JP2010256714A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent partial decrease in the pressure of a fixing nip of a belt fixing system, which uses a pair of belts. <P>SOLUTION: The first belt 31 wound around a first entrance side roller 33 and first exit side roller 34 has such rigidity that the belt runs while maintaining a curved state in which a belt portion extending between both the rollers 33 and 34 juts out, and forms a fixing nip by its being firmly pressed against the running portion of a second belt 32 wound around a second entrance side roller 35 and a second exit side roller 36. The entrance portion of the fixing nip is located between the middle of the first belt 31 and the middle of the second belt 32 separated from the first entrance side roller 33 and second entrance side roller 35 respectively. In the exit portion of the nip, the first belt 31 and second belt 32 are firmly pressed by the first exit side roller 34 and second exit side roller 36. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a fixing device that thermally fixes an unfixed image formed on a recording sheet to a recording sheet, and an image forming apparatus including the fixing device.

  In an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a complex machine of these, usually, a toner image corresponding to image data is formed and transferred to a recording sheet, and the transferred toner image is fixed on the recording sheet by a fixing device. It has a configuration to let you. As a fixing device, a belt fixing method in which a fixing nip is formed by using a heating belt wound around a pair of rollers and a pressure belt wound around the pair of rollers has been proposed.

  In the belt fixing method, usually, each of a pair of heating side rollers around which a heating belt is wound and each of a pair of pressure side rollers around which a pressure belt is wound are passed through the heating belt and the pressure belt. A fixing nip is formed between the belt running portions of the heating belt and the pressure belt facing each other by pressing the heating roller and the pressure roller facing each other.

  The recording sheet on which the toner image has been transferred passes through the entrance portion of the fixing nip between the heating belt and the pressure belt in a state of being pressed against each other by the heating side roller and the pressure side roller on the recording sheet entry side (entrance side). Then, after entering the free nip portion formed by the belt running portions pressed against each other, and passing through the free nip portion, the heating pressed against each other by the heating side roller and the pressure side roller on the recording sheet discharge side After passing through an outlet portion between the belt and the pressure belt, it is discharged from the fixing nip. While the recording sheet passes through the fixing nip, the toner image is heated by the heating belt and is pressed onto the recording sheet to be fixed on the recording sheet.

In such a belt fixing type fixing device, since the heat capacity of the heating belt for heating the toner image is small, warming up for raising the heating belt to a predetermined heating temperature can be performed in a short time, thereby saving power. The fixing operation can be performed at high speed.
The distribution of the nip pressure in such a fixing nip is shown in FIG. At the entrance of the fixing nip, the heating belt and the pressure belt are pressed against each other by the heating side roller and the pressure side roller, so the nip pressure is high. In the free nip portion where no is added, the nip pressure is significantly lower than that at the inlet portion.

In particular, in the portions adjacent to the inlet portion and the outlet portion in the free nip portion respectively, the heating belt and the pressure belt are pressed against each other at a high pressure by the heating side roller and the pressure side roller at each of the inlet portion and the outlet portion. The pressure contact force between the heating belt and the pressure belt is relaxed.
In the fixing nip having such a pressure distribution, when the recording sheet that has passed through the inlet portion that is at a high nip pressure by the heating side roller and the pressure side roller enters the free nip portion, the nip pressure applied to the recording sheet rapidly decreases. Therefore, the heating belt, the pressure belt, and the recording sheet, which are moving around, do not move together, and there is a risk that a speed difference may occur between them.

  In this case, the toner on the recording sheet adheres to the heating belt due to the high nip pressure at the entrance of the fixing nip, and then the pressure contact with the recording sheet occurs due to a pressure drop that lowers the nip pressure. There is a possibility that the toner position attached to the heating belt is deviated from the position originally attached to the recording sheet. As a result, an image shift occurs in which a part of the toner image on the recording sheet is shifted.

Due to the occurrence of pressure loss also in the vicinity of the exit of the free nip, there is a risk that image misalignment will occur in the toner image on the recording sheet.
In Patent Document 1, the rollers on the inlet side are mutually connected in a state where the fixing belt and the pressure belt are pressed against each other by the rollers on the outlet side around which the fixing belt (heating belt) and the pressure belt are wound. The fixing belt and the pressure belt are separated from each other on the upstream side in the conveyance direction of the recording sheet, and the inner peripheral surfaces of the fixing belt and the pressure belt in the vicinity of each roller on the outlet side are A fixing device configured to ensure nip pressure by pressing with a pad over a length is disclosed.

  In the fixing device having such a configuration, since the belts are not pressed against each other by the rollers at the entrance of the fixing nip where the recording sheet enters, there is a pressure drop in which the nip pressure rapidly decreases in the vicinity of the entrance of the fixing nip. It can be prevented from occurring.

