JP2001117291A - Device and method for image forming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device and an image forming method by which mono-chromatic both-sided images are formed with high through put. SOLUTION: Two switchback paths are provided, and maximum two sheets are successively inverted and carried. In rear face printing processing to form an image on the rear face of an unprocessed sheet, the successive processing of (n-1) sheets is executed and by keeping just one sheet at a stand-by state the successive processing of (n) sheets is executed for two sheets. Thus, image forming and switchback inversion are successively executed. Also, when the printing processing of the rear face is completed and also stand-by time passes, image forming processing for an opposite face (surface) for three sheets on whose single face the image is formed by surface printing processing is immediately and successively executed. Thus, the stand-by time for the image forming is shortened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus and an image forming method for forming a single-color image on both sides of a sheet.

[0002]

2. Description of the Related Art Many image forming apparatuses, such as printers, facsimile machines and copiers, having a structure capable of forming an image on both sides of a sheet have been conventionally provided. As such an image forming apparatus,
For example, there is an apparatus described in JP-A-10-83104. In this image forming apparatus, a black toner image is developed by a black developing device among the four color developing devices, and is primarily transferred to an intermediate transfer member. Then, the black toner image on the intermediate transfer body is secondarily transferred to a sheet fed from a feeding unit, thereby forming a monochrome image. Subsequently, after the sheet is conveyed to a fixing unit to fix the monochrome image on the sheet, the sheet is discharged to a discharge unit on the upper surface of the apparatus by a discharge roller.

Further, in order to form an image on the opposite side of a sheet having an image fixed on one side, a reversing unit is provided as a mechanism for reversing the sheet. The reversing unit is composed of two pairs of conveying rollers provided along the sheet discharge path and a flapper, and conveys the sheet subjected to the fixing process as described above to the discharge unit side by the pair of conveying rollers. After that, the sheet is reversely conveyed by rotating the pair of conveying rollers in reverse. At the time of the reverse conveyance, the flapper is switched, and is carried into the reverse conveyance path. Thereafter, the sheet is again conveyed to the position of the registration roller pair by the reversing path conveyance roller pair.

Then, in the same manner as described above, a monochrome image is formed on the opposite surface of the sheet to complete image formation on both sides of the sheet, and the sheet is discharged to a discharge section on the upper surface of the apparatus by discharge rollers.

[0005]

In the above prior art, monochrome double-sided printing is performed as described above. However, there is only one switchback path, and the following steps are performed until sheet reversal conveyance by the reversing unit is completed. Cannot be conveyed to the reversing unit, and it is necessary to idle the image forming process and wait for the reversing conveyance. For this reason, when a monochrome double-sided image is formed on a plurality of sheets, the standby time (idling time) of image formation, which is inevitable to execute the reverse conveyance of the sheet, is inevitably increased,
As a result, there is a problem that the number of pages on which an image can be formed per unit time, that is, the throughput is low.

The present invention has been made in view of the above problems, and has as its object to provide an image forming apparatus and an image forming method capable of forming a single-color double-sided image with high throughput.

[0007]

An image forming apparatus according to the present invention is an image forming apparatus for forming a monochromatic image on both sides of a sheet. In order to achieve the above object, an image forming apparatus for forming a monochromatic image on a sheet is provided. Means, sheet feeding means for feeding a sheet to the image forming means, re-feeding means for re-feeding the sheet to the sheet feeding means, and a sheet on which an image is formed only on one side by the image forming means. A switchback unit having n (n ≧ 2) switchback paths for inverting a switchback to the refeeding unit; and an image forming unit, the sheet feeding unit, the refeeding unit, and the switchback unit. Control means for controlling. The control unit performs the following n-sheet continuous processing and (n-1) -sheet continuous processing. One sheet is reversely conveyed from the switchback unit to the re-feeding unit, and the next sheet is processed. The one-sided processing that is executed at a stand-by time necessary for being able to be carried into the switchback means, and the one-sided processing is received after the elapse of the waiting time from the one-sided processing (2n-1). And (2) another surface processing for continuously forming an image on the other surface of each sheet by the image forming means.

