Curl straightening appliance and image forming apparatus

Abstract

To provide a curl correction device that can appropriately set a correction quantity.SOLUTION: A curl correction device includes: a curl correction device 800 that can correct a curl on a sheet S on which an image is formed; a loading table 500 on which the sheets S discharged from the curl correction device 800 are loaded; a curl detection part 880 that detects a curl state of sheet bundles S1 to S3 made up of the plurality of sheets S loaded on the loading table 500; and a control part that can adjust a curl correction quantity by the curl detection part 880. The control part forms the first sheet bundle S1 to be loaded on a loading part by the plurality of sheets each on which the curl correction has been executed with a first correction quantity, and executes the curl correction with a second correction quantity obtained by correcting the first correction quantity based on a detection result obtained by detecting the curl state of the first sheet bundle S1 by the curl detection part 880 for the sheets S subsequent to the first sheet bundle S1.SELECTED DRAWING: Figure 12

Description

Technical Field

[0001] The present invention relates to a curl correction device for correcting the curl of a sheet, and an image forming apparatus to which this curl correction device is applied.

Background Art

[0002] In recent years, image forming apparatuses such as copiers, printers, and facsimiles using an electrophotographic method have become widespread. In the electrophotographic method, it is known that various curls such as heat curl, toner curl, and conveyance path curl may occur on a sheet. For example, heat curl occurs by passing through a fixing device that applies heat to fix a toner image on a sheet, toner curl occurs due to contraction of the toner, and conveyance path curl occurs by passing through a sheet conveyance path in which the sheet is bent. Particularly in digital printing, since variable printing is possible, various curls occur according to the printed image.

[0003] If the curl generated on the sheet is large, there is a risk of various problems such as poor stacking at the discharge part of the image forming apparatus, dropping of the stacked sheet bundle when transporting it to a post-processing apparatus, or poor feeding, misalignment, and stapling failure of the sheet in the post-processing apparatus.

[0004] In order to solve such problems, there is known an image forming apparatus provided with a curl correction device for correcting the curl generated on a sheet (see Patent Document 1). This curl correction device is configured, for example, by providing a pair of rollers with different hardnesses under variable clamping pressure. By pressing the rollers with different hardnesses, a curved nip is formed, and by transporting the sheet with this roller pair, a forced deformation is applied to the sheet, and the curl of the sheet can be corrected. Also, the magnitude of the correction amount for correcting the curl can be arbitrarily adjusted by changing the clamping pressure of the roller pair. Such setting of the correction amount is performed by predicting the amount of curl generated from mechanical characteristic values such as the thickness of the sheet and conditions such as the amount of heat applied by the fixing device, and predicting an appropriate value according to the conditions.

[0005] In contrast, even with the same type of sheet, the amount of curl that occurs may differ depending on the manufacturing lot. Also, if the amount of moisture contained in the sheet changes depending on the storage conditions of the sheet or the environment in which the image forming apparatus is placed, differences in the amount of curl that occur will occur. Therefore, even if the amount of curl that occurs is predicted from various parameters and the amount of curl correction is set accordingly, there is a risk that the amount of correction will be too much or too little due to other reasons. To solve this, a configuration has been developed that detects the curl state of a single sheet that has passed through a curl correction device and corrects the amount of correction for the next sheet to be corrected based on the detected remaining curl amount (see Patent Document 2). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-137912 [Patent Document 2] Japanese Patent Publication No. 2006-16157 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the curl correction device described in Patent Document 2 detects the curl state of one sheet and corrects the amount of correction for the next sheet based on that. Therefore, if the area and density of the image differ between the sheet whose curl state was detected and the sheet to be corrected immediately afterward, the resulting correction amount may not be suitable for the sheet to be corrected. For example, if the curl state of a sheet that is curled downward is detected and the correction is set to correct it upward, there is a risk that the curl cannot be properly corrected if the next sheet is already curled upward.

[0008] The present invention aims to provide a curl correction device and an image forming device that can appropriately set the amount of correction. [Means for solving the problem]

[0009] One aspect of the present invention is a curl correction device comprising: a curl correction unit capable of correcting the curl of a sheet on which an image has been formed; a loading unit for loading sheets discharged from the curl correction unit; a detection unit for detecting the curl state of a sheet bundle consisting of a plurality of sheets loaded in the loading unit; and a control unit capable of adjusting the amount of curl correction by the curl correction unit, wherein the control unit forms a first sheet bundle loaded in the loading unit by a plurality of sheets on which curl correction has been performed by a first correction amount on each sheet, and performs curl correction on sheets following the first sheet bundle by a second correction amount obtained by correcting the first correction amount based on the detection result obtained by the detection unit for detecting the curl state of the first sheet bundle.

[0010] One aspect of the present invention is an image forming apparatus characterized by comprising an image forming unit for forming an image on a sheet, and a curl correction device for correcting the curl of the sheet on which the image has been formed by the image forming unit. [Effects of the Invention]

[0011] According to the present invention, the amount of correction can be appropriately set. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing an image forming apparatus according to the first embodiment. [Figure 2] These are perspective views showing curls that occur in the sheet, with (a) being a downward curl and (b) being an upward curl. [Figure 3] A cross-sectional view showing a curl straightening device according to the first embodiment. [Figure 4] A cross-sectional view showing the default state of the curl straightening device according to the first embodiment. [Figure 5] A cross-sectional view showing the state when the curl-correcting device according to the first embodiment corrects the lower curl. [Figure 6] A cross-sectional view showing the state when the curl-correcting device according to the first embodiment corrects an upper curl. [Figure 7]Block diagram showing the control system of the discharge device according to the first embodiment. [Figure 8] Cross-sectional view showing the discharge device according to the first embodiment. [Figure 9] Images taken by the curl detection unit according to the first embodiment, where (a) shows the case where the upper curl remains, and (b) shows the case where the lower curl remains. [Figure 10] Flowchart showing the operation of the curl correction device according to the first embodiment. [[ID=X]] [Figure 11] Cross-sectional view showing the case where the feedback of the correction by the curl correction device is short, where (a) is before feedback and (b) is after feedback. [Figure 12] Cross-sectional view showing the case where a sheet bundle is corrected using the curl correction device according to the first embodiment, where (a) is the first part, (b) is the second part, and (c) is the state where the third part is discharged. [Figure 13] Flowchart showing the operation of the curl correction device according to the second embodiment.

Mode for Carrying Out the Invention

[0013] <First Embodiment> Hereinafter, the first embodiment of the present invention will be described in detail with reference to FIGS. 1 to 12. In this embodiment, a tandem type full-color printer is described as an example of the image forming apparatus 1. However, the present invention is not limited to the tandem type image forming apparatus 1, and may be other types of image forming apparatuses, and is not limited to being full-color, and can also be applied to monochrome, monotone color, or inkjet printers. <00X0091> [Image Forming Apparatus] First, the overall configuration of the image forming apparatus 1 according to the present embodiment will be described using FIG. 1. FIG. 1 is a cross-sectional view of an intermediate transfer tandem type image forming apparatus 1 in which four-color image forming units are arranged side by side on an intermediate transfer belt 506. Hereinafter, the operation of the image forming apparatus 1 will be described using FIG. 1.

[0015] The sheet S is stored in a manner that it is loaded on the lift-up device 52 of the sheet feeding device 51, and is fed by the sheet feeding means 53 in accordance with the image forming timing of the image forming apparatus 1. Note that the sheet conveyance direction is indicated by V1. Examples of the sheet feeding means 53 include a method using frictional separation by a feed roller or the like, a method using separation adsorption by air, etc. In the image forming apparatus 1 of FIG. 1, it is assumed that an air-based sheet feeding method is used. The sheet S sent out by the sheet feeding means 53 passes through the conveyance path 54a and the conveyance unit 54 of the conveyance unit 54, and is conveyed to the skew correction unit 7. After the sheet S undergoes skew correction and timing correction in the skew correction unit 7, it is sent to the secondary transfer unit. The secondary transfer unit is a toner image transfer clamping unit for the sheet S formed by the substantially opposing secondary transfer inner roller 503 and the secondary transfer outer roller 56, and transfers the toner image to the sheet S by applying a predetermined pressing force and an electrostatic load bias.

[0016] Regarding the conveyance process of the sheet S up to the secondary transfer unit described above, the image forming process of the image that is sent to the secondary transfer unit at the same timing will be described. In the present embodiment, the image forming unit 513 is provided with four sets of yellow (Y), magenta (M), cyan (C), and black (K). Since these only differ in the color of the toner and have the same configuration, the yellow image forming unit will be described here as a representative.

[0017] The image forming unit 513 includes a photoreceptor 508, an exposure device 511, a developing device 510, a primary transfer device 507, and a photoreceptor cleaner 509. Based on the image information signal received, the exposure device 511 emits light on the rotating photoreceptor 508, whose surface has been uniformly charged in advance by a charging means, and an electrostatic latent image is formed via the diffraction means 512 and other appropriate means. The developing device 510 performs toner development on the electrostatic latent image formed on the photoreceptor 508, and a toner image is formed on the photoreceptor 508. Subsequently, a predetermined pressure and electrostatic load bias are applied by the primary transfer device 507, and the toner image is transferred onto the intermediate transfer belt 506. The small amount of residual toner remaining on the photoreceptor 508 is collected by the photoreceptor cleaner 509 and prepared again for the next image formation.

[0018] Next, the intermediate transfer belt 506 will be described. The intermediate transfer belt 506 is stretched by rollers such as the drive roller 504, tension roller 505, and secondary transfer inner roller 503, and is transported in the direction of arrow R1 in the figure. Therefore, the image formation processes for each color, which are processed in parallel by the aforementioned Y, M, C, and K image forming units 513, are performed at the timing when they are superimposed on the upstream toner image that has been primary transferred onto the intermediate transfer belt 506. As a result, a full-color toner image is ultimately formed on the intermediate transfer belt 506 and transported to the secondary transfer unit.

