Image forming apparatus

Abstract

To prevent wrinkles from forming in the sheet when correcting the curl of the sheet in the curl correction section. [Solution] The image forming apparatus (1) includes an image forming unit (90) for forming an image on a sheet (S), a re-transport path (PR) for guiding the sheet (S), on which an image has been formed on the first surface by the image forming unit (90), back to the image forming unit (90), a curl correction unit (70) interposed in the re-transport path (PR) for correcting the curl of the sheet (S), and a first skew correction unit (80) positioned upstream of the curl correction unit (70) in the transport direction in the re-transport path (PR) and capable of correcting the skew of the sheet (S).

Description

Technical Field

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

Background Art

[0002] For example, in order to remove the curl of a sheet, a curling device that applies a curl in the opposite direction of the curl has been proposed (see Patent Document 1). The curling device of this Patent Document 1 is configured such that a first roller and a second roller having a higher hardness than the first roller are pressed against each other, and the sheet is conveyed by a nip formed by these rollers to perform curling. In general, when an image is formed on a sheet, the sheet is likely to curl. Therefore, in the image forming apparatus described in Patent Document 1, in which the sheet after the first image formation is stacked on an intermediate tray and conveyed from the intermediate tray to a transfer charger again, the curling device is arranged to perform curling on the sheet stacked on the intermediate tray.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, as described above, in a case where curling (that is, curl correction) is performed on a curled sheet, the sheet is sandwiched and conveyed by a nip formed by a roller formed of an elastic member and a roller that is pressed against the former so as to bite into it. However, in such a structure, when trying to perform curling particularly strongly, the restraining force of the sheet by the nip becomes strong. Therefore, if a deviation occurs in the speed applied in the conveyance direction at different positions in the width direction of the sheet, there is a problem that wrinkles are likely to occur in the sheet.

[0005] Therefore, the present invention aims to provide an image forming apparatus that can prevent wrinkles from forming on the sheet when correcting the curl of the sheet in the curl correction section. [Means for solving the problem]

[0006] One aspect of the present invention is an image forming apparatus comprising: an image forming unit for forming an image on a sheet; a re-transport path for guiding the sheet, on which an image has been formed on a first surface by the image forming unit, to be re-transported to the image forming unit; a curl correction unit interposed in the re-transport path for correcting the curl of the sheet; and a first skew correction unit positioned upstream of the curl correction unit in the transport direction in the re-transport path and capable of correcting the skew of the sheet. [Effects of the Invention]

[0007] According to the present invention, when correcting the curl of the sheet in the curl correction section, it is possible to make it less likely for wrinkles to occur in the sheet. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing a printer according to the first embodiment. [Figure 2] This is a block diagram of the printer's control system. [Figure 3] (a) is a perspective view showing a sheet with a concave curled surface. (b) is a perspective view showing a sheet with a convex curled surface. [Figure 4] This is a schematic diagram showing a double-sided decal unit according to the first embodiment. [Figure 5] (a) is a schematic diagram showing the process of applying a concave curl sheet using a double-sided decal unit. (b) is a schematic diagram showing the process of applying a convex curl sheet using a double-sided decal unit. [Figure 6] This is a perspective view showing the penetration depth adjustment mechanism for the upstream curl correction roller pair. [Figure 7]This is a schematic diagram showing a state where the penetration amount is small in the penetration amount adjustment mechanism for the upstream curl correction roller pair. [Figure 8] This is a schematic diagram showing a state where the penetration amount is large in the penetration amount adjustment mechanism of the upstream curl correction roller pair. [Figure 9] (a) is a diagram showing the state in which the sheet has been transported to the pair of double-sided pre-resist rollers in the double-sided skew correction unit. (b) is a diagram showing the state in which the sheet has been transported to the pair of double-sided resist rollers in the double-sided skew correction unit. (c) is a diagram showing the state in which the sheet has started to be transported downstream from the pair of double-sided resist rollers in the double-sided skew correction unit. [Figure 10] (a) is an overhead view showing the state in which the sheet has been conveyed at an angle by the upstream curl correction roller pair. (b) is a schematic cross-sectional view showing the difference in velocity vectors that occur on the lagging and advancing sides in the width direction of the sheet. [Figure 11] This figure shows a three-dimensional simulation image of a sheet being transported to a double-sided decal unit while it is tilted. [Figure 12] (a) is a graph showing the results of a three-dimensional simulation when a sheet with a large degree of skew is transported to a double-sided decal unit. (b) is a graph showing the results of a three-dimensional simulation when a sheet with a small degree of skew is transported to a double-sided decal unit. [Figure 13] (a) is a graph showing the relationship between the amount of skew, the amount of curl correction penetration, and the occurrence of wrinkles when the sheet type is thin paper. (b) is a graph showing the relationship between the amount of skew, the amount of curl correction penetration, and the occurrence of wrinkles when the sheet type is ultra-thin paper. [Figure 14] This figure shows a curl correction table that records the relationship between the amount of curl correction penetration and the sheet type, ambient humidity, and image density. [Figure 15] This flowchart shows the control of the double-sided skew correction unit. [Figure 16] This is a schematic diagram showing a printer according to the second embodiment. [Figure 17] This is a schematic diagram showing a printer according to the third embodiment.

Mode for Carrying out the Invention

[0009] <First Embodiment> Hereinafter, the first embodiment will be described with reference to the drawings. First, the schematic configuration of the printer 1 as an image forming apparatus according to the first embodiment will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view showing the printer 1 according to the first embodiment.

[0010] [Schematic Configuration of Printer] The printer 1 according to the present embodiment is an electrophotographic full-color laser beam printer. As shown in FIG. 1, the printer 1 includes a housing 1a as a first housing incorporating a unit for feeding a sheet and forming an image, and a housing 1b as a second housing incorporating a unit for fixing and cooling.

[0011] The housing 1a has a feeding unit 10 as a feeding unit, a pulling unit 20, a registration unit 30 as a second skew correction unit, an image forming unit 90 as an image forming unit, a first duplex conveyance unit 60, and further has a duplex skew correction unit 80 as a first skew correction unit and a duplex decal unit 70 as a curl correction unit, which will be described in detail later. The housing 1b has a fixing unit 100 as a fixing unit, a cooling unit 110 as a cooling unit, a branch conveyance unit 120, a reverse conveyance unit 130 as a reversing unit, a second duplex conveyance unit 140, and a paper discharge decal unit 150.

[0012] The image forming unit 90 includes four process cartridges 99Y, 99M, 99C, and 99K that form toner images of four colors, namely yellow (Y), magenta (M), cyan (C), and black (K), and exposure devices 93Y, 93M, 93C, and 93K. Note that the four process cartridges 99Y, 99M, 99C, and 99K have the same configuration except for the color of the formed image. Therefore, only the configuration of the process cartridge 99Y and the image forming process will be described, and the descriptions of the process cartridges 99M, 99C, and 99K 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). Further, the image forming unit 90 is provided with an intermediate transfer belt 50 as an image carrier that rotates in the direction of arrow T 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, and 55K are provided, and outside the intermediate transfer belt 50, a secondary transfer outer roller 54 as a transfer roller is provided facing the secondary transfer inner roller 53.

