Image forming apparatus

Abstract

To provide an image forming apparatus that can switch rotation of a motor without being affected by a change in a conveyance speed of a sheet in a fuser.SOLUTION: An image forming apparatus 1 comprises: a sheet sensor 200 that detects a sheet S between an ejection roller 7 and a fixing nip N2 that is a nip position between a heating belt 61A and a pressure roller 62; a motor 9 that drives a photoconductor drum 51, the pressure roller 62, the ejection roller 7, and a re-conveying roller 8, and that, when rotating in a normal direction, rotates the ejection roller 7 in the normal direction to eject the sheet S toward an ejection tray 21, and when rotating in a reverse direction, rotates the ejection roller 7 in the reverse direction to guide the sheet S to a second conveyance path P2; and a control unit 10. The control unit 10 switches the motor 9 from the normal rotation to the reverse rotation on the basis of the fact that the sheet sensor 200 detects a rear end of the sheet S ejected toward the ejection tray 21, and switches the motor 9 from the reverse rotation to the normal rotation after the sheet S passes through the ejection roller 7 rotating in the reverse direction.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrophotographic image forming apparatus. [Background technology]

[0002] Conventionally, there is known an image forming apparatus capable of double-sided printing, which uses a common motor to drive discharge rollers that discharge a sheet toward a discharge tray, a transport section that transports the sheet along a transport path from a sheet cassette to the discharge rollers via an image forming section, and a re-transport roller that transports the sheet to the image forming section during double-sided printing (Patent Document 1). The discharge rollers rotate forward when the motor rotates forward to discharge the sheet toward the discharge tray, and rotate backward when the motor rotates backward to transport the sheet again toward the image forming section.

[0003] In this technology, the motor is switched from forward rotation to reverse rotation when a predetermined time has elapsed since the rear end of the sheet conveyed along the conveying path reaches the post-registration sensor and the post-registration sensor no longer detects the sheet. Also, when the leading end of the sheet conveyed by the re-conveying roller reaches the pre-registration sensor and the pre-registration sensor detects the sheet, the motor is switched from reverse rotation to forward rotation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-170646 A Summary of the Invention [Problem to be solved by the invention]

[0005] Generally, a fixing unit of an image forming apparatus includes a heating rotator such as a heating roller, a heating belt, or a heating film, and a pressure rotator such as a pressure roller, a pressure belt, etc. The fixing unit fixes a toner image transferred onto a sheet to the sheet by nipping and conveying the sheet at a fixing nip, which is a nip position between the heating rotator and the pressure rotator.

[0006] Incidentally, in a fixing unit, when a sheet is being conveyed, for example, the diameter of the pressure rotating body changes due to temperature, which changes the peripheral speed of the pressure rotating body, and the conveying speed of the sheet in the fixing unit may change. Therefore, when the motor is switched from forward rotation to reverse rotation based on the fact that the post-registration sensor no longer detects the sheet, the conveying speed of the sheet changes after the post-registration sensor no longer detects the sheet, and the position of the sheet at the time when the motor rotation is switched may deviate from the ideal position. This problem is particularly likely to occur in a system that drives a pressure rotating body, but it can also occur in a system that drives a heating rotating body, as the diameter of the heating rotating body changes due to temperature.

[0007] Therefore, it is desirable to be able to switch the rotation of the motor without being affected by changes in the sheet conveying speed in the fixing unit. [Means for solving the problem]

[0008] The image forming apparatus includes a housing, a toner image forming unit, a fixing unit, a discharge roller, a sheet tray, a first transport path, a second transport path, a re-transport roller, a sheet sensor, a motor, and a control unit. The housing has a discharge tray. The toner image forming unit has a photosensitive drum on which a toner image is formed, and transfers the toner image onto a sheet at a transfer position. The fixing unit has a heating rotor and a pressure rotor, and fixes the toner image transferred to the sheet onto the sheet by nipping and transporting the sheet at a fixing nip, which is a nip position between the heating rotor and the pressure rotor. The discharge roller is a discharge roller that can rotate forward and backward, and when rotating forward, discharges the sheet conveyed from the fixing unit toward a discharge tray. The sheet to be fed to the transfer position is set on the sheet tray. The first transport path is a path through which a sheet passes when it is transported from a sheet tray to a discharge roller via a transfer position and a fixing unit. The second transport path is a second transport path along which the sheet is guided by a reverse-rotating discharge roller, and branches off between the fixation unit and the discharge roller on the first transport path and merges upstream in the transport direction of the sheet toward the discharge roller on the first transport path, relative to the transfer position on the first transport path. The re-conveying roller is at least one re-conveying roller provided on the second transport path, and transports the sheet toward the transfer position. The sheet sensor detects the sheet on the first transport path between the fixing nip and the discharge roller. The motor drives the photosensitive drum, the pressure rotor, the discharge roller, and the re-conveyance roller, and when it rotates forward, it rotates the discharge roller forward, and when it rotates reverse, it rotates the discharge roller backward. The control unit controls the motor. The control unit performs control to switch the motor from forward rotation to reverse rotation based on the sheet sensor detecting the trailing end of the sheet being discharged toward the discharge tray. The control unit performs control to switch the motor from reverse rotation to forward rotation after the sheet passes the reversely rotating discharge rollers based on the sheet sensor detecting the rear end of the sheet being discharged toward the discharge tray.

[0009] By switching the motor rotation based on the detection of a sheet sensor that detects a sheet on the first transport path between the fixing nip and the discharge roller, the motor rotation can be switched without being affected by changes in the sheet transport speed in the fixing unit.

[0010] The re-conveying rollers may be provided in plurality on the second conveying path, and the control unit may be configured to control the motor to switch from reverse rotation to forward rotation when the sheet passes through the discharge roller and reaches at least one of the multiple re-conveying rollers that is located most downstream in the sheet conveying direction on the second conveying path.

[0011] When the sheet passes through the discharge rollers and reaches the most downstream re-conveying rollers, the motor is switched from reverse rotation to forward rotation, so that the sheet conveyed toward the transfer position after the motor is switched to forward rotation is not pulled by the discharge rollers rotating forward.

[0012] The control unit may be configured to control the motor to switch from forward rotation to reverse rotation when it determines that a first time has elapsed since the sheet sensor detected the trailing end of the sheet, and to control the motor to switch from reverse rotation to forward rotation when it determines that a second time, which is longer than the first time, has elapsed since the sheet sensor detected the trailing end of the sheet.

[0013] By switching the motor rotation based on the time after the sheet sensor detects the trailing edge of the sheet, the motor rotation can be switched regardless of the length of the sheet and without being affected by changes in the sheet conveying speed in the fixing unit.

[0014] The image forming apparatus may further include a transmission mechanism. The transmission mechanism transmits the driving force from the motor rotating in reverse to the re-conveying roller, causing the re-conveying roller to rotate in a specified direction to transport the sheet toward the transfer position. After the motor switches from reverse rotation to forward rotation, the transmission mechanism transmits the driving force from the motor rotating in the forward direction to the re-conveying roller for a specified period of time, causing the re-conveying roller to rotate in the specified direction a specified number of times.

[0015] By rotating the re-conveying roller a predetermined number of times after the motor switches from reverse rotation to forward rotation, the sheet can be transported toward the transfer position by the re-conveying roller while the re-conveying roller is rotating even after the motor switches to forward rotation. Also, by being able to transport the sheet toward the transfer position by the re-conveying roller while the motor is rotating forward, the photosensitive drum can be driven by the forward rotating motor while the sheet is being transported by the re-conveying roller, thereby making it possible to prepare for image formation.

[0016] The image forming apparatus may further include a registration roller. The registration roller is a registration roller provided on the first transport path between the sheet tray and the transfer position, and transports the sheet toward the transfer position. In this case, the second transport path can be configured to merge with the first transport path between the sheet tray and the registration roller. In this case, the predetermined number of rotations is a number of rotations sufficient for the sheet transported by the re-transport roller that has rotated the predetermined number of times to reach the registration roller.

[0017] The sheet transported by the re-conveying roller that has rotated a predetermined number of times reaches the registration roller, and after the re-conveying roller has rotated a predetermined number of times, the registration roller can transport the sheet toward the transfer position.

