JPH04256972A - Both-side printing device and its printing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic double-sided printing apparatus such as a printer or a copying machine capable of printing on both sides of transfer paper, and a printing method using the same.

0002

2. Description of the Related Art For example, a double-sided printing device such as a laser printer includes, in addition to a normal paper feed tray and its paper feed roller group, a reversing tray, a reversing path, a group of paper feed/discharge rollers for the reversing tray, and a reversing tray. It is common to have a group of conveying rollers. The driving rollers of the sheet feeding/discharging roller group for the reversing tray are capable of forward and reverse rotation, and are configured to transport the transfer paper in both directions: feeding the sheet to the reversing tray and sending it out from the reversing tray to the image transfer section.

[0003] In such a double-sided printing device, the transfer paper printed on one side is led to the reversing tray via the reversing path, and after being stopped there, the transfer paper is synchronized with the photoreceptor. Double-sided printing is performed by feeding the paper from the reversing tray to the image transfer section and printing on the other side as well.

0004

[Problems to be Solved by the Invention] However, in such a double-sided printing device, standard size cut paper is fed from the paper feed tray, but in the case of cut paper, two
There is a problem called double feeding, where more than one sheet of transfer paper is fed out in an overlapping manner.

[0005] If transfer paper is double-fed from the paper feed tray, one side of the paper will be printed normally and the paper will be transported to the reversing tray, but the double-fed paper will remain as it is from the reversing tray. Since the image is sent to the image transfer unit and printed on the other side, the resulting paper is not a double-sided paper, but two sheets of paper, each with two pages of images printed on one side. It will end up being put away.

However, in conventional printers (laser printers) capable of double-sided printing, in such cases,
Even though normal double-sided printing cannot be obtained, it cannot be determined that it is an error, and the printed material cannot be used as is, resulting in a waste of paper.

[0007] The present invention has been made in view of the above points, and it is possible to prevent double feeding of transfer paper from a reversing tray for double-sided printing to an image transfer unit as much as possible, and as a result, there is no unused paper on the reversing tray. The purpose is to make it possible to reuse any remaining transfer paper.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a paper feed tray, a reversing tray, a reversing path, and a paper feed roller for the paper feed tray that can be controlled independently of the main drive system. a paper feeding/discharging roller group for the reversing tray, a conveying roller group for the reversing path, and a paper presence/absence detector for detecting the presence or absence of transfer paper on the reversing tray, and the paper feeding/discharging roller group for the reversing tray is It is composed of at least a pair of driving rollers and a driven roller, and can transport the transfer paper in both forward and reverse directions by sandwiching the transfer paper between the two rollers, and when the transfer paper is transported in the forward direction by the drive roller into the reversing tray. The driven roller rotates freely, and when the transfer paper is conveyed in the opposite direction out of the reversing tray, the driven roller does not rotate unless a torque greater than a certain value, which is greater than the frictional force between the transfer papers, is applied. To provide an electrophotographic double-sided printing device.

[0009] Furthermore, in the double-sided printing device, when the paper presence/absence detector detects that extra transfer paper remains on the reversing tray, a new transfer paper is fed from the paper feed tray during the next printing. The present invention also provides a printing method in which transfer paper remaining on a reversing tray is fed and printed without paper.

Furthermore, in the above-mentioned double-sided printing device,
In order to perform double-sided printing, the transfer paper printed on one side is led to the reversing tray via the reversing path, and when the paper is fed from the reversing tray to the image transfer section and printing is performed on the other side of the transfer paper. If the paper presence/absence detector detects that transfer paper remains on the reversing tray, the paper feed roller group for the paper feed tray and the transport roller group on the reversing path are stopped, and the transfer paper remaining on the reversing tray is stopped. The present invention also provides a printing method in which a sheet of transfer paper is forcibly fed, a blank sheet is printed, and the paper is discharged, and then a group of paper feed rollers and a group of transport rollers that have been stopped are operated to print on the other side.

In this printing method, when blank printing is performed on the extra transfer paper remaining on the reversing tray and the transfer paper is discharged, it is preferable to use an offset stack and shift the paper discharge position.

0012

[Operation] The double-sided printing device according to the present invention can print on one side and then feed the multiple sheets of transfer paper overlappingly from the paper feed tray when performing double-sided printing. The sheet is conveyed through the reversing path, and sent to the reversing tray by the reversing tray paper feed/discharge roller group. At this time, the driving roller rotates in the normal direction to convey the transfer paper in the forward direction, so the driven roller rotates along with the transfer paper, and sends the double-fed transfer paper as it is onto the reversing tray.

