Image forming device

Abstract

To provide a configuration that can improve detection accuracy of both loop detection and remaining sheet detection.SOLUTION: A loop detection unit 151 detects that a sheet forms a loop upon contact of a loop detection flag 151a with the sheet. A remaining sheet detection unit 152 has a remaining sheet detection flag 152a that is brought into contact with a sheet at a position closer to an entrance of a fixing nip part than the loop detection flag 151a, and detects that a sheet remains in a fixing device upon contact of the remaining sheet detection flag 152a with the sheet. A rotary spring 154 is a common spring that applies urging force to the loop detection flag 151a and the remaining sheet detection flag 152a. The rotary spring 154 urges the loop detection flag 151a and remaining sheet detection flag 152a toward a sheet conveyance path between a secondary transfer unit and the fixing nip part.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a copier, printer, facsimile, or a multifunction machine having multiple functions of these. [Background technology]

[0002] In an image forming apparatus, an image formed on an image carrier such as a photosensitive drum or an intermediate transfer belt is transferred to a sheet in a transfer unit, and the sheet with the transferred image is transported to a fixing device where the image is fixed to the sheet. At this time, a loop is formed in the sheet to prevent the sheet from pulling against each other between the transfer unit and the fixing device, and the sheet transport speed between the transfer unit and the fixing device is controlled by detecting this loop with a sensor (Patent Document 1). Patent Document 1 also discloses that a loop detection unit for detecting the loop detects a sheet remaining in the fixing device. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-233372 Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, in the configuration described in Patent Document 1, the loop detection unit also detects residual sheets, but to detect small-sized sheets, it is necessary to position the contact portion of the loop detection unit that comes into contact with the sheet as close as possible to the entrance of the nip portion of the fixing device. On the other hand, in order to detect the amount of deformation of the sheet, loop detection requires that the contact portion be brought into contact with the sheet upstream of the position where residual sheet detection is performed.

[0005] For this reason, it is conceivable to provide separate contact portions for loop detection and residual sheet detection. However, if the contact portions are biased toward the sheet to detect the sheet, and if each contact portion is biased by a separate biasing means, the following problems may occur. For example, if the sheet has low rigidity, the sheet may be pushed by the biasing forces of the two contact portions and the biasing means, which may cause the sheet to deform. Furthermore, if the sheet is light in weight, the contact portions may be pushed by the sheet and become difficult to move, which may prevent the sensor from detecting the sheet.

[0006] An object of the present invention is to provide a configuration capable of improving the detection accuracy of both loop detection and residual sheet detection. [Means for solving the problem]

[0007] One aspect of the present invention is Sheet a transfer unit that transfers a toner image onto a transfer roller; a rotatable first rotating body having a heat source; and a second rotating body that forms a nip portion by contacting an outer circumferential surface of the first rotating body, Sheet a second rotating body that applies heat and pressure to fix the toner image while sandwiching and transporting the toner image; Sheet When the sheet is sandwiched and conveyed in the nip portion, it comes into contact with the first rotating body. Sheet The surface is in contact with the second rotating body. Sheet When the surface is the back side, Sheet In the conveying direction, between the nip portion and the transfer portion Sheet a loop detection unit for detecting a loop of the toner image transferred by the transfer unit; Sheet a loop detection unit having a first contact portion that contacts the rear surface of the sheet; and a remaining sheet detection unit that detects whether a sheet remains in the nip portion between the nip portion and the transfer portion in the conveying direction, and Sheet a remaining sheet detection unit having a second contact portion that can come into contact with the rear surface of the sheet; The first contact portion and the second contact portion are rotatably supported. The first contact portion and the second contact portion Common in Rotation axis of and , a common biasing unit that applies a biasing force to the first contact portion and the second contact portion, and biases the first contact portion and the second contact portion toward a sheet transport path between the transfer portion and the nip portion, The second contact portion is located downstream of the first contact portion in the conveying direction. Sheet and The first contact portion and the second contact portion are rotatable independently of each other within a predetermined rotation range. The image forming apparatus is characterized by the above. [Effects of the Invention]

[0008] According to the present invention, it is possible to improve the detection accuracy of both loop detection and residual sheet detection. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram of an image forming apparatus according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view showing a schematic configuration of a fixing device according to the embodiment. [Figure 3] 1A is a perspective view of a loop detection unit and a remaining sheet detection unit according to an embodiment, and FIG. 1B is a cross-sectional view of an engagement portion between a loop detection flag and a remaining sheet detection flag. [Figure 4] 1A is a diagram illustrating a state in which the sheet is looped, FIG. 1B is a diagram illustrating a state in which the sheet loop is beginning to be released, and FIG. 1C is a diagram illustrating a state in which the sheet loop has been released. [Figure 5] 10A is a diagram illustrating a state in which the remaining sheet detection flag is in contact with the trailing edge of the remaining sheet, and FIG. 10B is a diagram illustrating a state in which the remaining sheet is detected by the remaining sheet detection unit. [Figure 6] FIG. 2 is a control block diagram of the image forming apparatus according to the embodiment. [Figure 7] 10 is a flowchart of a sheet loop detection operation according to the embodiment. [Figure 8] 10 is a flowchart of a remaining sheet detection operation according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] The embodiment will be described with reference to Figures 1 to 8. First, the schematic configuration of an image forming apparatus according to the present embodiment will be described with reference to Figure 1.

[0011] [Image forming device] 1 is a schematic diagram of an image forming apparatus 1 to which this embodiment is applied. This image forming apparatus 1 is a full-color laser beam printer using an electrophotographic system, and forms and outputs an image on a sheet (recording material) S corresponding to electrical image information input from a host device C such as a personal computer or image reader to a controller A (control unit 200). The controller A exchanges various types of electrical information with the host device C and an operation panel (operation unit) B, and also comprehensively controls the image forming operation of the image forming apparatus in accordance with a predetermined control program and look-up table.

