JP2003104613A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of forming images on both surfaces of a sheet quickly without necessitating a large sized construction of the device. SOLUTION: A plurality of switchback transport paths 30 and 40 are branched from an inversive transport path 10 where a sheet with image(s) formed on one surface is inverted by an image forming device 105, and each sheet inverted by the inversive transport path 10 is transported toward a retransport path 20 which retransports the sheet to the image forming part 105. The sheets fed one by one to the inversive transport path 10 at certain intervals are distributed to the switchback transport paths 30 and 40, stopped once, and then fed to the retransport path 20 in the same sequence.

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 capable of forming images on both front and back surfaces of a sheet.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a page printer, a sheet having an image formed on one side is inverted and then conveyed again to an image forming section to form an image on the back side of the sheet. Such an image forming apparatus is provided with a double-sided unit for reversing a sheet having an image formed on one side and then conveying the sheet again to the image forming unit.

Here, as such a conventional double-sided unit, for example, as in a double-sided unit disclosed in Japanese Patent Laid-Open No. 182655/1983, a sheet discharging section, a switching gate, a switchback section, and a returning section are provided. A double-sided copying assisting means consisting of a reversing part is provided, and when the image is copied on the back side of the sheet after the image is copied on the front side, the copying return speed by the double-sided copying assisting means is increased and after the front side copying is completed In some cases, the efficiency of double-sided copying is improved by quickly reaching the predetermined return position where the back side of the sheet can be copied.

Further, as in the double-sided unit disclosed in Japanese Patent Laid-Open No. 62-161641, the sheet feeding interval of sheets on which images are formed on both sides or in multiples is adjusted according to the sheet conveying state or external means. In some cases, the conveyance control means is provided to prevent a decrease in the maximum printable number per minute during continuous printing.

Further, as in the double-sided unit disclosed in Japanese Patent Laid-Open No. Hei 6-35265, the sheet reversing path provided with the reversing conveying path is provided in two stages, so that the continuously fed sheets are temporarily stopped. A plurality of reversal paths in the re-conveyance path, such as a double-sided unit described in Japanese Unexamined Patent Publication No. 2000-143103, in which double-sided image formation can be performed at high speed without fail by reliably re-feeding paper. By arranging the parts separately, the sheets sequentially fed at a predetermined interval are introduced into the respective reversing parts and inverted, whereby the sheets are conveyed again to the image forming part at a sheet interval narrower than the predetermined interval and double-sided image formation is performed. Some speed things up.

[0006]

However, in an image forming apparatus provided with such a conventional double-sided unit, when a sheet after one-sided printing described in JP-A-58-182655 is conveyed to a reversing unit. When equipped with a speed-increasing device, or in Japanese Patent Laid-Open No. 62-16
In the case where the paper feed interval described in Japanese Patent No. 1641 is adjusted according to the sheet conveyance state, the double-sided image forming speed can be slightly increased, but the processing speed cannot be significantly improved.

Further, in the case where the sheet reversing path having the reversing conveying path described in Japanese Patent Laid-Open No. 6-35265 is provided in two stages, the apparatus itself becomes large and the conveying path increases. In order to increase the double-sided image forming speed, it becomes necessary to increase the sheet conveying speed.

Further, in the case where a device for providing a plurality of switchback paths is provided as disclosed in Japanese Patent Laid-Open No. 2000-143103, when the number of sheets continuously sent to the re-conveying path becomes three or more, the sheets are sent. Since it is necessary to set the sheet interval to be longer than the sheet reversal time, the number of sheets conveyed in the device is reduced, and if the number of sheets is 2 or less, efficient double-sided circulation conveyance cannot be performed, and the speed of double-sided image formation is increased. I can't plan.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an image forming apparatus capable of forming images on both sides of a sheet at high speed without increasing the size of the apparatus. It is intended.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a sheet having an image formed on one side by an image forming unit is reversed and again
In an image forming apparatus that conveys to the image forming unit and forms an image on the back surface of the sheet, a reversal conveyance path that reverses the sheet on which the image is formed on the one side, and a sheet that is reversed on the reversal conveyance path A re-conveyance path for re-conveying to the image forming unit, and a plurality of switchback conveyance paths for branching from a plurality of positions of the reversal conveyance path and reversing and conveying the sheet from the reversal conveyance path toward the re-conveyance path, respectively. , Is provided.

Further, according to the present invention, the sheets sequentially conveyed at a predetermined interval in the reversing conveyance path are sequentially distributed to the plurality of switchback conveyance paths and temporarily stopped, and then are conveyed in the conveyance order to the re-conveyance path. It is characterized in that a control means is provided.

The present invention is also characterized in that the sheet conveyed to the re-conveyance path is temporarily stopped.

Further, according to the present invention, while the sheet is temporarily stopped on the re-conveyance path, the succeeding sheets are temporarily stopped on the plurality of switchback conveyance paths, and after the sheet is conveyed from the re-conveyance path, The temporarily stopped sheet is conveyed to the re-conveyance path in the conveyance order.

Further, the present invention is characterized in that the sheets stopped on the plurality of switchback transport paths are transported to the re-transport path in the transport order at intervals shorter than the predetermined interval.

According to the present invention, a plurality of forward and backward reversible sheet conveying means for conveying a sheet are provided in the reverse conveying path, and the sheet is conveyed into the reverse conveying path by the normal rotation of the sheet conveying means. The sheet conveyed in the reverse conveyance path by the reverse rotation of the sheet conveying means is conveyed to the switchback conveyance path.

Further, according to the present invention, the plurality of sheet conveying means are constituted by a pair of rollers, and a predetermined roller pair among the plurality of sheet conveying means can be brought into contact with and separated from the sheet conveying means, and the sheet is conveyed into the reversing conveying path. When the sheet is conveyed to the switchback conveying path, the predetermined roller pair is separated from each other.

Further, the present invention is characterized in that the predetermined interval of the sheets sequentially conveyed to the reversal conveying path is changed according to the length of the sheet in the conveying direction.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is an overall configuration diagram of a copying machine which is an example of an image forming apparatus according to an embodiment of the present invention.

In FIG. 1, reference numeral 100 denotes a copying machine, which has a printer section 101 having an image forming section 105 and an image reading section 102. Further, the copying machine 100 is provided with an automatic document feeding device 103 above the image reading unit 102. With this automatic document feeding device 103, a document (not shown) is placed on the platen glass 102a on the image reading unit 102. The document is automatically fed, and the document is scanned by the image reading unit 102,
Digital information from the CCD camera 102b is stored in a storage device (not shown) as latent image data.

Further, in the copying machine 100, the scanner unit 104 forms a latent image on the photosensitive drum 106 of the image forming unit 105 based on the latent image data, and then develops with toner to form a latent image on the photosensitive drum 106. A toner image is formed.

On the other hand, 113A, 113B, 113C, 1
Reference numeral 13D denotes a paper feed cassette that is a sheet storage unit that is disposed in the printer unit 101 and that stores a large number of sheets, and these paper feed cassettes 113A, 113B, and
The sheets stored in 113C and 113D are fed by the sheet feeding units 114A, 114B, 114C and 114D, respectively.
After the sheets are fed one by one, the conveyance path 11 which is a sheet conveyance path
It is configured to be conveyed to the resist introducing section 116 at a predetermined timing through 5a and 115b.

A resist roller 117 is provided in the resist introducing section 116, and the sheet is conveyed to the transfer separation charger 118 at a predetermined timing after the skew is corrected by the resist roller 117. The transfer separation charger 118 transfers the toner image from the photosensitive drum 106.

Further, reference numeral 107 denotes a conveying section for conveying the sheet on which the toner image has been transferred to the fixing section 108, and the conveying section 10
A toner image is fixed on the sheet conveyed by the fixing roller 119 of the fixing unit 108. After the toner image is fixed, the sheet is distributed to the sheet discharge tray 110 or the duplex unit 112 by the sheet discharge flapper 111 provided in the discharge path 109.

Here, the paper discharge flapper 111 is switched to the paper discharge tray 110 side in the case of a one-sided copy in which an image is formed on only one side of the sheet under the control of a controller 80 shown in FIG. When performing double-sided copying in which images are formed on both sides or multiplex copying in which images are formed a plurality of times on one side, the duplex unit 112 is switched to.

On the other hand, FIG. 2 is a diagram showing a configuration of a double-sided unit 112 for reversing a sheet having an image formed on one side during double-sided copying and conveying the reversed sheet again to the image forming unit 105. .

Here, as shown in the figure, the double-sided unit 112 is provided with the first to fourth reversing roller pairs 1, 2, 3 and 4 which are sheet conveying means and is a switchback path which is a reversing conveying path. 10 and a re-conveyance path 20 including a re-conveyance roller 21 that conveys the reversed sheet to the image forming unit 105 again, and the re-conveyance path for the sheet branched from the switchback path 10 and reversed by the switchback path 10. Two
First and second transport rollers 31, 3 for transporting to 0
2 and a second switchback transport path 40 having third and fourth transport rollers 41 and 42, and a switchback path 1 for selectively switching sheets.
A first flapper 50 and a second flapper 60, which are introduced from 0 to the first or second switchback transport paths 30 and 40, are provided.

The switchback path 10 has the first to
The third sensors 71 to 73, the seventh sensor 77 on the re-conveyance path 20, the fourth and fifth sensors 74 and 75 on the first switchback conveyance path 30, and the second switchback conveyance path 40.
Is provided with a sixth sensor 76. In this embodiment, a reflective photosensor is used as each of these sensors.

Here, the first reversing roller pair 1 has a conveying force for conveying the sheet into the duplex unit and a conveying force for conveying the sheet from the switchback path 10 to the first switchback conveying path 30. It has a roller configuration that can be applied simultaneously.

That is, the first reversing roller pair 1 contacts the driving roller 1a and the driving roller 1a, respectively, and when the driving roller 1a rotates in the arrow direction, the driven roller 1b, which rotates in the arrow direction, follows. 1c, so that a conveying force for conveying the sheet in the duplex unit and a conveying force for conveying the sheet from the switchback path 10 to the first switchback conveying path 30 can be simultaneously given. ing.

The second, third, and fourth reversing roller pairs 2, 3, 4 include drive rollers 2a, 3a, 4a, and driven rollers 2b, which are in pressure contact with the drive rollers 2a, 3a, 4a.
3b, 4b and a separating mechanism (not shown) for separating the driven rollers 2b, 3b, 4b from the driving rollers 2a, 3a, 4a. The driven rollers 2b, 3b, 4b are driven by the separating mechanism to drive rollers 2a, 3a, 4a.
It is possible to prevent the conveying force of the roller pairs 2, 3 and 4 from being transmitted to the sheet by selectively separating the sheet.

In FIG. 2, reference numeral 80 denotes a controller, and the controller 80 controls the first to fourth reversing roller pairs 1 to 4 and the first and second flappers 50 and 60.
Are separated, driven in forward / reverse rotation, or swung.

Next, the operation of re-feeding the sheet having the image formed on one side thereof to the image forming section 105 in the double-sided unit 112 thus constructed will be described.

First, the sheet P is turned upside down using the first switchback transport path 30 of the duplex unit 112.
The operation when the paper is re-fed to the image forming unit 105 will be described.

First, a sheet having an image formed on one surface (front surface) thereof passes through the paper ejection path 109 and then the duplex unit 112.
When the first sensor 71 detects that the sheet has been conveyed in, the control device 80 controls the first reversing roller pair 1 and the second to fourth reversing roller pairs 2 to 4 depending on the size of the sheet in the conveying direction. The rollers required to convey the sheet are selected and driven in the forward direction. As a result, the seat has a switchback path 10
Be transported to.

Next, in this way, the switchback path 10
When the second sensor 72 detects the trailing edge of the sheet conveyed to the sheet, any one or all of the pair of reversing rollers 2, 3 and 4 which are selectively driven are temporarily stopped, and then the first flapper 50 is set to the first flapper 50. 1 switch back Switch to the transport side, after this,
The pair of reversing rollers once stopped is driven in the reverse direction.
As a result, the sheet is conveyed to the first switchback conveyance path 30 in a state of being turned upside down.

Next, when the fourth sensor 74 detects the leading edge of the sheet conveyed to the first switchback conveying path 30,
One or all of the reversing roller pairs 2, 3 and 4 that have been selectively driven are stopped, and the reversing roller pair that is selectively driven among the second and third reversing roller pairs 2 and 3 Separate.

Here, by separating the pair of reversing rollers in this manner, it is possible to prevent the sheet from being subjected to a conveying force, and thus the sheet to be conveyed to the switchback path 10 after that. The sheet can be conveyed.

Next, the first, second and fourth conveying rollers 31, 32 and 42 are driven in order at a predetermined timing. As a result, the sheet P conveyed to the first switchback conveyance path 30 is conveyed to the image forming unit 105 in the state of being turned upside down through the re-conveyance path 20.

The controller 80 controls the driving of the conveying rollers 31, 32, 42 at a predetermined timing after the sheet passes through the conveying rollers 31, 32, 42. Further, when the fifth sensor 75 detects the front end of the reversed sheet, and when it is determined that the preceding sheet exists on the re-conveyance path 20 based on the signal from the seventh sensor 77, the preceding sheet is detected. It is configured such that the driving of the first and second transport rollers 31 and 32 can be stopped and the sheet can be made to wait until the sheet passes through the re-transport unit 20.

Next, using the second switchback transport path 40 of the duplex unit 112, the sheet P is turned upside down.
The operation when the paper is re-fed to the image forming unit 105 will be described.

First, a sheet having an image formed on one side (front side) thereof passes through the paper discharge path 109 and the duplex unit 112.
When the first sensor 71 detects that the sheet has been conveyed inward, the first to fourth reversing roller pairs 1 to 4 are driven in the forward direction.
As a result, the sheet P is conveyed to the switchback path 10.

Next, when the third sensor 73 detects the trailing edge of the sheet, the third and fourth reversing roller pairs 3 and 4 are once stopped, and then the second flapper 60 is moved to the second switchback conveyance side. Switch, and then stop the 3rd and 4th
By driving the pair of reversing rollers 3 and 4 in the reverse direction and further driving the first conveying roller 41, the sheet P is conveyed to the second switchback conveying path 40 in the reversed state.

At this time, the first and second reversing roller pairs 1 and 2 are set so that the forward rotation drive and the conveyance force are applied so that the next sheet conveyed to the switchback path 10 can be conveyed. Keep it in contact.

Next, when the sixth sensor 76 detects the leading edge of the sheet P conveyed to the second switchback conveying path 40, the driven third and fourth reversing roller pairs 3, 4 are driven.
Is stopped and the third reversing roller pair 3 is separated so as not to apply a conveying force to the sheet, so that the next sheet conveyed to the switchback path 10 can be conveyed.

Next, the fourth conveyor roller 42 is driven at a predetermined timing. As a result, the sheet P transported to the second switchback transport path 40 is transported to the image forming unit 105 in the state of being turned upside down through the re-transport path 20. The controller 80 controls to stop the driving of the rollers 41 and 42 at a predetermined timing after the sheet passes through the third and fourth transport rollers 41 and 42.

Further, when the sixth sensor 76 detects the front end of the reversed sheet, the preceding sheet is reconveyed to the re-conveyance path 2.
0 exists on the re-conveying unit 2 when the preceding sheet exists.
The configuration is such that the driving of the third transport roller 41 can be stopped and the sheet can be made to wait until it passes 0.

Next, the copying machine 100 configured as described above.
In the following, a case of copying 6 single-sided originals on both sides of 3 sheets will be described.

First, the image data of six single-sided originals are stored in the storage device as latent image data by the automatic document feeder 103 and the image reading unit 102 as described above. Next, the latent image data corresponding to the contents of the first, third, and fifth originals stored in this storage device is developed in the image forming unit 105, and is sequentially transferred to the first to third sheets, and then the fixing unit. Fix at 108.

Next, the first to third sheets having the first, third, and fifth original images formed on one surface (front surface) are provided with a predetermined sheet interval, and a paper discharge flapper provided in the discharge path 109. By 111, it conveys to the duplex unit 112 continuously.

Here, when the first sensor 71 detects the leading end of the first sheet that has reached the duplex unit 112 when the three sheets are continuously conveyed in this way, the controller 8
0 first drives the first to fourth reversing roller pairs 1 to 4 in the forward direction, and further drives the second and third reversing roller pairs 2 and 3.
Are brought into contact with the sheet so that a conveying force can be applied. As a result, the first sheet is conveyed to the switchback path 10 by the conveying force of the reversing roller pairs 1 to 4.

Next, in this way, the switchback path 10
When the third sensor 73 detects the trailing edge of the first sheet that has been conveyed to the third sensor 73 as shown in FIG. 3A, the third and fourth reversing roller pairs 3, 4 is temporarily stopped, then the second flapper 60 is switched to the second switchback conveyance side, and the temporarily stopped third and fourth reversing roller pairs 3 and 4 are driven in the reverse direction, and then the third conveyance is performed. The roller 41 is driven.

As a result, the first sheet P1 is introduced into the second switchback transport path 40 as shown in FIG. 3 (b). The third sensor 73 detects the trailing edge of the first sheet P1 so as not to interfere with the conveyance of the second sheet P2 conveyed to the switchback path 10 while providing a predetermined sheet interval. From the 3rd at a predetermined timing
The reversing roller pair 3 is stopped and separated.

Next, when the sixth sensor 76 detects the leading edge of the first sheet P1 conveyed to the second switchback conveying path 40, the driving of the fourth reversing roller pair 4 is stopped, and after a predetermined time. By starting the driving of the fourth transport roller 42, the first sheet P1 is transported to the re-transport path 20.

In the present embodiment, a stepping motor is used as a drive source for the conveying roller and the reversing roller, and in order to prevent stepping out of the stepping motor, the fourth reversing roller pair 4 is thus driven. After the driving is stopped, the driving of the fourth conveyance roller 42 is started after a predetermined time.

Thereafter, the first sheet P1 is temporarily stopped at the re-feeding start position where the leading edge of the sheet is conveyed by a predetermined distance from the position on the re-conveyance path 20 detected by the seventh sensor 77.
In addition, during such a transport operation of the first sheet P1, the second and third sheets P2 and P3 are also transported. Then, after reaching the duplex unit 112, the switchback path 1
The rear end of the second sheet P2 conveyed to 0
Is detected, the pair of first and second reversing rollers 1 and 2 is once stopped, and then the first flapper 50 is switched to the first switchback conveyance side to stop the pair of first and second reversing rollers once. The pair 1 and 2 are driven in the reverse direction.

As a result, the second sheet P2 is introduced into the first switchback conveyance path 30 as shown in FIG. At this time, the first sheet P1 is at the refeed start position. Here, because of the conveyance of the third sheet P3 that is conveyed to the switchback path 10 next, when the fourth sensor 74 detects the leading edge of the inverted second sheet P2, the second sheet P3 is conveyed.
The driving of the reversing roller pair 2 is stopped and separated. At the timing when the rear end of the second sheet P2 has passed the fourth sensor 74, the driving of the second reversing roller pair 2 in the forward direction is started and the roller pair 2 is brought into contact.

As a result, the second reversing roller 10 which has been conveyed through the switchback path 10 only by the first reversing roller pair 1 is used.
The conveyance force of the second reversing roller pair 2 is applied to the sheet P2, and as a result, the reversed second sheet P
After passing through the second pair of reversing rollers 2, the first and second rollers 2
It is conveyed by the pair of reversing rollers 1 and 2.

Next, after reaching the duplex unit 112,
When the third sensor 73 detects the leading edge of the third sheet P3 conveyed to the switchback path 10, the third and fourth reversing roller pairs 3 and 4 are driven in the forward direction, and the third reversing roller pair is further driven. By bringing 3 into contact with the third sheet P3,
The inside of the switchback path 10 is conveyed. Then, thereafter, as with the first sheet P1, as shown in FIG. 4B, the rollers and the second flapper 60 are controlled so as to convey the third sheet P3 to the second switchback conveyance path 40. To do so.

On the other hand, the first sheet P1 conveyed to the re-conveyance path 20 has a sufficient sheet distance from the preceding sheet (third sheet in this embodiment) after reaching the re-feed position. After confirming that, the paper is re-fed and conveyed to the image forming unit 105.

Next, the second sheet P2 introduced into the first switchback transport path 30 is sequentially transported by the drive of the first and second transport rollers 31 and 32.
If the preceding first sheet P1 is still present in the re-conveyance unit 20 when the leading edge of No. 2 is detected by the fifth sensor 75, the driving of the conveyance rollers 31 and 32 is once stopped and the first sheet P1 is stopped.
The sheet interval is adjusted so that the sheet P1 reaches the re-feeding start position after passing through the transport roller 21 on the re-transport path 20, and the transport is controlled to restart.

Further, as shown in FIG. 4B, the third sheet P3 conveyed to the second switchback conveyance path 40 is
When the sixth sensor 76 detects the leading edge of the third sheet P3 and the preceding second sheet P2 is still present in the re-conveying section 20, the third conveying roller 41 is driven and conveyed. Once the driving of the third conveyance roller 41 is stopped,
The sheet interval is adjusted so that the sheet P2 reaches the refeeding start position after the sheet P2 has passed through the transport roller 21 on the re-transport path 20, and the transport is restarted.

Here, the second and third sheets P2 and P3 conveyed to the re-conveyance path 20 are first re-fed first as shown in FIGS. 5 (a) and 5 (b). Further, the sheet interval is adjusted so that the sheet interval between the second sheet P1 and the second sheet P2 is equal to or less than the predetermined sheet interval fed to the duplex unit 112, and the sheet is re-fed and conveyed to the image forming unit 105.

As described above, the first to third sheets P1 to P3 are sequentially conveyed while the double-sided unit 112 is provided with a predetermined interval, and the first to third sheets P1 to P3 are used as the first switchback conveyance path 30. By transporting each of them to the second switchback transport path 40, it is possible to increase the number of locations to be temporarily stopped for timing adjustment.

As a result, the sheets which are successively conveyed at the switchback path 10 at a predetermined interval are sequentially distributed to the first and second switchback conveying paths 30 and 40 and temporarily stopped, and then the sheets are surely reproduced again in the conveying order. The sheet can be sent to the transport path 20, and the sheet interval can be adjusted so that the sheet interval is equal to or less than the sheet interval when the sheet is fed to the duplex unit 112, and the sheet can be re-fed.

As a result, even in the image forming apparatus in which the conveyance interval between the sheets is narrow or the conveyance speed is high, the first reversed sheet and the image forming section are kept until the first reversed sheet reaches the image forming section 105. Since a large number of sheets before image formation can exist between the sheet 105 and 105, double-sided image formation can be performed at high speed.

By the way, the operation of copying 6 single-sided originals on both sides of 3 sheets has been described so far, but in the present embodiment, the number of originals or the number of copies increases and the number of output sheets increases. For example, if the number of sheets is five or more, the sheet interval between the fifth sheet for image formation on the front surface and the first sheet for image formation on the back surface becomes smaller than the predetermined sheet interval at the time of image formation. Because
In this case, it is possible to perform double-sided copying on all sheets at the same image forming sheet interval, and it is possible to further speed up double-sided image formation.

Although the case where the number of sheets to be simultaneously conveyed in the copying machine is three has been described so far, the present invention is not limited to this, and the number of sheets to be simultaneously conveyed in the copying machine according to the size of the sheet. By changing the value, the overall double-sided image forming speed can be increased and the productivity can be improved.

Next, a second embodiment of the present invention will be described in which the number of sheets to be simultaneously conveyed in the copying machine is changed according to the size of such sheets.

FIG. 6A shows a state in which the double-sided unit according to the present embodiment has a small size with a relatively short length in the carrying direction. In the present embodiment, this is the case. In the case of small size sheets, the number of sheets to be conveyed at the same time is five.

Further, FIG. 6B shows a state in the case of a middle size having a medium length in the carrying direction, and in the case of such a middle size sheet, the number of sheets to be carried simultaneously is 4 It is a sheet. Further, FIG. 7 shows a state in the case of a large size in which the length in the transport direction is relatively long, and in the case of such a large size sheet, the number of sheets to be simultaneously transported is three.

The number of sheets to be conveyed is, for example, as shown in FIG.
When the leading (first) sheet P1 passes through the second switchback conveyance path 40 and reaches the refeeding start position on the reconveyance path 20 as shown in (a) of FIG.
A predetermined timing time for transporting the sheet from the resist introducing unit 116 calculated from the size in the transport direction of the sheet is used to determine which number of sheets fed from A, 113B, 113C, and 113D can be subsequently transported. The value is calculated.

In this embodiment, the sheet size is 3
The calculation is roughly classified into types, and the above calculation is performed on the basis of the size in which the predetermined timing time for conveying the sheet from the resist introducing unit 116 is shortest in each size. If there is, the number of conveyed sheets is controlled to be the same.

In this way, the switchback transport path 30,
By controlling so as to change the number of sheets to be simultaneously conveyed in the image forming apparatus according to the size of the sheet to be conveyed in the conveyance direction, it is possible to always carry out optimum double-sided sheet conveyance regardless of the sheet size. The overall speed of double-sided image formation can be increased, and productivity can be improved.

Particularly, in the case of a small size sheet having a short conveyance sheet interval, it is possible to convey a larger number of sheets. Therefore, when forming a double-sided image on a large number of sheets, it is necessary to process it efficiently. You can

In the above description, the copying machine 100 provided with two switchback conveyance paths has been described, but if three or more switchback conveyance paths are provided, the speed of double-sided image formation can be improved. Can be raised.

[0077]

As described above, according to the present invention, a plurality of switchback transport paths are branched from the reverse transport path,
Sheets that are sequentially conveyed at a predetermined interval on the reverse conveyance path are distributed to the plurality of switchback conveyance paths and conveyed to the re-conveyance path, so that images can be printed on both sides of the sheet at high speed without increasing the size of the device. Can be formed.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a copying machine that is an example of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is an enlarged view showing a configuration of a duplex unit of the copying machine.

FIG. 3 is a first diagram illustrating a sheet flow when copying six single-sided originals onto both sides of three sheets in the copying machine.

FIG. 4 is a second diagram for explaining the flow of sheets when 6 single-sided originals are copied onto both sides of 3 sheets in the copying machine.

FIG. 5 is a third diagram illustrating the flow of sheets when copying six single-sided originals onto both sides of three sheets in the copying machine.

FIG. 6 is a diagram showing a sheet flow when the sheet size in the transport direction of the duplex unit according to the second embodiment of the present invention is a small size and a middle size.

FIG. 7 is a diagram showing a sheet flow when the sheet size of the duplex unit in the transport direction is a large size.

[Explanation of symbols]

1 to 4 1st to 4th reversing roller pairs 10 switchback pass 20 Re-transportation path 30 First switchback transport path 40 Second switchback transport path 80 controller 100 copiers 105 image forming unit 101 printer section 112 duplex unit 113A, 113B, 113C, 113D paper feed cassette 116 Resist Introduction Department

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 21/14 G03G 21/00 372 (72) Inventor Manabu Yamauchi 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Hiroto Nishihara 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. F-term (reference) 2H027 DA32 DC06 DC10 DC11 DE01 DE07 DE09 EC02 EC06 ED17 ED19 EE01 EE02 EE07 EF09 FA05 FA13 ZA07 2H028 BA06 BA09 BA14 BA16 BB04 2H072 AA07 AA16 AA24 AA32 CA01 CB01 CB03 CB05 JA02 3F053 BA03 BA29 EA02 EA05 EB02 EB04 EB07 EC02 EC14 LA02 LA07 LA03 3F100 AA02 CA11 CA13 CA15 CA17 DA17

Claims (8)

[Claims] 1. An image forming apparatus for reversing a sheet having an image formed on one side thereof by an image forming section and conveying the sheet to the image forming section again to form an image on the back side of the sheet. A reversing conveyance path for reversing the formed sheet, a reconveyance path for reconveying the sheet reversed on the reversing conveyance path to the image forming unit, and a reversing conveyance path branched from a plurality of positions of the reversing conveyance path. And a plurality of switchback conveyance paths that reverse and convey the sheet toward the re-conveyance path, respectively. 2. A control is performed such that sheets that are sequentially conveyed at a predetermined interval in the reversing conveyance path are sequentially distributed to the plurality of switchback conveyance paths and temporarily stopped, and then are conveyed to the reconveyance path in a conveyance order. The image forming apparatus according to claim 1, further comprising a control unit that controls the operation. 3. The sheet according to claim 1, wherein the sheet conveyed to the re-conveyance path is temporarily stopped.
The image forming apparatus described. 4. The sheet is temporarily stopped in the re-conveyance path, the succeeding sheets are temporarily stopped in the plurality of switchback conveyance paths, and the sheet is temporarily stopped after being conveyed from the re-conveyance path. The image forming apparatus according to claim 3, wherein the sheets are conveyed to the re-conveyance path in a conveyance order. 5. The sheet, which is stopped in the plurality of switchback transport paths, is transported to the re-transport path in a transport order at an interval shorter than the predetermined interval. The image forming apparatus according to item. 6. A plurality of forward / reverse rotatable sheet conveying means for conveying a sheet to the reverse conveying path is provided, and after the sheet is conveyed into the reverse conveying path by the forward rotation of the sheet conveying means, the sheet conveying means is provided. 6. The sheet conveyed in the reverse conveyance path by the reverse rotation of the sheet is conveyed to the switchback conveyance path.
The image forming apparatus according to item. 7. The sheet conveyed in the reversal conveying path is configured by forming a plurality of the sheet conveying means with a roller pair, and allowing a predetermined roller pair of the plurality of sheet conveying means to come into contact with and separate from the sheet. 7. The image forming apparatus according to claim 6, wherein the predetermined pair of rollers are separated from each other when they are conveyed to the switchback conveyance path. 8. The image according to claim 1, wherein a predetermined interval of the sheets sequentially conveyed to the reverse conveying path is changed according to the length of the sheet in the conveying direction. Forming equipment.