JP2006317730A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means for always keeping a leading writing position constant, irrespective of a recording paper which is supplied from a paper feed cassette to a main carrying path or a recording paper which is resupplied from a reversal carrying path to the main carrying path. <P>SOLUTION: In the image forming apparatus 1, a timing control of sending the recording paper P whose trailing end part is nipped by the paper feed unit 22 after being fed from the paper feed cassette 2 to a transfer nip position 12 by a pair of resist rollers 8 is performed based on the recording paper detection by a main carrying path sensor 23. Based on the recording paper detection by a reversal carrying path sensor 24, an operation control of making the recording paper P to be sent from the reversal carrying path Rh to the main carrying path Rs stand by in the reversal carrying path Rh until a printable state is attained in the transfer nip position 12, is performed, and also the timing control of feeding the paper to the transfer nip position 12 by the pair of resist rollers 8 is performed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus capable of forming an image on both sides of a recording sheet, and more particularly to an image forming apparatus that nips a recording sheet set in a paper feed cassette and feeds the recording sheet to a conveyance path.

  Conventionally, there are printers, facsimiles, copiers, etc. as image forming apparatuses for recording characters, figures, etc. on recording paper, but in recent years, from the viewpoint of resource saving, images that can be printed on both sides of recording paper A forming apparatus is used (see, for example, Patent Document 1). FIG. 4 is a block diagram showing an example of a conventional image forming apparatus capable of duplex printing. In this image forming apparatus 70, when a command for executing double-sided printing is issued, the surface of the photosensitive drum 71 is uniformly charged by the charging device 72, and light corresponding to image data to be recorded on the photosensitive drum 71 is emitted. Irradiation from the exposure device 73 forms an electrostatic latent image, and toner is supplied to the electrostatic latent image from the developing device 74 to form a toner image. On the other hand, with the start of the image forming operation, several sheets of recording paper P set in the paper feed cassette 75 are taken out from the uppermost paper by the paper feed roller 76 and nipped between the paper feed roller 76 and the friction pad 77. As a result, only the uppermost sheet is separated and fed into the main conveyance path 78. The fed recording paper P is fed by a registration roller 79 to a transfer nip position 81 where a transfer roller 80 is pressed against the photosensitive drum 71, and a toner image on the photosensitive drum 71 is transferred. Further, the recording paper P is transported downstream through the main transport path 78 and is heated and pressurized by the fixing device 82, whereby the toner image on the recording paper P is fixed.

  Thus, the recording paper P on which the printing on the first surface has been completed is sent to the reverse conveyance path 84 when the paper discharge roller 83 stops in a state where the recording paper P is nipped and starts reverse rotation. As shown in FIG. 4, the reverse conveyance path 84 communicates the upstream portion and the downstream portion of the main conveyance path 78, and a conveyance roller 85 is appropriately disposed on the path. The recording paper P is transported to the paper feed cassette 75 side with the printed first surface facing inward, and the front and back are reversed from the time of printing on the first surface via the junction 86. In this state, it is supplied again to the main transport path 78. The re-supplied recording paper P is fed to the transfer nip position 81 by the registration roller 79 as in the case of printing on the first side, whereby printing on the second side is performed. The recording paper P that has been printed on both sides in this way is discharged to the paper discharge tray 87 by the paper discharge roller 83 via the fixing device 82.

  In this image forming apparatus 70, as shown in FIG. 4, a main transport path sensor that detects the passage of the recording paper P at a position downstream of the junction 86 in the main transport path 78 and upstream of the registration rollers 79. 88 is arranged. When the recording paper P is detected by the main transport path sensor 88, based on the detection signal from the main transport path sensor 88, a control unit (not shown) applies the leading edge of the recording paper P to the registration roller 79 in a stopped state. The printing of the recording paper P is started by feeding the recording paper P to the transfer nip position 81 in synchronization with the so-called resist alignment control that corrects the skew by abutting and the formation of the latent image on the photosensitive drum 71. A so-called tip writing position is controlled to keep the position constant. In this registration alignment control and leading edge writing position control, the detected recording paper P is supplied from the paper feed cassette 75 to the main transport path 78 or re-supplied from the reverse transport path 84 to the main transport path 78. Regardless of what has been done, the same control is performed based on the detection signal from the main conveyance path sensor 88.

