JPH0934182A - Both-side image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a both-side image forming device capable of moving a draw-back roller and a pressure board with one solenoid and capable of surely feeding a transfer paper from an intermediate tray even through stack defect, etc., occur. SOLUTION: In this both-side image forming device provided with the intermediate tray 2 on which the transfer paper 1 having an image formed on its surface is discharged, and the draw-back roller 3 which is brought into press-contact with the transfer paper inside the tray 2, moves the transfer paper to a re-feeding part 70, and stacks the transfer paper, and also provided with the pressure board 4 pressuring the stacked transfer paper, and an access roller 5 re-feeding the transfer paper pressured by the pressure board for back-surface copying one by one from the bottom transfer paper at the re-feeding part, has one solenoid 13 and a cooperating mechanism 40 contrarily moving the draw-back roller and the pressure board up and down with the on/off of the solenoid 13. When a paper detecting sensor 16 detects that the re- feeding of paper has finished, and the number of surface copies is less than the number of back-surface copies, the draw-back roller performs moving operation again, and the paper re-feeding operation can be performed at the time when the paper detecting sensor detects the transfer paper.

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 capable of forming an image on both sides of a transfer sheet, and is applicable to a copying machine, a printer and the like.

[0002]

2. Description of the Related Art Some copying machines and the like are capable of forming an image on both sides of a transfer paper in order to effectively use the transfer paper. When forming images on both sides of the transfer paper, after forming the image on one side of the transfer paper, the transfer paper is inverted and fed to the image forming section, and the image is formed on the opposite side of the transfer paper. There is. FIG. 4 conceptually shows an example of a copying machine having a double-sided image forming apparatus.

In FIG. 4, there is a paper feed section 8 at the bottom,
The intermediate tray 2 is arranged thereon, and the image forming section 9 is arranged thereon. A large number of transfer papers 1 are provided in the paper supply unit 8.
Are loaded, and the transfer papers 1 are transferred one by one from the paper supply unit 8 to the image forming unit 9 through the transfer passage 20. Image forming unit 9
Is a well-known electrophotographic apparatus that has a photoconductor drum and executes the processes of charging, exposure, development, transfer, cleaning, and fixing.The image formed on the photoconductor drum and developed by toner is transferred by the transfer process. The image is transferred onto the surface of the transfer paper 1 and fixed in the fixing process. The transfer sheet 1 having the image formed on the surface in this way is discharged from the discharge passage 21, and the reversing passage 22 branches off from the middle of the discharge passage 21.
When trying to form an image on the back side of the transfer paper 1,
The transfer paper 1 is discharged to the intermediate tray 2 via the reversing passage 22.

The intermediate tray 2 has a pair of discharge rollers 14 for discharging the transfer sheet 1 from the reversing passage 22 to the intermediate tray 2, and a sheet re-feeding section 70. The transfer roller 1 is provided with a transfer roller 3 for transferring the transfer paper 1 discharged toward the re-feeding section 70. The re-feeding unit 70 has a calling roller 5 whose outer periphery is partially exposed from the upper surface of the intermediate tray 2, and a transfer sheet 1 which is located above the calling roller 5 and stacked on the intermediate tray 2. A pressing plate 4 which can press the calling roller 5, a separating roller 6 arranged on the right side in the drawing near the calling roller 5, and a pulley 24, 25 arranged vertically with the separating roller 6 as an intermediate. And a part of the separation belt 7 is in pressure contact with the outer circumference of the separation roller 6.

When an image is to be formed on both sides of the transfer sheet 1, the transfer sheet 1 having the image formed on the surface in the image forming section 9 is transferred onto the intermediate tray 2 through the discharge passage 21 and the reversing passage 22. The sheet is discharged face down, and the transfer roller 1 is pressed against the transfer sheet 1 on the intermediate tray 2 and is transferred to the sheet re-feeding section 70 to be stacked. Next, the pressure plate 4 is lowered to pressurize the transfer paper 1 and press it against the calling roller 5. Since the calling roller 5 is driven to rotate in the clockwise direction, the transfer paper 1 on the intermediate tray 2 is sequentially sent out from the lower side to the right side in the figure by the above pressing force, and the separating roller 6 and the separating belt 7 sheet by sheet. The transfer passage 26 is formed below the intermediate tray 2 after being separated and transferred along the partial arc-shaped contact surface between the separation roller 6 and the separation belt 7 and turned upside down.
Sent to. The transfer passage 26 merges with the transfer passage 20, the transfer paper 1 is transferred to the image forming unit 9 through the transfer passages 26 and 20, and an image is formed on the back surface of the transfer paper 1 by the image forming unit 9. . The transfer paper 1 on which images are automatically formed on the front and back sides in this manner is discharged from the discharge passage 21. The vertical movement of the pressing roller 3 and the pressure plate 4 used in the above-mentioned double-sided image forming apparatus is controlled at a predetermined timing.

