JP2012037847A - Fixing device and image forming device equipped with the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device capable of suppressing thermal deformation of a rotor used for fixing treatment and an image forming device equipped with the same.SOLUTION: A fixing device 14 comprises a first rotor 55 which can generate heat, a second rotor 44 for forming a nip part NP between the first rotor 55 and the second rotor 44, a third rotor 45 disposed inside the first rotor 55 and a heat generating source 54 for heating the first rotor 55. The third rotor 45 comprises a first layer 52 for pressurizing the first rotor 55 to the second rotor 44 at a position corresponding to the nip part NP so that the nip part is formed and a second layer 53 which is laminated on the first layer 52 and is contacted with the inner surface of the first rotor 55. The first layer 52 is formed of a material having a first linear expansion coefficient and a first Young's modulus. The second layer 53 is formed of a material having a second linear expansion coefficient and a second Young's modulus. The second linear expansion coefficient is smaller than the first linear expansion coefficient and the second Young's modulus is larger than the first Young's modulus.

Description

  The present invention relates to a fixing device that heat-fixes a toner image transferred onto a recording medium, and an image forming apparatus including the fixing device.

  An image forming apparatus such as a copying machine, a facsimile machine, or a printer has, as basic components, an image forming unit that forms an image on an image carrier (for example, a photosensitive drum) and a toner image on the image carrier as a recording medium. As an example, a transfer unit that transfers onto a sheet and a fixing device that heat-fixes the toner image transferred onto the sheet onto the sheet are included.

  In order to reduce the warm-up time at startup and to save energy, a number of fixing devices adopting a belt method that can set a small heat capacity are increasing. In addition, an electromagnetic induction heating (IH) method capable of rapid heating and high efficiency heating has attracted attention. In the electromagnetic induction heating method, an induction current is induced in a fixing roller or a fixing belt by a magnetic field generated by applying a high-frequency current to an induction coil, and the fixing roller or the fixing belt is attached using a resistance of the fixing roller or the fixing belt itself. Joule heat generation (induction heating) is performed, and the toner image is fixed on the sheet at the nip portion between the fixing roller (or the fixing belt) and the pressure roller by the Joule heat. A fixing device combining an electromagnetic induction heating method and a belt method has been commercialized.

  As a fixing device that combines an electromagnetic induction heating method and a belt method, for example, one disclosed in Patent Document 1 is known. The fixing device disclosed in Patent Document 1 includes a fixing belt that can generate heat by a heat source, a pressure roller that forms a nip portion for sandwiching and conveying a sheet on which a toner image is formed between the fixing belt, and a fixing belt. And a fixing roller (backup roller) that sandwiches the fixing belt together with the pressure roller at a position corresponding to the nip portion.

  In the fixing device of Patent Document 1, the contact area between the fixing roller and the fixing belt is reduced by bringing the fixing roller into contact with the inner surface of the fixing belt only at a position corresponding to the nip portion. Therefore, the amount of heat that moves from the fixing belt that generates heat to the fixing roller is suppressed. As a result, the power consumption for raising the temperature of the fixing belt is reduced, and the temperature raising time of the fixing belt is shortened.

JP 2007-108392 A

  In the fixing device of Patent Document 1, the fixing roller has an elastic layer such as silicone rubber that contacts the inner surface of the fixing belt. Although an elastic layer such as silicone rubber can suppress a decrease in the temperature of the fixing belt, it has a large linear expansion coefficient, so that it easily expands thermally when the temperature of the fixing belt rises. Since the fixing roller is thermally deformed due to thermal expansion, the nip portion is not stably formed, and the toner image is not fixed well on the paper.

  Further, since the outer diameter of the fixing roller is increased due to thermal expansion, the contact area between the fixing roller and the fixing belt is increased. For this reason, the amount of heat transfer from the fixing belt to the fixing roller increases, and it becomes difficult to shorten the temperature raising time of the fixing belt.

  In view of the above circumstances, an object of the present invention is to provide a fixing device capable of suppressing thermal deformation of a rotating body used for fixing processing and an image forming apparatus including the same.