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

[0015] The registration unit 30 has a pre-registration roller pair 31 that conveys the sheet S and a registration roller pair 32 that corrects the skew of the sheet S. 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.

[0016] On the other hand, the fixing unit 100 located in the housing 1b has a pair of heatable fixing rollers 101. The cooling unit 110 has an upper cooling belt 111a that rotates in the direction of arrow T by an upper cooling drive roller 112a. Similarly, it has a lower cooling belt 111b that rotates in the direction of arrow T by a lower cooling drive roller 112b. It also has a heat sink 113 for cooling the sheet.

[0017] [Image Formation Process] Next, the image formation operation of the printer 1 configured in this way will be described. For example, when an image signal is input to the exposure device 43 from an external computer 300 (see Figure 2), the exposure device 43 irradiates a laser beam corresponding to the image signal onto the photosensitive drum 91 of the process cartridge 99Y.

[0018] At this time, the surface of the photosensitive drum 91 is uniformly charged to a predetermined polarity and potential by a charging roller (not shown), and an electrostatic latent image is formed on the surface when laser light is irradiated from the exposure device 93Y via the mirror 94. The electrostatic latent image formed on the photosensitive drum 91 is developed by the developer 92, and a yellow (Y) toner image is formed on the photosensitive drum 91.

[0019] Similarly, laser light from exposure units 93M, 93C, and 93K is irradiated onto the photosensitive drums of process cartridges 99M, 99C, and 99K, 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 55K. 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 T2, which is a transfer section formed by the secondary transfer inner roller 53 and the 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 the feeding unit 10 and transported to the registration unit 30 by the extraction unit 20. The registration unit 30 has a pair of registration rollers 32 that correct the skewness of the sheet S, as described above. That is, the sheet S is corrected for misalignment and skewness by the registration unit 30 and transported to the secondary transfer nip T2 in time with the full-color toner image formed on the intermediate transfer belt 50 reaching the secondary transfer nip T2. Then, the full-color toner image on the intermediate transfer belt 50 is transferred to the first sheet surface (surface) of the sheet S by the secondary transfer bias applied to the secondary transfer outer roller 54. Any 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 57. The sheet S is then guided to the nip 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 nipped by the upper cooling belt 111a and the lower cooling belt 111b, which are endless belts, in the cooling unit 110, and the sheet S is transported by the rotation of the upper cooling drive roller 112a and the lower cooling drive roller 112b. The sheet S is then brought into contact with the heat sink 113 via the upper cooling belt 111a, and the sheet S is cooled by transferring heat to the heat sink 113.

[0022] Next, the branching transport unit 120 selects a route to either transport the sheet to the paper discharge decal unit 150 or to the inversion transport unit 130. After the sheet S is transported from the branching transport unit 120, the inversion transport unit 130 can invert the sheet S so that the first side of the sheet S, on which the image has been formed by the secondary transfer nip T2, is facing downwards. The inverted sheet S is then transported to either the second double-sided transport unit 140 or returned to the branching transport unit 120.

[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 paper discharge decal unit 150, 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 paper discharge decal unit 150 is discharged outside the machine or handed over to an optional paper discharge device (not shown).

[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 140, the double-sided skew correction unit 80, the double-sided decal unit 70, and the first double-sided transport unit 60, and guided to the registration unit 30. After this, an image is formed on the second side (back side) of sheet S at the secondary transfer nip T2, and the sheet S is discharged outside the machine via the branching transport unit 120 and the paper discharge decal unit 150, or handed over to an optional paper discharge device (not shown).

[0025] [Printer transport path] Here, we will describe the transport paths for the sheet S in printer 1. Printer 1 has roughly the following transport paths for the sheet S: a feeding path Pa1, a post-transfer transport path Pa2, an ejection path Pa3, an inversion path Pa4, and a post-inversion path Pa5. The sheet S, which is drawn out from the feeding unit 10 by the extraction unit 20 and fed, is transported along the feeding path Pa1 to the secondary transfer nip T2 of the image forming unit 90. The sheet S, on which the toner image has been transferred at the secondary transfer nip T2 of the image forming unit 90, is transported along the post-transfer transport path Pa2 to the branching transport unit 120. The sheet S that is ejected to the outside through the paper discharge decal unit 150 at the branching transport unit 120 is transported along the ejection path Pa3. On the other hand, the sheet S that is transported to the inversion transport unit 130 at the branching transport unit 120 is transported along the inversion path Pa4. Then, during double-sided printing, the sheet S, whose transport direction (front and back) has been reversed by the reversing transport unit 130, is transported along the reversed path Pa5 and then transported to merge with the feed path Pa1.

[0026] In other words, during double-sided printing, the sheet S on which the image has been formed on the first side is transported from the secondary transfer nip T2 of the image forming unit 90 through the re-transport path PR to the feed path Pa1. That is, the re-transport path PR, which re-transports the sheet S back to the secondary transfer nip T2 of the image forming unit 90, is configured to include the post-transfer transport path Pa2, the inversion path Pa4, and the post-inversion path Pa5. The double-sided skew correction unit 80 and the double-sided decal unit 70, which will be described in more detail later, are positioned to be interposed in the post-inversion path Pa5 of the re-transport path PR. The feed unit 10 is positioned to be interposed in the feed path Pa1.

[0027] [Printer control system configuration] Next, the configuration of the control system for printer 1 will be explained using Figure 2. Figure 2 is a block diagram of the printer's control system.

[0028] As shown in Figure 2, the control unit 200 of printer 1 has a CPU 201, which is connected to 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 curl correction control unit 208. The control unit 200 of printer 1 is also connected to an external computer 204, which receives various information and commands for printing.

[0029] The memory 202 includes so-called RAM and ROM. The operation unit 203 is an operation panel, which is not shown in Figure 1. The image forming control unit 205 is connected to the exposure devices 93 of each color and forms an image to be transferred to the sheet S by exposing the photosensitive drum 91.

[0030] The sheet transport control unit 206 transports the sheet S by controlling various rollers. In particular, the sheet transport control unit 206 is connected to the double-sided pre-registration drive motor M3, which drives the double-sided pre-registration roller pair 81 of the double-sided skew correction unit 80 (described in more detail later), and the double-sided registration drive motor M4, which drives the double-sided registration roller pair 82.

[0031] The sensor control unit 207 is connected to the tip position detection sensor 83, which is provided in the double-sided skew correction unit 80, which will be described in more detail later. The sensor control unit 207 is also connected to the photosensors 78-1 and 78-2, which are provided in the upstream curl correction unit 70A and the downstream curl correction unit 70B of the double-sided decal unit 70, which will be described in more detail later.

[0032] The curl correction control unit 208 is connected to curl correction roller drive motors M1-1 and M1-2, which drive the curl correction rollers 71a and 72a, respectively, provided in the upstream curl correction unit 70A and the downstream curl correction unit 70B. Furthermore, the curl correction control unit 208 is connected to penetration amount adjustment motors M2-1 and M2-2, which adjust the penetration amount of the driven correction rollers 71b and 72b into the curl correction rollers 71a and 72a.

