JP2009109847A - Image heating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image heating device capable of reducing a frequency to move a heat-resistant sheet related to the surface of a rotating body. <P>SOLUTION: The image heating device includes: the rotating body; the heat-resistant sheet that is movable forward in the rotating direction of the rotating body; a heating body that forms a first nip part by bringing the heat-resistant sheet in contact with the surface of the rotating body, and heats the rotating body at the first nip part through the heat-resistant sheet; and a backup member that forms a second nip part in contact with the surface of the rotating body; wherein a recording material with the image formed thereon is nipped and conveyed in the second nip part so as to heat the image. A water contact angle of the surface of the rotating body is larger than that of the surface coming in contact with the surface of the rotating body, of the heat-resistant sheet. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image heating apparatus suitable for use as a fixing device mounted on an image forming apparatus such as an electrophotographic copying machine or an electrophotographic printer.

  As an image heating device (fixing device) mounted on an electrophotographic copying machine, printer, or the like, the one disclosed in Patent Document 1 is known. The image heating apparatus includes a fixing roller, a heat-resistant sheet that contacts the fixing roller, a plate-like heater that sandwiches the heat-resistant sheet between the fixing roller and heats the fixing roller via the heat-resistant sheet, and contacts the fixing roller. And a pressure roller for forming a nip portion. Then, the toner image is heated and fixed on the recording material surface while the recording material carrying the toner image on the recording material surface is nipped and conveyed by the nip portion.

  Ideally, all the toner images carried on the recording material surface are appropriately heated and melted and fixed on the recording material surface. However, if there is a cold offset toner that does not completely melt or a hot offset toner that is too melted on the recording material surface, the offset toner is transferred to the outer peripheral surface (surface) of the fixing roller that contacts the recording material surface. .

Since the heat-resistant sheet is in contact with the surface of the fixing roller in the fixing device, offset toner or the like transferred from the recording material surface to the surface of the fixing roller can be attached to the heat-resistant sheet. Then, the heat-resistant sheet is wound together with the dirt (offset toner etc.) according to the degree of dirt on the heat-resistant sheet due to the offset toner, etc., to remove the dirt from the surface of the fixing roller and to make the surface of the fixing roller clean. I try to keep it.
JP 2003-186327 A

  The present invention is a further development of the above prior art. SUMMARY OF THE INVENTION An object of the present invention is to provide an image heating apparatus that can reduce the frequency with which a heat-resistant sheet is moved relative to the surface of a rotating body.

The configuration for achieving the above object includes a rotating body, a heat resistant sheet that moves in a forward direction with respect to the rotating direction of the rotating body, and the first nip portion by contacting the heat resistant sheet and the surface of the rotating body. And a heating member that heats the rotating body through the heat-resistant sheet at the first nip portion, and a backup member that forms a second nip portion in contact with the surface of the rotating body, In the image heating apparatus for heating an image while sandwiching and conveying a recording material carrying an image at the second nip portion,
The contact angle with respect to the water of the said rotary body surface is larger than the contact angle with respect to the water of the surface which contacts the said rotary body surface of the said heat resistant sheet.

In addition, a configuration for achieving the above object includes a rotating body, a heat-resistant sheet that moves in a forward direction with respect to the rotation direction of the rotating body, and the heat-resistant sheet and the surface of the rotating body are brought into contact with each other. A heating body that forms a nip portion and heats the rotating body through the heat-resistant sheet at the first nip portion, and a backup member that contacts the surface of the rotating body to form a second nip portion. In the image heating apparatus that heats the image while sandwiching and conveying the recording material carrying the image at the second nip portion,
The surface of the heat-resistant sheet that comes into contact with the surface of the rotating body is a region having a better sliding property than a region where the contact angle with water is smaller than the surface of the fixing roller. The regions smaller than the corners are alternately arranged along the moving direction of the heat-resistant sheet.

  According to the present invention, since the dirt adhering to the surface of the rotating body when the image is heated can be accumulated on the surface of the heat resistant sheet at the end of the first nip portion on the downstream side in the rotating body rotation direction, the heat resistant sheet is rotated. The frequency of movement with respect to the body surface can be reduced.

  The present invention will be described with reference to the drawings.

[Example 1]
(1) Example of Image Forming Apparatus FIG. 1 is a schematic diagram schematically illustrating an example of an image forming apparatus in which an image heating apparatus according to the present invention can be mounted as a fixing device. This image forming apparatus is an electrophotographic laser beam printer.

  The image forming apparatus shown in this embodiment includes a rotating drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 as an image carrier. The photosensitive drum 1 is configured by forming a photosensitive material such as OPC, amorphous Se, or amorphous Si on a cylindrical substrate such as aluminum or nickel.

  The photosensitive drum 1 is rotated at a predetermined peripheral speed (process speed) in the arrow direction in accordance with a print command. During the rotation, the outer peripheral surface (surface) of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by a charging roller 2 as a charging device.

  Next, image information is written and exposed to the uniformly charged surface by a laser scanner 3 as image exposure means. That is, the laser scanner 3 scans and exposes the uniformly charged surface of the surface of the photosensitive drum 1 with the laser L that is ON / OFF controlled (modulated) according to the time-series electric digital pixel signal of the image information. As a result, the exposed portion potential of the uniformly charged surface on the surface of the photosensitive drum 1 is attenuated, and an electrostatic latent image of image information is formed on the surface of the photosensitive drum 1.

  This electrostatic latent image is developed and visualized as a toner image (developed image) by toner (developer) by the developing device 4. As a development method, a jumping development method, a two-component development method, or the like is used, and image exposure and reversal development are often used in combination.

  On the other hand, in synchronization with the toner image formed on the surface of the photosensitive drum 1, the recording material P is moved between the surface of the photosensitive drum 1 and the outer peripheral surface (surface) of the transfer roller 5 as a transfer means by a recording material feeding mechanism (not shown). It is fed to the transfer nip portion A. That is, the top sensor 8 detects the leading edge of the recording material P so that the image forming position of the toner image on the surface of the photosensitive drum 1 matches the writing position of the leading edge of the recording material P, and based on the detection signal of the top sensor 8. Then, the recording material P is sent to the transfer nip portion A by the recording material transport mechanism.

