JP2020067474A - Fixing device - Google Patents

Abstract

To combine prevention of occurrence of adhesion of lubricant applied onto an inner surface of a fixing belt to a nip section to generate a defective image with improvement of separability in a fixing device having a heating source heating the fixing belt at the nip section.SOLUTION: A sheet member 16 having bending rigidity lower than that of a fixing belt 11 is provided in a nip section between the fixing belt 11 and a heating source 13.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a fixing device for fixing an unfixed toner image formed on a recording material in an image forming unit in an image forming apparatus such as an electrophotographic copying machine and a laser beam printer onto the recording material.

  As a fixing device, a heated fixing member and a pressure member are brought into pressure contact with each other to form a fixing nip portion, and a recording material carrying an unfixed toner image is nip-conveyed in the nip portion to heat and pressurize the toner image. Fix on top.

  In such a fixing device, the heat capacity is small because the time (warm-up time) from when the power is turned on to when the fixing member is heated to a predetermined temperature necessary for fixing the toner image and image formation can be started is shortened. A fixing device using a thin belt-shaped fixing member has been put into practical use.

  Patent Document 1 discloses a fixing device using a belt (hereinafter referred to as a fixing belt) as a fixing member. In this fixing device, a plate-shaped ceramic heater is arranged inside the fixing belt, and a nip portion is formed between the heater and a pressure roller as a pressure member via the fixing belt. Then, the fixing belt is driven and rotated by the pressure roller to convey the recording material to the nip portion, and the unfixed toner image is heated and pressed to be fixed on the recording material.

  Further, Japanese Patent Application Laid-Open No. 2004-242242 discloses a means for preventing a recording material from winding around a fixing belt and causing a jam when an image having a large amount of toner is fixed to a recording material having a low rigidity such as thin paper or hygroscopic paper. Has been done. That is, in order to improve the separation of the recording material from the fixing belt after passing through the nip, a protrusion protruding toward the pressure roller is provided downstream of the nip to reduce the radius of curvature of the belt track and easily separate the recording material from the belt. is doing. Similarly, in Patent Document 2, in order to prevent the lubricant such as heat-resistant grease interposed inside the fixing belt from being blocked by the above-mentioned protrusion and the grease circulation being lowered, the grease is provided on the protrusion so that the grease can pass through the protrusion. It has a slit.

  Further, in Patent Document 3, in a fixing device in which a ceramic heater and a heater holding member for holding the heater are arranged in the nip portion, in order to prevent the inner surface of the fixing belt from being strongly rubbed against the heater or the holding member and damaged, And the holding member is covered with a heat conduction plate having a curved shape.

JP-A-4-204980 JP, 2003-229234, A JP-A-11-16667

  However, when a protrusion is provided in the conventional fixing device in order to improve separability, gloss unevenness of an image defect occurs due to grease retention near the protrusion.

  The above problem will be described with reference to FIGS. 8 and 9.

  FIG. 8 shows a main part (near the nip) of a conventional fixing device. A ceramic heater 13 and a holding member 14 that holds the heater 13 and guides the traveling of the fixing belt are provided inside the fixing belt 11. The pressure roller 19 is pressed against the heater 13 and the holding member 14 via the fixing belt 11. To form the nip portion N. A protrusion 14a is provided on the upstream side of the heater 13 along the rotation direction of the fixing belt 11 (arrow B) so as to protrude toward the pressure roller 19. This is to prevent the inner surface of the fixing belt 11 from coming into contact with the upstream corner of the heater 13 and to prevent the inner surface from being damaged by rubbing against the corner. Further, a protrusion 14b that protrudes toward the pressure roller 19 is provided downstream of the nip. This is to reduce the radius of curvature of the fixing belt 11 downstream of the nip portion and improve the separability. The protruding portion 14b also has a role of preventing the inner surface of the fixing belt from coming into contact with the downstream corner portion of the heater 13, like the protruding portion 14a on the upstream side. A protective layer having a low coefficient of friction may be provided on the inner surface of the fixing belt 11 to improve durability.