After that, when the transfer paper is fed from the reversing tray to the image transfer section, the drive roller of the paper feed/discharge roller group for the reversing tray reverses and conveys the transfer paper in the opposite direction, so the driven roller The sheets of paper will not rotate unless a certain amount of torque is applied, which is greater than the frictional force between the sheets. Therefore, when multiple sheets of transfer paper overlap and try to pass between the drive roller and the driven roller, the frictional force between the sheets of paper will be Since it is smaller than the above-mentioned constant value, the driven roller does not rotate, and only the transfer paper printed on one side directly facing the drive roller is conveyed, and the other transfer paper remains on the reversing tray without being conveyed. Therefore, it is possible to reliably obtain double-sided printed paper in which both sides of a sheet of transfer paper are printed.

However, if the double fed sheets are separated in this way, blank sheets will remain on the reversing tray, making it impossible to continue printing. Therefore, if the presence or absence of paper remaining in this reversing tray is detected and there is paper remaining, if the next print sequence requires new paper (blank paper) from the paper tray, the Paper is not fed, but the remaining paper is fed from the reversing tray and printed. By this,
This allows printing to continue and makes effective use of remaining paper.

[0015] Furthermore, when the next print in the print sequence is to print on the other side of the transfer paper that was printed on one side, when a remaining paper is detected on the reversing tray, the remaining paper is forcibly removed. After paper is fed, a blank sheet is printed, and the sheet is discharged, printing is performed on the other side. This prevents the print sequence from being interrupted and
Moreover, the discharged transfer paper can be used again.

In this case, by discharging the remaining sheets in an offset stack and shifting the paper discharge position, the user can easily distinguish between blank sheets that have been discharged due to double feeding.
It becomes easier to determine that an error has occurred.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram of a printer showing an embodiment of the present invention.

This printer is an electrophotographic printer such as a laser printer capable of double-sided printing, and has a paper feed tray 1, a reversing tray 2, and a reversing path 3, each of which can be controlled independently of the main drive system. A paper feed roller group 4 for the paper tray, a paper feed/discharge roller group 5 for the reversing tray, a conveyance roller group 6 of the reversing path 3, and a paper sensor 7 that is a paper presence/absence detector that detects the presence or absence of transfer paper on the reversing tray 2. It is equipped with

Furthermore, a photosensitive drum 8 as an image carrier, a pair of registration rollers 9 and a registration sensor 10,
It includes a fixing device 11, a pair of paper discharge rollers 12, and a paper discharge tray 13, and a branch claw 14 is provided at the entrance of the reversing path 3, and a branch claw 15 is provided at the exit. 16 is an optical writing unit equipped with an optical system such as a laser diode, a polygon mirror, and a lens, and 17 is an integrated control (sequence control) of each part of this printer, and the image data created by the controller 18 is transferred to transfer paper. This is a printer control unit that prints data.

As is well known, a charging charger, a developing unit, a transfer charger, a separation charger, a cleaning unit, etc. are arranged around the photosensitive drum 8, but these are not shown.

By the way, the paper feed/discharge roller group 5 for the reversing tray in this printer consists of a pair of driving rollers 21 and driven rollers 22, and the transfer paper can be held between the rollers 21 and 22 and conveyed in both forward and reverse directions. When the transfer paper is conveyed by the drive roller 21 in the forward direction (direction of arrow A) into the reversing tray 2, the driven roller 22 freely rotates with it, and when the transfer paper leaves the reversing tray 2 in the reverse direction. When being transported in the direction of arrow B, the driven roller 22 does not rotate unless a torque of a certain value or more, which is greater than the frictional force between the transfer sheets, is applied.

For this reason, a one-way clutch and a friction member, for example, are interposed between the driven roller 22 and its fixed shaft.

To outline the operation of this embodiment, in the case of normal printing (single-sided printing), the controller 18
When a print request is received from , the printer control unit 17 sequentially controls each part, rotates the paper feed roller group 4 for the paper feed tray to feed new transfer paper from the paper feed tray 1, and rotates the paper feed roller group 4 for the paper feed tray to feed new transfer paper from the paper feed tray 1, and It is temporarily stopped at the position (detected by the registration sensor 10).