[0012] As shown in FIG. 1, the image forming apparatus 1 includes a main body 1A configured with image forming units 20Y, 20M, 20C, and 20K corresponding to the colors Y (yellow), M (magenta), C (cyan), and K (black), arranged in series as image forming means. In other words, the image forming apparatus employs a tandem system in which the process leading up to visualization is performed in parallel for each color. The image forming units 20Y, 20M, 20C, and 20K have essentially the same configuration, except for the toner colors. The arrangement order of the image forming units for the colors Y, M, C, and K is not limited to the illustrated example.

[0013] Each image forming unit 20Y, 20M, 20C, and 20K is equipped with the following process means. It includes a photosensitive drum 21, which serves as an image carrier and photosensitive body for carrying an electrostatic latent image on its surface corresponding to each color of Y, M, C, and K, a charging roller 22, which serves as a primary charging device, an exposure device 23, a developing device 24, and a cleaning device 25. The charging roller 22 applies a charging bias voltage of a set potential to the surface of the corresponding photosensitive drum 21 to uniformly charge it. The exposure device 23 exposes the charged surface of the photosensitive drum 21 to light, forming an electrostatic latent image on the surface of the photosensitive drum 21. The electrostatic latent image is developed with toner by the developing device 24, becoming a visible toner image.

[0014] The toner images of each color formed and carried on the surface of the photosensitive drum 21 of each image forming unit 20Y, 20M, 20C, and 20K are sequentially superimposed on the intermediate transfer belt 26 serving as an image carrier at a primary transfer nip portion (primary transfer portion) T1 of the primary transfer device 26a, thereby performing primary transfer. Any residual toner remaining on the photosensitive drum 21 after the primary transfer is removed by a cleaning device 25.

[0015] The intermediate transfer belt 26 is an endless belt that is supported by being stretched over a drive roller 27, a tension roller 28, and an opposing roller 29, and is driven by the drive roller 27 to rotate in the clockwise direction indicated by the arrow. A secondary transfer device 30 is disposed on the portion of the intermediate transfer belt 26 that is stretched over the opposing roller 29, facing the opposing roller 29 across the intermediate transfer belt 26. A secondary transfer roller 31 as a transfer member provided in the secondary transfer device 30 is pressed against the intermediate transfer belt 26, which is supported from the inside by the opposing roller 29, and forms a secondary transfer portion (secondary transfer nip portion) T2 between the secondary transfer roller 31 and the intermediate transfer belt 26.

[0016] As described above, the toner images on the intermediate transfer belt 26 onto which all the colors Y, M, C, and K have been primarily transferred are then collectively secondarily transferred onto a sheet S at the secondary transfer portion T2. ​​Examples of the sheet include paper, plastic film, and cloth.

[0017] The belt cleaning device 32 rubs a cleaning web against the intermediate transfer belt 26 to remove untransferred toner, paper dust, etc. remaining on the surface of the intermediate transfer belt 26 after passing through the secondary transfer portion T2.

[0018] Meanwhile, in the sheet feeding device 10, the sheets S are pulled out by the pickup roller 12 from the sheet storage cassette 11, and separated one by one by the separation device 13, and sent to the registration rollers 15. The registration rollers 15 receive the sheets S in a stopped state and make them wait, and then send the sheets S to the secondary transfer section T2 in time with the toner image on the intermediate transfer belt 26 (on the image carrier).

[0019] Furthermore, sheets having a large length in the conveying direction, such as long sheets, are fed from a long sheet feeding device 16 provided on the right side of the image forming apparatus 1. That is, in the long sheet feeding device 16, sheets S are pulled out from a manual feed tray 17 by a pickup roller 18, and separated one by one by a separation device 19, and sent to the registration rollers 15.

[0020] The sheet S carrying the toner image transferred at the secondary transfer portion T2 is transported to the fixing device 100 by a belt transport device (pre-fixing transport portion) 40. The belt transport device 40 is, for example, a transport belt that attracts and transports the sheet S, and is disposed between the secondary transfer portion T2 and the fixing device 100. As a conveying unit The belt conveying device 40 is arranged to feed the sheet S discharged from the secondary transfer portion T2 into the fixing device 100 while assisting the sheet S in its conveying position. In this embodiment, the belt conveying device 40 is formed of a belt stretched between rollers, but it may also be configured with only a guide member that guides the sheet S. The sheet S that has passed through the secondary transfer portion T2 is handed over to the belt conveying device 40, and is conveyed by the belt conveying device 40 to the fixing device 100. Note that the belt conveying device 40 may be omitted, and the sheet S that has passed through the secondary transfer portion T2 may be directly conveyed to the fixing device 100.

[0021] In the fixing device 100, the sheet S is clamped in the fixing nip N formed by a pair of rotating bodies, a fixing belt 105 (first rotating body) and a pressure belt 120 (second rotating body), and the toner image is fixed to the sheet S by applying heat and pressure to the unfixed toner image on the sheet.

[0022] After the fixing process, the sheet S is sent out from the fixing device 100 and is conveyed by a pair of conveying rollers 90 toward the discharge path 50 or the double-sided conveying path 60. In the single-sided printing mode, the sheet S on which the toner image has been fixed advances to the discharge path 50, passes through the decurl device 70 by the discharge rollers 51, and is discharged from the discharge outlet 80 onto the discharge tray 81 where it is stacked.

[0023] The decurling device 70 is a curl removing means and is composed of a pair of rollers: a metal roller 71 and a sponge roller 72 made of a metal shaft wrapped with a material such as urethane rubber. When the sheet S passes through, the sponge roller 72 is pressed into the metal roller 71 by a pressure mechanism (not shown). This forms a decurling nip. Department Form Decals The curl of the sheet is corrected by passing the sheet through the nip portion. In order to correct the upward and downward curls of the sheet S discharged from the image forming apparatus 1, a lower decurling unit 73 and an upper decurling unit 74 are provided.

