JP2014160221A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus configured to improve productivity in forming an image during continuous duplex printing.SOLUTION: During duplex printing, an image forming apparatus 1 performs both-side magnification correction which changes rotation speed of a polygon mirror 21 so that magnifications in a surface A and a surface B of a sheet S are made different from each other in a rotation direction of a photoreceptor 11, which is a sub-scanning direction. During continuous duplex printing with the both-side magnification correction, when an expression (1) 2×PCS>PSB+PD is satisfied, images are formed in circulative duplex mode. When an expression (2) 2×PCS<PSB+PD is satisfied, images are formed in alternate duplex mode.

Description

  The present invention relates to an image forming apparatus capable of continuous duplex printing. More specifically, an image that can be continuously printed on both sides by reversing the front and back of a sheet that has passed through a conveyance path provided with a transfer nip and a fixing nip and then passing the sheet again through the conveyance path. The present invention relates to a forming apparatus.

  A general image forming apparatus has a conveyance path for forming an image on a sheet. In the conveyance path, a transfer nip and a fixing nip are provided in this order in order to form an image on a sheet. The transfer nip is for transferring an image formed by the image forming unit onto a sheet. The fixing nip is for performing a fixing process for fixing the image onto the paper by heating the paper with the image transferred at the transfer nip while applying pressure.

  In addition, the image forming apparatus having the duplex printing function further includes a front / back reversing unit that reverses the front and back of the paper that has passed through the conveyance path, and a sheet that has been reversed by the front / back reversing unit, again upstream of the transfer nip of the conveyance path. And a double-sided path for transporting to.

  In duplex printing, an image is first formed on the surface of the sheet by passing the sheet through a transfer nip and a fixing nip of the conveyance path. Next, the front and back of the paper on which the image is formed is reversed by the front / back reversing unit, and the paper after the front / back reversal is conveyed to the conveyance path by passing through the double-sided path. Then, the sheet is again passed through the transfer nip and the fixing nip of the conveyance path, thereby forming an image on the back surface of the sheet. Therefore, a sheet on which double-sided printing is performed passes through the fixing nip twice.

  Here, when the paper passes through the fixing nip, moisture is lost due to heating by the fixing process, and the paper shrinks slightly. For this reason, the image forming unit includes an image forming apparatus that forms an image to be transferred to the back surface at a lower magnification than the image to be transferred to the front surface in accordance with the paper shrunk during the front surface fixing process. This is because when the images on both the front and back surfaces are formed with the same size by the image forming unit, the front image is smaller than the back image on the paper.

  Therefore, in the image forming unit of such an image forming apparatus, the front and back images are formed at different magnifications. Therefore, after forming the front image and before forming the back image, the electrostatic latent image is formed on the photosensitive member. The number of rotations of the polygon mirror that forms the image is switched. In addition, the polygon mirror rotation speed is switched after the back side image is formed and before the next sheet front side image is formed.

  For this reason, in continuous double-sided printing in which double-sided printing is performed continuously on a plurality of sheets, when the front and back images are formed at different magnifications, the interval between the sheets is longer than when the front and back images are formed at the same magnification. I have to take it. The rotation speed of the polygon mirror is switched by switching the rotation speed of the polygon motor that rotates the polygon mirror. This is because it takes a certain amount of time for the rotation speed of the polygon motor to stabilize at the rotation speed after switching.

  Therefore, when continuous duplex printing is performed with the front and back images at different magnifications, the productivity tends to decrease. Therefore, the present applicant has previously filed an image forming apparatus capable of suppressing a decrease in productivity even when continuous double-sided printing is performed with front and back images at different magnifications (Patent Document 1).

  The image forming apparatus disclosed in Patent Literature 1 performs image formation by the circulation double-side method when performing continuous double-sided printing with front and back images at different magnifications, and performs continuous double-sided printing with front and back images at the same magnification. Image formation is performed by an alternating double-sided method.

  The circulation duplex method is a method in which continuous duplex printing is performed for each number of sheets accommodated in a circulation path formed by a conveyance path and a duplex path. Specifically, for example, in an image forming apparatus that can store two sheets in a circulation path, in the circulation double-sided method, after images are continuously formed on the surface of two sheets, An image is continuously formed on the back surface. By repeating this procedure, continuous duplex printing is performed on a plurality of sheets.

  On the other hand, in the alternate double-sided method, first, a plurality of sheets are accommodated in the circulation path, and each time one sheet on which image formation on both sides is completed is discharged from the circulation path, the circulation path becomes empty. Continuous double-sided printing is performed while the next sheet is accommodated in the spot. Specifically, for example, in an image forming apparatus capable of accommodating two sheets in a circulation path, in the alternate double-sided method, after an image is continuously formed on the surface of two sheets, the first of the two sheets An image is formed on the back side of the paper and discharged from the circulation path. Then, the next sheet is accommodated in a portion of the circulation path that is vacant as the first sheet is discharged. Thereafter, each time a sheet on which image formation has been completed is discharged, the next sheet is accommodated in the vacant portion, thereby performing continuous duplex printing on a plurality of sheets.

  For this reason, in the example of the image forming apparatus that can accommodate two sheets in the circulation path, when continuous duplex printing is performed while switching the magnification of the front and back surfaces by the circulation duplex method, every time two images are formed. In addition, the rotation speed of the polygon mirror is switched. On the other hand, when performing continuous duplex printing while switching the magnification of the front and back surfaces by the alternate duplex method, the number of rotations of the polygon mirror is switched after image formation of the first surface on the first surface, and then image formation is performed 1 The number of rotations of the polygon mirror is switched each time the surface is moved. Therefore, when continuous double-sided printing is performed at different magnifications for the front and back images, it is said that the productivity reduction can be suppressed by forming the image with the circular double-sided method with a small number of polygon mirror rotations. Yes. In addition, when the front and back images have the same magnification, it is said that the alternate double-sided method that can perform continuous double-sided printing while keeping the paper interval short can increase the productivity.

JP 2011-197392 A

  In the above-described conventional image forming apparatus, when continuous double-sided printing is performed with images on the front and back sides having different magnifications, image formation is performed by the cyclic double-sided method. However, even when continuous double-sided printing is performed with the images on the front and back sides at different magnifications, the alternating duplex method produces more than the recirculating duplex method depending on the specifications of the device and the length in the paper transport direction. In some cases, it was possible to suppress a decrease in sex.

  The present invention has been made for the purpose of solving the problems of the prior art described above. That is, an object of the present invention is to provide an image forming apparatus having high image forming productivity in continuous duplex printing.

In order to solve this problem, an image forming apparatus according to an aspect of the present invention includes an image carrier for each color that supports a single color toner image while rotating during image formation, and one polygon mirror for all the image carriers. An exposure unit for performing an exposure process for writing an electrostatic latent image, a developing unit for each color for forming a single color toner image by applying toner to a position after the exposure process on each color image carrier, and a transfer nip for the formed toner image. A transfer unit that transfers the sheet to the sheet at the transfer nip, a sheet feeding unit that supplies the sheet to the transfer nip, a fixing unit that fixes the toner image to the sheet by heating the sheet that has passed through the transfer nip at the fixing nip, , An image forming apparatus having a reverse circulation path for reversing the sheet that has passed through the fixing nip and leading it to the entrance side of the transfer nip, and a discharge unit that discharges the sheet on which image formation has been completed. By changing the rotation speed of the polygon mirror, the formation of the single-color toner image on the second surface of the sheet on the image carrier is reduced in the circumferential direction of the image carrier compared to the formation of the single-color toner image on the first surface. A control unit that performs front / back magnification correction to be performed at the specified magnification, and the control unit performs continuous double-sided printing that continuously forms images on both sides of a plurality of sheets while performing front / back magnification correction.
PCS: When image formation on the first side and image formation on the second side are performed continuously, from the start of electrostatic latent image writing on the leading image carrier associated with the toner image of the first image formed, Writing of the electrostatic latent image on all the image carriers related to the toner image of the image formed earlier is completed, and the change of the rotation speed of the polygon mirror is completed, so that the toner image of the image formed later The time until the start of the writing of the electrostatic latent image on the leading image carrier,
PSB: Both of the two sheets on which both sides are image-formed, when two images on the first side are continuously formed, the first sheet enters the fixing nip and the subsequent sheet enters the fixing nip. Time until entry, and PD: Both of the two sheets on which images are formed on both sides after the previous sheet enters the fixing nip when two images on the second side are formed continuously. Depending on the relationship between the time until the subsequent sheet enters the fixing nip,
2 x PCS> PSB + PD
Is satisfied, the plurality of sheets having the toner image fixed on the first surface within the maximum number of sheets that can be accommodated in the reverse circulation path are accommodated in the reverse circulation path, and For all the contained sheets, image formation is performed by a circulation double-side method by repeating the fixing of the toner image on the second surface and discharging from the discharge portion.
2 x PCS <PSB + PD
Is satisfied, the maximum number of sheets or the number of circulation sheets equal to or less than that is accommodated in the reversal circulation path while the first sheet of toner image fixed is accommodated in the reversal circulation path. Each time one sheet on which the toner image on the second side is fixed is ejected from the ejection section, a new sheet is fed from the paper feeding section to transfer the toner image on the first side and Image formation is performed by alternating double-sided method by repeating fixing and accommodating in the reverse circulation path,
2 x PCS = PSB + PD
Is satisfied, the image forming apparatus is characterized in that image formation is performed by one of a circulation duplex method and an alternating duplex method.

