Fixing device and image forming apparatus

Abstract

To prevent wear of a belt when a posture of the belt is corrected by a support roll provided on the upstream side of a fixing part.SOLUTION: A fixing device has: an endless belt that is heated and circularly movable; a stationary part that is fixed in contact with an inner peripheral surface of the belt; a pressure roll that presses the belt against the stationary part to form a fixing part through which a recording material on which an unfixed image is formed is passed, and is driven to rotate; and a support roll that is arranged on the upstream side of the fixing part in a direction of movement of the belt, and rotatably supports the belt. In the fixing part, a first curved surface that is an arcuate curved surface is formed on the upstream side in the movement direction of a position where the fixing part is formed, and a second curved surface that is an arcuate curved surface is formed on the upstream side in the movement direction of the first curved surface. When a curvature radius of the first curved surface is defined as r1 (mm), and a curvature radius of the second curved surface as r2 (mm), a condition of r1>r2 is satisfied. The belt is stretched over the support roll and the first curved surface, with tension applied thereto.SELECTED DRAWING: Figure 3

Description

Technical Field

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

Background Art

[0002] In Patent Document 1, for a fixing member, when forming a fixing portion, an arcuate curved surface is formed such that a contact portion where the belt and the pressure roll contact is curved away from the pressure roll, and an insertion portion adjacent to an insertion side where a recording material is inserted into the contact portion is formed with an arcuate curved surface curved to the opposite side of the contact portion. When the thickness of the belt is t (mm), the radius of curvature of the curved surface of the inlet portion is r1 (mm), and the radius of curvature of the curved surface of the contact portion is r2 (mm), a fixing device is disclosed that satisfies the conditions of t(1 / r1 + 1 / r2) ≤ 0.130 and r1 ≤ r2.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] There is a fixing device that has a fixing portion fixed on the inner peripheral side of an endless belt that can circulate and move, and fixes an unfixed toner image formed on a recording material by a pressure roll that rotates while applying pressure to this fixing portion. In this fixing device, when the posture of the belt is disturbed, the unfixed toner is also disturbed. In order to suppress this disturbance, a technique is adopted in which a support roll that contacts and rotates with the belt is provided upstream of the fixing portion to correct the posture of the belt. However, when the degree of freedom of the posture of the belt is restricted for reasons such as the distance from this support roll to the fixing portion becoming short, the fixing portion is displaced due to vibration or the like, and friction is likely to occur between the end portion of the fixing portion and the belt, and wear of the belt is likely to occur. The object of the present invention is to suppress wear of a belt when the posture of the belt is corrected by a support roll provided on the upstream side of a fixing portion.

Means for Solving the Problems

[0005] The invention according to claim 1 includes an endless belt that can be heated and circulated, a fixing portion that is fixed in contact with the inner peripheral surface of the belt, a fixing portion that presses the belt against the fixing portion to form a non-fixed image, and a fixing portion that passes a recording material. A pressure roll that is rotationally driven, and a support roll that is disposed upstream of the fixing portion in the moving direction of the belt and rotatably supports the belt. The fixing portion has a first curved surface that is an arc-shaped curved surface formed on the upstream side in the moving direction from the position where the fixing portion is formed, and a second curved surface that is an arc-shaped curved surface formed on the upstream side in the moving direction from the first curved surface. When the radius of curvature of the first curved surface is r1 (mm) and the radius of curvature of the second curved surface is r2 (mm), the condition r1>r2 is satisfied, and the belt is stretched over the support roll and the first curved surface. A fixing device characterized by this. The invention according to claim 2 is the fixing device according to claim 1, characterized in that the first curved surface and the second curved surface are formed at an end of the fixing portion, and the second curved surface is formed continuously with the first curved surface. The invention according to claim 3 is the fixing device according to claim 2, characterized in that the first curved surface and the second curved surface are smoothly continuous. The invention according to claim 4 is the fixing device according to claim 1, characterized in that the radius of curvature of the second curved surface is larger than the radius of curvature at which the belt starts to deteriorate when the belt is curved. The invention according to claim 5 is the fixing device according to claim 4, characterized in that the radius of curvature of the second curved surface is determined as a radius of curvature at which the belt does not contact the second curved surface even when the belt flutters. The invention according to claim 6 is an image forming apparatus including an image forming means for forming an unfixed image on a recording material, and a fixing device according to any one of claims 1 to 5 for fixing the unfixed image formed by the image forming means on the recording material.