[0024] In double-sided printing mode, the toner image is fused to one side. Ta The sheet S proceeds to the reversing path 53, is switched back, and is sent to the double-sided conveying path 60, where it waits at the re-feed roller 61. Thereafter, the sheet S is sent to the secondary transfer portion T2 by the registration roller 15, where the toner image is secondarily transferred onto the back surface of the sheet, and the unfixed toner image is fixed by the fixing device 100.

[0025] In this way, the image forming apparatus 1 performs a series of image formation processes, including charging, exposure, development, transfer, and fixing, to form and discharge a color toner image on the sheet S. In the case of a monochrome image forming apparatus, only a black (K) photosensitive drum is present, and the toner image formed on that photosensitive drum is transferred to the sheet S by a transfer device.

[0026] The image forming apparatus 1 is also provided with an operation panel B as an operation unit, which includes various buttons and operation switches operated by the user, as well as a display unit that displays messages to the user and displays errors such as jams.

[0027] [Fusing device] Next, a fixing device 100, which is an image heating device of this embodiment, will be described with reference to FIG. 2. The fixing device 100 has a fixing belt 105 and a pressure belt 120 as a pair of rotating bodies. The pressure belt 120 as a pressure rotating body is brought into contact with the outer circumferential surface of the fixing belt 105 as a heating rotating body, thereby forming a fixing nip portion N for heating a toner image on a sheet. Such a fixing device 100 is an image heating device of a belt nip type, an electromagnetic induction heating (IH) type, or an oil-less fixing type. Specifically, the electromagnetic induction heating portion (heat source) The fixing belt 105 is heated by electromagnetic induction.

[0028] The configuration of the pair of rotating bodies is not limited to this. For example, both of the pair of rotating bodies may be rollers, or the belt on the heating side may be a film. Furthermore, one may be an endless belt and the other a roller. The heating method is also not limited to induction heating, and a halogen heater may be disposed inside the roller around which the belt is stretched. Furthermore, when a film is used as the heating rotating body, the film may be heated by a ceramic heater or the like, or when a roller is used as the heating rotating body, a halogen heater may be disposed inside the roller.

[0029] The fixing belt 105 and the pressure belt 120 are disposed inside a housing 110. That is, the fixing device 100 includes the housing 110, the fixing belt 105 and the pressure belt 120 as a pair of rotating bodies disposed inside the housing 110, a fixing entrance guide 150, a loop detection unit 151, and a remaining sheet detection unit 152. The housing 110 has openings at the entrance and exit for the sheet S.

[0030] A fixing entrance guide 150 is provided upstream in the conveyance direction of the sheet S from the fixing nip N formed by pressing the pressure belt 120 against the fixing belt 105. The fixing entrance guide 150 guides the sheet S, which carries an unfixed toner image at the secondary transfer unit T2, from the belt conveyance device 40 to the entrance of the fixing nip N. That is, the fixing entrance guide 150 is disposed between the entrance of the housing 110 and the fixing nip N, and guides the sheet S, which enters through the entrance of the housing 110 and carries the unfixed toner image, to the fixing nip N. The sheet S guided along the fixing entrance guide 150 is heated and pressurized while being nipped and conveyed between the fixing belt 105 and the pressure belt 120 in the fixing nip N. This fixes the toner image to the sheet S. A loop detection unit 151 detects whether a loop has been formed in the sheet S. A remaining sheet detection unit 152 detects whether a sheet remains in the fixing device 100. The loop detection unit 151 and the remaining sheet detection unit 152 are provided on the fixing entrance guide 150 .

[0031] [Loop detection and remaining sheet detection mechanism] Next, the loop detection and remaining sheet detection mechanisms will be described with reference to Figures 3(a) and 3(b). As shown in Figure 3(a), the loop detection unit 151 and the remaining sheet detection unit 152 are provided so that their respective loop detection flags 151a and remaining sheet detection flags 152a rotate about the same rotation shaft 153. The rotation shaft 153 is located upstream of the fixing nip portion N in the housing 110 in the sheet conveying direction. The remaining sheet detection unit 152 has a remaining sheet detection sensor S2 as a first sensor, and the loop detection unit 151 has a loop detection sensor S1 as a second sensor. Each of the configurations will be specifically described below.

[0032] [Loop detection section] The loop detection unit 151 has a loop detection flag 151a as a first contact portion, a cylindrical portion 151b, an engaging protrusion 151c as a first engaging portion, a light-shielding portion 151d, and a loop detection sensor S1. The loop detection flag 151a has a base end fixed to the cylindrical portion 151b, and is a plate-like or rod-like member extending in the radial direction of the cylindrical portion 151b. The cylindrical portion 151b is directly attached to the frame 111 of the fixing device 100. fixedAlternatively, it is fitted onto a rotary shaft 153 supported by a fixing entrance guide 150 provided on the frame 111 so as to be relatively rotatable. The frame 111 also supports rollers for tensioning the fixing belt 105 and the pressure belt 120.

[0033] The engaging protrusion 151c is formed to protrude in the direction of the rotation axis from a portion of the circumferential direction of the cylindrical portion 151b. The light-shielding portion 151d is fixed to the cylindrical portion 151b at a different circumferential position from the loop detection flag 151a, and extends in the radial direction of the cylindrical portion 151b. The loop detection flag 151a, cylindrical portion 151b, engaging protrusion 151c, and light-shielding portion 151d are integrally formed from, for example, resin. The loop detection flag 151a is rotatably supported on a rotation shaft 153 via the cylindrical portion 151b, and the engaging protrusion 151c and light-shielding portion 151d rotate (swing) together with the loop detection flag 151a around the rotation shaft 153.

[0034] The loop detection sensor S1 is a sensor capable of detecting the rotation position of the loop detection flag 151a, and in this embodiment, is a photointerrupter. That is, the loop detection sensor S1 has a light-emitting portion S11 and a light-receiving portion S12 capable of receiving light S1a emitted from the light-emitting portion S11. A light-shielding portion 151d is configured to pass between the light-emitting portion S11 and the light-receiving portion S12. The light S1a emitted from the light-emitting portion S11 is blocked by the light-shielding portion 151d, thereby enabling the loop detection sensor S1 to detect the rotation position of the loop detection flag 151a.