  The recording material P fed to the transfer nip A is nipped and conveyed by the photosensitive drum 1 and the transfer roller 5. In the conveying process, the toner image is transferred from the surface of the photosensitive drum 1 onto the surface of the recording material P by the transfer roller 5. The recording material P carrying an unfixed toner image transferred from the surface of the photosensitive drum 1 is conveyed to the fixing device 6. The fixing device 6 heats and fixes the unfixed toner image on the recording material P surface under heat and pressure. Then, the recording material P is discharged to a discharge tray (not shown).

  Residual toner remaining on the surface of the photosensitive drum 1 after the transfer of the toner image is removed from the surface of the photosensitive drum 1 by the cleaning device 7, and the surface of the photosensitive drum 1 is used for the next image formation.

  The above is the printing operation of the image forming apparatus.

  In the image forming apparatus of this embodiment, the photosensitive drum 1 and the charging roller 2, the developing device 4 and the cleaning device 7 as process means acting on the photosensitive drum 1 are integrally formed into a cartridge. A process cartridge 73 in which the photosensitive drum 1, the charging roller 2, the developing device 4, and the cleaning device 7 are integrated into a cartridge is detachably mounted on the image forming apparatus main body constituting the housing of the image forming apparatus.

(2) Fixing Device In the following description, with respect to the fixing device and members constituting the fixing device, the longitudinal direction refers to a direction orthogonal to the recording material conveyance direction on the surface of the recording material. The short side direction is a direction parallel to the recording material conveyance direction on the surface of the recording material. The short width is a dimension in the short direction. The longitudinal width is a dimension in the longitudinal direction.

  FIG. 2 is a schematic cross-sectional view of an example of the fixing device 6.

  The fixing device 6 roughly includes a fixing roller (rotating body) 10, a heat resistant sheet (cleaning sheet) 19, a heating member 20, a pressure member (backup member) 30, and the like. The heating member 20 includes a heater (heating member) 21 and a heat insulating stay holder (heating member holding member) 24. The pressure member 30 includes a sliding sheet (sliding member) 31, a heat insulating pressure holder (sliding member pressure member) 32, a flexible film (flexible rotating body) 33, and the like. The fixing roller 10, the heat resistant sheet 19, the heater 21, the heat insulating stay holder 24, the sliding sheet 31, the heat insulating pressure holder 32, and the flexible film 33 are all elongated members in the longitudinal direction.

1) Fixing roller 10
The fixing roller 10 includes the following members. That is, basically, after an outer peripheral surface of a metal core 11 made of aluminum or iron or the like is subjected to surface roughening such as blasting, an elastic layer (solid) made of silicon rubber is formed on the outer periphery of the core 11. Rubber layer) 12 is provided. The elastic layer 12 is not limited to a solid rubber layer, and a sponge rubber layer formed by foaming silicon rubber in order to provide a heat insulating effect, or a hollow filler is dispersed in the silicon rubber layer, and a gas is contained in the cured product. It is possible to use a foam rubber having a part and having a high heat insulating effect.

  If the fixing roller 10 has a large heat capacity and a small thermal conductivity, the heat of the heater 21 received from the outer peripheral surface (surface) of the fixing roller 10 is easily absorbed inside, and the surface temperature of the fixing roller 10 is difficult to increase. . For this reason, the elastic layer 12 is advantageous in terms of the rise time of the surface temperature of the fixing roller 10 by using a material having as low a heat capacity as possible and a low thermal conductivity and a high heat insulating effect.

  Here, the solid rubber layer of the silicone rubber has a thermal conductivity of 0.25 to 0.29 W / (m · k), and the sponge rubber and the bubble rubber have 0.11 to 0.16 W / (m · k). Sponge rubber and foam rubber show about half the value of solid rubber.

  The specific gravity related to the heat capacity is about 1.05 to 1.30 for solid rubber and about 0.75 to 0.85 for sponge rubber / bubble rubber. Therefore, as a preferable form of the elastic layer 12 of the fixing roller 10, a sponge rubber layer or a bubble rubber layer having a heat insulation effect with a thermal conductivity of about 0.15 W / (m · k) or less and a specific gravity of 0.85 or less. Is preferred.

  The smaller the outer diameter of the fixing roller 10, the smaller the heat capacity. However, if the fixing roller 10 is too small, it is difficult to increase the short width of the heating nip H, which will be described later, so an appropriate outer diameter is required. As for the thickness of the elastic layer 12, if it is too thin, heat will escape to the metal core 11, so an appropriate thickness is required.

  In consideration of the above, in the present embodiment, in order to form an appropriate heating nip H and to suppress the heat capacity, the elastic layer 12 is formed using a foam rubber having a wall thickness of 4 mm, and the fixing having an outer diameter of φ20 mm. Roller 10 was used. As a hollow filler used for forming the elastic layer 12 made of cellular rubber, any glass balloon, silica balloon, carbon balloon, and phenol balloon may be selected and used. Further, the present invention is not limited to this, and an arbitrary one of acrylonitrile balloon, vinylidene chloride balloon, alumina balloon, zirconia balloon, and shirasu balloon may be selected and used. The core metal 11 is not limited to a solid core metal but may be a hollow core metal.

  A fluororesin release layer 13 such as perfluoroalkoxy resin (PFA), polytetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene resin (FEP) is provided on the outer periphery of the elastic layer 12 described above. Form. As the releasable layer 13, the elastic layer 12 is coated with a GLS latex coating, the elastic layer 12 is coated with a tube, or the elastic layer 12 is coated with a paint on the outer periphery. It may be.

  The fixing roller 10 is held by an apparatus frame (not shown) so as to be rotatable via a bearing (not shown) at the core bar exposed portion at both ends of the longitudinal direction.

  Reference numeral 14 denotes a temperature detection element (first temperature detection means) such as a thermistor for detecting the temperature of the surface of the fixing roller 10. The temperature detection element 14 is provided for the purpose of controlling the temperature of the heater 21 or for monitoring the abnormal temperature rise of the heater 21. The temperature detection element 14 is supported by the apparatus frame via a support member and is in contact with the surface of the fixing roller 10 at the center in the longitudinal direction of the fixing roller 10.

2) Heating member 20
FIG. 3 is an explanatory diagram showing the configuration of the heater 21 and the temperature control system.

