JP2012144348A - Sheet feeding device and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet feeding device that can perform stable skew correction, and to provide an image forming apparatus.SOLUTION: Clamping pressure changing parts 505a, 505b are controlled based on a signal from paired sensors 502a, 502b for detecting a sheet before a resist roller. The sensors are provided between a plurality of pair of feeding rollers 503A, 503B and paired resist rollers 501, and intended for detecting a skew direction of a sheet S to be fed. The clamping pressure of the paired feeding rollers 503A that is located at a preceding side of the skew sheet S out of the paired feeding rollers 503A, 503B is made to be lower than those of the paired feeding rollers 503B that are located at following sides.

Description

  The present invention relates to a sheet conveying apparatus and an image forming apparatus, and more particularly to a configuration for correcting sheet skew.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a printer, a copier, or a facsimile, a skew feeding correction mechanism provided with a skew feeding correction unit that feeds a sheet to an image forming unit and corrects skew feeding of the sheet conveyed to the image forming unit Some have a sheet conveying apparatus having the following.

  Here, as such a skew correction mechanism, as the skew correction unit, a registration roller pair that corrects the skew of the sheet and aligns the image formed by the image forming unit and the leading edge of the sheet is used. There is something. When correcting the skew of the sheet, the leading edge of the sheet conveyed by the upstream conveying roller pair is abutted against the nip portion of the stopped registration roller pair.

  As a result, the sheet is bent, and the leading edge of the sheet follows the nip line of the registration roller pair due to the rigidity (rigidity) of the sheet, and the skew of the sheet is corrected. After that, the timing at which the registration roller pair starts to rotate is adjusted to align the image formed by the image forming unit with the leading edge of the sheet.

  However, in the skew correction mechanism of this configuration, when the sheet is a highly rigid sheet such as thick paper, for example, when the sheet hits the nip portion of the registration roller pair, the leading edge of the sheet penetrates the nip portion of the registration roller pair. There is. In this case, the front end position of the sheet is shifted and skew correction is insufficient. Further, when the sheet is a sheet having low rigidity such as thin paper, the bending cannot be formed well, and the sheet is buckled. As described above, the skew correction ability varies depending on the sheet type.

  Therefore, conventionally, the guide for loop formation can be swung in the foreground (width direction orthogonal to the sheet conveying direction), and this rocking guide can maintain a constant even if the loop amount changes in the foreground due to skew. There is one in which a pressing force is applied to a sheet (see Patent Document 1).

  In addition, there is a sensor in which a sensor for detecting the rigidity of a sheet is provided between the registration roller pair and the conveying roller. Then, the loop amount is adjusted by controlling the paper feed amount of the conveying roller according to the detection result of this sensor, that is, the sheet stiffness detected by the sensor, and the variation in the contact force to the registration roller pair is suppressed. (See Patent Document 2).

  In such a skew correction mechanism, a rotatable restricting plate is provided between the conveying roller and the registration roller pair, and the restricting plate is rotated according to the thickness of the sheet to move the sheet to the resist roller pair. The contact angle is adjusted. Further, the pressure detection sensor provided on the regulating plate detects the strength of the sheet when the loop is formed, and controls the paper feed amount of the conveying roller, thereby making the contact force to the registration roller pair uniform. I am doing so.

Japanese Patent Laid-Open No. 07-267427 JP 07-117899 A

  By the way, in the conventional sheet conveying apparatus provided with such a skew correction mechanism, the skew correction mechanism using the swing guide is configured such that the guide plate swings when the sheet contacts the registration roller pair to form a loop. It is the composition of moving. However, in such a configuration, only an equal pressing force is generated in the width direction with respect to the loop portion of the sheet, and it is difficult to strongly press the small side of the sheet loop, that is, the delayed side of the skewed sheet. Can not.

  Therefore, when the skew amount of the sheet is large, when the skewed sheet comes into contact with the registration roller pair, a phenomenon occurs in which only the preceding side of the sheet penetrates the nip of the registration roller pair. Further, since the swing guide comes into contact with the entire width direction of the sheet, a frictional resistance is generated between the sheet and the swing guide on the small loop side of the sheet. As a result, the force for abutting the small loop side against the registration roller pair becomes weaker than the force for abutting the large loop side against the registration roller pair, and the entire sheet tip cannot be abutted against the registration roller.