[0008] The n-sheet continuous processing means that n unprocessed sheets are continuously processed by the image forming means.
After an image is formed on one side of each sheet, each sheet is conveyed to a different switchback path to perform switchback reversal, and the (n-1) continuous processing means the image forming means. Means that, for (n-1) unprocessed sheets, an image is continuously formed on one surface of each sheet, and then each sheet is conveyed to a different switchback path to perform switchback reversal. .

Further, the image forming method according to the present invention comprises:
After a single color image is formed on one side of the sheet by the image forming means, the sheet is selectively switched back in one of n (n ≧ 2) switchback paths,
An image forming method for forming a single-color image on the opposite surface of the sheet by the image forming means. In order to achieve the above object, the n-sheet continuous processing and the (n-1) continuous processing are performed on one sheet. A one-side processing step in which the sheet is reversed and conveyed from the switchback means and is separated by a standby time necessary for the next sheet to be able to be carried into the switchback means; and After a lapse of only one, the one-side processing step was performed (2n-
1) The other surface processing step of continuously forming an image by the image forming means on the other surface of each sheet of the sheets.

In these inventions, n switchback paths (n ≧ 2) are provided for switchback reversing conveyance of the sheet on which an image is formed only on one side by the image forming means to the refeeding means. , A maximum of n sheets can be continuously conveyed reversely. In the one-side processing for forming an image on one side of the unprocessed sheet, a plurality of sheets are continuously switched back and inverted except during the standby time. Further, as soon as the one-side processing is completed and the standby time elapses, the image forming processing on the opposite side is continuously performed on the (2n-1) sheets on which an image is formed on one side by the one-side processing. Is executed.

Here, the sheet on which a double-sided image is to be formed is (2)
When more than (n-1) sheets are processed, the one-side processing and the other-side processing are repeated for every (2n-1) sheets.

[0012]

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a diagram showing an embodiment of an image forming apparatus according to the present invention. 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, 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 means for forming an image on a sheet. 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.

When an image signal is given 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 equipped with a multi-bin unit (not shown) on the upper side and a finisher (not shown) on the side as optional. Openings 13 are provided on the top and side surfaces of case 1 for transport to the bin unit and finisher.
14 are provided.

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. The image carrier unit 2 is a photoconductor 2 rotatable in the direction of the arrow in FIG.
1, a charging roller 22 as charging means, developing devices 23Y, 23C, 23M, 23K as developing means, and a cleaning unit 24 are arranged around the photoreceptor 21 in the direction of rotation thereof. ing. 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. This exposure unit 3
Scans and exposes the laser beam L onto the photosensitive member 21 in accordance with an image signal given from an external device such as a host computer to form an electrostatic latent image on the photosensitive member 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.

A cleaning section 24 is disposed at a position which is advanced from the primary transfer region R1 in the circumferential direction (the direction of the arrow in FIG. 1). 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 on the developing unit 23 side (the left-hand side in FIG. 1) of the primary transfer area R 1, and applies tension to the intermediate transfer belt 46 with the driving roller 41. 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 an 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 way is further fed by the sheet feeding / discharging unit 6 to the sheet feeding path 6.
The paper is fed to the standard paper discharge tray 11 and the multi-bin unit 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 for conveying the sheet S to the next transfer area R2 and the fixing unit 5, and the sheet S on which the image has been fixed is selectively discharged to the standard sheet discharge tray 11, the multi-bin unit or the finisher, or the sheet S conveyed is A paper discharge unit 64 that reversely conveys to the re-feeding unit side, a switching unit 65 that selectively switches the paper discharge paths 641a and 641b of the sheet S in the paper discharge unit 64, and a sheet that is conveyed from the paper discharge unit 64 S
And a paper re-feeding section 66 for feeding paper to the paper feeding section 63 again.