The heater 21 is a plate-shaped heater having a low heat capacity. It has an elongated substrate 21a made of an insulating ceramic substrate such as alumina or aluminum nitride, or a heat-resistant resin substrate such as polyimide, PPS, or liquid crystal polymer. The longitudinal width of the substrate 21a (the longitudinal width of the heater 21) is substantially equal to the longitudinal width of the fixing roller 10. Then, on the surface of the substrate 21a, the energization heat generation resistance layer 21b is formed by coating the surface of the substrate 21a in a linear or narrow strip shape having a thickness of about 10 μm and a width of about 1 to 5 mm by screen printing or the like. It is. For example, Ag / Pd (silver palladium), RuO ² , Ta ² N, or the like is used as the material of the energization heat generation resistance layer 21b.

  Further, a protective layer (not shown) for protecting the energization heating resistor layer 21b may be provided on the surface of the substrate 21a as long as the thermal efficiency is not impaired. The thickness of the protective layer is preferably thin enough to make the surface property of the energization heating resistor layer 21b good. As a material for the protective layer, a fluororesin layer such as perfluoroalkoxy resin (PFA), polytetrafluoroethylene resin (PTFE), tetrafluoroethylene-hexafluoropropylene resin (FEP), or the like can be used. As a material for the protective layer, a fluororesin layer such as ethylenetetrafluoroethylene resin (ETFE), polychlorotrifluoroethylene resin (CTEF), polyvinylidene fluoride (PVDF), or the like can be used. Then, these fluororesin layers are singly or mixed to cover the surface of the energization heat generating resistance layer 21b. Further, as the protective layer, it is conceivable to use a dry film lubricant made of graphite, diamond-like carbon (DLC), molybdenum disulfide, or the like, a glass coat, or the like. 21c is an electrode portion provided on the surface of the substrate 21a, and is electrically connected to the energization heating resistor layer 21b in the longitudinal direction of the substrate 21a.

  When aluminum nitride or the like having good thermal conductivity is used as the material of the substrate 21a, the energization heating resistor layer 21b may be formed on the back surface opposite to the surface of the substrate 21a.

  On the back surface of the substrate 21a, a temperature detection element (second temperature detection means) such as a thermistor for detecting the temperature of the substrate 21a raised in accordance with the heat generated by the energization heating resistor layer 21b at the center in the longitudinal direction of the substrate 21a. ) 23 is arranged. The temperature detection element 23 is also provided for the purpose of controlling the temperature of the heater 21 or monitoring the abnormal temperature rise of the heater 21.

  The heat insulating stay holder 24 that holds the heater 21 is made of a heat resistant resin material such as liquid crystal polymer, phenol resin, PPS, PEEK. As the heat conductivity of the heat-insulating stay holder 24 made of a heat-resistant resin is lower, the heat efficiency is higher when the surface of the fixing roller 10 is heated by the heater 21. Therefore, in order to reduce the thermal conductivity of the heat insulating stay holder 24, the heat resistant resin material of the heat insulating stay holder 24 may include a hollow filler, such as a glass balloon or a silica balloon. A groove 24 a is provided along the longitudinal direction on the lower surface of the heat insulating stay holder 24 on the surface side of the fixing roller 10. In the groove 24a, the substrate 21a of the heater 21 is held in a state where the energization heat generating resistance layer 21b is exposed from the groove 24a.

  The heat-insulating stay holder 24 is held by the apparatus frame at both longitudinal ends. Then, both end portions of the heat insulating stay holder 24 are pressed by a pressing means such as a pressure spring to energize the heat-resistant sheet 19 interposed between the surface of the fixing roller 10 and the energization heat generation resistance layer 21b of the heater 21. The surface of the fixing roller 10 is brought into contact with the heating resistance layer 21b. That is, the heat-insulating stay holder 24 is pressed by the pressing means so that the surface of the energized heat generating resistance layer 21b of the heater 21 is brought into contact with the surface (back surface) of the heat-resistant sheet 19 on the heater 21 side. Then, the surface of the heat resistant sheet 19 is brought into contact with the surface of the fixing roller 10 to form a heating nip portion (first nip portion) H for heating the surface of the fixing roller 10 between the surface of the heat resistant sheet 19 and the surface of the fixing roller 10. (Fig. 2).

3) Pressurizing member 30
The flexible film 33 is a cylindrical (endless belt-like) thin film having heat resistance and flexibility. The flexible film 33 is a resin film having a base layer of polyimide, polyamideimide, PEEK, PES, PPS, PFA, PTFE, FEP or the like having heat resistance and heat insulation. The thickness of the flexible film 33 is 20 μm or more and less than 150 μm in an appropriate range in consideration of strength and the like. In addition, a heat-resistant resin having good releasability such as PFA, PTFE, FEP, and silicone resin is applied to the outer peripheral surface of the base layer so that the offset toner attached to the surface of the fixing roller 10 does not accumulate on the outer peripheral surface (surface) of the base layer. The releasable layer may be formed by coating or mixing alone. Therefore, the heat capacity of the flexible film 33 is smaller than that of the fixing roller 10.

  The sliding sheet 31 provided inside the flexible film 33 is formed from a heat-resistant resin sheet. As a material for the resin sheet, heat-resistant felt, mica sheet, ceramic sheet, liquid crystal polymer, phenol resin, PPS, PEEK, polyimide, polyamideimide, fluororesin, PEEK, or the like is used. As the sliding sheet 31, a member having heat insulation is particularly suitable. The sliding sheet 31 may be coated on the surface of the sliding sheet 31 with a sliding layer such as a glass coat or a fluororesin that reduces frictional resistance.

  The adiabatic pressure holder 32 is formed of a heat resistant resin such as liquid crystal polymer, phenol resin, PPS, PEEK or the like in a substantially semicircular inverted saddle shape. A groove 32 a is provided on the upper surface of the heat insulating pressure holder 32 along the longitudinal direction. The sliding sheet 31 is fixedly held in the groove 32 a so as to be flush with the upper surface of the heat insulating pressure holder 32. A flexible film 33 is loosely fitted on the heat insulating pressure holder 32 on which the sliding sheet 31 is fixedly held. The longitudinal width of the flexible film 33 is substantially equal to the longitudinal width of the fixing roller 10. As will be described later, when the flexible film 33 rotates following the rotation of the fixing roller 10, the inner peripheral surface (inner surface) of the flexible film 33 is the surface of the sliding sheet 31 and the insulating pressure holder 32. It rotates while touching the outer periphery. Therefore, as the material of the heat insulating pressure holder 32, a material having heat insulating properties and sliding properties is suitable.

