Fixing device and image forming apparatus

Abstract

To continue appropriate temperature detection even if a temperature detection member deviates from a target installation position.SOLUTION: A fixing device 9 comprises: a hollow fixing member 20 that heats and fixes an image to a recording material P; a heating member 22 that heats the fixing member from an inner peripheral surface side of the fixing member; a temperature detection member 65 that faces a surface of the heating member on the opposite side of the inner peripheral surface side of the fixing member and detects temperature; and an urging member 65a that urges the temperature detection member to bring the temperature detection member into contact with the surface. The surface is formed of a curved surface having the center of curvature on the side of the temperature detection member when seen from a width direction of the recording medium. In a recording material conveyance direction, both end parts of the temperature detection member are in contact with the curved surface, and a central part of the temperature detection member is separated from the curved surface.SELECTED DRAWING: Figure 9

Description

[Technical field]

[0001] The present invention relates to a fixing device and an image forming apparatus. [Background technology]

[0002] Conventionally, a fixing device has been known that includes a fixing member, a heating member that heats the fixing member from the inner surface side of the fixing member, a temperature detection member that detects temperature by facing the surface of the heating member opposite the inner surface side of the fixing member, and a biasing member that biases the temperature detection member so that it abuts against the surface.

[0003] For example, Patent Document 1 discloses a fixing device in which a heater holder holds a planar heater (heating member) that heats an endless fixing belt (fixing member) on the inner periphery of the fixing belt, and the heater holder is supported by stays in the longitudinal direction (recording material width direction). This fixing device forms a fixing nip between the fixing belt and the pressure roller by biasing the heater holder toward the fixing nip side via the stay. The heater holder holds a heater on the fixing nip side, and holds a thermistor (temperature detection member) on the opposite side to the fixing nip. This temperature detection member detects the temperature by contacting the surface of the heater via an insulating sheet. Summary of the Invention [Problem to be solved by the invention]

[0004] In a conventional fixing device, there are cases where the temperature detection member deviates from the target installation position, making it difficult to detect the temperature appropriately. [Means for solving the problem]

[0005] In order to solve the above-mentioned problems, the present invention provides a fixing device including a hollow fixing member that heats and fixes an image on a recording material, a heating member that heats the fixing member from an inner peripheral surface side of the fixing member, a temperature detection member that faces a surface of the heating member opposite to the inner peripheral surface side of the fixing member and detects a temperature, and a biasing member that biases the temperature detection member so that the temperature detection member abuts against the surface, the surface being configured as a curved surface having a center of curvature on the temperature detection member side when viewed from a recording material width direction perpendicular to a recording material transport direction, and both end portions of the temperature detection member abut against the curved surface and a central portion of the temperature detection member is separated from the curved surface, a point on the curved surface where a direction of the urging force applied to the temperature detecting member by the urging member substantially coincides with a direction of a radius of curvature substantially coincides with a center position of the heating member in the recording material conveying direction. It is characterized by: Effect of the Invention

[0006] According to the present invention, even if the temperature detection member deviates from the target installation position, the temperature detection member can be returned to the target installation position, and appropriate temperature detection can be continued. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment. [Diagram 2] FIG. 2 is an explanatory diagram of the internal structure of the fixing device in the image forming apparatus as viewed from the width direction of the paper (width direction of the recording material). [Diagram 3] FIG. [Figure 4] FIG. [Diagram 5] FIG. [Figure 6] FIG. [Figure 7] FIG. [Figure 8] FIG. [Figure 9] FIG. 4 is an explanatory diagram of the internal structure (thermistor, etc.) of the fixing device as viewed from the belt width direction (recording material width direction). [Figure 10]FIG. 4 is an explanatory diagram of the internal structure (thermostat) of the fixing device as viewed from the belt width direction (recording material width direction). [Figure 11] FIG. 4 is a diagram illustrating a temperature distribution in the short side direction of the heater. [Figure 12] FIG. 13 is an explanatory diagram illustrating the force acting on both ends of the thermistor in the short side direction that contacts the rear surface (curved surface) of the heater due to the biasing force of a compression spring when the thermistor is located at a target installation position. [Figure 13] FIG. 13 is an explanatory diagram illustrating the force acting on both ends of the thermistor in the short direction that contacts the rear surface (curved surface) of the heater due to the biasing force of a compression spring when the thermistor is deviated from the target installation position. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] An embodiment in which the present invention is applied to a fixing device of an image forming apparatus will be described below. The image forming apparatus may be a copier, a facsimile, or a combination machine of these in addition to a printer.

[0009] FIG. 1 is a schematic diagram of an image forming apparatus according to the present embodiment. The image forming apparatus 100 of this embodiment includes four imaging units 1Y, 1M, 1C, and 1Bk that are image forming sections. Each imaging unit 1Y, 1M, 1C, and 1Bk is configured to be detachable from the image forming apparatus main body 103, and has the same configuration except that it contains different color developers of yellow, magenta, cyan, and black that correspond to the color separation components of a color image. Specifically, each imaging unit 1Y, 1M, 1C, and 1Bk includes a drum-shaped photoconductor 2 as a latent image carrier. Each imaging unit also includes a charging device 3 that charges the surface of the photoconductor 2, a developing device 4 that supplies toner as a developer to the surface of the photoconductor 2 to form a toner image, and a cleaning device 5 that cleans the surface of the photoconductor 2.

