JP2009166993A - Curl correction method for transfer material, and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To correct a curl of a transfer material generated by fixation while suppressing the disturbance of an image and the contamination of the transfer material in double-sided image formation. <P>SOLUTION: In a curl correction method for the transfer material 8, the curl of the transfer material 8 generated by fixation in a main transfer material conveying passage 14 is conveyed from the main transfer material conveying passage 14 to a conveying passage 18 for double-sided image formation by switchback, and the curl in an opposite direction to the curl of the transfer material 8 is given in the conveying passage 18 for double-sided image formation, and thereby the curl of the transfer material 8 is corrected. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a curling correction method for a transfer material that corrects curling of the transfer material caused by fixing, and an electrophotographic image forming apparatus used in this method, such as a copying machine, a facsimile machine, and a printer.

  2. Description of the Related Art Conventionally, in an image forming apparatus, an image is formed on a transfer material by heat-fixing a toner image transferred to a transfer material such as paper with a fixing device. As described above, when the toner image is heated and fixed by the fixing device, the transfer material after fixing is curled by the nip shape of the pair of rollers of the fixing device. For this reason, when performing double-sided image formation, if the transfer material curled after image formation on one side is transported to the photoconductor again to form an image on the other side, the transfer material is curled and the photoconductor In some cases, the transfer material may not be transported well, such as being wound around.

Therefore, curling is applied to the transfer material immediately after fixing at the time of image formation on one side by a curved transfer material conveyance path guide member so that the transfer material does not wrap around the photosensitive member. There has been proposed an image forming apparatus in which the transfer material is transported satisfactorily (see, for example, Patent Document 1).
JP 2000-206739 A.

  In the image forming apparatus described in Patent Document 1 described above, curling is imparted to the transfer material so that the image surface side is recessed immediately after fixing. Therefore, the transfer material is curled while the toner temperature is high and the toner is soft. For this reason, the image surface and the guide member of the transfer material conveyance path are rubbed strongly, and a rubbing mark (rib stripe) is generated on the image. In addition, since the toner adheres to the guide member, the adhered toner disturbs other image portions or soils the transfer material.

Further, when the transfer material enters the transfer nip portion, the leading edge of the transfer material does not follow the entrance of the transfer nip portion. For this reason, the toner of the transferred toner image is scattered and the image is disturbed. Further, the separation from the transfer nip portion is not good, and the transfer material sticks to the photoreceptor or the transfer belt. For this reason, there may be a case where the transfer material is not always transported satisfactorily in double-sided image formation.
Further, when the transfer material is conveyed from the double-sided image forming conveyance path to the main transfer material conveyance path, the transfer material is placed in a standby state at a curved portion in the double-sided image forming conveyance path for adjusting the timing. By this waiting, curl is imparted to the transfer material by the guide member of the curved portion.

  The present invention has been made in view of such circumstances, and its object is to correct the curling of the transfer material caused by fixing in double-sided image formation while suppressing image disturbance and contamination of the transfer material. It is an object of the present invention to provide a transfer material curl correction method and an image forming apparatus.

  In order to solve the above-described problems, in the transfer material curl correction method and the image forming apparatus according to the present invention, the transfer material curl generated by the fixing in the main transfer material conveyance path is transferred by the double-sided image formation conveyance path. The curling of the transfer material is corrected by imparting the curling in the direction opposite to the curling of the material. Further, the curl of the transfer material is corrected by applying the curl in the opposite direction to the curl of the transfer material with a decurler provided in the conveyance path for double-sided image formation.

In this case, since the conveyance path for double-sided image formation is separated from the fixing device, the curling of the transfer material is corrected with the toner temperature lowered and the toner almost solidified. Thereby, it is possible to suppress the occurrence of rubbing marks (rib stripes) on the toner image of the transfer material. In addition, it is possible to suppress the toner from adhering to the guide member and the decurler of the double-sided image forming conveyance path, and to prevent the toner image from being disturbed and the transfer material from becoming dirty.
In this way, in the double-sided image formation, the curling of the transfer material caused by the fixing can be corrected while suppressing the disturbance of the image and the contamination of the transfer material.

