Image forming apparatus

Abstract

To provide an image forming apparatus that can prevent a failure in conveyance of a sheet during fixing.SOLUTION: A pressure adjustment unit can change the contact pressure between a pair of fixing rotating bodies. A control unit checks the passage time of a sheet passing through a registration roller pair on the basis of an output value from a determination sensor, and when the passage time exceeds a predetermined reference passage time, drives the pressure adjustment unit to change the contact pressure between the pair of fixing rotating bodies, and thereby adjusts the ejection direction of the sheet that passes through a fixing nip and is ejected.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] A conventional image forming apparatus has an image forming unit, a pair of registration rollers, and a fixing unit. The image forming unit includes a photosensitive roller and a transfer rotor that forms a transfer nip, and transports paper, forms an image, and transfers the formed image to the paper. The pair of registration rollers transports the paper to the transfer nip. The fixing unit includes a fixing rotor that forms a fixing nip, and fixes the transferred image to the paper by passing through the fixing nip.

[0003] By matching the feed speed of the pair of registration rollers with the rotation speed of the photosensitive roller to transport the paper to the transfer nip, it is possible to reduce misalignment of the image transfer position. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 7-261485 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional image forming devices, the transport time from the registration roller pair to the transfer nip is long, which can slow down printing speed. Also, if the image transfer position shifts toward the leading edge of the paper, the leading edge of the paper on one side where the image has been transferred can come into contact with and wrap around the fixing rotor, causing paper jams, bent edges, and other transport problems.

[0006] The present invention has been made to solve the above-mentioned problems, and has an object to provide an image forming apparatus that can suppress paper transport failure during fixing. [Means for solving the problem]

[0007] To achieve the above object, an image forming apparatus according to a first aspect of the present invention includes a paper output tray to which paper is ejected, a printing unit that prints on the paper, a determination sensor that detects the arrival and passage of the paper, and a control unit that controls the printing unit. The printing unit includes an image forming unit, a pair of registration rollers, a fixing unit, and a paper transport path. The image forming unit includes a transfer rotor that forms a transfer nip, transports the paper, forms an image, and transfers the formed image to the paper. The pair of registration rollers transport the paper to the transfer nip. The fixing unit includes a fixing rotor that forms a fixing nip, and fixes the transferred image to the paper by passing through the fixing nip. The paper transport path passes through a paper feed nip, a transfer nip, and a fixing nip to reach the paper output tray. The fixing unit includes a heating unit and a pressure adjustment unit. The heating unit heats one of the pair of fixing rotors. The pressure adjustment unit is capable of changing the pressure contact force between the pair of fixing rotors. The control unit measures the passage time of the paper passing through the pair of resist rollers based on the output value of the judgment sensor, and when the passage time exceeds a predetermined reference passage time, it drives the pressure adjustment unit to change the pressure contact force between the pair of fixing rotors, thereby adjusting the discharge direction of the paper as it passes through the fixing nip and is discharged.

[0008] An image forming apparatus according to a second aspect of the present invention includes an output tray to which paper is discharged, a printing unit that prints on the paper, a determination sensor that detects the arrival and passage of the paper, and a control unit that controls the printing unit. The printing unit includes an image forming unit, a pair of registration rollers, a fixing unit, and a paper transport path. The image forming unit includes a transfer rotor that forms a transfer nip, transports the paper, forms an image, and transfers the formed image to the paper. The pair of registration rollers transport the paper to the transfer nip. The fixing unit includes a fixing rotor that forms a fixing nip, and fixes the transferred image to the paper by passing through the fixing nip. The paper transport path passes through a paper feed nip, a transfer nip, and a fixing nip to reach the output tray. The fixing unit has a heating unit. The heating unit heats one of the pair of fixing rotors. The control unit measures the passage time of the paper passing through the pair of resist rollers based on the output value of the judgment sensor, and when the passage time exceeds a predetermined reference passage time, reduces the transfer rate of the leading edge region of the image onto the paper to suppress the amount of toner applied to the paper.

[0009] An image forming apparatus according to a third aspect of the present invention includes an output tray to which paper is discharged, a printing unit that prints on the paper, a determination sensor that detects the arrival and passage of the paper, and a control unit that controls the printing unit. The printing unit includes an image forming unit, a pair of registration rollers, a fixing unit, and a paper transport path. The image forming unit includes a transfer rotor that forms a transfer nip, transports the paper, forms an image, and transfers the formed image to the paper. The pair of registration rollers transport the paper to the transfer nip. The fixing unit includes a fixing rotor that forms a fixing nip, and fixes the transferred image to the paper by passing through the fixing nip. The paper transport path passes through a paper feed nip, a transfer nip, and a fixing nip to reach the output tray. The fixing unit has a heating unit. The heating unit heats one of the pair of fixing rotors. The control unit measures the passage time of the paper passing through the pair of resist rollers based on the output value of the judgment sensor, and when the passage time exceeds a predetermined reference passage time, increases the passage speed of the paper through the fixing nip to reduce the adhesion of the toner to the paper. [Effects of the Invention]

[0010] The configuration of the present invention can provide an image forming apparatus that can suppress paper transport failure during fixing. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a side cross-sectional view showing the overall configuration of an image forming apparatus 100 according to a first embodiment of the present invention. [Figure 2] 1 is a side cross-sectional view of a fixing unit 6 of an image forming apparatus 100 according to a first embodiment of the present invention; [Figure 3] 3 is an enlarged side view of the fixing nip FN and its surroundings in FIG. 2. [Figure 4] FIG. 1 is an enlarged cross-sectional side view of a paper transport path of an image forming apparatus according to a first embodiment of the present invention. [Figure 5] FIG. 1 is a block diagram showing a hardware configuration of an image forming apparatus according to a first embodiment of the present invention. [Figure 6] 10 is a side cross-sectional view of a fixing unit 6 of an image forming apparatus 100 according to a second embodiment of the present invention. [Figure 7] 10 is a side cross-sectional view of a fixing unit 6 of an image forming apparatus 100 according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] First Embodiment <Configuration of image forming device> A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the overall configuration of an image forming apparatus 100. The image forming apparatus 100 includes an image reading unit 1, a printing unit 2, a paper output tray 20T, a determination sensor PS, and a control unit 110. The image reading unit 1 reads an original document and generates image data of the original document. The printing unit 2 includes a paper feed unit 3, an image forming unit 4, a fixing unit 6, and a paper transport path 20. The printing unit 2 transports a paper sheet P along the paper transport path 20, prints an image on the paper sheet P as it is being transported, and finally ejects the printed paper sheet P to the paper output tray 20T.

[0013] The paper feed unit 3 includes a pickup roller 31 and a pair of paper feed rollers 32. The pair of paper feed rollers 32 form a paper feed nip PN for feeding paper P to the paper transport path 20. In other words, the pair of paper feed rollers 32 corresponds to a "paper feed rotating body." The paper feed unit 3 pulls out paper P from the paper cassette 30 and causes it to enter the paper feed nip PN, supplying the paper P to the paper transport path 20 (performing primary paper feeding).

