JP2017044927A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate the occurrence of folding and bending of a fixing belt and eliminate breakage of the fixing belt.SOLUTION: A fixing device 19 comprises: an endless fixing belt 22; a pressure roller 23; a nip forming part 27 that receives a press contact pressure from the pressure roller 23 via the fixing belt 22; a heater 24 that heats the fixing belt 22; and a holding frame 29 that is inserted into the inside of an end of the fixing belt 22, includes an outer peripheral surface in contact with an inner peripheral surface of the end of the fixing belt 22, and holds the end of the fixing belt 22 from the outer peripheral surface. The outer peripheral surface includes a smooth surface continuing on the entire circumference to hold the end of the fixing belt 22. The nip forming part 27 and holding frame 29 are arranged side by side with a gap in the longitudinal direction of the fixing belt 22. The nip forming part 27 has a first convex guide curved surface 27d that is arranged at a position flush with the outer peripheral surface of the holding frame 29.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a fixing device and an image forming apparatus for fixing an image formed on a recording sheet by sandwiching the recording sheet in a nip region between an endless fixing belt and a pressure roller.

  An example of this type of fixing device is disclosed in Patent Document 1. Here, a substantially cylindrical support member is inserted inside the endless fixing belt, and the fixing belt is rotatably supported by the support member. Further, a concave portion parallel to the longitudinal direction of the fixing belt is formed on the outer periphery of the support member, and a nip forming member is fitted into the concave portion, and the pressure contact pressure by the pressure roller is received by the nip forming portion via the fixing belt. A nip region is formed between the fixing belt and the pressure roller. Then, the fixing belt is rotated with a heater provided inside the fixing belt, the fixing belt is rotated, the recording paper is sandwiched in the nip area, and the image (unfixed toner image) on the recording paper is heated. It is fixed by crimping.

JP 2012-128219 A

  However, in the configuration in which the nip forming member is fitted in the recess on the outer periphery of the support member as in Patent Document 1, a gap is formed between the edge of the recess and the nip forming member, and this gap is fixed on the outer periphery of the support member. It becomes an elongate groove | channel extended in the longitudinal direction of a belt. When the fixing belt rotates, the fixing belt may enter the elongated groove and bend locally, which causes the fixing belt to break.

  SUMMARY An advantage of some aspects of the invention is that the fixing belt is not bent and the fixing belt is not broken.

  A fixing device according to an aspect of the present invention is provided with an endless fixing belt, a pressure roller pressed against the fixing belt, and an inner side of the fixing belt. A nip forming portion received via the fixing belt, a heat generating portion for heating the fixing belt, and an inner peripheral surface of the end portion of the fixing belt inserted inside the fixing belt end portion in the longitudinal direction of the fixing belt An outer peripheral surface that is in contact with the outer peripheral surface, and the outer peripheral surface has a smooth surface that is continuous over the entire periphery thereof, and a holding frame that holds the end portion of the fixing belt. The fixing belt is provided side by side with a gap in the longitudinal direction of the fixing belt, and the nip forming portion receives a pressure contact pressure by the pressure roller via the fixing belt, and the fixing belt is more than the pressure receiving surface. Times A first guide curved surface provided on the upstream side in the direction and in contact with the inner peripheral surface of the fixing belt, and the first guide curved surface is flush with the outer peripheral surface of the holding frame. To a fixing device disposed between the positions of the fixing belt and the central portion in the radial direction.

  An image forming apparatus according to an aspect of the present invention includes the fixing device according to the present invention described above and an image forming unit that forms an image on a recording sheet, and the fixing apparatus includes the recording sheet and the fixing belt. This is an image forming apparatus that is sandwiched in a nip region between pressure rollers and thermocompression-bonds an image formed on the recording paper.

  According to the present invention, the fixing belt can be prevented from being bent, and the fixing belt can be prevented from being broken.

