Fixing apparatus and image forming apparatus

Abstract

To provide a fixing device and an image forming apparatus that can achieve both improvement in image quality and improvement in durability.SOLUTION: A fixing pad 56 has a nip forming surface for forming a fixing nip 80, and a curved surface located upstream of the nip forming surface in a conveyance direction of a recording medium S. When it is assumed that the distance in the conveyance direction between an upstream-side edge 81 in the conveyance direction of the fixing nip 80 and a boundary 73 between the nip forming surface and the curved surface of the fixing pad 56, takes a positive value when the boundary 73 is on a downstream in the conveyance direction of the upstream-side edge 81, and takes a negative value when the boundary 73 is on the upstream in the conveyance direction of the upstream-side edge 81, a value of the distance at the center in a width direction of the recording medium S is larger than a value of the distance at an end in the width direction. At the center in the width direction of the recording medium S, the boundary 73 between the nip forming surface and the curved surface is on the downstream in the conveyance direction of the upstream-side edge 81 of the fixing nip 80.SELECTED DRAWING: Figure 4

Description

Technical Field

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

Background Art

[0002] Regarding a fixing device, Japanese Patent Application Laid-Open No. 2020-91461 (Patent Document 1) discloses a nip forming member that forms a fixing nip by sandwiching a fixing belt between a pressure roller. The nip forming member has a flat nip forming portion that contacts the inner peripheral surface of the fixing belt, and a pair of bent portions that bend from both end portions of the nip forming portion in the belt rotation direction to the side opposite to the pressure roller side.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When the paper passing through the fixing device cannot maintain flatness in the paper width direction, wrinkles in the paper, bleeding of the image, etc. may occur, and the image quality may deteriorate. In addition, when the paper passes through the fixing nip, the release layer provided on the surface of the fixing belt may be scraped off, and the durability of the fixing device may decrease.

[0005] In the present disclosure, a fixing device and an image forming apparatus that can achieve both improvement in image quality and improvement in durability are proposed.

Means for Solving the Problems

[0006] In the present disclosure, the following fixing device is proposed. (Section 1) A fixing device for fixing an image of toner formed on a recording medium to the recording medium comprises a fixing belt, a fixing pad having a contact surface that contacts the fixing belt, and a pressure roller that forms a fixing nip by pressing the fixing pad via the fixing belt. The contact surface of the fixing pad has a nip-forming surface for forming a fixing nip and a curved surface that is upstream of the nip-forming surface in the transport direction of the recording medium and curves away from the pressure roller. Assuming that the distance in the transport direction between the upstream edge of the fixing nip in the transport direction and the boundary between the nip-forming surface and the curved surface of the fixing pad is positive when the boundary is downstream of the upstream edge in the transport direction and negative when the boundary is upstream of the upstream edge in the transport direction, the value of the distance at the center of the recording medium in the width direction is greater than the value of the distance at the edges in the width direction. At the center of the recording medium in the width direction, the boundary between the nip-forming surface and the curved surface is downstream of the upstream edge of the fixing nip in the transport direction.

[0007] (Clause 2) In the fixing apparatus described in paragraph 1, the boundary between the nip-forming surface and the curved surface at the widthwise end of the recording medium may be located upstream in the transport direction from the upstream edge of the fixing nip.

[0008] (3) In the fixing device described in paragraph 1 or 2, the outer diameter of the pressure roller at the center of the recording medium in the width direction may be larger than the outer diameter of the pressure roller at the end of the recording medium in the width direction.

[0009] (Clause 4) In the fixing device described in any one of paragraphs 1 to 3, the nip-forming surface may be a flat surface.

[0010] (Paragraph 5) In the fixing device described in any one of paragraphs 1 to 4, the position of the edge in the width direction of the recording medium may include the position through which the edge of the recording medium having the widest width among the recording media that can be fed through the fixing device passes.

[0011] (Paragraph 6) In the fixing device described in any one of paragraphs 1 to 4, the position of the edge in the width direction of the recording medium may include the position through which the edge of the recording medium that is fed to the fixing device most frequently passes.

[0012] This disclosure proposes the following image forming apparatus. (Clause 7) The image forming apparatus comprises an image forming unit that forms a toner image on a recording medium, and a fixing device according to any one of the above paragraphs 1 to 6, which fixes the toner image to the recording medium. [Effects of the Invention]

[0013] According to the fixing apparatus and image forming apparatus in accordance with this disclosure, it is possible to achieve both improved image quality and improved durability. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram of an image forming apparatus according to an embodiment. [Figure 2] Figure 1 is a schematic diagram showing the fixing device of the image forming apparatus. [Figure 3] This is a perspective view showing the outer shape of the fixing pad. [Figure 4] This is a schematic diagram showing the relationship between the shape of the contact surface of the fixing pad and the fixing nip. [Figure 5] Figure 4 is a cross-sectional view of the fixing pad along the VV line. [Figure 6] Figure 4 shows a cross-sectional view of the fixing pad along the line VI-VI. [Figure 7] This figure shows the amount of wear on the fixing belt at the center and edges in the width direction of the recording medium. [Figure 8] This is a schematic diagram showing the relationship between the shape of the contact surface of the fixing pad and the fixing nip according to the second embodiment. [Figure 9] This is a perspective view showing the external shape of the fixing pad according to the third embodiment. [Figure 10] This figure shows the external shape of the pressure roller according to the third embodiment. [Figure 11] It is a schematic diagram showing the relationship between the shape of the contact surface of the fixing pad and the fixing nip according to the third embodiment. [Figure 12] It is a schematic diagram showing a fixing device according to the fourth embodiment.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, embodiments of a fixing device and an image forming apparatus according to the present disclosure will be described with reference to the drawings. In the following description, the same parts and components are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated. It is also initially planned that any configuration can be extracted from each embodiment and they can be arbitrarily combined.

