the nip portion of the fixing device according to the second embodiment Schematic cross-sectional view showing the periphery of the nip portion of the fixing device according to the third embodiment

  Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Further, the use of the invention and the terms shown here are not limited thereto.

(First embodiment)
FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus including a fixing device according to an embodiment of the present invention. The image forming apparatus 1 is provided with a sheet feeding unit 2 disposed in a lower portion thereof, a sheet conveying unit 3 disposed on a side of the sheet feeding unit 2, and an upper side of the sheet conveying unit 3. The image forming unit 4 includes a fixing device 5 disposed on the discharge side of the image forming unit 4, and an image reading unit 6 disposed above the image forming unit 4 and the fixing device 5.

  The paper feed unit 2 includes a plurality of paper feed cassettes 7 that store paper 9 that is a recording medium. The paper 9 is sent to the paper transport unit 3 one by one.

  The paper 9 sent to the paper transport unit 3 is transported toward the image forming unit 4 via the paper transport path 10. The image forming unit 4 forms a toner image on a sheet 9 by an electrophotographic process. The image forming unit 4 is a photoconductor 11 supported so as to be rotatable in the direction of an arrow in FIG. A charging unit 12, an exposure unit 13, a developing unit 14, a transfer unit 15, a cleaning unit 16, and a charge eliminating unit 17 are provided.

  The charging unit 12 includes a charging wire to which a high voltage is applied. When a predetermined potential is applied to the surface of the photoconductor 11 by corona discharge from the charging wire, the surface of the photoconductor 11 is uniformly charged. Then, when light based on the image data of the original read by the image reading unit 6 is irradiated to the photoconductor 11 by the exposure unit 13, the surface potential of the photoconductor 11 is selectively attenuated, and the surface of the photoconductor 11 is irradiated. An electrostatic latent image is formed.

  Next, the developing unit 14 develops the electrostatic latent image on the surface of the photoconductor 11, and a toner image is formed on the surface of the photoconductor 11. This toner image is transferred by the transfer unit 15 to the sheet 9 supplied between the photoconductor 11 and the transfer unit 15.

  The sheet 9 on which the toner image has been transferred is conveyed toward the fixing device 5 disposed downstream of the image forming unit 4 in the sheet conveying direction. In the fixing device 5, the paper 9 is heated and pressed, and the toner image is melted and fixed on the paper 9. Next, the sheet 9 on which the toner image is fixed is discharged onto the discharge tray 21 by the discharge roller pair 20.

  After the transfer of the toner image onto the paper 9 by the transfer unit 15, the toner remaining on the surface of the photoconductor 11 is removed by the cleaning unit 16, and the residual charge on the surface of the photoconductor 11 is removed by the charge eliminating unit 17. Then, the photosensitive member 11 is charged again by the charging unit 12, and image formation is performed in the same manner.

  FIG. 2 is a cross-sectional view showing a schematic configuration of the fixing device 5 used in the image forming apparatus 1 described above.

  The fixing device 5 is a fixing method using a heat source of an electromagnetic induction heating method, and includes a fixing belt 26, a pressure roller 19, a pressing member 55, an induction heating unit 30 that heats the fixing belt 26, and an induction heating unit. 30, a thermistor 25, and a reed switch 27 that detects the presence or absence of rotation of the fixing belt 26. The pressure roller 19 is rotatably supported in the longitudinal direction (axial direction) of the apparatus main body (not shown) of the fixing device 5, and the induction heating unit 30, the pressing member 55, the thermistor 25, and the reed switch 27 are fixed to the apparatus main body. Supported.

  The fixing belt 26 is endless and has a circular cross section, and in order from the inner peripheral side, a conductive layer 26a, an elastic layer 26b made of silicon rubber, a fluororesin, and an unfixed toner image at a nip portion N. A release layer 26c that improves the releasability at the time of melting and fixing is laminated. The fixing belt 26 has, for example, a circular cross section with an outer diameter of 20 to 50 mm, the conductive layer 26 a is formed of copper, nickel, and stainless steel with a thickness of 20 to 50 μm, and the elastic layer 26 b is formed with a thickness of 100 to 500 μm. The release layer 26c is preferably formed with a thickness of 30 to 50 μm. Furthermore, a thin film such as a polyimide resin may be coated on the inner peripheral surface side of the conductive layer 26 a so as to slide smoothly with respect to the pressing member 55.

  The pressing member 55 includes a pressing pad 56 and an arc guide portion 59. The pressure pad 56 is disposed on the inner peripheral surface of the fixing belt 26 and faces the outer peripheral surface of the pressure roller 19 with the fixing belt 26 interposed therebetween. The pressing pad 56 presses the fixing belt 26 toward the pressure roller 19 side. The surface of the pressing pad 56 that contacts the fixing belt 26 is provided with a sliding portion with a low friction material such as fluororesin or polyimide resin, and the surface roughness Ra of the sliding surface is set to 10 to 100 μm, for example. You may make it reduce the sliding load at the time of conveyance of the paper 9. FIG. The arc guide portion 59 is provided on the opposite side of the pressing pad 56 so as to face the induction heating portion 30, and guides the fixing belt 26 so as to rotate along the induction heating portion 30.

  The pressure roller 19 includes a cylindrical cored bar 51, a magnet 52 disposed on the outer periphery of the cored bar 51, and an elastic layer 53 made of silicon rubber or the like formed on the magnet 52. The elastic layer 53 may be formed to have a thickness of 2 to 5 mm, for example, and a release layer made of a fluororesin tube covering the surface of the elastic layer 53 is provided, and a sheet for melting and fixing an unfixed toner image at the nip N. 9 may be improved.

  The pressure roller 19 is driven to rotate in the direction of the arrow by the motor 69, and the fixing belt 26 that is in pressure contact with the pressure roller 19 is driven to rotate in the direction of the arrow by the rotation of the pressure roller 19. As a result, a nip portion N is formed at a portion where the fixing belt 26 and the pressure roller 19 come into contact with each other while rotating in reverse. In the nip portion N, an unfixed toner image on the conveyed paper 9 is heated and pressurized to fix the toner image on the paper 9.

  The induction heating unit 30 includes an exciting coil 37, a bobbin 38, and a core 39, and heats the fixing belt 26 by electromagnetic induction. The induction heating unit 30 extends in the longitudinal direction (from the front surface to the back surface in FIG. 2) and is fixed. The outer periphery of the belt 26 is disposed to face the fixing belt 26 so as to surround a substantially semicircle.

  An exciting coil 37 made of copper wire or the like is wound around a bobbin 38 and spirally disposed around a semicircle on the outer periphery of the fixing belt 26 so as to circulate around the central portion of the core 39 in the longitudinal direction. . The exciting coil 37 is connected to a power source 41 and generates a magnetic flux by a high frequency current supplied from the power source 41. A magnetic flux generated from the induction heating unit 30 is generated in a direction parallel to the paper surface of FIG. 2 and penetrates the conductive layer 26 a of the fixing belt 26. An eddy current is generated around the magnetic flux of the conductive layer 26a. When the eddy current flows, Joule heat is generated by the electric resistance in the conductive layer 26a, and the conductive layer 26a generates heat.

  The thermistor 25 is arranged on the outer peripheral surface of the fixing belt 26 so as to face the center and both ends in the longitudinal direction, and detects the temperature of each region. The thermistor 25 and the power source 41 are connected to a control unit 43 including a CPU and storage elements such as a RAM and a ROM.

  The control unit 43 controls the power of the power source 41 based on the temperature detected by the thermistor 25 so that the fixing belt 26 reaches a predetermined temperature by the induction heating unit 30. When the fixing belt 26 is heated and heated to a predetermined temperature, the sheet 9 sandwiched by the nip portion N is heated and pressed by the pressure roller 19, whereby the powder on the sheet 9 is heated. The toner in the state is melted and fixed.

  The reed switch 27 has two magnetic leads and a contact portion where the tips of the magnetic leads are opposed to each other with a predetermined interval. When a magnetic field is applied from the outside, the magnetic leads are magnetized and contacted. If the magnetic field is erased, the contact portion is opened by the elasticity of the lead. As a result, the reed switch 27 outputs on and off signals. Further, the reed switch 27 is disposed opposite to the vicinity of the outer peripheral surface of the fixing belt 26, and outputs on and off signals to the control unit 43. Note that the reed switch 27 may be disposed to face the vicinity of the inner peripheral surface of the fixing belt 26.

  The fixing belt 26 is alternately magnetized with N and S poles in the direction of rotation. When the fixing belt 26 is stopped rotating, the magnetic lead of the reed switch 27 is magnetized by the magnetic pole of the fixing belt 26. The reed switch 27 outputs an on signal. Alternatively, depending on the rotation stop position of the fixing belt 26, the fixing belt 26 stops at a position where the magnetic pole of the fixing belt 26 does not face the magnetic lead, the magnetic lead is demagnetized, and the reed switch 27 outputs an off signal. On the other hand, when the fixing belt 26 is rotating, the magnetic lead is repeatedly magnetized and demagnetized at a predetermined timing according to the pitch of the N pole and S pole of the fixing belt 26, and the reed switch 27 is turned on. The off signal is output alternately. The magnetization of the fixing belt 26 will be described in detail later.

  The control unit 43 controls energization / non-energization from the power supply 41 to the excitation coil 37 based on a signal output from the reed switch 27. For example, when an on / off alternate signal is output from the reed switch 27 at a predetermined time, the control unit 43 determines that the fixing belt 26 is rotating, and the power source 41 applies an excitation coil 37 to the excitation coil 37. Control to energize. On the other hand, when the ON signal or the OFF signal is continuously output from the reed switch 27 at a predetermined time, it is determined that the fixing belt 26 has stopped rotating, and the excitation coil of the induction heating unit 30 is determined from the power source 41. It controls so that the electricity supply to 37 may be stopped. When the fixing belt 26 stops rotating, if a magnetic flux is generated from the induction heating unit 30 to the conductive layer 26a of the fixing belt 26, only a part of the fixing belt 26 generates heat. When energization from the power supply 41 to the exciting coil 37 is controlled based on the signal output from the reed switch 27, it is possible to prevent excessive heat generation only at a specific portion of the fixing belt 26. Note that the present invention is not limited to the electromagnetic induction heating method, and can be applied to control of energization of a power source by a heater heating method such as a halogen heater.

  The fixing belt 26 is magnetized as shown in FIG. FIG. 3 is a schematic sectional view showing the fixing belt 26 and the pressure roller 19 around the nip N.

  The fixing belt 26 includes the conductive layer 26a, the elastic layer 26b, and the release layer 26c as described above. The conductive layer 26 a constitutes a first magnetized portion in which N and S poles are alternately magnetized in the rotation direction of the fixing belt 26. The N pole and the S pole of the first magnetized portion are arranged at substantially equal intervals over the entire circumference in the rotation direction, and are formed over the entire width direction of the fixing belt 26 (the front and back direction in FIG. 3). The fixing belt 26 is sandwiched between the sliding portion 57 of the pressing member 55 and the elastic layer 53 of the pressure roller 19 so that the S pole of the first magnetized portion is located at the center of the nip portion N. Instead of configuring the first magnetized portion with the conductive layer 26a, a first magnetized portion is provided separately from the conductive layer 26a, the elastic layer 26b, and the release layer 26c, for example, the first magnetized portion. May be disposed between the conductive layer 26a and the elastic layer 26b.

  In the pressure roller 19, a magnet 52 as a second magnetized portion is disposed between the cored bar 51 and the elastic layer 53. The second magnetized portion is made of a permanent magnet in which N poles and S poles are alternately arranged in the circumferential direction, and is formed over the entire axial direction of the pressure roller 19 (longitudinal direction, the front and back direction in FIG. 3). . The N pole and S pole of the second magnetized portion are arranged at substantially equal intervals in the circumferential direction, and are configured to have the same pitch as the N pole and S pole of the first magnetized portion of the fixing belt 26. . In a state where the S pole of the first magnetized portion of the fixing belt 26 is located at the center of the nip portion N, the pressure roller 19 is rotated so that the N pole of the second magnetized portion is located at the center of the nip portion N. The position is adjusted in the direction and connected to a drive source such as a motor 69 (see FIG. 2). The second magnetized portion may be configured by magnetizing a magnetic material such as iron, cobalt, or nickel.

  By arranging the fixing belt 26 and the pressure roller 19 in this way, at the nip portion N, the S pole of the first magnetized portion of the fixing belt 26 is N of the second magnetized portion of the pressure roller 19. The N pole of the first magnetized portion faces the S pole of the second magnetized portion, and the first and second magnetized portions are opposite to each other in opposite polarities.

