Fixing device

Abstract

To suppress formation of wrinkles on a recording material, without causing a difference in fixability of a toner image with respect to the recording material, within a fixing nip part.SOLUTION: A fixing pad 303 is formed in a protrusion shape in which a central part in a width direction, of a downstream side rubbing surface 303b protrudes toward a downstream side in a conveyance direction with respect to both ends thereof, the downstream side rubbing surface rubbing against a fixing belt downstream of a fixing nip part N. A travel direction and conveyance force of the fixing belt change according to a protrusion amount of the downstream side rubbing surface 303b. Use of the fixing pad 303 directs a travel direction of the fixing belt, when passing the fixing nip part N, outward with respect to the center as a reference, causing a recording material P to be conveyed while being pulled toward both end sides from the center following the travel direction of the fixing belt. Further, the downstream side rubbing surface 303b hardly influences pressure distribution at the fixing nip part N. Accordingly, formation of wrinkles on the recording material can be suppressed, without causing a difference in fixability of a toner image with respect to the recording material, between at the central part and end parts in the width direction at the fixing nip part N.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a fixing device suitable for use in an image forming apparatus that utilizes electrophotographic technology, such as a printer, a copying machine, a facsimile machine, or a multifunction machine. [Background technology]

[0002] The image forming apparatus is equipped with a fixing device that applies heat and pressure to a recording material on which a toner image has been formed to fix the toner image to the recording material. The fixing device has, for example, a rotating endless belt, a roller that contacts the outer surface of the belt, and a pad that rubs against the inner surface of the belt. The pad sandwiches the belt between itself and the roller, and forms a fixing nip where heat and pressure are applied to fix the toner image while nipping and transporting the recording material.

[0003] Conventionally, a device has been proposed in which the fixing nip portion is shaped so that both ends in the width direction protrude further upstream in the conveyance direction of the recording material than the center portion (Patent Document 1). With this shape of the fixing nip portion, the pressure distribution in the fixing nip portion is greater at both ends in the width direction than at the center portion, and as the recording material enters the fixing nip portion, a difference in relative speed occurs between the end portion (entry portion) and the center portion (non-entry portion) of the recording material. As a result, the recording material is stretched from the center portion to the end portion as it passes through the fixing nip portion, making it less likely to wrinkle. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-292948 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in the past, in the fixing nip portion where the pressure distribution differs in the width direction, there was a risk of differences in the fixability of the toner image to the recording material between the center and ends in the width direction, which could result in poor fixing on the recording material.

[0006] The present invention has been made in consideration of the above problems, and aims to provide a fixing device that can suppress the occurrence of wrinkles on the recording material without causing a difference in the fixability of the toner image to the recording material between the center and end portions in the width direction of the fixing nip portion. [Means for solving the problem]

[0007] A fixing device according to one embodiment of the present invention comprises a rotating endless belt, a rotating body that contacts the outer peripheral surface of the belt, and a nip forming member that is arranged inside the belt sandwiching the rotating body and the belt, and that forms a nip that applies heat and pressure to fix the toner image to the recording material while sandwiching and transporting the recording material on which a toner image has been formed, and is characterized in that the nip forming member has a downstream side friction surface that rubs against the inner peripheral surface of the belt downstream of the nip portion in the transport direction of the recording material, among its friction surfaces that rub against the inner peripheral surface of the belt, and the downstream side friction surface is formed in a convex shape with its central portion protruding downstream in the transport direction more than both end portions in the width direction that intersects the transport direction.

[0008] A fixing device according to one embodiment of the present invention comprises a rotating endless belt, a rotating body that contacts the outer peripheral surface of the belt, and a nip forming member that is arranged inside the belt sandwiching the rotating body and the belt, and that forms a nip that applies heat and pressure to fix the toner image to the recording material while sandwiching and transporting the recording material on which a toner image has been formed, and the nip forming member has an upstream rubbing surface that rubs against the inner peripheral surface of the belt upstream of the nip surface in the transport direction of the recording material, and the upstream rubbing surface is formed in a concave shape with the center recessed further downstream in the transport direction than both end portions in the width direction that intersects the transport direction.

[0009] A fixing device according to one embodiment of the present invention comprises a rotating endless belt, a rotating body that contacts the outer peripheral surface of the belt, and a nip portion-forming member that is arranged inside the belt, sandwiching the rotating body and the belt, and that applies heat and pressure to form a nip portion in which a recording material on which a toner image has been formed is fixed to the recording material while sandwiching and transporting the recording material. The nip portion-forming member has a friction surface that rubs against the inner peripheral surface of the belt, the friction surface having a downstream side that rubs against the inner peripheral surface of the belt downstream of the nip portion in the transport direction of the recording material, and an upstream side that rubs against the inner peripheral surface of the belt upstream of the nip portion, the downstream side friction surface being formed in a convex shape with a central portion that protrudes downstream in the transport direction relative to both end portions in a width direction that intersects the transport direction, and the upstream side friction surface being formed in a concave shape with a central portion that is recessed downstream in the transport direction relative to both end portions in the width direction. [Effects of the Invention]

[0010] According to the present invention, it is possible to prevent wrinkles from occurring on the recording material without causing a difference in fixability of the toner image on the recording material between the central portion and the end portions in the width direction of the fixing nip portion. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram showing an image forming apparatus suitable for using the fixing device of the present embodiment. [Figure 2] FIG. [Figure 3] FIG. 4 is a view of the fixing pad of the first embodiment as seen from the opposite side to the pressure direction. [Figure 4] FIG. 4 is a diagram showing the running direction of a fixing belt. [Figure 5] 5A and 5B are diagrams illustrating the mechanism by which wrinkles occur in a recording material. [Figure 6] 10A and 10B are diagrams showing examples of wrinkles that occur on a recording material. [Figure 7] FIG. 10 is a view of the fixing pad of the second embodiment as viewed from the opposite side of the pressure direction. [Figure 8] FIG. 11 is a view of the fixing pad of the third embodiment as viewed from the opposite side of the pressure direction. DETAILED DESCRIPTION OF THE INVENTION

[0012] The fixing device of this embodiment will be described below: First, the schematic configuration of an image forming apparatus suitable for using the fixing device of this embodiment will be described with reference to FIG.