1 is a perspective view illustrating an appearance of an image forming apparatus to which a fixing device according to an embodiment of the present invention is applied. FIG. 2 is a cross-sectional view schematically illustrating an image forming unit and a fixing device in the image forming apparatus. 1 is a perspective view illustrating a schematic configuration of a fixing device according to an exemplary embodiment. FIG. 2 is an exploded perspective view illustrating a schematic configuration of a fixing device of the present embodiment. FIG. 2 is a cross-sectional view illustrating a fixing device of the present embodiment. FIG. 3 is a longitudinal sectional view showing an internal structure of the fixing belt by cutting a part of the fixing belt in the fixing device of the present embodiment. 2 is an exploded perspective view showing an internal structure of a fixing belt. FIG. 3 is a plan view showing a holding frame that holds both ends of the fixing belt. FIG. (A) is a plan view showing a nip forming part and a holding frame in the fixing device of the present embodiment, and (b) is a perspective view showing the nip forming part and the holding frame from below. (A) is a top view which shows the nip formation part and holding frame of a comparative example, (b) is a perspective view which shows the nip formation part and holding frame of a comparative example from the bottom, (c) is nip formation of a comparative example FIG. 6 is a diagram illustrating a state in which the fixing belt is bent at a gap between the portion and the holding frame. It is a cross-sectional view showing the fixing device of the present embodiment, and was used to explain the shape of the concave pressure receiving surface of the nip forming portion, the shape of the first convex guide curved surface, and the shape of the second convex guide curved surface. FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a perspective view showing an external appearance of an image forming apparatus to which a fixing device according to an embodiment of the present invention is applied. The apparatus main body 2 of the image forming apparatus 1 as shown in FIG. 1 is provided with a plurality of components for realizing various functions of the image forming apparatus 1. For example, the apparatus main body 2 is provided with an image forming unit, a fixing device, a data communication unit, an operation unit 3, a paper feeding unit 4, a discharge tray 5, and the like.

  The data communication unit receives image data from an external terminal such as a personal computer via a network. The image forming unit forms (prints) the image indicated by the image data received by the data communication unit on the recording paper supplied from the paper feeding unit 4. The recording paper on which the image has been formed is discharged to the discharge tray 5 after undergoing fixing processing by the fixing device.

  The operation unit 3 includes, for example, a start key, a determination key (enter key) for confirming an operation input, a numerical value input key for inputting a numerical value, and the like, and gives instructions from the operator for various operations and processes. Accept. The operation unit 3 includes a display unit that displays operation guidance to the operator.

  FIG. 2 is a cross-sectional view schematically showing an image forming unit and a fixing device in the image forming apparatus 1. The image forming unit 11 is for forming a toner image on the recording paper P supplied from the paper feeding unit 4 by an electrophotographic method. The image forming unit 11 rotates in the direction of an arrow, and the surface of the photoconductive drum 12. The neutralizing unit 13 for neutralizing, the cleaning unit 14 for cleaning the surface of the photosensitive drum 12, the charging unit 15 for uniformly charging the surface of the photosensitive drum 12, and the photosensitive drum 12 by exposure from an exposure unit (not shown). A developing device 16 that develops the electrostatic latent image generated on the surface of the toner into a toner image, a toner container 21 that supplies toner to the developing device 16, and the like.

  The photosensitive drum 12 is pressed against a transfer roller 18 provided below the photosensitive drum 12 to form a nip region between the photosensitive drum 12 and the transfer roller 18.

  When the recording paper P is supplied from the paper supply unit 4, the recording paper P is transported in the arrow direction A through the transport path 17. The recording paper P is passed through the nip area between the photosensitive drum 12 and the transfer roller 18, and the toner image on the surface of the photosensitive drum 12 is transferred. Subsequently, the recording paper P is conveyed to the fixing device 19, and the recording paper P is heated and pressurized by the fixing device 19, whereby the toner image on the recording paper P is fixed. Further, the recording paper P is transported from the fixing device 19 through a transport path 17 curved upward, and is discharged to a discharge tray 5 provided on the upper side of the image forming apparatus 1.

  Next, the fixing device 19 of this embodiment will be described in detail. FIG. 3 is a perspective view illustrating a schematic configuration of the fixing device 19, and FIG. 4 is an exploded perspective view illustrating a schematic configuration of the fixing device 19. FIG. 5 is a cross-sectional view showing the fixing device 19. 6 is a longitudinal sectional view showing the internal structure of the fixing belt 22 by cutting a part of the fixing belt 22 in the fixing device 19, and FIG. 7 is an exploded perspective view showing the internal structure of the fixing belt 22. FIG. FIG. 8 is a plan view showing a holding frame 29 that holds both ends of the fixing belt 22.