  Further, the transfer material is set in a standby state for the main transfer material conveyance path in a state where the leading end portion of the transfer material is nipped by the decurler. As a result, the front end portion of the transfer material enters the main transfer material conveyance path in a state where the forward curl is corrected to be substantially flat, and further enters the transfer nip portion of the transfer device along the entrance. Therefore, toner scattering at the transfer portion is prevented, and the toner image can be more effectively prevented from being disturbed. In addition, since the front end portion of the transfer material becomes flat, the separation from the transfer nip portion is improved, and the transfer material can be prevented from sticking to the transfer medium or the latent image carrier. In this way, the transfer material can be conveyed more favorably in double-sided image formation.

  Further, while the curled transfer material passes through the linear portion of the double-sided image forming conveyance path, the conveyance speed is controlled at a high speed, and when the curled transfer material passes through the curved portion of the double-sided image forming conveyance path, the conveyance speed is controlled. Controlled at low speed. By transferring the transfer material at a high speed in the linear portion, the transfer material reaches the curved portion with a certain temperature high due to heat from fixing. Thereby, the curl of the transfer material can be corrected more effectively.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1A is a diagram schematically and partially showing an example of an embodiment of an image forming apparatus according to the present invention.

As shown in FIG. 1A, the image forming apparatus 1 includes an endless intermediate transfer belt 2 that is a transfer medium. The transfer medium can also be constituted by a transfer roller. In the following description, it is assumed that the transfer medium is the intermediate transfer belt 2.
The intermediate transfer belt 2 is stretched around a belt driving roller 3 and a driven roller 4 to which a driving force of a motor (not shown) is transmitted. Further, the intermediate transfer belt 2 is given a predetermined tension by a tension roller 5. The intermediate transfer belt 2 is rotatably provided by a belt driving roller 3 in the counterclockwise direction indicated by an arrow α in FIG.

  Although not shown, the image forming apparatus 1 is, for example, yellow (Y), magenta (M), cyan (C), and black (K), as in the image forming apparatus described in JP-A-2005-10610. Image forming stations for each of the four colors. Each image forming station includes at least a latent image carrier that carries an electrostatic latent image and a developing device that develops the electrostatic latent image of the latent image carrier with toner. The image forming stations for each of the four colors are arranged in tandem in an arbitrary color order. Photosensitive members which are latent image carriers of the respective image forming stations are in contact with the intermediate transfer belt 2. Then, the toner images formed at the respective image forming stations are color-superposed on the intermediate transfer belt 2 from the respective photosensitive members and are primarily transferred to form a full-color toner image.

  A secondary transfer device 6 is provided on the belt driving roller 3 side of the intermediate transfer belt 2. The secondary transfer device 6 includes a secondary transfer roller 7. The secondary transfer roller 7 is in pressure contact with the intermediate transfer belt 2 wound around the belt driving roller 3. Then, the transfer material 8 such as paper is conveyed to the secondary transfer device 6 and is sandwiched between the intermediate transfer belt 2 and the secondary transfer roller 7 so that the toner image on the intermediate transfer belt 2 is transferred to the transfer material. 8 is transferred. At this time, the belt driving roller 3 functions as a backup roller against the pressure of the secondary transfer roller 7 at the time of transfer.

  A transfer material cassette 9 for storing the transfer material 8 is provided upstream of the secondary transfer device 6 in the transfer material conveyance direction β. Then, the transfer material 8 in the transfer material cassette 9 is picked up one by one by the pickup roller 10 and conveyed to the secondary transfer device 6.

  On the other hand, a fixing device 11 is provided on the downstream side of the secondary transfer device 6 in the transfer material conveyance direction β. The fixing device 11 includes a heating roller 12 and a pressure roller 13 pressed against the heating roller 12. Then, the transfer material 8 onto which the toner image has been transferred is conveyed to the fixing device 11 and heated and pressed between the heating roller 12 and the pressure roller 13, whereby the toner image is fixed on the transfer material 8. The transfer material 8 on which the toner image has been fixed and image formation has been completed is stored in a waste transfer material tray (not shown).

