JP2009198665A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device capable of quickly preventing the temperature at a portion through which a small-sized sheet does not pass of a fixing member from being excessively raised. <P>SOLUTION: This fixing device includes a fixing member 1 in which a conveyed sheet is pressed against the outer periphery, and an exciting coil 31 for induction heating of a heating layer of the fixing member 1 arranged facing the outer periphery 1a of the fixing member 1. This fixing device includes a first magnetic material core 32 arranged so as to cover the exciting coil 31 at a position equivalent to the opposite side of the fixing member 1 in relation to the exciting coil 31. This fixing device includes a second magnetic material core 35 arranged as another body of the first magnetic material core 32 at a position nearer than the first magnetic material core 32 in relation to the outer periphery of the fixing member 1. The Curie temperature of the second magnetic material core 35 is above a target fixing temperature and below a predetermined upper limit control temperature. The Curie temperature of the first magnetic material core 32 is above the upper limit control temperature. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fixing device, and more particularly to an electromagnetic induction heating type fixing device.

  The present invention also relates to an image forming apparatus provided with such a fixing device.

  Conventionally, as an electromagnetic induction heating type fixing device, for example, in Patent Document 1 (Japanese Patent Laid-Open No. 2001-23767), a Curie temperature of about 200 ° C., which is slightly higher than the fixing temperature (temperature for melting toner), is used. It has been proposed to have a ferrite (magnetic core) having a coil, a coil wound around the ferrite, and a roller induction-heated by a magnetic flux generated from the coil. When fixing toner on small-size paper, if the temperature of the part of the roller where the paper does not pass rises excessively and the ferrite exceeds the Curie temperature due to heat conduction, the permeability decreases, The induced current decreases and the calorific value decreases. As a result, the temperature of the roller is lowered.

Japanese Patent Application Laid-Open No. 2001-318545 proposes a fixing device including a magnetic core divided into a plurality of parts in the longitudinal direction of a roller. In this fixing device, a core having a Curie temperature of 250 ° C. is disposed as a central core, which is a small size paper passage area, and a core having a Curie temperature of 200 ° C. is disposed at both ends of the non-paper passage area. Yes. When fixing toner on small-size paper, the cores arranged at both end portions which are the non-sheet passing area work in the same manner as in Patent Document 1 so as to prevent excessive temperature rise at both end portions of the roller. It has become.
Japanese Patent Laid-Open No. 2001-23767 JP 2001-318545 A

  However, in the conventional fixing device, since the magnetic cores are all continuously integrated in a cross section perpendicular to the central axis of the roller, the heat capacity of the magnetic core is relatively large. For this reason, when the toner is fixed on a small size paper, when the temperature of the portion of the roller through which the paper does not pass (end portion) rises, the magnetic core disposed opposite to the portion is delayed in temperature rise. Occurs. For this reason, the conventional fixing device has a problem that it is not possible to quickly prevent an excessive temperature rise at both end portions of the roller.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a fixing device that can quickly prevent an excessive temperature rise in a portion of a fixing member through which a small size sheet does not pass (referred to as a “small size sheet non-passing region”). There is to do.

  Another object of the present invention is to provide an image forming apparatus provided with such a fixing device.

In order to solve the above problems, the fixing device of the present invention is:
A fixing member in which the conveyed sheet is pressed against the outer peripheral surface;
An exciting coil for inductively heating the heat generating layer of the fixing member, disposed opposite to the outer peripheral surface of the fixing member;
A first magnetic core disposed so as to cover the exciting coil at a position corresponding to the opposite side of the fixing member with respect to the exciting coil;
A second magnetic core disposed separately from the first magnetic core at a position closer to the outer peripheral surface of the fixing member than the first magnetic core;
The Curie temperature of the second magnetic core is not less than the target fixing temperature and not more than a predetermined upper limit control temperature,
The Curie temperature of the first magnetic core is higher than the upper limit control temperature.

  In the fixing device of the present invention, the second magnetic core is disposed separately from the first magnetic core at a position closer to the outer peripheral surface of the fixing member than the first magnetic core. ing. When the excitation coil is energized, the permeability of the second magnetic core as well as the first magnetic core is maintained as long as the temperature of the outer peripheral surface of the fixing member is maintained at or below the target fixing temperature. It is in a high state. Therefore, the magnetic flux generated by the exciting coil enters, for example, the heat generating layer of the fixing member from the second magnetic core, passes through the heat generating layer, reaches the end of the first magnetic core, and then the first magnetic core. It passes through a magnetic circuit that goes from the end of the magnetic core to the central portion and returns from the central portion to the second magnetic core. Thereby, the heat generating layer of the fixing member is effectively heated by electromagnetic induction.

  Here, when fixing the toner on the small-size sheet, if the temperature of the small-size sheet non-passing region on the outer peripheral surface of the fixing member excessively rises above the target fixing temperature, the second magnetic body is removed from the fixing member. Due to the radiant heat to the core, the temperature rises relatively quickly with respect to the second magnetic core disposed facing the portion. This is because the second magnetic core is disposed closer to the fixing member than the first magnetic core. In addition, since the second magnetic core is arranged separately from the first magnetic core, the heat capacity is smaller than when the second magnetic core is integrally continuous with the first magnetic core. Because it is. When the temperature of the second magnetic core exceeds the Curie temperature of the core, the induced current on the fixing member decreases due to the decrease in the magnetic permeability of the core, and the heat generation in the small size sheet non-passing region The amount decreases. Therefore, in this fixing device, it is possible to quickly prevent an excessive temperature rise in the small size sheet non-passing region of the fixing member. Note that when the temperature of the outer peripheral surface of the fixing member almost returns to the target fixing temperature, the temperature of the second magnetic core falls below the Curie temperature of the core. The layer is effectively heated.

  The “small size sheet” refers to a sheet having a width smaller than the width of the maximum size sheet planned for the fixing device in the width direction of the sheet passing through the fixing member.

  In the fixing device according to an embodiment, the second magnetic core is disposed in a gap formed by a ring of the exciting coil between the outer peripheral surface of the fixing member and the first magnetic core. Features.

