Image forming device

Abstract

To avoid trouble caused by shifting a sheet.SOLUTION: An image forming apparatus is provide with an image forming unit that forms an image on a sheet, a moving means that moves the roller in a width direction of the sheet that intersects the conveying direction of the sheet in the roller, and a control means for controlling not to move the roller in the width direction by the moving means if the sheet has a second length that is longer than a first length.SELECTED DRAWING: Figure 7

Description

[Technical Field]

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

[0002] Generally, in an image forming apparatus such as a copy machine, a sheet may shift laterally in the width direction of the sheet while being conveyed. If an image is formed on the sheet while the sheet is shifted laterally, the image will be printed offset from the center of the sheet, resulting in poor sheet quality. For this reason, a shift mechanism is known that detects the position of the edge of the sheet in the width direction and corrects the sheet's lateral shift (misalignment) before forming an image on the sheet. (See Patent Document 1) [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-143643 Summary of the Invention [Problem to be solved by the invention]

[0004] However, there are cases where problems occur when a sheet that is long in the conveying direction is shifted in the width direction. [Means for solving the problem]

[0005] The image forming apparatus of the present invention includes an image forming unit that forms an image on a sheet, a pair of reversing rollers that rotate in a first direction while sandwiching a sheet on which an image is formed by the image forming unit, and then rotate in a second direction opposite to the first direction to reverse and convey the sheet, a moving unit that moves the pair of reversing rollers in a width direction of the sheet that intersects with a conveying direction while the sheet is sandwiched between the pair of reversing rollers, an acquiring unit that acquires information about the length of the sheet in the conveying direction, and when a first sheet having a length in the conveying direction is to be reversed and conveyed, the moving unit is configured to move the pair of reversing rollers in a width direction of the sheet that intersects with a conveying direction. Invert The present invention is characterized in that it has a control means that controls the moving means based on information regarding the length of the sheet acquired by the acquisition means so that the roller pair is moved a first amount in the width direction, and when transporting a second sheet whose length in the transport direction is a second length longer than the first length, the moving means moves the reversing roller pair transporting the second sheet in the width direction a second amount smaller than the first amount. [Effects of the Invention]

[0006] According to the present invention, it is possible to avoid problems caused by shifting a long sheet in the width direction. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic view showing an entire printer according to a first embodiment. [Figure 2] FIG. [Figure 3] FIG. 2 is a control block diagram showing a control unit. [Figure 4] 10 is a flowchart showing a skew correction operation and a shift operation by the registration unit. [Figure 5] (a) is a plan view showing the state in which the sheet is skewed, (b) is a plan view showing the state in which the edge position of the sheet is detected, (c) is a plan view showing the state in which the sheet is being transported by a pair of registration rollers, and (d) is a plan view showing the sheet after being shifted. [Figure 6] FIG. [Figure 7] 10 is a flowchart showing a shift operation performed by the reversing conveying unit. [Figure 8] (a) is a schematic diagram showing how a sheet is transported toward the inverting shift section, (b) is a schematic diagram showing the state in which the sheet is stopped by the inverting shift section, and (c) is a schematic diagram showing how the inverted sheet is transported. [Figure 9] FIG. 4 is a perspective view showing a second double-sided conveying unit. [Figure 10] 10 is a flowchart showing a skew correction operation performed by the second double-sided conveying unit. [Figure 11] FIG. 10 is a perspective view showing a second double-sided conveying unit according to a second embodiment. [Figure 12] FIG. 2 is a control block diagram showing a control unit. [Figure 13] 10 is a flowchart showing a skew correction operation and a shift operation by the second double-sided conveying unit. [Figure 14] 10A and 10B are diagrams illustrating a state of a sheet being conveyed in a skewed state. DETAILED DESCRIPTION OF THE INVENTION

[0008] First Embodiment [Overall structure] First, a first embodiment of the present invention will be described. Image forming apparatus 1 of this embodiment is an electrophotographic full-color laser beam printer. As shown in Fig. 1, image forming apparatus 1 has a housing 1A as a first housing having a unit for feeding sheets and forming images, and a housing 1B as a second housing having a unit for fixing and cooling sheets, and housing 1B is connected to housing 1A.

[0009] The housing 1A has feeding units 10a and 10b, pulling units 20a and 20b, a registration unit 30, an image forming unit 90, and a first double-sided conveying unit 70. The housing 1B has a fixing unit 100, a cooling unit 110, a branching conveying unit 120, a reversing conveying unit 130, a second double-sided conveying unit 150, and a decurling unit 170.

[0010] The image forming unit 90 includes four process cartridges 99Y, 99M, 99C, and 99Bk that form toner images of four colors, yellow (Y), magenta (M), cyan (C), and black (K), respectively, and exposure devices 93, 96, 97, and 98. The four process cartridges 99Y, 99M, 99C, and 99Bk have the same configuration except for the colors of the images they form. Therefore, only the configuration and image forming process of the process cartridge 99Y will be described, and descriptions of the process cartridges 99M, 99C, and 99Bk will be omitted.

[0011] The process cartridge 99Y has a photosensitive drum 91, a charging roller (not shown), a developing unit 92, and a cleaner 95. The photosensitive drum 91 is constructed by coating the outer periphery of an aluminum cylinder with an organic photoconductive layer, and is rotated by a drive motor (not shown). The image forming unit 90 also has an intermediate transfer belt 50 that is rotated in the direction of arrow T1 by a drive roller 52. The intermediate transfer belt 50 is wound around a tension roller 51, the drive roller 52, and a secondary inner transfer roller 53. Primary transfer rollers 55Y, 55M, 55C, and 55Bk are provided inside the intermediate transfer belt 50, and a secondary outer transfer roller 54 is provided outside the intermediate transfer belt 50, facing the secondary inner transfer roller 53.

[0012] Feeding unit 10a has a lift plate 11a that moves up and down while stacking sheets S, a pickup roller 12a that feeds the sheets S stacked on lift plate 11a, and a separation roller pair 13a that separates the fed sheets one by one. Similarly, feeding unit 10b has a lift plate 11b that moves up and down while stacking sheets S, a pickup roller 12b that feeds the sheets S stacked on lift plate 11b, and a separation roller pair 13b that separates the fed sheets one by one.

[0013] The registration unit 30 has a pre-registration roller pair 31 that conveys the sheet, and a registration roller pair 32 that serves as a first moving unit and a first skew correction unit that corrects skew of the sheet. The registration unit 30 also has a registration sensor 33 that detects the position of the sheet S in the conveyance direction, and a CIS 34 that serves as a first detection unit that detects the position of the sheet in the width direction that intersects with the conveyance direction. The fixing unit 100 has a fixing roller pair 101 that can heat the sheet.

[0014] The cooling unit 110 has an upper cooling belt 111a that is rotated in the direction of arrow T2 by an upper cooling drive roller 112a, a lower cooling belt 111b that is rotated in the direction of arrow T2 by a lower cooling drive roller 112b, and a heat sink 113 that cools the sheet.