  In the above configuration, when the pressure roller 19 is rotated by the rotational drive of the motor 69, the fixing belt 26 pressed by the pressing member 55 is driven to rotate. When the sheet 9 is nipped at the nip portion N and conveyed by the rotation of the fixing belt 26 and the pressure roller 19, a magnetic force attracted by the first magnetized portion and the second magnetized portion is generated in the nip portion N. The attractive magnetic force acts to promote the rotational force of the fixing belt 26. Usually, the fixing belt 26 having relatively small rigidity is subjected to a load due to the pressure contact of the nip portion N, and the fixing belt 26 may be damaged when used for a long period of time. However, the first magnetized portion and the second magnetized portion Due to the attractive magnetic force, the fixing belt 26 rotates smoothly, so that the load applied to the fixing belt 26 is reduced and the durability of the fixing belt 26 can be improved.

  Further, as described above, the second magnetic pole portion is disposed in the circumferential direction of the pressure roller 19 so that the fixing device 5 is not increased in size.

(Second Embodiment)
FIG. 4 is a schematic cross-sectional view showing the periphery of the nip portion of the fixing device according to the second embodiment. In the second embodiment, an electromagnet is provided as the second magnetized portion. The first and second magnetized portions that are different from the first embodiment will be mainly described. Hereinafter, the same portions as those in the first embodiment will be described. Is omitted.

  The fixing belt 26 includes a conductive layer 26a, an elastic layer 26b, and a release layer 26c. The conductive layer 26 a constitutes a first magnetized portion in which N and S poles are alternately magnetized in the rotation direction of the fixing belt 26. The N pole and S pole of the first magnetized portion are arranged at substantially equal intervals over the entire circumference in the rotation direction, and are formed over the entire width direction of the fixing belt 26. The fixing belt 26 is sandwiched between the sliding portion 57 of the pressing member 55 and the pressure roller 19. Instead of configuring the first magnetized portion with the conductive layer 26a, a first magnetized portion is provided separately from the conductive layer 26a, the elastic layer 26b, and the release layer 26c, for example, the first magnetized portion. May be disposed between the conductive layer 26a and the elastic layer 26b.

  The pressure roller 19 includes a cylindrical cored bar 51 and an elastic layer 53 made of silicon rubber or the like. Further, a release layer made of a fluororesin tube covering the surface of the elastic layer 53 may be provided.

  The pressing member 55 includes a pressing pad 56 and a sliding portion 57, and an electromagnet 58 that is a second magnetic pole portion is disposed between the pressing pad 56 and the sliding portion 57. The pressing pad 56 presses the fixing belt 26 toward the pressure roller 19 side. The sliding portion 57 is made of a low friction material such as a fluorine resin or a polyimide resin, and contacts the inner peripheral surface of the fixing belt 26. Furthermore, the surface roughness Ra of the sliding portion 57 may be set to 10 to 100 μm, for example, so as to reduce the sliding load when the paper 9 is conveyed.

  The electromagnet 58 includes a magnetic material provided in the entire longitudinal direction of the pressing pad 56 and a coil wound around the longitudinal direction of the magnetic material. By passing a current through the coil of the electromagnet 58, the magnetic poles of the N pole and the S pole are generated in the paper conveyance direction. On the other hand, by switching the direction of the current flowing through the coil of the electromagnet 58, the arrangement of the N pole and the S pole A magnetic pole in which is replaced occurs in the paper transport direction. Two electromagnets 58 are arranged side by side in the paper conveyance direction, and are fixed to the pressing pad 56 and the sliding portion 57 with an adhesive or the like.

  When currents in the same direction flow through the coils of the two electromagnets 58 and 58, the electromagnets 58 and 58 are, for example, S poles in order from the upstream side (right side in FIG. 4) in the sheet conveyance direction at the nip portion N. Magnetic poles alternating with N poles, S poles and N poles are generated. When the currents flowing through the coils of the two electromagnets 58 and 58 are switched in the opposite direction, the nip portion N is switched to the N pole, the S pole, the N pole, and the S pole in order from the upstream side in the sheet conveyance direction. One electromagnet 58 may be provided, or three or more electromagnets 58 may be provided.

  The electromagnets 58 and 58 can switch the current direction of the coil so as to have a polarity opposite to that of the first magnetized portion of the fixing belt 26 facing the nip portion N.

  The coil current direction is switched by, for example, constants such as the pitch between the magnetic poles of the first magnetic pole portion of the fixing belt 26, the rotational speed of the fixing belt 26, and the arrangement distance of the reed switch 27 (see FIG. 2) with respect to the nip portion N. From the ON / OFF signal of the reed switch 27 output according to the rotation of the fixing belt 26, the magnetic pole of the fixing belt 26 in the nip portion N is determined, and then the fixing belt 26 in the determined nip portion N is determined. The current direction of the coil is controlled so that the magnetic poles of the electromagnets 58 and 58 are opposite in polarity to the magnetic poles.

