JP2000351511A - Device and method for forming image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a device and a method to form an image, capable of forming images on the two surfaces of a sheet through simple constitution. SOLUTION: A paper discharge part 64 comprises a second paper discharge route 641a extending from a fixing unit 5 to a standard paper discharge tray 11, and a first paper discharge route 641b extending between a paper refeed part 66 and a multibin unit and approximately in parallel to the paper discharge route 641a. Drive rollers 643c and 644c are rotatable forward and reversed and driven rollers 643a and 644a are respectively disposed on the paper discharge route 641a side and driven rollers 643b and 644b are respectively disposed on the paper discharge route 641b side. The sheet S is conveyable in directions reverse to each other in both the paper discharge routes 641a, 641b according to the rotation directions of the drive rollers 643c and 644c.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an image on both main surfaces of a copy sheet, a transfer sheet, a sheet on which an image is formed, and a sheet such as an OHP transparent sheet, and discharging the image to a discharge tray and a finisher. And an image forming method.

[0002]

2. Description of the Related Art Many image forming apparatuses, such as printers, facsimile machines, and copiers, having a configuration capable of forming an image on both main surfaces of a sheet have been conventionally provided. In this specification, an image forming mode in which an image is formed on both main surfaces of a sheet in this manner is referred to as a “double-sided mode”.

As an image forming apparatus capable of executing the double-sided mode, there is, for example, an apparatus described in Japanese Patent Application Laid-Open No. H10-166695. In this image forming apparatus, an intermediate tray for temporarily storing a sheet having an image formed on one side (hereinafter, referred to as a “single-sided image sheet”) is provided to enable image formation on both main surfaces of the sheet. I have. And
The single-sided image sheet is conveyed to an intermediate tray via an intermediate conveyance path by a reversing device disposed downstream of the fixing device,
Keep temporarily. Then, the sheet is taken out from the intermediate tray at an appropriate timing, and after turning over, an image is formed on the other main surface of the sheet.

In this image forming apparatus, a non-sort bin (corresponding to a discharge tray of the present invention) and a sorting bin (corresponding to a finisher of the present invention) are provided as sheet discharge destinations. The sheet on which images are formed on both sides as described above (hereinafter, referred to as a “double-sided image sheet”) is stored in a non-sort bin via a non-sort carry-in path and a sort bin via a sort carry-in path. Are selectively discharged.

[0005]

As described above, in the conventional image forming apparatus, the reversing device, the non-sorting carry-in path, and the sort carry-in path have only independent single functions. That is, the reversing device performs a function of reversing the single-sided image sheet and transporting the sheet to the intermediate tray side, the non-sorting carry-in path serves to discharge the sheet on which the image has been formed to the non-sorting bin, It merely serves to discharge the formed sheets to the sorting bin. Therefore, the number of components constituting the image forming apparatus is large, and this is one of the main factors of increase in the cost of the apparatus and complexity of the configuration.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides an image forming apparatus and an image forming method capable of forming an image on both main surfaces of a sheet with a simple configuration at a low cost. Aim.

[0007]

According to the present invention, there is provided an image forming means for forming an image on a sheet, a finisher for performing predetermined post-processing on the sheet on which the image is formed by the image forming means, and Means for receiving sheets on which images have been formed by the means, sheet feeding means for feeding sheets to the image forming means, and feeding the sheets on which one image has been formed on one main surface by the image forming means. A sheet re-feeding unit for re-feeding the sheet to a paper unit, and selectively discharging a sheet conveyed from the image forming unit to the discharge tray or the finisher, and opposing the discharge tray and the finisher. And a discharge unit that guides the sheet to the sheet re-feeding unit and forms an image on both main surfaces of the sheet, and then selects one of the discharge tray and the finisher. In order to achieve the above object, an image forming apparatus configured to discharge a sheet to a finisher and a second discharge path to guide a sheet to the finisher. A paper path, a switching means for selectively switching a sheet conveyance path to a first or second paper discharge path, and directing a sheet toward the finisher along the first paper discharge path on the first paper discharge path side. Transport means for transporting the sheet to the discharge tray along the second discharge path on the second discharge path side while reversely transporting the sheet, and reversely transporting the sheet. It consists of.

According to the present invention, the sheet conveyed to the first sheet discharge path is reversely conveyed to the re-feeding means in the first sheet discharge path, and the sheet conveyed to the second sheet discharge path is the second sheet. The sheet is reversed and conveyed to the sheet re-feeding means in the sheet discharge path. As described above, the switchback operation of the sheet is performed in the first and second discharge paths, and the first and second discharge paths also function as the switchback paths. For this reason, the throughput is improved by setting an optimal sheet conveyance sequence.

The number of paper discharge paths in the present invention is not limited to two, and a similar effect can be obtained when two or more paper discharge paths are provided. That is, a plurality of paper discharge paths are provided in one-to-one correspondence with the plurality of paper discharge positions, and the sheet is reversely conveyed along each paper discharge path toward the re-feeding unit. The switchback operation of the sheet can be performed in each sheet discharging path. Further, when a plurality of paper discharge paths function as switchback paths as described above, the throughput is improved by forming an optimal sheet conveyance sequence.

Here, the conveying means may be constituted as follows. That is, a first drive roller that is rotatable in the forward and reverse directions is disposed at a position sandwiched between the first and second paper discharge paths, and the first and second paper discharge paths are sandwiched between the first drive rollers. The first and second driven rollers may be arranged to face each other, and the transporting means can be configured with a minimum configuration.

In addition, the carry-in means may be arranged on the opposite side of the paper discharge tray with respect to the switching means, and the carry-out means may be arranged on the opposite side of the finisher with respect to the switch means.
In this case, the sheet is surely sent to the switching means by the carrying-in means, and the sheet conveyed from the switching means to the re-feeding means is surely carried out to the re-feeding means.

Here, by configuring the carrying-in means and the carrying-out means as follows, the number of components can be minimized. That is, a second drive roller rotatable in a predetermined direction is disposed at a position sandwiched between the first and second paper discharge paths,
The fourth and third driven rollers may be arranged to face the second drive roller so as to sandwich the first and second paper discharge paths. In this case, the second driving roller and the fourth driven roller constitute a carrying-out means, and the second driving roller and the third driven roller constitute a carrying-in means.

Further, the image forming method according to the present invention comprises:
An image forming method of discharging a sheet having images formed on both main surfaces by an image forming unit along a selected discharge path selected from a discharge path leading to a finisher and a discharge path leading to a discharge tray. A first step of feeding a reference sheet before image formation to the image forming means along a paper feed path to form an image on one main surface of the reference sheet, A second step of transporting the reference sheet, on which an image is formed only on the main surface, to one of the two discharge paths; and, after the second step, a transport direction of the reference sheet in the discharge path. A third step of reversing the reference sheet and transporting it to the paper supply path side via a re-feed path, thereby turning the reference sheet upside down and the image along the paper supply path. Feed the reference sheet The forming an image on the other main surface of the bets 4
And a fifth step of, after the fourth step, discharging a reference sheet having images formed on both main surfaces via the selected discharge path.
And a process.

According to the present invention, the sheet having an image formed on one principal surface is conveyed to one of a paper discharge path leading to a finisher and a paper discharge path leading to a paper discharge tray.
The sheet is reversed and conveyed on the sheet discharge path, and conveyed to the sheet supply path via the re-sheet supply path. Then, after an image is formed on the other main surface of the sheet, the sheet is discharged via a selected discharge path.
Thus, the switchback operation of the sheet is performed in the sheet discharge path. In addition, the switchback operation can be performed in any of the paper discharge paths, and the throughput is improved by forming an optimal sheet conveyance sequence.

Here, before the second step, an image is formed on one main surface of the preceding sheet immediately before the reference sheet, and the image forming path is different from the reference sheet discharge path in which the reference sheet is conveyed in the second step. When the reference sheet is conveyed to the other front sheet discharge path, the reference sheet is carried into the reference sheet discharge path, and at the same time, the front sheet is discharged from the previous sheet discharge path to the re-feed path. Can be. In this case, both transport operations can be performed simultaneously and collectively, and the throughput is further improved.

At the same time when the reference sheet is conveyed to the paper re-feeding path, the next sheet after the reference sheet, on which the image is formed on one main surface, is conveyed in the second step. The sheet may be conveyed to the other sheet discharge path for the other sheet, which is different from the sheet discharge path for the reference sheet. In this case, both the conveying operations can be performed collectively and simultaneously, and the throughput is further improved. .

Further, if the reference sheet having an image formed only on one main surface is conveyed to another discharge path different from the selected discharge path, the conveyance of the reference sheet to the re-feed path can be reduced. At the same time, it is possible to discharge the previous sheet, which is the sheet immediately before the reference sheet, on which images are formed on both main surfaces, along the selected sheet discharging path. Also in this case, both transport operations can be performed simultaneously and collectively, and the throughput is further improved.