  Also, the skew correction mechanism provided with a sensor for detecting the stiffness of the sheet can adjust the contact angle of the sheet to the registration roller pair, but can penetrate the leading side of a sheet such as thick paper having a large skew amount. Can not be prevented. Furthermore, it is necessary to provide a pressure sensor and to displace the regulating plate so as to be displaceable. As a result, the number of parts increases, the structure becomes complicated, and the cost increases.

  Accordingly, the present invention has been made in view of such a current situation, and an object thereof is to provide a sheet conveying apparatus and an image forming apparatus capable of performing stable skew correction.

  The present invention provides a sheet conveying apparatus having a skew correction unit that corrects skew of a sheet by abutting the leading edge of the sheet being conveyed, and is arranged in parallel in a width direction perpendicular to the sheet conveying direction. A plurality of conveyance roller pairs that convey the sheet while sandwiching the sheet toward the correction unit; and a detection unit that is provided between the plurality of conveyance roller pairs and the skew feeding correction unit and detects a leading edge of the conveyed sheet. A plurality of conveying roller pairs, a clamping pressure changing unit that changes the clamping pressure of each conveying roller pair, and a signal from the detection unit to determine the skew direction of the sheet, The clamping pressure changing unit is configured to make the clamping pressure of the conveying roller pair positioned on the preceding side of the skewed sheet smaller than the clamping pressure of the conveying roller pair positioned on the delayed side of the skewed sheet. A control unit for controlling And it is characterized in and.

  As in the present invention, stable skew feeding can be achieved by making the clamping pressure of the conveying roller pair positioned on the preceding side of the skewed sheet smaller than the clamping pressure of the conveying roller pair positioned on the delayed side. Correction can be performed.

1 is a diagram illustrating a schematic configuration of a color copying machine that is an example of an image forming apparatus including a sheet conveying device according to a first embodiment of the present invention. FIG. 3 is a side view illustrating a configuration of a skew feeding correction mechanism provided in the sheet conveying apparatus. The top view of the said skew feeding correction mechanism. The figure explaining the structure of the clamping pressure change part provided in the said skew feeding correction mechanism. FIG. 3 is a control block diagram of the sheet conveying apparatus. 6 is a flowchart for explaining a sheet skew correction operation of the skew correction mechanism. The figure explaining the other structure of the said clamping pressure change part. FIG. 6 is a diagram illustrating a configuration of a skew feeding correction mechanism provided in a sheet conveying device according to a second embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a color copying machine which is an example of an image forming apparatus provided with a sheet conveying apparatus according to a first embodiment of the present invention.

  In FIG. 1, reference numeral 200 denotes a color copying machine, and 201 denotes a color copying machine main body (hereinafter referred to as an apparatus main body). The apparatus main body 201 is provided with an image forming unit 202, a sheet feeding unit 203 that feeds the sheet S, and a sheet conveying device 204 that conveys the sheet S fed from the sheet feeding unit 203. A reader unit 205 is provided on the upper part of the apparatus main body 201.

  Here, the reader unit 205 includes a document table glass 31, a document pressing plate 32 that can be opened and closed with respect to the document table glass 31, a light source that irradiates the image surface of the document O, and the like, and a moving optical system that scans the document. 33 and a CCD 34 which is a photoelectric conversion element (solid-state imaging element).

  The image forming unit 202 is also provided with an electrophotographic photosensitive drum (hereinafter referred to as a photosensitive drum) as an image carrier that is rotated counterclockwise by a motor (not shown), a charger 2, and a laser scanner 3. It is installed. Further, the image forming unit 202 is provided with a cleaner device 7 for cleaning residual toner on the photosensitive drum, a developing unit 401, and the like.

  The photosensitive drum 1 is rotated counterclockwise at a predetermined speed, and the surface thereof is uniformly charged to a predetermined polarity and potential by a charger 2 as a charging means. The laser scanner 3 includes a laser output unit, a polygon mirror, an imaging lens, a folding mirror, and the like, and laser light (optical signal) modulated in accordance with an image information signal input from an image processing unit (not shown). ) To scan and expose the charged surface of the rotating photosensitive drum 1. Then, an electrostatic latent image is formed on the surface of the photosensitive drum 1 by performing scanning exposure with the laser scanner 3 in this way.