On the other hand, the optical writing section 16 and the aforementioned image forming sections (not shown) are operated to form a toner image on the photosensitive drum 8, and in synchronization with the image writing position, the pair of registration rollers 9 are operated. The conveyance of the transfer paper is restarted by , and the paper is fed to the image transfer section indicated by the arrow, and the image is transferred onto one side of the paper.

After the image has been transferred, the transfer paper is sent to a fixing device 11 to fix the toner image thereon, and then the paper is ejected onto a paper ejection tray 13 by a pair of paper ejection rollers 12. Note that the general processes of electrophotography (charging, exposure, development, transfer, cleaning) will not be explained.

In the case of double-sided printing, after the transfer paper printed on one side passes through the fixing device 11, the branch claw 14
By switching, the transfer paper is guided to the reversing path 3 in the upper part of the figure, and sent onto the reversing tray 2 by the conveyance roller group 6 and the reversing tray paper feed/discharge roller group 5.

After stopping there, the branch claw 15 is switched to reverse the drive roller 21 of the paper feed/discharge roller group 5, and the transfer paper on the reversing tray 2 is fed through the paper refeed path 19, and the registration roller The paper is once again stopped at the position of pair 9, and in synchronization with the image writing position on the photoconductor, it is fed to the image transfer section by the registration roller pair 9, and the image is transferred to the other side of the transfer paper. , it is fixed through a fixing device 11 and output onto a paper ejection tray 13 by a pair of paper ejection rollers 12 .

FIG. 2 is a block diagram showing an example of the configuration of the control section of this printer. The controller 18 creates image data (video data) based on character data and various commands sent from a host computer (external device) 50 such as a computer or word processor. The printer control unit 1 sequentially sends the information as serial data TXD at a predetermined timing.
7, and one page of image data is written in response to the gate signal LGATE in the main scanning direction, the gate signal FGATE in the sub-scanning direction, and the synchronization signal LSYNC in the main scanning direction sent from the printer control unit 17. The data is sequentially sent to the printer control unit 17 as data WDATA.

The printer control section 17 includes a microcomputer (hereinafter abbreviated as "CPU") 51, a main motor driver 52, a polygon motor driver 53, a paper feed clutch driver 54, a registration clutch driver 55, a laser drive circuit 56, and a synchronization signal. It is composed of a detection circuit 57, an optical writing control circuit 58, a double-sided drive circuit 59, etc., and controls the printing operation according to each command from the controller 18.

[0030] The CPU 51 is a microprocessor, RO
It consists of M, RAM, counter, I/O, etc., and sequentially processes the transmission data TXD from the controller 18 and signals from various sensors and switches including the paper sensor 7 and the registration sensor 10 shown in FIG. , which controls the printer control section 17 in an integrated manner. It also sends various information such as status information to the controller 18 as serial data RXD.

[0031] The main motor driver 52 is a CPU 51
A main motor (M1) 6 for rotating the photosensitive drum 8 and each roller shown in FIG. 1 in response to a signal MM from
Drive 1.

[0032] The polygon motor driver 53 is operated by the CPU 5.
One of two types of rotation speed corresponding to low density (for example, 240 DPI) or high density (for example, 300 DPI) is selected by the recording density (rotation speed) selection signal DPISEL from 1, and the signal PM from the CPU 51 is selected. The polygon motor (M2) 62 is driven to rotate the polygon mirror at the rotation speed selected accordingly. Furthermore, when the polygon motor 62 reaches the normal rotation speed, a signal PML indicating this is generated.
K is output to the CPU 51.

[0033] The paper feed clutch driver 54 is operated by the CPU 51.
The paper feed clutch (CL1) 63 is turned on by the signal FC from the main motor 61, and the driving force of the main motor 61 is transmitted to the paper feed roller group 4. The resist clutch driver 55 is
The resist clutch (C) is activated by the signal RC from the CPU51.
L2) 64 is turned on to transmit the driving force of the main motor 61 to the registration roller pair 9.

The laser drive circuit 56 sets the light emission intensity of the laser diode (LD) 65 in the optical writing section 16 shown in FIG. When the is sent,
The light emission of the laser diode 65 is controlled on and off accordingly.

The synchronization signal detection circuit 57 detects the synchronization signal from the synchronization detection sensor (PD) 66, amplifies it, shapes the waveform, and outputs the synchronization signal DETP to the optical write control circuit 58. The synchronization detection sensor 66 is composed of a phototransistor or the like, and detects the incident laser beam just before it scans the photoreceptor drum 8.