Advantages of the Invention

[0006] According to the invention of claim 1, when the posture of the belt is corrected by the support roll provided on the upstream side of the fixing portion, wear of the belt can be suppressed. According to the invention of claim 2, the contact area between the belt and the fixing portion can be reduced as compared with the case where the second curved surface is not provided continuously with the first curved surface. According to the invention of claim 3, wear of the belt can be suppressed as compared with the case where the first curved surface and the second curved surface are not smoothly continuous. According to the invention of claim 4, the lower limit value of the second curved surface can be determined. According to the invention of claim 5, the upper limit value of the second curved surface can be determined. According to the invention of claim 6, when the posture of the belt is corrected by the support roll provided on the upstream side of the fixing portion in the fixing device, an image forming apparatus capable of suppressing wear of the belt can be provided.

Brief Description of the Drawings

[0007]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0008] 〔Image Forming Apparatus 1〕 Hereinafter, with reference to the accompanying drawings, an image forming apparatus to which the present embodiment is applied will be described in detail. FIG. 1 is a diagram showing an image forming apparatus 1 according to the present embodiment. The image forming apparatus 1 is, for example, an electrophotographic image forming apparatus that forms a toner image on a recording material.

[0009] The image forming apparatus 1 includes image forming units 10Y, 10M, 10C, 10K (hereinafter sometimes simply referred to as "image forming unit 10") that form toner images for each color, a transfer unit 20 that transfers the toner image formed by the image forming unit 10 onto a recording material, a conveyance unit 40 that conveys the recording material, a fixing device 50 that fixes the unfixed toner image, which is an unfixed image, onto the recording material, and a control unit 100 that controls each unit of the image forming apparatus 1.

[0010] 〔Control Unit 100〕 The control unit 100 includes a CPU (Central Processing Unit) 101 that controls the entire image forming apparatus 1, a RAM (Random Access Memory) 102 used as a work area during calculation, and a ROM (Read Only Memory) 103 that is a memory for storing various programs and various settings executed by the CPU 101.

[0011] 〔Image Forming Unit 10〕 A plurality of the image forming units 10 shown in FIG. 1 are provided to form toner images for each color. In the present embodiment, a total of four image forming units 10 for yellow (Y), magenta (M), cyan (C), and black (K) are provided. 10Y, 10M, 10C, and 10K shown in FIG. 1 indicate the components corresponding to the respective colors. Note that since the image forming units 10 for each color are configured in the same manner except for the toner used, reference numerals are attached to each part of the image forming unit 10K in FIG. 1 to represent the image forming units 10 for each color.

[0012] Each color image forming unit 10 includes a photosensitive drum 11 on which an electrostatic latent image is formed while rotating in the clockwise direction (arrow A direction) in FIG. 1. Each color image forming unit 10 also includes a charger 12 that charges the surface of the photosensitive drum 11, an exposure device 13 that forms an electrostatic latent image on the photosensitive drum 11, a developing device 14 that develops the electrostatic latent image formed on the photosensitive drum 11, a primary transfer roll 15 that transfers the toner image formed on the photosensitive drum 11 to the intermediate transfer belt 21, and a drum cleaner 16 that removes the developer remaining on the photosensitive drum 11. Then, by applying a primary transfer electric field between the primary transfer roll 15 and the photosensitive drum 11, the toner image formed on the photosensitive drum 11 is transferred to the intermediate transfer belt 21 at the primary transfer position T1.

[0013] 〔Transfer unit 20〕 The transfer unit 20 includes an intermediate transfer belt 21 onto which the toner images of each color formed by the photosensitive drums 11 of the respective image forming units 10 are transferred, and belt support rolls 22a to 22f provided on the inner peripheral side of the intermediate transfer belt 21. The transfer unit 20 also includes a secondary transfer roll 23 that collectively transfers the toner image transferred onto the intermediate transfer belt 21 to the recording material. Further, the transfer unit 20 includes a cleaner 24 that removes toner and the like on the surface of the intermediate transfer belt 21.