[0015] Next, we will explain the image forming operation of the thus configured image forming apparatus 1. When an image signal is input to the exposure device 93 from a personal computer (not shown) or the like, the exposure device 93 irradiates the photosensitive drum 91 of the process cartridge 99Y with a laser beam corresponding to the image signal.

[0016] At this time, the surface of the photosensitive drum 91 is uniformly charged to a predetermined polarity and potential by a charging roller, and an electrostatic latent image is formed on the surface by irradiating the surface with laser light from an exposure device 93 via a mirror 94. The electrostatic latent image formed on the photosensitive drum 91 is developed by a developing device 92, and a yellow (Y) toner image is formed on the photosensitive drum 91.

[0017] Similarly, laser light is irradiated from exposure devices 96, 97, and 98 onto the photosensitive drums of process cartridges 99M, 99C, and 99Bk, forming magenta (M), cyan (C), and black (K) toner images on the photosensitive drums. The toner images of each color formed on the photosensitive drums are transferred to the intermediate transfer belt 50 by primary transfer rollers 55Y, 55M, 55C, and 55Bk. The full-color toner image is then transported to the secondary transfer nip N formed by the inner secondary transfer roller 53 and the outer secondary transfer roller 54 by the intermediate transfer belt 50, which is rotated by a drive roller 52. Toner remaining on the photosensitive drum 91 is collected by a cleaner 95. The image formation process for each color is performed at a timing to overlap the upstream toner image that was primarily transferred onto the intermediate transfer belt 50.

[0018] In parallel with this image forming process, a sheet S is fed from either one of feeding units 10a or 10b, and is conveyed to a registration unit 30 by either one of pulling units 20a or 20b. In the registration unit 30, a pre-registration roller pair 31 abuts the leading edge of the sheet S against the nip portion of a registration roller pair 32 that is stopped. This corrects the skew of the sheet S, and the sheet S is conveyed to a secondary transfer nip N serving as an image forming unit at a predetermined conveyance timing. A full-color toner image on an intermediate transfer belt 50 is transferred onto a first sheet surface (front surface) of the sheet S by a secondary transfer bias applied to a secondary transfer outer roller 54. Residual toner remaining on the intermediate transfer belt 50 is collected by a belt cleaner 56.

[0019] The sheet S onto which the toner image has been transferred is conveyed to the fixing unit 100 by the pre-fixing conveyance section 60. The sheet S is then guided to the nip portion of a pair of fixing rollers 101, where predetermined heat and pressure are applied to melt and fix the toner. After passing through the fixing unit 100, the sheet S is conveyed in the cooling unit 110 while being sandwiched between an upper cooling belt 111a and a lower cooling belt 111b, which are endless belts. The heat of the sheet S is then transferred to the heat sink 113 via the upper cooling belt 111a, and the sheet S is cooled.

[0020] Next, the branching conveying unit 120 selects a path for conveying the sheet S to either the decurling unit 170 or the reversing conveying unit 130. After the sheet S is conveyed to the reversing conveying unit 130, the sheet S can be reversed at the secondary transfer nip N so that the first sheet surface on which the image is formed faces downward, and then the sheet S can be conveyed to the decurling unit 170.

[0021] When an image is formed on only one side of the sheet S, the sheet S is conveyed from the branching conveying unit 120 to the decurling unit 170, where the curl of the sheet is corrected by a small-diameter hard roller and a large-diameter soft roller. Subsequently, the sheet S that has passed through the decurling unit 170 is discharged onto the discharge tray 171.

[0022] When images are formed on both sides of the sheet S, the sheet S is conveyed to the reversing conveying unit 130 by the branching conveying unit 120 and is switched back in the reversing conveying unit 130. The switched back sheet S is conveyed from the reversing conveying unit 130 to the second double-sided conveying unit 150 and the first double-sided conveying unit 70 and is guided to the registration unit 30. Thereafter, an image is formed on the second sheet surface (back surface) of the sheet S at the secondary transfer nip N, and the sheet S is discharged to the discharge tray 171 via the branching conveying unit 120 and the decurling unit 170.

[0023] The branch conveying unit 120, the inverting conveying unit 130, the second double-sided conveying unit 150 and the first double-sided conveying unit 70 constitute a re-conveying section 500 that inverts the sheet with an image formed on its first side and conveys the sheet again to the secondary transfer nip N.

[0024] In addition, in the image forming apparatus 1 according to this embodiment, as an example, the explanation will be given assuming that a centrally based sheet conveying method is adopted in which the center of the width direction perpendicular to the conveying direction in the conveying path 65 is aligned with the center of the width direction of the sheet to convey the sheet.

[0025] [Registration Unit] 1 and 2, the registration unit 30 is provided in a conveying path 65 that connects the pull-out unit 20a and the secondary transfer nip N. The registration unit 30 also has a registration roller pair 32, a pre-registration roller pair 31, a registration sensor 33, and a CIS (Contact Image Sensor) 34. The pre-registration roller pair 31 is disposed upstream of the registration roller pair 32 in the sheet conveying direction A, and the registration sensor 33 and the CIS 34 are provided between these roller pairs.

[0026] As shown in Fig. 2, the registration roller pair 32, which is a pair of rotating bodies, has an upper roller 32a serving as a first roller and a lower roller 32b serving as a second roller fixed to a rotary shaft 32S. An input gear 38 is fixed to the rotary shaft 32S, and the input gear 38 is driven by a registration drive motor 36 via an idler gear 39. The pre-registration roller pair 31 is driven by a pre-registration drive motor 35. Each roller of the pre-registration roller pair 31 and the registration roller pair 32 rotates about an axis extending in the width direction W.

[0027] A rack 41 is supported on the rotary shaft 32S so as to be rotatable relative to the rotary shaft 32S but immovable in the axial direction. The rack 41 receives a driving force from a shift motor 37 via a pinion gear 40, and shifts the rotary shaft 32S in the axial direction. Furthermore, a flange portion 42 formed integrally with the upper roller 32a is sandwiched by an input gear 38 of the lower roller 32b, so that the upper roller 32a shifts in the axial direction in conjunction with the lower roller 32b. The pair of registration rollers 32 sandwiching the sheet S moves in a width direction W perpendicular to the conveying direction A, so that the sheet moves in the width direction W, and the position of the sheet in the width direction W is corrected.

[0028] The idler gear 39 has a wider tooth width than the input gear 38. This is to maintain the gear meshing and enable the registration roller pair 32 to rotate even when the registration roller pair 32 and the input gear 38 move in the width direction.

[0029] The CIS 34 detects the position of the edge in the width direction W of the conveyed sheet S (hereinafter referred to as the edge position). The control unit 200 (see FIG. 3) calculates the amount of deviation between the design reference position of the sheet and the edge position detected by the CIS 34, and shifts the registration unit 30 by the amount of this deviation. This causes the position of the sheet S in the width direction W to match the transfer position in the image forming unit 90, resulting in a high-quality product.