[0010] Moreover, the image forming apparatus 100 includes an exposure device 6 that exposes the surface of each photoconductor 2 to light to form an electrostatic latent image, a paper feeder 7 that supplies paper P as a recording material, and a transfer device 8 that transfers the toner image formed on each photoconductor 2 to the paper P. The image forming apparatus 100 also includes a fixing device 9 that fixes the toner image transferred to the paper P, and a paper discharge device 10 that discharges the paper P to the outside of the apparatus.

[0011] The transfer device 8 has an endless intermediate transfer belt 11 as an intermediate transfer body stretched by a plurality of rollers. The transfer device 8 also has four primary transfer rollers 12 as primary transfer members that transfer the toner images on the photoconductors 2 to the intermediate transfer belt 11, and a secondary transfer roller 13 as a secondary transfer member that transfers the toner images transferred onto the intermediate transfer belt 11 to a sheet of paper P. Each of the primary transfer rollers 12 contacts the photoconductor 2 via the intermediate transfer belt 11. As a result, the intermediate transfer belt 11 and each photoconductor 2 contact each other, and a primary transfer nip is formed between them. Meanwhile, the secondary transfer roller 13 contacts one of the rollers that stretch the intermediate transfer belt 11 via the intermediate transfer belt 11. As a result, a secondary transfer nip is formed between the secondary transfer roller 13 and the intermediate transfer belt 11.

[0012] Further, a paper transport path 14 along which the paper P sent out from the paper feeder 7 is transported is formed within the image forming apparatus 100. A pair of timing rollers 15 is provided on the paper transport path 14 midway from the paper feeder 7 to the secondary transfer nip (secondary transfer roller 13).

[0013] Next, the printing operation of the image forming apparatus of this embodiment will be described. When an instruction to start a printing operation is given, in each of the imaging units 1Y, 1M, 1C, and 1Bk, the photoconductor 2 is rotated clockwise in FIG. 1, and the surface of the photoconductor 2 is charged to a uniform high potential by the charging device 3. Next, the exposure device 6 exposes the surface of each photoconductor 2 based on the image information of the document read by the document reading device or the print information instructed to be printed from the terminal, thereby lowering the potential of the exposed portion and forming an electrostatic latent image. Then, toner is supplied from the developing device 4 to this electrostatic latent image, and a toner image is formed on each photoconductor 2.

[0014] When the toner images formed on each photoconductor 2 reach the primary transfer nip (the position of the primary transfer roller 12) as the photoconductor 2 rotates, they are transferred onto the intermediate transfer belt 11, which rotates counterclockwise in FIG. 1, so as to overlap each other. The toner images transferred onto the intermediate transfer belt 11 are then transported to the secondary transfer nip (the position of the secondary transfer roller 13) as the intermediate transfer belt 11 rotates, and are transferred to the paper P that has been transported at the secondary transfer nip. This paper P is supplied from the paper supply device 7. The paper P supplied from the paper supply device 7 is temporarily stopped by the timing roller 15, and then transported to the secondary transfer nip in accordance with the timing at which the toner image on the intermediate transfer belt 11 reaches the secondary transfer nip. Thus, a full-color toner image is carried on the paper P. After the toner image is transferred, the toner remaining on each photoconductor 2 is removed by each cleaning device 5.

[0015] The paper P onto which the toner image has been transferred is transported to a fixing device 9, which fixes the toner image onto the paper P. Thereafter, the paper P is discharged outside the apparatus by a paper discharge device 10, completing a series of printing operations.

[0016] Next, the configuration of the fixing device 9 of this embodiment will be described. FIG. 2 is an explanatory diagram of the internal structure of the fixing device 9 in this embodiment, as viewed from the width direction of the paper P (the recording material width direction). The fixing device 9 of this embodiment includes a fixing belt 20 made of an endless belt member as a fixing member, a pressure roller 21 as an opposing member that contacts the outer circumferential surface of the fixing belt 20 to form a nip portion N that is a fixing nip, and a heating device 19 that heats the fixing belt 20. The heating device 19 includes a planar heater 22 as a heating member that heats the fixing belt from the inner circumferential surface side of the fixing belt 20, and a heater holder 23 as a holding member that holds the heater 22. The heating device 19 also includes a stay 24 as a support member that supports the heater holder 23 in the longitudinal direction (the recording material width direction, the direction perpendicular to the paper surface of FIG. 2).

[0017] The fixing belt 20 has a hollow base made of polyimide (PI) having an outer diameter of 25 mm and a thickness of 40 to 120 μm. A release layer made of fluorine-based resin such as PFA or PTFE and having a thickness of 5 to 50 μm is formed on the outermost surface of the fixing belt 20 in order to increase durability and ensure releasability. An elastic layer made of rubber or the like having a thickness of 50 to 500 μm may be provided between the base and the release layer. The base of the fixing belt 20 is not limited to polyimide, and may be a heat-resistant resin such as PEEK or a metal base such as nickel (Ni) or SUS. The inner peripheral surface of the fixing belt 20 may be coated with polyimide, PTFE, or the like as a sliding layer.