The optical write control circuit 58 is turned on for each line (horizontal scanning line) during its effective write period based on the signal FSYNC from the CPU 51 and the synchronization signal DETP from the synchronization signal detection circuit 57. The gate signal LGATE in the main scanning direction, the gate signal FGATE in the sub-scanning direction which is on during the valid writing period (corresponding to the length of the recording paper in the paper feeding direction) in the sub-scanning direction, and writing every line during that time. A synchronization signal LSYNC in the main scanning direction for position synchronization is output to the controller 18, respectively.

Furthermore, the write data WDATA from the controller 18 is internally generated using the write synchronization clock WCLK.
is synchronized and outputted to the laser drive circuit 56 as a modulation signal VIDEO.

The double-sided drive circuit 59 controls the transport roller group 6 of the reversing path 3 shown in FIG.
a double-sided conveyance motor (M3) 67 that rotates the reversing tray, a forward/reverse motor (M4) 68 that rotates the drive roller 21 of the reversing tray paper feed/discharge roller group 5 in forward and reverse directions, and a double-sided inlet solenoid (SOL1) that switches the branch claw 14. 69, and a reversing solenoid (SOL2) 7 that switches the branch claw 15.
Drive 0.

FIG. 3 shows the sequence of double-sided printing by this printer. Mode 1 shown in (a) is the simplest double-sided printing sequence, in which the back side is printed first, the paper is guided to the reversing tray 2, and then the paper is re-fed and the front side is printed. That is, double-sided printing is performed by simply printing alternately in the order of even pages→odd pages→even pages→odd pages. The reason why we print the back side first is to align the order of the pages.
When printed continuously and stacked on the paper discharge tray 13, the pages are stacked on the paper discharge tray in page order from the bottom.

[0040] This mode 1 is set to page-by-page (page-by-page).
This is called e-by-page) mode. The disadvantage of this mode is that it takes time to transport the transfer paper from printing one side to printing the other side, which slows down the printing speed. Therefore, when the transfer paper is completely guided to the reversing path 3 shown in FIG. 1, the conveyance speed by the conveyance roller group 6 is increased to shorten the conveyance time.

Mode 2 shown in (b) of FIG. 3 is mode 1
This was devised to improve the drawback of slow printing speed, and is particularly effective when double-sided printing is performed continuously.

In the double-sided printing sequence of mode 2, first, even-numbered pages 2 and 4 are sequentially printed on one side of different transfer sheets, and both of the transfer sheets are guided to the reversing path 3. . When the transfer paper on which two pages have been printed reaches the reversing tray 2, the conveying direction of the paper is immediately reversed, the paper is re-fed, and one page is printed on the other side of the transfer paper. At this time, the transfer paper on which four pages have been printed is present in the reversing path 3.

After that, even-numbered pages and odd-numbered pages are alternately printed, and printing is performed efficiently. This mode is called an interleave mode. This mode has faster printing speeds, but requires more complicated control and higher memory costs.

Next, the operation of the sheet feeding/discharging roller group 5 for the reversing tray in the embodiment shown in FIG. 1 will be explained with reference to FIG. As mentioned above, the paper feed/discharge roller group 5 for the reversing tray is composed of two upper and lower rollers. In this embodiment, the upper roller 21 is the driving roller, and the lower roller 2 is the driving roller.
2 is a driven (non-driven) roller.

When the reversing tray 2 functions as a paper ejection tray, the driving roller 2
1 rotates normally in the direction of arrow a, and conveys the transfer paper P in the direction of entering the reversing tray 2 (direction of arrow A). At this time, the driven roller 22 unconditionally rotates in the direction of arrow b. Therefore, even when two sheets of transfer paper P1 and P2 are fed in duplicate as shown in FIG.

In this case, the transfer paper is not completely ejected onto the reversing tray 2, but a portion of the rear end is partially ejected onto the roller 21.
, 22 and stops. When the reversing tray 2 functions as a paper feed tray and feeds paper to the image transfer section, the driving roller 2 is rotated as shown in FIG.
1 reverses in the direction of arrow c and conveys the transfer paper P in the direction out of the reversing tray 2 (in the direction of arrow B).

At this time, the driven roller 22 is designed not to rotate unless a rotational torque of a certain value or more is applied, but when only one sheet of transfer paper P is fed, the driven roller 22
Since the frictional forces between 1 and the transfer paper P and between the transfer paper P and the driven roller 22 are large, the driven roller 22 is rotated by a rotational torque of a certain value or more. Therefore, the paper is fed directly in the direction of arrow B.