[0030] The CIS 34 is disposed at a position offset to one side with respect to the center of the conveying path 65 in the width direction W. This is because it is necessary to detect the edge position of only one side of the sheet S in the position correction of the sheet S. The CIS 34 is configured to be able to detect the edge positions of the smallest and largest sheets among the sheet sizes permitted for use in the image forming apparatus 1. The CIS 34 is disposed as close as possible to the registration roller pair 32 so as not to reduce the detection accuracy of the CIS 34.

[0031] Furthermore, in the registration unit 30, the leading edge of the conveyed sheet S is abutted against the nip portion of the stopped registration roller pair 32, causing the sheet S to bend and align with the nip portion, thereby correcting skew. After the registration sensor 33 detects the leading edge of the sheet S, the sheet S is fed a predetermined amount by the pre-registration roller pair 31, and then conveyed by the registration roller pair 32 to the secondary transfer nip N.

[0032] Furthermore, a constant distance is maintained between the CIS 34 and the lower guide 65a facing the CIS 34, and a predetermined space is formed in the conveying path 65 by the lower guide 65a and the upper guides 65b and 65c so that the sheet can bend. The amount of conveyance of the sheet S by the pre-registration roller pair 31 is set so that an appropriate amount of bending is formed in the sheet S.

[0033] [Control Block] 3 is a control block diagram showing a control unit 200 of the image forming apparatus 1. The control unit 200 has a CPU 201, a memory 202, an operation unit 203, an image formation control unit 205, a sheet conveyance control unit 206, a sensor control unit 207, and a shift control unit 208. The CPU 201 executes predetermined control programs and the like to realize various processes performed by the image forming apparatus 1. The memory 202 is composed of, for example, RAM and ROM, and stores various programs and various data in predetermined storage areas. The operation unit 203, which serves as an acquisition unit, accepts input of various information related to sheets (for example, sheet size, sheet basis weight, sheet surface properties, etc.), as well as execution and cancellation of jobs.

[0034] The image formation control unit 205 issues instructions to the image forming unit 90, which includes the exposure devices 93, 96, 97, 98, etc., and controls the image forming operation. The sheet transport control unit 206 issues instructions to the pre-registration drive motor 35, the registration drive motor 36, the reversal drive motor 136, the second pre-registration drive motor 153, the second registration drive motor 154, etc., thereby controlling the transport operation of the sheet S. The sensor control unit 207 issues detection start and detection stop instructions to the registration sensor 33, the reversal sensor 138, the second registration sensor 157, etc., and receives the detection results of these sensors.

[0035] The shift control unit 208 receives the detection results of the CIS 34 and the inverting CIS 139, and issues instructions such as starting and stopping the drive of the shift motor 37 and the inverting shift motor 137, thereby controlling the movement of the sheet S in the width direction W, i.e., the shift operation. The CPU 201 can also be connected to an external computer 204 connected via a network, for example, and can receive various information about the sheet, print jobs, etc. from the computer 204.

[0036] [Skew correction and shift operation by registration unit] Next, the skew correction operation (first skew correction operation) and shift operation by the registration unit 30 will be described with reference to the flowchart shown in Fig. 4. First, when a print instruction is input from the operation unit 203 or the computer 204, the control unit 200 starts a print job (step S101). Note that the user can also use the operation unit 203 or the computer 204 to specify the number of copies to be printed and the type of sheet to be used for printing.

[0037] The control unit 200 starts feeding the sheet S (step S102) and determines whether printing is being performed on the first or second side of the sheet in the print job (step S103). If it is determined that printing is being performed on the first side of the sheet, the control unit 200 controls the image forming unit 90 to form a toner image on the intermediate transfer belt 50 at a predetermined image writing position g1 for the first side (step S104). The image writing position g1 here is a value based on the results of writing position adjustment performed at the time of shipment from the factory, and is stored in the memory 202 as a fixed value specific to the device body.

[0038] Specifically, the control unit 200 controls the exposure devices 93, 96, 97, and 98 to form electrostatic latent images at image writing positions g1 on the photosensitive drums of the process cartridges 99Y, 99M, 99C, and 99K. Then, as described above, the electrostatic latent images formed on the photosensitive drums are developed into toner images by the developing devices, and these toner images are transferred onto the intermediate transfer belt 50 by the primary transfer rollers 55Y, 55M, 55C, and 55K.

[0039] Meanwhile, the sheet S is conveyed to the pre-registration roller pair 31. Here, as shown in FIG. 5(a), the conveyed sheet S is rotated clockwise with respect to the conveying direction A, skewed, and shifted to the left with respect to the conveying direction A. The dotted squares shown in FIGS. 5(a) to 5(d) schematically show a state in which the leading edge of the sheet S, which has been conveyed without skew or lateral shift, abuts on the nip portion of the registration roller pair 32. The end position of the sheet in the width direction W at this time is taken as the zero point, and the left side is taken as the positive direction.

[0040] Next, based on the detection result of the registration sensor 33 (step S105), the control unit 200 causes the pre-registration roller pair 31 to feed the sheet S by a set feed amount. As a result, the sheet S is abutted against the stopped registration roller pair 32, as shown in FIG. 5(b), and a predetermined amount of flexure is formed (step S106). In this way, skew correction of the sheet S is performed, and the sheet S is nipped and conveyed by the registration roller pair 32, whose rotational drive has started, as shown in FIG. 5(c) (step S107). Skew correction of the sheet S using the registration sensor 33 is performed regardless of the length of the sheet in the conveying direction.

[0041] Then, the CIS 34 detects the edge position of the sheet S after the skew correction (step S108), and the control unit 200 calculates the shift amount of the sheet based on the detection result (L1). In this case, the shift amount can be obtained by subtracting the image writing position (g1) from the detection result (L1) of the CIS 34 (L1 - g1).

[0042] The control unit 200 moves the pair of registration rollers 32 that sandwich the sheet S in the width direction W by the shift amount (L1-g1) via the shift control unit 208 and the shift motor 37. This allows the sheet S to be moved in the width direction W by the shift amount (L1-g1) (step S109). This corrects the position of the sheet S in the width direction W so that it corresponds to the image writing position g1. The shift of the sheet S in the width direction using the registration sensor 33 is performed regardless of the length of the sheet in the conveying direction.

[0043] Then, the sheet shifted by the shift amount (L1-g1) by the registration roller pair 32 has the toner image on the intermediate transfer belt 50 transferred to it by the secondary transfer nip N (step S110). After that, the toner image is fused and fixed by the fixing unit 100 (step S111).

[0044] In the case of a single-sided job, the sheet S with the fixed toner image is discharged to the discharge tray 171 and the job is completed (step S112). In the case of a double-sided job, the sheet S is inverted for image formation on the second side. Next, the control unit 200 determines whether or not there is a subsequent sheet (step S113). If the control unit 200 determines that there is no subsequent sheet (step S113: No), the print job is completed (step S114). If the control unit 200 determines that there is a subsequent sheet (step S113: Yes), the control unit 200 returns the registration roller pair 32 to the home position (center position) (step S115). Then, the process returns to step S103.