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. 1) 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 feed 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 that has been subjected to the fixing process by the fixing unit 5 is fed to the sheet discharging section 64. The paper discharge section 64 has two paper discharge paths 641a,
641b, one of the paper ejection paths (second paper ejection path; second switchback path) 641a extends from the fixing unit 5 to the standard paper ejection tray 11, and the other paper ejection path (first paper ejection path). Route; first switchback route) 64
Reference numeral 1b extends between the re-feeding unit 66 and the multi-bin unit substantially in parallel with the paper discharge path 641a. Three drive rollers 64 are provided along these paper discharge paths 641a and 641b.
2c to 644c are separated by a predetermined interval,
41a and 641b. In addition, driven rollers 642a to 644a are disposed corresponding to the drive rollers 642c to 644c, respectively, and driven rollers 642b to 644b are disposed, respectively, in the discharge path 641b. .

The driving roller 642c provided in the vicinity of the fixing unit 5 and the sheet re-feeding section 66 among the three driving rollers 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 paper discharge unit 64 functions as a switchback unit that switches back and reversely conveys the sheet on which an image is formed only on one side by the image forming unit to the refeed unit 66, and includes two (n = 2) switchback paths. 641a and 641b. In this embodiment, both switchback paths 641a and 641b also function as a paper discharge path, and can discharge the sheet S on which images are formed on both sides as described later.

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.

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 paper discharge path 641b while the sheet S is conveyed as it is along the paper 41b and discharged to the multi-bin unit, the sheet S is discharged to the paper discharge path 641b.
b, and is discharged to the finisher via 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. 2 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 the tip 651b is directed toward the first driven roller 642b as shown by the 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 guides the sheet once conveyed to the second discharge path 641a to the re-feeding section 66, and is provided with a guide plate 657 for guiding one surface of the first 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 paper discharge path 641b shown 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 paper discharging device is configured by incorporating the switching unit 65 into the paper discharging unit 64 having the plural paper discharging paths 641a and 641b. 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. 1, the configuration of the re-feeding unit 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. 3 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 unit 79 including operation buttons and a display panel, an image carrier unit controller 72 for controlling the operation of the image carrier unit 2,
An exposure unit controller 73 that controls the operation of the exposure unit 3, a transfer unit controller 74 that controls the operation of the transfer unit 4, a fixing unit controller 75 that controls the operation of the fixing unit 5, and controls the operations of the paper supply / discharge unit 6. It is electrically connected to the paper supply / discharge unit controller 76 and gives a control command to each section of the apparatus to control the entire apparatus. 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, an operation of the image forming apparatus configured as described above, particularly, an image forming sequence most relevant to the features of the present invention will be described in detail.

In this image forming sequence, a monochrome image is formed on both sides of an A4 sheet and then discharged to a standard discharge tray. This will be described below with reference to FIGS. 4 and 6.

FIG. 4 is a schematic diagram showing a first embodiment (first image forming sequence) of the image forming method according to the present invention. FIG. 5 is a diagram schematically showing the sheet conveyance status at the timing T in FIG.

In FIG. 4, "intermediate transfer belt"
The column indicates a toner image primarily transferred to the intermediate transfer belt 46. Also, 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 of one A3 size toner image or two A4 size toner images. 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: front surface, B: back surface; parenthesis numbers: page numbers; parenthesis alphabetic characters: K: black The display contents other than “parenthesis characters” are shown in FIG. Commonly used in the columns of “gate roller”, “first discharge path” and “second discharge path”, and the sheet attributes shown in FIGS. 6 and 9 to 11 described later. Have been.

A: Back side printing processing (one side processing) a-1: (n-1) sheet continuous processing First, when an image signal is input from the host computer to the control unit 7, the image of the second page (one sheet) Image to be formed on the back of the eye)
The A4 size sheet S1 is transported from the cassette 61 or the like to the gate roller pair 637, and is kept on standby. And
The intermediate toner image is secondarily transferred to the sheet S1 at an appropriate timing, and is fixed by the fixing unit 5, and the second image is formed on the back side of the first sheet.
An image for the page is formed. Then, the sheet S1 on which the image of the second page is formed is conveyed to a second discharge path (second switchback path) 641a connected to the standard discharge tray 11 previously selected as a discharge position, and the switchback is performed. The sheet is inverted and conveyed to the sheet re-feeding section 66.