However, when two or more sheets of transfer paper are fed in duplicate as shown in (d), only the transfer paper that is in direct contact with the drive roller 21 (in the illustrated example, the upper transfer paper P1) is in the direction of arrow B. conveyed in the direction. This is between the transfer paper (P1 and P
2) is smaller than the necessary rotational torque of the driven roller 22, the driven roller 22 does not rotate, and the lower transfer paper P2 can be separated while being kept stationary.

Assuming that the size of the transfer paper is known in advance, double feeding sheets can be separated by controlling the rotation time of the drive roller 21. FIG. 5 shows an operational flow diagram of this embodiment. In the double-sided printing operation, whether there is any remaining paper on the reversing tray 2 can be determined based on the paper sensor 7 and the registration sensor 10 attached to the reversing tray 2 and information on the paper size.

L (mm); Distance W (mm) between paper sensor 7 and registration sensor 10; Length V (mm/mm) of transfer paper;
sec); Conveyance speed of transfer paper

[0051]W>L
In the case of , after the leading edge of the paper passes through the registration sensor 10
If the paper sensor 7 is still ON after (W-L)/V} seconds, it can be determined that there is remaining paper. In the case of W<L, if the paper sensor 7 is still ON when the leading edge of the paper reaches the registration sensor 10, it can be determined that there is a remaining paper.

[0052] The presence or absence of remaining paper is determined by checking the following paper feed tray 1.
This must be done before the paper feeding from the transfer paper or the paper ejection to the subsequent reversing tray 2 is started, or within a time period in which the conveyance of these transfer paper sheets can be independently controlled to stop. Further, it must be done before the subsequent print image is written on the photoreceptor drum 8.

[0053] When it is determined that there is paper remaining, paper feed tray 1
Immediately stop the paper feeding operation from the
Reverse tray 1 while maintaining a certain distance from the preceding transfer paper.
Feed the remaining paper from the Thereby, the preceding transfer paper is printed correctly.

Since the remaining paper on the reversing tray 2 is blank,
After the preceding double-sided printing is completed, if the subsequent printing is to be fed from the paper feed tray 1, the remaining paper on the reversing tray 2 can be used as is. Therefore, the paper is not fed from the paper feed tray 1, but is fed from the reversing tray 2 for printing, and the transfer paper is used effectively.

However, in the case of mode 2 shown in FIG. will be printed on a blank sheet and ejected to the paper ejection tray 13.

[0056] When printing out blank paper in this way, if there is an offset stacking mechanism in the paper ejection section, the offset stacking mechanism is used to shift the paper ejection position and eject the paper. If the blank paper is ejected at a different position from the printed transfer paper, it is convenient to see at a glance that there has been an error output, and to easily identify the blank paper. This blank paper can also be reset into the paper feed tray 1 and used for subsequent printing.

By the way, in the case of double-sided printing, the reversing tray is treated as a paper discharge tray and also as a paper feed tray. It is assumed that the printer control unit 17 always performs the printing operation in the following sequence.

[0058] This idea is applicable when conceptually considering the paper path when there are multiple paper feed trays and paper output trays, and there is a double-sided printing function, and a reversing tray is used as both the paper feed tray and the paper output tray. It is effective for That is, when printing, the paper feed tray and paper ejection tray are first specified, and then the print operation is executed.

FIG. 5 is a flowchart for mode 1 of double-sided printing in the above embodiment, but mode 2 can be considered in the same way. For double-sided printing in mode 1, the first page is set to the paper feed tray,
Printing is executed with the paper ejection specification set to the inverted tray, and the next page is printed with the paper feed specification set to the inverted tray and the paper output specified to the output tray, and this operation is repeated when double-sided printing is to be performed continuously.

[0060] If it is found that there are remaining sheets when paper is fed from the reversing tray, it is checked at that point whether there is a subsequent print request. If not, a forced print is executed in which the remaining paper is forcibly printed as a blank sheet, with the paper feed designation set to the inversion tray and the paper discharge designation set to the paper discharge tray, and the transfer paper is output to the paper discharge tray. At this time,
If the paper discharge tray of the printer has an offset stack function, it is possible to shift the paper discharge position and discharge the paper to make it easier to determine that an error has occurred.