[0014] The image forming unit 4 includes mechanism units 40Bk, 40Y, 40C, and 40M corresponding to the colors black (Bk), yellow (Y), cyan (C), and magenta (M), respectively.

[0015] Each of the mechanical units 40Bk, 40Y, 40C and 40M includes one photosensitive drum 41, one charging device 42, one developing device 43 and one cleaning device 44, and is arranged so that the photosensitive drum 41 contacts the intermediate transfer belt 51 described later.

[0016] The image forming unit 4 also includes an exposure device 45 that is used in common for each color. The image forming unit 4 forms a toner image of each color on the surface of the corresponding photosensitive drum 41.

[0017] The image forming unit 4 also includes an intermediate transfer belt 51, a primary transfer roller 52, and a secondary transfer roller 53.

[0018] The intermediate transfer belt 51 is stretched by a drive roller 54 and a driven roller 55. The primary transfer roller 52 is disposed inside the intermediate transfer belt 51, and sandwiches the intermediate transfer belt 51 between itself and the photosensitive drum 41. The secondary transfer roller 53 is disposed opposite the drive roller 54 across the intermediate transfer belt 51, and forms a transfer nip TN between itself and the intermediate transfer belt 51.

[0019] The transfer nip TN is a nip for transferring a toner image onto the paper P. In other words, the intermediate transfer belt 51 and the secondary transfer roller 53 correspond to a "transfer rotating body." After the image forming unit 4 performs the primary transfer of the toner image onto the surface of the intermediate transfer belt 51, the image forming unit 4 performs the secondary transfer of the toner image onto the paper P while transporting the paper P that has entered the transfer nip TN.

[0020] The fixing unit 6 has a pair of fixing rollers 61. One of the rollers in this fixing roller pair 61 has a built-in heat source. The other roller is in pressure contact with the one roller, forming a fixing nip FN between the one roller. The fixing nip FN is a nip for fixing the toner image transferred to the paper P. In other words, the fixing roller pair 61 corresponds to a "fixing rotor." The fixing unit 6 fixes the transferred toner image (image) to the paper P by applying heat and pressure to the paper P as it passes through the fixing nip FN while being transported.

[0021] <Configuration of the fixing unit> 2 is a side cross-sectional view of the fixing unit 6. The fixing unit 6 includes a pair of fixing rotors 61 that form a fixing nip FN, and has a pressing member 613, a pressing pad 614, an induction heating unit (heating unit) 618, and a pressure adjusting unit 619. In this embodiment, one of the pair of fixing rotors 61 is an endless fixing belt 611 that is rotatable in the transport direction of the paper P, and the fixing belt 611 rotates counterclockwise in FIG. 2. The other of the pair of fixing rotors 61 is a pressure roller 612 that is disposed opposite the outer peripheral surface of the fixing belt 611 and applies a rotational driving force to the fixing belt 611.

[0022] The pressing member 613 contacts the inside of the fixing belt 611 and presses the fixing belt 611 against the pressure roller 612. The pressing pad 614 is provided on the surface of the pressing member 613 facing the fixing belt 611, and has a sliding surface 614a on which the inner circumferential surface of the fixing belt 611 slides. The pressure roller 612 is pressed against the pressing pad 614 via the fixing belt 611, thereby forming a fixing nip FN. At the fixing nip FN, the fixing belt 611 curves convexly in a direction away from the pressure roller 612.

[0023] The fixing belt 611 is an endless belt formed by laminating multiple layers, including an induction heating layer provided on the innermost side in contact with the pressure pad 614 and a release layer provided on the outermost side in contact with the pressure roller 612. The fixing belt 611 is wound around a pressing member 613 and a support member 615, and a predetermined tension is applied to the fixing belt 611. The support member 615 is inscribed in the fixing belt 611 on the side opposite the pressing member 613 (the side facing the induction heating unit 618), and has an arch-shaped cross section. The portion of the fixing belt 611 that is not in contact with the pressing member 613 (pressure pad 614) is maintained in an arc shape with a predetermined distance from the induction heating unit 618. The shape of the support member 615 is not limited to that of the present embodiment.

[0024] For example, a metal layer plated or rolled with a metal such as nickel is used as the induction heating layer of the fixing belt 611. For example, a fluorine-based resin such as PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer) is used as the release layer, and is formed by applying paint or covering with a tube.

[0025] One end of the pressing member 613 is supported by a support member 615, and the other end comes into contact with the pressure roller 612 via the pressing pad 614 and the fixing belt 611, thereby forming a fixing nip FN through which the paper passes. The pressing member 613 is made of a metal such as aluminum, a heat-resistant resin, or the like.

[0026] The pressure pad 614 is made of a heat-resistant resin such as a liquid crystal polymer resin or an elastic material such as silicone rubber, and an elastomer may be disposed on the surface facing the fixing belt 611 in some cases.

[0027] The pressure roller 612 has, for example, a core metal 612a and an elastic layer 612b provided on the outside thereof. The core metal 612a is made of, for example, aluminum, and the elastic layer 612b is made of silicone rubber. The pressure roller 612 is rotated clockwise by a drive motor (not shown).

[0028] The induction heating unit 618 heats the fixing belt 611 by electromagnetic induction, and is composed of a coil bobbin 618a, a coil 618b, and a core unit consisting of a center core 618c, an arch core 618d, and a side core 618e, and is arranged opposite to and surrounding a portion of the arc-shaped outer surface of the fixing belt 611.

[0029] Coil bobbin 618a is formed into an arc-shaped cross section that fits along the outer surface of fixing belt 611. Coil 618b is arranged on coil bobbin 618a, and is formed by winding a litz wire, which is a bundle of thin wires, multiple times in the longitudinal direction of induction heating unit 618 (the direction perpendicular to the paper surface of FIG. 2). Coil 29 is connected to a power source (not shown).

[0030] The center core 618c is a ferrite core having a rectangular cross section and disposed in the center of the coil 618b. A pair of arch cores 618d and side cores 618e are disposed on both sides of the center core 618c.

[0031] Induction heating unit 618 supplies high-frequency current to coil 618b, thereby generating magnetic flux through center core 618c and arch cores 618d and side cores 618e on both sides. The magnetic flux emitted from induction heating unit 618 acts on the induction heating layer of fixing belt 611. As a result, eddy currents are generated around the magnetic flux in the induction heating layer, and Joule heat is generated by electrical resistance in the induction heating layer, causing fixing belt 611 to heat up.

[0032] When the fixing belt 611 is heated by the induction heating unit 618 and reaches a predetermined temperature, the paper P (see FIG. 1) held in the fixing nip FN is heated and pressed by the pressure roller 612, thereby melting and fixing the powder toner on the paper P.

[0033] The pressure adjustment unit 619, for example, rotatably supports both axial ends of the core metal 612a and swings the pressure roller 612 toward and away from the pressure pad 614. This allows the pressure adjustment unit 619 to move the pressure roller 612 toward and away from the pressure pad 614. The swing movement of the pressure roller 612 is performed, for example, by driving a motor (not shown). By pressing the pressure roller 612 against the pressure pad 614, the pressure contact force between the pair of fixing rotors 61 can be increased. Furthermore, by moving the pressure roller 612 in a direction away from the pressure pad 614, the pressure contact force between the pair of fixing rotors 61 can be decreased.