[0018]

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. FIG. 2 is a diagram showing the inside of the case of FIG. This image forming apparatus forms a full-color image by superimposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K), or uses only black (K) toner. To form a monochrome image. In this image forming apparatus, as shown in FIG. 2, an image carrier unit 2, an exposure unit 3, a transfer unit 4, and a fixing unit 5 are provided inside a case 1, and these form an image forming unit for forming an image on a sheet. Is configured. The case 1 also includes a sheet supply / discharge unit 6 for feeding a sheet to the image forming unit and discharging the image-formed sheet to a predetermined discharge position. Further, as shown in FIG. 1, a multi-bin unit 8 functioning as one of finishers is disposed at an upper position of the case 1 to sort sheets on which images are formed by the image forming unit, and to sort the sheets into corresponding bins 81. It can be accommodated.

When an image signal is supplied to the control unit 7 from an external device such as a host computer, the control unit 7 controls each unit of the device to form an image corresponding to the image signal on the sheet S. That is, in this image forming apparatus, a toner image is formed on the photoreceptor 21 of the image carrier unit 2 and further primary-transferred to the intermediate transfer belt 46 of the transfer unit 4. And this primary transfer image (toner image)
Is secondarily transferred to the sheet S fed to the transfer position by the sheet supply / discharge unit 6, and then the sheet S is conveyed to the fixing unit 5 and undergoes a fixing process. The sheet S on which the fixing process has been performed is fed to a predetermined sheet discharge position by the sheet discharge unit 64 of the sheet supply and discharge unit 6. Hereinafter, after describing the configuration of each unit of the apparatus, the characteristic parts of the present invention will be described in detail.

B-1. Case 1 The upper surface of the case 1 functions as a standard paper discharge tray 11, and a paper discharge port 12 opened on the upper surface side of the case 1.
The sheet S on which an image has been formed is ejected one by one to the standard ejection tray 11 via the. The image forming apparatus can be provided with an optional multi-bin unit 8 on the upper side and another finisher (not shown) on the side. The opening 1 is provided on the top and side surfaces of the case 1 for transport to the unit 8 and another finisher.
3 and 14 are provided respectively.

B-2. Image Carrier Unit 2 and Exposure Unit 3 In this image forming apparatus, the image carrier unit 2 is detachable from the case 1 as shown in FIG. The image carrier unit 2 includes a photoconductor 21 rotatable in a direction indicated by an arrow in FIG.
Along the direction of rotation thereof, a charging roller 22 as charging means, and developing devices 23Y and 23C as developing means.
23M, 23K, and the cleaning unit 24 are arranged, respectively. The charging roller 22 contacts the outer peripheral surface of the photoconductor 21 to uniformly charge the outer peripheral surface. The laser light L from the exposure unit 3 is directed toward the outer peripheral surface of the photoconductor 21 charged by the charging roller 22. Irradiated.
The exposure unit 3 scans and exposes the laser beam L onto the photoconductor 21 in accordance with an image signal given from an external device such as a host computer, and forms an electrostatic latent image on the photoconductor 21 corresponding to the image signal. .

The electrostatic latent image thus formed is developed
Is developed with toner. That is, in this embodiment, as the developing unit 23, a developing unit 23Y for yellow, a developing unit 23C for cyan, a developing unit 23M for magenta, and a developing unit 23K for black are arranged along the photoconductor 21 in this order. Have been. These developing units 23Y and 23
C, 23M, and 23K are configured to be able to freely contact and separate from the photoreceptor 21, respectively.
One of the developing units 3M, 23C, and 23B selectively contacts the photoconductor 21, and applies a toner of a corresponding color to the surface of the photoconductor 21 to make the electrostatic latent image on the photoconductor 21 visible. I do.

The toner image developed by the developing unit 23 is primarily transferred onto the intermediate transfer belt 46 of the transfer unit 4 in a primary transfer region R1 located between the black developing unit 23K and the cleaning unit 24. The transfer unit 4
Will be described later in detail.

The cleaning section 24 is disposed at a position which is advanced from the primary transfer area R1 in the circumferential direction (the direction of the arrow in the figure). This cleaning unit 24
A cleaner blade 241 for scraping off toner remaining on the outer peripheral surface of the photoreceptor 21, and a receiving portion 2 for receiving the toner scraped off by the cleaner blade 241.
42.

B-3. Transfer Unit 4 Next, the configuration of the transfer unit 4 will be described. In this embodiment, the transfer unit 4 includes the drive rollers 41 and 4
Driven rollers 42 to 45 and these rollers 41 to 4
5, and a secondary transfer roller 48 for secondarily transferring the intermediate toner image transferred to the intermediate transfer belt 46 to the sheet S. And
When the color image is transferred to the sheet S, the photosensitive member 21
A color image is formed by superimposing the toner images of each color formed thereon on the intermediate transfer belt 46, and is transferred to the sheet S to obtain a full-color image. When a monochrome image is to be transferred to the sheet S, only a black toner image is formed on the photoreceptor 21 on the intermediate transfer belt 46 and transferred to the sheet S to obtain a monochrome image.

More specifically, the transfer unit 4
Is configured as follows. That is, as shown in the figure, the drive roller 41 is arranged on the cleaning section 24 side (the right hand side in the figure) of the primary transfer area R1. A gear is fixed to an end of the drive roller 41, and the gear is meshed with a drive gear of the photoconductor 21, so that the drive roller 41 is driven to rotate at substantially the same peripheral speed as the photoconductor 21. Then, with the rotation of the drive roller 41, the intermediate transfer belt 46 is circulated and driven at substantially the same peripheral speed as the photosensitive member 21 in the direction of the arrow shown in the figure. In addition, the driving roller 41 includes
An electrode roller 47 is in contact with the outer peripheral side via the intermediate transfer belt 46, and a primary transfer voltage is applied to a conductive layer (described later) of the intermediate transfer belt 46 via the electrode roller 47.

On the other hand, a driven roller 45 is disposed in the primary transfer region R 1 on the side of the developing section 23 (the left hand side in the figure), and applies tension to the intermediate transfer belt 46 between the driven roller 41 and the driven roller 45. As a result, the belt portion located between the rollers 41 and 46 is pressed against the photosensitive member 21 to form a primary transfer portion.

The driven roller 42 among the remaining rollers is a tension roller, and applies tension to the intermediate transfer belt 46 by urging means (not shown). The driven roller 43 is a backup roller that forms the secondary transfer area R2.

A secondary transfer roller 48 is opposed to the backup roller 43 with the intermediate transfer belt 46 interposed therebetween. The secondary transfer roller 48 can be moved toward and away from the intermediate transfer belt 46 by a contact / separation mechanism (not shown), and is pressed against the intermediate transfer belt 46 during the secondary transfer. Then, in this pressed state, the secondary transfer roller 48
Is applied, and the toner image transferred to the intermediate transfer belt 46 is transferred to the sheet S fed to the secondary transfer region R2 by the paper supply / discharge unit 6 (secondary transfer).

The driven roller 44 is connected to the belt cleaner 4.
9 is a backup roller. The belt cleaner 49 includes a cleaner blade 491 that comes into contact with the intermediate transfer belt 46 to scrape off the toner remaining on the outer peripheral surface thereof, and a receiving portion 492 that receives the toner scraped by the cleaner blade 491. ing. The belt cleaner 49 can be moved toward and away from the intermediate transfer belt 46 by a contact / separation mechanism (not shown).

In this embodiment, the intermediate transfer belt 4
Reference numeral 6 denotes a three-layer structure including an insulating base made of a synthetic resin, a conductive layer formed on the insulating base, and a resistance layer formed on the conductive layer and pressed against the photoconductor 21. It has a structure. At the side edge of the intermediate transfer belt 46 having the three-layer structure, the resistance layer is removed in a strip shape, and the conductive layer is exposed in a strip shape. On the other hand, a primary transfer voltage is applied. The intermediate transfer belt 46 is endlessly formed by connecting both ends of the belt having the three-layer structure, and is controlled so that the toner image is primarily transferred while avoiding the joint. .

B-4. Fixing unit 5 Sheet S on which the toner image has been transferred by transfer unit 4
Is a predetermined feeding path 6 by a feeding / discharging unit 6 described later.
Although the sheet is fed along a dotted line 30 (a two-dot chain line in the figure), a fixing unit 5 is disposed downstream of the secondary transfer region R2. The fixing unit 5 is formed by two fixing units 51 and 52. In the main fixing unit 51, a heat roller 511 disposed above the paper feed path 630,
The heat roller 511 includes a pressing roller 512 pressed from below and fixes the toner image transferred onto the conveyed sheet S onto the sheet S. On the other hand, in the auxiliary fixing section 52, a heat roller 521 disposed below the sheet feeding path,
And a pressure roller 522 pressed from above on the sheet S.
The habit is alleviated. The sheet S on which an image has been formed in this manner is further fed by the sheet feeding / discharging unit 6
The paper is fed to the standard paper discharge tray 11, the multi-bin unit 8, and the like along 30.

B-5. The paper supply / discharge unit 6 includes a cassette 61 and a manual tray 62 capable of storing a plurality of sheets S in a stacked state, and a sheet S from the cassette 61 or the manual tray 62 along the paper supply path 630. The sheet feeding unit 63 that conveys the sheet S to the next transfer region R2 and the fixing unit 5, and the sheet S that has been fixed is selectively discharged to the standard sheet discharge tray 11, the multi-bin unit 8, or the finisher, or the sheet S that has been conveyed.
Paper discharge unit 64 that reversely feeds paper to the paper re-feeding unit side, and paper discharge unit 64
A switching unit 65 for selectively switching the sheet discharge paths 641a and 641b of the sheet S, and a sheet re-feeding unit 66 for feeding the sheet S conveyed from the sheet discharging unit 64 to the sheet feeding unit 63 again. It is configured.