  The heat insulating pressure holder 32 has both longitudinal ends fixed to the apparatus frame. The surface of the fixing roller 10 and the flexible film are pressed by pressurizing a bearing that rotatably holds the cored bar exposed portion of the fixing roller 10 against the heat insulating pressure holder 32 by a pressing means such as a pressure spring. 33 is in contact with the outer peripheral surface (surface). Thus, a nip portion (fixing nip portion, second nip portion) T for heating and fixing an unfixed toner image on the recording material P is formed between the surface of the fixing roller 10 and the surface of the flexible film 33 (see FIG. 2).

  Between the inner surface of the flexible film 33 and the surface of the sliding sheet 31 in contact with the inner surface of the flexible film 33, grease or the like is used to keep the frictional resistance between the inner surface of the flexible film 33 and the surface of the sliding sheet 31 small. A small amount of lubricant is interposed. It is desirable that the lubricant has a low thermal conductivity and can prevent the heat from the fixing roller 10 from being radiated to the sliding sheet 31 and the heat insulating pressure holder 32 through the flexible film 33.

  In this embodiment, the sliding sheet 31 and the adiabatic pressure holder 32 are handled as separate members. However, the sliding sheet 31 and the adiabatic pressure holder 32 are formed by integral molding, and the flexible film 33 is formed in the integral molded member. You may coat said sliding layer in the part which contact | connects an inner surface. As a result, the number of parts can be reduced and the cost can be reduced.

  The pressure member 30 may be an elastic pressure roller. As an elastic pressure roller, like the fixing roller 10, the outer peripheral surface of a metal core such as aluminum or iron is subjected to surface roughening treatment such as blasting, and then the outer peripheral surface of the core is made of silicon rubber. What provided the elastic layer (solid rubber layer) can be used. The elastic layer is not limited to a solid rubber layer, but a sponge rubber layer formed by foaming silicon rubber to give a heat insulation effect, or a hollow filler is dispersed in the silicon rubber layer, and a gas portion is formed in the cured product. It is possible to use a foam rubber having a heat insulating effect. The surface of the fixing roller 10 and the outer peripheral surface (surface) of the elastic pressure roller are brought into contact with each other by pressing the bearing of the fixing roller 10 against the elastic pressure roller by a pressing means such as a pressure spring. As a result, a nip portion (fixing nip portion) for heating and fixing an unfixed toner image on the recording material P can be formed between the surface of the fixing roller 10 and the surface of the elastic pressure roller.

4) Heat-resistant sheet 19
The heat resistant sheet 19 is formed of a resin sheet having a base layer of polyimide, polyamideimide, PEEK, PES, PPS or the like having heat resistance and flexibility. The heat-resistant sheet 19 is formed to a thickness of 25 μm or less in order to efficiently transfer the heat from the heater 21 to the surface of the fixing roller 10. The longitudinal width of the heat resistant sheet 19 is substantially equal to the longitudinal width of the fixing roller 10. In the rotation direction of the fixing roller 10, the tension roller 17 (heat resistant sheet feeding member) is upstream of the heating nip portion H, and the winding roller 18 (heat resistant sheet winding member) is downstream of the heating nip portion H. Are provided respectively. The longitudinal width of the tension roller 17 and the take-up roller 18 is substantially equal to the longitudinal width of the heat resistant sheet 19. One end of the heat resistant sheet 19 is wound around the tension roller 17 along the longitudinal direction of the tension roller 17, and the other end of the heat resistant sheet 19 is wound around the winding roller 18 along the longitudinal direction of the winding roller 18. ing. The winding amount of one end of the heat resistant sheet 19 around the tension roller 17 is larger than the winding amount of the other end of the heat resistant sheet 19 around the winding roller 18. The tension roller 17 and the take-up roller 18 are rotated in the same direction, and the other end side of the heat-resistant sheet 19 is taken up by the take-up roller 17. The tension roller 17 and the take-up roller 18 are not rotated during the printing operation of the image forming apparatus. Therefore, while the recording material P is nipped and conveyed between the surface of the fixing roller 10 and the surface of the flexible film 33 at the nip portion N as will be described later, the surface of the heat resistant sheet 19 is the surface of the fixing roller 10 with the surface of the fixing roller 10. Stopped in contact. Further, unlike the flexible film 33 used for the pressure member 30, the heat resistant sheet 19 is used in a stopped state without rotating during the printing operation, so that the tension roller 17 and the heating nip H A tension load is applied between them. Therefore, it is desirable that the heat-resistant sheet 19 is formed with a thickness of 5 μm or more that satisfies the tensile strength. Therefore, the thickness of the heat resistant sheet 19 is preferably about 5 μm to 25 μm in order to efficiently transfer the heat of the heater 21 to the surface of the fixing roller 10 and satisfy the strength.

  Further, the resin material of the heat-resistant sheet 19 may be mixed with a thermally conductive filler that improves thermal conductivity, such as a filler of BN, aluminum, alumina, or AlN.

  When the heat conductivity in the thickness direction of the heat-resistant sheet 19 is low, heat cannot be efficiently applied to the surface of the fixing roller 10, and the heater 21 is used to raise the fixing roller 10 to a predetermined fixing temperature (target temperature). The exothermic temperature must be higher. In that case, the heat generation temperature of the heater 21 may exceed the heat resistance temperature of the heat insulation stay holder 24. Therefore, the heat conductivity in the thickness direction of the heat-resistant sheet 19 is preferably 0.1 W / m · k or more in order to efficiently apply the heat of the heater 21 to the surface of the fixing roller 10. On the other hand, if the heat conductivity in the thickness direction of the heat-resistant sheet 19 is too high such as SUS, the heat of the heater 21 is dispersed not only in the fixing roller 10 but also in the entire heat-resistant sheet 19 and the heat efficiency is deteriorated. End up. Therefore, the heat conductivity in the thickness direction of the heat resistant sheet 19 is desirably 3 W / m · k or less. That is, the heat conductivity of the heat resistant sheet is preferably 0.1 W / m · k or more and 3 W / m · k or less.

