Fixing device

Abstract

To provide a configuration that can prevent shortening of the life of a belt 301.SOLUTION: A pad 303 is arranged on the inside of a belt 301 to sandwich the belt 301 with a pressure roller, and forms a nip part N for sandwiching and conveying a recording material between the belt 301 and the pressure roller. The pad 303 has, on the downstream side of the nip part N with respect to a conveyance direction of the recording material, a bent part 303a that bends the belt 301 so that the recording material passing through the nip part N is separated from the belt 301. The bent part 303a has a first part 303a1 that is located on the downstream side in the conveyance direction of a passage area through which a maximum size recording material passes, and a second part 303a2 that is located on the downstream side in the conveyance direction of non-passage areas on the outside of the passage area with respect to a width direction of the recording material intersecting the conveyance direction. The radius of curvature of the second part 303a2 is larger than the radius of curvature of the first part 303a1.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to a fixing device that fixes a toner image carried on a recording material onto the recording material. [Background technology]

[0002] A known fixing device configuration forms a nip between a belt and a rotating body such as a roller, which sandwiches and conveys the recording material, and heats and pressurizes the recording material as it passes through the nip (see Patent Documents 1 and 2). Patent Document 1 describes a pad member that is disposed inside the belt so as to sandwich the belt and face the rotating body, forming the nip. The pad member has a curved surface at the downstream end in the conveyance direction of the recording material at the nip, and the belt is curved by the curvature of this curved surface, so that the recording material that has passed through the nip is separated from the belt. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-114394 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-222339 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, in the case of a configuration in which the belt is curved at the downstream end of the pad member in the recording material transport direction as described above, if the belt is repeatedly rotated in a curved state, the belt will be prone to fatigue failure in areas where the recording material does not pass, and there is a risk that the belt's lifespan will be shortened.

[0005] An object of the present invention is to provide a configuration that can prevent the shortening of the life of a belt. [Means for solving the problem]

[0006] One aspect of the present invention is a fixing device that fixes a toner image carried on a recording material to the recording material, the fixing device comprising: an endless rotatable belt; a rotating body that rotates in contact with an outer circumferential surface of the belt; a pad member that is disposed inside the belt so as to sandwich the belt between itself and the rotating body, and that forms a nip portion between the belt and the rotating body to sandwich and transport the recording material; a sliding member that is disposed between the pad member and the belt, and that slides against the inner circumferential surface of the belt at the nip portion; and a heating roller that is disposed inside the belt and heats the belt, the pad member being disposed in such a manner that the pad member is in contact with the nip portion in the direction of transport of the recording material. a curved portion downstream of the nip portion, continuous with the downstream end of the nip portion, that brings the sliding member into contact with the belt or curves to be in direct contact with the belt, and that curves the belt so that the recording material that has passed through the nip portion separates from the belt; the curved portion has a first portion located downstream in the conveying direction of a passing area where the largest size recording material passes through the nip portion, and a second portion located downstream in the conveying direction of a non-passing area outside the passing area with respect to the width direction of the recording material that intersects the conveying direction, and the radius of curvature of the second portion is greater than the radius of curvature of the first portion.

[0007] One aspect of the present invention is a fixing device that fixes a toner image carried on a recording material to the recording material, the fixing device comprising: an endless rotatable belt; a rotating body that rotates in contact with the outer circumferential surface of the belt; a pad member that is disposed inside the belt so as to sandwich the belt between itself and the rotating body, and that forms a nip portion between the belt and the rotating body to sandwich and transport the recording material; a sliding member that is disposed between the pad member and the belt and slides against the inner circumferential surface of the belt at the nip portion; and a heating roller that is disposed inside the belt and heats the belt, the pad member being disposed downstream of the nip portion in the transport direction of the recording material and discontinuous with the downstream end of the nip portion, and that causes the sliding member to contact the belt or to directly contact the belt. a contact portion in which the recording material passes through the nip portion, and a separation portion, which bends the belt so that the recording material that has passed through the nip portion is separated from the belt, and the separation portion has a first portion located downstream in the conveying direction of a passing area where the largest size recording material passes through the nip portion, and a second portion located downstream in the conveying direction of a non-passing area outside the passing area in a width direction of the recording material that intersects with the conveying direction, and wherein directions perpendicular to the conveying direction and the width direction are defined as pressure directions in which the recording material is pressed at the nip portion, and when the distance from the downstream end of the nip portion to the contact portion in the pressure direction is defined as a pressure direction distance, the pressure direction distance of the second portion is shorter than the pressure direction distance of the first portion. [Effects of the Invention]

[0008] According to the present invention, it is possible to prevent the life of the belt from being shortened. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2A is a cross-sectional view showing the schematic configuration of a fixing device according to the first embodiment, and FIG. 2B is an enlarged schematic view showing a portion A in FIG. [Figure 3](a) A schematic diagram of the stay and pad according to the first embodiment as seen from above in the pressure direction, (b) a schematic cross-sectional view of the stay and pad cut along the width direction as seen from the downstream side in the conveying direction, and (c) a schematic cross-sectional view of the belt, stay, and pad according to the first embodiment cut at the center position in the longitudinal direction of the stay. [Figure 4] (a) A graph showing the longitudinal distribution of peak pressure generated between the curved portion of the pad and the belt in the first embodiment, (b) A schematic cross-sectional view of the belt, stay, and pad at the position A-np-Y1 in (a), and (c) A schematic cross-sectional view of the belt, stay, and pad at the position Ap-Yc in (a). [Figure 5] 1(a) is a cross-sectional view showing a schematic configuration of a belt, a stay, and a pad according to the first embodiment, FIG. 1(b) is an enlarged view of part B in FIG. 1(a), and FIG. 1(c) is a graph showing the relationship between the radius of curvature of the curved part of the stay and bending strain. [Figure 6] (a) A schematic cross-sectional view of the stay and pad according to the first embodiment, cut along the width direction and viewed from the downstream side in the conveying direction; (b) A schematic cross-sectional view of the belt, stay, and pad according to the first embodiment; (c) An enlarged view of part C of (b) at the position A-np-Yn of (a); (d) An enlarged view of part C of (b) at the position Ap-Yc of (a). [Figure 7] (a) Schematic diagram of the stay and pad according to Comparative Example 1 as viewed from the downstream side in the conveying direction, (b) a cross-sectional view of the belt, stay, and pad according to Comparative Example 1, (c) an enlarged view of part D in (b) at the position A-np-Yn in (a), and (d) an enlarged view of part D in (b) at the position Ap-Yc in (a). [Figure 8] FIG. 10(a) is a cross-sectional view showing a schematic configuration of a belt, a stay, and a pad according to a second embodiment, (b) is an enlarged view of part E in (a), and (c) is a graph showing the relationship between ΔZ and bending strain. [Figure 9] (a) Schematic diagram of the stay and pad according to the second embodiment as viewed from the downstream side in the conveying direction, (b) sectional view of the belt, stay, and pad according to the second embodiment, (c) enlarged view of part F in (b) at the position A-np-Yn in (a), (d) enlarged view of part F in (b) at the position Ap-Yc in (a). [Figure 10] (a) Schematic diagram of the stay and pad according to Comparative Example 2 as viewed from the downstream side in the conveying direction, (b) sectional view of the belt, stay, and pad according to Comparative Example 2, (c) enlarged view of part G in (b) at the position A-np-Yn in (a), (d) enlarged view of part G in (b) at the position Ap-Yc in (a). [Figure 11] 1 is a table showing the verification results of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0010] First Embodiment The first embodiment will be described with reference to Figures 1 to 7(d) First, the schematic configuration of an image forming apparatus according to this embodiment will be described with reference to Figure 1.

[0011] [Image forming equipment] Image forming apparatus 1 is an electrophotographic full-color printer having four image forming units Pa, Pb, Pc, and Pd corresponding to the four colors of yellow, magenta, cyan, and black. In this embodiment, the image forming units Pa, Pb, Pc, and Pd are arranged in tandem along the rotation direction of an intermediate transfer belt 204 (described later). Image forming apparatus 1 forms a toner image (image) on a recording material in response to an image signal from an image reading unit (document reading device) 2 connected to image forming apparatus main body 3 or from a host device such as a personal computer connected to image forming apparatus main body 3 so as to be able to communicate with the image forming apparatus main body 3. Examples of recording materials include sheet materials such as paper, plastic film, and cloth.