  In the above configuration, spaces 13a and 13b where the fixing belt 11 does not come into contact are formed in the upstream portion and the downstream portion of the heater 13. The grease G applied to the inner surface of the fixing belt 11 moves to the nip portion due to the rotation of the fixing belt 11, and the grease is present between the heater 13 and the protruding portions 14a and 14b and the fixing belt 11 to reduce friction. On the other hand, the grease that has entered the spaces 13a and 13b is likely to stay because the conveyance force from the fixing belt 11 is low. The accumulated grease may be heated on the heater 13 for a long time, the oil component contained in the grease may volatilize, the viscosity may decrease, and the grease may stick to the heater 13.

  FIG. 9 is a schematic view showing a state where the grease is fixed, and is a view of a part of the heater 13 seen from the pressure roller side. The heater 13 has a width H along the rotation direction of the fixing belt 11. Grease fixation G1 occurs on the upstream side of the heater 13, and grease fixation G2 occurs on the downstream side. When viewed in the direction of the rotation axis of the fixing belt 11 (direction perpendicular to the rotation direction, hereinafter referred to as the longitudinal direction for simplicity), if there is a region G3 where a large amount of grease adheres locally, the heater 13 is present only in that region. The amount of heat transferred from the fixing belt 11 to the fixing belt 11 is reduced. In this area, the amount of heat received from the fixing belt 11 while the recording material P is passing through the nip is reduced, so that the melting of the toner image is reduced and the gloss reduction F occurs on the image.

  In Patent Document 2, a slit is provided in the downstream projecting portion to prevent grease from staying in the vicinity of the projecting portion, but staying occurs in a region where there is no slit. In addition, it is not possible to prevent retention on the upstream side.

  In Patent Document 3, improvement in separability is not taken into consideration, and there is no protrusion on the downstream side. If a protrusion is provided on the downstream side to improve separability, it is expected that uneven gloss will occur due to grease retention.

  The present invention is intended to solve the above problems, and an object thereof is to prevent the gloss unevenness due to the adhesion of grease to the heater while providing a protruding portion to improve the separability.

In order to achieve the above object, the fixing device according to the present invention,
A rotating belt-shaped fixing member, a heating source arranged inside the fixing member to heat the fixing member, a holding member for holding the heating source, and a nip portion which is pressed against the heating source and the holding member via the fixing member. The holding member has a pressing member, and the holding member has a protruding portion 14b protruding toward the pressing member side from the heating source downstream of the nip portion along the rotation direction of the fixing member, and a lubricant is applied to the inner surface of the fixing member. In a fixing device that conveys a recording material carrying a toner image to the nip portion and fixes the toner image, a sheet member 16 having a bending rigidity smaller than that of the fixing member is provided between the fixing member and the heat source, and the sheet member 16 is pressed. The sheet member is in close contact with the inner surface of the fixing member in a range including the protrusion from the heat source along the circumferential direction of the fixing member.

  According to the fixing device of the present invention, the vicinity of the nip portion can be stably provided without being affected by the dimensional variation of the heater and the heater holding member and the variation of the rigidity of the fixing belt while securing the separability of the thin paper etc. by providing the protruding portion. It is possible to prevent the unevenness of gloss by eliminating the grease retention area.

Image forming device Fixing device of the first embodiment Sectional view of the main part of the fixing device Side view of fixing device Comparison table showing effects of the invention Main Parts of Fixing Device of Second Embodiment Fixing device according to third embodiment Conventional fixing device Diagram showing problems in the conventional fixing device (image defect due to sticking of lubricant)

  Hereinafter, modes for carrying out the present invention will be described with reference to the drawings.

<Image forming device>
A schematic configuration of an image forming apparatus to which the present invention is applied will be described.