[0013] [First embodiment] <Image forming device> The 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 apparatus 1 is a tandem type in which the image forming units Pa, Pb, Pc, and Pd are arranged along the rotation direction of an intermediate transfer belt 204, which will be described later. The image forming apparatus 1 forms a toner image (image) on a recording material in response to an image signal from a document reading device 2 connected to a device main body 3 of the image forming apparatus 1 or from a host device such as a personal computer connected to the device main body 3 so as to be able to communicate with the device main body 3. Examples of the recording material include sheet materials such as paper, plastic film, and cloth.

[0014] As shown in FIG. 1, image forming apparatus 1 includes document reading device 2 and device main body 3. Document reading device 2 reads a document placed on document glass 21. Light emitted from light source 22 is reflected by the document and forms an image on CCD sensor 24 via optical components 23, such as lenses. When this optical unit is scanned in the direction indicated by the arrow under the control of a reader control unit, it reads the document line by line and converts it into an electrical signal data stream. The image signal obtained by CCD sensor 24 is sent to device main body 3, where it is processed by control unit 30 in accordance with the image forming units described below. Control unit 30 also receives external inputs as image signals from external host devices, such as a print server.

[0015] The 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.

[0016] Image forming unit Pa forms a toner image of yellow (Y), image forming unit Pb forms a toner image of magenta (M), image forming unit Pc forms a toner image of cyan (C), and image forming unit Pd forms a toner image of black (Bk), respectively. Since these image forming units Pa to Pd have substantially the same configuration, the following description will be given taking image forming unit Pa, which forms a toner image of yellow (Y), as an example, and will not include a description of the other image forming units Pb to Pd. In image forming unit Pa, a toner image is formed on the surface of photosensitive drum 200a based on an image signal, as will be described below.

[0017] 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.

[0018] 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 secondary transfer station T2, which is composed of a pair of secondary transfer rollers 205 and 206. At secondary transfer station T2, 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 from intermediate transfer belt 204 to the recording material.

[0019] 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 T2.

[0020] The recording material onto which the toner image has been transferred at secondary transfer section T2 is transported to fixing device 8, where it is heated and pressed, thereby fixing the toner image carried on the recording material to the recording material. The recording material that has passed through fixing device 8 is discharged onto discharge tray 7. When forming images on both sides of the recording material, after the toner image has been transferred and fixed to the front side of the recording material, the recording material is turned over via reversing conveyance section 10, and the toner image is transferred and fixed to the back side of the recording material, and then the recording material is stacked on discharge tray 7.

[0021] 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.

[0022] <Fixing device> Next, the configuration of the fixing device 8 in this embodiment will be described with reference to FIG. 2. In this embodiment, a fixing device of a belt heating type using an endless belt is adopted. As shown in FIG. 2, in the fixing device 8, the recording material is conveyed in a conveying direction (direction of arrow X). In this specification, the width direction refers to a direction intersecting the conveying direction of the recording material in the fixing nip portion N, in other words, the direction of the rotation axis of the pressure roller 305.

[0023] The fixing device 8 includes an endless rotatable fixing belt 301 , a pressure roller 305 that contacts the outer peripheral surface of the fixing belt 301 , a heating roller 307 , a tension roller 308 , and a fixing pad unit 300 .

[0024] <Fixing belt> The fixing belt 301 is stretched between a heating roller 307, a tension roller 308, and a fixing pad unit 300. The fixing belt 301 has thermal conductivity, heat resistance, and the like, and is formed in a thin cylindrical shape. The fixing belt 301 has a three-layer structure, with a base layer, an elastic layer, and a release layer formed on a substrate such as rubber, in that order from the inner periphery. As an example, the base layer is 80 μm thick and made of polyimide resin (PI), the elastic layer is 300 μm thick and made of silicone rubber, and the release layer is 30 μm thick and made of tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin (PFA). In this embodiment, the Young's modulus of the base layer is set to 5.0 (GPa), and the Young's modulus of the elastic layer is set to 0.4 MPa. The outer diameter of the fixing belt 301 is set to, for example, 150 mm.

[0025] <Heating roller> Heating roller 307 is disposed on the inner circumferential side of fixing belt 301, and stretches fixing belt 301 together with fixing pad unit 300. Heating roller 307 is formed into a cylindrical shape from a metal such as aluminum or stainless steel with a thickness of, for example, 1 mm, and has a halogen heater 306 disposed inside as a heat source for heating fixing belt 301. Heating roller 307 is heated to a predetermined temperature by halogen heater 306, and thereby fixing belt 301 is heated by heating roller 307.

[0026] The heating roller 307 has a rotation center at one end or near the center in the direction of the rotation axis (width direction) and is provided so as to be able to swing relative to the fixing belt 301. By swinging the heating roller 307, the position (shift position) of the fixing belt 301 in the direction of the rotation axis is controlled. The heating roller 307 is also biased by a spring supported by the fixing device 8, and serves as a tension roller that applies a predetermined tension to the fixing belt 301.

[0027] <Pressure roller> Pressure roller 305 as a rotating body is rotatably supported by fixing device 8, has a gear (not shown) fixed to one end in the width direction, and is connected to a drive source (not shown) such as a motor via the gear to be rotated. When pressure roller 305 rotates, frictional force generated in a fixing nip portion N (described later) transmits the rotational force of pressure roller 305 to fixing belt 301, and fixing belt 301 rotates following pressure roller 305.

