JP2002362811A - Double-sided image forming device - Google Patents

Abstract

(57) [Problem] To form an image on both sides of a short sheet and a long sheet without lowering the throughput. A laser beam printer (1) includes an image forming unit (2) for forming an image on a sheet, and a sheet (10) on which an image is formed on the opposite side of the sheet on which an image is formed by the image forming unit (2). The sheet re-supplying / conveying unit 3 is provided with the sheet re-supplying / conveying unit 3 for supplying the sheet to the forming unit 2. In A, the width of the inversion path is wide, and the inversion path from the branch portion D is more curved than the inversion path B.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a double-sided image forming apparatus such as a printer, a facsimile, a copying machine, and a multifunction machine for forming images on both sides of a sheet.

[0002]

2. Description of the Related Art FIG. 6 shows a laser beam printer 100 as an example of a two-sided image forming apparatus. The laser beam printer 100 can form images on both sides of a sheet P. The laser beam printer 100 mainly supplies an image forming unit 2 for forming an image on a sheet and a sheet to the image forming unit 2 again for forming an image on the other surface of the sheet having an image formed on one surface. And a sheet re-supplying / conveying unit 103. The image forming section 2 includes a laser scanner unit 12, a photosensitive drum 8, and the like. Examples of the sheet include plain paper, a thin resin sheet which is a substitute for plain paper, thick paper, and paper for an overhead projector.

When the user presses a print start button and the controller board 11 receives a print signal, a photosensitive drum 8 provided in the process cartridge 7 is provided.
The toner image is formed after the laser beam is irradiated from the laser scanner unit 12. The sheets P stacked on the cassette 4 are separated and fed one by one by a feeding roller 6, and the nip between the photosensitive drum 8 and the transfer roller 9 provided in the process cartridge 7 and the fixing unit 1 are fixed.
The images are sequentially conveyed to 0 and an image is formed.

When the controller board 11 receives a signal for single-sided printing, the sheet P is sent to the sheet discharge section 1 as it is.
3 and is discharged onto the sheet discharge stacking table 16.
When the controller board 11 has received the signal for double-sided printing, the sheet P is continuously sent to the sheet re-supply / conveyance unit 103. Note that the cassette 4 and the feeding roller 6 constitute a feeding unit 5.

[0005] The sheet re-supply / conveyance section 103 includes a flapper 3
9, a post-fixing roller 17, a reversing path C including a pair of intermediate conveyance rollers 28, a lateral registration correction unit 23 including a skew roller 26, a pair of resupply rollers 22, and the like. The post-fixing roller 17 also serves as a switchback roller, and forms a switchback portion with the opposing switchback rollers 19.

When the controller board 11 receives the double-sided printing signal, the sheet discharge roller 14a rotates in the reverse direction after the trailing edge of the sheet passes through the leading edge of the flapper 39, and the flapper 3
9 moves to the position indicated by the broken line. Thereby, the sheet P
Is continuously conveyed to the sheet re-supply / conveyance unit 103.
The sheet P conveyed to the sheet re-supply / conveyance unit 103 is sent to the lateral registration correction unit 23 by the intermediate conveyance roller pair 28 through the reversing path C.

The sheet P that has reached the lateral registration correction unit 23 is
In order to receive a skew feeding force from the lateral registration correction roller 25 and the inclined skew rollers 26, the sheet approaches the reference plate 24 disposed on one side of the reversing path while being conveyed along the sheet conveyance direction. The sheet P arrives at and abuts on the reference plate 24, and the sheet end is conveyed along the reference plate 24, and the position of the sheet P is regulated to a correct position without skew or displacement.

[0008] The sheet P corrected in the lateral registration as described above.
Is again fed into the image forming unit 2 by the re-supply roller 22 to form an image on the second side, and the sheet discharging unit 1
3 is discharged to the sheet discharge stacking table 16 and stacked.

[0009]

However, the conventional laser beam printer 100 can print on both long media sheets such as LGL size and short media sheets such as A5 size. The following problems have occurred.

