JP2013020106A - Fixing device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fixing device having a configuration which prevents conveyance failure of a transfer material smaller in size than a transfer material with such a size that it can be held at the front edge by paper discharge rollers in an existing configuration and prevents deterioration of a toner image carried by the transfer material.SOLUTION: A fixing device 6 is constituted by a heat generating member 62 and a pressure member 63 which presses the heat generating member 62. It comprises a first pair of conveyance rollers 7 which are disposed on the downstream side from a fixing nip part N constituted by the heat generating member 62 and the pressure member 63 in a paper conveyance path and a second pair of conveyance rollers 64 which are disposed between the fixing nip part N and the first pair of conveyance rollers 7 and are contactable with and separable from each other. The second pair of conveyance rollers 64 is arranged in such a manner that they are contactable with and separable from each other via the paper conveyance path between them, and allows selection of a position in which they hold and convey a sheet S having passed the fixing nip part N or a position in which they release the holding and conveyance of the sheet.

Description

  The present invention relates to a fixing device and an image forming apparatus, and more particularly, to a conveyance mechanism for a sheet to be fixed.

As is well known, in an image forming apparatus using an electrophotographic system, a toner image transferred from an image carrier such as a photoconductor to a transfer paper is fixed by a melting / penetrating action by heat / pressure to obtain a copy. It is like that.
In order to improve the temperature rise, apart from the heat roller fixing method in which the fixing roller having a built-in heat source and the pressure roller for pressing the transfer paper are brought into abutment with each other, the apparatus used for fixing is configured. There is a configuration using a belt fixing system using a belt having a small heat capacity.

  In the configuration using the latter belt fixing method, the fixing roller facing the toner image on the transfer paper and the belt mounted between the heating roller provided at a position different from the fixing roller are opposed. A configuration combining a pressure roller that abuts has been proposed (for example, Patent Document 1).

FIG. 5 is a schematic diagram for explaining the above configuration.
In the figure, the fixing device A is provided with a fixing roller B on one side and a pressure roller C on the other side across the conveyance path of the transfer paper S.
A fixing belt E is wound around the fixing roller B between the fixing roller B and a heating roller D disposed at a position different from the fixing roller B. The fixing belt E is applied with tension by the tension roller F. It can be moved in conjunction with the rotation of the fixing roller B.

  A heater D1 is disposed inside the heating roller D, and the temperature of the fixing belt E heated by the heater C1 is controlled by a thermistor G in contact with the belt surface.

  Each of these rollers is disposed in the casing A1 of the fixing device A, and the transfer sheet S is heated when passing through the fixing nip formed at a position where the fixing roller B and the pressure roller C are in contact with each other. Under pressure, the toner image is fixed.

  After the fixing, the transfer sheet S is discharged with its discharge direction regulated by the separation claw H disposed behind the fixing nip portion.

  In the fixing device having the above-described configuration, the transfer paper that has passed through the fixing nip portion is discharged onto the paper discharge tray via the paper discharge roller disposed in front of the paper discharge tray. There is.

When the transfer paper size, especially the length in the transport direction, is shorter than the distance between the paper discharge roller and the fixing nip, the paper discharge roller In some cases, the front end in the transport direction is not pinched, and this causes a situation in which the transfer paper hits the front end against the transport guide instead of the paper discharge roller. For this reason, transfer paper cannot be transported, and transport failure occurs.
Thus, although it is conceivable to provide another pair of conveying rollers in the vicinity of the fixing nip (for example, Patent Document 2), this configuration causes the following new problem.

In other words, the transfer paper immediately after passing through the fixing nip may be rubbed with the toner image when it comes into contact with the paper discharge roller when the toner image is not cooled and fixed.
In particular, since the toner image immediately after passing through the fixing nip portion is not sufficiently cooled and cured, if the toner image contacts the roller that is always in contact in this state, the toner image may be rubbed into a defective image. There is.