[0028] The pressure roller 305 is a roller in which an elastic layer and a release layer are formed on a cylindrical core metal 305c. As an example, the core metal 305c is made of stainless steel (SUS) with a diameter of 72 mm, the elastic layer is made of conductive silicone rubber with a thickness of 8 mm, and the release layer is made of PFA with a thickness of 100 μm. The outer diameter of the pressure roller 305 is set to, for example, φ80 mm.

[0029] The pressure roller 305 is in contact with the outer peripheral surface of the fixing belt 301 so as to sandwich the fixing belt 301 between the pressure roller 305 and a fixing pad 303 (described later). The pressure roller 305 presses the fixing pad 303 (in the direction of arrow Z) by a drive source (not shown). In the case of this embodiment, the pressure roller 305 presses the fixing belt 301 toward the fixing pad 303 so that, for example, the pressure at the fixing nip portion N is "160 kgf," and the length of the fixing nip portion N in the conveying direction is "24.5 mm," and the length in the width direction is "326 mm."

[0030] <Fixing pad unit> The fixing pad unit 300 is disposed on the inner periphery of the fixing belt 301 and includes a fixing stay 302, a fixing pad 303, and a sliding member 304. The fixing stay 302 is a rigid member made of, for example, metal that extends in the width direction of the fixing belt 301, and supports the fixing pad 303 on the pressure roller 305 side. In this embodiment, the fixing belt 301 is wound around the fixing pad 303 supported by the fixing stay 302, along with a heating roller 307 and a tension roller 308, and the fixing belt 301 is sandwiched between the fixing pad 303 and the pressure roller 305, and the fixing pad 303 is pressed against the pressure roller 305. As a result, a wide fixing nip portion N is formed between the pressure roller 305 and the fixing belt 301, ensuring sufficient length in both the conveyance direction and the width direction. In the fixing nip portion N, a recording material carrying an unfixed toner image is sandwiched and conveyed, and heat and pressure are applied to the recording material during this process, thereby fixing the toner image to the recording material.

[0031] <Fixing pad> The fixing pad 303, which serves as a nip portion forming member, is non-rotatably provided on the inner peripheral side of the fixing belt 301 and presses against the inner peripheral surface of the fixing belt 301. The fixing pad 303 is a resin member that extends in the width direction, and its width length is longer than the width length of the largest recording material on which an image can be formed. The fixing pad 303 is formed by injection molding using a mold from a resin with good insulating and heat resistance, such as a liquid crystal polymer resin. In order to form the fixing nip portion N in the width direction, it is preferable that the side of the fixing pad 303 that sandwiches the fixing belt 301 between itself and the pressure roller 305 is curved in the width direction so that the center portion is curved more toward the pressure roller 305 than the ends.

[0032] In order to reduce the friction between the fixing belt 301 and the fixing pad 303, the fixing pad 303 is provided with a sliding member 304 that slides on the fixing belt 301. The sliding member 304 is disposed opposite the pressure roller 305 with the fixing belt 301 sandwiched therebetween. The surface of the sliding member 304 that slides on the fixing belt 301 has an embossed shape with irregularities of several microns. A lubricant such as silicone oil may be applied to the inner circumferential surface of the fixing belt 301 to allow the fixing belt 301 to slide smoothly on the sliding member 304. The sliding member 304 may be fixed to the fixing stay 302 or to the fixing pad 303. Alternatively, the sliding member 304 and the fixing pad 303 may be integrally configured.

[0033] Next, the characteristic shape of the fixing pad 303 in this embodiment will be described with reference to Figures 3 to 6 along with Figure 2. As shown in Figures 2 and 3, the fixing pad 303 has, among its friction surfaces that rub against the inner circumferential surface of the fixing belt 301, an upstream friction surface 303c that rubs against the fixing belt 301 upstream of the fixing nip portion N in the conveyance direction of the recording material P (the direction of the arrow X), and a downstream friction surface 303b that rubs against the fixing belt 301 downstream of the fixing nip portion N.

[0034] 3, the fixing pad 303 of this embodiment has a downstream rubbing surface 303b formed in a convex shape with the center of the width direction (Y direction) of the fixing nip N at its apex, and the center protruding downstream in the conveying direction beyond both ends. The downstream rubbing surface 303b protrudes downstream in the conveying direction from both ends in the width direction toward the center, and the center in the width direction substantially coincides with the center of the fixing nip N. The length of the downstream rubbing surface 303b in the width direction is longer than the maximum passing range J through which a recording material P having the maximum width on which an image can be formed in the fixing nip N can pass. Note that the shape of the downstream rubbing surface 303b is not limited to a quadratic curved shape that smoothly connects one end to the other end via the center as shown in the figure, but may also be a linear shape that connects the center to one end and the center to the other end with straight lines, respectively.

[0035] The fixing pad 303 is thick in the width direction at the center and thin at both ends in the width direction. That is, the length W in the width direction from the upstream rubbing surface 303c to the downstream rubbing surface 303b is long at the center of the fixing nip N and becomes shorter from the center toward the ends.

[0036] When the fixing pad 303 is viewed from the opposite side of the pressure direction (arrow Z direction) in which the pressure roller 305 presses the fixing pad 303, the protrusion amount D from the end position F in the width direction where the maximum passing range J and the downstream rubbing surface 303b intersect to the most downstream position G of the downstream rubbing surface 303b in the conveying direction is set to, for example, 0.3 mm. For example, when the maximum passing range J is 300 mm, the end position F is located 150 mm away from the center in the width direction toward the end, and protrudes 0.3 mm from the end position F downstream in the conveying direction.