5) Heat Fixing Operation of Fixing Device 6 A driving gear (not shown) is provided at one end of the exposed portion of the core metal 11 of the fixing roller 10, and a rotation driving mechanism (rotation driving means) (not shown) is provided in accordance with a print command. ) Rotates the drive gear. As a result, the fixing roller 10 is rotated at a predetermined peripheral speed (process speed) in the direction of the arrow. As the fixing roller 10 rotates, a rotational force acts on the flexible film 33 due to frictional resistance between the surface of the fixing roller 10 and the surface of the flexible film 33 at the nip portion N. The flexible film 33 slides in a state where the inner surface of the flexible film 33 is in close contact with the surface of the sliding sheet 31 at the nip portion N, and the rotational peripheral speed of the fixing roller 10 in the arrow direction around the heat insulating pressure holder 32. Driven at the same peripheral speed.

  Further, the temperature control unit (temperature control unit) 27 (FIG. 3) energizes the energization heat generating resistor layer 21 b through the electrode unit 21 c of the heater 21 in accordance with the print command. As a result, the energization heat generating resistance layer 21b generates heat, and the heater 21 rapidly rises in temperature. Heat generated by the energized heat generating resistance layer 21 b is transmitted to the surface of the fixing roller 10 through the heat resistant sheet 19. As a result, the surface of the fixing roller 10 is heated. That is, the heater 21 contacts the surface of the fixing roller 10 with the heat-resistant sheet 19 to form the heating nip portion H, and heats the surface of the fixing roller 10 through the heating nip portion H. The temperature control unit 27 takes in an output signal from the temperature detection element 14 (temperature detection signal on the surface of the fixing roller 10) and takes in an output signal from the temperature detection element 23 (temperature detection signal from the heater 21). Then, the temperature control unit 27 appropriately controls the duty ratio, wave number, and the like of the voltage applied to the energization heating resistor layer 21b of the heater 21 according to the output signals of the temperature detection element 14 and the temperature detection element 23. As a result, the temperature control unit 27 generates heat in the heater 21 and heats and adjusts the surface of the fixing roller 10 to a predetermined fixing temperature (target temperature).

  The recording material P carrying the unfixed toner image (image) t is fixed in a state where the rotation of the fixing roller 10 and the flexible film 33 is stable and the surface of the fixing roller 10 is heated and adjusted to a predetermined fixing temperature. It is introduced into the nip portion N along the inlet guide 15. Then, the recording material P is nipped and conveyed by the surface of the fixing roller 10 and the surface of the flexible film 33 at the nip portion N, so that the heat of the surface of the fixing roller 10 and the nip pressure of the nip portion N are applied to the recording material P and are undetermined. The received toner image t is heated and fixed on the surface of the recording material P (on the recording material). The recording material P separated from the surface of the fixing roller 10 and exiting the nip portion N is discharged from the fixing device 67 along the fixing discharge guide 16.

6) Relationship between releasability between the surface of the fixing roller 10 and the surface of the heat-resistant sheet 19 When printing is performed on the recording paper as the recording material P by the image forming apparatus, when the number of printed sheets increases, the nip portion N Cold offset toner, hot offset toner, paper dust, and the like adhere to the surface of the fixing roller 10. Hereinafter, the toner in the cold offset state adhered to the surface of the fixing roller 10 or the toner in the hot offset state that is too melted is referred to as offset toner. Then, a small amount of offset toner and paper dust adhering to the surface of the fixing roller 10 is accumulated in the heat resistant sheet 19 portion in the heating nip portion H. This will be described with reference to FIG.

  4A is a diagram illustrating a state in which dirt T such as offset toner and paper dust accumulates on the upstream side of the heating nip portion H in the fixing roller rotation direction as the fixing roller 10 rotates. FIG. 4B shows a state in which dirt T such as offset toner and paper dust moves from the upstream side in the fixing roller rotation direction to the downstream side in the fixing roller rotation direction and accumulates as the fixing roller 10 rotates. FIG.

  Dirt T such as a small amount of offset toner or paper dust adhering to the surface of the fixing roller 10 in the nip portion N is first dammed up and accumulated at the end of the heating nip portion H on the upstream side in the fixing roller rotation direction. (FIG. 4A). Since the offset toner is melted by the heat of the heater 21 due to the dirt T, the rubbing portion between the surface of the heat resistant sheet 19 and the surface of the fixing roller 10 in the heating nip portion H gradually moves as the fixing roller 10 rotates. Moves downstream in the fixing roller rotation direction. The dirt T is accumulated on the surface of the heat-resistant sheet 19 that has passed the heating nip H (FIG. 4B).

  Here, when the dirt T accumulated on the surface of the heat-resistant sheet 19 is returned to the surface of the fixing roller 10 and carried to the nip portion N, the dirt T is transferred to the recording material P and stains the recording material P.

  Therefore, in this embodiment, before the dirt T accumulated on the surface of the heat resistant sheet 19 is returned to the surface of the fixing roller 10, the tension roller 17 and the take-up roller 18 are rotated, and the heat resistant sheet 19 is moved in the direction of arrow C. Is slid by a predetermined amount (moved by a predetermined amount) (FIG. 4B). The tension roller 17 and the take-up roller 18 are rotated by a take-up drive mechanism (not shown) as a take-up drive unit. In other words, the tension roller 17 and the take-up roller 18 are rotated, and the heat-resistant sheet 19 is taken up by a certain amount by the take-up roller 18, whereby the heat-resistant sheet is forward (arrow C direction) with respect to the rotation direction of the fixing roller 10. 19 is slid a predetermined amount. When the heat-resistant sheet 19 is slid by a predetermined amount, the dirt T ′ moves from the end of the heating nip portion H on the downstream side in the fixing roller rotation direction by the amount of sliding of the heat-resistant sheet 19. In FIG. 4B, the stain T ′ moved together with the heat resistant sheet 19 can no longer be transferred to the surface of the fixing roller 10, so that it is possible to prevent problems such as staining the recording material P at the nip portion N.

  The amount by which the heat resistant sheet 19 is slid (moved) is preferably larger than the width of the heating nip H (the dimension of the contact area between the surface of the heat resistant sheet 19 and the surface of the fixing roller 10 in the moving direction of the heat resistant sheet 19). . The timing of sliding the heat-resistant sheet 19 is performed every predetermined number of prints, every number of prints set by the user, every time the toner consumption reaches a predetermined amount, or after the recording material P is jammed. It is set as follows. A predetermined amount of the heat-resistant sheet 19 is slid (updated) by the rotation of the tension roller 17 and the take-up roller 18. Further, the winding roller 18 is wound up by the winding roller 18 so that the surface to which the dirt T ′ adheres is inside, and even if the dirt T ′ attached to the heat resistant sheet 19 is peeled off from the heat resistant sheet 19, the fixing device. 6 and the image forming apparatus are less contaminated.