[0035] [Residual sheet detection unit] The residual sheet detection unit 152 includes a residual sheet detection flag 152a as a second contact portion, a cylindrical portion 152b, an engagement recess 152c as a second engagement portion, a light-shielding portion 152d, and a residual sheet detection sensor S2. The residual sheet detection flag 152a is a plate-like or rod-like member whose base end is fixed to the cylindrical portion 152b and extends radially from the cylindrical portion 152b. The residual sheet detection flag 152a is longer than the loop detection flag 151a and is positioned adjacent to the loop detection flag 151a. Therefore, the tip of the residual sheet detection flag 152a is positioned farther from the rotation center than the tip of the loop detection flag 151a. In other words, the tip of the residual sheet detection flag 152a can be positioned closer to the entrance of the fixing nip portion N than the tip of the loop detection flag 151a.

[0036] Furthermore, the tip of the remaining sheet detection flag 152a is formed to protrude further toward the loop detection flag 151a than the middle part, allowing it to come into contact with a sheet over a wider area than the tip of the loop detection flag 151a. The remaining sheet detection flag 152a and the loop detection flag 151a are arranged so as not to interfere with each other even when they rotate around the rotation shaft 153. The cylindrical portion 152b is fitted adjacent to the cylindrical portion 151b and is rotatable relative to the rotation shaft 153.

[0037] The engagement recess 152c is formed in a portion of the circumferential direction of the cylindrical portion 152b so as to be recessed in the direction of the rotation axis. The engagement protrusion 151c can enter the engagement recess 152c, and the engagement protrusion 151c and the engagement recess 152c engage with each other, allowing the loop detection flag 151a and the remaining sheet detection flag 152a to rotate together. As shown in FIG. 3B, the circumferential width of the engagement recess 152c is wider than the circumferential width of the engagement protrusion 151c, so the loop detection flag 151a and the remaining sheet detection flag 152a can rotate independently of each other within a predetermined rotation range (the difference in the circumferential width between the engagement protrusion 151c and the engagement recess 152c). The relationship between the engagement protrusion and the engagement recess may be reversed. That is, the loop detection unit 151 side may be the engagement recess, and the remaining sheet detection unit 152 side may be the engagement protrusion.

[0038] The light-shielding portion 152d is fixed to the cylindrical portion 152b at a position different from the remaining sheet detection flag 152a in the circumferential direction, and extends in the radial direction of the cylindrical portion 152b. In this embodiment, the light-shielding portion 152d is formed to be bifurcated, and the light S2a (described later) is blocked at two positions in the circumferential direction. Light blocking The remaining sheet detection flag 152a, cylindrical portion 152b, engaging recess 152c, and light-shielding portion 152d are integrally formed of, for example, resin. The remaining sheet detection flag 152a is rotatably supported on a rotation shaft 153 via the cylindrical portion 152b, and the engaging recess 152c and light-shielding portion 152d rotate (swing) around the rotation shaft 153 together with the remaining sheet detection flag 152a.

[0039] The remaining sheet detection sensor S2 is a sensor capable of detecting the rotational position of the remaining sheet detection flag 152a, and in this embodiment is a photointerrupter. That is, the remaining sheet detection sensor S2 has a light-emitting portion S21 and a light-receiving portion S22 capable of receiving light S2a emitted from the light-emitting portion S21. A light-shielding portion 152d is provided between the light-emitting portion S21 and the light-receiving portion S22 to allow passage. The light S2a emitted from the light-emitting portion S21 is blocked by the light-shielding portion 152d, thereby enabling the remaining sheet detection sensor S2 to detect the rotational position of the remaining sheet detection flag 152a.

[0040] The rotation shaft 153 is disposed below the fixing entrance guide 150, and the loop detection flag 151a and the residual sheet detection flag 152a can protrude above the fixing entrance guide 150 through an opening or a notch formed in the fixing entrance guide 150. This allows the loop detection flag 151a and the residual sheet detection flag 152a to come into contact with the sheet S that is conveyed from the secondary transfer portion T2 to the fixing entrance guide 150 via the belt conveying device 40. That is, when the sheet S is nipped and conveyed in the fixing nip portion N, the surface of the sheet S that contacts the fixing belt 105 is the front surface, and the surface of the sheet S that contacts the pressure belt 120 is the back surface. When the loop detection flag 151a comes into contact with the back surface of the sheet S, the loop detection unit 151 detects that the sheet S is looped between the fixing nip portion N and the secondary transfer portion T2 in the conveyance direction of the sheet S. When the remaining sheet detection flag 152a comes into contact with the back surface of the sheet S, the remaining sheet detection unit 152 detects that a sheet remains in the fixing nip portion N between the fixing nip portion N and the secondary transfer portion T2 in the conveyance direction of the sheet S.

[0041] In this embodiment, a rotary spring 154 is provided as a common biasing means for applying a biasing force to the loop detection flag 151a and the remaining sheet detection flag 152a. The rotary spring 154 biases the loop detection flag 151a and the remaining sheet detection flag 152a toward the sheet transport path between the secondary transfer portion T2 and the fixing nip portion N. Specifically, the rotary spring 154 is provided so as to bias the loop detection flag 151a in the direction of arrow A in FIG. 3(a).

[0042] The biasing direction of the rotary spring 154 is opposite to the direction in which the loop detection flag 151a is pressed by the sheet S transported from the secondary transfer portion T2 via the belt transport device 40. When the loop detection flag 151a is biased in the direction of arrow A by the rotary spring 154 and the engaging protrusion 151c engages with the engaging recess 152c, this biasing force is also transmitted to the remaining sheet detection flag 152a. As a result, the rotary spring 154, which is a common biasing means, can bias the loop detection flag 151a and the remaining sheet detection flag 152a toward the sheet transport path.