[0019] As described above, a full-color toner image is transferred onto the sheet S in the secondary transfer section using the sheet S transport process and image formation process. The sheet S is then transported to the fixing unit 58 by the pre-fixing sheet transport device 57. The fixing unit 58 melts and fixes the toner onto the sheet S by applying a predetermined pressure using substantially opposing rollers or belts, and generally by applying a heating effect from a heat source such as a heater. The sheet S with the resulting fixed image is then transported by the branching transport device 59 to either the discharge device 8 or, if double-sided image formation is required, to the inversion transport device 501. If the sheet is delivered to the discharge device 8, it passes through the curl correction device 800 and is discharged and loaded onto a loading platform 500, which is an example of a loading section. The loading platform 500 is configured to move vertically and moves vertically downward as sheets are loaded, operating to maintain a constant sheet height at the top. In Figure 1, the image forming apparatus 1 shows an example where the curl correction device 800 is placed in the discharge device 8 downstream of the branching conveying device 59 in the conveying direction, but this is not the only option. The curl correction device 800 will be described later.

[0020] The image forming apparatus 1 has a main unit control unit 10. The main unit control unit 10 has a CPU and memory and controls each part of the image forming apparatus 1 described above. The CPU outputs output signals to each electrical component to operate the electrical component at the desired timing and with the required control amount, based on detection signals input from each sensor and information stored in memory. The memory stores information data necessary for controlling each unit, and the CPU reads and writes information data stored in memory.

[0021] [The principle behind the formation of curls] Next, the curl that occurs in the sheet S will be explained using Figure 2. When the sheet S passes through the fuser 58, it is heated by a heat source such as a heater, and a temperature difference generally occurs between the surface, which becomes hot to melt the toner, and the back surface. As a result, moisture movement occurs inside the sheet S at the nip of the fuser 58, and the moisture content on the back surface becomes greater than that on the surface, which becomes hotter. After passing through the fuser 58, when the sheet S comes into contact with air, moisture evaporates from the hot sheet S, but more evaporation occurs from the back surface, which has a higher moisture content. If the material of the sheet S is pulp, the greater the evaporation, the greater the shrinkage tends to be. Therefore, a difference in shrinkage occurs between the front and back surfaces, and as shown in Figure 2(a), the downstream and upstream ends in the transport direction of the sheet S are pulled downwards compared to the center, resulting in a downward curl (hereinafter also called a heat curl) as the back surface, which has a greater shrinkage, pulls downwards.

[0022] Furthermore, the toner and sheet S, which are melted and fixed at high temperature in the fuser 58, cool down upon contact with air after passing through the fuser 58, causing them to shrink. Since the constituent materials of the toner and the material of the sheet S are generally different, there is a difference in the coefficient of linear expansion, resulting in different amounts of shrinkage on the front and back sides in response to the decrease in temperature. Generally, the amount of shrinkage of the toner layer on the surface is greater, so as shown in Figure 2(b), an upward curl (hereinafter also called toner curl) occurs, where the edges of the sheet S curl upward. Since the thickness of the toner layer changes with changes in image density, the amount of curl that occurs also differs.

[0023] Ultimately, the amount of curl is determined by the sum of heat curl and toner curl, so the amount of curl changes depending on various parameters such as the material, thickness, rigidity, moisture content, temperature, ambient temperature, ambient humidity, fixing temperature, and image density of the sheet S. The curl correction device 800 cancels out the curl by applying deformation in the opposite direction to the curl that has occurred in this way. Therefore, it is necessary to predict the amount of curl that will occur under these various conditions in advance and set an appropriate amount of correction (i.e., correction force) accordingly.

[0024] [Curl straightening appliance] The curl straightening device 800 in this embodiment will be described with reference to Figures 3 to 7. As shown in Figure 3, the curl straightening device 800 includes a frame (not shown), a transport path 830, an upward straightening section 840a located on the upstream side in the transport direction of the transport path 830, and a downward straightening section 840b located on the downstream side. The upward straightening section 840a and the downward straightening section 840b are examples of curl straightening sections and can straighten the curl of a sheet S on which an image has been formed.

[0025] The upward straightening section 840a has a sponge roller 801a and a metal roller 803a arranged substantially opposite each other. The metal roller 803a is rotatably fixed to the frame via a metal roller shaft 804a. The sponge roller 801a is rotatably fixed to the oscillating arm 805a via a sponge roller shaft 802a. One end of the oscillating arm 805a is pivotably connected to the frame via an oscillating shaft 806a, and a follower 807a is rotatably fixed to the other end via a follower shaft 808a. In addition, the cam 809a is rotatably fixed to the frame by an eccentric cam shaft 810a, and the surface of the cam 809a is in contact with the follower 807a. The angle of the cam 809a is controlled by a cam motor M1a (see Figure 7) which is commanded by a controller 100 (see Figure 7). As the relative distance between the camshaft 810a and the follower shaft 808a changes, the angle of the oscillating arm 805a changes, causing the sponge roller 801a to be displaced in the thickness direction of the sheet S. This makes it possible to change the pressing force between the sponge roller 801a and the metal roller 803a.