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. To adjust the number of sheets to be continuously processed and the standby time for image formation. That is, in this embodiment, the second paper discharge path 641
The switchback reversal of the sheet S1 is performed by a. If the next sheet is carried into the paper discharge unit (switchback means) 64 before the switchback reversal is completed,
Both sheets interfere. Therefore, following the (n-1) sheet continuous processing, a standby time necessary for one sheet to be reversely conveyed from the paper discharge unit 64 and the next sheet to be able to be carried into the paper discharge unit 64 is secured. Therefore, the intermediate toner image is formed only on the leading side of the intermediate transfer belt 46. 4 and FIG. 7 and FIG. 10 which will be described later, in order to clearly indicate that image formation is regulated or on standby for such a reason.
12 to 12 show a non-image forming portion (a square region surrounded by a broken line) X.

A-2: Elapse of standby time Since the intermediate toner image is formed only on the leading side of the intermediate transfer belt 46 as described above, from the image forming processing of the second page to the next image forming processing The sheet S2 on which the image forming process is regulated and waits, and an image is formed in the next n-sheet continuous process
While avoiding the interference with the sheet S1, the sheet S1 is switched back and reversed in the paper discharge path 641a and conveyed to the re-feeding section 66 in reverse.

A-3: n-sheet continuous processing When the standby time elapses, an intermediate toner image corresponding to the image of the fourth page (the image to be formed on the back surface of the second page)
An 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 formed side by side on the intermediate transfer belt 41. Then, at an appropriate timing, the sheet S2
Is fed, and the intermediate toner image of the fourth page is secondarily transferred to the sheet S2, and is fixed by the fixing unit 5 to form the image of the fourth page on the back side of the second sheet. Then, the sheet S2 on which the image of the fourth page is formed is conveyed to a first discharge path (switchback path) 641b connected to a multi-bin unit (not shown). Similarly, 6
The image of the page is continuously formed on the back surface of the next sheet S3.

The sheet S3 on which the image of the sixth page has been formed is conveyed to the other second sheet discharge path (second switchback path) 641a as shown in FIG. 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. Thus, according to this image forming sequence, the two sheets S2 and S3
Can be continuously performed.

B: Elapse of Standby Time (Standby Process) Following the back surface printing process for the three sheets S1 to S3,
An image is formed on the front side of each of the sheets S1 to S3. That is, an intermediate 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 rear end side of the intermediate transfer belt 46. The reason is that as described above, in the final stage of the n-sheet continuous processing, the switchback reversal of the sheet S3 is performed by the second discharge path 641a, but before the switchback reversal is completed, the next sheet S1 is changed. This is because both sheets S1 and S3 interfere with each other when the sheet S is conveyed to the sheet discharge section 64. Therefore, in order to avoid this, sheet 1
In order to secure a standby time necessary for the sheet to be reversed and conveyed from the paper discharge unit (switchback means) 64 to allow the next sheet to be carried into the paper discharge unit 64, only the rear end side of the intermediate transfer belt 46 is provided. An intermediate toner image is formed.

C: Front side printing processing (other side processing) Sheet S already conveyed to re-feeding section 66 earlier
5, the sheet S1 is conveyed to the gate roller pair 637 and is on standby as shown in FIG.
Is fed toward the secondary transfer region R2, and an intermediate toner image corresponding to the image of the first page is secondarily transferred onto the sheet S1, and is fixed by the fixing unit 5 to fix the first page on the front side of the first sheet. Is formed. Thus, the image formation on both sides of the sheet S1 is completed, and the sheet S1 on which the images are formed on both sides is discharged toward the standard discharge tray 11 selected in advance.

The image of the third page (the image to be formed on the second surface) and the image of the fifth page (the image to be formed on the third surface) are continuously printed on each sheet S.
2, S3, which has been discharged earlier
Following 1, the paper is discharged toward the standard paper discharge tray 11.

The number of sheets on which double-sided images are to be formed is 3 (=
When the number of sheets exceeds 2 × 2-1), the back surface printing process and the front surface printing process are repeated for every 3 (= 2 × 2-1) sheets until all the images up to the desired page are formed on the sheet.

