Image forming apparatus

Abstract

To achieve both improvement in a sheet conveyance performance and improvement in image quality in forming an image on a sheet.SOLUTION: An image forming apparatus comprises: an image carrier; transfer means (130) that has a transfer nip unit (N2); fixing means (50) that has a fixing nip unit (N); an endless belt (11) that has air permeability; conveying means (10) that has stretching members (12, 12a, 12b, 12c) that rotatably stretch the belt and an air suction unit (15) that can apply sucking force for a peripheral surface of the belt by switching the sucking force between a first sucking force that can hold a sheet and second sucking force that can separate the sheet, and rotates the belt to convey the sheet; and control means. When a leading end of the sheet reaches the fixing nip part while the sheet is sandwiched at the transfer nip part, the control means executes first switching processing of switching the sucking force applied to the peripheral surface from the first sucking force to the second sucking force while rotating the rotation of the belt.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet. [Background technology]

[0002] Conventionally, electrophotographic image forming apparatuses include a transfer unit that transfers an image onto a sheet and a fixing unit that fixes the image transferred onto the sheet. Japanese Patent Application Laid-Open Publication No. 2012-83416 discloses a configuration in which a conveying unit that sucks and conveys a sheet on a belt between the transfer unit and the fixing unit in the sheet conveying direction is provided. Japanese Patent Application Laid-Open Publication No. 2014-44232 discloses a configuration in which a loop detecting unit is provided between the transfer unit and the fixing unit, and the sheet conveying speed of the fixing unit is controlled based on the detection result. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-83416 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-44232 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in recent years, in order to achieve high image quality and high productivity, the devices of the transfer unit and the fixing unit have become larger, and as a result, the distance conveyed by the conveying means between the transfer unit and the fixing unit has become longer. In this configuration, for example, when loop control is performed on a long sheet that is longer than the distance between the transfer unit and the fixing unit and is a thin sheet with a small basis weight, the sheet is sucked in the suction unit in the sheet conveying direction because of the sheet's low rigidity, and there is a risk that the sheet will lift up in the tension unit that tensions the belt, and the behavior of the sheet cannot be stabilized, and there is a risk that loop control will not be possible.

[0005] SUMMARY OF THE INVENTION An object of the present invention is to improve the sheet conveyance performance and prevent a decrease in image quality in an image forming apparatus that loop-controls a long sheet between a transfer unit and a fixing unit. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems, one aspect of the present invention is an image forming apparatus that forms an image on a sheet, the image forming apparatus comprising: an image carrier that carries a toner image; a transfer unit that has a transfer nip portion that sandwiches and transports the sheet and transfers the toner image carried on the image carrier to the sheet sandwiched in the transfer nip portion; a fixing unit that has a fixing nip portion that sandwiches and transports the sheet and fixes the toner image transferred to the sheet by the transfer unit onto the sheet; an endless belt that is breathable; a tension member that rotatably stretches the belt; and a tension member that sucks air through the belt and switches the suction force of the circumferential surface of the belt between a first suction force that can hold the sheet on the circumferential surface and a second suction force that can separate the sheet from the circumferential surface. The sheet conveying device is provided with an air suction section capable of applying an air suction force, and a conveying means for rotating the belt to convey the sheet from the transfer means to the fixing means, and a control means for controlling the sheet conveying speed by the fixing means and the suction force of the conveying means and the rotation speed of the belt, wherein the control means conveys the sheet from the transfer means to the fixing means by applying the first suction force to the circumferential surface, and when the leading edge of the sheet reaches the fixing nip portion while being sandwiched in the transfer nip portion, performs a first switching process to rotate the belt and switch the suction force applied to the circumferential surface from the first suction force to the second suction force. [Effects of the Invention]

[0007] According to the present invention, in an image forming apparatus that loop-controls a long sheet between a transfer unit and a fixing unit, it is possible to improve the sheet conveyance performance and prevent a decrease in image quality. [Brief explanation of the drawings]

[0008] [Figure 1]FIG. 1 is a schematic diagram illustrating the configuration of an image forming apparatus according to a first embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing a secondary transfer unit, a belt transport unit, and a fixing unit according to the first embodiment. [Figure 3] FIG. 2 is a control block diagram of the image forming apparatus according to the first embodiment. [Figure 4] 6 is a flowchart showing the flow of an operation for conveying a sheet by the belt conveying unit according to the first embodiment. [Figure 5] FIG. 10 is a cross-sectional view showing a secondary transfer portion, a belt transport unit, and a fixing portion according to a second embodiment. [Figure 6] FIG. 10 is a control block diagram of an image forming apparatus according to a second embodiment. [Figure 7] 10 is a flowchart showing the flow of an operation for conveying a sheet by a belt conveying unit according to a second embodiment. [Figure 8] 10 is a flowchart showing the flow of an operation for conveying a sheet by a belt conveying unit according to a third embodiment. [Figure 9] 10 is a flowchart showing the flow of an operation for conveying a sheet by a belt conveying unit according to a fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, exemplary embodiments of the present invention will be described with reference to the drawings. [Example]

[0010] FIG. 1 is a schematic diagram of an image forming apparatus 100 according to a first embodiment. First, the configuration of the image forming apparatus 100 will be described with reference to FIG. 1. The image forming apparatus 100 includes a feeding unit 110 that feeds sheets, an image forming unit 920 that forms a toner image to be fixed on the sheet fed by the feeding unit 110, and a belt conveying unit 904 that conveys the sheet onto which the toner image has been transferred to the fixing unit 50. The image forming apparatus 100 further includes a post-conveying unit 903 that conveys the sheet onto which the toner image has been fixed by the fixing unit 50. The feeding unit 110 includes a sheet cassette 111 that stores sheets, a pickup roller 112 that picks up sheets from the sheet cassette 111, and a separation device 113 that separates and feeds the sheets picked up by the pickup roller 112. The feeding unit 110 further includes a feed roller 114 that conveys the sheets within a feeding path 901 along which the sheets separated and fed by the separation device 113 are conveyed, and a registration roller 115. The image forming unit 920 is a so-called tandem type image forming means in which electrophotographic image forming stations 200Y, 200M, 200C, and 200K that form toner images of Y (yellow), M (magenta), C (cyan), and K (black) are arranged in series.

[0011] The image forming stations 200Y, 200M, 200C, and 200K share the same configuration except for the toner color. Therefore, the configuration of image forming station 200Y will be described as an example, and descriptions of the configurations of image forming stations 200M, 200C, and 200K will be omitted. Note that in FIG. 1, the components of image forming station 200Y are designated with a "Y," the components of image forming station 200M with an "M," the components of image forming station 200C with a "C," and the components of image forming station 200K with a "K" at the end of their reference numerals. Image forming station 200Y includes a photosensitive drum 120Y, a primary charging device 121Y, an exposure device 122Y, and a developing device 123Y. Image forming unit 920 includes an intermediate transfer belt 125, an example of an image carrier, on which the toner images visualized by image forming stations 200Y, 200M, 200C, and 200K are carried. The intermediate transfer belt 125 is supported in a state in which it is stretched over a drive roller 126, a tension roller 127, and an inner transfer roller 128, and is rotated in the direction of arrow R2 in FIG.

[0012] The secondary transfer roller 131 presses against the intermediate transfer belt 125, which is supported from the inside of the intermediate transfer belt 125 by the inner transfer roller 128, forming a secondary transfer nip N2 between the intermediate transfer roller 131 and the intermediate transfer belt 125. The secondary transfer roller 131, the intermediate transfer belt 125, and the inner transfer roller 128 constitute a secondary transfer unit 130, which serves as the transfer means of this embodiment. A cleaning device 128' rubs a cleaning web against the intermediate transfer belt 125 to remove residual toner and paper dust remaining on the surface of the intermediate transfer belt 125 after it has passed through the secondary transfer nip N2. The fixing unit 50, located downstream of the secondary transfer unit 130 in the sheet conveyance direction FD, is a fixing unit that fixes the toner image to the sheet using heat and pressure. The fixing unit 50 includes a heating roller 52 equipped with a heater therein and an opposing roller 53 that is arranged to be able to abut against the heating roller 52 and forms a fixing nip N together with the heating roller 52. Fixing unit 50 also includes a heating roller temperature sensor 70 that detects the surface temperature of heating roller 52, and a pressure roller temperature sensor 71 that detects the surface temperature of opposing roller 53. Heating roller temperature sensor 70 and pressure roller temperature sensor 71 are provided to maintain the surface temperatures of heating roller 52 and opposing roller 53 at appropriate temperatures, respectively.

[0013] In the sheet conveying direction FD, a belt conveying unit 904 is disposed between the secondary transfer unit 130 and the fixing unit 50. The belt conveying unit 904 is configured by a first belt conveying unit 10 disposed on the upstream side of the sheet conveying direction FD, and a second belt conveying unit 20 disposed on the downstream side. The configuration of the belt conveying unit 904 will be described later.

[0014] The rear conveying unit 903 includes discharge rollers 911 that discharge the sheet discharged from the fixing unit 50 to the outside of the image forming apparatus 100. The rear conveying unit 903 further includes reversing rollers 912 that reverse and convey the sheet, and a double-sided conveying path 913 that conveys the sheet reversed by the reversing rollers 912 and merges it with the feeding path 901.

[0015] Next, a series of steps for forming an image on a sheet in the image forming apparatus 100 will be described. First, based on an image formation job input to the image forming apparatus 100, the exposure device 122Y exposes the photosensitive drum 120Y to light to form an electrostatic latent image on the surface of the photosensitive drum 120Y. The electrostatic latent image on the photosensitive drum 120Y is developed by the development device 123Y and made visible as a toner image. The toner image carried on the surface of the photosensitive drum 120Y is primarily transferred onto the intermediate transfer belt 125 by the primary transfer device 124Y. The toner images (multiple) carried on the surfaces of the photosensitive drums 120Y, 120M, 120C, and 120K are sequentially transferred onto the intermediate transfer belt 125 so as to overlap one another, forming a full-color toner image (added to restore a single toner image). The toner images primarily transferred onto the intermediate transfer belt 125 are then secondarily transferred onto the sheet S fed from the feed unit 110 at the secondary transfer nip N2, which serves as the transfer nip in this embodiment. The intermediate transfer belt 125 is driven to rotate by a drive roller 126 that rotates at a constant speed, and its circumferential velocity (rotational speed) is maintained at a constant transfer speed. Therefore, the sheet transport speed at the secondary transfer nip N2 is the circumferential velocity of the intermediate transfer belt 125. Hereinafter, the sheet transport speed at the secondary transfer unit 130 will be referred to as the "transfer speed VT." The transfer speed VT is the sheet transport speed when the toner image is transferred at the secondary transfer unit 130.

[0016] The registration rollers 115 receive the sheet S while stopping rotation and keep it waiting, and then send the sheet S toward the secondary transfer nip N2 in time with the toner image on the intermediate transfer belt 125. The sheet S carrying the toner image transferred at the secondary transfer nip N2 is transported from the secondary transfer nip N2 to the fixing unit 50 by the belt transport unit 904. In the fixing unit 50, the sheet S is sandwiched at the fixing nip N and heat and pressure are applied to the unfixed toner image to fix the toner image to the sheet. The sheet S sent out from the fixing unit 50 is discharged outside the image forming apparatus 100 by the discharge rollers 911.

[0017] When forming images on both sides of a sheet, the sheet sent out from the fixing unit 50 is conveyed to the reversing rollers 912, and after being reversed by the reversing rollers 912, is conveyed toward the double-sided conveying path 913. Then, the sheet is conveyed again to the feeding path 901 via the double-sided conveying path 913, and a toner image is formed on the second side (back side) of the sheet in the same manner as on the first side (front side).

[0018] Next, the detailed configuration of the belt transport unit 904 and its surroundings will be described with reference to Fig. 2. Fig. 2 is a cross-sectional view showing the secondary transfer section 130, the belt transport unit 904, and the fixing section 50. The belt transport unit 904, which serves as a transport means in this embodiment, includes a first belt transport section 10 and a second belt transport section 20. The first belt transport section 10 is disposed downstream of the secondary transfer nip section N2 in the sheet transport direction FD, and the second belt transport section 20 is disposed downstream of the first belt transport section 10 and upstream of the fixing nip section N.

[0019] In the sheet conveyance direction FD, a transfer guide 951 is provided between the belt conveyance unit 904 and the secondary transfer nip N2 to guide the sheet conveyed from the secondary transfer nip N2 toward the belt conveyance unit 904. In addition, a pre-fixing guide 952 is provided between the belt conveyance unit 904 and the fixing nip N in the sheet conveyance direction FD to guide the sheet conveyed by the belt conveyance unit 904 to the fixing nip N. As shown in FIG. 2, the first belt conveyance unit 10 is positioned lower than the secondary transfer nip N2 when viewed in the sheet width direction perpendicular to the sheet conveyance direction FD. In addition, the second belt conveyance unit 20 is positioned lower than the fixing nip N when viewed in the sheet width direction perpendicular to the sheet conveyance direction FD. With this configuration, the leading edge of the sheet that has passed through the secondary transfer nip N2 is conveyed along the transfer guide 951 toward the first belt conveyance unit 10.

[0020] The second belt conveying unit 20 conveys the sheet along the sheet conveying surface 21a toward the fixing unit 50. An imaginary line 21a' extending from the sheet conveying surface 21a downstream in the sheet conveying direction FD intersects with the nip line N' of the fixing nip N downstream of the fixing nip N in the sheet conveying direction FD. The nip line N' of the fixing nip N refers to a tangent line of the fixing nip N that contacts the heating roller 52 and the opposing roller 53. With this configuration, the sheet conveyed through the second belt conveying unit 20 is conveyed in a direction that intersects with the nip line N' from below to above in FIG. 2. Furthermore, a pre-fixing guide 952 is provided between the second belt conveying unit 20 and the fixing unit 50 in the sheet conveying direction FD. The pre-fixing guide 952 serves as a guide member in this embodiment. The pre-fixing guide 952 has a guide surface 952a that guides the leading edge of the sheet conveyed along the sheet conveying surface 21a toward the fixing nip N. When viewed in the width direction perpendicular to the sheet conveying direction FD, the guide surface 952a of the pre-fixing guide 952 is downstream of the second belt conveying unit 20 and intersects with the imaginary line 21a' of the sheet conveying surface 21a upstream of the fixing nip N. With this configuration, the sheet conveyed through the second belt conveying unit 20 is guided by the pre-fixing guide 952 to the fixing nip N in a state where the sheet intersects with the nip line N' from below to above in FIG.

[0021] The nip line N' of the fixing nip N exists on a plane formed by tangents to the fixing nip N that are in contact with the heating roller 52 and the opposing roller 53. In other words, the sheet is transported to the fixing nip N in a state where it crosses the nip line N' from below to above in FIG. 2, which makes it possible to prevent the heating roller 52 from coming into contact with unfixed toner on the sheet.

[0022] The first belt conveying unit 10 includes a first conveyor belt 11 serving as a first belt in this embodiment, a first drive roller 12 that rotatably stretches the first conveyor belt 11, and driven rollers 12a, 12b, and 12c. The first tensioning members in this embodiment are the first drive roller 12 and the driven rollers 12a, 12b, and 12c. The first belt conveying unit 10 also includes a first drive motor 14 that rotates the first drive roller 12 to rotate the first conveyor belt 11. The first conveyor belt 11 is an endless belt formed with numerous holes, and is a breathable member that allows air to pass between the inside and outside of the circumferential surface of the first conveyor belt 11 through the holes. A first suction fan 15 is disposed on the inside of the circumferential surface of the first conveyor belt 11 to attract sheets to the circumferential surface of the first conveyor belt 11. The first suction fan 15 can apply suction force to the circumferential surface of the first conveyor belt 11 by sucking air from the outside to the inside of the circumferential surface of the first conveyor belt 11 through a large number of holes formed in the first conveyor belt 11, thereby conveying a sheet onto the circumferential surface of the first conveyor belt 11. In this embodiment, the first suction fan 15 serves as the first air suction unit. Of the suction forces applied to the circumferential surface of the first conveyor belt 11 by driving the first suction fan 15, a suction force capable of holding a sheet on the circumferential surface of the first conveyor belt 11 is referred to as the first suction force in this embodiment. A suction force capable of separating a sheet from the circumferential surface of the first conveyor belt 11 is referred to as the second suction force in this embodiment. The suction force capable of holding a sheet on the circumferential surface of the first conveyor belt 11 is, for example, a suction force capable of attracting a sheet that is away from the circumferential surface of the first conveyor belt 11 toward the circumferential surface. The suction force capable of separating a sheet from the circumferential surface of the first conveyor belt 11 is, for example, a suction force capable of allowing a sheet attracted to the circumferential surface of the first conveyor belt 11 to move freely away from the circumferential surface. When viewed in the width direction perpendicular to the sheet conveying direction FD, the sheet that has passed through the secondary transfer nip portion N2 is conveyed to the upper surface of the first conveyor belt 11. As a result, the sheet is conveyed while being attracted to the upper surface of the first conveyor belt 11 by the suction force applied to the circumferential surface of the first conveyor belt 11 by the first suction fan 15. In this embodiment, the first drive motor 14 is driven so that the sheet conveying speed V1 by the first conveyor belt 11 is slightly faster than the transfer speed VT.This makes it possible to prevent buckling of the sheet due to the speed difference between the secondary transfer nip portion N2 and the first conveyor belt 11. Note that the conveying speed V1 of the sheet by the first conveyor belt 11 refers to the peripheral speed of the first conveyor belt 11.

[0023] The second belt conveying unit 20 includes a second conveyor belt 21 serving as the second belt of this embodiment, a second drive roller 22 that rotatably stretches the second conveyor belt 21, and driven rollers 22a, 22b, and 22c. The second tensioning members of this embodiment are the second drive roller 22 and the driven rollers 22a, 22b, and 22c. The second belt conveying unit 20 also includes a second drive motor 24 that rotates the second drive roller 22 to rotate the second conveyor belt 21. The second conveyor belt 21 is an endless belt formed with numerous holes and is a breathable member that allows air to pass between the inside and outside of the circumferential surface of the second conveyor belt 21 through the holes. A second suction fan 25 is disposed on the inside of the circumferential surface of the second conveyor belt 21 to attract the sheet to the circumferential surface of the second conveyor belt 21. The center of the second suction fan 25 may be located downstream of the center of the second conveyor belt 21 in the sheet conveyance direction FD. In this manner, the sheet can be conveyed to the fixing nip portion N while being brought close to the sheet conveying surface 21a. The second suction fan 25 can apply suction force for conveying the sheet to the circumferential surface of the second conveying belt 21 by sucking air from the outside to the inside of the circumferential surface of the second conveying belt 21 through a large number of holes formed in the second conveying belt 21. The second suction fan 25 serves as the second air suction unit in this embodiment. Of the suction forces applied to the circumferential surface of the second conveying belt 21 by driving the second suction fan 25, a suction force capable of holding the sheet on the circumferential surface of the second conveying belt 21 is referred to as a third suction force in this embodiment. A suction force capable of separating the sheet from the circumferential surface of the second conveying belt 21 is referred to as a fourth suction force in this embodiment. The suction force capable of holding the sheet on the circumferential surface of the second conveying belt 21 is, for example, a suction force capable of attracting a sheet that is away from the circumferential surface of the second conveying belt 21 to the circumferential surface. Furthermore, the suction force that can separate a sheet from the circumferential surface of the second conveyor belt 21 is, for example, a suction force that can separate a sheet that has been attracted to the circumferential surface of the second conveyor belt 21 from the circumferential surface and move freely. Note that the suction force that the first suction fan 15 applies to the circumferential surface of the first conveyor belt 11 and the suction force that the second suction fan 25 applies to the circumferential surface of the second conveyor belt 21 may be the same.When viewed in the width direction perpendicular to the sheet conveyance direction FD, the sheet that has passed through the first conveyor belt 11 is conveyed to the upper surface of the second conveyor belt 21. As a result, the sheet is conveyed while being attracted to the upper surface of the second conveyor belt 21 by the suction force applied to the circumferential surface of the second conveyor belt 21 by the second suction fan 25. In this embodiment, the second drive motor 24 drives the second conveyor belt 21 so that the sheet conveyance speed V2 by the second conveyor belt 21 is slightly faster than the sheet conveyance speed V1 by the first conveyor belt 11. In this way, buckling of the sheet can be prevented due to the speed difference between the first conveyor belt 11 and the second conveyor belt 21. However, if the first conveyor belt 11, first drive roller 12, and driven rollers 12a, 12b, and 12c that make up the first belt conveying section 10, and the second conveyor belt 21, second drive roller 22, and driven rollers 22a, 22b, and 22c that make up the second belt conveying section 20 are made of the same parts, then the relative speed variation due to part tolerances can be almost ignored, and the sheet conveying speed V1 by the first conveyor belt 11 and the sheet conveying speed V2 by the second conveyor belt 21 may be the same. Note that the sheet conveying speed V2 by the second conveyor belt 21 refers to the peripheral speed of the second conveyor belt 21.

[0024] Furthermore, a sheet detection sensor 116 that detects a sheet is provided between the registration roller 115 and the secondary transfer nip portion N2 in the sheet conveyance direction FD. The sheet detection sensor 116 detects the presence or absence of a sheet at a detection position P1 between the registration roller 115 and the secondary transfer nip portion N2 in the sheet conveyance direction FD. A signal output by the sheet detection sensor 116 is sent to the control unit 170 (see FIG. 3).

[0025] The sheet is transported from the second transport belt 21 to the fixing unit 50. In the fixing unit 50, the heating roller 52 is rotated by a heating roller drive motor 54 (see FIG. 3), such as a DC brushless motor. The sheet transport speed in the fixing nip N is variable. The sheet transport speed in the fixing nip N is the peripheral speed of the heating roller 52. Hereinafter, the transport speed in the fixing unit 50, i.e., the sheet transport speed in the fixing nip N, is referred to as the "fixing speed VF." Hereinafter, the fixing speed refers to the sheet transport speed when a toner image is fixed to the sheet in the fixing unit 50. In other words, in the fixing nip N formed between the heating roller 52 and the counter roller 53, the toner image is fixed while the sheet is transported at the fixing speed VF. In this embodiment, the sheet is transported at a fixing speed VF faster than the transfer speed VT. This prevents the formation of a convex loop on the sheet above the belt transport unit 904.

[0026] In this embodiment, the distance L1 between the secondary transfer nip N2 and the fixing nip N is set to 19 inches (483 mm) or more. If the length of a sheet in the conveying direction FD is 19 inches or less, the sheet is conveyed without being sandwiched between the secondary transfer nip N2 and the fixing nip N. In addition, the length from the second suction fan 25 to the fixing nip N in the sheet conveying direction FD is set to a distance that allows a sheet whose length in the sheet conveying direction FD is shorter than a predetermined length to be conveyed. Here, a sheet shorter than the predetermined length is, for example, a sheet whose length in the sheet conveying direction FD is approximately 148 mm, which is the shortest sheet in the conveying direction FD that can be used in the image forming apparatus 100.

[0027] Furthermore, in the sheet conveying direction FD, the first conveying belt 11 and the second conveying belt 21 have the same length (length B). In this embodiment, the first conveying belt 11 and the second conveying belt 21 have the same configuration and share common parts, but the lengths of the first conveying belt 11 and the second conveying belt 21 may be different. For example, the length of the first conveying belt 11 in the conveying direction FD may be set to 3 / 10 of the distance L1 between the secondary transfer nip N2 and the fixing nip N. Furthermore, the length of the second conveying belt 21 in the conveying direction FD may be set to 1 / 2 of the distance L1 between the secondary transfer nip N2 and the fixing nip N.

[0028] Next, a control configuration for conveying a sheet by the belt conveying unit 904 in the image forming apparatus 100 of this embodiment will be described with reference to FIG. 3. FIG. 3 is a block diagram showing the control configuration of the image forming apparatus 100 of this embodiment. The control unit 170, which serves as a control means in this embodiment, includes an arithmetic processing unit including a CPU 171 and a memory 172, an I / O port 173, and a communication interface 174 as a circuit for communicating data with external devices. In the control unit 170, the CPU 171 loads multiple programs stored in the memory 172, and the CPU 171 executes the loaded programs to control the operation of the image forming apparatus 100. The control unit 170 controls the feeding unit 110, the image forming unit 920, and the like in accordance with an image formation job sent from an external device. An operation unit 210 is connected to the control unit 170, and information regarding the sheet type, such as the sheet basis weight, size, whether plain paper or coated paper, is sent from the operation unit 210. Note that information regarding the sheet type may also be included in the information sent as an image formation job from an external device. Here, coated paper refers to a sheet whose surface is coated with a resin. Furthermore, the control unit 170 receives signals output from the sheet detection sensor 116, the heating roller temperature sensor 70, and the pressure roller temperature sensor 71. Based on the received signals, the control unit 170 controls the operation of the first drive motor 14, the second drive motor 24, the first suction fan 15, the second suction fan 25, the heating roller drive motor 54, the temperature adjustment unit 55, the drive roller drive motor 129, etc.

[0029] Next, the control flow of the belt conveying unit 904 in the image forming apparatus 100 of this embodiment will be described with reference to FIG. 4. FIG. 4 is a flowchart showing the operation flow of conveying a sheet by the belt conveying unit 904 of this embodiment. This flow starts when information about the size and basis weight of the sheet in an image formation job is input from the operation unit 210 of the image forming apparatus 100, or when an image formation job is submitted to the image forming apparatus 100 from an external device. This flow is also executed mainly by the control unit 170. When an image formation job starts, the first suction fan 15 of the first belt conveying unit 10 and the second suction fan 25 of the second belt conveying unit 20 are driven (S01).

[0030] For example, if the operation of the first suction fan 15 is OFF before the start of an image forming job, the suction force applied to the circumferential surface of the first conveyor belt 11 is sufficient to separate a sheet from the first conveyor belt 11. When the first suction fan 15 is driven and turned ON, an airflow is generated from the outside to the inside of the circumferential surface of the first conveyor belt 11, thereby applying a suction force sufficient to hold a sheet on the circumferential surface of the first conveyor belt 11. Also, for example, if the operation of the second suction fan 25 is OFF before the start of an image forming job, the suction force applied to the circumferential surface of the second conveyor belt 21 is sufficient to separate a sheet from the second conveyor belt 21. When the second suction fan 25 is driven and turned ON, an airflow is generated from the outside to the inside of the circumferential surface of the second conveyor belt 21, thereby applying a suction force sufficient to hold a sheet on the circumferential surface of the second conveyor belt 21. The OFF state of the first suction fan 15 does not necessarily mean a state in which the operation of the first suction fan 15 is stopped. That is, in this embodiment, when a suction force weaker than that capable of holding a sheet, for example, a suction force strong enough to separate a sheet, is applied to the circumferential surface of the first conveyor belt 11, the first suction fan 15 is in the OFF state. Also, the OFF state of the second suction fan 15 is not limited to a state in which the operation of the second suction fan 25 is stopped. That is, in this embodiment, when a suction force weaker than that capable of holding a sheet, for example, a suction force strong enough to separate a sheet, is applied to the circumferential surface of the second conveyor belt 21, the second suction fan 25 is in the OFF state.

[0031] Next, the control unit 170 acquires information about the length of the sheet in the conveying direction FD from information included in the image forming job, and determines whether it is longer than the distance L1 between the secondary transfer nip N2 and the fixing nip N (S02). If the length of the sheet in the conveying direction FD is shorter than the distance L1 in the conveying direction between the secondary transfer nip N2 and the fixing nip N (S02 / N), an image is formed on the sheet and the sheet is discharged outside the image forming apparatus 100. Then, if the image forming job has not ended (S09 / N), the control unit 170 returns to S02, and if the image forming job has ended (S09 / Y), the control unit 170 ends this flow.

[0032] If the length of the sheet in the conveying direction FD is longer than the distance L1 in the conveying direction between the secondary transfer nip N2 and the fixing nip N (S02 / Y), the control unit 170 starts conveying the sheet and then waits until the leading edge of the sheet is detected by the sheet detection sensor 116 (S03). When the leading edge of the sheet reaches the detection position P1 (see FIG. 2) of the sheet detection sensor 116 (S03 / Y), the control unit 170 causes the timer 175 to start measuring the elapsed time since the leading edge of the sheet reached the detection position P1 (S04). Then, the control unit 170 determines whether the leading edge of the sheet has reached the fixing nip N based on the measurement value of the timer 175 (S05). Specifically, the control unit 170 starts measuring the elapsed time with the timer 175 from the timing (S03 / Y) when the control unit 170 receives a signal from the sheet detection sensor 116 indicating that the sheet has reached the detection position P1. Then, the control unit 170 determines whether a first time period required for the leading edge of the sheet to reach the fixing nip N from the detection position P1 has elapsed based on the measurement value of the timer 175. If the measurement value of the timer 175 indicates that the first time period has elapsed, the control unit 170 determines that the leading edge of the sheet has reached the fixing nip N (S05 / Y). Note that if the measurement value of the timer 175 indicates that the first time period has not elapsed (S05 / N), the control unit 170 waits until the first time period has elapsed. The predetermined time period in this embodiment corresponds to the first time period.

[0033] When the leading edge of the sheet reaches the fixing nip N, the control unit 170 switches the first suction fan 15 from ON to OFF and the second suction fan 25 from ON to OFF (S06). That is, when the leading edge of the sheet reaches the fixing nip N while being sandwiched in the secondary transfer nip N2, the suction force applied to the circumferential surface of the first conveyor belt 11 is smaller than that before the leading edge of the sheet reaches the fixing nip N. Also, when the leading edge of the sheet reaches the fixing nip N while being sandwiched in the secondary transfer nip N2, the suction force applied to the circumferential surface of the second conveyor belt 21 is smaller than that before the leading edge of the sheet reaches the fixing nip N. The first switching process in this embodiment is a process of switching the first suction fan 15 of the first belt conveyor unit 10 from ON to OFF. That is, the first switching process can be performed when the length of the sheet in the conveyance direction FD is longer than the distance L1 in the conveyance direction between the secondary transfer nip N2 and the fixing nip N.

[0034] The control unit 170 then determines whether the trailing edge of the sheet has passed through the secondary transfer nip N2 based on the measurement value of the timer 175 (S07). Specifically, the control unit 170 starts measuring the timer 175 from the timing (S03 / Y) when the control unit 170 receives a signal from the sheet detection sensor 116 indicating that the sheet has reached the detection position P1 (S04). The control unit 170 then determines whether a second time period required for the trailing edge of the sheet to pass through the secondary transfer nip N2 has elapsed based on the measurement value of the timer 175. If the measurement value of the timer 175 has elapsed the second time period, the control unit 170 determines that the trailing edge of the sheet has passed through the secondary transfer nip N2 (S07 / Y). Note that if the measurement value of the timer 175 has not elapsed the second time period (S07 / N), the control unit 170 waits until the second time period has elapsed. Whether the trailing edge of the sheet has passed through the secondary transfer nip N2 may be determined based on the timing when the signal from the sheet detection sensor 116 changes from an ON state to an OFF state.

[0035] When it is determined that the trailing edge of the sheet has passed through the secondary transfer nip N2, the control unit 170 turns on the first suction fan 15 and the second suction fan 25 (S08). This provides the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 with an attraction force capable of holding the sheet. The second switching process in this embodiment is a process of turning on the first suction fan 15 of the first belt conveyor unit 10 from the off state. Then, if the image formation job has not ended (S09 / N), the process returns to S02, and if the image formation job has ended (S09 / Y), the process ends this flow.

[0036] In this embodiment, the sheet is conveyed from the secondary transfer unit 130 toward the fixing unit 50 so that the fixing speed VF is greater than the transfer speed VT. That is, when the leading edge of the sheet reaches the fixing nip N N while being sandwiched in the secondary transfer nip N2, the sheet is gradually pulled downstream in the sheet conveyance direction FD by the fixing nip N. At this time, the suction force applied to the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 is reduced to prevent abrupt sheet displacement due to the suction force of the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21. In this way, even when conveying a long sheet, whose length in the sheet conveyance direction FD is longer than the distance L1 between the secondary transfer nip N2 and the fixing nip N in the conveyance direction, abrupt sheet displacement can be prevented. In this embodiment, preventing abrupt sheet displacement can prevent transfer failures and sheet conveyance failures at the secondary transfer unit 130, thereby achieving both improved sheet conveyance performance and improved image quality. [Example]

[0037] In Example 1, the sheet was transported from the secondary transfer unit 130 to the fixing unit 50 with the fixing speed VF greater than the transfer speed VT. However, the relationship between the fixing speed VF and the transfer speed VT may vary depending on the basis weight and material of the sheet. Furthermore, for a so-called long sheet, i.e., a sheet having a length in the sheet transport direction FD of 1000 mm or more, if the fixing speed VF is faster than the transfer speed VT, the fixing unit 50 will pull the sheet. The fixing unit 50 is designed to exert a greater clamping force on the sheet than the clamping force on the sheet of the secondary transfer unit 130 in order to fix the toner to the sheet. Therefore, a sheet passing through the secondary transfer unit 130 may be pulled by the fixing unit 50, which may result in a transfer failure of the toner image carried on the intermediate transfer belt 125. On the other hand, if the transfer speed VT is faster than the fixing speed VF, the leading edge of the sheet may strike the fixing nip N, causing the sheet to be pushed back, potentially resulting in a transfer failure of the toner image carried on the intermediate transfer belt 125. Furthermore, if the transfer speed VT is faster than the fixing speed VF, a slack (loop) is formed in the sheet in the sheet conveyance direction FD between the secondary transfer unit 130 and the fixing unit 50. Depending on the amount of loop of the sheet between the secondary transfer unit 130 and the fixing unit 50, there is a concern that damage to the sheet, conveyance problems, and image defects may occur.

[0038] In contrast to this, in this embodiment, a loop detection unit is disposed to detect the amount of loop of the sheet at a detection position in the sheet conveyance direction FD between the secondary transfer unit 130 and the fixing unit 50. This makes it possible to control the amount of loop of the sheet between the secondary transfer unit 130 and the fixing unit 50.

[0039] FIG. 5 is a cross-sectional view showing the secondary transfer unit 130, the belt conveying unit 904, and the fixing unit 50 in the image forming apparatus 100 of this embodiment. The configuration of the image forming apparatus 100 is the same as that of the first embodiment (see FIG. 1). In FIG. 5, the same components as those in the first embodiment are denoted by the same reference numerals, and redundant description will be omitted. As shown in FIG. 5, the loop detection unit 16 is disposed downstream of the first suction fan 15 of the first belt conveying unit 10 in the sheet conveying direction FD and at the center of the belt conveying unit 904 in the direction perpendicular to the sheet conveying direction FD (width direction). The detection position PL of the loop detection unit 16 in this embodiment is disposed at a position where the formation of a loop in the sheet is most visible, for example, slightly downstream of the first suction fan 15 in the sheet conveying direction FD. However, the detection position PL of the loop detection unit 16 may be disposed anywhere between the secondary transfer nip N2 and the fixing nip N in the sheet conveying direction FD. In this embodiment, the detection position PL of the loop detection means 16 is arranged in the sheet conveyance direction FD at a position overlapping with the belt conveyance unit 904 when viewed in the width direction perpendicular to the conveyance direction FD. This makes it possible to accurately detect the loop amount of the entire sheet in the conveyance direction FD.

[0040] The loop detection means 16 has a loop detection flag 161 that protrudes from the conveyance surface 11a of the first conveyor belt 11, contacts the sheet S conveyed by the belt conveyance unit 904, and swings in accordance with the amount of loop of the sheet S. The position where the loop detection flag 161 and the sheet S conveyed by the belt conveyance unit 904 come into contact is an example of a detection position PL of the loop detection means 16. The loop detection flag 161 as a flag member in this embodiment swings in accordance with the height of the sheet S from the conveyance surface 11a. The loop detection means 16 has a loop detection sensor 162 (see FIG. 6) such as a photointerrupter that switches between a light-blocking state and a light-transmitting state in accordance with the swing angle of the loop detection flag 161 and outputs an ON or OFF signal. The loop detection sensor 162 is disposed at a central position between the sheet paths S1 and S2 in a height direction that is perpendicular to the sheet conveyance direction FD and perpendicular to the width direction perpendicular to the conveyance direction FD. The sheet path S1 refers to a conveyance path along which the sheet is conveyed at a position farthest from the conveyance surface 11a before the sheet becomes taut between the fixing unit 50 and the secondary transfer unit 130 in the sheet conveyance direction FD. The sheet path S2 refers to a conveyance path along which the sheet is conveyed at a position closest to the conveyance surface 11a before the sheet becomes too loose between the fixing unit 50 and the secondary transfer unit 130 in the sheet conveyance direction FD. The loop detection sensor 162 switches from a light-transmitting state to a light-blocking state and outputs an ON signal when the loop detection flag 161 swings from the center position in the height direction of the sheet path S2 toward the conveyance surface 11a. The position of the loop detection flag 161 in the direction approaching the conveyance surface 11a from the center position in the height direction of the sheet path S2 is the first position in this embodiment, and the ON signal is the first signal in this embodiment. Furthermore, when the loop detection flag 161 swings from the center position in the height direction of the sheet path S2 in a direction away from the conveying surface 11a, the loop detection sensor 162 switches from the light-blocking state to the light-transmitting state and outputs an OFF signal. The position of the loop detection flag 161 in a direction away from the center position in the height direction of the sheet path S2 in the direction away from the conveying surface 11a is the second position in this embodiment, and the OFF signal is the second signal in this embodiment.The first amount in this embodiment is the amount of loop of the sheet when the sheet is being conveyed in a direction away from the conveying surface 11a than the sheet path S2. The height of the sheet with respect to the conveying surface 11a when the sheet is being conveyed in a direction away from the conveying surface 11a than the sheet path S2 is the first height in this embodiment. The second amount in this embodiment is the amount of loop of the sheet when the sheet is being conveyed in a direction closer to the conveying surface 11a than the sheet path S2. The height of the sheet with respect to the conveying surface 11a when the sheet is being conveyed in a direction closer to the conveying surface 11a than the sheet path S2 is the second height in this embodiment.

[0041] Next, the control configuration when a sheet is conveyed by the belt conveying unit 904 in the image forming apparatus 100 of this embodiment will be described with reference to Fig. 6. Fig. 6 is a block diagram showing the control configuration of the image forming apparatus 100 of this embodiment. In the description of Fig. 6, the same components as those in the first embodiment (see Fig. 3) are given the same reference numerals, and duplicated explanations will be omitted.

[0042] In this embodiment, a signal output from the loop detection sensor 162 is input to the control unit 170. Based on the received signal, the control unit 170 controls the operations of the first drive motor 14, the second drive motor 24, the first suction fan 15, the second suction fan 25, the heating roller drive motor 54, the temperature adjustment unit 55, etc.

[0043] Next, a control flow of the belt conveying unit 904 in the image forming apparatus 100 of this embodiment will be described with reference to FIG. 7. FIG. 7 is a flowchart showing the operation flow of conveying a sheet by the belt conveying unit 904 of this embodiment. This flow is started when information regarding the size and basis weight of the sheet in the image forming job is input from the operation unit 210 of the image forming apparatus 100, or when an image forming job is submitted to the image forming apparatus 100 from an external device. This flow is mainly executed by the control unit 170. When an image forming job is started, the control unit 170 executes job start control (S11). In the image forming job start control of this embodiment, the control unit 170 sets the transfer speed VT, the sheet conveying speed V1 in the first belt conveying unit 10, the sheet conveying speed V2 in the second belt conveying unit 20, and the fixing speed VF so that V2>V1>VT and VF>VT. Then, the control unit 170 controls the heat roller drive motor 54, the first drive motor 14, the second drive motor 24, and the drive roller drive motor 129 so that sheet conveyance is started with V2>V1>VT and VF>VT. In this embodiment, since the fixing speed VF is variable as described above, the relationship VF>VT may change during execution of an image forming job. The control unit 170 also drives the first suction fan 15 of the first belt conveying unit 10 and the second suction fan 25 of the second belt conveying unit 20.

[0044] For example, if the operation of the first suction fan 15 is OFF before the start of an image forming job, the suction force applied to the circumferential surface of the first conveyor belt 11 is sufficient to separate a sheet from the first conveyor belt 11. When the first suction fan 15 is driven and turned ON, an airflow is generated from the outside to the inside of the circumferential surface of the first conveyor belt 11, thereby applying a suction force sufficient to hold a sheet to the circumferential surface of the first conveyor belt 11. Also, for example, if the operation of the second suction fan 25 is OFF before the start of an image forming job, the suction force applied to the circumferential surface of the second conveyor belt 21 is sufficient to separate a sheet from the second conveyor belt 21. When the second suction fan 25 is driven and turned ON, an airflow is generated from the outside to the inside of the circumferential surface of the second conveyor belt 21, thereby applying a suction force sufficient to hold a sheet to the circumferential surface of the second conveyor belt 21. The OFF state of the first suction fan 15 does not necessarily mean a state in which the operation of the first suction fan 15 is stopped. That is, in this embodiment, when a suction force weaker than that capable of holding a sheet, for example, a suction force strong enough to separate a sheet, is applied to the circumferential surface of the first conveyor belt 11, the first suction fan 15 is in the OFF state. Also, the OFF state of the second suction fan 15 is not limited to a state in which the operation of the second suction fan 25 is stopped. That is, in this embodiment, when a suction force weaker than that capable of holding a sheet, for example, a suction force strong enough to separate a sheet, is applied to the circumferential surface of the second conveyor belt 21, the second suction fan 25 is in the OFF state.

[0045] Next, the control unit 170 acquires information about the length of the sheet in the conveying direction FD from information included in the image forming job, and determines whether it is longer than the distance L1 between the secondary transfer nip N2 and the fixing nip N (S12). If the length of the sheet in the conveying direction FD is shorter than the distance L1 in the conveying direction between the secondary transfer nip N2 and the fixing nip N (S12 / N), an image is formed on the sheet and the sheet is discharged outside the image forming apparatus 100. Then, if the image forming job has not ended (S22 / N), the control unit 170 returns to S12, and if the image forming job has ended (S22 / Y), the control unit 170 ends this flow.

[0046] If the length of the sheet in the conveying direction FD is longer than the distance L1 in the conveying direction between the secondary transfer nip N2 and the fixing nip N (S12 / Y), the control unit 170 starts conveying the sheet and then waits until the leading edge of the sheet is detected by the sheet detection sensor 116 (S13). When the leading edge of the sheet reaches the detection position P1 (see FIG. 5) of the sheet detection sensor 116 (S13 / Y), the control unit 170 causes the timer 175 to start measuring the elapsed time since the leading edge of the sheet reached the detection position P1 (S14). Then, the control unit 170 determines whether the leading edge of the sheet has reached the fixing nip N based on the measurement value of the timer 175 (S15). Specifically, the control unit 170 starts measuring the elapsed time with the timer 175 from the timing (S13 / Y) when the control unit 170 receives a signal from the sheet detection sensor 116 indicating that the sheet has reached the detection position P1. Then, the control unit 170 determines whether a first time period required for the leading edge of the sheet to reach the fixing nip N from the detection position P1 has elapsed based on the measurement value of the timer 175. If the measurement value of the timer 175 has elapsed the first time period, the control unit 170 determines that the leading edge of the sheet has reached the fixing nip N (S15 / Y). Note that if the measurement value of the timer 175 has not elapsed the first time period (S15 / N), the control unit 170 waits until the first time period has elapsed. The predetermined time period in this embodiment corresponds to the first time period.

[0047] When the leading edge of the sheet reaches the fixing nip N, the control unit 170 switches the first suction fan 15 from ON to OFF and the second suction fan 25 from ON to OFF (S16). That is, when the leading edge of the sheet reaches the fixing nip N while being sandwiched in the secondary transfer nip N2, the suction force applied to the circumferential surface of the first conveyor belt 11 is smaller than that before the leading edge of the sheet reaches the fixing nip N. Also, when the leading edge of the sheet reaches the fixing nip N while being sandwiched in the secondary transfer nip N2, the suction force applied to the circumferential surface of the second conveyor belt 21 is smaller than that before the leading edge of the sheet reaches the fixing nip N. The first switching process in this embodiment is a process of switching the first suction fan 15 of the first belt conveyor unit 10 from ON to OFF. That is, the first switching process can be performed when the length of the sheet in the conveyance direction FD is longer than the distance L1 in the conveyance direction between the secondary transfer nip N2 and the fixing nip N.

[0048] As described above, in this embodiment, since the fixing speed VF is variable, the relationship VF>VT may change during the execution of an image forming job. In other words, when the leading edge of a sheet reaches the fixing nip N in a state where the sheet is sandwiched in the secondary transfer nip N2, the sheet may be pulled by the fixing nip N toward the downstream side in the sheet conveyance direction FD, or a loop may be formed in the sheet. At this time, the suction force applied to the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 is reduced to suppress a sudden displacement of the sheet due to the suction force of the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21.

[0049] When the suction force applied to the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 is reduced, the control unit 170 determines the amount of loop of the sheet based on the signal received from the loop detection sensor 162 (S17). When the loop detection sensor 162 outputs an ON signal (S17 / Y), the loop detection flag 161 swings in a direction approaching the conveyance surface 11a from the central position in the height direction of the sheet path S2. That is, it can be seen that a loop is formed in the sheet being conveyed on the conveyance surface 11a and the sheet is being conveyed along the sheet path S2 (see FIG. 5). In this case, the control unit 170 increases the driving amount of the heating roller driving motor 54 to make the speed relationship between the transfer speed VT and the fixing speed VF such that VF>VT (S18). As a result, since the sheet is pulled toward the fixing unit 50, the loop formed in the sheet is gradually eliminated, and it is possible to prevent the amount of loop of the sheet from becoming excessive. The first speed in this embodiment is the conveyance speed of the sheet in the secondary transfer unit 130, that is, the transfer speed VT. Further, the second speed in this embodiment is the conveyance speed of the sheet in the fixing unit 50 when it is faster than the conveyance speed of the sheet in the secondary transfer unit 130, that is, the fixing speed VF when the speed relationship between the transfer speed VT and the fixing speed VF is VF>VT.

[0050] On the other hand, when the loop detection sensor 162 outputs an OFF signal (S17 / N), the loop detection flag 161 swings in a direction away from the conveyance surface 11a from the central position in the height direction of the sheet path S2. That is, it can be seen that no loop is formed in the sheet being conveyed on the conveyance surface 11a or the loop is small, and the sheet is being conveyed along the sheet path S1 (see FIG. 5). In this case, the control unit 170 reduces the driving amount of the heating roller driving motor 54 to make the speed relationship between the transfer speed VT and the fixing speed VF such that VF<VT (S19). As a result, it is possible to prevent the sheet from being excessively pulled toward the fixing unit 50. The third speed in this embodiment is the conveyance speed of the sheet in the fixing unit 50 when it is slower than the conveyance speed of the sheet in the secondary transfer unit 130, that is, the fixing speed VF when the speed relationship between the transfer speed VT and the fixing speed VF is VF<VT.

[0051] The control unit 170 then determines whether the trailing edge of the sheet has passed through the secondary transfer nip N2 based on the measurement value of the timer 175 (S20). Specifically, the control unit 170 starts measuring the timer 175 from the timing (S13 / Y) when the control unit 170 receives a signal from the sheet detection sensor 116 indicating that the sheet has reached the detection position P1 (S14). The control unit 170 then determines whether a second time required for the trailing edge of the sheet to pass through the secondary transfer nip N2 has elapsed based on the measurement value of the timer 175. If the measurement value of the timer 175 has elapsed the second time, the control unit 170 determines that the trailing edge of the sheet has passed through the secondary transfer nip N2 (S20 / Y). Note that if the measurement value of the timer 175 has not elapsed the second time (S20 / N), the control unit 170 waits until the second time has elapsed. Whether the trailing edge of the sheet has passed through the secondary transfer nip N2 may be determined based on the timing when the signal from the sheet detection sensor 116 changes from an ON state to an OFF state.

[0052] When it is determined that the trailing edge of the sheet has passed through the secondary transfer nip N2, the control unit 170 turns on the first suction fan 15 and the second suction fan 25 (S21). This provides the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 with an attraction force capable of holding the sheet. The second switching process in this embodiment is a process of turning on the first suction fan 15 of the first belt conveyor unit 10 from an OFF state. Then, if the image formation job has not ended (S22 / N), the process returns to S02, and if the image formation job has ended (S22 / Y), the process ends this flow.

[0053] In this embodiment, the relationship between the transfer speed VT and the fixing speed VF is changed depending on the state of the sheet when the leading edge of the sheet reaches the fixing nip N while being sandwiched in the secondary transfer nip N2. Specifically, if the sheet is being pulled by the fixing nip N when the leading edge of the sheet reaches the fixing nip N, the fixing speed VF is slowed down to prevent the sheet from being excessively pulled toward the fixing unit 50. Furthermore, if the sheet has a large amount of loop when the leading edge of the sheet reaches the fixing nip N, the fixing speed VF is increased to prevent the sheet from becoming excessively looped. Furthermore, when detecting the amount of loop of the sheet, the suction force applied to the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21 is reduced to prevent a sudden displacement of the sheet due to the suction force of the circumferential surfaces of the first conveyor belt 11 and the second conveyor belt 21. This allows for more accurate detection of the amount of loop of the sheet.

[0054] In this way, in this embodiment, it is possible to suppress sudden displacement of the sheet and formation of excessive loops even when conveying a so-called long sheet, a sheet whose length in the sheet conveyance direction FD is longer than the distance L1 in the conveyance direction between the secondary transfer nip portion N2 and the fixing nip portion N. In this embodiment, by suppressing sudden displacement of the sheet and formation of excessive loops, it is possible to suppress transfer defects at the secondary transfer portion 130, sheet conveyance defects, and the like, and therefore it is possible to achieve both improved sheet conveyance performance and improved image quality. [Example]

[0055] In the third embodiment, the suction force of the first suction fan and the second suction fan and the belt conveying speed are clarified in the first embodiment. The rest is the same as in the first embodiment, so the explanation is omitted. Fig. 8 is a flowchart showing the flow of the operation of conveying a sheet by the belt conveying unit, and S01' and S06 are particularly distinctive.

[0056] When an image formation job is started, the first drive motor 14 of the first belt conveying unit 10 is driven to rotate the belt 11 at V1, and the first suction fan 15 is driven to set the suction force to the first suction force. Then, the second drive motor 24 of the second belt conveying unit 20 is driven to rotate the belt 21 at V2, and the second suction fan 25 is driven to set the suction force to the third suction force (S01). At this time, the third suction force is greater than the first suction force. The suction force is also strong enough to adsorb and convey a sheet passing over the belt.

[0057] When the leading edge of the sheet reaches the fixing nip N, the control unit 170 drives the first drive motor to rotate the belt at V1, and changes the suction force of the first suction fan 15 from the first suction force to the second suction force. Also, while driving the second drive motor to rotate the belt at V2, the control unit 170 changes the suction force of the second suction fan 25 from the third suction force to the fourth suction force (S06). At this time, the fourth suction force is greater than the second suction force. The second and fourth suction forces may be set even when the fans are not driven. Therefore, the suction forces have the following relationship: first suction force > third suction force >> fourth suction force ≥ second suction force.

[0058] This allows the sheet being conveyed and the belt to be driven at approximately the same speed, which reduces image contamination caused by toner stains and paper dust adhering to the belt due to friction between the sheet and belt and adhering to the sheet. [Example]

[0059] In the fourth embodiment, the suction force of the first suction fan and the second suction fan and the belt conveying speed are clarified in the second embodiment. The rest is the same as in the second embodiment, so the explanation is omitted. Fig. 9 is a flowchart showing the flow of the operation of conveying a sheet by the belt conveying unit, and S11' and S16 are particularly distinctive.

[0060] When an image formation job is started, the control unit 170 executes job start control (S11). In the image formation job start control of this embodiment, the control unit 170 drives the first drive motor of the first belt conveying unit 10 to rotate the belt at V1 and drives the first suction fan 15 to set the suction force to the first suction force. Then, the control unit 170 drives the second drive motor of the second belt conveying unit 20 to rotate the belt at V2 and drives the second suction fan 25 to set the suction force to the third suction force (S11'). At this time, the third suction force is greater than the first suction force.

[0061] When the leading edge of the sheet reaches the fixing nip N, the control unit 170 drives the first drive motor to rotate the belt at V1, and changes the suction force of the first suction fan 15 from the first suction force to the second suction force. Also, the control unit 170 drives the second drive motor to rotate the belt at V2, and changes the suction force of the second suction fan 25 from the third suction force to the fourth suction force (S16). At this time, the fourth suction force is greater than the second suction force. Therefore, the suction forces have the following relationship: first suction force > third suction force >> fourth suction force > second suction force.

[0062] This allows the sheet and the belt to be transported at approximately the same speed, preventing image contamination caused by toner stains or paper dust adhering to the belt and adhering to the sheet due to friction between the sheet and the belt.

[0063] <Other Examples> In the first and second embodiments, an example has been described in which the belt conveying unit 904 includes the first belt conveying section 10 and the second belt conveying section 20, but the belt conveying unit 904 may include only one belt conveying section (for example, the first belt conveying section 10). In this case, the first conveying belt 11 is an endless belt having breathability, the first driving roller 12 and the driven rollers 12a, 12b, and 12c are tension members, and the first suction fan 15 is an air suction section.

[0064] Furthermore, the belt transport unit 904 may include three or more belt transport sections. In this case, the sheet transport speed in the downstream belt transport section in the sheet transport direction is set faster than the sheet transport speed in the upstream belt transport section. This makes it possible to suppress buckling of the sheet due to the difference in sheet transport speed in the belt transport sections. Furthermore, the configurations of Examples 1 and 2 can also be applied to a direct transfer type printer in which toner is directly transferred from a photosensitive drum as an image carrier to a sheet by a primary transfer roller as a transfer means.

[0065] The control unit 170 in the first and second embodiments includes a central processing unit (CPU) 171 and a memory 172. The CPU 171 reads and executes programs stored in the memory 172 and, as described below, controls the device in cooperation with each functional unit that performs a specific function. The memory 172 includes a non-volatile storage medium such as a read-only memory (ROM) and a volatile storage medium such as a random access memory (RAM), and serves as a storage location for programs and data and as a work area when the CPU 171 executes the programs. The memory 172 is an example of a non-transitory storage medium that stores programs for controlling the image forming apparatus 100. Each function of the control unit 170 may be implemented on the control unit circuit as independent hardware such as an ASIC, or may be implemented in software as a functional unit of a program executed by the CPU 171 or another processing unit. [Explanation of symbols]

[0066] 10 First belt conveying section / 11 First conveying belt (belt, first belt) / 11a Conveying surface / 12 Drive roller (tensioning member, first tensioning member) / 12a, 12b, 12c Follower roller (tensioning member, first tensioning member) / 15 First suction fan (air suction section, first air suction section) / 16 Loop detection means / 20 Second belt conveying section / 21 Second conveying belt (second belt) / 21a Sheet conveying surface / 21a' Virtual line / 22 Drive roller (second tensioning member) / 22a, 22b, 22c Follower roller (second tensioning member) / 25 Second suction fan (second air suction section) / 50 Fixing section (fixing means) / 52 Heating roller / 53 Counter roller / 100 Image forming apparatus / 116 Sheet detection sensor (sheet detection means) / 120Y, 120M, 120C, 120K photosensitive member (image carrier) / 125 intermediate transfer belt (image carrier) / 130 secondary transfer unit (transfer means) / 161 loop detection flag (flag member, loop detection means) / 162 loop detection sensor (loop detection means) / 170 control unit (control means) / 904 belt transport unit (transport means) / 920 image forming unit / 952 pre-fixing guide (guide member) / 952a guide surface / FD transport direction / N fixing nip portion / N' nip line / N2 secondary transfer nip portion (transfer nip portion)

Claims

1. an image carrier that carries a toner image; a transfer means having a transfer nip portion for nipping and conveying a sheet, and for transferring a toner image carried on the image carrier onto the sheet nipped in the transfer nip portion; a fixing unit having a fixing nip portion that holds and conveys a sheet, and that fixes the toner image transferred onto the sheet by the transfer unit; a conveying means for conveying a sheet from the transfer means to the fixing means by rotating the belt; an air suction unit for sucking air through the belt and for switching the suction force of the circumferential surface of the belt between a first suction force capable of holding a sheet on the circumferential surface and a second suction force capable of separating the sheet from the circumferential surface; and a control unit that controls the sheet conveying speed by the fixing unit, and also controls the suction force of the conveying unit and the belt rotation speed, the control unit applies the first suction force to the circumferential surface of the sheet when the sheet is conveyed from the transfer unit to the fixing unit by the conveying unit; When the leading edge of the sheet reaches the fixing nip portion while being sandwiched in the transfer nip portion, a first switching process is executed to switch the suction force applied to the circumferential surface from the first suction force to the second suction force while the belt is being rotated. An image forming apparatus characterized by:

2. a loop detection unit that outputs a first signal when a loop amount of the sheet at a detection position between the transfer unit and the fixing unit in a sheet conveying direction is a first amount, and outputs a second signal when the loop amount is a second amount smaller than the first amount, the control means sets the sheet conveying speed by the transfer means to a first speed and conveys the sheet; when the first signal is output from the loop detection means in a state in which the sheet is sandwiched between the transfer nip portion and the fixing nip portion, the control means sets the sheet conveying speed by the fixing means to a second speed that is faster than the first speed; and when the second signal is output from the loop detection means, the control means sets the sheet conveying speed by the fixing means to a third speed that is slower than the first speed.

2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

3. the control unit changes the sheet conveying speed by the fixing unit to the second speed when the sheet conveying speed by the fixing unit is the third speed and the trailing edge of the sheet held between the transfer nip portion and the fixing nip portion has passed through the transfer nip portion.

3. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

4. the detection position is disposed at a position overlapping with the conveying means in the sheet conveying direction when viewed in a width direction perpendicular to the sheet conveying direction; 4. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

5. The loop detection means has a flag member that is at a first position when the height of the sheet transported on the belt from the transport surface of the belt at the detection position is a first height, and is at a second position when the height is a second height lower than the first height, and outputs the first signal when the flag member is at the first position and the second signal when the flag member is at the second position.

5. The image forming apparatus according to claim 2, wherein the image forming apparatus is a recording medium.

6. the control unit acquires information about the size of the sheet transported from the transfer unit to the fixing unit, and is capable of executing the first switching process when the length of the sheet in the transport direction is longer than the length from the transfer nip portion to the fixing nip portion in the sheet transport direction.

6. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. the control unit executes a second switching process to switch the suction force applied to the peripheral surface from the second suction force to the first suction force when the trailing edge of the sheet sandwiched between the transfer nip portion and the fixing nip portion passes through the transfer nip portion.

7. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

8. the belt, the tension member, and the air suction unit are a first belt, a first tension member, and a first air suction unit, respectively; The conveying means is disposed between the first belt and the fixing means in the sheet conveying direction and includes a breathable endless second belt, a second tension member disposed inside the second belt and tensioning the second belt so as to be rotatable, and a second air suction unit that can switch the suction force of the circumferential surface of the second belt between a third suction force that can hold the sheet on the circumferential surface of the second belt and a fourth suction force that can separate the sheet from the circumferential surface of the second belt, the control unit starts conveying the sheet from the transfer unit to the fixing unit by the conveying unit in a state in which the third suction force is applied to the circumferential surface of the second belt and the sheet conveying speed by the fixing unit is made faster than the sheet conveying speed by the transfer unit, when the leading edge of the sheet reaches the fixing nip portion while being sandwiched in the transfer nip portion, the suction force applied to the circumferential surface of the second belt is switched from the third suction force to the fourth suction force.

8. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

9. a center of the second air suction section is disposed downstream of a center of the second belt in a sheet conveying direction; 9. The image forming apparatus according to claim 8,

10. the fixing unit includes a heating roller for heating the sheet and an opposing roller for contacting the heating roller to form the fixing nip portion; the second belt is disposed downstream of the first belt in a sheet conveying direction, and has a sheet conveying surface that conveys the sheet; an imaginary line extending from the sheet conveying surface to the downstream side in the sheet conveying direction intersects with a nip line, which is a tangent line between the heating roller and the counter roller in the fixing nip portion, downstream of the fixing nip portion in the sheet conveying direction; 10. The image forming apparatus according to claim 8, wherein the image forming apparatus is a recording medium.

11. a guide member having a guide surface that intersects with the virtual line downstream of the second belt when viewed in a width direction perpendicular to the sheet conveying direction and that guides the leading edge of the sheet conveyed on the sheet conveying surface toward the fixing nip portion; 11. The image forming apparatus according to claim 10.

12. a sheet detecting means for detecting the presence or absence of a sheet at a detection position disposed upstream of the transfer nip portion in a sheet conveying direction, the control unit is capable of executing the first switching process when a predetermined time has elapsed since the leading edge of the sheet reached the detection position.

12. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

13. The second suction force is generated when the fan of the air suction unit is stopped.

13. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

Images