  As shown in FIGS. 3 to 7, the fixing device 19 includes a fixing belt 22, a pressure roller 23, a heater 24, a reflection plate 25, a support portion 26, a nip forming portion 27, a pair of rings 28, and a pair of holding frames 29. Etc.

  The fixing belt 22 is an endless belt, and is a sheet-like one in which a base material layer made of metal, synthetic resin, etc., an elastic layer made of silicone rubber, etc., a release layer made of synthetic resin, etc. are laminated. , Has flexibility.

  The pair of holding frames 29 includes a fixed plate 29a and a cylindrical portion 29b provided on the inner surface of the fixed plate 29a. A fixing plate 29a of each holding frame 29 is fixed to frames (not shown) on both sides of the fixing device 19, and each holding frame 29 is supported. Further, the cylindrical portion 29 b of each holding frame 29 is inserted inside the both end portions of the fixing belt 22 through the respective rings 28.

  As shown in FIG. 8, the cylindrical portion 29b of the holding frame 29 has an outer circumferential surface composed of a circumferential surface 29c, a flat surface 29d, and curved surfaces 29e and 29f that connect both sides of the flat surface 29d to the circumferential surface 29c. This outer peripheral surface is smoothly continuous. The outer peripheral surface of the cylindrical portion 29 b of the holding frame 29 is in contact with the inner peripheral surface of the end portion of the fixing belt 22 to hold the fixing belt 22.

  Further, the outer diameter of the cylindrical portion 29 b of each holding frame 29 is set to be smaller than the inner diameter of the fixing belt 22, and between the inner peripheral surface of the fixing belt 22 and the outer peripheral surface of the cylindrical portion 29 b of each holding frame 29. A gap is formed, and the fixing belt 22 is rotatable.

  Each ring 28 is a spacer interposed between both ends of the fixing belt 22 and the fixing plate 29 a of each holding frame 29. The fixing plate 29 a of each holding frame 29 regulates the positions of both ends of the fixing belt 22 via each ring 28 and holds the fixing belt 22 so as not to be displaced in the longitudinal direction of the fixing belt 22.

  Further, a heater 24 extending in the longitudinal direction of the fixing belt 22, a reflection plate 25, a support portion 26, and a nip forming portion 27 are disposed inside the fixing belt 22.

  The support portion 26 is made of metal, for example, and has sufficient strength and rigidity. Both end portions of the support portion 26 protrude to the outside of the holding frames 29 through the cylindrical portions 29b of the holding frames 29, and are fixed and supported by frames (not shown) on both sides of the fixing device 19. . The reflecting plate 25 is placed on and supported by the support portion 26, the heater 24 is disposed above the reflecting plate 25, and both end portions 24 a of the heater 24 are supported by the support portion 26. Each cord 31 is connected to both ends 24 a of the heater 24, and power is supplied to the heater 24 through each cord 31. The heater 24 is, for example, a halogen lamp, and generates heat by power feeding through each cord 31, and the radiant heat is directly applied to the fixing belt 22 and reflected by the reflecting plate 25 to be applied to the fixing belt 22. 22 is heated.

  The nip forming unit 27 includes a main body 27a and a sliding sheet 27b that covers the outer peripheral surface of the main body 27a. The main body 27a is made of, for example, metal and has sufficient strength and rigidity. The sliding sheet 27b is made of, for example, a fluororesin and has a low coefficient of friction. The sliding sheet 27b covers at least the entire lower surface of the main body 27a and the entire side surfaces along the longitudinal direction of the main body 27a. ing.

  The lower surface of the main body 27a of the nip forming portion 27 has a generally bowl-shaped concave surface, and two convex surfaces are formed between both edges of the bowl-shaped concave surface and each side surface of the main body 27a. The concave surface and each convex surface are smoothly continuous. The sliding sheet 27b covers the bowl-shaped concave surface and each convex surface of the main body 27a, and a concave pressure receiving surface 27c is formed on the lower surface of the nip forming portion 27. Both sides of the concave pressure receiving surface 27c are first convex. A curved guide curved surface 27d and a second convex guide curved surface 27e are formed.

  The pressure roller 23 is obtained by covering the outer peripheral surface of a metal cylinder or column with a layer made of synthetic resin or the like, and is provided in parallel with the fixing belt 22. Specifically, the shaft 23 a of the pressure roller 23 is supported so as to be parallel to the rotation center of the fixing belt 22. The curvature of the concave pressure receiving surface 27 c of the nip forming portion 27 is set to be smaller than the curvature of the outer peripheral surface of the pressure roller 23.