  Incidentally, the image forming apparatus 1 includes a main transfer material conveyance path 14 that conveys the transfer material from the transfer material cassette 9 to the waste transfer material tray through the secondary transfer device 6 and the fixing device 11. The main transfer material conveyance path 14 is provided with a pair of gate rollers 15 located upstream of the secondary transfer device 6 in the transfer material conveyance direction β. The gate roller pair 15 defines the feeding timing of the transfer material 8 to the secondary transfer device 6 for the toner image on the intermediate transfer belt 2.

  The main transfer material conveyance path 14 is provided with a transfer material feeding roller pair 16 located downstream of the fixing device 11 in the transfer material conveyance direction β. Further, a discharge transfer material roll pair 17 is provided in the main transfer material conveyance path 14 so as to be located downstream of the transfer material feeding roller pair 16 in the transfer material conveyance direction β. The transfer material feed roller pair 16 feeds the transfer material 8 that has been fixed to the waste transfer material roll pair 17, and the waste transfer material roll pair 17 transfers the transfer material 8 that has completed image formation. Is discharged to a waste transfer material tray.

  Further, the image forming apparatus 1 is provided with a double-sided image forming conveyance path 18 for forming images on both sides of the transfer material 8. The double-sided image forming conveyance path 18 is branched from the main transfer material conveyance path 14 in a switchback manner at a branch position 19 in the vicinity of the discharge transfer material roll pair 17. That is, when double-sided image formation is performed on the transfer material 8, the transfer material 8 that has been conveyed along the main transfer material conveyance path 14 stops when the rear end passes through the branch position 19, and then the rear end of the transfer material 8. Becomes the leading edge and is conveyed along the conveyance path 18 for double-sided image formation.

  The double-sided image forming conveyance path 18 is extended substantially linearly as an extension line of the linear main transfer material conveyance path 14 downstream of the discharge transfer material roll pair 17 in the transfer material conveyance direction β, and then relatively large. It is curved and extended with a radius of curvature. Further, the double-sided image forming conveyance path 18 has a linear portion 18 a that is curved and extends substantially parallel to the main transfer material conveyance path 14. Further, the double-sided image forming conveyance path 18 has a curved portion 18b extending in a semicircular arc shape from the linear portion 18a. Then, the curved portion 18 b is joined to the main transfer material conveyance path 14 at the merge position 20 upstream of the gate roller pair 15 in the transfer material conveyance direction β.

  A transfer material feeding roller pair 21 is provided on the linear portion 18 a of the double-sided image forming conveyance path 18. Further, a decurler 24 including a pair of inner and outer rollers 22 and 23 disposed on the outer side and the outer side of the curved portion 18b is provided on the curved portion 18b of the double-sided image forming conveyance path 18, respectively.

  As shown in FIG. 1B, in the fixing device 11 of the image forming apparatus 1 of this example, the heating roller 12 is formed to be harder than the pressure roller 13. For this reason, the heating roller 12 bites into the pressure roller 13 in an arc shape. That is, the nip shape between the heating roller 12 and the pressure roller 13 is a shape that protrudes toward the pressure roller 13. As a result, the fixed transfer material 8 that has passed through the fixing device 11 is curled so that the fixed toner image forming surface side is recessed (hereinafter, in the present invention, the transfer material 8 is fixed as described above). The case where the toner image forming surface is curled so as to be recessed is also referred to as forward curling).

  Further, as shown in FIG. 1C, in the decurler 24 of the image forming apparatus 1 of this example, the outer roller 23 is formed to be harder than the inner roller 22. For this reason, the outer roller 23 bites into the inner roller 22 in an arc shape. That is, the nip shape between the inner roller 22 and the outer roller 23 is a shape that protrudes toward the inner roller 22 side, that is, the curved portion 18b. As a result, the transfer material 8 that has passed through the decurler 24 curls so that the surface side that is the outside of the curved portion 18b is recessed. That is, the decurler 24 gives the transfer material 8 a curl in the reverse direction to the forward curl.