  In view of the problems in the conventional fixing device, the object of the present invention is to convey a transfer material of a size smaller than that of the transfer material that can be pinched by the discharge roller of the existing configuration. An object of the present invention is to provide a fixing device and an image forming apparatus having a configuration capable of preventing defects and preventing deterioration of a toner image carried on a transfer material.

In order to achieve this object, the present invention has the following configuration.
(1) In a fixing device including a heat generating member and a pressure member that pressurizes the heat generating member,
A first conveying roller pair disposed on the downstream side of the sheet conveying path from the fixing nip formed by the heat generating member and the pressure member;
A second conveying roller pair provided between the fixing nip portion and the first conveying roller pair and capable of contacting and separating from each other;
The second conveying roller pair is provided so as to be able to contact and separate with respect to the sheet conveying path, and is configured to nipping and conveying the sheet that has passed through the fixing nip portion, and to be configured to release the nipping and conveying of the sheet. A fixing device.

(2) The second conveying roller pair is disposed downstream of the fixing nip portion in the sheet conveying direction in the casing in which the heat generating member and the pressure member are disposed. (1) The fixing device according to (1).

(3) The heat generating member includes an endless belt member having flexibility, a support member that rotatably supports the belt member in contact with the pressure member, and the support member in the support member. A nip forming member that is provided at a position facing the pressure member and forms the fixing nip portion by pressurization of the pressure member, and a heating member of the fixing belt are provided. The fixing device according to (1) or (2).

(4) The distance between the center of the second conveyance roller pair and the center of the fixing nip is set to 50 to 150 mm along the conveyance direction of the sheet. ) Fixing device described in the above.

(5) A roller facing the image surface side of the sheet of the second pair of conveying rollers has a surface layer coated with a fluororesin (1), (2), (4) The fixing device according to any one of the above.

(6) The fixing device according to (5), wherein the fluororesin is PFA or PTFE.

(7) Of the second conveying roller pair, at least a roller facing the image surface side of the sheet is provided so as to be able to contact and separate from the facing roller. (5) The fixing device according to one of (5).

(8) In the second conveying roller pair, a roller facing the non-image surface of the sheet is used as a driving roller, and is described in one of (1) to (5), Fixing device.

(9) A hollow pipe is used as the roller facing the image surface of the paper in the second pair of conveying rollers, and a cooling medium is circulated along the axial direction of the hollow pipe. The fixing device according to any one of (1) to (5) and (7).

(10) Of the second pair of transport rollers, the roller facing the non-image surface of the paper is made of an elastic body. (1) to (5) and (8) The fixing device according to one.

(11) An image forming apparatus using the fixing device according to any one of (1) to (10).

  According to the present invention, a second transport roller pair is provided on the downstream side of the fixing nip portion in the paper transport direction separately from the first transport roller pair capable of transporting the paper that has passed through the fixing nip portion. The roller pair can select a state for nipping and conveying the paper and a state for releasing the nipping and conveying. For this reason, when transporting a small-size sheet whose front end in the transport direction is not nipped by the first transport roller pair when the nipping and transport of the rear end in the transport direction of the sheet is released at the fixing nip portion, the second transport Nipping and conveying by a roller pair can be performed. Accordingly, it is possible to prevent a conveyance failure when a small-size sheet that is difficult to be nipped and conveyed by the first conveyance roller pair is used.

  In particular, since the second conveyance roller pair is provided in the casing where the fixing nip portion is located, the position of the fixing nip portion and the second conveyance roller pair is different from the case where the second conveyance roller pair is provided in a position other than the casing. Even when the relationship can be set accurately, nipping and conveying of the leading edge of the sheet at the time of conveying a small-sized sheet can be performed reliably.

  Further, according to the present invention, the roller facing the image surface of the paper in the second conveying roller pair is provided with a configuration that can form a flow path for the cooling medium, so that the roller immediately after passing through the fixing nip portion. It is possible to prevent the occurrence of defective images by accelerating the cooling and curing of the image and preventing the image from being rubbed when the roller rubs.