[0026] When the sponge roller 801a and the metal roller 803a are pressed together by a predetermined pressure, the harder metal roller 803a bites into the less hard sponge roller 801a, forming a curved nip. As the sheet S passes through this nip, it is forcibly deformed along the smaller radius metal roller 803a, and thus curved. The greater the clamping pressure, the more the sponge roller 801a bites into the metal roller 803a and deforms, and the wider the width of the nip in the conveying direction becomes. As a result, the time during which the sheet S is forcibly deformed as it passes through the nip is increased, making it possible to correct the curl more strongly. In this way, the amount of correction applied to the sheet S can be adjusted by controlling the rotation angle of the cam 809a with the cam motor M1a.

[0027] Furthermore, the metal roller 803a is rotationally driven in the sheet S transport direction by the transport motor M2a (see Figure 7), and the sponge roller 801a rotates in response to the metal roller 803a by pressing against it. When the sheet S is held between the sponge roller 801a and the metal roller 803a, it gains transport force and is transported downstream in the transport direction.

[0028] The upward straightening section 840a has been described above. The downward straightening section 840b has the same configuration as the upward straightening section 840a, except that the top and bottom are reversed. Therefore, for the downward straightening section 840b, the description of the upward straightening section 840a will be used as a substitute by replacing the symbol a with the symbol b.

[0029] The specific operation of the curl straightening device 800 will now be explained. As shown in Figure 4, in both the upward straightening section 840a and the downward straightening section 840b, the default state is when the pressing force of the sponge rollers 801a, 801b and the metal rollers 803a, 803b is at its lowest. Even if the sheet S is passed through in this state, no forced deformation occurs in the sheet S.

[0030] Here, as shown in Figure 5, if a downward curl occurs in the sheet S, the angle of the cam 809a of the upward straightening section 840a is controlled to change the clamping pressure of the sponge roller 801a and the metal roller 803a to a predetermined value. As a result, a downwardly convex curved nip is formed, and the sheet S passing through the curl straightening device 800 is subjected to forced upward deformation, resulting in a flat output with the downward curl corrected. Conversely, as shown in Figure 6, if the sheet S is curled upward, the angle of the cam 809b of the downward straightening section 840b is controlled to change the clamping pressure of the sponge roller 801b and the metal roller 803b to a predetermined value. As a result, an upwardly convex curved nip is formed, and the sheet S passing through the curl straightening device 800 is subjected to forced downward deformation, resulting in a flat output with the upward curl corrected.

[0031] As described above, in the curl straightening device 800 of this embodiment, the pressing force of the sponge rollers 801a, 801b and metal rollers 803a, 803b of the upward straightening section 840a and the downward straightening section 840b is controlled. This makes it possible to apply a forced deformation that counteracts any desired amount and direction of curl, thereby straightening the curl.

[0032] As shown in Figures 1 and 7, the discharge device 8 has a controller 100, which is an example of a control unit. The controller 100 has a CPU 101 and a memory 102, and controls each part of the discharge device 8 based on instructions from the main control unit 10 (see Figure 1). The controller 100 is an example of a control unit and can adjust the amount of curl correction by the curl correction device 800. The CPU 101 outputs output signals to each electrical component to operate the electrical component at the desired timing and with the required control amount, based on detection signals input from the curl detection unit 880 and environmental sensor 840 (described later) and information stored in the memory 102. Electrical components here include, for example, cam motors M1a and M1b, and transport motors M2a and M2b. The memory 102 stores information data necessary for controlling each part, and the CPU 101 reads the information data stored in the memory 102 and writes it to the memory 102.

[0033] Here, we will explain the challenges in a system that detects the remaining curl amount of the top sheet S loaded on the loading platform 500 and feeds this back into the correction amount for the next sheet S. In this system, if the area and density of the image formed on the detected sheet S and the sheet S to be corrected are different, there is a possibility of miscalculating the correction amount. If the image density on the sheet S is low, for example, if it is a document containing only text, the sheet S has a tendency to curl downward due to the difference in the amount of moisture evaporation between the top and bottom surfaces. On the other hand, if the image density on the sheet S is high, for example, if it is a photograph, it has a tendency to curl upward due to the shrinkage of the toner. Therefore, for example, suppose a remaining curl is detected on a sheet S with a high image density image, and it is determined that the downward correction amount is insufficient. In this case, if the image density of the next sheet S is low, the correction amount will be corrected downward, so the correction will be applied in the same direction as the curl that occurred, and it may not be possible to correct the curl appropriately.

[0034] Furthermore, when the sheet S is rapidly heated by the fixing device 58, its temperature rises and its moisture content decreases as it evaporates. Over time, the temperature and moisture content gradually return to their original values. During this process, the amount of curl, which is correlated with temperature and moisture content, also changes over time. As a result, there is a difference between the amount of curl immediately after the sheet is discharged from the curl correction device 800 and the amount of curl after some time has passed since loading. Therefore, if the amount of correction is determined based on the amount of curl immediately after discharge, the correction will ignore the subsequent changes, and the desired reduction in curl may not be achieved.

[0035] Furthermore, even minute curl amounts that cannot be detected on a single sheet S can cause the stacking state to become unstable when multiple sheets S are printed consecutively and stacked. This is because the curl shape of the sheet S below supports the shape of the newly stacked sheet S, and this is amplified by the repeated addition of curl, making the stacking state of the top sheet S more unstable as the number of stacked sheets increases. Such problems are difficult to prevent by detecting the remaining curl amount of each sheet and feeding it back into the correction amount of the next sheet S. Therefore, in this embodiment, the curl state of the sheet bundle stacked on the stacking platform 500 is detected to appropriately correct the correction amount.

[0036] [Curl detection unit] The curl detection unit 880 in this embodiment will be described with reference to Figures 8 and 9. As shown in Figure 8, the curl detection unit 880 has a CCD camera 801, which is fixed to the discharge device 8 above the loading platform 500. That is, the curl detection unit 880 is positioned above the loading platform 500 and images the top sheet of the sheet bundle from above. The curl detection unit 880 is an example of a detection unit and detects the curl state of a sheet bundle consisting of multiple sheets S loaded on the loading platform 500. In particular, the curl detection unit 880 is an example of an imaging device that photographs the sheet bundle loaded on the loading platform.

[0037] When the timing for detecting a curled state is reached, the CCD camera 801 captures the state of the sheet S loaded on the loading platform 500, obtaining images as shown in Figures 9(a) and 9(b). Subsequently, the edges and corners of the top sheet are detected through image processing. At this time, the four detected corners are designated as C1, C2, C3, and C4, starting from the upper left and moving clockwise, with the left edge of the sheet S designated as E1 and the right edge as E2.

[0038] Next, the position of the detection edge E1 on the left side is compared with the straight line connecting C1 and C4, and the position of the detection edge E2 on the right side is compared with the straight line connecting C2 and C3. As shown in Figure 9(a), if the detection edges E1 and E2 are detected inward from the straight lines C1-C4 and C2-C3, it is determined that an upper curl remains. On the other hand, as shown in Figure 9(b), if the detection edges E1 and E2 are detected outward from the straight lines C1-C4 and C2-C3, it is determined that a lower curl remains. In this way, by processing the images obtained from the CCD camera 801, it is possible to detect any remaining curl in the stacked bundle of sheets S.

[0039] [Corrective action] Next, regarding the image forming apparatus 1 equipped with the curl correction device 800 in this embodiment, a control method for detecting the state of the loaded sheets S and correcting the correction amount of the curl correction device 800 will be explained using Figures 10 to 12. First, the user specifies the type of sheet S to be printed. For example, this can be set within broad categories such as plain paper, cardboard, and glossy paper, or settings can be created for each brand of recording material. Then, the user sets the file to be printed (a series of image and text data) and the number of copies, and starts printing.

[0040] The control flow will be explained using the flowchart shown in Figure 10 as an example, when printing at least two copies of a file containing multiple images and text. The controller 100 determines whether or not it has received a print start signal (step S1). If the controller 100 determines that it has not received a print start signal (NO in step S1), it makes the determination again. If the controller 100 determines that it has received a print start signal (YES in step S1), it sets an appropriate curl correction amount for the amount of curl that will occur (step S2). Here, the controller 100 sets the curl correction amount based on a pre-set table using information such as the physical properties of the sheet S set by the user, the temperature and humidity of the atmosphere from the environmental sensor 840, and the density of the printed image.

[0041] The controller 100 determines if there is a correction value (step S3). Since there is no correction value in the initial state, the controller 100 determines that there is no correction value (NO in step S3) and determines if the first print job is finished (step S5). The print data contains data on multiple pages, and in the initial state, it determines that the first print job is not finished (NO in step S5) and then determines if there is another sheet (step S10). Here again, in the initial state there is another sheet (YES in step S10), so it returns to step S2 and repeats the same process as the first loop until the first print job is finished.

[0042] On the other hand, after outputting the last sheet S of the first part, the branch in step S5 becomes YES, and the controller 100 uses the curl detection unit 880 to detect the remaining amount of curl of the sheet S on the loading platform 500 (step S6). Then, it determines whether the amount of curl of the sheet bundle loaded on the loading platform 500 is greater than or equal to the allowable value (step S7). This allowable value is the range in which it is acceptable even if the sheet is not strictly flat but slightly curled. If the amount of curl of the sheet bundle is not greater than or equal to the allowable curl amount (NO in step S7), the controller 100 maintains the correction value for the curl correction amount without updating it (step S9).