  The heater 24 is an example of a heat generating portion in the claims, and the concave pressure receiving surface 27c, the first convex guide curved surface 27d, and the second convex guide curved surface 27e of the nip forming portion 27 are pressure receiving in the claims. It is an example of a surface, a 1st guide curved surface, and a 2nd guide curved surface.

  Here, the pressure roller 23 is disposed below the fixing belt 22. Then, the shaft 23 a of the pressure roller 23 is urged upward by a spring or the like (not shown) to approach the fixing belt 22, and the pressure roller 23 is pressed against the fixing belt 22. At this time, the fixing belt 22 is dented and deformed in the pressure contact area of the pressure roller 23, and the pressure roller 23 is pressed against the concave pressure receiving surface 27 c of the nip forming portion 27 via the fixing belt 22, and the pressure roller 23. Is received by the concave pressure receiving surface 27 c of the nip forming portion 27. Thereby, the outer peripheral surface of the pressure roller 23 and the outer peripheral surface of the fixing belt 22 are pressed against each other in a wide range, and a nip region N is formed in this wide range.

  When the rotational driving force of a motor (not shown) is transmitted to the pressure roller 23 and the pressure roller 23 is rotationally driven in the direction of the arrow, the fixing belt 22 pressed against the pressure roller 23 is applied. It rotates following the direction opposite to the pressure roller 23. At this time, the fixing belt 22 is brought into sliding contact with the concave pressure receiving surface 27c of the nip forming portion 27, and the nip area N between the pressure roller 23 and the fixing belt 22 is maintained.

  Further, the heater 24 is heated to heat the fixing belt 22, and the temperature of the fixing belt 22 is set to a specified temperature suitable for fixing.

  In this state, when the recording paper P is conveyed to the nip area N, the recording paper P is heated and pressurized in the nip area N, and the toner image on the recording paper P is fixed by thermocompression bonding.

  By the way, in such a fixing device 19, the pressure roller 23 is rotationally driven, the fixing belt 22 is driven to rotate, and the fixing belt 22 is in sliding contact with the concave pressure receiving surface 27 c of the nip forming portion 27. For this reason, a load is always applied to the fixing belt 22, and the fixing belt 22 is broken unless the fixing belt 22 rotates smoothly.

  Therefore, in the present embodiment, the fixing belt 22 is smoothly rotated so that the load on the fixing belt 22 can be suppressed to prevent the fixing belt 22 from being broken.

  Next, a configuration capable of suppressing such a load on the fixing belt 22 will be described in detail.

  First, as apparent from FIGS. 5 and 8, the shape of the outer peripheral surface of the cylindrical portion 29 b of the holding frame 29 is such that the fixing belt 22 is deformed by being pressed against the concave pressure receiving surface 27 c of the nip forming portion 27. It is set according to the inner peripheral surface shape. That is, the outer peripheral surface of the cylindrical portion 29b of the holding frame 29 includes a circumferential surface 29c, a flat surface 29d, and curved surfaces 29e and 29f, and has a shape substantially similar to the inner peripheral surface shape of the fixing belt 22. ing. Further, the outer peripheral surface of the cylindrical portion 29 b is a smooth curved surface that is continuous from the outer peripheral surface of the cylindrical portion 29 b of the holding frame 29. Further, the separation distance between the outer peripheral surface of the cylindrical portion 29 b of the holding frame 29 and the inner peripheral surface of the fixing belt 22 is appropriately set. As a result, both ends of the fixing belt 22 in the longitudinal direction are rotatably supported by the outer peripheral surface without receiving a large load from the outer peripheral surface of the cylindrical portion 29 b of the holding frame 29.

  Further, a concave pressure receiving surface 27 c is formed on the lower surface of the nip forming portion 27, and the first convex guide curved surface 27 d and the second convex guide curved surface 27 e are provided upstream and downstream of the concave pressure receiving surface 27 c in the rotation direction of the fixing belt 22. Is forming. The concave pressure-receiving surface 27c, the first convex guide curved surface 27d, and the second convex guide curved surface 27e are smooth continuous curved surfaces. For this reason, the central portion of the fixing belt 22 (the portion inside the both end portions in the longitudinal direction of the fixing belt 22) extends from the first convex guide curved surface 27d to the second convex guide curved surface 27e via the concave pressure receiving surface 27c. Rotates and slides smoothly into the back. Accordingly, the central portion of the fixing belt 22 is not subjected to a large load.