FIG. 2 is a schematic diagram for explaining a configuration of an image forming apparatus using a fixing device according to the present invention. FIG. 3 is a schematic diagram for explaining an embodiment of a fixing device according to the present invention. FIG. 3 is a view corresponding to FIG. 2 for explaining the characteristics of the main configuration of the fixing device shown in FIG. 2. FIG. 4 is a perspective view for explaining another feature of the main part configuration shown in FIG. 3. FIG. 10 is a schematic diagram for explaining a conventional example relating to a conveyance path of a fixing device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments for carrying out the present invention according to embodiments shown in the drawings will be described below.
FIG. 1 is a schematic diagram showing an image forming apparatus using a fixing device according to the present invention.
The image forming apparatus shown in the figure is a tandem type printer in which image forming portions capable of forming a plurality of color images are arranged along the belt's stretched surface used as an intermediate transfer member. However, the present invention is not limited to this. It is also possible to target a copying machine, a facsimile machine, or a printing machine.

Hereinafter, the configuration of the image forming apparatus will be described.
In FIG. 1, an image forming apparatus 100 includes photosensitive drums 20Y, 20C, and 20M as image carriers capable of forming images as images corresponding to colors separated into yellow, magenta, cyan, and black, respectively. , 20BK is used in parallel.
In the image forming apparatus 100 having the configuration shown in FIG. 1, the visible images formed on the photosensitive drums 20Y, 20C, 20M, and 20BK face the photosensitive drums 20Y, 20C, 20M, and 20BK, and are in the direction of arrow A1. A primary transfer process is performed on an intermediate transfer member 11 (hereinafter referred to as a transfer belt) 11 that uses a movable endless belt, and each image is superimposed and transferred to a sheet S on which a recording sheet or the like is used. On the other hand, batch transfer is performed by executing a secondary transfer process.

  Around each photosensitive drum, an apparatus for performing image forming processing is arranged according to the rotation of the photosensitive drum. Now, the photosensitive drum 20BK that performs black image formation will be described as an object. A charging device 30BK, a developing device 40BK, a primary transfer roller 12BK, and a cleaning device 50BK that perform image forming processing along the rotation direction are arranged. As described later, the optical scanning device 8 is used for writing performed after charging.

  In the superimposing transfer to the transfer belt 11, the visible image formed on each photoconductive drum 20 </ b> Y, 20 </ b> C, 20 </ b> M, 20 </ b> BK is transferred to the same position on the transfer belt 11 while the transfer belt 11 moves in the A <b> 1 direction. As described above, voltage application by the primary transfer rollers 12Y, 12C, 12M, and 12BK arranged to face the photosensitive drums 20Y, 20C, 20M, and 20BK across the transfer belt 11 causes the upstream side in the A1 direction. The timing is shifted toward the downstream side.

  The photosensitive drums 20Y, 20C, 20M, and 20BK are arranged in this order from the upstream side in the A1 direction. Each of the photosensitive drums 20Y, 20C, 20M, and 20BK is provided in an image station for forming yellow, cyan, magenta, and black images.

  The image forming apparatus 100 is disposed so as to face four image stations that perform image forming processing for each color and above each of the photosensitive drums 20Y, 20C, 20M, and 20BK, and the transfer belt 11 and the primary transfer roller 12Y. , 12C, 12M, and 12BK, a secondary transfer roller 5 that is a transfer roller as a transfer member that is arranged to face the transfer belt 11 and is driven by the transfer belt 11 and rotated. An intermediate transfer belt cleaning device 13 disposed opposite to the transfer belt 11 for cleaning the transfer belt 11, and an optical scanning device 8 serving as an optical writing device disposed below these four image stations. have.