[0043] [Loop detection and remaining sheet detection] Next, the loop detection operation and the remaining sheet detection operation performed using the loop detection unit 151 and the remaining sheet detection unit 152 will be described with reference to Figures 4(a) to 5(b). In addition to a schematic side view of the belt conveying device 40 to the fixing nip N, Figures 4(a) to 5(b) also show a cross-sectional view of the engagement portion between the engagement protrusion 151c and the engagement recess 152c. This cross-sectional view of the engagement portion is a cross-sectional view seen from the same direction as the schematic side view of the belt conveying device 40 to the fixing nip N, and the positional relationship between the engagement protrusion 151c and the engagement recess 152c is the same as this schematic view.

[0044] When the sheet S passes over the fixing entrance guide 150, the loop detection flag 151a and the remaining sheet detection flag 152a are pressed by the sheet S and rotate around the rotation shaft 153. As described above, the loop detection flag 151a is rotated by the rotation spring 154 in the direction opposite to the pressing force of the sheet S (see FIG. 3). (a)A rotation force is applied in the direction of arrow A in Fig. 4(a) to (c) and Fig. 5(a) and (b) (clockwise direction).

[0045] The leading edge of the loop detection flag 151a, which is pressed against the sheet S, is arranged to be rotatable between a state L1 (FIG. 4A) in which a loop occurs due to the difference in the conveying speed of the sheet S conveyed by the intermediate transfer belt 26 and the belt conveying device 40 and the conveying speed of the sheet S conveyed by the fixing device 100, and a state L2 (FIG. 4C) in which the loop is eliminated. The belt conveying device 40 is designed to convey the sheet at approximately the same speed as the sheet conveying speed at the secondary transfer portion T2. ​​Similarly, the sheet conveying speed at the fixing device 100 is also designed to convey the sheet at approximately the same speed. However, in the fixing device 100, the fixing belt 105 and the pressure belt 120 are rotated by the rotation of the drive rollers among the rollers that stretch the belts. The fixing belt 105 and the pressure belt 120 rotate in response to the rotation of the drive rollers, causing slight slippage between the drive rollers and the belts. This causes an error in the belt rotation speed. The same applies to the belt conveying device 40. Therefore, there is a possibility that the sheet conveying speed in the fixing device 100 may differ from the sheet conveying speed in the secondary transfer portion T2.

[0046] On the other hand, in recent years, there has been an increasing demand for printing on long sheets of paper. In the conveyance direction of the recording material, there has been an increasing number of cases where long sheets of paper are printed, which are recording materials that are longer than the distance between the secondary transfer portion T2 and the fixing nip portion N. When the secondary transfer portion T2 and the fixing nip portion N simultaneously nip and convey the sheet, and the sheet conveyance speed in the fixing device 100 becomes faster than the sheet conveyance speed in the secondary transfer portion T2, the fixing device 100 pulls the sheet, and the desired area on the sheet is not conveyed. The image There is a risk that the image may not be transferred (misaligned transfer). To prevent this misalignment, a loop detection sensor detects the amount of loop in the sheet and controls the sheet conveyance speed in the fixing device. The details are described below.

[0047] The loop detection sensor S1 is configured so that when the light blocking portion 151d blocks the light S1a of the loop detection sensor S1, the loop detection sensor S1 turns ON, and when the light S1a of the loop detection sensor S1 passes through, the loop detection sensor S1 turns OFF. Therefore, as shown in Fig. 4(a), the position of the light blocking portion 151d relative to the tip of the loop detection flag 151a is specified so that when the tip of the loop detection flag 151a reaches the position of state L1 where a loop has occurred, the light blocking portion 151d turns ON the loop detection sensor S1.

[0048] 4(a), the remaining sheet detection flag 152a rotates counterclockwise in the figure together with the loop detection flag 151a due to the engagement between the engagement protrusion 151c and the engagement recess 152c. Therefore, the light-shielding portion 152d of the remaining sheet detection unit 152 also blocks the light S2a of the remaining sheet detection sensor S2, and the remaining sheet detection sensor S2 is also ON. Note that, at this time, the engagement positions of the engagement protrusion 151c and the engagement recess 152c may be set so that the tip of the remaining sheet detection flag 152a does not come into contact with the sheet S in state L1.

[0049] As will be described later, when the loop detection unit 151 detects a loop in the sheet, the sheet conveying speed by the fixing device 100 is increased to eliminate the loop in the sheet. At this time, as shown in FIG. 4(b), when the loop in the sheet begins to be eliminated, the loop detection flag 151a begins to rotate clockwise in the figure by the rotation spring 154, following the sheet. Then, as shown in the cross section of the engagement portion in the same figure, the engagement between the engagement protrusion 151c and the engagement recess 152c is released. At this time, the residual sheet detection flag 152a remains in the position shown in FIG. 4(a) due to its own weight and does not come into contact with the sheet.

[0050] That is, the engaging protrusion 151c of the loop detection flag 151a and the engaging recess 152c of the remaining sheet detection flag 152a are configured to engage with each other so as to be able to rotate independently within a predetermined rotation range. Therefore, as shown in Fig. 4(b), while the light blocking portion 151d of the loop detection unit 151 blocks the light S1a of the loop detection sensor S1, the tip of the remaining sheet detection flag 152a is positioned below the tip of the loop detection flag 151a (dotted line position) due to its own weight.

[0051] Next, as shown in state L2 in FIG. 4(c), when the sheet loop is eliminated, the light-shielding portion 151d of the loop detection unit 151 transmits the light S1a of the loop detection sensor S1, and the loop detection sensor S1 turns OFF. Then, as will be described later, the sheet conveying speed of the fixing device 100 is reduced to form a loop again. At this time, as shown in the cross section of the engagement portion in the same figure, the engagement protrusion 151c and the engagement recess 152c engage, and the remaining sheet detection flag 152a rotates clockwise and protrudes above the fixing inlet guide 150. However, even in this state, the tip of the remaining sheet detection flag 152a is located below the tip of the loop detection flag 151a.