[0034] 3 is an enlarged side view of the periphery of the fixing nip FN in FIG. 2. The sliding surface (opposing surface) 614a of the pressure pad 614, which contacts the inner surface of the fixing belt 611, has a flat portion 614b that is approximately parallel to the paper transport direction (the direction of arrow A in the figure) on the upstream side of the fixing nip FN, and an arc-shaped portion 614c that is convexly curved in a direction away from the pressure roller 612 downstream of the flat portion 614b in the paper transport direction. The pressure roller 612 is pressed against the arc-shaped portion 614c via the fixing belt 611, thereby forming the fixing nip FN. This configuration ensures a sufficient contact width (fixing nip width) between the pressure pad 614 and the pressure roller 612 without interfering with the paper transportability, thereby stabilizing the fixing performance.

[0035] Furthermore, the radius of curvature of the arc-shaped portion 614c is larger than the radius of curvature of the pressure roller 612. When the force pressing the pressure roller 612 toward the pressure pad 614 is increased and the pressure roller 612 is placed in the pressing position, the pressure roller 612 deforms, and the pressure roller 612 is pressed against the pressure pad 614 at the end of the arc-shaped portion 614c downstream in the paper transport direction. At this time, the discharge direction of the paper P that is discharged after passing through the fixing nip FN is tilted toward the pressure roller 612 in a cross section perpendicular to the rotation axis of the pressure roller 612. This allows the paper P that has passed through the fixing nip FN to be transported along the arc-shaped portion 614c toward the pressure roller 612 (direction of arrow B).

[0036] On the other hand, when the force pressing the pressure roller 612 toward the pressure pad 614 is reduced and the pressure roller 612 is returned to its normal position, the shape of the pressure roller 612 is restored, and the pressure roller 612 moves away from the pressure pad 614 at the end of the arc-shaped portion 614c that is downstream in the paper transport direction. The normal position of the pressure roller 612 is located farther from the pressure pad 614 than the pressing position. At this time, the paper P that has passed through the fixing nip FN can be transported away from the pressure roller 612 in a cross section perpendicular to the rotation axis of the pressure roller 612. In other words, by changing the pressure contact force between the pair of fixing rotors 61, the discharge direction of the paper P that passes through the fixing nip FN and is discharged can be adjusted.

[0037] <Paper transport path configuration> 4 is an enlarged view of the paper transport path. The paper transport path 20 provided in the image forming apparatus 100 includes a main transport path 2A and a reverse transport path 2B. In FIG. 4, the main transport path 2A is indicated by a solid line, and the reverse transport path 2B is indicated by a dashed line.

[0038] The main transport path 2A passes through a paper feed nip PN (paper feed unit 3), a transfer nip TN (image forming unit 4), and a fixing nip FN (fixing unit 6) before reaching the paper output tray 20T. When a print job is executed, the paper P fed to the main transport path 2A is transported so as to pass through the paper feed nip PN, transfer nip TN, and fixing nip FN in that order. The transport direction of the paper P at this time will be described below using the symbol D.

[0039] The reverse transport path 2B is a transport path used during double-sided printing. The reverse transport path 2B has a first transport path 21 and a second transport path 22. In Fig. 4, the first transport path 21 and the second transport path 22 are indicated by different dashed lines.

[0040] The first transport path 21 branches off from the main transport path 2A at a position downstream of the fixing nip FN in the transport direction D of the paper P, and leads to the paper output tray 20T. The second transport path 22 branches off from the first transport path 21 at a predetermined branch position P1, and leads to a predetermined junction position P2 (transport path The transfer nip TN merges with the main transport path 2A at a position upstream of the transfer nip TN in the transport direction D of the paper P. During double-sided printing, the single-sided printed paper P is transported in a first direction D1, which is the direction toward the paper output tray 20T, and a second direction D2, which is opposite to the first direction D1.

[0041] Further, the paper transport path 20 is provided with a plurality of transport roller pairs 71 to 77 for transporting the paper P along the paper transport path 20.

[0042] Between the paper feed nip PN and the transfer nip TN of the main transport path 2A, transport roller pairs 71 and 72 are provided in this order toward the transport direction D of the paper P. Of these, the transport roller pair 72 located downstream in the transport direction D of the paper P temporarily stops the paper P that has reached the transport roller pair 72, and then sends the paper P to the transfer nip TN (performing secondary paper feeding). Hereinafter, the transport roller pair 72 will be referred to as the registration roller pair 72.

[0043] A pair of transport rollers 73 is provided on the discharge tray 20T side of the main transport path 2A (the most downstream side in the transport direction D of the paper P). The pair of transport rollers 73 transports the paper P that has passed through the fixing nip FN along the main transport path 2A, and finally discharges the paper P onto the discharge tray 20T. Hereinafter, the pair of transport rollers 73 will be referred to as the discharge roller pair 73.

[0044] A transport roller pair 74 is provided on the discharge tray 20T side of the first transport path 21. As will be described in detail later, the transport roller pair 74 switches back the paper P (reverses the traveling direction of the paper P) during double-sided printing. Hereinafter, the transport roller pair 74 will be referred to as a reversing roller pair 74. In addition, transport roller pairs 75, 76, and 77 are provided in this order along the second direction D2 between the branching position P1 and the merging position P2 of the second transport path 22.

[0045] At the branching position of the main transport path 2A and the reverse transport path 2B (first transport path 21), a switching claw 78 is provided for switching the transport path of the paper P. The switching claw 78 is rotatably supported, and by rotating, blocks or opens the transport path from the main transport path 2A to the reverse transport path 2B.

[0046] Further, a plurality of determination sensors PS (PS1 to PS7) are provided on the paper transport path 20 to detect the presence or absence of paper P at predetermined positions on the transport path. The determination sensors PS are, for example, optical sensors that change their output value depending on the presence or absence of paper P at the corresponding detection positions. In other words, the determination sensors PS (PS1 to PS7) detect the arrival and passage of paper P.

[0047] The determination sensor PS1 is a sensor whose detection position is near the installation position of the paper feed roller pair 32 (in this embodiment, a position downstream of the paper feed nip PN in the transport direction D of the paper P).

[0048] The determination sensor PS2 is a sensor whose detection position is near the installation position of the transport roller pair 71 (in this embodiment, a position upstream of the installation position of the transport roller pair 71 in the transport direction D of the paper P).

[0049] The determination sensor PS3 is a sensor whose detection position is near the installation position of the pair of registration rollers 72 (in this embodiment, a position upstream of the installation position of the pair 72 in the transport direction D of the paper P).

[0050] The determination sensor PS4 is a sensor whose detection position is near the installation position of the drive roller 54 (in this embodiment, a position downstream of the transfer nip TN in the transport direction D of the paper P).