[0014] The toner images of each color are transferred from the respective photosensitive drums 11 to the outer peripheral surface of the intermediate transfer belt 21. As shown in FIG. 1, the intermediate transfer belt 21 is endless and is wound around a plurality of belt support rolls 22a to 22f. The intermediate transfer belt 21 circulates in the arrow B direction when at least one of the plurality of belt support rolls 22a to 22f is rotationally driven.

[0015] The secondary transfer roll 23 is disposed to face the belt support roll 22e with the intermediate transfer belt 21 interposed therebetween. The secondary transfer roll 23 rotates while pressing the intermediate transfer belt 21 against the belt support roll 22e. As shown in FIG. 1, the cleaner 24 is disposed on the outer peripheral side of the intermediate transfer belt 21. The cleaner 24 is disposed downstream of the secondary transfer roll 23 and upstream of the image forming unit 10 in the circumferential direction of the intermediate transfer belt 21.

[0016] 〔Transport Unit 40〕 The transport unit 40 includes a storage unit 41 for storing a recording material, and a feeding device 42 for feeding out the recording materials stored in the storage unit 41 one by one. The transport unit 40 also includes a pair of paper transport rolls 43a to 43c for transporting the recording material from the storage unit 41 to the secondary transfer position T2 of the transfer unit 20. Further, the transport unit 40 includes paper transport belts 45 and 46 for transporting the recording material from the secondary transfer position T2 to the fixing device 50. Additionally, the transport unit 40 includes a pair of paper discharge rolls 47a and 47b for discharging the recording material sent out from the fixing device 50 to the outside of the image forming apparatus 1.

[0017] 〔Fixing Device 50〕 Hereinafter, the fixing device 50 to which the present embodiment is applied will be described with reference to FIG. 2. FIG. 2 is a diagram showing the configuration of the fixing device 50. The fixing device 50 includes a fixing belt 51 which is an endless belt that can be heated and circulated, a fixing portion 52 that is fixed in contact with the inner peripheral surface of the fixing belt 51, inner support rolls 53, 54a to 54c that are provided in contact with the inner peripheral surface of the fixing belt 51 and rotatably support the fixing belt 51, and an outer support roll 55 that is provided in contact with the outer peripheral surface of the fixing belt 51 and rotatably supports the fixing belt 51. The fixing device 50 also includes a pressure roll 58 provided at a position facing the fixing portion 52, a displacement mechanism 63 for displacing the position of the pressure roll 58, and a pressurizing mechanism 64 for applying a pressing force toward the fixing portion 52 to the pressure roll 58. Further, the fixing device 50 includes a coating device 69 for applying a lubricant to the inner peripheral surface of the fixing belt 51, introduction guide members 65a and 65b for guiding the recording material to the fixing device 50, and discharge guide members 66a and 66b for guiding the recording material fixed by the fixing device 50 to the transport path.

[0018] The fixing belt 51 is composed of a plurality of layers, and in this embodiment, it is composed of three layers. In this embodiment, the fixing belt 51 is composed of, for example, a base material of an endless belt, an elastic layer formed on the surface (outer peripheral surface) side of the base material, and a release layer formed on the elastic layer. The base material is formed of a material such as polyimide resin. Further, the elastic layer is formed of an elastic member such as silicone rubber. Further, the release layer is formed of a material such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer).

[0019] Further, the fixing belt 51 is set to have a belt width larger than the maximum value of the width during the conveyance of the recording material. Further, the fixing belt 51 is set to have a belt length necessary for being wound around the fixing portion 52, the plurality of internal support rolls 53, 54a to 54c, and the external support roll 55.

[0020] The fixing portion 52 includes a rectangular tube-shaped support 521 arranged in a fixed state and a contact member 522 that contacts the inner peripheral surface of the fixing belt 51. The support 521 is arranged along the depth side from the front side of the apparatus in the longitudinal direction (the direction in which the rectangular tube extends), which is a direction orthogonal to the conveyance direction of the recording material. Further, the support 521 is provided to face the pressure roll 58. The support 521 is formed of a material such as a metal such as iron or aluminum, or a liquid crystal polymer. The contact member 522 is attached to the surface of the support 521 that faces the pressure roll 58. Further, the contact member 522 is formed of a material such as a fluororesin such as polytetrafluoroethylene (PTFE) or PFA. The shape of the contact member 522 will be described later with reference to FIG. 3.