  The developing unit 401 includes a rotary 41 that rotates counterclockwise as indicated by an arrow A, a black developer 4 for full-color development mounted on the rotary 41, and yellow, magenta, and cyan developers that are not shown. It has four color developing devices. Further, the developing unit 401 includes a clear developing device having clear toner that is, for example, a non-colored toner that is used in a clear single color mode to be described later.

  In addition, the developing unit 401 is disposed by switching the color developing units to the developing positions facing the photosensitive drum 1 by rotating the rotary 41 in a direction indicated by an arrow at a predetermined control timing. Then, by developing the electrostatic latent image for each color by each color developing device, toner images are sequentially formed on the photosensitive drum. The developing unit 401 includes a supply pipe 400 so that toner as a developer is supplied from a toner cartridge unit (not shown) as needed.

  In FIG. 1, reference numeral 8 denotes an endless intermediate transfer belt 5 that transfers four color toner images on top of each other, and then transfers the image onto the sheet S. 5 is an intermediate transfer belt unit provided with a primary transfer roller 6 and the like for transferring to a toner image. Here, the intermediate transfer belt 5 is an endless belt made of a dielectric material having flexibility, and is stretched around a plurality of rollers 5a to 5g. The intermediate transfer belt 5 is driven to rotate clockwise at substantially the same speed as that of the photosensitive drum 1, for example, using the roller 5a as a driving roller.

  Further, the outer surface of the intermediate transfer belt 5 is in contact with the photosensitive drum 1 between the rollers 5b and 5c, and this contact portion is a primary transfer nip portion T1. In the primary transfer nip T 1, a primary transfer roller 6 is disposed in contact with the inner surface of the intermediate transfer belt 5 on the side opposite to the photosensitive drum 1 side of the intermediate transfer belt 5.

  A primary transfer voltage is applied to the primary transfer roller 6, and the toner image of each color developed on the photosensitive drum 1 is applied to the intermediate transfer belt 5 by the application of the primary transfer voltage. Is transferred to. The residual toner on the intermediate transfer belt 5 is scraped off by a belt cleaning unit 16 that is a cleaning unit that cleans the intermediate transfer belt 5.

  Reference numeral 15 denotes a secondary transfer outer roller for transferring the toner image from the intermediate transfer belt 5 to the sheet S. The secondary transfer outer roller 15 is brought into contact with and separated from the intermediate transfer belt 5 by a pressure control mechanism (not shown). It is provided as possible. When the toner image is transferred to the sheet S, the secondary transfer outer roller 15 is intermediate with respect to one of the rollers 5a to 5g of which the intermediate transfer belt 5 is suspended. The transfer belt 5 is moved to the first position where the belt 5 is pressed.

  By moving to the first position in this way, a secondary transfer nip portion T2 is formed between the outer surface of the intermediate transfer belt 5 and the second transfer nip portion T2. Further, the secondary transfer outer roller 15 moves to a second position separated from the outer surface of the intermediate transfer belt 5 during standby when the toner image is not transferred onto the sheet S. A fixing unit 18 for fixing an unfixed image on the sheet is provided on the downstream side of the secondary transfer nip T2.

  The sheet feeding unit 203 is provided with sheet feeding cassettes 81 to 84 that store sheets S and are detachable from the apparatus main body 201. The sheets S stored in the paper feed cassettes 81 to 84 are sent out by the pickup roller 11. The sheet feeding unit 203 includes a multi-manual tray 85, and the sheets stored in the multi-manual tray 85 are sent out by the pickup roller 11 and the like.

  The sheet conveying device 204 includes a registration roller pair 501 and a conveying belt unit 17 that conveys a sheet onto which a toner image has been transferred to the fixing unit 18 as will be described later. Here, the registration roller pair 501 increases the posture position accuracy of the sheet S, and feeds the sheet S in a timely manner in accordance with the toner image on the intermediate transfer belt, and is positioned upstream of the secondary transfer nip T2. Is provided. A CPU 300 is an arithmetic control unit that controls an image forming operation of the color copying machine 200 and a skew correction operation of a skew correction mechanism 500 described later.