In order to print on both a short sheet such as an A5 size sheet and a long sheet such as an LGL size sheet, as shown in FIG. Roller 17 to lateral registration correction roller 2
5 is set in accordance with the length of the A5 size sheet, the distance (n) from the discharge roller pair 14 to the re-supply roller 22 becomes shorter when the LGL size sheet is conveyed on both sides. It becomes impossible to alternately supply the sheets of the size 4 from the cassette 4 and the supply of the sheets of the LGL size from the reversing path C, so that the number of images formed per unit time (throughput) is different from that of the other sheets. In some cases, it decreased.

Therefore, when the supply of the LGL size sheet from the cassette 4 and the supply from the reversing path C are performed alternately, as shown in FIG. The distance (m) becomes longer, and the sheet supply / conveyance unit 103 cannot convey a sheet of A5 size or the like. For this reason, the intermediate conveyance roller pair 28
Must be added. When the pair of intermediate conveyance rollers 28 is added, a clamping force is applied between the pair of intermediate conveyance rollers 28 between the lateral registration correction roller 25 having the function of removing the lateral registration of the sheet and the post-fixing roller 17. Another problem arises in that the lateral registration operation of a long size sheet such as an LGL size sheet by the rollers 26 cannot be reliably performed.

According to the present invention, a sheet having a short length such as A5 size can be provided without lowering the throughput.
For example, an object of the present invention is to provide a double-sided image forming apparatus in which an image is formed on both sides of a sheet having a long length such as an LGL size and the throughput and the printing performance at the time of resupply and conveyance are not reduced.

[0013]

In order to achieve the above object, a double-sided image forming apparatus according to the present invention comprises an image forming means for forming an image on a sheet, and an image forming means for forming an image on the sheet by the image forming means. Re-supplying means for supplying the sheet to the image forming means for forming an image on the opposite surface on which the image has been formed, the re-supplying means comprising a guide path having a different length for guiding the sheet. A guideway having a plurality of guideways having a longer length has a wider guideway, and is greatly curved from a branch portion of the guideways.

The re-supply means of the double-sided image forming apparatus of the present invention has a plurality of guide paths having different lengths for guiding the sheet, and is disposed upstream of a branch portion of the plurality of guide paths. An upstream transport unit that transports the sheet; and a downstream transport unit that is disposed downstream of a junction of the plurality of guide paths and transports the sheet. The upstream transport unit along the guide path. The length of each of the guide paths between the sheet and the downstream transport section is set to be equal to or less than the length of the minimum length sheet among the sheets guided along each of the guide paths.

[0015] In the double-sided image forming apparatus of the present invention, a guide port to a short guide path is formed at the branch portion, and a downstream edge of the guide port is formed in a shape cut off downstream. Have been.

The downstream edge of the guide port of the double-sided image forming apparatus of the present invention is formed in a shape cut off to the downstream side from the inflection point of the long guide path.

In the double-sided image forming apparatus of the present invention, the inner wall of the curved guide path inside the curved guide path has a central portion in a direction intersecting the sheet conveying direction.

In the double-sided image forming apparatus of the present invention, a guide port to a short guideway is formed at the branch portion, and a downstream side of the guide port and a curved inside of the long guideway. On the inner wall of the sheet, a plurality of ribs along the sheet conveying direction are arranged in a direction intersecting with the sheet conveying direction, and at a guide entrance, a center in a direction intersecting with the sheet conveying direction. The height of the nearby rib is the lowest,
The ribs are set higher gradually toward the outside.

In the double-sided image forming apparatus of the present invention, a guide port to a short guideway is formed at the branch portion, and a downstream side of the guide port and inside a curved portion of the long guideway. On the inner wall, a plurality of ridge members along the sheet conveyance direction are arranged in a direction intersecting with the sheet conveyance direction, and the height of the ridge members is set to be lower on the downstream side than on the upstream side. Is set high.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A laser beam printer which is a double-sided image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings.

The laser beam printer 1 can form images on both sides of a sheet. The laser beam printer 1 mainly includes an image forming unit (image forming unit) 2 for forming an image on a sheet and an image forming unit 2 for forming an image on the other surface of the sheet having an image formed on one surface. Resupplying / conveying unit (resupplying means) 3 for supplying sheets to the printer
It is composed of The image forming section 2 includes a laser scanner unit 12, a photosensitive drum 8, and the like.