  The image forming apparatus of the present invention forms an electrostatic latent image on an image carrier of each color by one polygon mirror. Also, during double-sided printing, the front and back magnification correction is performed so that the magnification in the circumferential direction of the image carrier is different between the first and second sides of the sheet by switching the rotation speed of the polygon mirror. When continuous double-sided printing is performed while correcting the front and back magnifications, if 2 × PCS> PSB + PD is satisfied, image formation is performed by the cyclic double-side method. On the other hand, when 2 × PCS <PSB + PD is satisfied, image formation is performed by the alternate double-sided method. Thereby, continuous double-sided printing can be performed with high productivity.

  An image forming apparatus according to another aspect of the present invention writes an electrostatic latent image by an image carrier of each color that carries a single color toner image while rotating during image formation, and a single polygon mirror on all the image carriers. An exposure unit for performing an exposure process, a developing unit for each color that forms a single color toner image by applying toner to portions after the exposure process in the image carrier of each color, and the formed toner image is transferred to a sheet at a transfer nip. The transfer unit, a sheet feeding unit that supplies a sheet to the transfer nip, a fixing unit that fixes the toner image to the sheet by heating the sheet that has passed through the transfer nip, and the fixing nip. An image forming apparatus having a reversal circulation path for reversing the front and back of the sheet and leading the sheet to the entrance side of the transfer nip, and a discharge unit for discharging the sheet on which image formation has been completed. Is changed so that the formation of the single-color toner image on the second surface of the sheet on the image carrier is performed at a reduced magnification in the circumferential direction of the image carrier compared to the formation of the single-color toner image on the first surface. A control unit that performs magnification correction and a threshold storage unit that stores a predetermined threshold for the length in the sheet conveyance direction, and the control unit continuously forms images on both sides of a plurality of sheets. When continuous double-sided printing is performed while correcting the front and back magnification, if the length of the sheet in the conveyance direction is less than the threshold, the first toner image is fixed within the maximum number of sheets that can be accommodated in the reverse circulation path. It is repeated that a plurality of completed sheets are accommodated in the reverse circulation path, and the toner image on the second surface is fixed on all the sheets accommodated in the reverse circulation path and discharged from the discharge portion. Recycling both sides method by doing When the image is further formed and the length in the sheet conveyance direction is equal to or greater than the threshold value, the reversal circulation path is used to transfer the maximum number of sheets or the number of circulation sheets equal to or less than that on which the first toner image has been fixed. Each time a sheet on which the toner image on the second surface is fixed is discharged from the discharge section among the sheets stored in the reversal circulation path, a new sheet is supplied. An image forming apparatus characterized in that image formation is performed by an alternating double-sided system by repeating feeding and fixing of a toner image on a first surface after being fed from a paper portion and accommodated in a reverse circulation path It is. The longer the length of the sheet used in the conveyance direction, the longer the sheet after the first sheet enters the fixing nip when image formation is continuously performed on the first side of the sheet that forms an image on both sides. Time to enter the fixing nip tends to be long. The frequency of the time is higher in the circular double-sided method than in the alternating double-sided method. Therefore, when the length of the sheet conveyance direction is short, the double-sided circulation method is more suitable for image formation than the other method when the length of the sheet conveyance direction is long and the alternate double-side method is longer than the other method. Productivity can be increased.

  An image forming apparatus according to another aspect of the present invention includes an image carrier for each color that supports a single color toner image while rotating during image formation, and exposure that writes an electrostatic latent image on all the image carriers using a single polygon mirror. An exposure unit for processing, a developing unit for each color that forms a single-color toner image by applying toner to locations after the exposure processing in the image carrier for each color, and a transfer for transferring the formed toner image to a sheet at a transfer nip A sheet feeding unit that supplies the sheet to the transfer nip, a fixing unit that fixes the toner image to the sheet by heating the sheet that has passed through the transfer nip, and a sheet that has passed through the fixing nip. Is an image forming apparatus having a reverse circulation path that reverses the front and back and guides it to the entrance side of the transfer nip, and a discharge unit that discharges a sheet on which image formation has been completed. By changing the front and back magnifications, the formation of the single-color toner image on the second surface of the sheet on the image carrier is performed at a reduced magnification in the circumferential direction of the image carrier compared to the formation of the single-color toner image on the first surface. A control unit that performs correction, and the control unit selects a color mode for forming a color image when performing continuous double-sided printing that continuously forms images on both sides of a plurality of sheets while correcting the front and back magnification. If there is a plurality of sheets of toner images fixed on the first surface within the maximum number of sheets that can be accommodated in the reverse circulation path, the sheets are accommodated in the reverse circulation path and accommodated in the reverse circulation path. For all the sheets that have been fixed, the toner image on the second side is fixed and discharged from the discharge section, and image formation is performed by the cyclic double-side method, and the monochrome mode for forming a monochrome image is selected. In this case, the number of circulating sheets, which is the maximum number or less, is accommodated in the reverse circulation path while the sheet on which the first toner image has been fixed is stored in the reverse circulation path. Among them, each time one sheet on which the toner image on the second surface is fixed is discharged from the discharge section, a new sheet is fed from the sheet feeding section to transfer and fix the toner image on the first surface. The image forming apparatus is characterized in that image formation is performed by an alternating double-sided system by repeating the above and accommodating in the reverse circulation path. In the color mode, the toner image for the first side and the toner image for the second side are alternately and continuously formed compared to the monochrome mode. Time to start tends to be long. The frequency of the time is higher in the alternating duplex method than in the circulating duplex method. Therefore, the productivity of image formation can be made higher in the circular duplex method in the color mode and in the alternate duplex method in the monochrome mode than in the other method.

In the image forming apparatus described above,
Color PCS: When the first side image formation and the second side image formation are successively performed by the multi-layer toner image formed by superimposing single-color toner images of the respective colors, the leading order related to the multi-layer toner image of the first image to be formed After the start of writing of the electrostatic latent image on the other image carrier, the writing of the electrostatic latent image on all the image carriers related to the multilayer toner image of the previously formed image is completed, and the rotation speed of the polygon mirror is further increased. The time until the start of electrostatic latent image writing on the leading image carrier associated with the multi-layer toner image of the image to be formed later can be started,
Monochrome PCS: Image carrier related to a single-layer toner image of an image formed first when image formation on the first surface and image formation on the second surface are successively performed with a single-layer toner image of a single-color toner image of one color After the start of writing of the electrostatic latent image on the body, the writing of the electrostatic latent image on the image carrier relating to the single-layer toner image of the previously formed image is completed, and the change in the rotation speed of the polygon mirror is completed. The time until the start of writing of the electrostatic latent image on the image carrier relating to the single-layer toner image of the image to be formed later,
PSB: Both of the two sheets on which both sides are image-formed, when two images on the first side are continuously formed, the first sheet enters the fixing nip and the subsequent sheet enters the fixing nip. Time until entry, and PD: Both of the two sheets on which images are formed on both sides after the previous sheet enters the fixing nip when two images on the second side are formed continuously. Depending on the relationship between the time until the subsequent sheet enters the fixing nip,
2 x Color PCS> PSB + PD
In monochrome mode,
2 x Monochrome PCS <PSB + PD
It is preferable that these relationships are satisfied.

In the image forming apparatus described above, it is preferable that the control unit performs image formation by an alternate double-side method when performing continuous double-sided printing without correcting the front and back magnification. This is because when continuous duplex printing is performed without correcting the front and back magnification, the alternate duplex method has higher image formation productivity than the circulating duplex method.

  Further, in the image forming apparatus described above, the optimum circulation which is a circulation number equal to or less than the maximum number of sheets that can be completed in the shortest time by performing continuous double-sided printing on the sheets of a predetermined number or more maximum number of sheets. The control unit has an optimum circulation number storage unit for storing the number of sheets, and the control unit obtains the optimum circulation number from the optimum circulation number storage unit when performing continuous duplex printing by the alternate duplex method, and obtains the circulation number as the obtained optimum circulation number. It is preferable to perform image formation. This is because in continuous duplex printing by the alternate duplex method, productivity may be higher when the number of circulating sheets, which is the number of sheets accommodated in the reverse circulation path, is smaller than the maximum number.