B-5-1. Paper Feed Unit 63 The paper feed unit 63 is provided with a pickup roller 631 so as to come into contact with the uppermost sheet S of the sheets S stored in the cassette 61 in order to carry out the sheets S from the cassette 61. And this pickup roller 63
A separation roller pair 632 is provided on the outlet side (the right-hand side in FIG. 2) of the cassette 61 in order to surely separate the sheets S carried out from the cassette 61 by one.
Thus, the sheets S stored in the cassette 61 are
The sheets can be taken out toward the secondary transfer region R2 without fail. The configuration for taking out the sheet S is the same on the manual feed tray 62 side. That is, the sheet S stored in the manual feed tray 62
Is the manual feed tray 62 by the pickup roller 633.
Are transported to the secondary transfer region R2 one by one by the separation roller pair 634.

The sheet S sent from the cassette 61 is fed to the gate roller pair 637 via the second and third relay roller pairs 635 and 636, while the manual tray 6
The sheet S sent out of the second relay roller pair 636
, And are fed to the gate roller pair 637 via the third relay roller pair 636. In this image forming apparatus, a cassette can be added to the lower part of the apparatus as an option. Sheets fed into the case 1 from an additional cassette (not shown) through the opening 15 can be provided in the same manner as described above. Opening 15 to feed to gate roller pair 637
, A first relay roller pair 638 is provided.

As described above, the gate roller pair 63 is removed from the cassette 61, the manual tray 62 or the additional cassette.
7 is fed to the gate roller pair 637.
Then, the sheet is fed at a predetermined timing to the secondary transfer region R2, that is, between the intermediate transfer belt 46 and the secondary transfer roller 48. As described above, in this embodiment, the paper feed unit 63 is formed by the components 631 to 638, and the cassette 61, the manual feed tray 62, the additional cassette, the secondary transfer area R2, and the paper discharge unit 6
The conveyance path of the sheet S up to 4 is a sheet feeding path 630.

B-5-2. Sheet discharging section 64 The toner image is transferred in the secondary transfer area R2, and the sheet S (image-formed sheet) subjected to the fixing process by the fixing unit 5 is fed to the sheet discharging section 64. This paper discharge unit 64 is 2
One discharge path (second discharge path) 641a extends from the fixing unit 5 to the standard discharge tray 11, and the other discharge path (first discharge path). The path 641b extends between the re-feed section 66 and the multi-bin unit 8 substantially in parallel with the sheet discharge path 641a. Three drive rollers 642c to 644c are arranged at predetermined intervals along these paper discharge paths 641a and 641b, and are sandwiched between the paper discharge paths 641a and 641b. Further, each drive roller 642c-
Driven rollers 642a to 644a are respectively disposed corresponding to the paper discharge path 641a, and driven rollers 642b to 644b are disposed respectively corresponding to the paper discharge path 641b.

Of the three drive rollers, the drive roller 642c provided in the vicinity of the fixing unit 5 and the sheet re-feeding section 66 is configured to rotate only in the counterclockwise direction on the sheet of FIG. As described above, the paper discharge unit 64
In the embodiment, the driving roller 642c and the driven roller 642a form a sheet discharge entrance roller pair 645, and the fixed sheet S
Functions as a carrying-in means for drawing the
The sheet S is transferred along the discharge path 641a to the standard discharge tray 1
Conveyed toward 1. On the other hand, the driving roller 642c and the driven roller 642b form a switchback exit roller pair 646.
And functions as an unloading unit that conveys the sheet conveyed along the first paper discharge path 641b from the paper discharge unit 64, and as described later, the second paper discharge path 641a or the first paper discharge path 641b. Sheet S temporarily waited at
Is transported toward the re-feeding section 66.

The remaining drive rollers 643c and 644c are
Unlike the drive roller 642c, the drive roller 642c can be rotated forward and backward, and constitutes the first and second paper discharge roller pairs 643, 644 at the positions where the drive rollers 643c, 644c are disposed. Then, the driving rollers 643c, 644
The sheet S can be conveyed in opposite directions on both the paper discharge paths 641a and 641b in accordance with the rotation direction of c. For example, the driving rollers 643c and 644c in FIG.
Is rotated counterclockwise, the sheet S can be conveyed toward the standard paper discharge tray 11 along the second paper discharge path 641a, and at the same time, the sheet S is re-fed along the first paper discharge path 641b. It can be transported to the part 66 side. When the rotation direction of the driving rollers 643c and 644c is reversed, the sheet S is conveyed in a direction opposite to the above.

As described above, in this embodiment,
The driving roller 643c and the driven roller 643a, and the driving roller 644c and the driven roller 644a transfer the sheet S along the second discharge path 641a to the standard discharge tray 111.
Second transport means for transporting the sheet S toward the (second paper discharge position) and reversely transporting the sheet S toward the re-feeding unit 66 are provided. The driving roller 643c and the driven roller 643b, and the driving roller 644c and the driven roller 6
44b, the sheet S is conveyed along the first paper discharge path 641b toward the multi-bin unit 8 (first paper discharge position), and the sheet S is reversely conveyed toward the re-feed unit 66. Conveying means are respectively constituted. By providing such first and second transport means, the paper discharge paths 641a,
A transport unit that enables bidirectional transport of the sheet S at 641b is configured.

Note that, in this embodiment, the first and second
Although the drive rollers are commonly used in forming the transport means, dedicated drive rollers may be provided for each. Further, in this embodiment, the first conveying means is a pair of two rollers (643b and 643c; 644b and 644c).
And the second transporting means is also composed of two pairs of rollers (643a and 643c; 644a and 644c), but the number of each pair is arbitrary, and at least one or more sets are provided for each sheet S. What is necessary is just to carry it.

In this embodiment, the first and second
Each of the discharge roller pairs 643 and 644 is connected to the discharge path 6
It functions as carrying-in / out means (transporting means) for simultaneously carrying the sheet S to the paper discharge side and the carrying to the switching section 65 in the sheets 41a and 641b, which is more effective than the case where the first and second carrying means are provided. The device configuration becomes simple and control becomes easy. In this embodiment, two loading / unloading means, that is, a first and a second pair of discharge rollers 643 and 644 are used.
However, the number of carrying-in / out means is arbitrary, and may be appropriately set according to the maximum sheet size to be conveyed.

By the way, in the paper discharge path 641a, a third paper discharge roller pair 64 is provided at a position near the standard paper discharge tray 11.
7 are provided. This third paper discharge roller pair 647 is
Drive rollers 647c sandwich the paper discharge path 641a.
And a driven roller 647a are rotatably arranged. The third discharge roller pair 647 is connected to the second discharge path 641.
a of the sheet S conveyed along the first paper discharge tray 1
1. Stack and discharge. Although not shown in FIG. 2, a corrugation roller is attached to at least one of the driving shafts of the driving roller 647c and the driven roller 647a, and a direction substantially orthogonal to the sheet discharging direction is applied to the sheet S. To reduce and remove sheet curl.

On the other hand, in the first paper discharge path 641b, a switching flap 648 is provided immediately above the driven roller 643b constituting the first paper discharge roller pair 643, and the paper discharge path 641b is provided.
The discharge destination of the sheet S conveyed along b can be switched. That is, when the switching flap 648 is located at the solid line position in FIG.
When the switching flap 648 is positioned on the discharge path 641b while the sheet S is conveyed as it is along the discharge bin 41 and is discharged toward the multi-bin unit 8, the sheet S is transferred to the discharge path 641b.
1b, and is discharged to the finisher through the opening 14.

Further, between the first discharge roller pair 643 configured as described above, the discharge entrance roller pair 645 and the switchback exit roller pair 646, the sheet S is transported by two discharge paths. A switching unit 65 for switching between the paths 641a and 461b is provided. By controlling the switching unit 65, the transport path of the sheet S is selectively switched. More specifically, the switching unit 65 is configured as follows.

B-5-3. Switching Section 65 FIG. 3 is an enlarged perspective view showing the switching section of FIG. The switching unit 65 includes a first switching flap 651 as shown in FIG.
And a second switching flap 652. And
In order to drive each switching flap 651, 652, respectively,
It is configured as follows.

The first switching flap 651 serves as a switching means for guiding the sheet S which has been subjected to the fixing process and conveyed by the pair of discharge rollers 645 to one of the discharge paths 641a and 641b. The sheet S once conveyed to the first sheet ejection path 641b is
It is intended to lead to. This first switching flap 651
Is fixed to a shaft 653b disposed between the paper ejection paths 641a and 641b. An arm 654b is fixed to one end of the shaft 653b, and a pin 656b of a solenoid 655b is connected to a tip of the arm 654b. Therefore, when the pin 656b is in the upwardly moved position, the first switching flap 651 is located at a position where its tip 651b is directed toward the first driven roller 642b as shown by a solid line in FIG. The sheet S conveyed by the roller pair 645 is directly transferred to the second discharge path 64
It is guided toward the standard paper discharge tray 11 along 1a. On the other hand, when the pin 656b moves down, the first switching flap 65
1 rotates counterclockwise, the leading end 651b is switched to a position to be directed to the second driven roller 642a side, and the sheet S conveyed by the second discharge inlet roller pair 645 is transferred to the other first roller. Opening 1 along discharge path 641b
Guide towards 3.