[0052] 5(b), if a sheet S remains in the fixing nip N due to a jam or the like, the trailing edge of the sheet S is detected by the remaining sheet detection flag 152a of the remaining sheet detection unit 152. In this embodiment, even if a small-sized sheet with a length in the sheet conveyance direction of about 150 mm, such as a postcard or envelope, remains in the fixing nip N, the leading edge of the remaining sheet detection flag 152a is extended to the vicinity of the entrance of the fixing nip N so that the remaining sheet can be detected. For this reason, the leading edge of the remaining sheet detection flag 152a is configured to rotate at a position closer to the fixing nip N than the leading edge of the loop detection flag 151a.

[0053] As shown in FIG. 5A, when the leading edge of the loop detection flag 151a is not in contact with the sheet, the biasing force of the pivot spring 154 is transmitted to the remaining sheet detection flag 152a via the engagement between the engagement protrusion 151c and the engagement recess 152c. The remaining sheet detection flag 152a then rotates clockwise until its leading edge comes into contact with the sheet S. FIG. 5A shows the state in which the remaining sheet detection flag 152a is in contact with the rear end of the sheet S being conveyed through the fixing nip N, but the light-shielding portion 152d of the remaining sheet detection unit 152 does not block the light S2a of the remaining sheet detection sensor S2. Furthermore, the light-shielding portion 151d of the loop detection unit 151 does not block the light S1a of the loop detection sensor S1.

[0054] Next, as shown in FIG. 5B, if a sheet S remains in the fixing nip N due to a jam or other reason, the sheet S follows the direction of the nip surface of the fixing nip N, causing the trailing edge of the sheet S to move downward from the dashed line position to the solid line position. Then, the remaining sheet detection flag 152a, which is in contact with the trailing edge of the sheet S, rotates counterclockwise, and the light-shielding portion 152d of the remaining sheet detection flag 152a blocks the light S2a of the remaining sheet detection sensor S2. Even in this state, the light-shielding portion 151d of the loop detection unit 151 does not block the light S1a of the loop detection sensor S1. When the light-shielding portion 152d blocks the light S2a of the remaining sheet detection sensor S2, the remaining sheet detection sensor S2 turns ON, and it is detected that a sheet S remains in the fixing nip N.

[0055] In this embodiment, as shown in FIGS. 5A and 5B, the engaging protrusion 151c of the loop detector 151 and the remaining sheet detector S2 are in contact with each other until the light blocking portion 152d blocks the light S2a of the remaining sheet detector S2. 152 Therefore, only during this time, the remaining sheet detection flag 152a is rotated clockwise together with the loop detection flag 151a by the rotation spring 154.

[0056] In this embodiment, the rotation angle θ1 of the loop detection flag 151a is set to 50 degrees, and the rotation angle θ2 of the remaining sheet detection flag 152a is set to 30 degrees. A region (predetermined angle range) of θ2-θ1=20 degrees is provided in which the engaging protrusion 151c of the loop detection unit 151 and the engaging recess 152c of the remaining sheet detection unit 152 do not engage. By setting the rotation angle of the remaining sheet detection flag 152a to be 20 degrees larger than the rotation angle of the loop detection flag 151a, the loop detection flag 151a can be rotated independently by the rotation angle of 20 degrees.

[0057] The loop detection flag 151a and the remaining sheet detection flag 152a are provided on the fixing entrance guide 150 and are biased toward the sheet transport path. As a result, the loop detection flag 151a and the remaining sheet detection flag 152a come into contact with the sheet surface (back surface) that comes into contact with the pressure belt 120. By providing the loop detection flag 151a and the remaining sheet detection flag 152a so that they come into contact with the back surface, it is possible to prevent the loop detection flag 151a and the remaining sheet detection flag 152a from coming into contact with unfixed toner.

[0058] [Control Unit] The configuration of the control unit that controls the fixing speed of the image forming apparatus of this embodiment will be described with reference to the block diagram of Fig. 6. The control unit 200 includes a CPU 201, a ROM 202, and a RAM 203, and controls the overall operation of the image forming apparatus 1. The CPU 201 controls the overall operation of the image forming apparatus 1 in accordance with a control program stored in the ROM 202. The ROM 202 stores programs executed by the CPU 201, as well as various default values ​​and data. The RAM 203 provides a work area for temporarily storing various data during control processing by the CPU 201, and also stores various flags and data referenced by the programs.

[0059] The image forming apparatus 1 includes the image forming units 20Y to 20K shown in FIG. 1, an intermediate transfer belt 26, a sheet feeding device 10, and the like, and performs the function of transferring an image corresponding to input image information onto a sheet S to form an unfixed toner image under the control of a control unit 200. A motor 212 drives and rotates the rollers of the fixing device 100 in accordance with instructions from the control unit 200. Specifically, The motor 212 The drive roller among the rollers that stretch the fixing belt 105 and the drive roller among the rollers that stretch the pressure belt 120 are driven, respectively, to convey the sheet held in the fixing nip N. Furthermore, a motor 213 rotates and drives the intermediate transfer belt 26 of the image forming unit in accordance with instructions from the control unit 200. Furthermore, a motor 214 rotates and drives the belt conveying device 40 of the image forming unit in accordance with instructions from the control unit 200. In addition to these, the image forming apparatus 1 also includes a rotation drive mechanism that rotates and drives the rollers of the image forming units 20Y to 20K, the sheet feeding device 10, and the decurling device 70 shown in FIG. 1, but these are not shown here.

[0060] The operation panel B is equipped with various buttons and operation switches operated by the user, as well as a display unit that displays messages to the user and displays errors such as jams. In the above description, there is one loop detection sensor S1 and one residual sheet detection sensor S2, but the number of sensors may be increased to improve the accuracy of detecting the position of the flag.

[0061] [Loop Control] Here, we will explain loop control, which detects the loop state of a sheet using the loop detection unit 151 described above and controls the sheet conveying speed of the fixing device 100. When the loop detection sensor S1 does not detect a loop of the sheet during sheet conveyance, the control unit 200 sets the sheet conveying speed by the fixing belt 105 and the pressure belt 120 to a first speed. On the other hand, when the loop detection sensor S1 detects a loop of the sheet, the control unit 200 sets the sheet conveying speed by the fixing belt 105 and the pressure belt 120 to a second speed that is faster than the first speed.

[0062] That is, when the sheet is not looped, the control unit 200 reduces the rotation speed of the motor 212 that drives the fixing device 100, thereby reducing the speed at which the sheet is conveyed by the fixing belt 105 and the pressure belt 120 to be lower than the speed at which the sheet is conveyed by the secondary transfer unit T2 and the belt conveying device 40. This causes a loop to be formed in the sheet conveyed by the belt conveying device 40 and the fixing nip N. On the other hand, when the sheet is looped, the control unit 200 reduces the rotation speed of the motor 212 that drives the fixing device 100. Ta2 By increasing the rotation speed of the fixing belt 105 and the pressure belt 120, the speed at which the sheet is conveyed by the secondary transfer unit T2 and the belt conveying device 40 is made faster than the speed at which the sheet is conveyed by the secondary transfer unit T2 and the belt conveying device 40. This ensures the sheet conveying speed without making the sheet loop too large.

[0063] Such loop control in this embodiment will be specifically described with reference to the flowchart in Fig. 7. First, when a job is started, a sheet S is fed from the sheet storage cassette 11 and conveyed toward the registration rollers 15 (S101). Next, an image is formed in each image forming unit in synchronization with the sheet feeding, and the image is transferred to the sheet S conveyed from the registration rollers 15 at the secondary transfer unit T2 (S102). In this way, The image The sheet S onto which the toner image has been transferred is conveyed by the rotation of the secondary transfer portion T2 and the belt conveying device 40, and reaches the fixing device 100.

[0064] At this time, the conveying speed of the fixing device 100 (the sheet conveying speed by the fixing belt 105 and the pressure belt 120) is set to VL (first speed) which is 2% slower than the sheet conveying speed at the secondary transfer portion T2. ​​As a result, as the conveyance of the sheet S progresses, a loop of the sheet is formed between the belt conveying device 40 and the fixing nip portion N of the fixing device 100.

[0065] Then, the control unit 200 determines whether a loop is formed in the sheet from the state of the loop detection unit 151 (S103). Specifically, it determines whether the sheet S passes through the fixing nip N in a state where the output of the loop detection sensor S1 is OFF, i.e., where the sheet S does not form a loop, or whether the sheet S passes through the fixing nip N in a state where a loop is formed between the belt conveying device 40 and the fixing nip N (the output of the loop detection sensor S1 is ON).

[0066] If the sheet S passes through the fixing nip N without forming a loop (No in S103), the rotation of the motor 212 is controlled to set the conveying speed of the fixing device 100 to VL (first speed) (S104). That is, if the sheet conveying speed of the fixing device is VL, this VL is maintained, and if the sheet conveying speed of the fixing device is VH (described below), the sheet conveying speed of the fixing device is reduced from VH to VL.

[0067] On the other hand, if a loop is formed in the sheet in S103 (Yes in S103), the rotation of the motor 212 is controlled for a certain time h to set the conveying speed of the fixing device 100 to VH (second speed), which is 2% faster than the sheet conveying speed at the secondary transfer portion T2 (S105). That is, the sheet S is conveyed at the sheet conveying speed of the fixing device 100 at VH for a certain time h (0.5 seconds in this embodiment) to eliminate the sheet loop. Thereafter, the controls of S103 to S105 are repeated until the post-secondary transfer sensor 220 (FIG. 1) provided downstream of the secondary transfer portion T2 detects that the trailing edge of the sheet S has passed through the secondary transfer portion T2 (S106). Note that the loop detection sensor may be configured to determine that a loop exists if the sensor signal remains on for a certain time (e.g., 0.1 seconds) to prevent erroneous detection.

[0068] [Residual sheet detection control] Next, a description will be given of residual sheet detection control, which uses the above-mentioned residual sheet detection unit 152 to detect a sheet remaining in the fixing nip N and notify an error. If the residual sheet detection unit 152 detects a sheet when the device is powered on, the control unit 200 outputs a message indicating that a sheet remains in the fixing device 100. That is, if the residual sheet detection sensor S2 is ON when the image forming apparatus 1 is powered on, the control unit 200 displays an error message on the operation panel B indicating that a sheet remains in the fixing device 100. The control unit 200 may also output the error message to an external terminal, such as a personal computer, connected to the image forming apparatus 1.

[0069] The remaining sheet detection control of this embodiment will be specifically described with reference to the flowchart in Fig. 8. Fig. 8 is a flowchart illustrating the process of detecting remaining sheets S when the image forming apparatus 1 is powered on and when the image forming apparatus 1 is in an idle state where no print job is being executed. When the image forming apparatus 1 is powered on, the control unit 200 checks whether the remaining sheet detection sensor S2 is in the ON state, i.e., whether it is detecting sheets S (S201). If the remaining sheet detection sensor S2 is in the ON state (Yes in S201), the control unit 200 determines that a remaining sheet exists in the fixing device 100 and notifies the user of an error indicating the presence of a remaining sheet (S202).

[0070] On the other hand, in S201, if the remaining sheet detection sensor S2 does not detect the sheet S when the power is turned on (No in S201), the process proceeds to normal initialization processing, reception of an image formation job, and execution processing of the image formation job (S203). Note that an image formation job is an operation from the start of image formation based on a print signal (image formation signal) for forming an image on a sheet to the completion of image formation. The initialization processing is a preparatory operation before the image formation operation, in which the rotation of the photosensitive drum is started and various voltages are sequentially started up and various voltages are adjusted, and is what is known as pre-rotation processing.

[0071] Next, the control unit 200 determines whether the image forming job is in a standby state where no image forming job is being executed (S204). If the image forming job is in a standby state (Yes in S204), the process proceeds to S201, where a determination is made as to whether there are any remaining sheets, in the same manner as when the power is turned on as described above, and whether to notify the user of a jam error. If the image forming job is not in a standby state in S204 (No in S204), the process returns to S203 and continues normal processing. This makes it possible to determine whether there are any remaining sheets S when the sheet S is not being conveyed.

[0072] In this embodiment, the detection accuracy of both loop detection and residual sheet detection can be improved. Specifically, because the residual sheet detection flag 152a contacts the sheet at a position closer to the entrance of the fixing nip N than the loop detection flag 151a, residual sheet detection can be performed even for small-sized sheets. Furthermore, because the loop detection flag 151a and the residual sheet detection flag 152a are biased by the same biasing means, the rotational spring 154, the biasing force on the sheet can be smaller than when each flag is biased by a spring. Therefore, even if the sheet has low rigidity, deformation of the sheet can be suppressed. Furthermore, even if the sheet is light in weight, the loop detection flag 151a and the residual sheet detection flag 152a can be prevented from being pressed by the sheet and becoming difficult to move, thereby enabling more reliable detection by the sensors.

[0073] In addition, in this embodiment, by providing a loop detection section 151 and a residual sheet detection section 152 in the fixing entrance guide 150, residual sheet detection can be performed at a position close to the fixing nip section N, and loop detection can be performed at a position where the loop change amount is larger upstream in the sheet transport direction than the residual sheet detection position.

[0074] In addition, a rotation spring 154 is provided on the loop detection flag 151a, and the loop detection flag 151a and the residual sheet detection flag 152a are arranged to be rotatable around the same rotation axis 153, and an area is provided where the engagement protrusion 151c and the engagement recess 152c do not engage, so that each can rotate independently within a predetermined rotation range without providing multiple rotation springs 154.

[0075] <Other embodiments> In the above-described embodiment, the first and second contact portions of the loop detection unit 151 and the remaining sheet detection unit 152 are configured as the rotatable loop detection flag 151a and the remaining sheet detection flag 152a. However, the first and second contact portions may be configured to slide by contacting the sheet, in addition to rotating. For example, the first and second contact portions may be arranged to be movable in the vertical direction in FIGS. 4(a) to 5(b), and the first and second contact portions may be urged upward by a common urging means, such as a spring.

[0076] In the above embodiment, an intermediate transfer method has been described in which a toner image is transferred from the intermediate transfer belt 26 as an image carrier to a sheet, but the present invention can also be applied to a direct transfer method in which a toner image is transferred directly from a photosensitive drum to a sheet. In this case, the photosensitive drum corresponds to the image carrier. [Explanation of symbols]

[0077] N Fixing nip (nip) S...Seat T2: Secondary transfer unit (transfer unit) 1. Image forming device 1A: Device body 26. Intermediate transfer belt (image carrier) 31 Secondary transfer roller (transfer member) 100 Fixing device 105 Fixing belt (rotating body) 120 Pressure belt (rotating body) 150···Fixed entrance guide (entrance guide) 151 Loop detection unit 151a Loop detection flag (first contact) 152...Residual sheet detection unit 152a: Residual sheet detection flag (second contact portion) 153 Rotating shaft 154... Rotating spring (urging means) 200 Control unit

Claims

1. A transfer unit that transfers a toner image onto a sheet; a first rotor that is rotatable and has a heat source; a second rotating body that forms a nip portion by contacting an outer peripheral surface of the first rotating body, and that applies heat and pressure to fix a toner image while sandwiching and conveying a sheet together with the first rotating body; a loop detection unit that detects whether a sheet is looped between the nip unit and the transfer unit in a sheet conveying direction when the sheet is nipped and conveyed in the nip unit, with the surface of the sheet that contacts the first rotating body being the front surface and the surface of the sheet that contacts the second rotating body being the back surface, the loop detection unit having a first contact portion that contacts the back surface of the sheet onto which a toner image has been transferred in the transfer unit; a remaining sheet detection unit that detects whether a sheet remains in the nip portion between the nip portion and the transfer portion in the conveying direction, the remaining sheet detection unit having a second contact portion that can come into contact with the back surface of the sheet onto which a toner image has been transferred by the transfer portion; a rotation axis common to the first contact portion and the second contact portion, which supports the first contact portion and the second contact portion so as to be rotatable; a common biasing unit that applies a biasing force to the first contact portion and the second contact portion, and biases the first contact portion and the second contact portion toward a sheet transport path between the transfer portion and the nip portion, the second contact portion is capable of contacting the rear surface of the sheet downstream of the first contact portion in the conveying direction, The first contact portion and the second contact portion are rotatable independently of each other within a predetermined rotation range. An image forming apparatus characterized by:

2. When a jam occurs and the second contact portion comes into contact with the rear surface, the remaining sheet detection portion detects that a sheet remains in the nip portion.

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

3. The loop detection unit detects that a loop has formed in the sheet when the first contact portion comes into contact with the back surface during image formation.

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

4. the remaining sheet detection unit has a first sensor that detects that a sheet remains in the nip portion, The loop detection unit has a second sensor that detects a loop in the sheet.

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

5. a control unit for controlling the rotation speed of the second rotor; When the second sensor does not detect a loop in the sheet during conveyance of the sheet, the control unit sets the rotation speed of the second rotating body to a first speed, and when the second sensor detects a loop in the sheet, sets the rotation speed of the second rotating body to a second speed that is faster than the first speed.

5. The image forming apparatus according to claim 4.

6. an entrance guide that guides the sheet conveyed from the transfer unit to an entrance of the nip portion; The remaining sheet detection unit and the loop detection unit are provided on the entrance guide.

6. The image forming apparatus according to claim 1, wherein the image forming apparatus comprises: a first fixing member;

Images