  According to the present invention, an image forming apparatus having high image forming productivity in continuous duplex printing is provided.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment. FIG. 2 is a diagram illustrating an outline of a control configuration of an image forming apparatus according to the present embodiment. It is a figure for demonstrating a circulation double-sided system. It is a figure for demonstrating an alternating double-sided system. Both are diagrams for explaining the pitch when images are continuously formed on the first surface of a sheet for duplex printing. Both are diagrams for explaining the pitch when images are continuously formed on the second surface of the sheet for duplex printing. FIG. 6 is a diagram for explaining a pitch when images are continuously formed on only one side of a sheet. Both are diagrams for explaining the pitch when images are alternately and continuously formed on the first surface and the second surface of a sheet for duplex printing. In the case where the front / back magnification correction is not performed, the pitch at which images are alternately and continuously formed on the first surface and the second surface of the paper for double-sided printing is determined by the exposure device. It is a figure showing an electric latent image. When performing front / back magnification correction, the electrostatic pitch formed on the photoconductor by the exposure device is the pitch at which images are alternately and continuously formed on the first and second sides of the paper for duplex printing. It is a figure showing a latent image. It is a figure for demonstrating the circulation double-sided system when not performing front and back magnification correction. It is a figure for demonstrating the alternate double-sided system in case front / back magnification correction is not performed. It is a figure for demonstrating the circulation double-sided system in the case of performing front and back magnification correction. It is a figure for demonstrating the alternate double-sided system in the case of performing front and back magnification correction. It is a flowchart for demonstrating the procedure which judges whether continuous double-sided printing is performed by any method of a cyclic | annular double-sided method and an alternating double-sided method. It is a flowchart for demonstrating the procedure which judges whether continuous double-sided printing is performed by either a double-sided printing system or an alternating double-sided system by paper size. It is a flowchart for demonstrating the procedure which judges whether continuous double-sided printing is performed by either a cyclic | annular double-sided system or an alternating double-sided system by the mode of the image to form. FIG. 10 is a diagram for explaining an alternate double-sided method in a case where the number of sheets accommodated in the circulation path is less than the maximum number of sheets that can be accommodated in the circulation path. It is a figure which shows the length of a circulation path. FIG. 10 is a diagram for explaining a case where a waiting time is generated due to a rate-determining time during which a sheet is transported once in a circulation path in the alternate duplex method.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments embodying the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows a schematic configuration of an image forming apparatus 1 of the present embodiment. The image forming apparatus 1 is a so-called tandem type color printer having an intermediate transfer belt 30. The intermediate transfer belt 30 is an endless belt member having conductivity, and both end portions in FIG. 1 are supported by rollers 31 and 32. At the time of image formation, the roller 31 on the right side in FIG. 1 is driven to rotate counterclockwise as indicated by an arrow in the figure. As a result, the intermediate transfer belt 30 and the roller 32 on the left side in FIG. 1 are each driven to rotate in the direction indicated by the arrow in the drawing.

  A secondary transfer roller 40 is provided on the outer peripheral surface of the portion of the intermediate transfer belt 30 supported by the right roller 31 in FIG. The secondary transfer roller 40 is pressed against the intermediate transfer belt 30 in a direction perpendicular to the axis (leftward in FIG. 1). A secondary transfer nip N1 for transferring the toner image on the intermediate transfer belt 30 onto the paper S is formed at a portion where the intermediate transfer belt 30 and the secondary transfer roller 40 are in contact with each other. At the time of image formation, the secondary transfer roller 40 is driven to rotate in the direction indicated by the arrow in FIG. 1 by the frictional force generated by the pressure contact with the rotating intermediate transfer belt 30.

  Also, a belt cleaner 41 is provided on the outer peripheral surface of the portion of the intermediate transfer belt 30 supported by the left roller 32 in FIG. The belt cleaner 41 of this embodiment has a roller shape as shown in FIG. 1 and has a conductive brush on the surface of the cored bar. The belt cleaner 41 is for collecting toner adhering to the surface of the intermediate transfer belt 30. That is, the belt cleaner 41 collects transfer residual toner that has not been transferred to the paper S in the secondary transfer nip N1.

  In the lower part of the intermediate transfer belt 30 in FIG. 1, the image forming units 10Y, 10M, 10C for yellow (Y), magenta (M), cyan (C), and black (K) are sequentially arranged from left to right. 10K is arranged. Each of the image forming units 10Y, 10M, 10C, and 10K is for forming a toner image of the corresponding color and transferring it onto the intermediate transfer belt 30. The image forming units 10Y, 10M, 10C, and 10K all have the same configuration. For this reason, in FIG. 1, the image forming unit 10 </ b> Y is represented by a reference numeral.

  The image forming units 10Y, 10M, 10C, and 10K include a photosensitive member 11 that is a cylindrical electrostatic latent image carrier, a charging device 12, a developing device 13, a primary transfer roller 14, and a peripheral transfer device. A photoreceptor cleaner 15 is provided. An exposure device 20 is disposed below the image forming units 10Y, 10M, 10C, and 10K for each color in FIG.

  At the time of image formation, the photoconductor 11 is driven to rotate clockwise as indicated by an arrow in FIG. For this reason, the moving directions of the surfaces of the respective photoreceptors 11 and the surface of the intermediate transfer belt 30 at the time of image formation are the same at locations where they are in contact with each other. The charging device 12 is for charging the surface of the photoreceptor 11 uniformly. The exposure device 20 is for irradiating the surface of the photoreceptor 11 with laser light based on the image data of the corresponding color to form an electrostatic latent image. The developing device 13 is for applying the toner contained therein to the surface of the photoreceptor 11.

  The primary transfer roller 14 is disposed at a position facing the photoreceptor 11 with the intermediate transfer belt 30 interposed therebetween. The primary transfer roller 14 is pressed against the intermediate transfer belt 30 in a direction perpendicular to the axis (downward in FIG. 1). Due to this pressure contact, primary transfer nips for transferring the toner images on the photosensitive drums 11 of the respective colors onto the intermediate transfer belt 30 are formed at the portions where the intermediate transfer belt 30 and the photosensitive drum 11 are in contact with each other. . The photoconductor cleaner 15 is for collecting toner that has not been transferred from the photoconductor 11 onto the intermediate transfer belt 30.

  In FIG. 1, the photoconductor cleaner 15 is plate-shaped and one end thereof is in contact with the outer peripheral surface of the photoconductor 11, but the present invention is not limited to this. Other cleaning members, for example, a fixed brush, a rotating brush, a roller, or a combination of a plurality of them can be used. Alternatively, if a cleaner-less system in which the untransferred toner on the photoconductor 11 is collected by the developing device 13 is employed, the photoconductor cleaner 15 may be omitted.

  Further, the exposure apparatus 20 of the present embodiment has one polygon mirror 21 and a polygon motor 22. The polygon mirror 21 is a polygonal mirror having a vertical rotation axis in FIG. 1 and having a plurality of mirror surfaces parallel to the rotation axis. The polygon mirror 21 reflects the laser beam while rotating, and scans the reflected laser beam in the main scanning direction that is the axial direction of the photoconductor 11 (the depth direction in FIG. 1). Note that the rotation direction of the photoconductor 11 is a sub-scanning direction. The polygon motor 22 is for rotating the polygon mirror 21. Further, the laser light reflected by the polygon mirror 21 is reflected by the mirrors 23Y, 23M, 23C, and 23K for each color, and then irradiated on the surface of the photoconductor 11 for each color.

  Further, hoppers 16Y, 16M, 16C, and 16K that store toners of the respective colors are disposed above the intermediate transfer belt 30 in FIG. The toners of the respective colors stored in these are appropriately supplied to the developing devices 13 of the respective colors.

  A paper feed tray 51 that stores sheets S by stacking is mounted at the bottom of the image forming apparatus 1. A paper feed path 60 is provided above the right side of the paper feed tray 51 in FIG. The sheets S stored in the sheet feeding tray 51 are sent one by one from the uppermost one to the sheet feeding path 60 by the sheet feeding roller 52, and the conveyance path 61 provided downstream of the sheet feeding path 60. It is to be transported to.

  In the transport path 61, a pair of registration rollers 53, a secondary transfer nip N1, and a fixing device 70 are arranged in this order. The registration roller 53 is for finely adjusting the timing at which the sheet S is sent to the secondary transfer nip N1. The fixing device 70 is for performing a fixing process on the sheet S of the toner image transferred from the intermediate transfer belt 30 to the sheet S in the secondary transfer nip N1.

  The fixing device 70 includes a heating roller 72 and a pressure roller 71 facing the heating roller 72 with the conveyance path 61 interposed therebetween. The heating roller 72 has a heater 73 inside. The pressure roller 71 is pressed against the heating roller 72 in a direction perpendicular to the axis. Due to the pressure contact, a fixing nip N <b> 2 for fixing the sheet S is formed between the heating roller 72 and the pressure roller 71.

  The heating roller 72 is driven to rotate counterclockwise during image formation. The pressure roller 71 is driven and rotated by the rotation of the heating roller 72. The fixing device 70 fixes the toner image carried by the paper S by heating and pressurizing the paper S passing through the fixing nip N2.

  Also, a switching lever 54 is provided on the downstream side of the fixing device 70 in the transport path 61. The switching lever 54 can rotate in the direction indicated by the arrow in FIG. By the rotation, the switching lever 54 can take the discharging position indicated by the solid line in FIG. 1 or the double-sided position indicated by the two-dot chain line. That is, when the switching lever 54 is at the discharge position indicated by the solid line in FIG. 1, the paper S that has been transported through the transport path 61 is then transported to the paper discharge path 62.

  On the other hand, when the switching lever 54 is in the double-sided position indicated by the two-dot chain line in FIG. 1, the paper S that has been transported through the transport path 61 is then transported to the reverse path 63. Accordingly, the switching lever 54 is for switching the subsequent transport destination of the paper S that has been transported on the transport path 61 to the paper discharge path 62 or the reverse path 63.

  A paper discharge roller 56 is disposed in the paper discharge path 62, and a paper discharge unit 57 for discharging the paper S on which image formation has been completed is provided further downstream. A reversing roller 58 is disposed on the downstream side of the reversing path 63. The reversing roller 58 reverses the transport direction of the paper S that has been transported through the reversing path 63 and transports the paper S to the duplex path 64.

  The double-sided path 64 is for transporting the paper S that has once passed through the transport path 61 to the transport path 61 again. The double-sided path 64 is connected to a position on the upstream side of the registration roller 53 in the transport path 61. Then, after the transport direction is reversed by the reversing roller 58, the sheet S transported through the double-sided path 64 and transported again to the transport path 61 is opposite to the front side when transported to the transport path 61 before that. It has become.