[0018] The transmission mechanism may include an input gear, a first pendulum gear, a second pendulum gear, an output gear, an intermediate gear, a swinging member, a locking mechanism, and an elastic member. The input gear rotates when a driving force is input from the motor. The first pendulum gear meshes with the input gear. The second pendulum gear meshes with the input gear. The output gear is an output gear that outputs a driving force to the re-conveyance roller, and is capable of meshing with the first pendulum gear. The intermediate gear has a first gear capable of meshing with the output gear and a second gear capable of meshing with the second pendulum gear, the first gear and the second pendulum gear being rotatable together about the same rotation axis. The first gear has a first toothed portion with which the output gear can mesh, and a first toothless portion where no teeth are provided. The second gear has a second toothed portion with which the second pendulum gear can mesh, and a second toothless portion where no teeth are provided. The oscillating member holds the first pendulum gear and the second pendulum gear rotatably about their respective rotation axes, and supports the first pendulum gear and the second pendulum gear so that they can revolve around the rotation axis of the input gear while meshing with the input gear. When the motor rotates in the reverse direction, the swing member swings to a first swing position where the first pendulum gear meshes with the output gear and the second pendulum gear is separated from the second gear. When the motor rotates forward, the swing member swings to a second swing position where the second pendulum gear can mesh with the second gear and the first pendulum gear is separated from the output gear. The locking mechanism is switchable between a locked state in which the intermediate gear is stopped at a first rotation position in which the first missing tooth portion faces the output gear and the second missing tooth portion faces the second pendulum gear, and an unlocked state in which rotation of the intermediate gear from the first rotation position is permitted, and switches from the locked state to the unlocked state when the motor switches from forward rotation to reverse rotation. When the locking mechanism is switched from a locked state to an unlocked state, the elastic member rotates the intermediate gear from a first rotation position to a second rotation position in which the first toothless portion faces the output gear and the second toothed portion can mesh with the second pendulum gear. When the motor switches from reverse rotation to forward rotation, the intermediate gear rotates by transmitting the driving force from the motor to the second toothed portion via the input gear and the second pendulum gear, and rotates the output gear until the first toothed portion meshes with the output gear and the first toothless portion is positioned in a rotational position opposite the output gear.

[0019] The image forming apparatus may further include a fixing frame that supports the heating rotator and the pressure rotator. In this case, the sheet sensor may include an optical sensor and a sensor actuator. The optical sensor has a light-emitting element and a light-receiving element capable of receiving light emitted by the light-emitting element. The sensor actuator is supported so as to be movable between a shielding position where it blocks light traveling from the light-emitting element to the light-receiving element and a retracted position where the light from the light-emitting element can be received by the light-receiving element, and moves from one of the shielding position and the retracted position to the other when a sheet being transported on the first transport path comes into contact with it. The optical sensor and the sensor actuator may be supported by the fixing frame.

[0020] The optical sensor and the sensor actuator are supported by the fixing frame, so that the sheet can be detected with high accuracy, and the rotation of the motor can be switched with high accuracy.

[0021] The heating rotator may be rotated following rotation of the pressure rotating body, and the pressure rotating body may be rotated by inputting a driving force from a motor.

[0022] The pressure rotating body may be a pressure roller having a rubber layer.

[0023] A pressure roller having a rubber layer is susceptible to changes in diameter due to heating, which changes the sheet transport speed in the fixing unit. However, by switching the motor rotation based on detection by the sheet sensor, the motor rotation can be switched without being affected by changes in the sheet transport speed in the fixing unit.

[0024] The toner image forming unit may have a transfer roller, and may be configured to transfer the toner image onto the sheet by nipping and transporting the sheet at a transfer nip, which is a nip position between the photosensitive drum and the transfer roller and serves as a transfer position. In this case, the image forming apparatus can be configured to further include a chute. The chute is disposed between the transfer nip and the fusing nip and guides the sheet beneath the sheet. The chute may be configured to be movable between a first position and a second position below the first position.

[0025] Because the chute can move between the first and second positions, even if the sheet conveying speed in the fixing unit changes and the sheet is bent between the transfer nip and the fixing nip, the bending of the sheet can be absorbed. This allows the sheet to be conveyed in an appropriate position. In addition, because the behavior of the sheet is stable, the sheet is less likely to shift position.

[0026] The image forming apparatus may further include a pressing mechanism and a solenoid actuator. The pressing mechanism is a mechanism for pressing the chute to move it from the first position to the second position. The solenoid actuator moves the pushing mechanism.

[0027] By operating the pressing mechanism with a solenoid actuator and controlling the solenoid actuator, the chute can be moved from the first position to the second position independently of the discharge rollers driven by the motor.

[0028] The second transport path may be provided so as to pass below the chute, and the pressing mechanism may be disposed above the chute.

[0029] The second transport path is provided so as to pass under the chute, and the pressing mechanism is disposed above the chute, so that the pressing mechanism, the chute, and a portion of the second transport path can be arranged vertically in a compact manner. This allows the image forming apparatus to be made more compact. In addition, the second transport path can be made shorter. Effect of the Invention

[0030] The motor rotation can be switched without being affected by changes in the sheet transport speed in the fixing unit. [Brief description of the drawings]

[0031] [Figure 1] FIG. 1 illustrates an image forming apparatus. [Diagram 2] FIG. 1 is a block diagram of an image forming apparatus. [Diagram 3] 1A and 1B are diagrams showing a shoot, in which FIG. 1A shows the shoot in a first position, and FIG. 1B shows the shoot in a second position. [Figure 4] FIG. [Diagram 5] FIG. 2 is a diagram showing the chute, the pressing mechanism, and the solenoid actuator, with the chute in a first position. [Figure 6]FIG. 13 is a diagram showing the chute, the pressing mechanism, and the solenoid actuator, with the chute in a second position. [Figure 7] 1A is a diagram showing a fixing unit and a sheet sensor, in which FIG. 1A shows a state in which a sensor actuator is located at a shielding position, and FIG. 1B shows a state in which the sensor actuator is located at a retracted position. [Figure 8] FIG. 2A is a diagram showing a sheet sensor, and FIG. 2B is a perspective view of a sensor actuator. [Figure 9] FIG. [Figure 10] 13A and 13B are diagrams showing the transmission mechanism when a print job is input and the motor rotates in the forward direction. [Figure 11] 13(a) and 13(b) are diagrams showing the transmission mechanism when the motor is rotating in reverse. [Figure 12] 13(a) and 13(b) are diagrams showing the transmission mechanism when the motor switches from reverse rotation to forward rotation. [Figure 13] 1A and 1B are diagrams showing the transmission mechanism when the intermediate gear is in mesh with the output gear. [Figure 14] 13A and 13B are diagrams showing the transmission mechanism in a state in which the meshing between the intermediate gear and the output gear is released. [Figure 15] FIG. 1A is a diagram explaining the timing for switching the motor from forward rotation to reverse rotation, and FIG. 1B is a diagram explaining the timing for switching the motor from reverse rotation to forward rotation. [Figure 16] 13 is a diagram showing a state in which the re-feed roller has been rotated a predetermined number of times by a transmission mechanism. FIG. [Figure 17] 4 is a flowchart showing the operation of a control unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Next, an embodiment will be described. 1, image forming apparatus 1 is capable of forming images on both a first side and a second side behind the first side of sheet S. Image forming apparatus 1 includes a housing 2, a sheet supply unit 3, registration rollers 4, a toner image forming unit 5, a fixing unit 6, discharge rollers 7, re-conveyance rollers 8, a motor 9, and a control unit 10. Housing 2 has a discharge tray 21 that supports sheet S on which an image has been formed. Discharge tray 21 is formed on the upper surface of housing 2.

[0033] In the image forming apparatus 1, a sheet S supplied from a sheet supply unit 3 is conveyed to a toner image forming unit 5 by a registration roller 4. In the image forming apparatus 1, a toner image is transferred to a first side of the sheet S by the toner image forming unit 5, and the toner image is fixed to the sheet S by a fixing unit 6. When an image is formed only on the first side of the sheet S, the image forming apparatus 1 discharges the sheet S to a discharge tray 21 by a discharge roller 7.

[0034] When forming an image on the second side of the sheet S, the image forming apparatus 1 transports the sheet S again to the toner image forming unit 5 by the discharge rollers 7 and the re-conveyance rollers 8. The image forming apparatus 1 transfers a toner image onto the second side of the sheet S in the toner image forming unit 5, fixes the toner image onto the sheet S by the fixing unit 6, and then discharges the sheet S onto the discharge tray 21 by the discharge rollers 7.

