JP2002244357A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve throughput in terms of A4 size paper. SOLUTION: This image forming device is equipped with a returning path 40 switching back paper carried toward a paper ejection part 11 by a carrying path 30 for carrying the paper on either surface of which an image is formed by an image forming part 20 to the ejection part 11 and returning the paper to the image forming part so that the image may be formed on the other surface of the paper, and the loop length of a loop-shaped path R formed by the carrying path and the returning path is set to be longer than (the length of A4 size paper in the width direction) × 2 + paper interval × 2 and shorter than (the length of A4 size paper in the width direction) × 2 + paper interval × 3. The image forming part is equipped with at least an intermediate transfer body 25 having peripheral length such as the length of A4 size paper in the width direction × 2, and the loop length of the loop-shaped path formed by the carrying path and the returning path is set to be longer than the peripheral length of the intermediate transfer body and shorter than the peripheral length of the intermediate transfer body + paper interval.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming and discharging an image on a sheet (a sheet-shaped recording medium (including an envelope)), and forming an image on the sheet mainly using an electrophotographic technique. The present invention relates to an image forming apparatus such as a printer, a facsimile, a copying machine, and the like for discharging. In particular, the present invention relates to an image forming apparatus capable of forming a monochrome image or a color image on both sides of a sheet.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus capable of forming an image on both sides of a sheet is disclosed in, for example, Japanese Patent No. 2692.
The one described in Japanese Patent No. 957 is known. In this image forming apparatus, as shown in FIG.
Forming unit 1 having photoreceptor 1a and the like for forming an image on
And a transport path 3 for transporting the sheet S, on which an image is formed on one side by the image forming unit 1, to a discharge tray 2 as a sheet discharge unit, and transporting the sheet S toward the sheet discharge unit 2 by the transport path 3. And a return path 4 for returning the sheet S to the image forming unit 1 in order to switch back the sheet S to form an image on the other side. Between the transport path 3 and the paper discharge unit 2, a drive roller 5 a capable of rotating in the normal and reverse directions is provided.
a driven roller 5b that can be moved toward and away from the paper discharge path 2 to convey the paper from the conveyance path 3 toward the paper discharge section 2 or the paper S partially discharged to the paper discharge section 2 to the return path 4. A first transporting unit 5 for transporting the light toward the sensor and a sensor 6 are provided. A first registration roller pair 7 is provided at a junction below the conveyance path 3 and the return path 4, and a second registration roller pair 8 is provided on the return path 4 separately from the registration roller pair 7. Have been. The transport path 3 includes a transport roller 9.
The return path 4 is provided with a return roller 9b.

The operation at the time of double-sided printing in such an image forming apparatus is at least as follows according to the publication. First, the first sheet S printed on one side is conveyed from the conveying path 3 by the first conveying unit 5 toward the sheet discharge unit 2, and when the trailing end is detected by the sensor 6, the first conveying unit 5, the first sheet S is transported to the return path 4. When the sheet S conveyed to the return path 4 is conveyed by the return roller 9b, the first conveying means 5 stops rotating, and the driven roller 5b is separated from the drive roller 5a. The first sheet S conveyed to the return path 4 comes into contact with the second registration roller pair 8 and temporarily stops, and the leading end registration and the skew state are corrected. Thereafter, the first sheet S waits for a print signal for printing on the other side to come at the position where it is slightly fed. Then, the second sheet S
Is printed on one side, and the transport roller 9a stops before the trailing edge passes the transport roller 9a, whereby the rear edge of the second sheet S is held and stopped by the transport roller 9a.

Thereafter, when a print signal for the other side of the first sheet S comes, the second pair of registration rollers 8 rotates, and the first sheet S is imaged via the first pair of registration rollers 7. The sheet is sent to the forming section 1, printed on the other side, and discharged to the sheet discharge tray 2. The above operation is repeated.

[0005]

According to the conventional image forming apparatus described above, when images are formed on both sides of a sheet,
Regardless of the size of the paper, it is necessary to temporarily stop at the second pair of registration rollers 8 and wait for a print signal for printing on the other side to come, so that the throughput (the number of prints per unit time) is reduced. It cannot be improved.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus which can solve the above problems and can improve the throughput of at least A4 size paper.

[0007]

According to another aspect of the present invention, there is provided an image forming apparatus for forming an image on a sheet, and a sheet on which an image is formed on one side by the image forming section. Transport path for transporting the paper toward the paper discharge section,
A return path for switching back the paper transported toward the paper discharge section by the transport path and returning the paper to the image forming section so as to form an image on the other side, wherein the transport path and the return path are provided. The loop length of the loop-shaped path formed by (1) is longer than (Length in the widthwise direction of A4 size paper) × 2 + paper interval × 2, and (Length in the widthwise direction of A4 size paper) × 2 + The paper interval is shorter than 3 times. The image forming apparatus according to claim 2, wherein the image forming unit forms an image on a sheet, a conveying path that conveys the sheet on which an image is formed on one side by the image forming unit to a sheet discharging unit, and the conveying path. And a return path for returning the paper conveyed toward the paper discharge unit to the image forming unit so as to switch back the paper conveyed toward the paper discharge unit and form an image on the other surface, wherein the image forming unit is at least an A4 size paper. And an intermediate transfer member having a perimeter of 2 × 2 in the width direction of the paper, and the loop length of a loop formed by the transport path and the return path is the same as that of the intermediate transfer member. The length is longer than the circumference of the body, and is shorter than the circumference of the intermediate transfer body + the sheet interval. According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, a drive roller capable of normal and reverse rotation is provided between the transport path and the sheet discharge portion, and the drive roller is capable of coming and coming from the drive roller. And a first transport unit configured to transport the paper from the transport path toward the paper discharge section or transport the paper partially discharged to the paper discharge section toward the return path. And a driving roller that rotates only one side and a conveyance path-side roller that abuts on the driving path and is driven by the driving roller on the side opposite to the sheet discharge unit with respect to the first conveyance unit. A second conveying means having a return path roller which abuts and is driven by the drive roller on the return path side, and wherein the return path roller in the second conveying means is opposed to the drive roller. Characterized in that it is configured to be separable Te. According to a fourth aspect of the present invention, in the image forming apparatus according to the first or second aspect, when forming a monochrome image on both sides of three or more A4 size sheets, the three sets are transported as a set. While the paper is conveyed in the transverse direction on the path and the return path, an image is first formed on an even page, then an image is formed on an odd page, and if the last page is an odd page, the last page is formed. And forming an image of the odd page. In the image forming apparatus according to the fifth aspect, in the image forming apparatus according to the first or second aspect, when forming a color image on both sides of an A4 size sheet, the sheet is transported through the transport path and the return path. While forming an image on an even page first, forming an image on an odd page first, and, if the last page is an odd page, forming an image on the odd page last. I do. The image forming apparatus according to claim 6,
The image forming apparatus according to claim 3, wherein two or more sheets of A3
When forming a monochrome image on both sides of a sheet of paper,
While transporting the paper in the transport path and the return path as a set of two sheets in the longitudinal direction, first, an image is formed on an even page, then an image is formed on an odd page, and the last page is an odd page. In this case, the image of the odd-numbered page is finally formed. In the image forming apparatus according to claim 7, in the image forming apparatus according to claim 3, when a color image is formed on both sides of an A3-size sheet, the sheet is transported along the transport path and the return path.
First, an image is formed on an even-numbered page, then an image is formed on an odd-numbered page, and when the last page is an odd-numbered page, an image on the odd-numbered page is formed last. The “sheet interval” is the interval between the trailing edge of the preceding sheet and the leading edge of the succeeding sheet, and refers to the interval necessary for sheet conveyance and image forming timing.