As described above, two switchback paths are provided, and a maximum of two sheets can be continuously reversed and conveyed, and a back surface for forming an image on the back surface of the unprocessed sheet is provided. In the printing process (one-sided process), the two sheets are continuously switched back and inverted except during the standby time, and the back side printing process is completed. The image forming process on the opposite surface (front surface) is continuously performed on the three sheets on which the image is formed on one surface by the process (one-side process), so that the sheets are switched back in the switchback path. The throughput can be improved as compared with the conventional example in which the image formation is on standby by the idling process each time the process is performed.

In this embodiment, in the backside printing process, after the (n-1) -sheet continuous processing is first executed, the n-sheet continuous processing is executed after the elapse of the standby time (FIG. 6).
(A): Two-sheet picking (pattern A)), as shown in FIG. 2B, the n-sheet continuous processing and the (n-1) -sheet continuous processing may be interchanged to perform the back side printing processing ( 2 sheets (pattern B)). Also, the back side printing process is repeated for every 3 (= 2 × 2-1) sheets, but in each back side printing process, as shown in FIG. Is executed first, and after the elapse of the standby time, a pattern A in which the n-sheet continuous processing is executed, and the elapse of the standby time after the n-sheet continuous processing is executed first as shown in FIG. , The pattern B for executing the (n-1) continuous processing may be selectively executed. 9 to 1 described later with respect to the modification in the back side printing process.
The same applies to the embodiment shown in FIG.

Although the image forming apparatus according to the above embodiment has two switchback paths 641a and 641b, one or more switchback paths may be added to the apparatus shown in FIG. 7 and FIG. 8, and a switchback path 82 for switching back the sheet by a pair of conveying rollers 81 provided corresponding to each bin is represented by n (n ≧ 2: in these figures, The present invention can be applied to a device provided with n = 4) devices. When the multi-bin unit is used as described above, the switchback path does not have to be configured by all the conveying roller pairs. For example, a multi-bin unit in which m (> n) conveying roller pairs are provided. In the above, n switchback paths may be configured by n transport roller pairs.

The image forming sequence when the number of switchback paths is "3", "4" or "n" as described above is shown in FIGS. 9 to 11, respectively. That is, FIG.
As shown in (a), when three switchback paths are provided (n = 3), the back printing process (one-side processing) is performed on the second and fourth pages of two sheets. (N-1) sheet continuous processing for forming an image, standby time lapse processing for waiting for image formation for one sheet, and n for forming images on pages 6, 8, and 10 for three sheets The continuous sheet processing is executed in this order. Then, after a lapse of the waiting time from the back side printing processing, 5 (= 2
× 3-1) A front surface printing process for continuously forming images of the first, third, fifth, seventh and ninth pages on the opposite surface (front surface) of each sheet is executed.

Further, as shown in FIG. 10A, when four switchback paths are provided (n = 4), the back side printing processing (one side processing) is performed for three sheets. To form images on pages 2, 4, and 6 (n-
1) Continuous sheet processing, standby time elapse processing for waiting for image formation for one sheet, and 8,1 for four sheets
The n-sheet continuous processing for forming the images on the 0th, 12th, and 14th pages is executed in this order. Then, after a lapse of the standby time from the back side printing process, 7 (= 2 × 4-
1) A front surface printing process for continuously forming images of the first, third, fifth, seventh, ninth, eleventh, and thirteenth pages on the opposite surface (front surface) of each sheet is executed.

Further, as shown in FIG. 11A, when n switchback paths are provided, the back side printing processing (one side processing) is performed on (n-1) sheets. 2, 4, 6,... (N-1) sheet continuous processing for forming an image of the (2n-2) page, standby time elapse processing for waiting for image formation for one sheet, and n sheets On the other hand, the n-th continuous processing for forming the image of the 2n, (2n + 2),..., (4n−3) pages is executed in this order. Then, after a lapse of the standby time from the back side printing process, the (2n-1) sheets are continuously
1,3, ..., (2n
-3) The front side printing process for forming the image of the page is executed.