[0043] If the curl amount of the sheet bundle exceeds the allowable curl amount (YES in step S7), the controller 100 calculates a correction value to compensate for any insufficient or excessive correction amount and records it in memory 102 (step S8). If a correction value is recorded here, the process proceeds to the YES branch in step S3 when printing the second copy. In step S4, the correction value is added to the correction amount calculated by the controller 100 in step S2, canceling any curl that could not be corrected in the first copy. This is repeated until the print job is finished (up to NO in step S10), and then printing is completed. If sheet bundles (copies) with different content are printed during a print job, the correction amount is set while taking into account the image information of the different copies, etc. This allows for more appropriate correction compared to not using a correction amount.

[0044] Here, as shown in Figure 12(a), the controller 100 forms a first sheet bundle S1 which is loaded onto the loading platform 500 by multiple sheets, each of which has undergone curl correction by a first correction amount. Here, the first sheet bundle S1 is the first sheet bundle in the image forming job. Then, as shown in Figure 12(b), the controller 100 performs curl correction on the sheets S that follow the first sheet bundle S1 by a second correction amount, which is obtained by correcting the first correction amount based on the detection result obtained by the curl detection unit 880, which detects the curl state of the first sheet bundle S1.

[0045] Here, the controller 100 forms a second sheet bundle S2, which is loaded onto the loading platform 500, by performing curl correction on each sheet following the first sheet bundle S1 with a second correction amount. Furthermore, the controller 100 performs curl correction on each sheet following the second sheet bundle S2 with a third correction amount, which is obtained by correcting the second correction amount based on the detection result obtained by the curl detection unit 880, which detects the curl state of the first sheet bundle S1 and the second sheet bundle S2. Then, as shown in Figure 12(c), a third sheet bundle S3 is formed, which is loaded onto the loading platform 500, by performing curl correction on each sheet with a third correction amount. Note that the first sheet bundle S1, the second sheet bundle S2, and the third sheet bundle S3 each form a section consisting of multiple pages with the same content.

[0046] As described above, according to the image forming apparatus 1 of this embodiment, the controller 100 detects the curl of the sheet bundles loaded on the loading platform 500 and corrects the amount of straightening based on that, so that the amount of straightening can be made appropriate. As a result, even if a predetermined curl straightening effect cannot be obtained due to differences in the manufacturing lot of the sheets S or changes in the moisture content, the loading platform 500 can detect the state of the sheets S and feed back the correction amount, making it possible to load them stably.

[0047] Furthermore, according to the image forming apparatus 1 of this embodiment, the loading state is detected at an appropriate timing when the sheets S are stacked on the loading platform 500 in a bundle, and the amount of straightening of the curl straightening device 800 is corrected. This makes it possible to maintain a stable loading state without overcorrecting the curl of the sheets S, and by responding to the amount of curl that changes over time. Thus, problems such as loading defects, dropping during transport in bundle form, feeding defects in the post-processing device, misalignment defects, and stapling defects can be suppressed.

[0048] Furthermore, according to the image forming apparatus 1 of this embodiment, since the feedback period is set to the unit of the print file, feedback can be applied to the same image set (distribution of image density). However, if the image density is different, the tendency of the output sheet S to curl will also be different. For example, if the feedback period is short, and as shown in Figures 11(a) and 11(b), multiple sheets S with high image density are stacked, the stacking state is detected, the correction amount is calculated, and if the image density of the sheet S to which this is fed back is low, the desired effect may not be obtained. In contrast, in this embodiment, by detecting in units of the number of print copies, as shown in Figures 12(a) to 12(c), the distribution of image density of the stacked sheets S at the time of detection matches the distribution of image density to which this is fed back, thus increasing the effectiveness.

[0049] In the above-described embodiment, the curl detection unit 880 was described as a case in which a CCD camera 801 is placed above the loading platform 500 to photograph the top sheet, but it is not limited to this. For example, it may be placed to the side so that the sheet bundle placed on the loading platform 500 can be photographed from the side, or it may be placed on the upper side so that the sheet bundle can be photographed from diagonally above. Alternatively, the curl detection unit 880 is not limited to a camera, and a non-contact distance sensor capable of measuring multiple points on the sheet surface may be used to detect curl from the relative difference in distance information from each sensor. Alternatively, auxiliary light or slit light may be irradiated onto the sheet bundle, and curl may be detected based on the deformation of the irradiated light.

[0050] Furthermore, in the embodiment described above, when printing multiple copies, the amount of curl of the sheet bundles stacked on the stacking platform 500 is detected after each copy is completed, but this is not limited to this. For example, the amount of curl may be detected every two or three copies.

[0051] <Second Embodiment> Next, a second embodiment of the present invention will be described in detail with reference to Figure 13. In this embodiment, the timing for detecting the amount of curl of the sheet bundle is not at the end of the section, but when the image forming section 513 is temporarily paused, which is a difference in configuration from the first embodiment. However, the other configurations are the same as in the first embodiment, so the same reference numerals are used and detailed explanations are omitted.