[0016] [First Embodiment] [Configuration of Image Forming Apparatus 100] FIG. 1 is a schematic diagram of an image forming apparatus 100 according to an embodiment. Hereinafter, the image forming apparatus 100 as a color printer will be described, but the image forming apparatus 100 is not limited to a color printer. For example, the image forming apparatus 100 may be a monochrome printer, a fax machine, or a multi-functional peripheral (MFP) having functions of a scanner, a printer, and a copier.

[0017] The image forming apparatus 100 includes image forming units 1Y, 1M, 1C, 1K, an intermediate transfer belt 30, a primary transfer roller 31, a secondary transfer roller 33, a cassette 37, a driven roller 38, a driving roller 39, a timing roller 40, a fixing device 50, a housing 90, a control unit 101, and an operation panel 102.

[0018] The housing 90 defines the outer shell of the image forming apparatus 100. The housing 90 houses the image forming units 1Y, 1M, 1C, 1K, the intermediate transfer belt 30, the primary transfer roller 31, the secondary transfer roller 33, the cassette 37, the driven roller 38, the drive roller 39, the timing roller 40, the fixing device 50, and the control unit 101.

[0019] The control panel 102 is mounted on the housing 90. The control panel 102 includes a display unit that notifies the user of various information and an input unit that accepts various user operations. More specifically, the control panel 102 includes, as an input unit, a group of various input keys including a numeric keypad and a touch sensor, and as a display unit, a liquid crystal display unit integrated with the touch sensor and various indicators such as LEDs (Light Emitting Diodes).

[0020] The image forming unit is comprised of image forming units 1Y, 1M, 1C, and 1K, an intermediate transfer belt 30, a primary transfer roller 31, a secondary transfer roller 33, a cassette 37, a driven roller 38, a drive roller 39, and a timing roller 40. This image forming unit forms a toner image on a recording medium S that is transported along a transport path 41, which will be described later.

[0021] Image forming units 1Y, 1M, 1C, and 1K are arranged in order along the intermediate transfer belt 30. Image forming unit 1Y receives toner from toner bottle 15Y and forms an image of yellow (Y) toner. Image forming unit 1M receives toner from toner bottle 15M and forms an image of magenta (M) toner. Image forming unit 1C receives toner from toner bottle 15C and forms an image of cyan (C) toner. Image forming unit 1K receives toner from toner bottle 15K and forms an image of black (BK) toner.

[0022] The image forming units 1Y, 1M, 1C, and 1K are each arranged along the intermediate transfer belt 30 in the order of the rotation direction of the intermediate transfer belt 30. Each image forming unit 1Y, 1M, 1C, and 1K comprises a photoreceptor 10 as an image carrier and a plurality of functional members. The plurality of functional members include, for example, a charging device 11, an exposure device 12, a developing device 13, and a cleaning device 17.

[0023] The charging device 11 uniformly charges the surface of the photoreceptor 10. The exposure device 12 irradiates the photoreceptor 10 with laser light in response to a control signal from the control unit 101, and exposes the surface of the photoreceptor 10 according to the input image pattern. As a result, an electrostatic latent image corresponding to the input image is formed on the photoreceptor 10.

[0024] The developing device 13 rotates the developing roller 14 and applies a developing bias to the developing roller 14, causing toner to adhere to the surface of the developing roller 14. As a result, the toner is transferred from the developing roller 14 to the photoreceptor 10, and a toner image corresponding to the electrostatic latent image is developed on the surface of the photoreceptor 10.

[0025] The photoreceptor 10 and the intermediate transfer belt 30 are in contact with each other at the portion where the primary transfer roller 31 is provided. The primary transfer roller 31 has a roller shape and is configured to be rotatable. When a transfer voltage opposite in polarity to the toner image is applied to the primary transfer roller 31, the toner image is transferred from the photoreceptor 10 to the intermediate transfer belt 30. The images of yellow (Y) toner, magenta (M) toner, cyan (C) toner, and black (BK) toner are sequentially superimposed and transferred from the photoreceptor 10 to the intermediate transfer belt 30. As a result, the images of the color toners are formed on the intermediate transfer belt 30.

[0026] The toner comprises a resin and a colorant dispersed in the resin. The resin can be any conventionally known material without particular limitation, such as polyester resin, acrylic resin, styrene-acrylic copolymer resin, urethane resin, vinyl chloride resin, or vinyl acetate resin. In addition to the resin and colorant, the toner also contains a release agent. The release agent contains a wax. The wax can be any conventionally known material without particular limitation, such as synthetic ester wax.

[0027] The intermediate transfer belt 30 is stretched over the driven roller 38 and the drive roller 39. The drive roller 39 is rotationally driven, for example, by a motor (not shown). The intermediate transfer belt 30 and the driven roller 38 rotate in conjunction with the drive roller 39. This transports the toner image on the intermediate transfer belt 30 to the secondary transfer roller 33.