  Further, as is apparent from the plan view of FIG. 9A, the first convex guide curved surface 27 d of the nip forming portion 27 is flush with the curved surface 29 e of the cylindrical portion 29 b of the holding frame 29. Specifically, when viewed in the longitudinal direction of the fixing belt 22, the first convex guide curved surface 27 d of the nip forming portion 27 swells outward as it approaches the nip region N, and the cylindrical shape of the holding frame 29 on the nip region N side. It overlaps the curved surface 29e of the portion 29b and is flush with the curved surface 29e. The central portion of the fixing belt 22 rotates while being in sliding contact with the first convex guide curved surface 27d of the nip forming portion 27, and the cylindrical portions of the holding frames 29 are provided at both ends of the fixing belt 22. It rotates while slidingly contacting the curved surface 29e of 29b. For this reason, both end portions and the central portion of the fixing belt 22 rotate while being bent with substantially the same curvature. This also reduces the load on the fixing belt 22.

  However, the first convex guide curved surface 27d may be disposed at a position slightly retracting inward (direction toward the center in the radial direction of the fixing belt 22) from the curved surface 29e of the cylindrical portion 29b of the holding frame 29. . That is, the first convex guide curved surface 27 d is arranged between a position that is flush with the outer peripheral surface of the holding frame 29 and a position that is from the central portion in the radial direction of the fixing belt 22.

  9B, the nip forming portion 27 and the cylindrical portion 29b of the holding frame 29 are provided on both sides of the boundary K parallel to the circumferential direction of the fixing belt 22 as apparent from the perspective view and FIG. Is distributed. Specifically, in the longitudinal direction of the fixing belt 22, the nip forming portion 27 is arranged slightly spaced (with a gap) from the cylindrical portion 29 b of the holding frame 29, and both in the longitudinal direction of the fixing belt 22. Are not overlapping or intricate. Therefore, no gap is formed between the nip forming portion 27 and the cylindrical portion 29 b of the holding frame 29 in the circumferential direction of the fixing belt 22.

  For example, as shown in FIGS. 10A and 10B, a part of the cylindrical portion 29b of the holding frame 29 is notched to form a notch 29g, and the end of the nip forming portion 27 is the notch. In the configuration fitted to 29 g, a gap G is formed between the nip forming portion 27 and the cylindrical portion 29 b of the holding frame 29 in the circumferential direction of the fixing belt 22.

  When such a gap G is formed, when the fixing belt 22 rotates, the fixing belt 22 enters the gap G and bends locally as shown in FIG. 10C, and the fixing belt 22 is broken. Sometimes.

  Further, as described above, the same inconvenience occurs in Patent Document 1. That is, in Patent Document 1, a recess is formed on the outer periphery of the cylindrical support member, and the nip forming member is fitted into the recess, so that a gap is formed between the edge of the recess and the nip forming member. This gap becomes an elongated groove extending in the longitudinal direction of the fixing belt. For this reason, the fixing belt enters the elongated groove and bends locally, and the fixing belt is broken.

  On the other hand, in the present embodiment, the nip forming portion 27 is slightly separated from the outer peripheral surface of the cylindrical portion 29b of the holding frame 29 in the longitudinal direction of the fixing belt 22 so that the two do not overlap with each other. Therefore, in the circumferential direction of the fixing belt 22, no gap is formed between the nip forming portion 27 and the cylindrical portion 29b of the holding frame 29, and the fixing belt 22 is not bent as shown in FIG. The fixing belt 22 is not broken.

  Next, the shape of the concave pressure receiving surface 27c of the nip forming portion 27, the shape of the first convex guide curved surface 27d, and the shape of the second convex guide curved surface 27e will be described in more detail.