  Next, an image forming operation of the color copying machine 200 having such a configuration will be described.

  The moving optical system 33 moves while illuminating the original O, and scans the image surface of the original O. Then, the original scanning light is imaged on the CCD 34 and is color-separated and read with the three primary colors of RGB (red, green, and blue).

  Thereafter, the read RGB signals are input to an image processing unit (not shown), subjected to various image processing in the image processing unit, and then output to the laser scanner 3 as image information signals. The laser scanner 3 modulates the image information signal into an optical signal, and irradiates the modulated optical signal on the photosensitive drum as a first color optical signal through the lens and each reflection mirror. At this time, the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by the charger 2 in advance, and an electrostatic latent image is formed by irradiation with an optical signal.

  Next, the electrostatic latent image is developed by a developing unit corresponding to the first color selected from among the plurality of developing units arranged in the developing unit 401, and a first color toner image is formed. Thereafter, the toner image formed on the photosensitive drum is transferred onto the intermediate transfer belt 5 by the primary transfer roller 6 in the primary transfer nip T1.

  Here, in the color mode, the intermediate transfer belt 5 onto which the toner image has been transferred further rotates so that the next toner image is formed and transferred. During this time, the developing unit 401 rotates the next designated color developing device by 90 ° in the direction of the arrow B so as to face the photosensitive drum 1, and prepares to develop the next electrostatic latent image.

  After the primary transfer of the first color is completed in this way, the second color, the third color, the fourth color and the latent image, development, and primary transfer are repeated on the intermediate transfer belt 5 as in the first color. The toner images of each color are sequentially superimposed.

  On the other hand, the pickup roller 11 of the paper feed cassette 81 selected in advance, for example, from the paper feed cassettes 81 to 84 or the multi manual feed tray 85 is driven at a predetermined control timing in parallel with such an image forming operation. The As a result, one sheet S stored in the paper feed cassette 81 is separated and sent out, and then sent to the skew correction mechanism 500 including the pair of conveyance rollers 503 and the pair of registration rollers 501 through the sheet path 13. .

  Note that when the sheet is conveyed from the conveyance roller pair 503, the registration roller pair 501 is stopped, and the sheet S conveyed to the registration roller pair 501 in the stopped state contacts the sheet S. Skew is corrected. Thereafter, the timing is adjusted by the registration roller pair 501 and the sheet is sent to the secondary transfer nip portion T2 constituted by the intermediate transfer belt 5 and the secondary transfer outer roller 15. At this time, the secondary transfer outer roller 15 has moved to the first position at a predetermined control timing.

  Next, the sheet S is nipped and conveyed through the secondary transfer nip portion T2. During this time, a predetermined secondary transfer voltage is applied to the secondary transfer outer roller 15 to electrostatically transfer the toner images of a plurality of colors on the intermediate transfer belt 5 onto the sheet S in a batch. An unfixed toner image is formed (transferred) thereon.

  Next, the sheet S thus transferred to the secondary transfer nip T2 and having the toner image transferred by the secondary transfer outer roller 15 is separated from the surface of the intermediate transfer belt 5 and is fixed by the conveyance belt unit 17 to the fixing unit. 18 is conveyed. The fixing unit 18 is heated and pressed to fuse the unfixed toner image onto the sheet S to be a fixed image. Thereafter, the sheet S on which the toner image is fixed is conveyed to the discharge roller pair 82 through the conveyance path 19, and is discharged onto the discharge tray 20 by the discharge roller pair 82.

  Here, the color copying machine 200 can select a duplex printing mode and a multiplex printing mode in addition to a thick paper mode to be described later. For this reason, sheet conveyance paths 25 and 26 for transferring and fixing an image on the sheet S again are provided. When the double-sided printing mode or the multiple printing mode is selected, the sheet having the single-sided image formed or the first-time image formed from the fixing unit 18 is not reversed by the sheet conveyance paths 25 and 26, or the sheet is not reversed. Re-transport to the secondary transfer nip T2.

