JP2016188916A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to adjust a heat diffusion property at the ends of a soaking member after suppressing a reduction in durability.SOLUTION: There is provided a fixing device comprising: an endless belt; a heat source that heats the belt; a nip forming member that is disposed inside the belt; and a pressure member that is in pressure contact with the nip forming member with the belt therebetween to form a fixing nip, the fixing device passing a recording material carrying an unfixed image through the fixing nip to fix the unfixed image on the recording material, where the nip forming member has a surface on the fixing nip side formed of a metal material; gaps are formed in end areas in the longitudinal direction of the metal material and outside the outer end parts of a paper feed area of a sheet of the maximum feedable size; the vertical sides of the gaps extend inclined with respect to a direction of rotation of the belt.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a fixing device that fixes an image on a recording material, and an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction machine having at least two functions thereof.

In recent years, there has been a strong market demand for energy saving and high speed for image forming apparatuses.
In an image forming apparatus, an unfixed toner image is transferred to a recording paper, printing paper, photosensitive paper, electrostatic recording paper, etc. by an image transfer method or a direct method by an image forming process such as electrophotographic recording, electrostatic recording, or magnetic recording. Formed on recording material. As a fixing device for fixing an unfixed toner image, a contact heating method fixing device such as a heat roller method, a film heating method, and an electromagnetic induction heating method is widely used.

Problems in recent years in such a fixing device include the following.
-When the power is turned on, etc., the warm-up time, which is the time required to reach a predetermined printable temperature (reload temperature) from the normal temperature state, and after receiving a print request, the printing operation is performed and the paper discharge is completed. Further shortening of the first print time, which is the time until the start (Problem 1).
As the speed of the image forming apparatus increases, the number of sheets passing per unit time increases and the required heat quantity increases, so the heat quantity is insufficient particularly at the beginning of continuous printing (problem 2).

  In order to solve the above problems, an endless belt having a low heat capacity is directly heated (not via a metal heat conductor), and even when mounted on a high-production image forming apparatus, it has good fixability. A fixing device that can be obtained has been proposed (for example, Patent Document 1).

  However, in the case of a fixing configuration using an endless belt having a low heat capacity, it has been difficult to maintain a uniform temperature distribution in the longitudinal direction during paper feeding. That is, in the area (sheet passing area) through which a small size recording material passes, heat is consumed to heat the recording material and unfixed toner placed on the recording material. In the non-sheet passing area, heat is consumed by the recording material or the like. Since the heat is accumulated on the heating roller and the belt, the temperature of the nip portion in the non-sheet passing region becomes higher than the nip portion temperature in the sheet passing region maintained at a predetermined temperature. Temperature rise occurs.

  To cope with this problem of temperature rise at the end, a part of the nip forming member that is in pressure contact with the fixing belt is used as a heat equalizing member made of a highly heat-conductive material. It has also been proposed to increase the temperature and prevent the end temperature from rising. This is because the heat transfer in the width direction of the fixing belt, that is, in the longitudinal direction / axial direction of the nip forming member is facilitated by the high thermal conductive material, so that the end region remains heated without being absorbed by the paper passing. Heat is moved in the longitudinal direction to make the temperature distribution as smooth as possible.

  However, the configuration using such a heat equalizing member has a problem that the temperature of the end portion becomes lower than that of the central portion at the time of fixing start-up as a trade-off for preventing the end temperature increase. That is, at the time of fixing start-up before passing paper, heat is radiated from the edge of the end portion region of the heat equalizing member, and the end portion does not readily rise in temperature compared to the central portion. For this reason, as a measure for restricting heat escape in the vicinity of the end portion in the longitudinal direction of the heat equalizing member, it is conceivable to provide a hole in the end portion in the longitudinal direction of the heat equalizing member to form a low heat conduction void region. However, if a hole is simply formed, the wear of the fixing belt is accelerated by the edge of the formed hole, and the durability may be reduced.

  Therefore, an object of the present invention is to make it possible to adjust the thermal diffusibility of the end portion of the heat equalizing member while suppressing a decrease in durability.

