JP2011081181A - Image forming apparatus - Google Patents

Abstract

An image forming apparatus capable of preventing toner stains from being rubbed against the trailing edge of a sheet passing through a transfer nip portion.
A photosensitive drum 31 rotates at a first peripheral speed. The transfer roller 34A is in pressure contact with the photosensitive drum 31 to form a transfer nip portion. The fixing unit 36 rotates at a second peripheral speed that is faster than the first peripheral speed. The sensor 240 detects the paper conveyed in the vicinity of the transfer roller 34A. The sensor 250 detects a sheet conveyed in the vicinity of the fixing unit 36. The CPU 300 rotates the transfer roller 34A at the first circumferential speed until the leading end of the sheet reaches the transfer nip portion, and then, when the leading end of the sheet reaches the transfer nip portion, the transfer roller 34A is faster than the first circumferential speed. Then, when the leading edge of the sheet reaches the fixing unit 36, the transfer roller 34A is rotated at the second circumferential speed.
[Selection] Figure 2

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In an electrophotographic image forming apparatus, image formation is performed by transferring a toner image formed on a photoreceptor onto a sheet. As this configuration, there is a configuration in which a transfer nip portion is formed between a photoconductor and a transfer roller provided facing the photoconductor. In this configuration, an image is formed on the paper through a process in which a transfer bias is applied to the transfer roller and the toner image formed on the photoconductor is transferred to the paper passing through the transfer nip portion.

  However, when the toner image is transferred onto the paper with the above configuration, the toner on the photoconductor aggregates due to the nip pressure between the photoconductor and the transfer roller, causing a dropout phenomenon in the transfer image.

  Therefore, in recent electrophotographic image forming apparatuses, in order to prevent a hollowing out phenomenon after transfer onto a sheet, toner aggregation on the photosensitive member is removed. Specifically, a configuration is adopted in which a difference in peripheral speed between the photosensitive member and the transfer roller is provided to provide a difference between the peripheral speed of the photosensitive member and the sheet conveyance speed.

  However, when a peripheral speed difference is provided between the photosensitive member and the transfer roller, the photosensitive member and the transfer roller are in direct contact with each other when there is no paper in the transfer nip portion. And the surface of the transfer roller was damaged.

  Therefore, paper detection means are provided on the upstream side and the downstream side of the transfer nip portion to detect whether or not there is paper in the transfer nip portion. When both paper detection means are off, there is a gap between the photosensitive member and the transfer roller. A technique for preventing the surface of the photoreceptor and the surface of the transfer roller from being damaged by eliminating the peripheral speed difference is disclosed (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 02-262175

  As described above, in order to prevent the toner from agglomerating on the photoconductor to prevent the transfer image on the paper from being lost, in a conventional electrophotographic image forming apparatus, there is a gap between the photoconductor and the transfer roller. There was a difference in peripheral speed. Usually, in order to prevent the dropout phenomenon of the transfer image on the paper, the peripheral speed of the transfer roller is made faster than the peripheral speed of the photoconductor, so that the paper transport speed is made faster than the peripheral speed of the photoconductor. Yes. The sheet conveyance speed at this time is a speed between the circumferential speed of the photoconductor and the circumferential speed of the transfer roller. Therefore, the peripheral speed of the transfer roller is faster than the paper conveyance speed.

  With this configuration, when the trailing edge of the paper passes through the transfer nip portion, the peripheral speed of the transfer roller is faster than the conveyance speed of the paper, so that the transfer roller rubs toner stains on the trailing edge of the paper. There was a problem.

  In the technique disclosed in Patent Document 1, the peripheral speed of the transfer roller does not become the same as the peripheral speed of the photosensitive member unless the sheet detecting means upstream and downstream of the transfer nip portion are turned off. That is, when the trailing edge of the paper passes through the transfer nip portion, the downstream paper detection means remains on, and the peripheral speed of the transfer roller continues to be higher than the paper conveyance speed. Yes. Therefore, the technique disclosed in Patent Document 1 cannot solve the problem of toner contamination on the trailing edge of the paper.

  In view of the above problems, an object of the present invention is to provide an image forming apparatus capable of preventing toner stains from being rubbed against the trailing edge of a sheet passing through a transfer nip portion.