  Thus, by switching the current direction of the coils of the electromagnets 58 and 58, a magnetic force attracted by the magnetic poles of the first magnetized portion of the fixing belt 26 and the electromagnets 58 and 58 is generated in the nip portion N. The attractive magnetic force acts to promote the rotational force of the fixing belt 26. As a result, the fixing belt 26 rotates smoothly, the load applied to the fixing belt 26 is reduced, and the durability of the fixing belt 26 can be improved.

  In the above embodiment, the magnet 52 as the second magnetized portion may be further disposed between the cored bar 51 and the elastic layer 53 of the pressure roller 19.

(Third embodiment)
FIG. 5 is a schematic cross-sectional view showing the periphery of the nip portion of the fixing device according to the third embodiment. In the third embodiment, the pressing member has an electromagnet in the configuration of the first embodiment.

  The fixing belt 26 includes a conductive layer 26a, an elastic layer 26b, and a release layer 26c, and further includes a first magnetized portion in which N and S poles are alternately magnetized in the rotation direction. The N pole and S pole of the first magnetized portion are arranged at substantially equal intervals over the entire circumference in the rotation direction, and are formed over the entire width direction of the fixing belt 26. The pressure roller 19 includes a core metal 51 and an elastic layer 53, and further includes a second magnetic pole portion in which N poles and S poles are alternately arranged in the circumferential direction between the core metal 51 and the elastic layer 53. . The second magnetic pole portion is formed over the entire area of the pressure roller 19 in the axial direction (longitudinal direction), and the N pole and the S pole of the second magnetized portion are arranged at substantially equal intervals in the circumferential direction. It is comprised so that it may become the same pitch as the N pole of a magnetic part, and a S pole.

  The electromagnet 61 constitutes a pressing member, and includes a magnetic material provided in the entire longitudinal direction of the nip portion N and a coil wound around the longitudinal direction of the magnetic material. By passing a current through the coil of the electromagnet 61, the magnetic poles of the N pole and the S pole are generated in the paper transport direction. On the other hand, by switching the direction of the current passed through the coil of the electromagnet 61, the arrangement of the N pole and the S pole A magnetic pole in which is replaced occurs in the paper transport direction. Two electromagnets 61 are arranged side by side in the paper conveyance direction, and are fixed to the apparatus main body at a position separated from the fixing belt 26.

  When currents in the same direction are passed through the coils of the two electromagnets 61 and 61, the electromagnets 61 and 61 are, for example, in order from the upstream side (the right side in FIG. 4) in the paper conveyance direction, N poles and alternating magnetic poles are generated. When the currents flowing through the coils of the two electromagnets 61 and 61 are switched in the opposite directions, the electrodes are switched to the N pole, the S pole, the N pole, and the S pole in order from the upstream side in the paper transport direction. One electromagnet 61 may be provided, or three or more electromagnets 61 may be provided.

  The electromagnets 61 and 61 can switch the current direction of the coil so that they have the same polarity as that of the first magnetized portion of the fixing belt 26 in the nip portion N. When the magnetic poles of the electromagnets 61 and 61 and the first magnetized portion of the fixing belt 26 have the same polarity, a repulsive magnetic field is generated between the electromagnets 61 and 61 and the first magnetized portion of the fixing belt 26. . By this repulsive magnetic field, the fixing belt 26 is pressed toward the pressure roller 19 without contacting the pressing members (electromagnets 61 and 61), and the sheet 9 is held at the nip portion N. When the pressure roller 19 is rotated by the rotational drive of the motor 69 (see FIG. 2), the fixing belt 26 pressed by the electromagnets 61 and 61 (pressing member) is driven to rotate. A magnetic force attracted by the first magnetized portion of the fixing belt 26 and the second magnetized portion of the pressure roller 19 is generated in the nip portion N, and this attracting magnetic force acts to promote the rotational force of the fixing belt 26. To do. As a result, the fixing belt 26 rotates smoothly, the load applied to the fixing belt 26 is reduced, and the durability of the fixing belt 26 can be improved.