JP 2001-335216 A

  However, in the conventional image forming apparatus 70, the recording paper P detected by the main transport path sensor 88 is supplied from the paper feed cassette 75 to the main transport path 78, or the reverse transport path 84 to the main transport path 78. There is a problem that the leading edge writing position on the recording paper P varies depending on whether it is re-supplied. That is, in the case of the recording paper P supplied from the paper feed cassette 75, the rear end portion is nipped between the paper feed roller 76 and the friction pad 77 while the front end portion is nipped by the registration roller 79 and conveyed. In contrast, in the case of the recording paper P re-supplied from the reverse conveyance path 84, such a load is hardly received. Accordingly, the time required for transporting the recording paper P to the transfer nip position 81 after the main transport path sensor 84 detects the recording paper P is reversed for the recording paper P supplied from the paper feed cassette 75. It becomes longer than the recording paper P re-supplied from the conveyance path 84. However, as described above, in the control of the leading end writing position, the same control is performed without distinguishing whether the paper is supplied from the paper feed cassette 75 or re-supplied from the reverse conveyance path 84, that is, the same. Since the sheet is fed to the transfer nip position 81 at the timing, the leading edge writing position varies.

  Therefore, it is necessary to change the control of the leading end writing position between the recording sheet P supplied from the paper feed cassette 75 and the recording sheet P supplied from the reverse conveyance path 84. As a means for avoiding this, a main transport path sensor 78 for detecting the recording paper P from the paper feed cassette 75 and a main transport path sensor 78 for detecting the recording paper P from the reverse transport path 84 are provided separately. Although not shown in detail in the drawing or the method of controlling the tip writing position separately, a film material made of resin or the like is affixed to the wall surface of the main conveyance path 78 to absorb errors caused by the load. For example, there is a method of performing the same control with the main transport path sensor 78 for the recording paper P supplied from any transport path with the same transport path, but all of them increase the number of parts, leading to an increase in cost. Q There is.

  The present invention has been made in view of such a problem, and is a recording sheet supplied from a paper feed cassette to a main transport path by a paper feed roller and a friction pad or re-supplied from a reverse transport path to the main transport path. Regardless of the recording paper to be used, a means for always keeping the leading writing position constant is provided.

  In order to achieve the above object, an image forming apparatus according to claim 1, wherein a main transport path from the paper feed cassette to the discharge tray through the transfer nip position, and an upstream side of the transfer nip position of the main transport path A reverse conveyance path that communicates the upstream branch position and the downstream downstream branch position, a paper feed unit that nips the recording paper set in the paper feed cassette and feeds it to the main conveyance path, and an upstream in the main conveyance path A pair of registration rollers provided at a predetermined position downstream from the side branch position and upstream from the transfer nip position; and a predetermined position downstream from the upstream branch position in the main transport path and upstream from the pair of registration rollers. In the image forming apparatus, comprising: a main transport path sensor that detects the passage of the recording paper at a position; and a reverse transport path sensor that detects the passage of the recording paper at a predetermined position of the reverse transport path. Based on the detection of the recording paper, the timing control for feeding the recording paper fed out from the paper feeding cassette and having its rear end nipped by the paper feeding unit to the transfer nip position by the registration roller pair is performed, and the reverse conveyance is performed. Based on the detection of the recording paper by the path sensor, the recording paper fed from the reverse conveyance path to the main conveyance path is made to wait in the reverse conveyance path until the transfer nip position is ready for printing, and the registration roller pair Timing control for feeding to the transfer nip position is performed.

  According to a second aspect of the present invention, the reverse conveyance path sensor is provided at a position immediately upstream of the upstream branch position in the reverse conveyance path.

  According to a third aspect of the present invention, the occurrence of a paper jam in the reverse conveyance path is detected based on the detection of the recording paper by the reverse conveyance path sensor.

  According to the image forming apparatus of the first aspect of the present invention, for the recording paper supplied from the paper feed cassette, the leading writing position is controlled based on the recording paper detection by the main transport path sensor, while the reverse transport is performed. For recording paper that is re-supplied from the path to the main transport path, whether or not a load is applied to the trailing edge of the recording paper by controlling the leading end writing position based on recording paper detection by the reverse transport path sensor Regardless, the tip writing position can always be kept constant.

  According to the image forming apparatus of the second aspect of the present invention, the conveyance distance to the registration roller or the transfer nip position after the recording sheet is detected by the reverse conveyance path sensor is shortened, and the recording sheet slips or the like. It is possible to minimize the error in the transport time that occurs, and to perform the resist alignment control and the tip writing position control more accurately.