[0035] The sheet supply unit 3 has a sheet tray 31, a pressure plate 32, and a supply mechanism 33. The sheet tray 31 is detachable from the housing 2. A sheet S is set in the sheet tray 31. The sheet S in the sheet tray 31 is supplied to a transfer nip N1 serving as a transfer position. The transfer nip N1 is a nip position between the photosensitive drum 51 and a transfer roller 55. The supply mechanism 33 has a pickup roller 33A, a separation roller 33B, and a conveying roller 33C.

[0036] In the sheet supply unit 3, the sheet S in the sheet tray 31 is brought to the pickup roller 33A by the pressure plate 32, and the sheet S is sent out from the sheet tray 31 by the pickup roller 33A. After that, the sheet supply unit 3 separates the sheet S into one sheet by the separation roller 33B, and supplies the sheet S toward the transfer nip N1 by the conveyance roller 33C.

[0037] The registration roller 4 aligns the position of the toner image formed on the photosensitive drum 51 with the leading edge of the sheet S, and then transports the sheet S toward the transfer nip N1. The registration roller 4 also corrects skew of the sheet S. The registration roller 4 is provided on the first transport path P1 between the sheet tray 31 and the transfer nip N1. Specifically, the registration roller 4 is provided on the first transport path P1 between the transport roller 33C and the transfer nip N1.

[0038] The toner image forming unit 5 has a photosensitive drum 51, a charger 52, an exposure device 53, a developing device 54, and a transfer roller 55. The charger 52 charges the surface of the photosensitive drum 51. The exposure device 53 emits a light beam to the surface of the charged photosensitive drum 51 to expose the surface of the photosensitive drum 51. As a result, an electrostatic latent image based on image data is formed on the surface of the photosensitive drum 51.

[0039] The developing unit 54 has a developing roller 54A, and supplies toner from the developing roller 54A to the surface of the photosensitive drum 51 after exposure. As a result, a toner image is formed on the surface of the photosensitive drum 51. The transfer roller 55 forms a transfer nip N1 between itself and the photosensitive drum 51. The toner image forming unit 5 transfers the toner image formed on the photosensitive drum 51 to the sheet S by nipping and transporting the sheet S at the transfer nip N1 between the photosensitive drum 51 and the transfer roller 55.

[0040] The fixing device 6 has a heating unit 61, a pressure roller 62, and an intermediate discharge roller 63. The heating unit 61 and the pressure roller 62 are in contact with each other at a fixing nip N2. The fixing nip N2 is a nip position between a heating belt 61A of the heating unit 61 and the pressure roller 62. The heating unit 61 and the pressure roller 62 are disposed to face each other with the fixing nip N2 in between.

[0041] The heating unit 61 has a heating belt 61A, a heater 61B, and a holder 61C. The heating belt 61A heats the sheet S. The heating belt 61A is heated by the heater 61B. The heating belt 61A contacts the pressure roller 62 at the fixing nip N2. The heating belt 61A is rotated by the rotation of the pressure roller 62. The heater 61B contacts the inner circumferential surface of the heating belt 61A. The holder 61C supports the heater 61B.

[0042] The pressure roller 62 is a roller in which the periphery of a shaft 62A is covered with a rubber layer 62B. The pressure roller 62 is driven to rotate by inputting a driving force from a motor 9. The fixing device 6 fixes the toner image transferred to the sheet S by the toner image forming unit 5 onto the sheet S by nipping and conveying the sheet S at a fixing nip N2 between the heating belt 61A and the pressure roller 62.

[0043] The intermediate discharge rollers 63 transport the sheet S toward the discharge rollers 7. The intermediate discharge rollers 63 are disposed downstream of the fixing nip N2 in the transport direction of the sheet S toward the discharge rollers 7 on the first transport path P1. Hereinafter, the transport direction of the sheet S toward the discharge rollers 7 on the first transport path P1 is also referred to as the "first transport direction."

[0044] In the embodiment, the heating belt 61A corresponds to the “heating rotor,” and the pressure roller 62 corresponds to the “pressure rotor.” In other words, the pressure rotor in the embodiment is the pressure roller 62 having the rubber layer 62B.

[0045] The discharge rollers 7 are rollers that discharge the sheet S toward the outside of the housing 2. Specifically, the discharge rollers 7 discharge the sheet S toward the discharge tray 21. The discharge rollers 7 are disposed downstream of the intermediate discharge rollers 63 in the first transport direction. The discharge rollers 7 are disposed at a position visible from outside the housing 2.

[0046] The discharge rollers 7 are capable of rotating forward and reverse. When the discharge rollers 7 rotate forward, the discharge rollers 7 discharge the sheet S transported from the fixing unit 6 toward the discharge tray 21. When the discharge rollers 7 rotate reversely, the discharge rollers 7 transport the sheet S toward the second transport path P2.

[0047] The image forming apparatus 1 has a first transport path P1 and a second transport path P2. The first transport path P1 and the second transport path P2 are formed in a housing 2. Specifically, the first transport path P1 and the second transport path P2 are formed by a transport guide (not shown).

[0048] The first transport path P1 is a path through which the sheet S passes when it is transported from the sheet tray 31 to the discharge rollers 7 via the transfer nip N1 and the fixing unit 6. The second transport path P2 is a path through which the sheet S passes when performing double-sided printing. The sheet S is guided to the second transport path P2 by the discharge rollers 7 rotating in the reverse direction. The second transport path P2 branches off from a branching point P3 on the first transport path P1 and merges with a merging point P4 on the first transport path P1.

[0049] When double-sided printing is performed, the sheet S is transported in the first transport path P1 from the discharge rollers 7 toward the branching point P3 in the opposite direction to the first transport direction, passes through the branching point P3, passes through the second transport path P2, and merges with the junction P4. Hereinafter, the transport direction of the sheet S from the discharge rollers 7 toward the junction P4 during double-sided printing is also referred to as the "second transport direction."

[0050] The branching portion P3 is located on the first transport path P1 between the fixing unit 6 and the discharge rollers 7. Specifically, the branching portion P3 is located on the first transport path P1 between the intermediate discharge rollers 63 and the discharge rollers 7. The second transport path P2 branches off from the first transport path P1 between the intermediate discharge rollers 63 of the fixing unit 6 and the discharge rollers 7.

[0051] The junction P4 is located upstream of the transfer nip N1 on the first transport path P1 in the first transport direction. The junction P4 is located between the sheet tray 31 and the transfer nip N1 on the first transport path P1. Specifically, the junction P4 is located between the sheet tray 31 and the registration roller 4 on the first transport path P1. More specifically, the junction P4 is located between the transport roller 33C and the registration roller 4 on the first transport path P1. The second transport path P2 merges between the transport roller 33C and the registration roller 4 on the first transport path P1.

[0052] The second transport path P2 branches off from the branching point P3, passes between the fixing device 6 and the toner image forming unit 5, and the sheet tray 31, and then joins the joining point P4.

[0053] The re-conveying rollers 8 are provided on the second transport path P2. The re-conveying rollers 8 transport the sheet S toward the transfer nip N1. A plurality of re-conveying rollers 8 are provided on the second transport path P2. Specifically, two re-conveying rollers 8 are provided on the second transport path P2. The re-conveying rollers 8 include a first re-conveying roller 81 and a second re-conveying roller 82.

[0054] The second re-conveying roller 82 is disposed in the second conveying direction downstream of the first re-conveying roller 81. In the embodiment, the second re-conveying roller 82 corresponds to "the re-conveying roller disposed most downstream in the sheet conveying direction in the second conveying path."

[0055] When an image is formed only on the first side of the sheet S, the discharge rollers 7 rotate forward to discharge the sheet S with the fixed toner image onto the discharge tray 21. When an image is also formed on the second side of the sheet S, the discharge rollers 7 rotate reversely to guide the sheet S to the second transport path P2 before the sheet S passes through the discharge rollers 7. The re-transport rollers 8 transport the sheet S guided to the second transport path P2 towards the transfer nip N1.

[0056] Thereafter, a toner image is transferred onto the second surface of the sheet S by the toner image forming unit 5, and the toner image is fixed by the fixing unit 6. After that, the sheet S is discharged onto a discharge tray 21 by a discharge roller 7 rotating in the forward direction.