[0021] The internal support roll 53 is a cylindrical roll made of aluminum or the like, and is rotatably disposed at a position farthest from the fixing portion 52. Further, the internal support roll 53 is provided with a halogen heater 56 in its internal space. By heating the internal support roll 53 with this halogen heater 56, the internal support roll 53 heats the fixing belt 51 from the inner peripheral surface side. Further, a temperature detector 67 for detecting the surface temperature is provided on the internal support roll 53. Further, the internal support roll 53 is also configured as a tension applying roll that applies tension to the fixing belt 51 by elastically pressing the fixing belt 51 from the inner peripheral surface side toward the outer peripheral surface side and supporting it.

[0022] The internal support roll 54a is disposed upstream of the fixing portion 52 in the moving direction (arrow C direction) of the fixing belt 51 and rotatably supports the fixing belt 51. Then, the internal support roll 54a corrects the posture of the fixing belt 51 before contacting the fixing portion 52. The internal support roll 54b is disposed downstream of the fixing portion 52 in the moving direction of the fixing belt 51 and rotatably supports the fixing belt 51. Then, the internal support roll 54b corrects the posture of the fixing belt 51 away from the fixing portion 52. The internal support roll 54c is rotatably disposed between the internal support roll 53 and the internal support roll 54a in the moving direction of the fixing belt 51. Then, the internal support roll 54c corrects the posture of the fixing belt 51 during running.

[0023] The external support roll 55 is a cylindrical roll made of aluminum or the like, contacts the outer peripheral surface of the fixing belt 51, and is arranged in a rotatable state. The external support roll 55 is provided between the inner support roll 54b and the inner support roll 53 in the moving direction of the fixing belt 51. A halogen heater 57 is provided in the internal space of the external support roll 55. By heating the external support roll 55 with this halogen heater 57, the external support roll 55 heats the fixing belt 51 from the outer peripheral surface side. Further, a temperature detector 68 for detecting the surface temperature is provided on the external support roll 55. Furthermore, a first driving device 61 composed of a driving motor or the like is provided on the external support roll 55. This first driving device 61 is configured to be able to rotationally drive the external support roll 55 at a required time. Thereby, the external support roll 55 rotationally drives the fixing belt 51 in the direction of arrow C.

[0024] The pressure roll 58 is arranged along the depth side from the front of the image forming apparatus 1 with the rotation axis of the pressure roll 58 in the longitudinal direction orthogonal to the conveyance direction of the recording material. The pressure roll 58 is a roll having a structure in which an elastic layer 582 made of silicone rubber or the like and a release layer 583 made of PFA or the like are sequentially laminated on the outer peripheral surface of a cylindrical roll base 581 made of aluminum or the like. The elastic layer 582 is a layer that elastically deforms when the pressure roll 58 presses the fixing belt 51 against the fixing portion 52 to form the fixing area NP. For example, its hardness is set to a value such that it is harder than the hardness of the fixing portion 52. Here, the fixing area NP refers to the area where the pressure roll 58 presses the fixing belt 51 against the fixing portion 52, and is indicated by the reference numeral NP in FIG. 2. In this fixing area NP, a fixing portion 59 is formed through which the recording material on which the unfixed image is formed passes.

[0025] The pressure roll 58 includes a second driving device 62 that rotates the pressure roll 58, and the pressure roll 58 is rotationally driven. The second driving device 62 is composed of a driving motor or the like. Thereby, the pressure roll 58 contacts and rotationally drives the outer peripheral surface of the fixing belt 51 pressed against the fixing portion 52, and rotates the fixing belt 51 in the direction indicated by arrow C.

[0026] The displacement mechanism 63 moves the pressure roll 58 to a first position in contact with the fixed part 52 and a second position spaced apart from the fixed part 52. FIG. 2 shows a state where the pressure roll 58 is in the second position. As the displacement mechanism 63, for example, a mechanism that displaces a support frame rotatably supporting the pressure roll 58 by displacement driving means such as a cam is used.

[0027] The pressing mechanism 64 applies a force toward the fixed part 52 to the pressure roll 58. As the pressing mechanism 64, one configured using a spring member or the like is applied, and it is configured to function in combination with the displacement mechanism 63.