The present invention is not limited to the above embodiment, and various changes other than those described above can be made without departing from the gist of the present invention. For example, in the above embodiment, the present invention is applied to an image forming apparatus capable of forming color and monochrome images, but the present invention can be applied to an image forming apparatus dedicated to monochrome. Further, in this embodiment, a monochrome image is formed using black toner. However, it is needless to say that the present invention can be applied to a case where a single color image is formed using another toner color alone. .

In the above embodiment, the back side printing process is executed as the one-sided process, and the front side printing process is executed as the other-sided process. In the direction processing, an image of an even page may be formed.

In the above embodiment, a monochrome double-sided image is formed on an A4 size sheet. However, the sheet size to which the present invention can be applied is not limited to this, and is arbitrary.

Further, in the above embodiment, the intermediate transfer belt 46 having the seam portion is used as the intermediate transfer medium. However, as the intermediate transfer medium, the intermediate transfer belt without the seam, the intermediate transfer drum used in the conventional example, and the like are used. May be adopted. Further, the present invention can be applied to an image forming apparatus that directly transfers a toner image on a photoconductor to a sheet without using an intermediate transfer medium.

[0074]

As described above, according to the present invention, the number of switchback paths (n ≧ n) is set so that the sheet on which an image is formed on one side only by the image forming means is switched back to the refeeding means. 2) Since it is provided, a maximum of n sheets can be continuously reversely conveyed. Also, in the one-sided process of forming an image on one side of the unprocessed sheet, a plurality of sheets can be continuously switched back and inverted except during the standby time, and the one-sided process is completed. As soon as the standby time has elapsed, an image is formed on one side by one-side processing (2).
(n-1) Since the image forming process on the opposite surface can be continuously performed on the sheets, the standby time (idling time) of the image formation, which is inevitable for performing the reverse conveyance of the sheet, is reduced. It can be shortened to improve throughput.

[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 an enlarged perspective view showing a switching unit of FIG.

FIG. 3 is a block diagram illustrating a schematic configuration of a control unit that controls the image forming apparatus of FIG. 1;

FIG. 4 is a schematic diagram showing a first embodiment (first image forming sequence) of the image forming method according to the present invention.

FIG. 5 is a diagram schematically illustrating a sheet conveyance state at a timing T in FIG. 4;

FIG. 6 is a schematic diagram showing an image forming sequence when there are two switchback paths.

FIG. 7 is a diagram showing another embodiment of the image forming apparatus according to the present invention.

FIG. 8 is a diagram showing another embodiment of the image forming apparatus according to the present invention.

FIG. 9 is a schematic diagram showing an image forming sequence when there are three switchback paths.

FIG. 10 is a schematic diagram illustrating an image forming sequence when the number of switchback paths is four.

FIG. 11 is a schematic diagram illustrating an image forming sequence when the number of switchback paths is n.

[Explanation of symbols]

 2 Image carrier unit 3 Exposure unit 4 Transfer unit 5 Fixing unit 6 Paper supply / discharge unit 7 Control unit (control means) 8 Multi-bin unit 63 Paper feed unit 64 Paper discharge unit (switchback) Means 66 Re-feeding section 71 Control section (control means) 81 Conveying roller pair 82 Switchback path 641a, 641b Paper discharge path (switchback path) S, S1 to S3 Sheet

Claims (8)