  In order to achieve the above object, the present invention provides an endless belt, a heat source for heating the belt, a nip forming member disposed inside the belt, and a pressure contact with the nip forming member across the belt. And a pressure member that forms a fixing nip, and a fixing device that fixes an unfixed image on a recording material by passing a recording material carrying an unfixed image through the fixing nip. The surface on the fixing nip side is made of a metal material, and a gap is formed outside the outer edge of the sheet passing region of the maximum size paper that is the end region in the longitudinal direction of the metal material. The vertical side of the gap extends in an inclined manner with respect to the belt rotation direction.

  According to the present invention, the gap is formed outside the outer end of the sheet passing region of the maximum size paper that is the end region in the longitudinal direction of the metal material constituting the surface on the fixing nip side of the nip forming member. Since the vertical side of the gap extends with an inclination with respect to the belt rotation direction, the portion where the gap contacts the belt is displaced in the belt rotation direction, and the wear load on the belt due to the edge of the gap is dispersed. In addition, it is possible to suppress a decrease in the durability of the belt, and it is possible to adjust the thermal diffusibility at the end of the nip forming member by the gap.

1 is a schematic diagram illustrating an overall configuration of an image forming apparatus according to an embodiment of the present invention. 1 is a cross-sectional view illustrating an embodiment of a fixing device. It is a figure which shows the structure of a nip formation member. It is an exploded view of a nip forming member. It is the top view which looked at the nip forming member from the nip side.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
FIG. 1 is a schematic diagram showing the overall configuration of an image forming apparatus according to an embodiment of the present invention.
The image forming apparatus 1 shown here is a color laser printer, and four image forming units 4Y, 4M, 4C, and 4K are provided in the center of the printer body. Each of the image forming units 4Y, 4M, 4C, and 4K stores developers of different colors of yellow (Y), magenta (M), cyan (C), and black (K) corresponding to the color separation components of the color image. It is the same composition except being.

  Specifically, each of the image forming units 4Y, 4M, 4C, and 4K includes a drum-shaped photoconductor 5 as a latent image carrier, a charging device 6 that charges the surface of the photoconductor 5, and a surface of the photoconductor 5. A developing device 7 that supplies toner and a cleaning device 8 that cleans the surface of the photoreceptor 5 are provided. In FIG. 1, only the photoconductor 5, the charging device 6, the developing device 7, and the cleaning device 8 included in the black image forming unit 4 </ b> K are denoted by reference numerals, and the other image forming units 4 </ b> Y, 4 </ b> M, and 4 </ b> C are denoted by reference numerals. Omitted.

  Below the image forming units 4Y, 4M, 4C, and 4K, an exposure device 9 that exposes the surface of the photoreceptor 5 is disposed. The exposure device 9 includes a light source, a polygon mirror, an f-θ lens, a reflection mirror, and the like, and irradiates the surface of each photoconductor 5 with laser light based on image data.

  A transfer device 3 is disposed above the image forming units 4Y, 4M, 4C, and 4K. The transfer device 3 includes an intermediate transfer belt 30 as a transfer body, four primary transfer rollers 31 as primary transfer means, and a secondary transfer roller 36 as secondary transfer means. Further, the transfer device 3 includes a secondary transfer backup roller 32, a cleaning backup roller 33, a tension roller 34, and a belt cleaning device 35.

  The intermediate transfer belt 30 is an endless belt and is stretched by a secondary transfer backup roller 32, a cleaning backup roller 33, and a tension roller 34. Here, when the secondary transfer backup roller 32 is driven to rotate, the intermediate transfer belt 30 runs (rotates) in the direction indicated by the arrow in the figure.

  Each of the four primary transfer rollers 31 sandwiches the intermediate transfer belt 30 with each photoconductor 5 to form a primary transfer nip. In addition, each primary transfer roller 31 is connected to a power source of a printer main body, and a predetermined DC voltage (DC) and / or AC voltage (AC) is applied to each primary transfer roller 31. .

  The secondary transfer roller 36 sandwiches the intermediate transfer belt 30 with the secondary transfer backup roller 32 to form a secondary transfer nip. Similarly to the primary transfer roller 31, the secondary transfer roller 36 is also connected to the power source of the printer main body, and a predetermined DC voltage (DC) and / or AC voltage (AC) is applied to the secondary transfer roller 36. It has come to be.