[0033] Note that the image formation control unit 205, sheet transport control unit 206, sensor control unit 207, and curl correction control unit 208 are function implementation units that operate when the CPU 201 executes a program stored in memory 202, and are not physical components. The operation of the printer 1, which functions through these function implementation units, will be described later.

[0034] [About double-sided decal units] (Regarding the occurrence of curls and the problems associated with them) Next, the cause of curling in sheet S will be explained using Figure 3. Figure 3(a) is a perspective view showing a sheet with a concave curled surface. Figure 3(b) is a perspective view showing a sheet with a convex curled surface.

[0035] As a result of the image forming operation of the printer 1 described above, the sheet S on which the toner image has been fixed is heated in the fixing unit 100, which alters the balance of in-plane moisture content between the first surface (front) and the opposite second surface (back) of the sheet S. This causes curling of the sheet S, such as a concave curl on the surface as shown in Figure 3(a), or a convex curl on the surface as shown in Figure 3(b). This curling is also caused by the sheet S being compressed by the nip of each transport path, each transport roller pair, the nip of the fixing roller pair 101, etc. Furthermore, this curling is also caused by the difference in toner shrinkage rate between the toner-rich and toner-scarce surfaces after the toner image has been heated and fixed, the difference in cooling rate between the first and second surfaces of the sheet S, etc.

[0036] When the sheet S has a large curl, especially during image formation on the second side in double-sided printing, if the sheet is transported to the secondary transfer nip T2 with a large curl, the toner image may not be accurately transferred to the sheet, potentially resulting in image defects. Furthermore, after passing through the secondary transfer nip T2, separation problems may occur, leading to paper jams. Additionally, at the nip of the fuser roller 101, the curl can worsen the sheet S's entry position into the nip, potentially causing wrinkles and fuser defects. These defects are more pronounced with thin sheets, as they have less rigidity and therefore a smaller tolerance for curl.

[0037] Therefore, in this embodiment, a double-sided decal unit 70 is provided in the re-transport path PR, specifically the inversion path Pa5. After the curl is corrected by the double-sided decal unit 70, the sheet S is transported to the secondary transfer nip T2 to transfer the image to the second surface.

[0038] (Configuration and operation of the double-sided decal unit 70) Next, the detailed configuration of such a double-sided decal unit 70 will be explained using Figures 4 to 8. Figure 4 is a schematic diagram showing a double-sided decal unit according to the first embodiment. Figure 5(a) is a schematic diagram showing the state of decaling a concave curl sheet with the double-sided decal unit. Figure 5(b) is a schematic diagram showing the state of decaling a convex curl sheet with the double-sided decal unit. Figure 6 is a perspective view showing the penetration amount adjustment mechanism of the upstream curl correction roller pair. Figure 7 is a schematic diagram showing the state of small penetration in the penetration amount adjustment mechanism of the upstream curl correction roller pair. Figure 8 is a schematic diagram showing the state of large penetration in the penetration amount adjustment mechanism of the upstream curl correction roller pair. Note that the paper discharge decal unit 150 has the same configuration as the double-sided decal unit 70 described below, so a detailed explanation will be omitted. The paper discharge decal unit 150 is provided to reduce curl when handing over to an unillustrated processing device or the like connected to the downstream side in the transport direction of the printer 1.

[0039] As shown in Figure 4, the double-sided decal unit 70 has an upstream curl correction section 70A located upstream in the conveying direction and a downstream curl correction section 70B located downstream of the upstream curl correction section 70A in the conveying direction. The upstream curl correction section 70A is equipped with an upstream curl correction roller pair 71, and the downstream curl correction section 70B is equipped with a downstream curl correction roller pair 72.

[0040] The upstream curl correction roller pair 71 has a curl correction roller 71a as a first roller, made of a metal member such as SUS, which is rotated by a curl correction roller drive motor M1-1. It also has a driven correction roller 71b as a second roller, made of a soft elastic member such as foamed urethane, which presses against the curl correction roller 71a to form a first nip portion N1. Similarly, the downstream curl correction roller pair 72 has a curl correction roller 72a as a third roller, made of a metal member such as SUS, which is rotated by a curl correction roller drive motor M1-2. It also has a driven correction roller 72b as a fourth roller, made of a soft elastic member such as foamed urethane, which presses against the curl correction roller 72a to form a second nip portion N2. The driven correction rollers 71b and 72b press against the curl correction rollers 71a and 72a by changing the amount of penetration according to the phase of the cam member, which will be described later.

[0041] Incidentally, the nip portion N of the upstream curl correction roller pair 71 is curved as the curl correction roller 71a bites into the driven correction roller 71b, as shown in Figure 4. The upstream curl correction roller pair 71, having such a curved nip portion N, corrects the curl of a sheet with a concave shape that is curved upwards, with both ends in the conveying direction being in the first curl direction, as shown in Figure 3(a). Furthermore, the downstream curl correction roller pair 72 corrects the curl of a sheet with a convex shape that is curved downwards, with both ends in the conveying direction being in the second curl direction, opposite to the first curl direction, as shown in Figure 3(b).

[0042] In order to increase the amount of curl correction, it is necessary to create a large curvature in the nip section N. For this reason, the roller used as the curl correction roller 71a is preferably smaller in diameter than the other conveying rollers, and in this embodiment, a roller with a diameter of φ8 mm is used. Furthermore, the driven correction roller 72a, which has a different hardness from the curl correction roller 71a, is preferably larger in diameter, and in this embodiment, a roller with a diameter of φ24 mm is used. The curl correction roller 72a and the driven correction roller 72b have a similar configuration.

[0043] When the sheet S is transported to the double-sided decal unit 70 with this configuration, the sheet S is transported to the nip portion of the upstream curl correction roller pair 71, where the concave curl is corrected. Next, the sheet S is transported to the nip portion of the downstream curl correction roller pair 72, where the convex curl is corrected. Finally, with the curl corrected in this state, the sheet S is handed over to the first double-sided transport unit 60.

[0044] In this embodiment, as shown in Figure 5(a), when correcting a concave curl, the amount of curvature of the nip portion N1 of the upstream curl correction roller pair 71 is increased, and the amount of curvature of the nip portion N2 of the downstream curl correction roller pair 72 is decreased. When correcting a convex curl, as shown in Figure 5(b), the amount of curvature of the nip portion N1 of the upstream curl correction roller pair 23 is decreased, and the amount of curvature of the nip portion N2 of the downstream curl correction roller pair 24 is increased.

[0045] Incidentally, the amount of curl varies depending on various parameters such as temperature and humidity, moisture content of the sheet, type of sheet, thickness of sheet, and image density, so the amount of curl correction is determined according to each parameter. Based on the determined correction amount, the control unit 200 changes the amount of penetration (pressing force) of the driven correction rollers 72b, 72b relative to the curl correction rollers 71a, 72a, in other words, the shape of the nip portion, by the amount of rotation of the cam member described later.