A plurality of sheets P are stacked on the cassette 4. The feeding unit 5 includes a feeding roller 6
For example, the sheets P are separated from the cassette 4 one by one and sent out. Process cartridge 7
Is integrally provided with a photosensitive drum 8 and a process unit (charging device, developing device, cleaning member) (not shown) acting on the photosensitive drum 8, and is detachably provided on an apparatus main body 29 of the laser beam printer 1.

The transfer roller 9 transfers a toner image formed on the photosensitive drum 8 to a sheet P passing between the transfer roller 9 and the photosensitive drum 8 facing the transfer roller 9. The fixing unit 10 semi-permanently fixes the toner image on the sheet on which the toner image has been transferred. The controller board 11 controls the laser beam printer 1. The controller board 11 also has a function of performing image forming processing on an image signal of a connected computer (not shown).

The laser scanner unit 12 irradiates the photosensitive drum 8 with a laser beam corresponding to an image signal generated by the controller board 11 to form a latent image. After the fixing, the roller 17 forms a nip with the first roller 18 and the sheet P
Is transported along the discharge guide 20 and the flapper 39. The sheet discharge section 13 has a sheet discharge roller 14a and a sheet discharge roller 14b facing each other. The sheet discharge loading table 16 is a sheet discharge roller 14
a) and a table for receiving the sheet discharged by the sheet discharge rollers 14b.

In the above-described configuration, when the image forming unit 2 receives a print command, the sheets P stacked on the cassette 4 are separated and fed one by one by a feed roller 6, and the photosensitive drum 8 in the process cartridge 7 is fed. And transfer roller 9
Sent to the nip between. The print command is sent when image information is sent from a personal computer (not shown).
Emitted through the controller board 11. A laser beam in which image information is converted into a bit image from the laser scanner unit 12 is irradiated onto the photosensitive drum 8 together with the print command.
A latent image is formed thereon, and a toner image is formed by a developing device (not shown).

The toner image formed on the photosensitive drum 8 is
The image is transferred to the sheet P at the nip between the photosensitive drum 8 and the transfer roller 9. Then, the sheet P on which the toner image has been transferred
Is sent to the fixing unit 10 and heated and pressed by the fixing unit 10. As a result, the toner image is fixed on the sheet P semi-permanently.

After the sheet P has passed through the fixing unit 10, the sheet P
The sheet is sent to the nip of the first roller 18 facing the roller 17 after the fixing, and is sent to the sheet ejection section 13 through the ejection guide 20 and the flapper 39. At this time, the position of the flapper 39 is at the position indicated by the solid line in FIG. Controller board 11
Receives the single-sided printing signal, the sheet P is nipped and conveyed by the sheet discharge rollers 14a and the sheet discharge rollers 14b and discharged to the sheet discharge stacking table 16. Here, the single-sided printing signal is a signal sent from a personal computer (not shown) to the controller board 11 in the same manner as the print signal and the image signal.

When the controller board 11 receives the double-sided printing signal, it rotates the discharge roller pair 14 in the reverse direction after the trailing edge of the sheet has passed the leading edge of the flapper 39, and the flapper 3
9 is moved to the position indicated by the broken line. Thereby, the sheet P
Is continuously sent to the sheet re-supplying / transporting unit 3. At this time, the rear end of the sheet is the front end, and the front end is the rear end. The discharge roller pair 14 is driven by a motor (not shown) independent of other rollers.

Next, the configuration of the sheet re-supplying and conveying section 3 will be described. The sheet re-supply / conveyance section 3 is configured around a sheet conveyance path from the discharge roller pair 14 to the junction 21. The junction 21 is a portion where the sheet conveyance path from the lateral registration correction unit 23 and the sheet conveyance path from the cassette 4 merge. In addition to the first roller 18 described above, a second roller, ie, a switchback roller (upstream transport section) 19, is opposed to and pressed against the post-fixing roller (upstream transport section) 17. The sheet P after the reversal is fed into the nip between the post-fixing roller 17 and the second switchback roller 19, and is nipped and conveyed.