  FIG. 11 is a cross-sectional view showing the fixing belt 22, the pressure roller 23, and the like. In FIG. 11, when the cylindrical shape is maintained, the rotation center of the fixing belt 22 is Q1, the rotation center of the pressure roller 23 is Q2, and a virtual straight line passing through these rotation centers Q1 and Q2 is S1. An intersection point of the concave pressure receiving surface 27c that intersects the straight line S1 is defined as Q3, and a tangent line of the concave pressure receiving surface 27c passing through the intersection point Q3 is defined as S2. At this time, the first convex guide curved surface 27d is a curved surface that is in contact with the tangent line S2 of the concave pressure receiving surface 27c and guides the central portion of the fixing belt 22 in the direction of the tangent line S2.

  Further, the curved surface 29e of the cylindrical portion 29b of the holding frame 29 is a curved surface that guides the end portion of the fixing belt 22 in the direction of the tangent S2.

  The fixing belt 22 is smoothly moved from the first convex guide curved surface 27d and the curved surface 29e to the nip region N by the first convex guide curved surface 27d of the nip forming portion 27 and the curved surface 29e of the cylindrical portion 29b of the holding frame 29. Led to. Here, the virtual straight line S1 and the tangent line S2 are orthogonal to each other, but the angle formed by the virtual straight line S1 and the tangent line S2 may be changed.

  On the other hand, the second convex guide curved surface 27 e of the nip forming portion 27 protrudes outward from the curved surface 29 f of the cylindrical portion 29 b of the holding frame 29. For this reason, on the downstream side of the nip region N, the fixing belt 22 protrudes outward from the curved surface 29f of the cylindrical portion 29b of each holding frame 29 and slidably contacts the second convex guide curved surface 27e of the nip forming portion 27. While rotating. Due to the shape protruding to the outside of the fixing belt 22, the curvature of the fixing belt 22 increases on the downstream side of the nip area N, and the recording paper P that has passed through the nip area N is quickly peeled off from the fixing belt 22.

  Here, the part of the fixing belt 22 in contact with the second convex guide curved surface 27e of the nip forming part 27 is Q4, and the virtual straight line passing through this part Q4 and the rotation center Q1 of the fixing belt 22 is S3. The maximum diameter φ of the rotation locus is the diameter on the virtual straight line S3, and this maximum diameter φ is set smaller than the inner diameter of the fixing belt 22 when the cylindrical shape is maintained. For this reason, even if the second convex guide curved surface 27e of the nip forming portion 27 protrudes outside the outer peripheral surface of the cylindrical portion 29b of the holding frame 29, the fixing belt 22 does not break. It has been experimentally confirmed that when the maximum diameter φ is equal to or larger than the inner diameter of the fixing belt 22 when the cylindrical shape is maintained, the fixing belt 22 is easily broken.

  As for the shape of the nip forming portion 27, the thickness (the length in the vertical direction (the direction in which the virtual straight line S1 extends) in FIG. 11) of the second convex guide curved surface 27e is set to be the largest as compared with the other portions. Further, in the direction in which the tangent S2 extends, the nip forming portion 27 is formed such that the portion downstream of the intersection Q3 in the recording paper conveyance direction is longer than the portion upstream of the point Q3 in the recording paper conveyance direction. ing. Accordingly, the recording paper that has passed through the nip area N is easily peeled off from the nip fixing belt 22, and the recording paper can be peeled off from the nip fixing belt 22 quickly.

  Thus, in this embodiment, the outer peripheral surface of the cylindrical portion 29b of the holding frame 29 is smoothly continuous, and the concave pressure receiving surface 27c, the first convex guide curved surface 27d, and the second convex guide of the nip forming portion 27 are provided. The curved surface 27e is smoothly continuous, and the first convex guide curved surface 27d of the nip forming portion 27 is flush with the curved surface 29e of the cylindrical portion 29b of the holding frame 29 or retracts inward from the curved surface 29e. Therefore, both end portions and the central portion of the fixing belt 22 are smoothly rotated while being bent with substantially the same curvature, and the load applied to the fixing belt 22 is suppressed.

  Further, in the longitudinal direction of the fixing belt 22, the nip forming portion 27 is disposed slightly spaced from the outer peripheral surface of the cylindrical portion 29 b of the holding frame 29, and the both do not overlap or entangle, and thus the fixing belt 22. In the circumferential direction, no gap is formed between the nip forming portion 27 and the cylindrical portion 29b of the holding frame 29, and the fixing belt 22 is not broken.