[0051] The determination sensor PS5 is a sensor whose detection position is near the installation position of the fixing roller pair 61 (in this embodiment, a position downstream of the fixing nip FN in the transport direction D of the paper P).

[0052] The determination sensor PS6 is a sensor whose detection position is near the branch position P1 (in this embodiment, downstream of the branch position P1 in the second direction D2).

[0053] The determination sensor PS7 is a sensor whose detection position is near the installation position of the transport roller pair 77 (in this embodiment, upstream of the installation position of the transport roller pair 77 in the second direction D2).

[0054] Returning to FIG. 1, the image forming apparatus 100 is equipped with an operation panel 200. The operation panel 200 includes a touch panel display 201. The touch panel display 201 displays a screen for receiving various settings from the user. The operation panel 200 is also provided with a plurality of hard keys 202. Examples of the hard keys 202 provided on the operation panel 200 include a start key for receiving job execution instructions from the user and a numeric keypad for receiving numeric inputs from the user. The operation panel 200 also displays information that a jam error or a size mismatch error has occurred. At this time, the operation panel 200 functions as a notification unit.

[0055] <Hardware configuration of image forming device> 5 is a block diagram showing the hardware configuration of an image forming apparatus 100. Image forming apparatus 100 includes a control unit 110 that controls the entire apparatus. Control unit 110 includes a CPU 111 that executes various processes, a memory 112 that stores control programs for operating CPU 111, and the like.

[0056] The control unit 110 is connected to the image reading unit 1 and the printing unit 2, and controls the reading operation of the image reading unit 1 and the printing operation of the printing unit 2. The control unit 110 is also connected to the operation panel 200, and controls the display operation of the operation panel 200 and detects operations performed on the operation panel 200.

[0057] Here, the control unit 110 controls the printing operation by controlling the transport operations of multiple processing units (rotating bodies for transporting the paper P) that are responsible for transporting the paper P. That is, the control unit 110 controls the driving of motors M1, M2, M3, and M4 that cause each processing unit to perform the transport operation. The control unit 110 also controls the on / off of clutches C1, C2, and C3.

[0058] The processing unit that performs the transport operation using the driving force of the motor M1 includes a pickup roller 31 and a pair of paper feed rollers 32. In other words, the processing unit that performs the transport operation using the driving force of the motor M1 is the paper feed unit 3. The pickup roller 31 and the pair of paper feed rollers 32 rotate by receiving the driving force of the motor M1 and feed (transport) the paper P.

[0059] The pickup roller 31 is connected to a motor M1 via a clutch C1, which allows only the pair of paper feed rollers 32 to rotate.

[0060] The processing section that performs the transport operation using the driving force of motor M2 includes intermediate transfer belt 51, secondary transfer roller 53, drive roller 54, transport roller pair 71, registration roller pair 72, and transport roller pairs 76 and 77. In the following description, the processing section that performs the transport operation using the driving force of motor M2 is referred to as image forming section 4. Each rotating body of image forming section 4 rotates by receiving the driving force of motor M2 and transports paper P.

[0061] The drive roller 54 rotates by receiving the driving force of the motor M2. When the drive roller 54 rotates, the intermediate transfer belt 51 rotates, and the secondary transfer roller 53 rotates following the rotation of the intermediate transfer belt 51. As a result, the paper P is transported.

[0062] When the image forming unit 4 operates to transport the paper P in the same direction as the transport direction D of the paper P when executing a job, the transport roller pairs 76 and 77 rotate so that the paper P proceeds toward the junction position P2, and when the image forming unit 4 operates to transport the paper P in the opposite direction to the transport direction D of the paper P when executing a job, the transport roller pairs 76 and 77 rotate so that the paper P proceeds toward the branch position P1.

[0063] The pair of registration rollers 72 is connected to a motor M2 via a clutch C2. This allows the pair of registration rollers 72 to rotate or stop rotating independently while the pair of transport rollers 71 is rotating. This allows secondary paper feeding to be performed at the appropriate timing.

[0064] The processing unit that performs the transport operation using the driving force of the motor M3 includes the fixing roller pair 61. That is, the processing unit that performs the transport operation using the driving force of the motor M3 is the fixing unit 6. The fixing roller pair 61 rotates by receiving the driving force of the motor M3 and transports the paper P.

[0065] The processing section that performs the transport operation using the driving force of motor M4 includes the reversing roller pair 74 and the transport roller pair 75. In other words, the processing section that performs the transport operation using the driving force of motor M4 is the reversing section 8. The reversing roller pair 74 and the transport roller pair 75 rotate by receiving the driving force of motor M4 and transport the paper P.

[0066] The discharge roller pair 73 is also configured to rotate by the driving force of the motor M4. In this configuration, for example, the reversing roller pair 74 and the transport roller pair 75 are connected to the motor M4 via the clutch C3. This allows only the discharge roller pair 73 to rotate while the rotation of the reversing roller pair 74 (transport roller pair 75) is stopped.

[0067] Furthermore, in order to control the printing operation, the control unit 110 detects the presence or absence of paper P at the corresponding detection positions based on the output values ​​of the multiple determination sensors PS. In other words, the control unit 110 detects the arrival of the leading edge of paper P and the passing of the trailing edge of paper P at each detection position. For example, when a print job is being executed, the control unit 110 determines whether a jam error has occurred based on the detection results. Details will be described later.

[0068] Furthermore, the control unit 110 is connected to a pressure adjustment unit 619, and can change the pressure contact force between the pair of fixing rotors 61 by driving a motor (not shown) to oscillate the pressure roller 612.

[0069] <Single-sided and double-sided printing> The transport path downstream of the fixing nip FN in the transport direction D of the paper P varies depending on whether a single-sided print job or a double-sided print job is executed. This will be specifically described below with reference to FIG.

[0070] First, when a single-sided print job is executed, the control unit 110 causes the single-sided printed paper P (paper P that has passed through the fixing nip FN) to be discharged directly to the paper output tray 20T. That is, the control unit 110 blocks the conveyance path from the main conveyance path 2A to the reverse conveyance path 2B using the switching claw 78. As a result, the single-sided printed paper P that has passed through the fixing nip FN is guided to the paper output tray 20T.

[0071] When a double-sided printing job is executed, the control unit 110 performs a switchback process (a process of switching back the paper P) using the reversing unit 8. Specifically, the control unit 110 opens the transport path from the main transport path 2A to the reverse transport path 2B by rotating the switching claw 78. This causes the single-sided printing paper P that has passed through the fixing nip FN to be drawn from the main transport path 2A into the reverse transport path 2B (first transport path 21).

[0072] At this time, the control unit 110 rotates the pair of reversing rollers 74 so that the single-sided printed paper P pulled into the first transport path 21 proceeds in the first direction D1. Then, the control unit 110 reverses the rotation of the pair of reversing rollers 74 at the timing when the rear end of the single-sided printed paper P passes the branch position P1. As a result, the single-sided printed paper P proceeds in the second direction D2 and is sent into the second transport path 22.