[0028] The introduction guide member 65 is provided at a position near the upstream side of the fixing area NP in the conveyance direction of the recording material. The introduction guide member 65 guides the recording material toward the entrance of the fixing area NP. The discharge guide member 66 is provided at a position near the downstream side of the fixing area NP in the conveyance direction of the recording material. The discharge guide member 66 guides the recorded material after fixing toward the paper discharge roll pair 47a (see FIG. 1). A discharge guide member 66b disposed on the side of the discharge guide member 66 close to the pressure roll 58 is provided in a tapered shape of a peeling claw (not shown) that peels the recording material discharged from the fixing area NP from the outer peripheral surface of the pressure roll 58 in a state of lightly contacting the outer peripheral surface of the pressure roll 58. Further, the discharge guide member 66b is attached to the frame or the like of the displacement mechanism 63 and is configured to be displaced in conjunction with the displacement operation of the displacement mechanism 63.

[0029] 〔Contact member 522〕 Next, with reference to FIG. 3, the shape of the contact member 522 to which the present embodiment is applied will be described. FIG. 3 is a diagram showing the shape of the contact member 522 that contacts the inner peripheral surface of the fixing belt 51. The contact member 522 has a contact surface 523 formed in a range indicated by the fixing area NP in FIG. 3, where the fixing belt 51 pressed by the pressure roll 58 (see FIG. 2) comes into contact. Further, the contact member 522 has a large-diameter surface 524 formed in a range indicated by H1 in FIG. 3, over which the fixing belt 51 from the internal support roll 54a is passed. Furthermore, the contact member 522 has a small-diameter surface 525 formed in a range indicated by H2 in FIG. 3, which avoids contact with the fixing belt 51. Still further, the contact member 522 has a downstream side surface 526 formed in a range indicated by H3 in FIG. 3, with a so-called R shape provided to reduce the friction applied to the fixing belt 51 leaving the contact member 522. The large-diameter surface 524 formed in the range indicated by H1 is an example of a first curved surface, and the small-diameter surface 525 formed in the range indicated by H2 is an example of a second curved surface.

[0030] The contact surface 523 is a concave portion that curves in a direction away from the pressure roll 58 and is an arcuate surface. The contact surface 523 is designed such that the magnitude of the curvature of the arc is substantially the same as the curvature of the outer diameter of the pressure roll 58. Thereby, the fixing area NP where the pressure roll 58 presses the fixing belt 51 has the width of the contact surface 523 in the conveyance direction of the recording material.

[0031] The large-diameter surface 524 is continuous with the contact surface 523 in the moving direction of the fixing belt 51 and is formed upstream of the contact surface 523. The large-diameter surface 524 is a convex portion protruding toward the fixing belt 51 side and is an arcuate curved surface. The center of the arc of the large-diameter surface 524 is shown in FIG. 3 as point P1. Also, the radius of curvature of the arc of the large-diameter surface 524 is shown in FIG. 3 as r1 (mm). The large-diameter surface 524 is provided such that any one of its tangents contacts the outer diameter of the internal support roll 54a. Thereby, with the fixing belt 51 under tension, the fixing belt 51 is passed from the internal support roll 54a over the large-diameter surface 524.

[0032] The small-diameter surface 525 is formed on the upstream side of the large-diameter surface 524 in the moving direction of the fixing belt 51. The small-diameter surface 525 is a convex portion protruding toward the fixing belt 51 side and is an arcuate curved surface. The center of the arc of the small-diameter surface 525 is shown in FIG. 3 as point P2. Also, the radius of curvature of the arc of the small-diameter surface 525 is shown in FIG. 3 as r2 (mm). The radius of curvature r2 of the small-diameter surface 525 is formed so as to satisfy the condition of r1 > r2. Further, in the present embodiment, the size of the radius of curvature r2 of the small-diameter surface 525 may be made larger than the radius of curvature at which the fixing belt 51 starts to deteriorate when the fixing belt 51 is curved. Here, the deterioration of the fixing belt 51 refers to an irreversible change that occurs when the fixing belt 51 is curved, for example, cracks or the like that occur in the polyimide resin used for the base material of the fixing belt 51. The radius of curvature at which the fixing belt 51 starts to deteriorate varies depending on various conditions such as the material used for the fixing belt 51 and the environment in which it is used, and this radius of curvature at which the fixing belt 51 starts to deteriorate is determined by experiments, simulations, or the like.