[0037] By forming the fixing pad 303 into the convex shape shown in FIG. 3, as shown in FIG. 4, the fixing belt 301 runs so that its running direction in the width direction (indicated by dotted line 602) faces outward from the center (here, the most downstream position G) as it passes through the fixing nip N. The running direction of the fixing belt 301 faces more outward as it moves away from the center toward the ends. Since the recording material P is transported following the running direction of the fixing belt 301, the recording material P is transported while being pulled from the center toward both ends and passes through the fixing nip N. The running direction and conveying force (vector) of the fixing belt 301 are determined by the amount of protrusion of the downstream-side rubbing surface 303b.

[0038] <Protrusion amount of downstream sliding surface> Next, the protrusion amount of the downstream-side rubbing surface 303b will be described. Before that, the mechanism by which wrinkles occur in the recording material will be described with reference to FIGS. 5 and 6. For example, when starting up the fixing device 8 from a cold state, the widthwise ends of the recording material P in the passage area L1 of the fixing nip N dissipate heat more easily than the center, resulting in a higher temperature in the center than the ends. In such a case, the outer diameter of the rotating pressure roller 305 at the center may expand slightly more than the outer diameter at the ends, causing the pressure roller 305 to assume a crowned shape. When the recording material P is conveyed with the pressure roller 305 deformed into a crowned shape, as shown in FIG. 5, the feed speed Vc at the center is faster than the feed speed Ve at the ends. This difference in feed speed causes the widthwise ends of the recording material P to move toward the center. As shown in FIG. 6, distortion occurs in the widthwise center of the recording material P upstream in the conveying direction, resulting in wrinkles 601 in the recording material P.

[0039] To detect the above-mentioned feed speed, a strip of recording material is prepared with multiple cuts made in the width direction and position markings are attached to the recording material. The time at the same strip position is measured at the point where the recording material enters the fixing nip N and the point where it is discharged from the fixing nip N, and by plotting the speed of the recording material passing through the fixing nip N in the longitudinal direction, the conveying speed at the center and the conveying speed at the edges can be measured and the difference between them can be obtained.

[0040] If the difference in the conveying speed described above becomes too large, wrinkles may occur on the recording material and uneven gloss may occur in the fixed toner image, so it is preferable to optimize the conveying speed of the recording material so as to satisfy the following formula 1. "n" in formula 1 is a coefficient less than 1, and is a predetermined value according to the stiffness of the recording material. For example, when the stiffness of the recording material is "0.30 mN" in Gurley stiffness, "n ≒ 0.99". Center conveying speed × n < Edge conveying speed ≦ Center conveying speed Formula 1

[0041] In this embodiment, as described above, the fixing pad 303 is formed in a convex shape, and the running direction of the fixing belt 301 is directed outward from the center, thereby optimizing the recording material conveyance speed to a speed that satisfies the above formula 1. To achieve this, the protrusion amount of the downstream rubbing surface 303b is adjusted by the length of the fixing nip portion N in the conveyance direction and the pressure force of the pressure roller 305. That is, if the length of the fixing nip portion N in the conveyance direction is short, the distance over which the recording material is pulled outward is short, making it more likely to wrinkle. Conversely, if the length of the fixing nip portion N in the conveyance direction is long, the distance over which the recording material is pulled outward is long, making it less likely to wrinkle. Therefore, when the length of the fixing nip portion N in the conveyance direction is short, the protrusion amount of the downstream rubbing surface 303b is increased, and when the length of the fixing nip portion N in the conveyance direction is long, the protrusion amount of the downstream rubbing surface 303b is decreased.

[0042] When the pressure of the pressure roller 305 is large, the length of the fixing nip portion N in the conveying direction at both ends in the width direction becomes longer than the length of the fixing nip portion N in the conveying direction at the center. This is because the movement of both ends of the pressure roller 305 is restricted by the metal plates, while the movement of the center portion is not restricted. Therefore, as the pressure is increased, the both ends of the pressure roller 305 do not move, and the center portion of the pressure roller 305 bends. As a result, the length of the fixing nip portion N in the conveying direction becomes longer at both ends than at the center portion. Therefore, when the pressure of the pressure roller 305 is large, the protrusion amount of the downstream friction surface 303b is reduced, and when the pressure of the pressure roller 305 is small, the protrusion amount of the downstream friction surface 303b is increased. In this embodiment, the difference in the length of the fixing nip portion N in the conveying direction between the ends and the center portion is within 1.5 mm.

[0043] When the recording material is subjected to a conveying force for a longer period of time at both ends of the fixing nip than at the center, the speed of the recording material becomes relatively faster at both ends than at the center. As a result, as described above, the running direction of the fixing belt 301 is oriented outward relative to the center, which prevents wrinkles from forming on the recording material. However, if the pressure applied by the pressure roller 305 is weak, wrinkles may form on the recording material. Therefore, in this embodiment, the fixing pad 303 is formed by adjusting the protrusion amount of the downstream friction surface 303b depending on the length of the fixing nip N in the conveying direction and the pressure applied by the pressure roller 305. When the pressure applied by the pressure roller 305 is low, the protrusion amount of the downstream friction surface 303b is increased. As described above, the protrusion amount (mm) of the downstream friction surface 303b is adjusted depending on the length of the fixing nip N in the conveying direction (mm) and the pressure applied by the pressure roller 305 (kgf). In this embodiment ... according to the length of the fixing nip N in the conveying direction (mm) and the pressure applied by the pressure roller 305 (kgf). [Table 1]

[0044] As can be seen from Table 1, the protrusion amount of the downstream friction surface 303b is preferably "0.1 mm or more and 0.6 mm or less." For example, depending on the pressure of the pressure roller 305, when the length of the fixing nip portion N in the conveying direction is "20 mm," the protrusion amount is "0.2 mm or more and 0.6 mm or less." When the length of the fixing nip portion N in the conveying direction is "24.5 mm," the protrusion amount is "0.15 mm or more and 0.55 mm or less." When the length of the fixing nip portion N in the conveying direction is "30 mm," the protrusion amount is "0.1 mm or more and 0.5 mm or less." Thus, when the length of the fixing nip portion N in the conveying direction is the same, the protrusion amount of the downstream friction surface 303b is a first protrusion amount when the pressure of the pressure roller 305 is a first pressure, and is a second protrusion amount smaller than the first protrusion amount when the pressure of the pressure roller 305 is a second pressure greater than the first pressure.