  In the fixing device of this embodiment, the second magnetic core is disposed in a gap formed by a ring of the exciting coil between the outer peripheral surface of the fixing member and the first magnetic core. When the excitation coil is energized, the permeability of the second magnetic core as well as the first magnetic core is maintained as long as the temperature of the outer peripheral surface of the fixing member is maintained at or below the target fixing temperature. It is in a high state. Therefore, the magnetic flux generated by the exciting coil enters, for example, the heat generating layer of the fixing member from the second magnetic core, and is shunted in the opposite directions in the heat generating layer to be respectively end portions of the first magnetic core. And passes through a magnetic circuit that goes from each end of the first magnetic core to the central portion, joins at the central portion, and returns to the second magnetic core. Thereby, the heat generating layer of the fixing member is effectively heated by electromagnetic induction.

  Here, when fixing the toner on the small-size sheet, if the temperature of the small-size sheet non-passing region on the outer peripheral surface of the fixing member excessively rises above the target fixing temperature, the second magnetic body is removed from the fixing member. Due to the radiant heat to the core, the temperature rises relatively quickly with respect to the second magnetic core disposed facing the portion. The reason is that the second magnetic core is disposed between the outer peripheral surface of the fixing member and the first magnetic core, that is, closer to the fixing member than the first magnetic core. Because. In addition, since the second magnetic core is arranged separately from the first magnetic core, the heat capacity is smaller than when the second magnetic core is integrally continuous with the first magnetic core. Because it is. When the temperature of the second magnetic core exceeds the Curie temperature of the core, the induced current on the fixing member decreases due to the decrease in the magnetic permeability of the core, and the heat generation in the small size sheet non-passing region occurs. The amount decreases. Therefore, in this fixing device, it is possible to quickly prevent an excessive temperature rise in the small size sheet non-passing region of the fixing member. When the temperature of the outer peripheral surface of the fixing member almost returns to the target fixing temperature, the temperature of the second magnetic core is lower than the Curie temperature of the core. The layer is effectively heated.

  Moreover, in the fixing device of this embodiment, the second magnetic core is disposed in a gap formed by the ring of the exciting coil between the outer peripheral surface of the fixing member and the first magnetic core. Yes. Therefore, providing the second magnetic core does not increase the size of the apparatus. For example, there is no need to increase the distance between the outer peripheral surface of the fixing member and the first magnetic core.

  On the outer peripheral surface of the fixing member, a release layer, an elastic layer, and the like are usually provided above and below the heat generating layer. These materials, such as a release layer and an elastic layer, may be deteriorated due to excessive temperature rise exceeding the target fixing temperature, although a heat-resistant resin or rubber is selected.

  Therefore, in the fixing device according to one embodiment, the upper limit control temperature for the Curie temperature of the second magnetic core is a temperature higher by 50 ° C. than the target fixing temperature.

  In the fixing device of this embodiment, the temperature of the small-size sheet non-passing region of the fixing member is generally suppressed to a temperature that is higher by 50 ° C. than the target fixing temperature. Therefore, it is possible to prevent the elastic layer and the release layer normally provided on the outer peripheral surface of the fixing member from being deteriorated due to excessive temperature rise.

In the fixing device of one embodiment,
The fixing member and the exciting coil are each arranged in an elongated shape in the width direction of the sheet,
The second magnetic cores are arranged in a plurality of rows in the width direction of the sheet, and a space or a heat insulating layer is provided between the second magnetic cores adjacent to each other.

  In the fixing device according to this embodiment, since the second magnetic cores are arranged in a plurality of rows in the width direction of the sheet, the heat generation layer of the fixing member is uniform in the longitudinal direction of the fixing member. Can be heated. Moreover, a space or a heat insulating layer is provided between the second magnetic cores adjacent to each other. Therefore, even if the temperature of a certain second magnetic core exceeds the Curie temperature of the core, the second magnetic core disposed adjacent to the core is not treated by the space or the heat insulating layer. , Heat conduction hardly occurs and does not adversely affect. As a result, the temperature uniformity in the longitudinal direction of the fixing member can be improved.

  A plurality of the first magnetic cores are arranged side by side in the width direction of the sheet, and each first magnetic core preferably has an elongated shape along the circumferential direction of the fixing member.

  In the fixing device according to an embodiment, the Curie temperature of the second magnetic core disposed in the minimum size sheet non-passing region where the minimum size sheet does not pass in the width direction of the sheet is the minimum size in the width direction of the sheet. It is characterized by being set lower than the Curie temperature of the second magnetic core disposed in the minimum size sheet passage region through which the sheet passes.

  The “minimum size sheet” refers to a sheet having a minimum size width planned for the fixing device in the width direction of the sheet passing through the fixing member.

  In the fixing device according to this embodiment, even when toner is fixed to a minimum size sheet, overheating of a portion of the fixing member where the minimum size sheet does not pass (minimum size sheet non-passing area) can be quickly prevented. can do.

  The fixing device according to an embodiment is characterized in that a nonmagnetic radiant heat absorption layer is provided on a surface corresponding to the fixing member side of the second magnetic core.

  In the fixing device of this embodiment, the nonmagnetic radiant heat absorption layer is provided, so that the radiant heat is rapidly absorbed from the fixing member to the second magnetic core. Therefore, when fixing the toner on the small size sheet, if the temperature of the small size sheet non-passing region on the outer peripheral surface of the fixing member excessively rises to the target fixing temperature or more, the fixing member takes the second magnetic core. Due to the radiant heat to the second magnetic core, the temperature rises more rapidly with respect to the second magnetic core disposed facing the portion. Therefore, it is possible to prevent an excessive temperature rise in the small size sheet non-passing region of the fixing member more quickly.

  An image forming apparatus of the present invention is an image forming apparatus provided with the above-described fixing device.

  In the image forming apparatus of the present invention, the fixing device can quickly prevent an excessive temperature rise in the small-size sheet non-passing area of the fixing member, so that the quality of the formed image can be improved. Further, this image forming apparatus is excellent in durability and safety.

  Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.

  FIG. 1 shows a cross-sectional configuration of a fixing device according to an embodiment for a color laser printer.

  This fixing device generally includes a fixing roller 1, a pressure roller 2, a magnetic flux generator 3, a high-frequency inverter 4 as a high-frequency power supply circuit, a control circuit 5 as a controller, and a changeover switch 7. Yes. 6 is a temperature sensor, 8 is a separation claw, and 90 is a sheet as a sheet.