[0045] If the control unit 200 determines in step S103 that the second side of the print job is being printed, the control unit 200 controls the image forming unit 90 to form a toner image at image writing position g2 for the second side (step S116). Note that the image writing position g2 for the second side may be the same position as or different from the image writing position g1 for the first side in the width direction. The skew correction operation by the registration roller pair 32 for a sheet on which an image is formed on the second side is the same as for a sheet on which an image is formed on the first side, and therefore a description thereof will be omitted (steps S117 to S119).

[0046] After the skew correction, the CIS 34 detects the edge position of the second side of the sheet S (step S120), and the control unit 200 calculates the shift amount of the sheet S based on the detection result (L2). In this case, the shift amount can be obtained by subtracting the image writing position (g2) from the detection result (L2) of the CIS 34 (L2 - g2).

[0047] The control unit 200 moves the pair of registration rollers 32, which sandwich the sheet S, in the width direction W by the shift amount (L2 - g2) via the shift control unit 208 and the shift motor 37, which constitutes another moving means. This allows the sheet S to be moved in the width direction W by the shift amount (L2 - g2) (step S121). For example, when the image writing position on the second side is g2 = g1 = 0, shifting the sheet S by the shift amount L2 moves the sheet S to the same position as the position before image formation on the first side. This causes the positions of the images formed on the first side and the second side to match, and these images are formed in the center of the sheet S, resulting in a high-quality product.

[0048] Then, the sheet shifted by the shift amount (L2-g2) by the registration roller pair 32 has the toner image on the intermediate transfer belt 50 transferred to it by the secondary transfer nip N (step S122). Thereafter, as in the processing of the first side, this toner image is fused and fixed by the fixing unit 100, and the sheet is discharged to the discharge tray 171 (steps S111 and S112).

[0049] Here, when printing the second side, the sheet is transported a long distance after the registration unit 30 corrects skew and lateral misalignment on the first side. Therefore, skew and lateral misalignment are often greater than when printing the first side due to variations in the components of each unit. This results in a larger shift amount for the registration roller pair 32. When the registration roller pair 32 shifts, the sheet S experiences a large amount of friction resistance against the transport guide member. Especially when the sheet S is large, the resistance is large because it is nipped by other rollers. As a result, when the registration roller pair 32 shifts due to this resistance, the sheet S may become skewed, the shift amount of the sheet S may be smaller than expected, or the sheet S may become wrinkled.

[0050] Furthermore, when the shift amount is large, it takes a long time to shift the registration roller pair 32 and for the registration roller pair 32 to return to the home position (center position) after the sheet S passes through the registration roller pair 32. This may result in a loss of productivity. In order to reduce the above-mentioned problems, in this embodiment, the sheet S is shifted (horizontal registration shift) also in the reverse conveying unit 130.

[0051] [Reversing transport unit] Next, the configuration of the reverse conveying unit 130 will be described. As shown in Fig. 6, the reverse conveying unit 130 as a reversing section has a conveying roller pair 131, a reverse shift section 132 as a second moving section, a reverse sensor 138, a reverse CIS 139 as a second detecting section, and a switching member 143. The reverse shift section 132 has a first reverse shift roller pair 132a and a second reverse shift roller pair 132b as reversing rollers, and the reverse sensor 138 and the reverse CIS 139 are provided between the conveying roller pair 131 and the first reverse shift roller pair 132a.

[0052] The conveying roller pair 131 is driven by a reverse drive motor 136 via a belt 136a. Rotation of the conveying roller pair 131 is transmitted to an idler gear 135 via a belt 136b. An input gear 134 is fixed to a rotation shaft 132S of the first reverse shift roller pair 132a, and the input gear 134 is driven by the idler gear 135. The first reverse shift roller pair 132a and the second reverse shift roller pair 132b are connected by a belt 136c and configured to move in conjunction with each other. Each roller of the first reverse shift roller pair 132a and the second reverse shift roller pair 132b rotates around an axis extending in the width direction W. For example, the first reverse shift roller pair 132a has a third roller and a fourth roller that rotate around an axis extending in the width direction W, and these third roller and fourth roller move in the width direction W while sandwiching a sheet.

[0053] A rack 141 is supported on the rotation shaft 132S so as to be rotatable relative to the rotation shaft 132S but immovable in the axial direction. The rack 141 receives a driving force from a reverse shift motor 137, which serves as a moving means, via a pinion gear 140, and shifts the rotation shaft 132S in the axial direction. The first reverse shift roller pair 132a and the second reverse shift roller pair 132b, which hold the sheet S therebetween, move in the width direction W, whereby the sheet moves in the width direction W and the position of the sheet in the width direction W is corrected. In this way, the shift operation by the reverse conveyance unit 130 is realized.

[0054] The idler gear 135 has a wider tooth width than the input gear 134. This is to maintain meshing of the gears and enable rotation of the reverse shift unit 132 even when the first reverse shift roller pair 132a and the input gear 134 move in the width direction.

[0055] The reversing CIS 139 is disposed at a position offset to one side with respect to the center of the reversing conveying path 165 in the width direction W, and detects the edge position of the conveyed sheet S in the width direction W. This is because, in correcting the position of the sheet S, it is sufficient to detect the edge position of only one side of the sheet S. Furthermore, the reversing CIS 139 is disposed as close as possible to the first reversing shift roller pair 132a so as not to reduce the detection accuracy of the reversing CIS 139.

[0056] [Shift operation by reverse transport unit] Next, the shift operation by the reverse conveying unit 130 will be described with reference to the flowchart shown in Fig. 7. When the print job is double-sided printing, the sheet S with an image formed on the first side is conveyed to the reverse conveying unit 130 by the branching conveying unit 120. As shown in Fig. 8(a), the switching member 143 of the reverse conveying unit 130 is biased in a positioned state by a biasing member (not shown).

[0057] The sheet S conveyed from the branch conveying unit 120 is conveyed to the conveying roller pair 131 and conveyed while pressing the switching member 143 against the biasing force of the biasing member. In S210, the control unit 200 makes a determination based on information regarding the length of the sheet in the conveying direction input to and acquired by the operation unit 203. That is, the control unit 200 determines whether the length of the sheet in the conveying direction input to the operation unit 203 is equal to or greater than a predetermined length S (S210). If it is determined that the sheet length is not equal to or greater than S, the reversing sensor 138 detects the position of the sheet S in the conveying direction A (step S201). Thereafter, the reversing CIS 139 detects the edge position of the sheet S (step S202). The control unit 200 calculates the shift amount of the sheet based on the detection result (L3) and the deviation amount (g3). The amount of deviation (g3) is the amount of deviation of the sheet S in the width direction W when the sheet S is conveyed from the reverse conveying unit 130 to the registration unit 30, and is obtained in advance, for example, when the image forming apparatus 1 is installed. The amount of shift of the sheet S can be calculated by subtracting the amount of deviation (g3) from the detection result (L3) of the reverse CIS 139 (L3-g3).

[0058] Next, as shown in Figure 8(b), based on the detection result of the inversion sensor 138, the control unit 200 stops driving the inversion drive motor 136 at a position where the rear end of the sheet S has advanced a predetermined distance from the switching member 143, thereby stopping the sheet S (step S203).