  FIG. 2 shows an outline of the control configuration of the image forming apparatus 1. The image forming apparatus 1 includes an engine unit 2 and a controller unit 3 in order to control each unit. The engine unit 2 includes a CPU 4 that performs control processing of the entire apparatus, and a nonvolatile memory 5 attached to the main body.

  In the non-volatile memory 5, for example, the photosensitive member pitch, which is the distance between the press contact portions between the photosensitive member 11 of each image forming unit 10 and the intermediate transfer belt 30 on the intermediate transfer belt 30, the conveyance of the paper S and the toner image, etc. Each value known by design in the apparatus, such as the system speed, which is the speed, and the length of the circulation path described later, is stored. In addition, each paper size that can be accommodated in the paper feed tray 51, and a magnification for performing front / back magnification correction, which will be described later, are also stored.

  The various units 6 include, for example, a toner bottle and an imaging unit. The nonvolatile memory 7 attached to each unit stores, for example, the remaining amount of toner in the memory attached to the toner bottle, and the number of printed sheets in the memory attached to the imaging unit.

  The CPU 4 controls each unit of the image forming apparatus 1 based on the storage in the nonvolatile memory 5 and the nonvolatile memory 7. For example, the sheet feeding timing of the sheet S from the sheet feeding tray 51 and the rotation of the polygon motor 22 of the exposure apparatus 20 are controlled.

  The controller unit 3 is connected to an external personal computer or the like and receives an instruction input. For example, an image forming job is generated in the image forming apparatus 1 by receiving an image forming command from a personal computer. Further, various information such as dot counter values are exchanged between the engine unit 2 and the controller unit 3.

  Next, an example of a normal image forming operation by the image forming apparatus 1 of this embodiment will be briefly described. The following description is an example of an image forming operation in the color mode in which a color image is formed on the paper S stored in the paper feed tray 51 using four color toners.

  When a normal color image is formed, first, the outer peripheral surface of the photoconductor 11 is charged almost uniformly by the charging device 12. Light that corresponds to the image data is projected onto the charged photoreceptor 11 by the exposure device 20 to form an electrostatic latent image. Subsequently, the electrostatic latent image is developed by the developing device 13, and a toner image is formed on the photoreceptor 11. Each color toner image formed on the photoreceptor 11 is transferred (primary transfer) onto the intermediate transfer belt 30 by the primary transfer roller 14. In other words, yellow (Y), magenta (M), cyan (C), and black (K) toner images are superimposed on the intermediate transfer belt 30 in this order.

  Further, the transfer residual toner that is not transferred to the intermediate transfer belt 30 and remains on the photoconductor 11 after passing the primary transfer roller 14 is scraped off by the photoconductor cleaner 15 and removed from the photoconductor 11. Is done. The superimposed four color toner images are conveyed to the secondary transfer nip N1 by the rotation of the intermediate transfer belt 30.

  On the other hand, the sheets S stored in the sheet feed tray 51 are drawn out one by one from the uppermost one to the sheet feed path 60. The drawn paper S is transported to the secondary transfer nip N1 along the paper feed path 60 and the transport path 61 in this order. The entry timing of the sheet S into the secondary transfer nip N1 is finely adjusted by the registration roller 53 so as to coincide with the entry timing of the toner image on the intermediate transfer belt 30 into the secondary transfer nip N1. As a result, the superimposed four color toner images are transferred (secondary transfer) onto the paper S in the secondary transfer nip N1. Note that the transfer residual toner remaining on the intermediate transfer belt 30 even after passing through the secondary transfer nip N1 is collected by the belt cleaner 41. As a result, the intermediate transfer belt 30 is removed.

  The sheet S on which the toner image is transferred in the secondary transfer nip N1 is further conveyed downstream of the conveyance path 61. That is, the sheet S is subjected to a fixing process when passing through the fixing nip N2. Further, the sheet S that has passed through the transport path 61 reaches the switching lever 54.

  When the image formation for the paper S is completed because the paper S has passed through the transport path 61, the switching lever 54 is at the discharge position indicated by the solid line in FIG. That is, the paper S passes through the discharge path 62 and is discharged to the paper discharge unit 57 by the paper discharge roller 56.

  On the other hand, on the sheet S on which the image is formed on the first side (hereinafter referred to as A side) by passing through the conveyance path 61, the second surface (hereinafter referred to as B side) opposite to the A side is thereafter applied. In the case of double-sided printing for forming an image, the switching lever 54 is in a double-sided position indicated by a two-dot chain line in FIG. For this reason, the paper S that has reached the switching lever 54 is conveyed to the reverse path 63 and reaches the reverse roller 58.

  Therefore, the sheet S is conveyed to the duplex path 64 after the conveyance direction is reversed by the reversing roller 58. The front and back of the sheet S are reversed by reversing the conveyance direction by the reversing roller 58. Then, the sheet S that has passed through the double-sided path 64 is conveyed again to the conveyance path 61 from the registration roller 53. As a result, images are formed on both the A side and B side of the paper S. The paper S on which images are formed on both sides then passes through the paper discharge path 62 and is discharged to the paper discharge unit 57.

  Here, in double-sided printing in which images are formed on both sides A and B of the paper S, the paper S passes through the transport path 61 twice. An image is formed on the A side of the paper S in the first passage through the transport path 61 of the paper S. Further, an image is formed on the B surface of the paper S in the second passage of the paper S conveyance path 61. That is, the sheet S is transferred to the A surface at the secondary transfer nip N1, the A surface is fixed at the fixing nip N2, and the toner image is transferred to the B surface at the secondary nip N1 and fixed. The fixing process of the B surface is received in this order at the nip N2.

  As described above, in the fixing nip N2, the fixing process is performed by applying pressure while heating the sheet S. Then, the sheet S is slightly shrunk by the fixing process at the fixing nip N2. This is because the moisture in the sheet S evaporates due to the heating of the fixing process.

  Further, the toner image is transferred to the A side of the sheet S before the fixing process is performed on the A side of the sheet. In contrast, the toner image is transferred to the B side of the sheet S after the fixing process is performed on the A side of the sheet. That is, the toner image on the A side is transferred to the paper S before being contracted by the fixing process on the A side. On the other hand, the toner image on the B side is transferred to the paper S that has been shrunk by the fixing process on the A side. For this reason, when both the A-side and B-side toner images of the sheet S are formed with the same size by the image forming unit 10, the A-side image is better than the B-side image after duplex printing. Will also get smaller.

  Therefore, the image forming apparatus 1 according to the present embodiment can perform front / back magnification correction, which is performed with different magnifications for the A-side image formation and the B-side image formation of the paper S during duplex printing. That is, when the front / back magnification correction is performed, the image forming unit 10 matches the toner image on the B side with the sheet S shrunk by passing through the fixing nip N2 once, and the toner image on the A side. Is formed at a small magnification. Note that the amount of shrinkage of the paper S due to the fixing process can be obtained in advance by experiments or the like.

  Specifically, the front / back magnification correction is performed by switching the magnification of the electrostatic latent image formed on the surface of the photoreceptor 11 by the exposure device 20. When the front / back magnification correction is performed, the exposure apparatus 20 sets the magnifications of the polygon mirror 21 in the main scanning direction and the sub-scanning direction to be different between the A side and the B side.

  Here, the dot interval of the electrostatic latent image in the sub-scanning direction varies depending on the scanning speed per line of the polygon mirror 21 with respect to the moving speed of the surface of the photoconductor 11. Therefore, the magnification in the sub-scanning direction is switched by increasing / decreasing the scanning speed per line of the polygon mirror 21 with respect to the moving speed of the surface of the photoconductor 11. For this reason, when switching the magnification in the sub-scanning direction, the rotation speed of the polygon mirror 21 is switched by switching the rotation speed of the polygon motor 22. The magnification in the main scanning direction is switched by switching the frequency related to the on / off of the laser beam for writing the electrostatic latent image.

  Further, the image forming apparatus 1 according to the present embodiment can perform image formation by any one of a circulation duplex method and an alternate duplex method when performing continuous duplex printing on a plurality of sheets S continuously. it can. Examples of the circulation double-side method and the alternating double-side method are shown in FIGS. 3 and 4, respectively. As shown in FIGS. 3 and 4, the circulation double-side method and the alternating double-side method are different in the method of transporting the paper S during continuous double-sided printing. First, the circulation double-sided method will be described with reference to FIG.

  FIG. 3 is a diagram showing a procedure in the case of performing continuous printing on a plurality of sheets S by the circulating double-side method. In FIG. 3, a route closed by a conveyance route 61, a reversing route 63, and a both-side route 64 through which the sheet S circulates during duplex printing is shown as a circulation route 65. The image forming apparatus 1 according to the present embodiment uses A4 size paper S, and when image formation is performed by lateral conveyance in which the long side of the paper S is conveyed in a direction perpendicular to the conveyance direction, a maximum of three sheets in the circulation path 65. S can be accommodated. Therefore, FIG. 3 shows a procedure in a case where continuous duplex printing is performed by the circulating duplex method while accommodating three sheets S. Also, the sheets S shown in FIG. 3 are numbered in the order of feeding.

  In the circulation double-sided method, as shown in steps 1 to 4 in FIG. 3, first, three sheets S1, S2, and S3 are sequentially fed and pass through the transport path 61, so An image is formed. In addition, the sheets S1, S2, and S3 that have passed through the transport path 61 pass through the reversing path 63, the transport direction is reversed by the reversing roller 58, and then transported to the duplex path 64.