Further, in this embodiment, the release property of the heat-resistant sheet 19 is separated from the surface of the fixing roller 10 so that more dirt T can be accumulated on the surface of the heat-resistant sheet 19 on the downstream side of the heating nip portion H in the rotation direction of the fixing roller. Lower than type. That is, the releasability of the heat-resistant sheet 19 is made lower than the releasability of the surface of the fixing roller 10 so that the dirt T on the downstream side of the heating nip H in the fixing roller rotation direction does not easily move to the fixing roller 10 side. The releasability has a correlation with the contact angle with water. The smaller the contact angle with water, the lower the releasability. Therefore, when the above content is expressed as a contact angle with water, a contact angle with water of a surface (surface) of the heat resistant sheet 19 that contacts the fixing roller 10 surface between the surface of the heat resistant sheet 19 and the surface of the fixing roller 10.
<The fixing roller 10 has a contact angle with water on the surface.

7) Comparative Experiment A recording material was tested for image smear using the fixing device of the example configured as follows and the fixing device of the comparative example.

  In the fixing device of the example, a heater 21 in which an energization heating resistor layer 21b having a width of 3 mm was formed on the surface of an alumina substrate 21a by screen printing was used. The surface of the energization heating resistor layer 21b is in contact with the back surface of the heat-resistant sheet 19 in a bare state.

  As the fixing roller 10, a foam rubber in which a hollow filler is dispersed in silicon rubber is formed as an elastic layer 12 on the outer periphery of an aluminum core 11 with a thickness of 4 mm, and a release layer 13 is formed on the outer periphery of the elastic layer 12. What formed the 30-micrometer-thick PFA tube 13 was used. The contact angle of the surface of the fixing roller 10 with respect to water is about 110 °.

  As the heat resistant sheet 19, a sheet made of polyimide resin having a thickness of 20 μm was used. The contact angle of water on the surface of the heat-resistant sheet 19 that contacts the fixing roller 10 is about 80 °, which is smaller than the contact angle of water on the surface of the fixing roller 10. The heat conductivity of the heat resistant sheet 19 is about 0.3 W / m · k.

  The fixing device of the comparative example has the same configuration as the fixing device of the embodiment except that the heat-resistant sheet 19 is different. As the heat-resistant sheet 19, a polyimide resin having a fluorine coating and a total thickness of 20 μm was used. The coating surface of the heat resistant sheet 19 is brought into contact with the surface of the fixing roller 10. The contact angle of the coating surface of the heat resistant sheet 19 with respect to water is about 110 °, which is the same as the surface of the fixing roller 10. Further, the heat conductivity of the heat-resistant sheet 19 is about 0.3 W / m · k, which is the same value as in this embodiment.

  Said contact angle was measured by measuring the angle which the tangent of the water droplet on a heat resistant sheet | seat and the heat resistant sheet | seat make using the contact angle measuring device (Drop Mater 700) by Kyowa Interface Science Co., Ltd. In addition, the measurement of the thermal conductivity was performed by a periodic superheating method thermal diffusivity measuring apparatus FTC-1.

  In the fixing device of the embodiment and the fixing device of the comparative example, the energization heat generation resistance of the heater 21 is set so that the temperature of the temperature detection element 14 in contact with the surface of the fixing roller 10 is 210 ° C. so that the same fixing performance can be obtained. The energization to the layer 21b was controlled. The conveying speed of the fixing roller 10 is also 200 mm / s, and the printing speed is about 35 sheets per minute. During the evaluation, the heat-resistant sheet 19 does not move.

  A comparison was made between the fixing device of the example and the fixing device of the comparative example in terms of the image smearing state when 10,000 sheets of recording materials holding unfixed toner images were continuously passed through the nip portion N. In this evaluation, the heat-resistant sheet 19 was slid after image smearing occurred on the recording material.

  The results are shown in Table 1.

  In the fixing device of the embodiment, the mold release property on the surface of the heat resistant sheet 19 is worse than the mold release property on the surface of the fixing roller 10, so that a large amount of dirt T is accumulated on the downstream side of the heating nip H in the fixing roller rotation direction. Can do. Therefore, the image of the recording material was not soiled by the dirt T accumulated on the surface of the heat-resistant sheet 19 up to 8000 continuous prints. That is, every time 8000 sheets are printed, the tension roller 17 and the take-up roller 18 are rotated so that the heat-resistant sheet 19 is slid with respect to the surface of the fixing roller 10. It is possible to prevent the dirt T from smearing the image of the recording material.

  On the other hand, in the fixing device of the comparative example, the image of the recording material is not soiled by the dirt T accumulated on the surface of the heat resistant sheet 19 until 4000 continuous prints, but the heat resistant sheet is between 4001 and 6000 sheets. 19 Image contamination due to contamination T accumulated on the surface occurred. Thereafter, the image rollers were improved by rotating the tension roller 17 and the take-up roller 18 and sliding the heat-resistant sheet 19, but accumulated again on the surface of the heat-resistant sheet 19 between 8001 and 10,000 sheets. An image defect due to the stain T occurred. In other words, every time 4000 sheets are printed, if the tension roller 17 and the take-up roller 18 are rotated and the heat-resistant sheet 19 is not slid with respect to the surface of the fixing roller 10, the image stain due to the stain T can be prevented. Can not.

  In the fixing device of this embodiment, the contact angle of water on the surface of the heat resistant sheet 19 that contacts the surface of the fixing roller 10 is smaller than the contact angle of the surface of the fixing roller 10 on water. A lot of dirt T can be accumulated. Therefore, the frequency with which the heat resistant sheet 19 is slid with respect to the surface of the fixing roller 10 can be reduced. Further, the mounting amount (usage amount) of the expensive heat-resistant sheet 19 can be reduced, and the fixing device 6 can be greatly reduced in cost.

[Example 2]
Another example of the fixing device will be described.

  In the present embodiment, the same members and portions as those of the fixing device 6 of the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. The same applies to Example 3.