[0046] Next, the penetration amount adjustment mechanism 70Aa, which serves as the first pressure amount adjustment unit for changing the penetration amount (pressing force) of the driven correction rollers 72b, 72b relative to the curl correction rollers 71a, 72a, will be explained using Figure 6. Note that Figure 6 shows the penetration amount adjustment mechanism 70Aa of the upstream curl correction unit 70A, but the penetration amount adjustment mechanism 70Ab of the downstream curl correction unit 70B, which serves as the second pressure adjustment unit, has a similar configuration, so its explanation will be omitted.

[0047] As shown in Figure 6, the penetration amount adjustment mechanism 70Aa has a rocking member 75 that rotatably holds the driven correction roller 71b. This rocking member 75 rocks with its pivot points 76a, 76b, and roller members 74a, 74b are rotatably provided at the rocking ends. Cam members 73a, 73b, which rotate by a drive unit, penetration amount adjustment motor M2-1, which is capable of forward and reverse rotation (i.e., rotation in a first direction and a second direction opposite to the first direction), are pressed against these roller members 74a, 74b. These cam members 73a, 73b have cam surfaces on their outer circumferential surfaces, where the height from the center of rotation gradually changes. In addition, the photosensor 78-1 detects the home position (HP) of the cam members 73a, 73b by detecting the HP detection flag 77 of the cam members 73a, 73b.

[0048] Here, as shown in Figure 7, the roller members 74a and 74b held by the oscillating member 75 are constantly in contact with the outer surfaces of the cam members 73a and 73b due to the reaction force of the driven correction roller 71b pressing against the curl correction roller 71a, or due to a pressing member (not shown). Then, for example, when the printer 1 is powered on, the control unit 200 drives the penetration amount adjustment motor M2-1 to rotate the cam members 73a and 73b in order to adjust the penetration amount (pressing force) of the driven correction roller 71b against the curl correction roller 71a according to the amount of curl correction.

[0049] The control unit 200 determines the rotation angle of the cam members 73a and 73b from a reference angle according to the curl correction amount when rotating the cam members 73a and 73b. Then, based on the signal from the photosensor 78-1, it detects that the cam members 73a and 73b are in the home position. Subsequently, it drives the penetration amount adjustment motor M2-1 to rotate the cam members 73a and 73b by a predetermined amount in stages from the home position, and adjusts the penetration amount (pressing force) of the upstream curl correction roller pair 71 in multiple stages. In other words, the cam members 73a and 73b move one of the upstream curl correction roller pair 71 relative to the other to adjust the nip pressure of the upstream curl correction roller pair 71. The downstream curl correction roller pair 72 adjusts the nip pressure in the same way.

[0050] Specifically, as shown in Figure 8, when the penetration amount adjustment motor M2-1 is driven and the cam members 73a and 73b rotate in the direction of arrow W, the oscillating member 75 oscillates in the directions of arrows B and C around the oscillating centers 76a and 76b via the roller members 74a and 74b. Consequently, the driven correction roller 71b moves in the direction of arrow D. As a result, the driven correction roller 71b presses against the curl correction roller 71a, and the curl correction roller 71a penetrates the driven correction roller 71b by a predetermined amount.

[0051] Incidentally, as described in Japanese Patent Publication No. 9-249345, it is conceivable to prevent wrinkles from forming during sheet decaling by not forming the curl correction roller and driven correction roller over the entire width. However, with such a configuration, especially when correcting curl in sheets with low rigidity such as thin paper, curl may remain at the edges in the width direction. This could lead to transport problems during double-sided printing, separation problems where the sheet wraps around the rollers in the secondary transfer nip or fixing roller pair, or failure to accurately transfer the toner image to the edges in the width direction at the secondary transfer nip. For this reason, the curl correction rollers 71a, 72a and driven correction rollers 72a, 72b of this embodiment use so-called single rollers in which the nip portion is longer in the width direction than the maximum sheet width that can be printed by the printer 1. This prevents curl from remaining at the edges in the width direction. However, with such a long roller in the width direction, differences in velocity vectors at different positions in the width direction are likely to occur. This problem will be discussed later.

[0052] [About the double-sided skew correction unit] Next, the configuration and operation of the double-sided skew correction unit 80, which can perform skew correction of the sheet S, will be explained with reference to Figure 9. Figure 9(a) shows the state in which the sheet has been transported to the double-sided pre-resist roller pair in the double-sided skew correction unit. Figure 9(b) shows the state in which the sheet has been transported to the double-sided resist roller pair in the double-sided skew correction unit. Figure 9(c) shows the state in which the sheet has started to be transported downstream from the double-sided resist roller pair in the double-sided skew correction unit. Note that the configuration and operation of the registration unit 30 are the same as those of the double-sided skew correction unit 80, so their explanation will be omitted.

[0053] The double-sided skew correction unit 80 includes a pair of double-sided pre-registration rollers 81 upstream in the conveying direction and a pair of double-sided registration rollers 82 downstream in the conveying direction. The pair of double-sided pre-registration rollers 81 is rotationally driven by the double-sided pre-registration drive motor M3, and the pair of double-sided registration rollers 82 is rotationally driven by the double-sided registration drive motor M4 (see Figure 2). In addition, a tip position detection sensor 83 is positioned immediately upstream of the pair of double-sided pre-registration rollers 81 to detect the leading edge of the sheet S and to detect the timing when the leading edge of the sheet S reaches the pair of double-sided registration rollers 82.

[0054] As described above, after an image is formed on the first surface of the sheet S, the sheet S is transported to the reversal transport unit 130 by the branch transport unit 120, where the direction of travel (transport direction) of the sheet is reversed (switchback reversal) (see Figure 1). The reversed sheet S is then transported to the second double-sided transport unit 140, and subsequently to the double-sided pre-registration roller pair 81 of the double-sided skew correction unit 80.

[0055] Here, let's assume that the conveyed sheet S is, for example, rotated clockwise with respect to the conveying direction A as shown in Figure 9(a), that is, it is moving at an angle. Then, first, the leading edge of the conveyed sheet S abuts against the nip portion of the stopped double-sided registration roller pair 82 and follows the double-sided registration roller pair 82. Subsequently, as shown in Figure 8(b), the control unit 200, based on the detection result of the leading edge position detection sensor 83, conveys the sheet S with the double-sided pre-registration roller pair 81 to a set feed amount, thereby forming a predetermined loop on the sheet S. Then, as shown in Figure 8(c), the rotation of the stopped double-sided registration roller pair 82 is started, and the sheet S is conveyed by the double-sided registration roller pair 82. As a result, the angled conveyance of the sheet S upstream of the loop is not affected by the loop, and the angle of the sheet S is corrected. In other words, the leading edge of the sheet S is transported to the double-sided decal unit 70 (see Figure 1) while remaining parallel to the double-sided registration roller pair 82, and the double-sided decal unit 70 can transport the sheet S with its skew corrected.