[0018] The pressure roller 21 has an outer diameter of, for example, 25 mm, and is composed of a solid iron core 21a, an elastic layer 21b formed on the surface of the core 21a, and a release layer 21c formed on the outside of the elastic layer 21b. The elastic layer 21b is made of silicone rubber and has a thickness of, for example, 3.5 mm. In order to improve the release property of the surface of the elastic layer 21b, it is desirable to form the release layer 21c made of a fluororesin layer having a thickness of, for example, about 40 μm.

[0019] The heater 22 has an elongated shape across the width direction of the fixing belt 20 (the recording material width direction) and is disposed so as to be in contact with the inner peripheral surface of the fixing belt 20. The heater 22 may be in non-contact with the fixing belt 20 or indirectly in contact with the fixing belt 20 via a low friction sheet or the like, but the direct contact of the heater 22 with the fixing belt 20 improves the efficiency of heat transfer to the fixing belt 20. The heater 22 can also be in contact with the outer peripheral surface of the fixing belt 20, but since there is a risk that the fixing quality will decrease if the outer peripheral surface of the fixing belt 20 is damaged by contact with the heater 22, it is preferable that the heater 22 be in contact with the inner peripheral surface of the fixing belt 20.

[0020] The heater holder 23 and the stay 24 are disposed on the inner circumferential side of the fixing belt 20. The stay 24 is made of a metal channel material, and both ends thereof are supported by both side walls of the fixing device 9. Since the surface of the heater holder 23 opposite to the heater 22 side is supported by the stay 24, the heater 22 and the heater holder 23 are maintained without being significantly deflected by the pressure force of the pressure roller 21, and a nip portion N is formed between the fixing belt 20 and the pressure roller 21.

[0021] The heater holder 23 is desirably made of a heat-resistant material because it is likely to become hot due to the heat of the heater 22. For example, when the heater holder 23 is made of a heat-resistant resin with low thermal conductivity such as LCP or PEEK, the heat transfer from the heater 22 to the heater holder 23 is suppressed, and the fixing belt 20 can be heated efficiently.

[0022] The pressure roller 21 and the fixing belt 20 are pressed against each other by a spring as a pressure means. As a result, a nip N is formed between the fixing belt 20 and the pressure roller 21. The pressure roller 21 also functions as a drive roller that is rotated by a driving force transmitted from a driving means provided in the image forming apparatus main body 103. Meanwhile, the fixing belt 20 is configured to rotate in accordance with the rotation of the pressure roller 21. During rotation, the fixing belt 20 slides against the heater 22. In order to improve the slidability of the fixing belt 20, a lubricant such as oil or grease may be interposed between the heater 22 and the fixing belt 20.

[0023] When the printing operation is started, the pressure roller 21 is rotated and the fixing belt 20 starts to rotate. Furthermore, the fixing belt 20 is heated by supplying power to the heater 22. Then, when the temperature of the fixing belt 20 reaches a predetermined target temperature (fixing temperature), the paper P carrying an unfixed toner image is transported between the fixing belt 20 and the pressure roller 21 (nip portion N) as shown in FIG. 2. As a result, the unfixed toner image is heated and pressurized to be fixed to the paper P.

[0024] FIG. 3 is a perspective view of the fixing device, and FIG. 4 is an exploded perspective view thereof. 3 and 4, the device frame 40 of the fixing device 9 includes a first device frame 25 consisting of a pair of side walls 28 and a front wall 27, and a second device frame 26 consisting of a rear wall 29. The pair of side walls 28 are disposed at one end side and the other end side of the width direction of the fixing belt 20 (hereinafter referred to as the "belt width direction"), and both end sides of the pressure roller 21 and the heating device 19 are supported by the side walls 28. A plurality of engagement protrusions 28a are provided on each side wall 28, and the engagement protrusions 28a are engaged with engagement holes 29a provided in the rear wall 29, whereby the first device frame 25 and the second device frame 26 are assembled together.

[0025] Each side wall portion 28 is provided with an insertion groove 28b for inserting the rotating shaft of the pressure roller 21 and the like. The insertion groove 28b opens on the rear wall portion 29 side and is a butting portion that does not open on the opposite side. A bearing 30 that supports the rotating shaft of the pressure roller 21 is provided at the end on the butting portion side. The pressure roller 21 is rotatably supported by both side wall portions 28 by mounting both ends of the rotating shaft to the bearings 30.

[0026] Further, a drive transmission gear 31 serving as a drive transmission member is provided on one end side of the rotation shaft of the pressure roller 21. The drive transmission gear 31 is disposed in a state where it is exposed outward from the side wall portions 28 with the pressure roller 21 supported by the both side wall portions 28. As a result, when the fixing device 9 is mounted on the image forming apparatus main body 103, the drive transmission gear 31 is connected to a gear provided on the image forming apparatus main body 103, and is in a state where it is possible to transmit a drive force from a drive source.

[0027] A pair of support members 32 for supporting the fixing belt 20 and the like are provided at both ends of the heating device 19 in the longitudinal direction. The support members 32 are an apparatus frame of the heating device 19, and are also part of an apparatus frame 40 of the fixing device 9. The fixing belt 20 is supported by the support members 32 in a so-called free belt manner, in which basically no tension is applied in the circumferential direction when the fixing belt 20 is not rotating. Each support member 32 is provided with a guide groove 32a, and is assembled to the side wall portion 28 by inserting the guide groove 32a along the edge of the insertion groove 28b of the side wall portion 28.