[0008]

According to the image forming apparatus of the first aspect, an image forming section for forming an image on a sheet, and conveyance for conveying the sheet on which an image is formed on one side by the image forming section toward a sheet discharge section. And a return path for returning the paper conveyed toward the paper discharge unit by the transport path to the image forming unit so as to switch back the paper and form an image on the other surface. The loop length of the loop path formed by the return path and the return path is longer than (Length in the short direction of A4 size paper) × 2 + paper interval × 2, and (Length in the short direction of A4 size paper). ) × 2 + paper interval × 3,
With the configuration, the two A4 size sheets are transported in the short direction by the loop-shaped path formed by the transport path and the return path, without being temporarily stopped.
An image can be formed on both sides. Therefore,
According to the image forming apparatus of the first aspect, the throughput is improved. According to the image forming apparatus of the second aspect, an image forming unit that forms an image on a sheet, a conveying path that conveys the sheet on which an image is formed on one side by the image forming unit to a sheet discharging unit, A return path for returning the paper conveyed toward the paper discharge unit by the transport path toward the image forming unit so as to form an image on the other side by switching back the paper, and the image forming unit includes at least:
An intermediate transfer member having a perimeter of A4 size paper in the widthwise direction × 2 is provided, and the loop length of the loop path formed by the transport path and the return path is the same as the above. It is longer than the circumference of the intermediate transfer body and shorter than the circumference of the intermediate transfer body + the paper interval. Therefore, the loop-shaped path formed by the transport path and the return path provides:
It is possible to form images on both sides of two A4 size sheets without stopping them while transporting them in the short direction. Therefore, according to the image forming apparatus of the second aspect, the throughput is improved. According to the image forming apparatus of the third aspect, in the image forming apparatus of the first or second aspect, a drive roller capable of normal and reverse rotation is provided between the transport path and the sheet discharge section, and the drive roller is in contact with the drive roller. A first transporting means comprising a driven roller which can be separated and transports the paper from the transport path toward the paper discharge section, or transports the paper partially discharged to the paper discharge section toward the return path; And a drive roller that rotates only one side on the side opposite to the paper discharge section with respect to the first transport means, and a transport path side that abuts and is driven by the drive roller on the transport path side. A second conveying means having a roller and a return path roller which abuts on and is driven by the drive roller on the return path side, and the return path roller in the second conveying means is driven by the driving roller. , So that the loop length is not increased unnecessarily ((the length in the short direction of the A4 size paper) × 2 + the paper interval × 3, or the intermediate transfer member). (Without making the length longer than the length of the circumference of the sheet + the length of the sheet interval), it is possible to form an image on both sides of an A3 size sheet. According to the image forming apparatus of the fourth aspect, in the image forming apparatus of the first or second aspect, when forming a monochrome image on both sides of three or more sheets of A4 size, the three sheets are set as one set. While transporting the paper in the short side direction in the transport path and the return path, first form an image on an even page, then form an image on an odd page, and if the last page is an odd page, Finally, since the image of the odd-numbered page is formed, the sheets are discharged in a state where they are aligned in the page order. According to the image forming apparatus of claim 5, in the image forming apparatus of claim 1 or 2,
When forming color images on both sides of 4 size paper,
While transporting the paper in the transport path and the return path, first, an image is formed on even-numbered pages, then an image is formed on odd-numbered pages, and when the last page is an odd-numbered page, the image is finally formed. Since the image of the odd-numbered page is formed, the sheets are discharged in a state where they are aligned in the page order. According to the image forming apparatus of the sixth aspect, in the image forming apparatus of the third aspect, when forming a monochrome image on both sides of two or more sheets of A3 size, the two sheets are transported as a set. While the paper is transported in the longitudinal direction on the path and the return path, first, an image is formed on even-numbered pages, then an image is formed on odd-numbered pages, and if the last page is an odd-numbered page, finally, Since the image of the odd-numbered page is formed, the sheets are discharged in a state where the images are arranged in the page order. According to the image forming apparatus of the seventh aspect, in the image forming apparatus of the third aspect, when a color image is formed on both sides of an A3-size sheet, the sheet is transported along the transport path and the return path. On the other hand, first, an image is formed on an even page, then an image is formed on an odd page, and if the last page is an odd page, the image of the odd page is formed last. The sheets are discharged in the aligned state.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic front view showing an embodiment of the image forming apparatus according to the present invention. This image forming apparatus is an apparatus capable of forming a full-color image on both sides of a sheet, and includes a case 10, an image forming section 20 housed in the case main body 10 for forming an image on a sheet, and an image forming section 20. A conveyance path 30 that conveys the sheet on which an image is formed on one side by the forming section 20 toward a sheet discharge section (discharge tray section) 11 on the upper surface of the case main body 10, and the conveyance path 30 toward the sheet discharge section 11. A return path 40 is provided for returning the conveyed sheet to the image forming unit 20 so as to switch back the conveyed sheet and form an image on the other side. At a lower portion of the case main body 10, a paper feed tray 12 that holds a plurality of sheets in a stacked state and a paper feed roller 13 that feeds the sheets one by one toward the image forming unit 20 are provided.

The image forming section 20 has a photoreceptor 21 having a photosensitive layer on the outer peripheral surface, a charging unit 22 for uniformly charging the outer peripheral surface of the photoreceptor 21, and a uniform charging by the charging unit 22. Exposure means 23 for selectively exposing the outer peripheral surface of the photoconductor 21 to form an electrostatic latent image;
Developing means 24 for applying a toner as a developer to the electrostatic latent image formed by the developing means 24 to form a visible image (toner image), and transferring the toner image developed by the developing means 24 to a sheet to be transferred Transfer means 25 for transferring the photosensitive member 21 after the transfer.
And cleaning means 26 for removing the toner remaining on the surface.