  The image forming apparatus of the present invention includes an image carrier, a transfer unit, a fixing unit, a first detection unit, a second detection unit, and a control unit. The image carrier rotates at the first peripheral speed. The transfer portion is pressed against the image carrier to form a transfer nip portion. The fixing unit rotates at a second peripheral speed that is faster than the first peripheral speed. The first detection unit detects a sheet conveyed in the vicinity of the transfer unit. The second detection unit detects a sheet conveyed in the vicinity of the fixing unit. The control unit controls the peripheral speed of the transfer unit based on information from the first detection unit and the second detection unit. The control unit rotates the transfer unit at the first peripheral speed until the leading end of the sheet reaches the transfer nip, and then, when the leading end of the sheet reaches the transfer nip, the transfer unit is faster than the first peripheral speed. Then, when the leading edge of the paper reaches the fixing unit, the transfer unit is rotated at the second peripheral speed.

  In this configuration, when the leading edge of the sheet reaches the fixing unit, the peripheral speed of the transfer unit is made equal to the peripheral speed of the fixing unit, so that the conveyance speed of the sheet and the peripheral speed of the transfer unit become the same. When the end passes through the transfer nip portion, the transfer portion can prevent the toner stain from being rubbed against the rear end of the sheet.

  The image forming apparatus according to the present invention can prevent the toner stains from being rubbed against the trailing edge of the paper passing through the transfer nip portion.

1 is a diagram illustrating a configuration of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating a part of an image forming unit of the image forming apparatus according to the first embodiment of the present invention. FIG. 2 is a block diagram of a control unit of the image forming apparatus according to the first embodiment of the present invention. 3 is a flowchart of a control unit of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a state in which the leading edge of the sheet has reached a sensor disposed upstream of a registration roller in the image forming unit of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a state where the leading edge of the paper has reached a registration roller in the image forming unit of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a state in which the leading edge of the paper has reached a transfer nip portion in the image forming portion of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a state in which the leading edge of the sheet has reached a sensor disposed upstream of the fixing unit in the image forming unit of the image forming apparatus according to the first embodiment of the present invention. FIG. 3 is a diagram illustrating a state in which the leading edge of the paper reaches the fixing unit in the image forming unit of the image forming apparatus according to the first embodiment of the present invention. It is a figure which shows a part of image forming part of the image forming apparatus which concerns on 2nd Embodiment of this invention. It is a figure which shows a part of image forming part of the image forming apparatus which concerns on 3rd Embodiment of this invention. It is a figure which shows a part of image forming part of the image forming apparatus which concerns on 4th Embodiment of this invention. It is a figure which shows a part of image forming part of the image forming apparatus which concerns on 5th Embodiment of this invention.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a diagram illustrating a configuration of an image forming apparatus 100 according to an embodiment of the present invention.

  The image forming apparatus 100 forms an image on a sheet in any one of the copier mode, the printer mode, and the FAX mode. Each mode is selected by the user. The image forming apparatus 100 can perform double-sided printing.

  The image forming apparatus 100 includes a document reading unit 10, a paper feeding unit 20, an image forming unit 30, a paper discharge unit 40, an operation panel unit (not shown), and the like. The document reading unit 10 is disposed on the upper part of the apparatus main body, and includes a platen glass 11, a document placing tray 12, a scanner optical system 13, and the like. The scanner optical system 13 includes a light source 14, reflection mirrors 15 </ b> A to 15 </ b> C, an optical lens 16, and a CCD (Charge Coupled Device) 17. The light source 14 irradiates the original placed on the platen glass 11 or the original conveyed on the original conveying path 18 from the original placing tray 12 with light. The reflection mirrors 15 </ b> A to 15 </ b> C reflect the reflected light from the document and guide it to the optical lens 16. The optical lens 16 focuses the reflected light guided by the reflection mirrors 15 </ b> A to 15 </ b> C on the CCD 17. The CCD 17 outputs an electrical signal corresponding to the reflected light.

  The paper feed unit 20 is disposed at the lower part of the apparatus main body and includes a paper feed tray 21 and a pickup roller 22. The paper feed tray 21 stores paper to be fed to the paper transport path 38 during image formation. The pickup roller 22 rotates to feed the paper placed on the paper feed tray 21 to the paper transport path 38.