[0012] The image forming apparatus 1 comprises an image reading unit 2 and an image forming apparatus main body 3. The image reading unit 2 reads an original placed on a platen glass 21. Light emitted from a light source 22 is reflected by the original and forms an image on a CCD sensor 24 via optical components 23 such as a lens. This optical unit scans in the direction of the arrow, converting the original into a line-by-line electrical signal data stream. The image signal obtained by the CCD sensor 24 is sent to the image forming apparatus main body 3, where it is subjected to image processing in accordance with each image forming unit (described later) by a control unit 30. The control unit 30 also receives external inputs as image signals from external host devices such as a print server.

[0013] The image forming apparatus main body 3 includes multiple image forming units Pa, Pb, Pc, and Pd, and each image forming unit forms an image based on the image signal. That is, the image signal is converted into a laser beam that is PWM (pulse width modulation) controlled by a control unit 30. A polygon scanner 31 serving as an exposure device scans the laser beam in accordance with the image signal. The laser beam is then irradiated onto photosensitive drums 200a to 200d serving as image carriers of each image forming unit Pa to Pd.

[0014] Note that Pa is a yellow (Y) image forming unit, Pb is a magenta (M) image forming unit, Pc is a cyan (C) image forming unit, and Pd is a black (Bk) image forming unit, each forming an image of the corresponding color. Since the image forming units Pa to Pd are substantially identical, the Y image forming unit Pa will be described in detail below, and descriptions of the other image forming units will be omitted. In the image forming unit Pa, a toner image is formed on the surface of the photosensitive drum 200a based on an image signal, as will be described below.

[0015] A charging roller 201a, which serves as a primary charger, charges the surface of the photosensitive drum 200a to a predetermined potential to prepare for the formation of an electrostatic latent image. A laser beam from a polygon scanner 31 forms an electrostatic latent image on the surface of the photosensitive drum 200a, which has been charged to a predetermined potential. A developing unit 202a develops the electrostatic latent image on the photosensitive drum 200a to form a toner image. A primary transfer roller 203a discharges electricity from the back surface of the intermediate transfer belt 204 and applies a primary transfer bias of opposite polarity to the toner, transferring the toner image on the photosensitive drum 200a onto the intermediate transfer belt 204. After transfer, the surface of the photosensitive drum 200a is cleaned by a cleaner 207a.

[0016] The toner image on intermediate transfer belt 204 is then conveyed to the next image forming station, where the toner images of each color formed at each image forming station are transferred in the order of Y, M, C, and Bk, forming a four-color image on the surface. The toner image that has passed through Bk image forming station Pd, which is located at the most downstream side in the rotation direction of intermediate transfer belt 204, is conveyed to a secondary transfer station made up of a pair of secondary transfer rollers 205 and 206. In the secondary transfer station, a secondary transfer electric field of opposite polarity to the toner image on intermediate transfer belt 204 is applied, thereby secondarily transferring the toner image onto the recording material.

[0017] The recording material is stored in a cassette 9, and the recording material fed from the cassette 9 is transported to a registration unit 208, which is made up of, for example, a pair of registration rollers, and waits at the registration unit 208. Thereafter, the timing of the registration unit 208 is controlled to align the position of the toner image on the intermediate transfer belt 204 with the position of the paper, and the recording material is transported to a secondary transfer unit.

[0018] The recording material onto which the toner image has been transferred in the secondary transfer section is transported to a fixing device 8, where the toner image carried on the recording material is fixed to the recording material by heating and pressing. The recording material that has passed through the fixing device 8 is discharged onto a discharge tray 7. When forming images on both sides of the recording material, after the toner image has been transferred and fixed onto the first side (front side) of the recording material, the recording material is turned over via a reversing conveyance section 10, and the toner image is transferred and fixed onto the second side (rear side) of the recording material, and the recording material is then stacked on the discharge tray 7.

[0019] As described above, the control unit 30 controls the entire image forming apparatus 1. The control unit 30 can also perform various settings based on input from the operation unit 4 of the image forming apparatus 1. The control unit 30 has a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). The CPU controls each unit by reading a program corresponding to a control procedure stored in the ROM. The RAM stores working data and input data, and the CPU performs control by referring to the data stored in the RAM based on the aforementioned programs.

[0020] [Fixing device] Next, the configuration of the fixing device 8 in this embodiment will be described with reference to Figures 2(a) and (b). In this embodiment, a fixing device of a belt heating type using an endless belt is adopted. In Figure 2(a), the X direction indicates the conveyance direction of the recording material P (not shown in the figure), the Y direction indicates the width direction of the recording material that intersects with the conveyance direction of the recording material (orthogonal in this embodiment), and the Z direction indicates the pressure direction in which the recording material is pressed at the nip portion N. In this embodiment, the X direction, Y direction, and Z direction are each orthogonal to each other.

[0021] The fixing device 8 includes a fixing belt (hereinafter referred to as "belt") 301, a stay 302, a pressure pad (hereinafter referred to as "pad") 303, a sliding member 304, a pressure roller 305, a heating roller 307, and a steering roller 308. The belt 301 is an endless, rotatable heating rotor. The pressure roller 305, which serves as a rotor, is a pressure rotor that contacts the outer circumferential surface of the belt 301 to form a nip N between the belt 301 and the pressure roller 305, which sandwiches and conveys the recording material.

[0022] The sliding member 304 slides against the inner circumferential surface of the belt 301 at the nip portion N. The pad 303, which serves as both a backup member and a pad member, is disposed inside the belt 301 so as to sandwich the sliding member 304 and the belt 301 between itself and the pressure roller 305, thereby backing up the sliding member 304. The sliding member 304 is disposed so as to cover the outer circumferential surface of the pad 303 facing the belt 301. The stay 302 is disposed inside the belt 301, on the opposite side of the pad 303 from the nip portion N, and supports the pad 303. The heating roller 307 is disposed inside the belt 301 so as to stretch the belt 301 and heat the belt 301. The steering roller 308 is disposed inside the belt 301 so as to stretch the belt 301. As described below, the steering roller 308 applies tension to the belt 301 and controls deviation of the belt 301. Each component will be described in detail below.

[0023] The belt 301 has thermal conductivity, heat resistance, and the like, and is a thin-walled cylindrical shape. In this embodiment, as shown in FIG. 2(b), the belt 301 has a three-layer structure including a base layer 301a, an elastic layer 301b formed on the outer periphery of the base layer 301a, and a release layer 301c formed on the outer periphery of the elastic layer 301b. The base layer 301a is, for example, 80 μm thick and made of polyimide resin (PI). The elastic layer 301b is, for example, 300 μm thick and made of silicone rubber. The release layer 301c is, for example, 30 μm thick and made of PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin) as a fluororesin. The belt 301 is stretched by a pad 303 and a heating roller 307. In this embodiment, the outer diameter of the belt 301 is 150 mm.

[0024] The pad 303 is disposed inside the belt 301 so as to face the pressure roller 305 with the belt 301 sandwiched therebetween, and forms a nip portion N between the belt 301 and the pressure roller 305 for sandwiching and conveying the recording material. In this embodiment, the pad 303 is a substantially plate-shaped member that is long in the width direction of the belt 301 (the longitudinal direction intersecting the rotation direction of the belt 301, the direction of the rotation axis of the heating roller 307). The pad 303 is pressed against the pressure roller 305 with the belt 301 sandwiched therebetween, thereby forming the nip portion N. The pad 303 is made of LCP (liquid crystal polymer) resin. A sliding member 304 is interposed between the pad 303 and the belt 301. Details of the sliding member 304 will be described later.

[0025] The pad 303 is supported by a stay 302 serving as a support member disposed inside the belt 301. That is, the stay 302 is disposed on the opposite side of the pad 303 from the pressure roller 305, and supports the pad 303. Such a stay 302 is a reinforcing member having long rigidity along the longitudinal direction of the belt 301, and contacts the pad 303 to back it up. That is, the stay 302 provides strength to the pad 303 when the pad 303 is pressed by the pressure roller 305, thereby ensuring the pressure at the nip portion N. The widthwise ends of the stay 302 are supported by a frame.