  FIG. 1 is an overall sectional view of a color image forming apparatus. First, second, third, and fourth image forming units Py, Pm, Pc, and Pk are arranged in the apparatus, and toner images of different colors are formed through the processes of latent image, development, and transfer. To be done. The photosensitive drums 1a, 1b, 1c and 1d, which are image bearing members, are rotatably supported, and on the outer periphery thereof, chargers 2a, 2b, 2c and 2d, developing devices 3a, 3b, 3c and 3d, and primary transfer means. 4a, 4b, 4c, and 4d are provided, and exposure devices 5a, 5b, 5c, and 5d are further provided below the image forming unit.

  The photosensitive drums 1a to 1d are rotationally driven by a drum motor (not shown), and are charged to a predetermined potential by the chargers 2a to 2d, and then laser beams corresponding to the image signals are emitted from the exposure devices 5a to 5d. An electrostatic latent image is formed on the photosensitive drums 1a to 1d by condensing and exposing the light on the generatrix of.

  The developing devices 3a to 3d are filled with predetermined amounts of yellow, magenta, cyan, and black toners as developers, respectively, and develop the latent images on the photosensitive drums 1a to 1d to develop yellow, magenta, and Visualize as cyan and black toner images.

  Then, the intermediate transfer body 25 stretched by the tension roller 21, the drive roller 22, and the backup roller 23 is rotationally driven in the direction of arrow F by the drive roller 22, and the primary transfer means 4a is charged with toner by a high voltage power source (not shown). A transfer voltage having a polarity opposite to the polarity is applied, and the first color yellow toner image on the photosensitive drum 1a is transferred to the intermediate transfer body 25 (primary transfer). Similarly, for the second color, the third color, and the fourth color, the toner images developed on the photosensitive drums are transferred onto the intermediate transfer body 25 in a superimposed manner, and a color image in which the four color toner images are superimposed is obtained.

  Transfer residual toner remains on the surfaces of the photosensitive drums 1a to 1d after the transfer, and the transfer residual toner is removed by the cleaning devices 6a, 6b, 6c and 6d and is recovered in a toner recovery container (not shown). After that, the residual charges on the photosensitive drums 1a to 1d are removed by the pre-exposure devices 7a, 7b, 7c and 7d to prepare for the next latent image formation.

  The recording material P is sent by a recording material conveying device (not shown) to the secondary transfer portion in contact with the secondary transfer means 24 and the backup roller 23 which is grounded, and the secondary transfer means 24 has a polarity opposite to that of the toner. By applying the secondary transfer voltage of the polarity, the four color toner images on the intermediate transfer body 25 are collectively transferred to the recording material P (secondary transfer).

  The recording material P on which the toner image has been transferred is conveyed to the fixing device 10, and the toner image is heated and melted and fixed to the recording material to form a full-color image.

  The transfer residual toner remaining on the intermediate transfer body 25 moves with the rotation of the intermediate transfer body 25, is removed by the cleaning blade 20, and is recovered in a recovery container (not shown).

<Fixing device>
FIG. 2 is a sectional view of a fixing device 10 using a fixing belt to which the present invention is applied.

  The fixing belt 11 is provided with an elastic layer made of silicone rubber having a thickness of about 200 μm on the outer peripheral surface of a cylindrical base layer made of stainless steel having a thickness of about 50 μm, and a tube having a thickness of about 30 μm made of PFA is coated on the outer peripheral surface of the elastic layer. It is a fixing member having an endless belt shape. A holding member 14 holds the heater and is formed of a liquid crystal polymer resin having heat resistance and rigidity.

  Reference numeral 13 denotes a heater as a heating source, which is a long and narrow plate-shaped ceramic substrate provided with a resistor that rapidly generates heat when energized along the longitudinal direction of the substrate, and the resistor is covered with a glass protective layer. It has a conductor electrode portion for applying a voltage to the end portion in the longitudinal direction.