[0052] In this embodiment, the control method for correcting the amount of curl correction of the curl correction device 800 by detecting the state of the loaded sheet S will be explained with reference to Figure 13. In the flowchart of this embodiment, the same reference numerals are used for processes similar to those shown in the flowchart of Figure 10, and detailed explanations are omitted.

[0053] After adding the correction value in step S4, or if there is no correction value in step S3, the image density of the printed sheet S is recorded in memory 102 each time printing is performed (step S11). Then, the controller 100 determines whether or not it is time for main unit adjustment (step S12). Main unit adjustment here includes all operations in which the image forming unit 513 temporarily stops printing, such as applying patches to stabilize the color tone of the image during continuous printing, replenishing toner, and adjusting the temperature of the fuser unit 58.

[0054] If the controller 100 determines that it is time to adjust the main unit (YES in step S12), it uses the curl detection unit 880 to detect the remaining curl amount of the sheets S on the stacking platform 500 (step S6). Then, it determines whether the curl amount of the sheet bundle stacked on the stacking platform 500 is greater than or equal to an allowable value (step S7). If the curl amount of the sheet bundle is greater than or equal to an allowable curl amount (YES in step S7), the controller 100 calculates the average value of the image density of the stacked sheets S (step S13). Based on the detected curl amount of the stacking platform and the calculated average value of the image density, the controller 100 calculates a correction value for the amount of curl correction for subsequent sheets S (step S14). This is repeated until the print job is finished (up to NO in step S10), and then the printing is terminated. In other words, when the image forming unit 513 is temporarily stopped due to a preparation operation during the execution of the image forming job, the controller 100 performs detection by the curl detection unit 880 and corrects the amount of correction for subsequent sheets.

[0055] Furthermore, image formation is temporarily stopped during the main unit adjustment timing because the sheet spacing of the sheets transported from the image forming apparatus 1 to the discharge apparatus 8 increases compared to when image formation is performed. Specifically, the sheet spacing of the sheets transported from the image forming unit 513 is the first spacing during image formation, and is a second spacing, which is longer than the first spacing, during the main unit adjustment timing. In this case, when the sheet spacing is the second spacing, the controller 100 performs detection by the curl detection unit 880 and corrects the amount of correction for subsequent sheets.

[0056] As described above, according to the image forming apparatus 1 of this embodiment, similar to the first embodiment, the controller 100 detects the curl of the sheet bundles loaded on the loading platform 500 and corrects the amount of straightening based on that, so that the amount of straightening can be made appropriate. As a result, even if a predetermined curl straightening effect cannot be obtained due to differences in the manufacturing lot of the sheets S or changes in the moisture content, the loading platform 500 can detect the state of the sheets S and feed back the correction amount, making it possible to load them stably.

[0057] Furthermore, according to the image forming apparatus 1 of this embodiment, since the feedback cycle is set to the timing of the main unit adjustment, the curl state can be detected at the moment printing stops. If the state of the sheets S on the stacking platform is detected while sheets S are being loaded one after another, the detection results may not be stable. Therefore, the highest detection accuracy is achieved when the ejection of sheets S stops. By aligning this timing with the state in which printing is stopped due to main unit adjustment, it is possible to perform highly accurate curl detection without reducing printing productivity as much as possible.

[0058] Furthermore, we will explain why the average image density of the loading platform is used when calculating the correction value for curl correction. As mentioned above, the tendency of the output sheet S to curl differs depending on the image density. Therefore, if the average image density of the sheets S loaded on the loading platform is low at the time of curl detection, and the correction amount is calculated using only the detection result, there is a possibility of overcorrection if the density of the images printed thereafter is high. When calculating the correction value, it is possible to obtain an appropriate correction result by using information on the bias in image density of the sheets S loaded on the loading platform 500 to adjust the correction value.