The belt cleaning device 35 includes a cleaning brush and a cleaning blade disposed so as to contact the intermediate transfer belt 30.
A bottle container 2 is provided in the upper part of the printer main body, and four toner bottles 2Y, 2M, 2C, and 2K containing replenishing toner are detachably attached to the bottle container 2. As is well known, a replenishment path is provided between each toner bottle 2Y, 2M, 2C, 2K and each developing device 7, and each development from each toner bottle 2Y, 2M, 2C, 2K is performed via this replenishment path. The toner is supplied to the device 7.

  On the other hand, at the lower part of the printer main body, a paper feed tray 10 containing paper P as a recording material, a paper feed roller 11 for carrying out the paper P from the paper feed tray 10, and the like are provided. Here, the recording material includes, in addition to plain paper, thick paper, postcard, envelope, thin paper, coated paper (coated paper, art paper, etc.), tracing paper, OHP sheet, and the like. Further, as is well known, a manual paper feed mechanism may be provided.

  In the printer main body, a transport path R is provided for discharging the paper P from the paper feed tray 10 through the secondary transfer nip to the outside of the apparatus. In the transport path R, a pair of registration rollers 12 serving as transport means for transporting the paper P to the secondary transfer nip is disposed upstream of the position of the secondary transfer roller 36 in the paper transport direction.

  Further, a fixing device 20 for fixing the unfixed image transferred onto the paper P is disposed downstream of the position of the secondary transfer roller 36 in the paper transport direction. Further, a pair of paper discharge rollers 13 for discharging the paper to the outside of the apparatus is provided downstream of the fixing device 20 in the paper conveyance direction of the conveyance path R. A discharge tray 14 for stocking sheets discharged outside the apparatus is provided on the upper surface of the printer main body.

  The basic operation of the printer according to this embodiment is as follows. When the image forming operation is started, the photoconductors 5 in the image forming units 4Y, 4M, 4C, and 4K are rotationally driven in the clockwise direction in the drawing, and the surface of each photoconductor 5 is set to a predetermined polarity by the charging device 6. Uniformly charged. The surface of each charged photoconductor 5 is irradiated with laser light from the exposure device 9 to form an electrostatic latent image on the surface of each photoconductor 5. At this time, the image information to be exposed on each photoconductor 5 is single-color image information obtained by separating a desired full-color image into color information of yellow, magenta, cyan, and black. In this way, toner is supplied to each electrostatic latent image formed on each photoconductor 5 by each developing device 7, whereby the electrostatic latent image is visualized (visualized) as a toner image. .

  When the image forming operation is started, the secondary transfer backup roller 32 is driven to rotate counterclockwise in the figure, and the intermediate transfer belt 30 is caused to run in the direction indicated by the arrow in the figure. Then, a constant voltage or a constant current controlled voltage having a polarity opposite to the charging polarity of the toner is applied to each primary transfer roller 31. As a result, a transfer electric field is formed in the primary transfer nip between each primary transfer roller 31 and each photoconductor 5.

  Thereafter, when each color toner image on the photoconductor 5 reaches the primary transfer nip as each photoconductor 5 rotates, the toner image on each photoconductor 5 is formed by the transfer electric field formed in the primary transfer nip. The images are sequentially superimposed and transferred onto the intermediate transfer belt 30. Thus, a full color toner image is carried on the surface of the intermediate transfer belt 30. Further, the toner on each photoconductor 5 that could not be transferred to the intermediate transfer belt 30 is removed by the cleaning device 8. Thereafter, the surface of each photoconductor 5 is neutralized, and the surface potential is initialized.

  In the lower part of the image forming apparatus, the paper feed roller 11 starts to rotate, and the paper P is sent out from the paper feed tray 10 to the transport path R. The sheet P sent to the transport path R is timed by the registration roller 12 and sent to the secondary transfer nip between the secondary transfer roller 36 and the secondary transfer backup roller 32. At this time, a transfer voltage having a polarity opposite to the toner charge polarity of the toner image on the intermediate transfer belt 30 is applied to the secondary transfer roller 36, thereby forming a transfer electric field in the secondary transfer nip.

  Thereafter, when the toner image on the intermediate transfer belt 30 reaches the secondary transfer nip as the intermediate transfer belt 30 rotates, the toner image on the intermediate transfer belt 30 is generated by the transfer electric field formed in the nip. Are collectively transferred onto the paper P. At this time, the residual toner on the intermediate transfer belt 30 that could not be transferred onto the paper P is removed by the belt cleaning device 35, and the removed toner is conveyed to a waste toner container placed in the printer body. To be recovered.

  Thereafter, the paper P is conveyed to the fixing device 20, and the toner image on the paper P is fixed to the paper P by the fixing device 20. Then, the paper P is discharged out of the apparatus by the paper discharge roller 13 and stocked on the paper discharge tray 14.

  The above description is an image forming operation when a full-color image is formed on a sheet. A single-color image is formed using any one of the four image forming units 4Y, 4M, 4C, and 4K. Two or three image forming units can be used to form a two-color or three-color image.

FIG. 2 is a schematic cross-sectional configuration diagram showing an embodiment of the fixing device 20.
The fixing device 20 is disposed in a fixing belt 21 that is an endless belt, a pressure roller 22 that is a pressure member that comes into contact with the fixing belt 21 from the outer periphery thereof, and the inside (in a loop) of the fixing belt 21. And a heater 23 for heating the fixing belt 21. Further, inside the fixing belt 21, a nip forming member 24 that forms a fixing nip with the pressure roller 22 via the fixing belt 21, and a nip formed by pressure from the pressure roller 22 that supports the nip forming member 24. And a stay 25 (support member) for preventing the member 24 from bending. Both ends of the stay 25 and the heater 23 are supported by holders 26. Instead of the fixed support by the holder 26, it may be fixed to the side plate of the fixing device 20. Further, a low friction material may be provided on the sliding surface of the nip forming member 24 with the fixing belt 21. The surface of the stay 25 facing the heater 23 is generally subjected to a mirror surface treatment for reflecting the light from the halogen heater 23 to the fixing belt 21. The stay 25 and the like are radiated from the heater 23. The useless energy consumption by being heated is suppressed. A reflecting member may be provided between the stay 25 and the heater 23 instead of the mirror surface treatment on the stay 25. The heater 23 has two configurations, one having a heat generation area at the center in the longitudinal direction corresponding to small size paper and one having heat generation areas at both ends in the longitudinal direction corresponding to large size paper. There can be one or three configurations. As the heat source, a halogen heater is generally used, but an induction heating device, a resistance heating element, a carbon heater, or the like may be used.

  As is well known, a temperature sensor for detecting the belt temperature is provided at an appropriate position on the outer peripheral side of the fixing belt 21, and the paper P is separated from the fixing belt 21 on the downstream side in the paper transport direction of the fixing device 20. Although a separating member is provided and a releasable pressure means for pressing the pressure roller 22 to the fixing belt 21 is provided, it is not directly related to the configuration of the present invention, and therefore, the description is omitted.

  The fixing belt 21 is composed of a thin and flexible endless member (including a film), and an inner peripheral base material formed of a metal material such as SUS or a resin material such as polyimide (PI). , A release layer on the outer peripheral side made of PFA, PTFE or the like. An elastic layer formed of a rubber material such as silicone rubber, foamable silicone rubber, or fluorine rubber may be interposed between the base material and the release layer.

  The pressure roller 22 includes a cored bar, an elastic layer made of foamable silicone rubber or fluororubber provided on the surface of the cored bar, and a release layer made of PFA, PTFE, etc. provided on the surface of the elastic layer. It is configured. At the location where the pressure roller 22 and the fixing belt 21 are in pressure contact, the elastic layer of the pressure roller 22 is crushed to form a fixing nip having a predetermined width. The pressure roller 22 is configured to be rotationally driven by a drive source such as a motor provided in the printer main body. When the pressure roller 22 is rotationally driven, the driving force is transmitted to the fixing belt 21 at the fixing nip, and the fixing belt 21 is driven to rotate.

  In the present embodiment, the pressure roller 22 is a solid roller, but may be a hollow roller. In that case, a heating source such as a halogen heater may be disposed inside the pressure roller 22. By setting the thickness of the elastic layer to 100 μm, when the unfixed toner is crushed and fixed, minute irregularities on the belt surface can be absorbed by the elastic deformation of the elastic layer, and the occurrence of uneven gloss can be avoided. The elastic layer may be solid rubber, but if there is no heat source inside the pressure roller, sponge rubber may be used. Sponge rubber is more preferable because heat insulation is enhanced and heat of the fixing belt 21 is less likely to be taken away.