  The heater 13 is adhesively fixed to a concave portion of the holding member 14 provided along the rotation axis direction (hereinafter, referred to as a longitudinal direction) of the fixing belt 11 using a heat resistant adhesive. The fixing belt 11 is loosely fitted onto the heater holder 14.

  The stay 15 is a high-rigidity support body formed of stainless steel plate in a U-shape, and the heater holder 14 is fixed to the stay 15 to reinforce the heater holder 14.

  Reference numeral 19 denotes a pressure roller as a pressure member, which is formed by forming a silicone rubber layer having a thickness of about 3 mm on the peripheral surface of a stainless steel core by injection molding and coating a PFA resin tube having a thickness of about 40 μm on the silicone rubber layer. . The holding member 14 and the heater 13 are pressed against the pressure roller 19 via the fixing belt 11 to form a fixing nip portion N having a predetermined width necessary for fixing.

  The pressure roller 19 is rotationally driven in the direction of arrow A at a predetermined peripheral speed by a driving device described later. As a result, the fixing belt 11 pressed against the outer surface of the pressure roller 19 is driven to rotate by receiving the frictional force from the pressure roller 19. The inner surface of the fixing belt 11 rotates while being in close contact with the fixing heater 13. The inner surface of the fixing belt 11 is coated with a silicone oil and a fluorine resin-containing grease as a heat-resistant lubricant in order to secure the slip properties of the sheet member 16 and the fixing belt 11 which will be described later.

  Reference numeral 18 is a thermistor as a temperature detecting means. The thermistor 18 is arranged in the vicinity of the center where recording materials of all sizes pass along the width direction of the belt, detects the temperature of the heater 13, and supplies power to the heater 13 so that the temperature detected by the central thermistor reaches a target temperature. To control.

  The pressure roller 19 and the belt 11 are rotationally driven, and the recording material P carrying the unfixed toner image is conveyed to the nip portion N while the heater 13 is controlled to the target temperature. The recording material P is heated by receiving heat from the 13 heater via the fixing belt 11, and at the same time, the toner image is fixed by being nipped and pressed in the nip portion. The recording material P that has passed through the nip portion is curvature-separated from the fixing belt 11 at the exit of the nip portion.

  Next, the features of the present invention will be described with reference to FIG.

  FIG. 3A shows a state before the fixing belt 11 and the pressure roller 19 are pressed.

  As shown in FIG. 3 (a), the holding member 14 is provided with a protrusion 14 a that protrudes from the heater 13 toward the pressure roller side on the upstream side of the heater 13 along the circumferential direction of the fixing belt 11. Further, on the downstream side of the heater 13, there is provided a projecting portion 14b projecting from the heater 13 to the pressure roller side. The upstream protrusion 14a prevents the fixing belt 11 from coming into contact with the upstream corner of the heater 13, and thereby prevents the inner surface of the fixing belt 11 from being rubbed and damaged at the corner. The downstream protrusion 14b separates the recording material P after fixing from the fixing belt 11 by reducing the radius of curvature of the trajectory of the fixing belt 11 at the position of the protrusion 14b when the pressure roller 19 and the fixing belt 11 are pressed. Make it easier.

  Grease G as a lubricant is applied to the inner surface of the fixing belt 11 in advance when the fixing device is assembled. The grease used is a mixture of heat-resistant fluororesin particles, silicone resin particles and the like with heat-resistant fluorine oil and silicone oil. The grease may be applied to the sheet member 16 described later.

  In the present invention, the sheet member 16 is provided between the fixing belt 11, the heater 13 and the holding member 14 in the nip portion so as to cover at least the heater 13 and the protruding portions 14a and 14b. That is, the range S is covered with the sheet member 16. In the first embodiment, the sheet member 16 has a semicircular cross section and is held by fixing the ends 16a and 16b to the holding member 14. Further, the sheet member 16 has substantially the same length as the fixing belt 11 in the longitudinal direction.