[0059] After the sheet S has stopped, the control unit 200 moves the reverse shift unit 132, which is clamping the sheet S, in the width direction W by the shift amount (L3-g3) via the shift control unit 208 and the reverse shift motor 137. This allows the sheet S to be moved in the width direction W by the shift amount (L3-g3) (step S204).

[0060] In parallel with this shift operation, the control unit 200 reverses the reverse drive motor 136 (step S205). As a result, a switchback is performed by the first reverse shift roller pair 132a and the second reverse shift roller pair 132b of the reverse shift unit 132. That is, the sheet S is conveyed in a first direction A1 (see FIG. 8(a)), and then conveyed in a second direction A2 (see FIG. 8(c)) opposite to the first direction A1.

[0061] During the switchback operation, the sheet S is guided while making sliding contact with the reversing guide 142, which serves as a guide member. At this time, the second side of the sheet S, opposite to the first side on which the image is formed, makes sliding contact with the reversing guide 142. Furthermore, no guide member is provided on the opposite side of the reversing guide 142, and the first side of the sheet S guided by the reversing guide 142 is not guided by another guide member. Then, as shown in FIG. 8(c), the sheet S is guided by the switching member 143 to the second double-sided conveying unit 150, where an image is formed on the second side.

[0062] Next, the control unit 200 determines whether or not there is a subsequent sheet (step S206). If the control unit 200 determines that there is no subsequent sheet (step S206: No), the shifting operation by the reverse conveying unit 130 ends. If the control unit 200 determines that there is a subsequent sheet (step S206: Yes), the control unit 200 returns the reverse shift unit 132 to the home position (center position) (step S207). Then, the process returns to step S201.

[0063] In S210, when the control unit 200 determines that the length of the sheet in the conveying direction is equal to or longer than the predetermined length S, the control unit 200 stops driving the reversing drive motor 136 (S212) based on the fact that the reversing sensor 138 detects the sheet (S211). Then, the reversing drive motor 136 is rotated in the reverse direction (S213), and the process proceeds to S206. In other words, when the control unit 200 determines that the length of the sheet is equal to or longer than the predetermined length S, the shift operation by the reversing conveying unit 130 is not performed.

[0064] In this embodiment, step S205 is performed after step S204, but this order may be reversed or they may be performed simultaneously.

[0065] [Second double-sided transport unit] Next, a description will be given of the configuration of the second double-sided conveying unit 150. As shown in Fig. 9, the second double-sided conveying unit 150 as a double-sided conveying section has a second registration roller pair 152 as a skew correction section, a second pre-registration roller pair 151, and a second registration sensor 157. The second pre-registration roller pair 151 is disposed upstream of the second registration roller pair 152 in the sheet conveying direction A, and the second registration sensor 157 is provided between these roller pairs.

[0066] The second registration roller pair 152, which is a pair of rotating bodies, has an upper roller 152a and a lower roller 152b fixed to a rotary shaft 152S. An input gear 156 is fixed to the rotary shaft 152S, and the input gear 156 is driven by a second registration drive motor 154 via an idler gear 155. The second pre-registration roller pair 151 is driven by a second pre-registration drive motor 153.

[0067] The second double-sided conveying unit 150 is provided in the housing 1B, and performs skew correction on the sheet S before the sheet S is discharged from the housing 1B to the housing 1A. Note that the second double-sided conveying unit 150 performs the skew correction operation on the sheet S, but does not perform the shift operation.

[0068] [Skew correction operation by the second duplex conveying unit] Next, the skew correction operation (second skew correction operation) by the second double-sided conveying unit 150 will be described with reference to the flowchart shown in Fig. 10. When the print job is double-sided printing, the sheet S with an image formed on its first side undergoes a shift operation in the reversing conveying unit 130 as described above. Then, the position of the sheet S sent from the reversing conveying unit 130 to the double-sided conveying unit 150 in the conveying direction A is detected by the second registration sensor 157 (step S301).

[0069] Next, based on the detection result of the second registration sensor 157, the control unit 200 causes the second pre-registration roller pair 151 to feed the sheet S by a set feed amount. As a result, the sheet S is abutted against the stopped second registration roller pair 152, and a predetermined amount of flexure is formed (step S302). In this way, skew correction of the sheet S is performed, and the sheet S is nipped and conveyed by the second registration roller pair 152, whose rotational driving has started (step S303). Skew correction of the sheet S using the second registration roller pair 152 is performed regardless of the length of the sheet in the conveying direction.

[0070] Next, the control unit 200 determines whether or not there is a subsequent sheet (step S304). If the control unit 200 determines that there is no subsequent sheet (step S304: No), the skew correction operation by the second double-sided conveying unit 150 ends. If the control unit 200 determines that there is a subsequent sheet (step S304: Yes), the process returns to step S301.

[0071] As described above, in this embodiment, in a double-sided printing job, after image formation on the first side of the sheet S, shift operations are performed at two locations: the reverse conveying unit 130 and the registration unit 30. This allows the shift amount of the sheet S to be distributed across the shift operations at these two locations. Furthermore, by performing the shift operation at the reverse conveying unit 130, the sheet S is not nipped by rollers other than the reverse shift section 132 that performs the shift operation. In other words, regardless of the size of the sheet S, the sheet S is not nipped by rollers other than the reverse shift section 132 and does not become resistive, allowing for stable shift operation.

[0072] When the length of the sheet in the conveying direction is equal to or greater than a predetermined length S (e.g., a first length), the reverse conveying unit 130 shifts the sheet. On the other hand, when the length of the sheet in the conveying direction is equal to or greater than the predetermined length S (e.g., a second length longer than the first length), the reverse conveying unit 130 does not shift the sheet. This avoids the problems described below that may be caused by shifting a long sheet. FIG. 14(a) shows a state in which a first sheet of a first length is conveyed in the conveying direction H in a skewed state. FIG. 14(b) shows a state in which a second sheet of a second length longer than the first length is conveyed in the conveying direction H, skewed at the same angle as the sheet of the first length in FIG. 14(a). The deviation in the width direction between a corner on one end of the second sheet in the conveying direction H and a corner on the other end of the second sheet in the conveying direction H is defined as deviation amount Z2. The shift amount Z2 is longer than the shift amount Z1 in the widthwise position between the corner of one end of the first sheet in the conveying direction and the corner of the other end of the first sheet in the conveying direction. Therefore, for example, if the second sheet is shifted widthwise by the reverse conveying unit 130 while nipping the vicinity of one end (leading end) of the second sheet, the corner of the other end (trailing end) of the second sheet may be significantly shifted widthwise from its correct position. In this case, a problem may occur in which the side edge of the second sheet, including the corner of the trailing end, comes into contact with other components of the image forming unit (components that the sheet is not intended to come into contact with), damaging the side edge of the second sheet. Therefore, in this embodiment, if the length of the sheet is equal to or longer than a predetermined length S (for example, a second length longer than the first length), the reverse conveying unit 130 does not shift the sheet. Therefore, this embodiment can avoid the above problem. In this embodiment, even if the length of the sheet S is equal to or longer than the predetermined length S, the skew is corrected by the second registration roller pair 152 and also by the registration roller pair 32. Therefore, the long sheet that has passed through the re-conveyance section 500 can be shifted in the width direction by the registration roller pair 32.