  The heater 23 is configured to generate heat by being output controlled by a power supply unit provided in the printer main body, and the output control is based on the temperature detection result of the belt surface by the temperature sensor provided on the outer periphery of the fixing belt 21. Based on. By such heater output control, the temperature of the fixing belt 21 (fixing temperature) can be set to a desired temperature.

  The nip forming member 24 arranged in the longitudinal direction in the width direction of the fixing belt 21 is fixedly supported by the stay 25, so that the nip forming member 24 is prevented from being bent and the shaft of the pressure roller 22 is prevented. A uniform nip width can be obtained across the direction. The nip forming member 24 is composed of a heat resistant member having high mechanical strength and a heat resistant temperature of 200 ° C. or higher. This prevents deformation of the nip forming member 24 due to heat in the toner fixing temperature range, ensures a stable fixing nip state, and stabilizes the output image quality.

  FIG. 3 is a cross-sectional view showing the detailed configuration of the nip forming member 24 as viewed from the paper conveyance direction (the cross-sectional area is the approximate center of the fixing nip of FIG. 2, the left-right direction is the longitudinal direction, and FIG. 4 is an exploded view thereof. FIG. 5 is a plan view of the nip forming member 24 of FIG. 3 viewed from the nip side.

The nip forming member 24 shown in FIGS. 3 and 4 includes a base 241, a fixing nip-side heat equalizing member 242, a stay-side heat equalizing member 243, and an intermediate heat equalizing member 244.
As shown in an exploded view in FIG. 4, the base member 241 includes a central member 241C, two end members 241T, and two connection members 241S. For the base material 241, PES (polyethersulfone), PPS (polyphenylene sulfide), PEN (polyethernitrile), PAI (polyamideimide), etc., which are general heat resistant resins, are used. The thermal conductivity is low.

  The soaking members 242, 243, 244 are formed of a metal material having high thermal conductivity such as copper (Cu) or aluminum (Al), and ensure high thermal conductivity. The nip-side heat equalizing member 242 is a member that directly receives the heat from the fixing belt 21 (in the case where a low friction member is interposed, via the low friction member). By providing the heat equalizing member 242 in the paper width direction, for example, even when small-size paper is continuously passed, even if the fixing belt temperature rises in the non-paper passing area, the heat is transferred in the longitudinal direction (paper width Direction) can be efficiently moved and diffused, and the belt end temperature rise can be suppressed.

  In the heat equalizing member 242 on the nip side in this example, a gap 242a is formed in the vicinity of both end portions in the longitudinal direction, and a convex portion 241a provided on the end member 241T of the base material 241 is fitted therein, thereby forming a nip. The entire surface (surface) on the nip side of the member 24 is configured to be flat. That is, the convex part 241a of the end member 241T is an auxiliary part piece for the gap 242a, and fills the step due to the gap 242a of the heat equalizing member 242.

  The stay-side heat equalizing member 243 is a heat equalizing member disposed on the surface of the nip forming member 24 opposite to the nip so as to contact the stay 25 (FIG. 2). The intermediate heat equalizing member 244 is a heat equalizing member disposed between the stay-side heat equalizing member 243 and the base material 241 (more precisely, the connecting member 241S). The position of the intermediate heat equalizing member 244 in the longitudinal direction (fixing belt rotation axis direction) corresponds to the end of the sheet when a small size sheet (for example, A5 length) is passed. In other words, the intermediate heat equalizing member 244 is arranged so that the end on the center side overlaps (overlaps) the end of the small-size paper passing area. Even when the intermediate temperature-uniforming member 244 is arranged to continuously pass small-size paper and the temperature rises in the vicinity of the end of the paper-passing region, the heat is efficiently distributed on the stay side. 243 can be diffused and absorbed in the sheet width direction and the nip forming member 24 thickness direction.

  In order to dispose the intermediate heat equalizing member 244, the thickness of the base material 241 (the vertical direction in the figure) is reduced at that portion, and in the illustrated example, the connecting member 241S and the intermediate heat equalizing member 244 are overlapped. The thickness is equal to the central member 241C of the base material.