[0045] The protrusion amount of the downstream-side sliding surface 303b in Table 1 can be calculated using the structural calculation finite element analysis software "ABAQUS." The simulation conditions are as follows: The fixing belt 301 had an outer diameter of 150 mm, an elastic layer thickness of 300 μm, a Young's modulus of the elastic layer of 0.4 MPa, a substrate thickness of 80 μm, and a Young's modulus of the substrate of 5 GPa. The pressure roller 305 had an outer diameter of 80 mm, an elastic layer thickness of 8 mm, and a Young's modulus of 0.1 to 0.6 MPa. The pressure of the pressure roller 305 was 130 to 190 kgf, the rotational speed of the pressure roller 305 was 450 mm / s, and the friction coefficient between the recording material and the pressure roller 305 was 0.3.

[0046] In this simulation, to set the desired length of the fixing nip N in the conveying direction, the Young's modulus of the elastic layer of the pressure roller 305 was changed in the range of 0.1 to 0.6 MPa depending on the pressure of the pressure roller 305. For example, to set the length in the conveying direction to 20 mm at a pressure of 190 kgf, the Young's modulus of the elastic layer was set to 0.6 MPa. Also, to set the length in the conveying direction to 30 mm at a pressure of 130 kgf, the Young's modulus of the elastic layer was set to 0.1 MPa.

[0047] By performing multiple regression analysis on the relationships in Table 1, the relationship between the length M (mm) of the fixing nip portion N in the conveying direction and the pressure W (kgf) of the pressure roller 305 with respect to the protrusion amount D (mm) of the downstream rubbing surface 303b is expressed as Equation 2. D=-0.00997×M-0.00667×W+1.648 ··· Formula 2

[0048] The protrusion amount D of the downstream rubbing surface 303b shown in Formula 2 is a relational expression in the combination of the configuration in this embodiment, and although the numerical value varies depending on a plurality of peripheral parameters, the magnitude relationship of the values and the approximate values are correct. However, if the protrusion amount D of the downstream rubbing surface 303b is too small, it becomes difficult for the traveling direction of the fixing belt 301 at the end to be directed toward the end side with respect to the traveling direction of the fixing belt 301 at the central portion. On the contrary, if the protrusion amount D of the downstream rubbing surface 303b is too large, the conveyance speed of the recording material at the end becomes larger than the conveyance speed of the recording material at the central portion, and there is a risk of rear-end flutter occurring in the recording material. In view of this point, in the case of this embodiment, the protrusion amount D of the downstream rubbing surface 303b satisfies Formula 3 shown below. "-0.00997×M - 0.00667×W + 1.548 < D < -0.00997×M - 0.00667×W + 1.748" ··· Formula 3

[0049] As described above, in this embodiment, a convex-shaped fixing pad 303 is used in which the downstream rubbing surface 303b that rubs against the fixing belt 301 downstream of the fixing nip portion N protrudes downstream in the conveyance direction at the central portion in the width direction more than at both end portions. The traveling direction and conveyance force (vector) of the fixing belt 301 change depending on the protrusion amount of the downstream rubbing surface 303b, and the protrusion amount of the downstream rubbing surface 303b is adjusted by the conveyance direction length of the fixing nip portion N and the pressing force of the pressing roller 305. When this convex-shaped fixing pad 303 is used, when the fixing belt 301 passes through the fixing nip portion N, the traveling direction in the width direction is directed outward with the center as a reference, and the recording material P is conveyed following the traveling direction of the fixing belt 301. Therefore, the recording material P is conveyed while being pulled from the center toward both end sides. Further, by making the downstream rubbing surface 303b downstream of the fixing nip portion N convex, it is difficult to affect the uniformity of the pressure distribution in the fixing nip portion N. Therefore, it is possible to suppress the occurrence of wrinkles in the recording material without causing a difference in the fixing property of the toner image with respect to the recording material between the central portion and the end portion in the width direction in the fixing nip portion N.

[0050] In addition to the above, the protrusion amount of the downstream rubbing surface 303b may be changed depending on the crown amount in the width direction of the pressure roller 305 formed in a crown shape. This is because the distribution of the width direction length of the fixing nip portion N in the width direction may change depending on the crown amount of the pressure roller 305. Therefore, if the pressure roller 305 is formed in an inverted crown shape, the protrusion amount of the downstream rubbing surface 303b may be reduced.

[0051] [Second embodiment] Next, a second embodiment will be described. In the first embodiment described above, the fixing pad 303 having a convex shape with the downstream rubbing surface 303b protruding is described. However, this is not limited to this, and a fixing pad 303A having a concave shape with the upstream rubbing surface 303c recessed may also be used. Therefore, as a second embodiment, a fixing pad 303A having a concave shape with the upstream rubbing surface 303c recessed will be described using FIG. 7 with reference to FIG. 2. Note that the second embodiment described below differs from the first embodiment described above only in the shape of the fixing pad, and other configurations may be the same. Therefore, configurations similar to those of the first embodiment described above will be assigned the same reference numerals, and descriptions thereof will be simplified or omitted.