  Next, as shown in steps 5 to 8, the sheets S1, S2, and S3 that have passed through the double-sided path 64 pass again through the transport path 61, whereby an image is formed on the B side thereof. . Then, the sheets S1, S2, and S3 on which the image is formed on the B side and the duplex printing is completed are sequentially conveyed to the paper discharge path 62 and discharged.

Subsequently, as shown in steps 8 to 10, the three sheets S1, S2, and S3 related to the first sheet feeding are discharged, and the next three sheets S4, S5, and S6 are sequentially fed. By passing through the conveyance path 61, an image is formed on those A surfaces. In addition, sheets S4 and S
Similarly to the sheets S1, S2 and S3, 5 and S6 pass through the reversing path 63 and are reversed to the conveying direction by the reversing roller 58 and then conveyed to the double-sided path 64. As a result, after step 10, the sheets S4, S5, and S6 also pass through the transport path 61, so that an image is formed on the B side, and then transported to the paper discharge path 62 and discharged. Paper.

  Also in the alternating duplex method, as shown in FIG. 4, the procedures 1 to 4 are the same as the circulating duplex method. That is, first, three sheets S1, S2, and S3 are sequentially fed and pass through the transport path, whereby an image is formed on the A side thereof. In addition, the sheets S1, S2, and S3 that have passed through the transport path 61 pass through the reversing path 63, the transport direction is reversed by the reversing roller 58, and then transported to the duplex path 64.

  Next, in the alternate double-sided method, as shown in Step 5 to Step 6, the sheet S1 that has passed through the double-sided path 64 passes through the transport path 61 again, whereby an image is formed on the B-side. In addition, the first paper S1 on which the image is formed on the B side and the double-sided printing is completed is conveyed to the paper discharge path 62 and discharged. Then, the first sheet S1 is discharged, and the next sheet S4 is fed to a portion of the circulation path 65 that is emptied by discharging the sheet S1. Then, when the fed paper S4 passes through the transport path, an image is formed on the A side.

  Thereafter, in steps 7 to 8 and steps 9 to 10, as in steps 5 to 6, every time one sheet S on which double-sided printing has been completed is discharged, the discharge of the circulation path 65 is performed. One sheet of the next sheet S is fed to the vacant part.

  That is, in the circulation double-side method described with reference to FIG. 3, first, images on the A side are continuously formed on the number of sheets S less than or equal to the number that can be accommodated in the circulation path 65, and these are accommodated in the circulation path 65. To do. Next, the sheet S is discharged while continuously forming images of the B side of the sheet S accommodated in the circulation path 65. Then, by repeating these procedures, continuous duplex printing is performed. For this reason, in the circulation double-sided method, the image formation on the A side and the image formation on the B surface are alternately performed continuously for each number of sheets S accommodated in the circulation path 65.

  On the other hand, the alternate double-side method described with reference to FIG. To house. After that, among the sheets S stored in the circulation path 65, every time one sheet S on which the image of the B side is formed is discharged, the next sheet S is fed repeatedly. This is a method for performing continuous duplex printing. For this reason, in the alternate double-sided method, first, image formation on the A side is continuously performed by the number of sheets S accommodated in the circulation path 65. After that, image formation on the B side and image formation on the A side are performed alternately.

  Next, the pitch of the paper S during continuous printing will be described with reference to FIGS. Each of the pitches of the sheet S described below is the time from when the leading edge of the preceding sheet S passes an arbitrary point on the transport path 61 until the leading edge of the next sheet S passes through that point. It is. First, a double-sided AA pitch, which is a pitch when images are continuously formed on the A-side of the paper S related to double-sided printing, will be described with reference to FIG. In FIG. 5, the time of the double-sided AA pitch is shown as PSB.

  When images are continuously formed on the A side of the sheet S for duplex printing, all the sheets S are transported from the paper feed path 60 to the transport path 61. Furthermore, as shown in FIG. 5, after passing through the transport path 61 and the reverse path 63, the transport direction is reversed by the reverse roller 58 and then transported to the double-side path 64. For this reason, PSB which is the time of the double-sided AA pitch is such that the leading edge of the next sheet S reaches the reversing roller 58 after the trailing edge of the previous sheet S is removed from the reversing roller 58 and conveyed to the duplex path 64. The pitch is determined as follows.

  Next, with reference to FIG. 6, the double-sided BB pitch, which is a pitch when images are continuously formed on the B-side of the paper S related to double-sided printing, will be described. In FIG. 6, the time of the double-sided BB pitch is shown as PD. When images are continuously formed on the B side of the sheet S for duplex printing, all the sheets S are conveyed from the duplex path 64 to the conveyance path 61. Furthermore, since image formation on both sides is completed by passing through the transport path 61, after passing through the transport path 61, it is transported to the paper discharge path 62 and discharged.

  Next, a single-sided AA pitch that is a pitch when images are continuously formed on only one side of the paper S will be described with reference to FIG. In FIG. 7, the time of one-side AA pitch is shown as PS. When images are continuously formed on only one side of the sheet S, all the sheets S are transported from the sheet feeding path 60 to the transport path 61. Further, since the image formation is completed by passing through the transport path 61, after passing through the transport path 61, it is transported to the paper discharge path 62 and discharged. For this reason, the single-sided AA pitch shown in FIG. 7 does not have the special restriction | limiting which concerns on the inversion roller 58 like the double-sided AA pitch shown in FIG. Therefore, the PS that is the time of the single-sided AA pitch is shorter than the PSB that is the time of the double-sided AA pitch.

  Further, with reference to FIG. 8, a double-sided AB pitch, which is a pitch when images are formed alternately and continuously on the A-side and B-side of the paper S related to double-sided printing, will be described. In other words, the double-sided AB pitch includes the pitch when image formation is performed on the A side of the previous paper S and the B side of the next paper S in this order, and the B side of the previous paper S and the next paper S. And the pitch when the A plane is performed in this order.

  In both (a) and (b) of FIG. 8, the time of the double-sided AB pitch is indicated by PS and PCS. The time when the double-sided AB pitch is the same PS as the single-sided AA pitch is when the front / back magnification correction is not performed. Further, the time of the double-sided AB pitch is PCS when the front / back magnification correction is performed. This point will be described in detail later.

  First, FIG. 8A shows a case where an image is formed on the A side of the previous sheet S and then an image is formed on the B side of the next sheet S. That is, the previous paper S is transported from the paper feed path 60 to the transport path 61, and the next paper S is transported from the duplex path 64 to the transport path 61. Further, the previous sheet S is transported to the reverse path 63 after passing through the transport path 61. That is, the preceding paper S is transported to the double-sided path 64 with the transport direction reversed by the reversing roller 58. On the other hand, because the next sheet S passes through the transport path 61 and the image formation on both sides is completed, after passing through the transport path 61, it is transported to the paper discharge path 62 and discharged.

  FIG. 8B shows a case where an image is formed on the A side of the next sheet S after an image is formed on the B side of the previous sheet S. That is, the previous paper S is transported from the duplex path 64 to the transport path 61, and the next paper S is transported from the paper feed path 60 to the transport path 61. Further, since the image formation on both sides is completed by passing through the transport path 61, the previous sheet S is transported to the paper discharge path 62 and discharged after passing through the transport path 61. On the other hand, the next sheet S is transported to the reverse path 63 after passing through the transport path 61. That is, the preceding paper S is transported to the double-sided path 64 with the transport direction reversed by the reversing roller 58.

  In other words, in the case where images are formed alternately and continuously on the A side and the B side of the paper S related to double-sided printing, one of the previous paper S and the next paper S is passed through the transport path 61. The paper is ejected. For this reason, the double-sided AB pitch shown in FIG. 8 does not have any particular restriction on the reversing roller 58 like the double-sided AA pitch shown in FIG. As described above, the double-sided AB pitch time is PS when the front / back magnification correction is not performed, and PCS when the front / back magnification correction is performed. This point will be described with reference to FIGS.

  FIG. 9 is a diagram showing the double-sided AB pitch when the front / back magnification correction is not performed for the electrostatic latent image formed on the photoconductor 11 by the exposure device 20. FIG. 9 shows a case where the images of the A side and the B side are formed in this order. However, the same applies to the case where the images of the B side and the A side are formed in this order. FIG. 9 shows a case where a color image is formed. Therefore, exposure by the exposure apparatus 20 is started in the order of yellow, magenta, cyan, and black on both the A side and the B side.

  Then, as shown in FIG. 9, the exposure of the B surface of each color is started immediately after the exposure of the A surface of the same color is completed. For this reason, for example, when the yellow exposure of the B side is started, it is still before the black exposure of the A side is completed. Further, the time of the double-sided AB pitch when the front / back magnification correction is not performed is the same PS as the single-sided AA pitch because there is no restriction as in the case of performing the front / back magnification correction described later in FIG. In FIG. 9, the time for each color from the completion of the exposure on the A surface to the start of the exposure on the B surface is indicated as T1.

  FIG. 10 is a diagram illustrating the double-sided AB pitch when the front / back magnification correction is performed for the electrostatic latent image formed on the photoconductor 11 by the exposure device 20. FIG. 10 also shows a case where the images of the A side and the B side are formed in this order. However, the same applies to the case where the images of the B side and the A side are formed in this order. FIG. 10 also shows a case where a color image is formed. In FIG. 10, the time of the double-sided AB pitch is shown as PCS.