[0073] Thereafter, the control unit 110 continues to rotate the reversing roller pair 74 and the conveying roller pair 75. As a result, the single-sided printed paper P sent into the second conveying path 22 proceeds in the second direction D2 along the second conveying path 22 and enters the main conveying path 2A from the junction position P2 (returns to the main conveying path 2A).

[0074] The single-sided printed paper P that has returned to the main transport path 2A is transported along the main transport path 2A, passing through the transfer nip TN and the fixing nip FN again in that order. At this time, the single-sided printed paper P has been turned over, so an image is printed on the back (unprinted side) of the single-sided printed paper P. Then, the paper P with images printed on both sides is transported along the first transport path 21 as is and discharged to the paper output tray 20T.

[0075] <Detection of jam errors> The control unit 110 measures the passing time, which is the time from when a certain one of the multiple determination sensors PS detects that the leading edge of the paper P has reached its detection position until when it detects that the trailing edge of the paper P has passed. When the measured passing time exceeds a reference passing time, it is determined that a jam error has occurred. The reference passing time in this case is the time required from when the leading edge of a sheet of paper P reaches the detection position of a certain determination sensor PS until the trailing edge of the paper P passes the same detection position, and is a time that can be calculated in advance based on the paper size (length in the transport direction) and the paper transport speed.

[0076] Furthermore, when a print job is executed, the control unit 110 starts timing from the start of paper feeding by the rotation of the pickup roller 31. Then, when the jam detection time has elapsed from the start of paper feeding, the control unit 110 checks whether the leading edge of the paper P has reached the detection position of a certain determination sensor PS. As a result, if the control unit 110 cannot confirm that the leading edge of the paper P has reached the detection position of the determination sensor PS, it determines that a jam error (paper jam in the paper transport path 20) has occurred. The jam detection time in this case is the time required for one sheet of paper P to move from the start of paper feeding to the detection position of the determination sensor PS, and is a time that can be calculated in advance based on the distance between the pickup roller 31 and the detection position of the determination sensor PS and the paper transport speed.

[0077] Furthermore, when a print job is executed, the control unit 110 may start timing after detecting that the leading edge of the paper P has reached the detection position of a certain determination sensor PS (referred to as a first sensor) among the multiple determination sensors PS. In this case, when the jam detection time has elapsed since the control unit 110 detected that the leading edge of the paper P has reached the detection position of a determination sensor PS (referred to as a second sensor) that is downstream of the first sensor in the transport direction of the paper P, the control unit 110 checks whether the leading edge of the paper P has reached the detection position of the determination sensor PS (referred to as a second sensor) that is located downstream of the first sensor in the transport direction of the paper P.

[0078] When the control unit 110 determines that a jam error has occurred, it interrupts the print job that is currently being executed. Then, the control unit 110 causes an error message indicating that a jam error has occurred to be displayed on the operation panel 200. In other words, the control unit 110 notifies the user that a jam error has occurred (causing the operation panel 200 to function as a notification unit).

[0079] When a user performs jam clearance work (work to remove jammed paper P from paper transport path 20), the side cover (not shown) of image forming apparatus 100 is opened. When this side cover is in the open state, paper transport path 20 is exposed. This allows jammed paper P to be removed from paper transport path 20. Alternatively, when paper cassette 30 is pulled out from image forming apparatus 100, paper feed unit 3 is exposed. Therefore, if jammed paper P remains in paper feed unit 3, the jammed paper P can be removed by pulling out paper cassette 30.

[0080] After detecting the occurrence of a jam error, the control unit 110 determines whether the user has completed the jam clearance process. For example, when the control unit 110 detects that the side cover, which was opened after the jam error occurred, has been closed, the control unit 110 determines that the jam clearance process is complete. Also, when the control unit 110 detects that the paper cassette 30, which was pulled out after the jam error occurred, has been returned to its original position, the control unit 110 determines that the jam clearance process is complete.

[0081] After the jam clearance work is completed, if no paper P remains in paper transport path 20, control unit 110 resumes the print job that was interrupted when it detected the occurrence of a jam error. At this time, control unit 110 stops displaying the error message.

[0082] <Detecting misalignment of image transfer position> The control unit 110 measures the passing time, which is the time from when the determination sensor PS3 detects that the leading edge of the paper P has reached the detection position until when it detects that the trailing edge of the paper P has passed. If the measured passing time exceeds a predetermined reference passing time, the control unit 110 determines that the paper P transported to the transfer nip TN may have slipped in the registration roller pair 72 or the transfer nip TN, causing the image transfer position to be misaligned.

[0083] In this embodiment, the determination sensor PS3 is used to determine whether or not slippage has occurred between the sheet P and the pair of registration rollers 72, but other determination sensors may be used. For example, the time may be measured from when the determination sensor PS3 detects that the leading edge of the sheet P has reached its detection position until the determination sensor PS4 detects that the leading edge of the sheet P has reached its detection position. If the measured passage time of the sheet P exceeds a predetermined reference passage time, it is determined that slippage of the sheet P has occurred at the pair of registration rollers 72 or the transfer nip TN when passing through the pair of registration rollers 72.

[0084] In addition, in this embodiment, the determination sensor PS3 is positioned downstream of the resist roller pair 72 in the paper transport direction, but the determination sensor PS3 may also be positioned upstream of the resist roller pair 72 in the paper transport direction, as long as it is positioned near the resist roller pair 72.

[0085] When the passing time exceeds a predetermined reference passing time, the control unit 110 drives the pressure adjusting unit 619 to change the pressure contact force between the pair of fixing rotors 61, thereby adjusting the discharge direction of the paper P that passes through the fixing nip FN and is discharged. In this embodiment, when the passing time exceeds the predetermined reference passing time, the control unit 110 determines that the image transfer position is misaligned, and drives the pressure adjusting unit 619 to press the pressure roller 612 toward the pressure pad 614, thereby increasing the pressure contact force between the pair of fixing rotors 61. As a result, the paper P that has passed through the fixing nip FN is transported closer to the pressure roller 612 (see FIG. 3).

[0086] Therefore, even if the image transfer position is shifted toward the leading edge of the paper P, it is possible to reduce the likelihood that the leading edge of one side of the paper P onto which the toner image (image) has been transferred will come into contact with and become wrapped around the fixing belt 611. This prevents paper P transported from the transfer nip TN from causing paper jams, bent edges, or other transport problems. Furthermore, even if the image transfer position is shifted, printing can be performed without reducing the printing speed, while suppressing the occurrence of image defects.

[0087] On the other hand, if the measured passage time has not exceeded the predetermined reference passage time, the control unit 110 determines that the image transfer position has not shifted. At this time, the control unit 110 drives the pressure adjustment unit 619 to return the pressure roller 612 from the pressing position to the normal position. This causes the pressure roller 612 to move away from the pressure pad 614, reducing the pressure contact force between the pair of fixing rotors 61. At this time, the paper P that has passed through the fixing nip FN is transported in a direction away from the pressure roller 612 compared to when the pressure roller 612 is in the pressing position.

[0088] During double-sided printing, if it is determined that the image transfer position on the first side has shifted, but a predetermined reference passing time has not yet elapsed during printing on the second side, the control unit 110 may drive the pressure adjustment unit 619 to move the pressure roller 612 to a position farther away from the pressure pad 614 than its normal position. This moves the pressure roller 612 away from the pressure pad 614, further reducing the pressure contact force between the pair of fixing rotors 61. At this time, the paper P that has passed through the fixing nip FN is transported in a direction closer to the fixing belt 611. This further reduces the likelihood that the leading edge of one side of the paper P, onto which the toner image (image) has been transferred, will come into contact with and become wrapped around the pressure roller 612 during printing on the second side.

[0089] Second Embodiment Next, a second embodiment of the present invention will be described. Figure 6 is a side cross-sectional view of a fixing unit 6 according to the second embodiment. For ease of explanation, the same parts as those in the first embodiment shown in Figures 1 to 5 above are denoted by the same reference numerals. In the second embodiment, the fixing unit 6 has a different configuration. The other parts are the same as those in the first embodiment.

[0090] In this embodiment, one of the pair of fixing rotors 71 is a fixing roller 713 that is rotatable in the transport direction of the paper P and has a fixing belt 711 disposed on its outer circumferential surface, and the fixing belt 711 rotates counterclockwise in Fig. 2. The other of the pair of fixing rotors 71 is a pressure roller 712 that is disposed opposite the fixing belt 711 and applies a rotational driving force to the fixing roller 713.

[0091] In this embodiment, the fixing belt 711 has a base layer (not shown), a fixing elastic layer (not shown), and a release layer (not shown). The fixing elastic layer is provided on the outer peripheral surface of the base layer. The release layer is provided on the outer peripheral surface of the fixing elastic layer. The base layer includes a ferromagnetic layer. The fixing elastic layer is made of, for example, silicone rubber. The release layer is made of, for example, a PFA tube.

[0092] Fixing roller 713 has fixing core metal 713b as a rotation shaft, and fixing elastic layer 713a. Fixing elastic layer 713a is disposed on the outer peripheral surface of fixing core metal 713b. Fixing roller 713 is supported so as to be rotatable around fixing core metal (core metal) 713b. Fixing core metal 713b is made of, for example, aluminum and constitutes the rotation shaft of fixing roller 713.

[0093] The fixing elastic layer 713a is provided on the outer peripheral surface of the fixing core metal 713b and is made of, for example, foamed silicone rubber.

[0094] The pressure roller 712 has a pressure core metal 712b as a rotation shaft, and a pressure elastic layer 712a. The pressure elastic layer 712a is disposed on the outer peripheral surface of the pressure core metal 712b. The pressure roller 712 is supported so as to be rotatable around the pressure core metal (core metal) 712b. The pressure core metal 712b is made of, for example, aluminum and constitutes the rotation shaft of the fixing roller 713.

[0095] The pressure elastic layer 712a is provided on the outer peripheral surface of the pressure core metal 712b and is made of, for example, foamed silicone rubber.

[0096] The fixing nip FN is formed by pressing the pressure roller 712 against the fixing roller 713. At this time, the fixing roller 713 has a lower hardness than the pressure roller 712, and a fixing nip FN with a concave shape is formed on the fixing roller 713 side. That is, the fixing belt 711 curves convexly in the direction away from the pressure roller 712 at the fixing nip FN.

[0097] The pressure adjusting unit 619, for example, rotatably supports both axial end portions of the pressure core metal 712b and can swingably move the pressure roller 712b toward and away from the fixing roller 713. The swinging movement of the pressure roller 712 is performed, for example, by driving a motor (not shown). By pressing the pressure roller 712 against the fixing roller 713, the pressure contact force between the pair of fixing rotors 71 can be increased. Furthermore, by moving the pressure roller 712 in a direction away from the fixing roller 713, the pressure contact force between the pair of fixing rotors 71 can be decreased.

[0098] When the pressure roller 712 is pressed toward the fixing roller 713 and positioned in the pressing position, the fixing elastic layer 713a of the fixing roller 713 is further deformed, and the fixing belt 711 is curved even more deeply in the direction away from the pressure roller 712 in a cross section perpendicular to the rotation axis of the pressure roller 712.

[0099] As a result, the downstream end of the fixing nip FN in the paper transport direction is tilted toward the pressure roller 712 in a cross section perpendicular to the rotation axis of the pressure roller 712. Therefore, the discharge direction of the paper P that is discharged after passing through the fixing nip FN is tilted toward the pressure roller 712 in a cross section perpendicular to the rotation axis of the pressure roller 712. As a result, the paper P that has passed through the fixing nip FN can be transported closer to the pressure roller 712.

[0100] On the other hand, when the force pressing pressure roller 712 toward fixing roller 713 is reduced and pressure roller 712 is returned to its normal position, the deformation of fixing roller 713 is restored. As a result, the end of fixing nip FN on the downstream side in the paper transport direction tilts away from pressure roller 712 in a cross section perpendicular to the rotation axis of pressure roller 712. At this time, paper P that passes through fixing nip FN and is discharged can be transported away from pressure roller 712 in a cross section perpendicular to the rotation axis of pressure roller 712.

[0101] If the passing time exceeds a predetermined reference passing time, the control unit 110 determines that the image transfer position is misaligned, and drives the pressure adjustment unit 619 to press the pressure roller 712 toward the fixing roller 713, thereby increasing the pressure contact force between the pair of fixing rotors 71. As a result, the paper P that has passed through the fixing nip FN is transported closer to the pressure roller 712.

[0102] Therefore, when the image transfer position is shifted toward the leading edge of the paper P, it is possible to reduce the possibility that one side of the paper P onto which the toner image (image) is transferred at the transfer nip TN will come into contact with and become wrapped around the fixing belt 711. This makes it possible to prevent paper jams, bent edges, and other transport problems caused by the paper P transported from the transfer nip TN.

[0103] On the other hand, if the measured passage time does not exceed the predetermined reference passage time, the control unit 110 determines that the image transfer position is not misaligned. At this time, the pressure adjustment unit 619 is driven to return the pressure roller 712 from the pressing position to the normal position. This causes the pressure roller 712 to move in a direction away from the fixing roller 713, reducing the pressure contact force between the pair of fixing rotors 71. At this time, the paper P that has passed through the fixing nip FN is transported in a direction away from the pressure roller 712 compared to when the pressure roller 712 is in the pressing position.

[0104] <Third embodiment> Next, a third embodiment of the present invention will be described. Figure 7 is a side cross-sectional view of a fixing unit 6 according to the third embodiment. For ease of explanation, the same parts as those in the first embodiment shown in Figures 1 to 5 above are denoted by the same reference numerals. In the second embodiment, the fixing unit 6 has a different configuration. The other parts are the same as those in the first embodiment.

[0105] In this embodiment, one of the pair of fixing rotors 91 is a fixing roller 913 that is rotatable in the transport direction of the paper P and has a fixing elastic layer 913a arranged on its outer circumferential surface, and the fixing roller 913 rotates counterclockwise in Fig. 2. The other of the pair of fixing rotors 91 is a pressure roller 912 that is arranged opposite the fixing roller 913, has a pressure elastic layer 912a arranged on its outer circumferential surface, and applies a rotational driving force to the fixing roller 913.