[0026] The stay 302 is made of a metal such as stainless steel, and has a substantially rectangular cross section (transverse cross section) perpendicular to the longitudinal direction of the stay 302, which intersects with the rotation direction of the belt 301. For example, the stay 302 is made of a 3 mm thick drawn material of SUS304 (stainless steel), and the transverse cross section is formed into a hollow, substantially square shape to ensure strength. Note that the stay 302 may also be formed into a substantially rectangular cross section by combining multiple metal plates and fixing them together by welding or the like. The material of the stay 302 is not limited to stainless steel as long as strength can be ensured.

[0027] Heating roller 307 is disposed inside belt 301, and stretches belt 301 together with pad 303 and steering roller 308. Heating roller 307 is formed into a cylindrical shape from a metal such as aluminum or stainless steel, and has a halogen heater 306 disposed therein as a heat source for heating belt 301. Heating roller 307 is heated to a predetermined temperature by halogen heater 306. In addition, heating roller 307 is supported and biased at its widthwise ends by a frame and bearings (not shown), allowing it to rotate.

[0028] The steering roller 308 has a rotation center at one end or near the center in the longitudinal direction, and rotates relative to the belt 301 to generate a tension difference between the front and rear of the belt 301, thereby controlling the position (shift position) of the belt 301 in the main scanning direction (width direction). The steering roller 308 is also biased by a spring supported by a frame (not shown), and functions as a tension roller that applies a predetermined tension SF to the belt 301. That is, the steering roller 308 as a tension applying member is disposed inside the belt 301, and stretches the belt 301 together with the pad 303 to apply the tension SF to the belt 301. The tension SF applied to the belt 301 is preferably about 40N to 120N.

[0029] In this embodiment, the heating roller 307 is formed, for example, from a stainless steel pipe having a thickness of 1 mm. While a single halogen heater 306 is sufficient, it is preferable to have multiple heaters in consideration of temperature distribution control in the longitudinal direction (rotation axis direction) of the heating roller 307. The multiple halogen heaters 306 have different light distributions in the longitudinal direction, and the lighting ratio is controlled according to the size of the recording material. In this embodiment, three halogen heaters 306 are provided. The heat source is not limited to a halogen heater, and other heaters capable of heating the heating roller 307, such as a carbon heater, may also be used. The belt 301 is heated by the heating roller 307 heated by the halogen heater 306, and is controlled to a predetermined target temperature according to the type of recording material based on temperature detection by a thermistor (temperature detection member) (not shown).

[0030] The pressure roller 305 rotates in contact with the outer circumferential surface of the belt 301 and also serves as a driving rotor that applies a driving force to the belt 301. In this embodiment, the heat roller 307 is also driven by a drive source (e.g., a drive motor) and applies a driving force to the belt 301. However, the application of a driving force to the heat roller 307 may be omitted. The pressure roller 305 is a roller comprising a core (shaft) 305c, an elastic layer 305b on the outer periphery of the core 305c, and a release layer 305a on the outer periphery of the elastic layer 305b. The core 305c is made of stainless steel, for example, with a diameter of 72 mm. The elastic layer 305b is made of conductive silicone rubber, for example, with a thickness of 8 mm. The release layer 305a is made of a fluororesin, such as PFA (tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin), with a thickness of 100 μm. Pressure roller 305 is rotatably supported by a frame (not shown) of fixing device 8, has a gear fixed to one end thereof, and is connected to a drive source (e.g., a drive motor, not shown) via the gear to be rotated. Pressure roller 305 presses belt 301 toward pad 303 by pressure mechanism 309. Pressure mechanism 309 has pressure arm 310 connected to core 305c, and pressure arm 310 receives force from a drive mechanism (not shown) to swing about swing center 311, whereby pressure roller 305 is pressed toward pad 303 with pressure force PF.

[0031] The fixing device 8 sandwiches the recording material P carrying a toner image in a nip portion N formed between the belt 301 and the pressure roller 305, and heats the toner image while transporting the recording material P. In this way, the fixing device 8 fixes the toner image to the recording material P while sandwiching and transporting the recording material P. Therefore, the fixing device 8 must be able to both apply heat and pressure and transport the recording material P. A driving source (not shown) presses the pressure roller 305 against the sliding member 304 via the belt 301. In this embodiment, the pressure force (NF) at the nip portion N during image formation is set to 1600 N, and the width of the nip portion N in the X direction (the transport direction of the recording material) is set to 24.5 mm and the width in the Y direction (the width direction of the recording material) is set to 326 mm.

[0032] [Sliding member] The sliding member 304 is fixed to the stay 302 via the pad 303 with screws or the like. The sliding member 304 may be integral with the pad 303. Alternatively, a portion of the sliding member 304 may be fixed to the stay 302 or the pad 303. For example, both ends of the sliding member 304 in the Y direction (width direction) may be fixed to the pad 303 with screws or the like. In this embodiment, the sliding member 304 is configured to cover the pad 303 regardless of whether it is inside or outside the nip portion N. Although not shown here, it is acceptable as long as a portion of the nip portion N is covered by the sliding member 304. In other words, a configuration in which the sliding member 304 is disposed only in the nip portion N is also acceptable.

[0033] The sliding member 304 is composed of a base material layer 304a and a sliding layer 304c. A plurality of protrusions 304b protruding toward the inner peripheral surface of the belt 301 are formed on the side of the base material layer 304a that slides against the belt 301. The sliding layer 304c is provided so as to cover the surface of the base material layer 304a that slides against the belt 301 (including the plurality of protrusions 304b). The convex portions formed by the protrusions 304b being covered with the sliding layer 304c are referred to as embossed portions 304d.

[0034] The base layer 304a only needs to have sufficient heat resistance and strength. Materials include stainless steel, copper, aluminum, and engineering plastics (PI (polyimide), PEEK (polyether ether ketone), LCP (liquid crystal polymer), etc.), and in this embodiment, metal materials such as stainless steel, copper, and aluminum are desirable. In this embodiment, PI with a thickness of 300 μm is used as the base layer 304a.

[0035] The plurality of protrusions 304b are provided from the base layer 304a toward the inner circumferential surface of the belt 301. The plurality of protrusions 304b are integrally formed from the same material as the base layer 304a and are arranged in the nip N along the recording material conveyance direction (X direction) and the width direction (Y direction) of the recording material, which intersects with the conveyance direction. The distance (interval) d between the centers of adjacent protrusions 304b in the conveyance direction and the distance (interval) d between the centers of adjacent protrusions 304b in the width direction are each 1.25 mm or more, preferably 1.4 mm or more. In this embodiment, to ensure uniform sliding performance with the belt 301, the intervals between the plurality of protrusions 304b are the same in the conveyance direction and the width direction, and each interval d is 1.4 mm. The plurality of protrusions 304b are distributed in the nip N and outside the nip N in the width direction (a passing region and a non-passing region, which will be described later).

[0036] In this way, by providing multiple protrusions 304b on the surface (sliding surface) of sliding member 304 that slides against belt 301, the contact area between sliding member 304 and belt 301 is reduced, thereby reducing the sliding resistance between sliding member 304 and belt 301. Protrusions 304b have flat tip surfaces and are formed in a substantially cylindrical shape.

[0037] The sliding layer 304c is preferably made of a coating agent such as a fluororesin (PTFE (polytetrafluoroethylene), PFA, etc.) to achieve low friction. In this embodiment, the sliding member 304 is formed by coating the surface of the base layer 304a, including the plurality of protrusions 304b, with a 20 μm-thick PTFE. In this embodiment, a lubricant is applied to the inner surface of the belt 301. This allows the belt 301 to slide smoothly against the sliding member 304. Silicone oil is used as the lubricant. In this embodiment, the sliding layer 304c is provided on the base layer 304a. However, an adhesive layer may be provided between the base layer 304a and the sliding layer 304c. The use of an adhesive layer can provide good adhesive strength between the base layer 304a and the sliding layer 304c when the base layer 304a is made of a metal material such as stainless steel, copper, or aluminum.