  For example, when the duplex printing mode is selected, the sheet that has exited the fixing unit 18 is first guided to the sheet conveyance path 21 by the switching member 26 and then reversed by the reversing roller 22. As a result, the sheet is conveyed rightward in the drawing by switching the switching member 27 with the sheet rear end side as the leading end, and is conveyed to the decurler 23. Next, the curl is corrected by the decurler 23, and then the position in the width direction is detected by the lateral registration detection sensor 24 and sent to the sheet conveyance path 25.

  The decurler 23 is set in the direction of curl correction so that when the sheet is fed again to the secondary transfer nip portion T2, the curl is downward (upwardly convex). This is because when the sheet is curled in the reverse upward direction (downward convex form), the leading edge of the sheet is caught at the entrance to the nip portion, is difficult to enter, is difficult to be conveyed, or the sheet jumps when the sheet enters and exits the nip. This is because a secondary transfer failure (image failure) due to a large size or the like occurs.

  Further, the position information of the sheet edge detected by the lateral registration detection sensor 24 is used for correcting the writing position in the main scanning direction of the image on the back surface. As a result, even if the position of the sheet in the main scanning direction changes during the double-sided conveyance pass, it is possible to form an image at the same main scanning position as the image on the first side. Thereafter, the sheet S passes through the fixing unit 18 again, passes through the conveyance path 19, passes through the discharge roller pair 82, and is discharged onto the discharge tray 20.

  When the sheet is reversed and discharged in the reverse direction, the switching member 26 guides the sheet to the sheet conveyance path 21. Next, the sheet is reversed by the reversing roller 22 and conveyed to the reversing path 30 by the switching member 28 with the trailing end side of the sheet as the leading end. Thereafter, the curler is corrected by the decurler 29 and discharged onto the discharge tray 20. Is done. The decurler 29 is set so as to discharge with a downward curl formed on the paper discharge tray. Here, the downward curl is formed and discharged in order to maintain a good stacking state of the sheets on the discharge tray 20.

  The decurler 29 is provided in a conveyance path used when the sheet is reversed and discharged in the reverse direction. The decurler 29 is reversed by the reversing roller 22 and then reversely conveyed upward in the figure without operating the switching member 27, and then the sheet conveying direction is switched to the discharge roller side by the switching member 28. It has a role of forming a downward curl on the paper tray and discharging it. The reason for forming the downward curl on the sheet in this way and discharging it is to keep the stacked state of the sheet on the sheet discharge tray 20 well.

  FIG. 2 is a side view showing the configuration of the skew feeding correction mechanism 500, and FIG. 3 is a plan view thereof. As shown in FIGS. 2 and 3, the skew feeding correction mechanism 500 includes a registration roller pair 501 that is a skew feeding correction unit, and a conveyance roller pair 503 (503 a and 503 b) that sandwich and convey a sheet toward the registration roller pair 501. And. In addition, an upper guide 504 is provided so that the sheet conveyed to the stopped registration roller pair 501 by the conveying roller pair 503 can bend upward (a loop).

  Furthermore, two pre-registration sheet detection sensor pairs 502 (502a and 502b) are provided between the conveyance roller pair 503 and the registration roller pair 501 at the front and rear sides on both sides in the width direction orthogonal to the sheet conveyance direction. ing. In the present embodiment, the two pre-registration sheet detection sensor pairs 502 detect the skew feeding direction of the conveyed sheet as will be described later. However, the line sensor is used as a detection unit in the width direction. The skew direction of the sheet can also be detected by providing.

  Two conveyance roller pairs 503 are arranged side by side in the width direction. Of the two rollers 503a and 503b constituting the two conveying roller pairs 503A and 503B, the upper roller 503a is pressed against the lower roller 503b so that the clamping pressure (pressure contact pressure) can be changed. Yes. The clamping pressure of the upper roller 503a with respect to the lower roller 503b can be changed by a clamping pressure changing unit 505 (505a, 505b) provided for each of the conveying roller pair 503A, 503B.

  As shown in FIG. 4, the clamping pressure changing unit 505 includes a spring 506 that biases the bearing 510 that supports the transport roller shaft 509 of the upper roller 503 a downward, a support member 507 that supports the spring 506, and a support member A cam 508 is provided to press 507 downward. Then, the cam 508 is driven and rotated by the cam drive motor M1, and the supporting member 507 and the bearing 510 are moved in the vertical direction against the spring 506, whereby the upper roller 503a is clamped with respect to the lower roller 503b ( (Pressure contact pressure) can be changed.