  Therefore, in the image forming apparatus 1 of this example, when image formation is performed on both surfaces of the transfer material 8, first, when an image is first formed on one side surface of the transfer material 8 and fixed by the fixing device 11, Thus, the transfer material 8 curls in order. The forward curled transfer material 8 is transferred from the main transfer material conveying path 14 to the double-sided image forming conveying path 18 through the branching position 19 by the switchback as described above in order to form an image on the other side surface. The transfer material 8 transferred to the double-sided image forming conveyance path 18 reaches the decurler 24. At this time, the forward curled transfer material 8 is conveyed to the decurler 24 with its toner image forming surface facing the inside of the curved portion 18b as shown in FIG.

  The transfer material 8 is on standby with its leading end nipped between the pair of rollers 22 and 23 of the decurler 24 in order to adjust the conveyance timing to the main transfer material conveyance path 14. As a result, the tip of the transfer material 8 is curled so that the surface opposite to the toner image forming surface is recessed. That is, the front end portion of the transfer material 8 curled forward is curled in the opposite direction to the forward curl. As a result, the forward curl of the transfer material 8 caused by the fixing device 11 is corrected, and the transfer material 8 is made almost flat. Further, when the transfer material 8 passes through the decurler 24, the entire forward curl of the transfer material 8 is also corrected, and the entire transfer material 8 is made substantially flat. The substantially flat transfer material 8 passes through the merge position and is conveyed again to the main transfer material conveyance path 14. Then, the feeding timing is defined by the gate roller pair 15 and is conveyed to the secondary transfer device 6. At this time, since the front end portion of the transfer material 8 is substantially flat, the transfer material 8 passes smoothly through the gate roller pair 15 and the secondary transfer device 6 without causing jamming.

  The transfer material 8 conveyed to the secondary transfer device 6 transfers the toner image from the intermediate transfer belt 2 to the other surface side of the transfer material 8 in the same manner as the image formation on the one surface side of the transfer material 8 described above. . Further, the toner image on the other surface side of the transfer material 8 is heated and fixed by the fixing device 11 and stored in the waste transfer material tray.

  According to the transfer material curl correcting method and the image forming apparatus 1 configured as described above, the fixing device is provided on the curved portion 18b in the vicinity of the joining position 20 with the main transfer material conveyance path 14 in the double-sided image forming conveyance path 18. 11 is provided with a decurler 24 for imparting curl to the transfer material 8 in a direction opposite to the forward curl of the transfer material 8 caused by the transfer material 11. Accordingly, the decurler 24 can correct the forward curl of the transfer material 8.

  Since the decurler 24 is provided in the curved portion 18 b of the double-sided image forming conveyance path 18 far from the fixing device 11, the forward curl of the transfer material 8 is corrected by the decurler 24 with the toner temperature lowered and the toner almost solidified. The Thereby, it is possible to suppress the occurrence of rubbing marks (rib stripes) on the toner image of the transfer material. In addition, the toner can be prevented from adhering to the pair of rollers 22 and 23 of the decurler 24. As a result, it is possible to prevent the toner image from being disturbed and the transfer material 8 from being soiled.

  Further, the transfer material 8 is set in a standby state to the main transfer material conveyance path 14 with the leading end portion of the transfer material 8 being nipped by the decurler 24. As a result, the transfer material 8 enters the main transfer material conveyance path 14 in a state in which the forward curl is corrected to be substantially flat and further passes through the gate roller pair 15 and then the transfer nip portion of the secondary transfer device 7. To enter along the entrance. Therefore, toner scattering at the transfer portion is prevented, and the toner image can be more effectively prevented from being disturbed. In addition, since the front end portion of the transfer material becomes flat, the separation from the transfer nip portion is improved, and sticking of the transfer material 8 to the intermediate transfer belt 2 can be prevented. In this way, the transfer material 8 can be conveyed more favorably in double-sided image formation.