[0038] Furthermore, the sliding member 304 of this embodiment is configured to cover the pad 303 regardless of whether it is inside or outside the nip portion N. That is, the entire surface of the pad 303 that faces the belt 301 is covered by the sliding member 304, except for the surface opposite the nip portion N. Furthermore, the multiple protrusions 304b are arranged over the entire area of ​​the sliding member 304. Although not shown here, it is acceptable for the embossed portion 304d to be present in part of the nip portion N. That is, it is acceptable for the embossed portion 304d to be arranged only in the nip portion N.

[0039] [Factors that reduce belt life] Next, using Figures 3(a) to 4(c), factors that cause fatigue failure (cracks) to occur in belt 301 and shorten the lifespan of belt 301 will be described. Figure 3(a) is a schematic diagram showing the positions of stay 302 and pad 303 from above in the pressure direction. Figure 3(b) is a diagram that schematically shows a cross section showing the positions of stay 302 and pad 303 from downstream in the conveying direction. Figure 3(c) is a diagram that schematically shows a cross section at position Ap-Yc in Figure 3(b) (the center position in the longitudinal direction of stay 302).

[0040] The area Ap in FIG. 3(a) is the area in the nip N through which the maximum size recording material usable in the image forming apparatus 1 passes (hereinafter referred to as the "passing area" or "AP area"). The area A-np in FIG. 3(a) is the area in the nip N through which the maximum size recording material does not pass (hereinafter referred to as the "non-passing area" or "A-np area"). The A-np area (non-passing area) is located outside the AP area (passing area) in the width direction. An embossed portion 304d may be present in the A-np area. In this embodiment, the width of the Ap area is set to 340 mm, and the A-np area is set to 15 mm. The curved portion 303a in FIG. 3(c) is the area curved to a specific curvature by tensioning the belt 301 from the inner surface by the steering roller 308. That is, the pad 303 has a curved portion 303a, which is continuous with the downstream end of the nip portion N on the downstream side of the nip portion N in the conveying direction of the recording material, and which curves so as to bring the sliding member 304 into contact with the belt 301 and curves the belt 301 so that the recording material that has passed through the nip portion N is separated from the belt 301. If the curved portion 303a does not have the sliding member 304, the curved portion 303a would come into direct contact with the belt 301, thereby curving the belt 301. The pad 303 is set to have a width greater than that of the pressure roller 305 in order to stably secure the Ap region through which the recording material passes in the nip portion N. That is, the pad 303 is configured to form an A-np region widthwise outside the AP region in order to stably secure the Ap region.

[0041] As described above, to improve the separation performance of the recording material from the belt 301, the belt 301 is stretched by the steering roller 308 and curved to a specific radius of curvature at the curved portion 303a downstream of the nip N. By reducing the radius of curvature of the belt 301 downstream of the nip N, energy is applied to the recording material during conveyance, exceeding the adhesive energy between the toner and the belt 301. This allows the recording material to be separated and conveyed from the belt 301 after toner fixation. On the other hand, if the belt 301 is curved with a radius of curvature greater than necessary, unnecessary large strain is generated in the belt 301. As a result, the belt 301 is repeatedly subjected to large strain when it rotates. This can cause fatigue failure in the belt 301, such as cracks in the base layer 301a of the belt 301, shortening the life of the belt 301.

[0042] Fig. 4(a) is a graph showing the longitudinal (widthwise) distribution of peak pressure generated between a curved portion 303a that curves the belt 301 to a specific curvature by stretching the belt 301 from the inside between a steering roller 308 and a pad 303, and the belt 301. Fig. 4(b) and Fig. 4(c) are schematic cross-sectional views showing the results of observation of the vicinity of the nip portion N at positions A-np-Y1 and Ap-Yc in Fig. 4(a), respectively.

[0043] As can be seen from the results in FIG. 4(a), in the Ap region, the pressure applied to the curved portion 303a of the pad 303 due to the tension SF applied to the belt 301 from the steering roller 308 is low. In contrast, in the A-np region, the pressure applied to the curved portion 303a of the pad 303 due to the tension SF applied to the belt 301 from the steering roller 308 is high. When the tension SF of the steering roller 308 is applied to the belt 301, the ability of the belt 301 to follow the curved portion 303a changes. Then, the tension SF applied to the belt 301 from the steering roller 308 becomes high in a certain area, and the peak pressure applied to the curved portion 303a becomes higher. In other words, when the ability of the belt 301 to follow the curved portion 303a becomes stronger, it can be assumed that the radius of curvature of the belt 301 at that part of the curved portion 303a becomes smaller.

[0044] 4(b) and 4(c) also confirm that, at the A-np-Y1 cross section where the peak pressure at curved portion 303a due to tension SF is large, the radius of curvature of belt 301 at curved portion 303a downstream of nip portion N is small, and at the Ap-Yc cross section where the peak pressure at curved portion 303a due to tension SF is small, the radius of curvature of belt 301 at curved portion 303a downstream of nip portion N is large. Therefore, although the radius of curvature of belt 301 curved by curved portion 303a, which is necessary to improve the separation performance of the recording material, is sufficiently small at the Ap-Yc cross section, the radius of curvature of belt 301 curved by curved portion 303a is even smaller at the A-np-Y1 cross section where no recording material passes.

[0045] Therefore, it was found that if no countermeasures were taken, excessive strain would occur in the curved belt 301 due to the curved portion 303a in the A-np region, where the recording material does not pass, and fatigue failure would occur when the belt 301 rotates. The cause of this was assumed to be due to deformation of the pad 303, heating roller 307, and steering roller 308 when a spring was placed at the end of the steering roller 308 to tension the belt 301. In other words, it is thought that when tension is applied to the belt 301 by the steering roller 308, the inside of the fixing device 8 deforms, causing a stronger tension to be applied to the A-np region than to the Ap region.

[0046] [Configuration of the curved part of the pad] As described above, it has been found that if no measures are taken for the curved portion 303a of the pad 303, the radius of curvature of the belt 301 curved by the curved portion 303a necessary to improve the separation performance of the recording material is sufficiently small on the Ap-Yc cross section, but the radius of curvature of the belt 301 curved by the curved portion 303a is even smaller on the A-np-Y1 cross section, where no recording material passes. To solve the above problem, in this embodiment, the shape of the pad 303 is configured so that the bending strain in the A-np region, where it is not necessary to improve the separation performance of the recording material, is smaller than the bending strain in the Ap region, where it is necessary to improve the separation performance of the recording material.

[0047] Next, using Figures 5(a) to (c), we will explain step by step how to calculate the strain generated in the belt 301 when the belt 301 is bent to a specific radius of curvature at the curved portion downstream of the nip portion N. First, the curved portion 303a where the pad 303 and the belt 301 come into contact and bend the belt 301 is measured with pressure-sensitive paper to identify the area of ​​the curved portion 303a. Next, the dimensions of each part are measured according to the shape of that area. Finally, the measured values ​​are substituted into a theoretical formula to calculate the strain at each part. Figure 5(a) shows a cross-sectional view of the belt 301, stay 302, and pad 303 around the nip portion N. Figure 5(b) is an enlarged view of part B surrounded by a dotted line in Figure 5(a).

[0048] As described above, the area of ​​the curved portion 303a where the pad 303 and the belt 301 come into contact and curve the belt 301 is first measured and identified using pressure-sensitive paper. For the measurement, a Keyence VR-3200 three-dimensional shape measuring instrument and a Fujifilm pressure-sensitive paper prescale were used. The Fujifilm pressure-sensitive paper prescale was used for ultra-low pressure (LLLW) to match the measurement pressure range (0.2 MPa to 0.6 MPa). In the fixing device 8 shown in Figure 2(a), the spring pressure of the steering roller 308 is released, and the belt 301 is removed along its length. Next, a prescale is placed and fixed over the entire length so that it covers the nip portion N and the curved portion shown in Figures 5(a) and (b). After the prescale is placed, the belt 301 is inserted into the fixing device 8. The spring pressure of the steering roller 308 is then released, and the steering roller 308 applies pressure to the belt 301. After applying pressure, the spring pressure of the steering roller 308 is released again, and the belt 301 is removed. Observation of the recovered prescale reveals that pressure is applied to the curved portion 303a where the pad 303 and belt 301 come into contact, bending the belt 301, causing only the contact area to turn red. The contact area of ​​the prescale after the color change was measured two-dimensionally using a Keyence VR-3200 three-dimensional shape measuring instrument, and this contact area was calculated as the curved portion 303a. It is desirable to set the magnification to 10x or higher during measurement. At this time, as shown in Figure 5(b), if the curved portion 303a exists continuously from the most downstream portion of the nip portion, the start point (the most downstream position of the nip portion N) is defined as R-1, and the end point of the curved portion 303a is defined as RE.