  The image forming unit 30 is disposed on the manual feed tray side below the document reading unit 10. The image forming unit 30 includes a laser scanning unit (hereinafter referred to as LSU) 37, a photosensitive drum 31, and a fixing unit 36. Around the photosensitive drum 31, a charging device 32, a developing device 33, a transfer device 34, and a cleaner unit 35 are arranged in this order along the direction of the arrow shown in FIG. Yes.

  When the start key provided on the operation panel unit is pressed, the image forming apparatus 100 rotates the pickup roller 22 and feeds the sheet to the sheet conveyance path 38. The fed paper is conveyed to a registration roller 51 provided on the paper conveyance path 38.

  The registration roller 51 stops rotating when the leading edge of the sheet arrives. The registration roller 51 starts to rotate at a timing when the leading edge of the sheet coincides with the leading edge of the toner image formed on the photosensitive drum 31 between the photosensitive drum 31 and the transfer device 34.

  The paper guide 50 is disposed between the photosensitive drum 31, the transfer device 34, and the fixing unit 36, and is provided to smoothly convey the sheet being conveyed to the fixing unit 36.

  The charging device 32 is a charging means for uniformly charging the surface of the photosensitive drum 31 to a predetermined potential. In addition to a charger type as shown in FIG. 1, a contact type roller type or brush type charger is used. Sometimes used.

  The LSU 37 has a function of forming an electrostatic latent image corresponding to the image data on the surface thereof by exposing the charged photosensitive drum 31 according to the input image data. The LSU 37 includes a laser emitting unit, a reflection mirror, and the like. The LSU 37 is provided with a polygon mirror that scans the laser beam and optical elements such as a lens and a mirror for guiding the laser beam reflected by the polygon mirror to the photosensitive drum 31. As the LSU 37, a method using, for example, an EL or LED writing head in which other light emitting elements are arranged in an array can also be employed.

  The developing device 33 visualizes the electrostatic latent image formed on the photosensitive drum 31 with toner. The toner attached to the surface of the MG roller 33A is attracted and attached to the surface of the photosensitive drum 31 according to the potential gap on the surface of the photosensitive drum 31, and the electrostatic latent image is visualized as a toner image. Is done.

  The transfer device 34 includes a transfer roller 34 </ b> A that faces the photosensitive drum 31. The photosensitive drum 31 and the transfer roller 34A are pressed against each other at a predetermined nip to form a transfer nip portion, and a voltage for transferring the toner onto the paper is applied to the transfer roller 34A (the toner charging polarity (−)). Is high voltage of reverse polarity (+).

  The cleaner unit 35 removes and collects toner remaining on the surface of the photosensitive drum 31 after development and image transfer.

  The fixing unit 36 includes a heating roller 36A and a pressure roller 36B, and the heating roller 36A and the pressure roller 36B rotate with the sheet interposed therebetween. The heating roller 36A is set to have a predetermined fixing temperature by a control unit based on a signal from a temperature detector (not shown), and the toner is thermocompression bonded to the sheet together with the pressure roller 36B. It has a function of fusing, mixing, and pressing the transferred toner image and thermally fixing the sheet.

  The paper discharge unit 40 is disposed above the paper feed tray 21 and includes a paper discharge roller 41 and a paper discharge tray 42. The paper discharge roller 41 discharges the paper transported on the paper transport path 38 to the paper discharge tray 42. The paper discharge roller 41 is capable of reversible rotation. When image formation is performed on both sides of the paper, the paper is held in a state where the paper on the front surface that has been transported on the paper transport path 38 is sandwiched. The paper rotates in the direction opposite to the discharging direction and is transported to the paper transport path 39. As a result, the front and back sides of the sheet are reversed, the back side faces the photosensitive drum 31, and the toner image is transferred to the back side. The paper discharge tray 42 stacks and stores the paper discharged from the paper discharge rollers 41.

  FIG. 2 shows the registration roller 51, the photosensitive drum 31, the transfer roller 34A, the heating roller 36A, and the pressure roller 36B in the image forming unit 30. FIG. 3 shows a part of a control block related to the image forming unit 30.

  The photosensitive drum 31 rotates at a specific peripheral speed (corresponding to the first peripheral speed of the present invention). In the present embodiment, the first peripheral speed is 100%, and other peripheral speeds are relatively represented. Further, the heating roller 36A and the pressure roller 36B (fixing unit 36) rotate at a second peripheral speed that is faster than the first peripheral speed. In the present embodiment, the second peripheral speed is set to 102%.

