Image forming apparatus

Abstract

To provide an image forming apparatus that can produce high-quality results. [Solution] The system comprises an image forming unit that forms an image on a sheet, a first moving unit provided upstream of the image forming unit in the sheet transport direction, which moves the sheet in the width direction perpendicular to the transport direction while holding the sheet, and a re-transporting unit that reverses the front and back sides of the sheet on which the image has been formed on the first surface by the image forming unit, and transports the sheet back to the image forming unit, wherein the re-transporting unit has a second moving unit that moves the sheet in the width direction while holding the sheet.

Description

Technical Field

[0001] The present invention relates to an image forming apparatus for forming an image on a sheet.

Background Art

[0002] Generally, in an image forming apparatus such as a copying machine, a sheet may be laterally displaced in the width direction of the sheet while being conveyed. When an image is formed on the sheet in a state where the sheet is laterally displaced, the image is printed shifted from the center of the sheet, and the quality of the sheet is not good. For this reason, a shift mechanism is known that detects the position of the end portion in the width direction of the sheet and corrects the lateral displacement (position shift) of the sheet before forming an image on the sheet.

[0003] Conventionally, an image forming apparatus has been proposed that detects the position of the end portion in the width direction of the first sheet and corrects the image forming position on the third sheet with respect to the photosensitive member based on the amount of deviation from the reference position of the end portion of the first sheet (see Patent Document 1). In order to correct the image forming position in advance based on the deviation amount of the sheet two sheets before, this image forming apparatus reduces the shift amount of the sheet, aiming for high image quality and high productivity of the image forming apparatus.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the image forming apparatus described in Patent Document 1 shifts the sheet in the width direction using only a register roller positioned in front of the transfer step where the image is transferred to the sheet. Therefore, if the amount of sheet shift by the register roller is large, the sheet may become skewed or the amount of shift may vary greatly when it shifts in the width direction. As a result, the image formed on the sheet is misaligned, and a low-quality product is supplied.

[0006] Therefore, the present invention aims to provide an image forming apparatus that can produce high-quality results. [Means for solving the problem]

[0007] The present invention relates to an image forming apparatus, comprising: an image forming unit for forming an image on a sheet; a reversing unit for reversing the leading and trailing ends of a sheet on which an image has been formed on the first surface of the sheet by the image forming unit, the reversing unit comprising: a pair of reversing rollers that grip the sheet and rotate in a first direction, and then rotate in a second direction opposite to the first direction to reverse the sheet; a first moving unit that moves the sheet in a width direction perpendicular to the sheet transport direction while the sheet is gripped by the pair of reversing rollers; a first pair of registration rollers provided upstream of the image forming unit in the transport direction, which corrects the skewness of the sheet by abutting the leading edge of the sheet; and a second moving unit that moves the sheet in the width direction while the sheet is gripped by the pair of registration rollers. The device comprises a moving section and a skew correction section having a second pair of registration rollers provided between the reversing section and the skew correction section in the transport direction, which corrects the skew of the sheet by abutting the leading edge of the sheet that has been reversed by the reversing section and transports the sheet, and a control section, wherein the control section controls the sheet so that when forming an image on the second surface of the sheet opposite to the first surface, (1) the position of the sheet in the width direction is corrected by the first moving section, and then the skew of the sheet is corrected by the second pair of registration rollers, (2) the skew of the sheet is corrected by the first pair of registration rollers, and then the position of the sheet in the width direction is corrected by the second moving section, and (3) the sheet is transported to the image forming section. [Effects of the Invention]

[0008] According to the present invention, an image forming apparatus capable of producing high-quality results can be provided. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic diagram showing the printer according to the first embodiment. [Figure 2] A perspective view showing the registration unit. [Figure 3] A control block diagram showing the control unit. [Figure 4] A flowchart illustrating the skew correction and shift operations performed by the registration unit. [Figure 5] (a) is a plan view showing the sheet in an oblique position, (b) is a plan view showing the sheet's edge position being detected, (c) is a plan view showing the sheet being transported by a pair of registration rollers, and (d) is a plan view showing the sheet after a shift operation. [Figure 6] A perspective view showing the reversal transport unit. [Figure 7] A flowchart illustrating the shift operation by the reversing transport unit. [Figure 8] (a) is a schematic diagram showing the sheet being transported toward the reversal shift section, (b) is a schematic diagram showing the sheet stopped by the reversal shift section, and (c) is a schematic diagram showing the sheet being transported after being reversed. [Figure 9] A perspective view showing the second double-sided transport unit. [Figure 10] A flowchart illustrating the skew correction operation by the second double-sided transport unit. [Figure 11] A perspective view showing a second double-sided transport unit according to a second embodiment. [Figure 12] A control block diagram showing the control unit. [Figure 13] A flowchart illustrating the skew correction and shift operations performed by the second double-sided transport unit. [Modes for carrying out the invention]

[0010] <First Embodiment> [Overall structure] First, a first embodiment of the present invention will be described. The image forming apparatus 1 of this embodiment is an electrophotographic full-color laser beam printer. As shown in Figure 1, the image forming apparatus 1 has a housing 1A as a first housing having a unit for feeding and transporting 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.

[0011] The housing 1A has a feeding unit 10a, 10b, a pulling-out unit 20a, 20b, a registration unit 30, an image forming unit 90, and a first duplex conveyance unit 70. The housing 1B has a fixing unit 100, a cooling unit 110, a branching conveyance unit 120, a reversing conveyance unit 130, a second duplex conveyance unit 150, and a decal unit 170.