[0049] Next, we will explain how to measure the dimensions of each part of the curved portion 303a and how to calculate the bending strain generated in the belt 301. To calculate the strain generated in the belt 301, the radius of curvature R of the curved portion of the belt 301 is calculated from the shape of the pad 303 at the curved portion 303a. First, the shape of the pad 303 is measured using a three-dimensional shape measuring instrument, VR-3200, manufactured by Keyence Corporation. A magnification of 10x or more is desirable during measurement. The pad 303 is attached to the measuring instrument with the surface on the N-side of the nip portion facing up, and the three-dimensional shape is acquired. The longitudinal position (widthwise position) to be measured is determined, and a shape profile in the pressure direction along the conveyance direction at that longitudinal position is output. From the profile shape, the center position of the section R-1 to RE in the curved portion 303a calculated by the pressure-sensitive paper measurement described above is defined as RC. A circle passing through the three points R-1, RC, and RE is drawn, and its radius of curvature is defined as r. r is measured at three or more locations in each region (Ap region, A-np region), and the average value is taken as the radius of curvature R of the curved portion 303a.

[0050] Finally, the measured radius of curvature R is substituted into the following theoretical formula to calculate the bending strain E. In the calculation, the thickness of the base layer 301a of the belt 301 is defined as t, and t is uniformly set to a fixed value of 0.095 and substituted into the following formula.

number

[0051] 6(a) to 6(d) are cross-sectional views of the nip portion N of the fixing device 8 equipped with the pad 303 of this embodiment. FIG. 6(a) is a schematic cross-sectional view of the stay 302 and the pad 303 cut along the width direction and viewed from the downstream side in the conveying direction. FIG. 6(b) is a cross-sectional view at the Ap-Yc position in FIG. 6(a). FIG. 6(c) is a cross-sectional view enlarging the portion C enclosed by the dotted square in FIG. 6(b) at the A-np-Yn position in FIG. 6(a). FIG. 6(d) is a cross-sectional view enlarging the portion C in FIG. 6(b) at the Ap-Yc position in FIG. 6(a).

[0052] As shown in FIGS. 6(c) and 6(d), in this embodiment, the shape of the pad 303 is set so that the bending strain E at the A-np-Yn position is smaller than the bending strain E at the Ap-Yc position. Here, a region of the curved portion 303a located downstream in the conveying direction of the AP region, which is a passing region through the nip N for recording materials of the maximum size, is defined as a first portion 303a1. Also, a region of the curved portion 303a located downstream in the conveying direction of the A-np region, which is a non-passing region outside the passing region in the width direction (longitudinal direction), is defined as a second portion 303a2. That is, as shown in FIG. 6(c), the region downstream in the conveying direction of the nip N at the A-np-Yn position is defined as the second portion 303a2, and as shown in FIG. 6(d), the region downstream in the conveying direction of the nip N at the Ap-Yc position is defined as the first portion 303a1. The radius of curvature R of first portion 303a1 of curved portion 303a is R1, and the radius of curvature R of second portion 303a2 of curved portion 303a is R2. In this embodiment, the radius of curvature R2 of second portion 303a2 is larger than the radius of curvature R1 of first portion 303a1 (R2>R1).

[0053] Specifically, the radius of curvature R2 of the second portion 303a2 (A-np-Yn position) was set to 8 mm, and the bending strain E was set to 1.2%. The radius of curvature R1 of the first portion 303a1 (Ap-Yc position) was set to 4 mm, and the bending strain E was set to 2.4% (Example 1). The boundary region between the Ap region and the A-np region was shaped so that the radius of curvature R calculated in a cross section cut along the pressure direction gradually increases from the end of the Ap region toward the A-np region in the longitudinal direction (the direction of the arrow in FIG. 6(a)). That is, the curved portion 303a is formed so that the radius of curvature R increases continuously from the first portion 303a1 toward the second portion 303a2. While the above description has been given on one side in the longitudinal direction, a similar shape is also provided for the A-np region on the opposite side in the longitudinal direction.

[0054] [Comparative Example 1] 7(a) to 7(d) are cross-sectional views of the nip portion N of a fixing device 8 equipped with a pad 3030 of Comparative Example 1 for verifying the effects of this embodiment. FIG. 7(a) is a schematic cross-sectional view of the stay 302 and the pad 3030 cut along the width direction and viewed from the downstream side in the conveying direction. FIG. 7(b) is a cross-sectional view at the Ap-Yc position in FIG. 7(a). FIG. 7(c) is a cross-sectional view enlarging the portion D surrounded by the dotted square in FIG. 7(b) at the A-np-Yn position in FIG. 7(a). FIG. 7(d) is a cross-sectional view enlarging the portion D in FIG. 7(b) at the Ap-Yc position in FIG. 7(a).

[0055] In Comparative Example 1, as shown in FIGS. 7(c) and 7(d), the shape of the pad 3030 was set so that the bending strain E at the A-np-Yn position and the bending strain E at the Ap-Yc position, which are caused by the shape of the pad 3030, were equivalent. Also in Comparative Example 1, the region of the curved portion 3030a located downstream of the AP region in the conveying direction is designated as the first portion 3030a1 (FIG. 7(d)). The region of the curved portion 3030a located downstream of the A-np region in the conveying direction is designated as the second portion 3030a2. In Comparative Example 1, the radius of curvature R2 of the second portion 3030a2 is set to be the same as the radius of curvature R1 of the first portion 3030a1 (R2=R1).

[0056] Specifically, the radius of curvature R2 of the second portion 3010a2 (A-np-Yn position) was set to 4 mm, and the bending strain E was set to 2.4%. The radius of curvature R1 of the first portion 3030a1 (Ap-Yc position) was set to 4 mm, and the bending strain E was set to 2.4%. While this description focuses on one side in the longitudinal direction, a similar shape was also provided for the A-np region on the opposite side in the longitudinal direction. As described with reference to FIG. 4, the tension SF from the steering roller 308 is actually higher at the A-np-Yn position than at the Ap-Yc position. Therefore, although the calculated bending strain E is equivalent at the Ap-Yc position and the A-np-Yn position, it can be assumed that the bending strain actually generated at the A-np-Yn position is larger than that at the Ap-Yc position.

[0057] In this embodiment, the radius of curvature R2 of the second portion 303a2 of the curved portion 303a is larger than the radius of curvature R1 of the first portion 303a1. Therefore, the bending strain E of the second portion 303a2 can be reduced compared to Comparative Example 1, in which R2 and R1 are the same. As described above, excessive strain occurs in the belt 301 curved by the curved portion 303a (i.e., the second portion 303a2) in the A-np region, and fatigue failure is likely to occur when the belt 301 rotates. In contrast, in this embodiment, the radius of curvature of the second portion 303a2 is increased, thereby reducing the bending strain E generated in the belt 301 curved by the second portion 303a2 and suppressing fatigue failure of the belt 301. As a result, the life of the belt 301 can be prevented from being shortened.

[0058] <Second embodiment> The second embodiment will be described with reference to Figures 8(a) to 10(d). In the first embodiment described above, a configuration was described in which, in pad 303, curved portion 303a for separating the recording material from belt 301 exists continuously from the most downstream position of nip portion N. In contrast, in this embodiment, in pad 303A, separation portion 303b for separating the recording material from belt 301 has contact portion 303d that is provided discontinuously with downstream end 303c, which is the most downstream position of nip portion N. Since the other configurations and functions are the same as those of the first embodiment described above, the same reference numerals are used for the same configurations, and description and illustration will be omitted or simplified. The following description will focus on the points that are different from the first embodiment.

[0059] In this embodiment, the pad 303A serving as a pad member is provided downstream of the nip portion N in the recording material conveyance direction, discontinuously from the downstream end 303c of the nip portion N. The pad 303A has a contact portion 303d that brings the sliding member 304 into contact with the belt 301 or that comes into direct contact with the belt 301. That is, the pad 303A of this embodiment has a step downstream of the nip portion N. The downstream end 303c of the nip portion N and the contact portion 303d form a separation portion 303b that curves the belt 301 so that the recording material that has passed through the nip portion N separates from the belt 301. The downstream end 303c of the nip portion N may be the downstream end of the protrusion 304b that is located furthest downstream in the conveyance direction. If the separation portion 303b does not have the sliding member 304, the contact portion 303d comes into direct contact with the belt 301, thereby curving the belt 301.

[0060] Next, using FIGS. 8(a) to 8(c), a method for calculating the strain generated in the belt 301 when the belt 301 is bent to a specific radius of curvature at the separation portion 303b downstream of the nip portion N will be described. The method for calculating the strain is the same as that described in the first embodiment. First, the separation portion 303b, where the pad 303A and the belt 301 come into contact and bend the belt 301, is measured with pressure-sensitive paper to identify the area of ​​the separation portion 303b. Next, the dimensions of each portion are measured according to the shape of the area. Finally, the measured values ​​are substituted into a theoretical formula to calculate the strain at each portion. FIG. 8(a) shows a cross-sectional view of the belt 301, stay 302, and pad 303A around the nip portion N. FIG. 8(b) is an enlarged view of the area E surrounded by the dotted line in FIG. 8(a).

[0061] As in the first embodiment, the separation portion 303b was measured using a three-dimensional shape measuring machine, Keyence VR-3200, and a pressure-sensitive paper prescale, Fujifilm Corporation. The measurement method was the same as in the first embodiment. The contact area (the area where the color changed) of the collected prescale was then measured two-dimensionally using the three-dimensional shape measuring machine, Keyence VR-3200, and this contact area was calculated as the separation portion 303b. It is desirable to set the magnification at 10 times or more during measurement.

[0062] In this case, as shown in FIG. 8(b), if there are one or more discontinuous contact portions 303d of the separation portion 303b separate from the downstream end 303c of the nip portion N, the starting point (the most downstream position of the nip portion N) is defined as R-1, the multiple regions of the platen where the color changes (multiple contact portions 303d) are defined as 303b-2, 303b-3, ... 303b-E, in order from the most downstream position of the nip portion N, and the rotational center positions of each region are defined as R-2, R-3, ... RE. Note that the position of R-1 may also be the downstream end of the embossed portion 304d in the nip portion N that is the most downstream in the conveying direction. In the example shown in FIG. 8(b), the separation portion 303b is configured with two locations: R-1 at the downstream end 303c of the nip portion N and RE at the contact portion 303d. However, the same applies if there are one or more contact portions between R-1 and RE.

[0063] Next, we will explain how to measure the dimensions of each part of separation portion 303b and how to calculate the bending strain generated in belt 301. The conveying direction distance ΔX and pressure direction distance ΔZ of the part that curves belt 301 are calculated from the shape of pad 303A in separation portion 303b. Here, the conveying direction distance ΔX is the distance between downstream end 303c of nip portion N and contact portion 303d in the conveying direction. The pressure direction distance ΔZ is the distance between the downstream end of nip portion N and contact portion 303d in the pressure direction. As described above, the pressure direction is the direction in which pressure is applied to the recording material at nip portion N, and is a direction perpendicular to both the conveying direction and the width direction.

[0064] The pressure direction distance ΔZ is the distance between the downstream end of the nip portion N and the contact portion 303d in the pressure direction. However, the nip portion is coated with a sliding layer 304c to improve sliding performance with the belt. The pressure direction distance ΔZ may be the distance from the tip of the sliding layer to the contact portion 303d, or the distance excluding the sliding layer 304c.

[0065] The sliding layer 304c is worn down by sliding with the belt. Therefore, it is preferable that the distance from the tip of the protrusion 304b is set so that the pressure direction distance ΔZ is guaranteed even after the fixing device has been used a predetermined number of times. On the other hand, it is also preferable that the pressure direction distance ΔZ is set to the distance including the sliding layer so that the pressure direction distance ΔZ is guaranteed even when the fixing device is new.

[0066] To calculate the conveying direction distance ΔX and the pressure direction distance ΔZ, first, the shape of the pad 303A was measured using a Keyence VR-3200 three-dimensional shape measuring instrument. A magnification of 10x or greater is desirable during measurement. The pad was attached to the measuring instrument with the nip N side facing up, and the three-dimensional shape was acquired. The longitudinal position (widthwise position) to be measured was determined, and a shape profile in the pressure direction along the conveying direction at that longitudinal position was output. From the profile shape, the coordinates of R-1 and RE in the separation section 303b calculated using the pressure-sensitive paper measurement described above were confirmed. Based on these coordinates, the conveying direction distance Δx and the pressure direction distance Δz shown in Figure 8(b) were calculated. Measurements were taken at three or more locations in each region (Ap region, A-np region), and the average values ​​were used as the conveying direction distance ΔX and the pressure direction distance ΔZ.

[0067] Finally, the measured conveyance direction distance ΔX and pressure direction distance ΔZ are substituted into the following theoretical formula to calculate the bending strain E. In the calculation, the thickness of the base layer 301a of the belt 301 is defined as t, and t is uniformly set to a fixed value of 0.095 and substituted into the following formula.

number

[0068] 9(a) to 9(d) are cross-sectional views of the nip portion N of the fixing device 8 equipped with the pad 303A of this embodiment. FIG. 9(a) is a schematic cross-sectional view of the stay 302 and the pad 303A cut along the width direction and viewed from the downstream side in the conveying direction. FIG. 9(b) is a cross-sectional view taken at the Ap-Yc position in FIG. 9(a). FIG. 9(c) is a cross-sectional view enlarging the portion F enclosed by the dotted square in FIG. 9(b) at the A-np-Yn position in FIG. 9(a). FIG. 9(d) is a cross-sectional view enlarging the portion F in FIG. 9(b) at the Ap-Yc position in FIG. 9(a).

[0069] As shown in FIGS. 9C and 9D, in this embodiment, the shape of the pad 303A is set so that the bending strain E at the A-np-Yn position is smaller than the bending strain E at the Ap-Yc position. Here, in the separation section 303b, a region located downstream in the conveying direction of the AP region, which is a passing region through the nip N for recording materials of the maximum size, is defined as a first portion 303b1. In addition, in the separation section 303b, a region located downstream in the conveying direction of the A-np region, which is a non-passing region outside the passing region in the width direction (longitudinal direction), is defined as a second portion 303b2. That is, as shown in FIG. 9C, the region downstream in the conveying direction of the nip N at the A-np-Yn position is defined as the second portion 303b2, and as shown in FIG. 9D, the region downstream in the conveying direction of the nip N at the Ap-Yc position is defined as the first portion 303b1. The pressure direction distance ΔZ of the first portion 303b1 of the separation portion 303b is ΔZ1, and the pressure direction distance ΔZ of the second portion 303b2 of the separation portion 303b is ΔZ2. In this embodiment, the pressure direction distance ΔZ2 of the second portion 303b2 is shorter than the pressure direction distance ΔZ1 of the first portion 303b1 (ΔZ2<ΔZ1). The pressure direction distance ΔZ is preferably greater than 0 mm and equal to or less than 1.5 mm.