2 (a) to 2 (d) are diagrams showing other examples of the embodiment of the present invention.
The image forming apparatus 1 of each example shown in FIGS. 2A to 2C is different from the image forming apparatus 1 of the above example in the fixing device 11 and the decurler 24.
That is, as shown in FIGS. 2A to 2C, the fixing device 11 of the image forming apparatus 1 of these examples is composed of a belt fixing device having a belt 25. In that case, in the example shown in FIG. 2A, the belt 25 is used on the pressure side. In this example, the pressure roller 13 around which the belt 25 is wound is formed to be harder than the heating roller 12. For this reason, the pressure roller 13 bites into the heating roller 12 in an arc shape. That is, the nip shape between the heating roller 12 and the pressure roller 13 is a shape that protrudes toward the heating roller 12. Therefore, the transfer material 8 on which the toner image is fixed by the fixing device 11 is curled so that the surface opposite to the surface on which the toner image is fixed is recessed (hereinafter, in the present invention, the transfer material 8 is thus described. The case of curling so that the surface opposite to the fixed toner image forming surface is concave is also referred to as reverse curling).

  In the example shown in FIG. 2B, a belt 25 is used on the heating side. In this example, the pressure roller 13 is formed to be harder than the heating roller 12 around which the belt 25 is wound. For this reason, the pressure roller 13 bites into the heating roller 12 in an arc shape as in the example shown in FIG. That is, the nip shape between the heating roller 12 and the pressure roller 13 is a shape that protrudes toward the heating roller 12. Therefore, the transfer material 8 on which the toner image is fixed by the fixing device 11 is reversely curled.

  Further, in the example shown in FIG. 2C, the belt 25 is used on the pressure side as in the example shown in FIG. Also in this example, the pressure roller 13 is formed to be harder than the heating roller 12 around which the belt 25 is wound. For this reason, the pressure roller 13 bites into the heating roller 12 in an arc shape as in the example shown in FIG. That is, the nip shape between the heating roller 12 and the pressure roller 13 is a shape that protrudes toward the heating roller 12. Therefore, the transfer material 8 on which the toner image is fixed by the fixing device 11 is reversely curled.

In the case of the image forming apparatus 1 using the fixing device 11 that reversely curls the transfer material 8 after fixing as described above, as shown in FIG. 2D, the curved portion 18b of the double-sided image forming conveyance path 18 is provided. The decurler 24 provided in the inner roller 22 is formed to be harder than the outer roller 23. For this reason, contrary to the above example, the inner roller 22 bites into the outer roller 23 in an arc shape. That is, the nip shape between the inner roller 22 and the outer roller 23 is a shape that protrudes toward the outer roller 23 side. Therefore, similarly to the above-described example, the reverse curled transfer material 8 is curled in a direction opposite to the reverse curl (that is, the forward curl direction) by the decurler 24, and the reverse curl is corrected.
Other configurations and other functions and effects of the image forming apparatus 1 of these examples are the same as those of the above-described example.

  In the image forming apparatus 1 of this example, the reverse curl of the transfer material 8 caused by the fixing can be corrected by controlling the conveyance of the transfer material 8 in the double-sided image forming conveyance path 18. That is, while the reversely curled transfer material 8 passes through the linear portion 18a of the double-sided image forming conveyance path 18, the conveyance speed is controlled at a high speed and also while passing through the curved portion 18b of the double-sided image forming conveyance path 18. The transport speed is controlled to a low speed. In this case, since the curved shape of the curved portion 18b is a concave shape opposite to the reverse curl of the transfer material 8, the transfer material 8 is conveyed at a low speed in the curved portion 18b, so that the transfer material 8 is curved. The guide member is curled in the opposite direction to the reverse curl (that is, the forward curl direction). Thereby, the reverse curl of the transfer material 8 is corrected. In particular, when the transfer material 8 is transported at a high speed by the linear portion 18a, the transfer material 8 reaches the curved portion 18b and the standby position in a state where the temperature of the transfer material 8 is high to some extent by heat due to fixing. Thereby, the reverse curl of the transfer material 8 is more effectively corrected. In this case, the temperature of the transfer material 8 is lower than that in the case where the transfer material is curled immediately after passing through the fixing device as described in Patent Document 1 described above. Therefore, the problems caused by the curling of the transfer material described in Patent Document 1 described above hardly occur.