The transport path from the discharge roller pair 14 to the nip between the second switchback roller 19 and the post-fixing roller 17 uses the same transport path as the first surface discharge transport path. Here, the post-fixing roller 17 is used to prevent the sheet P from skewing during sheet conveyance as shown in FIG.
A plurality of sheets are arranged in a direction crossing the sheet conveying direction, that is, in the sheet width direction. A plurality of first rollers 18 and switchback rollers 19, which are second rollers, are arranged in the sheet width direction similarly to the post-fixing roller 17. Returning to FIG. 1, the lower conveyance guide 27 guides most of the lower side of the sheet conveyance surface.

A nip is formed downstream of the lateral registration correction unit 23 by the re-supply roller 22 and the roller 30 facing the re-supply roller 22. A plurality of re-supply rollers 22 are arranged in a direction intersecting the sheet conveyance direction so that the sheet P subjected to the lateral registration correction does not skew again. Horizontal registration correction unit 2
The upper and lower frame members 31 including a guide connecting the third transport lower guide 27 and the re-supply roller 22 simultaneously hold the roller 30 facing the re-supply roller 22. Further, the re-supply roller 22 is arranged within the range of the length of the sheet of the minimum size from the lateral registration correction roller 25. Downstream of the re-supply roller 22, there is a junction 21 with the sheet supply conveyance path from the cassette 4.

Next, the operation of the sheet re-supplying and conveying section 3 will be described.

When the controller board 11 receives the double-sided printing signal, as described above, after the trailing edge of the sheet P has passed through the leading edge of the flapper 39, the controller 14 rotates the discharge roller pair 14 in the reverse direction to move the flapper 39 to the position indicated by the broken line. Move. Sheet P
In the state in which the rear end of the front end is reversed, the paper is nipped and conveyed by the post-fixing roller 17 and the switchback roller 19 which is the second roller pressed against the post-fixing roller 17, and is subjected to a reversing path (guide path) A or a reversing path. The sheet is conveyed to the lateral registration correcting section 23 through the (guide path) B. Here, the reversing path A and the reversing path B bifurcate immediately after the nip between the post-fixing roller 17 and the switchback roller 19 and merge again before entering the lateral registration correction unit 23. Reverse path A, Reverse path B
Is defined by the width of the sheet P passing through each pass, and the pass width β of the reverse pass B which is the inner pass
Is smaller than the width α of the outer inversion path A, and has a relationship of α> β.

The distance (m) from the nip between the post-fixing roller 17 and the switchback roller 19 to the nip between the lateral registration correction roller (downstream transport section) 25 and the skew roller (downstream transport section) 26. Are set within the range of the minimum sheet length of a sheet that can be conveyed in each reversing pass via the reversing pass A and the reversing pass B.

Further, the branching portion D of the inversion path shown in FIGS.
The reversing path A is curved outward (to the left in FIG. 1) immediately after the nip between the post-fixing roller 17 and the switchback roller 19, and the reversing path B is formed between the post-fixing roller 17 and the switchback roller 19. The entrance (guide port) Ba of the reversing path B is formed directly below the nip, and opens at the center of the reversing path A in the width direction.

For example, if the sheet conveyance width of the reversing path B is A5
If the sheet conveyance distance (m) is set slightly shorter than the A5 size length in the size width, the A5 size sheet
As shown in (a), after passing through the nip between the post-fixing roller 17 and the switchback roller 19, the roller 17 goes straight down and enters the reversing path B. If the sheet conveyance distance (m) of the reversing path A is set to be slightly shorter than the sheet length of the LTR size longer than the A5 size sheet, the LTR size that is wider and longer than the A5 size sheet is set. , LGL size and A4 size sheets are shown in FIG.
As shown in (b), after passing through the nip between the roller 17 and the switchback roller 19 after the fixing, the sheet is scooped at the left and right ends of the entrance Ba of the reversing path B without entering the reversing path B to form the reversing path A. It is guided and conveyed to the lateral registration correction unit 23.

The reversing path A and the reversing path B are formed symmetrically with respect to the center in the width direction of the sheet, and are adapted to the laser beam printer 1 for conveying the sheet center.

Thereafter, the sheet P corrected in the lateral registration is sent again to the image forming section 2 by the re-supply roller 22, and the image is formed on the second surface of which the writing position has been adjusted based on the reference surface 24a of the reference plate 24. Then, the sheet is sent to the sheet discharge section 13 and discharged to the sheet discharge stacking table 16 to be stacked.

As shown in FIG. 4, if the edge (downstream edge) Bc of the entrance Ba of the reverse path B is formed in a V-shape,
Unlike the edge (downstream edge) Bb of the entrance Ba shown in FIG. 2, which is substantially orthogonal to the sheet conveyance direction,
The leading end of the sheet toward the reversing path A is less likely to be caught on the edge Bc, and sheet jamming can be prevented. Note that the V-shaped valley portion Bd is located downstream of the most protruding portion (inflection point) E of the inversion path A.

The shape of the valley need not be V-shaped. U
It may be in the shape of a letter. In other words, the shape of the valley only needs to be such that the center in the sheet width direction bites downstream rather than both sides.

As shown in FIG. 5, a plurality of ribs (protrusion members) 40a, 40b, 40c, 40d, 4 extending in the sheet conveying direction are provided on the inner wall Aa inside the curve of the reversing path A.
0e is provided in the concave portion Ab having a uniform depth along the inner wall Aa in the width direction of the sheet. These ribs 40a, 40b, 4
The heights of the entrances of 0c, 40d, and 40e are the lowest at the rib 40a near the center in the width direction of the sheet, are higher as the distance from the center is increased, and are higher as going to the downstream side. Of these ribs, the outer rib 40e
Is formed substantially flush with the inner wall Aa. In this way, as in the case of FIG. 4, the leading end of the sheet heading toward the reversing path A is less likely to be caught on the edge Bc, and sheet jamming can be prevented.

The height of the rib may be increased as it goes away from the vicinity of the center in the direction intersecting the sheet conveying direction. Alternatively, only the height on the downstream side may be higher than the height on the upstream side.

Further, the inversion path is branched into two at the branch part D, but it may be branched into three or more. In this case, the farther the reversing path is from the reversing path B, the larger the curved sheet and the wider the sheet is guided. Also,
In the case of branching into three or more, instead of branching into three or more at one location, first branch into two branches, and downstream of the branched reverse path, further branch into a large curved reverse path. Alternatively, the reversing path that is greatly curved on the downstream side may be branched into two branches.

As described above, the laser beam printer 1 of the present embodiment branches the reversal path immediately after the nip between the post-fixing roller 17 and the switchback roller 19 into two paths to reduce the width of the A5 size sheet. The narrow sheet is set to the inner inversion path B, and the wider sheet LGL (LT
R) A sheet of size, A4 size, etc.
Since it is possible to sort each of them, the A
It is possible to secure an appropriate sheet conveyance path length for a short medium sheet such as a 5 size sheet and a long medium sheet such as an LGL size, and convey the short medium sheet and the long medium sheet. Further, the laser beam printer 1 of the present embodiment can prevent the lateral registration correction performance from being deteriorated on the second side of a long medium sheet, and can alternately convey the long medium sheet with a long medium sheet that forms an image on one side. Thus, it is possible to prevent a decrease in throughput.

Further, the laser beam printer 1 of the present embodiment has a plurality of ribs whose height is increased as they move away from the vicinity of the center in the sheet conveyance direction, or ribs whose downstream side is higher than the upstream side. Is provided on the downstream side of the guide opening Ba, the sheet on which the image is formed on the first surface is reversed by the sheet discharge roller pair 14, passes through the post-fixing roller 17, and the curl or waving occurs at the tip of the second surface. In this state, when the sheet is automatically distributed to the reversing path A and rushes, it is possible to prevent the edge from being broken due to the hooking and to prevent the sheet from being jammed, so that the image can be smoothly formed on the second side.

[0046]

According to the double-sided image forming apparatus of the present invention, the re-supply means has a plurality of guide paths having different lengths for guiding sheets, and the guide path having a longer guide path has a wider width. Is wide and is greatly curved from the branch portion of the plurality of guide paths, so that an appropriate sheet transport path length between the short media sheet and the long media sheet is secured without adding a flapper or an intermediate transport roller. It is possible to convey a short media sheet and a long media sheet.

[Brief description of the drawings]

FIG. 1 is a schematic front sectional view of a laser beam printer which is a double-sided image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a perspective view of a main part of the laser beam printer of FIG.