As described above, in the double-sided image forming apparatus, the intermediate tray is used to turn over the transfer paper, and after the transfer paper having the image formed on the front surface is stacked on the intermediate tray, the image is formed on the back surface. In order to do so, the transfer paper is carried out from the intermediate tray. As an example of this intermediate stack device,
Japanese Utility Model Publication No. 62-89236, Japanese Utility Model Publication No. 63-1263.
No. 38, No. 63-18574, No. 6
Various types of systems such as those described in JP-A-2-187690 have been proposed.

FIG. 5 specifically shows an example of an intermediate stack section used in a conventional double-sided image forming apparatus. In FIG. 5, the shifting roller 3 is rotatably provided at one end of a lever 57 that can rotate in a vertical plane about a shaft 58, and is rotationally driven counterclockwise in the drawing by an appropriate drive source. One end 55 of a lever 53, which is rotatably supported in a vertical plane about a shaft 54, can come into contact with the lever 57, and when the lever 53 rotates clockwise, the lever 57 rotates clockwise. It has become. The lever 53 is urged to rotate counterclockwise by a tension spring 56 hung on the other end. The lever 53 has one end near the shaft 54 connected to the plunger 51 of the solenoid 10 via a connecting member 52. Therefore, when the solenoid 10 is off, the biasing force of the spring 56 causes the levers 53 and 57 to rotate in the counterclockwise direction, the separation roller 3 is separated from the transfer sheet 1 on the intermediate tray 2, and when the solenoid 10 is turned on, the driving is performed. The levers 53 and 57 are rotationally driven in the clockwise direction against the above-mentioned urging force by the force, and the shifting roller 3 comes into pressure contact with the transfer paper 1 on the intermediate tray 2.

The pressure plate 4 is provided at one end of a lever 64 rotatable in a vertical plane about a shaft 65. The other end 63 of the lever 64 cooperates with one end 66 of another lever 61 rotatable about a shaft 62 in a vertical plane. The lever 64 is urged to rotate counterclockwise by a spring 67 hung on the other end. The other end of the other lever 61 is connected to the plunger 59 of the solenoid 11 via a connecting member 60. Therefore, when the solenoid 11 is off, the biasing force of the spring 67 causes the lever 64 to rotate counterclockwise, the pressure plate 4 is separated from the transfer sheet 1 on the intermediate tray 2, and when the solenoid 11 is turned on, its driving force is increased. Then, the lever 61 is rotated counterclockwise and the lever 64 is rotated clockwise against the above-mentioned urging force, and the pressure plate 4 calls the transfer paper 1 on the intermediate tray 2 to call the roller 5.
It is designed to be pressed toward.

The reversing passage 22 is provided with an entrance sensor 15 for detecting the transfer sheet 1 transferred to the reversing passage 22. Further, a paper detection sensor 16 for detecting the presence or absence of a transfer sheet in the re-feeding unit 70 is provided near the re-feeding unit 70. When images are to be formed on both sides of the transfer paper 1, the transfer paper 1 on the surface of which the image is formed by the image forming unit 9 is already described.
Is discharged onto the intermediate tray 2 with the image surface facing down through the reversing passage 22, the solenoid 10 is turned on at a predetermined timing, and the transfer roller 1 is moved to the re-feeding section 70 by pressing the roller 3 toward the transfer sheet 1. To stack. Next, the solenoid 10 is turned off, the solenoid 11 is turned on instead of the solenoid 10, and the pressure plate 4 is lowered to press the transfer paper 1 and press it against the calling roller 5 which is rotationally driven in the clockwise direction. By this pressing force, the transfer paper 1 is sequentially sent out from the lower side to the right side of the drawing, separated one by one by the separation roller 6 and the separation belt 7, and along the partial arc-shaped contact surface between the separation roller 6 and the separation belt 7. While being transferred, it is turned upside down and sent to the transfer passage 26. The transfer paper 1 is transferred to the image forming unit 9 via the transfer path 26 and the transfer path 20, and an image is formed on the back surface of the transfer paper 1 by the image forming unit 9.