[0012] The image forming unit 90 includes four process cartridges 99Y, 99M, 99C, 99Bk that form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively, and exposure devices 93, 96, 97, 98. The four process cartridges 99Y, 99M, 99C, 99Bk have the same configuration except that the colors of the formed images are different. Therefore, only the configuration of the process cartridge 99Y and the image forming process will be described, and the description of the process cartridges 99M, 99C, 99Bk will be omitted.

[0013] The process cartridge 99Y has a photosensitive drum 91, a charging roller (not shown), a developing device 92, and a cleaner 95. The photosensitive drum 91 is formed by applying an organic photoconductive layer to the outer periphery of an aluminum cylinder and rotates by a drive motor (not shown). The image forming unit 90 is provided with an intermediate transfer belt 50 that rotates in the direction of arrow T1 by a drive roller 52. The intermediate transfer belt 50 is wound around a tension roller 51, a drive roller 52, and a secondary transfer inner roller 53. Inside the intermediate transfer belt 50, primary transfer rollers 55Y, 55M, 55C, 55Bk are provided, and outside the intermediate transfer belt 50, a secondary transfer outer roller 54 is provided facing the secondary transfer inner roller 53.

[0014] The sheet feeding unit 10a has a lift plate 11a that moves up and down while loading the sheet S, a pickup roller 12a that feeds the sheet S loaded on the lift plate 11a, and a separation roller pair 13a that separates the fed sheets one by one. Similarly, the sheet feeding unit 10b has a lift plate 11b that moves up and down while loading the sheet S, a pickup roller 12b that feeds the sheet S loaded on the lift plate 11b, and a separation roller pair 13b that separates the fed sheets one by one.

[0015] The registration unit 30 has a pre-registration roller pair 31 that conveys the sheet and a registration roller pair 32 that corrects the skew of the sheet. Further, the registration unit 30 has a registration sensor 33 that detects the position of the sheet S in the conveyance direction and a CIS 34 that detects the position of the sheet in the width direction. The fixing unit 100 has a fixing roller pair 101 capable of heating the sheet.

[0016] The cooling unit 110 has an upper cooling belt 111a that rotates in the direction of arrow T2 by an upper cooling drive roller 112a. Further, the cooling unit 110 has a lower cooling belt 111b that rotates in the direction of arrow T2 by a lower cooling drive roller 112b and a heat sink 113 that cools the sheet.

[0017] Next, the image forming operation of the image forming apparatus 1 configured as described above will be described. When an image signal is input from a personal computer or the like (not shown) to the exposure device 93, laser light corresponding to the image signal is irradiated onto the photosensitive drum 91 of the process cartridge 99Y from the exposure device 93.

[0018] At this time, the surface of the photosensitive drum 91 is uniformly charged to a predetermined polarity and potential in advance by a charging roller, and an electrostatic latent image is formed on the surface by irradiating laser light from the exposure device 93 via the mirror 94. The electrostatic latent image formed on the photosensitive drum 91 is developed by the developing device 92, and a yellow (Y) toner image is formed on the photosensitive drum 91.

[0019] Similarly, laser light from exposure units 96, 97, and 98 is irradiated onto each photosensitive drum of process cartridges 99M, 99C, and 99Bk, forming magenta (M), cyan (C), and black (K) toner images on each drum. The toner images of each color formed on each drum 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 by the intermediate transfer belt 50, which is rotated by the drive roller 52, to the secondary transfer nip N, which is formed by the secondary transfer inner roller 53 and secondary transfer outer roller 54. The toner remaining on the photosensitive drum 91 is collected by the cleaner 95. The image formation process for each color is performed at a timing that overlaps with the upstream toner image that has been primary transferred onto the intermediate transfer belt 50.

[0020] In parallel with this image formation process, the sheet S is fed from either the feeding unit 10a or 10b, and the sheet S is transported to the registration unit 30 by either the extraction unit 20a or 20b. In the registration unit 30, the pre-registration roller pair 31 abuts the leading edge of the sheet S against the nip portion of the registration roller pair 32 that is holding it in place. This corrects the skew of the sheet S, and it is transported to the secondary transfer nip N, which serves as the image formation section, at a predetermined transport timing. The full-color toner image on the intermediate transfer belt 50 is transferred to the first sheet surface (front surface) of the sheet S by the secondary transfer bias applied to the secondary transfer outer roller 54. The remaining toner on the intermediate transfer belt 50 is collected by the belt cleaner 56.

[0021] The sheet S onto which the toner image has been transferred is transported to the fixing unit 100 by the pre-fixing transport unit 60. There, the sheet S is guided to the nip portion of the fixing roller pair 101, and predetermined heat and pressure are applied to melt and fix the toner. After passing through the fixing unit 100, the sheet S is transported in the cooling unit 110 while being held between the upper cooling belt 111a and the lower cooling belt 111b, which are endless belts. The heat from the sheet S is then transferred to the heat sink 113 via the upper cooling belt 111a, and the sheet S is cooled.

[0022] Next, the branching transport unit 120 selects a route for the sheet S, determining whether it will be transported to the decal unit 170 or the inversion transport unit 130. After the sheet S has been transported to the inversion transport unit 130, it is also possible to invert the sheet S so that the first sheet surface on which the image was formed by the secondary transfer nip N is facing downwards, and then transport the sheet S to the decal unit 170.