[0052] 7, the fixing pad 303A of the second embodiment has an upstream rubbing surface 303c formed in a concave shape with the center of the fixing nip N in the width direction (Y direction) at its apex, the center recessed further downstream in the conveying direction than both ends. The upstream rubbing surface 303c recesses further downstream in the conveying direction from both ends in the width direction toward the center, and the center in the width direction substantially coincides with the center of the fixing nip N. The widthwise length of the upstream rubbing surface 303c is longer than the maximum passing range J through which a recording material P having the maximum width on which an image can be formed in the fixing nip N can pass. Note that the shape of the upstream rubbing surface 303c is not limited to a linear shape connecting the center to one end and the center to the other end as shown in the figure, but may also be a quadratic curve smoothly connecting one end to the other end via the center.

[0053] The fixing pad 303A is formed such that the central portion in the width direction is thin in the conveyance direction, and both end portions in the width direction are thick in the conveyance direction. That is, the conveyance direction length W from the downstream rubbing surface 303b to the upstream rubbing surface 303c is short at the central portion of the fixing nip portion N and becomes longer from the central portion toward the end portions.

[0054] When viewed from the opposite side of the pressing direction (arrow Z direction) for pressing the fixing pad 303A by the pressing roller 305 with respect to the conveyance direction, the recess amount E from the end position O in the width direction where the maximum passage range J described above intersects with the upstream rubbing surface 303c to the most downstream position Q of the upstream rubbing surface 303c is set to, for example, "0.3 mm". For example, when the maximum passage range J is "300 mm", the end position O is a position separated from the center by "150 mm" toward the end portion in the width direction, and protrudes "0.3 mm" downstream in the conveyance direction from the end position O.

[0055] By forming the fixing pad 303A in a concave shape, when the fixing belt 301 passes through the fixing nip portion N, the running direction (indicated by the dotted line 602) in the width direction runs so as to face outward with respect to the center (here, the most downstream position Q). The farther the fixing belt 301 is from the center toward the end portion, the more the running direction faces outward. Since the recording medium P is conveyed following the running direction of the fixing belt, the recording medium P is conveyed while being pulled from the center toward both end sides and passes through the fixing nip portion N. The running direction and the conveying force (vector) of the fixing belt 301 are influenced by the recess amount of the upstream rubbing surface 303c.

[0056] The recess amount E (mm) of the upstream rubbing surface 303c is adjusted according to the conveyance direction length M (mm) of the fixing nip portion N and the pressing force (kgf) of the pressing roller 305, similarly to the protrusion amount D of the downstream rubbing surface 303b described above, and becomes the above-described Table 1 in the present embodiment. Therefore, the recess amount E of the upstream rubbing surface 303c satisfies the following formula 4. "-0.00997×M - 0.00667×W + 1.548 < E < - (0.00997×M + 0.00667×W + 1.748)" ··· Formula 4

[0057] As described above, in the second embodiment, the fixing pad 303A is formed in a concave shape by recessing the upstream-side rubbing surface 303c. In the case of this shape, as with the fixing pad 303 of the first embodiment, the running direction of the fixing belt 301 faces outward from the center as it passes through the fixing nip N. Therefore, the recording material is conveyed while being pulled from the center to both ends and passes through the fixing nip N, thereby achieving the same effect as in the first embodiment, that is, the occurrence of wrinkles in the recording material can be suppressed.

[0058] Note that, when the fixing pad 303 is formed in a convex shape with the downstream rubbing surface 303b protruding, as in the first embodiment, there is a risk that the separation ability of the recording material discharged from the fixing nip portion N will decrease. For this reason, a separating member formed in a crown shape may be provided on the downstream side of the fixing nip portion N. In contrast, when the fixing pad 303A is formed in a concave shape with the upstream rubbing surface 303c recessed, as in the second embodiment, the separation ability of the recording material discharged from the fixing nip portion N is unlikely to decrease, so it is not necessary to provide a separating member on the downstream side of the fixing nip portion N.

[0059] [Third embodiment] Next, a third embodiment will be described. In the third embodiment, the downstream-side friction surface 303b is protruded to form a convex shape, and the upstream-side friction surface 303c is recessed to form a concave shape, which is different from the first and second embodiments. The fixing pad 303B of this third embodiment will be described using FIG. 8 with reference to FIG. 2. The third embodiment described below differs from the first and second embodiments only in the shape of the fixing pad, and other configurations may be the same. Therefore, the same components as those of the first and second embodiments will be denoted by the same reference numerals, and descriptions thereof will be simplified or omitted.

[0060] 8, the downstream rubbing surface 303b is formed in a convex shape with the center of the width direction of the fixing nip N as the apex, with the center protruding further downstream in the conveying direction than both ends, and the upstream rubbing surface 303c is formed in a concave shape with the center recessed further downstream in the conveying direction than both ends. Even with this shape, the running direction of the fixing belt 301 faces outward from the center as it passes through the fixing nip N. Therefore, the recording material is conveyed while being pulled from the center to both ends and passes through the fixing nip N, which prevents wrinkles from occurring on the recording material.

[0061] In this embodiment, the protrusion amount (mm) of the downstream-side friction surface 303b and the recess amount (mm) of the upstream-side friction surface 303c are both shown in Table 2. As can be seen from Table 2, the protrusion amount of the downstream-side friction surface 303b and the recess amount of the upstream-side friction surface 303c are preferably "0.05 mm or more and 0.3 mm or less." [Table 2]

[0062] The above values ​​are the results of a simulation using the structural calculation finite element analysis software "ABAQUS" under the same parameter conditions as in the first embodiment. A multiple regression analysis was performed on the relationships in Table 3, and the relationship between the length M (mm) of the fixing nip portion N in the conveying direction and the pressure W (kgf) of the pressure roller 305 for the protrusion amount D (mm) of the downstream-side friction surface 303b and the recess amount E (mm) of the upstream-side friction surface 303c is shown in Equation 5. D(E)=-0.00498×M-0.00333×W+0.824 ··· Formula 5