[0106] Specifically, the fixing roller 913 has a fixing core metal 913b and a fixing elastic layer 913a. The fixing core metal 913b is cylindrical and made of a metal material such as aluminum or iron. The fixing elastic layer 913a is made of, for example, silicone rubber and is provided on the outer circumferential surface of the fixing core metal 913b. The outer circumferential surface of the fixing elastic layer 913a is provided with, for example, a fluororesin layer (not shown). The fluororesin layer reduces sticking of printing paper.

[0107] The heating unit 918 is configured by a heater such as a halogen lamp, and is provided inside the fixing core metal 913b.

[0108] The pressure roller 912 has a pressure core 912b and a pressure elastic layer 912a. The pressure core 912b is cylindrical and made of a metal material such as aluminum or iron. The pressure elastic layer 912a is made of silicone rubber and is provided on the outer circumferential surface of the pressure core 912b. In addition, a fluororesin layer (not shown) is provided on the outer circumferential surface of the pressure elastic layer 912a. The fluororesin layer is made of a fluororesin such as PTFE or PTA.

[0109] The fixing nip FN is formed by the pressure roller 912 being pressed against the fixing roller 713 via the fixing belt 711. At this time, the pressure roller 912 has a lower hardness than the fixing roller 913, and the fixing nip FN is formed with a concave shape on the pressure roller 912 side. That is, the pressure elastic layer 912a is concave in the direction away from the fixing roller 913 at the fixing nip FN.

[0110] The pressure adjusting unit 619, for example, rotatably supports both axial end portions of the pressure core metal 912b and can swingably move the pressure roller 912 toward and away from the fixing roller 913. The swinging movement of the pressure roller 912 is performed, for example, by driving a motor (not shown). By pressing the pressure roller 912 against the fixing roller 913, the pressure contact force between the pair of fixing rotors 91 can be increased. Furthermore, by moving the pressure roller 912 in a direction away from the fixing roller 913, the pressure contact force between the pair of fixing rotors 91 can be decreased.

[0111] When pressure roller 912 is moved away from fixing roller 913 and placed in the weak pressing position, pressure elastic layer 912a is restored from its deformation. As a result, the depression in pressure elastic layer 912a becomes shallower in the direction approaching fixing roller 913 in a cross section perpendicular to the rotation axis of pressure roller 912.

[0112] As a result, the downstream end of the fixing nip FN in the paper transport direction is tilted away from the fixing roller 913 in a cross section perpendicular to the rotation axis of the pressure roller 912. Therefore, the discharge direction of the paper P that is discharged after passing through the fixing nip FN is tilted away from the fixing roller 913 in a cross section perpendicular to the rotation axis of the pressure roller 912. At this time, the paper P that has passed through the fixing nip FN can be transported closer to the pressure roller 912.

[0113] On the other hand, when the force pressing the pressure roller 912 toward the fixing roller 913 is increased and the pressure roller 912 is returned to its normal position, the depression in the pressure elastic layer 912a deepens in the direction away from the fixing roller 913. As a result, the end of the fixing nip FN on the downstream side in the paper transport direction is tilted toward the fixing roller 913 in a cross section perpendicular to the rotation axis of the pressure roller 912. At this time, the paper P that passes through the fixing nip FN and is discharged can be transported closer to the fixing roller 913 in a cross section perpendicular to the rotation axis of the pressure roller 912.

[0114] If the passing time exceeds a predetermined reference passing time, the control unit 110 determines that the image transfer position is misaligned, and drives the pressure adjustment unit 619 to move the pressure roller 912 in a direction away from the fixing roller 913, thereby reducing the pressure contact force between the pair of fixing rotors 91. As a result, the paper P that has passed through the fixing nip FN is conveyed closer to the pressure roller 912.

[0115] Therefore, when the image transfer position is shifted toward the leading edge of the paper P, it is possible to reduce the possibility that one side of the paper P onto which the toner image (image) has been transferred at the transfer nip TN will come into contact with and become wrapped around the fixing roller 913. This makes it possible to prevent paper jams, bent edges, and other transport problems caused by the paper P transported from the transfer nip TN.

[0116] On the other hand, if the measured passage time has not exceeded the predetermined reference passage time, the control unit 110 determines that the image transfer position has not shifted. At this time, the pressure adjustment unit 619 is driven to return the pressure roller 912 from the weak pressure position to the normal position. This causes the pressure roller 912 to move closer to the fixing roller 913, increasing the pressure contact force between the pair of fixing rotors 91. At this time, the paper P that has passed through the fixing nip FN is transported in a direction away from the pressure roller 912 compared to when the pressure roller 712 is in the weak pressure position.

[0117] Furthermore, the present invention is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present invention. For example, in the above-described embodiment, the control unit 110 determines that the image transfer position is misaligned when the passing time exceeds a predetermined reference passing time, but the present invention is not limited to this. For example, the control unit 110 may drive the pressure adjustment unit 619 to change the pressure contact force between the pair of fixing rotors when the passing time exceeds the reference passing time and the print rate of the leading edge region of the image is equal to or greater than a predetermined value. This reduces the driving of the pressure adjustment unit 619 and shortens the printing time when the leading edge of one side of the paper P is unlikely to come into contact with and wrap around the fixing belt 611.

[0118] Furthermore, when the passing time exceeds a predetermined reference passing time, the control unit 110 drives the pressure adjusting unit 619 to change the pressure contact force between the pair of fixing rotors, but the present invention is not limited to this. For example, when the passing time exceeds a predetermined reference passing time, the control unit 110 may reduce the transfer rate of the leading edge region of the image onto the paper P to suppress the amount of toner applied to the paper P. Alternatively, the passing speed of the paper P through the fixing nip FN may be increased to reduce the adhesion force of the toner to the paper P. In this way, by omitting the pressure adjusting unit 619, the fixing unit 6 can be simplified, reducing the manufacturing cost of the image forming apparatus 100, while also achieving the same effects as the above-described embodiment. [Explanation of symbols]

[0119] 1 Image reading unit 2 Printing Department 2A Main transport path 2B Reversing conveyance path 3 Paper feed section 4 Image forming unit 6 Fixing section 8 Reversal section 20 Paper transport path 20T via output tray 20T paper output tray 21 First conveying path 22 Second conveying path 29 Coil 30 Paper cassette 31 Pickup roller 32 Paper feed roller pair 40Bk, 40Y, 40C Mechanism 41 Photosensitive drum 42 Charging device 43 Developing device 44 Cleaning equipment 45 Exposure equipment 51 Intermediate transfer belt 52 Primary transfer roller 53 Secondary transfer roller 54 Drive roller 55 Driven roller 61 Fixing roller pair 71~77 Transport roller pair 78 Switching claw 91 Fixing Rotor 100 Image forming device 110 control section 111 CPU 112 Memory 200 Operation Panel 201 Touch Panel Display 202 Hard Key 611 Fixing belt 612 Pressure roller 612a Core metal 612b Elastic layer 613 Pressing member 614 Pressure pad 614a Sliding surface 614b Flat area 614c Arc-shaped part 615 Support member 618 Induction heating section (heating section) 618a Coil bobbin 618b Coil 618c Center Core 618d Arch Core 618e Side Core 619 Pressure adjustment unit 711 Fixing belt 712, 912 pressure roller 712a, 912a Pressurized elastic layer 712b, 912b pressure core 713, 913 Fuser roller 713a, 913a Fixing elastic layer 713b, 913b fixing core 918 Heating section FN Fixing Nip P paper P1 Branch position P2 merging position PN Paper feed nip PS1~PS7 Judgment Sensor TN Transfer Nip

Claims

1. a paper output tray into which paper is discharged; a printing unit that prints on paper; A judgment sensor that detects the arrival and passage of paper; a control unit that controls the printing unit, The printing unit an image forming unit including a transfer rotating body that forms the transfer nip, which conveys paper, forms an image, and transfers the formed image onto the paper; a pair of registration rollers that transport paper to the transfer nip; a fixing unit including a pair of fixing rotors that form a fixing nip, and that fixes the transferred image onto the paper by applying heat and pressure to the paper passing through the fixing nip; a paper transport path that passes through the registration roller pair, the transfer nip, and the fixing nip and reaches the paper discharge tray, The fixing unit is a heating unit that heats one of the pair of fixing rotors; a pressure adjusting unit that can change the pressure contact force between the pair of fixing rotors, The control unit measuring a time for the sheet to pass through the pair of registration rollers based on an output value of the determination sensor; When the passing time exceeds a predetermined reference passing time, the image forming apparatus drives the pressure adjusting unit to change the pressure contact force between the pair of fixing rotors, thereby adjusting the ejection direction of the paper that passes through the fixing nip and is ejected.

2. The control unit When the passing time exceeds the reference passing time and the printing rate of the leading edge region of the image is equal to or greater than a predetermined value, the image forming apparatus drives the pressure adjustment unit to change the pressure contact force between the pair of fixing rotors, thereby adjusting the ejection direction of the paper that passes through the fixing nip and is ejected.

3. One of the pair of fixing rotors is an endless fixing belt that can rotate along the paper transport direction, the other of the pair of fixing rotors is a pressure roller that is disposed opposite to the outer peripheral surface of the fixing belt and applies a rotational driving force to the fixing belt; the fixing belt is curved convexly in a direction away from the pressure roller at the fixing nip, The fixing unit is a pressing member that contacts the inside of the fixing belt and presses the fixing belt against the pressure roller; a pressing pad provided on a surface of the pressing member facing the fixing belt, the pressing pad having a sliding surface on which an inner peripheral surface of the fixing belt slides, the pressure roller is pressed against the pressure pad via the fixing belt to form the fixing nip; the pressure adjusting unit is capable of moving the pressure roller toward and away from the pressure pad, 2. The image forming apparatus according to claim 1, wherein when the passing time exceeds a predetermined reference passing time, the control unit drives the pressure adjustment unit to press the pressure roller toward the pressure pad, thereby increasing the pressure contact force between the pair of fixing rotors, and transports the paper that has passed through the fixing nip closer to the pressure roller.

4. one of the pair of fixing rotors is a fixing roller that is rotatable along the paper transport direction and has a fixing belt disposed on its outer circumferential surface; the other of the pair of fixing rotors is a pressure roller that is disposed opposite the fixing roller and applies a rotational driving force to the fixing roller; the fixing belt is curved convexly in a direction away from the pressure roller at the fixing nip, The fixing unit is The pressure roller is pressed against the fixing roller to form the fixing nip, the pressure adjusting unit is capable of moving the pressure roller so as to be able to approach and separate from the fixing roller, 2. The image forming apparatus according to claim 1, wherein when the passing time exceeds a predetermined reference passing time, the control unit drives the pressure adjustment unit to press the pressure roller toward the fixing roller, thereby increasing the pressure contact force between the pair of fixing rotors, and transports the paper that has passed through the fixing nip closer to the pressure roller.

5. one of the pair of fixing rotors is a fixing roller that is rotatable along the paper transport direction and has a fixing elastic layer disposed on its outer circumferential surface, the other of the pair of fixing rotors is a pressure roller that is disposed opposite the fixing roller, has a pressure elastic layer disposed on its outer peripheral surface, and applies a rotational driving force to the fixing roller; The fixing unit is The pressure roller is pressed against the fixing roller to form the fixing nip, the pressure elastic layer is recessed in a direction away from the fixing roller at the fixing nip, the pressure adjusting unit is capable of moving the pressure roller so as to be able to approach and separate from the fixing roller, 2. The image forming apparatus according to claim 1, wherein when the passing time exceeds a predetermined reference passing time, the control unit drives the pressure adjustment unit to move the pressure roller in a direction away from the fixing roller, thereby reducing the pressure contact force between the pair of fixing rotors, and transports the paper that has passed through the fixing nip closer to the pressure roller.

6. a paper output tray into which paper is discharged; a printing unit that prints on paper; A judgment sensor that detects the arrival and passage of paper; a control unit that controls the printing unit, The printing unit an image forming unit including a transfer rotating body that forms the transfer nip, which conveys paper, forms an image, and transfers the formed image onto the paper; a pair of registration rollers that transport paper to the transfer nip; a fixing unit including a pair of fixing rotors that form a fixing nip, and that fixes the transferred image onto the paper by applying heat and pressure to the paper passing through the fixing nip; a paper transport path that passes through the registration roller pair, the transfer nip, and the fixing nip and reaches the paper discharge tray, The fixing unit is a heating unit for heating one of the pair of fixing rotors, The control unit measuring a time for the sheet to pass through the pair of registration rollers based on an output value of the determination sensor; When the passing time exceeds a predetermined reference passing time, the image forming apparatus reduces the transfer rate of the leading edge region of the image onto the paper, thereby suppressing the amount of toner applied to the paper.

7. a paper output tray into which paper is discharged; a printing unit that prints on paper; A judgment sensor that detects the arrival and passage of paper; a control unit that controls the printing unit, The printing unit an image forming unit including a transfer rotating body that forms the transfer nip, which conveys paper, forms an image, and transfers the formed image onto the paper; a pair of registration rollers that transport paper to the transfer nip; a fixing unit including a pair of fixing rotors that form a fixing nip, and that fixes the transferred image onto the paper by applying heat and pressure to the paper passing through the fixing nip; a paper transport path that passes through the registration roller pair, the transfer nip, and the fixing nip and reaches the paper discharge tray, The fixing unit is a heating unit for heating one of the pair of fixing rotors, The control unit measuring a time for the sheet to pass through the pair of registration rollers based on an output value of the determination sensor; When the passing time exceeds a predetermined reference passing time, the image forming apparatus increases the passing speed of the paper through the fixing nip to reduce the adhesive force of the toner on the paper.

Images