  The fixing device 6 shown in the present embodiment has the same configuration as the fixing device 6 of the first embodiment except for the following features. That is, the fixing device 6 of the present embodiment makes the surfaces of the substrate 21 a and the energization heat generating resistance layer 21 b of the heater 21 contact the inner surface of the heat resistant sheet 19 and rotates the fixing roller 10 when the heat resistant sheet 19 is slid. The power supply to the heater 21 is controlled by stopping.

  Most of the dirt T adhering to the surface of the heat-resistant sheet 19 on the downstream side of the heating nip H in the rotation direction of the fixing roller is toner. Therefore, the adhesion force of the dirt T also depends on the temperature. In other words, the adhesion of the toner is large because the toner is more likely to be solidified at a low temperature. Therefore, if the heat-resistant sheet 19 is slid when the temperature of the heat-resistant sheet 19 is lower than that of the fixing roller 10, the dirt T can be reliably adhered to the heat-resistant sheet 19 side.

  FIG. 5 is an enlarged view of the downstream side in the fixing roller rotation direction of the heating nip portion H where the dirt T is adhered. The area where the dirt T adheres (dirt adhesion area) is divided into an area A where the surface of the heater 21 and the inner surface of the heat-resistant sheet 19 are in contact with an area B where the heater 21 and the heat-resistant sheet 19 are not in contact. A graph showing the temperature relationship between the adhesive sheet 19 and the fixing roller 10 is shown in FIG.

  FIG. 6A shows the temperature relationship between the heat-resistant sheet 19 and the fixing roller 10 when the energization heating resistor layer 21 b of the heater 21 is energized. Since the fixing roller 10 has a larger heat capacity than the heater 21, the temperature gradually decreases as the distance from the heating nip H increases. On the other hand, since the heat resistance sheet 19 has a smaller heat capacity than the fixing roller 10, the temperature of the portion of the heater 21 that is not in contact with the surface of the substrate 21a rapidly decreases. Therefore, the temperature of the heat resistant sheet 19 is higher than that of the fixing roller 10 in the region A, and the temperature of the fixing roller 10 is higher than that of the heat resistant film 19 in the region B. Accordingly, in the region A, dirt tends to adhere to the fixing roller 10 side more than when the heat-resistant film 19 and the fixing roller 10 have the same temperature.

  FIG. 6B shows the temperature relationship between the heat-resistant sheet 19 and the fixing roller 10 when the energization heating resistor layer 21b of the heater 21 is not energized for a long time. Both the heat resistant sheet 19 and the fixing roller 10 are close to room temperature. In this case, since the contact angle with respect to water of the surface of the heat resistant sheet 19 in contact with the surface of the fixing roller 10 is smaller than the contact angle with respect to the water on the surface of the fixing roller 10, the heat resistant sheet with a smaller contact angle than that of the fixing roller 10. 19 tends to adhere to the dirt T. However, since both the heat-resistant sheet 19 and the fixing roller 10 have a low temperature close to room temperature, the adhesion force of the dirt T to the fixing roller 10 is sufficiently large. If the heat-resistant sheet 19 is slid in such a state, the dirt T may remain on the surface of the fixing roller 10.

  FIG. 6C shows the temperature relationship between the heat-resistant sheet 19 and the fixing roller 10 after a while has passed since the energization of the heater 21 to the energization heat generating resistance layer 21b. Since the heater 21 has a smaller heat capacity than the fixing roller 10, the temperature decreases immediately after the energization of the energization heat generating resistance layer 21b. Accordingly, the temperature of the heat resistant sheet 19 is lower than that of the fixing roller 10, and the adhesion force of the dirt T to the surface of the heat resistant sheet 19 is greater than the surface of the fixing roller 10. That is, in the case of the temperature relationship shown in FIG. 6C, the adhesion force of the dirt T in the areas A and B is surely greater on the surface of the heat resistant film 19 than on the surface of the fixing roller 10. Further, since the absolute value of the adhesion force between the surface of the fixing roller 10 and the dirt T is small, it is most suitable for removing the dirt T by sliding the heat resistant sheet 19.

  In order to create the temperature relationship as shown in FIG. 6C, the following control is performed. In a state where the rotation driving mechanism stops the rotation of the fixing roller 10, the temperature control unit 27 energizes the energization heat generation resistance layer 21 b of the heater 21, and the value of the temperature detection element 23 provided on the back surface of the heater 21 is The energization to the heater 21 is controlled so as to keep the state of 200 ° C. for 5 seconds. As a result, the temperature relationship as shown in FIG. Therefore, at this time, the tension roller 17 and the take-up roller 18 are rotated to slide the heat-resistant sheet 19.

  In the fixing device 6 of this embodiment, when the heat resistant sheet 19 is slid, the temperature of the heat resistant sheet 19 in the dirt adhesion region is controlled to be lower than the temperature of the fixing roller 10, so that the dirt T is surely removed. It can be removed from the heating nip H. Thereby, the frequency with which the heat-resistant sheet 19 is slid with respect to the surface of the fixing roller 10 can be reduced. Therefore, the surface of the fixing roller 10 can always be kept free from contamination, and image contamination can be prevented. Further, the mounting amount (usage amount) of the expensive heat-resistant sheet 19 can be reduced, and the fixing device 6 can be greatly reduced in cost.

[Example 3]
Another example of the fixing device will be described.

  FIG. 7 is a schematic cross-sectional view of an example of a fixing device according to this embodiment. FIG. 8 is a view showing the surface of the heat-resistant sheet 19. FIG. 9 is a diagram illustrating the relationship between the width L2 of the region 19a included in the heat resistant sheet 19 and the width of the heating nip H, and the relationship between the width L1 of the region 19b included in the heat resistant sheet 19 and the width of the dirt adhesion region.

  In the fixing device 6 shown in the present embodiment, the surface of the substrate 21 a of the heater 21 and the energization heat generation resistance layer 21 b is in contact with the inner surface of the heat resistant sheet 19 as in the fixing device 6 of the second embodiment. The fixing device 6 shown in the present embodiment has the same configuration as the fixing device 6 of the first embodiment except that the heat-resistant sheet 19 is configured as follows.