[0063] The protrusion amount D of the downstream-side friction surface 303b and the recess amount E of the upstream-side friction surface 303c shown in Equation 4 are relational expressions based on the combination of the configurations in this embodiment, and although the numerical values ​​may vary depending on multiple peripheral parameters, the magnitude relationship of the values ​​and the approximate values ​​are correct. In this embodiment, the protrusion amount D of the downstream-side friction surface 303b satisfies Equation 6 shown below, and the recess amount E of the upstream-side friction surface 303c satisfies Equation 7 shown below. 「-0.00498×M - 0.00333×W + 0.724 < D < -0.00498×M - 0.00333×W + 0.924」 ··· Formula 6 「-0.00498×M - 0.00333×W + 0.724 < E < -0.00498×M - 0.00333×W + 0.924」 ··· Formula 7

[0064] As described above, in the second embodiment, the fixing pad 303B is formed in a convex shape in which the downstream rubbing surface 303b protrudes and a concave shape in which the upstream rubbing surface 303c is recessed. Also in this shape, when the fixing belt 301 passes through the fixing nip portion N, the traveling direction is outward with respect to the center. Therefore, since the recording material is conveyed while being pulled from the center to both end sides and passes through the fixing nip portion N, the same effects as those of the above-described first and second embodiments, in which the occurrence of wrinkles in the recording material can be suppressed, can be obtained.

[0065] <Examination experiment> The inventors of the present application confirmed experimentally the effects obtained by the differences in the protruding amount D of the downstream rubbing surface 303b and the recessed amount E of the upstream rubbing surface 303c. In the experiment, a 100% black single-color image was continuously formed on the entire surface of both sides of a recording material of Oji Paper OK Top Coat + 79.1 g, width 297 mm × length 420 mm. The experimental results are shown in Table 3.

Table 3

[0066] As can be seen from Table 3, wrinkles occurred on the recording material in the conventional example, but no wrinkles occurred on the recording material in the first to third embodiments described above. However, in the first to third embodiments described above, if the conveying force of the fixing belt 301 differs significantly between the center and the edges in the width direction, excessive stress is applied to the recording material, which may result in uneven gloss in the toner image. According to the experimental results shown in Table 3, in Comparative Example 1, the protrusion amount is 0.1 mm, which is smaller than the 0.3 mm in the first embodiment. As in the first embodiment, no wrinkles occurred on the recording material, but uneven gloss occurred. In Comparative Example 2, the recess amount is 0.3 mm, the same as in the second embodiment, but the pressure force of the pressure roller 305 is 130 kgf, which is smaller than the 160 kgf in the second embodiment. As in the second embodiment, no wrinkles occurred on the recording material, but uneven gloss occurred in the toner image. In Comparative Example 3, the protrusion amount and the length in the conveying direction of the fixing nip portion N (referred to as nip width) are smaller than those in the third embodiment, and as in the third embodiment, no wrinkles occur in the recording material, but gloss unevenness occurs in the toner image. In the above Comparative Examples 1 to 3, the protrusion amount D and the recess amount E do not satisfy the above-mentioned formulas 3, 4, 6, and 7, respectively.

[0067] [Other embodiments] The above-described embodiment is not limited to a configuration in which fixing belt 301 is heated, but can also be applied to a configuration in which a belt-shaped pressure belt is used instead of pressure roller 305 and this pressure belt is heated by a heater, etc. In such a case, a pressure pad may be disposed inside the pressure belt, and this pressure pad may have the same configuration as fixing pad 303 described above.

[0068] The present technology can also be configured as follows. (1) A rotating endless belt, a rotating body that contacts the outer peripheral surface of the belt; a nip portion forming member that is provided inside the belt with the rotating body and the belt sandwiched therebetween, and that applies heat and pressure to a recording material on which a toner image has been formed while sandwiching and conveying the recording material, thereby forming a nip portion where the toner image is fixed to the recording material; The nip portion forming member has a downstream rubbing surface that rubs against the inner peripheral surface of the belt downstream of the nip portion with respect to the conveyance direction of the recording material among the rubbing surfaces that rub against the inner peripheral surface of the belt, The downstream rubbing surface is formed in a convex shape in which the central portion protrudes downstream in the conveyance direction from both end portions with respect to the width direction intersecting the conveyance direction. A fixing device characterized by this. (2) The downstream rubbing surface protrudes downstream in the conveyance direction from both ends in the width direction toward the center, and the center in the width direction substantially coincides with the center of the nip portion. The fixing device according to (1) above, characterized by this. (3) The downstream rubbing surface has a length in the width direction that is longer than the passing range through which the recording material having the maximum width in the nip portion can pass. The fixing device according to (1) or (2) above, characterized by this. (4) The nip portion forming member is pressurized against the rotating body. When viewed from the opposite side of the pressurizing direction of the nip portion forming member, with respect to the conveyance direction, from the end position in the width direction where the passing range and the downstream rubbing surface intersect to the most downstream position of the downstream rubbing surface, the protruding amount D, the length M (mm) of the nip portion in the conveyance direction, and the pressing force W (kgf) of the rotating body, The protruding amount satisfies "-0.00997×M - 0.00667×W + 1.548 < D < -0.00997×M - 0.00667×W + 1.748". The fixing device according to (3) above, characterized by this. (5) The protruding amount is 0.1 mm or more and 0.6 mm or less. The fixing device according to (4) above, characterized by this. (6) An endless rotating belt, <00ZZZ00>A rotating body that contacts the outer peripheral surface of the belt, A nip portion forming member that is provided between the rotating body and the belt on the inner side of the belt so as to sandwich them, and forms a nip portion that applies heat and pressure while sandwiching and conveying a recording material on which a toner image is formed to fix the toner image on the recording material. The nip portion forming member has an upstream-side rubbing surface that rubs against the inner peripheral surface of the belt upstream of the nip portion with respect to the conveyance direction of the recording material among the rubbing surfaces that rub against the inner peripheral surface of the belt. The upstream-side rubbing surface is formed in a concave shape in which the central portion is recessed downstream in the conveyance direction from both end portions with respect to the width direction that intersects the conveyance direction. A fixing device characterized by this. (7) The upstream-side rubbing surface is recessed downstream in the conveyance direction from both ends in the width direction toward the center, and the center in the width direction substantially coincides with the center of the nip portion. The fixing device according to (⑥) above, characterized by this. (8) The upstream-side rubbing surface has a length in the width direction that is longer than a passing range through which a recording material having a maximum width in the nip portion can pass. The fixing device according to (⑥) or (⑦) above, characterized by this. (9) The nip portion forming member is pressurized against the rotating body. When viewed from the opposite side in the pressurizing direction of the nip portion forming member, with respect to the conveyance direction, from the end position in the width direction where the passing range and the upstream-side rubbing surface intersect, to the most downstream position of the upstream-side rubbing surface, the recess amount E, the length M (mm) of the nip portion in the conveyance direction, and the pressurizing force W (kgf) of the rotating body. The recess amount satisfies "-0.00997×M - 0.00667×W + 1.548 < E < -0.00997×M - 0.00667×W + 1.748". The fixing device according to (⑧) above, characterized by this. (10) The recess amount is 0.1 mm or more and 0.6 mm or less. The fixing device according to (⑨) above, characterized by this. (11) An endless rotating belt A rotating body that abuts against the outer peripheral surface of the belt, and a nip portion forming member that is provided inside the belt so as to sandwich the rotating body and the belt, and forms a nip portion that sandwiches and conveys a recording material on which a toner image is formed and applies heat and pressure to fix the toner image on the recording material. The nip portion forming member, has a downstream rubbing surface that rubs against the inner peripheral surface of the belt downstream of the nip portion with respect to the conveyance direction of the recording material and an upstream rubbing surface that rubs against the inner peripheral surface of the belt upstream of the nip portion among the rubbing surfaces that rub against the inner peripheral surface of the belt, where the downstream rubbing surface is formed in a convex shape in which the central portion protrudes downstream in the conveyance direction from both end portions with respect to the width direction intersecting the conveyance direction, and the upstream rubbing surface is formed in a concave shape in which the central portion is recessed downstream in the conveyance direction from both end portions with respect to the width direction. [[ID=I3]]A fixing device characterized by the above. (12) The nip portion forming member is pressed against the rotating body, when viewed from the opposite side of the pressing direction of the nip portion forming member, with respect to the conveyance direction, from the end position in the width direction where the passage range through which the recording material having the maximum width can pass in the nip portion intersects the downstream rubbing surface to the most downstream position of the downstream rubbing surface, the protruding amount D, from the end position in the width direction where the passage range intersects the upstream rubbing surface to the most downstream position of the upstream rubbing surface, the recessed amount E, the length M (mm) of the nip portion in the conveyance direction, and the pressing force W (kgf) of the rotating body, the protruding amount satisfies "-0.00498×M - 0.00333×W + 0.724 < D < -0.00498×M - 0.00333×W + 0.924", the recessed amount satisfies "-0.00498×M - 0.00333×W + O.724 < E < -0.00498×M - 0.00333×W + 0.924". [[ID=I4]] The fixing device according to (11), characterized by the above. (13) The protruding amount and the recessed amount are 0.05 mm or more and 0.3 mm or less. The fixing device according to (12) above. [Explanation of symbols]

[0069] 8... fixing device, 301... belt (fixing belt), 303... nip portion forming member (fixing pad), 303b... downstream side rubbing surface, 303c... upstream side rubbing surface, 305... rotating body (pressure roller), N... nip portion (fixing nip portion)

Claims

1. A rotating endless belt; A rotating member that contacts an outer peripheral surface of the belt; a nip portion forming member that is provided inside the belt and non-rotatably sandwiches the belt with the rotating member, contacting the inner circumferential surface of the belt to form a nip portion in which the toner image is fixed to the recording material by applying heat and pressure while sandwiching and transporting the recording material, the nip portion forming member has a contact area that contacts the inner circumferential surface of the belt, the contact area has a downstream end portion that contacts the belt downstream of the nip portion in a conveying direction of the recording material, The downstream end portion has a center portion located downstream in the conveying direction relative to both end portions in a width direction intersecting with the conveying direction. A fixing device comprising:

2. The downstream end portion extends downstream in the transport direction from the both end portions toward the central portion.

2. The fixing device according to claim 1,

3. The downstream end portion has a center portion that is approximately aligned with a center of the nip portion in the width direction.

2. The fixing device according to claim 1,

4. The central portion is located furthest downstream in the conveying direction among the downstream ends.

4. The fixing device according to claim 3.

5. With regard to the width direction, when the central portion approximately coincides with the central position of the maximum width of the recording material that can pass through the fixing device, the length D in the transport direction from an end position that intersects with the maximum width of the recording material at the downstream end to the central portion, the length M (mm) of the nip portion in the transport direction, and the pressure W (kgf) of the rotating member satisfy "-0.00997 x M - 0.00667 x W + 1.548 < D < -0.00997 x M - 0.00667 x W + 1.748." 4. The fixing device according to claim 3.

6. A length D in the conveying direction from the end position to the center portion at the downstream end portion is 0.1 mm or more and 0.6 mm or less.

6. The fixing device according to claim 5,

7. A length of the contact area in the width direction is longer than a maximum width of a recording material that can pass through the fixing device.

2. The fixing device according to claim 1,

8. The contact area has an upstream end that contacts the belt upstream of the nip portion in the conveying direction, The upstream end portion has a center portion located upstream of both end portions in a width direction intersecting with the transport direction.

2. The fixing device according to claim 1,

Images