The second switching flap 652 is for guiding the sheet once conveyed to the second paper discharge path 641a to the re-feeding section 66, and is provided with a guide plate 657 for guiding one surface of the first paper discharge path 641b. It is fixed to a shaft 653a disposed outside. An arm 654a is fixed to one end of the shaft 653a, and a pin 656a of a solenoid 655a is connected to a tip of the arm 654a. Therefore, when the pin 656a is at the position where the pin 656a is moved upward, the second switching flap 652 is at a position retracted from the first sheet discharge path 641b indicated by a solid line in FIGS. On the other hand, the pin 65
When 6a moves down, it turns clockwise to move its tip portion 6a.
52a enters the first discharge path 641b, and guides the sheet once conveyed to the second discharge path 641a to the re-feed section 66.

The guide plate 657 has a second switching flap 65.
2 is provided, and when the second switching flap 652 rotates clockwise, the leading end portion 652a enters the notch 657a to allow the drive roller 642c and the driven roller 642b to rotate. Nip part 64
Head to 6b. On the other hand, when the first switching flap 651 rotates counterclockwise, the tip 651b of the first switching flap 651 is notched 657.
a.

The rotation driving mechanism of the switching flap 648 includes first and second switching flaps 651, 65.
2 is the same as that of FIG.

As described above, in this embodiment, the switching unit 65 is incorporated in the paper discharging unit 64 having the plurality of paper discharging paths 641a and 641b to constitute a paper discharging device. According to this paper discharging device, The sheet S on which an image has been formed can be selectively conveyed to one of the two discharge paths 641a and 461b, and the discharge paths 641a and 461b.
The sheet S waiting to be conveyed to the sheet re-feeding unit 66 can be selectively conveyed. The characteristic operation will be described later in detail while showing a specific image forming sequence.

B-5-4. Returning to FIG. 2, the configuration of the re-feeding section 66 will be described.
The re-feeding section 66 transports the sheet S transported from the switchback exit roller pair (conveying means) 646 to the gate roller pair 637 of the paper feeding section 63 along the re-feeding path 664. It is composed of three pairs of re-feed rollers 661 to 663 arranged along the re-feed path 664. In each of the pair of re-feed rollers 661 to 663, a drive roller is provided below the re-feed path 664, and a driven roller is provided so as to face and contact the drive roller. In this way, the sheet S conveyed from the sheet discharge unit 64 is moved along the re-feed path 664 by the gate roller pair 6.
By returning to 37, the non-image forming surface of the sheet S in the sheet feeding unit 63 faces the intermediate transfer belt 46, and the image can be secondarily transferred to the surface.

B-6. Control Unit 7 FIG. 4 is a block diagram illustrating a schematic configuration of a control unit that controls the image forming apparatus of FIG. This control unit 7
Includes a CPU 711 for performing various arithmetic processes and control commands for controlling the entire apparatus, a RAM 712 for temporarily storing control data, arithmetic results, and the like.
RO for storing arithmetic programs and the like performed by CPU 711
M713. The control unit 71 is connected to an external device such as a host computer via a communication interface 77, and can input image signals and the like from the external device via the communication interface 77. The image signal is also directly input to the image memory 78 via the communication interface 77, so that image data based on the image signal can be stored.

The control unit 71 includes a communication interface 77
Besides, an image memory 78, an operation display section 79 including operation buttons and a display panel, and a multi-bin unit controller 8 for controlling the operation of the multi-bin unit 8
0, an image carrier unit controller 72 for controlling the operation of the image carrier unit 2, an exposure unit controller 73 for controlling the operation of the exposure unit 3, a transfer unit controller 74 for controlling the operation of the transfer unit 4, and the fixing unit 5 Are electrically connected to a fixing unit controller 75 for controlling the operation of the paper feed / discharge unit 6 and a feed / discharge unit controller 76 for controlling the operation of the paper feed / discharge unit 6. In this embodiment, since the features of the present invention are closely related to the operation of conveying the sheet S, the configuration of the sheet supply / discharge unit controller 76 will be described in further detail.

The paper supply / discharge unit controller 76 includes:
As shown in the figure, a motor driver 761, three clutch drivers 762 to 764, a solenoid driver 76
5 and a sensor group 766 that is disposed in each section of the paper supply / discharge unit 6 and detects the sheet S. Among them, the motor driver 761 is for controlling the driving of a driving motor 67 which is a driving source of the paper supply / discharge unit 6, and the driving force of the driving motor 67 is applied to the paper supply unit 63 and the paper
4 and the sheet S is conveyed to the re-feed section 66 to convey the sheet S.

In the sheet feeding section 63, the sheet discharging section 64, and the sheet re-feeding section 66, the driving force is transmitted to a part of the rollers via a clutch (not shown). 63
The clutch driver 762 controls the paper feed side clutch group 68a provided in the paper discharge unit, the clutch driver 763 controls the paper discharge side clutch group 68b provided in the paper discharge unit 64, and the The clutch driver 764 controls ON / OFF of the paper feed side clutch group 68c, respectively.

Further, in the paper discharge section 64, the three switching flaps 641, 642, 648 are provided as described above, and are rotated by a flap switching solenoid group (solenoids 655a, 655b, etc.). In this embodiment, a solenoid driver 765 is provided to control the flap switching solenoid group.

C. Operation of Image Forming Apparatus Next, the operation of the image forming apparatus configured as described above, particularly, five types of image forming sequences most relevant to the features of the present invention will be described in detail.

C-1. First Image Forming Sequence In the first image forming sequence, a monochrome image is formed on both main surfaces of an A3 sheet, and then the sheet is discharged to the standard sheet discharge tray 11 as a sheet discharge destination. This will be described with reference to FIGS.

FIG. 5 is a schematic diagram showing a first embodiment (first image forming sequence) of the image forming method according to the present invention. FIGS. 6 and 7 are diagrams schematically showing a sheet conveyance state at timings T1 and T2 in FIG. FIG. 8 is a partially enlarged view of FIG. Note that FIG. 5 (and FIGS. 9, 12, 14,
15, 17, and 18), the column of “intermediate transfer belt” indicates a toner image primarily transferred to the intermediate transfer belt 46. In addition, the boundary line in the same column indicates a joint portion of the intermediate transfer belt 46. In this embodiment, the intermediate transfer belt 46 has a maximum A3 size for one sheet.
Alternatively, two toner images of A4 size can be primarily transferred. The attributes of the primary transferred toner image are indicated by four alphanumeric characters. The contents of each display are as follows.

Numerical characters: number of sheets; English characters: F ... surface,
B: back side; parentheses: page number; parenthesis: K
... Black, Y. Yellow, M. Magenta, C. Cyan; The display contents other than “Bracket English characters” are “Gate roller”, “First paper discharge path” and “Second paper discharge” in FIG. The attribute of the sheet shown in the “path” column is also used in common.

(A) First, when an image signal is input to the control unit 7 from the host computer, an electrostatic image corresponding to the image of the second page (the image to be formed on the back surface of the first sheet) is formed on the photosensitive member 21. After forming the latent image, a black developing device 2
3K selectively contacts the photoconductor 21 to form a black toner image on the surface of the photoconductor 21. Then, the toner image is primarily transferred to the intermediate transfer belt 46. For convenience of description below, the black toner image thus formed on the intermediate transfer belt 46 is referred to as an “intermediate toner image”, and the process of forming the intermediate toner image is referred to as “intermediate toner image forming process”. Called.

In parallel with the intermediate toner image forming process, the A3 size sheet S1 is conveyed from the cassette 61 or the like to the gate roller pair 637, and is kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form a second page image on the back side of the first sheet.

(B) The sheet S1 on which the image of the second page is formed is conveyed to the second paper discharge path 641a connected to the standard paper discharge tray 11 which is previously selected as the paper discharge position. When the sheet S1 is conveyed to the second discharge path 641a, the switching flap 651 of the switching unit 65 rotates counterclockwise in FIG.
Of the driven roller 642a.
Subsequently, the first and second discharge roller pairs 643,6
The rotation direction of the sheet S1 is switched, and the sheet
6, the sheet S1 is turned over, and the sheet S1 is conveyed toward the gate roller pair 637. Note that, while the switchback of the sheet S1 is performed as described above, the image forming process is idled for one time.

(C) Subsequently, before forming the image of the first page, the image of the fourth page (the image to be formed on the back of the second sheet) is formed on the back of the next sheet S2. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46, and in parallel with the intermediate toner image forming process, the next sheet S2 is conveyed from the cassette 61 or the like to the gate roller pair 637 and is made to stand by. Keep it. Then, the intermediate toner image is secondarily transferred onto the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the fourth page on the back side of the second sheet.

(D) The sheet S2 on which the image of the fourth page is formed is connected to the other non-selected discharge position different from the selected discharge position, that is, the multi-bin unit 8, as shown in FIG. The sheet is conveyed to the first sheet discharge path 641b.