[0056] [Regarding the mechanism of wrinkle formation when correcting the curl of a skewed sheet] Next, the mechanism of wrinkle generation during curl correction of a skewed sheet S will be explained using Figures 10 to 13. Figure 10(a) is an overhead view showing the state in which the sheet has been conveyed skewed to the upstream curl correction roller pair. Figure 10(b) is a schematic cross-sectional diagram showing the difference in velocity vectors that occur on the lagging and advancing sides in the width direction of the sheet. Figure 11 is a three-dimensional simulation image when a skewed sheet is conveyed to a double-sided decal unit. Figure 12(a) is a graph showing the results of a three-dimensional simulation when a sheet with a large degree of skew is conveyed to a double-sided decal unit. Figure 12(b) is a graph showing the results of a three-dimensional simulation when a sheet with a small degree of skew is conveyed to a double-sided decal unit. Figure 13(a) is a graph showing the relationship between the amount of skew, the amount of curl correction penetration, and the generation of wrinkles when the sheet type is thin paper. Figure 13(b) is a graph showing the relationship between the amount of skew, the amount of curl correction penetration, and the generation of wrinkles when the sheet type is ultra-thin paper.

[0057] As mentioned above, with thin paper and other sheets that have less rigidity (lack of stiffness) than regular paper, it is necessary to correct the curl with high precision to reduce image formation defects and transport defects on the second side. However, when correcting the curl of sheets that lack stiffness, such as thin paper, care must be taken to avoid the formation of wrinkles.

[0058] As shown in Figure 10(a), assume that the sheet is tilted when it is being conveyed to the upstream curl correction roller pair 71 (or downstream curl correction roller pair 72). In this case, as shown in Figure 10(b) between the lagging velocity vector B1 and the advancing velocity vector B2, a difference in velocity vectors occurs. Even such a small difference in velocity vectors increases the stress on the sheet in plane, causing wrinkles to form. In particular, if the upstream curl correction roller pair 71 (or downstream curl correction roller pair 72) is a single roller that is long in the width direction, differences in velocity vectors are likely to occur at both ends of the sheet S.

[0059] The effect of sheet S being skewed when it is conveyed to the upstream curl correction roller pair 71 and the downstream curl correction roller pair 72 was analyzed by three-dimensional simulation (hereinafter referred to as "three-dimensional SIM") as shown in Figure 11. Figure 11 shows the simulation when a skewed sheet S is conveyed to the upstream curl correction roller pair 71 and the downstream curl correction roller pair 72.

[0060] The results obtained by the three-dimensional SIM shown in Figure 11 are shown in Figures 12(a) and 12(b). The graphs shown in Figures 12(a) and 12(b) represent the velocity distribution of the sheet at the nip of the upstream curl correction roller pair 71 at a given time. In these graphs, the horizontal axis represents the coordinate values ​​in the width direction of the sheet S, and the vertical axis represents the contact velocity at each coordinate value.

[0061] As shown in the graph in Figure 12(a), when the sheet S being conveyed by the upstream curl correction roller pair 71 is significantly skewed, there is a point where the speed of the sheet S drops sharply. In actual experiments, it was found that wrinkles occur at this point. In contrast, when the sheet being conveyed by the upstream curl correction roller pair 71 is small, as shown in the graph in Figure 12(b), the speed of the sheet S remains stable throughout its width, and no wrinkles occur. These phenomena were also observed with the downstream curl correction roller pair 72.

[0062] As described above, when the sheet S is conveyed to the upstream curl correction roller pair 71 or the downstream curl correction roller pair 72, if the sheet S is tilted, even a slight difference in velocity vectors will increase the stress on the sheet S within its plane, causing wrinkles to form. This mechanism was confirmed by the results obtained from three-dimensional SIM. It was also found that the formation of wrinkles is particularly pronounced in thin paper.

[0063] Figures 13(a) and 13(b) are graphs showing whether or not wrinkles occur in relation to the amount of penetration in the upstream curl correction roller pair 71 (hereinafter referred to as "curl correction penetration amount") and the amount of skew of the sheet S conveyed to the upstream curl correction roller pair 71. That is, in this graph, the horizontal axis is the penetration amount, and the vertical axis is the amount of skew of the sheet S. The curl correction penetration amount is the amount of penetration of the curl correction roller 71a into the driven correction roller 71b. In this embodiment, this penetration amount can be adjusted in 8 steps from "0" to "7", for example, with a larger step indicating a larger penetration amount. This graph is similar for the downstream curl correction roller pair 72, so the following explanation will use the upstream curl correction roller pair 71 as an example. However, the curl correction penetration amount of the downstream curl correction roller pair 72 is in the opposite direction of penetration (i.e., the direction of unevenness) relative to the sheet S compared to the curl correction penetration amount of the upstream curl correction roller pair 71, so in this embodiment, it will be explained in 8 steps in the negative direction from "0" to "-7".

[0064] As shown in Figures 13(a) and 13(b), it can be seen that wrinkles occur as the amount of curl correction penetration in the upstream curl correction roller pair 71 and the amount of skew of the sheet S conveyed to the upstream curl correction roller pair 71 increase. Furthermore, it can be seen that the area of ​​the graph where wrinkles occur is larger when the sheet S is ultra-thin paper rather than thin paper, i.e., when the stiffness of the sheet S is low.

[0065] For example, when the amount of skew of the sheet S being conveyed to the upstream curl correction roller pair 71 is 3 mm, wrinkles do not occur in thin paper even when the curl correction penetration amount of the upstream curl correction roller pair 71 is "4". In contrast, with ultra-thin paper, wrinkles may occur even when the curl correction penetration amount of the upstream curl correction roller pair 71 is "1". In other words, it was confirmed that the thinner the sheet S is, the lower its rigidity becomes, and the more easily wrinkles occur.

[0066] Here, we will also explain the differences between a typical conveyor roller pair and a decal correction roller pair (i.e., the upstream curl correction roller pair 71 and the downstream curl correction roller pair 72). In a typical conveyor roller pair, the rollers do not penetrate deeply into the nip, so a difference in velocity vectors is unlikely to occur when the sheet S is conveyed to the nip. Therefore, the phenomenon of wrinkles occurring due to skew is considered to be a phenomenon unique to the decal correction roller pair. In a typical conveyor roller pair, the curl correction penetration amount of the upstream curl correction roller pair 71 corresponds to states of "0" or "1" in the graphs shown in Figures 13(a) and 13(b). Therefore, even if the amount of skew of the sheet S conveyed to the nip in a typical conveyor roller pair is large, the possibility of wrinkles occurring is low.

[0067] As explained above, with thin paper and ultra-thin paper, it is difficult to achieve both wrinkle reduction and high-precision curl correction due to the effect of sheet S skew. On the other hand, as mentioned earlier, curl is more likely to occur with sheets that have low rigidity, such as thin paper and ultra-thin paper, and it is necessary to correct the curl with high precision in order to reduce image formation defects and transport defects on the second surface of sheet S.