The developing means 24 is a rotary developing device, and a developing roller 24Y for yellow, a developing roller 24C for cyan, a developing roller 24M for magenta, a developing roller 24M for black, and a main body 24a which is a rotating body. Developing roller 24
K are provided rotatably, and the main body 24a
Is rotated at a 90 ° pitch in the direction of the arrow, so that the surface of the photoconductor 21 can be selectively developed.

The transfer means 25 is an intermediate transfer member 25 which performs a primary transfer T1 of the toner image formed on the photoreceptor 21 and further performs a secondary transfer T2 of the toner image on a sheet to be transferred.
a. Intermediate transfer member 25a of this embodiment
Is composed of an intermediate transfer belt. The intermediate transfer belt 25a includes a driving roller 25b, a driven roller 25c,
The tension roller 25d and the guide roller 25e are stretched and circulated in the direction indicated by the arrow in the figure. The primary transfer portion T1 is formed between the guide roller 25e and the driven roller 25c, and the secondary transfer portion T2 is formed at a pressure contact portion between the driven roller 25c and the secondary transfer roller 27. The secondary transfer roller 27, as shown by the arrow,
The secondary transfer portion T2 is formed so as to be able to contact and separate from the driven roller 25c (therefore, to the intermediate transfer belt 25a), and to make contact therewith. Therefore, when a color image is formed, a plurality of color toner images are superimposed on the intermediate transfer belt 25a in a state where the secondary transfer roller 27 is separated from the intermediate transfer belt 25a to form a color image. Then, the secondary transfer roller 27 abuts on the intermediate transfer belt 25a, and the image is transferred onto the sheet by supplying the sheet to the abutting portion (secondary transfer portion T2). Reference numeral 25f denotes a cleaning unit capable of coming into contact with and separating from the intermediate transfer belt 25a, and removes toner remaining on the intermediate transfer belt 25a after the secondary transfer.

The transport path 30 and the return path 40 form a substantially drop-shaped loop-shaped path R, and the loop length of the loop-shaped path R is (length in the widthwise direction of A4 size paper). ×
It is configured to be longer than 2 + sheet interval × 2, and shorter than (length in the short direction of A4 size sheet) × 2 + sheet interval × 3. Further, the intermediate transfer belt 25a has at least a circumferential length of A4 size paper in the transverse direction × 2 (in this embodiment, a transverse direction length of A4 size paper × 2 + a paper interval × 2 circumferential length). ), And the loop-shaped path R
Is longer than the circumferential length of the intermediate transfer belt 25a and shorter than the circumferential length of the intermediate transfer belt 25a + the sheet interval. The “sheet interval” is an interval between the trailing edge of the preceding sheet and the leading edge of the succeeding sheet, and is an interval necessary for sheet transport and image forming timing.

A first detector 51 for the paper, a pair of gate rollers 52, the above-described secondary transfer portion T2, and a toner image are fixed on the paper in the transport path 30 from below along the paper transport direction. Fixing roller pair 53 and second paper detector 54
And are arranged. In the return path 40, a first return roller pair 55, a third sheet detector 56, and a second return roller pair 57 are disposed from above along the sheet transport direction. A paper guide (not shown) is provided in the loop-shaped path R including the transport path 30 and the return path 40.

Between the transport path 30 and the paper discharge unit 11,
A first transport unit 60 is provided, and a second transport unit 70 is provided on a side of the first transport unit 60 opposite to the sheet discharge unit 11.

The first transport means 60 comprises a drive roller 61 which can be rotated forward and backward and a driven roller 62 which can be brought into contact with and separated from the drive roller 61. As will be described later, the paper from the transport path 30 is discharged to a paper discharge section. 11, or the paper partially discharged to the paper discharge unit 11 is transported toward the return path 40.

The second conveying means 70 includes a driving roller 71 which rotates only one side (in this embodiment, clockwise in FIG. 1), and a conveying path side which abuts on and is driven by the driving roller 71 on the conveying path 30 side. A roller 72 and a return path roller 73 that abuts on and is driven by the drive roller 71 on the return path 40 side.
Is configured to be able to contact and separate from the drive roller 71.
A branch guide 74 is provided between the second transport means 70 and the first transport means 60.

Next, the operation of the above-described image forming apparatus will be described. A case where an image is formed on only one side of a sheet, a case where an image is formed on both sides of only one sheet, and a case where an image is continuously formed on both sides of a plurality of sheets will be described separately. In either case, the image forming unit 20 is operated next, and the paper is supplied from the paper supply tray 12 by the paper supply roller 13 at a predetermined timing. Controlled and supplied to the secondary transfer unit T2, the image is transferred by the secondary transfer unit T2, the image is fixed by the fixing roller pair 53, and then the image is transferred by the second transfer unit 70 in common. Therefore, this point is omitted in the following description.

When an image is formed only on one side of a sheet, the sheet conveyed by the second conveying means 70 is further discharged to the discharge tray section 11 by the first conveying means 60.

When an image is formed on both sides of only one sheet, the image is formed on one side, and the second conveying means 70
After the trailing end of the sheet conveyed by the first passage unit 60 (the drive roller 61)
Before passing through the pressure contact portion between the sheet and the driven roller 62), the driving roller 61 of the first conveying means 60 rotates in the reverse direction, and the sheet is guided by the branch guide 74 and is conveyed (switched back) to the return path 40, and again. After being supplied to the secondary transfer section T2 and forming an image on the other surface, the sheet is discharged to the sheet discharge tray section 11 by the second transfer means 70 and the first transfer means 60. The timing at which the driving roller 61 of the first conveying means 60 is reversed is determined by the timing of a stepping motor (not shown) serving as a driving source of the driving roller 61 after the trailing edge of the sheet is detected by the second detector 54. It can be controlled by the number of steps.

When an image is formed on both sides of a plurality of sheets, whether the sheet size is A4 or A3,
The operation differs depending on whether the image to be formed is a single-color image or a full-color image. When the size of the sheet is B5, the operation is the same as that of the case of A4, and when the size of the sheet is B4, the operation is the same as that of the case of A3. Therefore, the A4 size and the A3 size will be described as representatives.

<Case where a single color image is continuously formed on both sides of a plurality of A4 size sheets> In this case, as shown in FIGS. 2 to 6 and FIG. Are formed as a set, first, an image is formed on an even-numbered page while the sheet P is conveyed in the short direction in the conveyance path 30 and the return path 40,
Next, an image is formed on the odd-numbered page, and if the last page is the odd-numbered page, the image of the odd-numbered page is formed last. The numbers in FIG. 9 indicate page numbers.