  The fixing roller 1 and the pressure roller 2 are respectively cylindrical members extending perpendicularly to the paper surface of FIG. 1 and are arranged vertically in parallel with each other in FIG. It is supported by. The pressure roller 2 is urged toward the fixing roller 1 by a pressure mechanism (not shown) using a spring or the like. Thereby, the lower part of the fixing roller 1 and the upper part of the pressure roller 2 are pressed against each other with a predetermined pressure (described later) to form a nip part (fixing nip part). The pressure roller 2 is rotationally driven at a predetermined peripheral speed in a clockwise direction indicated by an arrow in the drawing by a drive mechanism (not shown). The fixing roller 1 is rotated by the rotation of the pressure roller 2 by the frictional force with the pressure roller 2 at the nip portion. Note that the fixing roller 1 may be driven to rotate and the pressure roller 2 may be driven to rotate. That is, the relationship between driving and driven may be reversed.

  As shown in FIG. 2, the fixing roller 1 includes a cored bar 11 as a support layer, a heat insulating layer 12, a heat generating layer 13, and an elastic layer 14 provided in order from the center side toward the outer peripheral surface 1 a side. A five-layer structure with the release layer 15 is provided. The hardness of the fixing roller 1 is, for example, 30 to 90 degrees in terms of ASKER-C hardness.

  The metal core 11 as the support layer is made of aluminum having an outer diameter of 26 mm and a thickness of 4 mm in this example. The material of the metal core 11 may be a heat-resistant molded pipe such as iron or PPS (polyphenylene sulfide), for example, as long as strength can be secured. However, in order to prevent the metal core 11 from generating heat, it is desirable to use a nonmagnetic material that is less affected by electromagnetic induction heating.

  The heat insulating layer 12 is mainly provided to bring the heat generating layer 13 into a heat insulating state. As will be described later, by reducing the heat capacity of the heat generating layer 13 and insulating the heat generating layer 13 with the heat insulating layer 12, the elastic layer 14 and the release layer 15 on the outer peripheral surface 1a side can be rapidly heated. . As a material of the heat insulating layer 12, a sponge material (heat insulating structure) of heat-resistant and elastic rubber material or resin material is used. As a result, the heat insulating layer 12 not only plays a role of heat insulation, but also increases the nip width by allowing the heat generating layer 13 to bend, and reduces the hardness of the fixing roller 1 to improve paper discharge performance and paper separation performance. Fulfill. For example, when the heat insulating layer 12 is made of a silicon sponge material, the thickness is set to 2 mm to 10 mm, preferably 3 mm to 7 mm, and the hardness is set to 20 degrees to 60 degrees, preferably 30 degrees to 50 degrees with an Asker rubber hardness meter. . The heat insulating layer 12 may have a two-layer structure of a rubber material and a sponge body.

  The heat generating layer 13 is provided in order to generate heat by electromagnetic induction due to the magnetic flux from the magnetic flux generating unit 3. In this example, the heat generating layer 13 is composed of an endless nickel electroformed belt layer having a thickness of 40 μm. The thickness of the heat generating layer 13 is desirably 10 μm to 100 μm, and more desirably 20 μm to 50 μm. The reason why the thickness of the heat generating layer 13 is set to 100 μm or less, more desirably 50 μm or less is to reduce the heat capacity of the heat generating layer 13 and increase the rate of temperature increase. As the material of the heat generating layer 13, a material having a relatively high magnetic permeability μ and an appropriate resistivity ρ, such as a magnetic material (magnetic metal) such as magnetic stainless steel, may be used. Furthermore, even if it is a nonmagnetic material, conductive materials, such as a metal, can be used as a material of the heat generating layer 13 by making it a thin film. In addition, the structure of the heat generating layer 13 may be obtained by dispersing particles that generate heat by electromagnetic induction in a resin. With this configuration, the separability can be improved.

  The elastic layer 14 is provided in order to improve adhesion between the sheet and the surface of the fixing roller (important for dealing with a color image) by elasticity in the thickness direction. In this example, the elastic layer 14 is a rubber material or a resin material having heat resistance and elasticity, and specifically, is made of a heat resistant elastomer such as silicon rubber or fluorine rubber that can withstand use at a fixing temperature. . Various fillers may be mixed into the elastic layer 14 for the purpose of thermal conductivity, reinforcement, and the like. Examples of the thermally conductive particles used as the filler include diamond, silver, copper, aluminum, marble, and glass. Practically, silica, alumina, magnesium oxide, boron nitride, and beryllium oxide are included.

  The thickness of the elastic layer 14 is preferably 10 μm to 800 μm, for example, and more preferably 100 μm to 300 μm. If the thickness of the elastic layer 14 is less than 10 μm, it is difficult to obtain the desired elasticity in the thickness direction. On the other hand, when the thickness exceeds 800 μm, the heat generated in the heat generating layer hardly reaches the outer peripheral surface of the fixing film, and the thermal efficiency tends to deteriorate.

  When the elastic layer 14 is made of silicon rubber, the hardness is preferably 1 to 80 degrees, more preferably 5 to 30 degrees as JIS hardness. Within this JIS hardness range, it is possible to prevent poor toner fixability while preventing a decrease in strength of the elastic layer and poor adhesion. Specifically, this silicon rubber is a one-component, two-component or three-component or more silicone rubber, LTV (Low Temperature Vulcanization) type, RTV (Room Temperature Vulcanization) type, or HTV (High Temperature Vulcanization) type. Silicon rubber, condensation type, or addition type silicon rubber can be used. In this example, silicon rubber having a JIS hardness of 10 degrees and a thickness of 200 μm was used as the material of the elastic layer 14.

  The outermost release layer 15 is provided to improve the release property of the outer peripheral surface 1a. The material of the release layer 15 needs to be able to withstand use at a fixing temperature and have a releasability with respect to the toner. For example, silicon rubber, fluorine rubber, PFA (tetrafluoroethylene / perfluoroalkyl vinyl ether) Polymers), fluororesins such as PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene / hexafluoropropylene copolymer), and PFEP (perfluoroethylene / hexafluoropropylene copolymer) are preferably used. The thickness of the release layer 15 is preferably 5 μm to 100 μm, and more preferably 10 μm to 50 μm. Further, in order to improve the interlayer adhesion, an adhesion treatment with a primer or the like may be performed. In addition, a conductive material, an abrasion resistant material, and a good heat conductive material can be added to the release layer 15 as a filler as necessary.