  In the present embodiment, the two sandwiching pressure changing sections 505a and 505b are operated independently, whereby each of the transport roller pairs 503A and 503B arranged side by side in the width direction. The clamping pressure of 503A and 503B can be changed independently. Then, the sheet conveying force of the pair of conveying rollers 503A and 503B can be changed in the width direction by independently changing the clamping pressure of the pair of conveying rollers 503A and 503B.

  FIG. 5 is a control block diagram of the sheet conveying apparatus 204 according to the present embodiment. As shown in FIG. 5, the detection signals of the pre-registration sheet detection sensor (front side) 502 a and the pre-registration sheet detection sensor (back side) 502 b constituting the pre-registration sheet detection sensor pair 502 are input to the CPU 300. The CPU 300 (calculation control unit) is connected to a cam drive motor M1 provided in the clamping pressure changing units 505a and 505b and a registration roller drive motor M2 for driving the registration roller pair 501.

  The leading edge of the sheet passing through the sheet conveying path (the sheet conveying direction downstream end) is detected by the pre-registration sheet detection sensor (front side) 502a and the pre-registration sheet detection sensor (back side) 502b. The CPU 300 as the control unit determines the sheet skew direction based on the detection signals from the two (plural) sensors 502a and 505b, that is, the sheet detection timing of the sensors 502a and 505b.

  Here, after this, when the skewed sheet comes into contact with the stopped registration roller pair 501, the sheet follows the nip of the registration roller pair 501 while being bent. The leading side forms a larger loop than the delayed side. As a result, the leading side of the sheet may pierce the registration roller pair 501 in some cases.

  Therefore, in the present embodiment, the CPU 300 sets the clamping pressure of the conveying roller pair 503 positioned on the preceding side of the skewed sheet according to the determined skewing direction of the sheet to the delayed side of the skewed sheet. It is made smaller than the clamping pressure of the conveying roller pair 503 positioned. For example, the CPU 300 operates the nipping pressure changing unit 505 of the conveying roller pair 503 located on the preceding side of the sheet so as to reduce the nipping pressure of the conveying roller pair 503. As a result, the skewed sheet can be pressed against the stopped pair of registration rollers 501 more strongly than the smaller side of the loop than the larger side, and as a result, the leading edge of the sheet can easily follow the nip of the registration roller pair 501.

  Next, the sheet skew correction operation of the skew correction mechanism 500 according to the present embodiment will be described with reference to the flowchart shown in FIG. When the image forming operation is started, the sheet is transported from the sheet transport path 25 in any of the first to fourth paper feed cassettes 81 to 84, the plurality of paper feed units of the multi-manual tray 85, or at the time of duplex printing. The sheet S is conveyed to the conveyance roller pair 503. Then, the sheet S is conveyed by the conveying roller pair 503 toward the registration roller pair 501 (S001).

  At this time, the leading edge of the sheet passes through the sheet detection sensor 502 disposed in the middle of the conveying roller pair 503 and the registration roller pair 501 (S002). Then, signals are input to the CPU 300 from the sensors 502a and 505b that have detected the leading edge of the sheet, and the CPU 300 determines the sheet skew direction based on the sheet detection timing of each of the sensors 502a and 505b. Here, the CPU 300 releases the nipping pressure between the pair of conveyance rollers 503A and 503B on the front and back sides in the width direction independently according to the skew direction of the sheet thus determined, or the sheet and conveyance roller pair 503A, The clamping pressure is reduced to a pressure at which 503B slips.

  For example, when the pre-registration sheet detection sensor (front side) 502a detects a sheet first (Y in S003), the CPU 300 reduces the clamping pressure of the conveying roller pair 503B located on the preceding side of the sheet. Activating 505a. Specifically, the front cam drive motor M1a is driven (S004). When the sheet S is conveyed in the skew state (skew direction) as shown in FIG. 3, the pre-registration sheet detection sensor (back side) 502b detects the sheet first. When the pre-registration sheet detection sensor (back side) 502b detects the sheet first (N in S003), the CPU 300 changes the nipping pressure so as to reduce the nipping pressure of the conveying roller pair 503B located on the preceding side of the sheet. The part 505b is operated. Specifically, the front cam drive motor M1b is driven (S005).