  Then, as shown in FIG. 10, when the front / back magnification correction is performed, the exposure of the B side yellow is started after the exposure of the A side black is completed. That is, after the exposure of all the colors on the A side is completed, the exposure on the B side is started. When performing front / back magnification correction, as described above, the magnification of the polygon mirror 21 is different between the A plane and the B plane in order to set different magnifications in the sub-scan line direction between the A plane and the B plane. It is necessary to set the rotation speed. Therefore, the exposure of the B surface cannot be started until the exposure of all the colors on the A surface is completed. Further, in FIG. 10, the time from the completion of the yellow exposure on the A side to the completion of the black exposure on the A side is indicated as T2.

  In FIG. 10, the time from the completion of exposure of all the colors on the A side to the start of exposure on the B side is shown as T3. This T3 is longer than T1 shown in FIG. In T3 shown in FIG. 10, the polygon motor 22 has a rotational speed different from the rotational speed of the polygon mirror 21 before the exposure of the B surface is started after the exposure of all the colors on the A surface is completed. Includes time to switch. That is, the time until the rotation speed of the polygon motor 22 is switched and stably rotated at the rotation speed after switching is included. On the other hand, at T1 in FIG. 9 where the front / back magnification correction is not performed, it is not necessary to switch the rotation speed of the polygon motor 22.

  Therefore, the PCS that is the time of the double-sided AB pitch when the front / back magnification correction is performed is different from the PS that is the time of the double-sided AB pitch when the front / back magnification correction is not shown in FIG. It is long by as much as. Even when the double-sided AB pitch is used when front / back magnification correction is performed, the PCS is relatively short when a monochrome image is formed. This is because the double-sided AB pitch of a monochrome image when performing front / back magnification correction is shorter than the color image by a PCS by T2.

  Next, based on each pitch described above, a comparison of time required when continuous duplex printing is performed in both the circulation duplex method and the alternate duplex method without performing front / back magnification correction will be described. First, FIG. 11 shows a case where continuous double-sided printing is performed on six sheets S by the circulation double-side method without performing front / back magnification correction. FIG. 11 shows a case where continuous duplex printing is performed while three sheets S are accommodated in the circulation path 65.

  As shown in FIG. 11, in the circulation double-sided method, after the images on the A side of the three sheets S1, S2, and S3 are continuously formed, the images on the B surface of these sheets S1, S2, and S3 are continuously formed. To do. Further, the A side image is continuously formed on the next three sheets S4, S5 and S6, and then the B side image of these sheets S4, S5 and S6 is continuously formed. Therefore, when continuous double-sided printing is performed on the six sheets S without using the front / back magnification correction, the respective pitches are as shown in FIG. It can be seen that the total number of times of each pitch is 4 times for PSB, 3 times for PS, and 4 times for PD.

  On the other hand, FIG. 12 shows a case where continuous duplex printing is performed on six sheets of paper S by the alternate duplex method without performing front / back magnification correction. FIG. 12 also shows a case where continuous duplex printing is performed while three sheets S are accommodated in the circulation path 65. As shown in FIG. 12, in the alternate double-sided method, the A side images of the three sheets S are continuously formed, and then the B side images and the A side images are alternately formed. In the alternate double-sided method, as shown in FIG. 12, the B-side images are continuously formed for the three sheets S that are accommodated in the circulation path 65 from the last sheet S6. Therefore, when continuous double-sided printing is performed on the six sheets S without using the front / back magnification correction, the respective pitches are as shown in FIG. It can be seen that the total number of times for each pitch is 2 for PSB, 7 for PS, and 2 for PD.

  Here, as described above, PS is shorter than PSB. Therefore, it can be seen that when the front / back magnification correction is not performed, the alternate duplex method can complete all image formation in a shorter time than the circulating duplex method. As shown in FIG. 11, in the circulation double-sided system, every time three sheets are accommodated in the circulation path 65, a PSB having a long pitch is required twice.

  Next, a description will be given of a comparison of time required when continuous duplex printing is performed in both the circulating duplex method and the alternate duplex method while performing front / back magnification correction. First, FIG. 13 shows a case where continuous double-sided printing is performed on six sheets of paper S by the circulating double-side method while performing front and back magnification correction. FIG. 13 also shows a case where continuous duplex printing is performed while three sheets S are accommodated in the circulation path 65. Therefore, as shown in FIG. 13, the order of image formation on the A and B sides of each sheet S is the same as in FIG.

  Further, when continuous double-sided printing is performed on the six sheets S with the circulation double-side method while performing front and back magnification correction, each pitch is as shown in FIG. That is, the double-sided AB pitch, which was PS in FIG. 11 when the front / back magnification correction is not performed, is PCS in FIG. 13 when the front / back magnification correction is performed. It can be seen that the total number of times for each pitch is 4 for PSB, 3 for PCS, and 4 for PD.

  On the other hand, FIG. 14 shows a case where continuous duplex printing is performed on six sheets of paper S by the alternate duplex method while performing front / back magnification correction. FIG. 14 also shows a case where continuous duplex printing is performed while three sheets S are accommodated in the circulation path 65. Therefore, as shown in FIG. 14, the order in which images are formed on the A and B sides of each sheet S is the same as in FIG. In addition, when continuous double-sided printing is performed on the six sheets S with the alternate double-sided method while correcting the front and back magnification, each pitch is as shown in FIG. That is, the double-sided AB pitch, which was PS in FIG. 12 when the front / back magnification correction is not performed, is PCS in FIG. 14 when the front / back magnification correction is performed. It can be seen that the total number of times for each pitch is 2 for PSB, 7 for PCS, and 2 for PD.

When the front and back magnification correction is performed, the circulation double-side method and the alternate double-side method are compared according to the total number of times of each pitch. It can be seen that all image formation can be completed in a short time.
2 x PCS> PSB + PD (1)
Further, when the following equation (2) obtained by reversing the inequality sign of equation (1) holds, it can be seen that the alternate double-sided method can complete all image formation in a shorter time.
2 x PCS <PSB + PD (2)
Furthermore, it can be seen that when 2 × PCS and PSB + PD are the same time, all image formation can be completed in the same time in both the double-sided circulation method and the alternating double-sided method. The relationship between the PCS, PSB, and PD is the same even when the number of sheets S related to continuous duplex printing and the number of sheets S accommodated in the circulation path 65 are different from the above.

  Next, a procedure for determining whether continuous double-sided printing in the image forming apparatus 1 is to be executed by the circulating double-sided method or the alternating double-sided method will be described with reference to the flowchart of FIG. FIG. 15 shows a case where a plurality of duplex printing jobs are input to the image forming apparatus 1.

  First, when a plurality of double-sided printing jobs are input to the image forming apparatus 1, the CPU 4 determines whether or not to perform front / back magnification correction in the print job (S101). When the front / back magnification correction is not performed (S101: NO), image formation is performed by the alternate double-sided method (S107). This is because when the front / back magnification correction is not performed, the alternate duplex method can complete all image formation in a shorter time than the circulating duplex method.

  On the other hand, when the front / back magnification correction is performed (S101: YES), the PSB (S102), PD (S103), and PCS (S104) are determined based on the photosensitive member pitch, system speed, length of the paper S to be used, and the like. Acquire each pitch. Then, it is determined whether or not the above-described equation (1) 2 × PCS> PSB + PD is satisfied based on each acquired pitch (S105). When Expression (1) is satisfied (S105: YES), image formation is performed by the circulating double-side method while correcting the front and back magnification (S106). This is because, when the front / back magnification correction is performed, when the formula (1) is established, the circulation double-side method can complete all image formation in a shorter time than the alternate double-side method.

  On the other hand, if equation (1) does not hold (S105: NO), image formation is performed by the alternate double-sided method while correcting the front and back magnification (S107). Here, when Expression (1) does not hold, there are cases where Expression (2) 2 × PCS <PSB + PD is satisfied and 2 × PCS and PSB + PD are the same. In this embodiment, when 2 × PCS and PSB + PD are the same, image formation is performed by the alternate double-sided method. However, when 2 × PCS and PSB + PD are the same, image formation may be performed by a circulating double-sided method. This is because image formation can be completed in the same time in both the circulation double-side method and the alternating double-side method. When the front / back magnification correction is performed, when the formula (2) is satisfied, the alternate duplex method can complete all image formation in a shorter time than the cyclic duplex method. Therefore, continuous duplex printing with high productivity can be performed.

  In addition, the time of each pitch such as PSB, PD, PCS can be calculated in advance for each size of the paper S used for image formation. This is because each pitch is determined from each value known in the design of the apparatus, such as the photosensitive member pitch and the system speed, and the length of the sheet S in the transport direction.

  Each pitch takes a longer time as the length of the paper S in the transport direction is longer. Among the pitches, the PSB tends to become longer as the length of the sheet S in the transport direction becomes longer than the other pitches. Therefore, the longer the length of the sheet S in the transport direction, the longer the time required to complete all image formation in the circulating duplex method than in the alternate duplex method. As described above, the circulation double-sided method requires more PSBs than the alternating double-sided method. Therefore, when the front / back magnification correction is performed, it is possible to determine whether to perform continuous double-sided printing according to the size of the paper S to be used, which is the cyclic double-sided method or the alternating double-sided method.

  FIG. 16 shows a procedure for determining whether to perform continuous double-sided printing according to the paper size by either the circulating double-sided method or the alternating double-sided method. As shown in FIG. 16, first, the CPU 4 determines whether or not to perform front / back magnification correction (S201). When the front / back magnification correction is not performed (S201: NO), image formation is performed by the alternate double-sided method (S205).