In parallel with this, the image of the first page (the image to be formed on the first surface) is formed on the surface of the sheet S1. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46 and the second paper discharge path 6
The sheet S1 conveyed from the sheet feeder 41a is conveyed toward the gate roller pair 637 via the sheet re-feeding unit 66 and the sheet feeding unit 63. Thus, the sheet S1 is turned upside down, and the sheet S1 is conveyed to the gate roller pair 637 to complete the preparation for forming the image of the first page. Then, the intermediate toner image is secondarily transferred onto the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form an image of the first page on the front side of the first sheet (FIG. 6).

(E) The double-sided image sheet S1 on which images are formed on both sides is discharged toward the standard discharge tray 11, which is previously selected as a discharge destination, as shown in FIG. At this time, since the paper discharge unit 64 is configured as described above in detail, the sheet S1 is conveyed to the standard paper discharge tray 11 on the second paper discharge path 641a side, and at the same time, the first paper discharge path 641b side is used. Then, the sheet S2 is reversely conveyed toward the re-feeding section 66 (FIG. 8). As described above, according to the image forming sequence, the two sheets S1 and S2 can be simultaneously conveyed, the processing time required for image formation can be reduced, and the throughput can be improved.

The sheet S2 conveyed to the sheet re-feeding section 66 is conveyed to the gate roller pair 637 via the sheet re-feeding section 66 and the sheet feeding section 63. Thus, the sheet S2 is turned upside down, and the sheet S2 is conveyed to the gate roller pair 637 and waits. During the re-feeding of the sheet S2 in this manner, the image forming process idles once.

(F) After the first idle rotation, an intermediate toner image corresponding to the image of the third page (the image to be formed on the second surface) is formed on the intermediate transfer belt 46. And
The intermediate toner image is secondarily transferred to the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5, and is transferred to the surface of the second sheet.
An image for the page is formed.

(G) Then, the second sheet S2, on which images are formed on both sides as described above, is switched through the switching section 65 and the second sheet discharging path 461a to the standard sheet discharging position previously selected as the sheet discharging position. The sheet is discharged to the paper tray 11.

(H) Following the above step (g), an image of the sixth page is formed on the back surface of the next sheet S3, and is conveyed to the second paper discharge path 461a. That is, in the same manner as in the step (a), the intermediate toner image corresponding to the image of the sixth page (the image to be formed on the back surface of the third sheet) is primarily transferred to the intermediate transfer belt 46, and the gate roller pair 637 is used. Then, the next sheet S3 is put on standby, the intermediate toner image is secondarily transferred to the back surface of the sheet S3 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the sixth page on the third back surface side. I do. Then, the sheet S3 is conveyed to the second sheet discharge path 641a with a slight gap (ΔD) from the rear end of the preceding sheet S2.

After that, the above step (b) is repeated until all the images up to the desired page are formed on the sheet.
To (h) are repeated.

As described above, according to the first image forming sequence, both the paper discharge paths 641a and 641b function as switchback paths, and the switchback path has become an essential component in the conventional image forming apparatus. No special reversing device is required, and as a result, images can be formed on both main surfaces of the sheet S with a simple configuration.

Further, the following effects are obtained by making the paper discharge paths 641a and 641b also function as switchback paths. For example, when the throughput is not considered at all, the sheet S may be switched back only by the selected paper discharge path selected as the paper discharge destination (the paper discharge path 641a corresponds to this image sequence). However, for example, as described in Japanese Patent Publication No. 61-11865, when the discharge of the double-sided image sheet and the switchback of the single-sided image sheet are executed in one discharge path, the two do not interfere with each other. Therefore, it is not possible to form an optimal transport sequence, and the number of sheets that can be processed in a short time, that is, the throughput is reduced. On the other hand, in the image forming sequence, the switchback operation is performed at the non-selected paper discharge position that is not selected as the paper discharge destination, that is, at the paper discharge path 641b corresponding to the multi-bin unit 8, and The sheet can be transported in a simple transport sequence, and the throughput can be improved.

In particular, in the step (e), two sheets S1, S2
Since the transfer of 2 is performed at the same time, the throughput can be further improved. That is, according to this image forming sequence, a reference sheet having an image formed on one main surface (corresponding to the back surface in the second image forming sequence), for example, sheet S2, is conveyed to the re-feeding path 66, The previous sheet S1, which is the sheet immediately before the sheet S2 and has images formed on both main surfaces, is discharged to the standard discharge tray 11 via the selected discharge path 641a (FIG. 7, 8), the reference sheet S2 and the previous sheet S1 can be transported simultaneously, and the processing capacity per unit time, that is, the throughput can be improved.

Further, in this way, the two sheets are discharged
When the sheets are conveyed at the same time, there is a problem that the sheets are stained due to the rubbing between the sheets.
a, 641b, the above problem does not occur, and the sheet can be prevented from being stained.

Further, a continuous sheet, for example, sheet S
In accordance with the discharge of the preceding sheet S1 toward the selected standard discharge tray 11 among the sheets S1 and S2, the succeeding sheet S2 following the preceding sheet S1 is selectively discharged corresponding to the standard discharge tray 11. Since the sheet is conveyed toward the paper path 641a, the interval ΔD between successive sheets can be narrowed to perform image formation with excellent throughput.

C-2. Second Image Forming Sequence This second image forming sequence is for forming a monochrome image on both main surfaces of an A4 sheet and then discharging the same to the standard paper output tray 11. Hereinafter, FIGS. It will be described with reference to FIG.

FIG. 9 is a schematic view showing a second embodiment (second image forming sequence) of the image forming method according to the present invention. FIGS. 10 and 11 are diagrams schematically showing the sheet conveyance status at timings T3 and T4 in FIG.

(I-1) First, when an image signal is input from the host computer to the control unit 7, an intermediate toner image of the image of the second page (the image to be formed on the back surface of the first page) is formed. , A4 size sheet S1 is conveyed from cassette 61 or the like to gate roller pair 637, and is kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form a second page image on the back side of the first sheet.

The intermediate transfer belt 46 according to this embodiment can simultaneously transfer two A4 size intermediate toner images. However, in this image sequence, if necessary, the number of simultaneously formed intermediate toner images can be increased. Is switched.

(I-2) The sheet S1 on which the image of the second page has been formed is conveyed to the second paper discharge path 641a connected to the standard paper discharge tray 11 previously selected as the paper discharge position.

(J-1) The sheet S1 is moved to the second paper discharge path 641a
2, the switching flap 651 of the switching section 65 rotates counterclockwise in FIG. 2, and the leading end 651 b is switched to a position directed to the second driven roller 642 a side. Then, the first and second discharge roller pairs 643, 64
4 is switched, and the sheet S1 is re-supplied by the sheet re-feeding unit 66.
Transported in reverse.

In parallel with this, the image of the fourth page (the image to be formed on the second back surface) is formed on the back surface of the next sheet S2. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46, and the next sheet S2 is conveyed from the cassette 61 or the like to the gate roller pair 637 in parallel with the intermediate toner image forming process.
Keep waiting. Then, the intermediate toner image is secondarily transferred onto the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the fourth page on the back side of the second sheet.

(J-2) The sheet S2 on which the image of the fourth page is formed is connected to another multi-bin unit (not shown) different from the standard paper discharge tray 11 previously selected as the paper discharge position. The sheet is conveyed to the first sheet discharge path 641b.

At the same time, the image of the sixth page (the image to be formed on the back of the third sheet) is formed on the back of the next sheet S3. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46 adjacent to the intermediate toner image on the fourth page, and the next sheet S3 is placed in the cassette 61 in parallel with the intermediate toner image forming process. For example, the sheet is conveyed to the gate roller pair 637 and kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S3 at an appropriate timing, and is fixed by the fixing unit 5, so that an image of the sixth page is formed on the back side of the third sheet.

(K-1) As shown in FIG. 10, the sheet S3 on which the image of the sixth page is formed is connected to the standard paper discharge tray 11 which is previously selected as the paper discharge position.
The sheet is conveyed to the sheet discharge path 641a. At this time, since the sheet discharging unit 64 is configured as described above in detail, the sheet S2 is re-fed on the first sheet discharging path 641b while the sheet S3 is conveyed on the second sheet discharging path 641a. The sheet is reversely conveyed toward the section 66. As described above, according to this image forming sequence, as in step (e) of the first image forming sequence, the two sheets S2 and S3 can be simultaneously conveyed, and the processing time required for image formation is reduced. Can be shortened,
Throughput can be improved.

The sheet S1 which has been already conveyed to the re-feeding section 66 earlier is conveyed to the gate roller pair 637 as shown in FIG.

(K-2) Next, an intermediate toner image corresponding to the image of the first page (the image to be formed on the first surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form an image of the first page on the front side of the first sheet. Thus, the image formation on both sides of the sheet S1 is completed.

At the same time, the sheet S2 being conveyed to the sheet re-feeding section 66 is conveyed to the gate roller pair 637 and made to stand by.

(L-1) The sheet S1 on which images are formed on both sides in this way is discharged to the standard discharge tray 11 which is selected in advance.

At the same time, an intermediate toner image corresponding to the image of the third page (the image to be formed on the second surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5 to form an image of the third page on the front side of the second sheet. Thus, the image formation on both sides of the sheet S2 is completed.

In step (k-1), the second paper discharge path 641a
The third sheet S3 conveyed to the re-feeding section 66 at an appropriate timing is reversed and further conveyed to the gate roller pair 637 following the sheet S2, where it is put on standby.

(L-2) The sheet S2 on which images are formed on both sides is discharged toward the standard discharge tray 11 following the previously discharged sheet S1.

In parallel with this, an intermediate toner image corresponding to the image of the fifth page (the image to be formed on the third surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S3 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the fifth page on the third surface side. Thus, the image formation on both sides of the sheet S3 is completed.

(M-1) The sheet S3 on which images are formed on both sides in this manner is discharged toward the standard discharge tray 11 following the previously discharged sheet S2.

In parallel with this, an intermediate toner image corresponding to the image of the eighth page (the image to be formed on the back surface of the fourth sheet) is formed on the intermediate transfer belt 46, and in parallel with this intermediate toner image forming process. Then, the next sheet S4 is placed in the cassette 6
The sheet is conveyed from 1 or the like to the gate roller pair 637 and kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S4 at an appropriate timing, and is
The image of the eighth page is formed on the back side of the sheet.

(M-2) Then, the sheet S4 on which an image is formed only on one main surface (back surface) is separated from the rear end of the preceding sheet S3 with a slight gap (ΔD) by a second gap. The sheet is conveyed to the sheet discharge path 641a.

Thereafter, the above steps (j-j) are repeated until all the images up to the desired page are formed on the sheet.
1) to (m-2) are repeated.

As described above, the same operation and effect as in the first image forming sequence can be obtained in the second image forming sequence. That is, according to this image forming sequence, both the paper discharge paths 641a and 641b function as switchback paths, and an image can be formed on both main surfaces of the sheet S with a simple configuration. In addition, the switchback operation is performed at a non-selected paper discharge position that is not selected as a paper discharge destination, that is, at the paper discharge path 641b corresponding to the multi-bin unit 8, and the sheet is conveyed in the above-described optimal conveyance sequence. And the throughput can be improved.

In particular, in the step (k-1), two sheets S2,
Since the transfer of S3 is performed at the same time, the throughput can be further improved. That is, according to this image forming sequence, the reference sheet, for example, the sheet S3 is conveyed to the second paper discharge path 641a (step (k-
Prior to 1)), the sheet S2 (corresponding to the preceding sheet) preceding the other sheet discharge path is conveyed in advance, and at the same time as the reference sheet S3 is conveyed to the second sheet discharge path 641a. The sheet S2 is reversely conveyed from the first discharge path 641b to the re-feeding unit 66. As described above, according to this image forming sequence, as in step (e) of the first image forming sequence, the two sheets S2 and S3 can be simultaneously conveyed, and the processing time required for image formation is reduced. It can be shortened and the throughput can be improved.
When the preceding sheet S2 is viewed as a reference sheet,
Simultaneously with the conveyance of the reference sheet S2 to the re-feeding section 66, the sheet S3 one sheet after the reference sheet S2 is transferred to the sheet discharge path 641b for the other sheet which is different from the sheet discharge path 641b for the reference sheet S2. This means that the paper has been carried into the paper discharge path 641a.

Further, the sheets do not come into sliding contact with each other at all, and the contamination of the sheets, which has been a problem when two sheets are simultaneously conveyed in one sheet discharge path, does not occur.

Further, a continuous sheet, for example, sheet S
Following the discharge of the preceding sheet S4 of the sheets S4 and S4 toward the selected standard discharge tray 11, the succeeding sheet S4 following the preceding sheet S3 is selectively discharged corresponding to the standard discharge tray 11. Since the sheet is conveyed toward the paper path 641a, the interval ΔD between successive sheets can be narrowed to perform image formation with excellent throughput.

C-3. Third Image Forming Sequence In the third image forming sequence, a monochrome image is formed on both main surfaces of an A4 sheet and then discharged to a multi-bin unit. Hereinafter, FIGS. 10, 12, and 13 will be described.
This will be described with reference to FIG.

FIG. 12 is a schematic diagram showing a third embodiment (third image forming sequence) of the image forming method according to the present invention. FIG. 13 is a timing chart of FIG.
FIG. 5 is a diagram schematically illustrating a sheet conveyance state in FIG.

(N-1) First, when an image signal is input from the host computer to the control unit 7, the image of the second page (formed on the back surface of the first sheet) is processed in the same manner as in step (p-1). Secondary transfer of the intermediate toner image of the
The image is fixed by the fixing unit 5 to form a second page image on the back side of the first sheet.

(N-2) The sheet S1 on which the image of the second page has been formed is conveyed to a first discharge path 641b connected to a multi-bin unit (not shown) selected as a discharge position. . At this time, the switching flap 651 of the switching unit 65 is rotated in the counterclockwise direction in FIG. 2, and the tip 651b is switched to a position in which the tip 651b is directed to the second driven roller 642a.

(O-1) The sheet S1 is moved to the first paper discharge path 641b
2, the switching flap 651 of the switching section 65 is rotated clockwise in FIG. 2 to return the tip 651 b to a position directed toward the first driven roller 642 b. Then, the rotation direction of the first and second paper discharge roller pairs 643 and 644 is switched to reversely convey the sheet S1 to the re-feeding unit 66.

At the same time, the image of the fourth page (the image to be formed on the back surface of the second sheet) is formed on the back surface of the next sheet S2. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46, and the next sheet S2 is conveyed from the cassette 61 or the like to the gate roller pair 637 in parallel with the intermediate toner image forming process.
Keep waiting. Then, the intermediate toner image is secondarily transferred onto the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the fourth page on the back side of the second sheet.

(O-2) The sheet S2 on which the image of the fourth page has been formed is conveyed toward the first paper discharge path 641b if it is already empty.

At the same time, the image of the sixth page (the image to be formed on the back of the third sheet) is formed on the back of the next sheet S3. That is, an intermediate toner image corresponding to this image is formed on the intermediate transfer belt 46 adjacent to the intermediate toner image on the fourth page, and the next sheet S3 is placed in the cassette 61 in parallel with the intermediate toner image forming process. For example, the sheet is conveyed to the gate roller pair 637 and kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S3 at an appropriate timing, and is fixed by the fixing unit 5, so that an image of the sixth page is formed on the back side of the third sheet.

(P-1) The sheet S3 on which the image of the sixth page has been formed in this manner is, as shown in FIG. 10, the other standard paper discharge different from the multi-bin unit 8 previously selected as the paper discharge position. Second paper discharge path 6 leading to tray 11
It is transported to 41a. At this time, since the paper discharge unit 64 is configured as described above in detail, the second paper discharge path 641a
The first sheet discharge path 641 at the same time as the conveyance of the sheet S3 on the side
On the side b, the sheet S2 is reversely conveyed toward the re-feeding section 66. Thus, according to this image forming sequence,
As in the step (e) of the first image forming sequence and the step (k-1) of the second image forming sequence, the two sheets S2 and S2
3 can be performed at the same time, the processing time required for image formation can be reduced, and the throughput can be improved.

The sheet S1 already conveyed to the sheet re-feeding section 66 earlier is conveyed to the gate roller pair 637 and stands by, as shown in FIG.

(P-2) Next, an intermediate toner image corresponding to the image of the first page (the image to be formed on the first surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form an image of the first page on the front side of the first sheet. Thus, the image formation on both sides of the sheet S1 is completed.

At the same time, the sheet S2 being conveyed to the sheet re-feeding section 66 is conveyed to the gate roller pair 637 and made to stand by.

(Q-1) The sheet S1 on which images are formed on both sides is discharged to the standard discharge tray 11 which is selected in advance.

At the same time, an intermediate toner image corresponding to the image of the third page (the image to be formed on the second surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5 to form an image of the third page on the front side of the second sheet. Thus, the image formation on both sides of the sheet S2 is completed.

In the step (p-1), the second paper discharge path 641a
The third sheet S3 conveyed to the re-feeding section 66 at an appropriate timing is reversed and further conveyed to the gate roller pair 637 following the sheet S2, where it is put on standby.

(Q-2) The sheet S2 on which images are formed on both sides in this way is discharged toward the multi-bin unit following the previously discharged sheet S1.

At the same time, an intermediate toner image corresponding to the image of the fifth page (the image to be formed on the third surface) is formed on the intermediate transfer belt 46. Then, the intermediate toner image is secondarily transferred to the sheet S3 at an appropriate timing, and is fixed by the fixing unit 5 to form the image of the fifth page on the third surface side. Thus, the image formation on both sides of the sheet S3 is completed.

(R-1) The sheet S3 on which images are formed on both sides is discharged toward the standard discharge tray 11 following the previously discharged sheet S2.

In parallel with this, an intermediate toner image corresponding to the image of the eighth page (the image to be formed on the back of the fourth sheet) is formed on the intermediate transfer belt 46, and in parallel with the intermediate toner image forming process. Then, the next sheet S4 is placed in the cassette 6
The sheet is conveyed from 1 or the like to the gate roller pair 637 and kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S4 at an appropriate timing, and is
The image of the eighth page is formed on the back side of the sheet.