[0073] Furthermore, the sheet S switched back by the reverse shift unit 132 has its second side, on which no image is formed, guided by the reverse guide 142. And, because no guide member is provided on the opposite side of the reverse guide 142, the first side, which is the image side of the sheet S, is not guided by the guide member. Since the image side, on which an image is formed, has high frictional resistance, only the second side, which is not the image side, is guided by the reverse guide 142, thereby reducing the resistance caused by the sliding between the sheet S and the reverse guide 142. Therefore, there is little resistance in the shift operation of the reverse shift unit 132.

[0074] In addition, the reverse shift unit 132 simultaneously shifts the first reverse shift roller pair 132a and the second reverse shift roller pair 132b in the width direction W. In this way, by performing the shift operation while the sheet S is nipped by two pairs of rollers, it is possible to reduce the occurrence of slippage between the sheet S and the rollers during the shift operation, which can cause the sheet S to skew, and to perform a stable shift operation. Therefore, skew and lateral deviation of the sheet S can be reduced, and a high-quality product can be obtained. In particular, in this embodiment, skew and lateral deviation of the sheet S can be reduced when forming an image on the second side of the first sheet of a job. Therefore, a high-quality product can be obtained more quickly than with an apparatus that corrects the position of a subsequent sheet based on the position of a preceding sheet.

[0075] Furthermore, since the shift amounts of the reversing conveying unit 130 and the registration unit 30 are reduced, the time required to return the roller pair to the home position after the shift operation is shortened, thereby improving productivity.

[0076] Furthermore, the registration unit 30 is provided in the housing 1A, and the reverse conveying unit 130 is provided in the housing 1B. In this way, by performing shift operations in separate housings, lateral misalignment in each housing can be corrected. Then, after lateral misalignment is corrected in each housing, the sheet S is conveyed to another housing, so the amount of shift of the sheet S in each housing can be reduced. Therefore, the length in the width direction W of the guide members forming each conveying path can be reduced, resulting in cost reduction and space savings.

[0077] Furthermore, in this embodiment, in a double-sided printing job, after image formation on the first side of the sheet S, skew correction operations are performed at two locations: the double-sided conveying unit 150 and the registration unit 30. This allows the amount of skew correction for the sheet S to be distributed to the skew correction operations at these two locations, reducing the amount of skew correction at each location. The skew correction operation bends the sheet S, so if the amount of skew correction is large, the sheet S may become distorted and wrinkles may occur. However, in this embodiment, the amount of skew correction can be reduced, thereby preventing wrinkles in the sheet S.

[0078] Furthermore, the registration unit 30 is provided in the housing 1A, and the duplex conveying unit 150 is provided in the housing 1B. In this way, by performing the skew correction operation in separate housings, skew can be corrected in each housing. Then, after skew correction in each housing, the sheet S is conveyed to another housing, so the amount of skew correction for the sheet S in each housing can be reduced. Therefore, the skew correction capacity required for each housing can be specified, and a skew correction mechanism with an optimal amount of skew correction can be selected, within the range of excess or deficiency.

[0079] <Second embodiment> Next, a second embodiment of the present invention will be described, in which the shift operation is not performed in the reverse conveying unit 130, but the skew correction operation and shift operation are performed in the second double-sided conveying unit 180. For this reason, the same configurations as those in the first embodiment will be omitted from the illustrations or will be described with the same reference numerals in the drawings.

[0080] [Second double-sided transport unit] First, the configuration of the second double-sided conveying unit 180 according to the second embodiment will be described. As shown in Fig. 11, the second double-sided conveying unit 180 has a second registration roller pair 182 serving as a second moving section and a second skew correction section, and a second pre-registration roller pair 181. The second double-sided conveying unit 180 also has a second registration sensor 187 and a second CIS 188. The second pre-registration roller pair 181 is disposed upstream of the second registration roller pair 182 in the sheet conveying direction A, and the second registration sensor 187 and the second CIS 188 are provided between these roller pairs.

[0081] The second registration roller pair 182, which is a pair of rotating bodies, has an upper roller 182a serving as a third roller, and a lower roller 182b serving as a fourth roller fixed to a rotating shaft 182S. An input gear 186 is fixed to the rotating shaft 182S, and the input gear 186 is driven by a second registration drive motor 184 via an idler gear 185. The second pre-registration roller pair 181 is driven by a second pre-registration drive motor 183. Each roller of the second pre-registration roller pair 181 and the second registration roller pair 182 rotates about an axis extending in the width direction W.

[0082] A rack 191 is supported on the rotary shaft 182S so as to be rotatable relative to the rotary shaft 182S but immovable in the axial direction. The rack 191 receives a driving force from a second shift motor 189 via a pinion gear 190, and shifts the rotary shaft 182S in the axial direction. Furthermore, a flange portion 192 formed integrally with the upper roller 182a is sandwiched by an input gear 186 of the lower roller 182b, so that the upper roller 182a shifts in the axial direction in conjunction with the lower roller 182b. The second registration roller pair 182 sandwiching the sheet S moves in the width direction W, so that the sheet moves in the width direction W, and the position of the sheet in the width direction W is corrected.

[0083] The idler gear 185 has a wider tooth width than the input gear 186. This is to maintain the gear meshing and allow the second registration roller pair 182 to rotate even when the second registration roller pair 182 and the input gear 186 move in the width direction.

[0084] Similarly to the CIS 34 (see FIG. 2), the second CIS 188 serving as the second detection unit is disposed at a position offset to one side with respect to the center of the conveyance path in the width direction W. The second CIS 188 is disposed as close as possible to the second registration roller pair 182 so as not to reduce the detection accuracy of the second CIS 188.

[0085] [Control Block] 12 is a control block diagram showing the control unit 200 of the image forming apparatus 1 according to the second embodiment. A sheet conveyance control unit 206 issues instructions to the pre-registration drive motor 35, the registration drive motor 36, the reverse drive motor 136, the second pre-registration drive motor 183, the second registration drive motor 184, etc., thereby controlling the conveyance operation of the sheet S. A sensor control unit 207 issues instructions to start and stop detection to the registration sensor 33, the second registration sensor 187, etc., and receives the detection results of these sensors.

[0086] The shift control unit 208 receives the detection results of the CIS 34 and the second CIS 188, and issues instructions to start and stop driving the shift motor 37 and the second shift motor 189, thereby controlling the movement of the sheet S in the width direction W, i.e., the shift operation.