  By the way, as for each heat equalizing member, the larger the thickness (the thicker), the greater the effect of preventing the end temperature rise. However, if the thickness is too large, the warm-up is delayed and the energy efficiency is also lowered. For this reason, in this example, the thickness of the stay-side heat equalizing member 243 is 2 mm, the thickness of the intermediate heat equalizing member 244 is 1.5 mm, and the thickness of the nip-side heat equalizing member 242 is 0.6 mm. In addition, the base member 241 has a thickness of the connecting member 241S of 1 mm, and the central member 241C has a thickness of 2.5 mm which is the thickness of the connecting member 241S + the thickness of the intermediate heat equalizing member 244. The thickness of the convex portion 241a of the base material (the thickness of only the convex portion) is 0.6 mm, which is the same as the thickness of the heat equalizing member 242 on the nip side.

The nip-side heat equalizing member 242 has a length in the axial direction (longitudinal direction = paper width direction = left-right direction in the drawing) of 344 mm, and as can be seen from FIG. And a trapezoidal gap 242a having a center width of 7.00 mm is provided. The gap 242a in the end region of the heat equalizing member 242 is arranged in a range where it contacts the pressure roller 22 via the fixing belt 21, and the lateral side on the downstream side is longer than the upstream side in the belt rotation direction. Is formed. By forming a trapezoidal gap with a longer lateral side on the downstream side of the belt, the fixing belt 21 bites into the vertical side of the gap in the fixing nip even when the nip side surface of the nip forming member 24 is not completely flat. By avoiding the operation, the load on the belt can be reduced. In addition, the vertical side near the center of the gap 242a (extends inclined with respect to the belt rotation direction), that is, the vertical side near the center of the convex portion 241a of the base 241 is the maximum size paper that can be passed. It is provided so as to be located on the outer side (end side) than the outer end portion of the sheet passing area. An intersection 242a ₁ (starting point) of the horizontal side and the vertical side closer to the center of the trapezoidal gap 242a in the belt rotation direction is located in the fixing nip region. This starting point is preferably at a position 1 mm or more from the end of the fixing nip on the upstream side in the belt rotation direction, in other words, from the side where the belt enters the fixing nip. On the other hand, the intersection 242a _u (end point) of the horizontal side and the vertical side near the center of the gap 242a in the belt rotation direction is outside the fixing nip region, preferably at a position 1 mm or more outside the fixing nip from the side where the belt comes off. It is in. By setting the starting point and the ending point in this way, an operation in which the fixing belt bites into the side of the gap in the fixing nip is avoided.

  3 and 4, the thickness direction is exaggerated with respect to the length direction. 3 and 4 are schematic diagrams for explaining the configuration of the nip forming member 24, and the dimensions of each member including the length direction are drawn differently from those obtained by reducing the actual member as it is. . 3 and 4, the base material 241 is divided into a plurality of members, and an intermediate heat equalizing member 244 is provided. However, the base material and each heat equalizing member are each divided. A plate-shaped member can be used, and an integral member having a convex portion or a concave portion may be used.

DESCRIPTION OF SYMBOLS 20 Fixing device 21 Fixing belt 22 Pressure roller 23 Heater 24 Nip forming member 25 Stay 26 Holder 242, 243, 244 Heat equalizing member 241 Base material

JP2013-137470A

Claims (7)

An endless belt, a heat source for heating the belt, a nip forming member disposed inside the belt, and a pressure member that presses against the nip forming member to form a fixing nip with the belt interposed therebetween. In a fixing device for fixing an unfixed image on a recording material by passing a recording material carrying an unfixed image through the fixing nip,
The surface on the fixing nip side of the nip forming member is made of a metal material, and the gap is outside the outer end portion of the sheet passing region of the maximum size paper that is the end region in the longitudinal direction of the metal material. The fixing device is characterized in that the vertical sides of the gaps are inclined with respect to the belt rotation direction.   The fixing device according to claim 1, wherein the inclined vertical side extends from the upstream side to the downstream side in the belt rotation direction so as to approach the center side of the nip forming member.   The fixing device according to claim 1, wherein the gap starts from a position inside the fixing nip of 1 mm or more from a side where the belt enters the fixing nip.   3. The fixing device according to claim 1, wherein the gap has an end point at a position outside the fixing nip of 1 mm or more from a side where the belt comes out of the fixing nip.   The fixing device according to claim 1, wherein an auxiliary part having a lower thermal conductivity than that of a metal material is provided in the gap and fills a step due to the gap.   The fixing device according to claim 1, wherein a low friction member is interposed between the nip forming member and the belt.   An image forming apparatus comprising the fixing device according to claim 1.

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