(R-2) Then, the sheet S4 on which an image is formed only on one main surface (rear surface) is separated from the rear end of the preceding sheet S3 by a slight gap (ΔD). The sheet is conveyed to the sheet discharge path 641b (FIG. 13).

Thereafter, the above steps (o-) are performed until all the images up to the desired page are formed on the sheet.
1) to (r-2) are repeated.

As described above, also in the third image forming sequence, the same operation and effects as those of the first and second image forming sequences can be obtained. That is, according to this image forming sequence, both the paper discharge paths 641a and 641b function as switchback paths, and an image can be formed on both main surfaces of the sheet S with a simple configuration.
Also, a non-selected discharge position not selected as a discharge destination,
That is, the paper discharge path 641a corresponding to the standard paper discharge tray 11
, The switchback operation is performed, and the sheet can be conveyed in the optimum conveyance sequence as described above, and the throughput can be improved. In particular, in the step (p-1), the two sheets S2 and S3 are simultaneously conveyed, so that the throughput can be further improved.

Further, the sheets do not come into sliding contact with each other at all, and the contamination of the sheets, which has been a problem when two sheets are simultaneously conveyed in one sheet discharge path, does not occur.

Further, a continuous sheet, for example, sheet S
Following the discharge of the preceding sheet S4 of the sheets S4 and S4 toward the selected standard discharge tray 11, the succeeding sheet S4 following the preceding sheet S3 is selectively discharged corresponding to the standard discharge tray 11. Since the sheet is conveyed toward the paper path 641a, the interval ΔD between successive sheets can be narrowed to perform image formation with excellent throughput.

C-4. Fourth Image Forming Sequence In the fourth image forming sequence, a color image is formed on both main surfaces of the A4 sheet and then discharged to the standard paper output tray 11. Hereinafter, FIGS. It will be described with reference to FIG.

FIGS. 14 and 15 are schematic views showing a fourth embodiment (fourth image forming sequence) of the image forming method according to the present invention. FIG. 16 is a diagram schematically showing the sheet conveyance status at the timing T6 in FIG.

(S-1) First, when an image signal is input from the host computer to the control unit 7, the image of the second page (the image to be formed on the back surface of the first sheet) is formed on the photosensitive member 21.
After the formation of the electrostatic latent image corresponding to the above, the yellow developing unit 23Y is selectively brought into contact with the photoconductor 21 to form a yellow toner image on the surface of the photoconductor 21. Then, the toner image is primarily transferred to the intermediate transfer belt 46.

(S-2) Similarly, the yellow toner image of the image of the fourth page (the image to be formed on the back surface of the second page) is formed side by side with the yellow toner image of the second page.

(T-1) Further, a cyan toner image, a magenta toner image and a black toner image are superimposed on each toner image to form a color intermediate toner image on the intermediate transfer belt 46.

Along with the formation of the intermediate toner image, A
The four-size sheet S1 is conveyed from the cassette 61 or the like to the gate roller pair 637 and is kept on standby. Then, the intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form a second page image on the back side of the first sheet.

(T-2) The sheet S1 on which the image of the second page has been formed is conveyed to the first paper discharge path 641b connected to the multi-bin unit 8.

In parallel with this, the image of the fourth page (the image to be formed on the back surface of the second sheet) is formed on the back surface of the next sheet S2. That is, after the sheet S1, the next sheet S2 is transferred from the cassette 61 or the like to the gate roller pair 637.
The intermediate toner image on the intermediate transfer belt 46 is secondarily transferred to the sheet S2 at an appropriate timing, and is fixed by the fixing unit 5, and is transferred to the back side of the second sheet.
An image for the page is formed.

(U-1) The sheet S2 on which the image of the fourth page is formed is connected to the standard paper output tray 11 which is previously selected as the paper output position, as shown in FIG.
The sheet is conveyed to the sheet discharge path 641a. At this time, since the sheet discharging unit 64 is configured as described above in detail, the sheet S1 is re-fed on the first sheet discharging path 641b at the same time when the sheet S2 is conveyed on the second sheet discharging path 641a. The sheet is reversely conveyed toward the section 66. Thus, according to this image forming sequence, the step (e) of the first image forming sequence, the second
As in the step (k-1) of the image forming sequence and the step (p-1) of the third image forming sequence, the two sheets S1, S2
Can be simultaneously performed, the processing time required for image formation can be reduced, and the throughput can be improved.

In parallel with this, a yellow toner image corresponding to the image of the first page (the image to be formed on the surface of the first sheet) is formed on the intermediate transfer belt 46.

(U-2) Similarly, the yellow toner image of the image of the third page (the image to be formed on the surface of the second sheet) is formed side by side with the yellow toner image of the first page.
At the same time, the sheet S1 being conveyed to the sheet re-feeding section 66 is conveyed toward the gate roller pair 637 and is made to stand by.

(V-1) Further, a color intermediate toner image is formed on the intermediate transfer belt 46 by superimposing a cyan toner image, a magenta toner image, and a black toner image on each toner image.

The intermediate toner image thus formed is secondarily transferred onto the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5 to form a first page image on the front side of the first sheet. Thus, the image formation on both sides of the sheet S1 is completed.

In the step (u-1), the second paper discharge path 641a
The second sheet S2 conveyed to the re-feeding section 66 at an appropriate timing is conveyed reversely, and further conveyed to the gate roller pair 637 following the sheet S1 and is put on standby.

(V-2) The sheet S 1 on which images are formed on both sides is discharged toward the standard discharge tray 11, and the intermediate toner formed on the intermediate transfer belt 46 in parallel with the discharge. The image is secondarily transferred to the sheet S2 and fixed by the fixing unit 5 to form a third page image on the front side of the second sheet.

(W-1) The sheet S2 on which images are formed on both sides in this manner is discharged toward the standard discharge tray 11 following the previously discharged sheet S1.

In parallel with this, a yellow toner image corresponding to the image of the sixth page (the image to be formed on the back of the third sheet) is formed on the intermediate transfer belt 46.

After that, the above steps (s-) are repeated until all images up to the desired page are formed on the sheet.
2) to (w-1) are repeated.

As described above, also in the fourth image forming sequence, the same operation and effect as those in the first to third image forming sequences can be obtained. That is, according to this image forming sequence, both the paper discharge paths 641a and 641b function as switchback paths, and an image can be formed on both main surfaces of the sheet S with a simple configuration.
Also, a non-selected discharge position not selected as a discharge destination,
That is, the paper discharge path 641 corresponding to the multi-bin unit 8
Since the switchback operation is performed in b, the sheet can be transported in the optimal transport sequence as described above, and the throughput can be improved.

In particular, in the step (u-1), two sheets S1,
Since the transfer of S2 is performed at the same time, the throughput can be further improved.

Further, the sheets do not come into sliding contact with each other at all, and the contamination of the sheets, which has been a problem when two sheets are conveyed simultaneously in one sheet discharge path, does not occur.

C-5. Fifth Image Forming Sequence In the fifth image forming sequence, a color image is formed on both main surfaces of an A4 sheet and then discharged to the multi-bin unit 8. 17 and 18 are schematic views showing a fifth embodiment (fifth image forming sequence) of the image forming method according to the present invention.

As is apparent from comparing these figures with the drawings (FIGS. 14 and 15) showing the fourth image forming sequence, the fifth image forming sequence is the fourth image forming sequence.
The only difference from this image forming sequence is that the sheet discharge path is the first discharge path 641b, and the other points are substantially the same. For this reason,
Here, a detailed description of the fifth image forming sequence will be omitted.

Also in the fifth image forming sequence, the paper discharge paths 641a and 641b both function as switchback paths, and an image can be formed on both main surfaces of the sheet S with a simple configuration. In addition, the switchback operation is performed at a non-selected paper discharge position that is not selected as a paper discharge destination, that is, at a paper discharge path 641a corresponding to the standard paper discharge tray 11, and the sheet is conveyed in the above-described optimal conveyance sequence. And the throughput can be improved. In particular, two sheets S1, S2
Are carried out at the same time, whereby the throughput can be further improved.

In addition, the sheets do not come into sliding contact with each other at all, and the dirt on the sheets, which is a problem when two sheets are simultaneously conveyed in one sheet discharge path, does not occur.

D. Others The present invention is not limited to the above embodiments,
Various changes other than those described above can be made without departing from the gist of the invention. For example, in the image forming apparatus according to the embodiment, the two paper ejection paths 641
a and 641b, the sheet is conveyed in a switchback manner. However, the application of the present invention is not limited to the image forming apparatus having two paper discharge paths, and is not limited to three.
The present invention can also be applied to an image forming apparatus having two or more paper discharge paths. In this case, the same effect as that of the above-described embodiment can be obtained by performing switchback conveyance of sheets in each paper discharge path.

In the above-described embodiment, the multi-bin unit is provided as a finisher. However, the type of the finisher is not limited to this. And a punch unit for forming a punch hole.

[0156]

As described above, according to the image forming apparatus and the image forming method according to the present invention, a plurality of discharge paths for discharging sheets also function as switchback paths.
The reversing device dedicated to switchback, which is an essential component in the conventional image forming apparatus, is not required. As a result, images can be formed on both main surfaces of the sheet with a simple configuration. In addition, each paper discharge path also functions as a switchback path, and the throughput can be improved by forming an optimal sheet conveyance sequence.