[0087] [Skewing correction and shifting operations by the second duplex conveying unit] Next, the skew correction operation (second skew correction operation) and shift operation by the second double-sided conveying unit 180 will be described with reference to the flowchart shown in Fig. 13. When the print job is double-sided printing, the sheet S with an image formed on its first side is switched back in the reversing conveying unit 130. In this embodiment, the reversing conveying unit 130 does not perform a shift operation. Then, the position of the sheet S sent from the reversing conveying unit 130 to the double-sided conveying unit 180 in the conveying direction A is detected by the second registration sensor 187 (step S401).

[0088] Next, the control unit 200 causes the second pre-registration roller pair 181 to feed the sheet S by a set feed amount based on the detection result of the second registration sensor 187. As a result, the sheet S is abutted against the stopped second registration roller pair 182, and a predetermined amount of slack is formed (step S402). In this way, skew correction of the sheet S is performed, and the sheet S is nipped and conveyed by the second registration roller pair 182, which has started to be rotated (step S403).

[0089] The control unit 200 determines whether the length of the sheet in the conveyance direction input to the operation unit 203 is equal to or greater than a predetermined length S (S410). If it is determined that the length of the sheet is not equal to or greater than S, the second CIS 188 detects the edge position of the sheet S (step S404). The control unit 200 calculates the shift amount of the sheet based on this detection result (L4) and the misalignment amount (g4). The misalignment amount (g4) is the amount of misalignment of the sheet S in the width direction W when the sheet S is conveyed from the second duplex conveying unit 180 to the registration unit 30, and is obtained in advance, such as when the image forming apparatus 1 is installed. The shift amount of the sheet S can be calculated by subtracting the misalignment amount (g4) from the detection result (L4) of the second CIS 188 (L4-g4).

[0090] Then, the control unit 200 moves the second registration roller pair 182, which sandwiches the sheet S, in the width direction W by the shift amount (L4-g4) via the shift control unit 208 and the second shift motor 189 as a moving means. This allows the sheet S to be moved in the width direction W by the shift amount (L4-g4) (step S405).

[0091] Next, the control unit 200 determines whether or not there is a subsequent sheet (step S406). If the control unit 200 determines that there is no subsequent sheet (step S406: No), the skew correction operation and shift operation by the second double-sided conveying unit 180 are terminated. If the control unit 200 determines that there is a subsequent sheet (step S406: Yes), the control unit 200 returns the second registration roller pair 182 to the home position (center position) (step S407). Then, the process returns to step S401.

[0092] In S410, if the control unit 200 determines that the length of the sheet is equal to or greater than the predetermined length S, the process proceeds to S406. In other words, if the control unit 200 determines that the length of the sheet is equal to or greater than the predetermined length S, the second double-sided conveying unit 180 does not perform the shift operation.

[0093] As described above, in this embodiment, in a double-sided printing job, after forming an image on the first side of the sheet S, the skew correction operation and the shift operation are performed at two locations, the second double-sided conveying unit 180 and the registration unit 30. Therefore, it is possible to achieve the same effects as in the first embodiment.

[0094] Furthermore, since the second double-sided conveying unit 180 is positioned near the exit from the housing 1B to the housing 1A, the amount of skew and the position in the width direction W of the sheet S discharged from the housing 1B can be made clearer than in the first embodiment.

[0095] <Other embodiments> In the first embodiment, the shift operation is performed in the reverse conveying unit 130, and the skew correction operation is performed in the second double-sided conveying unit 150. In the second embodiment, the shift operation and the skew correction operation are performed in the second double-sided conveying unit 180, but this is not limited to this. That is, it is sufficient that at least one of the shift operation and the skew correction operation is performed in the re-conveying section 500. Furthermore, it is not limited to which unit performs these shift operation and skew correction operation. For example, the skew correction operation and the shift operation may be performed in the reverse conveying unit 130, or only the shift operation may be performed in the first double-sided conveying unit 70.

[0096] In the above-described embodiment, a mode in which no sheet shift is performed at all has been exemplified as one mode of limiting the shift of the sheet in the width direction of the sheet by the reverse conveying unit 130 or the second duplex conveying unit 150 when the length of the sheet is length S or more. However, when the length of the sheet is length S or more, the amount of shift of the sheet by the reverse conveying unit 130 or the second duplex conveying unit 150 may be limited to within a predetermined set amount. In other words, when the length of the sheet is less than length S, the amount of sheet shift is not limited, and the sheet is allowed to shift beyond a predetermined set amount based on the detection result of the CIS. On the other hand, when the length of the sheet is length S or more, the amount of sheet shift is limited so as not to exceed a predetermined set amount.

[0097] Furthermore, in this embodiment, even when the length of the sheet in the conveying direction is equal to or greater than the predetermined length S, the sheet is shifted in the width direction by the registration roller pair 32. However, when the length of the sheet in the conveying direction input to the operation unit 203 is equal to or greater than the predetermined length S, the shift of the sheet in the width direction by the registration roller pair 32 may be restricted.

[0098] One problem that can occur when moving a long sheet in the width direction is that the long sheet may become skewed as it shifts in the width direction. Because the contact area between the long sheet and the conveying guide is larger than the contact area between the long sheet and the conveying guide, when moving the long sheet in the width direction, the long sheet is subject to large frictional resistance between the long sheet and the conveying guide, and is therefore likely to become skewed.

[0099] In the first embodiment, both the first reverse shift roller pair 132a and the second reverse shift roller pair 132b of the reverse shift unit 132 are configured to be movable in the width direction W, but this is not limiting. For example, only one of the first reverse shift roller pair 132a and the second reverse shift roller pair 132b may be configured to be movable in the width direction W. Furthermore, the second reverse shift roller pair 132b may be omitted, and the sheet S may be moved in the width direction W while being sandwiched by only the first reverse shift roller pair 132a.

[0100] In addition, a CCD sensor or a CMOS sensor may be used instead of CIS34, inversion CIS139, and second CIS188, and if the position of the sheet in the width direction can be detected by these sensors, it is not necessary to detect the position of the edge of the sheet in the width direction.

[0101] In addition, instead of the method of correcting the skew of a sheet by hitting the sheet against the registration roller pair 32 or the second registration roller pair 182, a method of hitting the sheet against a shutter member provided upstream in the conveying direction of the roller pair may be applied.

[0102] In addition, although the above-described embodiments have been described using an electrophotographic image forming apparatus 1, the present invention is not limited to this. For example, the present invention can also be applied to an inkjet image forming apparatus that forms an image on a sheet by ejecting ink liquid from nozzles.

