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 using electrophotographic technology, such as a printer, a copier, a facsimile machine, or a multifunction machine.

Background Art

[0002] An image forming apparatus includes a fixing device that applies heat and pressure to a recording material on which a toner image is formed to fix the toner image to the recording material. The fixing device has, for example, a rotating endless belt, a roller that abuts against the outer peripheral surface of the belt, and a pad that rubs against the inner peripheral surface of the belt. The pad sandwiches the belt between the roller and forms a fixing nip portion that applies heat and pressure while sandwiching and conveying the recording material to fix the toner image.

[0003] Conventionally, a device has been proposed in which the shape of the fixing nip portion has both end sides in the width direction protruding upstream in the conveyance direction of the recording material from the central portion (Patent Document 1). In the case of this shape of the fixing nip portion, the pressure distribution in the fixing nip portion becomes larger at both ends than at the central portion in the width direction, and a relative speed difference occurs between the end portion (entry portion) and the central portion (non-entry portion) with respect to the recording material when the recording material enters the fixing nip portion. Along with this, when the recording material passes through the fixing nip portion, it is stretched from the central portion toward the end portion side, so wrinkles are less likely to occur on the recording material.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, conventionally, in a fixing nip portion having a different pressure distribution in the width direction, a difference in the fixing property of the toner image with respect to the recording material is likely to occur between the central portion and the end portion in the width direction, and there has been a risk of fixing failure on the recording material.

[0006] The present invention has been made in view of the above problems, and aims to provide a fixing device that can suppress the occurrence of wrinkles in the recording material without causing a difference in the fixing performance of the toner image to the recording material between the central part and the edges 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 includes a rotating endless belt and a rotating member that contacts the outer circumferential surface of the belt, A heating rotating body that contacts the inner circumferential surface of the belt and heats the belt, and a tensioning member that contacts the inner circumferential surface of the belt and tensions the belt, Inside of the aforementioned belt Not It is mounted on the rotating surface and contacts the inner circumferential surface of the belt. It is designed to , Between the rotating member and Holding and transporting recording materials Send It comprises a nip-forming member that forms a nip portion, The belt is wrapped around the heating rotating body, the tensioning member, and the nip portion forming member. The nip portion forming member is ,before Regarding the transport direction of the recording material hand, The aforementioned nip portion Downstream The belt inner circumferential surface downstream contact Having a section, downstream contact The department is, The downstream end of the recording material in the transport direction is With respect to the width direction intersecting the aforementioned transport direction hand The central portion is located downstream in the conveying direction compared to both ends. [Effects of the Invention]

[0010] According to the present invention, it is possible to suppress the formation of wrinkles in the recording material without causing a difference in the fixing performance of the toner image to the recording material between the central part and the edges in the width direction of the fixing nip portion. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic diagram showing a suitable image forming apparatus using the fixing device of this embodiment. [Figure 2] A schematic diagram showing the fixing device. [Figure 3] A view of the fixing pad of the first embodiment from the opposite side of the pressure direction. [Figure 4]A diagram showing the direction of travel of the anchoring belt. [Figure 5] A diagram explaining the mechanism by which wrinkles form in recording material. [Figure 6] A diagram showing an example of wrinkles that can form on recording material. [Figure 7] A view of the fixing pad of the second embodiment, seen from the opposite side of the pressure direction. [Figure 8] A view of the fixing pad of the third embodiment, seen from the opposite side of the pressure direction. [Modes for carrying out the invention]

[0012] The fixing device of this embodiment will now be described. First, a schematic configuration of an image forming apparatus suitable for using the fixing device of this embodiment will be described with reference to Figure 1.

[0013] [First Embodiment] <Image forming apparatus> The image forming apparatus 1 is an electrophotographic full-color printer having four image forming units Pa, Pb, Pc, and Pd, corresponding to four colors: yellow, magenta, cyan, and black. This embodiment is a tandem-type image forming apparatus 1 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 reader 2 connected to the main body 3 of the image forming apparatus 1 or from a host device such as a personal computer that is communicatively connected to the main body 3. Examples of recording materials include paper, plastic film, and sheet materials such as cloth.