[0023] When an image is to be formed on only one side of the sheet S, the sheet S is transported from the branching transport unit 120 to the decal 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 decal unit 170 is discharged into the discharge tray 171.

[0024] When forming an image on both sides of sheet S, sheet S is transported to the inversion transport unit 130 by the branching transport unit 120, where it is switched back. The switched back sheet S is then transported from the inversion transport unit 130 to the second double-sided transport unit 150 and the first double-sided transport unit 70, and guided to the registration unit 30. After this, an image is formed on the second sheet surface (back side) at the secondary transfer nip N, and the sheet S is discharged to the discharge tray 171 via the branching transport unit 120 and the decal unit 170.

[0025] The branching transport unit 120, the reversing transport unit 130, the second double-sided transport unit 150, and the first double-sided transport unit 70 constitute a re-transport section 500 that reverses the front and back sides of the sheet on which the image is formed on the first surface and transports the sheet back to the secondary transfer nip N.

[0026] In this embodiment, the image forming apparatus 1 employs a central-reference sheet transport method, as an example, in which the sheet is transported by aligning the center in the width direction perpendicular to the transport direction in the transport path 65 with the center in the width direction of the sheet.

[0027] [Registration Unit] As shown in Figures 1 and 2, the registration unit 30 is provided in the transport path 65 connecting the extraction unit 20a and the secondary transfer nip N. The registration unit 30 also includes a pair of registration rollers 32, a pair of pre-registration rollers 31, a registration sensor 33, and a CIS (Contact Image Sensor) 34. The pair of pre-registration rollers 31 is positioned upstream of the pair of registration rollers 32 in the sheet transport direction A, and the registration sensor 33 and CIS 34 are provided between these pairs of rollers.

[0028] As shown in Figure 2, the rotating registration roller pair 32 has an upper roller 32a as the first roller and a lower roller 32b as the second roller fixed to the rotating shaft 32S. An input gear 38 is fixed to the rotating 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 around an axis extending in the width direction W.

[0029] A rack 41 is supported on the rotating shaft 32S so as to be rotatable relative to the rotating shaft 32S but immovable in the axial direction. The rack 41 receives driving force from the shift motor 37 via a pinion gear 40 and shifts the rotating shaft 32S in the axial direction. The upper roller 32a also shifts in the axial direction in conjunction with the lower roller 32b, as the flange portion 42, which is integrally provided with the upper roller 32a, is clamped by the input gear 38 of the lower roller 32b. As the registration roller pair 32, with the sheet S clamped, moves in the width direction W perpendicular to the conveying direction A, the sheet moves in the width direction W, and the position of the sheet in the width direction W is corrected.

[0030] Furthermore, the idler gear 39 has a wider tooth width than the input gear 38. This is to maintain gear meshing and allow rotation of the registration roller pair 32 even when the registration roller pair 32 and the input gear 38 move in the width direction.

[0031] The CIS34 detects the position of the edge of the sheet S in the width direction W (hereinafter referred to as the edge position) as it is being transported. The control unit 200 (see Figure 3) calculates the amount of deviation between the sheet's design reference position and the edge position detected by the CIS34, and shifts the registration unit 30 by this amount of deviation. As a result, the position of the sheet S in the width direction W matches the transfer position in the image forming unit 90, and a high-quality output is obtained.

[0032] Furthermore, the CIS34 is positioned off-center to one side of the transport path 65 in the width direction W. This is because, in correcting the position of the sheet S, it is only necessary to detect the edge position on one side of the sheet S. In addition, the CIS34 is configured to detect the edge positions of both the smallest and largest sheet sizes among the sheet sizes permitted for use in the image forming apparatus 1. Moreover, the CIS34 is positioned as close as possible to the registration roller pair 32 to avoid reducing the detection accuracy of the CIS34.

[0033] Furthermore, the registration unit 30 corrects the skewness by bringing the leading edge of the conveyed sheet S against the nip portion of the stationary registration roller pair 32, causing it to bend and align the leading edge of the sheet S with the nip portion. The sheet S is fed by a predetermined amount by the pre-registration roller pair 31 after the registration sensor 33 detects the leading edge of the sheet S, and then conveyed by the registration roller pair 32 to the secondary transfer nip N.

[0034] Furthermore, the gap between the CIS34 and the lower guide 65a facing the CIS34 is kept at a constant distance, 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 flex. The amount of sheet S conveyed by the pre-registration roller pair 31 is set so that an appropriate amount of flex is formed in the sheet S.

[0035] [Control Block] Figure 3 is a control block diagram showing the control unit 200 of the image forming apparatus 1. The control unit 200 includes a CPU 201, a memory 202, an operation unit 203, an image forming control unit 205, a sheet transport control unit 206, a sensor control unit 207, and a shift control unit 208. The CPU 201 realizes various processes performed by the image forming apparatus 1 by executing predetermined control programs, etc. The memory 202 is composed of, for example, RAM or ROM, and stores various programs and various data in a predetermined storage area. The operation unit 203 accepts input of various information related to the sheet (for example, sheet size, sheet basis weight, sheet surface properties, etc.) and accepts execution and cancellation of jobs.

[0036] The image forming control unit 205 issues instructions to the image forming unit 90, including 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, registration drive motor 36, inversion drive motor 136, second pre-registration drive motor 153, and second registration drive motor 154, etc. This controls the transport operation of the sheet S. The sensor control unit 207 issues instructions to start and stop detection for the registration sensor 33, inversion sensor 138, and second registration sensor 157, etc., and receives the detection results of each of these sensors.