  According to the image forming apparatus of the third aspect of the present invention, there is no need to separately provide a sensor for detecting the occurrence of a paper jam in the reverse conveyance path, so that the number of parts can be reduced, the apparatus can be downsized, and Cost can be reduced.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. This image forming apparatus is a copy / facsimile multifunction peripheral having a copy function of a document, a facsimile communication function, an Internet facsimile communication function, etc., on one side of a recording sheet based on image data read from the document or image data received by facsimile Alternatively, images are formed on both sides. Of course, the present image forming apparatus can be configured as a copy-only machine having only a copy function or a facsimile-only machine having only a facsimile communication function.

  FIG. 1 is a schematic longitudinal sectional view showing a configuration of an image forming apparatus 1 according to the present embodiment. The image forming apparatus 1 includes a main transport path Rs from a paper feed cassette 2 through a photosensitive drum 3 to a discharge tray 4, an upstream branch position 5 on the upstream side of the main transport path Rs, and a downstream branch position on the downstream side. 6 includes a reverse conveyance path Rh that communicates with 6 and a manual conveyance path Rt that communicates between the manual feed tray 7 and the upstream branch position 5.

  As shown in FIG. 1, the image forming apparatus 1 includes a paper feed cassette 2 for storing recording paper P, and a pair of registration rollers 8 arranged on the downstream side of the upstream branch position 5 in the main transport path Rs. A photosensitive drum 3 disposed on the downstream side of the registration roller 8 and having a photosensitive layer formed thereon; a charging device 9 for charging the surface of the photosensitive drum 3; and an exposure device for irradiating the surface of the photosensitive drum 3 with light. 10, a developing device 11 that supplies toner to the surface of the photosensitive drum 3, a transfer roller 13 that is pressed against the photosensitive drum 3 to form a transfer nip position 12, and a cleaning device 14 that cleans the surface of the photosensitive drum 3. A fixing device 15 that heats and presses the recording paper P that is transported through the main transport path Rs, and a pair of paper discharge rollers 16 that are disposed at the most downstream portion of the main transport path Rs and discharge the recording paper P to the paper discharge tray 4; A branch claw 17 disposed at the downstream branch position 6, a pair of transport rollers 18 that are appropriately disposed in the reverse transport path Rh and transport the recording paper P, and set when the recording paper P is manually fed. And a manual feed tray 7.

  As shown in FIG. 1, the paper feed cassette 2 is a box shape that can store recording paper P of various sizes, and is provided at the bottom of the apparatus so as to be able to be drawn out, so that the recording paper P can be loaded as needed. It is like that. A guide 19 for holding the recording paper P at the paper feed position is disposed inside the paper feed cassette 2. The guide 19 is provided so as to be able to swing within a predetermined range. When the guide 19 is biased upward by a spring or the like (not shown), the guide 19 holds a plurality of recording sheets P in a stacked state. One end of the paper is always positioned at the paper feed position. A paper feed unit 22 including a paper feed roller 20 and a friction pad 21 is provided at the paper feed position, and the uppermost paper positioned at the paper feed position by the guide 19 is pressed against the paper feed roller 20. The paper feed roller 20 is adapted to carry the recording paper P into the main transport path Rs by rotating while being pressed against the uppermost paper. The friction pad 21 has a friction coefficient with respect to the recording paper P lower than the friction coefficient with respect to the recording paper P of the paper feed roller 20 and higher than the friction coefficient between the recording papers P. The friction pad 21 is disposed below the paper feed roller 20 so as to be movable up and down and is urged upward by a spring or the like (not shown). As a result, the friction pad 21 is pressed against the roller surface of the paper feed roller 20, and the recording paper P passing through the pressed portion is separated and supplied one by one. Similarly, a paper feed unit 22 including a paper feed roller 20 and a friction pad 21 is also provided in the manual feed path Rt, and the recording paper P set on the manual feed tray 7 is printed one by one from the top paper. It is separated and supplied to the manual feed path Rt. Note that the paper feed unit 22 is not limited to this embodiment as long as it can nip the recording paper P set in the paper feed cassette 2 and feed it to the main transport path Rs. For example, the paper feed unit 22 rotates in the opposite direction. It is also possible to comprise a pair of friction rollers (not shown).