JP 2007-78998 A

  In the fixing device disclosed in Patent Document 1, each of the fixing belt and the pressure belt slides with high pressure applied by each pad in order to ensure nip pressure. Since a large frictional force is generated between the belt and the belt, a large driving force is required to move the fixing belt and the pressure belt around at a predetermined speed. For this reason, the energy for driving each of the fixing belt and the pressure belt increases. Further, since the wear is promoted by the frictional force between the belts and the pads, the durability of each belt is lowered at an early stage, and there is a possibility that the belts cannot be used stably over a long period of time.

  Further, in the vicinity of the exit portion of the nip by the pad, there is a limit to disposing each pad close to each of the rollers, so that it cannot be pressed by each pad. For this reason, the part which is not pressed by each pad arises in the exit part of the nip by a pad. The portion that is not pressed by each pad is adjacent to the entrance side of the portion where the fixing belt and the pressure belt are pressed against each other with high pressure by each roller. (See FIG. 3 of Patent Document 1). As a result, as described above, the fixing belt and the recording sheet are not conveyed together, and there is a possibility that image misalignment may occur in the toner image to be fixed.

  The present invention has been made in view of such conventional problems, and an object of the present invention is to prevent image deviation from occurring in a toner image on a recording sheet in a belt fixing method using a pair of belts. An object of the present invention is to provide a fixing device that does not require a large driving force to move the belt around and that can stably use the belt for a long period of time, and an image forming apparatus including the fixing device.

  In order to achieve the above object, a fixing device according to the present invention includes a first belt wound around a pair of first rollers so as to be able to move around and a second belt wound around the pair of second rollers so as to be turned around. A fixing nip formed by pressure contact with a belt, and a recording sheet passing through the fixing nip to fix an unfixed image on the recording sheet by thermocompression bonding, wherein the first belt If the belt is placed in a state where it is not in pressure contact with the second belt, the belt portion straddling the pair of first rollers has a rigidity such that the belt portion bulges outward, and is recorded in the fixing nip. The end portion on the sheet entry side is present at an intermediate portion of each of the first belt and the second belt away from the first roller and the second roller, while the recording sheet is discharged from the fixing nip. End of the is characterized by the presence at the site that is wound around the first roller and the second roller positioned on the discharge side of each of said first belt and said second belt.

  The image forming apparatus according to the present invention is characterized in that the fixing device is provided as a fixing unit.

  In the fixing device of the present invention, the belt running portion bulging out of the first belt having rigidity and the belt running portion of the second belt are pressed against each other to form a fixing nip. Since the end (entrance portion) on the recording sheet entrance side of the recording sheet exists at an intermediate portion away from the first roller and the second roller in each of the first belt portion and the second belt portion, the entrance to the fixing nip. There is no risk of a pressure drop in which the nip pressure rapidly decreases in the portion.

  Further, the end (exit portion) on the recording sheet discharge side of the fixing nip is present at a portion of the first belt and the second belt that is wound around the first roller and the second roller on the end portion side. However, since the rigid first belt and the second belt are in pressure contact with each other, it is possible to prevent the nip pressure from rapidly decreasing even in a portion adjacent to the exit portion in the fixing nip. As a result, it is possible to prevent image deviation from occurring in the toner image on the recording sheet.

  Further, since the fixing nip portion is formed by pressing the first belt against the second belt using the rigidity of the first belt itself, the belt slides on each belt to increase the nip pressure in the fixing nip portion. It is not necessary to provide a member that comes into contact. As a result, a large driving force is not required to rotate the first belt and the second belt, and energy for driving the first belt and the second belt can be reduced. In addition, since wear on the inner peripheral surfaces of the first belt and the second belt can be suppressed, the belt can be used stably over a long period of time.

Preferably, if the second belt is placed in a state where it is not in pressure contact with the first belt, a belt portion straddling the pair of second rollers is parallel.
Preferably, if the second belt is placed in a state where it is not in pressure contact with the first belt, the belt has a rigidity so that the belt portion extending between the pair of second rollers bulges outward. It is characterized by.

Preferably, the second belt is recessed in a concave shape when the first belt is pressed.
Preferably, the mutually pressed portions of the first belt and the second belt are flat.
Preferably, the fixing nip portion is provided with vibration suppressing means for suppressing the first belt from vibrating in a direction in which the first belt contacts and separates from the second belt.

Preferably, the vibration suppressing means includes a second belt side guide body that guides an inner peripheral surface of the second belt so that the second belt is in contact with the first belt.
Preferably, the vibration suppressing means further includes a first belt side guide body that guides an inner peripheral surface of the first belt.
Preferably, side edges on both sides of the first belt and the second belt are located on both sides of the recording sheet conveyance area in the fixing nip, respectively, and the first belt side guide body and the second belt. Each of the side guide bodies guides the side edge portions of the first belt and the second belt.

  Preferably, each of the first belt side guide body and the second belt side guide body is a rotating body that rotates following the movement of the first belt and the second belt.