  As shown in FIG. 3, the pressure roller 2 is composed of a core metal 21 made of aluminum having a thickness of 3 mm and a silicon sponge rubber having a thickness of 3 mm to 10 mm, which are provided in order from the center side toward the outer peripheral surface 2a. And a release layer 25 made of a fluorine-based resin having a thickness of 10 μm to 50 μm, such as PTFE or PFA.

  The material of the core metal 21 may be a heat-resistant molded pipe such as iron or PPS (polyphenylene sulfide), for example, as long as the strength can be secured. However, in order to prevent the metal core 21 from generating heat, it is desirable to use a nonmagnetic material that is less affected by electromagnetic induction heating.

  The thickness of the heat insulating layer 22 made of silicon sponge rubber can be appropriately changed in accordance with the use conditions in the range of 3 mm to 10 mm. The heat insulating layer 22 may have a two-layer structure of silicon rubber and silicon sponge.

  The outermost release layer 25 is provided to improve the release property of the outer peripheral surface 2a.

  The pressure roller 2 is pressed against the fixing roller 1 shown in FIG. 1 with a pressure of 300 N to 500 N to form a nip portion. In this case, the nip width is about 5 mm to 15 mm. Depending on circumstances, the nip width may be changed by changing the load.

  The magnetic flux generator 3 includes a coil bobbin 33 having a substantially semi-cylindrical cross section disposed so as to cover the upper portion of the fixing roller 1 in FIG. 1 and an excitation coil disposed in a layered manner along the outer peripheral surface 33b of the coil bobbin 33 31, a demagnetizing coil 34 arranged in a layered manner on the exciting coil 31, and a magnet as a first magnetic core having a substantially trapezoidal cross section arranged so as to cover the exciting coil 31 and the demagnetizing coil 34 A magnetic core as a second magnetic core provided separately from the magnetic core 32 at a position closer to the outer peripheral surface 1a of the fixing roller 1 than the magnetic core 32 35.

  A central portion 33a of the coil bobbin 33 in FIG. 1 is deformed radially outward in a U-shaped cross section. The magnetic core 35 has a rectangular cross section, and is fitted and accommodated inside a U-shaped central portion 33 a of the coil bobbin 33. That is, the magnetic core 35 is disposed in a gap formed by the ring of the exciting coil 31. As a result, the magnetic core 35 is magnetically coupled to the heat generating layer 13 of the fixing roller 1 and is also opposed to the magnetic core 32 so as to be magnetically coupled. According to this arrangement, providing the magnetic core 35 does not increase the size of the apparatus. For example, it is not necessary to increase the distance between the outer peripheral surface 1 a of the fixing roller 1 and the magnetic core 32.

  FIG. 4 shows the coil bobbin 33, the exciting coil 31, the magnetic core 32, and the like as viewed from above in FIG. As shown in FIG. 4, the coil bobbin 33, the excitation coil 31, and the magnetic core 32 are long members having a length dimension that substantially corresponds to the dimension in the longitudinal direction (axial direction) X of the fixing roller 1. The configuration related to the X direction of the magnetic core 35 will be described later.

  The coil bobbin 33 is provided to support the exciting coil 31, the degaussing coil 34, and the magnetic cores 32 and 35. The coil bobbin 33 is preferably made of a nonmagnetic material. In this example, the coil bobbin 33 is made of a heat-resistant resin (for example, polyimide) having a thickness of 1 mm to 3 mm.

  The exciting coil 31 is provided to generate a magnetic flux upon receiving power supply from the high-frequency inverter 4. The exciting coil 31 is formed by winding a wire bundle into an oval shape a plurality of times. The conductor bundle includes an outward path portion 31a and a backward path portion 31b extending along the longitudinal direction X of the fixing roller 1, and curved portions 31c that connect the forward path portion 31a and the backward path portion 31b at both ends 1c and 1d of the fixing roller 1. 31d. One conductive wire bundle is formed by bundling a few hundred dozens of strands (copper wires having a diameter of about 0.18 mm to 0.20 mm, which are insulated and coated with enamel) in order to increase the energization efficiency. It is a known stranded wire having a diameter of about several mm. Thereby, the drive frequency of 10 kHz to 100 kHz and the power of 100 W to 2000 W can be received from the high frequency inverter 4. In this example, in consideration of the case where heat was transferred to the winding, a material coated with a heat resistant resin was used.

  The magnetic core 32 is provided for increasing the efficiency of the magnetic circuit and for magnetic shielding. In this example, the magnetic core 32 includes a pair of edge portions 32P and 32P extending in the longitudinal direction X and a plurality of trapezoidal portions 32D (shown in FIG. 1) integrally formed across the edge portions 32P and 32P. It has a cross section.) In this example, the trapezoidal portions 32 </ b> D are arranged at a dense pitch near both ends with respect to the longitudinal direction X, and are arranged at a coarse pitch inside except for the neighborhood of both ends with respect to the longitudinal direction X. As the material of the magnetic core 32, a magnetic material with high permeability and low loss is used. In the case of an alloy such as permalloy, an eddy current loss in the core increases at a high frequency, so a laminated structure may be used. Further, when a resin material in which magnetic powder is dispersed is used, the magnetic permeability is relatively low, but the shape can be freely set. In this example, the magnetic core 32 is produced by sintering a resin material in which ferrite powder is dispersed. In this example, the Curie temperature of the magnetic core 32 (referred to as Tc1) is set to about 250 ° C. by adjusting the concentration of the ferrite powder in the resin material.

  FIG. 5 schematically shows the disposition of the magnetic core 35, the excitation coil 31 and the demagnetization coil 34 in the longitudinal direction X of the fixing roller 1, particularly the correspondence relationship with the region through which the paper passes. Yes.

  As shown in FIG. 5, the degaussing coil 34 is shorter than the excitation coil 31, but is formed by winding a wire bundle in an elliptical shape a plurality of times in the same manner as the excitation coil 31. In this example, a pair of degaussing coils 34 are disposed so as to overlap with both ends of the exciting coil 31 in the X direction. The demagnetizing coils 34 are configured in the same manner, and include an outward path portion 34a and a backward path portion 34b that extend along the longitudinal direction X of the fixing roller 1, and curved portions 34c and 34d that connect the forward path portion 34a and the backward path portion 34b. Have.