  Even if the transfer material 8 is conveyed at a low speed by the curved portion 18b as described above, the transfer material 8 is conveyed at a high speed on the linear portion. Compared to the case where the forming conveyance path 18 is conveyed at a constant speed, there is almost no change. Therefore, there is no delay in transport of the transfer material 8 to the main transfer material transport path 14. Thereby, even if the curling portion 18b corrects the reverse curl of the transfer material 8, the time required for forming the double-sided image on the transfer material 8 hardly increases.

  The control of the transfer speed of the transfer material 8 in the double-sided image forming transfer path 18 is performed by a control device of the image forming apparatus. In that case, the control device, when the transfer material 8 starts switchback, the size of the transfer material 8, the distance from the switchback start position to the linear portion 18a, the length of the linear portion 18a, and the curved portion 18b. The conveyance speed of the transfer material 8 is controlled on the basis of the length.

  In this example, the transfer material 8 stands by with its leading end nipped between the pair of rollers 22 and 23 of the decurler 24. However, the transfer material 8 has its leading end joined with the decurler 24. It can also be made to stand by so that it may be located in the curved part 18b between 20. In this case, the reverse curl of the entire transfer material 8 is corrected by the decurler 24, and the reverse curl of the front end portion of the transfer material 8 is corrected by the guide member of the curved portion 18b.

  Further, as shown in FIG. 1A, the portion 14a of the main transfer material conveyance path 14 between the transfer material feed roller pair 16 and the discharge transfer material roll pair 17 is opposite to the reverse curl (that is, the same as the forward curl). Direction). Therefore, in the image forming apparatus 1 having the reverse curling fixing device 11 of this example, the reverse curled transfer material 8 is also corrected by this portion 14a. That is, the image forming apparatus 1 of these examples includes a curl correction unit including a portion 14a for correcting reverse curl on the downstream side in the transfer material conveyance direction β from the fixing device 11. In this case, the curl correction by the portion 14a needs to be curl correction that does not cause a problem in curling described in Patent Document 1 described above.

FIG. 3 is a diagram schematically showing still another example of the image forming apparatus of the present invention.
As shown in FIG. 3, the image forming apparatus 1 in this example is one on the side of the merge position 20 from the decurler 24 in the curved portion 18b of the double-sided image forming conveyance path 18 in the image forming apparatus 1 shown in FIG. The part is formed in the linear part 18b ₁ . The standby position of the transfer material 8 is set so that the tip of the transfer material 8 is positioned at the position A of the linear portion 18b ₁ . Then, when the transfer material 8 waits at this standby position, the forward curl at the tip of the transfer material 8 is corrected by the guide member of the linear portion 18b ₁ .
Other configurations and other functions and effects of the image forming apparatus 1 of this example are the same as those of the example shown in FIG. Incidentally, the linear portion 18b ₁ of the image forming apparatus 1 of this embodiment can be applied to an image forming apparatus 1 cause reverse curl in the example shown in FIGS. 2 (a) through 2 (d).

FIG. 4 is a diagram schematically showing still another example of the image forming apparatus of the present invention.
Although the example shown in FIG. 1 includes the intermediate transfer belt 2, the image forming apparatus 1 of this example does not include the intermediate transfer belt 2 as shown in FIG. 4. That is, in the image forming apparatus 1 of this example, the toner image of the photosensitive member 26 that is a latent image carrier is directly transferred to the transfer material 8. In this case, since the image forming apparatus 1 of this example does not include two transfer devices, the secondary transfer device 6 of the example shown in FIG. 1 is merely a transfer device.

Other configurations and other functions and effects of the image forming apparatus 1 of this example are the same as those of the example shown in FIG. Note that the image forming apparatus 1 of this example that does not include the intermediate transfer belt 2 can also be applied to the image forming apparatus 1 that causes reverse curl of the example shown in FIGS. 2 (a) to 2 (d).
The present invention can be applied to any image forming apparatus within the scope of the matters described in the claims.