[0033] Also, in the present embodiment, the size of the radius of curvature r2 may be determined as the size of the radius of curvature at which the fixing belt 51 does not contact the small-diameter surface 525 even when there is flutter of the fixing belt 51. For example, when the radius of curvature r2 of the small-diameter surface 525 is large, when the fixing belt 51 in contact with the large-diameter surface 524 flutters, it is likely to contact the small-diameter surface 525. Therefore, the size of the radius of curvature r2 is determined as a value smaller than the value at which the fixing belt 51 does not contact the small-diameter surface 525. Here, the flutter of the fixing belt 51 is the disturbance of the fixing belt 51 that occurs when the fixing belt 51 moves in a circulating manner, for example, the fixing belt 51 is in a wavy state.

[0034] Further, in the present embodiment, the large-diameter surface 524 and the small-diameter surface 525 are formed at the end of the fixing portion 52, and the large-diameter surface 524 is formed continuously with the small-diameter surface 525. Note that the large-diameter surface 524 does not have to be continuous with the small-diameter surface 525. For example, a flat surface or a curved surface may be formed between the large-diameter surface 524 and the small-diameter surface 525.

[0035] Further, in the present embodiment, the large-diameter surface 524 and the small-diameter surface 525 are smoothly continuous. Here, being smoothly continuous means that at the boundary between the large-diameter surface 524 and the small-diameter surface 525, the tangent line is uniquely determined. Note that the large-diameter surface 524 and the small-diameter surface 525 may be angular instead of being smoothly continuous.

[0036] The downstream side surface 526 is on the downstream side of the contact surface 523 in the moving direction of the fixing belt 51 and is formed at the end of the contact member 522.

[0037] 〔Processing performed by the fixing device 50〕 Next, with reference to FIGS. 2 and 4, the operation of the fixing device 50 in the present embodiment will be described. FIG. 4 is a diagram showing a state in which the fixing belt 51 is pressed against the fixing portion 52 by the pressure roll 58. The pressure roll 58 presses the fixing belt 51 against the contact surface 523 of the contact member 522 and rotates clockwise in the figure.

[0038] When the main power supply of the image forming apparatus 1 (see FIG. 1) is turned on in the fixing device 50 (see FIG. 1) of the present embodiment, the necessary power is also supplied to the fixing device 50 accordingly, and the warm-up operation is started. In the warm-up operation of the fixing device 50, first, the displacement mechanism 63 operates to separate the pressure roll 58 from the fixing belt 51, and the halogen heaters 56 and 57 operate to heat the inner support roll 53 and the outer support roll 55. When it is determined that the surface temperature of the halogen heaters 56 and 57 has reached a predetermined temperature or higher, the displacement mechanism 63 moves the pressure roll 58 to a position where it presses against the fixing portion 52, and the first driving device 61 and the second driving device 62 are respectively rotationally driven. As a result, the fixing belt 51 circulates and moves, and is heated over the entire circumference of the fixing belt 51.

[0039] During fixing, in the fixing device 50, as shown in FIG. 4, the pressure roll 58 presses the fixing belt 51 against the fixing portion 52 to form the fixing portion 59. At this time, the elastic layer 582 of the pressure roll 58 is elastically deformed in a state where the portion passing in contact with the fixing portion 52 is indented. Further, in the fixing device 50, the fixing belt 51 is transmitted with rotational power from the pressure roll 58 that contacts the outer peripheral surface of the fixing belt 51 and rotates in the direction indicated by the arrow C. At this time, the external support roll 55 is not rotationally driven and is rotatable, and rotates following the fixing belt 51. Therefore, during the fixing operation, the fixing belt 51 receives only the rotational power of the pressure roll 58 and rotates at a required speed in the direction indicated by the arrow C.

[0040] At this time, the recording material on which the unfixed toner image is formed is guided by the lower introduction guide member 65b and passes through the fixing area NP between the fixing belt 51 and the pressure roll 58. When passing through this fixing area NP, the unfixed toner image is fixed to the recording material by receiving heat and pressure treatment. The recording material after fixing is peeled off from the pressure roll 58 by the discharge guide member 66 and discharged from the fixing device 50.