[0070] Specifically, the conveying direction distance ΔX of the second portion 303b2 (A-np-Yn position) was set to 1.2 mm, the pressure direction distance ΔZ2 was set to 0.2 mm, and the bending strain E was set to 1.3%. Furthermore, the conveying direction distance ΔX of the first portion 303b1 (Ap-Yc position) was set to 1.2 mm, the pressure direction distance ΔZ2 was set to 0.9 mm, and the bending strain E was set to 4.8% (Example 2). That is, the conveying direction distance ΔX of the second portion 303b2 and the conveying direction distance ΔX of the first portion 303b1 were set to be the same. Furthermore, the shape of the boundary region between the Ap region and the A-np region was determined so that the pressure direction distance ΔZ calculated in a cross section taken along the pressure direction continuously decreases as one moves from the end of the Ap region toward the A-np region in the longitudinal direction (the direction of the arrow in FIG. 9(a)). That is, the separation portion 303b is formed so that the pressure direction distance ΔZ continuously decreases from the first portion 303b1 toward the second portion 303b2. Note that, although the above description has been given on one side in the longitudinal direction, a similar shape is also provided for the A-np region on the opposite side in the longitudinal direction.

[0071] Comparative Example 2 10(a) to 10(d) are cross-sectional views of the nip portion N of the fixing device 8 equipped with the pad 3031 of Comparative Example 2 for verifying the effects of this embodiment. FIG. 10(a) is a schematic cross-sectional view of the stay 302 and the pad 3031 cut along the width direction and viewed from the downstream side in the conveying direction. FIG. 10(b) is a cross-sectional view taken at the Ap-Yc position in FIG. 10(a). FIG. 10(c) is a cross-sectional view enlarging the portion G enclosed by the dotted square in FIG. 10(b) at the A-np-Yn position in FIG. 10(a). FIG. 10(d) is a cross-sectional view enlarging the portion G in FIG. 10(b) at the Ap-Yc position in FIG. 10(a).

[0072] In Comparative Example 2, as shown in Figures 10(c) and (d), the shape of the pad 3031 was set so that the bending strain E at the A-np-Yn position and the bending strain E at the Ap-Yc position, which are caused by the shape of the pad 3031, were equivalent. Also in Comparative Example 2, the region of the separation section 3031b located downstream of the AP region in the transport direction is defined as the first section 3031b1 (Figure 10(d)). Also, the region of the separation section 3031b located downstream of the A-np region in the transport direction is defined as the second section 3031b2. In Comparative Example 2, the pressure direction distance ΔZ2 of the second section 3031b2 is set to be the same as the pressure direction distance ΔZ1 of the first section 3031b1 (ΔZ2 = ΔZ1).

[0073] Specifically, the conveying direction distance ΔX of the second portion 3011b2 (A-np-Yn position) was set to 1.2 mm, the pressure direction distance ΔZ2 was set to 0.9 mm, and the bending strain E was set to 4.8%. The conveying direction distance ΔX of the first portion 3011b1 (Ap-Yc position) was set to 1.2 mm, the pressure direction distance ΔZ1 was set to 0.9 mm, and the bending strain E was set to 4.8%. While this description focuses on one side of the longitudinal direction, a similar shape was also provided for the A-np region on the opposite side of the longitudinal direction. As described with reference to FIG. 4, the tension SF from the steering roller 308 is actually higher at the A-np-Yn position than at the Ap-Yc position. Therefore, although the calculated bending strain E is equivalent at the Ap-Yc position and the A-np-Yn position, it can be assumed that the bending strain occurring at the A-np-Yn position is actually larger than that occurring at the Ap-Yc position.

[0074] In this embodiment, the pressure direction distance ΔZ2 of the second portion 303b2 of the separation portion 303b is shorter than the pressure direction distance ΔZ1 of the first portion 303b1. Therefore, the bending strain E of the second portion 303b2 can be reduced compared to Comparative Example 2, in which ΔZ2 and ΔZ1 are the same. As described above, excessive strain occurs in the belt 301 curved by the separation portion 303b (i.e., the second portion 303b2) in the A-np region, and fatigue failure is likely to occur when the belt 301 rotates. In contrast, in this embodiment, the pressure direction distance of the second portion 303b2 is shortened, thereby reducing the bending strain E generated in the belt 301 curved by the second portion 303b2 and suppressing fatigue failure of the belt 301. As a result, the life of the belt 301 can be prevented from being shortened.

[0075] [verification] A belt durability evaluation test and a paper (recording material) separation performance evaluation test were conducted to verify the effectiveness of Examples 1 and 2 and Comparative Examples 1 and 2 described in the first and second embodiments. Below, the procedures for each verification test will be explained, and then the verification results using Examples 1 and 2 and Comparative Examples 1 and 2 will be explained.

[0076] [Belt durability evaluation test procedure] In the belt durability evaluation test, a fixing device 8 equipped with the pads of Examples 1 and 2 and Comparative Examples 1 and 2 was installed in a printer (product name: Canon Inc. imagePRESS V1000), and the life of the belt 301 was evaluated. The peripheral speed of the pressure roller 305 installed in the fixing device 8 was set to 450 mm / sec, and the temperature control temperature of the halogen heater 306 was set to 190°C.

[0077] In the belt durability evaluation test, 10,000 sheets of paper with a predetermined image formed on them were printed continuously, the printer was shut down, the belt 301 was checked, and then 10,000 sheets of paper with the predetermined image formed on them were printed continuously again. This procedure was repeated. The number of sheets printed until cracks appeared in the belt 301 was defined as the lifespan (number of sheets that could be printed). Damage to the widthwise ends of the belt 301 was determined by visual inspection of the widthwise ends of the belt 301. A lifespan exceeding 3,000K (3,000 x 1,000) sheets was defined as having achieved the target. The temperature and humidity conditions during measurement were 23°C and 30%. The base layer 301a of the belt 301 used in this test was made of polyimide and had a thickness of 95 μm. A4-size CS-680 (Canon Inc.) paper was used.

[0078] [Paper separation performance evaluation test procedure] In the paper separation performance evaluation test, a fixing device 8 equipped with the pads of Examples 1 and 2 and Comparative Examples 1 and 2 was installed in a printer (product name: Canon Inc. imagePRESS V1000), and the paper conveying capacity of the fixing device 8 of each example was evaluated. The peripheral speed of the pressure roller 305 installed in the fixing device 8 was set to 450 mm / sec, and the temperature control temperature of the halogen heater 306 was set to 190°C. In the paper separation performance evaluation test, 20 sheets of paper carrying images with the maximum amount of toner were passed through the nip N in succession to confirm that no conveyance problems occurred. No conveyance problems were defined as having achieved the target. The temperature and humidity conditions during measurement were 30°C and 80%. The base layer 301a of the belt 301 used in this test was made of polyimide and had a thickness of 95 μm. The paper used was OK topcoat+ with a basis weight of 73 g / m. 2 (Oji Paper Co., Ltd.) A4 size was used.

[0079] [Verification result 1] Figure 11 shows the results of a belt durability evaluation test and a paper separation performance evaluation test conducted for Example 1 and Comparative Example 1. In Figure 11, the evaluation "Good" indicates a lifespan exceeding 3000K (3000 x 1000) sheets, while "Poor" indicates a lifespan of 3000K (3000 x 1000) sheets or less. In addition, the evaluation "Good" in the paper separation performance evaluation test indicates a case where no paper transport problems occurred.

[0080] The results of the belt durability evaluation test showed that Example 1 could withstand 10 million sheets, while Comparative Example 1 could withstand 3 million sheets. This is thought to be because the bending strain of the second portion 303a2 downstream of the A-np region in Example 1 was smaller than that in Comparative Example 1. Also, while cracks occurred at the widthwise ends of the belt 301 in Comparative Example 1, cracks occurred in the widthwise center of the belt 301 in Example 1. It is presumed that the bending strain at the widthwise ends (A-np region) of the belt 301 in Example 1 was reduced, which relatively increased the strain in the widthwise center, causing cracks in the center. Furthermore, in the paper separation performance evaluation test, both functions were met, confirming that changing the shape of the second portion of the pad did not affect the recording material conveyance function.