[0057] As shown in FIG. 2, the motor 9 drives the supply mechanism 33, the registration roller 4, the photosensitive drum 51, the developing roller 54A, the pressure roller 62, the intermediate discharge roller 63, the discharge roller 7, and the re-conveying roller 8. The motor 9 can rotate forward and reverse. The rotation direction of the motor 9 is controlled by the control unit 10. The motor 9 rotates the discharge roller 7 forward when rotating forward, and rotates the discharge roller 7 reversely when rotating reverse. The supply mechanism 33, the registration roller 4, the developing roller 54A, the pressure roller 62, and the intermediate discharge roller 63 rotate only when the motor 9 rotates forward.

[0058] The image forming apparatus 1 further includes a transmission mechanism 300. The transmission mechanism 300 transmits the driving force from the motor 9 rotating in the reverse direction to the re-conveying roller 8 to rotate the re-conveying roller 8 in a predetermined direction. The predetermined direction is a rotation direction for conveying the sheet S toward the transfer nip N1. After the motor 9 switches from the reverse rotation to the forward rotation, the transmission mechanism 300 transmits the driving force from the motor 9 rotating in the forward direction to the re-conveying roller 8 for a predetermined time to rotate the re-conveying roller 8 in the predetermined direction a predetermined number of times.

[0059] For example, the photosensitive drum 51 may be configured to rotate in a direction to transport the sheet S between the transfer roller 55 and the sheet S toward the fixing device 6 when the motor 9 rotates forward, and to rotate in the reverse direction when the motor 9 rotates reverse. Alternatively, the photosensitive drum 51 may be configured to rotate only when the motor 9 rotates forward.

[0060] The image forming apparatus 1 further includes a chute 110, a pressing mechanism 120, and a solenoid actuator .

[0061] As shown in FIG. 3, the chute 110 is a member that guides the sheet S from below. The chute 110 constitutes the first transport path P1. The chute 110 is disposed between the transfer nip N1 and the fixing nip N2 in the transport direction of the sheet S toward the discharge rollers 7 on the first transport path P1. The fixing unit 6 has a fixing guide 64 that guides the sheet S to the fixing nip N2. The chute 110 is disposed between the transfer roller 55 and the fixing guide 64 in the transport direction of the sheet S toward the discharge rollers 7 on the first transport path P1.

[0062] The chute 110 is movable between a first position shown in Fig. 3(a) and a second position shown in Fig. 3(b). The second position is a position lower than the first position. Specifically, the upstream end of the chute 110 in the first transport direction is supported rotatably with respect to the housing 2. This allows the downstream end of the chute 110 in the first transport direction to move up and down between the first position and the second position with respect to the housing 2.

[0063] When the chute 110 is located at the second position, the downstream end in the first transport direction is located closer to the upstream end of the fixing guide 64 in the first transport direction than when the chute 110 is located at the first position. When the chute 110 is located at the first position, the chute 110 guides the sheet S toward the fixing nip N2. When the chute 110 is located at the second position, the chute 110, together with the fixing guide 64, ensures a space for the sheet S present between the transfer nip N1 and the fixing nip N2 to bend downward.

[0064] 4, the image forming apparatus 1 further includes a spring SP. The spring SP biases the chute 110 from the second position to the first position. As an example, the spring SP is a torsion spring. The chute 110 includes a chute body 111, a plurality of guide ribs 112, and a contact portion 113.

[0065] The chute body 111 is plate-shaped. A number of guide ribs 112 protrude from the top surface of the chute body 111. The multiple guide ribs 112 are arranged at intervals in the width direction of the sheet S. The width direction of the sheet S is a direction perpendicular to the first transport direction. Hereinafter, the width direction of the sheet S will also be simply referred to as the "width direction." The contact portion 113 is located at one end of the chute body 111 in the width direction. The contact portion 113 is located at an end portion of the chute body 111 downstream in the first transport direction.

[0066] 5, the pressing mechanism 120 is a mechanism for pressing the chute 110 to move it from the first position to the second position. The pressing mechanism 120 has a first member 121, a second member 122, a third member 123, and a tension spring . The first member 121 extends in the width direction. The first member 121 is capable of reciprocating in the width direction. One end of the first member 121 is engaged with the second member 122.

[0067] The second member 122 extends in the width direction. One end of the second member 122 is rotatably supported by the housing 2. The second member 122 is rotatable between a first rotation position shown in Fig. 5 and a second rotation position shown in Fig. 6. The other end of the second member 122 moves downward when the second member 122 rotates from the first rotation position to the second rotation position.

[0068] The third member 123 extends vertically. The third member 123 is rotatably supported on the other end of the second member 122. The third member 123 is movable vertically. When the third member 123 moves downward, it presses the contact portion 113 of the chute 110 against the biasing force of the spring SP. As an example, the tension spring 124 is a tension coil spring. The tension spring 124 biases the second member 122 from the second pivot position toward the first pivot position.

[0069] The solenoid actuator 130 is an actuator for moving the pressing mechanism 120. The solenoid actuator 130 is fixed to a side frame (not shown) of the housing 2. The solenoid actuator 130 is disposed outside the contact portion 113 of the chute 110 in the width direction. The solenoid actuator 130 is capable of pulling the first member 121 in the width direction. The solenoid actuator 130 is controlled by the control unit 10.

[0070] When the chute 110 is in the first position shown in Fig. 5 and the solenoid actuator 130 is turned ON, the first member 121 is pulled by the solenoid actuator 130 and moves in a direction approaching the solenoid actuator 130, as shown in Fig. 6. Then, the second member 122 rotates from the first rotation position to the second rotation position, whereby the third member 123 moves downward and the chute 110 moves from the first position to the second position.

[0071] When the solenoid actuator 130 is turned OFF while the chute 110 is in the second position, the tension spring 124 rotates the second member 122 from the second rotation position to the first rotation position, and the first member 121 engaged with the second member 122 moves in a direction away from the solenoid actuator 130, as shown in Fig. 5. As the second member 122 rotates to the first rotation position, the third member 123 moves up and away from the chute 110, and the chute 110 moves from the second position to the first position by the biasing force of the spring SP.

[0072] The pressing mechanism 120 is disposed above the chute 110. The second transport path P2 is provided so as to pass below the chute 110. In other words, the second transport path P2 is provided so as to pass below the chute 110 in the vertical direction, opposite to the upper side where the pressing mechanism 120 is disposed.

[0073] 7, the image forming apparatus 1 further includes a sheet sensor 200. The fixing unit 6 further includes a fixing frame 65. The fixing frame 65 is a member that constitutes the frame of the fixing device 6, and supports the heating unit 61 including the heating belt 61A, and the pressure roller 62.

[0074] The sheet sensor 200 detects the sheet S on the first transport path P1 between the fixing nip N2 and the discharge rollers 7. Specifically, the sheet sensor 200 detects the sheet S on the first transport path P1 between the fixing nip N2 and the intermediate discharge rollers 63. The sheet sensor 200 has an optical sensor 210, a substrate 220, and a sensor actuator 230.

[0075] 8(a), the optical sensor 210 has a light-emitting element 211 and a light-receiving element 212. The light-emitting element 211 and the light-receiving element 212 face each other in the width direction. The light-emitting element 211 emits light toward the light-receiving element 212. The light-receiving element 212 can receive light emitted by the light-emitting element 211. The optical sensor 210 is mounted on a substrate 220.

[0076] 8(a) and (b), sensor actuator 230 has shaft 231, first lever 232, and second lever 233. Shaft 231 extends in the width direction. Shaft 231 is supported by housing 2 so as to be rotatable about an axis extending in the width direction.

[0077] The first lever 232 protrudes from one end of the shaft 231 in the width direction in a direction perpendicular to the width direction. The first lever 232 is capable of contacting the sheet S transported on the first transport path P1. The second lever 233 protrudes from the other end of the shaft 231 in the width direction in a direction perpendicular to the width direction. The second lever 233 is plate-shaped.

[0078] The sensor actuator 230 is supported so as to be movable between a blocking position shown in Fig. 7(a) and a retracted position shown in Fig. 7(b). The blocking position is a position where the second lever 233 enters between the light emitting element 211 and the light receiving element 212 to block light from the light emitting element 211 toward the light receiving element 212. The retracted position is a position where the second lever 233 retracts from between the light emitting element 211 and the light receiving element 212 to allow the light from the light emitting element 211 to be received by the light receiving element 212.