  As the material of the sheet member 16, a metal or alloy such as stainless steel, nickel, or copper, or a sheet of heat-resistant resin such as polyimide (PI), polyphenylene sulfide (PPS), or polyether ether ketone (PEEK) is used.

Further, as the sheet member 16, a sheet whose bending rigidity along the circumferential direction of the fixing belt 11 is smaller than that of the fixing belt 11 is used. In Example 1, as described above, the base layer of the fixing belt 11 is made of stainless steel (SUS) having a thickness of 50 μm. Since the other layers (rubber layer and PFA layer) forming the fixing belt 11 have smaller bending rigidity than the base layer, the bending rigidity of the entire fixing belt 11 is determined by the bending rigidity of the base layer. Therefore, the sheet member 16 is made of stainless steel having a thickness smaller than that of the base layer and having a thickness of 20 μm. Generally, the flexural rigidity of a sheet-shaped member is determined by the product of the Young's modulus E unique to the sheet material and the geometrical moment of inertia I of the cross section perpendicular to the direction along the bending. Accordingly, the Young's modulus in the circumferential direction of the fixing belt 11 is Ef, the second moment of area in the cross section parallel to the longitudinal direction of the fixing belt 11 is If, the Young's modulus in the circumferential direction of the sheet member 16 is Es, and the longitudinal direction of the sheet member 16 is Let Is a moment of inertia of area in a cross section parallel to, and satisfy the relation of Ef × If> Es × Is. Generally, the second moment of area I of the sheet member is determined by (b × h ³ ) / 12, where b is the longitudinal width of the sheet and h is the sheet thickness. Therefore, the thickness Ts of the sheet member 16 can be calculated from the thickness Tf of the fixing belt 11. You can make it smaller.

  FIG. 3B shows the fixing belt 11 and the pressure roller 19 when pressure is applied. At the time of pressurization, the pressure roller 19 is pressed against the heater 13, so that the fixing belt 11 is in close contact with the heater 13 with the sheet member 16 interposed therebetween. At this time, since the bending rigidity of the sheet member 16 is smaller than that of the fixing belt 11, the sheet member 16 is pressed and deformed by the fixing belt 11 and comes into close contact with the inner surface of the fixing belt 11. Since the fixing belt 11 is deformed in the range S including the protruding portions 14a and 14b, the sheet member 16 can be brought into close contact with the inner surface of the fixing belt 11 in the range S. The length of the circular arc portion of the sheet member 16 is set to a length that can ensure a close contact state when pressure is applied.

  With the above configuration, the grease G moves between the fixing belt 11 and the sheet member 16 and therefore does not enter the spaces 13a and 13 formed upstream and downstream of the heater 13. Further, the fixing belt 11 and the sheet member 16 are in close contact with each other within the range S, and the grease G does not stay because it is constantly circulated in the rotation direction of the fixing belt 11 due to the frictional force from the fixing belt 11. Further, the range S is wider than the width H, which is the heating region in the circumferential direction of the heater 13, and there is no grease that has stayed in the width H, so that the adhesion of grease can be prevented.

  FIG. 4 is a view of the fixing device 10 as viewed from the recording material conveyance direction, and shows the positions of the fixing belt 11 and the sheet member 16 in the longitudinal direction.

  The fixing belt 11 is supported by fitting support members 12a and 12b inside both ends. A holding member 14 is provided inside the fixing belt 11 so as to bridge the support members 12 a and 12 b, and the heater 13 is adhesively fixed onto the holding member 14. The supporting members 12a and 12b have protrusions 12c and 12d along about a half of the circumference of the fixing belt 11, and the fixing belt 11 which is biased at the time of rotation abuts against the protrusions 12c or 12d so that the fixing belt 11 moves in the longitudinal direction of the fixing belt 11. The position is restricted. The supporting members 12a and 12b are slidably supported on the frames 31a and 31b of the fixing device 10, and the pressure roller 19 is rotatably supported on the frames 31a and 31b. Then, the support members 12a and 12b are urged toward the pressure roller 19 by the pressure springs 32a and 32b, so that the holding member 14 and the heater 13 are pressed against the pressure roller 19 via the fixing belt 11.