  On the other hand, when the front / back magnification correction is performed (S201: YES), the length of the paper S in the transport direction is acquired from the size of the paper S to be used (S202). Further, it is determined whether or not the acquired paper length is less than a predetermined threshold value for the paper length (S203). The threshold for the paper length is determined to be a value that allows all image formation to be completed in a shorter time in the recurring duplex method than in the alternate duplex method if the paper length of the paper S to be used is less than the threshold. Yes. Note that the threshold for the sheet length may be stored in advance in the nonvolatile memory 5. Further, when the paper feed tray 51 can feed a plurality of sizes of sheets S, a threshold value may be stored in the nonvolatile memory 5 for each sheet length of those sizes.

  Therefore, if the acquired sheet length is less than the threshold value (S203: YES), image formation is performed by the circulating double-side method while performing front / back magnification correction (S204). On the other hand, if the acquired sheet length is equal to or greater than the threshold (S203: NO), image formation is performed by the alternate double-side method while performing front / back magnification correction (S205). Thus, continuous double-sided printing with high productivity can be performed also by the procedure shown in FIG.

  Further, as described above with reference to FIG. 10, in continuous double-sided printing when performing front / back magnification correction, the PCS is shorter when forming a monochrome image than when forming a color image. The longer the PCS, the longer the time required to complete all image formation in the alternate duplex method than in the cyclic duplex method. This is because the number of times of PCS is larger in the alternate duplex method than in the circulation duplex method. For this reason, depending on the device configuration, when performing front / back magnification correction, all of the time is shorter than the other method, using the circular double-side method when forming a color image, and the alternating double-side method when forming a monochrome image. In some cases, image formation can be completed.

An example of such a device configuration is shown below.
Paper size: A4
Paper transport direction: Sideways system speed: 310mm / s
PCS (color): 2000ms
(Monochrome): 1100ms
PSB: 1550ms
PD: 1200ms

  That is, in the continuous double-sided printing in the case where the front and back magnification correction is performed for the above example, if the image to be formed is a color image, Expression (1) 2 × PCS> PSB + PD is established. On the other hand, when the image to be formed is a monochrome image, Expression (2) 2 × PCS <PSB + PD is established.

  Therefore, in the configuration as in the above example, when the front / back magnification correction is performed, it is alternated with the cyclic duplex method depending on whether the color mode for forming a color image or the monochrome mode for forming a monochrome image. It is also possible to determine which of the two-sided methods is used for continuous double-sided printing.

  FIG. 17 shows the determination procedure. As shown in FIG. 17, first, the CPU 4 determines whether to perform front / back magnification correction (S301). When the front / back magnification correction is not performed (S301: NO), image formation is performed by the alternate double-sided method (S304).

  On the other hand, when performing front / back magnification correction (S301: YES), it is determined whether or not the color mode is for forming a color image (S302). In the case of the color mode (S302: YES), image formation is performed by the circulating double-side method while correcting the front / back magnification (S303). On the other hand, in the monochrome mode for forming a monochrome image (S302: NO), image formation is performed by the alternate double-sided method while performing front / back magnification correction (S304). Thus, continuous double-sided printing with high productivity can be performed also by the procedure shown in FIG. The case where 2 × PCS and PSB + PD are the same may be included in either the color mode or the monochrome mode. In this case, the productivity of image formation is the same regardless of whether the continuous duplex printing is performed by either the circulating duplex method or the alternate duplex method.

  Further, in the above description, the same number of sheets S are stored in the circulation path 65 in both the circulation double-side method and the alternating double-side method. Here, in this embodiment, in the circulation double-sided system, the image formation is performed with the maximum number of sheets S accommodated in the circulation path 65 as the maximum number of sheets S that can be accommodated in the circulation path 65, so that the productivity can be maximized. it can. In this embodiment, front / back magnification correction is always performed when image formation is performed by the circulation double-sided method. For this reason, as the number of sheets S accommodated in the circulation path 65 increases, the frequency of switching the rotation speed of the polygon motor 22 that causes a reduction in image formation productivity can be reduced.

  On the other hand, in the alternate double-sided system, productivity may be increased by performing image formation with a number smaller than the maximum number of sheets S that can be accommodated in the circulation path 65. FIG. 18 shows a case where continuous duplex printing is performed on six sheets of paper S by the alternate duplex method without performing front / back magnification correction. FIG. 18 shows a case where continuous duplex printing is performed while two sheets S are accommodated in the circulation path 65. This is different from FIG. 12 described as the case where the three sheets S are accommodated in the circulation path 65.

  In the alternate double-sided method in which two sheets S are accommodated in the circulation path 65, images of the A side of the two sheets S are continuously formed, and then the B side image and the A side image are alternately formed. To form. In the example of FIG. 18, the B-side images are continuously formed for the two sheets S, which are the number of sheets S accommodated in the circulation path 65 from the last sheet S6. Therefore, when continuous duplex printing is performed by the alternate duplex method while accommodating two sheets S in the circulation path 65 without performing front / back magnification correction on the six sheets S, each pitch is shown in FIG. As shown in FIG. It can be seen that the total number of times for each pitch is 1 for PSB, 9 for PS, and 1 for PD.

  On the other hand, in FIG. 12, which is described as containing three sheets S in the circulation path 65, the total number of each pitch is PSB twice, PS seven times, and PD twice. As described above, PS is shorter than PSB. Therefore, it can be seen that productivity can be increased by performing image formation by the alternate double-side method with a number smaller than the maximum number of sheets S that can be accommodated in the circulation path 65.

  However, in continuous duplex printing using the alternate duplex method, the productivity is not necessarily increased as the number of sheets S accommodated in the circulation path 65 is decreased. FIG. 19 shows the circulation pitch PA, which is the time required for the sheet S to make one round of the circulation path 65. If the circulation pitch PA is too long, continuous duplex printing by the alternate duplex method may be performed by reducing the number of sheets S accommodated in the circulation path 65, thereby reducing productivity.

  FIG. 20 shows a case where continuous double-sided printing is performed on six sheets S by the alternate double-sided method without performing front / back magnification correction when the circulation pitch PA is too long. 20 also shows a case where continuous duplex printing is performed while two sheets S are accommodated in the circulation path 65, as in FIG. Further, FIG. 20 shows the circulating pitch PA at a position where the leading edge is aligned with the leading edge of the A surface of each sheet S. Further, in FIG. 20, the leading edge of the B surface of each sheet S is shown to be behind the trailing edge of the circulation pitch PA shown for the A surface of the sheet S.

  As shown in FIG. 20, before the image formation on the B side of the sheets S1, S3, S5, and S6, the waiting times T4, T5, T6, and T7 caused by the circulation pitch PA after the previous image formation, respectively. Has occurred. That is, it can be seen that the circulation pitch PA is rate-limiting because the circulation pitch PA is too long. Then, as shown in FIG. 20, when a waiting time due to the circulation pitch PA becoming rate-limiting occurs, the image forming productivity can be increased when the sheet S accommodated in the circulation path 65 is large. There are things you can do.

  Considering the above, when continuous duplex printing is performed by the alternate duplex method, the number of sheets S accommodated in the circulation path 65 is set to the optimum accommodated number that is the most image forming productivity. preferable. The optimum number of sheets S can be obtained from each value known in the design, such as the length of the circulation path 65 and the system speed, and the sheet length which is the length in the transport direction of the sheet S to be used. Therefore, it is preferable to store the obtained optimal number of sheets S of paper S in the nonvolatile memory 5 in advance. When continuous duplex printing is performed by the alternate duplex method, image formation may be performed while accommodating the optimum number of sheets S stored in the nonvolatile memory 5 in the circulation path 65. In addition, when the paper feed tray 51 can feed a plurality of sizes of paper S, the optimum storage multiple may be stored in the nonvolatile memory 5 for each paper length of those sizes.

  As described in detail above, in the image forming apparatus 1 of the present embodiment, an electrostatic latent image is formed on the photoconductor 11 of each color by one polygon mirror 21. In double-sided printing, the magnification of the polygon mirror 21 in the sub-scanning direction can be corrected by changing the rotation speed of the polygon motor 22 so that the magnification in the A-side and B-side of the paper S is different. . When continuous double-sided printing is performed while correcting the front and back magnification, if the formula (1) 2 × PCS> PSB + PD is satisfied, image formation is performed by the cyclic double-side method. On the other hand, when the formula (2) 2 × PCS <PSB + PD is satisfied, image formation is performed by the alternate double-sided method. Thereby, the image forming apparatus 1 with high image forming productivity in continuous duplex printing is realized.

  Note that this embodiment is merely an example, and does not limit the present invention. Therefore, the present invention can naturally be improved and modified in various ways without departing from the gist thereof. For example, the present invention can be applied not only to a color printer but also to a copying machine. Further, for example, the present invention can be applied to a direct transfer type image forming apparatus in which a formed toner image is directly transferred onto a sheet from a photoreceptor, such as having a sheet carrying belt instead of an intermediate transfer belt.

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 2 ... Engine part 4 ... CPU
DESCRIPTION OF SYMBOLS 11 ... Photoconductor 13 ... Developing device 20 ... Exposure device 21 ... Polygon mirror 30 ... Intermediate transfer belt 40 ... Secondary transfer roller 61 ... Conveying route 64 ... Double-sided route 65 ... Circulating route 70 ... Fixing device N1 ... Secondary transfer nip N2 ... Fixing nip S ... Paper

Claims (6)

An image carrier for each color carrying a single color toner image while rotating during image formation;
An exposure unit for performing an exposure process of writing an electrostatic latent image on all the image carriers by a single polygon mirror;
A developing portion for each color that forms a single-color toner image by applying toner to a location after the exposure processing in the image carrier for each color;
A transfer portion for transferring the formed toner image to a sheet at a transfer nip; and
A paper feeding unit for feeding a sheet to the transfer nip;
A fixing unit that performs a fixing process of the toner image on the sheet by heating the sheet that has passed through the transfer nip at the fixing nip; and
A reverse circulation path for reversing the front and back of the sheet that has passed through the fixing nip and leading to the entrance side of the transfer nip;
An image forming apparatus having a discharge unit for discharging a sheet on which image formation has been completed.
By changing the rotation speed of the polygon mirror, the formation of the single-color toner image on the second surface of the sheet on the image carrier is more effective than the formation of the single-color toner image on the first surface in the circumferential direction of the image carrier. Has a control unit that performs front / back magnification correction to be performed at a reduced magnification.
The controller is
When performing continuous double-sided printing that forms images on both sides of a plurality of sheets while correcting the front and back magnification,
PCS: From the start of writing of an electrostatic latent image on the image carrier in the head order related to the toner image of the image formed earlier when image formation on the first surface and image formation on the second surface are performed continuously , The writing of the electrostatic latent image on all the image carriers related to the toner image of the image formed earlier is completed, and the change of the rotational speed of the polygon mirror is completed, and the image formed later The time until the start of writing of the electrostatic latent image on the image carrier in the head order of the toner image,
PSB: Both of the two sheets on which images are formed on both sides, when the two sheets of the first sheet are continuously formed, the first sheet enters the fixing nip and the subsequent sheet is fixed. Time until rushing into the nip, and PD: Both of the two sheets on which images are formed on both sides, the previous sheet rushes into the fixing nip when two images on the second side are continuously formed The time from when the subsequent sheet enters the fixing nip,
Depending on the relationship
2 x PCS> PSB + PD
If is satisfied,
Storing, within the reverse circulation path, a plurality of sheets having the toner image on the first surface within the maximum number of sheets that can be accommodated within the reverse circulation path;
For all the sheets accommodated in the reversal circulation path, image formation is performed by a circulation double-side method by repeating fixing of the toner image on the second surface and discharging from the discharge portion,
2 x PCS <PSB + PD
If is satisfied,
While the maximum number of sheets or the number of circulation sheets equal to or less than the number of sheets on which the first toner image has been fixed is accommodated in the reverse circulation path,
Each time one sheet on which the toner image on the second surface is fixed is discharged from the discharge unit among the sheets stored in the reverse circulation path,
A new sheet is fed from the sheet feeding unit, and the toner image on the first surface is transferred and fixed, and the image is formed by an alternating double-sided system by repeatedly storing the toner image in the reverse circulation path.
2 x PCS = PSB + PD
If is satisfied,
An image forming apparatus characterized in that image formation is performed by any one of the circulation duplex method and the alternating duplex method. An image carrier for each color carrying a single color toner image while rotating during image formation;
An exposure unit for performing an exposure process of writing an electrostatic latent image on all the image carriers by a single polygon mirror;
A developing portion for each color that forms a single-color toner image by applying toner to a location after the exposure processing in the image carrier for each color;
A transfer portion for transferring the formed toner image to a sheet at a transfer nip; and
A paper feeding unit for feeding a sheet to the transfer nip;
A fixing unit that performs a fixing process of the toner image on the sheet by heating the sheet that has passed through the transfer nip at the fixing nip; and
A reverse circulation path that reverses the front and back of the sheet that has passed through the fixing nip and guides it to the entrance side of the transfer nip;
An image forming apparatus having a discharge unit for discharging a sheet on which image formation has been completed.
By changing the rotation speed of the polygon mirror, the formation of the single-color toner image on the second surface of the sheet on the image carrier is more effective than the formation of the single-color toner image on the first surface in the circumferential direction of the image carrier. A control unit that performs front / back magnification correction to be performed at a reduced magnification,
A threshold storage unit storing a predetermined threshold for the length of the sheet in the conveyance direction,
The controller is
When performing continuous double-sided printing that forms images on both sides of a plurality of sheets while correcting the front and back magnification,
If the length of the sheet in the conveyance direction is less than the threshold,
Storing, within the reverse circulation path, a plurality of sheets having the toner image on the first surface within the maximum number of sheets that can be accommodated within the reverse circulation path;
For all the sheets accommodated in the reversal circulation path, image formation is performed by a circulation double-side method by repeating fixing of the toner image on the second surface and discharging from the discharge portion,
If the length of the sheet in the conveyance direction is equal to or greater than the threshold,
While the maximum number of sheets or the number of circulation sheets equal to or less than the number of sheets on which the first toner image has been fixed is accommodated in the reverse circulation path,
Each time one sheet on which the toner image on the second surface is fixed is discharged from the discharge unit among the sheets stored in the reverse circulation path,
A new sheet is fed from the sheet feeding unit to transfer and fix the toner image on the first side, and to store in the reverse circulation path, thereby forming an image by an alternating double-side method. An image forming apparatus. An image carrier for each color carrying a single color toner image while rotating during image formation;
An exposure unit for performing an exposure process of writing an electrostatic latent image on all the image carriers by a single polygon mirror;
A developing portion for each color that forms a single-color toner image by applying toner to a location after the exposure processing in the image carrier for each color;
A transfer portion for transferring the formed toner image to a sheet at a transfer nip; and
A paper feeding unit for feeding a sheet to the transfer nip;
A fixing unit that performs a fixing process of the toner image on the sheet by heating the sheet that has passed through the transfer nip at the fixing nip; and
A reverse circulation path that reverses the front and back of the sheet that has passed through the fixing nip and guides it to the entrance side of the transfer nip;
An image forming apparatus having a discharge unit for discharging a sheet on which image formation has been completed.
By changing the rotation speed of the polygon mirror, the formation of the single-color toner image on the second surface of the sheet on the image carrier is more effective than the formation of the single-color toner image on the first surface in the circumferential direction of the image carrier. Has a control unit that performs front / back magnification correction to be performed at a reduced magnification.
The controller is
When performing continuous double-sided printing that forms images on both sides of a plurality of sheets while correcting the front and back magnification,
If the color mode for forming a color image is selected,
Storing, within the reverse circulation path, a plurality of sheets having the toner image on the first surface within the maximum number of sheets that can be accommodated within the reverse circulation path;
For all the sheets accommodated in the reversal circulation path, image formation is performed by a circulation double-side method by repeating fixing of the toner image on the second surface and discharging from the discharge portion,
If the monochrome mode for forming a monochrome image is selected,
While the maximum number of sheets or the number of circulation sheets equal to or less than the number of sheets on which the first toner image has been fixed is accommodated in the reverse circulation path,
Each time one sheet on which the toner image on the second surface is fixed is discharged from the discharge unit among the sheets stored in the reverse circulation path,
A new sheet is fed from the sheet feeding unit to transfer and fix the toner image on the first side, and to store in the reverse circulation path, thereby forming an image by an alternating double-side method. An image forming apparatus. The image forming apparatus according to claim 3,
Color PCS: When the first side image formation and the second side image formation are successively performed by the multi-layer toner image formed by superimposing single-color toner images of the respective colors, the leading order related to the multi-layer toner image of the first image to be formed The writing of the electrostatic latent image on all the image carriers related to the multilayer toner image of the previously formed image has been completed since the beginning of the writing of the electrostatic latent image on the image carrier. The time until the start of electrostatic latent image writing on the image carrier in the head order relating to the multilayer toner image of the image to be formed later,
Monochrome PCS: The image related to the single-layer toner image of the first image formed when the first-side image formation and the second-side image formation are continuously performed by the single-layer toner image of the single color toner image. From the start of writing of the electrostatic latent image on the carrier, the writing of the electrostatic latent image on the image carrier relating to the single-layer toner image of the image formed earlier is completed, and the rotation speed of the polygon mirror is further reduced. The time from the completion of the change until the start of writing of the electrostatic latent image on the image carrier relating to the single-layer toner image of the image to be formed later,
PSB: Both of the two sheets on which images are formed on both sides, when the two sheets of the first sheet are continuously formed, the first sheet enters the fixing nip and the subsequent sheet is fixed. Time until rushing into the nip, and PD: Both of the two sheets on which images are formed on both sides, the previous sheet rushes into the fixing nip when two images on the second side are continuously formed The time from when the subsequent sheet enters the fixing nip,
Depending on the relationship
In the color mode,
2 x Color PCS> PSB + PD
In the monochrome mode,
2 x Monochrome PCS <PSB + PD
An image forming apparatus characterized by satisfying the above relationships. The image forming apparatus according to claim 1, wherein:
The controller is
An image forming apparatus, wherein when performing the continuous double-sided printing without performing the front-back magnification correction, image formation is performed by the alternate double-sided method. The image forming apparatus according to any one of claims 1 to 5,
Optimum storing the optimum circulation number that is the circulation number equal to or less than the maximum number of sheets that can be completed in the shortest time for continuous double-sided printing by the alternate duplex method for a predetermined number of sheets or more. It has a circulation number storage unit,
When the controller performs the continuous duplex printing by the alternate duplex method,
Obtaining the optimum circulation number from the optimum circulation number storage unit;
An image forming apparatus for performing image formation with the circulation number as the acquired optimum circulation number.

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