  The heat resistant sheet 19 has regions 19a and 19b alternately arranged on the surface of the heat resistant sheet 19 in contact with the surface of the fixing roller 10 in the sheet sliding direction as shown in FIG. The region 19 b is a region where the contact angle with respect to water is smaller than the surface of the fixing roller 10. The region 19a is a region having better sliding properties than the region 19b having a contact angle with water smaller than the surface of the fixing roller 10. The width L1 of the region 19a in the sheet slide direction is longer than the width of the heating nip portion H in the sheet slide direction. In addition, the width L2 of the region 19b in the sheet sliding direction is longer than the dirt adhesion region in the sheet sliding direction.

  Further, a hole for positioning along the longitudinal direction of the heat-resistant sheet 19 at the center of the width L1 of the region 19a is provided inside one end of the heat-resistant sheet 19 in a direction orthogonal to the sheet slide direction (hereinafter referred to as the longitudinal direction). 19c is provided at equal intervals with a width of L1 + L2. Then, the heat-resistant sheet 19 is set on the tension roller 17 and the take-up roller 18 so that the boundary between the region 19a and the region 19b coincides with the downstream end of the heating nip portion H in the fixing roller rotation direction (FIG. 9). Thus, the region 19a is positioned in the heating nip portion H, and the region 19b is positioned at the downstream end of the heating nip portion H in the fixing roller rotation direction. By positioning the region 19a in the heating nip portion H, the slidability of the heat-resistant sheet 19 with respect to the surface of the fixing roller 10 is improved in the heating nip portion H, and damage to the surface of the fixing roller 10 due to durability can be suppressed. . On the other hand, by positioning the region 19b at the downstream end of the heating nip H in the fixing roller rotation direction, a large amount of dirt T can be accumulated on the surface of the heat resistant sheet 19 at the downstream end of the fixing roller rotation direction.

  Further, as shown in FIG. 7, the photo sensor 100 is provided at a position through which the positioning hole 19 c provided in the heat resistant sheet 19 passes. When the accumulated dirt T is pulled away from the heating nip H, the rotation of the tension roller 17 and the take-up roller 18 is stopped when the positioning hole 19c passes through the position of the photosensor 100. Thereby, the boundary of the area | region 19a and the area | region 19b can be made to correspond with the downstream edge part of the heating nip part H correctly.

  In the fixing device 6 of this embodiment, the surface of the heat-resistant sheet 19 that is in contact with the surface of the fixing roller 10 is in contact with water and a region 19a that has a better sliding property than the region 19b that has a smaller contact angle with water than the surface of the fixing roller 10. An area 19 b having a smaller corner than the fixing roller surface 10 is included. Accordingly, it is possible to sufficiently accumulate the dirt T at the downstream end of the heating nip portion H in the fixing roller rotation direction while reducing the damage to the surface of the fixing roller 10 in the region 19a of the heat resistant sheet 19. became. Thereby, the frequency with which the heat-resistant sheet 19 is slid with respect to the surface of the fixing roller 10 can be reduced. Therefore, the surface of the fixing roller 10 can always be kept free from contamination, and image contamination can be prevented. Further, the mounting amount (usage amount) of the expensive heat-resistant sheet 19 can be reduced, and the fixing device 6 can be greatly reduced in cost.

Schematic configuration schematic diagram of an example of an image forming apparatus Cross-sectional model diagram of an example of a fixing device according to Embodiment 1 FIG. 3 is an explanatory diagram showing the configuration of the heater 21 and the temperature control system. (A) is a diagram showing how dirt accumulates on the upstream side in the fixing roller rotation direction of the heating nip, and (b) shows that dirt moves from the upstream side of the heating nip in the fixing roller rotation direction to the downstream side in the fixing roller rotation direction. Diagram showing how to accumulate FIG. 9 is an explanatory diagram of a fixing device according to a second embodiment, and is an enlarged view of a downstream side in a fixing roller rotation direction of a heating nip portion where dirt is attached. FIG. 9 is an explanatory diagram of a fixing device according to a second embodiment, and is a diagram illustrating a temperature distribution on the downstream side of the heating nip portion in the fixing roller rotation direction. Cross-sectional model diagram of an example of a fixing device according to Embodiment 3 The figure showing the heat resistant sheet | seat surface of the fixing apparatus which concerns on Example 3. FIG. The relationship between the width of the slidable region having the heat-resistant sheet of the fixing device according to Example 3 and the width of the heating nip, and the width of the region having a smaller contact angle with water than the surface of the fixing roller. Of the relationship between the width of the surface and the soiled area

Explanation of symbols

10: fixing roller, 19: heat-resistant sheet, 19a: area with good slidability, 19b: area where the contact angle with water is smaller than the contact angle with water on the surface of the rotating body, 21: heater, 30: pressure member , H: heating nip, N: nip, P: recording material, t: toner image

Claims (5)

A rotating body, a heat-resistant sheet that moves in a forward direction with respect to the rotation direction of the rotating body, a first nip portion is formed by contacting the heat-resistant sheet and the surface of the rotating body, and the first nip portion A heating member that heats the rotating body via the heat-resistant sheet; and a backup member that contacts the surface of the rotating body to form a second nip portion, and carries an image at the second nip portion. In an image heating apparatus that heats an image while nipping and conveying a recording material,
An image heating apparatus, wherein a contact angle with respect to water on the surface of the rotating body is larger than a contact angle with respect to water on a surface of the heat resistant sheet that contacts the surface of the rotating body.   The image heating apparatus according to claim 1, wherein the heat resistance of the heat resistant sheet is 0.1 W / m · k or more and 3 W / m · k or less.   The image heating apparatus according to claim 1, wherein when the heat resistant sheet is moved, a temperature of the rotating body is higher than a temperature of the heat resistant sheet. A rotating body, a heat-resistant sheet that moves in a forward direction with respect to the rotation direction of the rotating body, a first nip portion is formed by contacting the heat-resistant sheet and the surface of the rotating body, and the first nip portion A heating member that heats the rotating body via the heat-resistant sheet; and a backup member that contacts the surface of the rotating body to form a second nip portion, and carries an image at the second nip portion. In an image heating apparatus that heats an image while nipping and conveying a recording material,
The surface of the heat-resistant sheet that comes into contact with the surface of the rotating body is a region having a better sliding property than a region where the contact angle with water is smaller than the surface of the fixing roller. An image heating apparatus, wherein regions smaller than the corners are alternately arranged along the moving direction of the heat-resistant sheet.   5. The image heating apparatus according to claim 4, wherein the slidable region of the heat-resistant sheet is in contact with the first nip portion.