  The sheet member 16 is provided between the holding member 14 and the fixing belt 11 so as to cover the holding member 14, and is supported by fixing the circumferential ends 16a and 16b to the holding member 14 as shown in FIG. . The length of the sheet member 16 is set such that the end portion of the sheet member 16 protrudes from the fixing belt 11 even when the fixing belt 11 is offset. The grease G applied to the inner surface of the fixing belt 11 may slightly leak from both ends of the fixing belt 11 at the time of pressurization, but since the sheet member 16 protrudes from the fixing belt 11, the grease G is heated by the heater 13. Never infiltrate L. That is, it is possible to prevent the grease from flowing into the spaces 13a and 13 (FIG. 3) from both ends.

<Effect>
FIG. 5 shows the effect of the present invention. FIG. 5 shows the results of examining the occurrence of gloss unevenness in an image forming apparatus using the fixing device in which the bending rigidity is changed by changing the material or thickness of the fixing belt 11 and the sheet member 16. When the gloss unevenness did not occur, it was indicated by ◯, when it occurred, it was indicated by x, and when it was slight, it was indicated by Δ.

  From this, it can be seen that in Example 1, by providing the sheet member 16 and making the bending rigidity of the sheet member 16 smaller than the bending rigidity of the base layer of the fixing belt 11, no gloss unevenness occurs. As the base layer of the fixing belt 11, various materials and thicknesses are adopted in consideration of the target life and cost. 1-2 and 1-3 show cases in which the material and thickness of the base layer are changed. Even if the base layer has different flexural rigidity, occurrence of gloss unevenness can be caused by selecting a flexural rigidity of the sheet member 16 smaller than that of the base layer. You can see that it can be prevented.

  In the conventional example, the sheet member 16 is not provided, and uneven gloss occurs. The comparative example has the sheet member 16, but shows a case where the bending rigidity of the sheet member 16 is larger than that of the base layer, and the gloss unevenness was improved as compared with the conventional example, but it was not sufficiently improved. When the fixing device of the comparative example was examined after the test, there was a portion where the adhesion was insufficient between the sheet member 16 and the fixing belt 11, and grease retention was observed at that portion, which caused gloss unevenness.

  As described above, according to the present invention, even in the configuration in which the protrusion is provided downstream of the nip for improving the separability, the grease and the protrusion are covered with the sheet member having a small bending rigidity to prevent the grease from protruding. It was possible to prevent staying in the vicinity and prevent uneven gloss.

  FIG. 6 shows another embodiment (state under pressure) of the shape of the sheet member. The same members as in the first embodiment are designated by the same symbols as in the first embodiment.

  In the first embodiment, the configuration is shown in which the surface of the holding member 14 on the pressure roller 19 side is entirely covered with the sheet member 16. In order to prevent the grease from sticking to the heater, which is the object of the present invention, at least the heater width should be covered with the sheet member.

In FIG. 6, the upstream end 16 d of the sheet member 160 is fixed near the heater 13. The end portion 16d is inserted into a groove (not shown) provided in the holding member 140 and fixed with a heat resistant adhesive. A cover member 160 covers the upstream corner of the heater 13 so that the upstream corner does not come into contact with the inner surface of the fixing belt 11. As a result, the protruding portion 14a at the nip upstream portion shown in FIG. 3 is not necessary, so that the protruding portion 14a is eliminated so that the nip upstream portion of the holding member 140 is in the same plane as the lower surface of the sheet member 160. On the other hand, a protrusion 140b is provided in the downstream portion of the nip to improve the separability, and the sheet member 160 covers the protrusion 140b. Further, the downstream end of the sheet member 160 may be a free end without being fixed to the holding member 140.
Even with the simple configuration as described above, the heater 13 and the fixing belt 11 can be brought into close contact with each other in the range S at the time of pressurization, and grease retention can be eliminated.