  In order to achieve the above object, a fixing device according to the present invention is capable of generating heat, and includes a first rotating body that rotates in a predetermined direction and a nip portion through which a recording medium carrying a toner image passes. A second rotator formed between the first rotator and the first rotator while being in contact with the inner surface of the first rotator at a position corresponding to at least the nip portion. A third rotating body that rotates in the same direction; and a heat source that generates heat from the first rotating body, wherein the third rotating body includes the nip between the first rotating body and the second rotating body. A first layer that presses the first rotating body toward the second rotating body at a position corresponding to the nip portion so that a portion is formed, and the first layer is laminated on the first layer. A second layer in contact with the inner surface, the first layer having a first linear expansion coefficient and a first Yan The second layer is formed of a material having a second linear expansion coefficient and a second Young's modulus, and the second linear expansion coefficient is greater than the first linear expansion coefficient. The second Young's modulus is smaller than the first Young's modulus.

  According to the image forming apparatus of the present invention, since the second linear expansion coefficient of the second layer is smaller than the first linear expansion coefficient of the first layer, the second layer has a characteristic that is less likely to thermally expand than the first layer. Have. Further, since the second Young's modulus of the second layer is larger than the first Young's modulus of the first layer, the second layer has a characteristic that is less likely to be deformed than the first layer. When the heat from the first rotating body that generates heat moves in the order of the second layer that is in contact with the inner surface of the first rotating body and the first layer that is stacked on the second layer, the first layer will thermally expand. And However, due to the above characteristics of the second layer, the second layer can suppress the thermal expansion of the first layer. Therefore, thermal deformation of the third rotating body is suppressed. As a result, the nip portion is stably formed, and the toner image is satisfactorily fixed on the recording medium.

  In a preferred embodiment of the present invention, the third rotating body contacts the inner surface of the first rotating body only at a position corresponding to the nip portion, and sandwiches the first rotating body together with the second rotating body. .

  In the fixing device according to the present invention, in the present embodiment, since the thermal deformation of the third rotating body is suppressed, the third rotating body is brought into contact with the inner surface of the first rotating body only at a position corresponding to the nip portion. Even in the case of the configuration, the contact area between the third rotating body and the first rotating body at the position corresponding to the nip portion can be maintained constant. In other words, the third rotating body is prevented from contacting the inner surface of the first rotating body at a position other than the position corresponding to the nip portion. Therefore, the amount of heat that moves from the first rotating body to the third rotating body is suppressed. As a result, it is easy to quickly raise the temperature of the first rotating body, and the temperature raising time of the first rotating body is shortened.

  In another preferred embodiment of the present invention, the first rotating body is an endless fixing belt having a width direction dimension in a direction orthogonal to a sheet passing direction in which the recording medium is passed through the nip portion, The fixing belt includes a heat generation layer that generates heat from the heat generation source. The fixing belt includes a sheet passing area that contacts the fixing belt when the recording medium passes through the nip portion, and the recording medium includes the recording medium. A non-sheet-passing area that does not contact the fixing belt is set in the width direction of the fixing belt according to the size of the recording medium, and the third rotating body is a roller member that extends along the width direction of the fixing belt. is there.

  While the paper passing area is an area in contact with the recording medium, the non-paper passing area is an area not in contact with the recording medium, so that a temperature difference occurs in the width direction of the fixing belt. Therefore, a temperature difference also occurs in the axial direction in the roller member that is the third rotating body. However, since the thermal deformation of the roller member is suppressed in the fixing device according to the present invention, the outer diameter of the roller member varies in the axial direction due to thermal deformation even when a temperature difference occurs in the axial direction. Is suppressed. Thereby, a stable nip portion is formed in the axial direction of the roller member.

  In still another preferred embodiment of the present invention, the heat generating source is a coil that heats the first rotating body by inducing an induced current in the first rotating body by a magnetic field.

  The fixing device according to the present invention employs an electromagnetic induction heating method using a coil. Since the electromagnetic induction heating method has a high heating capacity of the first rotating body, the temperature of the third rotating body is easily increased. However, since the fixing device according to the present embodiment can suppress thermal deformation of the third rotating body, an electromagnetic induction heating method with high heating capability can be employed. Thereby, a 1st rotary body can be heated up rapidly.

  An image forming apparatus according to the present invention includes an image carrier on which a toner image is formed, an image forming unit that forms the toner image on the image carrier, and a transfer unit that transfers the toner image onto a recording medium. A fixing device that fixes the transferred toner image on the recording medium, and the fixing device having the above-described configuration is used as the fixing device.

  In the fixing device according to the present invention and the image forming apparatus including the fixing device, the second linear expansion coefficient of the second layer of the fixing roller is smaller than the first linear expansion coefficient of the first layer, and the second young of the second layer is used. By appropriately selecting the material of the second layer so that the rate is larger than the first Young's modulus of the first layer, thermal deformation of the fixing roller is suppressed. Thereby, formation of the stable nip part, shortening of the temperature rising time of a 1st rotary body, etc. are achieved.

1 schematically shows an internal structure of an image forming apparatus according to an embodiment of the present invention. 2 is a longitudinal sectional view of a fixing device. FIG. FIG. 3 is a cross-sectional view illustrating a layer configuration of a fixing roller, a fixing belt, and a pressure roller. FIG. 6 is a schematic diagram for explaining a variation in outer diameter in the axial direction of the fixing roller. FIG. 10 is a diagram illustrating the results of an experiment performed on a heating time of a fixing belt and generation of paper wrinkles. FIG. 6 is a longitudinal sectional view showing another configuration of the fixing device.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus 1 according to an embodiment. The image forming apparatus 1 is a printer, a copier, a facsimile apparatus, and the like that perform printing by transferring a toner image onto the surface of a print medium such as paper as an example of a recording medium based on image information input from the outside. It can take the form of a multifunction device having functions.

  An image forming apparatus 1 shown in FIG. 1 is, for example, a tandem color printer. The image forming apparatus 1 includes a square box-shaped apparatus main body 2 that forms (prints) a color image on a sheet therein, and a discharge for discharging a sheet on which the color image is printed on the upper surface of the apparatus main body 2. A tray 3 is provided. In the lower part of the apparatus main body 2, a paper feed cassette 5 for storing paper is disposed. Further, a stack tray 6 on which manual paper is placed is disposed on the right side surface of the apparatus main body 2 in FIG. An image forming unit 7 is provided on the upper part of the apparatus main body 2, and the image forming unit 7 forms an image on a sheet based on image data such as characters and patterns transmitted from the outside of the apparatus.

  In the left position of the image forming unit 7 in FIG. 1, a first transport path 9 for transporting the sheet fed from the paper feed cassette 5 to the image forming unit 7 is disposed. A second transport path 10 that guides the paper placed on the stack tray 6 to the first transport path is disposed above the paper feed cassette 5. Each of the first conveyance path 9 and the second conveyance path 10 is provided with a conveyance roller pair 43 for conveying a sheet. In the upper left part of the apparatus main body 2, a fixing device 14 that performs a fixing process on a sheet on which an image is formed by the image forming unit 7, and a sheet that has been subjected to the fixing process are conveyed to a discharge tray 3. 3 conveyance paths 11 are arranged.

  The paper feed cassette 5 can be inserted into and removed from the apparatus main body 2 and has a storage unit 16. The storage unit 16 can selectively store at least two types of sheets having different sizes in the paper feeding direction. The paper stored in the storage unit 16 is fed out to the first transport path 9 one by one by the paper feed roller 17 and the pair of roller rollers 18.

  The stack tray 6 can be opened and closed, and a sheet is placed on the manual feed surface 19 thereof. The sheets placed on the manual feed surface 19 are fed out to the second conveyance path 10 one by one by the pickup roller 20 and the pair of roller rollers 21.

  The first conveyance path 9 and the second conveyance path 10 join before the registration roller pair 22. The paper supplied to the registration roller pair 22 waits here, performs skew adjustment and timing adjustment, and then is sent to the secondary transfer unit 23. The full color toner image on the intermediate transfer belt 40 is secondarily transferred to the sheet by the secondary transfer unit 23 on the sent sheet. Thereafter, the sheet on which the toner image is fixed by the fixing device 14 is reversed in the fourth conveyance path 12 as necessary, and the full-color toner image is secondarily transferred to the opposite side surface by the secondary transfer unit 23. Is done. Then, after the toner image on the opposite surface is fixed by the fixing device 14, the toner image is discharged to the discharge tray 3 by the discharge roller pair 24 through the third conveyance path 11.

  The image forming unit 7 is formed by four image forming units 26 to 29 that form toner images of black (B), yellow (Y), cyan (C), and magenta (M), and image forming units 26 to 29. And an intermediate transfer unit 30 that synthesizes and carries the toner images of the respective colors.

  Each of the image forming units 26 to 29 includes a photosensitive drum 32 (image carrier), a charger 33 disposed so as to face the peripheral surface of the photosensitive drum 32, and a downstream side of the charger 33. A laser scanning unit 34 that irradiates a laser beam to a specific position on the peripheral surface of the photosensitive drum 32, and a downstream side of the laser beam irradiation position from the laser scanning unit 34 that faces the peripheral surface of the photosensitive drum 32. And a cleaner 36 disposed downstream of the developing device 35 and facing the peripheral surface of the photosensitive drum 32.

  The photosensitive drums 32 of the image forming units 26 to 29 are rotated in the counterclockwise direction in the drawing by a drive motor (not shown). The developing device 35 of each of the image forming units 26 to 29 includes a developing container 51 that stores a two-component developer that includes black toner, yellow toner, cyan toner, and magenta toner.

  The intermediate transfer unit 30 is disposed on the driving roller 38 disposed near the image forming unit 26, the driven roller 39 disposed near the image forming unit 29, and the driving roller 38 and the driven roller 39. The intermediate transfer belt 40 is attached to the tension roller 42, the driving roller 38, the driven roller 39, the intermediate transfer belt 40 disposed across the tension roller 42, and the photosensitive drum 32 of each of the image forming units 26 to 29. And four transfer rollers 41 arranged so as to be capable of being pressed through.

  In the intermediate transfer unit 30, the toner images of the respective colors are transferred from the photosensitive drum 32 onto the intermediate transfer belt 40 at the position of the transfer roller 41 of each of the image forming units 26 to 29, and full color toner is transferred. An image is formed. In the present embodiment, the secondary transfer unit 23 and the intermediate transfer unit 30 constitute a transfer unit.

  A transport path 47 is provided on the upstream side and the downstream side of the fixing device 14 in the paper transport direction. The sheet conveyed through the secondary transfer unit 23 is guided to the fixing device 14 through the upstream conveyance path 47. Then, the sheet subjected to the fixing process is guided to the third conveyance path 11 through the conveyance path 47 on the downstream side.

  The third transport path 11 guides the paper on which the fixing process has been performed by the fixing device 14 to the discharge tray 3. The third transport path 11 is provided with a transport roller pair 49 for transporting the paper to the discharge tray 3 and the discharge roller pair 24 is disposed at the outlet thereof.

  Hereinafter, the fixing device 14 will be described with reference to FIG. FIG. 2 is a longitudinal sectional view of the fixing device 14. The fixing device 14 performs a fixing process for fixing the toner image on the paper by heating and pressing the toner image transferred onto the paper. The fixing device 14 includes a pressure roller 44 (second rotating body), a fixing belt 55 (first rotating body), a fixing roller 45 (third rotating body), and a coil unit 50.

  The pressure roller 44 is a roller member that can rotate counterclockwise in FIG. 2, and includes an aluminum cylindrical core material 46, an elastic layer 48 made of silicone rubber laminated on the core material 46, and A surface release layer (not shown) made of PFA tube laminated on the elastic layer 48. The thickness of the elastic layer 48 is, for example, 4 mm, and the thickness of the surface release layer is, for example, 50 μm. A heat source such as a halogen heater may be disposed in the internal space of the core member 46. The elastic layer 48 can be heated by a heat source.

  The fixing belt 55 is an endless belt formed in a hollow cylindrical shape, and can be rotated clockwise in FIG. The fixing belt 55 is disposed in parallel to the pressure roller 44. The pressure roller 44 is pressed against the fixing belt 55 by an unillustrated urging member, and forms a nip portion NP between the fixing belt 55 through which the paper carrying the transferred toner image passes. Yes. The fixing belt 55 has a dimension in the width direction in a direction orthogonal to the conveyance direction T of the sheet passing through the nip portion NP.

  The fixing roller 45 is a roller member that is disposed in the fixing belt 55 in parallel with the pressure roller 44 and is rotatable in the clockwise direction that is the same as the rotation direction of the fixing belt 55. The fixing roller 45 rotates while contacting the inner surface of the fixing belt 55 at least at a position corresponding to the nip portion NP. In the present embodiment, the fixing roller 45 rotates while contacting the inner surface (a heat generating layer 64 described later) of the fixing belt 55 only at a position corresponding to the nip portion NP. As a result, the fixing belt 55 rotates while being sandwiched between the fixing roller 45 and the pressure roller 44 at the nip portion NP.

  Each of the pressure roller 44 and the fixing roller 45 has an axial dimension that is set larger than the maximum width dimension of the sheet that is passed through the nip portion NP, and the fixing belt 55 has a width dimension that is the maximum dimension of the sheet. It is set larger than the large dimension. Further, the fixing belt 55 and the fixing roller 45 may be driven to rotate by rotating the pressure roller 44, or the fixing belt 55 and the pressure roller 44 may be driven to rotate by rotating the fixing roller 45. Also good.

  Next, the layer configuration of the fixing roller 45 and the fixing belt 55 will be described with reference to FIG. 3 in addition to FIG. FIG. 3 is a cross-sectional view showing the layer structure of the fixing roller 45, the fixing belt 55 and the pressure roller 44. The fixing roller 45 includes a cylindrical core member 51 made of nonmagnetic stainless steel (for example, SUS303), an elastic layer 52 (first layer) stacked on the core member 51, and a heat resistant surface layer stacked on the elastic layer 52. 53 (second layer). The thickness of the elastic layer 52 is, for example, 8 mm, the thickness of the heat resistant surface layer 53 is, for example, 50 μm, and the outer diameter of the fixing roller 45 is, for example, 39 mm.

The elastic layer 52 is made of a material having a first linear expansion coefficient and a first Young's modulus, for example, foamed silicone rubber. In this case, the first linear expansion coefficient is 44 × 10 ⁻⁵ / ° C., and the first Young's modulus is 0.3 N / mm ² . The elastic layer 52 repels the pressing force of the pressure roller 44 against the fixing belt 55 due to its elasticity, and presses the fixing belt 55 toward the pressure roller 44 at a position corresponding to the nip portion NP. Therefore, the elastic layer 52 of the fixing roller 45 and the elastic layer 48 of the pressure roller 44 are elastically deformed with the fixing belt 55 interposed therebetween. As a result, a nip portion NP in which a nip width extending in the paper transport direction T is sufficiently secured is formed.

On the other hand, the heat resistant surface layer 53 is made of a heat resistant resin material having a second linear expansion coefficient and a second Young's modulus, for example, polyimide. In this case, the second linear expansion coefficient is 6 × 10 ⁻⁵ / ° C., and the second Young's modulus is 3400 N / mm ² .

  In the present embodiment, the second linear expansion coefficient of the heat resistant surface layer 53 is smaller than the first linear expansion coefficient of the elastic layer 52, and the second Young's modulus of the heat resistant surface layer 53 is larger than the first Young's modulus of the elastic layer 52. Thus, the material of the elastic layer 52 and the heat resistant surface layer 53 is selected. Since the second linear expansion coefficient is smaller than the first linear expansion coefficient, the heat resistant surface layer 53 has a characteristic that it is less likely to thermally expand than the elastic layer 52. Further, since the second Young's modulus is larger than the first Young's modulus, the heat resistant surface layer 53 has a characteristic that is less likely to deform than the elastic layer 52.

  The fixing belt 55 includes a heat generating layer 64 made of electroformed nickel positioned on the fixing roller 45 side, an elastic layer 65 made of silicone rubber laminated on the heat generating layer 64 on the side opposite to the fixing roller 45, and an elastic layer 65. And a surface release layer 66 made of a PFA tube positioned on the pressure roller 44 side. The thickness of the heat generating layer 64 is, for example, 40 μm, the thickness of the elastic layer 65 is, for example, 300 μm, and the thickness of the surface release layer 66 is, for example, 30 μm. The inner diameter of the fixing belt 55 is 40 mm, for example, and is 1 mm larger than the outer diameter of the fixing roller 45.

  The coil unit 50 induction-heats the heat generation layer 64 of the fixing belt 55, and includes a coil 54 (heat generation source), an arch core 58, and a pair of side cores 59.

  The coil 54 is disposed in a winding shape along the width direction of the fixing belt 55 at a position opposite to the pressure roller 44. The coil 54 is supported by a bobbin (not shown) while being separated from the fixing belt 55 by a predetermined distance. The winding area of the coil 54 is set to a size that exceeds the dimension in the width direction of the fixing belt 55. The coil 54 is connected to an AC bias power source V. When an AC bias is applied to the coil 54, the coil 54 generates a magnetic flux.

  The arch core 58 and the pair of side cores 59 are, for example, ferrite cores that form a magnetic path through which the magnetic flux generated by the coil 54 passes. The arch core 58 has an arch shape that extends over a region beyond the winding region of the coil 54. The arch core 58 is held by a core holder made of a heat-resistant resin (not shown) (for example, PPS, PET, LCP).

  The arch core 58 has a pair of free ends 58a extending along the extending direction with the coil 54 interposed therebetween, and each of the pair of side cores 59 is connected to the corresponding free end 58a. The side core 59 is also held by a core holder made of a heat resistant resin (not shown).

  A thermistor 60 is disposed at a position opposite to the pressure roller 44 across the fixing belt 55. The thermistor 60 detects the surface temperature of the fixing belt 55. The detected surface temperature is transmitted to a control unit (not shown), and the control unit controls the AC bias power supply V based on the surface temperature to adjust the magnetic flux density of the magnetic field generated by the coil 54.

  Next, a fixing operation and its operation by the fixing device 14 having the above configuration will be described. When an AC bias is applied to the coil 54 from the AC bias power supply V, the coil 54 generates a magnetic flux. The magnetic flux passes through a magnetic path formed between the arch core 58, the side core 59, and the heat generating layer 64 of the fixing belt 55. When the magnetic flux passes through the heat generating layer 64, an induced current is induced. When the induced current flows through the heat generating layer 64, Joule heat is generated by the resistance of the heat generating layer 64 itself, that is, the heat generating layer 64 is induction heated. The entire heat generating layer 64 generates heat as the fixing belt 55 rotates, and the temperature of the fixing belt 55 rises.

  As the fixing belt 55 rises in temperature, the heat of the fixing belt 55 moves from the position corresponding to the nip portion NP to the fixing roller 45. Specifically, the heat of the heat generating layer 64 of the fixing belt 55 moves in the order of the heat resistant surface layer 53 in contact with the heat generating layer 64 in the fixing roller 45 and the elastic layer 52 laminated on the heat resistant surface layer 53. Since the elastic layer 52 is formed of foamed silicone rubber, the elastic layer 52 has a property of suppressing a decrease in the temperature of the heat generating layer 64 but tends to expand due to heat.

  However, in this embodiment, the heat resistant surface layer 53 has a second linear expansion coefficient smaller than the first linear expansion coefficient of the elastic layer 52 and a second Young's modulus larger than the first Young's modulus of the elastic layer 52. Therefore, it has the characteristic that it is less likely to thermally expand than the elastic layer 52 and the characteristic that it is less likely to deform than the elastic layer 52. Therefore, the heat resistant surface layer 53 can suppress the thermal expansion of the elastic layer 52. Thereby, thermal deformation of the fixing roller 45 is suppressed. As a result, a nip portion NP having a sufficient nip width is secured.

  When the paper carrying the toner image is passed through the nip portion NP, the paper receives heat from the heat generating layer 64 of the fixing belt 55, and the toner image is satisfactorily fixed on the paper. Although the heat resistant surface layer 53 has a characteristic that is less likely to be deformed than the elastic layer 52, the thickness of the heat resistant surface layer 53 (for example, 50 μm) is extremely smaller than the thickness of the elastic layer 52 (for example, 8 mm). The elastic deformation is not hindered, and therefore the formation of the nip portion NP is not hindered.

  Further, in this embodiment, since the thermal deformation of the fixing roller 45 is suppressed, even when the fixing roller 45 is in contact with the inner surface of the fixing belt 55 only at a position corresponding to the nip portion NP, The contact area between the fixing roller 45 and the fixing belt 55 at the position corresponding to the nip portion NP can be maintained constant. In other words, the fixing roller 45 is prevented from contacting the inner surface of the fixing belt 55 at a position other than the position corresponding to the nip portion NP. Therefore, the amount of heat transferred from the heat generating layer 64 of the fixing belt 55 to the fixing roller 45 is suppressed. As a result, it is easy to raise the temperature of the fixing belt 55, and the temperature raising time of the fixing belt 55 is shortened.