[0079] The sensor actuator 230 moves from one of the shielding position and the retracted position to the other when the sheet S transported on the first transport path P1 comes into contact with the sensor actuator 230. In the embodiment, the sensor actuator 230 moves from the shielding position to the retracted position when the sheet S transported on the first transport path P1 comes into contact with the sensor actuator 230. Specifically, the sensor actuator 230 swings from the shielding position to the retracted position when the sheet S transported on the first transport path P1 comes into contact with the first lever 232. When the sheet S is no longer in contact with the first lever 232, the sensor actuator 230 swings from the retracted position to the shielding position due to the biasing force of the spring.

[0080] The optical sensor 210 and the sensor actuator 230 are supported by the fixing frame 65. Specifically, the optical sensor 210 is supported by the fixing frame 65 via a substrate 220. In addition, the sensor actuator 230 has a shaft 231 supported by the fixing frame 65 so as to be rotatable.

[0081] 9, the transmission mechanism 300 has an input gear 310, a first pendulum gear 320, a second pendulum gear 330, an output gear 340, an intermediate gear 350, a swinging member 360, a locking mechanism 370, and an elastic member 380. The transmission mechanism 300 is capable of transmitting a driving force from the motor 9 to the re-conveyance roller 8.

[0082] The input gear 310 is a gear that rotates when a driving force is input from the motor 9. The first pendulum gear 320 is a gear that meshes with the input gear 310. The second pendulum gear 330 is a gear that meshes with the input gear 310.

[0083] The output gear 340 is a gear that outputs a driving force to the re-conveying roller 8. The output gear 340 can mesh with the first pendulum gear 320. When the output gear 340 meshes with the first pendulum gear 320, the output gear 340 rotates by transmitting a driving force from the motor 9, which rotates in the reverse direction, via the input gear 310 and the first pendulum gear 320. As a result, the re-conveying roller 8 rotates in a predetermined direction to convey the sheet S toward the transfer nip N1.

[0084] The intermediate gear 350 has a first gear 351 and a second gear 352 that are rotatable together around the same rotation axis. The first gear 351 is capable of meshing with the output gear 340. Specifically, the first gear 351 has, on its outer circumferential surface, a first toothed portion 351A that is capable of meshing with the output gear 340, and a first toothless portion 351B that is not provided with teeth.

[0085] The second gear 352 can mesh with the second pendulum gear 330. Specifically, the second gear 352 has, on its outer circumferential surface, a second toothed portion 352A with which the second pendulum gear 330 can mesh, and a second toothless portion 352B where no teeth are provided.

[0086] When the motor 9 switches from reverse rotation to forward rotation, the intermediate gear 350 rotates by transmitting the driving force from the motor 9 to the second toothed portion 352A via the input gear 310 and the second pendulum gear 330. As a result, the first toothed portion 351A meshes with the output gear 340. The intermediate gear 350 rotates the output gear 340 until the first toothless portion 351B is positioned at a rotation position facing the output gear 340 after the first toothed portion 351A meshes with the output gear 340. As a result, the re-conveyance roller 8 rotates in a predetermined direction only while the intermediate gear 350 is rotating.

[0087] The oscillating member 360 is a member that holds the input gear 310, the first pendulum gear 320, and the second pendulum gear 330. Specifically, the oscillating member 360 holds the input gear 310, the first pendulum gear 320, and the second pendulum gear 330 so that they can rotate about their respective rotation axes.

[0088] The oscillating member 360 is supported by the housing 2 so as to be rotatable about the rotation axis of the input gear 310. As a result, the oscillating member 360 supports the first pendulum gear 320 and the second pendulum gear 330 so as to be revolvable around the rotation axis of the input gear 310 while the first pendulum gear 320 and the second pendulum gear 330 remain meshed with the input gear 310.

[0089] The swing member 360 can swing between a second swing position shown in Fig. 10 and a first swing position shown in Fig. 11. The first swing position is a position where the first pendulum gear 320 meshes with the output gear 340 and the second pendulum gear 330 is separated from the second gear 352. The second swing position is a position where the second pendulum gear 330 can mesh with the second gear 352 and the first pendulum gear 320 is separated from the output gear 340.

[0090] The oscillating member 360 swings to the first oscillating position when the motor 9 rotates in the reverse direction. On the other hand, the oscillating member 360 swings to the second oscillating position when the motor 9 rotates in the forward direction. In particular, the oscillating member 360 swings from the first oscillating position to the second oscillating position when the motor 9 switches from forward rotation to reverse rotation. On the other hand, the oscillating member 360 swings from the second oscillating position to the first oscillating position when the motor 9 switches from reverse rotation to forward rotation.

[0091] The lock mechanism 370 can be switched between a locked state shown in Fig. 10 and an unlocked state shown in Fig. 11. In the locked state, the intermediate gear 350 is stopped at a first rotation position shown in Fig. 10. In the first rotation position, the first toothless portion 351B of the first gear 351 faces the output gear 340, and the second toothless portion 352B of the second gear 352 faces the second pendulum gear 330.

[0092] The unlocked state is a state in which rotation from the first rotation position of the intermediate gear 350 is permitted. The lock mechanism 370 switches from the locked state to the unlocked state when the motor 9 switches from forward rotation to reverse rotation.

[0093] The locking mechanism 370 has a pawl 371 and a rib 372 engageable with the pawl 371. The pawl 371 is provided on the swinging member 360. The rib 372 is provided on the intermediate gear 350. When the locking mechanism 370 is in the locked state shown in Fig. 10, the pawl 371 engages with the rib 372, thereby stopping the intermediate gear 350 at the first rotation position. When the locking mechanism 370 is in the unlocked state shown in Fig. 11, the pawl 371 disengages from the rib 372, thereby allowing the intermediate gear 350 to rotate.

[0094] When the lock mechanism 370 switches from a locked state to an unlocked state, the elastic member 380 rotates the intermediate gear 350 from a first rotation position shown in Fig. 10 to a second rotation position shown in Fig. 11. The second rotation position is a position where the first toothless portion 351B of the first gear 351 faces the output gear 340, and the second toothed portion 352A of the second gear 352 faces the second pendulum gear 330 and can mesh with the second pendulum gear 330. The elastic member 380 is, for example, a torsion spring.

[0095] Intermediate gear 350 has cam portion 353. Cam portion 353 protrudes from first gear 351 in the direction in which the rotation shaft of intermediate gear 350 extends. Cam portion 353 protrudes on the opposite side to second gear 352. Cam portion 353 has first cam surface 353A and second cam surface 353B on its outer circumferential surface. First cam surface 353A is an arc-shaped surface centered on the rotation shaft of intermediate gear 350. Second cam surface 353B is a flat surface extending to connect both ends of arc-shaped first cam surface 353A.

[0096] The elastic member 380 has an arm 381. When the intermediate gear 350 is located at the first rotation position shown in FIG. 10, the arm 381 elastically contacts the boundary portion 353C between the first cam surface 353A and the second cam surface 353B, and biases the intermediate gear 350 clockwise. When the lock mechanism 370 switches from the locked state to the unlocked state shown in FIG. 11, the arm 381 biases the boundary portion 353C, thereby rotating the intermediate gear 350 from the first rotation position to the second rotation position. When the intermediate gear 350 is located at the second rotation position, the arm 381 contacts the second cam surface 353B.

[0097] Here, the operation of the transmission mechanism 300 will be described. 10, when the motor 9 is stopped, the swinging member 360 is located at the second swinging position, the locking mechanism 370 is in a locked state in which the claw 371 and the rib 372 are engaged with each other, and the intermediate gear 350 is located at the first rotation position. When a print job including a print instruction, image data, etc. is input, the image forming apparatus 1 rotates the motor 9 in the forward direction.

[0098] When the motor 9 rotates forward, the input gear 310 rotates clockwise, and the first pendulum gear 320 and the second pendulum gear 330 rotate counterclockwise. The intermediate gear 350 does not rotate because the second toothless portion 352B of the second gear 352 faces the second pendulum gear 330 and therefore does not receive the driving force from the motor 9. The output gear 340 does not rotate because the first pendulum gear 320 is separated from the output gear 340 and faces the first toothless portion 351B of the first gear 351 and therefore does not receive the driving force from the motor 9. At this time, the re-conveyance roller 8 does not rotate and is stopped.

[0099] As shown in FIG. 11, when the motor 9 switches from forward rotation to reverse rotation, the input gear 310 rotates counterclockwise, and the swinging member 360 swings from the second swinging position to the first swinging position. As a result, the first pendulum gear 320 meshes with the output gear 340. In addition, as the swinging member 360 swings to the first swinging position, the locking mechanism 370 changes from a locked state to an unlocked state in which the engagement between the claw 371 and the rib 372 is released. As a result, the intermediate gear 350 rotates from the first rotation position to the second rotation position by the biasing force of the elastic member 380, and the second toothed portion 352A of the second gear 352 faces the second pendulum gear 330. The output gear 340 remains facing the first toothless portion 351B of the first gear 351.