  FIG. 7 shows an embodiment in which the present invention is applied to another fixing device.

  The present invention is not limited to the configuration in which the heater is arranged in the nip portion. FIG. 7 shows a fixing device in which the halogen lamp 130 as a heating source heats the nip portion N by radiant heat. A nip forming member 141 made of a metal plate such as stainless steel is arranged in the nip portion, and the nip forming member 141 is reinforced by a stay 150. Reference numeral 131 is a reflecting plate that reflects radiant heat from the halogen lamp 130 to the nip side. The nip portion N is formed by pressing the pressure roller 19 and the nip forming member 141 with the fixing belt 11 in between by a pressure mechanism (not shown). The nip forming member 141 has a protruding portion 141b protruding toward the pressure roller 19 side for improving separability downstream of the nip portion.

  In the third embodiment, the sheet member 161 having a small bending rigidity is provided between the nip forming member 141 and the fixing belt 11. As a result, the grease G can be prevented from staying in the vicinity of the protrusion 141b, the staying grease can be prevented from sticking to the nip forming member 141, and the occurrence of gloss unevenness can be prevented.

10 fixing device, 11 fixing belt, 13 heating source, 13a, 13b grease staying part,
14 holding members, 14a, 14b protrusions, 16 sheet members, 19 pressure members,
G grease, N nip

Claims (6)

A rotating belt-shaped fixing member, a heating source arranged inside the fixing member to heat the fixing member, a holding member for holding the heating source, and a nip portion which is pressed against the heating source and the holding member via the fixing member. The holding member has a pressing member, and the holding member has a protruding portion 14b protruding toward the pressing member side from the heating source downstream of the nip portion along the rotation direction of the fixing member, and a lubricant is applied to the inner surface of the fixing member. In the fixing device that conveys the recording material carrying the toner image to the nip portion and fixes the toner image,
A sheet member 16 having a bending rigidity smaller than that of the fixing member is provided between the fixing member and the heat source, and the sheet member has an inner surface of the fixing member in a range including the protruding portion from the heat source along the circumferential direction of the fixing member in a pressurized state. A fixing device characterized by being closely attached to.   The Young's modulus in the circumferential direction of the fixing member is Ef, the second moment of area in the cross section parallel to the rotation axis of the fixing member If, the Young's modulus in the circumferential direction of the sheet member is Es, and the cross section parallel to the rotation axis of the sheet member is The fixing device according to claim 1, wherein Ef × If> Es × Is, where Is is the second moment of area.   The fixing device according to claim 2, wherein the base layer of the fixing member and the sheet member are made of stainless steel, and the thickness of the sheet member is smaller than the thickness of the base layer.   The fixing device according to claim 1, wherein the sheet member is supported so that it can be bent and deformed following the fixing member when pressed.   The fixing device according to claim 1, wherein an end portion of the sheet member protrudes from an end portion of the fixing member at an end portion in the rotation axis direction of the fixing member. It has a rotating belt-shaped fixing member, a nip forming member arranged inside the fixing member, a pressure member that is pressed against the nip forming member via the fixing member to form a nip portion, and a heating source that heats the nip portion. The nip forming member has a protruding portion that protrudes toward the pressure member downstream of the nip portion along the rotation direction of the fixing member, the lubricant is applied to the inner surface of the fixing member, and the toner image is formed on the nip portion. In the fixing device that conveys the carried recording material and fixes the toner image,
A sheet member having a bending rigidity smaller than that of the fixing member is provided between the fixing member and the nip forming member, and the sheet member is sandwiched by the fixing member and the nip forming member along the circumferential direction of the fixing member in a pressurized state. In the fixing device, the sheet member is in close contact with the inner surface of the fixing member.