[0068] Therefore, in this embodiment, as described above, the double-sided skew correction unit 80 is positioned upstream of the double-sided decal unit 70 in the re-transport path PR in the transport direction of the sheet S. As a result, as shown in Figures 13(a) and 13(b), the amount of skew of the sheet S is brought close to "0.0", allowing for high-precision curl correction in the double-sided decal unit 70 while making it less likely for wrinkles to occur on the sheet. In particular, during double-sided printing, the double-sided decal unit 70 is positioned downstream of the fixing unit 100 (cooling unit 110) in the transport direction, so that curls generated in the fixing unit 100 can be decaled by the double-sided decal unit 70.

[0069] Furthermore, while curling is likely to occur when the sheet S is heated by the fixing unit 100 and water vapor is released unevenly to one side, curling can also occur when the sheet S is cooled unevenly to one side by the cooling unit 110. Therefore, in the case of a printer 1 equipped with a cooling unit 110, it is preferable that the double-sided decal unit 70 is positioned downstream of the cooling unit 110 in the transport direction.

[0070] Furthermore, if, for example, the double-sided skew correction unit 80 is located upstream of the reversal transport unit 130 in the transport direction, there is a risk that the sheet S, whose skew has been corrected by the double-sided skew correction unit 80, may become skew again in the reversal transport unit 130. However, during double-sided printing, the double-sided skew correction unit 80 is located downstream of the reversal transport unit 130 in the transport direction. This reduces the likelihood of the sheet S, whose skew has been corrected by the double-sided skew correction unit 80, becoming skew again and being sent to the double-sided decal unit 70.

[0071] Furthermore, in this first embodiment, the double-sided skew correction unit 80, the double-sided decal unit 70, and the image forming unit 90 are all located inside the same housing 1a. By having the double-sided decal unit 70 and the image forming unit 90 located in the same housing 1a, it is possible to reduce the chances of the sheet S, whose curl has been corrected, becoming skewed again before being sent to the image forming unit 90. In particular, if the double-sided skew correction unit 80 and the double-sided decal unit 70 were located in different housings, depending on the installation environment of the housings, there is a risk that the sheet S may become skewed when passing through the connecting parts of the housings. However, by having the double-sided skew correction unit 80 and the double-sided decal unit 70 located in the same housing 1a, it is possible to reduce the chances of the sheet S, whose skew has been corrected by the double-sided skew correction unit 80, becoming skewed again before being sent to the double-sided decal unit 70.

[0072] [Control of the penetration amount adjustment mechanism for the double-sided decal unit 70] Next, the control of the penetration amount adjustment mechanism of the double-sided decal unit 70 will be explained using Figure 14. Figure 14 is a curl correction table that records the relationship between the curl correction penetration amount and the sheet type, ambient humidity, and image density.

[0073] As described above, the double-sided decal unit 70 includes an intrusion amount adjustment mechanism 70Aa for the upstream curl correction section 70A and an intrusion amount adjustment mechanism (not shown) for the downstream curl correction section 70B. The intrusion amount adjustment mechanism 70Aa for the upstream curl correction section 70A can adjust the curl correction intrusion amount in eight steps from "0" to "7", and the intrusion amount adjustment mechanism for the downstream curl correction section 70B can adjust the curl correction intrusion amount in eight steps from "0" to "-7". When the curl correction intrusion amount is between "0" and "7", the intrusion amount adjustment mechanism for the downstream curl correction section 70B maintains the curl correction intrusion amount at "0". Also, when the curl correction intrusion amount is between "0" and "-7", the intrusion amount adjustment mechanism 70Aa for the upstream curl correction section 70A maintains the curl correction intrusion amount at "0".

[0074] Here, the amount of curl of the sheet S varies depending on various parameters such as temperature and humidity, moisture content of the sheet S, type of sheet S, thickness of sheet S, and image density, so the amount of curl correction penetration is determined according to each parameter. Based on the determined amount of curl correction penetration, the control unit 200 controls the penetration amount adjustment mechanism 70Aa of the upstream curl correction unit 70A and the penetration amount adjustment mechanism of the downstream curl correction unit 70B. In other words, the control unit 200 changes the amount of penetration (pressing force) of the driven correction roller 72b (72b) relative to the curl correction roller 71a (72a).

[0075] In this case, the control unit 200 refers to the curl correction table shown in Figure 14 and sets the curl correction penetration amount to one of 15 steps from "-7" to "7". The curl correction table in Figure 14 is table data for changing the curl correction penetration amount according to the sheet thickness (sheet type) and image density in low humidity or high humidity environments. In other words, the control unit 200 sets the curl correction penetration amount to be smaller in absolute value the lower the humidity environment, or conversely, larger in absolute value the higher the humidity environment. The printer 1 is equipped with, for example, a humidity sensor (not shown), and the control unit 200 determines the humidity environment inside the printer 1 based on the detection result of the humidity sensor.

[0076] Furthermore, in this embodiment, as shown in Figure 14, the amount of curl correction penetration is set according to the thickness of the sheet, for example, ultra-thin paper, thin paper, regular paper, and thick paper. That is, the control unit 200 is set so that the thinner the sheet (the lower the rigidity), the larger the absolute value of the amount of curl correction penetration.

[0077] Furthermore, in this embodiment, as shown in Figure 14, the amount of curl correction penetration is set according to the image density, for example, low density, medium density, and high density. That is, the control unit 200 sets the amount of curl correction penetration to increase in the positive direction (i.e., increase the penetration amount of the upstream curl correction unit 70A) because the lower the density of the image formed on the first surface, the more likely the sheet S is to curl in a concave shape (see Figure 5(a)). Conversely, the control unit 200 sets the amount of curl correction penetration to increase in the negative direction (i.e., increase the penetration amount of the downstream curl correction unit 70B) because the higher the density of the image formed on the first surface, the more likely the sheet S is to curl in a convex shape (see Figure 5(b)).

[0078] As described above, the amount of curl of the sheet S on which the image is formed on the first surface tends to be greater in high humidity environments, greater with thinner sheets, and greater with higher image density. Therefore, the amount of curl correction intrusion is controlled to increase accordingly. As can be seen from this curl correction table, it is necessary to increase the amount of curl correction for thinner paper, and as mentioned above, it is necessary to reduce wrinkle formation.

[0079] [Control of the double-sided skew correction unit 80] Next, the control of the double-sided skew correction unit 80 will be explained using Figure 15. Figure 15 is a flowchart showing the control of the double-sided skew correction unit.

[0080] As shown in Figure 15, the control unit 200 first starts a print job in response to receiving print execution instruction information (print job information) from the user, for example via the computer 204 (S101). The user can also specify the number of copies to print and the type of sheet to be used for printing, for example, via the computer 204.

[0081] Then, when printing the first side (first page) of sheet S is performed (S102), it is determined whether the print job is for double-sided printing or not (S103). If the print job is not for double-sided printing (NO in S103), the process proceeds to step S110 to determine whether there is a subsequent sheet (to print the next sheet). If there is a subsequent sheet (NO in S109), the process returns to step S102 and prints the first side of the next sheet S. If there is no subsequent sheet (YES in S109), the print job is terminated (S110), and this control is ended.