[0100] When the motor 9 rotates in the reverse direction in this state, the input gear 310 rotates counterclockwise, and the first pendulum gear 320 and the second pendulum gear 330 rotate clockwise. The output gear 340 rotates counterclockwise as the driving force from the reversely rotating motor 9 is transmitted through the input gear 310 and the first pendulum gear 320. As a result, the re-conveyance roller 8 rotates in a predetermined direction to convey the sheet S toward the transfer nip N1. The intermediate gear 350 does not rotate because the driving force from the motor 9 is not transmitted thereto, since the first toothless portion 351B faces the output gear 340 and the second gear 352 is separated from the second pendulum gear 330.

[0101] 12, when the motor 9 switches from reverse rotation to forward rotation, the input gear 310 rotates clockwise and the oscillating member 360 oscillates from the first oscillating position to the second oscillating position. As a result, the first pendulum gear 320 moves away from the output gear 340, while the second pendulum gear 330 meshes with the second toothed portion 352A of the intermediate gear 350. Note that, as the oscillating member 360 oscillates to the second oscillating position, the claw 371 moves to a position where it can engage with the rib 372, but does not engage with the rib 372 because the intermediate gear 350 has rotated from the first rotation position to the second rotation position.

[0102] When the motor 9 rotates forward in this state, the input gear 310 rotates clockwise, and the first pendulum gear 320 and the second pendulum gear 330 rotate counterclockwise. The driving force from the motor 9 rotating forward is transmitted to the second toothed portion 352A via the input gear 310 and the second pendulum gear 330, causing the intermediate gear 350 to rotate clockwise from the second rotation position.

[0103] 13, when the intermediate gear 350 rotates clockwise, the first toothed portion 351A of the first gear 351 meshes with the output gear 340. As a result, the driving force from the motor 9 rotating forward is transmitted to the output gear 340 via the input gear 310, the second pendulum gear 330, and the intermediate gear 350, and the output gear 340 rotates counterclockwise. As a result, the re-conveyance roller 8 rotates in a predetermined direction to convey the sheet S toward the transfer nip N1.

[0104] 14, the intermediate gear 350 continues to rotate the output gear 340 until the first toothless portion 351B is positioned at a rotation position facing the output gear 340. After the output gear 340 faces the first toothless portion 351B, the driving force from the motor 9 is no longer transmitted to it and it stops. After the motor 9 switches from reverse rotation to forward rotation, the re-conveyance roller 8 rotates a predetermined number of times in a predetermined direction by the driving force transmitted from the motor 9 during a predetermined time from when the intermediate gear 350 starts to rotate until it stops, and then it stops.

[0105] The intermediate gear 350 rotates clockwise from the rotation position shown in FIG. 14 due to the biasing force of the elastic member 380, and stops at the first rotation position as the rib 372 engages with the claw 371 (see FIG. 10).

[0106] The control unit 10 includes a CPU, RAM, ROM, input / output circuits, etc., and executes control by performing arithmetic processing based on programs and data stored in the ROM, etc. The control unit 10 controls the motor 9. Specifically, the control unit 10 controls the rotation direction of the motor 9. When the control unit 10 receives a print job, it rotates the motor 9 in the forward direction.

[0107] The control unit 10 controls the motor 9 to switch from forward rotation to reverse rotation based on the sheet sensor 200 detecting the trailing end of the sheet S being discharged toward the discharge tray 21. In detail, as shown in Fig. 15(a), the control unit 10 controls the motor 9 to switch from forward rotation to reverse rotation when the trailing end of the sheet S is located between the discharge rollers 7 rotating forward and the branching portion P3 on the first transport path P1. Specifically, the control unit 10 controls the motor 9 to switch from forward rotation to reverse rotation when it is determined that a first time T1 has elapsed since the sheet sensor 200 detected the trailing end of the sheet S.

[0108] 15(b), the control unit 10 controls the motor 9 to switch from reverse rotation to forward rotation after the sheet S passes the reversely rotating discharge rollers 7, based on the sheet sensor 200 detecting the rear end of the sheet S being discharged toward the discharge tray 21. The control unit 10 controls the motor 9 to switch from reverse rotation to forward rotation when the sheet S passes the reversely rotating discharge rollers 7 and reaches at least the second re-conveying roller 82 out of the multiple re-conveying rollers 8 (81, 82).

[0109] Specifically, the control unit 10 performs control to switch the motor 9 from reverse rotation to forward rotation when it determines that a second time T2 has elapsed since the sheet sensor 200 detected the rear end of the sheet S. The second time T2 is a time longer than the first time T1.

[0110] After the motor 9 switches from reverse rotation to forward rotation, the transmission mechanism 300 transmits the driving force from the motor 9 rotating forward to the re-conveyance roller 8 for a predetermined time to rotate the re-conveyance roller 8 a predetermined number of rotations in a predetermined direction. The predetermined number of rotations is a number of rotations sufficient for the sheet S, which has been conveyed by the re-conveyance roller 8 that has rotated the predetermined number of rotations, to reach the registration roller 4.

[0111] In the embodiment, from the time when the motor 9 shown in Fig. 15(b) switches from reverse rotation to forward rotation, as shown in Fig. 16, the re-conveying rollers 8 (81, 82) rotate a predetermined number of times, so that the leading edge of the sheet S reaches the registration rollers 4. As a result, after the re-conveying rollers 8 rotate a predetermined number of times and stop, the registration rollers 4 can convey the sheet S toward the transfer nip N1.

[0112] In the embodiment, the length of the transport path of the sheet S from the discharge rollers 7 to the second re-conveyance rollers 82 is longer than the length (355.6 mm) of a legal size sheet S. Also, the length of the transport path of the sheet S from the discharge rollers 7 to the first re-conveyance rollers 81 is shorter than the length (297 mm) of an A4 size sheet S.

[0113] Next, the operation of the control unit 10 will be described. Specifically, the control of the motor 9 by the control unit 10 will be described. In the following, the state in which the sheet sensor 200 detects the sheet S will be referred to as "ON", and the state in which the sheet sensor 200 does not detect the sheet S will be referred to as "OFF".

[0114] 17, when the control unit 10 receives a print job, it rotates the motor 9 in the forward direction (S11). Next, the control unit 10 determines whether or not a sheet S has been supplied from the sheet tray 31 (S12). If a sheet S has been supplied (S12, Yes), the control unit 10 determines whether or not double-sided printing is required (S13).

[0115] If double-sided printing is selected (S13, Yes), the control unit 10 determines whether the sheet sensor 200 has switched from ON to OFF (S14). A toner image is transferred to the first side of the sheet S supplied from the sheet tray 31 by the toner image forming unit 5, and the toner image is fixed by the fixing device 6 to form an image.

[0116] When the sheet sensor 200 is switched from ON to OFF (S14, Yes), the control unit 10 judges whether or not a first time T1 has elapsed since the sheet sensor 200 was switched OFF (S15). When the first time T1 has elapsed (S15, Yes), the control unit 10 switches the motor 9 from forward rotation to reverse rotation (S16). As a result, the sheet S is guided to the second conveying path P2 by the reversely rotating discharge rollers 7, and is conveyed toward the transfer nip N1 by the re-conveying rollers 8 rotating in a predetermined direction within the second conveying path P2.

[0117] Next, the control unit 10 judges whether or not a second time T2 has elapsed since the sheet sensor 200 was switched OFF (S17). If the second time T2 has elapsed (S17, Yes), the control unit 10 switches the motor 9 from reverse rotation to forward rotation (S18). As a result, the sheet S is conveyed toward the transfer nip N1 by the re-conveyance roller 8, which rotates a predetermined number of times in a predetermined direction, and the registration roller 4, which has resumed driving due to the forward rotation of the motor 9. Thereafter, a toner image is transferred to the second side of the sheet S by the toner image forming unit 5, and the toner image is fixed by the fixing unit 6 to form an image.

[0118] If there is a next page (S19, Yes), the control unit 10 executes the process from step S12 onward. If there is no next page (S19, No), the control unit 10 determines whether printing is complete (S20). If printing is complete (S20, Yes), the control unit 10 stops the motor 9 (S21) and ends the process.