(A) is a diagram schematically and partially showing an example of an embodiment of an image forming apparatus according to the present invention, (b) is a partially enlarged view of a fixing device, and (c) is a partially enlarged view of a decurler. is there. (A) thru | or (c) is the elements on larger scale of the fixing apparatus of the other example of embodiment of this invention, respectively, (d) is the elements on larger scale of a decurler. It is a figure which shows typically the further another example of embodiment of this invention. It is a figure which shows typically the further another example of embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Intermediate transfer belt, 6 ... Secondary transfer apparatus (transfer apparatus), 8 ... Transfer material, 11 ... Fixing device, 12 ... Heating roller, 13 ... Pressure roller, 14 ... Main transfer material conveyance Path 14a... Part of main transfer material conveyance path (curl correction part) 18... Double-sided image forming conveyance path 18 a... Linear part 18 b curved part 18 b ₁ linear part 22 inner roller 23 ... outer roller, 24 ... decurler, 26 ... photoconductor

Claims (8)

The transfer material curl generated by fixing in the main transfer material conveyance path is conveyed from the main transfer material conveyance path to the double-sided image formation conveyance path by a switchback,
A curl correction method for a transfer material, wherein the curl of the transfer material is corrected by imparting a curl in a direction opposite to the curl of the transfer material to the transfer material through the double-sided image forming conveyance path. The transfer material curl generated by fixing in the main transfer material conveyance path is conveyed from the main transfer material conveyance path to the double-sided image formation conveyance path by a switchback,
The transfer material is characterized by correcting the curl of the transfer material by applying a curl in a direction opposite to the curl of the transfer material with a decurler provided in the conveyance path for double-sided image formation. Curl correction method.   2. The transfer according to claim 1, wherein the curling of the transfer material is also corrected by imparting a curl in a direction opposite to the curl of the transfer material to the transfer material through the double-sided image forming conveyance path. Curling correction method for materials.   The curling of the front end portion of the transfer material is corrected by placing the transfer material in a standby state with respect to the main transfer material conveyance path while the front end portion of the transfer material is nipped by the decurler. The method for correcting curl of a transfer material according to claim 2 or 3.   During transfer of the transfer material in the double-sided image forming transport path, the transfer material transport speed at a linear portion formed on the branch side of the double-sided image forming transport path with the main transfer material transport path. At a high speed, and the transfer speed of the transfer material at the curved portion formed on the side where the double-sided image forming transfer path meets the main transfer material transfer path is lower than the transfer speed at the linear portion. 5. The curl correction method for a transfer material according to claim 1, wherein the curl of the transfer material is corrected by controlling the curl. A latent image carrier that carries an electrostatic latent image, a developing device that develops the electrostatic latent image of the latent image carrier with toner, and a first that transfers a toner image on the latent image carrier to a transfer medium. A transfer device; a second transfer device that transfers a toner image on the transfer medium onto a transfer material; a fixing device that fixes the toner image transferred onto the transfer material; and the transfer material as a second transfer device; A main transfer material conveyance path for conveying to the fixing device; and the main transfer material conveyance path downstream from the fixing device in the transfer material conveyance direction and branched from the main transfer material conveyance path to the transfer material conveyance direction upstream from the second transfer device. At least a double-sided image forming conveyance path that joins a transfer material conveyance path and the transfer material is conveyed from the main transfer material conveyance path by a switchback,
An image forming apparatus comprising: a curl correction unit configured to correct curl of the transfer material caused by fixing by the fixing device in the double-sided image forming conveyance path. A latent image carrier that carries an electrostatic latent image, a developing device that develops the electrostatic latent image of the latent image carrier with toner, and a transfer device that transfers the toner image on the latent image carrier to a transfer material, A fixing device that fixes the toner image transferred onto the transfer material, a main transfer material conveyance path that conveys the transfer material to the transfer device and the fixing device, and the main transfer material downstream in the transfer material conveyance direction from the fixing device. For double-sided image formation that branches from the transfer material conveyance path, joins the main transfer material conveyance path upstream of the transfer device in the transfer material conveyance direction, and the transfer material is conveyed by switchback from the main transfer material conveyance path And at least a conveyance path,
An image forming apparatus comprising: a curl correction unit configured to correct curl of the transfer material caused by fixing by the fixing device in the double-sided image forming conveyance path.   The curl correction unit is at least one of a decurler that corrects curl of the transfer material and a curved part that is formed in the double-sided image forming conveyance path and corrects curl of the transfer material. 8. The image forming apparatus according to 6 or 7.

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