  The CPU 300 is connected to the ROM 310, the RAM 320, the sensor 240, the sensor 250, the timer 330, the clutch 212, and the solenoid 213. The CPU 300 corresponds to the control unit of the present invention. The sensor 240 corresponds to the first detection unit of the present invention. The sensor 250 corresponds to the second detection unit of the present invention. The CPU 300 reads out a program stored in the ROM 310 and controls each unit in a comprehensive manner. The RAM 320 is used as a working area for the CPU 300.

  A sensor 240 is provided on the upstream side of the registration roller 51. The sensor 240 is a non-contact type sensor that is provided so as to face the conveyance path 38. In this embodiment, the sensor 240 is a non-contact type, but a contact type sensor may be used. The sensor 240 detects a sheet conveyed on the conveyance path 38 on the upstream side of the registration roller 51, that is, in the vicinity of the transfer roller 34A. The sensor 240 detecting the paper is one of the conditions for starting the rotation of the registration roller 51. The transfer roller 34A corresponds to the transfer portion of the present invention.

  A sensor 250 is provided on the upstream side of the fixing unit 36. Similar to the sensor 240, the sensor 250 is a non-contact sensor that is provided facing the conveyance path 38. In the present embodiment, the sensor 250 is a non-contact type, but a contact type sensor may be used. The sensor 250 detects a sheet conveyed on the conveyance path 38 in the vicinity of the fixing unit 36. The sensor 250 detecting the sheet is one of the conditions for changing the peripheral speed of the transfer roller 34A.

  The timer 330 starts when the CPU 300 sets the time based on the detection signal of the sensor 240 and transmits a signal to the CPU 300 when the time is up, thereby triggering the registration roller 51 to start rotating. The timer 330 starts when the CPU 300 sets the time based on the detection signal of the sensor 250 and transmits a signal to the CPU 300 when the time is up, thereby triggering a change in the peripheral speed of the transfer roller 34A. As described above, the CPU 300 controls the peripheral speed of the transfer roller 34 </ b> A based on the detection signals from the sensors 240 and 250.

  A clutch 212 is used to turn the registration roller 51 on and off. The CPU 300 turns on and off the clutch 212 based on the time-up of the timer 330 and controls the rotation of the registration roller 51.

  A drive mechanism 210 is disposed in the vicinity of the transfer roller 34A. The drive mechanism 210 includes a support rod 214, a drive gear 215, a spring 211, and a solenoid 213. The support bar 214 is supported so as to be swingable at the center in the long axis direction. The drive gear 215 is provided at one end of the support bar 214 and is rotated by a driving force from a motor. The drive gear 215 rotates the transfer roller 34A by being connected to an interlocking gear 232 provided on the transfer roller 34A. The spring 211 is coupled to the support rod 214 on the drive gear 215 side with respect to the center of the support rod 214 in the long axis direction, and urges the support rod 214 in a direction in which the drive gear 215 is separated from the interlocking gear 232. .

  The solenoid 213 is coupled to the support bar 214 on the side opposite to the side where the drive gear 215 is provided from the center of the support bar 214 in the long axis direction. FIG. 2 shows a state where the solenoid 213 is on. When the solenoid 213 is turned on, the support rod 214 is pulled, so that the support rod 214 swings in a direction opposite to the direction in which the spring 211 is urged by the spring 211, and the drive gear 215 is connected to the interlocking gear 232. At this time, the transfer roller 34A rotates at a third peripheral speed that is faster than the first peripheral speed. In the present embodiment, the third peripheral speed is 104%. When the transfer roller 34A rotates at the third peripheral speed, the conveyance speed of the sheet passing through the transfer nip portion is 102%. When the solenoid 213 is turned off, the support rod 214 swings in the direction urged by the spring 211, and the drive gear 215 is separated from the interlocking gear 232. At this time, the transfer roller 34 </ b> A rotates following the photosensitive drum 31 or rotates following the paper conveyed in the conveyance path 38.

  FIG. 4 is a flowchart showing the control contents of the CPU 300 of this embodiment.