[0037] The shift control unit 208 receives detection results from 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 connect to an external computer 204, for example, via a network, and can receive various information about the sheet and print jobs from the computer 204.

[0038] [Diagonal correction and shift operation by the registration unit] Next, the skew correction operation (first skew correction operation) and shift operation by the registration unit 30 will be explained according to the flowchart shown in Figure 4. First, when a print command is input from the operation unit 203 or the computer 204, the control unit 200 starts the print job (step S101). The user can also specify the number of copies to print and the type of sheet to be used for printing from the operation unit 203 or the computer 204.

[0039] The control unit 200 starts feeding the sheet S (step S102) and determines whether it is printing the first or second side of the sheet in the print job (step S103). If it is determined that it is printing 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 on the first side (step S104). Here, the image writing position g1 is a value based on the writing position adjustment result performed at the time of factory shipment, etc., and is stored in the memory 202 as a fixed value specific to the device.

[0040] Specifically, the control unit 200 controls the exposure devices 93, 96, 97, and 98 to form an electrostatic latent image at the image writing position g1 on each photosensitive drum of process cartridges 99Y, 99M, 99C, and 99K. Then, as described above, the electrostatic latent images formed on each photosensitive drum are developed as toner images by the developing device, and these toner images are transferred to the intermediate transfer belt 50 by the primary transfer rollers 55Y, 55M, 55C, and 55K.

[0041] Meanwhile, the sheet S is transported to the pre-registration roller pair 31. Here, the transported sheet S is assumed to be rotating clockwise with respect to the transport direction A and shifted to the left with respect to the transport direction A, as shown in Figure 5(a). The dotted rectangles shown in Figures 5(a) to (d) schematically represent the state in which the leading edge of the sheet S, which has been transported without skew or lateral shifting, contacts the nip portion of the registration roller pair 32. Furthermore, the position of the end 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.

[0042] Next, based on the detection result of the registration sensor 33 (step S105), the control unit 200 feeds the sheet S by a set amount using the pre-registration roller pair 31. As a result, the sheet S is brought into contact with the stopped registration roller pair 32, as shown in Figure 5(b), and a predetermined amount of deflection is formed (step S106). In this way, the skew of the sheet S is corrected, and the sheet S is then gripped and transported by the registration roller pair 32, whose rotational drive has been started, as shown in Figure 5(c) (step S107).

[0043] Then, after the skew correction has been performed, the edge position of the sheet S is detected by the CIS34 (step S108), and the control unit 200 calculates the shift amount of the sheet based on this 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 CIS34 (L1-g1).

[0044] The control unit 200 moves the pair of registration rollers 32 that are holding the sheet S in the width direction W by a shift amount (L1-g1) via the shift control unit 208 and the shift motor 37. This allows the sheet S to be moved by a shift amount (L1-g1) in the width direction W (step S109). As a result, the position of the sheet S in the width direction W is corrected to correspond to the image writing position g1.

[0045] Then, the sheet, which has been 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, this toner image is melted and fixed by the fixing unit 100 (step S111).

[0046] In the case of a single-sided job, the sheet S with the fixed toner image is ejected to the output tray 171 and the job is completed (step S112). However, in the case of a double-sided job, the sheet S is inverted to form the image 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 terminated (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). After that, the process returns to step S103.

[0047] If the control unit 200 determines in step S103 that it is printing the second side of the print job, the control unit 200 controls the image forming unit 90 to form a toner image at the image writing position g2 of the second side (step S116). The image writing position g2 of the second side may be the same as or different from the image writing position g1 of the first side in the width direction. The skew correction operation by the registration roller pair 32 for the sheet on which the image is formed on the second side is the same as for the sheet on which the image is formed on the first side, and therefore the explanation is omitted (steps S117 to S119).

[0048] Then, after the skew correction is performed, the CIS34 detects the position of the second edge of the sheet S (step S120), and the control unit 200 calculates the shift amount of the sheet S based on this 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 CIS34 (L2-g2).

[0049] The control unit 200 moves the pair of registration rollers 32 that are holding the sheet S in the width direction W by a shift amount (L2-g2) via the shift control unit 208 and the shift motor 37. This allows the sheet S to be moved by a shift amount (L2-g2) in the width direction W (step S121). For example, if the image writing position for the second surface is g2=g1=0, the sheet S is shifted by a shift amount L2, moving it to the same position as the first surface before image formation. As a result, the positions of the image formed on the first surface and the image formed on the second surface coincide, and these images are formed in the center of the sheet S, resulting in a high-quality output.

[0050] Then, the sheet, which has been shifted by a 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). Subsequently, similar to the processing of the first surface, this toner image is melted and fixed by the fixing unit 100 and discharged to the discharge tray 171 (steps S111, S112).

[0051] Here, the second printing involves transporting the sheet over a longer distance after the registration unit 30 has corrected for skew and lateral misalignment on the first printing. As a result, variations in the components of each unit often lead to greater skew and lateral misalignment than in the first printing. Consequently, the shift amount of the registration roller pair 32 increases. When the registration roller pair 32 shifts, the sheet S experiences significant friction against the transport guide member, and this resistance is particularly high when the sheet S is large, as it is nipped by other rollers. Consequently, when the shift amount is large, these resistances can cause the sheet S to skew, the shift amount of the sheet S to be smaller than expected, or the sheet S to wrinkle when the registration roller pair 32 shifts.

