Image forming device

Abstract

To appropriately execute a cleaning operation of an electrifying member to prevent a reduction in image quality.SOLUTION: An image forming apparatus has: acquisition units UY, UM, UC, UK that, based on information on the print pixel area acquired from image data, for each of a plurality of areas on image carriers 1 in the direction of the axis of rotation of the image carriers 1, acquires the integrated value of index values corresponding to the amounts of toner in toner images passing through transfer units N1; and a control unit 50 that can execute a cleaning operation to clean electrifying members 2 during a non-image formation period. The control unit 50 controls the timing for executing the cleaning operation based on the difference in integrated value between adjacent areas in the plurality of areas.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile machine that uses an electrophotographic method or an electrostatic recording method. [Background technology]

[0002] In conventional image forming devices such as electrophotographic printers, a toner image formed on a photosensitive member as an image carrier is transferred to a recording sheet or the like as a receiving member. In intermediate transfer image forming devices, the toner image formed on the photosensitive member is first transferred to an intermediate transfer member as a receiving member, and then secondarily transferred to a recording material. The transfer of the toner image from the photosensitive member to the receiving member is performed by applying a transfer voltage to a transfer member such as a transfer roller that is arranged opposite the photosensitive member and sandwiches the receiving member therebetween.

[0003] In such image forming devices, residual toner is removed from the photoconductor. Conventionally, residual toner is generally removed from the photoconductor and collected by a cleaning device equipped with a cleaning member such as a cleaning blade and a cleaning container equipped with the cleaning member. In contrast, a cleanerless system has been proposed that does not require a dedicated cleaning device to collect residual toner from the photoconductor (Patent Document 1). With this system, residual toner is collected in a developing device, so it can be collected without a dedicated cleaning device. This eliminates the need for a dedicated space to hold the collected toner, enabling the image forming device to be made smaller and less expensive.

[0004] Residual toner on the photosensitive member may adhere to a charging member, such as a charging roller, that is placed in contact with the photosensitive member. When toner adheres to the charging member, the charging ability of the photosensitive member changes, causing a change in the latent image potential at the portion of the charging member where the toner adheres, resulting in a change in image density. Adhesion of residual toner to the charging member is particularly noticeable in the cleanerless image forming apparatus.

[0005] Patent Document 1 proposes a configuration in which a cleaning operation for the charging member is performed at a predetermined timing as a maintenance operation for suppressing this change in image density. Specifically, in this cleaning operation, toner adhering to the charging member is expelled onto a photosensitive member, and then the toner expelled onto the photosensitive member is transferred to an intermediate transfer member and collected by a collection member such as a cleaning blade that removes the toner on the intermediate transfer member.

[0006] In Patent Document 1, a method of determining the timing for cleaning the charging member is proposed, in which the determination is made based on the cumulative number of printed sheets, the cumulative number of printed pixels, or the cumulative exposure time.

[0007] Furthermore, Patent Document 2 proposes a method for determining the timing of cleaning the charging member based on a pixel count value obtained by counting the number of printed pixels for each position in the longitudinal direction of the charging member. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-126202 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-11159 Summary of the Invention [Problem to be solved by the invention]

[0009] It is desirable to perform the cleaning operation of the charging member only when necessary and sufficient. For example, if the cleaning operation of the charging member is performed at short intervals, the throughput of the image forming apparatus (the number of images that can be output per unit time) will decrease, and if it is performed at long intervals, image defects will occur and image quality will decrease.

[0010] The present invention is a further development of the above-mentioned conventional technology, and has an object to suppress deterioration of image quality by suitably performing a cleaning operation of the charging member. [Means for solving the problem]

[0011] The above object is achieved by an image forming apparatus according to the present invention. In summary, a typical configuration of the present invention comprises an image forming unit having a rotatable image carrier, a charging member that contacts the image carrier to charge the image carrier, and a developing member that supplies toner to the image carrier, and forming a toner image on the image carrier based on image data, and transferring the toner image from the image carrier to a transferee that contacts the image carrier in a transfer unit, an acquisition unit that acquires, based on information about print pixel areas acquired from the image data, an integrated value of an index value corresponding to the amount of toner in a toner image that passes through the transfer unit for each of a plurality of regions on the image carrier in the direction of the rotation axis of the image carrier, and a control unit that can execute a cleaning operation to clean the charging member when no image is being formed, the cleaning operation comprising a first cleaning operation that is executed based on information about the integrated value and a second cleaning operation that is executed based on information about the number of prints of a recording material, and the control unit is configured to select the region among the plurality of regions having the largest integrated value. Small to a specified percentage The integrated value becomes the smallest An area including the area is defined as an exclusion area, and in the effective area obtained by excluding the exclusion area from the plurality of areas, the integrated value in the first region, and the first region In the direction of the rotation axis Adjacent 2 the integrated value in the region Both exceeded the predetermined threshold. In this case, the first cleaning operation is controlled to be executed, and the timing for executing the first cleaning operation is and , the timing for executing the second cleaning operation The difference between The image forming apparatus is characterized by controlling the image forming apparatus so that the image forming apparatus is [Effects of the Invention]

[0013] According to the present invention, by suitably performing the cleaning operation of the charging member, it is possible to suppress the deterioration of image quality. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic block diagram showing a control mode of the image forming apparatus. [Figure 3] FIG. 4 is a flowchart showing an outline of an operation sequence of a print job. [Figure 4] FIG. 10 is a timing chart for explaining the charging roller cleaning operation. [Figure 5] 10 is a schematic diagram for explaining a calculation method A for determining whether or not to perform the charging roller cleaning operation in the first embodiment. FIG. [Figure 6] FIG. 10 is a schematic diagram for explaining a calculation method A for determining whether or not to perform the charging roller cleaning operation in the second embodiment. [Figure 7] FIG. 11 is a schematic diagram for explaining a calculation method A for determining whether or not to perform the charging roller cleaning operation in the third embodiment. [Figure 8] FIG. 10 is a schematic diagram for explaining a calculation method A for determining whether or not to perform the charging roller cleaning operation in the fourth embodiment. [Figure 9] 10A and 10B are schematic diagrams for explaining another example of control of the charging roller cleaning operation. [Figure 10] 10A and 10B are schematic diagrams for explaining still another example of control of the charging roller cleaning operation. DETAILED DESCRIPTION OF THE INVENTION

[0015] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0016] [Example 1] <Configuration and Operation of Image Forming Apparatus> 1 is a schematic cross-sectional view of an image forming apparatus 100 according to this embodiment. The image forming apparatus 100 according to this embodiment is a tandem-type color laser printer that employs an intermediate transfer system and is capable of forming full-color images using an electrophotographic system. The image forming apparatus 100 according to this embodiment also employs a cleanerless system (drum cleanerless system) that does not have a dedicated cleaning means for cleaning the photosensitive member.

[0017] The image forming apparatus 100 includes image forming units SY, SM, SC, and SK that form images in the colors yellow (Y), magenta (M), cyan (C), and black (K), respectively. Elements in the image forming units SY, SM, SC, and SK that have the same or corresponding functions or configurations will be generally described by omitting the Y, M, C, or K suffix to the reference numerals indicating that the element is provided for a specific color. In this embodiment, the image forming unit S includes photosensitive drums 1 (1Y, 1M, 1C, and 1K), charging rollers 2 (2Y, 2M, 2C, and 2K), exposure devices 3 (3Y, 3M, 3C, and 3K), developing devices 4 (4Y, 4M, 4C, and 4K), and primary transfer rollers 14 (14Y, 14M, 14C, and 14K), which will be described later.

[0018] The photosensitive drum 1, a rotatable drum-type (cylindrical) photosensitive member (electrophotographic photosensitive member) serving as an image carrier, is driven to rotate at a predetermined peripheral speed (process speed) in the direction of arrow R1 (counterclockwise) in FIG. 1 by a drum drive device 31 (FIG. 2) serving as a drive means. The drum drive device 31 includes a drum drive motor and other components serving as a drive source. When a control unit (described below) such as a controller receives an image signal to initiate a print job (described below), the photosensitive drum 1 is driven to rotate. In this embodiment, the photosensitive drum 1 is an organic photosensitive (OPC) drum having a photoconductive layer on a conductive support. The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 2, a roller-type charging member serving as a charging means. The charged surface of the photosensitive drum 1 is scanned and exposed by an exposure device 3 serving as an exposure means in accordance with an image signal, and an electrostatic latent image (electrostatic image) of a color component corresponding to each image forming unit S is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 as a developing means, which supplies toner as a developer, and a toner image (toner image, developer image) is formed on the photosensitive drum 1.

[0019] In this embodiment, the charging roller 2 is in contact with the surface of the photosensitive drum 1 with a predetermined pressure, and rotates in response to the rotation of the photosensitive drum 1 due to friction with the surface of the photosensitive drum 1. In this embodiment, the charging roller 2 is a 1.5 mm thick metal roller with a volume resistivity of 1×10 on a metal shaft (rotating shaft) with a diameter of 5.5 mm. 6 The charging roller 2 is configured with an elastic layer formed of a conductive elastic material with a resistance of approximately [Ωcm]. During the charging process, a predetermined charging voltage (charging bias), which is a negative DC voltage, is applied to the rotating shaft of the charging roller 2 from a charging power supply (high-voltage power supply circuit) E1 (FIG. 2). In this embodiment, a DC voltage of -1100 [V] is applied to the rotating shaft of the charging roller 2 during the charging process. At this time, the surface potential (charging potential, background potential) of the photosensitive drum 1 was measured using a Trek Model 344 surface potential meter and found to be approximately -500 [V]. The position on the photosensitive drum 1 where charging is performed in the rotational direction of the photosensitive drum 1 is the charging position. The charging roller 2 charges the surface of the photosensitive drum 1 by discharging in at least one of the minute gaps formed between the photosensitive drum 1 and the charging roller 2, which are formed upstream and downstream of the contact portion (charging nip) between the photosensitive drum 1 and the charging roller 2 in the rotational direction of the photosensitive drum 1. However, for simplicity, it may be assumed that the charging process is carried out in the charging nip portion.

[0020] The exposure device 3 includes a laser driver, a laser diode, a polygon mirror, an optical lens system, and the like. Based on image information input to the image forming apparatus 100 from a host device 200 (FIG. 2), such as a personal computer, the exposure device 3 irradiates the photosensitive drum 1 with laser light to form an electrostatic latent image on the uniformly charged surface of the photosensitive drum 1. The exposure device 3 scans the photosensitive drum 1 with the laser light in a direction substantially parallel to the rotation axis of the photosensitive drum 1. The exposure device 3 also scans the photosensitive drum 1 with the laser light in a direction substantially parallel to the movement direction of the surface of the photosensitive drum 1 as the photosensitive drum 1 rotates. Hereinafter, the direction of the rotation axis of the photosensitive drum 1 (a direction substantially perpendicular to the movement direction of the surface) is also referred to as the "main scanning direction." The direction substantially perpendicular to the "main scanning direction" is also referred to as the "sub-scanning direction." In this embodiment, the exposure amount is adjusted so that the surface potential (latent image potential, exposed portion potential) of the photosensitive drum 1 after exposure with the maximum light amount of the exposure device 3 is −100 V. The position on the photosensitive drum 1 where exposure by the exposure device 3 is performed in the rotation direction of the photosensitive drum 1 is the exposure position.

[0021] The developing device 4 has a developing roller 41 as a developing member (developer carrier) and a developing container 42 that contains a non-magnetic one-component developer (toner) as the developer. The developing roller 41 is configured by providing an elastic layer made of an electrically conductive elastic material on an electrically conductive core (rotating shaft). During the developing process, the developing roller 41 comes into contact with the photosensitive drum 1 at a predetermined contact width. During the developing process, the developing roller 41 is driven to rotate at a circumferential speed higher than the circumferential speed of the photosensitive drum 1 by a developing drive device 32 (FIG. 2) as a driving means. At this time, the developing roller 41 is driven to rotate so that the moving direction of the surface of the developing roller 41 is the same as the moving direction of the surface of the photosensitive drum 1 at the portion (contact portion) facing the photosensitive drum 1. The developing drive device 32 has a developing drive motor and the like as a driving source. The developing roller 41 carries toner charged to the normal charging polarity (negative polarity in this embodiment), transports it to the opposing portion (contact portion) with the photosensitive drum 1, and supplies toner onto the photosensitive drum 1 in accordance with the electrostatic latent image on the photosensitive drum 1. During the development process, a predetermined development voltage (developing bias), which is a negative DC voltage, is applied to the rotation shaft of the developing roller 41 by a development power supply (high-voltage power supply circuit) E2 (FIG. 2). In this embodiment, during the development process, a DC voltage of −300 V is applied to the rotation shaft of the developing roller 41. The position on the photosensitive drum 1 where the developing roller 41 supplies toner in the rotation direction of the photosensitive drum 1 (the position where the developing roller 41 abuts against the photosensitive drum 1) is the development position.

[0022] Here, the developing roller 41 is provided so as to be capable of contacting and separating from the photosensitive drum 1. That is, the developing device 4 and the main body of the image forming apparatus 100 are provided with a contact / separation mechanism 34 (FIG. 2) that controls the contact / separation state between the developing roller 41 and the photosensitive drum 1. The contact / separation mechanism 34 brings the developing roller 41 into contact with the photosensitive drum 1 during the development process, etc. Furthermore, the contact / separation mechanism 34 separates the developing roller 41 from the photosensitive drum 1 when, for example, the operation of the image forming apparatus 100 is stopped.

[0023] The toner in this example is a negatively charged non-magnetic toner produced by suspension polymerization, with a volume average particle size of approximately 6.0 μm. To improve the surface properties of the toner in this example, silicon oxide particles with a volume average particle size of approximately 20 nm are uniformly adhered to the toner surface, at approximately 1.5% of the toner weight. While the toner used in this example is produced by suspension polymerization, the present invention is not limited to this method. For example, toner produced by other polymerization methods, such as pulverization or emulsion polymerization, may also be used.

[0024] In this embodiment, the normal charge polarity of the toner contained in the developer container 42 is negative. In this embodiment, the electrostatic latent image is reversely developed using toner charged to the same polarity as the charge polarity of the photoconductor by the charging member. That is, in reverse development, toner charged to the same polarity as the charge polarity of the photoconductor adheres to exposed areas (image areas) on the photoconductor where the absolute value of the potential has been reduced by exposure after uniform charging. However, the present invention is not limited to this, and the electrostatic latent image may be normally developed using toner charged to the opposite polarity to the charge polarity of the photoconductor. That is, in normal development, toner charged to the opposite polarity to the charge polarity of the photoconductor adheres to areas (image areas) other than the areas where the absolute value of the potential has been reduced by exposure after uniform charging.

[0025] An intermediate transfer belt 10, which is an endless belt serving as an intermediate transfer body, is disposed facing the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer belt 10 is stretched by a plurality of stretching members (stretching rollers), including a drive roller 11, a tension roller 12, and a secondary transfer opposing roller 13. The intermediate transfer belt 10 rotates (circulates or moves circulatingly) in the direction of arrow R2 (clockwise) in FIG. 1 when the drive roller 11 is rotationally driven by a belt drive device 33 (FIG. 2) serving as a driving means. At this time, the intermediate transfer belt 10 rotates at approximately the same peripheral speed as the peripheral speed of each photosensitive drum 1 so that the moving direction of the surface of the intermediate transfer belt 10 is the same as the moving direction of the surface of each photosensitive drum 1 at the portions facing (contacting) each photosensitive drum 1. The belt drive device 33 includes a belt drive motor and the like as a driving source. On the inner circumferential surface side of the intermediate transfer belt 10, primary transfer rollers 14, which are roller-type primary transfer members serving as primary transfer means, are arranged corresponding to each photosensitive drum 1. The primary transfer rollers 14 press the intermediate transfer belt 10 toward the photosensitive drums 1, forming primary transfer portions (primary transfer nip portions) N1, which are contact portions between the photosensitive drums 1 and the intermediate transfer belt 10. The primary transfer rollers 14 rotate in response to the rotation of the intermediate transfer belt 10. The position on the photosensitive drum 1 where primary transfer occurs in the rotation direction of the photosensitive drum 1 (corresponding to the primary transfer portion N1) is the primary transfer position. The toner image formed on the photosensitive drum 1 as described above is transferred (primary transfer) onto the rotating intermediate transfer belt 10 (onto the transfer recipient) at the primary transfer portion N1 by the action of the primary transfer rollers 14. During the primary transfer process, a predetermined primary transfer voltage (primary transfer bias), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the primary transfer roller 14 by a primary transfer power supply (high-voltage power supply circuit) E3 (FIG. 2). In this embodiment, a DC voltage of +500 V is applied to the primary transfer roller 14 during the primary transfer process. For example, when a full-color image is formed, the toner images of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially transferred to the intermediate transfer belt 10 at each primary transfer portion N1 so as to be superimposed on top of each other.As a result, a four-color toner image corresponding to the desired full-color image is formed on the intermediate transfer belt 10.

[0026] Of the toner (transfer residual toner) remaining on the photosensitive drum 1 without being primarily transferred onto the intermediate transfer belt 10 at the primary transfer portion N1, the positively charged toner adheres to the charging roller 2. Meanwhile, the negatively charged toner of the transfer residual toner is transported to the opposing portion of the developing device 4 through the contact portion with the charging roller 2 as the photosensitive drum 1 rotates. At this time, the charging and exposing processes are again performed on the surface of the photosensitive drum 1, and an electrostatic latent image according to the image information is formed. The toner transported to the opposing portion of the developing device 4 is almost negatively charged. Therefore, some of this toner is collected by the developing device 4 due to the electric field formed by the potential difference between the surface potential of the photosensitive drum 1 (-500 [V] for the non-exposed portion and -100 [V] for the exposed portion) and the voltage applied to the developing roller 41 (-300 [V]). That is, in the non-exposed portion, the direction of the electric field is such that the negatively charged toner moves from the photosensitive drum 1 onto the developing roller 41, and therefore the negatively charged toner on the photosensitive drum 1 moves onto the developing roller 41 and is collected in the developing device 4. In this way, in this embodiment, the residual toner, such as the transfer residual toner, on the photosensitive drum 1 is collected by the cleanerless system.

[0027] A secondary transfer roller 15, a roller-type secondary transfer member serving as a secondary transfer means, is disposed on the outer peripheral surface of the intermediate transfer belt 10, facing the secondary transfer opposing roller 13. The secondary transfer roller 15 contacts the intermediate transfer belt 10, which is backed up by the secondary transfer opposing roller 13, with a pressure of 50 N, forming a secondary transfer portion (secondary transfer nip portion) N2, which is the contact portion between the intermediate transfer belt 10 and the secondary transfer roller 15. The secondary transfer roller 15 rotates in response to the rotation of the intermediate transfer belt 10. At the secondary transfer portion N2, the toner image formed on the intermediate transfer belt 10 is transferred (secondarily transferred) by the action of the secondary transfer roller 15 onto a recording material P, such as a recording sheet, which is being conveyed while being sandwiched between the intermediate transfer belt 10 and the secondary transfer roller 15. For example, when forming a full-color image, the four-color toner images formed on the intermediate transfer belt 10 are collectively secondarily transferred onto the recording material P at the secondary transfer portion N2. During the secondary transfer process, a predetermined secondary transfer voltage (secondary transfer bias), which is a DC voltage of opposite polarity (positive polarity in this embodiment) to the normal charging polarity of the toner, is applied to the secondary transfer roller 15 by a secondary transfer power supply (high-voltage power supply circuit) E4 (FIG. 2). In this embodiment, a DC voltage of +2500 V is applied to the secondary transfer roller 15 during the secondary transfer process. The recording material (transfer material, recording medium, sheet) P is stored in a recording material storage unit 16, and is fed out of the recording material storage unit 16 one sheet at a time by a feed roller 18 or the like as a feeding means, and is transported to the secondary transfer unit N2 in synchronization with the toner image on the intermediate transfer belt 10.

[0028] The recording material P onto which the toner image has been transferred is conveyed to a fixing device 30 serving as a fixing means. The fixing device 30 applies heat and pressure to the recording material P bearing the unfixed toner image, thereby fixing (melting and solidifying) the toner image onto the recording material P. For example, when forming a full-color image, the four colors of toner on the recording material P are melted, mixed, and fixed onto the recording material P. The recording material P (printed image) onto which the toner image has been fixed is discharged (output) to the outside of the main body of the image forming apparatus 100.

[0029] A belt cleaning device 17 serving as an intermediate transfer member cleaning unit is disposed on the outer peripheral surface of the intermediate transfer belt 10, downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (the most upstream primary transfer portion N1Y) in the rotation direction of the intermediate transfer belt 10. In this embodiment, the belt cleaning device 17 is disposed opposite the secondary transfer opposing roller 13. Toner (residual toner) remaining on the intermediate transfer belt 10 without being secondarily transferred onto the recording material P at the secondary transfer portion N2 is removed from the intermediate transfer belt 10 and collected by the belt cleaning device 17. The belt cleaning device 17 includes a cleaning member such as a cleaning blade disposed in contact with the outer peripheral surface of the intermediate transfer belt 10 and a cleaning container. The belt cleaning device 17 uses the cleaning member to scrape off deposits such as residual toner from the surface of the rotating intermediate transfer belt 10 and collect the deposits in the cleaning container.

[0030] <Control mode> FIG. 2 is a schematic block diagram showing the control mode of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 is provided with a control unit (control circuit) 50 as a control means. The control unit 50 is configured with a CPU as an arithmetic control means, and ROM and RAM as storage means. The CPU controls the overall operation of each unit of the image forming apparatus 100 in accordance with a program stored in the ROM, using the RAM as a work area. The control unit 50 is connected to, for example, a drum drive unit 31, a development drive unit 32, a belt drive unit 33, a contact / separation mechanism 34, various power sources E1 to E4, an exposure unit 3, and the like. The control unit 50 also includes an image signal processing circuit 51, image memories MY, MM, MC, MK, a temporary counter array RY, RM, RC, RK, a cumulative counter array UY, UM, UC, UK, and a cumulative print counter W, which will be described in detail later. Although not shown in FIG. 2, in this embodiment, the charging power supply E1, the developing power supply E2, and the primary transfer power supply E3 are provided independently for each of the image forming units SY, SM, SC, and SK.

[0031] A host device 200 such as a personal computer is connected to the control unit 50 via an interface. The control unit 50 controls the image forming operation so that an image corresponding to image data (electrical image information) input from the host device 200 is formed on a recording material P and output. The control unit 50 also controls the cleaning operation of the charging roller 2 (hereinafter simply referred to as the "charging roller cleaning operation"), which will be described in detail later, and controls the determination of whether to perform the charging roller cleaning operation.

[0032] Here, the image forming apparatus 100 executes a print job, which is a series of operations for forming and outputting an image on one or multiple recording materials P, initiated by a single start command from the host device 200. A print job generally includes an image formation process, a pre-rotation process, a sheet-to-sheet process (when forming images on multiple recording materials P), and a post-rotation process. The image formation process is a period during which electrostatic images of the images to be actually formed and output on the recording materials P are formed, toner images are formed, and the toner images are primarily and secondary transferred. This period is referred to as the image formation period (image formation operation period). More specifically, the timing of the image formation process differs depending on the positions where the electrostatic image formation, toner image formation, and primary and secondary transfer of the toner images are performed. The pre-rotation process is a period from when a start command is input until the actual start of image formation, during which preparatory operations are performed prior to the image formation process. The sheet-to-sheet process is a period corresponding to the interval between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period during which a cleaning operation (preparatory operation) is performed after the image formation process. Non-image formation time (non-image formation period) is a period other than image formation time, and includes the pre-rotation process, the sheet interval process, the post-rotation process, and the pre-multiple rotation process, which is a preparatory operation when the image forming apparatus is turned on or when it returns from a sleep state. In this embodiment, the control unit 50 can perform a charging roller cleaning operation, which will be described in detail later, during non-image formation time.

[0033] <Print job operation sequence> FIG. 3 is a flowchart showing an outline of the operation sequence of a print job in this embodiment.

[0034] The control unit 50 starts a print job when a print job start signal is sent from the host device 200 connected to the image forming apparatus 100. First, the control unit 50 receives image data from the host device 200 (S101). The image data is bitmap data of three colors, RGB, namely red (R), green (G), and blue (B), in an 8-bit two-dimensional array with a resolution of 600 DPI. Next, the control unit 50 performs image data processing in the image signal processing circuit 51 (S102). In this image data processing, the RGB image data is color-separated and overwritten into image memories MY, MM, MC, and MK as image data DY, DM, DC, and DK of four colors, namely yellow (Y), magenta (M), cyan (C), and black (K). Next, the control unit 50 starts an image forming operation (S103). When the image forming operation starts, the image data DY, DM, DC, and DK are converted into laser drive signals by the image signal processing circuit 51 and sent to the exposure devices 3 for the corresponding colors, thereby forming images as described above.

[0035] Although the image data described above is bitmap data, even if it is raster data, it is converted by the image signal processing circuit 51 into image data of four colors of YMCK in the same way as bitmap data.

[0036] Next, when the transfer of the image data to the exposure device 3 is completed, the control unit 50 performs a "determination as to whether to perform a charging roller cleaning operation" (S104). Details of this determination as to whether to perform a charging roller cleaning operation will be described later. If the control unit 50 determines that a charging roller cleaning operation is necessary, it performs a "charging roller cleaning operation" (S105). Details of this charging roller cleaning operation will be described later. On the other hand, if the control unit 50 determines that a charging roller cleaning operation is not necessary, the process proceeds to S106. Next, the control unit 50 checks whether "next image data exists" (S106). If the control unit 50 determines that unprocessed image data exists, the process returns to S102 and the above-mentioned operations are repeated. On the other hand, if the control unit 50 determines that processing of all image data has been completed and that there is no next image data, it ends the print job.

[0037] In this way, in this embodiment, the charge roller cleaning operation is performed, for example, during the inter-sheet step when no image is being formed. However, the present invention is not limited to this, and the charge roller cleaning operation may be performed during the post-rotation step, pre-rotation step, pre-multiple pre-rotation step, etc., as long as no image is being formed.

[0038] <Toner adhesion and accumulation on the charging roller> The adhesion and accumulation of toner on the surface of the charging roller 2 in this embodiment will be described.

[0039] Due to the influence of transfer at the primary transfer portion N1, the residual toner on the photosensitive drum 1 contains a certain proportion of toner charged with the polarity opposite to the normal charging polarity (positive polarity in this embodiment). Here, the residual toner on the photosensitive drum 1 refers to toner that adheres to the image area on the photosensitive drum 1 but is not transferred by primary transfer and remains on the photosensitive drum 1 ("residual transfer toner"), and toner that adheres to the background potential portion on the photosensitive drum 1 ("fogging toner").

[0040] Therefore, when the residual toner passes through the contact area with the charging roller 2 as the photosensitive drum 1 rotates, the electric field formed by the potential difference between the voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 causes positively charged toner in the residual toner to move onto the charging roller 2. In other words, toner charged with a polarity opposite to the normal charge polarity in the residual toner (hereinafter also referred to as "oppositely charged toner") moves onto the charging roller 2. Toner adhesion to the surface of the charging roller 2 occurs every time the residual toner passes through the contact area with the charging roller 2 as the photosensitive drum 1 rotates, so toner accumulates on the surface of the charging roller 2 as image formation operations and the like are repeated. Therefore, the distribution in the main scanning direction of the amount of toner adhesion to the charging roller 2 correlates with the distribution in the main scanning direction of the print pixel area of ​​the toner image on the photosensitive drum 1 that has passed through the primary transfer unit N1.

[0041] When toner adheres to the surface of the charging roller 2, the charging performance of the charging roller 2 changes, which changes the surface potential of the photosensitive drum 1 and leads to fluctuations in image density. Furthermore, the greater the amount of toner that adheres to the surface of the charging roller 2, the greater the change in charging performance and the greater the change in the surface potential of the photosensitive drum 1. Therefore, although the impact of a single image forming operation is small, if a certain amount of toner accumulates on the surface of the charging roller 2, the surface of the photosensitive drum 1 cannot be uniformly charged, and fluctuations in image density cannot be ignored.

[0042] In the image forming apparatus 100 of this embodiment, the amount of residual toner after transfer when a 100% yellow image was formed was measured to be approximately 2.0%. The method for measuring the amount of residual toner is as follows: The residual toner adhering to the photosensitive drum 1 after passing through the primary transfer portion N1 is collected with polyester tape and attached to a piece of high-quality paper. For reference, a piece of polyester tape is also attached to the high-quality paper, alongside the polyester tape mentioned above. The optical density of the polyester tape on the high-quality paper is measured using a whiteness meter TC-6DS manufactured by Tokyo Denshoku Kogyo Co., Ltd., through a filter complementary to the toner color. The difference between the two measurement results is then taken as the amount of residual toner.

[0043] A 100% yellow belt-shaped image extending in the sub-scanning direction was continuously printed on an A4-sized recording material S, and a sensory evaluation was conducted to determine the change in image density due to the adhesion of transfer residual toner to the charging roller 2. As a result, in the case of an image that was uniform in the main scanning direction (an image in which the above-mentioned belt-shaped images were evenly spread in the main scanning direction), density fluctuations were noticeable from the 100th page. In contrast, in the case of an image with a distribution in the main scanning direction (an image in which the above-mentioned belt-shaped images were formed unevenly in the main scanning direction), density fluctuations were noticeable from the 30th page.

[0044] Furthermore, the developing device 4 coats the developing roller 41 with toner charged negatively, which is the normal charging polarity. However, some of the toner becomes reversely charged toner, which is charged positively, which is the opposite polarity to the normal charging polarity. This reversely charged toner may adhere to the background potential portion of the photosensitive drum 1. This reversely charged toner adhering to the background potential portion of the photosensitive drum 1 is called "fogging toner." The fogging toner, which is reversely charged toner, is not primarily transferred onto the intermediate transfer belt 10 and adheres to the charging roller 2, just like the transfer residual toner. Since the fogging toner adheres uniformly to the entire surface of the photosensitive drum 1 when no image is being formed, it also adheres uniformly to the entire surface of the charging roller 2. In the image forming apparatus 100 of this embodiment, the amount of fogging toner measured in each image forming station SY, SM, SC, and SK was approximately 0.5%. The method for measuring the amount of fogging toner is the same as the method for measuring the amount of residual toner after transfer, except that the position where the toner is collected with polyester tape is between the developing roller 41 (developing position) and the primary transfer section N1 (primary transfer position) in the rotation direction of the photosensitive drum 1.

[0045] All-white images were continuously printed on A4-sized recording material P, and a sensory evaluation was conducted to see if there was any change in image density due to fogging toner adhering to the charging roller 2 of each image forming unit S. As a result, density fluctuations were noticeable from the 200th page.

[0046] <Charging roller cleaning operation> In this embodiment, the charging roller cleaning operation is performed by forming an electric field between the photosensitive drum 1 and the charging roller 2, and between the photosensitive drum 1 and the primary transfer roller 14, in the direction opposite to that during the image formation operation (more specifically, during the charging process and primary transfer).

[0047] The toner on the charging roller 2 is positively charged and electrostatically adheres to the surface of the charging roller 2. Therefore, by reversing the direction of the electric field between the charging roller 2 and the photosensitive drum 1 relative to the direction during image formation, the toner adhering to the surface of the charging roller 2 can be removed (cleaned). Here, the voltage applied to the charging roller 2 is preferably 100 V or more and Va V or less on the side of the polarity opposite to the normal charging polarity of the toner relative to the surface potential of the photosensitive drum 1, where Va V is the discharge inception voltage between the charging roller 2 and the photosensitive drum 1. This is because, if the voltage is less than 100 V, an electric field strong enough to move the positively charged toner from the charging roller 2 to the photosensitive drum 1 is not formed. On the other hand, if the voltage exceeds Va V, a discharge occurs between the charging roller 2 and the photosensitive drum 1, causing some of the positively charged toner on the charging roller 2 to turn negatively charged, preventing it from moving to the photosensitive drum 1.

[0048] In this way, the electric field between the charging roller 2 and the photosensitive drum 1 is directed in the opposite direction to that during image formation, so the charging roller cleaning operation (cleaning process, recovery process) is performed during non-image formation. Also, to prevent fluctuations in image density, when a certain amount of toner accumulates on the surface of the charging roller 2, it is necessary to clean the charging roller 2 before the surface of the photosensitive drum 1 can no longer be uniformly charged.

[0049] FIG. 4 is a timing chart of the charging roller cleaning operation in this embodiment. Here, we take as an example a case where the charging roller cleaning operation is performed in the post-rotation process for the next print job. FIG. 4 also shows the operation timing of each unit in one representative image forming unit S. The charging roller cleaning operation is controlled by the control unit 50.

[0050] When the image forming operation (more specifically, the charging, exposing, and developing processes for the image formed on the final recording material P in a print job) is completed, the developing roller 41 is first separated from the surface of the photosensitive drum 1. Then, at time T immediately after the trailing edge of the final toner image on the intermediate transfer belt 10 in the transport direction passes through the primary transfer portion N1K of the image forming station SK, which is the most downstream in the rotation direction of the intermediate transfer belt 10, the voltage applied to the primary transfer roller 14 is switched from +500 [V] to -900 [V]. The same charging voltage of -1100 [V] as during the image forming operation is applied to the charging roller 2, and the potential difference between the surface of the photosensitive drum 1 charged to -500 [V] and the primary transfer roller 14 to which a voltage of -900 [V] is applied does not exceed the discharge start voltage (discharge threshold). Therefore, the surface potential of the photosensitive drum 1 immediately before reaching the contact point with the charging roller 2 after passing through the primary transfer portion N1 is maintained at approximately -500 [V]. Here, the voltage applied to the primary transfer roller 14 of each image forming station S is switched after the primary transfer of the final image is completed in the most downstream image forming station SK. However, the present invention is not limited to this, and the voltage applied to the primary transfer roller 14 may be switched after the primary transfer of the final image is completed in each image forming station S.

[0051] Next, the voltage applied to the charging roller 2 is switched from −1100 [V], the same as during image formation, to 0 [V]. As a result, the electric field (potential gradient) formed by the potential difference between the potential of the charging roller 2 (0 [V]) and the surface potential of the photosensitive drum 1 (−500 [V]) is reversed from that during image formation. As a result, the positively charged toner adhering to the surface of the charging roller 2 moves onto the photosensitive drum 1. The positively charged toner that has moved onto the photosensitive drum 1 is then transported to the primary transfer portion N1 as the photosensitive drum 1 rotates. At the primary transfer portion N1, the potential difference between the surface potential of the photosensitive drum 1 (−500 [V]) and the voltage applied to the primary transfer roller 14 (−900 [V]) forms an electric field in a direction that moves the positively charged toner from the photosensitive drum 1 onto the intermediate transfer belt 10. This electric field moves the positively charged toner on the photosensitive drum 1 onto the intermediate transfer belt 10. The positively charged toner that has moved onto the intermediate transfer belt 10 is then transported to the area facing the belt cleaning device 17 through the secondary transfer portion N2 as the intermediate transfer belt 10 rotates, and is collected by the belt cleaning device 17. At least when the positively charged toner that has moved onto the intermediate transfer belt 10 reaches the secondary transfer portion N2, a DC voltage (e.g., +300 V) of the same polarity as that during image formation (positive in this embodiment) is applied to the secondary transfer roller 15. As a result, the positively charged toner on the intermediate transfer belt 10 passes through the secondary transfer portion N2 while remaining on the intermediate transfer belt 10. The secondary transfer roller 15 may be separated from the intermediate transfer belt 10 at least when the positively charged toner that has moved onto the intermediate transfer belt 10 reaches the secondary transfer portion N2.

[0052] Through the above steps, the toner adhering to the surface of the charging roller 2 can be removed (cleaned), and the charging performance of the charging roller 2 for the photosensitive drum 1 is restored.

[0053] In this embodiment, the dark decay rate of the surface potential of the photosensitive drum 1 is about 1.3 [V / sec], and the effect of the cleaning process of the charging roller 2 is negligible.

[0054] Furthermore, the execution time of the charge roller cleaning operation in this embodiment is the time it takes for the charge roller 2 to rotate three times so that the toner adhering to the charge roller 2 is sufficiently expelled. The execution time of the charge roller cleaning operation is preferably set to a time between the time it takes for the charge roller 2 to rotate one to ten times. Of these, the time it takes for the charge roller 2 to rotate two to four times is even more preferable.

[0055] Furthermore, the greater the magnitude of the electric field between the charging roller 2 and the photosensitive drum 1 and the electric field between the photosensitive drum 1 and the intermediate transfer belt 14 during the charging roller cleaning operation, the higher the expulsion efficiency. However, if these electric fields are too strong, discharge occurs, changing the polarity of the toner and reducing the expulsion efficiency. Furthermore, if the magnitude of these electric fields is too strong, discharge may cause surface deterioration of the photosensitive drum 1 and the intermediate transfer belt 14. During the charging roller cleaning operation, it is preferable that the surface potential of the photosensitive drum 1 be −200V to −800V, and the voltage applied to the primary transfer roller 14 be −400V to −1600V.

[0056] <Determining whether to clean the charging roller> In this embodiment, the determination of whether to perform the charging roller cleaning operation is based on the results of two calculation methods. One is based on image data (calculation method A), and the other is based on the cumulative number of printed sheets (calculation method B). The control of the determination of whether to perform the charging roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job.

[0057] (Calculation method A) First, calculation method A will be described. In calculation method A, the necessity of the charge roller cleaning operation is determined based on the amount of oppositely charged toner adhering to the charge roller 2 due to transfer residue remaining on the photosensitive drum 1. FIG. 5 is a schematic diagram for explaining the data processing process in calculation method A for determining whether to perform the charge roller cleaning operation in this embodiment. In calculation method A, the distribution in the main scanning direction of the amount of oppositely charged toner adhering to the charge roller 2 due to transfer residue is predicted based on image data as information about the toner image. In this embodiment, calculation method A determines whether to perform the charge roller cleaning operation in each of the image forming units SY, SM, SC, and SK for the colors yellow, magenta, cyan, and black.

[0058] First, the image data DY, DM, DC, and DK of the four colors YMCK stored in the image memories MY, MM, MC, and MK are binarized. The reason for this binarization is that, among the information about the amount of toner, the adhesion of oppositely charged toner due to transfer residue to the charging roller 2 has a higher correlation with the printing area than the absolute amount of toner.

[0059] Next, sub-scanning direction integration processing is performed. The control unit 50 has four temporary counter arrays RY, RM, RC, and RK, each of which has the same length as the number of pixels in the image memory in the scanning direction. Image data DY, DM, DC, and DK are integrated in the sub-scanning direction and overwritten as one-dimensional data DY2, DM2, DC2, and DK2 in the main scanning direction in the temporary counter arrays RY, RM, RC, and RK corresponding to each color. This one-dimensional data DY2, DM2, DC2, and DK2 corresponds to the main scanning direction distribution of the print pixel area of ​​the toner image on the photosensitive drum 1 that has passed through each of the primary transfer units N1Y, N1M, N1C, and N1K. This one-dimensional data DY2, DM2, DC2, and DK2 correlates to the amount of transfer residual toner (more specifically, the amount of reversely charged toner in the transfer residual toner) remaining on the photosensitive drum 1 after passing through each of the primary transfer units N1Y, N1M, N1C, and N1K.

[0060] Next, an accumulation process is performed. The control unit 50 has four color accumulation counter columns UY, UM, UC, and UK, each with a length equal to the number of pixels in the image memory in the scanning direction. The one-dimensional data DY2, DM2, DC2, and DK2 are added to the current values ​​of the accumulation counter columns UY, UM, UC, and UK corresponding to each color, and are then overwritten and stored as one-dimensional data DY3, DM3, DC3, and DK3 in the accumulation counter columns UY, UM, UC, and UK corresponding to each color. This one-dimensional data DY3, DM3, DC3, and DK3 correlates with the accumulated amount of toner adhering to the respective charging rollers 2Y, 2M, 2C, and 2K. The one-dimensional data DY2, DM2, DC2, and DK2 or the one-dimensional data DY3, DM3, DC3, and DK3 shown in FIG. 5 are examples of information related to the print image area obtained from image data. In this embodiment, the CPU, temporary counter row, cumulative counter row, etc. of the control unit 50 function as an acquisition unit that acquires information regarding the amount of toner in the toner image passing through the transfer unit, corresponding to each of multiple areas in the main scanning direction, based on the print pixel area.

[0061] Finally, a threshold check process is performed. In the threshold check process, the difference (Δ) between the count values ​​of the integrating counter arrays UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) between adjacent regions and the maximum value (max) alone are checked to see if they exceed predetermined thresholds. In this embodiment, the adjacent regions in the multiple regions along the rotation axis of the photosensitive drum 1 are adjacent regions in the multiple regions in the main scanning direction (corresponding to each storage region of the integrating counter array) divided by the number of pixels in the main scanning direction. The difference in values ​​between the adjacent regions is, for example, the difference between the count values ​​of the integrating counter arrays corresponding to the Nth and N+1th pixels in the main scanning direction, or the difference between the count values ​​of the integrating counter arrays corresponding to the N+1th and N+2th pixels. N is a natural number ranging from 1 to the maximum number of pixels in the main scanning direction. If it is determined that at least one of the above exceeds the threshold, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the integrating counter arrays UY, UM, UC, and UK are reset to their initial values ​​(0 in this embodiment). In this embodiment, if any one of the differences in the count values ​​of the integrating counter arrays between the plurality of adjacent regions exceeds the threshold, it is determined that the difference in the values ​​between the adjacent regions has exceeded the threshold. However, the present invention is not limited to this, and it can be determined that the threshold position has been exceeded if a predetermined number or more (at least one) of the differences in the values ​​between the plurality of adjacent regions exceeds the threshold. On the other hand, if it is determined that none of the above exceeds the threshold, the process proceeds to the next step (S106 in FIG. 3).

[0062] Performing threshold inspection processing on the difference in count values ​​between adjacent regions in the integrating counter row is equivalent to focusing on the relative difference in the amount of oppositely charged toner adhering in the main scanning direction, ignoring the amount that is uniformly adhered in the main scanning direction, with respect to the oppositely charged toner adhering to the charging roller 2 due to transfer residue. Also, performing threshold inspection processing on the maximum count value in the integrating counter row is equivalent to focusing on the absolute amount, including the amount that is uniformly adhered in the main scanning direction, with respect to the oppositely charged toner adhering to the charging roller 2 due to transfer residue.

[0063] In this embodiment, the threshold value for the count value of the integrating counter array is set to a value corresponding to the limit at which image density fluctuations due to adhesion of oppositely charged toner to the charging roller 2 resulting from transfer residue are visible. In this embodiment, the threshold value for the count value of the integrating counter array is set in terms of the number of pages, with the length of an A4-sized recording material P in the sub-scanning direction as a unit. In this embodiment, the threshold value for the difference in values ​​between adjacent regions (KΔ) is set to 20 pages, and the threshold value for the maximum value alone (Kmax) is set to 100 pages. However, these threshold values ​​are not limited to the values ​​in this embodiment, and can be set appropriately so as to sufficiently suppress image density fluctuations depending on the device configuration, the amount (proportion) of transfer residue toner generated, the amount (proportion) of oppositely charged toner in the transfer residue toner, etc.

[0064] (Calculation method B) In calculation method B, the need for charge roller cleaning operation is determined based on the amount of oppositely charged toner due to fogging that adheres to the charge roller 2. The fogging toner adheres almost uniformly in the main scanning direction to the charge roller 2 of the same image forming unit S where the fogging occurred. The amount of fogging toner that adheres to the charge roller 2 correlates with the cumulative number of printed pages. In calculation method B, the amount of oppositely charged toner due to fogging that adheres to the charge roller 2 is predicted based on the cumulative number of printed pages. Note that the determination of whether to perform charge roller cleaning operation using calculation method B is made independently for each of the image forming units SY, SM, SC, and SK for the yellow, magenta, cyan, and black colors.

[0065] The control unit 50 is provided with an accumulative print counter W that holds the accumulated number of printed sheets. The accumulative print counter W counts up by one each time image formation on one side of one sheet of recording material P is completed. In the threshold inspection process in calculation method B, it is inspected whether the count value of the accumulative print counter W (accumulated number of printed sheets) exceeds a preset threshold. If it is determined that the threshold is exceeded, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the accumulative print counter W is reset to its initial value (0 in this embodiment). On the other hand, if it is determined that the threshold is not exceeded, the process proceeds to the next step (S106 in FIG. 3).

[0066] In this embodiment, the threshold value of the cumulative print counter W is set to a value corresponding to the limit at which image density fluctuations caused by adhesion of oppositely charged toner to the charging roller 2 due to fogging become visible. In this embodiment, the threshold value of the cumulative print counter W is set to 200 pages for A4 size recording material P.

[0067] In this embodiment, calculation methods A and B were used independently. That is, in S104 of FIG. 3, a determination is made as to whether or not to perform the charge roller cleaning operation using both calculation methods A and B, and if it is determined that the operation is necessary using either method, the charge roller cleaning operation is performed in S105 of FIG. 3. However, the present invention is not limited to this, and a calculation method that combines these calculation methods may also be used. For example, if the charge roller cleaning operation is performed based on calculation method A, the cumulative print counter W used in calculation method B may be reset.

[0068] Furthermore, the charge roller cleaning operation can be performed independently by the image forming units SY, SM, SC, and SK for the yellow, magenta, cyan, and black colors when necessary. In this case, the accumulating counter column corresponding to the image forming unit S that performed the charge roller cleaning operation is reset. Alternatively, when it is determined that the charge roller cleaning operation is necessary for any of the image forming units SY, SM, SC, and SK for the yellow, magenta, cyan, and black colors, the charge roller cleaning operation can be performed synchronously by these image forming units SY, SM, SC, and SK. In this case, the accumulating counter columns corresponding to each image forming unit S are reset synchronously.

[0069] <Effects of the Example> Next, the effects of this embodiment will be described in comparison with a comparative example. The comparative example differs from this embodiment in that only the determination of whether to perform the charging roller cleaning operation is made based on the cumulative number of printed sheets (corresponding to calculation method B in this embodiment). In the comparative example, the threshold value of the cumulative print counter W is set to 20 pages. Except for this point, the configuration and operation of the image forming apparatus 100 in the comparative example are substantially the same as the configuration and operation of the image forming apparatus 100 in this embodiment.

[0070] In the comparative example, since information about the toner image is not taken into consideration, the charging roller cleaning operation is always performed every 20 pages regardless of what kind of image is formed.

[0071] In contrast, in this embodiment, information about the toner image (image data in this embodiment) is taken into consideration, so the charge roller cleaning operation is performed at an appropriate timing depending on the image data of the formed image. Also, in this embodiment, when an image with a uniform image pattern in the main scanning direction, in which changes in image density are unlikely to be visually noticeable, is formed evenly, the number of printed sheets until it is determined that a charge roller cleaning operation is necessary is substantially large. Therefore, excessive charge roller cleaning operations are prevented, and a decrease in the throughput of the image forming apparatus 100 is suppressed. In this embodiment, when image patterns that are uneven in the main scanning direction, in which changes in image density are likely to be visually noticeable, are not printed consecutively, the charge roller cleaning operation is performed at most every 200 pages.

[0072] Thus, according to this embodiment, the control unit 50 controls the charging roller cleaning operation to be performed at a timing when the number of recording materials P output is smaller when an image forming operation is performed in which a band-shaped image extending in a direction approximately perpendicular to the rotational axis direction of the photosensitive drum 1 is formed uniformly in the rotational axis direction and continuously formed on multiple recording materials P, and output.

[0073] Another feature of this embodiment is that the timing of the charge roller cleaning operation is determined based on the deviation in the integrated values ​​of index values ​​correlated with the amount of toner carried in each of multiple regions in the main scanning direction of the photosensitive drum 1, particularly the difference in values ​​between adjacent regions in the main scanning direction distribution. In other words, the timing of the charge roller cleaning operation is determined based on not only the maximum count value of the integration counter array but also the difference in values ​​between adjacent regions. Considering human visual characteristics, image density fluctuations due to toner adhesion to the charge roller 2 are less noticeable when they are uniform in the main scanning direction. By using the difference in count values ​​between adjacent regions in the integration counter array, it is possible to determine whether a situation is prone to noticeable image density fluctuations, ignoring the contribution of toner uniformly adhering to the entire area of ​​the charge roller 2. In this embodiment, by determining whether a situation is prone to noticeable image density fluctuations in this way, the timing of the charge roller cleaning operation can be appropriately determined.

[0074] With this configuration, according to this embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress a decrease in throughput and a fluctuation in image density. Therefore, according to this embodiment, by performing the charging roller cleaning operation appropriately, it is possible to suppress a decrease in image quality.

[0075] In this embodiment, image density fluctuations are suppressed by controlling the timing of the charge roller cleaning operation. However, similar effects can be achieved by controlling the execution time of the charge roller cleaning operation. For example, as shown in FIG. 9, the timing of the charge roller cleaning operation can be kept constant, and the charge roller cleaning operation can be extended to suppress image density fluctuations when the difference between adjacent count values ​​in the integrating counter array or the maximum value exceeds a corresponding predetermined threshold. Specifically, for example, the charge roller cleaning operation can be performed every 50 pages (or every post-rotation process). If the threshold is not exceeded, the normal charge roller cleaning operation is performed for one rotation of the charge roller 2. If the threshold is exceeded, the normal charge roller cleaning operation is performed for five rotations of the charge roller 2. In this way, adjusting the duration of each charge roller cleaning operation based on information about the toner image can minimize throughput degradation and effectively suppress image density fluctuations. Furthermore, multiple thresholds may be used instead of a single threshold. In other words, the execution time of the charge roller cleaning operation can be extended in stages (or continuously) depending on the information about the toner image. Also, both the timing of the charging roller cleaning operation and the duration of the charging roller cleaning operation may be controlled.

[0076] As shown in FIG. 10 , the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 during the charge roller cleaning operation can also be controlled. Specifically, for example, the charge roller cleaning operation is performed every 50 pages (or every post-rotation process). If the difference between the count values ​​of adjacent regions in the integrating counter array and the maximum value exceed corresponding predetermined thresholds, the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 are set to the same values ​​as in the above-described embodiment. On the other hand, if the thresholds are not exceeded, the absolute values ​​of the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 are set to smaller values ​​than in the above-described embodiment. Note that the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 may be changed stepwise (or continuously) depending on the difference between the count values ​​of adjacent regions in the integrating counter array and the maximum value. In this way, the magnitude of the electric field between the charge roller 2 and the photosensitive drum 1 and the magnitude of the electric field between the photosensitive drum 1 and the intermediate transfer belt 10 during the charge roller cleaning operation can be controlled based on information about the toner image (image data in this embodiment). In other words, when the difference or maximum value between adjacent regions in the count value of the integrating counter array is small, the unevenness of the amount of toner adhesion to the charging roller 2 and the amount of toner adhesion are small. Therefore, the electric field required to expel toner from the charging roller 2 to the photosensitive drum 1 and from the photosensitive drum 1 to the intermediate transfer belt 10 and thereby suppress image density fluctuations is small. Therefore, in this case, the electric field between the charging roller 2 and the photosensitive drum 1 and the electric field between the photosensitive drum 1 and the intermediate transfer belt 10 can be reduced. By using this control, an unnecessarily large electric field is not applied to the photosensitive drum 1 or the intermediate transfer belt 10, which would cause discharge, thereby reducing deterioration of the photosensitive drum 1 and the intermediate transfer belt 10. Note that, during the charging roller cleaning operation, it is also possible to control either the magnitude of the electric field between the charging roller 2 and the photosensitive drum 1 or the magnitude of the electric field between the photosensitive drum 1 and the intermediate transfer belt 10, and this produces a corresponding effect.Here, the example has been described in which control is performed to weaken the electric field in order to reduce deterioration of the photosensitive drum 1 and the intermediate transfer belt 10, but when the difference between the count values ​​of adjacent regions in the integrating counter row or the maximum value is large, control may be performed to strengthen the electric field, thereby increasing the discharge capacity of the cleaning operation of the charging roller 2. Also, it is possible to control both the magnitude of the electric field in the charging roller cleaning operation and at least one of the timing or duration of the charging roller cleaning operation.

[0077] [Example 2] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0078] In this embodiment, the threshold check process of calculation method A in determining whether or not to perform the charging roller cleaning operation is different from that in embodiment 1. In this embodiment, the effective range of the integrating counter row is defined, and the threshold check process is performed on the difference between the maximum value and the minimum value.

[0079] <Determining whether to clean the charging roller> In this embodiment, as in the first embodiment, the determination of whether to perform the charge roller cleaning operation is based on the results of two calculation methods. One is based on image data (reversely charged toner due to retransfer) (calculation method A), and the other is based on the cumulative number of printed pages (reversely charged toner due to fogging) (calculation method B). The control of the determination of whether to perform the charge roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job. Note that calculation method B is the same as in the first embodiment, so here we will explain calculation method A, which is characteristic of this embodiment, and omit a description of calculation method B.

[0080] (Calculation method A) As in the first embodiment, in calculation method A, the necessity of the charge roller cleaning operation is determined based on the amount of oppositely charged toner adhering to the charge roller 2 due to transfer residue remaining on the photosensitive drum 1. FIG. 6 is a schematic diagram for explaining the data processing process in calculation method A for determining whether to perform the charge roller cleaning operation in this embodiment. As in the first embodiment, calculation method A predicts the distribution in the main scanning direction of the amount of oppositely charged toner adhering to the charge roller 2 due to transfer residue based on image data as information about the toner image. In this embodiment, calculation method A determines whether to perform the charge roller cleaning operation in each of the image forming units SY, SM, SC, and SK for the colors yellow, magenta, cyan, and black.

[0081] First, the image data DY, DM, DC, and DK of four colors of YMCK stored in the image memories MY, MM, MC, and MK are binarized.

[0082] Next, the sub-scanning direction accumulation process is performed: the image data DY, DM, DC, and DK are accumulated in the sub-scanning direction and are overwritten and stored in the temporary counter rows RY, RM, RC, and RK corresponding to each color as one-dimensional data DY2, DM2, DC2, and DK2 in the main scanning direction.

[0083] Next, an accumulation process is performed. That is, the one-dimensional data DY2, DM2, DC2, and DK2 are added to the current values ​​of the accumulation counter columns UY, UM, UC, and UK corresponding to each color, and the accumulated values ​​are stored as one-dimensional data DY3, DM3, DC3, and DK3 in the accumulation counter columns UY, UM, UC, and UK corresponding to each color. The accumulation counter columns UY, UM, UC, and UK for each color are counter columns with lengths equal to the number of pixels in the scanning direction of the image memory. This one-dimensional data DY3, DM3, DC3, and DK3 correlates to the accumulated amount of toner adhering to the respective charging rollers 2Y, 2M, 2C, and 2K. The one-dimensional data DY2, DM2, DC2, and DK2 or the one-dimensional data DY3, DM3, DC3, and DK3 shown in FIG. 6 are examples of information related to the print image area obtained from image data. In this embodiment, the CPU, temporary counter row, cumulative counter row, etc. of the control unit 50 function as an acquisition unit that acquires information regarding the amount of toner in the toner image passing through the transfer unit, corresponding to each of multiple areas in the main scanning direction, based on the print pixel area.

[0084] Next, a range definition process is performed. In the range definition process, an effective range is defined for the count values ​​(one-dimensional data DY3, DM3, DC3, DK3) of the integrating counter arrays UY, UM, UC, and UK. In this embodiment, the effective range of the integrating counter arrays UY, UM, UC, and UK is defined by excluding an area where a toner image is unlikely to be formed. That is, in this embodiment, the effective range is the range of the integrating counter arrays UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) excluding a predetermined area where a toner image is unlikely to be formed. The area where a toner image is unlikely to be formed mostly refers to the margin area at the widthwise ends (typically both ends) of the recording material P, but also refers to an area within the image area where an image is unlikely to be formed, for example, at the widthwise ends of the recording material P. The width direction of the recording material P is substantially perpendicular to the conveyance direction of the recording material P, i.e., substantially parallel to the rotational axis direction of the photosensitive drum 1. The length of the recording material P in the width direction of the recording material P is simply referred to as the "width" of the recording material P, and the width of the image in the same direction is simply referred to as the "width" of the image. The image area is the area where a toner image can be formed. Specifically, in this embodiment, of the range of the integrating counter columns UY, UM, UC, and UK corresponding to the size of the recording material P selected in the print job, the area with the smallest count value, where a toner image is unlikely to actually be formed, is excluded, and the remaining 90% of the area is defined as the effective range of the integrating counter columns UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3). This essentially excludes the integrating counter columns UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) corresponding to the width direction ends of the recording material P where a toner image is unlikely to be formed. In other words, it is possible to determine whether to perform the charging roller cleaning operation based on values ​​of the accumulating counter columns UY, UM, UC, UK (one-dimensional data DY3, DM3, DC3, DK3) in a range narrower than the size (width) of the recording material P in the direction of the rotation axis of the photosensitive drum 1.

[0085] In this embodiment, regardless of the size (width) of the recording material P, the effective range of the integrating counter arrays UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) is set to 10% of the area from the smallest count value and 90% of the area from the largest count value. However, the margin area at the widthwise end of the recording material P is often uniform regardless of the size (width) of the recording material P, and the narrower the recording material P, the greater the proportion of the margin. Therefore, for example, the value of the effective range may be changed depending on the size (width) of the recording material P. Typically, when the width of the recording material P is a first width, the proportion of the excluded area within the range of the integrating counter array corresponding to the first width is set to a first proportion (%), and when the width of the recording material is a second width smaller than the first width, the proportion of the excluded area within the range of the integrating counter array corresponding to the second width is set to a second proportion (%) greater than the first proportion (%). For example, the percentage of the area to be excluded in the case of LTR size (portrait feeding) is set to 9%, and the percentage of the area to be excluded in the case of A4 size (portrait feeding) is set to 10%.

[0086] Finally, a threshold check process is performed. In the threshold check process, two count values ​​(one-dimensional data DY3, DM3, DC3, DK3) of the integrating counter arrays UY, UM, UC, and UK are checked to see if they exceed predetermined thresholds: the difference (Δ) between the maximum and minimum values ​​within the effective range, and the maximum value (max) within the effective range. If it is determined that at least one of these values ​​exceeds the threshold, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the integrating counter arrays UY, UM, UC, and UK are reset to their initial values ​​(0 in this embodiment). On the other hand, if it is determined that none of the above values ​​exceeds the threshold, the process proceeds to the next step (S106 in FIG. 3).

[0087] In this embodiment, the threshold value for the count value of the integrating counter array is set to a value corresponding to the limit at which image density fluctuations due to the adhesion of oppositely charged toner to the charging roller 2 resulting from transfer residue are visible. As in the first embodiment, in this embodiment, the threshold value for the count value of the integrating counter array is set in terms of the number of pages, with the length of an A4-sized recording material P in the sub-scanning direction as a unit. In this embodiment, the threshold value (KΔ) for the difference between the maximum and minimum values ​​is set to 20 pages, and the threshold value (Kmax) for the maximum value alone is set to 100 pages. However, these threshold values ​​are not limited to the values ​​in this embodiment, and can be set appropriately so as to sufficiently suppress image density fluctuations depending on the device configuration, the amount (proportion) of transfer residue toner generated, the amount (proportion) of oppositely charged toner in the transfer residue toner, etc.

[0088] The percentage (%) of the excluded area in the accumulating counter array, which is the predetermined condition that the excluded area must satisfy, is not limited to the value in this embodiment. This is set appropriately depending on the device configuration, the size of the margin area, the image to be formed, and the like so as to sufficiently suppress image density fluctuations. Also, in this embodiment, a predetermined percentage of the area with the smallest count value within the range of the accumulating counter array corresponding to the size of the recording material P is excluded, but it is also possible to exclude, for example, an area with a count value equal to or less than a predetermined value (for example, equal to or less than one page in the above-mentioned page count).

[0089] <Effects of the Example> Next, the effects of this embodiment compared with the comparative example will be described. Here, the description of the same effects as those described in the first embodiment will be omitted, and the effects characteristic of this embodiment will be described.

[0090] In this embodiment, information about the toner image (image data in this embodiment) is taken into consideration, so the charging roller cleaning operation is performed at an appropriate timing depending on the image data of the formed image. Also, in this embodiment, when an image with a uniform image pattern in the main scanning direction, in which changes in image density are difficult to see, is formed evenly, the number of printed sheets until it is determined that the charging roller cleaning operation needs to be performed is substantially large. Therefore, excessive charging roller cleaning operations are prevented, and a decrease in the throughput of the image forming apparatus is suppressed.

[0091] Another feature of this embodiment is that the timing of the charge roller cleaning operation is determined based on the deviation of the integrated value of the index value correlating with the amount of toner carried in each of multiple regions of the photosensitive drum 1 in the main scanning direction. Specifically, the difference between the maximum and minimum values ​​within an effective range excluding a portion of the width of the recording material P or the width of the image where a toner image is unlikely to form is used to determine the timing of the charge roller cleaning operation. In other words, the timing of the charge roller cleaning operation is determined based on not only the maximum count value but also the difference between the maximum and minimum count values ​​of the integrated counter array. Considering human visual characteristics, when image density fluctuations due to toner adhesion to the charge roller 2 are uniform in the main scanning direction or at the widthwise edges of the recording material P or the image where a toner image is unlikely to form, the image density fluctuations are less noticeable. Furthermore, because the image data at the widthwise edges of the recording material P or the image are small, the minimum value is likely to be used when calculating the difference between the maximum and minimum values. In other words, even though the image density fluctuations are still unlikely to occur in the high-tolerance regions where a toner image is likely to form, it may be determined that the charge roller cleaning operation needs to be performed. Therefore, by using the difference between the maximum and minimum count values ​​in the effective range of the integrating counter array, it is possible to determine whether a situation in which a toner image is likely to be formed is prone to noticeable image density fluctuations, while ignoring the contribution of toner uniformly adhering to the entire area of ​​the charging roller 2. In this embodiment, by determining whether a situation in which a noticeable image density fluctuation is likely to be formed is prone to occur in this manner, it is possible to appropriately determine the timing of performing the charging roller cleaning operation. Note that, according to the method of this embodiment, even if the area in which a toner image is likely to be formed is the center rather than the end in the width direction of the recording material P, the count value of the integrating counter array in that area can be prevented from being used as the minimum value. This makes it possible to effectively suppress image density fluctuations in areas other than the area in which a toner image is likely to be formed.

[0092] With this configuration, according to this embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress a decrease in throughput and a fluctuation in image density. Therefore, according to this embodiment, by performing the charging roller cleaning operation appropriately, it is possible to suppress a decrease in image quality.

[0093] The calculation method (determination method) of this embodiment may be combined with the control of the duration of the charging roller cleaning operation and the magnitude of the electric field in the charging roller cleaning operation, as described in the first embodiment.

[0094] [Example 3] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0095] In this embodiment, a laser drive signal converted from image data is used as information about the toner image, unlike in Embodiment 1. Fig. 7 is a schematic diagram for explaining the data processing process in calculation method A for determining whether to perform the charging roller cleaning operation in this embodiment.

[0096] In this embodiment, the laser drive signal is used, and the determination of whether to perform the charging roller cleaning operation is performed simultaneously with the image formation operation. The laser drive signal is 1-bit one-dimensional data that controls laser illumination and is sequentially transmitted from the image signal processing circuit 51 to the exposure device 3 during image formation. The sub-scanning direction integration process for the laser drive signals for each color—yellow (Y), magenta (M), cyan (C), and black (K)—is performed during the main scanning period of the laser for that color, and the results are stored in the temporary counter rows RY, RM, RC, and RK, respectively. This results in one-dimensional data DY4, DM4, DC4, and DK4, which represent the main scanning direction distribution of laser illumination (emission time) for each image. Here, like the one-dimensional data DY2, DM2, DC2, and DK2 in the first embodiment, the one-dimensional data DY4, DM4, DC4, and DK4 correlate with the amount of transfer residual toner (more specifically, the amount of oppositely charged toner in the transfer residual toner) remaining on the photosensitive drum 1 after passing through the primary transfer portion N1.

[0097] The subsequent processing can be the same as the processing after the accumulation processing in Embodiments 1 and 2. One-dimensional data DY4, DM4, DC4, and DK4, or one-dimensional data DY5, DM5, DC5, and DK5 shown in Fig. 7 are examples of information related to the area of ​​the printed image obtained from image data. In this embodiment, as in Embodiment 1, the CPU, temporary counter row, and accumulation counter row of the control unit 50 function as an acquisition unit that acquires information related to the amount of toner in the toner image passing through the transfer unit, corresponding to each of multiple regions in the main scanning direction, based on the area of ​​the printed pixels.

[0098] Thus, in this embodiment, the information regarding the printing pixel area is the emission time of light that the exposure device 3, which irradiates light onto the photosensitive drum 1, irradiates onto the photosensitive drum 1 in accordance with the toner image formed on the photosensitive drum 1.

[0099] As described above, according to this embodiment, the same effects as those of the first and second embodiments can be obtained, and by using the laser drive signal as information related to the toner image, the memory and calculation amount required for image data processing can be reduced.

[0100] [Example 4] Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the image forming apparatus of embodiment 1. Therefore, in the image forming apparatus of this embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of embodiment 1 are assigned the same reference numerals as those of embodiment 1, and detailed descriptions thereof will be omitted.

[0101] In this embodiment, the decision to perform the charging roller cleaning operation is made based on the amount of toner adhering to the charging roller 2 due to "retransfer," a phenomenon in which toner from a toner image formed in the upstream image forming unit S adheres to the photosensitive drum 1 of the downstream image forming unit S.

[0102] <Toner adhesion to charging roller> First, we will explain toner adhesion to the charging roller 2 due to retransfer. The image forming apparatus 100 in this embodiment employs an inline system. Therefore, when a toner image formed on the intermediate transfer belt 10 at the upstream image forming unit S passes through the downstream primary transfer unit N1, a phenomenon known as "retransfer" occurs, in which some of the toner in the toner image transfers to the photosensitive drum 1 of the downstream image forming unit S. This occurs because, at the downstream primary transfer unit N1, the toner on the intermediate transfer belt 10 becomes reversely charged toner, charged to the opposite polarity from its normal polarity, due to a discharge caused by a potential difference between the background potential of the photosensitive drum 1 and the primary transfer roller 14. The toner that adheres to the photosensitive drum 1 due to retransfer is called "retransferred toner." A voltage of -1100 V is applied to the charging roller 2 during image formation, attracting the retransferred toner, which is the reversely charged toner. Therefore, the distribution of toner adhesion in the main scanning direction on the charging roller 2 correlates with the distribution in the main scanning direction of the print pixel area of ​​the toner image on the intermediate transfer belt 10 that has passed through the primary transfer unit N1. In an in-line image forming apparatus, the oppositely charged toner adhering to the charging member may be significantly reversely charged due to retransfer.

[0103] In the image forming apparatus of this embodiment, the amount of retransferred toner when forming a 100% yellow image was measured at the black image forming unit SK and was approximately 1.0%. The method for measuring the amount of retransferred toner is the same as the method for measuring the amount of residual toner after transfer described in Example 1.

[0104] A 100% yellow belt-shaped image extending in the sub-scanning direction was continuously printed on an A4-sized recording material S, and a sensory evaluation was conducted to determine the change in image density due to the adhesion of retransferred toner to the charging roller 2. The evaluation of the change in image density was conducted using the black image forming unit SK. As a result, for an image that was uniform in the main scanning direction (an image in which the above-mentioned belt-shaped images were evenly laid out in the main scanning direction), density fluctuations were noticeable from the 100th page. In contrast, for an image with a distribution in the main scanning direction (an image in which the above-mentioned belt-shaped images were formed unevenly in the main scanning direction), density fluctuations were noticeable from the 30th page.

[0105] The adhesion of the oppositely charged toner to the charging roller 2 due to fogging described in the first embodiment and the charging roller cleaning operation are the same as those in the first embodiment, and therefore will not be described here.

[0106] <Determining whether to clean the charging roller> In this embodiment, as in the first embodiment, the determination of whether to perform the charge roller cleaning operation is based on the results of two calculation methods. One is based on image data (reversely charged toner due to retransfer) (calculation method A), and the other is based on the cumulative number of printed pages (reversely charged toner due to fogging) (calculation method B). The control of the determination of whether to perform the charge roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job. Note that calculation method B is the same as in the first embodiment, so here we will explain calculation method A, which is characteristic of this embodiment, and omit a description of calculation method B.

[0107] (Calculation method A) In calculation method A, the necessity of charge roller cleaning operation is determined based on the amount of oppositely charged toner adhering to charge roller 2 due to re-transfer. Fig. 8 is a schematic diagram for explaining the data processing process in calculation method A for determining whether to perform charge roller cleaning operation in this embodiment. In calculation method A, the distribution in the main scanning direction of the amount of oppositely charged toner adhering to charge roller 2 due to re-transfer is predicted based on image data as information about the toner image. In this embodiment, calculation method A determines whether to perform charge roller cleaning operation for each of the image forming units SM, SC, and SK for magenta, cyan, and black.

[0108] First, the image data DY, DM, and DC for the three colors YMC stored in the image memories MY, MM, and MC are binarized. The reason for this binarization is that retransfer, which is the cause of reversely charged toner adhering to the charging roller 2, has a higher correlation with the print area than with the absolute amount of toner among the information about the amount of toner. Furthermore, since retransfer is a phenomenon in which toner from a toner image formed in the upstream image forming unit S adheres to the photosensitive drum 1 of the downstream image forming unit S, the image from the most downstream image forming unit SK does not contribute to retransfer. Therefore, the black image data DK is not processed.

[0109] Next, color superimposition processing is performed. The result DK6 of the sum of the yellow image data DY, magenta image data DM, and cyan image data DC is stored in image memory MK. The result DC6 of the sum of the yellow image data DY and magenta image data DM is stored in image memory MC. The yellow image data DY is stored in image memory MM (image data DY is renamed image data DM6). The data in image memory MY is reset and will no longer be used. In this embodiment, the sum is performed element by element for each image data. At this point, the image data DM6, DC6, and DK6 stored in image memories MM, MC, and MK correspond to the binarized image data of the toner images on intermediate transfer belt 10 that have passed through primary transfer units N1M, N1C, and N1K, respectively.

[0110] Next, sub-scanning direction integration processing is performed. The control unit 50 has three temporary counter columns RM, RC, and RK, each of which has the same length as the number of pixels in the image memory in the scanning direction. Image data DM6, DC6, and DK6 are integrated in the sub-scanning direction and overwritten as one-dimensional data DM7, DC7, and DK7 in the main scanning direction in the temporary counter columns RM, RC, and RK corresponding to each color. This one-dimensional data DM7, DC7, and DK7 corresponds to the main scanning direction distribution of the print pixel area of ​​the toner image on the intermediate transfer belt 10 that has passed through each of the primary transfer stations N1M, N1C, and N1K. This one-dimensional data DM7, DC7, and DK7 correlates to the amount of retransferred toner moving to the photosensitive drum 1 at each of the primary transfer stations N1M, N1C, and N1K.

[0111] Next, the accumulation process is performed. The control unit 50 has three color accumulation counter columns UM, UC, and UK, each with a length equal to the number of pixels in the image memory in the scanning direction. The one-dimensional data DM7, DC7, and DK7 are accumulated to the current values ​​of the accumulation counter columns UM, UC, and UK corresponding to each color, and are then overwritten and stored as one-dimensional data DM8, DC8, and DK8 in the accumulation counter columns UM, UC, and UK corresponding to each color. This one-dimensional data DM8, DC8, and DK8 correlates with the accumulated amount of toner adhering to the respective charging rollers 2M, 2C, and 2K. The one-dimensional data DM7, DC7, and DK7 or the one-dimensional data DM8, DC8, and DK8 shown in FIG. 8 are examples of information related to the print image area acquired from the image data. In this embodiment, the CPU, temporary counter column, accumulation counter column, and the like of the control unit 50 function as an acquisition unit that acquires information related to the amount of toner in the toner image passing through the transfer unit for each of multiple regions in the main scanning direction based on the print pixel area.

[0112] Finally, a threshold check process is performed. In the threshold check process, the difference (Δ) between the values ​​in adjacent regions and the maximum value (max) alone in the count values ​​(one-dimensional data DM8, DC8, DK8) of the integrating counter columns UM, UC, and UK are checked to see if they exceed the corresponding predetermined thresholds. If it is determined that at least one of these values ​​exceeds the threshold, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the integrating counter columns UM, UC, and UK are reset to their initial values ​​(0 in this embodiment). On the other hand, if it is determined that none of the above values ​​exceeds the threshold, the process proceeds to the next step (S106 in FIG. 3).

[0113] Performing threshold inspection processing on the difference in count values ​​between adjacent regions in the integrating counter row is equivalent to focusing on the relative difference in the amount of oppositely charged toner adhering in the main scanning direction, ignoring the amount that is uniformly adhered in the main scanning direction, with respect to the oppositely charged toner that is adhering to the charging roller 2 due to re-transfer. Also, performing threshold inspection processing on the maximum count value in the integrating counter row is equivalent to focusing on the absolute amount, including the amount that is uniformly adhered in the main scanning direction, with respect to the oppositely charged toner that is adhering to the charging roller 2 due to re-transfer.

[0114] In this embodiment, the threshold value for the count value of the integrating counter array is set to a value corresponding to the limit at which image density fluctuations due to adhesion of oppositely charged toner to the charging roller 2 resulting from retransfer are visible. As in the first embodiment, in this embodiment, the threshold value for the count value of the integrating counter array is set in number of pages, with the length of an A4-sized recording material P in the sub-scanning direction as a unit. In this embodiment, the threshold value (KΔ) for the difference in values ​​between the adjacent regions is set to 20 pages, and the threshold value (Kmax) for the maximum value alone is set to 100 pages. However, these threshold values ​​are not limited to the values ​​in this embodiment, and can be set appropriately so as to sufficiently suppress image density fluctuations depending on the device configuration, the amount (proportion) of retransferred toner generated, etc.

[0115] As in Example 1, in this example, calculation methods A and B were used independently. That is, in S104 of FIG. 3, a determination is made as to whether or not to perform the charge roller cleaning operation using both calculation methods A and B, and if it is determined that the operation is necessary using either method, the charge roller cleaning operation is performed in S105 of FIG. 3. However, the present invention is not limited to this, and a calculation method that combines these calculation methods may also be used. For example, if the charge roller cleaning operation is performed based on calculation method A, the cumulative print counter W used in calculation method B may be reset.

[0116] Furthermore, the charge roller cleaning operation can be performed independently by the image forming units SM, SC, and SK for magenta, cyan, and black, when necessary. In this case, the accumulating counter column corresponding to the image forming unit S that performed the charge roller cleaning operation is reset. Alternatively, the charge roller cleaning operation can be performed synchronously by the image forming units SM, SC, and SK for magenta, cyan, and black, when it is determined that the charge roller cleaning operation is necessary. In this case, the accumulating counter column corresponding to each image forming unit S is reset synchronously.

[0117] <Effects of the Example> Next, the effects of this embodiment compared with the comparative example will be described. Here, the description of the same effects as those described in the first embodiment will be omitted, and the effects characteristic of this embodiment will be described.

[0118] In this embodiment, information about the toner image (image data in this embodiment) is taken into consideration, so the charging roller cleaning operation is performed at an appropriate timing depending on the image data of the formed image. Also, in this embodiment, when an image with a uniform image pattern in the main scanning direction, in which changes in image density are difficult to see, is formed evenly, the number of printed sheets until it is determined that the charging roller cleaning operation needs to be performed is substantially large. Therefore, excessive charging roller cleaning operations are prevented, and a decrease in the throughput of the image forming apparatus is suppressed.

[0119] Another feature of this embodiment is that the amount of toner adhering to the charge roller 2 of a certain image forming station S is estimated based on image data of an image formed in the image forming station S upstream of that image forming station S. In other words, the data added to the cumulative print counter is image data of the image forming station S upstream of the target image forming station S. In the inline system, the toner adhering to the charge roller 2 of a certain image forming station S can be significantly retransferred toner from a toner image formed on the intermediate transfer belt 10 in the image forming station S upstream of that image forming station S. In other words, the amount of toner adhering to the charge roller 2 of a certain image forming station S can be estimated based on image data of a toner image formed in the image forming station S upstream of that image forming station S. In this embodiment, by estimating the amount of toner adhering to the charge roller 2 in this manner, the timing of the charge roller cleaning operation can be appropriately determined.

[0120] With this configuration, according to this embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress a decrease in throughput and a fluctuation in image density. Therefore, according to this embodiment, by performing the charging roller cleaning operation appropriately, it is possible to suppress a decrease in image quality.

[0121] In this embodiment, with regard to the amount of toner adhesion to the charging roller 2 due to retransfer, threshold inspection processing was performed on the difference in count values ​​between adjacent regions of the integrating counter row, as in embodiment 1. However, the present invention is not limited to this, and it is also possible to define an effective range of the integrating counter row as in embodiment 2, and perform threshold inspection processing on the maximum and minimum values ​​within that effective range.

[0122] In addition to the method of binarizing the image data stored in the image memory described in this embodiment, a method using a laser drive signal converted from the image data described in the third embodiment may also be used to obtain information about the toner image.

[0123] Furthermore, the calculation method (determination method) of this embodiment may be combined with the control of the duration of the charging roller cleaning operation and the magnitude of the electric field in the charging roller cleaning operation, as described in the first embodiment.

[0124] Furthermore, the amount of oppositely charged toner adhering to the charging roller 2 due to each of transfer residue and retransfer, or transfer residue, retransfer and fogging, may be predicted (calculated), and the charging roller cleaning operation may be performed if it is determined to be necessary based on either prediction result. Alternatively, methods for predicting (calculating) the amount of oppositely charged toner adhering to the charging roller 2 due to each of transfer residue and retransfer, or transfer residue, retransfer and fogging, may be combined. For example, when the charging roller cleaning operation is performed based on one prediction method, the count value of the cumulative counter for the other prediction method may be reset.

[0125] [others] Although the present invention has been described above with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

[0126] In the above-described embodiment, the multiple regions in the rotation axis direction of the photosensitive drum are multiple regions in the main scanning direction divided by the number of pixels in the main scanning direction, but the present invention is not limited to this. The rotation axis direction of the photosensitive drum may be divided into multiple regions at predetermined lengths, such as at intervals of a predetermined number of pixels in the main scanning direction.

[0127] In the above-described embodiment, the image forming apparatus has been described as employing an intermediate transfer system, but the present invention can also be applied to a direct transfer image forming apparatus. As is well known to those skilled in the art, an inline image forming apparatus employing a direct transfer system has a recording material carrier formed of an endless belt or the like instead of the intermediate transfer member in the above-described embodiment. The toner image formed on the photosensitive drum of each image forming station is then directly transferred to a recording material carried and transported on the recording material carrier, similar to the primary transfer in an intermediate transfer image forming apparatus. Applying the present invention in accordance with the above-described embodiment to such an image forming apparatus can achieve the same effects as those of the above-described embodiment.

[0128] The present invention can also be applied to a monochrome image forming apparatus having a single image forming unit, in which case the cleaning operation of the charging member can be controlled in the same manner as in the above embodiment, based on an index value corresponding to the amount of toner in the toner image passing through a transfer unit that directly transfers the toner image from the photosensitive drum to the recording material.

[0129] Furthermore, the photosensitive member is not limited to a drum-shaped member (photosensitive drum) but may be an endless belt-shaped member (photosensitive belt). Furthermore, the intermediate transfer member and recording material carrier are not limited to an endless belt-shaped member but may be, for example, a drum-shaped member formed by stretching a film over a frame. Furthermore, in an electrostatic recording type image forming apparatus, the image carrier may be a drum-shaped or endless belt-shaped electrostatic recording dielectric.

[0130] Furthermore, the adhesion of residual toner to the charging member is particularly pronounced in cleanerless image forming apparatuses. Therefore, the present invention is particularly effective in cleanerless image forming apparatuses. However, the present invention is not limited to this, and can also be applied to image forming apparatuses equipped with dedicated cleaning means for removing residual toner from the image carrier. In this case, a cleaning operation can be performed to effectively remove residual toner that has not been completely removed by the cleaning means and remains on the charging member, thereby preventing a deterioration in image quality. [Explanation of symbols]

[0131] 1 Photosensitive drum 2 Charging roller 3 Exposure equipment 4. Developing device 10 Intermediate transfer belt 14 Primary transfer roller 15 Secondary transfer roller 17 Belt cleaning device 100 Image forming device

Claims

1. an image forming section including a rotatable image carrier, a charging member that contacts the image carrier to charge the image carrier, and a developing member that supplies toner to the image carrier, and which forms a toner image on the image carrier based on image data and transfers the toner image from the image carrier onto a transfer target that contacts the image carrier in a transfer section; an acquisition unit that acquires an integrated value of index values ​​corresponding to the amount of toner in a toner image that passes through the transfer unit for each of a plurality of regions on the image carrier in the direction of the rotation axis of the image carrier based on information about a print pixel area acquired from the image data; a control unit capable of executing a cleaning operation for cleaning the charging member when no image formation is being performed; and the cleaning operation is composed of a first cleaning operation that is executed based on information about the integrated value, and a second cleaning operation that is executed based on information about the number of prints on the recording material, The control unit a region among the plurality of regions that includes a region in which the integrated value is within a predetermined ratio from the minimum and in which the integrated value is the minimum is designated as an exclusion region, and in an effective region obtained by excluding the exclusion region from the plurality of regions, when both the integrated value in a first region and the integrated value in a second region adjacent to the first region in the rotation axis direction exceed predetermined threshold values, the first cleaning operation is controlled to be executed; an image forming apparatus configured to control the timing of executing the first cleaning operation and the timing of executing the second cleaning operation so as to be different from each other;

2. a plurality of image forming units are provided along a moving direction of the transfer medium, the plurality of image forming units including a first image forming unit and a second image forming unit disposed downstream of the first image forming unit in the moving direction of the transfer medium, and toner images can be sequentially transferred onto the transfer medium at the transfer unit of the first image forming unit and the transfer unit of the second image forming unit; the acquiring unit acquires the integrated value of index values ​​corresponding to the amount of toner in the toner image formed on the image carrier of the second image forming unit and passing through the transfer unit of the second image forming unit, based on information about the print pixel area acquired from the image data related to the toner image formed by the second image forming unit; The image forming apparatus according to claim 1 , wherein the control unit controls the cleaning operation in the second image forming unit based on the integrated value acquired in relation to the second image forming unit.

3. 3. The image forming apparatus according to claim 1, wherein the control section controls the cleaning operation so that an electric field is formed between the image carrier and the charging member in a direction opposite to that during image formation.

4. 4. An image forming apparatus according to claim 1, wherein the information regarding the printing pixel area is information regarding the emission time of light irradiated onto the image carrier by an exposure device that irradiates light onto the image carrier in accordance with a toner image formed on the image carrier.

5. 5. The image forming apparatus according to claim 1, wherein toner remaining on the image carrier without being transferred onto the transfer medium during image formation is collected by the developing member.

6. 6. The image forming apparatus according to claim 1, wherein the toner is a one-component developer.

Images