[0041] In the present embodiment, as described above, the contact member 522 is formed with a large-diameter surface 524 around which the fixing belt 51 is wound, and a small-diameter surface 525 is formed on the upstream side of the large-diameter surface 524 in the moving direction of the fixing belt 51. Thereby, when the fixing portion 52 is displaced, since the large-diameter surface 524 with a large radius of curvature r1 presses the fixing belt 51, the fixing belt 51 is less likely to bend compared to the case where the radius of curvature at the position where the fixing belt 51 is wound is small. Thereby, even when the fixing portion 52 is displaced, contact of the fixing belt 51 on the upstream side of the large-diameter surface 524 is suppressed.

[0042] Further, in the present embodiment, the magnitude of the radius of curvature r2 of the small-diameter surface 525 is larger than the radius of curvature at which the fixing belt 51 starts to deteriorate when the fixing belt 51 is curved. Therefore, even if the fixing belt 51 contacts the small-diameter surface 525, deterioration of the fixing belt 51 due to bending is suppressed.

[0043] 〔Comparative Example〕 Here, as a comparative example, a case where the contact member 522 of the fixing portion 52 does not have the large-diameter surface 524 will be described. FIG. 5 is a diagram showing a comparative example. In this comparative example, a fixing portion 92 corresponding to the fixing portion 52 of the present embodiment is shown. Different from the fixing portion 52 of the present embodiment, this fixing portion 92 does not have a portion corresponding to the large-diameter surface 524.

[0044] FIG. 5 shows the state of the pressure roller 58 when the fixing operation is performed. The pressure roller 58 presses the fixing belt 51 against the fixing portion 92 and rotates clockwise in the figure. The fixing portion 92 has a rounded corner portion 92c provided with a so-called R shape at the upstream end in the moving direction of the fixing belt 51. In this comparative example, the fixing belt 51 is stretched between the inner support roller 54a and the rounded corner portion 92c. When the fixing belt 51 moves, friction occurs between the inner peripheral surface of the fixing belt 51 and the rounded corner portion 92c.

[0045] In the enlarged view of FIG. 5, the position before the displacement of the fixing portion 92 is shown by a dashed line, and the state after the displacement of the fixing portion 92 due to the influence of vibration of the image forming apparatus or the like is shown by a solid line. After the fixing portion 92 is displaced, the inner peripheral side of the fixing belt 51 contacts the side surface of the fixing portion 92 along the rounded corner portion 92c. When the rounded corner portion 92c is displaced, the smaller the curvature of the rounded corner portion 92c, the easier it is for the fixing belt 51 to bend along the rounded corner portion 92c. And when the fixing belt 51 bends along the rounded corner portion 92c, cracks or the like may occur in the fixing belt 51. This crack becomes deterioration of the fixing belt 51. Further, when the fixing belt 51 circulates and moves while being in contact with the side surface of the fixing portion 92, friction occurs between the side surface of the fixing portion 92 and the fixing belt 51, which may cause wear of the fixing belt 51.

[0046] As described above, the shape of the contact member 522 of the fixing device 50 to which the present embodiment is applied includes a small-diameter surface 525 at the upstream end of the fixing portion 52 in the moving direction of the fixing belt 51, and continuously with the small-diameter surface 525, a large-diameter surface 524 is provided on the downstream side of the small-diameter surface 525. And the fixing belt 51 is stretched over the large-diameter surface 524. Therefore, in the fixing device 50 to which the present embodiment is applied, when the fixing portion 52 is displaced, the large-diameter surface 524 having an arc shape with a large curvature radius r1 presses the fixing belt 51, and compared with the case where the fixing belt 51 is stretched over a curved surface with a small curvature radius, the bending of the fixing belt 51 is suppressed, and the wear of the fixing belt 51 is suppressed.