  Further, in the fixing device 14 of the present embodiment, when a sheet having a small width direction size (a size in a direction orthogonal to the transport direction T (FIG. 2)) is continuously passed, the temperature in the width direction of the fixing belt 55 is increased. A difference occurs. Specifically, as shown in FIG. 4, when a sheet P1 having a small width direction passes through the nip portion NP, the intermediate portion in the width direction of the fixing belt 55 is a sheet passing region that contacts the sheet P1 at the nip portion NP. On the other hand, both end portions in the width direction of the fixing belt 55 become non-sheet passing regions A2 that do not contact the paper P1. In the non-sheet-passing area A2, heat fixing is not performed, and heat does not move from the non-sheet-passing area A2 to the sheet P1, so when continuous printing is performed on the sheet P1, between the sheet-passing area A1 and the non-sheet-passing area A2 A temperature difference occurs.

  Due to the temperature difference between the sheet passing area A1 and the non-sheet passing area A2, an area B1 corresponding to the sheet passing area A1 in the fixing roller 45 and an area B2 corresponding to the non-sheet passing area A2 in the fixing roller 45 Even between them, a temperature difference occurs in the axial direction. For this reason, the degree of thermal expansion in the area B2 of the fixing roller 45 is larger than the degree of thermal expansion in the area B1, and the outer diameter D1 of the fixing roller 45 in the area B1 and the outer diameter D2 of the fixing roller 45 in the area B2. Is different. As described above, when the outer diameter varies in the axial direction of the fixing roller 45, the nip width of the nip portion NP varies along the axial direction of the fixing roller 45. When the paper P2 having a larger nip width than the paper P1 is passed through the nip portion NP after continuous printing of the paper P1 with the nip width varied, the paper P2 is wrinkled or the toner image is printed on the paper. Fixing failure that is not fixed on P2 occurs.

  However, in the present embodiment, as described above, since the thermal deformation of the fixing roller 45 is suppressed, the outer diameter of the fixing roller 45 is reduced even when a temperature difference occurs in the axial direction of the fixing roller 45. The variation in the axial direction is suppressed. As a result, the nip width is prevented from varying along the axial direction of the fixing roller 45. As a result, even if the paper P2 is passed through the nip portion NP after continuous printing of the paper P1, the generation of wrinkles on the paper P2 is suppressed and the toner image is satisfactorily fixed on the paper P2.

  Further, the fixing device 14 according to the present embodiment employs an electromagnetic induction heating method using a coil 54. Since the electromagnetic induction heating method has a high heating capability of the fixing belt 55, even when the fixing roller 45 is in contact with the fixing belt 55 only at a position corresponding to the nip portion NP, the temperature of the fixing roller 45 is easily increased. However, since the fixing device 14 according to the present embodiment can suppress thermal deformation of the fixing roller 45, an electromagnetic induction heating method with high heating capability can be employed. Thereby, the temperature of the fixing belt 55 can be quickly raised.

Next, an experiment conducted on the heating time of the fixing belt 55 and generation of paper wrinkles will be described. In the experiment, an example using the fixing roller 45 having the heat-resistant surface layer 53 and a comparative example 1 using the fixing roller 45 that does not have the heat-resistant surface layer 53, that is, the elastic layer 52 directly contacts the heat generating layer 64 of the fixing belt 55. And Comparative Example 2 using a fixing roller 45 having a heat-resistant surface layer. The heat resistant surface layer of the fixing roller 45 of Comparative Example 2 has a Young's modulus (0.28 N / mm ² ) which is smaller than the linear expansion coefficient of the elastic layer, but the Young's modulus (0.30 N / mm ² ) of the elastic layer. It was formed from a solid silicone rubber slightly smaller than ² ). The temperature raising time of the fixing belt 55 was obtained by measuring the time until the surface temperature of the fixing belt 55 reached 180 ° C. Further, regarding the generation of wrinkles on the paper, it was visually confirmed whether or not the A3 paper was wrinkled when 100 sheets of A5 paper were continuously printed and then A3 paper was printed. FIG. 5 shows the results of the experiment.

  As shown in FIG. 5, the temperature rising times of the example, comparative example 1, and comparative example 2 were 24 seconds, 30 seconds, and 29 seconds, respectively. The linear expansion coefficient (second linear expansion coefficient) of the heat-resistant surface layer 53 is smaller than the linear expansion coefficient (first linear expansion coefficient) of the elastic layer 52, and the Young's modulus (second Young's modulus) of the elastic layer 52 is ( In the embodiment larger than the first Young's modulus, the heat deformation of the fixing roller 45 can be suppressed, so that the temperature raising time can be shortened by 6 seconds compared to Comparative Example 1 and 5 seconds shorter than Comparative Example 2. did it. Although Comparative Example 2 has a heat resistant surface layer, the Young's modulus is lower than the Young's modulus of the elastic layer, that is, the heat resistant surface layer of Comparative Example 2 is more easily deformed by heat than the elastic layer. The fixing roller 45 is in contact with the inner surface of the fixing roller 45 even at a position other than the position corresponding to the nip portion NP.

  Further, regarding the generation of wrinkles on the paper, the wrinkles did not occur in the example, whereas the wrinkles occurred in the comparative example 1 and the comparative example 2. This is probably because the outer diameter of the fixing roller 45 did not vary in the axial direction in the example, whereas the outer diameter of the fixing roller 45 varied in the axial direction in Comparative Example 1 and Comparative Example 2.

  In the fixing device 14 according to the present embodiment described above, the case where the fixing roller 45 is disposed in the fixing belt 55 and contacts the inner surface of the fixing belt 55 only at a position corresponding to the nip portion NP has been described. The present invention is not limited to this configuration, and a configuration in which the fixing belt 70 is wound around the outer peripheral surface of the fixing roller 71 as shown in FIG. Even in this configuration, by setting the linear expansion coefficient of the heat resistant surface layer of the fixing roller 71 to be smaller than the linear expansion coefficient of the elastic layer, and setting the Young's modulus of the heat resistant surface layer to be larger than the Young's modulus of the elastic layer, Thermal deformation of the fixing roller 71 can be suppressed.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 14 Fixing apparatus 44 Pressure roller (2nd rotary body)
45 Fixing roller (third rotating body)
50 Coil unit 52 Elastic layer (first layer)
53 Heat-resistant surface layer (second layer)
54 Coil 55 Fixing belt (first rotating body)
NP nip

Claims (5)

A first rotating body capable of generating heat and rotating in a predetermined direction;
A second rotating body that forms a nip portion through which a recording medium carrying a toner image is passed with the first rotating body;
A third rotating body that is arranged in the first rotating body and rotates in the same direction as the first rotating body while contacting the inner surface of the first rotating body at a position corresponding to at least the nip portion;
A heat source for generating heat in the first rotating body;
With
The third rotating body moves the first rotating body to the second rotating body at a position corresponding to the nip portion so that the nip portion is formed between the first rotating body and the second rotating body. A first layer that is pressed toward the first layer, and a second layer that is laminated on the first layer and that contacts the inner surface of the first rotating body,
The first layer is made of a material having a first linear expansion coefficient and a first Young's modulus;
The second layer is made of a material having a second linear expansion coefficient and a second Young's modulus,
The fixing device wherein the second linear expansion coefficient is smaller than the first linear expansion coefficient, and the second Young's modulus is larger than the first Young's modulus. The fixing device according to claim 1,
The fixing device, wherein the third rotating body contacts the inner surface of the first rotating body only at a position corresponding to the nip portion, and clamps the first rotating body together with the second rotating body. The fixing device according to claim 1 or 2,
The first rotating body is an endless fixing belt having a width direction dimension in a direction orthogonal to a paper passing direction in which the recording medium is passed through the nip portion;
The fixing belt has a heat generation layer that generates heat by the heat generation source,
The fixing belt has a sheet passing area where the recording medium contacts the fixing belt when the recording medium passes through the nip portion, and a non-sheet passing area where the recording medium does not contact the fixing belt. Is set in the width direction of the fixing belt according to
The fixing device, wherein the third rotator is a roller member extending along the width direction of the fixing belt. The fixing device according to any one of claims 1 to 3,
The fixing device, wherein the heat source is a coil that heats the first rotating body by inducing an induced current in the first rotating body by a magnetic field. An image carrier on which a toner image is formed;
An image forming unit that forms the toner image on the image carrier;
A transfer portion for transferring the toner image onto a recording medium;
A fixing device for fixing the transferred toner image on the recording medium;
With
An image forming apparatus in which the fixing device according to claim 1 is used as the fixing device.

Images