  The optical scanning device 8 in this embodiment is equipped with a semiconductor laser as a light source, a coupling lens, an fθ lens, a toroidal lens, a mirror, a rotary polygon mirror, and the like, and each of the photosensitive drums 20Y, 20C, 20M, and 20BK. On the other hand, writing light Lb corresponding to each color (in FIG. 1, for convenience, the reference is applied only to the image station of the black image, but the same applies to the other image stations), and the photosensitive drum 20Y is emitted. , 20C, 20M, and 20BK, an electrostatic latent image is formed.

  The image forming apparatus 100 includes a sheet feeding device 61 equipped with a sheet feeding cassette 61A loaded with sheets S conveyed toward the photosensitive drums 20Y, 20C, 20M, and 20BK and the transfer belt 11, and a sheet feeding The sheet S conveyed from the apparatus 61 is directed to a transfer portion between the photosensitive drums 20Y, 20C, 20M, and 20BK and the transfer belt 11 at a predetermined timing in accordance with the toner image formation timing by the image station. A registration roller pair 4 that is fed out and a sensor (not shown) that detects that the leading edge of the sheet S has reached the registration roller pair 4 are provided.

  The image forming apparatus 100 includes a fixing device 6 as a roller fixing type fixing unit for fixing the toner image on the paper S to which the toner image is transferred, and the fixed paper S outside the main body of the image forming apparatus 100. A discharge roller 7 that discharges, a discharge tray 17 that is disposed at the top of the main body of the image forming apparatus 100 and stacks the paper S discharged to the outside of the main body of the image forming apparatus 100 by the discharge roller 7, There are provided toner bottles 9Y, 9C, 9M, and 9BK that are located on the lower side and are filled with toners of yellow, cyan, magenta, and black.

The transfer belt unit 10 includes a drive roller 72 and a driven roller 73 around which the transfer belt 11 is wound, in addition to the transfer belt 11 and the primary transfer rollers 12Y, 12C, 12M, and 12BK.
The driven roller 73 also has a function as a tension urging unit for the transfer belt 11. For this reason, the driven roller 73 is provided with an urging unit using a spring or the like. Such a transfer belt unit 10, the primary transfer rollers 12Y, 12C, 12M, and 12BK, the secondary transfer roller 5, and the cleaning device 13 constitute a transfer device 71.

The paper feeding device 61 includes a paper feeding cassette 61A that is detachably disposed in a space portion provided at the lower part of the main body of the image forming apparatus 100. The paper feeding device 61 serves as a feeding means that comes into contact with the upper surface of the uppermost paper S. A feed roller 3 is arranged.
The feeding roller 3 is rotationally driven in the counterclockwise direction to feed the uppermost sheet S toward the registration roller pair 4 in cooperation with a friction separator that faces the feeding roller 3. Yes.

As shown in detail in FIG. 2, the fixing device 6 includes a heat generating member 62 and a pressure roller 63 that presses the heat generating member 62.
The heat generating member 62 includes a flexible endless fixing belt 62A, a support member 62B that rotatably supports the fixing belt 62A in contact with the pressure roller 63, and a heating member 62B. A heat source comprising a nip forming member 62C that is provided at a position facing the pressure roller 63 and forms a fixing nip portion N by the pressure of the pressure roller 63, and a halogen heater disposed inside the rotation locus of the fixing belt 62A 62D.
The pressure roller 63 has a structure in which a silicone rubber layer is laminated on the surface of a hollow metal roller, and a fluororesin layer such as PFA or PTFE is provided on the surface layer as a release layer. The pressure roller 63 is a roller that is rotationally driven by a drive motor (not shown), and further uses an elastic body such as a spring (not shown) to be pressed toward the nip forming member 62C as an urging means. I have.

When the fixing device 6 is driven to rotate while being pressed and biased toward the nip forming member 62 </ b> C, the fixing nip portion N is pressed by the pressure roller 63, so that the fixing belt 62 </ b> A rotates in conjunction with it. And can move to the fixing nip N while being heated by the heat source 62D.
The sheet S entering the fixing nip portion N by the rotation of the fixing belt 62A receives a heat path pressure while being nipped and conveyed between the fixing belt 62A and the pressure roller 63 in the fixing nip portion N, and a toner image is formed. It is fixed.