[0047] (Appendix) (((1))) An endless belt that can be heated and circulated, A fixing portion that is in contact with and fixed to the inner peripheral surface of the belt, A fixing portion that presses the belt against the fixing portion and allows a recording material on which an unfixed image is formed to pass through, and a pressure roll that is rotationally driven, A support roll that is disposed upstream of the fixing portion in the moving direction of the belt and rotatably supports the belt, having, The fixing portion has a first curved surface that is an arc-shaped curved surface formed on the upstream side in the moving direction from the position where the fixing portion is formed, and a second curved surface that is an arc-shaped curved surface is formed on the upstream side in the moving direction from the first curved surface. If the radius of curvature of the first curved surface is r1 (mm) and the radius of curvature of the second curved surface is r2 (mm), r1>r2 satisfies the condition of, The belt is stretched over the support roll and the first curved surface in a state where tension is applied, A fixing device characterized by the above. (((2))) The first curved surface and the second curved surface are formed at the end of the fixing portion, The fixing device according to (((1))), wherein the second curved surface is formed continuously with the first curved surface. (((3))) The fixing device according to ((2)), wherein the first surface and the second surface are smoothly continuous. (((4))) The fixing device according to any one of ((1)) to ((3)), wherein the radius of curvature of the second surface is larger than the radius of curvature at which the belt starts to deteriorate when the belt is curved. (((5))) The fixing device according to any one of ((1)) to ((4)), wherein the radius of curvature of the second surface is defined as a radius of curvature at which the belt does not contact the second surface even when the belt flutters. (((6))) Image forming means for forming an unfixed image on a recording material, The fixing device according to any one of ((1)) to ((5)) for fixing the unfixed image formed by the image forming means to the recording material, An image forming apparatus comprising:

[0048] According to the fixing device of ((1)), when the posture of the belt is corrected by a support roll provided upstream of the fixing portion, wear of the belt can be suppressed. According to the fixing device of ((2)), the contact area between the belt and the fixing portion can be reduced as compared with the case where the second surface is not provided continuously with the first surface. According to the fixing device of ((3)), wear of the belt can be suppressed as compared with the case where the first surface and the second surface are not smoothly continuous. According to the fixing device of ((4)), the lower limit value of the second surface can be determined. According to the fixing device of ((5)), the upper limit value of the second surface can be determined. According to the image forming apparatus of ((6)), when the posture of the belt is corrected by a support roll provided upstream of the fixing portion in the fixing device, an image forming apparatus capable of suppressing wear of the belt can be provided.

Description of Signs

[0049] 1…Image forming apparatus, 10…Image forming unit, 20…Transfer unit, 50…Fixing device, 51…Fixing belt, 52…Fixing unit, 53, 54a to 54c…Internal support roll, 55…External support roll, 58…Pressing roll, 523…Contact surface, 524…Large-diameter surface, 525…Small-diameter surface

Claims

1. An endless belt that can be heated and circulated, A fixing part that is in contact with and fixed to the inner peripheral surface of the belt, A fixing part that presses the belt against the fixing part and forms a pressure roll that rotates to allow a recording material on which an unfixed image is formed to pass through, A support roll that is arranged upstream of the fixing part in the moving direction of the belt and rotatably supports the belt, having, The fixing part has a first curved surface that is an arc-shaped curved surface formed on the upstream side in the moving direction from the position where the fixing part is formed, and a second curved surface that is an arc-shaped curved surface is formed on the upstream side in the moving direction from the first curved surface. When the radius of curvature of the first curved surface is r1 (mm) and the radius of curvature of the second curved surface is r2 (mm), r1 > r2 satisfies the condition of, The belt is stretched over the support roll and the first curved surface, A fixing device characterized by the above.

2. The first curved surface and the second curved surface are formed at the end of the fixing part, The fixing device according to claim 1, wherein the second curved surface is formed continuously with the first curved surface.

3. The fixing device according to claim 2, wherein the first curved surface and the second curved surface are smoothly continuous.

4. The fixing device according to claim 1, wherein the radius of curvature of the second curved surface is larger than the radius of curvature at which the belt starts to deteriorate when the belt is curved.

5. The fixing device according to claim 4, wherein the radius of curvature of the second curved surface is determined as a radius of curvature at which the belt does not contact the second curved surface even when the belt flutters.

6. Image forming means for forming an unfixed image on a recording material, The fixing device according to any one of claims 1 to 5 for fixing the unfixed image formed by the image forming means to the recording material, An image forming apparatus comprising the above.

Images