  Further, the first convex guide curved surface 27 d is a curved surface that guides the central portion of the fixing belt 22 in the direction of the tangent S 2 of the concave pressure receiving surface 27 c, and the curved surface 29 e of the cylindrical portion 29 b of the holding frame 29 is the end of the fixing belt 22. The fixing belt 22 is smoothly guided from the first convex guide curved surface 27d and the curved surface 29e to the nip region N because the curved surface guides the portion in the direction of the tangent S2.

  In addition, the fixing belt 22 has a maximum diameter φ at a portion Q4 of the fixing belt 22 that is in contact with the second convex guide curved surface 27e of the nip forming portion 27. The maximum diameter φ is maintained in a cylindrical shape. Since it is set smaller than the inner diameter of the fixing belt 22 at that time, the fixing belt 22 does not break.

  In the above embodiment, the nip forming portion 27 is slightly separated from the cylindrical portion 29 b of the holding frame 29 in the longitudinal direction of the fixing belt 22, but the nip forming portion 27 is the cylindrical portion 29 b of the holding frame 29. You may make it contact | abut to the outer peripheral surface.

  The image forming apparatus according to the present invention has been described using a monochrome printer, but this is only an example, and other color printers and other electronic devices such as multifunction peripherals, copiers, facsimile machines, and the like. An image forming apparatus may be used.

  The configuration and processing described with reference to FIGS. 1 to 11 are only an embodiment of the present invention, and the present invention is not limited to the configuration and processing.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Apparatus main body 3 Operation part 4 Paper feed part 5 Output tray 11 Image forming part 19 Fixing apparatus 22 Fixing belt 23 Pressure roller 24 Heater 25 Reflecting plate 26 Support part 27 Nip forming part 27c Concave pressure receiving surface 27d 1st Convex guide curved surface 27e Second convex guide curved surface 28 Ring 29 Holding frame 29c Circumferential surface 29d Flat surfaces 29e, 29f Curved surface

Claims (7)

An endless fixing belt,
A pressure roller pressed against the fixing belt;
A nip forming part provided inside the fixing belt and receiving the pressure contact pressure by the pressure roller via the fixing belt;
A heat generating part for heating the fixing belt;
Inserted inside the fixing belt end in the longitudinal direction of the fixing belt, and has an outer peripheral surface in contact with the inner peripheral surface of the end of the fixing belt, and the outer peripheral surface is a smooth surface continuous to the entire periphery. And a holding frame that holds the end of the fixing belt.
The nip forming part and the holding frame are provided side by side with a gap in the longitudinal direction of the fixing belt,
The nip forming portion is provided on the upstream side in the rotation direction of the fixing belt with respect to the pressure receiving surface that receives the pressure contact pressure by the pressure roller via the fixing belt, and the inner side of the fixing belt. A first guide curved surface in contact with the circumferential surface;
The fixing device is configured such that the first guide curved surface is disposed between a position that is flush with an outer peripheral surface of the holding frame and a position that is closer to a central portion in a radial direction of the fixing belt.   The fixing device according to claim 1, wherein the first guide curved surface is disposed at a position flush with an outer peripheral surface of the holding frame.   The first guide curved surface is in contact with the tangent of the pressure receiving surface passing through the intersection when the intersection of the pressure receiving surface intersecting with a virtual straight line passing through the rotation center of the pressure roller and the rotation center of the fixing belt is defined. The fixing device according to claim 1, wherein the fixing device is a curved surface that guides the fixing belt in a direction of the tangent line.   The fixing device according to claim 3, wherein an outer peripheral surface of the holding frame includes a curved surface that guides an end portion of the fixing belt in a direction of the tangent.   The fixing device according to claim 2, wherein a maximum diameter of a rotation locus of the fixing belt is set smaller than an inner diameter of the fixing belt when the cylindrical shape is maintained. The nip forming portion is provided downstream of the pressure receiving surface in the rotational movement direction of the fixing belt, and has a second guide curved surface in contact with the inner peripheral surface of the fixing belt,
The fixing device according to claim 5, wherein a maximum diameter of a rotation locus of the fixing belt is a diameter on an imaginary straight line passing through a portion of the fixing belt in contact with the second guide curved surface and a rotation center of the fixing belt. A fixing device according to any one of claims 1 to 6,
An image forming unit that forms an image on recording paper,
The fixing device is an image forming apparatus in which the recording paper is sandwiched in a nip region between the fixing belt and the pressure roller, and an image formed on the recording paper is thermocompression bonded.