In FIG. 5, with respect to the X direction, an area (maximum size sheet passage area) through which the maximum size sheet passes in the direction of the arrow Z is W _MAX , and an area through which smaller size sheets pass (small size sheet). (Passing area) is indicated by W _I , and a central area (minimum size sheet passing area) through which the minimum size sheet 90s passes is indicated by W _MIN . A region (minimum size sheet non-passing region) W ₀ through which the minimum size sheet 90 s does not pass is formed at both ends of the outer peripheral surface of the fixing roller 1 in the X direction. Degaussing coil 34 described above, the small size corresponding to the sheet non-passing area W _0, so as to have a margin is provided so as to occupy a slightly wider region W _D than that.

The magnetic core 35 is provided to increase the efficiency of the magnetic circuit and to improve the temperature distribution (described later) of the fixing roller 1 in the longitudinal direction. In this example, the magnetic cores 35 are divided into a plurality of portions in the X direction and arranged in one row. In Figure 5, the X-direction, among the plurality of magnetic core 35, magnetic core disposed at the outermost end in the minimum size sheet non-passing within region W ₀ is arranged 35E, on the inner magnetic The body core is represented by 35F. The minimum size magnetic core disposed in the sheet passing area W _MIN is represented by 35G. Spaces 39E, 39F, and 39G are provided between the second magnetic cores adjacent to each other in the X direction.

In this example, each of the magnetic cores 35E, 35F, and 35G is made by sintering a resin material in which ferrite powder is dispersed, as with the magnetic core 32 described above. Curie temperature of the placed magnetic core 35G to the minimum size sheet passage region W _MIN, similar to the Curie temperature Tc1 of the magnetic core 32 is set to about 250 ° C. (which is also referred to as Tc1.). Meanwhile, the minimum size sheet non-passing area W ₀ in the arranged magnetic core 35E, (referred to as Tc2.) Curie temperature of 35F by modulating lower the concentration of ferrite powder in the resin material, It is set to about 200 ° C. As illustrated in FIG. 7, the magnetic permeability μi of these magnetic cores 35E and 35F rapidly decreases when the temperature T exceeds about 200 ° C. (≈Tc2) and does not work as a magnetic body.

In this fixing device, as shown in the sectional view of FIG. 1, when the exciting coil 31 is energized, as long as the surface temperature of the fixing roller is maintained at a predetermined target fixing temperature T _T (≈180 ° C.) or lower, Along with the magnetic core 32 (including the edge portion 32P and the trapezoidal portion 32D), the magnetic core 35 (including 35E, 35F, and 35G) has a high magnetic permeability. Therefore, the magnetic flux generated by the exciting coil 31 enters the heat generating layer 13 of the fixing roller 1 from the magnetic core 35 and is shunted in the opposite directions in the heat generating layer 13 to each edge 32P of the magnetic core 32. Each of the edge portions 32P reaches the corresponding end portion 32b of the trapezoidal portion 32D, and from each of the end portions 32b toward the central portion 32a, joins at the central portion 32a and returns to the magnetic core 35. Pass through. Thereby, an eddy current flows through the heat generating layer 13 of the fixing roller 1 by electromagnetic induction, and the heat generating layer 13 of the fixing roller 1 is effectively heated. In contrast, when the fixing roller surface temperature with a minimum size sheet non-passing region W ₀ exceeds Tc2, the magnetic core 35E, 35F is not longer work as the magnetic body. Therefore, the magnetic circuit passing through the magnetic core 35 described above is substantially cut off, the heat generating layer 13 of the fixing roller 1 is not heated so much, and the surface temperature of the fixing roller is lowered.

Here, for example, in the case of fixing the toner to the minimum size of the paper 90s, when the temperature of the minimum size sheet non-passing area W ₀ of the outer peripheral surface 1a of the fixing roller 1 is excessively increased above the target fixing temperature T _T is , the magnetic core 35E from the fixing roller 1, by radiant heat to 35F, disposed opposite to the region _{W 0} the magnetic core 35E, the 35F, relatively quickly increase in temperature occurs. The reason is that the magnetic cores 35E and 35F are located between the outer peripheral surface 1a of the fixing roller 1 and the magnetic core 32, that is, closer to the fixing roller 1 than the magnetic core 32, like the magnetic core 35G. It is because it is arranged. Moreover, since the magnetic cores 35E and 35F are arranged separately from the magnetic core 32 similarly to the magnetic core 35G, as compared with the case where the magnetic cores 32 are integrally continuous, This is because the heat capacity is small. When the temperature of the magnetic cores 35E and 35F exceeds the Curie temperature Tc2 of the core, the induced current on the fixing roller 1 decreases due to the decrease in the magnetic permeability of the core, and the minimum size sheet non-passing area W _0. The calorific value of is reduced. Therefore, in this fixing device, it is possible to quickly prevent an excessive temperature rise in the minimum size sheet non-passing area W ₀ of the fixing roller 1. Incidentally, the fixing roller surface temperature returns to approximately the target fixing temperature T _T, the magnetic core 35E, the temperature of 35F is below the Curie temperature Tc2 of the core, the original electromagnetic induction again, heat generating layer of the fixing roller 1 is Heated effectively.

The heating and temperature control of the fixing roller 1 are performed by the control circuit 5 shown in FIG. The temperature sensor 6 is a thermistor, for example, and is disposed so as to contact the outer peripheral surface 1 a of the fixing roller 1. A detection signal indicating the fixing roller surface temperature of the temperature sensor 6 is input to the control circuit 5. The control circuit 5 controls the high frequency inverter 4 based on the detection signal of the temperature sensor 6 to increase or decrease the power supply from the high frequency inverter 4 to the exciting coil 31. Thus, it is automatically controlled so that the fixing roller surface temperature reaches a target fusing temperature T _T. Thereby, even if the sheet 90 is deprived of heat, the surface temperature of the fixing roller can be maintained. Target fixing temperature T _T is variably set by the transport speed of the specifications and paper equipment. In this example, it is assumed that the target fixing temperature T _T is about 180 ° C. The temperature sensor 6 is not limited to a contact type such as a thermistor, but may be a non-contact type such as an infrared detection sensor.