[Claims] 1. An image forming apparatus for forming a single-color image on both sides of a sheet, an image forming unit for forming a single-color image on a sheet, a sheet feeding unit for feeding a sheet to the image forming unit, and a sheet feeding unit A sheet re-feeding unit for re-feeding the sheet to the paper unit; and n number of switchback paths (n ≧ 2) for switching back the sheet on which an image is formed only on one side by the image forming unit to the sheet re-feeding unit. ), And control means for controlling the image forming means, the paper feeding means, the paper re-feeding means and the switchback means. The control means comprises: -1) In the continuous sheet processing, one sheet is reversely conveyed from the switchback unit to the refeeding unit so that the next sheet can be carried into the switchback unit. A one-sided process that is executed at a necessary standby time interval, and after the one-sided process has elapsed, the (2n-1) sheets that have been subjected to the one-sided process are successively processed. An image forming apparatus for performing another surface processing for forming an image on the other surface of the sheet by the image forming unit. The n-sheet continuous processing includes:
After forming an image on one side of each of the n unprocessed sheets continuously by the image forming unit, the respective sheets are conveyed to different switchback paths to perform switchback reversal. The (n-1) sheet continuous processing is such that after the (n-1) unprocessed sheets are successively formed on one surface of each sheet by the image forming means, the respective sheets are switched to different switchback paths. This is the process of transporting and inverting the switchback. 2. The sheet on which a double-sided image is to be formed is (2n-
2. The image forming apparatus according to claim 1, wherein when the number of sheets exceeds one, the control unit repeats the one-side processing and the other-side processing every (2n−1) sheets. 3. The image forming apparatus according to claim 1, wherein one of the n switchback paths also functions as a paper discharge path for discharging a sheet having an image formed on both sides. 4. After forming a monochromatic image on one side of a sheet by an image forming means, said sheet is selectively switched back and inverted in one of n (n ≧ 2) switchback paths. An image forming method for forming a monochromatic image on the opposite surface of the sheet by the image forming means, wherein the following n-sheet continuous processing and (n-1) -sheet continuous processing are performed such that one sheet is reversely conveyed from the switchback means. A one-side processing step of executing the next sheet at a standby time necessary to be able to be carried into the switchback means, and further after the standby time has elapsed from the one-side processing, the one-side processing The (2n-1) sheets that have undergone the processing step are provided with a second-side processing step of continuously forming an image by the image forming unit on the other surface of each sheet. Image forming method according to. In the n-sheet continuous processing, the image forming unit continuously forms an image on only one surface of each of the n unprocessed sheets, and then conveys each sheet to a different switchback path to perform switchback. The (n-1) continuous processing is performed after the (n-1) unprocessed sheets are continuously formed by the image forming unit on only one surface of each sheet. This is a process of transporting each sheet to a different switchback path and inverting the switchback. 5. The sheet on which a double-sided image is to be formed is (2n-
5. The image forming method according to claim 4, wherein when the number of sheets exceeds one, the control unit repeats the one-side processing step and the other-side processing step for every (2n−1) sheets. 6. A switch-back unit after forming a monochrome image on one surface of each of n (n ≧ 2) unprocessed sheets by the image forming unit continuously.
A first step of transporting each sheet to each of the switchback paths and inverting the switchback, and after the first step, inverting and transporting one sheet from the switchback means and switching the next sheet to the switchback means. Wait for the waiting time necessary to be able to carry into the second
And after the second step, (n-1)
A third step of forming a single-color image on one side of each sheet continuously for the unprocessed sheets, and then transporting each sheet to a different switchback path to reverse the switchback; and after the third step, A fourth step of waiting for the standby time; and after the fourth step, the (2n-1) sheets on which a monochromatic image is formed only on one side are continuously formed on the other side of each sheet. A fifth step of forming an image by means. 7. A switchback means after forming (n-1) (where n ≧ 2) unprocessed sheets of a single color image on one side of each sheet continuously by the image forming means. a first step in which each sheet is conveyed to a different switchback path among the n switchback paths and switchback is reversed, and after the first step, one sheet is reversed and conveyed from the switchback means and the next step is performed. A second waiting state for a waiting time necessary for the sheet to be able to be carried into the switchback means;
After the second step, the image forming unit continuously forms a single-color image on one surface of each of the n unprocessed sheets by the image forming unit, and then conveys each sheet to a different switchback path. A third step of performing switchback reversal, a fourth step of waiting for the standby time after the third step, and (2n-1) sheets having a single-color image formed only on one surface after the fourth step And a fifth step of continuously forming an image on the other surface of each sheet by the image forming means. 8. The sheet on which a double-sided image is to be formed is (2n-
8. The image forming method according to claim 6, wherein when the number of sheets exceeds one, the first to fifth steps are repeated.