[0119] Next, the effects of the image forming apparatus 1 will be described. A sheet sensor 200 for detecting the sheet S is provided on the first transport path P1 between the fixing nip N2 and the discharge roller 7. This allows the sheet sensor 200 to detect the trailing end of the sheet S after it has passed through the fixing nip N2. The control unit 10 switches the rotation of the motor 9 based on the detection by the sheet sensor 200, and can switch the rotation of the motor 9 when the sheet S is located at an ideal position without being affected by changes in the transport speed of the sheet S in the fixing unit 6.

[0120] The peripheral speed of the pressure rotating body may change, for example, even when the same print job is being processed, as the diameter of the pressure rotating body changes due to temperature. That is, the conveying speed of the sheet in the fixing unit may change for each sheet even when the same print job is being processed. Therefore, when the motor is switched from forward rotation to reverse rotation based on the change of the post-registration sensor from a state in which the sheet is detected to a state in which it is not detected, the position of the sheet at the time when the rotation of the motor is switched may vary for each sheet. On the other hand, in the image forming apparatus 1, the rotation of the motor 9 can be switched without being affected by the change of the conveying speed of the sheet S in the fixing unit 6, so that the position of the sheet S at the time when the rotation of the motor 9 is switched can be prevented from being shifted from the ideal position.

[0121] The pressure roller 62 having the rubber layer 62B is susceptible to changes in diameter due to heating, which changes the conveying speed of the sheet S in the fixing device 6. However, by switching the rotation of the motor 9 based on the detection of the sheet sensor 200, the rotation of the motor 9 can be switched without being affected by changes in the conveying speed of the sheet S in the fixing device 6.

[0122] When the sheet S passes through the discharge rollers 7 and reaches the second re-conveying rollers 82, which are the most downstream, the motor 9 is switched from reverse rotation to forward rotation, so that the sheet S, which is transported toward the transfer nip N1 after the motor 9 is switched to forward rotation, is not pulled by the discharge rollers 7 rotating forward.

[0123] If the sheet has not passed the discharge roller when the motor switches from reverse rotation to forward rotation, the sheet may be pulled and stopped by the discharge roller that attempts to discharge the sheet toward the discharge tray and the re-conveyance roller that attempts to convey the sheet toward the transfer nip, and it may be determined that a sheet jam has occurred. Image forming apparatus 1 can prevent such sheet jams from occurring.

[0124] By switching the rotation of the motor 9 based on the times T1 and T2 after the sheet sensor 200 detects the rear end of the sheet S, the rotation of the motor 9 can be switched regardless of the length of the sheet S and without being affected by changes in the conveying speed of the sheet S in the fixing device 6.

[0125] By rotating the re-conveying roller 8 a predetermined number of times after the motor 9 switches from reverse rotation to forward rotation, the sheet S can be conveyed toward the transfer position by the re-conveying roller 8 while the re-conveying roller 8 is rotating even after the motor 9 switches to forward rotation. In addition, by allowing the re-conveying roller 8 to convey the sheet S toward the transfer nip N1 while the motor 9 is rotating forward, while the sheet S is being conveyed by the re-conveying roller 8, the photosensitive drum 51 and the like can be driven by the motor 9 rotating forward to prepare for image formation.

[0126] The sheet S transported by the re-conveying roller 8 that has rotated a predetermined number of times reaches the registration roller 4, and after the re-conveying roller 8 has rotated a predetermined number of times, the sheet S can be transported by the registration roller 4 toward the transfer nip N1.

[0127] If the sheet has not yet reached the registration roller when the re-conveyance roller stops, the sheet may be stopped and it may be determined that a sheet jam has occurred. According to the image forming apparatus 1, it is possible to prevent such a sheet jam from occurring.

[0128] Since the optical sensor 210 and the sensor actuator 230 of the sheet sensor 200 are supported by the fixing frame 65, the sheet S can be detected with high accuracy by the sheet sensor 200. This allows the rotation of the motor 9 to be switched with high accuracy.

[0129] Since the chute 110 is movable between the first position and the second position, even if the conveying speed of the sheet S in the fixing device 6 changes and the sheet S bends between the transfer nip N1 and the fixing nip N2, the bending of the sheet S can be absorbed. This allows the sheet S to be conveyed in an appropriate posture. In addition, since the behavior of the sheet S is stable, the position of the sheet S is less likely to shift. This makes it possible to prevent the sheet S from getting stuck in the first conveying path P1, causing a sheet jam.

[0130] Furthermore, because the behavior of the sheet S is stable, it is not necessary to change the first time T1 and the second time T2 for each sheet S or for each print job. This makes it possible to transport the sheet S with simple control.

[0131] By operating the pressing mechanism 120 with the solenoid actuator 130 and controlling the solenoid actuator 130, the chute 110 can be moved from the first position to the second position independently of the discharge rollers 7 driven by the motor 9 and the like.

[0132] Since the second transport path P2 is provided to pass under the chute 110 and the pressing mechanism 120 is disposed above the chute 110, the pressing mechanism 120, the chute 110, and a part of the second transport path P2 can be arranged compactly one above the other. This makes it possible to reduce the size of the image forming apparatus 1. Also, for example, the second transport path P2 can be made shorter compared to a case in which the pressing mechanism is disposed below the chute and the second transport path is provided to pass further below the pressing mechanism.

[0133] Although the embodiment has been described above, the image forming apparatus can be modified as appropriate as exemplified below.

[0134] In the embodiment, two re-conveying rollers 8 are provided on the second transport path P2, but for example, three or more re-conveying rollers may be provided on the second transport path, or only one re-conveying roller may be provided. In other words, it is sufficient for the image forming apparatus to have at least one re-conveying roller.

[0135] Furthermore, when continuously conveying sheets S having a length longer than a predetermined length to form images, the control unit 10 may set the interval between the rear end of the previous sheet S and the front end of the following sheet S (sheet interval) to be larger than when continuously conveying sheets S having a length equal to or shorter than the predetermined length to form images. For example, when continuously conveying legal-size sheets S to form images, the control unit 10 may set the sheet interval to be larger than when continuously conveying A4-size sheets S to form images.

[0136] According to this, when a long size sheet S is continuously transported and an image is formed, the temperature of the fixing nip N2 is less likely to drop, so that the temperature change of the fixing nip N2 can be reduced. As a result, the change in the transport speed of the sheet S in the fixing device 6 can be reduced.

[0137] In the above embodiment, the heating belt 61A is exemplified as the heating rotator, but the heating rotator may be, for example, a heating roller, a heating film, etc. Also, in the above embodiment, the pressure roller 62 is exemplified as the pressure rotator, but the pressure rotator may be, for example, an endless belt sandwiched between the heating rotator and an elastic pad.

[0138] The specific configuration of the transmission mechanism described in the embodiment is an example. For example, the lock mechanism of the transmission mechanism may be a solenoid actuator that can be switched between a locked state in which the intermediate gear is engaged with the intermediate gear to stop the intermediate gear at the first rotation position and an unlocked state in which the intermediate gear is separated from the intermediate gear to allow rotation of the intermediate gear from the first rotation position. The transmission mechanism may be configured as disclosed in, for example, Japanese Patent Application Laid-Open No. 2017-173375. The image forming apparatus may be configured without the transmission mechanism.

[0139] The specific configuration of the sheet sensor described in the above embodiment is just an example. For example, the sheet sensor may be configured to detect the sheet on the first transport path between the intermediate discharge rollers and the discharge rollers.

[0140] In the above embodiment, the pressing mechanism 120 is disposed above the chute 110, but for example, the pressing mechanism may be disposed below the chute. Also, in the above embodiment, the chute 110 is provided to be rotatable, but for example, the chute may be capable of linear translational movement up and down. Also, the image forming apparatus may be configured without including a solenoid actuator, a pressing mechanism, and a chute.

[0141] In the embodiment, the second transport path P2 joins the first transport path P1 between the sheet tray 31 and the transfer nip N1, but the second transport path may join the sheet tray, for example. In other words, the second transport path may transport the sheet to the sheet tray.

[0142] In the above embodiment, the first time T1 and the second time T2 are values ​​that do not change. However, for example, at least one of the first time T1 and the second time T2 may be changed for each sheet or for each print job.