[0028] The cleaning device 17 is pressed against the photoreceptor 10. The cleaning device 17 collects toner remaining on the surface of the photoreceptor 10 after the toner image has been transferred.

[0029] A recording medium S is set in the cassette 37. The recording medium S is used to record images formed in the image forming apparatus 100. The recording medium S may be a paper recording medium such as plain paper, photographic paper, cardboard, or envelope, or it may be a resin recording medium such as OHP (overhead projector) paper. The recording medium S is fed one sheet at a time from the cassette 37 along the transport path 41 to the secondary transfer roller 33 by the timing roller 40.

[0030] The secondary transfer roller 33 has a roller shape and is configured to be rotatable. The secondary transfer roller 33 applies a transfer voltage opposite in polarity to the toner image to the recording medium S being transported. As a result, the toner image is attracted from the intermediate transfer belt 30 to the secondary transfer roller 33, and the toner image on the intermediate transfer belt 30 is transferred to the recording medium S. The timing of the transport of the recording medium S to the secondary transfer roller 33 is adjusted by the timing roller 40 to match the position of the toner image on the intermediate transfer belt 30. The timing roller 40 transfers the toner image on the intermediate transfer belt 30 to the appropriate position on the recording medium S.

[0031] The fuser unit 50 pressurizes and heats the recording medium S as it passes through it. This fixes the toner image onto the recording medium S. In this way, the fuser unit 50 fixes the toner image on the recording medium S as it is transported along the transport path 41 onto the recording medium S. The recording medium S on which the toner image has been fixed is discharged into the tray.

[0032] Although the image forming apparatus 100 employing a tandem printing method has been described above, the printing method of the image forming apparatus 100 is not limited to the tandem method. The arrangement of each component within the image forming apparatus 100 can be appropriately changed according to the printing method adopted. The image forming apparatus 100 may employ a rotary method or a direct transfer method as its printing method. In the case of a rotary method, the image forming apparatus 100 consists of one photoreceptor 10 and a plurality of developing devices 13 configured to be rotatable on the same axis. During printing, the image forming apparatus 100 guides each developing device 13 sequentially to the photoreceptor 10 to develop the image of each color toner. In the case of a direct transfer method, the image forming apparatus 100 directly transfers the toner image formed on the photoreceptor 10 to the recording medium S.

[0033] <Configuration of the fixing device 50> Next, the structure of the fixing device 50 will be described in detail. Figure 2 is a schematic diagram showing the fixing device 50 of the image forming apparatus 100 shown in Figure 1.

[0034] As shown in Figure 2, the fixing device 50 mainly comprises a pressure roller 51, an endless fixing belt 52, a heating roller 53 having a fixing heater 54, and a fixing pad 56.

[0035] The pressure roller 51 is positioned on the outside of the fixing belt 52. The pressure roller 51 faces the fixing pad 56. The pressure roller 51 presses the fixing pad 56 by sandwiching the fixing belt 52 between itself and the fixing pad 56 due to the biasing force of a biasing member (not shown). As a result, the fixing belt 52 is pressed against the fixing pad 56. The pressure roller 51 presses the fixing pad 56 through the fixing belt 52.

[0036] As the pressure roller 51 compresses and deforms to conform to the shape of the fixing pad 56, a fixing nip is formed between the pressure roller 51 and the fixing belt 52, which is the contact area between the pressure roller 51 and the fixing belt 52.

[0037] The pressure roller 51 is composed of, for example, a core metal, an elastic layer, and a release layer. The core metal is made of a metal such as aluminum or steel and has a hollow cylindrical shape. The elastic layer covers the outer surface of the core metal. The elastic layer is made of an elastic material such as silicone rubber. The release layer is provided to cover the elastic layer. The release layer is made of, for example, PFA (tetrafluoroethylene perfluoroalkyl vinyl ether copolymer) or PTFE (polytetrafluoroethylene).

[0038] The pressure roller 51 rotates due to the torque from the drive motor. As the pressure roller 51 rotates, the fixing belt 52 rotates in a circular motion. As the pressure roller 51 rotates, the fixing belt 52 rotates in the opposite direction to the rotation of the pressure roller 51. The heating roller 53 rotates in conjunction with the circular motion of the fixing belt 52.

[0039] The pressure roller 51 transports the recording medium S by driving the fixing belt 52 to rotate. The fixing pad 56 and the pressure roller 51, via the fixing belt 52, grip the recording medium S, which is carrying unfixed toner T, with fixing nip and heat and pressurize the recording medium S while transporting it in a predetermined transport direction. As the recording medium S passes through the fixing nip, the toner T is heated and pressurized, causing the toner T formed on the recording medium S to melt and the image of the toner T to be fixed to the recording medium S.

[0040] The axial length of the pressure roller 51 (in Figure 2, the direction perpendicular to the paper surface) is longer than the width of the widest recording medium S that can be fed through the fixing device 50. The same applies to the fixing belt 52, heating roller 53, and fixing pad 56. The width of the recording medium S refers to the dimension of the recording medium S in a direction perpendicular to the transport direction and perpendicular to the thickness direction of the recording medium S (in Figure 2, the direction perpendicular to the paper surface). The transport direction of the recording medium S along the transport path 41 is indicated by an arrow in Figure 2.