During the fixing operation, the pressure roller 2 is rotationally driven, and the fixing roller 1 is also rotated following this. Along with this, the surface temperature of the fixing roller 1 is automatically controlled to the target fixing temperature T _T and the heat generating layer 13 of the fixing roller 1 by electromagnetic induction by the generated magnetic flux of the magnetic flux generating section 3 generates heat. In this state, a sheet 90 as a sheet having an unfixed toner image 91 formed on one side is fed into a nip formed by the fixing roller 1 and the pressure roller 2 by a conveyance mechanism (not shown). In this case, the surface of the sheet 90 on which the unfixed toner image 91 is formed contacts the fixing roller 1. The sheet 90 fed to the nip formed by the fixing roller 1 and the pressure roller 2 is heated by the fixing roller 1 when passing through the nip. As a result, the unfixed toner image 91 is fixed on the paper 90. The sheet 90 passing through the nip portion is separated from the fixing roller 1 and discharged. Should the paper 90 stick to the fixing roller outer peripheral surface 1a after passing through the nip portion, the separation claw 8 arranged in contact with the fixing roller outer peripheral surface 1a causes the paper 90 to be fixed to the fixing roller outer peripheral surface 1a. To prevent jamming.

FIG. 6 shows the temperature distribution in the longitudinal direction (X direction) of the fixing roller 1 in various cases. _{T T} in FIG. 6 shows the aforementioned target fixing temperature (≒ 180 ℃), Tc1 represents the Curie temperature of the magnetic core 32,35G, also, Tc2 represents the magnetic core 35E, the Curie temperature of 35F ing. _TL indicates a fixability lower limit temperature (≈170 ° C.) set from the viewpoint of ensuring good fixability. T _U represents a heat resistant upper limit temperature allowed in view of heat resistance (≒ 250 ℃). In this example, T _U ≈Tc1 is set. Note that Tc1 may be set to 240 ° C. from the viewpoint of durability and safety.

In FIG. 5, when a sheet of the maximum size W _MAX is passed through the nip portion between the fixing roller 1 and the pressure roller 2, the changeover switch 7 is controlled by the control circuit 5 in FIG. The coil 34 is “open”. At this time, the effect of degaussing by the degaussing coil 34 does not occur.

6 indicates a temperature distribution in the longitudinal direction (X direction) of the fixing roller 1 when the paper size is W _MAX and the degaussing coil 34 is “open”. In this temperature distribution 81, the surface temperature of the fixing roller is between _TL and Tc2 over the entire region in the longitudinal direction of the fixing roller 1 (the entire region of the paper size W _MAX ).

On the other hand, when a sheet 90s of the minimum size W _MIN as shown in FIG. 5 is passed, the changeover switch 7 is controlled by the control circuit 5 and the degaussing coil 34 is “closed”. At this time, the degaussing coil 34 generates a magnetic field (reverse magnetic field) in a direction that prevents the change of the magnetic field by the exciting coil 31, and exhibits a demagnetizing effect. As a result, the excessive temperature rise of the fixing roller 1 is reduced in the region W _D where the demagnetizing coil 34 is disposed, that is, in the minimum size sheet non-passing region W ₀ .

A solid line 83 in FIG. 6 shows, as a reference example, the temperature in the longitudinal direction (X direction) of the fixing roller 1 when the magnetic core 35 is not provided and the paper size is W _MIN and the demagnetizing coil 34 is “closed”. Distribution is shown. In the temperature distribution 83, the fixing roller surface temperature in the range of paper sizes W _MIN, the fixing roller surface temperature is between T _L and T _U. Accordingly, the fixing operation is normally performed. However, in the area F just outside the minimum size sheet passing area W _MIN (the area where the minimum size sheet non-passing area W ₀ and the small size sheet passing area W _{I in} FIG. 5 overlap), the surface temperature of the fixing roller is since has reached approximately T _U, from the viewpoint of durability and safety, it can be said that there is no margin. Here, if the magnetic core 35F is provided so as to face the region F as in the present embodiment, even if the temperature distribution is as shown by the solid line 83 as a result of the temperature control, the fixing roller 1 is used. The temperature of the magnetic core 35F exceeds Tc2 due to the radiant heat from. Then, the magnetic circuit passing through the magnetic core 35F is substantially cut off, and the heat generating layer 13 of the fixing roller 1 is not heated so much, and the surface temperature of the fixing roller is lowered in the region F. Accordingly, it is possible to prevent an excessive temperature rise of the fixing roller 1.

6 is a longitudinal direction (X direction) of the fixing roller 1 when the magnetic core 35 is not provided and the paper size is W _I and the demagnetizing coil 34 is “closed” as a reference example. Shows the temperature distribution. The In the temperature distribution 82, in the region F, which corresponds to the end of the small-size sheet passage region W _I, fixing roller surface temperature is below T _L. For this reason, the fixing operation is not normally performed, and the image quality is deteriorated. Further, it can be said just outside the region E of the small-size sheet passage region W _I, since the fixing roller surface temperature has reached approximately T _U, from the viewpoint of durability and safety, and can not afford. Here, as in the present embodiment, the area F, opposite to the magnetic core 35F in E, if provided 35E, can afford against T _U fixing roller surface temperature in the region E is reduced . Accordingly, it is possible to perform control for turning on more power while paying attention to the region F. Therefore, it is possible to prevent an excessive temperature rise of the fixing roller 1 and to improve the uniformity of the temperature distribution in the longitudinal direction of the fixing roller 1.

As for the magnetic core 35, spaces 39E, 39F, 39G,... Are provided between the magnetic cores 35E, 35F, 35G, 39G,. Accordingly, the temperature of, for example, the region E of the fixing roller 1 excessively rises to the target fixing temperature T _T or more, and the temperature of the magnetic core 39E disposed facing the region E exceeds the Curie temperature Tc2 of this core. However, heat conduction is unlikely to occur in the magnetic core 39F disposed adjacent to the core due to the space 39E. Therefore, in this fixing device, the temperature of the magnetic core 39E does not adversely affect 39F arranged adjacent to each other. As a result, the uniformity of the temperature distribution in the longitudinal direction of the fixing roller 1 can be further improved. In addition, a heat insulating layer may be interposed in the spaces 39E, 39F, 39G,.