[0082] On the other hand, if it is determined in step S103 that double-sided printing is required (YES in S103), first, based on the information of the sheet type S, it is determined whether the basis weight of the sheet S is less than or equal to a predetermined value as a second threshold (for example, 75 gsm) (S104). If the basis weight of the sheet S is not less than or equal to the predetermined value (NO in S104), the process proceeds directly to step S108, and the second side of the sheet S is printed. In other words, by not performing skew correction of the sheet S by the double-sided skew correction unit 80, it is possible to prevent the generation of noise caused by the sheet S hitting something, and also improve productivity by not reducing the transport speed of the sheet S.

[0083] On the other hand, if the basis weight of sheet S is below a predetermined value (YES in S104), it is determined whether the humidity is above a predetermined value as a third threshold (for example, if the temperature is 25 degrees, the humidity is 60%) (S105). If the humidity is not above the predetermined value (NO in S105), the process proceeds directly to step S108, and the second side of sheet S is printed.

[0084] On the other hand, if the humidity is above a predetermined value (YES in S105), it is determined whether the curl correction penetration amount (i.e., pressing force) of the double-sided decal unit 70 is above a predetermined value as the first threshold (for example, "4" to "6" for thin paper, and "1" to "5" for ultra-thin paper) (S106). If the penetration amount is not above the predetermined value (NO in S106), the process proceeds to step S108, and the second side of the sheet S is printed. The curl correction penetration amount of the double-sided decal unit 70 is determined before the decal is applied by the double-sided decal unit 70, and the value obtained by referring to the curl correction table above is used.

[0085] If the amount of intrusion is greater than a predetermined value (YES in S106), the double-sided skew correction unit 80 performs skew correction before the double-sided decal unit 70 performs curl correction (decal). In other words, if the conditions in steps S104 to S106 above are met, the skew is corrected in the double-sided skew correction unit 80, and then the curl correction is performed in the double-sided decal unit 70 (S107). Then, the process proceeds to step S108, and the second side of sheet S is printed.

[0086] The process then proceeds to step S110 to determine whether there is a subsequent sheet (to print the next sheet). If there is a subsequent sheet (NO in S109), the process returns to step S102 and prints the first side of the next sheet S. If there is no subsequent sheet (YES in S109), the print job is terminated (S110), and this control process ends.

[0087] As described above, in this embodiment, when the sheet S is thin paper or ultra-thin paper that is prone to wrinkling, and when the humidity is high and the amount of curl correction penetration is large, the double-sided skew correction unit 80 is performed before the double-sided decal unit 70 performs curl correction. This makes it less likely for wrinkles to occur when the double-sided decal unit 70 performs curl correction.

[0088] In this embodiment, the skew correction is performed in the double-sided skew correction unit 80 when performing double-sided printing on thin paper or ultra-thin paper that is particularly prone to wrinkling. However, skew correction can also be performed in the double-sided skew correction unit 80 on ordinary paper or thick paper that is less prone to wrinkling. However, in the case of ordinary paper or thick paper, it is necessary to note that the operating noise may worsen due to loop formation during skew correction, roller slip marks may occur during loop formation, or friction marks with the guide may occur during loop formation.

[0089] <Second Embodiment> Next, a second embodiment, which is a modified version of the first embodiment described above, will be explained with reference to Figure 16. Figure 16 is a schematic diagram showing a printer according to the second embodiment. In this explanation of the second embodiment, the same reference numerals are used for parts similar to those in the first embodiment, and their explanations are omitted.

[0090] In the first embodiment described above, a double-sided skew correction unit 80 and a double-sided decal unit 70 are arranged side by side immediately upstream of the first double-sided transport unit 60 in the transport direction of the inverted path Pa5 in the housing 1a (see Figure 1). However, if the skew of the sheet transported to the double-sided decal unit 70 can be suppressed, the double-sided skew correction unit 80 does not necessarily need to be placed immediately upstream of the double-sided decal unit 70. It is sufficient for it to be placed after the image formation of the first surface, that is, in the transport path downstream of the fixing unit 100, and upstream of the double-sided decal unit 70.

[0091] Areas where sheet S is prone to skew include areas from the secondary transfer nip T2 to the fixing unit 100, where an unfixed toner image is formed on sheet S and sheet S cannot be gripped by rollers or guides. Other areas prone to skew include areas with high nip pressure, such as the fixing unit 100, and areas with a small number of roller gripping points, such as the inversion transport unit 130. The double-sided skew correction unit 80 is effectively installed downstream of these areas prone to skew.

[0092] Therefore, in this second embodiment, as shown in Figure 16, the double-sided skew correction unit 80 is positioned immediately downstream of the reversal conveying unit 130 in the conveying direction. Specifically, in the housing 1b, the double-sided skew correction unit 80 is positioned between the reversal conveying unit 130 and the second double-sided conveying unit 140 in the conveying direction. Even with this arrangement, the sheet S whose skew has been corrected by the double-sided skew correction unit 80 can be conveyed to the double-sided decal unit 70, and while high-precision curl correction can be performed in the double-sided decal unit 70, wrinkles can be less likely to occur in the sheet.

[0093] <Third Embodiment> Next, a third embodiment, which is a modified version of the second embodiment described above, will be explained with reference to Figure 17. Figure 17 is a schematic diagram showing a printer according to the third embodiment. In this explanation of the third embodiment, the same reference numerals as those used in the first and second embodiments will be used, and their explanations will be omitted.

[0094] In the second embodiment described above, a double-sided skew correction unit 80 is placed immediately downstream of the inversion transport unit 130 in housing 1b, and a double-sided decal unit 70 is placed immediately upstream of the transport direction of the first double-sided transport unit 60 in housing 1a (see Figure 16). However, the double-sided decal unit 70 does not necessarily have to be placed immediately upstream of the transport direction of the first double-sided transport unit 60; in other words, it can be placed anywhere after the image of the first surface is formed. In short, it is sufficient to place it in the re-transport path PR downstream of the fixing unit 100, and in the transport path upstream of the registration unit 30 (feed-in path Pa1) where the second surface sheet S is re-transported.

[0095] Therefore, in this third embodiment, as shown in Figure 17, the double-sided skew correction unit 80 and the double-sided decal unit 70 are arranged immediately downstream of the reversing conveying unit 130 in the conveying direction. Specifically, in the housing 1b, the double-sided skew correction unit 80 and the double-sided decal unit 70 are arranged between the reversing conveying unit 130 and the second double-sided conveying unit 140 in the conveying direction. Even with this arrangement, the sheet S whose skew has been corrected by the double-sided skew correction unit 80 can be conveyed to the double-sided decal unit 70, and while high-precision curl correction can be performed in the double-sided decal unit 70, wrinkles can be less likely to occur in the sheet. Furthermore, because the double-sided skew correction unit 80 and the double-sided decal unit 70 are arranged in the same housing 1b, it is possible to reduce the chances of the sheet S whose skew has been corrected by the double-sided skew correction unit 80 becoming skewed again and being sent to the double-sided decal unit 70.