[0028] Further, a pair of springs 33 serving as urging members are provided between each of the support members 32 and the rear wall portion 29. The support members 32 are urged toward the pressure roller 21 by each of the springs 33, so that the fixing belt 20 is pressed against the pressure roller 21, and a nip portion N is formed between the fixing belt 20 and the pressure roller 21.

[0029] FIG. 5 is a perspective view of the heating device 19, and FIG. 6 is an exploded perspective view thereof. 5 and 6, a rectangular accommodating recess 23a for accommodating the heater 22 is provided on the surface of the heater holder 23 on the fixing belt 20 side (nip portion N side). The heater 22 is held in the accommodating recess 23a by being sandwiched together with the heater holder 23 by a connector (described later).

[0030] The pair of support members 32 includes a C-shaped belt support portion 32b that is inserted into the inner periphery of the fixing belt 20 to support the fixing belt 20, and a flange-shaped belt regulation portion 32c that contacts an end face of the fixing belt 20 to regulate movement (deviation) in the belt width direction. The pair of support members 32 also have support recesses 32d into which both end sides of the heater holder 23 and the stay 24 are inserted to support them.

[0031] Fig. 7 is a plan view of heater 22, and Fig. 8 is an exploded perspective view thereof. In the following description, the fixing belt 20 side (nip portion N side) of heater 22 is referred to as the "front side," and the heater holder 23 side is referred to as the "back side." As shown in Figs. 7 and 8, the heater 22 is configured by stacking a plurality of constituent layers. Specifically, the heater 22 includes a plate-shaped base layer 50, a first insulating layer 51 provided on the front side of the base layer 50, a conductor layer 52 provided on the front side of the first insulating layer 51, and a second insulating layer 53 covering the front side of the conductor layer 52. The heater 22 also includes a third insulating layer 54 provided on the back side of the base layer 50. The conductor layer 52 includes a pair of heating parts 60 formed of planar resistance heating elements, a pair of electrode parts 61 provided on one end side of each heating part 60 in the longitudinal direction, and a plurality of power supply lines 62 connecting between the electrode parts 61 and the heating parts 60 and between the heating parts 60. Also, as shown in Fig. 7, at least a portion of each electrode part 61 is exposed without being covered by the second insulating layer 53 in order to ensure connection with a connector described later.

[0032] Each heating portion 60 can be formed, for example, by applying a paste of silver palladium (AgPd) and glass powder to the base layer 50 by screen printing or the like, and then firing the base layer 50. Other materials for the heating portion 60 include silver alloy (AgPt) and ruthenium oxide (RuO 2 ) may be used. In this embodiment, the heat generating parts 60 are provided so as to extend in parallel with each other in the longitudinal direction of the base layer 50. One end (right end in FIG. 7) of each heat generating part 60 is electrically connected to each other via a power supply line 62, and the other end (left end in FIG. 7) of each heat generating part 60 is electrically connected to the electrode part 61 via a separate power supply line 62. The power supply line 62 is made of a conductor having a smaller resistance value than the heat generating part 60. The power supply line 62 and the electrode part 61 may be made of silver (Ag) or silver palladium (AgPd), and the power supply line 62 and the electrode part 61 are formed by screen printing such a material.

[0033] The base layer 50 is made of a metal material such as stainless steel (SUS), iron, or aluminum. In addition to metal materials, ceramics, glass, and the like can also be used as the material of the base layer 50. When an insulating material such as ceramics is used for the base layer 50, the first insulating layer 51 between the base layer 50 and the conductor layer 52 can be omitted. On the other hand, metal materials are excellent in durability against rapid heating and are easy to process, so they are suitable for reducing costs. Among metal materials, aluminum and copper are particularly preferable in that they have high thermal conductivity and are less likely to cause temperature unevenness. Stainless steel also has the advantage of being cheaper to manufacture than these materials.

[0034] Each of the insulating layers 51, 53, and 54 is made of heat-resistant glass, and may be made of ceramic, polyimide (PI), or the like.

[0035] Also, a highly thermally conductive layer having a higher thermal conductivity than the base layer 50 may be provided on the rear surface of the base layer 50. In this case, the heat of the heater 22 is dispersed through the highly thermally conductive layer, thereby making it possible to suppress temperature unevenness of the heater 22. Also, in order to effectively suppress temperature unevenness, it is desirable that the highly thermally conductive layer be disposed over the entire area (longitudinal and lateral directions) in which the heat generating portion 60 is provided.

[0036] In this embodiment, the heating section 60, the electrode section 61, and the power supply line 62 are made of an alloy such as silver or palladium, and have a PTC characteristic. The PTC characteristic is a characteristic in which the resistance value increases as the temperature increases (when a constant voltage is applied, the heater output decreases). By making the heating section 60 have the PTC characteristic, it is possible to quickly start up at low temperatures with high output and to suppress excessive temperature rise at high temperatures with low output. For example, if the TCR coefficient of the PTC characteristic is about 300 to 4000 ppm / degree, it is possible to reduce costs while ensuring the resistance value required for the heater. More preferably, the TCR coefficient is 500 to 2000 ppm / degree. The TCR coefficient can be calculated by measuring the resistance value at 25 degrees and 125 degrees. For example, if the temperature rises by 100 degrees and the resistance value increases by 10%, the TCR coefficient is 1000 ppm / degree.