[0061] Forced printing (blank paper printing) refers to printing that does not involve the transfer of image data from the controller. If there is a subsequent print request when it is determined that there is paper remaining on the reversing tray, the paper source specification will be changed to the reversing tray,
Paper ejection is also specified as a reversing tray, and paper that should normally be fed from the paper feed tray is temporarily fed from the reversing tray and printing is performed, making effective use of remaining paper.

[0062]

[Effects of the Invention] As explained above, the double-sided printing device according to the present invention feeds the transfer paper from the reversing tray to the image transfer section even if double-feeding of transfer paper occurs when performing double-sided printing. Sometimes, only one sheet of transfer paper with prints on one side can be fed, which prevents the prints on the back and front sides from being printed on two sheets of transfer paper, ensuring double-sided printing. can be done.

Furthermore, if the printing method according to the present invention is carried out, even if paper remains on the reversing tray due to separation of double-fed transfer papers, it will not be treated as a jam and the printing operation will not be interrupted. You can continue without Then, the remaining paper is fed and used for the next printing, so the transfer paper can be used efficiently.

[0064] Furthermore, if the remaining paper (blank paper) on the reversing tray cannot be reused due to the print sequence,
You can print out a blank sheet of paper and continue subsequent printing operations, and you can also use the ejected blank sheet of paper. Furthermore, if the blank paper is outputted at an offset position on the paper ejection tray, it becomes easier for the user to determine whether an error has been output, and it becomes easier to identify the blank paper.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram of a printer showing an embodiment of the present invention.

FIG. 2 is a block diagram showing details of a printer control section in FIG. 1;

FIG. 3 is an explanatory diagram of different modes of a double-sided printing sequence according to the embodiment shown in FIGS. 1 and 2;

4 is an explanatory diagram for explaining the operation of the reversing tray paper feed/discharge roller group 5 in FIG. 1. FIG.

FIG. 5 is an operation flow diagram in the double-sided print mode of the embodiment shown in FIGS. 1 and 2;

[Explanation of symbols]

1 Paper feed tray 2 Reversing tray
3 Reversing path 4 Paper feed roller group for paper feed tray 5 Paper feed/discharge roller group for reversing tray 6 Conveyance roller group 7 Paper sensor (
paper presence/absence detector) 8 Photosensitive drum 9 Registration roller pair 10 Registration sensor 11 Fixing device 12 Paper ejection roller pair 13 Paper ejection trays 14, 15 Branch claw 16 Optical writing section
17 Printer control unit 18 Controller 21 Drive roller
22 Followed roller

Claims (4)

[Claims] 1. A paper feed tray, a reversing tray, a reversing path, a group of paper feed rollers for the paper feed tray, a group of paper feed/discharge rollers for the reversing tray, each of which can be controlled independently of a main drive system, and the comprising a conveying roller group on a reversing path and a paper presence/absence detector for detecting the presence or absence of transfer paper on the reversing tray,
The sheet feeding/discharging roller group for the reversing tray is composed of at least a pair of driving rollers and a driven roller, and is capable of pinching the transfer sheet and conveying it in both forward and reverse directions,
When the transfer paper is conveyed by the driving roller in the forward direction into the reversing tray, the driven roller freely rotates with it, and when the transfer paper is conveyed in the reverse direction out of the reversing tray, the driven roller rotates freely. An electrophotographic double-sided printing device characterized in that it does not rotate unless a torque greater than a certain value, which is greater than the frictional force between sheets of paper, is applied. 2. The double-sided printing apparatus according to claim 1, when the paper presence/absence detector detects that excess transfer paper remains on the reversing tray, the paper feed tray is removed at the time of next printing. A printing method characterized in that printing is performed by feeding the transfer paper remaining in the reversing tray without feeding a new transfer paper from the reversing tray. 3. In the double-sided printing apparatus according to claim 1, in order to perform double-sided printing, a transfer paper printed on one side is guided to the reversing tray via the reversing path, and from the reversing tray the image transfer section If the paper presence/absence detector detects that there is transfer paper remaining in the reversing tray when printing on the other side of the transfer paper, the feeder for the paper feed tray The paper roller group and the transport roller group on the reversing path are stopped, the transfer paper remaining in the reversing tray is forcibly fed, a blank sheet is printed, and the paper is discharged. A printing method characterized by performing the above-mentioned double-sided printing by operating a group of conveyance rollers. 4. In the printing method according to claim 3, when blank printing is performed on the excess transfer paper remaining on the reversing tray and the transfer paper is ejected, an offset stack is used to shift the ejection position of the paper. A printing method characterized by: