Image forming apparatus and adjustment method

Abstract

To prevent an image magnification in a conveyance direction from being easily changed regardless of an image area ratio of a toner image transferred to a surface of a sheet conveyed to a transfer nip.SOLUTION: An image forming apparatus is provided with adjustment means (motor 92, moving mechanism 94) that adjust at least one of a difference in a linear velocity of a secondary transfer belt 71 and an intermediate transfer belt 8 at a secondary transfer nip (transfer nip) and a contact pressure of the secondary transfer belt 71 on the intermediate transfer belt 8 at the secondary transfer nip. The image forming apparatus is further provided with a control unit 90 (control means) that, on the basis of a difference in image magnification in a conveyance direction of a toner image transferred to a surface of a sheet P conveyed to the secondary transfer nip, controls the adjustment means (motor 92, moving mechanism 94) to reduce a difference in an image magnification.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile machine, or a combination machine thereof, and an adjustment method performed therein. [Background technology]

[0002] Conventionally, in image forming devices such as copiers and printers, a transfer nip has been formed by contacting a transfer rotating body such as a transfer belt or a transfer roller with an image carrier such as an intermediate transfer belt or a photosensitive drum (see, for example, Patent Document 1).

[0003] More specifically, in the image forming apparatus in Patent Document 1 and the like, toner images formed on a plurality of photosensitive drums are primarily transferred onto the surface of an intermediate transfer belt (image carrier) in a superimposed manner. The toner images carried on the intermediate transfer belt are then secondarily transferred onto a sheet of paper transported to the position of a secondary transfer nip (transfer nip). The sheet of paper onto which the toner images have been secondarily transferred is then transported towards a fixing device, where a fixing process is carried out, and the sheet is then discharged from the main body of the image forming apparatus.

[0004] On the other hand, Patent Document 1 discloses a technique for adjusting the image position on the front side and also adjusting the image position on the back side during double-sided printing. Summary of the Invention [Problem to be solved by the invention]

[0005] In a conventional image forming apparatus, when a toner image formed on an image carrier is transferred to the surface of a sheet transported to a transfer nip, the image magnification in the transport direction of the transferred image may change depending on the image area ratio of the toner image to be transferred. Therefore, even if characteristic values ​​such as the rotation speed of the transfer rotor are determined so that the image magnification of the transferred image is equal to or less than a threshold value by focusing on a specific image area ratio, when a toner image with a different image area ratio is transferred to the sheet, the image magnification may not be equal to or less than the threshold value, and an image that is stretched or expanded in the transport direction may be formed.

[0006] This invention has been made to solve the above-mentioned problems, and aims to provide an image forming apparatus and an adjustment method in which the image magnification in the transport direction is less likely to change regardless of the image area ratio of the toner image transferred to the surface of the sheet transported to the transfer nip. [Means for solving the problem]

[0007] The image forming apparatus of the present invention includes an image carrier on which a toner image is carried, a transfer rotating body that contacts the image carrier to form a transfer nip and transfers the toner image on the image carrier to a sheet transported to the transfer nip, a relative linear speed difference between the transfer rotating body and the image carrier at the transfer nip, and a linear speed difference between the transfer rotating body and the image carrier at the transfer nip. of and a contact pressure adjusting means for adjusting at least one of the contact pressure and the contact pressure of the sheet conveyed to the transfer nip. No. 1 Transferred to the surface of the sheet First image area ratio Image magnification in the conveying direction of the toner image and an image magnification in a conveying direction of a toner image conveyed to the transfer nip and transferred onto a surface of a second sheet, the toner image having a second image area ratio higher than the first image area ratio. and a control means for controlling the adjustment means based on the difference so that the difference in the image magnification becomes smaller. Effect of the Invention

[0008] According to the present invention, it is possible to provide an image forming apparatus and an adjustment method in which the image magnification in the transport direction is unlikely to change regardless of the image area ratio of the toner image transferred to the surface of the sheet transported to the transfer nip. [Brief description of the drawings]

[0009] [Figure 1] 1 is an overall configuration diagram showing an image forming apparatus according to an embodiment of the present invention; [Diagram 2] FIG. 2 is an enlarged view showing a part of an image forming unit; [Diagram 3] FIG. 2 is a schematic diagram showing an intermediate transfer belt and its vicinity. [Figure 4] FIG. 2 is a diagram illustrating a configuration of a secondary transfer device. [Diagram 5] FIG. 1A is a diagram showing an image formed on the front surface of a sheet in an adjustment mode; and FIG. 1B is a diagram showing an image formed on the back surface of the sheet. [Figure 6] 10 is a flowchart showing a control in an adjustment mode. [Figure 7] 1A is a graph showing the relationship between the linear velocity difference and the image magnification difference in the secondary transfer nip, and FIG. 1B is a graph showing the relationship between the contact pressure and the image magnification difference in the secondary transfer nip. [Figure 8] 2 is a schematic top view showing a line sensor and a sheet on which a gradation image pattern is formed. FIG. [Figure 9] 13 is a flowchart showing control in an adjustment mode in the first modification. [Figure 10] 13 is a flowchart showing control in an adjustment mode in Modification 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and the duplicated description will be appropriately simplified or omitted.

[0011] First, the overall configuration and operation of an image forming apparatus 100 will be described with reference to FIGS. FIG. 1 is a diagram showing the configuration of a printer as an image forming apparatus, and FIG. 2 is an enlarged view showing a part of the image forming section. 1, an intermediate transfer belt 8 (intermediate transfer body) serving as an image carrier is provided in the center of the image forming apparatus main body 100. In addition, imaging units 6Y, 6M, 6C, and 6K corresponding to each color (yellow, magenta, cyan, and black) are arranged in parallel to face the intermediate transfer belt 8.

[0012] 2, the imaging unit 6Y corresponding to yellow is composed of a photoconductor drum 1Y as a photoconductor, a charging unit 4Y, a developing unit 5Y, a cleaning unit 2Y, a lubricant supplying device 3, a discharging unit, etc., which are arranged around the photoconductor drum 1Y. Then, an image forming process (charging process, exposure process, developing process, transfer process, cleaning process, discharging process) is performed on the photoconductor drum 1Y, and a yellow image is formed on the photoconductor drum 1Y.

[0013] The other three image forming units 6M, 6C, and 6K have almost the same configuration as the image forming unit 6Y corresponding to yellow, except for the toner colors used, and form images corresponding to the respective toner colors. In the following, the explanation of the other three image forming units 6M, 6C, and 6K will be omitted as appropriate, and only the image forming unit 6Y corresponding to yellow will be explained.

[0014] 2, the photoconductor drum 1Y is rotated counterclockwise by a drive motor, and the surface of the photoconductor drum 1Y is uniformly charged at the position of the charging unit 4Y (charging step). Thereafter, the surface of the photosensitive drum 1Y reaches the irradiation position of the laser light L emitted from the exposure unit 7, and an electrostatic latent image corresponding to yellow is formed by exposure scanning in the width direction (the direction perpendicular to the paper surface of Figures 1 and 2, which is the main scanning direction) at this position (this is the exposure process).

[0015] Thereafter, the surface of the photoconductor drum 1Y reaches a position facing the developing unit 5Y, where the electrostatic latent image is developed to form a yellow toner image (developing step). Thereafter, the surface of the photoconductor drum 1Y reaches a position facing the intermediate transfer belt 8 and the primary transfer roller 9Y, and the toner image formed on the surface of the photoconductor drum 1Y at this position is primarily transferred onto the surface of the intermediate transfer belt 8 (the primary transfer process). At this time, a small amount of untransferred toner remains on the photoconductor drum 1Y.

[0016] Thereafter, the surface of the photoreceptor drum 1Y reaches a position facing the cleaning unit 2Y, and the untransferred toner remaining on the photoreceptor drum 1Y at this position is collected into the cleaning unit 2Y by the cleaning blade 2a (cleaning process). Here, a lubricant supplying device 3 (photoconductor lubricant supplying device) consisting of a lubricant supplying roller 3a, a solid lubricant 3b, a compression spring 3c, etc. is installed inside the cleaning section 2Y. The lubricant supplying roller 3a, which rotates clockwise in FIG. 2, scrapes off the lubricant little by little from the solid lubricant 3b, and the lubricant is supplied to the surface of the photoconductor drum 1Y by the lubricant supplying roller 3a. Finally, the surface of the photoconductor drum 1Y reaches a position facing a charge removing unit, where the residual potential on the photoconductor drum 1Y is removed. Thus, a series of image forming processes carried out on the photosensitive drum 1Y is completed.

[0017] The above-mentioned image forming process is performed in the other image forming units 6M, 6C, and 6K in the same manner as in the yellow image forming unit 6Y. That is, laser light L based on image information is irradiated from the exposure unit 7 arranged above the image forming units toward the photosensitive drums 1M, 1C, and 1K of the image forming units 6M, 6C, and 6K. In detail, the exposure unit 7 emits laser light L from a light source, and irradiates the laser light L onto the photosensitive drums via multiple optical elements while scanning the laser light L with a polygon mirror that is rotated. Note that the exposure unit 7 may be one in which multiple LEDs are arranged in the width direction. Thereafter, the toner images of each color formed on the photoconductor drums 1M, 1C, and 1K through the developing process by the developing units 5M, 5C, and 5K are primarily transferred onto the intermediate transfer belt 8 in a superimposed manner. In this way, a color image is formed on the intermediate transfer belt 8.

[0018] Here, the intermediate transfer belt 8 serving as an image carrier is stretched and supported by a plurality of roller members 16 to 22, 80, and is moved endlessly in the direction of the arrow in FIG. 3 by the rotational drive of one roller member (drive roller 16) by a drive motor Mt1 (see FIG. 3). The four primary transfer rollers 9Y, 9M, 9C, and 9K form primary transfer nips by sandwiching the intermediate transfer belt 8 between the photoconductor drums 1Y, 1M, 1C, and 1K, respectively. A transfer voltage (primary transfer bias) having a polarity opposite to that of the toner is applied to the primary transfer rollers 9Y, 9M, 9C, and 9K. Then, the intermediate transfer belt 8 travels in the direction of the arrow and passes through the primary transfer nips of the primary transfer rollers 9Y, 9M, 9C, and 9K in sequence. In this way, the toner images of each color on the photoconductor drums 1Y, 1M, 1C, and 1K are primarily transferred onto the surface of the intermediate transfer belt 8 in a superimposed manner (this is the primary transfer process).

[0019] Thereafter, the intermediate transfer belt 8 onto which the toner images of each color are superimposed and primarily transferred reaches a position facing the secondary transfer belt 71 as a transfer rotator. At this position, a secondary transfer opposing roller 80 sandwiches the intermediate transfer belt 8 and the secondary transfer belt 71 between itself and the secondary transfer roller 72 to form a secondary transfer nip (transfer nip). The four-color toner images formed on the intermediate transfer belt 8 are then secondarily transferred onto a sheet P such as paper conveyed to the position of this secondary transfer nip (secondary transfer process). At this time, untransferred toner that has not been transferred to the sheet P remains on the intermediate transfer belt 8.

[0020] Thereafter, the intermediate transfer belt 8 reaches the position of the intermediate transfer cleaning unit 10. At this position, any deposits such as untransferred toner adhering to the surface of the intermediate transfer belt 8 are removed. Furthermore, the intermediate transfer belt 8 reaches the position of a lubricant supplying device 30 serving as an intermediate transfer lubricant supplying device. At this position, a lubricant is supplied to the surface of the intermediate transfer belt 8. Thus, a series of transfer processes carried out on the intermediate transfer belt 8 is completed.

[0021] Here, referring to FIG. 1, the sheet P transported to the position of the secondary transfer nip is transported from a paper feed section 26 arranged below the device main body 100 via a paper feed roller 27, a pair of registration rollers 28, etc. More specifically, a plurality of sheets P such as transfer paper are stored in a stack in the paper feed section 26. When the paper feed roller 27 is rotated counterclockwise in FIG. 1, the topmost sheet P is fed toward between the rollers of the pair of registration rollers 28 via the first transport path K1.

[0022] The sheet P conveyed to the registration roller pair 28 (timing roller pair) is temporarily stopped at the roller nip position of the registration roller pair 28, which has stopped rotating. Then, in synchronization with the color image on the intermediate transfer belt 8, the registration roller pair 28 is rotated and the sheet P is conveyed toward the secondary transfer nip. In this way, the desired color image is transferred onto the sheet P.

[0023] Thereafter, the sheet P onto which the color image has been transferred at the position of the secondary transfer nip (transfer nip) is transported by the secondary transfer belt 71, and after being separated from the secondary transfer belt 71, is transported by the transport belt 60 to the position of the fixing unit 50. Then, at this position, the color image transferred to the surface is fixed onto the sheet P by heat and pressure from the fixing belt and pressure roller (fixing process). Thereafter, the sheet P passes through the second transport path K2 and is discharged to the outside of the apparatus by the pair of discharge rollers. The sheet P discharged to the outside of the apparatus by the pair of discharge rollers is sequentially stacked on a stack unit as an output image. In this way, a series of image forming processes (printing operations) in the image forming apparatus is completed. In addition, a line sensor 95 used in the adjustment mode is installed downstream of the fixing unit 50 and upstream of the branching point between the second transport path K2 and the third transport path K3, but this will be described in detail later.

[0024] As shown in FIG. 1, the image forming apparatus 100 in this embodiment is provided with a double-sided conveying device 40 that conveys the sheet P toward the secondary transfer nip (transfer nip) in order to transfer the toner image on the intermediate transfer belt 8 to the back side of the sheet P after the toner image has been transferred to the front side at the secondary transfer nip. Specifically, when a "duplex printing mode" is selected in which printing is performed on both sides (front and back) of the sheet P, the sheet P after the fixing process on the front side is not discharged as it is as when the above-mentioned "single-sided printing mode" is selected, but is guided to the third conveying path K3 in the duplex conveying device 40, and after its conveying direction is reversed, it is conveyed again toward the position of the secondary transfer nip (secondary transfer device 70) via the fourth conveying path K4. Then, at the position of the secondary transfer nip, an image is formed (secondary transfer) on the back side of the sheet P by the image forming process (image forming operation) similar to that described above, and then the sheet undergoes a fixing process in the fixing unit 50 and is discharged from the image forming apparatus main body 100 via the second conveying path K2.

[0025] Next, the configuration and operation of the developing section 5Y (developing device) in the image forming section will be described in more detail with reference to FIG. The developing unit 5Y is composed of a developing roller 51Y facing the photosensitive drum 1Y, a doctor blade 52Y facing the developing roller 51Y, two conveying screws 55Y arranged in the developer container, and a concentration detection sensor 56Y that detects the toner concentration in the developer. The developing roller 51Y is composed of a magnet fixed inside and a sleeve that rotates around the magnet. The developer container contains a two-component developer G consisting of a carrier and a toner.

[0026] The developing section 5Y thus configured operates as follows. The sleeve of the developing roller 51Y rotates in the direction of the arrow in FIG. 2. The developer G carried on the developing roller 51Y by the magnetic field formed by the magnet moves on the developing roller 51Y as the sleeve rotates. Here, the developer G in the developing unit 5Y is adjusted so that the ratio of toner in the developer G (toner concentration) falls within a predetermined range. Specifically, when a toner concentration sensor installed in the developing unit 5Y detects a low toner concentration, new toner is replenished from the toner container 58 into the developing unit 5Y so that the toner concentration falls within the predetermined range. Thereafter, the toner supplied from the toner container 58 to the developer storage section circulates between the two separated developer storage sections (movement in the direction perpendicular to the paper surface of FIG. 2) while being mixed and stirred with the developer G by the two transport screws 55Y. The toner in the developer G is attracted to the carrier due to frictional charging with the carrier, and is carried on the developing roller 51Y together with the carrier by the magnetic force formed on the developing roller 51Y.

[0027] The developer G carried on the developing roller 51Y is transported in the direction of the arrow in FIG. 2 and reaches the position of the doctor blade 52Y. Then, the developer G on the developing roller 51Y is adjusted to an appropriate amount at this position, and then transported to a position facing the photoconductor drum 1Y (the developing area). Then, the toner is attracted to the latent image formed on the photoconductor drum 1Y by the electric field formed in the developing area. Thereafter, the developer G remaining on the developing roller 51Y reaches above the developer storage section as the sleeve rotates, and is separated from the developing roller 51Y at this position. The toner container 58 is detachably (replaceably) installed in the developing unit 5Y (image forming apparatus 100). When the new toner contained therein becomes empty, the toner container 58 is removed from the developing unit 5Y (image forming apparatus 100) and replaced with a new toner.

[0028] Next, the intermediate transfer belt device according to the present embodiment will be described in detail with reference to FIG. Referring to FIG. 3, the intermediate transfer belt device is composed of an intermediate transfer belt 8 as an image carrier, four primary transfer rollers 9Y, 9M, 9C, 9K, a drive roller 16, a driven roller 17, a pre-transfer roller 18, a tension roller 19, a cleaning opposing roller 20, a lubricant opposing roller 21, a backup roller 22, an intermediate transfer cleaning section 10, a lubricant supplying device 30 as an intermediate transfer lubricant supplying device, a secondary transfer opposing roller 80, a secondary transfer device 70, etc.

[0029] The intermediate transfer belt 8 contacts four photoconductor drums 1Y, 1M, 1C, and 1K, which respectively carry toner images of each color, to form a primary transfer nip. The intermediate transfer belt 8 is stretched and supported mainly by eight roller members (a drive roller 16, a driven roller 17, a pre-transfer roller 18, a tension roller 19, a cleaning opposed roller 20, a lubricant opposed roller 21, a backup roller 22, and a secondary transfer opposed roller 80).

[0030] In this embodiment, the intermediate transfer belt 8 is a belt member made of a single layer or multiple layers of PVDF (vinyldiene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), or the like, with a conductive material such as carbon black dispersed therein. 6 ~10 13 Ωcm, the surface resistivity of the back side of the belt is 10 7 ~10 13 The intermediate transfer belt 8 is adjusted to have a thickness in the range of 20 to 200 μm. In this embodiment, the intermediate transfer belt 8 is adjusted to have a thickness of about 60 μm and a volume resistivity of 10 9 It is set to about Ωcm. In this embodiment, an elastic layer made of a rubber material or the like is provided as an intermediate layer on the intermediate transfer belt 8. This makes it difficult for the transferability at the secondary transfer nip to decrease even when a sheet P having an uneven surface is passed through the intermediate transfer belt 8. If necessary, a release layer can be coated on the surface of the intermediate transfer belt 8. In this case, the material used for the coating can be a fluororesin such as ETFE (ethylene-tetrafluoroethylene copolymer), PTFE (polytetrafluoroethylene), PVDF (vinyldiene fluoride), PEA (perfluoroalkoxy fluororesin), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PVF (vinyl fluoride), but is not limited thereto.

[0031] The primary transfer rollers 9Y, 9M, 9C, and 9K are in contact with the corresponding photoconductor drums 1Y, 1M, 1C, and 1K, respectively, via the intermediate transfer belt 8. In more detail, the yellow transfer roller 9Y is in contact with the yellow photoconductor drum 1Y via the intermediate transfer belt 8, the magenta transfer roller 9M is in contact with the magenta photoconductor drum 1M via the intermediate transfer belt 8, the cyan transfer roller 9C is in contact with the cyan photoconductor drum 1C via the intermediate transfer belt 8, and the black transfer roller 9K is in contact with the black photoconductor drum 1K via the intermediate transfer belt 8. The primary transfer rollers 9Y, 9M, 9C, and 9K are elastic rollers in which a conductive sponge layer with an outer diameter of about 16 mm is formed on a core metal with a diameter of about 10 mm, and the volume resistance is about 10 6 ~10 12 Ω (preferably 10 7 ~10 9 It is adjusted to be in the range of Ω.

[0032] The drive roller 16 is disposed downstream of the four photosensitive drums in the running direction of the intermediate transfer belt, so as to abut against the inner circumferential surface of the intermediate transfer belt 8 with the intermediate transfer belt 8 wrapped around it at a wrap angle of about 120 degrees. The drive roller 16 is driven to rotate in the clockwise direction in FIG. 3 by a drive motor Mt1 controlled by the control unit 90. This causes the intermediate transfer belt 8 to run in a predetermined running direction (clockwise in FIG. 3).

[0033] The driven roller 17 is disposed upstream of the four photosensitive drums in the running direction of the intermediate transfer belt 8, and is disposed so as to abut against the inner peripheral surface of the intermediate transfer belt 8 with the intermediate transfer belt 8 wrapped around it at a wrapping angle of about 180 degrees. The portion of the intermediate transfer belt 8 from the driven roller 17 to the drive roller 16 is set to be substantially horizontal. The driven roller 17 is rotated clockwise in FIG. 3 as the intermediate transfer belt 8 runs.

[0034] The tension roller 19 contacts the outer circumferential surface of the intermediate transfer belt 8. The pre-transfer roller 18, the cleaning opposed roller 20, the lubricant opposed roller 21, the backup roller 22, and the secondary transfer opposed roller 80 contact the inner circumferential surface of the intermediate transfer belt 8. Between the secondary transfer opposing roller 80 and the lubricant opposing roller 21, an intermediate transfer cleaning unit 10 (cleaning blade) is provided so as to come into contact with the cleaning opposing roller 20 via the intermediate transfer belt 8. Between the cleaning facing roller 20 and the tension roller 19, a lubricant supplying device 30 (intermediate transfer lubricant supplying device) is installed so as to come into contact with the lubricant facing roller 21 via the intermediate transfer belt 8. The lubricant supplying device 30 is composed of a lubricant supplying roller, a solid lubricant, a compression spring, etc., similar to the lubricant supplying device 3 for the photosensitive drum. The lubricant supplying roller rotates counterclockwise in FIG. 3 to scrape off a small amount of lubricant from the solid lubricant, and the lubricant is supplied to the surface of the intermediate transfer belt 8 by the lubricant supplying roller. All of the roller members 17 to 22 and 80 except for the driving roller 16 are rotated as the intermediate transfer belt 8 moves.

[0035] 4, the secondary transfer opposing roller 80 is in contact with the secondary transfer roller 72 via the intermediate transfer belt 8 and the secondary transfer belt 71. The secondary transfer opposing roller 80 is a cylindrical core metal made of stainless steel or the like, and has a volume resistance of 10 7 ~10 8An elastic layer 83 (having a thickness of about 5 mm) made of NBR rubber having a resistance of about Ω and a hardness (JIS-A hardness) of about 48 to 58 degrees is formed.

[0036] In this embodiment, the secondary transfer opposing roller 80 is electrically connected to a power supply unit 93 (bias output means), and a secondary transfer bias of a high voltage of about -5 kV is applied from the power supply unit 93. The secondary transfer bias applied to the secondary transfer opposing roller 80 is for secondarily transferring the toner image primarily transferred onto the surface of the intermediate transfer belt 8 onto the sheet P conveyed to the secondary transfer nip, and is a secondary transfer bias (DC voltage) of the same polarity as that of the toner (negative polarity in this embodiment). As a result, the toner carried on the toner carrying surface (outer peripheral surface) of the intermediate transfer belt 8 is electrostatically moved from the secondary transfer opposing roller 80 side toward the secondary transfer device 70 side by the secondary transfer electric field.

[0037] Referring to Figure 4, the secondary transfer device 70 is composed of a secondary transfer belt 71 as a transfer rotating body, a secondary transfer roller 72, a separation roller 73, a tension roller 74, a brush opposing roller 75, a brush roller 78, a first blade opposing roller 76, a first blade 85, a lubricant application roller 79, a second blade opposing roller 77, a second blade 86, etc.

[0038] The secondary transfer belt 71 (transfer rotating body) is an endless belt stretched and supported by six rollers (secondary transfer roller 72, separation roller 73, tension roller 74, brush opposing roller 75, first blade opposing roller 76, and second blade opposing roller 77), and is made of approximately the same material as the intermediate transfer belt 8. The secondary transfer belt 71 (transfer rotating body) is a belt member that abuts against the intermediate transfer belt 8 (image carrier) to form a secondary transfer nip as a transfer nip, and transports the sheet P sent out from the secondary transfer nip. In this embodiment, the secondary transfer belt 71 may be provided with an elastic layer made of a rubber material or the like as an intermediate layer.

[0039] The secondary transfer roller 72 forms a secondary transfer nip between the intermediate transfer belt 8 and the secondary transfer belt 71 and the secondary transfer opposing roller 80. The secondary transfer roller 72 is formed (covered) with an elastic layer having a hardness (Asker C hardness) of about 40 to 50 degrees on a hollow core metal made of stainless steel, aluminum, or the like. The elastic layer of the secondary transfer roller 72 can be formed in a solid or foamed sponge shape by dispersing conductive filler such as carbon or by incorporating an ionic conductive material in a rubber material such as polyurethane, EPDM, or silicone. In this embodiment, the elastic layer has a volume resistance of 10 6.5 ~10 7.5 It is set to about Ω. In this embodiment, the secondary transfer roller 72 is grounded. 4 by a motor 92 controlled by a controller 90, causing the secondary transfer belt 71 to rotate (run) in the counterclockwise direction in Fig. 3. Accordingly, the roller members 73 to 77 in contact with the inner peripheral surface of the secondary transfer belt 71 are driven to rotate in the counterclockwise direction in Fig. 3, and the brush roller 78 and lubricant application roller 79 in contact with the outer peripheral surface of the secondary transfer belt 71 are driven to rotate in the clockwise direction in Fig. 3. The motor 92 is a variable speed motor, and is configured so that the rotation speed of the secondary transfer roller 72 (the rotation speed of the secondary transfer belt 71) can be adjusted by control of the control unit 90. 4 by a moving mechanism 94, the secondary transfer roller 72 can adjust the contact pressure (nip pressure) at the secondary transfer nip. As the moving mechanism 94, a cam mechanism that supports both end shaft portions of the secondary transfer roller 72 can be used.

[0040] The separation roller 73 is disposed at a position downstream of the secondary transfer nip in the transport direction of the sheet P. The sheet P sent out from the secondary transfer nip is transported along the secondary transfer belt 71 traveling in the counterclockwise direction in FIG. 4, and then separated from the secondary transfer belt 71 at the position of the separation roller 73 by the secondary transfer belt 71, which has a curved surface formed along the outer periphery of the separation roller 73 (curvature separation). To remove the toner adhering to the surface of the secondary transfer belt 71, a cleaning bias having a polarity opposite to that of the toner is applied to the brush roller 78. The first blade 85 comes into contact with the surface of the secondary transfer belt 71 to remove foreign matter such as toner and paper powder adhering to the surface of the secondary transfer belt 71 . The lubricant application roller 79 applies a lubricant to the surface of the secondary transfer belt 71 in order to reduce wear of the first blade 85 and the like. The second blade 86 comes into contact with the surface of the secondary transfer belt 71 to make the lubricant applied to the surface of the secondary transfer belt 71 into a thin layer.

[0041] Hereinafter, the configuration and operation of image forming apparatus 100, which is characteristic of this embodiment, will be described in detail with reference to FIGS. 3, 4, etc., the image forming apparatus 100 is provided with a secondary transfer belt 71 as a transfer rotating body that forms a transfer nip (secondary transfer nip) by contacting the intermediate transfer belt 8 as an image carrier that carries a toner image. This secondary transfer belt 71 (transfer rotating body) is for transferring the toner image on the intermediate transfer belt 8 to the sheet P that is transported to the secondary transfer nip.

[0042] In addition, in the image forming apparatus 100, a relative linear speed difference of the secondary transfer belt 71 (transfer rotating body) with respect to the intermediate transfer belt 8 (image carrier) in the secondary transfer nip (transfer nip) and a linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 in the secondary transfer nip are calculated. of An adjustment means is provided for adjusting at least one of the contact pressure and the contact pressure. In detail, this adjustment means is configured to adjust at least one of the rotation speed (running speed) of the secondary transfer belt 71 (transfer rotating body) and the contact pressure of the secondary transfer belt 71 against the intermediate transfer belt 8 (image carrier).

[0043] Specifically, the image forming apparatus 100 in this embodiment is provided with a motor 92 (see FIG. 4) capable of adjusting the number of rotations of the secondary transfer roller 72. This motor 92 is a variable-speed motor, and functions as an adjustment means for adjusting the rotation speed of the secondary transfer belt 71 by adjusting the number of rotations of the secondary transfer roller 72 under the control of the control unit 90, thereby adjusting the rotation speed of the secondary transfer belt 71 and adjusting the linear speed difference of the secondary transfer belt 71 relative to the intermediate transfer belt 8 at the secondary transfer nip. That is, the linear speed difference (L1-L2) between the linear speed L1 of the intermediate transfer belt 8 at the secondary transfer nip and the linear speed L2 of the secondary transfer belt 71 at the secondary transfer nip is adjusted under the control of the control unit 90 as necessary. Furthermore, the image forming apparatus 100 in this embodiment is provided with a moving mechanism 94 (see FIG. 4) capable of moving the secondary transfer roller 72. This moving mechanism 94 is a cam mechanism or the like, and functions as an adjustment means for adjusting the vertical position of the secondary transfer roller 72 under the control of the control unit 90 to adjust the contact pressure of the secondary transfer belt 71 against the intermediate transfer belt 8 at the secondary transfer nip. That is, the contact pressure F (nip pressure) at the secondary transfer nip is adjusted as necessary under the control of the control unit 90. In addition, as previously described with reference to FIG. 1, the image forming apparatus 100 in this embodiment is provided with a double-sided conveying device 40 that conveys the sheet P toward the secondary transfer nip in order to transfer the toner image on the intermediate transfer belt 8 to the back surface of the sheet P after the toner image has been transferred to the front surface at the secondary transfer nip.

[0044] Here, the image forming apparatus 100 in this embodiment is provided with a control unit 90 as a control means that controls an adjustment means (at least one of a motor 92 and a moving mechanism 94) to reduce the difference in image magnification in the transport direction of the toner image transferred to the surface of the sheet P transported to the secondary transfer nip (transfer nip) (this is the image magnification difference in the transport direction of the toner image on the sheet P caused by the difference in the image area ratio of the toner image transferred to the surface of the sheet P). In other words, the image forming apparatus 100 in this embodiment is provided with a control unit 90 as a control means that controls an adjustment means (at least one of the motor 92 and the moving mechanism 94) to reduce the amount of transport deviation in the transport direction of the sheet P caused by the difference in the image area ratio of the toner image transferred to the surface of the sheet P transported to the secondary transfer nip (transfer nip). The "image area ratio" is the ratio of the image portion (the portion on which a toner image is formed) per unit area on the sheet P. Therefore, when the sheet P is blank and no image is formed on it, the image area ratio is 0%, when a solid image is formed on the entire surface of the sheet P, the image area ratio is 100%, and when a halftone image is formed on the entire surface of the sheet P, the image area ratio is 25%. Moreover, the "image magnification in the transport direction" of the toner image transferred to the surface of sheet P is the rate at which the toner image (image) carried on the image carrier expands and contracts in the transport direction before and after the image is transferred onto the sheet (the rate at which the length of the image in the transport direction changes). Specifically, in the case of this embodiment, the change in the length (distance) in the transport direction of the image secondarily transferred onto sheet P with respect to the image formed on intermediate transfer belt 8 is the change in the "image magnification in the transport direction." Therefore, the "difference in image magnification in the transport direction" of the toner image transferred to the surface of sheet P is the difference (Z1-Z2) between the "image magnification in the transport direction Z1" on the surface of the first sheet and the "image magnification in the transport direction Z2" on the surface of the second sheet.

[0045] In detail, as shown in FIG. 5(A), detection marks R1, R2 (R1', R2') are transferred to different positions in the transport direction (sub-scanning direction) and a first image pattern M having a small image area ratio is transferred to a first sheet (the front surface PA of a single sheet P), and the distance in the transport direction between the detection marks R1, R2 (R1', R2') is detected as a first distance H1 (H1') by detection means 91, 95. Also, as shown in FIG. 5(B), detection marks R1, R2 (R1', R2') are transferred to different positions in the transport direction so as to be in the same position as the first sheet (front surface PA), and a second image pattern N having a larger image area ratio than the first image pattern M is transferred to a second sheet (which is the back surface PB of one sheet P). The distance in the transport direction between the detection marks R1, R2 (R1', R2') is detected as a second distance H2 (H2') by detection means 91, 95. Then, the control unit 90 (control means) controls the adjustment means 92, 94 so that the distance difference H1-H2 (H1'-H2') between the first distance H1 (H1') and the second distance H2 (H2') detected by the detection means 91, 95 becomes equal to or less than a predetermined value A.

[0046] As described above, in the control using Figure 5, the distance (H0) in the transport direction (sub-scanning direction) of the image (detection marks R1, R2) formed on the intermediate transfer belt 8 is constant regardless of the image area ratio, so the change in distance (distance difference) in the transport direction of the image (detection marks R1, R2) secondarily transferred onto the sheet P is used as the change in "image magnification in the transport direction" (image magnification difference). That is, since there is a correlation between the distance difference (H1-H2) and the image magnification difference (H1 / H0-H2 / H0), the control is based on the distance difference (H1-H2). Therefore, it can be said that the above control is based on the image magnification difference (H1 / H0-H2 / H0). The relationship between the image magnification difference (H1 / H0-H2 / H0) described here and the difference (Z1-Z2) between the "image magnification in the conveying direction Z1" on the first sheet surface (front surface PA) described earlier and the "image magnification in the conveying direction Z2" on the second sheet surface (back surface PB) is Z1-Z2=H1 / H0-H2 / H0 (where Z1=H1 / H0, Z2=H2 / H0).

[0047] This series of controls is performed at a timing different from that of a normal printing operation (for example, during warm-up performed before a printing operation) in order to adjust the linear speed difference and contact pressure at the secondary transfer nip so that the image magnification in the transport direction is unlikely to change regardless of the image area ratio of the image formed on the sheet P at the secondary transfer nip. Hereinafter, this type of control will be referred to as the "adjustment mode" as appropriate.

[0048] More specifically, in the image forming apparatus 100 according to the present embodiment, a line sensor 95 (see FIG. 1) is provided so as to extend in the width direction (main scanning direction) downstream of the fixing unit 50. The line sensor 95 is a plurality of photosensors arranged side by side in the width direction. The control unit 90 also has a calculation unit 91 (see Figure 4) that calculates the first distance H1 (H1') and the second distance H2 (H2') based on information about the detection marks R1, R2 (R1', R2') optically detected by the line sensor 95. That is, the line sensor 95 and the calculation unit 91 function as a detection unit that detects the first distance H1 (H1') and the second distance H2 (H2'). Based on these detection results, the image magnification difference (transport deviation amount) in the transport direction (sub-scanning direction) of the image formed on the sheet P is obtained.

[0049] As shown in Figure 5, in this embodiment, the first sheet on which the first image pattern M is formed and the second sheet on which the second image pattern N is formed are the same sheet P, and the first image pattern M is formed on the front surface PA of the sheet P, and the second image pattern N is formed on the back surface PB of the sheet P. That is, the double-sided printing mode previously described using Figure 1 is performed, and detection marks R1, R2, R1', R2' and a first image pattern M are formed on the front surface PA of a single sheet P, and detection marks R1, R2, R1', R2' and a second image pattern N are formed on the back surface PB.

[0050] A series of operations in the adjustment mode will now be described. In the explanation of the operation, the relationship with the flowchart shown in FIG. 6 will be described as appropriate. First, when the adjustment mode is executed, one sheet P is fed from the paper feed unit 26, and an image (four detection marks R1, R2, R1', R2' and a first image pattern M) shown in Fig. 5(A) is formed on the front surface PA of the sheet P at the position of the secondary transfer nip. This image is formed by the image forming process described above. The four detection marks R1, R2, R1', and R2' are cross-shaped images formed at the four corners of the sheet surface. The first image pattern M has a lower image area ratio than the second image pattern N, and the image area ratio of the first image pattern M in this embodiment is set to 0%. That is, only the four detection marks R1, R2, R1', and R2' are formed on the front surface PA. Then, the sheet P on which the four detection marks R1, R2, R1', R2' (and the first image pattern M) are formed on the front surface PA reaches the position of the line sensor 95 after the fixing process. Then, the line sensor 95 reads the detection marks R1, R2 (R1', R2') that are separated in the sub-scanning direction (conveying direction), and the calculation unit 91 multiplies the reading time difference by the conveying speed of the sheet P to obtain the first distance H1 (H1') (FIG. 6: step S1). The first distance H1 that is finally obtained is the average value of the first distance H1 obtained based on the detection marks R1, R2 on one end side in the main scanning direction and the first distance H1' obtained based on the detection marks R1', R2' on the other end side in the main scanning direction.

[0051] Thereafter, the sheet P, on whose front surface the four detection marks R1, R2, R1', R2' (and the first image pattern M) have been formed, is again transported to the position of the secondary transfer nip by the double-sided conveying device 40. Then, at the position of the secondary transfer nip, an image (the four detection marks R1, R2, R1', R2' and the second image pattern N) shown in FIG. 5(B) is formed on the back surface PB of the sheet P. This image is formed by the imaging process described above. The four detection marks R1, R2, R1', R2' are cross-shaped images formed at the four corners of the sheet surface, similar to those in FIG. 5(A). The second image pattern N has a higher image area ratio than the first image pattern M, and the image area ratio of the second image pattern N in this embodiment is set to 25% or more. In particular, in this embodiment, the second image pattern N is a halftone image, and the image area ratio is set to 25%. This second image pattern N is formed at a position excluding the four detection marks R1, R2, R1', R2' and their surroundings. Then, the sheet P on which the four detection marks R1, R2, R1', R2' (and the second image pattern N) are formed on the back surface PB reaches the position of the line sensor 95 after the fixing process. Then, the line sensor 95 reads the detection marks R1, R2 (R1', R2') spaced apart in the sub-scanning direction (conveying direction) on the back surface PB, and the calculation unit 91 multiplies the reading time difference by the conveying speed of the sheet P to obtain the second distance H2 (H2') (FIG. 6: step S2). The second distance H2 finally obtained is the average value of the second distance H2 obtained based on the detection marks R1, R2 on one end side in the main scanning direction and the second distance H2' obtained based on the detection marks R1', R2' on the other end side in the main scanning direction.

[0052] Then, the control unit 90 determines whether the distance difference (|H1-H2|) between the first distance H1 and the second distance H2 is equal to or smaller than a predetermined value A (FIG. 6: step S3). Note that this predetermined value A is a value that is set in advance based on the allowable value of the image magnification difference. As a result, if it is determined in step S3 of FIG. 6 that the distance difference (|H1-H2|) is equal to or less than the predetermined value A, it is determined that the state is unlikely to cause a problem in which the image magnification in the transport direction changes due to differences in image area ratio, and the rotational speed of the secondary transfer belt 71 or the contact pressure (nip pressure) of the secondary transfer nip is not adjusted (FIG. 6: step S4), and the adjustment mode is terminated. On the other hand, if it is determined in step S3 of Fig. 6 that the distance difference (|H1-H2|) is not equal to or less than the predetermined value A, it is determined that the state is prone to a problem in which the image magnification in the transport direction changes due to a difference in image area ratio, and the rotation speed of the secondary transfer belt 71 and the contact pressure (nip pressure) of the secondary transfer nip are adjusted (Fig. 6: step S5). Then, the flow from step S1 onwards is repeated. At this time, each time the flow from step S1 onwards is repeated, another sheet P for adjustment is fed from the paper feed unit 26. In the adjustment mode, only the rotation speed of the secondary transfer belt 71 may be adjusted, or only the contact pressure (nip pressure) of the secondary transfer nip may be adjusted, or both may be adjusted. In the present embodiment, the relative linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 (image carrier) at the secondary transfer nip is adjusted by adjusting the rotation speed of the secondary transfer belt 71 (transfer rotating body). In contrast, the relative linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 at the secondary transfer nip can also be adjusted by adjusting the running speed of the intermediate transfer belt 8 or by adjusting the speeds of both the secondary transfer belt 71 and the intermediate transfer belt 8.

[0053] Hereinafter, a mechanism by which the image magnification in the transport direction changes due to differences in the image area ratio of the image transferred at the secondary transfer nip, and a mechanism by which this defect is eliminated by adjustment using an adjustment unit will be described. When passing through the secondary transfer nip (transfer nip), the sheet P tries to follow the speed (linear speed) of either the intermediate transfer belt 8 (image carrier) or the secondary transfer belt 71 (transfer rotation, etc.). However, at that time, the friction state of the surface of the sheet P changes depending on the image area ratio transferred onto the sheet P. Therefore, the friction state of the intermediate transfer belt 8 or the secondary transfer belt 71 with respect to the sheet P at the secondary transfer nip changes depending on the image area ratio, causing a deviation in the conveying speed (conveying amount) of the sheet P and resulting in a difference in the image magnification (sub-scanning magnification) in the conveying direction. Such image magnification difference in the transport direction (transport misalignment) can be reduced by adjusting the contact pressure (nip pressure) between the intermediate transfer belt 8 and the secondary transfer belt 71 at the secondary transfer nip, or the linear speed difference between the intermediate transfer belt 8 and the secondary transfer belt 71. This is because the conveying speed (conveying amount) of the sheet P varies depending on the rotational speed (linear speed) of the intermediate transfer belt 8, the rotational speed (linear speed) of the secondary transfer belt 71, and the contact pressure at the secondary transfer nip.

[0054] To further supplement the explanation, in the secondary transfer nip, when the frictional force between the sheet P and the intermediate transfer belt 8 is greater than the frictional force between the sheet P and the secondary transfer belt 71, the sheet P is transported at a speed close to the linear speed of the intermediate transfer belt 8. In contrast, when the frictional force between the sheet P and the intermediate transfer belt 8 is smaller than the frictional force between the sheet P and the secondary transfer belt 71, the sheet P is transported at a speed close to the linear speed of the secondary transfer belt 71. Furthermore, when the contact pressure (nip pressure) in the secondary transfer nip increases, the shape of the secondary transfer nip changes. For example, when the nip pressure increases, in the secondary transfer nip formed by the secondary transfer roller 72 (secondary transfer belt 71) and the secondary transfer opposing roller 80 (intermediate transfer belt 8), the roller members 72 and 80 (or only the softer one) are deformed. Such deformation changes the posture of the sheet P in the secondary transfer nip, changing the frictional state between the sheet P and the secondary transfer belt 71 and between the sheet P and the intermediate transfer belt 8, and fluctuating the conveying speed of the sheet P. The friction state also changes when toner is carried on the sheet P. When toner is carried on the sheet P, the friction coefficient of the sheet surface decreases, and slippage is likely to occur between the sheet P and the intermediate transfer belt 8, or between the sheet P and the secondary transfer belt 71. Therefore, a difference occurs in the conveying speed (conveying amount) of the sheet P at the secondary transfer nip between a sheet P on which no image is formed and a sheet P on which a solid image is carried. This difference causes a difference in image magnification in the conveying direction due to the image area ratio.

[0055] FIG. 7A is a graph showing the linear speed difference (V1-V2) between the linear speed V1 of the intermediate transfer belt 8 and the linear speed V2 of the secondary transfer belt 71 on the horizontal axis, and the image magnification difference (first distance H1-second distance H2) in the conveying direction when an image with a low image area ratio is output and when an image with a high image area ratio is output on the vertical axis. At this time, the contact pressure F of the secondary transfer nip is constant. From FIG. 7A, it can be seen that when the linear speed difference (V1-V2) is changed, the friction state changes as described above, and the image magnification difference (H1-H2) also changes. And, in the relationship H1>H2, it can be seen that when the linear speed difference (V1-V2) is reduced, the image magnification difference (H1-H2) is reduced. Such a phenomenon is common regardless of the thickness of the sheet P (regardless of whether it is thin paper or thick paper). On the other hand, FIG. 7B is a graph in which the horizontal axis indicates the contact pressure F (nip pressure) in the secondary transfer nip, and the vertical axis indicates the image magnification difference (first distance H1-second distance H2) in the conveying direction when an image with a low image area ratio is output and when an image with a high image area ratio is output. At this time, the linear speed difference (V1-V2) in the secondary transfer nip is constant. From FIG. 7B, it can be seen that when the contact pressure F is changed, the friction state changes as explained above, and the image magnification difference (H1-H2) also changes. It can also be seen that, in the relationship H1>H2, when the contact pressure F is increased, the image magnification difference (H1-H2) becomes smaller. This phenomenon is common regardless of the thickness of the sheet P (whether it is thin paper or thick paper). The adjustment mode in this embodiment is carried out based on these phenomena.

[0056] In this manner, in this embodiment, the adjustment mode is performed at a predetermined timing, and the linear speed difference and contact pressure at the secondary transfer nip are adjusted so as to reduce the image magnification difference (transport deviation amount) that occurs due to the difference in image area ratio of the image formed on the sheet P at the secondary transfer nip. This makes it possible to optimize the image magnification in the transport direction, regardless of the image area ratio of the toner image transferred to the surface of the sheet P transported to the secondary transfer nip. In other words, compared to a case where the rotation speed of the secondary transfer belt is adjusted so that the image magnification of the transferred image is limited to a predetermined image area ratio and is equal to or less than a threshold value, it is possible to reduce the image magnification difference (transport deviation amount) even when outputting images with different image area ratios. Therefore, it is possible to reduce the problem of an image being formed that is stretched or expanded in the transport direction, regardless of different image area ratios.

[0057] In particular, when at least one of the intermediate transfer belt 8 (image carrier) and the secondary transfer belt 71 (transfer rotating body) is a belt member having an elastic layer, image magnification differences (amount of transport deviation) are likely to occur when images with different image area ratios are transferred, making the configuration of the present invention useful. In addition, in this embodiment, a first image pattern M with a low image area ratio is formed on the front surface of one sheet P, a second image pattern N with a high image area ratio is formed on the back surface thereof, and the detection marks R1, R2, R1', and R2' on the front surface and back surface are detected, respectively, to perform the adjustment mode. Therefore, it is possible to reduce the consumption of sheets P compared to the case where the sheet P (first sheet) on which the first image pattern M with a low image area ratio is formed and the sheet P (second sheet) on which the second image pattern N with a high image area ratio is formed are separated, and the adjustment mode is performed by detecting the detection marks R1, R2, R1', and R2' for each sheet P. In addition, in this embodiment, the image area ratio of the second image pattern N is set to 25%. As the image area ratio increases, slippage of the sheet P in the secondary transfer nip is more likely to occur, but when the image area ratio is 25% or more, there is no significant change in the degree of slippage (the influence of friction becomes saturated). Therefore, in this embodiment, the image area ratio of the second image pattern N is set to 25% so that toner consumption is as small as possible in a state where a large slippage is likely to occur.

[0058] In this embodiment, after the adjustment mode is executed, the writing timing and exposure distribution in the main scanning direction and sub-scanning direction in the exposure unit 7 are adjusted. In detail, after the adjustment mode described above using Figure 6 etc. is completed, gradation patterns PY, PK, PM, PC as shown in Figure 8 are formed on sheet P by an image creation process, and when sheet P on which these gradation patterns PY, PK, PM, PC are formed is transported to the position of line sensor 95, the gradation patterns PY, PK, PM, PC are read by line sensor 95. Here, the gradation patterns PY, PK, PM, and PC are image patterns formed so that the image density in the main scanning direction is equal for each color (yellow, magenta, cyan, and black) and the image density in the sub-scanning direction differs stepwise. More specifically, the gradation image pattern PY formed in yellow, the gradation image pattern PK formed in black, the gradation image pattern PM formed in magenta, and the gradation image pattern PC formed in cyan are formed on one sheet P (adjustment sheet) at a timing different from that of the normal image forming operation. The gradation image patterns PY, PK, PM, and PC of the four colors are each formed by forming four belt-shaped patterns (gradation parts P1 to P4) at intervals in the sub-scanning direction so that the image density (image area ratio) in the main scanning direction is equal. Moreover, each of the gradation parts P1 to P4 is formed so that the image density (image area ratio) differs stepwise. Specifically, the image densities (image area ratios) are formed in the order of increasing order, namely, P1, P2, P3, and P4, being 20%, 40%, 70%, and 100%. The positions of these gradation image patterns PY, PK, PM, and PC in the main scanning direction and the sub-scanning direction are detected by line sensor 95, and the writing timing of the exposure unit 7 for each color in the main scanning direction and the writing timing of the exposure unit 7 in the sub-scanning direction are adjusted based on the detection results. Furthermore, the image densities of the gradation image patterns PY, PK, PM, and PC are detected by line sensor 95, and the exposure distribution of the exposure unit 7 for each color in the main scanning direction and the sub-scanning direction is adjusted based on the detection results. These adjustments are also performed when the gradation image patterns PY, PK, PM, and PC are formed on the back surface of the sheet P and printing is performed on the back surface.

[0059] In this way, the adjustment method performed in the image forming apparatus 100 in this embodiment (the image forming method in the image forming apparatus 100) is as follows: (1) A step of transferring toner images M and N having different image area ratios onto the front surface PA and back surface PB of a sheet P transported to a secondary transfer nip (transfer nip) (or onto the surfaces of two sheets transported to the transfer nip); (2) A step of controlling an adjustment means (motor 92, movement mechanism 94) based on a difference in image magnification in the conveying direction between the toner images R1, R2, R1', and R2' (detection marks) on the front surface PA and back surface PB on which the toner images M and N having different image area ratios are formed (or on the surfaces of two sheets on which the toner images M and N having different image area ratios are formed), so as to reduce the difference in image magnification; The present invention is characterized by the above.

[0060] <Variation 1> In the image forming apparatus 100 in variant example 1, the control unit 90 (control means) controls the adjustment means (motor 92 or moving mechanism 94) so ​​that at least one of the rotational speed of the secondary transfer belt 71 and the contact pressure of the secondary transfer nip is increased when the distance difference (|H1-H2|) between the first distance H1 and the second distance H2 detected by the line sensor 95 and the calculation unit 91 (detection means) exceeds a predetermined value A and when the first distance H1 is greater than the second distance H2 (H1>H2). In response to this, when the distance difference (|H1-H2|) between the first distance H1 and the second distance H2 detected by the line sensor 95 and the calculation unit 91 (detection means) exceeds a predetermined value A and the first distance H1 is smaller than the second distance H2 (H1≦H2), the control unit 90 controls the adjustment means (motor 92 or moving mechanism 94) so ​​that at least one of the rotational speed of the secondary transfer belt 71 and the contact pressure of the secondary transfer nip is reduced. By carrying out such control, in the adjustment mode, it is possible to carry out fine adjustment according to the magnitude relationship between the first distance H1 and the second distance H2. In addition, in the first modification, when the distance difference (|H1-H2|) does not become equal to or less than the predetermined value A even after performing the adjustment mode for controlling the adjustment means 92, 94 based on the distance difference (|H1-H2|) a predetermined number of times (five times in the first modification), the control unit 90 (control means) displays a warning to that effect on the display panel 200 (see FIG. 1). Specifically, the adjustment mode is stopped, and an error message to the effect that the adjustment mode has ended unsuccessfully is displayed on the display panel 200. By carrying out such control, it is possible to prevent the inconvenience of the adjustment mode continuing indefinitely.

[0061] FIG. 9 is a flowchart showing control in the adjustment mode in the first modification. First, the detection marks R1, R2, R1', and R2' formed on the front surface PA of the sheet P are read by the line sensor 95, and the first distance H1 is calculated by the calculation unit 91 based on the information (step S1). Next, the detection marks R1, R2, R1', and R2' formed on the back surface PB of the sheet P are read by the line sensor 95, and the second distance H2 is calculated by the calculation unit 91 based on the information (step S2). Thereafter, when the counter of the control unit 90 counts up the number of times n that the adjustment mode has been repeated (step S10), it is determined whether the number of times n does not exceed 5 (a predetermined number of times) (step S11). As a result, if it is determined that the number of times n that the adjustment mode has been repeated exceeds 5, the adjustment mode is deemed to have ended unsuccessfully, an error is displayed on the display panel 200, and the set value of the rotation speed of the secondary transfer belt 71 (or the contact pressure of the secondary transfer nip) that minimizes the distance difference (|H1-H2|) is stored (step S12). Then, the printing operation that is performed thereafter is performed based on the stored set value. On the other hand, if it is determined in step S11 that the number of times n of the adjustment mode does not exceed 5, it is determined whether the distance difference (|H1-H2|) calculated from the results of steps S1 and S2 is 0.5 mm or less (step S13). As a result, if it is determined that the distance difference (|H1-H2|) is 0.5 mm or less, it is determined that the image magnification difference due to the difference in image area ratio is at an acceptable level, and the rotation speed of the secondary transfer belt 71 (or the contact pressure of the secondary transfer nip) is not adjusted, and the setting value of the rotation speed of the secondary transfer belt 71 (or the contact pressure of the secondary transfer nip) at that time is saved (step S14). Then, the printing operation performed thereafter is performed based on the saved setting value. On the other hand, if it is determined in step S13 that the distance difference (|H1-H2|) is greater than 0.5 mm, the first distance H1 and the second distance H2 at that time are stored (step S15), and it is determined whether the first distance H1 is greater than the second distance H2 (step S16). As a result, if it is determined that the first distance H1 is greater than the second distance H2, it is determined that the image magnification difference due to the difference in image area ratio is at a problematic level and the linear speed V2 of the secondary transfer belt 71 is slow, so the rotation speed of the secondary transfer belt 71 is increased by 0.4%, and the set value of the rotation speed of the secondary transfer belt 71 at that time is stored (step S17). Then, the flow from step S1 onwards is repeated. On the other hand, if it is determined in step S16 that the first distance H1 is not greater than the second distance H2, it is determined that the image magnification difference due to the difference in image area ratio is at a problematic level and the linear speed V2 of the secondary transfer belt 71 is fast, so the rotation speed of the secondary transfer belt 71 is decelerated by 0.4% and the set value of the rotation speed of the secondary transfer belt 71 at that time is stored (step S18). Then, the flow from step S1 onwards is repeated. In addition, when the control flow of FIG. 9 is repeated, if neither H1(n)-H2(n) nor H1(n+1)-H2(n+1) satisfies the condition of step S13, and further, the magnitude relationship of step S16 is reversed, it is preferable to set the adjustment range of the rotation speed of the secondary transfer belt 71 to a small range.

[0062] <Variation 2> In the image forming apparatus 100 in the second modified example, the control unit 90 (control means) controls the motor 92 and the movement mechanism 94 (adjustment means) so that the contact pressure F is adjusted when the distance difference (|H1-H2|) as the image magnification difference (transport deviation amount) exceeds a predetermined amount B, and controls the motor 92 (adjustment means) so that the linear speed difference at the secondary transfer nip is adjusted when the distance difference (|H1-H2|) as the image magnification difference is equal to or less than the predetermined amount B. That is, in the adjustment mode, the movement mechanism 94 (contact pressure) is adjusted when the image magnification difference is roughly adjusted, and the motor 92 (linear speed difference) is adjusted when the image magnification difference is finely adjusted. This is because the contact pressure at the secondary transfer nip has a larger adjustment amount for the image magnification difference relative to the change amount than the linear velocity difference at the secondary transfer nip. Such control improves the efficiency of the adjustment mode.

[0063] FIG. 10 is a flowchart showing control in the adjustment mode in the second modification. The flow up to steps S1, S2, S10, S11, and S12 is the same as that in FIG. 9. Then, as shown in FIG. 10, if it is determined in step S11 that the number of times n of the adjustment mode does not exceed 5 times, it is determined whether the distance difference (|H1-H2|) obtained from the results of steps S1 and S2 is 0.2 mm or less (step S23). As a result, if it is determined that the distance difference (|H1-H2|) is 0.2 mm or less, it is determined that the image magnification difference due to the difference in image area ratio is at an acceptable level, and the rotation speed of the secondary transfer belt 71 and the contact pressure of the secondary transfer nip are not adjusted, and the set values ​​of the rotation speed of the secondary transfer belt 71 and the contact pressure of the secondary transfer nip at that time are stored (step S24). Then, the printing operation performed thereafter is performed based on the stored set values. On the other hand, if it is determined in step S23 that the distance difference (|H1-H2|) is greater than 0.2 mm, the first distance H1 and the second distance H2 at that time are stored (step S25), and it is determined whether the distance difference (|H1-H2|) obtained from the results of steps S1 and S2 is 0.5 mm or less (step S26). If it is determined that the distance difference (|H1-H2|) is 0.5 mm or less, it is determined whether the first distance H1 is greater than the second distance H2 (step S27). If it is determined that the first distance H1 is greater than the second distance H2, it is better to fine-tune the image magnification difference due to the difference in image area ratio, and the linear speed V2 of the secondary transfer belt 71 is assumed to be slow, so the rotation speed of the secondary transfer belt 71 is increased by 0.3%, and the set value of the rotation speed of the secondary transfer belt 71 at that time is stored (step S28). Then, the flow from step S1 onwards is repeated. On the other hand, if it is determined in step S27 that the first distance H1 is not greater than the second distance H2, it is better to fine-tune the image magnification difference due to the difference in image area ratio, and the linear speed V2 of the secondary transfer belt 71 is assumed to be faster, so the rotation speed of the secondary transfer belt 71 is decelerated by 0.3%, and the set value of the rotation speed of the secondary transfer belt 71 at that time is stored (step S29). Then, the flow from step S1 onwards is repeated. On the other hand, if it is determined in step S26 that the distance difference (|H1-H2|) is greater than 0.5 mm, it is determined whether the first distance H1 is greater than the second distance H2 (step S30). As a result, if it is determined that the first distance H1 is greater than the second distance H2, it is better to roughly adjust the image magnification difference due to the difference in image area ratio, and the contact pressure F of the secondary transfer nip is assumed to be small, so the contact pressure F of the secondary transfer nip is increased by 50%, and the set value of the contact pressure F at that time is stored (step S31). Then, the flow from step S1 onwards is repeated. On the other hand, if it is determined in step S30 that the first distance H1 is not greater than the second distance H2, it is better to roughly adjust the image magnification difference due to the difference in image area ratio, and the contact pressure F of the secondary transfer nip is determined to be large, so that the contact pressure F of the secondary transfer nip is reduced by 50%, and the set value of the contact pressure F at that time is stored (step S32). Then, the flow from step S1 onwards is repeated.

[0064] As described above, the image forming apparatus 100 in this embodiment is provided with the intermediate transfer belt 8 (image carrier) on which a toner image is carried, and the secondary transfer belt 71 (transfer rotating body) which contacts the intermediate transfer belt 8 to form a secondary transfer nip (transfer nip) and transfers the toner image on the intermediate transfer belt 8 to the sheet P conveyed to the secondary transfer nip. In addition, the relative linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 at the secondary transfer nip and the linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 at the secondary transfer nip are of The control unit 90 (control unit) is provided to control the adjustment unit (motor 92, movement mechanism 94) so ​​as to reduce the difference in image magnification based on the difference in image magnification in the transport direction of the toner image transferred to the surface of the sheet P transported to the secondary transfer nip. This makes it possible to make the image magnification in the transport direction less likely to change, regardless of the image area ratio of the toner image transferred onto the surface of the sheet P transported to the secondary transfer nip.

[0065] In this embodiment, the present invention is applied to a repulsive transfer type image forming apparatus 100 in which a power supply unit 93 is configured to apply a secondary transfer bias to the secondary transfer opposing roller 80. In contrast, the present invention can also be applied to an attractive transfer type image forming apparatus in which a power supply unit is configured to apply a secondary transfer bias to the secondary transfer roller 72. In that case, the secondary transfer bias has an opposite polarity to that of the repulsive transfer type. The present invention can also be applied to an image forming apparatus in which the repulsive transfer type and the attractive transfer type are used in combination. In the present embodiment, the present invention is applied to the image forming apparatus 100 that uses the secondary transfer belt 71 as the transfer rotator. However, the present invention can also be applied to an image forming apparatus that uses a secondary transfer roller as the transfer rotator. In this embodiment, the present invention is applied to an image forming apparatus 100 using an intermediate transfer belt 8 (intermediate transfer body) as an image carrier and a secondary transfer belt 71 as a transfer rotator. In contrast, the present invention can also be applied to a device that does not include an intermediate transfer body such as an intermediate transfer belt or intermediate transfer drum, but includes a photosensitive drum (photosensitive body) as an image carrier and a transfer rotator that contacts the photosensitive drum to form a transfer nip and transfers the toner image on the photosensitive drum to a sheet transported to the transfer nip, that is, a so-called direct transfer type image forming apparatus. As the transfer rotator, a transfer belt supported by multiple rollers, a transfer roller, etc. can be used. In the present embodiment, the present invention is applied to the image forming apparatus 100 that forms color images. However, the present invention can also be applied to an image forming apparatus that forms only monochrome images. Even in such cases, the same effects as those of this embodiment can be obtained.