[0037] In addition, in this embodiment, the length (length in the longitudinal direction) of the heat generating section 60 is longer than the paper width. This can prevent poor fixing due to a drop in temperature near the end of the paper width direction immediately after start-up. On the other hand, if the length of the heat generating section 60 is too long, there is a risk of excessive temperature rise in non-paper passing areas during continuous paper passing, so the length of the heat generating section 60 needs to be set appropriately. Specifically, in this embodiment, it is desirable to set the heat generating section 60 to a range of 0.5 mm to 7 mm larger on one side in the width direction (heat generating length 217 mm to 230 mm) than the maximum paper size (maximum recording material passing width) of 216 mm, which is the letter size that can be passed. More desirably, the heat generating section 60 is set to a range of 1 mm to 5 mm larger on one side in the width direction than the maximum paper size (heat generating length 219 mm to 226 mm). In this embodiment, the length of the heat generating section 60 is 221 mm.

[0038] Next, the control of the fixing device 9 in this embodiment will be described. FIG. 9 is an explanatory diagram of the internal structure of the fixing device 9 in this embodiment, as viewed from the belt width direction (recording material width direction). However, FIG. 9 shows a location where a thermistor 65 serving as a temperature detection member is disposed in the belt width direction of the fixing device 9.

[0039] 9, in the fixing device 9 of the present embodiment, a thermistor 65 is disposed as a temperature detection member for detecting the temperature, facing the surface (back surface) of the heater 22 opposite to the inner circumferential surface side of the fixing belt 20. Specifically, the thermistor 65 is attached to a base 65b via a compression spring 65a as a biasing member, and is supported by the stay 24 by fitting the base 65b inside the stay 24. Then, by combining the heater holder 23 holding the heater 22 with the stay 24, the thermistor 65 comes into contact with the back surface of the heater 22 by the biasing force of the compression spring 65a through an opening 23b provided in the heater holder 23. The detected temperature of the thermistor 65 is sent to a temperature control unit 64.

[0040] The temperature control unit 64 controls the heater power supply 63 that supplies power to the heater 22 in accordance with the temperature detected by the thermistor 65, and performs fixing temperature control so that the temperature of the fixing belt 20 becomes a target temperature. The temperature control unit 64 can be configured by a microcomputer including a CPU, ROM, RAM, an I / O interface, etc. The temperature control unit 64 may control the temperature based only on the temperature detected by the thermistor 65, but may also take other parameters into consideration.

[0041] FIG. 10 is an explanatory diagram of the internal structure of the fixing device 9 in this embodiment, as viewed from the belt width direction (recording material width direction). However, FIG. 10 shows a location where a thermostat 66 serving as a temperature detection member is disposed in the belt width direction of the fixing device 9 (a location different from the location where the thermistor 65 is disposed in the belt width direction).

[0042] 10, the fixing device 9 of the present embodiment also includes a thermostat 66 as another temperature detection member that detects the temperature of the heater 22 facing the surface (back surface) opposite to the inner circumferential surface side of the fixing belt 20. However, in the belt width direction of the fixing device 9, the thermostat 66 is disposed at a position different from that of the thermistor 65.

[0043] Similar to the thermistor 65, the thermostat 66 is attached to a pedestal 66b via a compression spring 66a serving as a biasing member, and is supported by the stay 24 by fitting the pedestal 66b inside the stay 24. When the heater holder 23 holding the heater 22 is assembled with the stay 24, the thermostat 66 comes into contact with the rear surface of the heater 22 by the biasing force of the compression spring 66a through an opening 23b provided in the heater holder 23.

[0044] The thermostat 66 is connected on the power path from the heater power supply 63 to the heater 22. When the temperature of the heater 22 rises above a predetermined allowable upper limit temperature, the thermostat 66 detects this and switches off the power supply from the heater power supply 63 to the heater 22, thereby stopping heating of the heater 22. This controls the temperature of the heater 22 so that it does not exceed a predetermined allowable upper limit temperature, ensuring safety.

[0045] The thermistor 65 and thermostat 66 are positioned in the longitudinal direction (recording material width direction) and lateral direction (recording material conveyance direction) of the heater 22 by mounting the respective pedestals 65b, 66b to the stay 24. The thermistor 65 and thermostat 66 are held in the target installation position by static friction force caused by contacting the rear surface of the heater 22 with the biasing force of compression springs 65a, 66a.

[0046] Here, the positions of the thermistor 65 and thermostat 66 may deviate from the target installation positions due to, for example, installation errors during installation or the application of some kind of impact. Even in this case, the heater 22 has a configuration in which a pair of heating parts 60 made of planar resistance heating elements extend in the longitudinal direction (belt width direction) of the heater 22, so that the temperature distribution in the longitudinal direction of the heater 22 is approximately uniform. Therefore, even if the longitudinal positions (belt width direction positions) of the thermistor 65 and thermostat 66 deviate from the target installation positions, there is almost no effect on the temperature detection results.

[0047] However, if the positions of the thermistor 65 and thermostat 66 in the short-side direction (recording material conveying direction) deviate from the target installation positions, the temperature detection result is likely to be significantly affected. In detail, as shown in Figs. 9 and 10, the pair of heat generating parts 60 equipped with the heater 22 are not present over the entire area in the short-side direction, but are present in a part in the short-side direction. Therefore, the temperature distribution in the short-side direction is not uniform, and for example, as shown in Fig. 11, the temperature is highest in the center in the short-side direction and gradually decreases toward the ends in the short-side direction. Therefore, if the positions of the thermistor 65 and thermostat 66 in the short-side direction deviate from the target installation positions, the detected temperature will be different from the temperature that should be detected, which will have a significant effect on the temperature detection result.