  In the configuration shown in FIG. 2, the nip forming member 62 </ b> C is pressed by the pressure roller 63 to be recessed in the nip forming member 62 </ b> C side, and the recessed portion forms the fixing nip portion N. When the paper S passes through this portion, the toner image that is carried by the paper S being easily subjected to heat and pressure can be fixed on the surface of the paper S.

  In the configuration shown in FIG. 2, since the fixing nip portion N is recessed toward the nip portion forming member 62 </ b> C, the peripheral surface of the pressure roller 63 is not deformed. The tip can be easily moved along the peripheral surface of the pressure roller 63. Accordingly, the sheet S moving in the tangential direction on the circumferential surface of the pressure roller 63 enables curvature separation using the circumferential curvature of the pressure roller 63.

  On the other hand, the cleaning device 13 provided in the transfer device 71 has a cleaning brush and a cleaning blade disposed so as to face and abut the transfer belt 11, although detailed illustration is omitted. The transfer belt 11 is cleaned by scraping and removing foreign matters such as residual toner on the belt 11 with a cleaning brush and a cleaning blade. The cleaning device 13 also has a discharge means (not shown) for carrying out and discarding the residual toner removed from the transfer belt 11. In the image forming apparatus 100 having the configuration shown in FIG. 1, the images formed on the photosensitive drums are sequentially transferred onto the transfer belt 11 so that the color images are superimposed on each other by the secondary transfer roller 5. However, instead of this, a sheet S is carried on the transfer belt 11, and the sheet S is opposed to each photosensitive drum so that images of each color are directly superimposed on the sheet S. It is also possible to do.

The features of the present invention will be described below for the image forming apparatus 100 having the above-described configuration.
A feature of the present invention is that a small-size sheet can be reliably conveyed in a sheet conveying mechanism in the fixing device.

In FIG. 2, when the paper discharge roller 7 is a first discharge roller pair, the fixing device 6 includes a second conveyance roller pair 64 between the paper discharge roller 7 and the fixing nip N. Is disposed in the housing 6A.
The second conveyance roller pair 64 is located downstream of the fixing nip portion N in the conveyance direction of the paper S in the housing 6A, and is provided so as to be able to contact and separate using a configuration described later.

  The center of the second conveyance roller pair 64 is set to 50 to 150 mm from the center of the fixing nip N along the conveyance direction of the sheet S (a distance indicated by a symbol L in FIG. 2). The reason for this is that when the trailing edge in the conveyance direction of a small size sheet having a conveyance direction length of around 100 mm is separated from the fixing nip N, the leading edge of the sheet is pinched by the discharge roller 7 which is the first conveyance roller pair. This is to prevent the absence.

  Since the second transport roller pair 64 is provided in the housing 6A located together with the fixing nip portion N, the second transport roller pair 64 can be positioned with respect to the fixing nip portion N in the same housing. Unlike the case where it is provided at a position other than the body 6A, the positional relationship between the fixing nip portion N and the second conveying roller pair 64 is accurately, in other words, the position error is stored in the housing. The accuracy of the relationship can be ensured.

On the other hand, as shown in FIG. 3, the second conveying roller pair 44 is a roller 64 </ b> B opposite to the roller 64 </ b> A on the side facing the image surface of the sheet S across the conveyance path of the sheet S, in other words, the sheet S. The roller 64 </ b> B facing the non-image surface is contacted and separated.
As a configuration for contact and separation, a configuration in which the outer peripheral surface of the eccentric cam 65 is brought into contact with the rotation shaft 64A1 of the roller 64A on the side facing the image surface of the paper S is used. In order to change the position of the rotation shaft 64A1 according to the cam profile of the eccentric cam 65, the rotation shaft 64A1 is provided with pressing means using a spring or the like (not shown).
As shown in FIG. 3, the drive position of the eccentric cam 65 is executed by rotational phase control of the eccentric cam 65 using the drive motor M via the control unit 66.