Moreover, in the above-described example, spaces 39E and 39F between the magnetic cores 35E, 35F, and 35G are provided according to the paper sizes W _I and W _MIN . Therefore, the magnetic cores 35E and 35F facing the areas E and F work independently of each other with respect to the excessive temperature rise in the areas E and F generated according to the paper sizes W _I and W _MIN . Therefore, excessive temperature rise in the sheet non-passing areas E and F can be more appropriately prevented for each paper size W _I and W _MIN .

In the above-described example, three types of W _MAX , W _I , and W _MIN sheets are used. However, the present invention is not limited to this. More types of paper sizes may be used. In that case, it is desirable to divide the magnetic core 35 in the X direction according to the paper size. Note that the minimum sheet size W _MIN may be a postcard size.

In the above-described example, the target fixing temperature T _T is set to about 180 ° C., the Curie temperature Tc 1 of the magnetic cores 32 and 35 G is set to about 250 ° C., and the Curie temperature Tc 2 of the magnetic cores 35 E and 35 F is set to about 200 ° C. Each setting error is assumed to be ± 10 ° C.) However, the present invention is not limited to this. The target fixing temperature T _T is about 180 ° C. for the high-speed processing specification and about 160 ° C. for the low-speed processing specification, depending on the specification (or operation mode) of the fixing device and the image forming apparatus provided with the fixing device. It can be set variably.

In this case, the Curie temperature Tc2 may be set within the temperature range between the target fixing temperature T _T and the upper limit control temperature that is 50 ° C. higher than the target fixing temperature T _T. In the above example, since the target fixing temperature T _T was about 180 ° C., the Curie temperature Tc2 may be set within a range of up to 230 ° C. from 180 ° C.. Thereby, it is possible to prevent the elastic layers 12 and 14 and the release layer 15 provided on the outer peripheral surface 1a of the fixing roller 1 from being deteriorated due to excessive temperature rise.

  Further, from the viewpoint of achieving the effects of the invention, the Curie temperature Tc1 only needs to exceed the upper limit control temperature 230 ° C. Actually, it is desirable to distinguish with a margin of at least about 10 ° C. with respect to the Curie temperature Tc2. Therefore, when the upper limit control temperature of the Curie temperature Tc2 is 230 ° C., the Curie temperature Tc1 is preferably about 240 ° C. or higher.

Although not shown, a nonmagnetic radiant heat absorption layer may be provided on the surface corresponding to the fixing roller 1 side of the magnetic cores 35E, 35F, and 35G. As this width radiation heat absorption layer, for example, using a coating agent in which carbon particles having an average particle diameter of 0.1 μm or less having radiation heat absorption are dispersed in FEP fluororesin having heat resistance and releasability, A black coating layer having a thickness of about 20 μm can be used. When such a radiant heat absorption layer is provided, the radiant heat is quickly absorbed from the fixing roller 1 to the magnetic cores 35E, 35F, and 35G. Therefore, for example, when fixing toner on a small-size sheet, if the temperature of the small-size sheet non-passing area E on the outer peripheral surface 1a of the fixing roller 1 rises excessively to the target fixing temperature _TT or more, the fixing roller Due to the radiant heat from 1 to the magnetic core 35E, the temperature of the magnetic core 35E rises more rapidly. Therefore, it is possible to prevent an excessive temperature rise in the small size sheet non-passing region of the fixing member more quickly.

  In this embodiment, the fixing roller is provided as the fixing member. However, the present invention is not limited to this, and a fixing belt may be provided as the fixing member. The material and configuration of the fixing member are not limited to those in the present embodiment, and may be appropriately changed within a range where the present invention can be applied.

  In this embodiment, the degaussing coil 34 is provided, but the present invention is not limited to this. The demagnetizing coil 34 may be omitted, and the excessive temperature rise in the small size sheet non-passing region may be prevented exclusively by the second magnetic core.

  As the sheet, not only paper (paper) but also a resin sheet for OHP (overhead projector) can be used.

  FIG. 8 illustrates a configuration of an image forming apparatus 1501 including the fixing device 1520 according to an embodiment.

  First, a schematic configuration of the image forming apparatus 1501 will be described. The image forming apparatus 1501 includes an intermediate transfer belt 1502 as a belt member at a substantially central portion inside the image forming apparatus 1501. The intermediate transfer belt 1502 is supported on the outer periphery of the rollers 1504 and 1505 and is driven to rotate in the direction of arrow A. The intermediate transfer belt drive roller 1505 is connected to a drive motor (not shown), and the roller 1504 is driven to rotate as the intermediate transfer belt drive roller 1505 rotates.

  Below the lower horizontal portion of the intermediate transfer belt 1502, four image forming units 1506Y, 1506M, 1506C, and 1506K respectively corresponding to yellow (Y), magenta (M), cyan (C), and black (K) colors. Are arranged along the intermediate transfer belt 1502.

  Each of the image forming units 1506Y, 1506M, 1506C, and 1506K has a photosensitive drum 1507Y, 1507M, 1507C, and 1507K, respectively. Around each of the photosensitive drums 1507Y, 1507M, 1507C, and 1507K, the photosensitive members 1508, the print head unit 1509, the developing device 1510, and the intermediate transfer belt 1502 are sandwiched in order along the rotation direction. Primary transfer rollers 1511Y, 1511M, 1511C, and 1511K facing the drums 1507Y, 1507M, 1507C, and 1507K, and a cleaner 1512 are arranged, respectively.

  A secondary transfer roller 1503 is pressed against a portion of the intermediate transfer belt 1502 supported by the intermediate transfer belt driving roller 1505, and a nip portion between the secondary transfer roller 1503 and the intermediate transfer belt 1502 is a secondary transfer region. 1530.

  A fixing device 1520 including a fixing roller 1521, a pressure roller 1522, and a magnetic flux generator 1523 is disposed at a downstream position of the conveyance path 1541 behind the secondary transfer region 1530. A pressure contact portion between the fixing roller 1521 and the pressure roller 1522 is a fixing nip portion 1531. Although not shown in FIG. 8, the fixing device 1520 includes a demagnetizing coil 34 between the fixing roller 1521 and the magnetic flux generator 1523, as in the fixing device shown in FIG. 5. A high frequency inverter 4 and a changeover switch 7 are provided. The fixing device 1520 opens and closes the demagnetizing coil 34 by turning on and off the change-over switch 7 in accordance with a signal representing a sheet size received from a control unit (not shown) of the image forming apparatus.