[0059] Furthermore, this technology can also be configured as follows. (1) A curl correction unit capable of correcting the curl of the sheet on which the image is formed, A loading section for loading sheets discharged from the curl straightening section, A detection unit for detecting the curl state of a sheet bundle consisting of multiple sheets loaded on the loading section, The system includes a control unit capable of adjusting the amount of curl correction performed by the curl correction unit, The control unit, A first sheet bundle is formed by multiple sheets, each of which has undergone curl correction with a first correction amount, and which are then stacked in the stacking section. For sheets following the first sheet bundle, curl correction is performed by a second correction amount, which is obtained by correcting the first correction amount based on the detection result obtained by the detection unit that detects the curl state of the first sheet bundle. Curl straightening appliance. (2) The control unit forms a second sheet bundle which is loaded into the loading section by a plurality of sheets, each of which subsequent sheets in the first sheet bundle has undergone curl correction by the second correction amount. The curl straightening device described in (1) above. (3) The control unit performs curl correction on each sheet following the second sheet bundle by a third correction amount obtained by correcting the second correction amount based on the detection result obtained by the detection unit detecting the curl state of the first sheet bundle and the second sheet bundle, and forms a third sheet bundle to be loaded into the loading unit with a plurality of sheets that have undergone curl correction by the third correction amount. The curl straightening device described in (2) above. (4) The first sheet bundle and the second sheet bundle each form a section consisting of multiple pages with the same content. The curl correction device described in (2) or (3) above. (5) The sheets transported from the image forming unit, which forms images on the sheets, have a first interval and a second interval that is longer than the first interval. The control unit, when the sheet spacing is the second spacing, performs detection by the detection unit and corrects the amount of correction for the subsequent sheet. A curling orthodontic appliance as described in any one of (1) to (3) above. (6) The first sheet bundle is the initial sheet bundle in the image forming job, The control unit forms the first sheet bundle which is stacked in the stacking section by a plurality of sheets, each of which has undergone curl correction by the first correction amount. A second sheet bundle is formed by stacking multiple sheets in the loading section, each of which subsequent sheets have undergone curl correction using a second correction amount obtained by correcting the first correction amount. A curl straightening device according to any one of (1) to (5) above. (7) The detection unit is an imaging device that photographs the sheet bundles loaded on the loading unit. A curl straightening appliance according to any one of (1) to (6) above. (8) The imaging device is positioned above the loading section and images the top sheet of the sheet bundle from above. The curl straightening device described in (7) above. (9) An image forming unit that forms an image on a sheet, The device comprises a curl correction device according to any one of (1) to (8) above, which corrects the curl of a sheet on which an image has been formed by the image forming unit, Image forming apparatus. (10) The control unit, when the image forming unit is temporarily stopped due to a preparation operation during the execution of an image forming job, performs detection by the detection unit and corrects the amount of correction for the subsequent sheet. The image forming apparatus described in (9) above. [Explanation of Symbols]

[0060] 1…Image forming apparatus, 100…Controller (control unit), 500…Loading platform (loading unit), 513…Image forming unit, 800…Curl correction device, 840a…Upward correction unit (curl correction unit), 840b…Downward correction unit (curl correction unit), 880…Curl detection unit (detection unit, imaging device), S…Sheet, S1…First sheet bundle, S2…Second sheet bundle, S3…Third sheet bundle

Claims

1. A curl correction unit capable of correcting the curl of the sheet on which the image is formed, A loading section for loading sheets discharged from the curl straightening section, A detection unit for detecting the curl state of a sheet bundle consisting of multiple sheets loaded on the loading section, The system includes a control unit capable of adjusting the amount of curl correction performed by the curl correction unit, The control unit, A first sheet bundle is formed by multiple sheets, each of which has undergone curl correction with a first correction amount, and which are then stacked in the stacking section. For sheets following the first sheet bundle, curl correction is performed by a second correction amount obtained by correcting the first correction amount based on the detection result obtained by the detection unit, which detects the curl state of the first sheet bundle. A curl correction device characterized by the following features.

2. The control unit forms a second sheet bundle which is loaded into the loading section by a plurality of sheets, each of which subsequent sheets to the first sheet bundle has undergone curl correction by the second correction amount. The curl correction device according to feature 1.

3. The control unit performs curl correction on each sheet following the second sheet bundle by a third correction amount obtained by correcting the second correction amount based on the detection result obtained by the detection unit detecting the curl state of the first sheet bundle and the second sheet bundle, and forms a third sheet bundle to be loaded into the loading unit with a plurality of sheets that have undergone curl correction by the third correction amount. The curl correction device according to feature 2.

4. The first sheet bundle and the second sheet bundle each form a section consisting of multiple pages with the same content. The curl correction device according to feature 2 or 3.

5. The sheets transported from the image forming unit, which forms images on the sheets, have a first interval and a second interval that is longer than the first interval. The control unit, when the sheet spacing is the second spacing, performs detection by the detection unit and corrects the amount of correction for the subsequent sheet. The curl correction device according to feature 1.

6. The first sheet bundle is the initial sheet bundle in the image forming job, The control unit forms the first sheet bundle, which is stacked in the stacking section, by using a plurality of sheets, each of which has undergone curl correction by the first correction amount. A second sheet bundle is formed by stacking multiple sheets in the stacking section, each of which subsequent sheets have undergone curl correction using a second correction amount obtained by correcting the first correction amount. The curl correction device according to feature 1.

7. The detection unit is an imaging device that photographs the sheet bundles loaded on the loading unit. The curl correction device according to feature 1.

8. The imaging device is positioned above the loading section and images the top sheet of the sheet bundle from above. The curl correction device according to feature 7.

9. An image forming unit that forms an image on a sheet, The curl correction device according to claim 1, comprising correcting the curl of a sheet on which an image has been formed by the image forming unit, An image forming apparatus characterized by the following features.

10. The control unit, when the image forming unit is temporarily stopped due to a preparation operation during the execution of an image forming job, performs detection by the detection unit and corrects the amount of correction for the subsequent sheet. The image forming apparatus according to feature 9.

Images