[0066] In this embodiment, the adjustment means is configured to adjust the relative linear speed difference of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 (image carrier) at the transfer nip by adjusting the rotation speed of the secondary transfer belt 71 (transfer rotating body). In contrast to this, the adjustment means can also be configured to adjust the relative linear speed difference of the transfer rotating body with respect to the image carrier at the transfer nip by adjusting the rotation speed of the image carrier (or the image carrier and the transfer rotating body). In the present embodiment, the secondary transfer belt 71 (transfer rotating body) is moved to adjust the amount of the secondary transfer belt 71 with respect to the intermediate transfer belt 8 (image carrier) at the transfer nip. of The adjustment means is configured to adjust the contact pressure. In response to this, the image carrier (or the image carrier and the transfer rotating body) is moved to adjust the transfer rotating body relative to the image carrier in the transfer nip. of The adjustment means may be configured to adjust the contact pressure. Furthermore, in this embodiment, the detection marks R1, R2, R1', and R2' are detected by the line sensor 95, but the sensor for detecting the detection marks R1, R2, R1', and R2' is not limited to this, and for example, the detection marks R1, R2, R1', and R2' may be detected by the line sensor 95 using photosensors respectively installed at widthwise positions (both ends in the widthwise direction) corresponding to the detection marks R1, R2, R1', and R2'. In addition, in this embodiment, the image area ratio of the first image pattern M is set to 0% and the image area ratio of the second image pattern N is set to 25%, but these image area ratios are not limited to these values ​​and may be any value as long as there is a certain degree of difference. Even in such cases, the same effects as those of this embodiment can be obtained.

[0067] In this embodiment, the first image pattern M with a low image area ratio is formed on the front surface of one sheet P, the second image pattern N with a high image area ratio is formed on the back surface thereof, and the detection marks R1, R2, R1', and R2' on the front surface and back surface are detected, respectively, to perform the adjustment mode. In contrast, the sheet P (first sheet) on which the first image pattern M with a low image area ratio is formed and the sheet P (second sheet) on which the second image pattern N with a high image area ratio is formed can be separated, and the detection marks R1, R2, R1', and R2' can be formed on each sheet P, and the adjustment mode can be performed by detecting them. In addition, in this embodiment, the detection marks R1, R2, R1', R2' for the front side and the detection marks R1, R2, R1', R2' for the back side are formed on the intermediate transfer belt 8 so that the lengths H1, H1' in the transport direction of the detection marks R1, R2, R1', R2' before transfer formed on the front side of the sheet P match the lengths H2, H2' in the transport direction of the detection marks R1, R2, R1', R2' before transfer formed on the back side (H1, H1' = H2, H2'). In contrast to this, even if the lengths H1, H1' in the transport direction of the detection marks R1, R2, R1', R2' before transfer formed on the front side of the sheet P do not match the lengths H2, H2' in the transport direction of the detection marks R1, R2, R1', R2' before transfer formed on the back side, for example, if they are formed at a predetermined distance ratio (for example, H1, H1' = 2 × H2, H2'), it is possible to determine the distance difference (|H1-2 × H2|) of the detection marks on the sheet P taking into account the original distance ratio, and perform an adjustment mode based on the image magnification difference in the transport direction of the image caused by the difference in image area rate. Even in such cases, the same effects as those of this embodiment can be obtained.

[0068] It is clear that the present invention is not limited to the present embodiment, and that the present embodiment may be modified as appropriate within the scope of the technical concept of the present invention in addition to the modifications suggested in the present embodiment. Furthermore, the number, position, shape, etc. of the components are not limited to the present embodiment, and may be any number, position, shape, etc. suitable for implementing the present invention. [Explanation of symbols]

[0069] 1Y, 1M, 1C, 1K photoconductor drum (photoconductor), 8 intermediate transfer belt (image carrier), 40 Double-sided conveying device, 70 Secondary transfer device, 71 Secondary transfer belt (transfer rotating body), 72 Secondary transfer roller, 80 Secondary transfer opposing roller, 90 Control section (control means), 91 Arithmetic unit (calculation means), 92 Motor (adjustment means), 94 Movement mechanism (adjustment means), 100 Image forming apparatus (image forming apparatus main body), P sheet, PA front side, PB back side, R1, R1´, R2, R2´ detection marks, M first image pattern, N second image pattern, H1, H1´ 1st distance, H2, H2´ 2nd distance. [Prior art documents] [Patent documents]

[0070] [Patent Document 1] JP 2019-98734 A

Claims

1. an image carrier on which a toner image is carried; a transfer rotating body that contacts the image carrier to form a transfer nip and transfers a toner image on the image carrier to a sheet conveyed to the transfer nip; an adjusting unit that adjusts at least one of a relative linear speed difference of the transfer rotary body with respect to the image carrier at the transfer nip and a contact pressure of the transfer rotary body with respect to the image carrier at the transfer nip; a control means for controlling the adjusting means based on a difference between an image magnification in a transport direction of a toner image having a first image area ratio that is transported to the transfer nip and transferred to the surface of a first sheet, and an image magnification in a transport direction of a toner image having a second image area ratio that is higher than the first image area ratio that is transported to the transfer nip and transferred to the surface of a second sheet, so as to reduce the difference in image magnification; An image forming apparatus comprising:

2. a detection unit detects a distance in the conveying direction of the detection marks as a first distance with respect to the first sheet on which the detection marks are transferred at different positions in the conveying direction and the first image pattern having the first image area ratio is transferred; the detection marks are transferred to different positions in a transport direction so as to be in the same positions as the first sheet, and a second image pattern having the second image area ratio is transferred to the second sheet, the distance of the detection marks in the transport direction being detected as a second distance by the detection means; 2. The image forming apparatus according to claim 1, wherein the control unit controls the adjustment unit so that the difference between the first distance and the second distance detected by the detection unit becomes equal to or smaller than a predetermined value.

3. The image area ratio of the first image pattern is 0%, 3. The image forming apparatus according to claim 2, wherein an image area ratio of the second image pattern is 25% or more.

4. A double-sided conveying device is provided for conveying the sheet toward the transfer nip in order to transfer the toner image on the image carrier to the back side of the sheet after the toner image has been transferred to the front side by the transfer nip. The first sheet and the second sheet are one and the same sheet, 4. The image forming apparatus according to claim 2, wherein the first image pattern is formed on a front surface of the one sheet, and the second image pattern is formed on a back surface of the one sheet.

5. the adjusting unit is configured to adjust at least one of a rotation speed of the transfer rotor and a contact pressure of the transfer rotor with respect to the image carrier, The control means When a difference between the first distance and the second distance detected by the detection means exceeds the predetermined value, and the first distance is greater than the second distance, the adjustment means is controlled so that at least one of the rotation speed and the contact pressure is increased, An image forming apparatus as described in any one of claims 2 to 4, characterized in that when the distance difference between the first distance and the second distance detected by the detection means exceeds the specified value and the first distance is smaller than the second distance, the adjustment means is controlled so that at least one of the rotational speed and the contact pressure is reduced.

6. The image forming apparatus according to any one of claims 2 to 5, characterized in that the control means displays a warning to the effect that the distance difference does not become equal to or less than the predetermined value even after performing an adjustment mode for controlling the adjustment means based on the distance difference a predetermined number of times.

7. The image forming apparatus according to any one of claims 1 to 6, characterized in that, when controlling the adjustment means, the control means controls the adjustment means so that the contact pressure is adjusted when the difference in image magnification exceeds a predetermined amount, and controls the adjustment means so that the linear speed difference is adjusted when the difference in image magnification is equal to or less than the predetermined amount.

8. An image forming apparatus according to any one of claims 1 to 7, characterized in that after an adjustment mode for controlling the adjustment means is executed, the writing timing and exposure distribution in the main scanning direction and sub-scanning direction in the exposure section are adjusted.

9. 9. The image forming apparatus according to claim 1, wherein at least one of the image carrier and the transfer rotary member is a belt member having an elastic layer.

10. an image carrier on which a toner image is carried; a transfer rotating body that contacts the image carrier to form a transfer nip and transfers a toner image on the image carrier to a sheet conveyed to the transfer nip; an adjusting unit that adjusts at least one of a relative linear speed difference of the transfer rotary body with respect to the image carrier at the transfer nip and a contact pressure of the transfer rotary body with respect to the image carrier at the transfer nip; An adjustment method performed in an image forming apparatus comprising: a step of transferring toner images having different image area ratios onto the front and back surfaces of a sheet conveyed to the transfer nip, or onto the surfaces of two sheets conveyed to the transfer nip; a step of controlling the adjusting means based on a difference in image magnification in a conveying direction between the toner images on the front and back sides on which the toner images having different image area ratios are formed, or on the front surfaces of the two sheets on which the toner images having different image area ratios are formed, so that the difference in image magnification is reduced; An adjustment method comprising:

Images