  A paper feed cassette 1517 is detachably disposed at the bottom of the printer 1. Sheets P (including the aforementioned maximum-size sheet 90 and small-size sheet 90 s) stacked and housed in the sheet feed cassette 1517 are conveyed one by one from the uppermost sheet by the rotation of the sheet feed roller 1518. 1540.

  An AIDC (image density) sensor 1519 also serving as a registration sensor is installed between the image forming unit 1506K on the most downstream side of the intermediate transfer belt 1502 and the secondary transfer region 1530. This registration sensor 1519 measures the interval between the patterns of the respective colors formed on the intermediate transfer belt 1502 and compares the interval with a predetermined reference value to adjust the start timing of writing the image of each color. belongs to.

  Next, a schematic operation of the image forming apparatus 1501 having the above configuration will be described. When an image signal is input from an external device (for example, a personal computer) to an image signal processing unit (not shown) of the image forming apparatus 1501, the image signal processing unit converts the image signal into yellow, cyan, magenta, and black. The digital image signal is generated, and the print head unit 1509 of each of the image forming units 1506Y, 1506M, 1506C, and 1506K is caused to emit light based on the input digital signal. Thereby, electrostatic latent images for the respective colors are formed on the surfaces of the photosensitive drums 1507Y, 1507M, 1507C, and 1507K, respectively.

  The electrostatic latent images formed on the photosensitive drums 1507Y, 1507M, 1507C, and 1507K are respectively developed by the developing units 1510 to become toner images of the respective colors. Then, the toner images of the respective colors are primary-transferred by being sequentially superimposed on the intermediate transfer belt 1502 moving in the arrow A direction by the action of the primary transfer rollers 1511Y, 1511M, 1511C, and 1511K.

  The superimposed toner image formed on the intermediate transfer belt 1502 in this way reaches the secondary transfer region 1530 as the intermediate transfer belt 1502 moves. In the secondary transfer region 1530, the superimposed color toner images are secondarily transferred onto the paper P collectively by the action of the secondary transfer roller 1503.

  Next, the toner image secondarily transferred to the paper P reaches the fixing nip 1531. In the fixing nip portion 1531, the toner image is fixed on the paper P by the action of the fixing roller 1521 and the pressure roller 1522 that generate heat by the magnetic flux generation portion 1523.

  At the time of this fixing, like the fixing device shown in FIG. 1, the fixing device 1520 can more quickly prevent an excessive temperature rise in the small size sheet non-passing area of the fixing roller 1521. Therefore, this image forming apparatus can improve the quality of the formed image, and is excellent in durability and safety.

  The paper P on which the toner image is fixed is discharged to a paper discharge tray 1513 via a paper discharge roller 1514.

  The image forming apparatus may be any one of a monochrome / color copying machine, a printer, a FAX, and a complex machine of these.

1 is a diagram illustrating a schematic configuration of a fixing device according to an embodiment of the present invention. It is a figure which shows the cross-sectional structure of the fixing roller of the said fixing apparatus. It is a figure which shows the cross-sectional structure of the pressure roller of the said fixing device. FIG. 2 is a diagram showing the fixing device of FIG. 1 as viewed from above. It is a figure which shows typically the correspondence with the arrangement | positioning of a magnetic body core, an exciting coil, and a demagnetizing coil about the longitudinal direction of a fixing roller, especially the area | region through which a paper passes. It is a figure which illustrates temperature distribution of the longitudinal direction of a fixing roller. It is a figure which illustrates the relationship between the temperature of a magnetic body core, and a magnetic permeability. 1 is a diagram illustrating a configuration of an image forming apparatus including a fixing device according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1521 Fixing roller 13 Electromagnetic induction heat generating layer 2, 1522 Pressure roller 3, 1523 Magnetic flux generation part 5 Control circuit 6 Temperature sensor 7 Changeover switch 31 Excitation coil 32, 35, 35E, 35F, 35G Magnetic body core 34 Demagnetizing coil 39E , 39F, 39G space

Claims (7)

A fixing member in which the conveyed sheet is pressed against the outer peripheral surface;
An exciting coil for inductively heating the heat generating layer of the fixing member, disposed opposite to the outer peripheral surface of the fixing member;
A first magnetic core disposed so as to cover the exciting coil at a position corresponding to the opposite side of the fixing member with respect to the exciting coil;
A second magnetic core disposed separately from the first magnetic core at a position closer to the outer peripheral surface of the fixing member than the first magnetic core;
The Curie temperature of the second magnetic core is not less than the target fixing temperature and not more than a predetermined upper limit control temperature,
The fixing device according to claim 1, wherein the Curie temperature of the first magnetic core exceeds the upper limit control temperature. The fixing device according to claim 1,
The fixing device, wherein the second magnetic core is disposed in a gap formed by a ring of the exciting coil between an outer peripheral surface of the fixing member and the first magnetic core. The fixing device according to claim 1 or 2,
The fixing device, wherein the upper limit control temperature for the Curie temperature of the second magnetic core is higher by 50 ° C. than the target fixing temperature. In the fixing device according to any one of claims 1 to 3,
The fixing member and the exciting coil are each arranged in an elongated shape in the width direction of the sheet,
The fixing is characterized in that the second magnetic cores are arranged in a plurality of rows in the width direction of the sheet, and a space or a heat insulating layer is provided between the second magnetic cores adjacent to each other. apparatus. The fixing device according to claim 4.
The Curie temperature of the second magnetic core disposed in the minimum size sheet non-passing area where the minimum size sheet does not pass in the width direction of the sheet is the minimum size sheet passing area through which the minimum size sheet passes in the width direction of the sheet A fixing device, wherein the fixing device is set lower than a Curie temperature of the second magnetic core disposed on the surface. The fixing device according to any one of claims 1 to 5,
A fixing device, wherein a nonmagnetic radiant heat absorption layer is provided on a surface corresponding to the fixing member side of the second magnetic core.   An image forming apparatus comprising the fixing device according to claim 1.

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