  An operation panel 67 capable of specifying a paper type is connected to the control unit 66, and when an image formation command for a small-size paper S is input according to the selected paper type, The rotational phase of the eccentric cam 65 capable of bringing the roller 64A facing the image surface side of the sheet S into contact with the facing roller 64B is set by the rotation amount of the drive motor M.

  Regarding the contact / separation operation described above, the rollers of the second transport roller pair are in contact with each other only for small-size paper transport. Therefore, the rotation phase of the eccentric cam 65 is controlled by the controller 66 so that the roller 64A is separated from the opposing roller 64B. Moreover, it is also possible to make both the rollers abut and separate from each other without using only the roller 64A as a contact / separation target. With such a configuration, the operation time when leaving the paper conveyance path can be halved compared to the case where one roller is the target.

  Since the present embodiment is configured as described above, when it is input on the operation panel 67 that the image formation is intended for the small-sized sheet S, the controller 66 detects the roller 64A in the second roller pair 64. The rotational phase of the eccentric cam 65 is set so as to contact the roller 64B facing each other.

As a result, the second conveyance roller pair 64 can nipping and conveying the leading edge of the sheet S conveyed from the fixing nip portion N, so that the conveyance direction deviates until the leading edge of the sheet reaches the discharge roller 7. Or a conveyance failure caused by abutting against the conveyance guide is prevented.
In this way, the second roller pair 64 is brought into contact with each other for a small-sized sheet so that the sheet can be nipped and conveyed, thereby preventing the above-described conveyance failure and using a general-purpose size other than the small size. When transporting a certain A4 size or letter size, the rollers of the second roller pair 64 are separated from each other, thereby avoiding contact with the toner image that has passed through the fixing nip portion, and rubbing the toner image that is not sufficiently cooled and cured. You can prevent it from happening.
The structural characteristics used for the second transport roller 64 used in this embodiment will be described later.

Next, another embodiment of the main configuration of the present invention will be described.
The present embodiment is characterized in that a toner cooling mechanism is provided in the second conveying roller pair 64.
FIG. 4 is a perspective view showing the configuration of the second conveying roller pair 64. In FIG. 4, the roller 64A on the side facing the face drawing of the sheet S in the second conveying roller pair 64 is SUS or SUM or the like. A hollow metal pipe is used. Thereby, the heat capacity of the roller is reduced.
Then, as shown by the arrows, a cooling medium such as air is circulated through the roller 64A along the axial direction.

The cooling medium is caused to flow in the axial direction over the entire outer peripheral surface of the roller 64A using a fan or the like not shown, or is circulated not only outside but also inside the roller 64A.
Thus, when the sheet S immediately after passing through the fixing nip N comes into contact with the roller 64A, the toner fixed on the sheet S is efficiently cooled and cured. Therefore, image degradation due to rubbing of the image surface can be suppressed.

In the above embodiment, the roller 64A facing the image surface of the paper S among the rollers of the second roller pair has the following configuration.
As described above, SUS or SUM or the like is used as the core metal of the roller, and the surface thereof is covered with a 10-200 μm PFA tube. As long as the releasability is good, the covering portion may be PTFE, and is not limited to the covering, and may be configured by painting.

The pressure roller 63 is provided with silicone rubber. However, as shown in FIG. 4, it is also possible to provide a plurality of pieces with arbitrary axial lengths. The following conditions are adopted.
In general, ethylene propylene rubber and polyurethane rubber are used in image forming apparatuses. Ethylene propylene rubber is excellent in heat resistance but inferior in oil resistance, and polyurethane rubber is excellent in oil resistance but inferior in heat resistance. The advantages and disadvantages must be selected in consideration of the installation position from the fixing heat source, the internal temperature of the fixing set temperature, or the amount of wax contained in the toner used.