[0143] As an example, a time width of a first time T1 during which the motor 9 can be switched from forward rotation to reverse rotation when the rear end of the sheet S is located between the discharge roller 7 rotating forward and the branch point P3 on the first transport path P1 may be measured in advance by conducting an experiment or the like, and the first time T1 may be changed within that range. Also, a time width of a second time T2 during which the motor 9 can be switched from reverse rotation to forward rotation when the sheet S has at least reached the second re-transport roller 82 may be measured in advance by conducting an experiment or the like, and the second time T2 may be changed within that range.

[0144] The first time T1 and the second time T2 may be changed depending on, for example, the length, type, and conveying speed of the sheet S, the external environment, and the like. By varying the first time T1 and the second time T2, it is possible to prevent the sheet S from jamming in the first transport path P1.

[0145] In the above embodiment, the image forming apparatus 1 is a monochrome printer, but the image forming apparatus may be, for example, a color printer capable of forming color images. The image forming apparatus may also be a copier, a multifunction machine, or the like.

[0146] The elements described in the above embodiment and modified examples may be implemented in any combination. [Explanation of symbols]

[0147] 1. Image forming device 2. Chassis 5 Toner image forming section 6 Fixing unit 7 Discharge roller 8 Re-feed roller 9. Motor 10 Control section 21 Output tray 31 Sheet Tray 51 Photosensitive drum 61A Heating Belt 62 Pressure roller 200 Sheet Sensor N1 Transfer Nip N2 Fuser Nip P1 First transport route P2 Second transport route S Seat

Claims

1. A housing having a discharge tray; a toner image forming section having a photosensitive drum on which a toner image is formed and transferring the toner image to a sheet at a transfer position; a fixing unit having a heating rotator and a pressure rotator, and fixing a toner image transferred onto a sheet to the sheet by nipping and conveying the sheet at a fixing nip, which is a nip position between the heating rotator and the pressure rotator; a discharge roller that is rotatable in both directions and discharges the sheet conveyed from the fixing unit toward the discharge tray when the discharge roller rotates in the forward direction; a sheet tray in which a sheet to be supplied to the transfer position is set; a first transport path through which the sheet passes when the sheet is transported from the sheet tray to the discharge roller via the transfer position and the fixing unit; a second transport path along which the sheet is guided by the discharge roller rotating in the reverse direction, the second transport path branching off from between the fixing unit and the discharge roller on the first transport path and joining the first transport path upstream of the transfer position on the first transport path in a transport direction of the sheet toward the discharge roller on the first transport path; at least one re-conveying roller provided on the second transport path, the re-conveying roller configured to transport the sheet toward the transfer position; a sheet sensor configured to detect a sheet on the first transport path between the fixing nip and the discharge roller; a motor that drives the photosensitive drum, the pressure rotor, the discharge roller, and the re-conveying roller, the motor rotating in a forward direction to rotate the discharge roller in a forward direction and rotating in a reverse direction to rotate the discharge roller in a reverse direction; A control unit that controls the motor, The control unit, based on the sheet sensor detecting the rear end of the sheet discharged toward the discharge tray, Controlling the motor to switch from forward rotation to reverse rotation; 13. An image forming apparatus comprising: a discharge roller that is rotated in a reverse direction and a motor that is rotated in a forward direction;

2. a plurality of re-transport rollers are provided on the second transport path, 2. The image forming apparatus according to claim 1, wherein the control unit controls the motor to switch from reverse rotation to forward rotation when the sheet passes through the discharge roller and reaches at least one of the plurality of re-conveying rollers that is located most downstream in the sheet conveying direction in the second conveying path.

3. The control unit is When it is determined that a first time has elapsed since the sheet sensor detected the rear end of the sheet, control is performed to switch the motor from forward rotation to reverse rotation; 3. The image forming apparatus according to claim 1, further comprising: a control for switching the motor from reverse rotation to forward rotation when it is determined that a second time, which is longer than the first time, has elapsed since the sheet sensor detected the rear end of the sheet.

4. 3. The image forming apparatus according to claim 2, further comprising a transmission mechanism for transmitting a driving force from the motor rotating in the reverse direction to the re-conveying roller to rotate the re-conveying roller in a predetermined direction to transport the sheet toward the transfer position, the transmission mechanism transmitting a driving force from the motor rotating in the forward direction to the re-conveying roller for a predetermined period of time after the motor switches from reverse rotation to forward rotation, to rotate the re-conveying roller a predetermined number of rotations in the predetermined direction.

5. a registration roller provided on the first transport path between the sheet tray and the transfer position, the registration roller transporting the sheet toward the transfer position; the second transport path joins the first transport path between the sheet tray and the registration roller; 5. The image forming apparatus according to claim 4, wherein the predetermined number of rotations is a number of rotations sufficient for the sheet transported by the re-transport roller that has rotated the predetermined number of rotations to reach the registration roller.

6. The transmission mechanism includes: an input gear that rotates when a driving force from the motor is input; a first pendulum gear that meshes with the input gear; a second pendulum gear that meshes with the input gear; an output gear that outputs a driving force to the re-conveying roller, the output gear being capable of meshing with the first pendulum gear; an intermediate gear including a first gear capable of meshing with the output gear and a second gear capable of meshing with the second pendulum gear, the first gear and the second pendulum gear being integrally rotatable about a same rotation axis, the first gear has a first toothed portion with which the output gear can mesh and a first toothless portion on which no teeth are provided, the second gear includes an intermediate gear having a second toothed portion with which the second pendulum gear can mesh and a second toothless portion on which no teeth are provided; a swinging member that holds the first pendulum gear and the second pendulum gear rotatably about their respective rotation shafts, and supports the first pendulum gear and the second pendulum gear so as to be revolvable about the rotation shaft of the input gear while meshing with the input gear, When the motor rotates in the reverse direction, the first pendulum gear meshes with the output gear, and the second pendulum gear swings to a first swing position away from the second gear, a swinging member, when the motor rotates in a forward direction, such that the second pendulum gear is able to mesh with the second gear and the first pendulum gear swings to a second swing position where it is separated from the output gear; a locking mechanism switchable between a locked state in which the intermediate gear is stopped at a first rotation position in which the first tooth missing portion faces the output gear and the second tooth missing portion faces the second pendulum gear, and an unlocked state in which the intermediate gear is permitted to rotate from the first rotation position, the locking mechanism being switched from the locked state to the unlocked state when the motor switches from forward rotation to reverse rotation; an elastic member that rotates the intermediate gear from the first rotation position to a second rotation position in which the first toothless portion faces the output gear and the second toothed portion is capable of meshing with the second pendulum gear when the locking mechanism is switched from the locked state to the unlocked state; having 5. The image forming apparatus according to claim 4, wherein when the motor switches from reverse rotation to forward rotation, the intermediate gear rotates by transmitting driving force from the motor to the second toothed portion via the input gear and the second pendulum gear, and rotates the output gear until the first toothed portion meshes with the output gear and the first toothless portion is positioned in a rotational position opposite the output gear.

7. a fixing frame supporting the heating rotator and the pressure rotator, The sheet sensor includes: an optical sensor having a light emitting element and a light receiving element capable of receiving light emitted by the light emitting element; a sensor actuator supported movably between a blocking position for blocking light from the light emitting element toward the light receiving element and a retracted position for allowing the light from the light emitting element to be received by the light receiving element, the sensor actuator moving from one of the blocking position and the retracted position to the other when a sheet being transported on the first transport path comes into contact with the sensor actuator; 2. The image forming apparatus according to claim 1, wherein the optical sensor and the sensor actuator are supported by the fixing frame.

8. The heating rotor is rotated in accordance with the rotation of the pressurizing rotor, 2. The image forming apparatus according to claim 1, wherein the pressure rotating member is rotated by a driving force input from the motor.

9. 9. The image forming apparatus according to claim 1, wherein the pressure rotating member is a pressure roller having a rubber layer.

10. the toner image forming unit has a transfer roller, and transfers a toner image onto a sheet by nipping and conveying the sheet at a transfer nip, which is a nip position between the photosensitive drum and the transfer roller, as the transfer position; The image forming apparatus further includes a chute disposed between the transfer nip and the fixing nip, the chute guiding the sheet below the sheet; 2. The image forming apparatus according to claim 1, wherein the chute is movable between a first position and a second position that is lower than the first position.

11. a pressing mechanism for pressing the chute to move it from the first position to the second position; The image forming apparatus according to claim 10 , further comprising a solenoid actuator for moving the pressing mechanism.

12. The second conveying path is provided to pass under the chute, 12. The image forming apparatus according to claim 11, wherein the pressing mechanism is disposed above the chute.

Images