[0010]

As described above, according to the conventional double-sided image forming apparatus, in order to move the pressing roller 3 and the pressure plate 4 up and down, the solenoid for moving the pressing roller 3 and the pressure plate 4 move. A separate solenoid was needed for each, and multiple solenoids were needed.

When another transfer paper 1 is discharged to the intermediate tray 2 while the transfer paper 1 is stacked on the intermediate tray 2, the transfer paper 1 already discharged is stacked by the discharged transfer paper 1. Paper 1 was sometimes pushed back. Further, when the transfer paper 1 is caught during the moving operation of the transfer paper 1 by the rotation drive of the closing roller 3 and a stack defect occurs, the transfer paper 1 is still left on the intermediate tray. Could not be sent from the intermediate tray, and stable transfer paper could not be conveyed.

The present invention has been made in order to solve the above-mentioned problems of the prior art, and makes it possible to move the shifting roller and the pressure plate by a single solenoid, which is an inexpensive double-sided image forming apparatus. The purpose is to provide.

The present invention can prevent transfer paper from remaining on the intermediate tray by ensuring that the transfer paper can be delivered from the intermediate tray even if a stack defect or the like occurs. An object is to provide a double-sided image forming apparatus.

[0014]

According to the first aspect of the present invention,
The intermediate tray from which the transfer paper on the surface of which the image is formed in the image forming unit is discharged, and the transfer paper in the intermediate tray is pressed from above to be transferred one by one to the re-feeding unit and stacked. The re-feeding unit includes a pressing roller that presses the stacked transfer papers from above, and the transfer papers that are pressed by the pressure plate by being driven to rotate one by one from the bottom. In a double-sided image forming apparatus equipped with a calling roller that re-feeds paper for back side copying, one solenoid and the on / off operation of this one solenoid cause the up and down movements of the shifting roller and the pressure plate to be performed in a contradictory manner. It is characterized by having a cooperation mechanism.

When one solenoid is turned on or off, the shifting roller is moved downward and pressed against the transfer sheets on the intermediate tray to transfer the transfer sheets one by one to the re-feeding section and stack them. It avoids the trouble of the stacking operation by moving antithetical upward and separating from the transfer paper. When the pressure plate moves downward and is in pressure contact with the transfer sheet on the intermediate tray, the shifting roller moves antithetically upward and separates from the transfer sheet, which hinders the operation of discharging the transfer sheet to the intermediate tray. In addition, the pressure plate prevents the stacked transfer sheets from being pulled back to the transfer sheet during the discharging operation during the discharging operation of the transfer sheets to the intermediate tray.

According to a second aspect of the present invention, when the intermediate tray has a paper detection sensor, and when the paper detection sensor detects the end of re-feeding, when the number of back side copies is smaller than the number of front side copies, the transfer operation is performed again by the shifting roller. When the paper detection sensor detects the transfer paper, the re-feeding operation is performed.

According to the third aspect of the present invention, the intermediate tray has a paper detection sensor and a back fence which is driven by an actuator such as a motor and can push and move the transfer paper in the intermediate tray. If the number of back side copies is less than the number of front side copies when the end of re-feed is detected, move the back fence toward the re-feed section and perform the re-feed operation when the paper detection sensor detects the transfer paper. Is characterized by.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a double-sided image forming apparatus according to the present invention will be described below with reference to the drawings. It should be noted that common reference numerals are used for the same components as those of the conventional example shown in FIG. FIG. 1 shows a specific structure of a part of an intermediate tray 2 embodying the present invention in the double-sided image forming apparatus which has been schematically described with reference to FIG. In FIG. 1, a shift roller 3 is arranged near the center of the intermediate tray 2 in the left-right direction. The approach roller 3 is rotatably provided at the right end portion of a lever 27 that is rotatable in a vertical plane about a shaft 30 provided on the left side of the approach roller 3, and is counterclockwise in the figure by an appropriate drive source. Driven to rotate. Lever 2
The left end 12b of the lever 12, which is rotatably supported in a vertical plane about the shaft 12a located on the right side of the approach roller 3, can contact the upper edge bent portion 27a of the lower end 7 from below. When the lever 12 rotates clockwise, the left end portion 12b pushes up the upper edge bent portion 27a of the lever 57 to rotate the lever 27 counterclockwise. The lever 12 is rotationally biased in the clockwise direction by a tension spring 34 hung on its right arm. Further, the lever 12 is connected to the plunger 35 of the solenoid 13 via a connecting member 36 at a portion near the left end thereof.

A lever 32 is also mounted on the right side of the lever 12 so as to be rotatable about a shaft 32a in a vertical plane. The pressure plate 4 is attached to the right end of the lever 32. The lever 32 has a bent upper edge 32b,
The right end portion 12c of the lever 12 is positioned below and below the bent upper edge portion 32b. The lever 32 is rotationally biased clockwise by a biasing means such as a spring (not shown). Note that the lever 12 and the lever 32 may be linked so that the lever 32 is rotationally biased by the elasticity of the spring 34.

When the solenoid 13 is off, the lever 12 is rotated clockwise by the urging force of the spring 34, and the lever 1
By the rotation of 2, the left end portion 12b rotates the lever 27 in the counterclockwise direction as described above, and the shifting roller 3 is separated from the transfer sheet 1 on the intermediate tray 2. On the other hand, when the lever 12 is rotated in the clockwise direction, the right end portion 12c of the lever 32 escapes downward from the bent upper edge portion 32b of the lever 32, and the lever 32 is rotated in the clockwise direction by an appropriate biasing means, so that the tip end portion of the lever 32 is rotated. The pressure plate 4 calls the transfer paper 1 on the intermediate tray 2 and the roller 5
Press against When the solenoid 13 is turned on, the driving force thereof drives the lever 12 to rotate counterclockwise against the biasing force, and the left end 12b of the lever 12 pushes the lower edge of the lever 27 downward to move the lever 27 clockwise. The transfer roller 1 is rotated to press the transfer roller 3 against the transfer paper 1 on the intermediate tray 2, the transfer paper 1 is transferred to the right side by the rotational force of the transfer roller 3, and the transfer paper 1 is stacked on the intermediate tray 2. On the other hand, lever 1
By the counterclockwise rotation of 2, the right end portion 12c pushes up the bent upper edge portion 32b of the lever 32 against the biasing force,
The pressure plate 4 is separated from the transfer paper 1 on the intermediate tray 2.
As described above, the levers 12, 27, and 32 cooperate with each other to form the cooperation mechanism 40. By the cooperation mechanism 40, the vertical movement of the approach roller 3 and the pressure plate 4 is performed in a contradictory manner. Has become.

The intermediate tray 2 is provided with a reversing passage 22 in the upper right portion in FIG.
After that, the transfer paper 1 is discharged to the intermediate tray 2. The reversing passage 22 is provided with an entrance sensor 15 for detecting the transfer sheet 1 transferred to the reversing passage 22. In addition, the intermediate tray 2 has a reversing passage 22.
Pair of discharge rollers for discharging the transfer paper 1 from the sheet to the intermediate tray 2
4. It has a re-feeding section 70. The sheet re-feeding unit 70 has a part of the outer periphery of which is exposed from the upper surface of the intermediate tray 2, the separating roller 6 disposed on the right side in the drawing near the calling roller 5, and the separating roller 6 as an intermediate portion. It has a separating belt 7 which is hung on the pulleys 24 and 25 arranged above and below and a part of which is pressed against the outer circumference of the separating roller 6. A paper detection sensor 16 for detecting the presence or absence of the transfer sheet 1 on the intermediate tray 2, especially the presence or absence of the transfer sheet in the re-feeding section 70 is provided near the re-feeding section 70.

The above-described intermediate tray 2 and the components around it are attached to the plate 75 in the vertical direction as a base. Although not shown in the figure,
The plates 75 are fixed in a pair in the front and rear, the intermediate tray 2 is fixed between the plates 75 forming a pair, and the rollers for transferring the transfer paper are rotatably supported.

When images are to be formed on both sides of the transfer paper 1, the transfer paper 1 on the surface of which the image is formed by the image forming section 9 is turned down from the reversing path 22 to the intermediate tray 2.
To be discharged. When the transfer paper 1 is entering the discharge path 22 toward the intermediate tray 2, the inlet sensor 15 causes the transfer paper 1 to move.
Is detected, the solenoid 13 is turned off by the detection output of the entrance sensor 15. As a result, FIG.
As shown in FIG. 5, the lever 32 is rotated clockwise by the urging force to lower the pressure plate 4, and the pressure plate 4 presses the transfer paper 1. The linkage mechanism 40 moves the pressing roller 3 away from the transfer sheet 1 in an antithetical manner with respect to the operation of the pressure plate 4 so that the transfer sheet 1 does not enter the intermediate tray 2. In addition, as described above, the pressing plate 4 presses the transfer paper 1 already stacked on the intermediate tray 2, so that the transfer paper 1 discharged onto the intermediate tray 2 through the discharge passage 22 is already stacked. It prevents the transfer paper 1 being pushed from being pushed back.

Transfer paper 1 from discharge passage 22 to intermediate tray 2
When is discharged, the detection signal of the transfer sheet 1 by the entrance sensor 15 is stopped, and the solenoid 13 is turned on by stopping this detection signal. When the solenoid 13 is turned on, as shown in FIG. 2, the pressing roller 3 is pressed against the transfer paper 1, whereas the pressure plate 4 is reciprocally separated from the transfer paper 1 upward. Since the shifting roller 3 is driven to rotate in the counterclockwise direction, the transfer sheet 1 on the intermediate tray 2 is transferred to the right side of the drawing and stacked on the re-feeding section 70.

When re-feeding the transfer paper 1 to form an image on the back side of the transfer paper 1, the solenoid 13 is turned off and the transfer paper 1 is pressed toward the calling roller 5 by the pressure plate 4 as shown in FIG. The transfer roller 1 is rotationally driven to sequentially transfer the transfer sheet 1 from the lower side to the right side. The transferred transfer paper 1 is separated one by one by the separation roller 6 and the separation belt 7, and is turned upside down while being transferred along the partial arcuate contact surface between the separation roller 6 and the separation belt 7, and the transfer path 2
Sent to 6. The transfer paper 1 is transferred to the image forming unit 9 via the transfer path 26 and the transfer path 20, and an image is formed on the back surface of the transfer paper 1 by the image forming unit 9. Further, at this time, since the shifting roller 3 moves upward in a direction contradictory to the operation of the pressure plate 4 and is separated from the transfer sheet 1, it is possible to avoid the shifting roller 3 from interfering with the re-feeding operation. There is.

The re-feeding operation is repeated by the ON signal of the paper detection sensor 16 while the transfer paper 1 is present on the intermediate tray 2, and ends when the paper detection sensor 16 is turned off. When it is turned off, that is, when the end of refeeding is detected, the transfer paper 1
When the number of images formed on the front side of the sheet, that is, the number of sheets copied on the back side, that is, the number of sheets re-fed is smaller than the number of sheets entered into the intermediate tray 2, the feeding operation for forming the image on the back side is surely performed. Since it is considered that there was not, the solenoid 13 is turned on again and the approach roller 3 is lowered. If there is a transfer sheet 1 that has not reached the paper detection sensor 16 due to a stack defect, the transfer sheet 1 is moved to the re-feeding unit 70 by the shifting roller 3. As a result, when the paper detection sensor 16 is turned on, the solenoid 13 is turned off and the re-feeding operation is performed.

An apparatus as shown in FIG. 3 is adopted as a double-sided image forming apparatus capable of performing a re-feeding operation when the number of backside copies is smaller than the number of images formed on the front side of the transfer paper 1. Good. The example shown in FIG. 3 is shown in FIGS.
A back fence 17 driven by a stepping motor or the like is added to the example shown in FIG. Back fence 1
Numeral 7 is attached to the intermediate tray 2 so as to be movable in the left-right direction, and is arranged on the leading end side of the transfer paper 1 discharged onto the intermediate tray 2 in the discharge direction, and the number of copies on the back surface is larger than the number of images formed on the front surface. If the number of lines is small, use a stepping motor, etc.
It is moved to the position indicated by 7A. By moving the back fence 17 in this way, it is possible to transfer the defective transfer paper 1 to the re-feeding unit 70,
As a result, when the paper detection sensor 16 is turned on, the solenoid 13 can be turned off to perform the sheet re-feeding operation, and it is possible to prevent the transfer sheet 1 having a defective stack from remaining on the intermediate tray 2. .

[0028]

According to the first aspect of the invention, an intermediate tray from which a transfer sheet having an image formed on the surface is discharged, and one transfer sheet by pressing the transfer sheet in the intermediate tray from above. Each of the re-feeding units has a pressing roller that moves to the re-feeding unit and stacks them, and the re-feeding unit presses the stacked transfer sheets from above and a pressure plate that is rotated by being driven by the pressure plate. In a double-sided image forming apparatus equipped with a call roller for re-feeding transfer sheets one by one from the bottom so as to be used for back side copying, one solenoid, and a turning roller and a pressure plate by turning on / off this one solenoid. Since it is possible to move the moving roller and the pressure plate up and down with a single solenoid, it is necessary to use separate solenoids for the moving roller and the pressure plate. No longer, it is possible to provide an inexpensive two-sided image forming apparatus.

According to the second aspect of the present invention, when the paper detection sensor detects the end of re-feeding, if the number of back-side copies is smaller than the number of images formed on the front side of the transfer paper, they are re-aligned. Use the roller to transfer the transfer paper,
As a result, when the paper detection sensor detects the transfer paper, the re-feed operation is performed, so even if the transfer paper remains in the intermediate tray due to a stacking failure, etc., it can be reliably re-fed. Therefore, the transfer paper can be stably transported.

According to the third aspect of the invention, when the paper detection sensor detects the end of re-feeding, if the number of back side copies is smaller than the number of images formed on the front side of the transfer paper, the back fence is used. Since the transfer paper is transferred to the re-feeding unit, even if the transfer paper remains in the intermediate tray due to a stacking failure, etc., it can be reliably re-fed and the transfer paper can be stably transported. You can

[Brief description of drawings]

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

FIG. 2 is a front view showing a different operation mode of the same embodiment.

FIG. 3 is a front view showing another embodiment of the double-sided image forming apparatus according to the present invention.

FIG. 4 is a front view conceptually showing an example of a copying machine having a double-sided image forming apparatus.

FIG. 5 is a front view showing an example of a conventional double-sided image forming apparatus.

[Explanation of symbols]

 1 Transfer Paper 2 Intermediate Tray 3 Crawler Roller 4 Pressurizing Plate 5 Calling Roller 9 Image Forming Section 13 Solenoid 16 Paper Detection Sensor 17 Back Fence 40 Cooperating Section 70 Refeeding Section

Claims (3)

[Claims] 1. An intermediate tray from which a transfer sheet having an image formed on its surface in an image forming section is discharged, and a transfer sheet refeeding section one by one by pressing the transfer sheet in the intermediate tray from above. The re-feeding unit has a pressing plate that presses the stacked transfer papers from above and a transfer paper that is rotationally driven to press the transfer papers that are pressed by the pressure plate. In a double-sided image forming apparatus equipped with a calling roller that re-feeds sheets one by one from the bottom so as to be used for back side copying, one solenoid and the vertical movement of the pressure roller and the pressure roller are turned on and off by turning this solenoid on and off. A double-sided image forming apparatus, which has a cooperation mechanism for performing it in a contradictory manner. 2. The intermediate tray has a paper detection sensor, and when the paper detection sensor detects the end of re-feeding and the number of back side copies is smaller than the number of front side copies, the transfer operation is performed again by the shifting roller, and the paper detection sensor. 2. The double-sided image forming apparatus according to claim 1, wherein when the transfer paper is detected, the paper feeding operation is performed again. 3. The intermediate tray has a paper detection sensor and a back fence that is driven by an actuator such as a motor and can push and move the transfer paper in the intermediate tray. The back fence is moved toward the re-feed unit when the number of back-side copies is smaller than the number of front-side copies when detected, and the re-feed operation is performed when the paper detection sensor detects a transfer sheet. 1. The double-sided image forming apparatus described in 1.