1 is a schematic diagram illustrating an overall schematic configuration of a printer in which a fixing device according to an embodiment of the present invention is mounted as a fixing unit. 2 is a schematic diagram for explaining a configuration of a fixing device according to an embodiment of the present invention provided as a fixing unit in the printer. FIG. It is a schematic diagram for demonstrating the structure of the 1st belt used for the fixing device. FIG. 2 is a perspective view illustrating a configuration of a main part of the fixing device. 3 is a graph showing a distribution of nip pressure of a fixing nip in the fixing device. FIG. 6 is a schematic diagram for explaining a configuration of a fixing device according to another embodiment of the present invention. FIG. 10 is a schematic diagram for explaining a configuration of a fixing device according to still another embodiment of the present invention. 6 is a graph showing a nip pressure distribution of a fixing nip in a conventional fixing device.

Hereinafter, embodiments of a fixing device and an image forming apparatus according to the present invention will be described.
<First Embodiment>
FIG. 1 is a schematic diagram illustrating a configuration of a printer which is an example of an image forming apparatus provided with a fixing device according to an embodiment of the present invention. This printer forms a monochrome toner image on a recording sheet such as a recording sheet or an OHP sheet.

The image forming apparatus shown in FIG. 1 has a photosensitive drum 11 that is rotationally driven in a direction indicated by an arrow A. Around the photosensitive drum 11, a toner image is formed on a recording sheet S by electrophotography. A charging device 12, an exposure device 13, a developing device 14, and a transfer roller 15 for forming are provided in that order along the circumferential movement direction.
In this printer, image data input from an external device is converted into a laser diode drive signal by a control unit (not shown), and the laser diode provided in the exposure device 13 is driven by the drive signal. As a result, the exposure device 13 irradiates the surface of the photosensitive drum 11 with the laser beam L corresponding to the image data. The surface of the photosensitive drum 11 is charged to a predetermined potential by the charging device 12 in advance, and is exposed to the laser light L emitted from the exposure device 13, thereby electrostatic latent images on the surface of the photosensitive drum 11. An image is formed. The electrostatic latent image is developed with toner in the developing device 14 and visualized as a toner image.

  Below the photosensitive drum 11, there is provided a recording sheet cassette 21 that can store a plurality of recording sheets S such as recording paper and OHP sheets, and the recording sheet S in the recording sheet cassette 21 is 1 by a paper feed roller 22. It is fed out one by one. The recording sheet S fed out from the recording sheet cassette 21 is conveyed toward the photosensitive drum 11 via the timing roller pair 23.

  A transfer roller 15 that rotates in the direction of arrow B is provided on the side of the photosensitive drum 11 in a state of being pressed against the photosensitive drum 11. The recording sheet S is pressed against the transfer roller 15 and the photosensitive drum 11. Pass through the transfer nip formed by When the recording sheet S passes through the transfer nip portion, the toner image formed on the photosensitive drum 11 is formed on the recording sheet S by the action of the transfer electric field generated by the transfer voltage applied to the transfer roller 15. Transcribed. The recording sheet S to which the toner image is transferred is peeled off from the photosensitive drum 11 by the peeling claw 16 and conveyed to the fixing device 30.

  The fixing device 30 includes an endless first belt 31 and a second belt 32 that rotate around each other, and a fixing nip is formed by the first belt 31 and the second belt 32 being pressed against each other. Yes. Heater lamps 37 are provided inside the circular movement areas of the first belt 31 and the second belt 32, respectively. Each heater lamp 37 is controlled based on the surface temperature of the first belt 31 measured by the thermistor 39 so that the surface temperature becomes a predetermined temperature.

  The unfixed toner image on the recording sheet S conveyed to the fixing device 30 is brought into pressure contact with the first belt 31 and the second belt 32, which are rotated by the heater lamps 37 with the surface heated to a predetermined temperature. The recording sheet S is fixed on the recording sheet S by heating and pressurizing (thermocompression bonding) while the recording sheet S passes through the fixing nip formed by the above. The recording sheet S on which the toner image is fixed is peeled off from the outer peripheral surface of the first belt 31 by the peeling claw 38, conveyed toward the paper discharge roller 24, and discharged onto the paper discharge tray 19 by the paper discharge roller 24. The

  The photosensitive drum 11 after the toner image is transferred is erased by the eraser 18 after the toner remaining on the surface is removed by the cleaner 17. The surface of the photosensitive drum 11 from which the residual charge has been erased is charged by the charging device 12 according to an instruction for the next image formation, and a toner image is formed on the recording sheet by repeating the same operation as described above. Is done.

  FIG. 2 is a schematic diagram for explaining a specific configuration of the fixing device 30. The first belt 31 is wound around a first inlet side roller 33 located on the upstream side in the conveyance direction of the recording sheet S and a first outlet side roller 34 located on the downstream side in the conveyance direction of the recording sheet S. The second belt 32 is wound around a second inlet-side roller 35 positioned upstream in the conveyance direction of the recording sheet S and a second outlet-side roller 36 positioned downstream in the conveyance direction of the recording sheet S.

  FIG. 3 shows that the first belt 31 wound around the first inlet side roller 33 and the first outlet side roller 34 is separated from the second belt 32 before being assembled in a state of being pressed against the second belt 32. It is a schematic diagram at the time of putting in the state which is not made to contact members other than the 1 entrance side roller 33 and the 1st exit side roller 34. FIG. The first belt 31 is in a cylindrical shape with a circular cross section in a natural state where no tensile force is applied, but as shown in FIG. 3, the first inlet side roller 33 and the first outlet side roller 34 By being wound in a state where a predetermined tension (belt tension) is applied, it has a predetermined rigidity (flexibility) so as to move around while maintaining a substantially elliptical cylindrical cross section. ing.

  The first inlet side roller 33 and the first outlet side roller 34 around which the first belt 31 is wound have the same radius r, and the respective axis centers are spaced apart (interaxial distance) D. Arranged in a state. The inter-axis distance D has a relationship of L> 2D + 2πr, where L is the circumference of the first belt 31. Accordingly, the pair of belt running portions running in opposite directions in a state of being opposed to each other between the first inlet side roller 33 and the first outlet side roller 34 try to return to the cylindrical state before the belt tension is applied. Due to the force (restoring force), the curved state bulges outward with the same maximum protrusion amount (deflection amount) δ from the common tangent L1 of the first inlet side roller 33 and the first outlet side roller 34, respectively.

  As shown in FIG. 2, the second belt 32 is wound around a second inlet side roller 35 and a second outlet side roller 36 having the same radius r. The second inlet side roller 35 and the second outlet side roller 36 are arranged with a predetermined inter-axis distance in a state of being aligned in the vertical direction (Y-axis direction) along the conveyance direction of the recording sheet S. Yes. The recording sheet S is conveyed upward along one belt traveling portion of the second belt 32 that is close to the first belt 31.

  The second belt 32 has lower rigidity than the first belt 31, and is wound around the second inlet side roller 35 and the second outlet side roller 36, whereby the second inlet side roller 35 and the second outlet side are wound. Each belt running portion between the side rollers 36 is in a state where a predetermined tensile force (belt tension) is applied, and when placed in a state where it is not in pressure contact with the first belt 31, the second inlet side roller 35 and Travel along the common tangent line of the second outlet side roller 36.

  The first outlet side roller 34 around which the first belt 31 is wound is arranged in a state of being aligned in the horizontal direction (X-axis direction) with respect to the second outlet side roller 36, and the second outlet side roller 36 is The first belt 31 and the second belt 32 passing between the first outlet side roller 34 and the first outlet side roller 34 are directed toward the first outlet side roller 34 by an urging means (not shown) so as to be pressed against each other with high pressure. It is pressed.

  On the other hand, on the upstream side in the conveyance direction of the recording sheet S, the first inlet side roller 33 around which the first belt 31 is wound is opposite to the second inlet side roller 35 around which the second belt 32 is wound. The belt running portion wound along the first inlet side roller 33 and the belt running portion wound along the second inlet side roller 35 are arranged with a predetermined interval therebetween. It is in a state of being separated from each other.

  In the first belt 31, the belt running portion that bulges toward the second belt 32 starts press contact with the second belt 32 at an intermediate portion away from the portion wound along the first inlet side roller 33. The start position of the pressure contact is an entrance portion where the recording sheet S enters the fixing nip. In this embodiment, the entrance portion of the fixing nip where the first belt 31 starts to press contact with the second belt 32 is slightly closer to the second entrance side roller 35 than the center of the belt running portion of the second belt 32. Is in position.

The pressure contact between the first belt 31 and the second belt 32 continues until it passes between the first outlet side roller 34 and the second outlet side roller 36, and therefore, the fixing nip also has the first outlet side roller 34. And it continues until it passes between the 2nd exit side rollers 36.
For example, the first belt 31 rotates in the direction indicated by the arrow M in FIG. 2 when the first outlet roller 34 is rotationally driven by a drive motor (not shown). The second belt 32 pressed against the first belt 31 follows the circular movement of the first belt 31 and rotates in the direction indicated by the arrow N in FIG. 2 (the direction opposite to the circular movement direction of the first belt 31). It is supposed to move.

  Note that the second outlet is not limited to the configuration in which the first belt 31 is driven to rotate by the rotational driving of the first outlet roller 34 and the second belt 32 is moved in a circular motion following the first belt 31. A configuration in which the side roller 36 is rotated by a drive motor to cause the second belt 32 to move around, and the first belt 31 is moved around following the rotation of the second belt 32. The two inlet-side rollers 35 are driven to rotate, and the first belt 31 and the second bell 32 are moved around, respectively. Both the first inlet-side roller 33 and the first outlet-side roller 34 or the second inlet-side roller It is good also as a structure which rotationally drives both 35 and the 2nd exit side roller 36, and also the structure which rotationally drives all the four rollers 33-36.

  The recording sheet S is conveyed along the belt running portion facing the first belt 31 in a state of being separated from the first belt 31 below the second belt 32, and the first belt 31 and the second belt 32 are pressed against each other. The first belt 31 and the second belt 32 enter the free nip portion where the first belt 31 and the second belt 32 are in pressure contact with each other through the entrance portion of the fixing nip formed by the above. Therefore, the entrance portion of the fixing nip is the entrance portion of the free nip portion. The recording sheet S passes through the free nip portion, passes through the exit portion where the first belt 31 and the second belt 32 are pressed with high pressure by the first exit side roller 34 and the second exit side roller 36, It is discharged from the fixing nip.

  A cylindrical heater lamp 37 formed in a straight line is arranged in a state parallel to the first inlet-side roller 33 and the first outlet-side roller 34 inside the circumferential movement area of the first belt 31. The heater lamp 37 heats the first belt 31 that moves around to a predetermined temperature. Also, a cylindrical heater lamp 37 formed in a straight line is also arranged in a state parallel to the second inlet side roller 35 and the second outlet side roller 36 inside the circumferential movement area of the second belt 32. Yes. The heater lamp 37 heats the second belt 32 moving around to a predetermined temperature.

As each heater lamp 37, a halogen heater, a carbon heater, a xenon lamp heater, or the like can be used. Moreover, it replaces with such a heater lamp 37, and the heating wire heater formed linearly can also be used.
At the entrance of the fixing nip, a vibration suppressing member 40 is provided for suppressing the vibration of the first belt 31 in pressure contact with the second belt 32 in the direction in which the first belt 31 is in contact with and away from the second belt 32. Yes. As shown in FIG. 4, the vibration suppressing member 40 is a pair of second belt side rollers that guide the second belt 32 in contact with the inner peripheral surface of the belt running portion of the second belt 32 to which the first belt 31 is pressed. (Second belt side guide body) 42 and the second belt side roller 42 in contact with the inner peripheral surface of the first belt 31 with the first belt 31 and the second belt 32 facing each other. A first belt side roller (first belt side guide body) 41 for guiding the first belt 31;

  The pair of second belt side rollers 42 are rotatably attached to the respective end portions of the support shaft 44 extending linearly along the width direction of the second belt 32, and recording on the inner peripheral surface of the second belt 32. Each of the sheet S is guided in contact with the side edges in the width direction on both outer sides of the conveyance area of the sheet S. The pair of first belt-side rollers 41 are also rotatably attached to the respective end portions of the support shaft 43 extending linearly along the width direction of the first belt 31, and recording on the inner peripheral surface of the first belt 31 is performed. Each of the sheet S is guided in contact with the side edges in the width direction on both outer sides of the conveyance area of the sheet S. Each of the second belt side rollers 42 rotates following the circumferential movement of the second belt 32, and each of the first belt side rollers 41 rotates following the circumferential movement of the first belt 31.

  The first belt 31 and the second belt 32 are in a state where the side edge portions on both sides in the width direction are sandwiched between the first belt side roller 41 and the second belt side roller 42 facing each other. The first belt 31 is prevented from vibrating in the direction in which the first belt 31 is in contact with or separated from the second belt 32. As a result, the recording sheet S conveyed along the second belt 32 smoothly enters the entrance portion of the fixing nip (free nip portion).

The first belt-side roller 41 and the second belt-side roller 42 that are opposed to each other via the first belt 31 and the second belt can suppress the vibration of the first belt 31. The pressure only needs to be applied, and the pressure in contact with the first belt 31 and the second belt is set to an appropriate value.
The first belt 31 is, for example, a base having a predetermined thickness made of a material such as Ni (nickel), iron, SUS (stainless steel), W (tungsten), PI (polyimide) having a large elastic modulus (Young's modulus). The cross section is endlessly formed into a circular cylinder having a predetermined outer diameter. The higher the Young's modulus E, the more preferable the base material is because the pressure applied to the second belt 32 can be increased. The Young's modulus E is preferably 100,000 N / mm ² or more.

  The thickness of the substrate is, for example, about 20 to 400 μm, preferably 100 to 200 μm. The surface (outer peripheral surface) of the substrate is provided with a fluorine-based tube or coating of PFA, PTFE, ETFE or the like having a thickness of about 5 to 100 μm, or a surface layer of silicone rubber or the like, so that it can be released from the recording sheet S May be given. Further, the surface layer may be conductive. As the fluorine-based tube, trade names “PFA350-J”, “451HP-J”, “951HP Plus” manufactured by Mitsui DuPont Fluorochemical Co., Ltd. can be preferably used.

An intermediate layer may be provided between the substrate and the surface layer. As the intermediate layer, a material having elasticity and high heat resistance such as silicone rubber and fluorine rubber is preferable. The thickness of the intermediate layer is preferably about 0.05 to 2 mm.
The first belt 31 is preferably formed in a cylindrical shape with an outer diameter of about 30 to 150 mm.

  The 1st entrance side roller 33 and the 1st exit side roller 34 have the cylindrical core metal whose outside diameter is about 10-50 mm. The core metal is made of, for example, Al (aluminum), SUS (stainless steel), iron, or the like. The core metal may be in the shape of a pipe. In this case, the thickness is 0.1 mm or more, but considering bending (bending) when the first belt 31 is wound, the thickness is 1 mm or more. Thickness is preferred.