  After releasing or reducing the clamping pressure of the conveying roller pair 503 in this way, the sheet is conveyed to the nip of the registration roller pair 501 by the conveying roller pair 503. As a result, while the sheet is skewed, either the front side or the back side of the leading edge comes into contact with the nip of the registration roller pair 501 in the stopped state first (S006). Thereafter, a loop is formed below the upper guide 504 while the leading edge of the sheet follows the nip of the registration roller pair 501 depending on the rigidity of the sheet (S007).

  At this time, the clamping pressure of the conveying roller pair 503 on the side leading in the width direction of the sheet is released or reduced, so that the smaller side of the loop is stronger than the larger side in the width direction. Pressed against the nip of the roller pair 501. As a result, the front end of the sheet can easily follow the nip of the registration roller pair 501, and the skew of the sheet can be corrected even when the skew is large.

  Next, after correcting the skew of the sheet in this way, the nipping pressure changing unit 505 is operated at a predetermined timing, and the nipping pressure of the conveying roller pair 503 located on the preceding side of the sheet is returned to the original size. (S008). Then, after the clamping pressure of the conveying roller pair 503 is returned in this way, the registration roller pair 501 is driven in accordance with the timing (S009), and the sheet is conveyed to the secondary transfer unit.

  As described above, in the present embodiment, the holding pressure of the conveying roller pair positioned on the preceding side of the skewed sheet is made smaller than the holding pressure of the conveying roller pair positioned on the delayed side. . As a result, when the loop is formed, a conveying force larger than the conveying force on the large loop side can be applied to the small loop side, and the leading end of the sheet can easily follow the nip of the registration roller pair 501. As a result, a stable loop can be formed, and stable skew correction can be performed.

  The color copying machine 200 according to the present embodiment includes a thick paper mode for forming an image on thick paper having a large rigidity. In the thick paper mode, the user sets information such as the size and basis weight of the sheet on which an image is formed. Alternatively, the thickness of the sheet on which an image is formed is detected by a paper thickness detection sensor provided in the conveyance path.

If the sheet is determined to be, for example, thick paper (basis weight 106 g / mm ² or more) based on information such as the set sheet size and basis weight, and the detected sheet thickness, the leading edge of the sheet is a registration roller. Before reaching the tip, the clamping pressure of the transport roller is uniformly reduced. As a result, it is possible to prevent leading edge penetration due to rigidity when the thick paper comes into contact with the registration roller, and it is possible to reduce variation in skew correction ability due to sheet thickness.

  As described above, in the present embodiment, when the skew feeding correction of the sheet is performed, the sheet forming pressure of the conveying rollers arranged in parallel is independently changed according to the skew feeding direction of the sheet, thereby forming a loop. Sometimes it is possible to apply a conveying force only to the side where the loop is small. As a result, the front end of the sheet can be easily aligned with the registration roller pair or the bus member of the shutter member, and stable loop formation and thus skew correction can be performed.

  In the above description, the sandwiching pressure changing unit 505 has been described with respect to the configuration in which the sandwiching pressure of the conveying roller pair 503 is changed by rotating the cam. However, the present invention is not limited to this. For example, as shown in FIG. 7, the solenoid 514 is linked to a bearing 510 that supports the transport roller shaft 509, and the link member 512 is swung around the swing shaft 512a by turning on / off the solenoid, thereby transporting the roller. The clamping pressure of the pair 503 may be changed.

  Next, a second embodiment of the present invention will be described. FIG. 8 is a diagram illustrating the configuration of the skew feeding correction mechanism provided in the sheet conveying apparatus according to the present embodiment. In FIG. 8, the same reference numerals as those in FIG. 2 described above indicate the same or corresponding parts.

  In FIG. 8, reference numeral 520 denotes a conveying roller pair, and a shutter member 515 as a skew feeding correcting portion is rotatably attached to the shaft 521a of the upper roller 521 among the rollers 521 and 522 constituting the conveying roller pair 520. It has been. Here, the shutter member 515 includes an abutting portion 521b that abuts on the sheet conveyed from the conveying roller pair 503, and is urged in a direction opposite to the sheet conveying direction by an urging member (not shown).