[0048] 9 and 10, the back surface of the heater 22 against which the thermistor 65 and thermostat 66 come into contact is configured as a curved surface having a center of curvature on the side of the thermistor 65 and thermostat 66 when viewed from the belt width direction (recording material width direction). Both end portions of the thermistor 65 and thermostat 66 in the short side direction come into contact with the curved back surface of the heater 22, and the central portions of the thermistor 65 and thermostat 66 in the short side direction are separated from each other.

[0049] FIG. 12 is an explanatory diagram illustrating the force acting on both ends in the short-side direction of thermistor 65 that contacts the rear surface (curved surface) of heater 22 due to the biasing force of compression spring 65a. The same is true for the thermostat 66, so a description thereof will be omitted.

[0050] In this embodiment, as shown in Fig. 12, biasing forces F1 and F2 in the contact direction are applied to both ends of the thermistor 65 by the compression spring 65a. The direction (contact direction) of the biasing forces F1 and F2 coincides with the radial direction (up-down direction in Fig. 12) of the fixing belt 20 passing through the target setting position (the central position of the heater 22 in the short-side direction in this embodiment). In this embodiment, both ends of the thermistor 65 in the short-side direction are in contact with the back surface of the heater 22, which is a curved surface. Therefore, forces acting toward the central portion of the thermistor 65 along the surface of the curved portion at the contact point, i.e., tangential forces Fθ1 and Fθ2 acting toward the central portion along the tangential directions L1 and L2 of the curved portion, act on both ends of the short-side direction.

[0051] 12, the thermistor 65 is positioned at a balance position where the tangential forces Fθ1, Fθ2 acting on both ends of the thermistor 65 in the short side direction are approximately equal, and the thermistor 65 stops at that position. In this embodiment, the balance position is set to the target installation position (a position where the center position of the thermistor 65 in the short side direction coincides with the center position of the heater 22 in the short side direction in this embodiment), so the thermistor 65 is stably positioned at the target installation position in the short side direction.

[0052] 13, when the thermistor 65 is displaced from the target installation position (in FIG. 13, the thermistor 65 is displaced to the right side of the target installation position), the balance of the tangential forces Fθ1', Fθ2' acting on both ends of the thermistor 65 is lost. Specifically, one of the ends of the thermistor 65 (the right end in FIG. 13) moves away from the target installation position (the central position in the short-side direction of the heater 22), and the tangential force Fθ2' acting on that end portion becomes large. In contrast, the other end (the left end in FIG. 13) moves closer to the target installation position, and the tangential force Fθ1' acting on that end portion becomes small. Therefore, a force acts on the thermistor 65 as a whole, which moves the central position in the short-side direction of the thermistor 65 toward the central position in the short-side direction of the heater 22, which is the target installation position.

[0053] As a result, the thermistor 65 moves to a balance position (target installation position) where the tangential forces Fθ1', Fθ2' acting on both ends are approximately equal while sliding both ends along the rear surface (curved surface) of the heater 22. Therefore, according to this embodiment, even if the thermistor 65 deviates from the target installation position in the short side direction, the thermistor 65 can return to the target installation position and continue appropriate temperature detection.

[0054] In this embodiment, the curvature of the back surface (curved surface) of the heater 22 is appropriately set so that tangential forces Fθ1, Fθ2 act to return the thermistor 65 and thermostat 66 to their target installation positions even if they deviate from their target installation positions in the short-side direction. Usually, the larger the curvature, the larger the tangential forces Fθ1, Fθ2 acting on both ends of the thermistor 65 and thermostat 66 in the short-side direction. If the curvature of the back surface (curved surface) of the heater 22 is approximately the same as the curvature of the inner peripheral surface of the fixing belt 20 (the curvature of the inner peripheral surface of the belt portion excluding the nip portion N), the tangential forces Fθ1, Fθ2 acting on both ends of the thermistor 65 and thermostat 66 in the short-side direction may be insufficient. Therefore, it is preferable that the curvature of the back surface (curved surface) of the heater 22 is larger than the curvature of the inner peripheral surface of the fixing belt 20 (the curvature of the inner peripheral surface of the belt portion excluding the nip portion N).

[0055] The rear surface (curved surface) of the heater 22 is an arc-shaped curved surface having a certain curvature at least in the area where both ends of the thermistor 65 and the thermostat 66 in the short side direction may come into contact, but is not limited to this. For example, the curvature may be configured to increase toward each end of the heater 22 in the short side direction.

[0056] In this embodiment, the surface side of the heater 22 (the side that contacts the inner circumferential surface of the fixing belt 20) has a substantially flat shape. This reduces the curvature of the nip N, and suppresses problems such as envelope wrinkles caused by the difference in linear speed between the inside and outside of the nip N. Of course, the surface side of the heater 22 (the side that contacts the inner circumferential surface of the fixing belt 20) may have a curved shape that conforms to the inner circumferential surface of the fixing belt 20.