  Further, the fixing belt 64A is configured as an endless belt (or film) in which a metal material such as nickel or SUS or a resin material such as polyimide is used for the belt base material. On those substrates, in the case of induction heating, it has a heat generating layer such as copper or silver. Further, the surface layer of the belt has a release layer such as a PFA or PTFE layer, and has a release property so that toner does not adhere. There may be an elastic layer formed of a silicone rubber layer or the like between the belt substrate and the PFA or PTFE layer. When there is no silicone rubber layer, the heat capacity is reduced and the fixability is improved, but when the unfixed image is crushed and fixed, the subtle irregularities on the belt surface are transferred to the image, and the solid part of the image is crusty. There is a problem that marks remain. In order to improve this, it is necessary to provide a silicone rubber layer of 100 μm or more. Due to the deformation of the silicone rubber layer, subtle irregularities are absorbed and the skin image is improved.

Note that the pressure roller 63 facing the non-image surface of the paper S is a driving roller in this embodiment, and the toner carried on the paper is rubbed even when slippage occurs with the paper. It is supposed not to be done.
As this drive system, a transmission structure from either the drive source of the transfer belt 11 or the main body side other than the fixing device 6 is used.
In general, since the process linear velocity of the fixing device is different due to thermal expansion of the belt member and the pressure member immediately after the start-up of the device and during continuous operation, the driving of the conveying roller pair is taken from the main body side which is not affected by them. preferable.

6 fixing device 6A housing 62 heating member 62A fixing belt 62B support member 62C nip forming member 62D heat source 63 pressure roller 64 second roller pair 64A roller 64A1 rotating shaft 64B non-image surface of paper Opposing roller 65 Eccentric cam 66 Control unit 67 Operation panel 100 Image forming apparatus M Drive motor

JP 2009-104001 A JP 2010-128300 A

Claims (11)

In a fixing device constituted by a heat generating member and a pressure member that pressurizes the heat generating member,
A first conveying roller pair disposed on the downstream side of the sheet conveying path from the fixing nip formed by the heat generating member and the pressure member;
A second conveying roller pair provided between the fixing nip portion and the first conveying roller pair and capable of contacting and separating from each other;
The second conveying roller pair is provided so as to be able to contact and separate with respect to the sheet conveying path, and is configured to nipping and conveying the sheet that has passed through the fixing nip portion, and to be configured to release the nipping and conveying of the sheet. A fixing device.   2. The second conveying roller pair is disposed on the downstream side of the fixing nip portion in the sheet conveying direction in a casing in which the heat generating member and the pressure member are disposed. The fixing device described.   The heat generating member includes a flexible endless belt member, a support member that rotatably supports the belt member in contact with the pressure member, and the pressure member in the support member. 2. A nip forming member that is provided at a position opposite to the pressure member and forms the fixing nip portion by pressurization of the pressure member, and a heating member of the fixing belt. Or the fixing device according to 2.   3. The fixing device according to claim 1, wherein the distance between the center of the second conveyance roller pair and the center of the fixing nip is set to 50 to 150 mm along the conveyance direction of the sheet. .   5. The roller of the second conveying roller pair facing the image surface side of the sheet is coated with a fluororesin on a surface layer thereof. Fixing device.   The fixing device according to claim 5, wherein PFA or PTFE is used as the fluororesin.   6. Of the second conveying roller pair, at least a roller facing the image surface side of the sheet is provided so as to be able to contact and separate from the facing roller. The fixing device according to one of the above.   6. The fixing device according to claim 1, wherein a roller facing the non-image surface of the sheet in the second pair of conveying rollers is used as a driving roller.   A hollow pipe is used as a roller facing the image surface of the sheet in the second pair of conveying rollers, and a cooling medium is circulated along an axial direction of the hollow pipe. The fixing device according to any one of 1 to 5 and 7.   9. The fixing device according to claim 1, wherein an elastic body is used for a roller facing the non-image surface of the sheet in the second pair of conveying rollers. 10. .   An image forming apparatus using the fixing device according to claim 1.

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