  An elastic layer such as silicone rubber, fluororubber, or sponge may be provided on the surface of the metal core. The elastic layer is preferably made of a material having high heat resistance. The thickness of the elastic layer is preferably about 0.05 to 20 mm, more preferably about 2 to 10 mm. In order to impart slidability (low friction) to the surface of the elastic layer, a fluorine-based tube such as PFA, PTFE, or ETFE, or a skin layer such as a coating may be provided. The skin layer may be conductive.

  The thickness of the skin layer is preferably as thick as possible in consideration of durability, but is preferably about 5 to 200 μm, more preferably about 20 to 100 μm in consideration of warm-up time. As the fluorine-based tube, trade names “PFA350-J”, “451HP-J”, “951HP Plus” manufactured by Mitsui DuPont Fluorochemical Co., Ltd. can be preferably used.

The outer diameters of the first inlet side roller 33 and the first outlet side roller 34 are preferably about 10 to 70 mm.
The second belt 32 of the present embodiment has lower rigidity than the first belt 31, and is laminated on a heat resistant layer having heat resistance such as polyimide resin and the surface (outer peripheral surface) of the heat resistant layer. It has a laminated structure with a release layer. The release layer is made of a material excellent in releasability with the toner, and for example, a fluororesin is used. The thickness of the heat-resistant layer is, for example, 30 to 100 μm, and the thickness of the release layer is, for example, about 30 μm. The second belt 32 is not limited to such a two-layer structure, and one or more layers may be provided between the heat-resistant layer and the release layer.

The 2nd entrance side roller 35 and the 2nd exit side roller 36 have the same composition as the 1st entrance side roller 33 and the 1st exit side roller 34, for example.
In the present embodiment, the belt tension at which the belt running portion between the second inlet side roller 35 and the second outlet side roller 36 in the second belt 32 can run along the common tangent line of the rollers 35 and 36. The distance between the axes of the second inlet side roller 35 and the second outlet side roller 36 is set.

In the fixing device 30 configured as described above, the recording sheet S to which the toner image has been transferred is conveyed upward along the belt running portion of the second belt 32 that extends linearly in a state of being separated from the first belt 31. It enters the free nip portion of the fixing nip from the entrance portion of the fixing nip, which is the position where the first belt 31 starts to press.
In this case, at the entrance portion of the fixing nip, the side edges of the first belt 31 and the second belt 32 on both sides in the width direction are guided together by the first belt side roller 41 and the second belt side roller 42. Accordingly, the vibration of the first belt 31 is prevented, and the recording sheet S conveyed along the second belt 32 smoothly enters the fixing nip.

  The recording sheet S that has entered the fixing nip passes through the free nip portion formed by the belt running portion bulging outside the first belt 31 being pressed against the second belt 32, and further, The toner is discharged from the fixing nip through the outlet portion having a high nip pressure by the outlet side roller 34 and the second outlet side roller 36. The toner image on the recording sheet S is heated by each heater lamp 37 while passing through the entrance portion of the fixing nip (the entrance portion of the free nip portion), the inside of the free nip portion, and the exit portion (roller nip portion). The first belt 31 and the second belt 32 are melted and pressed onto the recording sheet S to be fixed on the recording sheet S.

  Each of the pair of first belt side rollers 41 and the pair of second belt side rollers 42 provided at the entrance portion of the fixing nip is located on both outer sides of the recording belt S conveyance area on the first belt 31 and the second belt 32. Since the first belt 31 and the second belt 32 in the heated state are in contact with only the side edge portions, the amount of heat taken away by the first belt side roller 41 and the second belt side roller 42 is small, and the recording sheet The amount of heat taken away from S itself is also small. Therefore, the heating efficiency of the recording sheet S due to the provision of the pair of first belt side rollers 41 and the pair of second belt side rollers 42 is suppressed.

  FIG. 5 is a graph showing the nip pressure at the fixing nip. The entire free nip portion including the entrance portion of the free nip portion, which is an entrance portion in the fixing nip, is only the nip pressure obtained by the belt running portion bulging outside the first belt 31 being pressed against the second belt. Therefore, the nip pressure sequentially increases in the free nip portion from the entrance portion to the vicinity of the exit portion in the fixing nip. The nip pressure at the outlet portion is further applied with the pressure by the first outlet side roller 34 and the second outlet side roller 36. For this reason, the nip pressure at the outlet portion is smaller than the nip pressure at the inlet portion. It is very high.

  In the free nip portion in the vicinity of the exit portion in the fixing nip, the first belt 31 and the second belt 32 are pressed against each other with high pressure by the first exit side roller 34 and the second exit side roller 36 in the adjacent exit portion. As a result, the pressure for pressing the first belt 31 and the second belt 32 together is relieved, so the nip pressure is slightly reduced. However, even in the vicinity of the outlet portion, the belt running portion bulging out of the first belt 31 is pressed against the second belt 32, so that the decrease in nip pressure is suppressed and the maximum nip in the free nip portion is suppressed. The nip pressure is maintained at the same level as the pressure.

  As described above, in the fixing nip, the nip pressure gradually increases from the entrance portion to the vicinity of the exit portion. Since the recording sheet S is pressed by such a nip pressure, the peak of the nip pressure is caused on the entrance side. The recording sheet S does not fluctuate on the entrance side as in the configuration having the recording sheet S, and the recording sheet S integrally passes through the free nip portion at a conveyance speed substantially the same as the circumferential movement speed of the first belt 31. be able to. Therefore, there is no possibility that an image shift occurs in the toner image during the passage.

  Thereafter, the recording sheet S passes between the first belt 31 and the second belt 32 which are pressed against each other at a high pressure by the first outlet side roller 34 and the second outlet side roller 36, so that the recording sheet S is in a molten state. The toner image is pressed and fixed on the recording sheet S with high pressure, and is discharged from the fixing nip exit. As a result, the optimum nip pressure can be obtained without using a conventional pressing pad, the belt is not worn by the pad and the life of the belt is extended, and the occurrence of image misalignment is also avoided. Furthermore, a high-quality toner image with good gloss can be obtained.

  In the fixing device 30 of the present embodiment having the nip pressure distribution shown in FIG. 5, it was confirmed that the toner image can be fixed on the recording sheet S without causing image shift. On the other hand, in the conventional fixing device having the nip pressure distribution as shown in FIG. 8, it is confirmed that an image shift of about 0.2 mm may occur in the toner image fixed on the recording sheet S. It was.

  As described above, in the fixing device 30 according to the present exemplary embodiment, there is no possibility that the nip pressure obtained by the pressure contact between the first belt 31 and the second belt 32 is rapidly reduced in the fixing nip. It is possible to reliably prevent the occurrence of image shift in the toner image on the recording sheet S, and to improve the image quality of the fixed image. Further, since the free nip portion of the fixing nip is not provided with a member such as a pad for increasing the nip pressure obtained by the pressure contact between the first belt 31 and the second belt 32, the first belt 31 and the first belt 31 are provided. The driving force for driving the two belts 32 can be reduced. Thereby, there is no possibility that the recording sheet S cannot be conveyed in the fixing nip. In addition, since deterioration such as wear of the first belt 31 and the second belt 32 can be suppressed, the first belt 31 and the second belt 32 can be used stably over a long period of time.

<Second Embodiment>
FIG. 6 is a schematic diagram illustrating a schematic configuration of a fixing device 30 according to another embodiment of the present invention. In the fixing device 30, like the first belt 31, the second belt 32 maintains a cylindrical shape with a circular cross section when not wound around the second inlet side roller 35 and the second outlet side roller 36. It has a predetermined rigidity and is wound around the second inlet side roller 35 and the second outlet side roller 36 by applying a predetermined belt tension, thereby maintaining a cylindrical shape having a substantially elliptical cross section. It has rigidity (flexibility) that can be moved around.

  In the present embodiment, the second belt 32 has the same configuration as the first belt 31 except that the second belt 32 has a lower rigidity than the first belt 31. As described above, since the second belt 32 is lower in rigidity than the first belt 31, the belt running portion bulging outward in the first belt 31 is outward in the second belt 32 in the fixing nip. The belt running portion of the second belt 32 is depressed in a concave shape by being pressed against the bulging belt running portion. Other configurations are the same as those of the embodiment shown in FIG.

  In the fixing device 30 having such a configuration, since the second belt 32 has a predetermined rigidity, the pressure contact force with the first belt 31 is further increased at the free nip portion in the fixing nip, so that the nip pressure is increased. Can be further enhanced. Accordingly, it is possible to suppress the occurrence of pressure loss in which the nip pressure decreases near the exit portion of the fixing nip, and to further improve the image quality of the image fixed on the recording sheet S.

<Third Embodiment>
FIG. 7 is a schematic diagram illustrating a schematic configuration of a fixing device 30 according to another embodiment of the present invention. In the fixing device 30, the belt running portion bulging outward in the first belt 31 is pressed against the belt running portion bulging outward in the second belt 32 in the fixing nip. The rigidity of the second belt 32 is substantially the same as the rigidity of the first belt 31 so that the belt running parts of the first belt 31 and the belt running parts of the second belt 32 are flat. Other configurations are the same as those of the fixing device 30 shown in FIG.

  In the fixing device 30 having such a configuration, the nip pressure at the free nip portion of the fixing nip can be further increased as compared with the case of the fixing device 30 shown in FIG. Further, it can be surely prevented, and the image quality of the image fixed on the recording sheet S can be further improved. In addition, since the recording sheet S is transported along the flat portion between the first belt 31 and the second belt 32, it is possible to more stably convey a kind of sheet that is difficult to bend such as cardboard. Can be transported.

<Modification>
In each of the above embodiments, the vibration suppressing member 40 that suppresses the vibration of the first belt 31 includes the first belt 31 and the first belt 31 by the pair of second belt side rollers 42 and the pair of first belt side rollers 41. The side edges of the two belts 32 on both sides in the width direction are sandwiched. However, the present invention is not limited to such a configuration, and the first belt 32 is guided by a pair of second belt-side rollers 42, whereby The vibration of the belt 31 may be suppressed. In addition, when the first belt 31 has such rigidity that vibration can be suppressed, such a vibration suppression member 40 may not be provided.