  The CPU 300 waits until printing is started (N in S10). When CPU 300 determines that printing has started (Y in S10), CPU 300 waits until an ON signal is transmitted from sensor 240 (N in S20). When CPU 300 determines that the ON signal from sensor 240 has been transmitted (Y of S20), CPU 300 sets time T1 in timer 330 (S30) and starts time T1 (S40). The state at this time is shown in FIG. In FIG. 5, the leading edge of the paper 260 in the conveyance path 38 reaches the sensor 240. When the leading edge of the paper 260 reaches the sensor 240, a series of control from S20 to S40 proceeds. At this time, since the leading edge of the paper 260 has not reached the transfer nip portion, the CPU 300 controls the transfer roller 34 </ b> A to be driven and rotated by the photosensitive drum 31. That is, since the drive gear 215 is not connected to the interlocking gear 232, the rotational driving force of the drive gear 215 is not transmitted to the transfer roller 34A. The time T1 indicates the time from when the leading edge of the paper 260 reaches the sensor 240 until the rotation of the registration roller 51 starts.

  Then, the CPU 300 waits until the time T1 is increased (N in S50). When the CPU 300 determines that the time T1 has been increased (Y in S50), the CPU 300 turns on the clutch 212 (S60) and starts the rotation of the registration roller 51. The state at this time is shown in FIG. Even if the leading edge of the sheet 260 reaches the registration roller 51, the sheet 260 is in a standby state at this position until the time T1 set in the timer 330 is increased. This is because the rotation of the registration roller 51 is started so that the image forming area of the sheet 260 and the toner image formed on the photosensitive drum 31 are synchronized.

  Then, the CPU 300 sets a time T2 in the timer 330 (S70), and starts the time T2 (S80). CPU 300 stands by until time T2 is up (N in S90). When CPU 300 determines that time T2 has expired (Y in S90), it turns on solenoid 213 (S100). The state at this time is shown in FIG. Since the time from when the registration roller 51 starts to rotate until the leading edge of the paper 260 reaches the transfer nip portion between the photosensitive drum 31 and the transfer roller 34A corresponds to the time T2, the CPU 300 determines that the time T2 If it is determined that the motor is up, the solenoid 213 is turned on to connect the drive gear 215 and the interlocking gear 232.

  When the driving gear 215 and the interlocking gear 232 are connected, the transfer roller 34A starts driving rotation. At this time, the transfer roller 34A rotates at the third peripheral speed. Then, the paper 260 passes through the transfer nip portion at a speed faster than the first peripheral speed and lower than the third peripheral speed. As described above, when the paper 260 is transported faster than the first peripheral speed, the aggregated toner on the photosensitive drum 31 generated by the nip pressure of the transfer nip portion is released, and thus a hollowing out phenomenon occurs. The toner is transferred onto the paper 260 instead.

  CPU 300 waits until an ON signal is transmitted from sensor 250 (N in S110). When CPU 300 determines that the ON signal is transmitted from sensor 250 (Y in S110), CPU 300 sets time T3 in timer 330 (S120), and starts time T3 (S130). The state at this time is shown in FIG. In FIG. 8, the leading edge of the paper 260 in the conveyance path 38 reaches the sensor 250. When the leading edge of the paper 260 reaches the sensor 250, a series of control from S110 to S130 proceeds. The time T3 indicates the time from when the leading edge of the paper 260 reaches the sensor 250 until it reaches the fixing unit 36.

  The CPU 300 waits until the time T3 is up (N in S140). When CPU 300 determines that time T3 is up (Y in S140), CPU 300 turns off solenoid 213 (S150) and turns off clutch 212 (S160). The state at this time is shown in FIG. As described above, the time T3 corresponds to the time from when the leading edge of the paper 260 reaches the sensor 250 until it reaches the fixing unit 36. Accordingly, when the time T3 is increased, the leading edge of the paper 260 has reached the fixing nip portion. Since the heating roller 36A and the pressure roller 36B form a fixing nip portion with a considerably strong pressure, the conveyance speed of the paper 260 reaching the fixing portion 36 is governed by the peripheral speeds of the heating roller 36A and the pressure roller 36B. Is done. As described above, the heating roller 36A and the pressure roller 36B are rotated at the second peripheral speed that is faster than the first peripheral speed.