[0057] In this embodiment, the back surface of the heater 22 itself is configured to be a curved surface, but the back surface of the heater 22 itself may be a flat surface, and a separate member having a curved surface may be interposed between the back surface (flat surface) of the heater 22 and the thermistor 65 or the thermostat 66. In this case, the separate member is also a component that constitutes the heating member together with the heater 22, and the curved surface of the separate member constitutes the back surface of the heating member.

[0058] Furthermore, in this embodiment, the target installation position is set at the center position of the heater 22 in the short side direction, but it may be set at a position deviated from the center position of the heater 22 in the short side direction.

[0059] In addition, the thermostat 66 having the configuration of this embodiment also provides the following effects.

[0060] As described above, the thermostat 66 is used for controlling the heater 22 to stop heating when the temperature of the heater 22 exceeds a predetermined allowable upper limit temperature. However, in recent years, the target temperature (target fixing temperature) of the fixing belt 20 has become higher, and the difference between the upper limit of the normal temperature control range of the fixing temperature and the allowable upper limit temperature has become smaller.

[0061] In the conventional configuration where the back surface of the heater 22 is flat, the contact area with the thermostat 66 is wide, and heat is easily transferred from the heater 22 to the thermostat 66 excessively. Therefore, even if the fixing temperature is controlled within the normal temperature control range, the thermostat 66 may erroneously detect that the temperature has exceeded the allowable upper limit. When the thermostat 66 stops heating the heater 22, the image forming operation stops, resulting in downtime.

[0062] On the other hand, in the conventional configuration in which the thermostat 66 is positioned completely separated from the rear surface of the heater 22, there is a risk that variations in the separation distance, etc., may cause the thermostat 66 to heat up slowly, resulting in a delay in the heater 22 stopping heating in the event of an abnormality.

[0063] As in this embodiment, the back surface of the heater 22 that contacts the thermostat 66 is made curved, and both ends of the thermostat 66 in the short side direction are in contact with the back surface (curved surface) of the heater 22, while the central portion in the short side direction is spaced apart, thereby easily eliminating the above-mentioned problems.

[0064] That is, in this embodiment, both ends of the thermostat 66 in the short side direction are in contact with each other, and the central portion in the short side direction is separated from the heater 22, so that the heater 22 comes into less contact with the thermostat 66 compared to the conventional configuration in which the back surface of the heater 22 is flat. Therefore, excessive heat transfer from the heater 22 to the thermostat 66 is suppressed, and even if the target temperature (target fixing temperature) of the fixing belt 20 is high, the thermostat 66 is prevented from erroneously detecting the fixing temperature during normal temperature control. Therefore, the occurrence of downtime can be suppressed.

[0065] In addition, as in this embodiment, both ends of the thermostat 66 in the short side direction are in contact with each other and the central portion in the short side direction is spaced apart, so that the thermostat 66 can be stably held at an appropriate distance from the rear surface of the heater 22. Therefore, compared to the conventional configuration in which the thermostat 66 is disposed completely spaced apart from the rear surface of the heater 22, the occurrence of a situation in which the temperature rise of the thermostat 66 is delayed and the stopping of heating of the heater 22 is delayed in the event of an abnormality is suppressed.

[0066] The above description is merely an example, and each of the following aspects provides unique effects. [First aspect] The first aspect is a fixing device 9 including a hollow fixing member (e.g., fixing belt 20) that heats and fixes an image onto a recording material (e.g., paper P), a heating member (e.g., heater 22) that heats the fixing member from its inner circumferential surface, a temperature detection member (e.g., thermistor 65, thermostat 66) that detects temperature by facing the surface of the heating member opposite the inner circumferential surface of the fixing member, and a biasing member (e.g., compression springs 65a, 66a) that biases the temperature detection member so that it abuts against the surface, wherein the surface is formed as a curved surface having a center of curvature on the temperature detection member side when viewed from the recording material width direction (longitudinal direction, belt width direction) perpendicular to the recording material transport direction (short direction), and wherein, in the recording material transport direction, both end portions of the temperature detection member abut against the curved surface, and the central portion of the temperature detection member is spaced apart from the curved surface. In this embodiment, the surface of the heating member that the temperature detection member contacts is configured as a curved surface with a center of curvature on the temperature detection member side when viewed from the recording material width direction. The temperature detection member is configured so that both end portions contact this curved surface, and the center portion of the temperature detection member is separated from it. With this configuration, at both end portions of the temperature detection member that contact the curved surface, a force toward the center portion of the temperature detection member along the surface of the curved portion at each contact point (a tangential force toward the center portion along the tangential direction of the curved portion) acts due to the biasing force of the biasing member. At this time, the temperature detection member comes to rest by being positioned at a balance position where the tangential forces acting on both end portions are approximately equal. Therefore, by setting the balance position as the target installation position, the temperature detection member is stably positioned at the target installation position. Here, there are cases where the temperature detection member is placed at a position different from the target installation position during installation, or is subjected to some kind of impact, causing the temperature detection member to move away from the target installation position. In this case, the temperature detection member detects a different temperature than when it was placed at the target installation position, making it difficult to detect the temperature appropriately. According to this aspect, when the temperature detection member is displaced from the target installation position, the balance of the tangential forces acting on both ends of the temperature detection member is lost. Specifically, one of the two ends of the temperature detection member moves away from the target installation position, resulting in a larger tangential force acting on that end, and the other moves closer to the target installation position, resulting in a smaller tangential force acting on that end. Therefore, a force that moves the temperature detection member toward the target installation position acts on the temperature detection member as a whole. As a result, the temperature detection member moves to a balance position (target installation position) where the tangential forces acting on both ends are approximately equal while sliding both ends along the curved surface, and stops at the balance position. Therefore, according to this aspect, even if the temperature detection member is displaced from the target installation position, the temperature detection member can return to the target installation position and continue appropriate temperature detection.

[0067] [Second aspect] The second aspect is characterized in that, in the first aspect, a point on the curved surface where the direction of the urging forces F1, F2 that the temperature detection member receives from the urging member approximately coincides with the direction of the radius of curvature, approximately coincides with the central position of the heating member in the recording material transport direction. According to this aspect, even if the temperature detection member deviates from the target installation position, which is the central position of the heating member, the temperature detection member can return to the target installation position (the central position of the heating member) and continue proper temperature detection.

[0068] [Third aspect] The third aspect is characterized in that, in the second aspect, the biasing member biases the temperature detection member along a direction that coincides with the direction of the biasing force from a predetermined biasing fulcrum (for example, a point where compression springs 65a, 66a are supported by bases 65b, 66b), and the predetermined biasing fulcrum is located at a position farther from the surface than the center of curvature. According to this, when the temperature detection member deviates from the central position of the heating member, which is the target installation position, a tangential force can be easily exerted to return the temperature detection member to the target installation position.

[0069] [Fourth aspect] A fourth aspect is characterized in that, in any one of the first to third aspects, the temperature detection member is a member (e.g., a thermistor 65) whose detected temperature is used to control the temperature of the heating member within a target temperature range. According to this aspect, even if the temperature detection member deviates from the center position of the heating member, which is the target installation position, appropriate control for controlling the temperature of the heating member within the target temperature range can be continued.

[0070] [Fifth aspect] A fifth aspect is characterized in that, in any one of the first to third aspects, the temperature detection member is used for controlling the heating member to stop heating when the detected temperature of the temperature detection member exceeds an allowable upper limit temperature of the heating member (e.g., a thermostat 66). According to this aspect, even if the temperature detection member moves away from the central position of the heating member, which is the target installation position, appropriate control can be continued to stop heating of the heating member when the allowable upper limit temperature of the heating member is exceeded.

[0071] [Sixth aspect] A sixth aspect is an image forming apparatus equipped with a fixing means for fixing an image formed on a recording material onto the recording material, characterized in that the fixing means is a fixing device according to any one of the first to fifth aspects. This makes it possible to provide an image forming apparatus that continues to properly detect temperature even if the temperature detection member is displaced from the target installation position. [Explanation of symbols]

[0072] 1: Imaging unit 2: Photoreceptor 3: Charging device 4: Developing device 5: Cleaning device 6: Exposure equipment 7: Paper feeder 8: Transfer device 9: Fixing device 10: Paper ejection device 11: Intermediate transfer belt 12: Primary transfer roller 13: Secondary transfer roller 14: Paper transport path 15: Timing roller 19: Heating device 20: Fixing belt 21: Pressure roller 22: Heater 23: Heater holder 23a: Recessed portion 23b: Opening 24: Stay 50: Base material layer 51: First insulating layer 52: Conductive layer 53: Second insulating layer 54: Third insulating layer 60: Heating part 61: Electrode part 62:Power line 63: Heater power supply 64: Temperature control unit 65: Thermistor 65a: Compression spring 65b: Pedestal 66: Thermostat 66a: Compression spring 66b: Pedestal 100: Image forming apparatus 103: Image forming apparatus body [Prior art documents] [Patent documents]

[0073] [Patent Document 1] JP 2020-098312 A

Claims

1. a hollow fixing member that heats and fixes an image onto a recording material; a heating member that heats the fixing member from an inner peripheral surface side of the fixing member; a temperature detection member that detects a temperature of the heating member and faces a surface of the heating member opposite to an inner circumferential surface of the fixing member; a biasing member that biases the temperature detecting member so that the temperature detecting member abuts against the surface; A fixing device comprising: the surface is a curved surface having a center of curvature on the temperature detection member side when viewed from a recording material width direction perpendicular to a recording material conveying direction, In the recording material conveying direction, both end portions of the temperature detection member are in contact with the curved surface, and a center portion of the temperature detection member is separated from the curved surface, A fixing device characterized in that a point on the curved surface where the direction of the urging force received by the urging member on the temperature detection member approximately coincides with the direction of the radius of curvature approximately coincides with the center position of the heating member in the recording material transport direction.

2. In the fixing device according to claim 1, The biasing member biases the temperature detection member along a direction that coincides with the direction of the biasing force from a predetermined biasing fulcrum, The fixing device according to claim 1, wherein the predetermined urging fulcrum is provided at a position farther from the surface than the center of curvature.

3. 3. The fixing device according to claim 1, The fixing device according to claim 1, wherein the temperature detected by the temperature detecting member is used for controlling the temperature of the heating member to be within a target temperature range.

4. 3. The fixing device according to claim 1, The fixing device, wherein the temperature detection member is used for control to stop heating of the heating member when the temperature detected by the temperature detection member exceeds an allowable upper limit temperature of the heating member.

5. An image forming apparatus including a fixing unit for fixing an image formed on a recording material onto the recording material, 5. An image forming apparatus comprising the fixing device according to claim 1 as said fixing means.

Images