[0052] Furthermore, when the shift amount is large, more time is required to shift the registration roller pair 32, and also for the registration roller pair 32 to return to its home position (center position) after the sheet S has passed through the registration roller pair 32. This may lead to a decrease in productivity. To mitigate the above problems, in this embodiment, the sheet S is also shifted (lateral register shift) in the reversing conveying unit 130.

[0053] [Reversing and conveying unit] Next, the configuration of the reversal conveying unit 130 will be described. As shown in Figure 6, the reversal conveying unit 130, which functions as a reversal unit, includes a conveying roller pair 131, a reversal shift unit 132, a reversal sensor 138, a reversal CIS 139, and a switching member 143. The reversal shift unit 132 includes a first reversal shift roller pair 132a and a second reversal shift roller pair 132b, and the reversal sensor 138 and the reversal CIS 139 are provided between the conveying roller pair 131 and the first reversal shift roller pair 132a.

[0054] The conveyor roller pair 131 is driven by the reversing drive motor 136 via belt 136a. The rotation of the conveyor roller pair 131 is transmitted to the idler gear 135 via belt 136b. An input gear 134 is fixed to the rotation axis 132S of the first reversing shift roller pair 132a, and the input gear 134 is driven by the idler gear 135. The first reversing shift roller pair 132a and the second reversing shift roller pair 132b are connected by belt 136c and configured to operate in conjunction. Each roller of the first reversing shift roller pair 132a and the second reversing shift roller pair 132b rotates around an axis extending in the width direction W. For example, the first reversing 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 and fourth rollers move in the width direction W while gripping a sheet.

[0055] A rack 141 is supported on the rotating shaft 132S so as to be rotatable relative to the rotating shaft 132S but immovable in the axial direction. The rack 141 receives driving force from the reversing shift motor 137 via a pinion gear 140, and shifts the rotating shaft 132S in the axial direction. As the first pair of reversing shift rollers 132a and the second pair of reversing shift rollers 132b, which are gripping the sheet S, move in the width direction W, the sheet moves in the width direction W, and the position of the sheet in the width direction W is corrected. This enables the shift operation by the reversing conveying unit 130.

[0056] Furthermore, the idler gear 135 has a wider tooth width than the input gear 134. This is to maintain gear meshing and enable rotation of the reversal shift section 132 even when the first reversal shift roller pair 132a and the input gear 134 move in the width direction.

[0057] The inversion CIS 139 is positioned offset to one side relative to the center of the inversion 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 on only one side of the sheet S. Furthermore, the inversion CIS 139 is positioned as close as possible to the first inversion shift roller pair 132a in order to avoid reducing the detection accuracy of the inversion CIS 139.

[0058] [Shift operation by reverse transport unit] Next, the shift operation by the reversal transport unit 130 will be explained according to the flowchart shown in Figure 7. When the print job is double-sided printing, the sheet S with the image formed on the first side is transported to the reversal transport unit 130 by the branch transport unit 120. The switching member 143 of the reversal transport unit 130 is biased in a positioned state by a biasing member (not shown), as shown in Figure 8(a).

[0059] The sheet S, transported from the branching transport unit 120, is transported to the transport roller pair 131 and is transported while pressing against the switching member 143 against the biasing force of the biasing member. Subsequently, the position of the sheet S in the transport direction A is detected by the inversion sensor 138 (step S201). After that, the end position of the sheet S is detected by the inversion CIS 139 (step S202). The control unit 200 calculates the shift amount of the sheet based on this detection result (L3) and the amount of displacement (g3). The amount of displacement (g3) is the amount by which the sheet S is shifted in the width direction W when transported from the inversion transport unit 130 to the registration unit 30, which was acquired in advance when the image forming apparatus 1 was installed, etc. The shift amount of the sheet S can be obtained by subtracting the amount of displacement (g3) from the detection result (L3) of the inversion CIS 139 (L3-g3).

[0060] Next, as shown in Figure 8(b), the control unit 200 stops driving the reversal drive motor 136 when the rear end of the sheet S has advanced a predetermined distance from the switching member 143, based on the detection result of the reversal sensor 138, and stops the sheet S (step S203).

[0061] After the seat S is stopped, the control unit 200 moves the reversing shift unit 132, which is holding the seat S, in the width direction W by a shift amount (L3-g3) via the shift control unit 208 and the reversing shift motor 137. This allows the seat S to be moved in the width direction W by a shift amount (L3-g3) (step S204).

[0062] In parallel with this shifting operation, the control unit 200 reverses the reversing drive motor 136 (step S205). This causes a switchback by the first pair of reversing shift rollers 132a and the second pair of reversing shift rollers 132b of the reversing shift unit 132. That is, the sheet S is transported in the first direction A1 (see Figure 8(a)) and then in the second direction A2 (see Figure 8(c)), which is opposite to the first direction A1.

[0063] During the switchback operation, the sheet S is guided by sliding contact with the reversal guide 142, which acts as a guide member. At this time, the second surface of the sheet S, opposite to the first surface on which the image is formed, slides against the reversal guide 142. Furthermore, there is no guide member on the opposite side of the reversal guide 142, so the first surface of the sheet S guided by the reversal guide 142 is not guided by any other guide member. Then, as shown in Figure 8(c), the sheet S is guided by the switching member 143 to the second double-sided transport unit 150, where the second image is formed.

[0064] 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 shift operation by the reversal transport unit 130 ends. If the control unit 200 determines that there is a subsequent sheet (step S206: Yes), the control unit 200 returns the reversal shift unit 132 to the home position (center position) (step S207). After that, the process returns to step S201.

[0065] In this embodiment, step S205 was performed after step S204, but the order may be reversed or performed simultaneously.

[0066] [Second double-sided transport unit] Next, the configuration of the second double-sided transport unit 150 will be described. As shown in Figure 9, the second double-sided transport unit 150, as a double-sided transport section, includes a second registration roller pair 152, a second pre-registration roller pair 151, and a second registration sensor 157. The second pre-registration roller pair 151 is positioned upstream of the second registration roller pair 152 in the sheet transport direction A, and the second registration sensor 157 is provided between these roller pairs.

[0067] The second registration roller pair 152, which is a rotating body pair, has an upper roller 152a and a lower roller 152b fixed to the rotating shaft 152S. An input gear 156 is fixed to the rotating shaft 152S, and the input gear 156 is driven by the second registration drive motor 154 via an idler gear 155. The second pre-registration roller pair 151 is driven by the second pre-registration drive motor 153.

[0068] The second double-sided transport unit 150 is installed in the housing 1B and corrects the skew of the sheet S before it is discharged from housing 1B to housing 1A. The second double-sided transport unit 150 performs skew correction on the sheet S but does not perform shifting.

[0069] [Diagonal correction operation by the second double-sided transport unit] Next, the skew correction operation (second skew correction operation) by the second duplex transport unit 150 will be explained according to the flowchart shown in Figure 10. When the print job is for double-sided printing, the sheet S on which the image is formed on the first side is shifted in the inversion transport unit 130 as described above. Then, the sheet S sent from the inversion transport unit 130 to the duplex transport unit 150 has its position in the transport direction A detected by the second registration sensor 157 (step S301).

[0070] Next, based on the detection result of the second registration sensor 157, the control unit 200 feeds the sheet S by a set amount using the second pre-registration roller pair 151. As a result, the sheet S is brought into contact with the stopped second registration roller pair 152, and a predetermined amount of deflection is formed (step S302). In this way, the skew of the sheet S is corrected, and the sheet S is gripped and transported by the second registration roller pair 152, whose rotational drive has been started (step S303).

[0071] 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 transport unit 150 ends. If the control unit 200 determines that there is a subsequent sheet (step S304: Yes), the process returns to step S301.

[0072] As described above, in this embodiment, in a double-sided printing job, after the image is formed on the first side of the sheet S, a shift operation is performed at two locations: the inversion transport unit 130 and the registration unit 30. Therefore, the amount of shift of the sheet S can be distributed to these two shift operations. Furthermore, because the shift operation is performed by the inversion transport unit 130, the sheet S is not nipped by rollers other than the inversion shift unit 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 inversion shift unit 132, which creates resistance, and the shift operation can be performed stably.

[0073] Furthermore, when the sheet S is switched back by the reversal shift unit 132, the second surface on which no image is formed is guided by the reversal guide 142. Since there is no guide member on the opposite side of the reversal guide 142, the first surface of the sheet S, which is the image surface, is not guided by a guide member. Because the image surface on which the image is formed has high frictional resistance, only the second surface, which is not the image surface, is guided by the reversal guide 142, thereby reducing frictional resistance between the sheet S and the reversal guide 142. For this reason, there is less resistance even during the shifting operation of the reversal shift unit 132.

[0074] In addition, the reversal shift unit 132 simultaneously shifts the first reversal shift roller pair 132a and the second reversal shift roller pair 132b in the width direction W. By performing the shift operation with the sheet S nipped by the two pairs of rollers in this way, the likelihood of slippage between the sheet S and the rollers during the shift operation causing skew is reduced, enabling stable shift operation. Therefore, skew and lateral displacement of the sheet S are reduced, and high-quality results can be obtained. In particular, in this embodiment, when forming an image on the second surface of the first sheet of the job, skew and lateral displacement of the sheet S can be reduced. For this reason, high-quality results can be obtained more quickly compared to devices that correct the position of subsequent sheets based on the position of preceding sheets.

[0075] Furthermore, since the shift amounts of both 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 housing 1A, and the reversal transport unit 130 is provided in housing 1B. In this way, by performing the shift operation in separate housings, lateral displacement in each housing can be corrected. After correcting the lateral displacement in each housing, the sheet S is transported to the other housing, thus reducing the amount of shift of the sheet S in each housing. As a result, the length in the width direction W of the guide members forming each transport path can be reduced, leading to cost reduction and space saving.

[0077] Furthermore, in this embodiment, in a double-sided printing job, after the image is formed on the first side of the sheet S, skew correction operations are performed at two locations: the double-sided transport unit 150 and the registration unit 30. Therefore, the amount of skew correction for the sheet S can be distributed to these two locations, reducing the amount of skew correction at each location. Skew correction operations cause the sheet S to bend, so if the amount of skew correction is large, the sheet S may become distorted and wrinkles may occur. However, in this embodiment, since the amount of skew correction can be reduced, wrinkles in the sheet S can be suppressed.

[0078] Furthermore, the registration unit 30 is provided in housing 1A, and the double-sided transport unit 150 is provided in housing 1B. In this way, by performing the skew correction operation in separate housings, skew can be corrected in each housing. After skew correction is performed in each housing, the sheet S is transported to another housing, thus reducing the amount of skew correction for the sheet S in each housing. Therefore, the required skew correction capacity in each housing can be defined, and a skew correction mechanism with the optimal amount of skew correction within the bounds of excess or deficiency can be selected.

[0079] <Second Embodiment> Next, a second embodiment of the present invention will be described. In the second embodiment, the reversing transport unit 130 does not perform a shift operation, and the second double-sided transport unit 180 performs both the oblique correction operation and the shift operation. For this reason, the same configuration as in the first embodiment will be omitted from the illustration or will be described using the same reference numerals in the figures.

[0080] [Second double-sided transport unit] First, the configuration of the second double-sided transport unit 180 according to the second embodiment will be described. As shown in Figure 11, the second double-sided transport unit 180 has a second registration roller pair 182 and a second pre-registration roller pair 181. The second double-sided transport unit 180 also has a second registration sensor 187 and a second CIS 188. The second pre-registration roller pair 181 is positioned upstream of the second registration roller pair 182 in the sheet transport 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 rotating body pair, has an upper roller 182a as a third roller and a lower roller 182b as a fourth roller fixed to the rotating shaft 182S. An input gear 186 is fixed to the rotating shaft 182S, and the input gear 186 is driven by the second registration drive motor 184 via an idler gear 185. The second pre-registration roller pair 181 is driven by the second pre-registration drive motor 183. Each roller of the second pre-registration roller pair 181 and the second registration roller pair 182 rotates around an axis extending in the width direction W.

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

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

[0084] Furthermore, the second CIS188, like the CIS34 (see Figure 2), is positioned off-center to one side of the conveyor path in the width direction W. In addition, the second CIS188 is positioned as close as possible to the second registration roller pair 182 in order to avoid reducing the detection accuracy of the second CIS188.

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

[0086] The shift control unit 208 receives detection results from the CIS 34 and the second CIS 188, and issues instructions such as starting and stopping the drive of the shift motor 37 and the second shift motor 189, thereby controlling the movement of the seat S in the width direction W, i.e., the shift operation.

[0087] [Diagonal correction and shifting operations by the second double-sided transport unit] Next, the skew correction operation (second skew correction operation) and shift operation by the second duplex transport unit 180 will be explained according to the flowchart shown in Figure 13. When the print job is for duplex printing, the sheet S on which the image has been formed on the first side is switched back in the inversion transport unit 130. In this embodiment, no shift operation is performed in the inversion transport unit 130. The sheet S sent from the inversion transport unit 130 to the duplex transport unit 180 has its position in the transport direction A detected by the second registration sensor 187 (step S401).

[0088] Next, based on the detection result of the second registration sensor 187, the control unit 200 feeds the sheet S by a set amount using the second pre-registration roller pair 181. As a result, the sheet S is brought into contact with the stopped second registration roller pair 182, and a predetermined amount of deflection is formed (step S402). In this way, the skew of the sheet S is corrected, and the sheet S is gripped and transported by the second registration roller pair 182, whose rotational drive has been started (step S403).

[0089] Subsequently, the edge position of the sheet S is detected by the second CIS 188 (step S404). The control unit 200 calculates the sheet shift amount based on this detection result (L4) and the amount of displacement (g4). The amount of displacement (g4) is the amount by which the sheet S is shifted in the width direction W when it is transported from the second double-sided transport unit 180 to the registration unit 30, which was acquired in advance when the image forming apparatus 1 was installed. The sheet shift amount can then be determined by subtracting the amount of displacement (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 is holding the sheet S, in the width direction W by a shift amount (L4-g4) via the shift control unit 208 and the second shift motor 189. This allows the sheet S to be moved in the width direction W by a 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 diagonal correction operation and shift operation by the second double-sided transport 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). After that, the process returns to step S401.

[0092] As described above, in this embodiment, in a double-sided printing job, after the image is formed on the first side of the sheet S, skew correction and shift operations are performed at two locations: the second double-sided transport unit 180 and the registration unit 30. Therefore, the same effects as in the first embodiment can be achieved.

[0093] Furthermore, since the second double-sided transport unit 180 is positioned near the exit from housing 1B to housing 1A, the amount of skew and the position in the width direction W of the sheet S discharged from housing 1B can be determined more clearly than in the first embodiment.

[0094] <Other Embodiments> In the first embodiment, a shift operation was performed in the reversing transport unit 130 and a skew correction operation was performed in the second double-sided transport unit 150. In the second embodiment, both the shift operation and the skew correction operation were performed in the second double-sided transport unit 180, but the embodiment is not limited to these. That is, it is sufficient for at least one of the shift operation and the skew correction operation to be performed in the re-transport unit 500. Furthermore, it is not limited 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 reversing transport unit 130, or only the shift operation may be performed in the first double-sided transport unit 70.

[0095] Furthermore, in the first embodiment, both the first reverse shift roller pair 132a and the second reverse shift roller pair 132b of the reverse shift section 132 were configured to be movable in the width direction W, but this is not limited to this. 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. Alternatively, the second reverse shift roller pair 132b may be omitted, and the sheet S may be held between the first reverse shift roller pair 132a and moved in the width direction W.

[0096] Furthermore, CCD sensors or CMOS sensors may be used instead of CIS34, inverted CIS139, and second CIS188. If these sensors can detect the position in the width direction of the sheet, it is not necessary to detect the position of the edges in the width direction of the sheet.

[0097] Alternatively, instead of correcting the sheet's skewness by abutting it against the registration roller pair 32 or the second registration roller pair 182, a method may be applied in which the sheet abuts against a shutter member provided upstream in the conveying direction of the roller pair.

[0098] Furthermore, although the invention has been described using an electrophotographic image forming apparatus 1 in all of the above-described embodiments, the present invention is not limited thereto. 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 a nozzle.

[0099] The present invention can also be realized by supplying a program that implements one or more of the functions of the above embodiments to a system or device via a network or storage medium, and by having one or more processors in the computer of that system or device read and execute the program. It can also be realized by a circuit (e.g., an ASIC) that implements one or more functions. [Explanation of symbols]

[0100] 1: Image forming apparatus / 1A: First housing / 1B: Second housing / 32: Second moving unit, second oblique correction unit (registration roller pair) / 32a: First roller (upper roller) / 32b: Second roller (lower roller) / 34: Second detection unit (CIS) / 65: Transport path / 130: Reversal unit (reversal transport unit) / 132: First moving unit (reversal shift unit) / 132a: Third roller, fourth roller (first reversal shift roller pair) / 139: First detection unit (reversal CIS) / 142: Guy Component (reversal guide) / 150: Double-sided transport section (second double-sided transport unit) / 152: Skew correction section (second registration roller pair) / 182: First moving section, first skew correction section (second registration roller pair) / 182a: Third roller (upper roller) / 182b: Fourth roller (lower roller) / 188: First detection section (second CIS) / 500: Re-transport section / A: Transport direction / A1: First direction / A2: Second direction / N: Image forming section (secondary transfer nip) / W: Width direction

Claims

1. An image forming unit that forms an image on a sheet, An inversion unit for inverting the leading and trailing ends of a sheet on which an image has been formed on the first surface by the image forming unit, comprising: a pair of inversion rollers that grip the sheet and rotate it in a first direction, and then rotate it in a second direction opposite to the first direction to invert the sheet; and a first moving unit that moves the sheet in the width direction perpendicular to the sheet transport direction while the sheet is gripped by the pair of inversion rollers, thereby moving the sheet in the width direction. A diagonal correction unit having a first pair of registration rollers provided upstream of the image forming unit in the transport direction and correcting the skewness of the sheet by abutting the leading edge of the sheet, and a second moving unit that moves the sheet in the width direction while the sheet is held between the first pair of registration rollers, A conveying unit having a pair of second registration rollers provided between the reversing unit and the skew correction unit in the conveying direction, which corrects the skew of the sheet by abutting the leading edge of the sheet that has been reversed by the reversing unit, It comprises a control unit and, When forming an image on the second surface of the sheet opposite to the first surface, the control unit controls the sheet so that (1) the position of the sheet in the width direction is corrected by the first moving unit, and then the skew of the sheet is corrected by the second registration roller pair, (2) the skew of the sheet is corrected by the first registration roller pair, and then the position of the sheet in the width direction is corrected by the second moving unit, and (3) the sheet is transported to the image forming unit. An image forming apparatus characterized by the following features.

2. The reversing roller pair comprises a first reversing roller pair and a second reversing roller pair provided downstream of the first reversing roller pair in the conveying direction. The control unit controls the sheet so that when the sheet is moved in the width direction by the first moving unit, the sheet is moved in the width direction while being held between the first reversing roller pair and the second reversing roller pair. The image forming apparatus according to feature 1.

3. The aforementioned reversal unit is A pair of conveyor rollers for transporting sheets, A reversal position detection unit that detects the rear end of the sheet being transported in order to reverse the sheet, A side edge detection unit that detects the position of the side edge of the sheet in the width direction, It has, As you move from upstream to downstream in the conveying direction, the conveying roller pair, the reversal position detection unit, the side end detection unit, the first reversal roller pair, and the second reversal roller pair are arranged in that order. The image forming apparatus according to feature 2.

4. A first side edge detection unit is positioned upstream of the reversing roller pair in the transport direction and detects the position of the side edge of the sheet in the width direction of the sheet, The system further includes a second side edge detection unit, which is positioned upstream of the first registration roller pair in the transport direction and detects the position of the side edge of the sheet in the width direction of the sheet, The control unit moves the sheet held between the first registration roller pair by the second moving unit in the width direction based on the detection result of the second side end detection unit, and moves the sheet held between the reversing roller pair by the first moving unit in the width direction based on the detection result of the first side end detection unit. The image forming apparatus according to any one of claims 1 to 3.

5. The transport path further includes a transport path that guides the sheet being transported in the aforementioned transport direction, When forming an image on the second surface of the sheet, the control unit moves the sheet in the width direction by the first moving unit and then moves the sheet further by the second moving unit, thereby aligning the center of the transport path with the center of the sheet. The image forming apparatus according to any one of claims 1 to 4.

6. A first housing having the image forming unit and the oblique correction unit, The system further comprises a second housing having the aforementioned reversing section and connected to the first housing, The image forming apparatus according to any one of claims 1 to 5.

7. The first registration roller pair each has a first roller and a second roller that rotate around an axis extending in the width direction, The aforementioned pair of reversing rollers each has a third roller and a fourth roller that rotate around an axis extending in the width direction, The control unit moves the sheet in the width direction by the second moving unit while the sheet is held between the first roller and the second roller, and moves the sheet in the width direction by the first moving unit while the sheet is held between the third roller and the fourth roller. The image forming apparatus according to any one of claims 1 to 6.

8. The control unit moves the sheet in the width direction by the first moving part when the reversing roller pair is rotating in the second direction. The image forming apparatus according to any one of claims 1 to 7.

Images