  When the leading edge of the sheet S conveyed by the conveying roller pair 503 hits the contact portion 521b of the shutter member 515, the sheet S bends and presses the shutter member 515 while forming a loop. As a result, the leading edge of the sheet follows the shutter member 515 while being bent, and the skew of the sheet is corrected. After the skew is corrected in this way, the sheet passes while moving the shutter member 515 from the position where it comes into contact with the sheet against the urging force to the position where the sheet is allowed to pass while contacting the sheet. Go.

  Here, when the skew amount is large, the leading side of the sheet forms a larger loop than the delayed side. As a result, the shutter member 515 may be rotated before the leading edge of the sheet follows the shutter member 515. Therefore, also in this embodiment, the nipping pressure changing unit 505 of the conveying roller pair 503 located on the preceding side of the sheet is operated to reduce the nipping pressure of the conveying roller pair 503.

  As a result, the sheet skewed against the shutter member 515 stopped at the position where it abuts against the sheet can be pressed more strongly on the smaller side of the loop than on the larger side, and as a result, the leading end of the sheet can easily follow the shutter member 515. Become. As a result, the skew of the sheet can be corrected with high accuracy.

DESCRIPTION OF SYMBOLS 200 ... Color copier, 201 ... Color copier main body, 202 ... Image forming part, 204 ... Sheet conveying apparatus, 300 ... CPU, 500 ... Skew correction mechanism, 501 ... Registration roller pair, 502 (502a, 502b) ... Register Front sheet detection sensor pair, 503 (503A, 503B) ... conveying roller pair, 505 (505a, 505b) ... clamping pressure changing unit, 515 ... shutter member, 520 ... conveying roller pair, S ... sheet

Claims (7)

In the sheet conveying apparatus having a skew correction unit that corrects the skew of the sheet by abutting the leading edge of the conveyed sheet,
A plurality of pairs of conveyance rollers that are arranged in parallel in the width direction orthogonal to the sheet conveyance direction and convey the sheet while sandwiching the sheet toward the skew feeding correction unit;
A detection unit that is provided between the plurality of conveyance roller pairs and the skew feeding correction unit and detects a leading edge of a conveyed sheet;
A clamping pressure changing unit that changes a clamping pressure of each of the plurality of conveyance roller pairs;
Based on the signal from the detection unit, the skew direction of the sheet is determined, and the sandwiching pressure of the conveying roller pair positioned on the preceding side of the skewed sheet among the plurality of conveying roller pairs is skewed. A control unit for controlling the clamping pressure changing unit so as to be smaller than the clamping pressure of the conveying roller pair located on the delayed side of
A sheet conveying apparatus comprising: The detection unit is arranged in parallel in the width direction, and includes a plurality of detection sensors that detect downstream ends of the conveyed sheet in the sheet conveyance direction,
The sheet conveying apparatus according to claim 1, wherein the control unit determines a sheet skew direction in accordance with detection timings of the plurality of detection sensors, and controls the nipping pressure changing unit.   The sheet conveying apparatus according to claim 1, wherein the nipping pressure changing unit is capable of independently changing nipping pressures of the plurality of conveying roller pairs.   2. The skew feeding correcting portion is a pair of registration rollers that abuts against a sheet conveyed by the plurality of conveying rollers in a stopped state and corrects the skew feeding of the sheet while bending the sheet. 4. The sheet conveying apparatus according to any one of items 1 to 3.   The skew feeding correction portion has a contact portion that comes into contact with the sheet conveyed by the plurality of conveyance rollers, and corrects the skew feeding of the sheet while bending the sheet in contact with the contact portion. 4. The sheet conveying apparatus according to claim 1, wherein the sheet conveying device is a shutter member that is pressed by the corrected sheet and passes the sheet while rotating. 5. It has a mode to convey a rigid sheet,
6. The method according to claim 1, wherein when the mode is selected, before the sheet reaches the skew feeding correction portion, the clamping pressure of the plurality of conveyance roller pairs is reduced. Sheet conveying device.   An image forming apparatus comprising: an image forming unit; and the sheet conveying device according to claim 1.

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