Further, a first drive roller rotatable in forward and reverse directions is disposed at a position sandwiched between the first and second discharge paths, and the first and second discharge paths are connected to the first drive roller. In the case where the first and second driven rollers are arranged to face each other so as to sandwich the sheet and constitute the conveying means, bidirectional conveyance of the sheet can be performed with a small number of parts.

By configuring the mechanism for transporting the sheet by the driving roller and the two driven rollers in this manner, the number of parts of the mechanism can be suppressed, and the first and second paper discharges are described. The present invention can be applied to the carrying-in / out means for transporting the sheet in both directions in the path, the carrying-in means for drawing the sheet into the switching means near the switching means, and the carrying-out means for carrying out the sheet from the switching means. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an image forming apparatus according to the present invention.

FIG. 2 is a view showing the inside of the case of FIG. 1;

FIG. 3 is an enlarged perspective view showing a switching unit of FIG. 2;

FIG. 4 is a block diagram illustrating a schematic configuration of a control unit that controls the image forming apparatus of FIG. 1;

FIG. 5 is a schematic diagram showing a first embodiment (first image forming sequence) of the image forming method according to the present invention.

FIG. 6 is a diagram schematically illustrating a sheet conveyance state at a timing T1 in FIG. 5;

FIG. 7 is a diagram schematically illustrating a sheet conveyance state at a timing T2 in FIG. 5;

FIG. 8 is a partially enlarged view of FIG. 7;

FIG. 9 is a schematic diagram showing a second embodiment (second image forming sequence) of the image forming method according to the present invention.

FIG. 10 is a diagram schematically illustrating a sheet conveyance state at a timing T3 in FIG. 9;

FIG. 11 is a diagram schematically illustrating a sheet conveyance state at a timing T4 in FIG. 9;

FIG. 12 is a schematic diagram showing a third embodiment (third image forming sequence) of the image forming method according to the present invention.

FIG. 13 is a diagram schematically illustrating a sheet conveyance state at a timing T5 in FIG. 12;

FIG. 14 is a schematic diagram showing a fourth embodiment (fourth image forming sequence) of the image forming method according to the present invention.

FIG. 15 is a schematic diagram showing a fourth embodiment (fourth image forming sequence) of the image forming method according to the present invention.

FIG. 16 is a diagram schematically illustrating a sheet conveyance state at a timing T6 in FIG. 14;

FIG. 17 is a schematic diagram showing a fifth embodiment (fifth image forming sequence) of the image forming method according to the present invention.

FIG. 18 is a schematic diagram showing a fifth embodiment (fifth image forming sequence) of the image forming method according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 2 ... Image carrier unit 3 ... Exposure unit 4 ... Transfer unit 5 ... Fixing unit 6 ... Paper supply / discharge unit 7 ... Control unit 8 ... Multibin unit 11 ... Standard paper discharge tray 63 ... Paper supply part 64 ... Paper discharge part 65 ... Switching section 66... Re-feed section 71... Control section 630... Sheet feeding path 641 a... (Second) sheet discharging path 641 b ... (first) sheet discharging path 642 a ... (third) driven roller 642 b. Driving rollers 642c to 644c Driving rollers 643, 644 Discharge roller pair (conveying means) 643a, 644a (Second) driven rollers 643b, 644b (First) Driving roller 645: Discharge entrance roller pair (Carrying means) ) 646: Switchback exit roller pair (unloading means) 664: Refeed path S, S1 to S4: Sheet

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H028 BA06 BA09 BA16 BB02 3F049 DA12 LA02 LA05 LA07 LB01 LB08 3F053 EA02 EA04 EA05 EB04 EC02 LA02 LA05 LA07 LB01 LB08

Claims (9)

[Claims] 1. An image forming means for forming an image on a sheet, a finisher for performing predetermined post-processing on the sheet on which the image is formed by the image forming means, and a sheet on which an image is formed by the image forming means , A sheet feeding unit that feeds sheets to the image forming unit, and a sheet feeding unit that re-feeds a sheet having an image formed on one main surface by the image forming unit to the sheet feeding unit. A sheet feeding unit, and a sheet conveyed from the image forming unit;
Sheet discharging means for selectively discharging a sheet to the sheet discharging tray or the finisher, and reversely transporting the sheet to a side opposite to the sheet discharging tray and the finisher to guide the sheet to the sheet re-feeding means; An image forming apparatus that forms an image on a sheet, and discharges the sheet to a selected one of the sheet discharge tray and the finisher, wherein the sheet discharge means includes: a first sheet discharge path for guiding a sheet to the finisher; A second paper discharge path for guiding a sheet to the paper discharge tray; a switching unit for selectively switching a sheet conveyance path to a first or second paper discharge path; and the first paper discharge on the first paper discharge path side. A sheet is conveyed toward the finisher along the path side and the sheet is conveyed reversely, while a sheet is conveyed along the second discharge path toward the discharge tray on the second discharge path side. And again An image forming apparatus comprising the conveying means for inverting conveying the over bets. 2. The conveyance means, comprising: a first drive roller disposed at a position sandwiched between the first and second paper discharge paths, the first drive roller being rotatable in forward and reverse directions; A first driven roller disposed opposite to the first driving roller; and a second driven roller disposed opposite to the first driving roller so as to sandwich the second paper discharge path. The image forming apparatus according to claim 1. 3. The apparatus according to claim 2, wherein the switching unit is provided on a side opposite to the sheet discharge tray, and transfers the conveyed sheet to the second sheet.
A carry-in unit that conveys the paper toward the switching unit along a paper discharge path; and a conveyance unit that is provided on a side opposite to the finisher with respect to the switching unit, and is conveyed from the switching unit along the first paper discharge path. An unloading means for transporting the received sheet outside the apparatus;
The image forming apparatus according to claim 1, further comprising: And a second drive roller disposed in a position sandwiched between the first and second paper discharge paths and rotatable in a predetermined direction, and configured to sandwich the second paper discharge path. And a third driven roller disposed opposite to the second drive roller. The unloading means includes a second drive roller and a second driven roller sandwiching the first paper discharge path. 4. The image forming apparatus according to claim 3, comprising a fourth driven roller disposed opposite to the driving roller. 5. An image forming means for forming an image on a sheet, a sheet feeding means for feeding a sheet to the image forming means,
A sheet re-feeding unit for re-feeding a sheet on which an image is formed on one main surface by the image forming unit to the sheet feeding unit; Sheet discharging means for selectively discharging the sheet to any one of the positions, and reversely transporting the sheet to a side opposite to the plurality of sheet discharging positions to guide the sheet to the sheet re-feeding means. An image forming apparatus that forms an image on a sheet and discharges the sheet to a selected sheet discharging position, wherein the sheet discharging means is provided in one-to-one correspondence with each of the plurality of sheet discharging positions; A plurality of paper discharge paths for guiding the sheet to a corresponding paper discharge position, switching means for selectively switching the sheet conveyance path from the plural paper discharge paths, and a paper discharge corresponding to the sheet along each paper discharge path. To the position and reverse the sheet An image forming apparatus comprising: 6. A sheet on which images are formed on both main surfaces by the image forming means is discharged along a selected discharge path selected from a discharge path leading to a finisher and a discharge path leading to a discharge tray. A first step of feeding a reference sheet before image formation to the image forming unit along a paper feed path to form an image on one main surface of the reference sheet; A second step of transporting the reference sheet having an image formed only on its surface to one of the two discharge paths; and after the second step, reversing the transport direction of the reference sheet in the discharge path. A third step of reversing the reference sheet by transporting the reference sheet to the paper supply path side via a paper supply path; Paper to the reference sheet A fourth step of forming an image on a major surface, after the fourth step, a reference sheet on which an image on both principal surfaces are formed, and a fifth step of discharging the paper through the selection discharge path,
An image forming method comprising: 7. The method according to claim 7, wherein one of the reference sheets is provided before the second step.
An image is formed on one principal surface of the immediately preceding front sheet, and is conveyed to another front sheet discharge path different from the reference sheet discharge path on which the reference sheet is conveyed in the second step. A sixth step, wherein in the second step, the reference sheet is conveyed into the reference sheet discharge path, and at the same time, the front sheet is directed from the front sheet discharge path to the re-feed path. 7. The image forming method according to claim 6, wherein the image forming method is carried out. 8. The third step is a step of simultaneously feeding the reference sheet to a re-feeding path, a sheet immediately after the reference sheet, and a sheet on which an image is formed on one main surface.
7. The image forming method according to claim 6, wherein in the second step, the reference sheet is conveyed into another sheet discharge path for a subsequent sheet different from the sheet discharge path for a reference sheet. 9. The method according to claim 9, wherein the second step is to convey the reference sheet having an image formed only on the one main surface to another discharge path different from the selected discharge path. In the third step, simultaneously with the conveyance of the reference sheet to the re-feeding path, the previous sheet, which is one sheet before the reference sheet, and on which the images are formed on both main surfaces, is transferred to the selective discharge path. The image forming method according to claim 6, wherein the sheet is discharged along the path.