  The coil current direction is switched, for example, by the pitch between the magnetic poles of the first magnetic pole portion of the fixing belt 26, the rotation speed of the fixing belt 26, the arrangement distance of the reed switch 27 (see FIG. 2) with respect to the nip portion N, and the like. The magnetic pole of the fixing belt 26 in the nip portion N is determined from the ON / OFF signal of the reed switch 27 output according to the rotation of the fixing belt 26, and then the fixing belt in the determined nip portion N is determined. The current direction of the coil is controlled so that the magnetic poles of the electromagnets 61 and 61 have the same polarity with respect to the 26 magnetic poles. Further, in order to facilitate the rotation of the fixing belt 26 toward the nip portion N, a guide member 62 is disposed upstream of the nip portion N in the rotation direction of the fixing belt 26. Further, an arc guide portion 59 (see FIG. 2) may be disposed in the apparatus main body so that the fixing belt 26 rotates along the induction heating portion 30 (see FIG. 2). Or you may make it the structure which attaches the circular arc guide part 59 and the electromagnets 61 and 61 integrally.

  The present invention can be used for a fixing device used in a copying machine, a printer, a facsimile, a multi-function machine thereof, and an image forming apparatus including the fixing device.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 5 Fixing apparatus 19 Pressure roller 25 Thermistor 26 Fixing belt 26a Conductive layer (1st magnetization part)
26b Elastic layer 26c Release layer 27 Reed switch 30 Induction heating unit (heating unit)
41 Power Supply 43 Control Unit 51 Core Bar 52 Magnet (Second Magnetization Unit)
53 Elastic layer 55 Pressing member 56 Pressing pad 57 Sliding part 58 Electromagnet (second magnetized part)
59 Circular guide 61 Electromagnet (pressing member)
62 Guide member 69 Motor N Nip part

Claims (9)

An endless fixing belt rotatably supported;
A heating unit for heating the fixing belt;
A pressure roller that rotationally drives the fixing belt in a state of being pressed against the outer peripheral surface of the fixing belt, and rotates the fixing belt.
A nip portion formed between the fixing belt and the pressure roller for fixing the toner image by heating and pressing an unfixed toner image on the recording medium;
A pressing member disposed on an inner peripheral surface of the fixing belt and press-contacting the recording medium and the fixing belt with the pressure roller;
The fixing belt is provided with a first magnetized portion in which N poles and S poles are alternately magnetized in the rotation direction,
A second magnetized portion that is arranged opposite to the first magnetized portion in the nip portion and is alternately magnetized with N and S poles;
The fixing device, wherein the second magnetized portion is configured to be a magnetic pole having a polarity opposite to that of the first magnetized portion passing through the nip portion.   2. The fixing device according to claim 1, wherein the second magnetized portion has N and S poles alternately arranged in a circumferential direction of the pressure roller.   The pressing member has an electromagnet disposed away from the fixing belt, and the electromagnet has a current having a polarity that is the same as that of the first magnetized portion passing through the nip portion. The fixing device according to claim 2, wherein the direction is switched.   A reed switch that is disposed in opposition to the vicinity of the fixing belt and outputs an on / off signal according to the passage of the first magnetized portion as the fixing belt rotates, and an on / off signal of the reed switch A control unit that detects a magnetic pole of the first magnetized portion passing through the nip portion based on the current direction and switches a current direction so that the magnetic pole of the electromagnet has the same polarity with respect to the detected magnetic pole; The fixing device according to claim 3, further comprising a fixing device.   The second magnetized portion is composed of an electromagnet disposed on the pressing member, and the electromagnet has a current polarity so as to have a polarity opposite to that of the first magnetized portion passing through the nip portion. The fixing device according to claim 1, wherein the direction can be switched.   A reed switch that is disposed in opposition to the vicinity of the fixing belt and outputs an on / off signal according to the passage of the first magnetized portion as the fixing belt rotates, and an on / off signal of the reed switch A control unit that detects a magnetic pole of the first magnetized portion that passes through the nip portion based on the magnetic pole, and switches a current direction so that the magnetic pole of the electromagnet has a reverse polarity with respect to the detected magnetic pole. The fixing device according to claim 5, further comprising:   A reed switch that is disposed in opposition to the vicinity of the fixing belt and outputs an on / off signal according to the passage of the first magnetized portion as the fixing belt rotates, and an on / off signal of the reed switch And a controller that detects whether or not the fixing belt is rotated based on the rotation of the fixing belt, and switches the power of the heating unit on and off based on whether or not the fixing belt is rotated. The fixing device according to claim 1.   The fixing device according to claim 1, wherein the heating unit includes an induction heating unit that generates heat from the eddy current generated in the conductive layer of the fixing belt.   An image forming apparatus comprising the fixing device according to claim 1.

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