Hereinafter, a specific description will be given. (1) See FIG. One side (in this case, the second side) P1b of the first sheet P1
An image is formed on (2), and after the trailing end P1r of the sheet P1 has passed through the leading end 74a of the branch guide 74 and before passing through the first conveying means 60 (the pressure contact portion between the driving roller 61 and the driven roller 62). Then, the driving roller 61 of the first conveying means 60 rotates in the reverse direction. At this point, the second sheet P2 has already been supplied from the paper feed tray 12 (see FIG. 1) to the transport path 30, and the second sheet P2 has one side (fourth side in this case).
The image is being formed on P2b (4).

(2) See FIG. 2 (2). In the above (1), the drive roller 61 of the first transport means 60 rotates in the reverse direction, and the trailing edge (leading edge in the traveling direction) P1r of the first sheet P1 becomes the drive roller 71 of the second transport means 70 and the return path roller 7
At the point where the roller reaches the pressure contact portion with the third transport unit 3 and can be returned by the second transport unit 70, the driven roller 62 of the first transport unit 60 separates from the drive roller 61 (see the arrow r). Note that the separation (and abutment described later) timing can be controlled by the number of steps of the stepping motor, which is the driving source of the driving roller 61, from the reverse rotation. When the driven roller 62 of the first conveying means 60 is separated from the driving roller 61, the second sheet P1 exists between the driving roller 61 and the driven roller 62 even though the first sheet P1 exists. The leading end P2t of the sheet P2 is moved between the drive roller 61 and the driven roller 62 by the discharge tray unit 1.
1 (see FIG. 1). An image is continuously formed on one side (the fourth side in this case) P2b (4) of the second sheet P2.

(3) See FIG. 3 (3). The leading end P2t of the second sheet P2 passes between the drive roller 61 and the driven roller 62 toward the sheet discharge tray section 11, and the first sheet P1 is continuously transported in the return path 40, and After the rear end P1t in the traveling direction has passed between the driving roller 61 and the driven roller 62, the second sheet P2
Before the trailing end P2r passes through the contact portion between the drive roller 71 of the second transport means 70 and the transport path side roller 72, the driven roller 62 contacts the drive roller 61 (see arrow s). As a result, even after the rear end P2r of the second sheet P2 has passed the contact portion between the drive roller 71 of the second conveying means 70 and the conveying path side roller 72, the second sheet P2 is transferred to the first conveying means. At 60, the sheet can be transported. Note that the contact timing of the driven roller 62 is determined by the leading end P in the traveling direction of the first sheet P1.
1r can also be obtained by being detected by the third detector 56. At this point, the third sheet P3 has already been supplied from the paper feed tray 12 to the transport path 30, and the third sheet P3 forms an image on one side (sixth side in this case) P3b (6). It is in the process of moving to the secondary transfer portion T2. The first sheet P1 continues to be transported in the return path 40 by the second transport unit 70 and the first return roller pair 55.

(4) See FIG. 3 (4). From the state shown in FIG. 3C, after the second sheet P2 is further conveyed toward the discharge tray section 11 and the trailing end P2r has passed the leading end 74a of the branch guide 74, the drive roller 61 rotates in the reverse direction. The trailing edge (leading edge in the traveling direction) P2r of the second sheet P2 reaches the pressure contact portion between the drive roller 71 of the second transport means 70 and the return path roller 73, and is ready to be returned by the second transport means 70. At this point, the driven roller 62 of the first transport unit 60 separates from the drive roller 61 (see arrow r). When the driven roller 62 of the first transport unit 60 is separated from the driving roller 61, the third sheet P2 exists between the driving roller 61 and the driven roller 62 even though the second sheet P2 exists. Leading edge P3 of paper P3
t can pass between the drive roller 61 and the driven roller 62 toward the sheet discharge tray section 11. First sheet P1
Are a first return roller pair 55 and a second return roller pair 57
Is continued to be transported in the return path 40. Third sheet P
An image is continuously formed on one side (sixth side in this case) P3b (4) of P3.

(5) See FIG. 4 (5). From the state shown in FIG. 4D, the leading end P3t of the third sheet P3 passes between the drive roller 61 and the driven roller 62 toward the discharge tray section 11, and the second sheet P2
Continues to be transported in the return path 40, and the rear end P2 in the traveling direction.
After t has passed between the driving roller 61 and the driven roller 62, the rear end P3r of the third sheet P <b> 3 sets the contact portion between the driving roller 71 of the second conveying unit 70 and the conveying path side roller 72. Before passing, the driven roller 62 contacts the drive roller 61 (see arrow s). As a result, the rear end P3r of the third sheet P3 is moved by the drive roller 71 of the second transport unit 70.
The third sheet P3 can be transported by the first transport unit 60 even after passing through the contact portion between the first transport unit 60 and the transport path side roller 72. At this point, the first sheet P1 has already been returned to the return path 40.
Are fed again to the transport path 30 through the gate roller pair 52, and the first sheet P1 is transferred to the secondary transfer portion T2 to form an image on the other surface (in this case, the first surface) P1f (1).
Then, an image is formed on the first surface P1f (1). The second sheet P2 is transferred to the second transport unit 70 and the first
The sheet is continuously transported in the return path 40 by the return roller pair 55.

(6) See FIG. 4 (6). From the state shown in FIG. 5 (5), the third sheet P3 is further conveyed toward the discharge tray section 11, and the rear end P3r passes through the leading end 74a of the branch guide 74 and then passes through the first conveying means 60. Before, the driving roller 6 of the first conveying means 60
1 is reversed and the trailing edge of the third sheet P3 (the leading edge in the traveling direction)
When P3r reaches the pressure contact portion between the drive roller 71 of the second transport means 70 and the return path roller 73 and is ready to be returned by the second transport means 70, the first transport means 60
Follower roller 62 is separated from drive roller 61 (see arrow r). When the driven roller 62 of the first conveying means 60 is separated from the driving roller 61, the first sheet P3 exists between the driving roller 61 and the driven roller 62 even though the third sheet P3 exists. Leading edge P of paper P1 in traveling direction
1r can pass between the driving roller 61 and the driven roller 62 toward the paper discharge tray section 11. Second sheet P
2 is a first return roller pair 55 and a second return roller pair 5
7 keeps being conveyed in the return path 40.

(7) See FIG. 5 (7). Both surfaces (second surface P1b (2) and first surface P1f (1))
The first sheet P1 on which an image is formed is a fixing roller pair 5
The third sheet P3 continues to be conveyed toward the discharge tray section 11 by the third and second conveying means 70, and continues to be conveyed in the return path 40 by the second conveying means 70. The leading end P2r in the traveling direction is the gate roller pair 52.
Reach

(8) See FIG. 5 (8). From the state shown in FIG. 7 (7), the first sheet P1 is
The third sheet P3 continues to be further conveyed toward the discharge tray unit 11 by the conveying means 70, and continues to be conveyed in the return path 40. The rear end P3t in the traveling direction is between the driving roller 61 and the driven roller 62. And the driven roller 62 is driven before the trailing end P1t of the first sheet P1 in the traveling direction of the first sheet P1 passes through the contact portion between the driving roller 71 of the second conveying means 70 and the conveying path side roller 72. It comes into contact with the roller 61 (see arrow s). As a result, even after the trailing end P1t of the first sheet P1 in the traveling direction has passed the contact portion between the driving roller 71 of the second conveying means 70 and the conveying path side roller 72, the first sheet P1 is discharged. The transport is continued toward the tray section 11 (see FIG. 1). At this point, the second sheet P2 has already been supplied from the return path 40 to the transport path 30 via the pair of gate rollers 52.
An image is formed on the other surface (third surface in this case) P2f (3) in the secondary transfer portion T2. Note that the third sheet P3 is
The first return roller pair 55 and the like continue to be transported in the return path 40.

(9) See FIG. 6 (9). The first sheet P1 and the second sheet P2 on which images are formed on both sides are discharged to the discharge tray unit 11, and the third sheet P3 is also on the other side (the fifth side in this case) P3f (5). The paper is conveyed toward the paper discharge tray 11 while an image is formed on the paper. In addition, any of the sheets in the discharge tray section 11
The odd-numbered pages are ejected so as to face downward.

(10) See FIG. 6 (10). In the process of transporting the third sheet P3 toward the discharge tray section 11, the fourth sheet P4, which is the first sheet in the next set of three sheets, is moved from the sheet tray 12 to the transport path 30. And the operation from the above (1) is repeated. If the last page of a series of image formation is an odd page, an image of the odd page is formed last. For example, if the last page is the seventh page, the fourth sheet P
In No. 4, the image of the seventh page is formed on one side P4f (7), and is discharged to the discharge tray unit 11 as it is. As described above, when a single-color image is continuously formed on both sides of a plurality of A4 size sheets, basically, the sheet P is transported in the transport path 30 and the return path 40 as a set of three sheets. While transporting in the short direction, first form an image on the even page,
Next, an image is formed on the odd-numbered page, and if the last page is the odd-numbered page, the image of the odd-numbered page is formed last.

<Case of Continuously Forming Full-Color Images on Both Sides of Multiple A4 Size Papers> In this case, FIGS.
As shown in FIGS. 10 and 19B, while the sheet P is conveyed one by one in the conveyance path 30 and the return path 40 one by one, an image is first formed on an even-numbered page, and then an odd-numbered page is formed. If an image is formed on the page and the last page is an odd page, an image on the odd page is formed last.

Hereinafter, a specific description will be given. (1) See FIG. One side (in this case, the second side) P1b of the first sheet P1
(2), a full-color image is formed, and after the trailing end P1r of the sheet P1 passes through the leading end 74a of the branch guide 74 and before passing through the first conveying means 60 (the pressure contact portion between the driving roller 61 and the driven roller 62). Then, the driving roller 61 of the first conveying means 60 rotates in the reverse direction.

(2) See FIG. 7 (2). When the drive roller 61 of the first transport means 60 rotates in the reverse direction from the state shown in FIG. 1A, the first sheet P1 enters the return path 40 from the rear end (leading edge in the traveling direction) P1r, and the second transport The sheet is conveyed in the return path 40 by the means 70, the first return roller pair 55, and the second return roller pair 57.

(3) See FIG. 8 (3). The first sheet P1 is again supplied to the secondary transfer portion T2, and a full-color image is formed on the other surface (in this case, the first surface) P1f (1).

(4) See FIG. 8 (4). In the process in which the first sheet P1 on which double-sided printing is performed is discharged to the discharge tray section 11, the second sheet P2 is
From one side (in this case, the fourth
Surface) A color image is formed on P2b (4).

(5) See FIG. 9 (5). The first sheet P1 is discharged to the discharge tray unit 11, and the second sheet P2 having an image formed on one side is discharged to the discharge tray unit 1.
It is transported toward 1.

(6) See FIG. 9 (6). From the state shown in FIG. (5), the second sheet P2 is further conveyed toward the discharge tray unit 11, and the rear end P2r
Transport means 60 (the pressure contact portion between the drive roller 61 and the driven roller 62) after passing through the tip 74a of the branch guide 74
Before passing through, the driving roller 61 of the first transport means 60 rotates in reverse, and the second sheet P2 enters the return path 40 from the rear end (leading end in the traveling direction) P2r and is transported in the return path 40. , The front end P2r of the gate roller pair 52
Reach

(7) See FIG. 10 (7). From the state shown in FIG. 6B, the second sheet P2 is again supplied to the transport path 30, and a full-color image is formed on the other surface (third surface in this case) P2f (3) in the secondary transfer portion T2. In the course of being discharged to the paper discharge tray section 11, the third sheet P3 is supplied from the paper supply tray 12 to the transport path 30, and
A color image is formed on one surface (sixth surface in this case) P3b (6).

(8) See FIG. 10 (8). From the state shown in FIG. 7 (7), the second sheet P2 is discharged to the discharge tray unit 11, and the third sheet P3 having an image formed on one side is conveyed toward the discharge tray unit 11, After the trailing end P3r has passed the leading end 74a of the branch guide 74, the first transporting means 60 (the driving roller 61 and the driven roller 62
Before passing through the pressure contact portion, the driving roller 61 of the first conveying means 60 rotates in the reverse direction, and the third sheet P3 enters the return path 40 from the rear end (leading end in the traveling direction) P3r. A similar operation is repeated. If the last page of a series of image formation is an odd page, an image of the odd page is formed last.

<Case Where Monochromatic Images are Continuously Formed on Both Sides of Plural A3 Size Papers> In this case, FIGS.
As shown in FIG. 4 and FIG. 19C, basically, an image is first formed on an even-numbered page while the sheet P is transported in the transport direction along the transport path 30 and the return path 40 as a set of two sheets. Then, an image is formed on the odd-numbered page, and if the last page is an odd-numbered page, the image of the odd-numbered page is formed last.

Hereinafter, a specific description will be given. (1) See FIG. 11 (1). One side (in this case, the second side) P1b of the first sheet P1
An image is formed in (2), and is conveyed toward the paper discharge tray unit 11.

(2) See FIG. 11 (2). In the process in which the first sheet P1 is conveyed toward the discharge tray section 11, the second sheet P2 is supplied from the sheet supply tray 12 to the conveyance path 30, and one side (fourth side in this case) P2b
An image is formed on (4).

(3) See FIG. 12 (3). The first sheet P1 further from the state shown in FIG.
Is transported toward the paper discharge tray section 11 and the rear end P1r
Transport means 60 (the pressure contact portion between the drive roller 61 and the driven roller 62) after passing through the tip 74a of the branch guide 74
Before passing through, the driving roller 61 of the first conveying means 60 rotates in the reverse direction, and the rear end (front end in the traveling direction) P1 of the first sheet P1
When r reaches the pressure contact portion between the drive roller 71 of the second transport means 70 and the return path roller 73 and is ready to be returned by the second transport means 70, the driven roller 62 of the first transport means 60 is driven. It separates from the roller 61 (see arrow r). When the driven roller 62 of the first conveying means 60 is separated from the driving roller 61, the second sheet P1 exists between the driving roller 61 and the driven roller 62 even though the first sheet P1 exists. The leading end P2t of the sheet P2 can pass between the driving roller 61 and the driven roller 62 toward the sheet discharge tray unit 11. An image is continuously formed on one side (the fourth side in this case) P2b (4) of the second sheet P2.

(4) See FIG. 12 (4). From the state of FIG. 3C, the first sheet P1 is further returned in the return path 40, and before the leading end P1r in the traveling direction reaches the gate roller pair 52, the return path roller 73 of the second transport means 70. Is separated from the drive roller 71 (arrow r).
1), the rotation of the first return roller pair 55 and the second return roller pair 57 is temporarily stopped, whereby the conveyance of the first sheet P1 is temporarily stopped. The driven roller 62 of the first conveying means 60 is kept separated from the driving roller 61, and the driving roller 71, the fixing roller pair 53, the secondary transfer roller 27, and the gate roller pair 52 of the second conveying means 70.
Is rotated, the second sheet P2 continues to be conveyed toward the discharge tray unit 11. The separation of the return path roller 73 and the stop timing of the first return roller pair 55 and the second return roller pair 57 are detected by the third detector 56 at the rear end (leading end in the traveling direction) P1r of the first sheet P1. It can be controlled by the number of steps of the step motor after the operation. Further, the first return roller pair 55 and the second return roller pair 5
7 can be stopped by interposing a clutch in the drive system.

(5) See FIG. 13 (5). While the second sheet P2 on which an image is formed on one side is further conveyed from the state shown in FIG. 4D, the first return roller pair 55 and the second return roller pair 57 rotate again at a predetermined timing. Then, the first sheet P1 is transported until the leading end P1r in the traveling direction reaches the gate roller pair 52. The return path roller 73 remains separated. After the first sheet P1 is conveyed in the return path 40 and its rear end P1t in the traveling direction has passed between the driving roller 61 and the driven roller 62, the rear end P2r of the second sheet P2 is Second
The driven roller 62 contacts the drive roller 61 before passing through the contact portion between the drive roller 71 of the transfer means 70 and the transfer path side roller 72. As a result, even after the rear end P2r of the second sheet P2 has passed through the contact portion between the drive roller 71 of the second conveying means 70 and the conveyance path side roller 72, the second sheet P2 remains in the discharge tray section. 11 is continued to be conveyed.

(6) See FIG. 13 (6). From the state shown in FIG. 5, the second sheet P2 continues to be further conveyed, and the first sheet P1 is again sent to the secondary transfer portion T2 via the gate roller pair 52 and the other surface ( In this case, an image is also formed on the first surface P1f (1). The conveyance of the first sheet P1 is resumed, and after the trailing end P1t of the first sheet P1 has passed through the second conveying means 70, the return path roller 73 of the second conveying means 70 is driven by the driving roller 71.
Abut. This contact timing can be controlled by the number of steps of the stepping motor from the time when the rotation of the first return roller pair 55 and the second return roller pair 57 is restarted.
Then, the rear end P2r of the second sheet P2 is
4 and before passing through the first conveying means 60 (the pressure contact portion between the driving roller 61 and the driven roller 62), the driving roller 61 of the first conveying means 60 reversely rotates and the second sheet P2 is conveyed toward the return path 40, and the leading end P2r in the traveling direction of the second sheet P2 reaches the pressure contact portion between the drive roller 71 of the second conveying means 70 and the return path roller 73, and is conveyed by the second conveying means 70. The driven roller 62 of the first transporting means 60 is separated from the driving roller 61 at the time when it can be returned. Thus, despite the presence of the second sheet P2 between the driving roller 61 and the driven roller 62, the leading end P1 of the first sheet P1 in the traveling direction is provided.
r can pass between the driving roller 61 and the driven roller 62 toward the sheet discharge tray section 11.

(7) See FIG. 14 (7). The first sheet P1 and the second sheet P2 continue to be conveyed from the state shown in FIG. 6B, and the leading end P2r in the traveling direction of the second sheet P2 reaches the gate roller pair 52.

(7) See FIG. 14 (8). The first sheet P1 is further conveyed from the state shown in FIG.
The second sheet P2 is supplied to the transport path 30 again, an image is formed on the other surface (third surface in this case) P2f (3) in the secondary transfer portion T2, and is transported to the paper discharge tray portion 11. In the process, the third sheet P3 is moved from the sheet feeding tray 12 to the transport path 30.
And the same operation is repeated thereafter. If the last page of a series of image formation is an odd page, an image of the odd page is formed last. For example, if the last page is the fifth page, the third sheet P
In No. 3, the image of the fifth page is formed on one side P3f (5), and is discharged to the discharge tray unit 11 as it is. As described above, when a single-color image is continuously formed on both sides of a plurality of A3 size sheets, basically, the sheets P are transported in the transport path 30 and the return path 40 as a set of two sheets. While transporting in the longitudinal direction, first form an image on the even page,
Next, an image is formed on the odd-numbered page, and if the last page is the odd-numbered page, the image of the odd-numbered page is formed last.

<Case where Full-Color Images are Continuously Formed on Both Sides of Multiple A3 Size Sheets> In this case, FIG.
As shown in FIGS. 18 and 19D, while the paper P is transported one by one in the transport path 30 and the return path 40 in the longitudinal direction, an image is first formed on an even-numbered page, and then an odd-numbered page is formed. If an image is formed on the page and the last page is an odd page, an image on the odd page is formed last.

Hereinafter, a specific description will be given. (1) See FIG. 15 (1). One side (in this case, the second side) P1b of the first sheet P1
While the full-color image is being formed in (2), the sheet is conveyed toward the paper discharge tray unit 11.

(2) See FIG. 15 (2). From the state shown in FIG. 1A, the first sheet P1 is further conveyed, and the rear end P1r is connected to the leading end 74 of the branch guide 74.
a, the driving roller 61 of the first conveying means 60 reversely rotates before passing through the first conveying means 60 (the pressure contact portion between the driving roller 61 and the driven roller 62).

(3) See FIG. 16 (3). When the drive roller 61 of the first transport means 60 rotates in the reverse direction, the first sheet P1 is moved to the rear end (the front end in the traveling direction).
It enters the return path 40 from P1r, and is transported in the return path 40 by the second transport means 70, the first return roller pair 55, and the second return roller pair 57.

(4) See FIG. 16 (4). From the state of FIG. 3C, the first sheet P1 is further returned in the return path 40, and before the leading end P1r in the traveling direction reaches the gate roller pair 52, the return path roller 73 of the second transport means 70. Is separated from the drive roller 71 (arrow r).
1). Thereafter, the leading end P1r in the traveling direction reaches the gate roller pair 52, and the first sheet P1 is supplied to the transport path 30 again at a predetermined timing.

(5) See FIG. 17 (5). The first sheet P1 is again supplied to the secondary transfer portion T2, and a full-color image is formed on the other surface (in this case, the first surface) P1f (1).

(6) See FIG. 17 (6). The first sheet P <b> 1 is further conveyed toward the discharge tray unit 11.

(7) See FIG. 10 (7). In the process of discharging the first sheet P1 to the discharge tray section 11, the second sheet P2 is transferred from the sheet feed tray 12 to the transport path 30.
And its one side (the fourth side in this case) P2b (4)
And a similar operation is repeated. If the last page of a series of image formation is an odd page, an image of the odd page is formed last.

As described above, according to this image forming apparatus, the following operational effects can be obtained. (A) An image forming unit 20 that forms an image on a sheet P, a conveying path 30 that conveys a sheet on which an image is formed on one side by the image forming unit 20 to a sheet discharging unit 11, and a conveying path 30 A return path 40 for switching back the sheet conveyed toward the sheet discharge section 11 and returning the sheet to the image forming section 20 to form an image on the other side; The loop length of the formed loop-shaped path R is (length in the widthwise direction of A4 size paper) × 2
+ Paper interval x 2 and shorter than (A4 size paper length in the short direction) x 2 + paper interval x 3, so the loop formed by the transport path 30 and the return path 40 The path R allows two A4 size sheets to be transported in the short direction and to form an image on both sides without pausing. Therefore, according to this image forming apparatus, the throughput is improved. Here, suppose that the loop length of the loop path R is (the length in the short direction of the A4 size paper) × 2 + the paper interval ×
If the length is shorter than the length P2, the rear end P in the traveling direction of the preceding first sheet P1 will be described with reference to FIG.
1t, the leading edge P2r of the trailing second sheet P2 in the traveling direction P2r is too close, and the leading edge P2r of the trailing second sheet P2 in the traveling direction before the gate roller pair 52 stops does not move. When it reaches the gate roller pair 52,
As a result, there arises a problem that the timing of the second sheet P2 toward the image forming unit (secondary transfer unit T2) by the gate roller pair 52 cannot be obtained. Conversely, the loop length of the loop-shaped path R is (the length in the short direction of the A4 size paper) × 2 + the paper interval ×
If the sheet is longer than 3 and the sheet is returned to the image forming section via the return path 40 so that an image can be formed on the other side, the leading end of the sheet in the traveling direction may be a predetermined (the optimum sheet interval for image formation). ) Since it does not reach the gate roller pair 52 at the timing, it is necessary to delay the image formation. That is, unless the print productivity is sacrificed, the supply timing of the paper to the image forming unit will not match. This problem can be solved by increasing the speed at which the paper is conveyed in the return path 40, but this causes another problem that the apparatus becomes complicated and large. On the other hand, according to the image forming apparatus of this embodiment, the loop length of the loop path R formed by the transport path 30 and the return path 40 is (length in the widthwise direction of A4 size paper). )
× 2 + paper interval × 2, and shorter than (length in the short direction of A4 size paper) × 2 + paper interval × 3, so that it is formed by the transport path 30 and the return path 40. By the loop path R, it is possible to form images on both sides without temporarily stopping two A4 size sheets while efficiently transporting them in the short direction thereof, thereby improving the throughput. . (B) The image forming section 20 is configured to include at least the intermediate transfer body 25a having a circumference of A4 size paper in the widthwise direction × 2, and the transport path 30 and the return path 4
0, the loop length of the loop path R is longer than the circumference of the intermediate transfer body 25a and shorter than the circumference of the intermediate transfer body 25a + the sheet interval. And the return path 40 formed by the return path 40, it is possible to form an image on both sides of the two A4 size sheets without stopping them while transporting them in the short direction. . Therefore, according to this image forming apparatus, the throughput is improved. here,
If the loop length of the loop-shaped path R is shorter than the circumference of the intermediate transfer body 25a or longer than the circumference of the intermediate transfer body 25a + the paper interval, the above-described problem occurs. However, according to the image forming apparatus of this embodiment, the loop length of the loop path R formed by the transport path 30 and the return path 40 is the same as that of the intermediate transfer member 2.
5A, and shorter than the circumference of the intermediate transfer body 25a + the sheet interval, the two A4 papers are formed by the loop path R formed by the transport path 30 and the return path 40. An image can be formed on both sides without temporarily stopping while efficiently transporting a sheet of paper in the short direction, thereby improving the throughput. (C) Between the transport path 30 and the paper discharge unit 11, a drive roller 61 that can rotate forward and reverse, and a driven roller 62 that can come and go to and away from the drive roller 61 are provided. A first transport unit 60 is provided for transporting the paper partially discharged to the paper discharging unit 11 or for transporting the paper partially discharged to the paper discharging unit 11 toward the return path 40. On the side opposite to the paper discharge unit 11, a driving roller 71 that rotates only one side, a conveyance path side roller 72 that abuts on and is driven by the driving roller 71 on the conveyance path 30 side, and a driving roller 71. On the return path 40 side, the return path roller 73 abuts and follows the return path.
And a second transporting means 70 having
Since the return path roller 73 of the second transport means 70 is configured to be able to contact and separate from the drive roller 71, the loop length is not increased unnecessarily ((Length in the widthwise direction of A4 size paper). 2) + 2 × paper interval × 3 or the length of the intermediate transfer body + the length of the paper interval), and also to form images on both sides of A3 size paper Becomes possible. (D) When forming a monochrome image on both sides of three or more sheets of A4 size, the three sheets are grouped as a set, and the sheets are conveyed in the short direction in the conveyance path 30 and the return path 40. When the image is formed on the page, then the image is formed on the odd page, and when the last page is the odd page, the image on the odd page is formed last. Will discharge the paper. (E) When forming a color image on both sides of A4 size paper, an image is first formed on an even-numbered page, and then an image is formed on an odd-numbered page while the paper is conveyed on the conveyance path 30 and the return path 40. Is formed, and when the last page is an odd-numbered page, the image of the odd-numbered page is formed last, so that the sheets are discharged in a state where they are aligned in the page order. (F) When forming a monochrome image on both sides of two or more sheets of A3 size, first the even numbered pages are conveyed along the conveyance path 30 and the return path 40 as a set of two sheets. If the image is formed on the second page, then the image is formed on the odd page, and if the last page is the odd page, the image on the odd page is formed last. The paper will be discharged. (G) When a color image is formed on both sides of A3-size paper, an image is first formed on an even-numbered page, and then an image is formed on an odd-numbered page while the paper is conveyed on the conveyance path 30 and the return path 40. Is formed, and when the last page is an odd-numbered page, the image of the odd-numbered page is formed last, so that the sheets are discharged in a state where they are aligned in the page order.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and can be appropriately modified within the scope of the present invention.

[0061]

According to any one of the first and second image forming apparatuses, it is possible to improve the throughput for A4 size paper. According to the image forming apparatus of the third aspect, it is possible to form an image on both sides of an A3-size sheet. In any of the image forming apparatuses according to the fourth to seventh aspects, the sheets are discharged in a state where they are aligned in the page order.

[0062]

[Brief description of the drawings]

FIG. 1 is a schematic front view showing an embodiment of an image forming apparatus according to the present invention.

FIGS. 2 (1) and (2) are explanatory diagrams of operation.

3 (3) and (4) are explanatory views of the operation.

FIGS. 4 (5) and (6) are operation explanatory diagrams.

5 (7) and (8) are explanatory views of the operation.

FIGS. 6 (9) and (10) are operation explanatory diagrams.

FIGS. 7 (1) and (2) are operation explanatory diagrams.

8 (3) and (4) are explanatory diagrams of the operation.

FIGS. 9 (5) and (6) are explanatory diagrams of the operation.

FIGS. 10 (7) and (8) are operation explanatory diagrams.

11 (1) and (2) are explanatory views of the operation.

FIGS. 12 (3) and (4) are operation explanatory diagrams.

13 (5) and (6) are explanatory views of the operation.

14 (7) and (8) are explanatory views of the operation.

15 (1) and (2) are explanatory views of the operation.

16 (3) and (4) are explanatory views of the operation.

17 (5) and (6) are explanatory views of the operation.

FIG. 18 is an explanatory view of the operation.

19A is a diagram showing a print pattern for A4 size single color, FIG. 19B is a diagram showing a print pattern for A4 size color, and FIG. 19C is a diagram showing a print pattern for A3 size single color. FIG. 4D is a diagram showing a print pattern in the case of A3 size color.

FIG. 20 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 P paper 11 paper discharge tray section (paper discharge section) 20 image forming section 25a intermediate transfer belt (intermediate transfer body) 30 transport path 40 return path R loop path 60 first transport means 61 drive roller 62 driven roller 70 second transport Means 71 Driving roller 72 Conveying path side roller 73 Return path side roller

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 15/16 G03G 15/16 3F053 21/14 21/00 372 3F100 F term (Reference) 2H027 DB08 DC04 DC05 DC07 DE07 DE10 ED16 ED24 ED30 EE04 EE06 FA05 FA13 FA35 FD08 ZA07 2H028 BA06 BA09 BA16 BB02 BB04 BB06 2H030 AD08 AD17 BB02 BB24 BB42 BB44 BB56 2H072 AB14 CB01 2H200 GA02 GA23 GA33 GA02 GA04 GA12 HBAB J12 J14 PA26 3F053 BA03 BA18 BA27 EA03 EA04 EA05 EB04 EC02 EC14 ED02 ED25 3F100 AA02 BA05 CA12 CA15 DA04 DA11 EA02

Claims (7)

[Claims] 1. An image forming section for forming an image on a sheet, a conveying path for conveying a sheet on which an image is formed on one side by the image forming section to a sheet discharging section, and the sheet discharging section using the conveying path. A return path for returning the paper conveyed toward the image forming unit to the image forming unit in order to switch back the paper conveyed toward the image forming unit and form an image on the other side, and a loop path formed by the transport path and the return path. The loop length of (A4 size paper in the width direction) × 2
The image forming apparatus is configured to be longer than (+ the paper interval × 2) and shorter than (the length in the short direction of the A4 size paper) × 2 + the paper interval × 3. 2. An image forming section for forming an image on a sheet, a conveying path for conveying a sheet on which an image is formed on one side by the image forming section to a sheet discharging section, and the sheet discharging section by the conveying path. And a return path for returning the paper conveyed toward the image forming unit to the image forming unit in order to switch back the paper conveyed toward the image forming unit to form an image on the other surface, wherein the image forming unit is configured to at least carry a short length of A4 size paper. And an intermediate transfer member having a circumferential length of a direction length × 2, and a loop length of a loop-shaped path formed by the transport path and the return path is larger than a circumferential length of the intermediate transfer body. The image forming apparatus is configured to be shorter than the circumference of the intermediate transfer member + the sheet interval. 3. A paper feed unit comprising: a drive roller capable of rotating forward and backward; and a driven roller capable of moving toward and away from the drive roller. A first transport unit that transports the paper toward the paper return unit or transports the paper partially discharged to the paper discharge unit toward the return path, and the paper discharge unit with respect to the first transport unit. On the opposite side, a driving roller that rotates only one side, a conveying path side roller that abuts on the driving path side to follow the driving roller, and a return path side that abuts and follows the driving roller on the return path side. And a second conveying means having a roller, and a return path side roller of the second conveying means is configured to be able to contact and separate from the driving roller. Placing the image forming apparatus. 4. When forming a monochrome image on both sides of three or more sheets of A4 size paper, first, as a set of three sheets, the sheets are conveyed in the short direction in the conveyance path and the return path. An image is formed on an even-numbered page, an image is formed on an odd-numbered page, and when the last page is an odd-numbered page, an image on the odd-numbered page is formed last. 3. The image forming apparatus according to 1 or 2. 5. When forming a color image on both sides of A4 size paper, an image is first formed on an even-numbered page, and then an image is formed on an odd-numbered page while the paper is conveyed on the conveyance path and the return path. 3. The image forming apparatus according to claim 1, wherein an image is formed, and when the last page is an odd page, the image of the odd page is formed last. 6. When forming a monochrome image on both sides of two or more sheets of A3 size, first, an even number of sheets are conveyed along the conveyance path and the return path as a set of two sheets. 4. An image is formed on a page, then an image is formed on an odd page, and when the last page is an odd page, an image on the odd page is formed last. The image forming apparatus as described in the above. 7. When forming a color image on both sides of an A3-size sheet, an image is first formed on an even-numbered page, and then an image is formed on an odd-numbered page while the sheet is conveyed through the conveyance path and the return path. 4. The image forming apparatus according to claim 3, wherein an image is formed, and when the last page is an odd page, the image of the odd page is formed last.