[0096] <Possibility of other embodiments> In the first to third embodiments described above, the curl correction rollers 71a, 72a and driven correction rollers 72b, 72b in the double-sided decal unit 70 were described as being composed of small-diameter hard rollers and large-diameter soft rollers. However, the invention is not limited to this, and a configuration combining small-diameter hard rollers and an endless belt is also acceptable, or any configuration that can form a curved nip to correct the curl of the sheet S. Regardless of the configuration, since wrinkles may occur in the curved nip due to the slight difference in velocity vectors as described above, it is effective to correct the skew of the double-sided decal unit 70 using the double-sided skew correction unit 80 before conveying the sheet S.

[0097] Furthermore, in the first to third embodiments, a configuration was described in which the double-sided skew correction unit 80 corrects the skew by abutting the leading edge of the sheet S against the nip portion of the double-sided registration roller pair 82 and making it conform to the sheet. However, the configuration is not limited to this, and for example, the skew may be corrected by abutting the side edge of the sheet against a stopper plate located on one side in the width direction perpendicular to the conveying direction, or by abutting the leading edge of the sheet against a stopper member such as a shutter. Moreover, the skew of the sheet may be corrected by the difference in rotational speed of a plurality of independently driven rollers. In other words, any configuration that can correct the skew of the sheet is acceptable.

[0098] Furthermore, although the first to third embodiments described a printer 1 consisting of two housings, it may also consist of one housing or three or more housings. In addition, the printer 1 is not limited to a single device, but may be an image forming system consisting of multiple devices. For example, an image forming system can be considered that includes a print module as an image forming device and a determination module that inverts a sheet on which an image has been formed on the first surface by the print module and re-transports it to the print module.

[0099] Furthermore, although the first to third embodiments describe a printer 1 equipped with a cooling unit 110, the invention is not limited to this configuration, and a configuration in which the sheet S heated by the fuser unit 100 is naturally cooled without a cooling unit 110 is also acceptable.

[0100] Furthermore, in the first to third embodiments, the image forming unit 90 was described in which a photoreceptor is exposed to a laser to form a toner image, meaning that the printer 1 is a laser beam printer. However, the invention is not limited to this, and other types of printers, such as inkjet printers, may also be used. In this case, the fixing unit may be a drying unit that fixes the image by drying the ink. [Explanation of Symbols]

[0101] 1…Printer (image forming apparatus) / 1a…Housing (first housing) / 1b…Housing (second housing) / 10…Feeding unit (feeding section) / 30…Registration unit (second skew correction section) / 70…Double-sided decal unit (curl correction section) / 70Aa…Penetration amount adjustment mechanism (first pressure adjustment section) / 70Ab…Penetration amount adjustment mechanism (second pressure adjustment section) / 71a…Curl correction roller (first roller) / 71b…Driven correction roller (second roller) / 72a…Curl correction roller La (3rd roller) / 72b... Driven correction roller (4th roller) / 80... Double-sided skew correction unit (1st skew correction section) / 90... Image forming unit (image forming section) / 100... Fixing unit (fixing section) / 110... Cooling unit (cooling section) / 130... Inversion transport unit (inversion section) / 200... Control unit / N1... 1st nip section / N2... 2nd nip section / PR... Re-transport path / Pa1... Feeding path / Pa5... Path after inversion / S... Sheet / T2... Secondary transfer nip (transfer section)

Claims

1. An image forming unit that forms an image on a sheet, A retransport path that guides the sheet on which an image has been formed on the first surface by the image forming unit to be retransported back to the image forming unit, Interposed in the aforementioned re-transport path is a curl correction unit that corrects the curl of the sheet, The system includes a first skew correction unit, which is located upstream of the curl correction unit in the transport direction in the re-transport path and is capable of correcting the skew of the sheet. An image forming apparatus characterized by the following:

2. The retransport path includes a reversal unit that reverses the direction of travel of the sheet on which an image has been formed on the first surface by the image forming unit, thereby reversing the front and back sides of the sheet. The retransport path includes a post-inversion path that guides the sheet, whose front and back sides have been inverted by the inversion unit, to the image forming unit. The first oblique correction unit is positioned to interpose with the reversed path, The image forming apparatus according to feature 1.

3. A feeding unit that feeds the sheet to the image forming unit, A feeding path that guides the sheet fed from the feeding unit to the image forming unit, The system includes a second skew correction unit interposed in the feeding path and capable of correcting the skew of the sheet fed from the feeding unit, The aforementioned re-transport path guides the sheet to the image forming unit by merging with the aforementioned feeding path. The image forming apparatus according to feature 2.

4. The first housing in which the feeding unit is located, The system comprises a second housing on which the reversing section is located, The curl correction unit is located in the first housing. The image forming apparatus according to feature 3.

5. The first oblique correction unit is located in the first housing. The image forming apparatus according to feature 4.

6. The first oblique correction unit is located in the second housing, The image forming apparatus according to feature 4.

7. The first housing in which the feeding unit is located, The system comprises a second housing on which the reversing section is located, The curl correction unit and the first skew correction unit are arranged in the second housing. The image forming apparatus according to feature 3.

8. The image forming unit comprises a transfer unit that transfers an image to a sheet, and a fixing unit positioned downstream of the transfer unit in the sheet transport direction and fixing the transferred image to the sheet. The image forming apparatus according to feature 1.

9. The image forming unit is located downstream of the fixing unit in the sheet transport direction and has a cooling unit for cooling the sheet on which the image has been fixed. The image forming apparatus according to feature 8.

10. The curl correction unit is, A first roller in contact with the first surface of the sheet, A second roller having a lower hardness than the first roller, which, when pressed toward the first roller, forms a first nip that sandwiches the sheet between itself and the first roller, A third roller in contact with the second surface of the sheet opposite to the first surface, The device has a fourth roller that is less hard than the third roller and, when pressed toward the third roller, forms a second nip that sandwiches the sheet between itself and the third roller. The image forming apparatus according to feature 1.

11. The system includes a control unit that controls the first skew correction unit and the curl correction unit, The curl correction unit is, A first pressing adjustment unit that can adjust the pressing force that presses one of the first roller and the second roller toward the other of the first roller and the second roller, It has a second pressing adjustment unit that can adjust the pressing force that presses one of the third roller and the fourth roller toward the other of the third roller and the fourth roller, The control unit controls the first pressure adjustment unit and the second pressure adjustment unit according to the amount of curl of the sheet. The image forming apparatus according to feature 10.

12. The control unit does not perform the skew correction by the first skew correction unit if the pressing force adjusted by the first pressing adjustment unit or the second pressing adjustment unit is smaller than the first threshold. The image forming apparatus according to feature 11.

13. The system includes a control unit that controls the first oblique correction unit, The control unit does not perform the skew correction by the first skew correction unit if the basis weight of the sheet is greater than the second threshold. The image forming apparatus according to feature 1.

14. The system includes a control unit that controls the first oblique correction unit, The control unit does not perform the skew correction by the first skew correction unit when the humidity is less than the third threshold. The image forming apparatus according to feature 1.

Images