[0014] As shown in FIG. 1, the image forming apparatus 1 includes a document reading apparatus 2 and an apparatus main body 3. The document reading apparatus 2 reads a document placed on the document table glass 21. Light irradiated from the light source 22 is reflected by the document and imaged on the CCD sensor 24 through an optical system member 23 such as a lens. When such an optical system unit is scanned in the direction of the arrow under the control of the reader control unit, the document is read line by line and converted into an electrical signal data series. The image signal obtained by the CCD sensor 24 is sent to the apparatus main body 3, and image processing is performed by the control unit 30 according to each image forming unit described later. Further, the control unit 30 also receives an external input from an external host device such as a print server as an image signal.

[0015] The apparatus main body 3 includes a plurality of image forming units Pa, Pb, Pc, Pd. In each image forming unit, image formation is performed based on the above-described image signal. That is, the image signal is converted into a laser beam PWM (pulse width modulation control) by the control unit 30. The polygon scanner 31 as an exposure device scans a laser beam corresponding to the image signal. Then, the laser beam is irradiated onto the photosensitive drums 200a to 200d as image carriers of the respective image forming units Pa to Pd.

[0016] Note that the image forming unit Pa forms a toner image of yellow (Y), the image forming unit Pb forms a toner image of magenta (M), the image forming unit Pc forms a toner image of cyan (C), and the image forming unit Pd forms a toner image of black (Bk). Since these image forming units Pa to Pd have substantially the same configuration, the image forming unit Pa that forms a toner image of yellow (Y) will be described as an example below, and the description of the other image forming units Pb to Pd will be omitted. In the image forming unit Pa, the photosensitive drum 200a has a toner image formed on its surface based on the image signal as described below.

[0017] The charging roller 201a, acting as a primary charger, charges the surface of the photosensitive drum 200a to a predetermined potential, preparing it for electrostatic latent image formation. An electrostatic latent image is formed on the surface of the photosensitive drum 200a, which has been charged to the predetermined potential, by a laser beam from the polygon scanner 31. The developer 202a develops the electrostatic latent image on the photosensitive drum 200a to form a toner image. The primary transfer roller 203a discharges from the back of the intermediate transfer belt 204, applying a primary transfer bias with the opposite polarity to the toner, and transfers the toner image on the photosensitive drum 200a onto the intermediate transfer belt 204. After the transfer, the surface of the photosensitive drum 200a is cleaned by the cleaner 207a.

[0018] Furthermore, the toner image on the intermediate transfer belt 204 is transported to the next image forming section, and the toner images of each color formed in the respective image forming sections are transferred sequentially in the order of Y, M, C, and Bk, forming a four-color image on its surface. The toner image that has passed through the Bk image forming section Pd, which is the furthest downstream in the rotational direction of the intermediate transfer belt 204, is transported to the secondary transfer section T2, which consists of a pair of secondary transfer rollers 205 and 206. In the secondary transfer section T2, a secondary transfer electric field with the opposite polarity to the toner image on the intermediate transfer belt 204 is applied, thereby secondary transferring the toner image from the intermediate transfer belt 204 to the recording material.

[0019] The recording material is housed in a cassette 9. The recording material fed from the cassette 9 is transported to a registration unit 208, which consists of, for example, a pair of registration rollers, and waits in the registration unit 208. Subsequently, the registration unit 208 is controlled in timing to align the toner image on the intermediate transfer belt 204 with the position of the paper, and then transports the recording material to the secondary transfer unit T2.

[0020] The recording material onto which the toner image has been transferred in the secondary transfer unit T2 is transported to the fuser unit 8, where it is heated and pressurized to fix the toner image onto the recording material. The recording material that has passed through the fuser unit 8 is discharged to the discharge tray 7. When image formation is performed on both sides of the recording material, once the transfer and fixing of the toner image on the front surface of the recording material is complete, the front and back surfaces of the recording material are reversed via the inversion transport unit 10, and the toner image is transferred and fixed to the back surface of the recording material before it is loaded onto the discharge tray 7.

[0021] The control unit 30 controls the entire image forming apparatus 1 as described above. The control unit 30 can also perform various settings based on input from the operation unit 4 of the image forming apparatus 1. This control unit 30 includes a CPU (Central Processing Unit), ROM (Read Only Memory), and RAM (Random Access Memory). The CPU controls each part while reading programs corresponding to control procedures stored in the ROM. The RAM stores working data and input data, and the CPU controls the apparatus by referring to the data stored in the RAM based on the aforementioned programs.

[0022] <Fusing device> Next, the configuration of the fixing device 8 in this embodiment will be described with reference to Figure 2. In this embodiment, a belt heating type fixing device using an endless belt is employed. As shown in Figure 2, in the fixing device 8, the recording material is conveyed in the conveying direction (arrow X direction). In this specification, the width direction refers to the direction intersecting the conveying direction of the recording material in the fixing nip section 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 surface of the fixing belt 301, a heating roller 307, a tensioning roller 308, and a fixing pad unit 300.

[0024] <Fixing belt> The fixing belt 301 is stretched by a heating roller 307, a tensioning roller 308, and a fixing pad unit 300. The fixing belt 301 has thermal conductivity and heat resistance, and is formed in a thin-walled cylindrical shape. The fixing belt 301 has a three-layer structure in which a base layer, an elastic layer, and a release layer are formed on a substrate such as rubber, in order from the inner circumference. As an example, the base layer is made of polyimide resin (PI) with a thickness of "80 μm", the elastic layer is made of silicone rubber with a thickness of "300 μm", and the release layer is made of tetrafluoroethylene-perfluoroalkoxyethylene copolymer resin (PFA) with a thickness of "30 μm". 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> The heating roller 307 is positioned on the inner circumference of the fixing belt 301 and tensions the fixing belt 301 together with the fixing pad unit 300. The heating roller 307 is formed in a cylindrical shape from a metal such as aluminum or stainless steel with a thickness of, for example, 1 mm, and a halogen heater 306 is disposed inside it as a heat source for heating the fixing belt 301. When the heating roller 307 is heated to a predetermined temperature by the halogen heater 306, the fixing belt 301 is heated by the heating roller 307.

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

[0027] <Pressure roller> The pressure roller 305, as a rotating body, is rotatably supported by the fixing device 8, and a gear (not shown) is fixed to one end in the width direction. The gear is connected to a drive source (not shown) such as a motor, and the roller is rotationally driven. When the pressure roller 305 rotates, the rotational force of the pressure roller 305 is transmitted to the fixing belt 301 by the frictional force generated at the fixing nip section N (described later), and the fixing belt 301 rotates in accordance with the pressure roller 305.

[0028] The pressure roller 305 is a roller in which an elastic layer and a release layer are formed on top of a cylindrical core metal 305c. For 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 surface of the fixing belt 301, sandwiching the fixing belt 301 between itself and the fixing pad 303, which will be described later. The pressure roller 305 then applies pressure to the fixing pad 303 using a drive source (not shown) (arrow Z direction). In this embodiment, for example, the pressure at the fixing nip section N is "160 kgf", and the length of the fixing nip section N in the transport direction is "24.5 mm" and the length in the width direction is "326 mm", so that the pressure roller 305 applies pressure to the fixing belt 301 toward the fixing pad 303.

[0030] <Fuser pad unit> The fixing pad unit 300 is positioned on the inner circumference side 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, for example, made of metal, that extends in the width direction of the fixing belt 301, and the fixing stay 302 supports the fixing pad 303 on the pressure roller 305 side. In this embodiment, the fixing pad 303 supported by the fixing stay 302 is placed on the fixing belt 301 together with the heating roller 307 and the tension roller 308, and is pressed by the pressure roller 305 with the fixing belt 301 in between. As a result, a wide nip fixing nip section N is formed between the pressure roller 305 and the fixing belt 301, ensuring both the transport direction length and the width direction length. In the fixing nip section N, the recording material carrying the unfixed toner image is held and transported, and heat and pressure are applied to the recording material at that time, fixing the toner image to the recording material.

[0031] <Fixing pad> The fixing pad 303, which serves as a nip-forming member, is non-rotatably mounted on the inner circumference of the fixing belt 301 and presses against the inner surface of the fixing belt 301. The fixing pad 303 is a resin member that extends in the width direction, and its width direction length is longer than the width direction length of the maximum size of 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-resistant properties, such as liquid crystal polymer resin. In order to form a fixing nip 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 has a shape in which the central part is curved toward the pressure roller 305 side than the ends in the width direction.

[0032] To reduce the frictional force between the fixing belt 301 and the fixing pad 303, the fixing pad 303 is provided with a sliding member 304 that slides against the fixing belt 301. The sliding member 304 is positioned opposite the pressure roller 305, with the fixing belt 301 in between. The surface of the sliding member 304 that slides against the fixing belt 301 has an embossed shape with irregularities of several microns. In addition, 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 against 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 constructed.

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

[0034] As shown in Figure 3, the fixing pad 303 of this embodiment is formed in a convex shape, with the downstream abrasive surface 303b having its apex at the center in the width direction (Y direction) of the fixing nip portion N, and the central portion protruding further downstream in the transport direction than both ends. The downstream abrasive surface 303b protrudes further downstream in the transport direction from both ends in the width direction towards the center, and the center in the width direction approximately coincides with the center of the fixing nip portion N. Furthermore, the length of the downstream abrasive surface 303b in the width direction is longer than the maximum passage range J through which the recording material P of the maximum width for which an image can be formed in the fixing nip portion N can pass. Note that the shape of the downstream abrasive surface 303b is not limited to being formed as a curved quadratic curve that smoothly connects one end through the center to the other end as shown in the figure, but may also be formed as a straight line shape, for example, connecting the center to one end and the center to the other end with straight lines.

[0035] Furthermore, the fixing pad 303 is formed so that the central part in the width direction is thicker in the conveying direction, and both ends in the width direction are thinner in the conveying direction. That is, the length W in the conveying direction from the upstream sliding surface 303c to the downstream sliding surface 303b is longer in the central part of the fixing nip N, and becomes shorter from the central part towards the ends.

[0036] When viewing the fixing pad 303 from the opposite side of the pressure direction (arrow Z direction) in which the fixing pad 303 is pressurized by the pressure roller 305, the amount of protrusion D from the end position F in the width direction where the maximum passage range J and the downstream sliding surface 303b intersect, to the downstreammost position G of the downstream sliding surface 303b, is set to, for example, "0.3 mm". For example, if the maximum passage range J is "300 mm", the end position F is located "150 mm" away from the center in the width direction, and protrudes "0.3 mm" downstream from the end position F in the conveying direction.

[0037] By forming the fixing pad 303 into the convex shape shown in Figure 3, as shown in Figure 4, the fixing belt 301 travels such that its travel direction in the width direction (shown by the dotted line 602) is directed outward from the center (in this case, the furthest downstream position G) when passing through the fixing nip section N. The further the fixing belt 301 moves from the center towards the ends, the more its travel direction is directed outward. Since the recording material P is conveyed following the travel direction of the fixing belt 301, the recording material P is conveyed while being pulled from the center towards both ends as it passes through the fixing nip section N. The travel direction and conveying force (vector) of the fixing belt 301 are influenced by the amount of protrusion of the downstream sliding surface 303b.

[0038] <Amount of protrusion of the downstream sliding surface> Next, we will explain the amount of protrusion of the downstream sliding surface 303b. Before that, we will explain the mechanism by which wrinkles occur in the recording material using Figures 5 and 6. For example, when starting up a cold fixing device 8, the ends in the width direction of the passing region L1 through which the recording material P passes in the fixing nip section N dissipate heat more easily than the center, so the temperature in the center is higher than that of the ends. In such a case, the outer diameter of the rotating pressure roller 305 in the center may bulge slightly more than the outer diameter of the ends, causing the pressure roller 305 to take on a crown shape. When the recording material P is transported with the pressure roller 305 deformed into a crown shape, as shown in Figure 5, the feed speed Vc in the center becomes faster than the feed speed Ve in the ends. Due to this difference in feed speed, the width direction ends of the recording material P move towards the center, and as shown in Figure 6, distortion occurs in the width direction center upstream in the transport direction of the recording material P, causing wrinkles 601 to occur in the recording material P.

[0039] As a method for detecting the feed rate described above, a strip-shaped recording material with multiple cuts in the width direction is prepared, and position markings are placed on the recording material. The time at the same position of the strip is measured at the point where it enters the fixing nip section N and the point where it is discharged from the fixing nip section N, and by plotting the speed of the recording material passing through the fixing nip section N in the longitudinal direction, the transport speed at the center and the transport speed at the edges can be measured, and the difference between them can be obtained.

[0040] If the difference in transport speed described above becomes large, wrinkles may form in the recording material and uneven gloss may occur in the toner image after fixing. Therefore, it is preferable to optimize the transport speed of the recording material so as to satisfy Equation 1 shown below. In Equation 1, "n" is a coefficient less than 1 and is a predetermined value corresponding to the stiffness of the recording material. For example, when the stiffness of the recording material is "0.30 mN" in Gahl type stiffness, "n ≈ 0.99". Central conveying speed × n < End conveying speed ≤ Central conveying speed ... Equation 1

[0041] In this embodiment, as described above, the fixing pad 303 is formed in a convex shape, and the direction of travel of the fixing belt 301 is directed outward with respect to the center, thereby optimizing the transport speed of the recording material to a speed that satisfies the above equation 1. To achieve this, the amount of protrusion of the downstream friction surface 303b is adjusted by the transport direction length of the fixing nip section N and the pressure applied by the pressure roller 305. That is, if the transport direction length of the fixing nip section N is short, the distance over which the recording material is pulled outward is shortened, making it easier for wrinkles to form on the recording material. Conversely, if the transport direction length of the fixing nip section N is long, the distance over which the recording material is pulled outward is increased, making it less likely for wrinkles to form on the recording material. Therefore, when the transport direction length of the fixing nip section N is short, the amount of protrusion of the downstream friction surface 303b is increased, and when the transport direction length of the fixing nip section N is long, the amount of protrusion of the downstream friction surface 303b is decreased.

[0042] When the pressure applied by the pressure roller 305 is high, the length of the fixing nip portions N at both ends in the width direction in the transport direction becomes longer than the length of the central portion in the transport direction. This is because the movement of the ends of the pressure roller 305 is restricted by sheet metal, while the movement of the central portion is not restricted. Therefore, as the pressure is increased, the ends of the pressure roller 305 do not move, while the central portion of the pressure roller 305 flexes. As a result, the length of the fixing nip portion N in the transport direction becomes longer at both ends than at the central portion. Therefore, when the pressure applied by the pressure roller 305 is high, the amount of protrusion of the downstream sliding surface 303b is reduced, and when the pressure applied by the pressure roller 305 is low, the amount of protrusion of the downstream sliding surface 303b is increased. In this embodiment, the difference in the length of the fixing nip portion N in the transport direction between the ends and the central portion is within "1.5 mm".

[0043] When the recording material is subjected to conveying force for a relatively longer period of time at both ends than at the center within the fixing nip section, the speed of the recording material at both ends becomes relatively faster than at the center. As a result, as described above, the direction of travel of the fixing belt 301 is directed outward from the center, which suppresses the formation of wrinkles in the recording material. However, if the pressure of the pressure roller 305 is small, wrinkles may form in the recording material. Therefore, in this embodiment, the amount of protrusion of the downstream sliding surface 303b is adjusted according to the length of the fixing nip section N in the conveying direction and the pressure applied by the pressure roller 305 to form the fixing pad 303. When the pressure applied by the pressure roller 305 is low, the amount of protrusion of the downstream sliding surface 303b is increased. As described above, the amount of protrusion (mm) of the downstream sliding surface 303b is adjusted according to the length of the fixing nip section N in the conveying direction (mm) and the pressure applied by the pressure roller 305 (kgf), and in this embodiment, it is as shown in Table 1 below. [Table 1]

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

[0045] The protrusion amount of the downstream friction surface 303b in Table 1 can be calculated using the structural calculation finite element method 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 base material thickness of "80 μm", and a Young's modulus of the base material of "5 Gpa". The pressure roller 305 had an outer diameter of "φ80 mm", an elastic layer thickness of "8 mm", and a Young's modulus of the elastic layer of "0.1 to 0.6 MPa". In addition, the pressing force of the pressure roller 305 was set to "130 to 190 kgf", the rotation speed of the pressure roller 305 was set to "450 mm / s", and the coefficient of friction between the recording material and the pressure roller 305 was set to "0.3".

[0046] In this simulation, in order to set the length of the fixing nip section N in the transport direction to the desired length, the Young's modulus of the elastic layer of the pressure roller 305 is changed within the range of "0.1 to 0.6 MPa" according to the pressure applied by the pressure roller 305. For example, to form a length of "20 mm" in the transport direction with a pressure of "190 kgf", the Young's modulus of the elastic layer was set to "0.6 MPa". Also, to form a length of "30 mm" in the transport direction with a pressure of "130 kgf", the Young's modulus of the elastic layer was set to "0.1 MPa".

[0047] Multiple regression analysis was performed on the relationship shown in Table 1, and the relationship between the protrusion amount D (mm) of the downstream sliding surface 303b, the length M (mm) of the fixing nip section N in the transport direction, and the pressure W (kgf) applied by the pressure roller 305 is shown in 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 may vary 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 center 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 center portion, and there is a possibility 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 to both end sides. Further, by making the downstream rubbing surface 303b downstream of the fixing nip portion N convex-shaped, 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] Furthermore, the amount of protrusion of the downstream sliding surface 303b may be changed according to the amount of crown in the width direction of the pressure roller 305, which is formed in a crown shape, in addition to the explanation given above. This is because the distribution of the width direction length of the fixing nip portion N may change with respect to the width direction depending on the amount of crown of the pressure roller 305. Therefore, if the pressure roller 305 is formed in an inverted crown shape, the amount of protrusion of the downstream sliding surface 303b may be reduced.

[0051] [Second Embodiment] Next, a second embodiment will be described. In the first embodiment described above, a fixing pad 303 was described with a convex shape by making the downstream sliding surface 303b side protrude. However, it is not limited to this, and a fixing pad 303A may be used with a concave shape by making the upstream sliding surface 303c side recessed. Therefore, as a second embodiment, a fixing pad 303A with a concave shape by making the upstream sliding surface 303c side recessed will be described using Figure 7 with reference to Figure 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 components may be the same. Therefore, components the same as in the first embodiment described above will be denoted by the same reference numerals and their descriptions will be simplified or omitted.

[0052] As shown in Figure 7, the fixing pad 303A of the second embodiment has an upstream abrasive surface 303c that is concave, with the center of the fixing nip portion N in the width direction (Y direction) being the apex, and the central part being recessed downstream in the transport direction compared to both ends. The upstream abrasive surface 303c is recessed downstream in the transport direction from both ends in the width direction towards the center, and the center in the width direction approximately coincides with the center of the fixing nip portion N. Furthermore, the length of the upstream abrasive surface 303c in the width direction is longer than the maximum passage range J through which the recording material P of the maximum width for which an image can be formed in the fixing nip portion N can pass. Note that the shape of the upstream abrasive surface 303c is not limited to a straight line shape connecting the center to one end and the center to the other end, as shown in the figure, but may also be formed as a curved quadratic curve that smoothly connects one end through the center to the other end.

[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 passing 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 passing 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 traveling direction (indicated by the dotted line 602) in the width direction travels 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 side, the more outward the traveling direction is. Since the recording material P is conveyed following the traveling direction of the fixing belt 301, the recording material P is conveyed while being pulled from the center toward both end sides and passes through the fixing nip portion N. The traveling 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 protruding amount D of the downstream rubbing surface 303b described above, and becomes the above-described Table 1 in this 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 with the upstream sliding surface 303c recessed. In this shape as well, similar to the fixing pad 303 of the first embodiment, the fixing belt 301 travels outward from the center as it passes through the fixing nip section N. Therefore, the same effect as in the first embodiment is obtained, as the recording material is conveyed while being pulled from the center to both ends as it passes through the fixing nip section N, thereby suppressing the formation of wrinkles in the recording material.

[0058] In the first embodiment, if the fixing pad 303 is formed in a convex shape with the downstream friction surface 303b protruding, there is a risk that the separation of the recording material discharged from the fixing nip N will decrease. For this reason, a crown-shaped separating member may be provided downstream of the fixing nip N. In contrast, in the second embodiment, if the fixing pad 303A is formed in a concave shape with the upstream friction surface 303c recessed, the separation of the recording material discharged from the fixing nip N is less likely to decrease, and therefore, a separating member does not need to be provided downstream of the fixing nip N.

[0059] [Third Embodiment] Next, a third embodiment will be described. Compared to the first and second embodiments described above, the third embodiment is a fixing pad 303B in which the downstream sliding surface 303b side is made to protrude and form a convex shape, and the upstream sliding surface 303c side is made to recess and form a concave shape. The fixing pad 303B of this third embodiment will be described with reference to Figure 2 and Figure 8. Note that the third embodiment described below differs from the first and second embodiments described above only in the shape of the fixing pad, and other components may be the same. Therefore, components similar to those in the first and second embodiments described above will be denoted by the same reference numerals and their descriptions will be simplified or omitted.

[0060] The fixing pad 303B shown in Figure 8 has a convex shape with the center of the fixing nip section N in the width direction as its apex, and the downstream sliding surface 303b having a central portion that protrudes further downstream in the conveying direction than both ends, while the upstream sliding surface 303c has a concave shape with a central portion that is recessed further downstream in the conveying direction than both ends. In this shape as well, when the fixing belt 301 passes through the fixing nip section N, its direction of travel is directed outward with respect to the center. Therefore, the recording material is conveyed while being pulled from the center to both ends as it passes through the fixing nip section N, thus suppressing the formation of wrinkles in the recording material.

[0061] In this embodiment, both the protrusion amount (mm) of the downstream sliding surface 303b and the recess amount (mm) of the upstream sliding surface 303c are as shown in Table 2 below. As can be seen from Table 2, the protrusion amount of the downstream sliding surface 303b and the recess amount of the upstream sliding 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 method analysis software "ABAQUS" under the same parameter conditions as the first embodiment. Multiple regression analysis was performed on the relationship in Table 3, and Equation 5 shows the relationship between the protrusion amount D (mm) of the downstream sliding surface 303b and the recess amount E (mm) of the upstream sliding surface 303c, and the conveying direction length M (mm) of the fixing nip section N and the pressure W (kgf) applied by the pressure roller 305. D(E)=-0.00498×M-0.00333×W+0.824 ··· Formula 5

[0063] The protrusion amount D of the downstream sliding surface 303b and the recess amount E of the upstream sliding surface 303c shown in Equation 4 are relational expressions for the configuration combination in this embodiment, and although the numerical values ​​may fluctuate depending on several peripheral parameters, the relative magnitudes and approximate values ​​are correct. In this embodiment, the protrusion amount D of the downstream sliding surface 303b satisfies Equation 6 shown below, and the recess amount E of the upstream sliding 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 with the downstream rubbing surface 303b protruding and a concave shape with the upstream rubbing surface 303c 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 toward both ends and passes through the fixing nip portion N, the same effects as those of the first and second embodiments described above, which can suppress the occurrence of wrinkles in the recording material, can be obtained.

[0065] <Examination experiment> The inventors of the present application confirmed the effects obtained due to the differences in the protruding amount D of the downstream rubbing surface 303b and the recessed amount E of the upstream rubbing surface 303c through experiments. 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, in the conventional example, wrinkles occurred in the recording material, but in the first to third embodiments described above, no wrinkles occurred in the recording material. However, in the first to third embodiments described above, if the transport force of the fixing belt 301 differs significantly between the center and the edges in the width direction, excessive stress is placed on the recording material, which may cause uneven gloss in the toner image. According to the experimental results shown in Table 3, Comparative Example 1 has a protrusion amount of "0.1 mm" which is smaller than the "0.3 mm" of the first embodiment, and although no wrinkles occurred in the recording material, similar to the first embodiment, uneven gloss occurred. Comparative Example 2 has the same recess amount of "0.3 mm" as the second embodiment, but the pressure of the pressure roller 305 is "130 kgf", which is smaller than the "160 kgf" of the second embodiment, and although no wrinkles occurred in the recording material, similar to the second embodiment, uneven gloss occurred in the toner image. Comparative Example 3 is a case where the protrusion amount and the transport direction length (referred to as nip width) of the fixing nip portion N are smaller than in the third embodiment. Similar to the third embodiment, no wrinkles are formed in the recording material, but uneven gloss is present in the toner image. In Comparative Examples 1 to 3, the protrusion amount D and the recess amount E do not satisfy the above-described equations 3, 4, 6, and 7.

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

[0069] 8... Fixing device, 301... Belt (fixing belt), 303... Nip forming member (fixing pad), 303b... Downstream sliding surface, 303c... Upstream sliding surface, 305... Rotating body (pressure roller), N... Nip section (fixing nip section)

Claims

1. A rotating endless belt, A rotating member that contacts the outer surface of the belt, A heating rotating body that contacts the inner circumferential surface of the belt and heats the belt, A tensioning member that contacts the inner circumferential surface of the belt and tensions the belt, The system includes a nip-forming member that is non-rotatingly provided on the inside of the belt and is provided so as to contact the inner circumferential surface of the belt, and which forms a nip portion between itself and the rotating member for gripping and transporting the recording material, The belt is wrapped around the heating rotating body, the tensioning member, and the nip portion forming member. The nip portion forming member has a downstream contact portion that contacts the inner circumferential surface of the belt downstream of the nip portion with respect to the transport direction of the recording material, The downstream contact portion has a downstream end with respect to the transport direction of the recording material such that the central part is located further downstream in the transport direction than both ends, with respect to the width direction intersecting the transport direction. A fixing device characterized by the following features.

2. The downstream end extends downstream in the conveying direction from both ends toward the central part, The fixing device according to feature 1.

3. The downstream end has a central portion that substantially coincides with the center of the nip portion in the width direction. The fixing device according to feature 1.

4. The central portion is the downstream end that is located furthest downstream in the direction of transport. The fixing device according to feature 3.

5. With respect to the width direction, when the center position of the maximum width of the recording material that can pass through the fixing device substantially coincides with the central portion, and the length D in the transport direction from the end position which 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 pressing force W (kgf) of the rotating member satisfy the condition "-0.00997 × M - 0.00667 × W + 1.548 < D < -0.00997 × M - 0.00667 × W + 1.748", The fixing device according to feature 3.

6. At the downstream end, the length D in the transport direction from the end position to the central part is 0.1 mm or more and 0.6 mm or less. The fixing device according to feature 5.

7. With respect to the width direction, the length of the downstream contact portion is longer than the maximum width of the recording material that can pass through the fixing device. The fixing device according to feature 1.

8. The nip portion forming member has an upstream contact portion that is provided upstream of the nip portion and contacts the inner circumferential surface of the belt, With respect to the conveying direction, the upstream end of the upstream contact portion has a central portion located downstream of both ends in the conveying direction with respect to the width direction intersecting the conveying direction. The fixing device according to feature 1.

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