  FIG. 2 is a partially enlarged sectional view in which the vicinity of the pair of registration rollers 8 in FIG. 1 is enlarged. As shown in the figure, the main transport path sensor 23 is positioned directly downstream of the upstream branch position 5 in the reverse transport path Rh at a position downstream of the upstream branch position 5 in the main transport path Rs and upstream of the registration rollers 8. A reverse conveyance path sensor 24 is disposed at each upstream position. The main transport path sensor 23 and the reverse transport path sensor 24 are not shown in detail in the drawing, but are optical sensors, for example, and are not detected by the light receiver because the light emitted from the light emitter is passed through the recording paper P. Is detected.

  Hereinafter, the operation at the time of duplex printing in the image forming apparatus 1 will be described. When a command for executing double-sided printing is issued, first, printing of the first side of the recording paper P is executed. That is, the surface of the photosensitive drum 3 is uniformly charged by the charging device 9, and light corresponding to image data to be recorded on the photosensitive drum 3 is irradiated from the exposure device 10 to form an electrostatic latent image. A toner image is formed on the electrostatic latent image by supplying toner or the like from the developing device 11.

  On the other hand, with the start of the image forming operation, the uppermost sheet among the recording sheets P stored in the sheet feeding cassette 2 is fed into the main transport path Rs by the sheet feeding roller 20 and the friction pad 21. When the first recording sheet P is detected by the main transport path sensor 23 (main transport path sensor ON), a control unit (not shown) controls registration alignment based on a detection signal from the main transport path sensor 23. I do. That is, the control unit starts the rotation of the registration roller 8 after a predetermined time T1 elapses from the detection of the recording paper P by the main conveyance path sensor 23. During this time, the recording paper P abuts against the resist roller 8 in a stopped state and bends, so that the skew generated in the process of transporting the recording paper P is corrected. Further, the control unit controls the leading edge writing position of the recording paper P based on the detection signal from the main transport path sensor 23. That is, the control unit transfers the recording paper P supplied from the paper feed cassette 2 to the main transport path Rs from the position of the main transport path sensor 23 to the transfer nip position with the rear end thereof being nipped by the paper feed unit 22. The time T2 required to carry up to 12 is set in advance. The control unit takes the time T2 into consideration, and sends the recording paper P to the transfer nip position 12 so as to synchronize with the image formation on the photosensitive drum 3. As a result, the leading end writing position of the recording paper P is kept constant.

  At the transfer nip position 12, the recording paper P is pressed against the photosensitive drum 3 by the transfer roller 13. At this time, a voltage having a polarity opposite to that of the photosensitive drum 3 is applied to the transfer roller 13. The upper toner image is transferred to the recording paper P. After the toner image is transferred, the surface of the photosensitive drum 3 is charged again by the charging device 9 after the toner or paper dust remaining on the surface is removed by the cleaning device 14. The recording paper P to which the toner image has been transferred is conveyed further downstream in the main conveyance path Rs, and is heated and pressurized by the fixing device 15, whereby the toner image on the recording paper P is fixed. The recording paper P that has been printed on the first surface in this way is located downstream because the branching claw 17 disposed at the downstream branching position 6 opens the main transport path Rs as shown in FIG. The paper is conveyed to the paper discharge roller 16 beyond the side branch position 6. When the recording paper P reaches the paper discharge roller 16, the paper discharge roller 16 stops in a state where the recording paper P is nipped, and the branching claw 17 opens the reverse conveyance path Rh as shown by a dotted line in FIG. When the paper discharge roller 16 starts reverse rotation, the recording paper P is switched back and sent to the reverse conveyance path Rh.

  The recording paper P sent to the reverse conveying path Rh is conveyed toward the upstream branching position 5 by the conveying roller 18 with the printed first surface facing the inside of the apparatus. When the leading edge of the recording paper P is detected by the reverse conveyance path sensor 24 (reverse conveyance path sensor ON), the control unit that receives this detection signal performs standby control of the recording paper P. That is, the control unit stops the conveyance roller 18 after a predetermined time T3 has elapsed after the reverse conveyance path sensor 24 detects the recording paper P. As a result, the recording paper P is stopped in a state where the leading end is positioned immediately before the upstream branching position 5 as shown in FIG. The first recording sheet P is then near the exit of the reverse conveying path Rh until the second recording sheet P fed out from the paper feed cassette 2 to the main conveying path Rs finishes printing on the first surface. It is supposed to wait at. The control unit also performs jam detection based on a detection signal from the reverse conveyance path sensor 24. That is, if the reverse conveyance path sensor 24 does not detect the passage of the recording paper P even at the scheduled time, or if the reverse conveyance path sensor 24 does not turn OFF even after a predetermined time has elapsed after the ON, It is determined that a paper jam has occurred in the transport path Rh, the image forming operation is stopped, and the occurrence of a jam is notified to the user.

  When the second recording paper P finishes printing on the first side, the first recording paper P waiting near the exit of the reverse conveyance path Rh is in a state in which the front and back are reversed from those at the time of printing on the first side. That is, in the state where the second surface is directed to the photosensitive drum 3 side, it is supplied again to the main transport path Rs through the upstream branch position 5. Then, the first recording paper P is fed to the transfer nip position 12 under the control of registration alignment and the control of the leading edge writing position by the control unit.

  Here, unlike the printing on the first surface, the resist alignment control and the tip writing position control during the second surface printing are performed based on the detection signal from the reverse conveyance path sensor 24. More specifically, the registration alignment control is performed by starting the rotation of the registration roller 8 after a predetermined time T4 has elapsed after the reverse conveyance path sensor 24 detects the recording paper P. This T4 is set to be slightly larger than the time required to transport the recording paper P re-supplied from the reverse transport path Rh to the main transport path Rs from the position of the reverse transport path sensor 24 to the position of the registration roller 8. .

  On the other hand, the control of the leading edge writing position will be described. The control unit conveys the recording paper P supplied from the reverse conveyance path Rh to the main conveyance path Rs from the position of the reverse conveyance path sensor 24 to the transfer nip position 12. The time T5 required for this is preset. The control unit takes the time T5 into consideration, and sends the recording paper P to the transfer nip position 12 so as to synchronize with the image formation on the photosensitive drum 3. As described above, the control of the leading edge writing position at the time of printing on the second side is not the same control as at the time of printing on the first side, but is different from that at the time of printing on the first side based on the recording paper detection of the reverse conveyance path sensor 24. Thus, even when the recording paper P fed to the transfer nip position 12 is the recording paper P supplied from the paper feed cassette 2 to the main transport path Rs and the rear end portion is under load. Even when the recording paper P is re-supplied from the reverse conveyance path Rh to the main conveyance path Rs and the rear end portion receives almost no load, it is possible to always keep the leading edge writing position constant. It has become.

  The first recording sheet P fed to the transfer nip position 12 is transferred with the toner image on the photosensitive drum 3 on the second surface in the same manner as in the first surface printing, and then the toner image by the fixing device 15. Is established. At this time, the branching claw 17 is switched to a position where the main transport path Rs is opened after the second recording sheet P is switched back and sent to the reverse transport path Rh. Accordingly, the first recording paper P is transported further downstream beyond the downstream branch position 6 and is discharged to the paper discharge tray 4 by the paper discharge roller 16. On the other hand, the second recording sheet P on which the first surface is printed is stopped in a state where the leading end is positioned immediately before the upstream branching position 5 in the same manner as the first recording sheet P. After waiting until the recording paper P of the eye completes the printing of the first surface, it is supplied again to the main transport path Rs and the printing of the second surface is performed. Thereafter, at the transfer nip position 12, printing is performed in the order of the first surface of the fourth sheet, the second surface of the third sheet, the first surface of the fifth sheet, the second surface of the fourth sheet,. ing. The order of feeding the recording paper P to the transfer nip position 12 is not limited to the present embodiment, and the design can be changed as appropriate.

  As described above, in this embodiment, the reverse conveyance path sensor 24 includes the registration alignment and leading edge writing position control sensor, the standby position control sensor, and the jam in the reverse conveyance path Rh during the second surface printing. The sensor is also used as three sensors. However, it is not an essential component of the present invention to use the reverse conveyance path sensor 24 as all three sensors. For example, the reverse conveyance path sensor 24 can be used for resist alignment and leading edge writing during the second side printing. It is also possible to use a sensor for position control and a sensor for standby position control, and separately install a sensor for jam detection in the reverse conveyance path Rh. Further, the reverse conveyance path sensor 24 is used as a sensor for controlling registration alignment and leading edge writing position at the time of printing on the second surface and as a jam detection sensor in the reverse conveyance path Rh. It is also possible to install a sensor for control.

  The installation position of the reverse conveyance path sensor 24 is not limited to the present embodiment, and can be an arbitrary position along the reverse conveyance path Rh. However, if the reverse conveyance path sensor 24 is arranged immediately upstream of the upstream branch position 5 as in the present embodiment, the reversal is performed when the resist alignment control and the leading edge writing position are controlled during the second surface printing. Since the distance for transporting the recording paper P to the registration roller 8 and the transfer nip position 12 after the transport path sensor 24 is turned on is shortened, errors caused by slippage of the recording paper P in the transport path are minimized. Therefore, there is an advantage that the resist alignment and the tip writing position can be controlled more accurately. Further, it does not interfere with the conveyance of the recording paper P supplied from the paper feed cassette 2 or the manual feed tray 7. On the other hand, when importance is attached to the role of the reverse conveyance path sensor 24 as a jam detection sensor, as shown in FIG. 3, if the reverse conveyance path sensor 24 is installed at a predetermined position on the upstream side in the reverse conveyance path Rh, It is possible to more reliably detect the occurrence of clogging.

  In this embodiment, the case where the recording paper P is supplied from the paper feed cassette 2 to the main transport path Rs has been described. However, as described above, the recording paper P set on the manual feed tray 7 is also fed by the paper feed roller. 20 and the friction pad 21 are fed out to the main transport path Rs, and therefore, the load on the rear end of the recording paper P causes a shift in the leading end writing position compared to the recording paper P supplied from the reverse transport path Rh. There is a problem that arises. Accordingly, in the case of the manual feed tray 7 as well, in the same way as in the case of the paper feed cassette 2, the registration alignment at the time of printing on the first side and the control of the leading writing position are performed based on the detection signal from the main conveyance path sensor 23 By performing registration alignment and front end writing position control on the second side printing based on a detection signal from the reverse conveyance path sensor 24, the front end writing position can be kept constant at all times.

  The present invention is also applicable to a copy-only machine having only a copying function and a facsimile-only machine having only a facsimile communication function.

1 is a schematic longitudinal sectional view showing a configuration of an image forming apparatus 1 according to an embodiment of the present invention. FIG. 2 is a partially enlarged cross-sectional view in which the vicinity of a pair of registration rollers in FIG. FIG. 2 is a partially enlarged cross-sectional view in which the vicinity of a pair of registration rollers in FIG. FIG. 10 is a block diagram illustrating a configuration of an image forming apparatus according to a conventional example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Paper feed cassette 4 Paper discharge tray 5 Upstream branch position 6 Downstream branch position 8 Registration roller pair 12 Transfer nip position 20 Paper feed roller 21 Friction pad 22 Paper feed unit 23 Main transport path sensor 24 Reverse transport path Sensor Rs Main transport path Rh Reverse transport path P Recording paper

Claims (3)

The main conveyance path from the paper feed cassette through the transfer nip position to the paper discharge tray, and the inversion that communicates the upstream upstream branch position and the downstream downstream branch position across the transfer nip position of the main conveyance path. A transport path, a paper feed unit that nips the recording paper set in the paper feed cassette and feeds it to the main transport path, and a predetermined downstream side of the upstream branch position in the main transport path and upstream of the transfer nip position. A registration roller pair provided at a position, a main conveyance path sensor that detects the passage of the recording paper at a predetermined position downstream of the upstream branch position in the main conveyance path and upstream of the registration roller pair, and reverse conveyance In an image forming apparatus comprising a reverse conveyance path sensor that detects the passage of a recording sheet at a predetermined position on the path,
Based on the detection of the recording paper by the main conveyance path sensor, timing control is performed to feed the recording paper fed out from the paper feeding cassette and having its rear end nipped by the paper feeding unit to the transfer nip position by the pair of registration rollers. I,
Based on the detection of the recording paper by the reverse conveyance path sensor, the operation control for waiting the recording paper fed from the reverse conveyance path to the main conveyance path in the reverse conveyance path until the transfer nip position is ready for printing, and the resist An image forming apparatus characterized in that timing control for feeding to a transfer nip position by a pair of rollers is performed.   The image forming apparatus according to claim 1, wherein the reverse conveyance path sensor is provided immediately upstream of the upstream branch position in the reverse conveyance path.   3. The image forming apparatus according to claim 1, wherein occurrence of a paper jam in the reverse conveyance path is detected based on recording paper detection by the reverse conveyance path sensor.

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