  In the state of FIG. 9, since the solenoid 213 is off, the drive gear 215 is separated from the connection gear 232. Accordingly, since the transfer roller 34A is in a free state, the transfer roller 34A rotates following the sheet conveyed through the transfer nip portion. Therefore, since the peripheral speed of the transfer roller 34A rotates at the same speed as the sheet conveyed through the transfer nip portion, it is possible to prevent the transfer roller 34A from rubbing toner stains on the rear end of the sheet passing through the transfer nip portion.

  In the state shown in FIG. 9, since the clutch 212 is off, the registration roller 51 is in a free state. Accordingly, the registration roller 51 is driven to rotate following the sheet conveyed between the registration rollers 51.

  Then, the CPU 300 determines whether or not the print job is finished (S170). If the CPU 300 determines that the print job has not ended (N in S170), the CPU 300 returns to the flow of S20. On the other hand, when the CPU 300 determines that the print job is finished (Y in S170), the control flow of the present embodiment is finished.

  In the first embodiment of the present invention, the CPU 300 rotates the transfer roller 34 </ b> A following the photosensitive drum 31 until the leading edge of the paper 260 reaches the transfer nip portion, and then the leading edge of the paper 260 reaches the fixing portion 36. Then, the transfer roller 34A is driven and rotated by the conveyed paper, but the present invention is not limited to this. For example, as shown in FIG. 10, the drive gear 215 may be always connected to the interlocking gear 232, and the transfer roller 34A may continue to rotate. That is, the CPU 300 drives and rotates the transfer roller 34 </ b> A at the first peripheral speed that is the same peripheral speed as the photosensitive drum 31 until the leading edge of the sheet 260 reaches the transfer nip, and then the leading end of the sheet 260 is fixed to the fixing unit 36. , The transfer roller 34A may be driven to rotate at a second peripheral speed that is the same peripheral speed as the fixing unit 36. As a drive source at this time, for example, a stepping motor is used.

  In the first embodiment of the present invention, the sensor 250 is disposed upstream of the fixing unit 36, but the present invention is not limited to this. For example, as shown in FIG. 11, the sensor 251 may be disposed on the downstream side of the fixing unit 36. By arranging the sensor 251 at this position, the flow from S120 to S140 can be omitted, so the control flow of the CPU 300 can be simplified.

  FIG. 12 is a diagram illustrating a part of the configuration of the image forming unit 30 of the secondary transfer method. The control flow by the CPU 300 as described above can also be used when the roller 34B having an endless belt stretched instead of the photosensitive drum 31 is disposed. FIG. 13 is a diagram showing a part of the configuration of the transfer belt type image forming unit 30. Even when the transfer belt 34C is disposed instead of the transfer roller 34A, the control flow of the embodiment by the CPU 300 as described above can be used.

  Finally, the description of the above-described embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

31-photosensitive drum 34A-transfer roller 36-fixing unit 100-image forming apparatus 240-sensor 250-sensor 300-CPU

Claims (3)

An image carrier that rotates at a first peripheral speed;
A transfer portion that presses against the image carrier to form a transfer nip portion;
A fixing unit that rotates at a second peripheral speed that is faster than the first peripheral speed;
A first detection unit for detecting paper conveyed in the vicinity of the transfer unit;
A second detection unit for detecting a sheet conveyed in the vicinity of the fixing unit;
A control unit that controls the peripheral speed of the transfer unit based on information from the first detection unit and the second detection unit;
With
The control unit rotates the transfer unit at the first peripheral speed until the leading end of the sheet reaches the transfer nip, and then, when the leading end of the sheet reaches the transfer nip, the control unit moves the transfer unit to the first rotation. An image forming apparatus that rotates at a third peripheral speed that is faster than the speed, and then rotates the transfer unit at the second peripheral speed when the leading edge of the sheet reaches the fixing unit.   The control unit rotates the transfer unit following the image carrier until the leading edge of the sheet reaches the transfer nip, and then, when the leading edge of the sheet reaches the fixing unit, the transfer unit moves to the sheet to be conveyed. The image forming apparatus according to claim 1, wherein the image forming apparatus is driven to rotate.   The control unit drives and rotates the transfer unit at the first peripheral speed until the leading end of the sheet reaches the transfer nip, and then moves the transfer unit to the second circumferential when the leading end of the sheet reaches the fixing unit. The image forming apparatus according to claim 1, wherein the image forming apparatus is driven to rotate at a high speed.

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