[0103] The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions. [Explanation of symbols]

[0104] 1. Image forming device 1A First enclosure 1B Second cabinet 32 First moving unit, first skew correction unit (pair of registration rollers) 32a First roller (upper roller) 32b Second roller (lower roller) 34 First detection unit (CIS) 65 Transport path 130 Reversing section (reversing transport unit) 132 Second moving part (reverse shift part) 132a Third roller, fourth roller (first pair of reverse shift rollers) 139 Second detection unit (inverted CIS) 142 Guide member (reversal guide) 150 Double-sided conveying section (second double-sided conveying unit) 152 skew correction unit (second registration roller pair) 182 Second moving unit, second skew correction unit (second registration roller pair) 182a Third roller (upper roller) 182b 4th roller (lower roller) 188 Second detection unit (second CIS) 500 Re-feeding unit A Conveying direction A1 1st direction A2 2nd direction N Image forming section (secondary transfer nip) W width direction

Claims

1. an image forming unit that forms an image on a sheet; a pair of reversing rollers that rotate in a first direction while holding a sheet on which an image is formed by the image forming unit, and then rotate in a second direction opposite to the first direction to reverse and convey the sheet; a moving means for moving the pair of reversing rollers in a width direction of the sheet that intersects with a conveying direction while the sheet is sandwiched between the pair of reversing rollers; an acquisition unit for acquiring information about the length of the sheet in the conveying direction; and a control means for controlling the moving means based on the information regarding the length of the sheet acquired by the acquiring means so that, when a first sheet having a first length in the conveying direction is to be reversed and conveyed, the moving means moves the pair of reversing rollers conveying the first sheet by a first movement amount in the width direction, and, when a second sheet having a second length in the conveying direction that is longer than the first length is to be conveyed, the moving means moves the pair of reversing rollers conveying the second sheet by a second movement amount in the width direction that is smaller than the first movement amount. An image forming apparatus characterized by:

2. An image forming unit that forms an image on a sheet; a pair of reversing rollers that rotate in a first direction while holding a sheet on which an image is formed by the image forming unit, and then rotate in a second direction opposite to the first direction to reverse and convey the sheet; a moving means for moving the pair of reversing rollers in a width direction of the sheet that intersects with a conveying direction while the sheet is sandwiched between the pair of reversing rollers; an acquisition unit for acquiring information about the length of the sheet in the conveying direction; and a control means for controlling the moving means based on the information regarding the length of the sheet acquired by the acquisition means so that, when a first sheet having a first length in the conveying direction is to be reversed and conveyed, the moving means moves the pair of reversing rollers conveying the first sheet in the width direction, and, when a second sheet having a second length in the conveying direction that is longer than the first length is to be conveyed, the moving means does not move the pair of reversing rollers conveying the second sheet in the width direction. An image forming apparatus characterized by:

3. a skew correcting means for correcting skew of the sheet after the sheet has been reversed and conveyed by the pair of reversing rollers; After the skew of the sheet is corrected by the skew correction unit, the sheet is conveyed toward the image forming unit.

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

4. a skew correction unit that corrects skew of the sheet whose conveying direction has been reversed by the pair of reversing rollers; another moving means for moving the sheet whose skew has been corrected by the skew correcting means in the width direction, When conveying the first sheet, after the image forming unit forms an image on a first surface of the first sheet and before the image forming unit forms an image on a second surface of the first sheet, the pair of reversing rollers conveying the first sheet are moved in the width direction by the moving means, the skew correction unit corrects the skew of the first sheet, and the other moving means moves the first sheet in the width direction, in this order; When the second sheet is conveyed, after the image forming unit forms an image on a first surface of the second sheet and before the image forming unit forms an image on a second surface of the second sheet, the skew correction unit performs skew correction of the second sheet and the other moving unit moves the second sheet in the width direction, in that order.

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

5. A double-sided conveying section that conveys the sheet conveyed by the pair of reversing rollers toward the image forming section, The skew correction means is disposed in the double-sided conveying section.

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

6. a detection means for detecting a position in the width direction of the sheet conveyed by the pair of reversing rollers; the control means causes the moving means to move the sheet in the width direction of the pair of reversing rollers based on the detection result of the position in the width direction of the sheet detected by the detection means; 6. The image forming apparatus according to claim 1, wherein the image forming apparatus is a recording medium.

7. an image forming unit that forms an image on a sheet; an inversion unit that conveys the sheet in a first direction and then conveys the sheet in a second direction opposite to the first direction; a double-sided conveying unit that includes a moving unit that moves the sheet in the width direction while sandwiching the sheet, and that conveys the sheet conveyed by the reversing unit toward the image forming unit; an acquisition unit for acquiring information about the length of the sheet in the conveying direction; a control means for controlling, based on information about the length of the sheet acquired by the acquisition means, so that when a first sheet having a first length is conveyed, the moving unit moves the first sheet in the width direction, and when a second sheet having a second length longer than the first length is conveyed, the moving unit does not move the sheet in the width direction; having An image forming apparatus characterized by:

8. Another moving unit is provided upstream of the image forming unit in the sheet conveying direction, and moves the sheet in a width direction perpendicular to the conveying direction while clamping the sheet, thereby moving the sheet in the width direction; 8. The image forming apparatus according to claim 7,

9. when conveying the first sheet, the control means controls the other moving unit to move the first sheet in the width direction after the image forming unit forms an image on a first surface of the first sheet and before the image forming unit forms an image on a second surface of the first sheet, and the moving unit controls the first sheet to move in the width direction; When the second sheet is conveyed, after the image forming unit forms an image on the first surface of the second sheet and before the image forming unit forms an image on the second surface of the second sheet, the control unit controls the other moving unit not to move the sheet in the width direction, but to move the sheet in the width direction.

9. The image forming apparatus according to claim 8,

10. a first detection unit that detects a position of the sheet in the width direction; a second detection unit that detects the position of the sheet in the width direction, the moving unit moves the sheet in the width direction based on the detection result of the first detection unit, the other moving unit moves the sheet in the width direction based on the detection result of the second detection unit.

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

11. The double-sided conveying unit performs a skew correction operation to correct skew of the sheet by abutting the leading edge of the sheet, the double-sided conveying unit performs the skew correction operation in the case of a sheet having the first length and in the case of a sheet having the second length.

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

12. an image forming unit that forms an image on a sheet; a pair of reversing rollers that rotate in a first direction while holding a sheet on which an image is formed by the image forming unit, and then rotate in a second direction opposite to the first direction to reverse and convey the sheet; a moving means for moving the pair of reversing rollers in a width direction of the sheet that intersects with a conveying direction while the sheet is sandwiched between the pair of reversing rollers; an acquisition unit for acquiring information about the length of the sheet in the conveying direction; an image forming apparatus characterized by having a control means that controls the moving means based on information regarding the length of the sheet acquired by the acquisition means so that when a first sheet having a sheet length in the conveying direction of a first length is to be reversed and conveyed, the moving means moves the pair of reversing rollers conveying the first sheet in the width direction, and when a second sheet having a sheet length in the conveying direction of a second length longer than the first length is to be conveyed, the moving means limits the movement of the pair of reversing rollers conveying the second sheet in the width direction.

13. when the second sheet is to be conveyed, the control unit causes the moving unit not to move the pair of reversing rollers conveying the second sheet in the width direction; 13. The image forming apparatus according to claim 12,

14. when the second sheet is conveyed, the control means limits the amount of movement of the pair of reversing rollers conveying the second sheet in the width direction by the movement means so that it does not exceed a predetermined amount; when the first sheet is conveyed, the control unit allows the pair of reversing rollers conveying the first sheet to move in the width direction by the moving unit by more than the predetermined amount; 13. The image forming apparatus according to claim 12,

Images