[0081] [Verification result 2] FIG. 11 shows the results of a belt durability evaluation test and a paper separation performance evaluation test for Example 2 and Comparative Example 2. The results of the belt durability evaluation test showed that Example 2 achieved a durability of 8 million sheets, while Comparative Example 2 achieved a durability of 1.2 million sheets. This is thought to be because the bending strain of the second portion 303b2 downstream of the A-np region in Example 2 was smaller than that in Comparative Example 2. Furthermore, while cracks occurred in the belt at the widthwise ends (A-np region) of the belt 301 in Comparative Example 2, cracks occurred in the widthwise center of the belt 301 in Example 2. It is presumed that the bending strain at the widthwise ends (A-np region) of the belt 301 in Example 2 was reduced, which relatively increased the strain in the widthwise center, resulting in the cracks in the center. Furthermore, in the paper separation performance evaluation test, both functions were met, confirming that changing the shape of the second portion of the pad did not affect the recording material conveyance function.

[0082] As described above, it was found that in Examples 1 and 2, the life of belt 301 can be shortened compared to Comparative Examples 1 and 2 without degrading the paper separation performance. That is, according to the first and second embodiments, the life of belt 301 can be shortened without degrading the recording material separation performance.

[0083] <Other embodiments> In the above-described embodiments, the pads 303 and 303A are configured as a single unit, but the pads 303 and 303A may be configured as multiple members. For example, the pads 303 and 303A may be separated into two separate members. Furthermore, in the above-described embodiments, the configuration of the pad 303 downstream of the nip portion N in the transport direction has been described, but the pad 303 upstream of the nip portion N in the transport direction may also have the same configuration as the downstream pad.

[0084] In addition, in each of the above-described embodiments, a configuration in which tension is applied to belt 301 by steering roller 308 as a tension applying member has been described, but a configuration in which pads and rollers are arranged to apply tension to the belt may also be used without such a tension applying member. For example, the present invention is also applicable to a configuration in which belt 301 is stretched by only heating roller 307 and pads 303 and 303A. [Explanation of symbols]

[0085] 8. Fixing device 301 Belt 301a...Base layer 303, 303A Pad (backup member) 303a···Bend 303a1...Part 1 303a2...Part 2 303b...Separation part 303b1...Part 1 303b2...Part 2 303c...downstream end 303d...Contact part 304...Sliding member 304b...Protrusion 304c···Sliding layer 305 Pressure roller (rotating body) 306 Halogen heater (heat source) 307 Heating roller 308 Steering roller (tensioning member)

Claims

1. A fixing device that fixes a toner image carried on a recording material to the recording material, an endless rotatable belt; a rotating body that rotates in contact with the outer peripheral surface of the belt; a pad member disposed inside the belt so as to sandwich the belt between the belt and the rotary body, the pad member forming a nip portion between the belt and the rotary body for sandwiching and conveying a recording material; a sliding member provided between the pad member and the belt, the sliding member sliding on an inner circumferential surface of the belt at the nip portion; a heating roller disposed inside the belt and configured to heat the belt; the pad member has a curved portion, continuous with the downstream end of the nip portion on the downstream side of the nip portion in the conveying direction of the recording material, that causes the sliding member to contact the belt or that curves so as to be in direct contact with the belt, and that curves the belt so that the recording material that has passed through the nip portion is separated from the belt; the curved portion has a first portion located downstream in the conveying direction of a passing area through which a maximum size recording material passes through the nip portion, and a second portion located downstream in the conveying direction of a non-passing area that is outside the passing area in a width direction of the recording material that intersects the conveying direction, The radius of curvature of the second portion is greater than the radius of curvature of the first portion. A fixing device characterized by:

2. The curved portion is formed so that the radius of curvature increases continuously from the first portion toward the second portion.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

3. The belt has a base layer, When the thickness of the base layer is t, the radius of curvature of the curved portion is R, and E calculated by the following formula is the bending strain, [Equation 1] The bending strain E of the second portion is smaller than the bending strain E of the first portion.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

4. the sliding member has a plurality of protrusions provided on the side that slides against the belt so as to protrude toward the inner circumferential surface of the belt, The plurality of protrusions are distributed in the width direction.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

5. The plurality of protrusions are distributed in the passing region and the non-passing region in the width direction.

5. The fixing device according to claim 4.

6. The sliding member has a sliding layer that covers the surface that slides against the belt and includes the plurality of protrusions.

5. The fixing device according to claim 4.

7. The belt further includes a tension applying member for applying tension to the belt.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

8. The tension applying member is a steering roller that is disposed inside the belt, stretches the belt, and controls the position of the belt in the width direction.

8. The fixing device according to claim 7,

9. The rotating body is a pressure roller that presses the belt toward the sliding member.

2. The fixing device according to claim 1, wherein the fixing device is a fixing device for fixing a toner image onto a recording medium.

10. A fixing device that fixes a toner image carried on a recording material to the recording material, an endless rotatable belt; a rotating body that rotates in contact with the outer peripheral surface of the belt; a pad member disposed inside the belt so as to sandwich the belt between the belt and the rotary body, the pad member forming a nip portion between the belt and the rotary body for sandwiching and conveying a recording material; a sliding member provided between the pad member and the belt, the sliding member sliding on an inner circumferential surface of the belt at the nip portion; a heating roller disposed inside the belt and configured to heat the belt; the pad member is provided on the downstream side of the nip portion in the conveying direction of the recording material, discontinuously from the downstream end of the nip portion, and has a contact portion that brings the sliding member into contact with the belt or that comes into direct contact with the belt; a separation portion that bends the belt so that the recording material that has passed through the nip portion is separated from the belt, the separation portion being formed by the downstream end of the nip portion and the contact portion; the separation section has a first section located downstream in the conveying direction of a passing area through which the maximum size recording material passes the nip section, and a second section located downstream in the conveying direction of a non-passing area that is outside the passing area in a width direction of the recording material that intersects the conveying direction, a direction perpendicular to the conveying direction and the width direction is defined as a pressure direction in which the recording material is pressed at the nip portion; When the distance from the downstream end of the nip portion to the contact portion in the pressure direction is defined as a pressure direction distance, The distance in the pressure direction of the second portion is shorter than the distance in the pressure direction of the first portion. A fixing device characterized by:

11. When the distance between the downstream end of the nip portion and the contact portion in the conveying direction is defined as a conveying direction distance, The distance in the conveying direction of the second portion is the same as the distance in the conveying direction of the first portion. The fixing device according to claim 10 .

12. The separation portion is formed such that the pressure direction distance continuously decreases from the first portion toward the second portion. The fixing device according to claim 10 .

13. The belt has a base layer, When the thickness of the base layer is t, the pressure direction distance is ΔZ, the conveying direction distance which is the distance between the downstream end of the nip portion and the contact portion in the conveying direction is ΔX, and E calculated by the following formula is the bending strain: [Equation 2] The bending strain E of the second portion is smaller than the bending strain E of the first portion. The fixing device according to claim 10 .

14. the sliding member has a plurality of protrusions provided on the side that slides against the belt so as to protrude toward the inner circumferential surface of the belt, The downstream end of the nip portion is the downstream end of the protrusion that is located furthest downstream in the conveying direction among the plurality of protrusions.

14. The fixing device according to claim 13.

15. the sliding member has a plurality of protrusions provided on the side that slides against the belt so as to protrude toward the inner circumferential surface of the belt, The plurality of protrusions are distributed in the width direction. The fixing device according to claim 10 .

16. The plurality of protrusions are distributed in the passing region and the non-passing region in the width direction.

16. The fixing device according to claim 15.

17. The sliding member has a sliding layer that covers a surface that slides against the belt and includes the plurality of protrusions, and the pressure direction distance is the distance from the sliding layer that covers the protrusions located at the downstream end of the nip portion to the contact portion.

16. The fixing device according to claim 15.

18. The belt further includes a tension applying member for applying tension to the belt. The fixing device according to claim 10 .

19. The tension applying member is a steering roller that is disposed inside the belt, stretches the belt, and controls the position of the belt in the width direction.

19. The fixing device according to claim 18.

20. The rotating body is a pressure roller that presses the belt toward the sliding member. The fixing device according to claim 10 .

21. The sliding member has a sliding layer that covers a surface that slides against the belt and includes the plurality of protrusions, and the pressure direction distance is a distance from the protrusion located at the downstream end of the nip portion to the contact portion.

16. The fixing device according to claim 15.

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