[0041] The fixing belt 52 is formed in an endless shape. The fixing pad 56 and the heating roller 53 are positioned inside the circumferential path of the fixing belt 52. The fixing belt 52 is rotatably wrapped around the fixing pad 56 and the heating roller 53.

[0042] The fixing belt 52 has a base layer, an elastic layer covering the base layer, and a release layer covering the elastic layer. The base layer may be made of a metal such as nickel, stainless steel (SUS), or copper, or a resin such as polyimide, polyamide, or polyimidoamide. The elastic layer may be made of a highly heat-resistant material such as silicone rubber or fluororubber. The release layer may be a material that provides release properties, such as a fluororesin or a fluorine-based coating.

[0043] The heating roller 53 is positioned inside the fixing belt 52. The heating roller 53 is positioned away from the fixing pad 56. The heating roller 53 has a core made of a metal material such as aluminum or SUS. The core has a hollow cylindrical shape. The outer surface of the core may be coated with PTFE or the like to suppress the occurrence of scratches on the outer surface caused by foreign matter. The outer surface of the core may also be coated with a heat-resistant release layer.

[0044] A fixing heater 54, which is an example of a heating element that heats the fixing belt 52, is positioned inside the core metal. The heating roller 53 has the fixing heater 54 built into it. The fixing heater 54 heats the core metal from the inside. The heat generated by the fixing heater 54 is transmitted to the fixing belt 52 via the heating roller 53, and further transmitted to the recording medium S that passes through the fixing nip via the fixing belt 52.

[0045] The fixing heater 54 is, for example, a halogen heater. When a halogen heater is used as the fixing heater 54, the inner surface of the core metal may be painted black. The heating element that heats the fixing belt 52 may be an induction heater or a carbon heater in addition to a halogen heater, and a system that generates heat using the heating roller 53 and the fixing belt 52 as resistance heating elements may also be used.

[0046] The fixing pad 56 is positioned inside the fixing belt 52. The fixing pad 56 is positioned opposite the pressure roller 51 with the fixing belt 52 in between. The fixing pad 56 is made of a heat-resistant resin such as polyphenylene sulfide, polyimide, or liquid crystal polymer. The fixing pad 56 is non-rotatable, and the rotating fixing belt 52 slides against the surface of the fixing pad 56.

[0047] <Configuration of fixing pad 56> Figure 3 is a perspective view showing the outer shape of the fixing pad 56. The fixing pad 56 has a contact surface 60 that contacts the inner circumferential surface of the fixing belt 52, and a support surface 61 opposite to the contact surface 60. The contact surface 60 faces the inner circumferential surface of the fixing belt 52. The pressure roller 51 elastically deforms to conform to the surface shape of the contact surface 60. The fixing pad 56 is held inside the fixing belt 52 by being supported by the support surface 61 in contact with a support member (not shown).

[0048] Figure 4 is a schematic diagram showing the relationship between the shape of the contact surface 60 of the fixing pad 56 and the fixing nip 80. Figure 5 is a cross-sectional view of the fixing pad 56 along the VV line shown in Figure 4. Figure 6 is a cross-sectional view of the fixing pad 56 along the VI-VI line shown in Figure 4. Figures 5 and 6 schematically illustrate the fixing pad 56, a part of the pressure roller 51, and the transport path of the recording medium S fed through the fixing device 50. In Figures 5 and 6, the fixing belt 52 is omitted from the illustration. The non-contact portion 64 shown in Figures 5 and 6 is the part of the outer surface of the fixing pad 56 that faces the inner circumferential surface of the fixing belt 52 but does not come into contact with the fixing belt 52 (see also Figure 2).

[0049] Referring to Figures 3 to 6, the contact surface 60 has a nip-forming surface 71. The nip-forming surface 71 is a portion of the surface of the fixing pad 56 for forming a fixing nip 80 between the pressure roller 51 and the fixing belt 52. The nip-forming surface 71 may be a flat surface. Here, "flat" includes the nip-forming surface 71 being a strictly planar surface and also being approximately flat. For example, a surface with a small curvature and a slight curve may be included in "flat".

[0050] The contact surface 60 has a curved surface 72. The curved surface 72 is located upstream of the nip-forming surface 71 in the transport direction of the recording medium S (the "paper feeding direction" shown in Figure 4). The curved surface 72 is curved away from the pressure roller 51. The curved surface 72 is formed to move away from the pressure roller 51 with respect to a virtual plane that extends from the nip-forming surface 71. The curved surface 72 is curved with respect to the paper feeding direction so that it approaches the pressure roller 51 as it moves downstream in the paper feeding direction. The curved surface 72 is curved in a convex shape toward the paper feeding path of the recording medium S. The curved surface 72 may have a cylindrical shape. The nip-forming surface 71 may constitute the tangent plane of the curved surface 72.

[0051] The contact surface 60 has a boundary 73 between the nip-forming surface 71 and the curved surface 72. The boundary 73 constitutes the upstream end of the nip-forming surface 71 in the transport direction of the recording medium S. The boundary 73 constitutes the downstream end of the curved surface 72 in the transport direction of the recording medium S.

[0052] The width direction of the recording medium S being transported to the fuser 50 is the vertical direction in Figure 4, and the direction perpendicular to the paper plane in Figures 5 and 6. The boundary 73 is bent with respect to the width direction of the recording medium S. The fuser pad 56 has a pad center portion 76 at the center of the recording medium S in the width direction, and a pad end portion 77 at the edge of the recording medium S in the width direction. The boundary 73 at the pad end portion 77 is located upstream of the boundary 73 at the pad center portion 76 in the transport direction of the recording medium S.

[0053] The pad edge 77 includes positions through which the edges Se1 and Se2 of the recording medium S pass in the width direction of the recording medium S. The pad edge 77 may also include positions through which the edges Se1 and Se2 of the recording medium S with the widest width among the recording medium S that can be fed into the fuser 50 pass. The pad edge 77 may also include positions through which the edges Se1 and Se2 of the recording medium S that are fed into the fuser 50 most frequently pass.

[0054] For example, if the image forming apparatus 100 is an A4 machine, the pad end 77 may include a position through which the edge of the paper width passes when A4 size paper is fed vertically, or a position through which the edge of the paper width passes when Letter size or Legal size paper is fed vertically. If the image forming apparatus 100 is an A3 machine, the pad end 77 may include a position through which the edge of the paper width passes when Letter size paper is fed horizontally, or a position through which the edge of the paper width passes when A4 size paper is fed vertically, or a position through which the edge of the paper width passes when SRA3 size paper is fed vertically.

[0055] The upstream edge 81 of the fuser nip 80, which is formed between the pressure roller 51 and the fuser belt 52, is the upstream edge in the direction of transport of the recording medium S. The upstream edge 81 is the starting end where pressure is applied to the recording medium S and toner T as they pass through the fuser nip 80. The downstream edge 82 of the fuser nip 80 is the downstream edge 82 in the direction of transport of the recording medium S. The downstream edge 82 is the ending end where pressure is applied to the recording medium S and toner T as they pass through the fuser nip 80.

[0056] In this embodiment, the upstream edge 81 and the downstream edge 82 extend in the width direction of the recording medium S. The length of the fixing nip 80 in the transport direction of the recording medium S (shown as "Nip width" in Figure 4) is constant at the center and ends in the width direction of the recording medium S. The fixing device 50 of this embodiment is configured to obtain a uniform Nip width in the width direction of the recording medium S. The Nip width does not necessarily have to be uniform in the width direction of the recording medium S.

[0057] At the center of the pad 76, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is downstream in the direction of transport of the recording medium S from the upstream edge 81 of the fixing nip 80. At the end of the pad 77, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is upstream in the direction of transport of the recording medium S from the upstream edge 81 of the fixing nip 80.

[0058] In this specification, the distance between the upstream edge 81 of the fixing nip 80 and the boundary 73 of the fixing pad 56 in the transport direction of the recording medium S is positive when the boundary 73 is downstream of the upstream edge 81 in the transport direction, and negative when the boundary 73 is upstream of the upstream edge 81 in the transport direction. The distance Da shown in Figure 4 is the distance between the boundary 73 and the upstream edge 81 at the center of the pad 76. The distance Db is the distance between the boundary 73 and the upstream edge 81 at the end of the pad 77.

[0059] In this embodiment, at the center portion 76 of the pad, the boundary 73 is downstream in the conveying direction from the upstream edge portion 81, so the distance Da takes a positive value. At the end portion 77 of the pad, the boundary 73 is upstream in the conveying direction from the upstream edge portion 81, so the distance Db takes a negative value. The value of distance Da at the center portion 76 of the pad is greater than the value of distance Db at the end portion 77 of the pad.

[0060] Figure 5 shows a cross-section of the fixing pad 56 and the pressure roller 51 at the center 76 of the pad. As shown in Figure 5, at the center 76 of the pad, the boundary 73 is downstream in the conveying direction from the upstream edge 81. The boundary 73 is inside the fixing nip 80. Part of the curved surface 72 is inside the fixing nip 80. At the entrance to the fixing nip 80, the curved surface 72 forms the fixing nip 80. Inside the fixing nip 80, the pressure roller 51 deforms to match the shape of the fixing pad 56. Therefore, at the center 76 of the pad, the fixing nip 80 takes on a curved shape near its entrance. The recording medium S is bent along the curved surface 72 at the entrance to the fixing nip 80 at the center of the width direction of the recording medium S.

[0061] Figure 6 shows a cross-section of the fixing pad 56 and the pressure roller 51 at the pad end 77. As shown in Figure 6, at the pad end 77, the boundary 73 is upstream in the conveying direction from the upstream side edge 81. The boundary 73 is outside the fixing nip 80. The entire curved surface 72 is outside the fixing nip 80. The nip forming surface 71 forms the entire fixing nip 80. Inside the fixing nip 80, the pressure roller 51 deforms to match the shape of the fixing pad 56. Therefore, at the pad end 77, the fixing nip 80 has a flat shape. The recording medium S passes through the fixing nip 80 while maintaining a flat shape at its widthwise end.

[0062] Figure 7 shows the amount of wear on the fixing belt 52 at the center and edges in the width direction of the recording medium S. The horizontal axis of Figure 7 indicates the position of the fixing belt 52 in the width direction of the recording medium S. The vertical axis of Figure 7 indicates the amount of wear on the fixing belt 52. The dashed lines in Figure 7 indicate the position corresponding to the edge of the recording medium S.

[0063] As shown in Figure 7, at the ends of the fixing belt 52, the amount of wear on the surface of the fixing belt 52 increases due to abrasion caused by the roughness of the cut surface of the recording medium S. To reduce the amount of wear on the fixing belt 52, it is desirable to eliminate the speed difference between the fixing belt 52 and the recording medium S. For this reason, as shown in Figure 6, the shape of the fixing nip 80 at the pad end 77 is made flat, and the paper path of the recording medium S is made as straight as possible.

[0064] On the other hand, in the central part 76 of the pad, as shown in Figure 5, the shape of the fixing nip 80 is such that the recording medium S is bent near the entrance of the fixing nip 80. By bending the recording medium within the fixing nip 80, the rigidity in the width direction of the recording medium S is improved. As a result, the flatness of the recording medium S is maintained, and the occurrence of wrinkles and image blurring of the recording medium S is suppressed.

[0065] [Second Embodiment] Figure 8 is a schematic diagram showing the relationship between the shape of the contact surface 60 of the fixing pad 56 and the fixing nip 80 according to the second embodiment. The fixing pad 56 according to the second embodiment has the same external shape as shown in Figure 3 as in the first embodiment. In the first embodiment, an example was described in which the distance Db between the boundary 73 at the pad end 77 and the upstream edge 81 is a negative value. The distance Db does not have to be a negative value, as long as it is smaller than the distance Da between the boundary 73 at the pad center 76 and the upstream edge 81.

[0066] In the second embodiment, the force with which the pressure roller 51 presses the fixing pad 56 is increased compared to the first embodiment. The amount of pressure applied by the pressure roller 51 to the fixing pad 56 is increased compared to the first embodiment. As a result, the Nip width shown in Figure 8 is larger than the Nip width in the first embodiment shown in Figure 4. Consequently, at both the pad center 76 and the pad edge 77, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is downstream in the transport direction of the recording medium S from the upstream edge 81 of the fixing nip 80. The distance Da shown in Figure 8 takes a positive value, and the distance Db takes a positive value.

[0067] In the second embodiment, the boundary 73 at the pad edge 77 is located upstream in the transport direction of the recording medium S compared to the boundary 73 at the pad center 76. Therefore, the absolute value of distance Da is greater than the absolute value of distance Db. Similar to the first embodiment, the value of distance Da at the pad center 76 is greater than the value of distance Db at the pad edge 77.

[0068] [Third Embodiment] Figure 9 is a perspective view showing the external shape of the fixing pad 56 according to the third embodiment. Unlike the fixing pad 56 of the first embodiment, the boundary 73 between the nip-forming surface 71 and the curved surface 72 on the contact surface 60 of the fixing pad 56 according to the third embodiment is not bent. The boundary 73 extends in the width direction of the recording medium S. The boundary 73 is at the same position throughout its entirety in the transport direction of the recording medium S. The fixing pad 56 has the same shape in the width direction of the recording medium S.

[0069] Figure 10 shows the external shape of the pressure roller 51 according to the third embodiment. The pressure roller 51 has a central roller portion 96 in the width direction of the recording medium S and a roller end portion 97 at the width direction of the recording medium S. The pressure roller 51 of the third embodiment does not have a uniform outer diameter in the width direction of the recording medium S. Specifically, the outer diameter of the pressure roller 51 at the central roller portion 96 is larger than the outer diameter of the pressure roller 51 at the roller end portion 97.

[0070] Figure 11 is a schematic diagram showing the relationship between the shape of the contact surface 60 of the fixing pad 56 and the fixing nip 80 according to the third embodiment. In the third embodiment, the upstream edge 81 and the downstream edge 82 have a curved shape. The upstream edge 81 in the pad center 76 is located upstream in the direction of transport of the recording medium S compared to the upstream edge 81 in the pad end 77. The downstream edge 82 in the pad center 76 is located downstream in the direction of transport of the recording medium S compared to the downstream edge 82 in the pad end 77.

[0071] The Nip width shown in Figure 11 is maximum at the center 76 of the pad and decreases as it moves away from the center 76 in the width direction of the recording medium S. The Nip width at the center 76 of the pad is larger than the Nip width at the edge 77 of the pad.

[0072] At the center of the pad 76, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is downstream in the direction of transport of the recording medium S from the upstream edge 81 of the fixing nip 80. At the end of the pad 77, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is upstream in the direction of transport of the recording medium S from the upstream edge 81 of the fixing nip 80.

[0073] In the central part 76 of the pad, the boundary 73 is downstream in the conveying direction from the upstream edge 81, so the distance Da takes a positive value. In the end part 77 of the pad, the boundary 73 is upstream in the conveying direction from the upstream edge 81, so the distance Db takes a negative value. Similar to the first embodiment, the value of distance Da in the central part 76 of the pad is greater than the value of distance Db in the end part 77 of the pad.

[0074] [Fourth Embodiment] In the first embodiment, an upper-pad configuration in which the fixing pad 56 is positioned above the fixing nip 80 was described with reference to Figure 2. As another example, a lower-pad configuration in which the fixing pad 56 is positioned below the fixing nip 80 may also be used. Figure 12 is a schematic diagram showing a fixing device 50 according to the fourth embodiment.

[0075] As shown in Figure 12, the lower pad type fixing device 50 mainly comprises a pressure roller 51, an endless fixing belt 52, a heating roller 53 having a fixing heater 54, a fixing pad 56, and an endless fixing belt 57.

[0076] The fixing belt 52 is rotatably wrapped around the fixing pad 56. The fixing pad 56 is positioned inside the circumferential path of the fixing belt 52. The fixing pad 56 is positioned inside the fixing belt 52. The pressure roller 51 is positioned outside the fixing belt 52. The pressure roller 51 faces the fixing pad 56.

[0077] The fixing belt 57 is rotatably wrapped around the pressure roller 51 and the heating roller 53. The pressure roller 51 and the heating roller 53 are positioned inside the circumferential path of the fixing belt 57. The fixing belt 57, like the fixing belt 52, has a base layer, an elastic layer covering the base layer, and a release layer covering the elastic layer.

[0078] The fixing pad 56 is positioned facing the pressure roller 51, with fixing belts 52 and 57 sandwiched between them. The pressure roller 51 presses the fixing pad 56 by sandwiching the fixing belts 52 and 57 between them, due to the biasing force of a biasing member (not shown). As a result, fixing belt 52 is pressed against the fixing pad 56. The pressure roller 51 presses the fixing pad 56 through the fixing belts 52 and 57. As the pressure roller 51 compresses and deforms to conform to the shape of the fixing pad 56, a fixing nip, which is the contact area between fixing belt 52 and fixing belt 57, is formed between fixing belt 52 and fixing belt 57.

[0079] The pressure roller 51 rotates due to torque from the drive motor. As the pressure roller 51 rotates, the fixing belt 57 revolves. As the pressure roller 51 rotates, the fixing belt 52 rotates in the opposite direction to the rotation of the pressure roller 51. The fixing pad 56 is immobile, and the rotating fixing belt 52 slides against the surface of the fixing pad 56. The pressure roller 51 transports the recording medium S by driving the fixing belts 52 and 57 to rotate.

[0080] The heating roller 53 is located inside the fixing belt 57. The heating roller 53 rotates in conjunction with the rotational motion of the fixing belt 57. The heating roller 53 is located away from the pressure roller 51. The heating roller 53 has a core made of metal material and a fixing heater 54 located inside the core. The heat generated by the fixing heater 54 is transmitted to the fixing belt 57 via the heating roller 53, and further transmitted to the recording medium S passing through the fixing nip via the fixing belt 57.

[0081] In the lower pad type fixing device 50, the fixing nip is shaped in such a way that the recording medium S is bent at the entrance of the fixing nip 80 in the central part 76 of the pad, and the fixing nip 80 is made flat at the pad end 77, thereby reducing the amount of wear on the fixing belt 52 and suppressing the occurrence of wrinkles on the recording medium S and blurring of images.

[0082] [Mechanism of Action and Effects] Although there is some overlap with the explanation above, the characteristic configuration and effects of the fixing device 50 of the embodiment are summarized as follows.

[0083] As shown in Figures 4, 8, and 11, in the central part 76 of the pad, the boundary 73 between the nip-forming surface 71 and the curved surface 72 of the fixing pad 56 is downstream in the transport direction of the recording medium S from the upstream edge 81 of the fixing nip 80. The curved surface 72 of the fixing pad 56 is located within the fixing nip 80. In the central part 76 of the pad, at the entrance to the fixing nip 80, the recording medium S being transported along the curved surface 72 is bent in the transport direction. This increases the rigidity of the recording medium S in the width direction, making it more resistant to deformation in the width direction. Since the flatness of the recording medium S is maintained in the central part 76 of the pad, the occurrence of wrinkles and image blurring of the recording medium is suppressed, and image quality can be improved.

[0084] Envelopes, in particular, are fed into the fixing device 50 in a double-layered state, making them prone to wrinkles and image blurring. On the other hand, envelopes generally have a narrower paper width than the recording medium S, which is fed most frequently. By configuring the pad so that the envelope can be bent at the center 76, it is possible to suppress the occurrence of wrinkles and image blurring in envelopes with a narrow paper width.

[0085] Furthermore, when the recording medium S passes the fuser nip 80, friction occurs between the release layer on the surface of the fuser belt 52 and the recording medium S, causing the release layer to be scraped off. As the scraping of the release layer progresses, the release properties of the toner T from the fuser belt 52 decrease. If the toner T passes the fuser nip 80 without being separated from the fuser belt 52, the toner T remaining on the fuser belt 52 will adhere to the recording medium S when it next circulates to the fuser nip 80, resulting in image contamination. In particular, the amount of scraping of the release layer on the fuser belt 52 is large at positions corresponding to the edges Se1 and Se2 of the recording medium S.

[0086] Therefore, the distance between the upstream edge 81 of the fixing nip 80 and the boundary 73 of the fixing pad 56 is made larger in the center 76 of the pad and smaller in the pad edge 77. By making the shape of the fixing nip 80 closer to flat at the pad edge 77, the speed difference between the fixing belt 52 and the recording medium S can be reduced. This reduces the stress on the fixing belt 52 when the fixing belt 52 and the recording medium S pass over the fixing nip 80, thereby reducing friction between the release layer of the fixing belt 52 and the recording medium S. This suppresses wear of the release layer.

[0087] Therefore, the fixing device 50 of the embodiment can achieve both improved image quality and improved durability by defining the positions of the boundary 73 of the fixing pad 56 and the upstream edge 81 of the fixing nip 80.

[0088] As shown in Figures 4 and 11, at the pad end 77, the boundary 73 of the fixing pad 56 may be upstream in the conveying direction of the upstream side edge 81 of the fixing nip 80. At the pad end 77 where the amount of release layer abrasion of the fixing belt 52 is large, the entire curved surface 72 of the fixing pad 56 is positioned outside the fixing nip 80. This prevents the shape of the curved surface 72 from affecting the shape of the fixing nip 80 at the pad end 77. At the pad end 77, the shape of the fixing nip 80 can be made closer to flat. Therefore, abrasion of the release layer of the fixing belt 52 can be suppressed.

[0089] As shown in Figure 10, the outer diameter of the pressure roller 51 at the center 96 of the roller may be larger than the outer diameter of the pressure roller 51 at the end 97 of the roller. By combining a pressure roller 51 of this shape with a fixing pad 56 having a nip-forming surface 71 and a curved surface 72, it is possible to realize a configuration in which the boundary 73 at the center 76 of the pad is located downstream in the conveying direction from the upstream edge 81, and the distance between the upstream edge 81 and the boundary 73 at the center 76 of the pad is greater than the distance at the end 77 of the pad. A fixing pad 56 having the outer shape shown in Figure 9 may also be used, in which case the outer shape of the fixing pad 56 can be simplified, thereby reducing the manufacturing cost of the fixing pad 56.

[0090] As shown in Figures 5 and 6, the nip-forming surface 71 may be a flat surface. In this way, the shape of the fixing nip 80 can be made flat, corresponding to the nip-forming surface 71. This makes it possible to achieve a shape that puts the least stress on the fixing belt 52 when the fixing belt 52 and the recording medium S pass over the fixing nip 80. As a result, the wear of the release layer on the fixing belt 52 can be suppressed, and the durability can be further improved.

[0091] As shown in Figures 4 and 11, the pad end 77 may include the position where the edges Se1 and Se2 of the recording medium S with the widest width that can be fed into the fuser 50 pass. The pad end 77 may also include the position where the edges Se1 and Se2 of the recording medium S that are fed into the fuser 50 most frequently pass. At the position where the edges Se1 and Se2 of the recording medium S pass, the amount of wear on the release layer of the fuser belt 52 is large. By making the shape of the fuser nip 80 at the position where the edges Se1 and Se2 of the recording medium S pass closer to flat, the wear on the release layer of the fuser belt 52 can be suppressed and the durability can be improved.

[0092] As shown in Figure 1, the image forming apparatus 100 includes an image forming unit that forms an image of toner T on a recording medium S, and a fixing device 50 with one of the above-described surfaces that fixes the image of toner T to the recording medium S. This makes it possible to realize an image forming apparatus 100 that achieves both improved image quality and improved durability of the fixing device 50.

[0093] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the claims rather than the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended. [Explanation of Symbols]

[0094] 50 Fixing device, 51 Pressure roller, 52, 57 Fixing belt, 53 Heating roller, 54 Fixing heater, 56 Fixing pad, 60 Contact surface, 61 Support surface, 64 Non-contact area, 71 Nip forming surface, 72 Curved surface, 73 Boundary, 76 Pad center, 77 Pad edge, 80 Fixing nip, 81 Upstream edge, 82 Downstream edge, 96 Roller center, 97 Roller edge, 100 Image forming apparatus, S Recording medium, Se1, Se2 Edge, T Toner.

Claims

1. A fixing device for fixing a toner image formed on a recording medium onto the recording medium, Fixing belt and A fixing pad having a contact surface that contacts the fixing belt, The system includes a pressure roller that presses the fixing pad via the fixing belt to form a fixing nip, The contact surface has a nip-forming surface for forming the fixing nip, and a curved surface located upstream of the nip-forming surface in the transport direction of the recording medium and curving away from the pressure roller. The distance in the conveying direction between the upstream edge of the fixing nip and the boundary between the nip forming surface and the curved surface is positive when the boundary is downstream of the upstream edge in the conveying direction, and negative when the boundary is upstream of the upstream edge in the conveying direction. The value of the distance at the center of the recording medium in the width direction is greater than the value of the distance at the edge in the width direction. In the central portion, the boundary is located downstream in the transport direction from the upstream edge. The boundary extends in the width direction at the central portion and extends at an inclination with respect to the width direction at both ends in the width direction, wherein the fixing device.

2. The fixing device according to claim 1, wherein at the end portion, the boundary is located upstream of the upstream side edge in the conveying direction.

3. The fixing device according to claim 1, wherein the outer diameter of the pressure roller at the central portion is larger than the outer diameter of the pressure roller at the end portion.

4. The fixing device according to any one of claims 1 to 3, wherein the nip-forming surface is a flat surface.

5. The fixing device according to any one of claims 1 to 3, wherein the end portion includes a position through which the edge of the recording medium having the widest width among the recording media that can be fed through the fixing device passes.

6. The fixing device according to any one of claims 1 to 3, wherein the end portion includes a position through which the edge of the recording medium that is fed to the fixing device most frequently passes.

7. An image forming unit that forms a toner image on a recording medium, An image forming apparatus comprising a fixing device according to claim 1 for fixing the image of the toner onto the recording medium.

Images