  In the above-described embodiment, the heater lamp 37 as the heating unit of the fixing device 30 is provided in the circumferential movement area of the first belt 31 and the second belt 32. The heater lamp 37 may be provided only in the region. Further, other heating means may be provided together with the heater lamp 37 or instead of the heater lamp 37. For example, one or both of the first inlet side roller 33 and the first outlet side roller 34 is formed into a pipe shape, and a heater lamp, a heating wire heater, or the like is disposed in the axial center portion of any one of the core bars. Also good. Similarly, either one or both of the second inlet side roller 35 and the second outlet side roller 36 is formed into a pipe shape, and a heater lamp, a heating wire heater, or the like is disposed in the axial center portion of any of the core bars. May be.

  Note that the heating means is not limited to a halogen heater, a carbon heater, a xenon lamp heater, a heating wire heater, or the like. For example, an induction heating element is provided on the first belt 31 and electromagnetic waves are provided facing the first belt 31. An excitation coil that is an induction heating type heating unit may be provided, and the induction heating element provided on the first belt 31 may be configured to generate heat by electromagnetic induction using the excitation coil.

Further, the first belt 31 is arranged to face the surface of the recording sheet S on which the toner image is fixed. However, the second belt 32 is made to face the surface of the recording sheet S on which the toner image is fixed. It may be arranged.
Further, the image forming apparatus to which the fixing device according to the present invention is applied is not limited to a monochrome image forming apparatus provided with only one developing device, and includes four image forming units along an intermediate transfer belt that moves around. A tandem color digital printer that arranges and forms a full-color image, or four developing devices are arranged around the rotation axis, and these four developing devices are sequentially opposed to the electrostatic latent image carrier to produce a full-color image. A so-called four-cycle image forming apparatus may be used. The configuration of the present invention can be applied not only to a printer but also to a copying machine, a FAX, an MFP (Multiple Function Peripheral), and the like.

  The present invention relates to a fixing device that heats and presses and fixes an unfixed image using a pair of belts, by suppressing a decrease in nip pressure at a fixing nip formed by the belts being pressed against each other. , Preventing the occurrence of image shift.

11 Photosensitive drum 12 Charging device 13 Exposure device 14 Developing device 15 Transfer roller 30 Fixing device 31 First belt 32 Second belt 33 First inlet side roller 34 First outlet side roller 35 Second inlet side roller 36 Second outlet side Roller 37 Heater lamp 40 Vibration suppressing member 41 First belt side roller 42 Second belt side roller

Claims (11)

A fixing nip is formed by press-contacting a first belt wound around a pair of first rollers so as to be able to circulate and a second belt wound around a pair of second rollers so as to be able to circulate, A fixing device that fixes an unfixed image on the recording sheet by thermocompression bonding when the recording sheet passes through the fixing nip,
If the first belt is placed in a state where it is not in pressure contact with the second belt, the first belt has a rigidity that is a curved state in which a belt portion extending between the pair of first rollers bulges outward,
An end portion of the fixing nip on the recording sheet entering side is present at an intermediate portion of each of the first belt and the second belt away from each first roller and each second roller,
On the other hand, the end of the fixing nip on the discharge side of the recording sheet is present at a portion of the first belt and the second belt that is wound around the first roller and the second roller located on the discharge side. A fixing device.   2. The fixing device according to claim 1, wherein if the second belt is placed in a state where it is not in pressure contact with the first belt, a belt portion straddling the pair of second rollers is parallel.   If the second belt is placed in a state where it is not in pressure contact with the first belt, the second belt has a rigidity sufficient to form a curved state in which a belt portion extending between the pair of second rollers bulges outward. The fixing device according to claim 1.   The fixing device according to claim 3, wherein the second belt is recessed in a concave shape when the first belt is pressed.   The fixing device according to claim 3, wherein portions of the first belt and the second belt that are in pressure contact with each other are flat.   6. The vibration suppression unit is provided at the fixing nip portion to suppress vibration of the first belt in a direction in which the first belt contacts and separates from the second belt. 6. The fixing device according to 1.   The said vibration suppression means has a 2nd belt side guide body which guides the internal peripheral surface of a said 2nd belt so that a said 2nd belt may contact | connect a said 1st belt. Fixing device.   The fixing device according to claim 7, wherein the vibration suppressing unit further includes a first belt side guide body that guides an inner peripheral surface of the first belt.   Side edges on both sides of the first belt and the second belt are located on both sides of the recording sheet conveyance area in the fixing nip, respectively, and the first belt side guide body and the second belt side guide body. The fixing device according to claim 8, wherein each of the first and second belts guides the side edge portions of the first belt and the second belt.   The first belt-side guide body and the second belt-side guide body are each a rotating body that rotates following the movement of the first belt and the second belt. Fixing device.   An image forming apparatus comprising the fixing device according to claim 1 as a fixing unit.

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