FIG. 3 is a sectional view of FIG. 2; (A) It is the sectional view on the arrow aa in FIG. (B) It is the bb arrow sectional drawing in FIG.

FIG. 4 is a perspective view of another embodiment of a main part of the laser beam printer.

FIG. 5 is a perspective view of another embodiment of the main part of the laser beam printer.

FIG. 6 is a schematic configuration diagram showing a conventional laser beam printer.

FIG. 7 is a schematic configuration diagram showing a conventional laser beam printer.

[Explanation of symbols]

A Inversion path (guideway) Aa Inner wall inside curve B Inversion path (guideway) Ba Inlet (guideway) Bb, Bc Entrance edge (downstream edge) Bd Valley portion C Inversion path D Branching point E Inversion path A most protruding part of A (inflection point) P sheet 1 laser beam printer (double-sided image forming apparatus) 2 image forming unit (image forming unit) 3 sheet resupply / conveying unit (resupplying unit) 4 cassette 5 feeding unit 8 Photosensitive drum 10 Fixing unit 11 Controller board 12 Laser scanner unit 14 Discharge roller pair 17 Post-fixing roller (upstream transport section) 19 Switchback roller (upstream transport section) 21 Confluence point 24 Reference plate 24a Reference surface 25 Lateral registration correction roller 26 Oblique roller (downstream transport section) 27 Lower transport guide (downstream transport section) 29 Main unit of laser beam printer 40 a, 40b, 40c, 40d, 40e Rib (protrusion member)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masafumi Maeda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Masahide Taue 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon F term in reference (reference) 2H028 BA06 BA09 BB02 BB04 BD03 3F053 EA02 EA04 EA05 EB01 EC05 EC07 LA02 LA05 LA07 LB03 3F100 AA02 BA17 CA12 DA04 EA02 EA13

Claims (7)

[Claims] 1. An image forming means for forming an image on a sheet, and the image forming means for forming an image on a surface of the sheet on which an image has been formed by the image forming means on a side opposite to the image formed on the sheet. Re-supplying means for supplying the sheet, the re-supplying means has a plurality of guide paths having different lengths for guiding the sheet, and the longer the guide path, the longer the guide path becomes. A double-sided image forming apparatus having a large width and being greatly curved from a branch portion of the plurality of guide paths. 2. The re-supply means includes a plurality of guide paths having different lengths for guiding the sheets, and is disposed upstream of a branch portion of the plurality of guide paths and upstream of conveying the sheets. A transport unit, and a downstream transport unit disposed downstream of a junction of the plurality of guide paths and transporting the sheet, wherein the upstream transport unit and the downstream transport unit along the guide path are provided. The length of each of the guide paths is set to be less than or equal to the minimum length of the sheet guided along each of the guide paths. Image forming device. 3. A guide port for a guide path having a short length is formed in the branch portion, and a downstream edge of the guide port is formed in a shape cut off on the downstream side. The double-sided image forming apparatus according to claim 1. 4. The method according to claim 3, wherein a downstream edge of the guide port is formed so as to be cut out to a downstream side from an inflection point of the long guideway. The double-sided image forming apparatus as described in the above. 5. The inner wall of the curved guide path, wherein a central portion in a direction intersecting with the sheet conveying direction is recessed.
A two-sided image forming apparatus according to any one of the preceding claims. 6. A guide port for a guideway having a short length is formed at the branch portion, and the inner wall on the downstream side of the guideport and inside the curve of the guideway having a long length is provided with: A plurality of ribs along the sheet conveyance direction are arranged in a direction intersecting the sheet conveyance direction, and the height of the rib near the center in the direction intersecting with the sheet conveyance direction at the guide entrance is set. The double-sided image forming apparatus according to any one of claims 1 to 5, wherein the rib is set to be lower at the lowest position and gradually higher toward the outside. 7. A guide port for a guideway having a short length is formed in the branch portion, and an inner wall on a downstream side of the guideport and inside a curved portion of the guideway having a long length includes: A plurality of ridge members along the sheet conveyance direction are arranged in a direction intersecting the sheet conveyance direction, and the height of the ridge members is set higher on the downstream side than on the upstream side. The double-sided image forming apparatus according to any one of claims 1 to 5, wherein: