JP2023000953A - Image forming apparatus - Google Patents

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

The present invention relates to an image forming apparatus such as a printer, a copier, a facsimile machine, etc., using an electrophotographic system, an electrostatic recording system, or the like.

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic printer, a toner image formed on a photosensitive member as an image bearing member is transferred onto a recording sheet as a transfer receiving member. In an intermediate transfer type image forming apparatus, a toner image formed on a photosensitive member is primarily transferred to an intermediate transfer member as a transfer target, and then secondarily transferred to a recording material. The transfer of the toner image from the photoreceptor to the transfer material is performed by applying a transfer voltage to a transfer member such as a transfer roller arranged corresponding to the photoreceptor with the transfer material interposed therebetween.

In such an image forming apparatus, residual toner on the photoreceptor is removed. Conventionally, residual toner is generally removed from the photoreceptor and collected by a cleaning device having a cleaning member such as a cleaning blade and a cleaning container provided with the cleaning member. On the other hand, there has been proposed a system (cleanerless system) that does not have a dedicated cleaning means for collecting the residual toner on the photoreceptor (Patent Document 1). According to this method, since the residual toner is collected in the developing device, the residual toner can be collected without providing a dedicated cleaning means. Therefore, a dedicated space for holding the collected toner becomes unnecessary, and the size and cost of the image forming apparatus can be reduced.

By the way, residual toner on the photoreceptor may adhere to a charging member such as a charging roller disposed in contact with the photoreceptor. When the toner adheres to the charging member, the charging ability for charging the photoreceptor changes, and the latent image potential of the portion corresponding to the position on the charging member where the toner adheres changes, resulting in a change in image density. Adhesion of residual toner to the charging member becomes significant in the cleanerless image forming apparatus.

Japanese Patent Application Laid-Open No. 2002-200003 proposes a configuration in which a cleaning operation of the charging member is performed at a predetermined timing as a maintenance operation for suppressing the change in image density. Specifically, in this cleaning operation, the toner adhering to the charging member is ejected onto the photoreceptor, then the toner ejected onto the photoreceptor is transferred to the intermediate transfer body, and the cleaning blade removes the toner on the intermediate transfer body. is recovered by a recovery member such as

Japanese Patent Application Laid-Open No. 2002-200003 proposes a method of determining the timing of performing cleaning operation of the charging member based on the cumulative number of printed sheets, the cumulative number of printed pixels, or the cumulative exposure time.

Further, Japanese Patent Application Laid-Open No. 2002-200003 proposes a method of determining the timing for performing the cleaning operation of the charging member according to a pixel number count value obtained by counting the number of printed pixels for each position in the longitudinal direction of the charging member. .

Japanese Patent Application Laid-Open No. 2004-126202 JP 2015-11159 A

It is desired that the charging member is cleaned sufficiently. For example, if the cleaning operation of the charging member is performed at short intervals, the throughput (the number of images that can be output per unit time) of the image forming apparatus is reduced, and if it is performed for a long time, image defects occur and the image quality deteriorates.

SUMMARY OF THE INVENTION The present invention is a further development of the above-described prior art, and an object of the present invention is to suppress deterioration in image quality by suitably performing cleaning operation of a charging member.

The above object is achieved by an image forming apparatus according to the present invention. In summary, a representative configuration of the present invention includes a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and supplies toner to the image carrier. a developing member for forming a toner image on the image carrier based on image data, and transferring the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer section; and toner passing through the transfer unit for each of a plurality of regions on the image carrier in the direction of the rotational axis of the image carrier based on information about the print pixel area obtained from the image data. an acquisition unit that acquires an integrated value of index values corresponding to the amount of toner in an image; The image forming apparatus is characterized in that the timing of executing the cleaning operation is controlled based on the difference in the integrated value between adjacent areas in a plurality of areas.

Another representative configuration of the present invention includes a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and supplies toner to the image carrier. a developing member for forming a toner image on the image carrier based on image data, and transferring the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer section; and toner passing through the transfer unit for each of a plurality of regions on the image carrier in the direction of the rotational axis of the image carrier based on information about the print pixel area obtained from the image data. an acquisition unit that acquires an integrated value of index values corresponding to the amount of toner in an image; Based on the integrated value in a valid range area excluding an area that satisfies a predetermined condition among a plurality of areas and having a larger integrated value than the integrated value in the excluded area, The image forming apparatus is characterized in that the timing of executing the cleaning operation is controlled.

According to the present invention, deterioration of image quality can be suppressed by appropriately performing the cleaning operation of the charging member.

1 is a schematic cross-sectional view of an image forming apparatus; FIG. 3 is a schematic block diagram showing a control mode of the image forming apparatus; FIG. FIG. 4 is a flow chart diagram showing an outline of an operation sequence of a print job; FIG. 5 is a timing chart for explaining the charging roller cleaning operation; FIG. 10 is a schematic diagram for explaining a calculation method A in determining whether or not to perform a charging roller cleaning operation according to the first embodiment; FIG. 11 is a schematic diagram for explaining a calculation method A in determining whether to perform a charging roller cleaning operation in Example 2; FIG. 11 is a schematic diagram for explaining a calculation method A in determining whether or not to perform a charging roller cleaning operation in Example 3; FIG. 11 is a schematic diagram for explaining a calculation method A in determining whether or not to perform a charging roller cleaning operation in Example 4; FIG. 9 is a schematic diagram for explaining another example of control of the charging roller cleaning operation; FIG. 9 is a schematic diagram for explaining still another example of control of the charging roller cleaning operation;

Hereinafter, the image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

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

The image forming apparatus 100 has image forming units SY, SM, SC, and SK that form yellow (Y), magenta (M), cyan (C), and black (K) images, respectively. Elements having the same or corresponding functions or configurations in the image forming units SY, SM, SC, and SK are indicated by the suffixes Y, M, C, In some cases, the K is omitted for a general description. In this embodiment, the image forming section S includes photosensitive drums 1 (1Y, 1M, 1C, 1K), charging rollers 2 (2Y, 2M, 2C, 2K), exposure devices 3 (3Y, 3M, 3C, 3K), which will be described later. ), developing devices 4 (4Y, 4M, 4C, 4K), primary transfer rollers 14 (14Y, 14M, 14C, 14K), and the like.

A photosensitive drum 1, which is a rotatable drum-shaped (cylindrical) photosensitive member (electrophotographic photosensitive member) as an image bearing member, is driven by a drum driving device 31 (FIG. 2) as driving means in the direction indicated by an arrow R1 in FIG. It is rotationally driven at a predetermined peripheral speed (process speed) in the direction (counterclockwise direction). The drum driving device 31 has a drum driving motor and the like as a driving source. When a control unit (described later) such as a controller receives an image signal to start a print job (described later), the photosensitive drum 1 is driven to rotate. In this embodiment, the photosensitive drum 1 is an organic photoreceptor (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 with a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as charging means. The charged surface of the photosensitive drum 1 is scanned and exposed according to an image signal by an exposure device 3 as an exposure means, and an electrostatic latent image (static image) of a color component corresponding to each image forming portion S is formed on the photosensitive drum 1 . electron image) is formed. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by supplying toner as a developer by a developing device 4 as developing means, and a toner image (toner image, developer image) is formed on the photosensitive drum 1 . image) is formed.

In this embodiment, the charging roller 2 is in contact with the surface of the photosensitive drum 1 with a predetermined pressing force, and rotates following 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 has a thickness of 1.5 [mm] and a specific volume resistivity of about 1×10 ⁶ [Ωcm] on a metal shaft (rotating shaft) with a diameter of 5.5 [mm]. is provided with an elastic layer formed of a conductive elastic body. 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 by 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, when the surface potential (charging potential, background potential) of the photosensitive drum 1 was measured with a surface potential meter Model 344 manufactured by Trek Corporation, it was about -500 [V]. A position on the photosensitive drum 1 where the charging process is performed in the rotation direction of the photosensitive drum 1 is a charging position. The charging roller 2 has minute gaps between the photosensitive drum 1 and the charging roller 2 formed upstream and downstream of a contact portion (charging nip portion) between the photosensitive drum 1 and the charging roller 2 in the rotation direction of the photosensitive drum 1. The surface of the photosensitive drum 1 is charged by the discharge generated by at least one of them. However, for the sake of simplicity, it may be hypothesized that the charging process is performed at the charging nip portion.

The exposure device 3 has a laser driver, a laser diode, a polygon mirror, an optical lens system, and the like. The exposure device 3 irradiates the photosensitive drum 1 with a laser beam based on image information input to the image forming apparatus 100 from a host device 200 (FIG. 2) such as a personal computer, thereby uniformly charging the photosensitive drum. An electrostatic latent image is formed on the surface of 1. The exposure device 3 scans the photosensitive drum 1 with a laser beam substantially parallel to the rotation axis direction. Further, the exposure device 3 scans the surface of the photosensitive drum 1 with laser light substantially parallel to the moving direction of the photosensitive drum 1 as the photosensitive drum 1 rotates. Hereinafter, the direction of the rotational axis of the photosensitive drum 1 (the direction substantially perpendicular to the moving direction of the surface) is also referred to as the "main scanning direction". A direction substantially perpendicular to the "main scanning direction" is also referred to as a "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]. A position on the photosensitive drum 1 where exposure is performed by the exposure device 3 in the rotation direction of the photosensitive drum 1 is an exposure position.

The developing device 4 has a developing roller 41 as a developing member (developer carrier) and a developing container 42 containing a non-magnetic one-component developer (toner) as a developer. The developing roller 41 is configured by providing an elastic layer made of a conductive elastic material on a conductive core metal (rotating shaft). During the developing process, the developing roller 41 contacts the photosensitive drum 1 with a predetermined contact width. During the developing process, the developing roller 41 is rotationally driven at a peripheral speed faster than the peripheral speed of the photosensitive drum 1 by a developing driving device 32 (FIG. 2) as a driving means. At this time, the developing roller 41 is rotationally driven 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 facing (contacting portion) with the photosensitive drum 1 . The development drive device 32 has a development drive motor and the like as a drive source. The developing roller 41 carries toner charged to a regular charging polarity (negative polarity in this embodiment) and conveys it to a portion (contact portion) facing the photosensitive drum 1 to form an electrostatic latent image on the photosensitive drum 1 . Toner is supplied onto the photosensitive drum 1 according to the condition. During the development process, a predetermined development voltage (development bias), which is a negative DC voltage, is applied to the rotating shaft of the development roller 41 by a development power supply (high voltage power supply circuit) E2 (FIG. 2). In this embodiment, a DC voltage of -300 [V] is applied to the rotating shaft of the developing roller 41 during the developing process. A position on the photosensitive drum 1 where the toner is supplied by the developing roller 41 in the rotational direction of the photosensitive drum 1 (a contact position of the developing roller 41 with respect to the photosensitive drum 1) is a developing position.

Here, the developing roller 41 is provided so as to be able to come into contact with and separate from the photosensitive drum 1 . That is, the apparatus bodies of the developing device 4 and the image forming apparatus 100 are provided with a contact/separation mechanism 34 (FIG. 2) for controlling the contact/separation state between the developing roller 41 and the photosensitive drum 1 . The contact/separation mechanism 34 brings the developing roller 41 and the photosensitive drum 1 into contact during the developing process. Further, the contact/separation mechanism 34 separates the developing roller 41 from the photosensitive drum 1 when the operation of the image forming apparatus 100 is stopped.

The toner in this embodiment is a negatively charged non-magnetic toner produced by a suspension polymerization method, and has a volume average particle diameter of about 6.0 μm. In order to modify the surface property of the toner in this embodiment, silicon oxide particles having a volume average particle diameter of about 20 nm are uniformly adhered to the surface of the toner in an amount of about 1.5% of the toner weight. . Although the toner produced by the suspension polymerization method was used in this example, the present invention is not limited to this, and for example, the toner may be produced using other polymerization methods such as a pulverization method and an emulsion polymerization method. It is also possible to use the toner that has been prepared.

Further, in this embodiment, the normal charge polarity of the toner accommodated in the developing container 42 is negative. In this embodiment, the electrostatic latent image is reversely developed with the toner charged to the same polarity as the charging polarity of the photosensitive member by the charging member. In other words, in reversal development, toner charged to the same polarity as the charge polarity of the photoreceptor is placed on the exposed area (image area) on the photoreceptor, where the absolute value of the potential is lowered by exposure after being uniformly charged. adheres. However, the present invention is not limited to this, and the electrostatic latent image may be normally developed with a toner charged in a polarity opposite to that of the photoreceptor. In other words, in normal development, toner charged to the opposite polarity to the charge polarity of the photoreceptor is present in areas (image areas) other than areas where the absolute value of the potential has decreased due to exposure after being uniformly charged. adhere to.

An intermediate transfer belt 10, which is an endless belt serving as an intermediate transfer member, is arranged so as to face the four photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer belt 10 is stretched by a driving roller 11, a tension roller 12, and a secondary transfer counter roller 13 as a plurality of stretching members (stretching rollers). The intermediate transfer belt 10 rotates (circulates, circulates) in the direction of arrow R2 (clockwise) in FIG. )do. At this time, the intermediate transfer belt 10 is arranged so that the moving direction of the surface of the intermediate transfer belt 10 is in the same direction as the moving direction of the surface of each photosensitive drum 1 at the portion (contact portion) facing each photosensitive drum 1 . It rotates at a peripheral speed substantially the same as the peripheral speed of each photosensitive drum 1 . The belt driving device 33 has a belt driving motor or the like as a driving source. A primary transfer roller 14 , which is a roller-type primary transfer member as primary transfer means, is arranged on the inner circumferential surface side of the intermediate transfer belt 10 so as to correspond to each photosensitive drum 1 . The primary transfer roller 14 presses the intermediate transfer belt 10 toward the photosensitive drum 1 to form a primary transfer portion (primary transfer nip portion) N1, which is a contact portion between the photosensitive drum 1 and the intermediate transfer belt 10 . The primary transfer roller 14 rotates following the rotation of the intermediate transfer belt 10 . A position on the photosensitive drum 1 (corresponding to the primary transfer portion N1) where the primary transfer is performed in the rotational direction of the photosensitive drum 1 is the primary transfer position. The toner image formed on the photosensitive drum 1 as described above is transferred (primary transfer image) onto the rotating intermediate transfer belt 10 (transfer target) by the action of the primary transfer roller 14 at the primary transfer portion N1. ) is done. During the primary transfer process, the primary transfer roller 14 is supplied with a DC voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) by the primary transfer power supply (high voltage power supply circuit) E3 (FIG. 2). A predetermined primary transfer voltage (primary transfer bias) is applied. 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 forming a full-color image, yellow, magenta, cyan, and black toner images formed on the respective photosensitive drums 1 are sequentially superimposed on the intermediate transfer belt 10 at the respective primary transfer portions N1. be transcribed. As a result, a four-color toner image corresponding to the intended full-color image is formed on the intermediate transfer belt 10 .

Of the toner remaining on the photosensitive drum 1 (residual transfer toner) that has not been primarily transferred onto the intermediate transfer belt 10 at the primary transfer portion N<b>1 , positively charged toner adheres to the charging roller 2 . Among the transfer residual toner, negatively charged toner is transported to a portion facing the developing device 4 through a contact portion with the charging roller 2 as the photosensitive drum 1 rotates. At this time, the surface of the photosensitive drum 1 is again subjected to the charging process and the exposure process to form an electrostatic latent image according to the image information. The toner conveyed to the portion facing the developing device 4 has a substantially negative polarity. Therefore, a part of this toner is applied to the surface potential of the photosensitive drum 1 (−500 [V] in the non-exposed portion and −100 [V] in the exposed portion) and the voltage applied to the developing roller 41 (−300 [V] ) are collected in the developing device 4 by the electric field formed by the potential difference between the . That is, in the non-exposed portion, the direction of the electric field is the direction in which the negatively charged toner moves from the photosensitive drum 1 onto the developing roller 41, so that the negatively charged toner on the photosensitive drum 1 is developed. It moves onto the roller 41 and is collected in the developing device 4 . As described above, in this embodiment, residual toner such as transfer residual toner on the photosensitive drum 1 is collected by a cleanerless method.

A secondary transfer roller 15 , which is a roller-type secondary transfer member as a secondary transfer means, is disposed at a position facing the secondary transfer opposing roller 13 on the outer peripheral surface side of the intermediate transfer belt 10 . The secondary transfer roller 15 is in contact with the intermediate transfer belt 10 backed up by the secondary transfer opposite roller 13 with a pressure of 50N. A transfer portion (secondary transfer nip portion) N2 is formed. The secondary transfer roller 15 rotates following the rotation of the intermediate transfer belt 10 . The toner image formed on the intermediate transfer belt 10 is transferred to a recording sheet which is sandwiched and conveyed between the intermediate transfer belt 10 and the secondary transfer roller 15 by the action of the secondary transfer roller 15 at the secondary transfer portion N2. is transferred (secondary transfer) onto the recording material P such as the For example, when forming a full-color image, four-color toner images formed on the intermediate transfer belt 10 are secondary-transferred collectively onto the recording material P at the secondary transfer portion N2. During the secondary transfer process, a secondary transfer power supply (high voltage power supply circuit) E4 (FIG. 2) is applied to the secondary transfer roller 15 with a direct current having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment). A predetermined secondary transfer voltage (secondary transfer bias) is applied. In this embodiment, a DC voltage of +2500 [V] is applied to the secondary transfer roller 15 during the secondary transfer process. A recording material (transfer material, recording medium, sheet) P is accommodated in a recording material accommodating portion 16, and is sent out one by one from the recording material accommodating portion 16 by a feeding roller 18 or the like as a feeding means, and intermediate transfer is performed. The toner image on the belt 10 is synchronized with the timing and conveyed to the secondary transfer portion N2.

The recording material P onto which the toner image has been transferred is conveyed to a fixing device 30 as fixing means. The fixing device 30 fixes (melts, fixes) the toner image on the recording material P by heating and pressing the recording material P bearing the unfixed toner image. For example, when forming a full-color image, four color toners on the recording material P are melted and mixed and fixed on the recording material P. FIG. The recording material P (printed image) on which the toner image is fixed is discharged (output) to the outside of the apparatus main body of the image forming apparatus 100 .

On the outer peripheral surface side of the intermediate transfer belt 10, an intermediate transfer member cleaning unit is provided downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1 (most upstream primary transfer portion N1Y) in the rotational direction of the intermediate transfer belt 10. A belt cleaning device 17 is arranged as means. In this embodiment, the belt cleaning device 17 is arranged to face the secondary transfer counter roller 13 . The toner remaining on the intermediate transfer belt 10 without being secondarily transferred onto the recording material P at the secondary transfer portion N2 (transfer residual toner) is removed from the intermediate transfer belt 10 by the belt cleaning device 17 and collected. . The belt cleaning device 17 has a cleaning member such as a cleaning blade arranged in contact with the outer peripheral surface of the intermediate transfer belt 10, and a cleaning container. The belt cleaning device 17 scrapes off deposits such as transfer residual toner from the surface of the rotating intermediate transfer belt 10 with a cleaning member, and stores it in a cleaning container.

<Control mode>
FIG. 2 is a schematic block diagram showing a control mode of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 is provided with a control section (control circuit) 50 as control means. The control unit 50 includes a CPU as arithmetic control means and a ROM or RAM as storage means. The CPU comprehensively controls the operation of each unit of the image forming apparatus 100 according to the programs stored in the ROM, using the RAM as a work area. The control unit 50 is connected to, for example, the drum drive device 31, the development drive device 32, the belt drive device 33, the contact/separation mechanism 34, various power sources E1 to E4, the exposure device 3, and the like. The control unit 50 includes an image signal processing circuit 51, image memories MY, MM, MC, MK, temporary counter arrays RY, RM, RC, RK, cumulative counter arrays UY, UM, UC, UK, A cumulative print counter W and the like are provided. Although not shown in FIG. 2, in this embodiment, the charging power source E1, the developing power source E2, and the primary transfer power source E3 are provided independently for each of the image forming sections SY, SM, SC, and SK. It is

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 the recording material P and output. In addition, the control unit 50 controls the cleaning operation of the charging roller 2 (hereinafter also simply referred to as the “charging roller cleaning operation”), which will be described later in detail, and the execution determination of the charging roller cleaning operation.

Here, the image forming apparatus 100 executes a print job, which is a series of operations for forming and outputting an image on a single or a plurality of recording materials P, which is started by one start instruction from the host device 200 . A print job generally includes an image forming process, a pre-rotation process, an inter-paper process when forming images on a plurality of recording materials P, and a post-rotation process. The image forming process is a period during which an electrostatic image of an image to be actually formed and output on the recording material P is formed, a toner image is formed, and the primary transfer and secondary transfer of the toner image are performed. "period, during image forming operation" refers to this period. More specifically, the timing of image formation differs depending on the position where each step of electrostatic image formation, toner image formation, primary transfer, and secondary transfer of the toner image is performed. The pre-rotation process is a period from when the start instruction is input to when the image formation is actually started, during which preparatory operations are performed before the image forming process. The paper interval process is a period corresponding to the interval between recording materials P when image formation is continuously performed on a plurality of recording materials P (continuous image formation). The post-rotation process is a period during which an arrangement operation (preparation operation) is performed after the image forming process. The non-image forming period (non-image forming period) is a period other than the image forming period, and includes the pre-rotation process, the paper interval process, the post-rotation process, and further, when the power of the image forming apparatus is turned on or from the sleep state. A pre-multi-rotation step, etc., which is a preparatory operation at the time of return, is included. In this embodiment, the control unit 50 can perform a charging roller cleaning operation, which will be described later in detail, during non-image formation.

<Print job operation sequence>
FIG. 3 is a flow chart showing an outline of the print job operation sequence in this embodiment.

When a print job start signal is transmitted from the host device 200 connected to the image forming apparatus 100, the control unit 50 starts the print job. First, the control unit 50 performs "reception of image data" from the host device 200 (S101). The image data is a two-dimensional array with a resolution of 600 DPI and 8 bits, and is bitmap data of three colors of RGB, red (R), green (G), and blue (B). 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 into image data DY, DM, DC, and DK of four YMCK colors of yellow (Y), magenta (M), cyan (C), and black (K). It is overwritten and stored in the memories MY, MM, MC, and MK. Next, the control unit 50 starts an "image forming operation" (S103). When the image forming operation is started, 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. Accordingly, image formation is performed as described above.

Although the image data described above is bitmap data, even if it is raster data, it is converted into YMCK four-color image data by the image signal processing circuit 51 in the same manner as the bitmap data.

Next, when the transfer of the image data to the exposure device 3 is completed, the control section 50 performs "determining whether to perform the charging roller cleaning operation" (S104). The details of the determination of whether or not to clean the charging roller will be described later. When the controller 50 determines that the charging roller cleaning operation is necessary, the controller 50 carries out the "charging roller cleaning operation" (S105). The details of this charging roller cleaning operation will be described later. On the other hand, when the controller 50 determines that the charging roller cleaning operation is not necessary, the process proceeds to S106. Next, the control unit 50 confirms "presence or absence of next image data" (S106). If the control unit 50 determines that there is unprocessed image data, the control unit 50 returns the process to S102 and repeats the above operation. On the other hand, when the control unit 50 determines that the processing of all image data is completed and there is no next image data, the print job is terminated.

As described above, in this embodiment, for example, the charging roller cleaning operation is performed during the paper-to-paper process during non-image formation. However, the present invention is not limited to this, and the charging roller cleaning operation may be performed during non-image formation, for example, during the post-rotation process, the pre-rotation process, the pre-multi-rotation process, or the like. good.

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

Due to the influence of transfer at the primary transfer portion N1, the residual toner on the photosensitive drum 1 contains toner charged to a polarity opposite to the normal charge polarity (positive polarity in this embodiment) at a certain rate. Here, the residual toner on the photosensitive drum 1 refers to the toner remaining on the photosensitive drum 1 (“transfer residual toner”) after the toner adhering to the image portion on the photosensitive drum 1 has not been primarily transferred. It is the toner ("fogging toner") that adheres to the upper background potential area.

Therefore, when the remaining toner passes through the contact portion 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 the residual toner to , the positively charged toner in the remaining toner moves onto the charging roller 2 . In other words, the toner charged to the opposite polarity of the normal charge polarity (hereinafter also referred to as “reversely charged toner”) in the remaining toner moves onto the charging roller 2 . Toner adheres to the surface of the charging roller 2 every time residual toner passes through the contact portion with the charging roller 2 as the photosensitive drum 1 rotates. Toner accumulates. Therefore, the main scanning direction distribution of the amount of toner adhering to the charging roller 2 correlates with the main scanning direction distribution of the print pixel area of the toner image on the photosensitive drum 1 that has passed through the primary transfer portion N1.

When toner adheres to the surface of the charging roller 2, the charging performance of the charging roller 2 changes, so the surface potential of the photosensitive drum 1 changes, leading to image density fluctuations. Also, the greater the amount of toner adhering 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 influence of one image forming operation is small, when 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 image density fluctuations cannot be ignored.

In the image forming apparatus 100 of this embodiment, when the amount of transfer residual toner when forming a 100% yellow image was measured, it was around 2.0%. The method for measuring the amount of residual toner after transfer is as follows. After passing through the primary transfer portion N1, the transfer residual toner adhering to the photosensitive drum 1 is picked up with a polyester tape and pasted on high-quality paper. Also, as a reference, the polyester tape itself is aligned with the above polyester tape and pasted on woodfree paper. The optical densities of the polyester tapes on these fine papers are measured using a whiteness meter TC-6DS manufactured by Tokyo Denshoku Kogyo Co., Ltd. through a complementary color filter of the toner color. Then, the difference between the two measurement results is taken as the amount of transfer residual toner.

A strip-shaped 100% yellow image extending in the sub-scanning direction was continuously printed on an A4-size recording material S, and a change in image density due to adhesion of transfer residual toner to the charging roller 2 was sensory evaluated. As a result, in the case of an image that is uniform in the main scanning direction (an image in which the belt-shaped images are uniformly laid out in the main scanning direction), density fluctuations can be recognized from the 100th page. On the other hand, in the case of an image having a distribution in the main scanning direction (an image formed by forming the belt-like image with a bias in the main scanning direction), density fluctuations could be recognized from the 30th page.

In addition, the developing device 4 coats the developing roller 41 with toner that is negatively charged, which is the regular charging polarity. It becomes reversely charged toner. This oppositely charged toner may adhere to the background potential portion of the photosensitive drum 1 . The oppositely charged toner adhering to the background potential portion of the photosensitive drum 1 is called "fogging toner". Fog toner, which is oppositely charged toner, is not primarily transferred onto the intermediate transfer belt 10 and adheres to the charging roller 2 in the same manner as the transfer residual toner. The fogging toner uniformly adheres to the entire surface of the charging roller 2 because it adheres uniformly to the entire surface of the photosensitive drum 1 when no image is formed. In the image forming apparatus 100 of this embodiment, the amount of fogging toner measured at each of the image forming stations SY, SM, SC, and SK was around 0.5%. In the method of measuring the amount of fogging toner, the position where the toner is collected with a polyester tape is between the developing roller 41 (development position) and the primary transfer portion N1 (primary transfer position) in the rotation direction of the photosensitive drum 1. Other than that, the method is the same as the method for measuring the amount of transfer residual toner.

All-white images were continuously printed on a recording material P of A4 size, and change in image density caused by fogging toner adhering to the charging roller 2 of each image forming station S was sensory evaluated. As a result, density fluctuations could be recognized from page 200 onwards.

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

The toner on the charging roller 2 is positively charged and adheres electrostatically to the surface of the charging roller 2 . Therefore, by setting the direction of the electric field between the charging roller 2 and the photosensitive drum 1 in the opposite direction to that during the image forming operation, the toner adhering to the surface of the charging roller 2 can be removed (cleaned). At this time, the voltage applied to the charging roller 2 is the normal charging polarity of the toner with respect to the surface potential of the photosensitive drum 1, where Va [V] is the discharge start voltage between the charging roller 2 and the photosensitive drum 1. is preferably 100 [V] or more and Va [V] or less on the opposite polarity side. This is because when 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, when Va [V] is exceeded, discharge occurs between the charging roller 2 and the photosensitive drum 1, and a portion of the positively charged toner on the charging roller 2 is negatively charged toner. This is because it becomes impossible to move onto the photosensitive drum 1 instead of .

As described above, since the direction of the electric field between the charging roller 2 and the photosensitive drum 1 is opposite to that during the image forming operation, the charging roller cleaning operation (cleaning process, recovery process) is performed in non-image forming operations. Performed during image formation. Also, in order to prevent image density fluctuations, it is necessary to clean the charging roller 2 before the surface of the photosensitive drum 1 can no longer be uniformly charged when a certain amount of toner or more accumulates on the surface of the charging roller 2 . There is

FIG. 4 is a timing chart of the charging roller cleaning operation in this embodiment. Here, the case where the charging roller cleaning operation is performed for the next print job in the post-rotation process is taken as an example. FIG. 4 representatively shows the operation timing of each section in one image forming section S. As shown in FIG. Control of this charging roller cleaning operation is executed by the control unit 50 .

When the image forming operation (more specifically, the charging process, exposure process, and development process for the final image formed on the recording material P in the print job) is completed, first, the developing roller 41 is 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 conveying direction passes through the primary transfer portion N1K of the image forming portion SK that is the most downstream in the rotation direction of the intermediate transfer belt 10, primary transfer is performed. The voltage applied to the roller 14 is switched from +500 [V] to -900 [V]. The surface of the photosensitive drum 1 charged to -500 [V] by applying the charging voltage of -1100 [V], which is the same as during the image forming operation, to the charging roller 2, and the primary surface to which the voltage of -900 [V] is applied. The potential difference with the transfer roller 14 does not exceed the discharge start voltage (discharge threshold). Therefore, the surface potential of the photosensitive drum 1 immediately before reaching the contact portion with the charging roller 2 after passing 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 last image is completed in the image forming station SK on the most downstream side. 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.

Next, the voltage applied to the charging roller 2 is switched from -1100 [V] to 0 [V], which is the same as in the image forming operation. 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 opposite to that during the image forming operation. be the direction. 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 conveyed to the primary transfer portion N1 as the photosensitive drum 1 rotates. At the primary transfer portion N1, the positively charged toner is transferred due to the potential difference between the surface potential (-500 [V]) of the photosensitive drum 1 and the voltage (-900 [V]) applied to the primary transfer roller 14. An electric field is formed in the direction of movement from the photosensitive drum 1 onto the intermediate transfer belt 10 . This electric field causes the positively charged toner on the photosensitive drum 1 to move onto the intermediate transfer belt 10 . As the intermediate transfer belt 10 rotates, the positively charged toner that has moved onto the intermediate transfer belt 10 is conveyed through the secondary transfer portion N2 to a portion facing the belt cleaning device 17. recovered by At least when the positively charged toner that has moved onto the intermediate transfer belt 10 reaches the secondary transfer portion N2, the secondary transfer roller 15 has the same polarity as during the image forming operation (positive polarity in this embodiment). ) is applied (for example, +300 [V]). As a result, the positively charged toner on the intermediate transfer belt 10 passes through the secondary transfer portion N<b>2 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.

By the above steps, the toner adhering to the surface of the charging roller 2 can be removed (cleaned), and the charging performance of the photosensitive drum 1 by the charging roller 2 is recovered.

Note that the dark decay speed of the surface potential of the photosensitive drum 1 in this example was about 1.3 [V/sec], and the influence of the dark decay rate on the cleaning process of the charging roller 2 was almost negligible.

Also, the execution time of the charging roller cleaning operation in this embodiment is the time during which the charging roller 2 rotates three times so that the toner adhering to the charging roller 2 is sufficiently discharged. The execution time of the charging roller cleaning operation is preferably a time between the time when the charging roller 2 rotates once and the time when it rotates 10 times. Among them, the time during which the charging roller 2 rotates twice to the time during which it rotates four times is more preferable.

Further, the greater the magnitude of the electric field between the charging roller 2 and the photosensitive drum 1 and the magnitude of the electric field between the photosensitive drum 1 and the intermediate transfer belt 14 in the charging roller cleaning operation, the higher the ejection efficiency. However, if these electric fields are too large, discharge will occur and the polarity of the toner will change, resulting in a decrease in ejection efficiency. Further, if the magnitude of these electric fields is too large, there is a possibility that the surfaces of the photosensitive drum 1 and the intermediate transfer belt 14 will deteriorate due to discharge. In the charging roller cleaning operation, the surface potential of the photosensitive drum 1 is preferably -200V to -800V, and the voltage applied to the primary transfer roller 14 is preferably -400V to -1600V.

<Judgment on execution of charging roller cleaning operation>
In this embodiment, the determination of whether to perform the charging roller cleaning operation is made 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 whether or not to perform the charging roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job.

(Calculation method A)
First, calculation method A will be described. Calculation method A determines whether or not the charging roller cleaning operation is necessary based on the amount of reversely charged toner adhered to the charging roller 2 due to transfer residue left on the photosensitive drum 1 . FIG. 5 is a schematic diagram for explaining the data processing process in the calculation method A for determining whether the charging roller cleaning operation is to be performed according to the present embodiment. In the calculation method A, the main scanning direction distribution of the adhesion amount of the oppositely charged toner caused by the transfer residue on the charging roller 2 is predicted based on the image data as the information about the toner image. In this embodiment, in the calculation method A, the execution of the charging roller cleaning operation is determined in each of the image forming units SY, SM, SC, and SK for each of yellow, magenta, cyan, and black.

First, the YMCK four-color image data DY, DM, DC, and DK stored in the image memories MY, MM, MC, and MK are binarized. The reason why the binarization process is performed in this way is that the adherence of the oppositely charged toner due to the transfer residue on the charging roller 2 has a higher correlation with the printing area than the absolute amount of toner among the information regarding the amount of toner. to have.

Next, sub-scanning direction integration processing is performed. The controller 50 is provided with temporary counter arrays RY, RM, RC, and RK for four colors, which are counter arrays each having the same length as the number of pixels in the scanning direction of the image memory. The image data DY, DM, DC, and DK are integrated in the sub-scanning direction, and overwritten and stored as one-dimensional data DY2, DM2, DC2, and DK2 in the main scanning direction in temporary counter rows RY, RM, RC, and RK corresponding to each color. be done. The one-dimensional data DY2, DM2, DC2, and DK2 correspond to the main scanning direction distribution of the print pixel areas of the toner images on the photosensitive drums 1 that have passed through the respective primary transfer portions N1Y, N1M, N1C, and N1K. The one-dimensional data DY2, DM2, DC2, and DK2 are the transfer residual toner remaining on the photosensitive drum 1 after passing through the respective primary transfer portions N1Y, N1M, N1C, and N1K (more specifically, the transfer residual toner). It correlates with the amount of oppositely charged toner).

Next, integration processing is performed. The controller 50 is provided with accumulating counter arrays UY, UM, UC, and UK for four colors, which are counter arrays each having the same length as the number of pixels in the scanning direction of the image memory. The one-dimensional data DY2, DM2, DC2, and DK2 are integrated with the current values of the accumulation counter rows UY, UM, UC, and UK corresponding to each color, and the one-dimensional data DY3, DM3, DC3, and DK3 are obtained as accumulation counters corresponding to each color. It is overwritten and stored in columns UY, UM, UC, and UK. The one-dimensional data DY3, DM3, DC3, and DK3 are correlated with the integrated amount of toner adhering to each of the charging rollers 2Y, 2M, 2C, and 2K. One-dimensional data DY2, DM2, DC2, and DK2 or one-dimensional data DY3, DM3, DC3, and DK3 shown in FIG. 5 are examples of information regarding the print image area obtained from the image data. In the present embodiment, the CPU of the control unit 50, the row of temporary counters, the row of cumulative counters, and the like correspond to each of a plurality of areas in the main scanning direction based on the print pixel area, and the toner image passing through the transfer unit is counted. It functions as an acquisition unit that acquires information about the amount of toner.

Finally, a threshold checking process is performed. In the threshold inspection process, in the count values (one-dimensional data DY3, DM3, DC3, and DK3) of the accumulation counter arrays UY, UM, UC, and UK, the difference (Δ) between adjacent regions and the maximum single value (max) and exceed predetermined threshold values set in advance correspondingly. In this embodiment, the adjacent regions in the plurality of regions in the rotational axis direction of the photosensitive drum 1 are adjacent regions in the plurality of regions in the main scanning direction (corresponding to each storage region of the integration counter row) partitioned by the number of pixels in the main scanning direction. area. Further, the difference in values between the adjacent regions is, for example, the difference between the count values of the accumulation counter rows corresponding to the N-th and N+1-th pixels in the main scanning direction, the accumulation counters corresponding to the N+1-th and N+2-th pixels. Such as the difference between count values in a column. Note that N is a natural number from 1 to the maximum number of pixels in the main scanning direction. Then, when it is determined that at least one of the above values exceeds the threshold value, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the accumulation counter arrays UY, UM, UC, UK is reset to an initial value (0 in this embodiment). In this embodiment, if any one of the differences in the values between the adjacent regions in the count value of the accumulation counter string exceeds the threshold, it is determined that the difference in the values between the adjacent regions exceeds the threshold. do. However, the present invention is not limited to this, and a threshold is obtained when a predetermined number or more (at least one) of the differences in values between the adjacent regions exceeds the threshold. It can be determined that the position has been exceeded. 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).

Performing the threshold inspection process on the difference in the count value between the adjacent areas in the count value of the accumulation counter row is related to the reversely charged toner caused by the transfer residue adhering to the charging roller 2 and uniformly adhering in the main scanning direction. This is equivalent to ignoring the amount of adhesion and paying attention to the relative difference in adhesion amount in the main scanning direction. In addition, performing the threshold inspection process on the maximum value of the count value of the accumulation counter row is related to the reversely charged toner caused by the transfer residue adhering to the charging roller 2, and the amount uniformly adhering in the main scanning direction is removed. It is equivalent to paying attention to the absolute amount included.

In this embodiment, as the threshold value for the count value of the accumulation counter row, a value corresponding to the limit at which image density fluctuation due to adherence of oppositely charged toner to the charging roller 2 caused by transfer residue is visually recognized is set. In this embodiment, the threshold value for the count value of the accumulation counter row is set by the number of pages in units of the length of the recording material P of A4 size in the sub-scanning direction. In this embodiment, the threshold value (KΔ) for the value difference between adjacent regions is set to 20 pages, and the threshold value for the single maximum value (Kmax) is set to 100 pages. However, these threshold values are not limited to the values in this embodiment, and may vary depending on the device configuration, the amount (percentage) of residual toner generated after transfer, or the amount (percentage) of reversely charged toner in the residual toner after transfer. , is appropriately set so as to sufficiently suppress image density fluctuations.

(Calculation method B)
In calculation method B, whether or not the charging roller cleaning operation is necessary is determined based on the amount of oppositely charged toner adhered to the charging roller 2 due to fogging. The fogging toner adheres substantially uniformly in the main scanning direction to the charging roller 2 of the same image forming station S where the fogging has occurred. The amount of fogging toner adhering to the charging roller 2 correlates with the cumulative number of printed sheets. In calculation method B, the adhesion amount of oppositely charged toner due to fogging on the charging roller 2 is predicted based on the cumulative number of printed sheets. It should be noted that the determination of whether to perform the charging roller cleaning operation by calculation method B is made independently for each of the image forming units SY, SM, SC, and SK for each of yellow, magenta, cyan, and black.

The control unit 50 is provided with an accumulated print counter W that holds the accumulated number of printed sheets. The cumulative print counter W is incremented by one each time image formation on one side of a sheet of recording material P is completed. In the threshold inspection process in the calculation method B, it is inspected whether or not the count value (accumulated number of printed sheets) of the accumulated print counter W exceeds a preset threshold. If it is determined that the threshold value is exceeded, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the cumulative print counter W is reset to the initial value (0 in this embodiment). be done. 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).

In this embodiment, as the threshold value of the cumulative print counter W, a value corresponding to the limit at which image density fluctuation due to adhesion of oppositely charged toner to the charging roller 2 caused by fogging is visually recognized is set. In this embodiment, the threshold value of the accumulated print counter W is set to 200 pages for the recording material P of A4 size.

Here, in this example, calculation method A and calculation method B were used independently. That is, in S104 of FIG. 3, both calculation methods A and B are used to determine whether or not the charging roller cleaning operation should be performed. A roller cleaning operation is performed. However, the present invention is not limited to this, and for example, a calculation method combining these calculation methods may be used. For example, when the charging roller cleaning operation is performed based on the calculation method A, a method such as resetting the cumulative print counter W used in the calculation method B can be used.

Also, the charging roller cleaning operation can be independently executed in the image forming units SY, SM, SC, and SK for each of yellow, magenta, cyan, and black, when necessary. In this case, the accumulation counter row corresponding to the image forming station S that has executed the charging roller cleaning operation is reset. Alternatively, if any of the image forming units SY, SM, SC, and SK for each of yellow, magenta, cyan, and black is judged to require the charging roller cleaning operation, these image forming units SY, SM, SC and SK can be executed synchronously. In this case, the accumulation counter rows corresponding to the respective image forming units S are synchronously reset.

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

In the comparative example, since information about the toner image is not considered, the charging roller cleaning operation is always performed every 20 pages no matter what kind of image is formed.

On the other hand, in this embodiment, since information (in this embodiment, image data) regarding the toner image is considered, the charging roller cleaning operation is performed at an appropriate timing according to the image data of the formed image. Further, in this embodiment, when an image having a uniform image pattern in the main scanning direction is formed on average, in which a change in image density is difficult to see, the charging roller cleaning operation is determined to be necessary. The number of prints is substantially increased. Therefore, excessive implementation of the charging roller cleaning operation is suppressed, and a decrease in throughput of the image forming apparatus 100 is suppressed. In the present embodiment, when image patterns that are biased in the main scanning direction and in which image density changes are easily visible are not printed continuously, the charging roller cleaning operation is performed every 200 pages at maximum.

As described above, according to the present embodiment, the control unit 50 continuously forms a band-shaped image extending in the direction substantially orthogonal to the rotational axis direction of the photosensitive drum 1 uniformly in the direction of the rotational axis line. A plurality of continuous images formed by forming strip-shaped images extending in a direction substantially perpendicular to the rotation axis direction with a deviation in the rotation axis direction than in the case of performing an image forming operation of forming and outputting on the recording material P. Control is performed so that the charging roller cleaning operation is performed at a timing when the number of recording materials P to be output is smaller when the image forming operation of forming and outputting on the recording material P is performed.

In addition, one of the features of this embodiment is the deviation of the integrated value of the index value that correlates with the amount of toner carried on each of a plurality of areas of the photosensitive drum 1 in the main scanning direction, particularly the adjacent distribution in the main scanning direction. It is to use the difference in values between the areas to determine the execution timing of the charging roller cleaning operation. That is, regarding the count value of the accumulation counter row, not only the maximum value but also the difference between the values of the adjacent regions is used to determine the execution timing of the charging roller cleaning operation. Considering the human visual characteristics, if the image density fluctuation due to the adhesion of toner to the charging roller 2 is uniform in the main scanning direction, it is difficult to perceive it. Then, by using the difference between the count values of the accumulation counter row between adjacent areas, the contribution of the toner uniformly adhered to the entire area of the charging roller 2 is ignored, and a situation in which perceptible image density fluctuations are likely to occur. It is possible to judge whether or not In this embodiment, it is possible to appropriately determine the timing of performing the charging roller cleaning operation by determining whether or not the situation is such that perceivable image density fluctuations are likely to occur.

With such a configuration, according to the present embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress image density fluctuation while suppressing a decrease in throughput. Therefore, according to the present embodiment, deterioration of image quality can be suppressed by appropriately performing the charging roller cleaning operation.

In this embodiment, the timing of the charging roller cleaning operation is controlled to suppress image density fluctuations, but similar effects can be obtained by controlling the execution time of the charging roller cleaning operation. For example, as shown in FIG. 9, the timing of the charging roller cleaning operation is constant, and the difference or the maximum value between adjacent areas in the count values of the cumulative counter row exceeds a predetermined threshold corresponding to each. If it exceeds, it can be controlled to extend the charging roller cleaning operation in order to suppress the image density fluctuation. Specifically, for example, the charging roller cleaning operation is performed every 50 pages (or every post-rotation process), and the charging roller 2 rotates once in the normal charging roller cleaning operation when the threshold value is not exceeded. If the threshold value is exceeded, control is performed so that the charging roller 2 rotates five times. In this manner, by adjusting the time for one charging roller cleaning operation in consideration of the information regarding the toner image, it is possible to suppress the reduction in throughput to the minimum and to suitably suppress fluctuations in image density. can. Also, the threshold may be set at multiple levels instead of one level. That is, the execution time of the charging roller cleaning operation may be extended stepwise (or continuously) according to the information regarding the toner image. Also, both the timing of the charging roller cleaning operation and the time of the charging roller cleaning operation may be controlled.

Further, as shown in FIG. 10, it is also possible to control the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 during the charging roller cleaning operation. Specifically, for example, the charging roller cleaning operation is performed every 50 pages (or each post-rotation process), and the difference and the maximum value of the count values of the integration counter row between adjacent regions correspond to each. , the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 are set in the same manner as in the above-described embodiment. On the other hand, when the threshold value is not exceeded, the absolute values of the surface potential of the photosensitive drum 1 and the potential of the voltage applied to the primary transfer roller 14 are each set smaller than in the above embodiment. It should be noted that the surface potential of the photosensitive drum 1 and the voltage applied to the primary transfer roller 14 are changed stepwise (or continuously) according to the difference or the maximum value between adjacent regions in the count value of the cumulative counter row. You can change it. In this way, based on the information about the toner image (image data in this embodiment), the magnitude of the electric field between the charging roller 2 and the photosensitive drum 1, the photosensitive drum 1 and the intermediate transfer belt 10 during the charging roller cleaning operation. The magnitude of the electric field between and can be controlled. In other words, when the difference between adjacent regions in the count values of the accumulation counter row or the maximum value is small, the bias in the amount of toner adhered to the charging roller 2 and the amount of toner adhered are small. Therefore, it can be said that the electric field required to discharge the toner from the charging roller 2 to the photosensitive drum 1 and the toner from the photosensitive drum 1 to the intermediate transfer belt 10 to suppress the image density fluctuation is small. Therefore, in that 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 such control, an unnecessary large electric field is not applied to the photosensitive drums 1 and the intermediate transfer belt 10 to cause discharge, so deterioration of the photosensitive drums 1 and the intermediate transfer belt 10 can be reduced. . It should be noted that 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 in the charging roller cleaning operation may be controlled. Well, the corresponding effect can be obtained. Here, an example of control in the direction of weakening the electric field was described in order to reduce the deterioration of the photosensitive drum 1 and the intermediate transfer belt 10. If the electric field is large, control may be performed to strengthen the electric field so as to increase the discharging ability of the charging roller 2 during the cleaning operation. At least one of the charging roller cleaning operation timing and the charging roller cleaning operation time, and both the magnitude of the electric field in the charging roller cleaning operation may be controlled.

[Example 2]
Another embodiment of the present invention will now 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 the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

The present embodiment differs from the first embodiment in the threshold inspection process of the calculation method A in determining the execution of the charging roller cleaning operation. In this embodiment, the effective range of the accumulation counter string is defined, and the difference between the maximum value and the minimum value is subjected to threshold inspection processing.

<Judgment on execution of charging roller cleaning operation>
In this embodiment, as in the first embodiment, the determination of whether to clean the charging roller is made based on the results of two calculation methods. One is based on image data (oppositely charged toner caused by retransfer) (calculation method A), and the other is based on the cumulative number of printed sheets (oppositely charged toner caused by fogging) (calculation method B). is. The control of whether or not to perform the charging roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job. Since the calculation method B is the same as that of the first embodiment, the calculation method A, which is characteristic of this embodiment, will be described here, and the description of the calculation method B will be omitted.

(Calculation method A)
As in the first embodiment, in the calculation method A, the necessity of the charging roller cleaning operation is determined based on the amount of reversely charged toner adhered to the charging roller 2 due to the transfer residue remaining on the photosensitive drum 1. . FIG. 6 is a schematic diagram for explaining the data processing process in the calculation method A for determining whether the charging roller cleaning operation is to be performed according to the present embodiment. As in the first embodiment, in the calculation method A, the main scanning direction distribution of the adhesion amount of the oppositely charged toner caused by the transfer residue on the charging roller 2 is predicted based on the image data as the information regarding the toner image. In this embodiment, in the calculation method A, the execution of the charging roller cleaning operation is determined in each of the image forming units SY, SM, SC, and SK for each of yellow, magenta, cyan, and black.

First, the YMCK four-color image data DY, DM, DC, and DK stored in the image memories MY, MM, MC, and MK are binarized.

Next, sub-scanning direction integration processing is performed. In other words, the image data DY, DM, DC, and DK are integrated in the sub-scanning direction and stored in 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. Stored by overwriting.

Next, integration processing is performed. That is, the one-dimensional data DY2, DM2, DC2, and DK2 are multiplied by the current values of the accumulation counter rows UY, UM, UC, and UK corresponding to each color, resulting in one-dimensional data DY3, DM3, DC3, and DK3 corresponding to each color. It is overwritten and stored in the accumulation counter columns UY, UM, UC, and UK. The four-color accumulation counter arrays UY, UM, UC, and UK are counter arrays each having the same length as the number of pixels in the scanning direction of the image memory. The one-dimensional data DY3, DM3, DC3, and DK3 are correlated with the integrated amount of toner adhering to each of the charging rollers 2Y, 2M, 2C, and 2K. One-dimensional data DY2, DM2, DC2, and DK2 or one-dimensional data DY3, DM3, DC3, and DK3 shown in FIG. 6 are examples of information regarding the print image area obtained from the image data. In the present embodiment, the CPU of the control unit 50, the row of temporary counters, the row of cumulative counters, and the like correspond to each of a plurality of areas in the main scanning direction based on the print pixel area, and the toner image passing through the transfer unit is counted. It functions as an acquisition unit that acquires information about the amount of toner.

Next, range definition processing is performed. In the range defining process, the effective range is defined among the count values (one-dimensional data DY3, DM3, DC3, DK3) of the cumulative counter arrays UY, UM, UC, and UK. In this embodiment, the effective range of the accumulation counter arrays UY, UM, UC, and UK is defined by excluding areas where the possibility of forming a toner image is low. In other words, in this embodiment, the effective range is defined as the accumulation counter array UY, UM, UC, UK (one-dimensional data DY3, DM3, DC3 , DK3). Further, the area in which the toner image is unlikely to be formed is mostly a blank area at the edge (typically, both edge) of the recording material P in the width direction, but is not included in the image area. also indicates, for example, a region where an image is difficult to be formed at the edge of the recording material P in the width direction. The width direction of the recording material P is a direction substantially orthogonal to the conveying direction of the recording material P, that is, a direction substantially parallel to the rotation 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. Also, the image area is an area in which a toner image can be formed. Specifically, in the present embodiment, the possibility that a toner image is actually formed within the ranges of the accumulation counter arrays UY, UM, UC, and UK corresponding to the size of the recording material P selected in the print job 10% of the area with the lowest count value is excluded, and the remaining 90% of the area is defined as the effective range of the accumulation counter array UY, UM, UC, UK (one-dimensional data DY3, DM3, DC3, DK3) do. As a result, the accumulation counter arrays UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) corresponding to the widthwise end portions of the recording material P on which the possibility of forming a toner image is substantially low. ) is excluded. That is, substantially, the range narrower than the size (width) of the recording material P in the rotation axis direction of the photosensitive drum 1 in the accumulation counter arrays UY, UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3) It becomes possible to determine whether to perform the charging roller cleaning operation based on the value of .

Here, in this embodiment, regardless of the size (width) of the recording material P, 10% of the area with the smallest count value is excluded, and 90% of the area with the largest count value The effective range is UM, UC, and UK (one-dimensional data DY3, DM3, DC3, and DK3). However, in many cases, the margin area at the end of the recording material P in the width direction is uniform regardless of the size (width) of the recording material P, and the narrower the size of the recording material P, the greater the ratio of the margin. . Therefore, the value of the effective range may be changed according to the size (width) of the recording material P, for example. Typically, when the width of the recording material P is the first width, the ratio of the area excluded from the range of the accumulation counter row corresponding to the first width is set as the first ratio (%), and the recording is performed. When the width of the material is a second width smaller than the first width, the ratio of the area excluded from the range of the cumulative counter row corresponding to the second width is set to be greater than the first ratio (%) A second percentage (%) that is larger than For example, the percentage of the excluded area in the case of LTR size (longitudinal feed) is set to 9%, and the percentage of the excluded area in the case of A4 size (longitudinal feed) is set to 10%.

Finally, a threshold checking process is performed. In the threshold inspection process, among the count values (one-dimensional data DY3, DM3, DC3, and DK3) of the accumulation counter columns UY, UM, UC, and UK, the difference (Δ) between the maximum value and the minimum value within the effective range and , the maximum value alone (max) within the valid range, and whether or not they exceed predetermined threshold values set in advance correspondingly. Then, when it is determined that at least one of the above values exceeds the threshold value, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the accumulation counter arrays UY, UM, UC, UK is reset to an initial value (0 in this embodiment). 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).

In this embodiment, as the threshold value for the count value of the accumulation counter row, a value corresponding to the limit at which image density fluctuation due to adherence of oppositely charged toner to the charging roller 2 caused by transfer residue is visually recognized is set. As in the first embodiment, in the present embodiment, the threshold value for the count value of the accumulation counter row is set by the number of pages in units of the length of the recording material P of A4 size in the sub-scanning direction. In this embodiment, the threshold value (KΔ) for the difference between the maximum value and the minimum value is 20 pages, and the threshold value (Kmax) for the maximum value alone is 100 pages. However, these threshold values are not limited to the values in this embodiment, and may vary depending on the device configuration, the amount (percentage) of residual toner generated after transfer, or the amount (percentage) of reversely charged toner in the residual toner after transfer. , is appropriately set so as to sufficiently suppress image density fluctuations.

Note that the ratio (%) of the excluded area in the accumulation counter row as the predetermined condition that the excluded area satisfies is not limited to the value in this embodiment. This is appropriately set according to the apparatus configuration, the size of the blank area, the image to be formed, etc., so that the image density fluctuation can be sufficiently suppressed. Further, in this embodiment, from the range of the accumulation counter array corresponding to the size of the recording material P, a predetermined ratio of areas with smaller count values are excluded. 1 page or less in number) may be excluded.

<Effect of Example>
Next, the effect of this example compared with the comparative example will be described. Here, the description of the same effects as those described in the first embodiment is omitted, and the characteristic effects of the present embodiment will be described.

In this embodiment, since information (in this embodiment, image data) regarding the toner image is considered, the charging roller cleaning operation is performed at an appropriate timing according to the image data of the formed image. Further, in this embodiment, when an image having a uniform image pattern in the main scanning direction is formed on average, in which a change in image density is difficult to see, the charging roller cleaning operation is determined to be necessary. The number of prints is substantially increased. Therefore, excessive implementation of the charging roller cleaning operation is suppressed, and a decrease in throughput of the image forming apparatus is suppressed.

In addition, one of the features of this embodiment is the deviation of the integrated value of the index value that correlates with the amount of toner carried on each of a plurality of areas of the photosensitive drum 1 in the main scanning direction, particularly when the toner image is formed. The difference between the maximum value and the minimum value within an effective range excluding some regions of the width of the recording material P and the width of the image, which are less likely, is used to determine the execution timing of the charging roller cleaning operation. That is, regarding the count value of the accumulation counter row, not only the maximum value but also the difference between the maximum value and the minimum value is used to determine the execution timing of the charging roller cleaning operation. Considering the characteristics of human vision, when the image density fluctuation due to the adhesion of toner to the charging roller 2 is uniform in the main scanning direction, and when the toner image is difficult to be formed on the recording material P or at the edge in the width direction of the image, the image may not be imaged. Density fluctuations are difficult to perceive. In addition, since the image data has a small value at the edges of the recording material P and the width direction of the image, there is a high possibility that the difference between the maximum value and the minimum value will be used as the minimum value. In other words, it may be determined that it is necessary to perform the charging roller cleaning operation even though there is still a low possibility that image density fluctuations will occur in high areas where toner images are formed. Therefore, by using the difference between the maximum value and the minimum value in the effective range among the count values of the accumulation counter row, the toner that is uniformly adhered to the entire area of the charging roller 2 with respect to the area where the toner image is likely to be formed is calculated. By ignoring the contribution of , it is possible to determine whether or not the situation is such that perceptible image density fluctuations are likely to occur. In this embodiment, it is possible to appropriately determine the timing of performing the charging roller cleaning operation by determining whether or not the situation is such that perceivable image density fluctuations are likely to occur. According to the method of the present embodiment, even if the area in which the toner image is often not formed is not the edge in the width direction of the recording material P but the central part, the integration of the area It is possible to prevent the count value of the counter column from being used as the minimum value. In addition, it is possible to suitably suppress image density fluctuations in areas other than areas in which the toner image is often not formed.

With such a configuration, according to the present embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress image density fluctuation while suppressing a decrease in throughput. Therefore, according to the present embodiment, deterioration of image quality can be suppressed by appropriately performing the charging roller cleaning operation.

The calculation method (determination method) of the present embodiment may be combined with the control of the charging roller cleaning operation time and the electric field magnitude in the charging roller cleaning operation described in the first embodiment.

[Example 3]
Another embodiment of the present invention will now 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 the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In the present embodiment, unlike the first embodiment, a laser driving signal converted from image data is used as the information regarding the toner image. FIG. 7 is a schematic diagram for explaining the data processing process in the calculation method A for determining whether the charging roller cleaning operation is to be performed according to the present embodiment.

In this embodiment, since the laser drive signal is used, the determination of whether to clean the charging roller is performed concurrently with the image forming operation. The laser driving signal is 1-bit one-dimensional data for controlling laser lighting, and is sequentially transmitted from the image signal processing circuit 51 to the exposure device 3 during the image forming operation. The sub-scanning direction integration processing of the laser driving signals for each color of yellow (Y), magenta (M), cyan (C), and black (K) is performed in the main scanning cycle of the laser for each color, and each temporary counter Stored in columns RY, RM, RC, RK. As a result, one-dimensional data DY4, DM4, DC4, and DK4, which are distributions in the main scanning direction of laser lighting (emission time) for each image, are obtained. Here, the one-dimensional data DY4, DM4, DC4, and DK4 are, similarly to the one-dimensional data DY2, DM2, DC2, and DK2 of the first embodiment, residual toner remaining on the photosensitive drum 1 after passing through the primary transfer portion N1 ( More specifically, it correlates with the amount of oppositely charged toner in the transfer residual toner.

Subsequent processing can be the same as the processing subsequent to the integration processing in the first and second embodiments. 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 regarding the print image area obtained from the image data. In this embodiment, as in the first embodiment, the CPU of the control unit 50, the temporary counter array, and the cumulative counter array correspond to each of a plurality of areas in the main scanning direction based on the print pixel area, and the transfer unit is shifted. It functions as an acquisition unit that acquires information about the amount of toner in a passing toner image.

Thus, in this embodiment, the information about the print pixel area is obtained by irradiating the photosensitive drum 1 corresponding to the toner image formed on the photosensitive drum 1 by the exposure device 3 for irradiating the photosensitive drum 1 with light. It is the emission time of light.

As described above, according to the present embodiment, the same effects as in Embodiments 1 and 2 can be obtained, and the amount of memory and calculation required for image data processing can be reduced by using the laser driving signal as information regarding the toner image. can do.

[Example 4]
Another embodiment of the present invention will now 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 the first embodiment. Accordingly, in the image forming apparatus of the present embodiment, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals as those of the first embodiment, and detailed description thereof is omitted. do.

In this embodiment, the amount of toner adhering to the charging roller 2 due to "retransfer", which is a phenomenon in which the toner of the toner image formed in the upstream image forming section S adheres to the photosensitive drum 1 of the downstream image forming section S Based on this, it is determined whether or not to perform the charging roller cleaning operation.

<Toner Adhesion to Charging Roller>
First, toner adhesion to the charging roller 2 due to retransfer will be described. The image forming apparatus 100 of this embodiment employs an in-line system. Therefore, when the toner image formed on the intermediate transfer belt 10 in the upstream image forming section S passes through the downstream primary transfer section N1, part of the toner of the toner image is exposed in the downstream image forming section S. A phenomenon called "re-transfer" may occur in which the toner is transferred to the drum 1 . This is because, at the downstream primary transfer portion N1, the toner on the intermediate transfer belt 10 is discharged due to the potential difference between the background potential portion of the photosensitive drum 1 and the primary transfer roller 14, and the polarity is opposite to the normal polarity. This is due to the oppositely charged toner that is charged to the polarity. The toner adhering to the photosensitive drum 1 as a result of retransfer in this way is called "retransferred toner". A voltage of −1100 [V] is applied to the charging roller 2 during image formation, which attracts the retransferred toner, which is the oppositely charged toner. Therefore, the main scanning direction distribution of the amount of toner adhered to the charging roller 2 correlates with the main scanning direction distribution of the print pixel area of the toner image on the intermediate transfer belt 10 that has passed through the primary transfer portion N1. In an in-line image forming apparatus, the oppositely charged toner that adheres to the charging member may be the oppositely charged toner due to retransfer.

In the image forming apparatus of this embodiment, the amount of re-transferred toner when forming a 100% yellow image was about 1.0% when measured at the image forming station SK for black. The method for measuring the amount of retransferred toner is the same as the method for measuring the amount of transfer residual toner described in the first embodiment.

A strip-shaped 100% yellow image extending in the sub-scanning direction was continuously printed on the recording material S of A4 size, and the change in image density due to adhesion of the re-transferred toner to the charging roller 2 was sensory evaluated. The image density change was evaluated in the image forming unit SK for black. As a result, in the case of an image that is uniform in the main scanning direction (an image in which the belt-shaped images are uniformly spread in the main scanning direction), density fluctuations can be recognized from the 100th page. On the other hand, in the case of an image having a distribution in the main scanning direction (an image formed by forming the belt-like image with a bias in the main scanning direction), density fluctuations could be recognized from the 30th page.

Note that the adhesion of the oppositely charged toner to the charging roller 2 due to fogging and the cleaning operation of the charging roller described in the first embodiment are the same as those in the first embodiment, and thus descriptions thereof are omitted here.

<Judgment on execution of charging roller cleaning operation>
In this embodiment, as in the first embodiment, the determination of whether to clean the charging roller is made based on the results of two calculation methods. One is based on image data (oppositely charged toner caused by retransfer) (calculation method A), and the other is based on the cumulative number of printed sheets (oppositely charged toner caused by fogging) (calculation method B). is. The control of whether or not to perform the charging roller cleaning operation is executed by the control unit 50 in the operation sequence of the print job. Since the calculation method B is the same as that of the first embodiment, the calculation method A, which is characteristic of this embodiment, will be described here, and the description of the calculation method B will be omitted.

(Calculation method A)
Calculation method A determines whether or not the charging roller cleaning operation is necessary based on the amount of oppositely charged toner adhered to the charging roller 2 due to retransfer. FIG. 8 is a schematic diagram for explaining the data processing process in the calculation method A for determining whether the charging roller cleaning operation is to be performed according to the present embodiment. In the calculation method A, the distribution in the main scanning direction of the adhesion amount of the oppositely charged toner due to the re-transfer to the charging roller 2 is predicted based on the image data as the information about the toner image. In this embodiment, in the calculation method A, it is determined whether the charging roller cleaning operation is to be performed in each of the image forming units SM, SC, and SK for magenta, cyan, and black.

First, the YMC three-color image data DY, DM and DC stored in the image memories MY, MM and MC are binarized. The reason why the binarization process is performed in this way is that the retransfer, which is the cause of the generation of the oppositely charged toner adhering to the charging roller 2, has a higher correlation with the printing area than the absolute amount of toner among the information regarding the amount of toner. because it has Further, retransfer is a phenomenon in which the toner of the toner image formed in the upstream image forming station S adheres to the photosensitive drum 1 of the downstream image forming station S, so the image of the image forming station SK, which is the most downstream, is retransferred. do not contribute to Therefore, the black image data DK is not processed.

Next, color superposition processing is performed. The result DK6 of the addition operation of the yellow image data DY, the magenta image data DM and the cyan image data DC is stored in the image memory MK. Further, DC6, which is the sum of the yellow image data DY and the magenta image data DM, is stored in the image memory MC. Also, yellow image data DY is stored in the image memory MM (the image data DY is called image data DM6). The data in the image memory MY are reset and will not be used from now on. Note that in this embodiment, the sum operation is performed for each image data element by element. At this point, the image data DM6, DC6, and DK6 stored in the image memories MM, MC, and MK are the image data of the toner images on the intermediate transfer belt 10 that have passed through the respective primary transfer portions N1M, N1C, and N1K. Corresponds to the valued.

Next, sub-scanning direction integration processing is performed. The controller 50 is provided with temporary counter arrays RM, RC, and RK for three colors, which are counter arrays each having the same length as the number of pixels in the scanning direction of the image memory. The image data DM6, DC6 and DK6 are accumulated in the sub-scanning direction, and overwritten and stored in temporary counter rows RM, RC and RK corresponding to each color as one-dimensional data DM7, DC7 and DK7 in the main scanning direction. The one-dimensional data DM7, DC7, and DK7 correspond to the main scanning direction distribution of the print pixel areas of the toner images on the intermediate transfer belt 10 that have passed through the respective primary transfer portions N1M, N1C, and N1K. The one-dimensional data DM7, DC7, and DK7 are correlated with the amount of retransferred toner transferred to the photosensitive drum 1 at the primary transfer portions N1M, N1C, and N1K.

Next, integration processing is performed. The control unit 50 is provided with accumulation counter arrays UM, UC, and UK for three colors, which are counter arrays each having the same length as the number of pixels in the scanning direction of the image memory. The one-dimensional data DM7, DC7, and DK7 are integrated with the current values of the accumulating counter rows UM, UC, and UK corresponding to each color, and the one-dimensional data DM8, DC8, and DK8 are obtained by accumulating the accumulating counter rows UM, UC, and UK corresponding to each color. is overwritten and stored in The one-dimensional data DM8, DC8, and DK8 are correlated with the integrated amounts of toner adhering to the respective charging rollers 2M, 2C, and 2K. One-dimensional data DM7, DC7, and DK7, or one-dimensional data DM8, DC8, and DK8 shown in FIG. 8 are examples of information relating to the print image area obtained from the image data. In the present embodiment, the CPU of the control unit 50, the row of temporary counters, the row of cumulative counters, and the like correspond to each of a plurality of areas in the main scanning direction based on the print pixel area, and the toner image passing through the transfer unit is counted. It functions as an acquisition unit that acquires information about the amount of toner.

Finally, a threshold checking process is performed. In the threshold inspection process, the difference (Δ) between adjacent regions in the count values (one-dimensional data DM8, DC8, and DK8) of the accumulation counter arrays UM, UC, and UK, and the maximum single value (max) are checked to see if they exceed predetermined thresholds, which are respectively preset. Then, when it is determined that at least one of the above exceeds the threshold value, the charging roller cleaning operation is performed as described above (S105 in FIG. 3), and the integration counter rows UM, UC, and UK are initialized is reset to a value (0 in this example). 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).

Performing the threshold inspection process on the difference in the count value between the adjacent areas in the count value of the accumulation counter row means that the oppositely charged toner caused by the retransfer that adheres to the charging roller 2 is uniformly adhered in the main scanning direction. This is equivalent to ignoring the amount of adhesion and paying attention to the relative difference in adhesion amount in the main scanning direction. Further, performing the threshold inspection process on the maximum value of the count value of the accumulation counter row is related to the reversely charged toner caused by the retransfer that adheres to the charging roller 2, and the amount that adheres uniformly in the main scanning direction is eliminated. It is equivalent to paying attention to the absolute amount included.

In this embodiment, as the threshold value for the count value of the accumulation counter row, a value corresponding to the limit at which image density fluctuation due to adhesion of oppositely charged toner to the charging roller 2 caused by retransfer is visually recognized is set. As in the first embodiment, in the present embodiment, the threshold value for the count value of the accumulation counter row is set by the number of pages in units of the length of the recording material P of A4 size in the sub-scanning direction. In this embodiment, the threshold value (KΔ) for the value difference between adjacent regions is set to 20 pages, and the threshold value for the single maximum value (Kmax) is set to 100 pages. However, these thresholds are not limited to the values in this embodiment, and are appropriately set according to the device configuration, the amount (ratio) of retransfer toner generated, etc., so as to sufficiently suppress image density fluctuations. It is what is done.

As in Example 1, in this example, calculation method A and calculation method B were used independently. That is, in S104 of FIG. 3, both calculation methods A and B are used to determine whether or not the charging roller cleaning operation should be performed. A roller cleaning operation is performed. However, the present invention is not limited to this, and for example, a calculation method combining these calculation methods may be used. For example, when the charging roller cleaning operation is performed based on the calculation method A, a method such as resetting the cumulative print counter W used in the calculation method B can be used.

Further, the charging roller cleaning operation can be performed independently in each of the magenta, cyan, and black image forming units SM, SC, and SK when necessary. In this case, the accumulation counter row corresponding to the image forming station S that has executed the charging roller cleaning operation is reset. Alternatively, if any of the magenta, cyan, and black image forming units SM, SC, and SK determines that the charging roller cleaning operation is necessary, these image forming units SM, SC, and SK are synchronized. can be run In this case, the accumulation counter rows corresponding to the respective image forming units S are synchronously reset.

<Effect of Example>
Next, the effect of this example compared with the comparative example will be described. Here, the description of the same effects as those described in the first embodiment is omitted, and the characteristic effects of the present embodiment will be described.

In this embodiment, since information (in this embodiment, image data) regarding the toner image is considered, the charging roller cleaning operation is performed at an appropriate timing according to the image data of the formed image. Further, in this embodiment, when an image having a uniform image pattern in the main scanning direction is formed on average, in which a change in image density is difficult to see, the charging roller cleaning operation is determined to be necessary. The number of prints is substantially increased. Therefore, excessive implementation of the charging roller cleaning operation is suppressed, and a decrease in throughput of the image forming apparatus is suppressed.

One of the features of this embodiment is that the amount of toner adhering to the charging roller 2 of a given image forming station S is converted into image data of an image formed by an image forming station S upstream of that image forming station S. is to estimate based on In other words, the data to be added to the cumulative print counter is the image data of the image forming section S upstream of the target image forming section S. In the in-line method, the toner adhering to the charging roller 2 of a certain image forming station S is re-transferred toner of the toner image formed on the intermediate transfer belt 10 in the image forming station S upstream of the image forming station S. Sometimes. That is, the amount of toner adhering to the charging roller 2 of a certain image forming station S can be estimated based on the image data of the toner image formed by the image forming station S upstream of the image forming station S. In this embodiment, by estimating the amount of toner adhering to the charging roller 2 in this way, it is possible to appropriately determine the timing of performing the charging roller cleaning operation.

With such a configuration, according to the present embodiment, by performing the charging roller cleaning operation at an appropriate timing, it is possible to suppress image density fluctuation while suppressing a decrease in throughput. Therefore, according to the present embodiment, deterioration of image quality can be suppressed by appropriately performing the charging roller cleaning operation.

In this embodiment, regarding the amount of toner adhering to the charging roller 2 due to retransfer, the threshold value inspection process is performed on the difference between the count values of the accumulation counter row between adjacent areas in the same manner as in the first embodiment. rice field. However, the present invention is not limited to this. Similar to the second embodiment, the effective range of the accumulation counter string is defined, and the maximum and minimum values within the effective range are subjected to the threshold inspection process. may

In addition to the method of binarizing the image data stored in the image memory described in the present embodiment, a laser driving signal converted from the image data described in the third embodiment was used as the information about the toner image. method may be used.

Further, the calculation method (determination method) of this embodiment may be combined with the control of the charging roller cleaning operation time and the electric field magnitude in the charging roller cleaning operation described in the first embodiment.

In addition, it predicts (calculates) the adhesion amount of oppositely charged toner to the charging roller 2 due to transfer residue and retransfer, or transfer residue, retransfer and fogging, and determines that it is necessary based on one of the prediction results. The charging roller cleaning operation may be performed when the charging roller is cleaned. Alternatively, a method of predicting (calculating) the amount of oppositely charged toner adhered to the charging roller 2 due to 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 of the prediction methods, a method such as resetting the count value of the integration counter in the other prediction method can be used.

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

In the above-described embodiment, the plurality of regions in the rotation axis direction of the photosensitive drum are the plurality of regions in the main scanning direction partitioned 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 a plurality of areas for each predetermined length such as each predetermined number of pixels in the main scanning direction.

In the above-described embodiments, the image forming apparatus adopts the intermediate transfer method, but the present invention can also be applied to a direct transfer type image forming apparatus. As is well known to those skilled in the art, an in-line image forming apparatus that employs a direct transfer system has a recording material carrier composed of an endless belt or the like instead of the intermediate transfer member in the above-described embodiments. . Then, the toner image formed on the photosensitive drum of each image forming section is directly transferred onto the recording material carried and conveyed by the recording material carrier in the same manner as the primary transfer in the intermediate transfer type image forming apparatus. . By applying the present invention to such an image forming apparatus according to the above-described embodiment, the same effects as those of the above-described embodiment can be obtained.

The present invention can also be applied to a monochrome image forming apparatus having a single image forming section. In this case, based on the index value corresponding to the amount of toner in the toner image passing through the transfer section that directly transfers the toner image from the photosensitive drum to the recording material, the cleaning operation of the charging member is controlled according to the above embodiment. Do it.

Further, the photosensitive member is not limited to a drum-shaped one (photosensitive drum), and may be an endless belt-shaped one (photosensitive belt). Further, the intermediate transfer member and the recording material carrier are not limited to the endless belt type, and may be, for example, a drum type formed by stretching a film on a frame. In the case of an electrostatic recording type image forming apparatus, the image carrier may be a drum-shaped or endless belt-shaped electrostatic recording dielectric.

Also, the adhesion of residual toner to the charging member becomes significant in a cleanerless image forming apparatus. Therefore, it can be said that the present invention is particularly effective in a cleanerless image forming apparatus. However, the present invention is not limited to this, and the present invention can also be applied to an image forming apparatus provided with a dedicated cleaning means for removing residual toner from the image carrier. In this case, it is possible to suitably perform a cleaning operation for cleaning residual toner that has not been completely removed by the cleaning means and has adhered to the charging member, thereby suppressing deterioration in image quality.

REFERENCE SIGNS LIST 1 photosensitive drum 2 charging roller 3 exposure device 4 developing device 10 intermediate transfer belt 14 primary transfer roller 15 secondary transfer roller 17 belt cleaning device 100 image forming apparatus

Claims (24)

a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The image forming apparatus according to claim 1, wherein the control section controls a timing of performing the cleaning operation based on a difference in integrated value between adjacent areas in the plurality of areas. The control unit performs the control to perform the cleaning operation when a predetermined number or more of the differences in the integrated values between the adjacent regions exceeds a predetermined threshold value. The image forming apparatus according to claim 1. a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The control unit controls the area of the effective range excluding an area that satisfies a predetermined condition among the plurality of areas, and the area of the effective range in which the integrated value is larger than the integrated value in the excluded area. An image forming apparatus, wherein the timing of executing the cleaning operation is controlled based on the integrated value. The control unit performs the control such that the cleaning operation is performed when a difference between the maximum value and the minimum value of the integrated values in the area of the effective range exceeds a predetermined threshold. 4. The image forming apparatus according to claim 3. 5. The image forming apparatus according to claim 3, wherein the area of the effective range is an area of a range obtained by excluding a predetermined ratio of areas having smaller integrated values among the plurality of areas. 6. The method according to claim 5, wherein the control unit changes the predetermined ratio according to the size of the recording material as the transfer target or the size of the recording material onto which the toner image is transferred from the transfer target. Image forming device. 7. The cleaning operation according to any one of claims 1 to 6, wherein the control unit further performs control to perform the cleaning operation when a maximum value of the integrated values in the plurality of areas exceeds a predetermined threshold value. The image forming apparatus according to any one of items 1 to 3. The control unit performed an image forming operation in which strip-shaped images extending in a direction substantially perpendicular to the rotation axis direction were formed uniformly in the rotation axis direction and continuously formed on a plurality of recording materials and output. A case where an image forming operation is performed in which band-shaped images extending in a direction substantially perpendicular to the rotation axis direction are formed with deviation in the rotation axis direction, and the images are successively formed on a plurality of recording materials and output. 8. The image forming apparatus according to any one of claims 1 to 7, wherein control is performed such that the cleaning operation is performed at a timing when the number of output recording materials is smaller. a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The image forming apparatus, wherein the control unit controls a time for performing the cleaning operation based on a difference in the integrated value between adjacent areas in the plurality of areas. When a predetermined number or more of the differences in the integrated values between the adjacent regions exceed a predetermined threshold value, the control unit performs the cleaning operation compared with a case where the threshold value is not exceeded. 10. The image forming apparatus according to claim 9, wherein the control is performed so as to lengthen the execution time. a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The controller controls the electric field between the charging member and the image carrier and the image carrier and the transfer target during the cleaning operation, based on the difference in the integrated value between the adjacent regions in the plurality of regions. An image forming apparatus that controls at least one of an electric field between and. When a predetermined number or more of the differences in the integrated values between the adjacent regions exceed a predetermined threshold, the control unit compares the cleaning operation with a case where the threshold is not exceeded. 3. The control is performed so as to increase at least one of an electric field between the charging member and the image bearing member and an electric field between the image bearing member and the transferred member. 12. The image forming apparatus according to 11. a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The control unit controls the area of the effective range excluding an area that satisfies a predetermined condition among the plurality of areas, and the area of the effective range in which the integrated value is larger than the integrated value in the excluded area. An image forming apparatus, wherein the time for performing the cleaning operation is controlled based on the integrated value. When the difference between the maximum value and the minimum value of the integrated values in the area of the effective range exceeds a predetermined threshold, the control unit compares the cleaning operation with a case where the threshold is not exceeded. 14. The image forming apparatus according to claim 13, wherein the control is performed so as to lengthen the time for executing the. a rotatable image carrier, a charging member that contacts the image carrier and charges the image carrier, and a developing member that supplies toner to the image carrier, and is based on image data. an image forming unit that forms a toner image on the image carrier and transfers the toner image from the image carrier onto a transfer-receiving member in contact with the image carrier in a transfer unit;
Based on the information about the print pixel area obtained from the image data, the amount of toner of the toner image passing through the transfer unit is adjusted for each of a plurality of areas on the image carrier in the rotation axis direction of the image carrier. an acquisition unit that acquires an integrated value of corresponding index values;
a control unit capable of executing a cleaning operation for cleaning the charging member during non-image formation;
has
The control unit controls the area of the effective range excluding an area that satisfies a predetermined condition among the plurality of areas, and the area of the effective range in which the integrated value is larger than the integrated value in the excluded area. At least one of an electric field between the charging member and the image bearing member and an electric field between the image bearing member and the transfer receiving member in the cleaning operation is controlled based on the integrated value. image forming apparatus. When the difference between the maximum value and the minimum value of the respective integrated values in the area of the effective range exceeds a predetermined threshold, the control unit compares the cleaning operation when the difference does not exceed the threshold. 3. The control is performed so as to increase at least one of an electric field between the charging member and the image bearing member and an electric field between the image bearing member and the transferred member. 15. The image forming apparatus according to 15 above. 17. The area of the effective range according to any one of claims 13 to 16, wherein the area of the effective range is an area of a range obtained by excluding a predetermined ratio of areas from the smaller integrated value among the plurality of areas. image forming device. 18. The method according to claim 17, wherein the control section changes the predetermined ratio according to the size of the recording material as the transfer-receiving material or the recording material onto which the toner image is transferred from the transfer-receiving material. Image forming device. A plurality of the image forming units are provided along the moving direction of the transfer-receiving body, and the plurality of image forming units comprise a first image forming unit and the first image-forming unit in the moving direction of the transfer-receiving body. and a second image forming section arranged downstream of the image forming section, wherein the transfer section of the first image forming section and the transfer section of the second image forming section form the transfer material on the transferred material. It is possible to sequentially transfer toner images,
The obtaining section forms a toner image on the image carrier of the second image forming section based on the information regarding the print pixel area obtained from the image data regarding the toner image formed by the second image forming section. acquiring the integrated value of the index value corresponding to the amount of toner of the toner image passing through the transfer portion of the second image forming portion;
19. The image forming apparatus according to any one of claims 1 to 18, wherein the control section performs the control regarding the cleaning operation in the second image forming section based on the integrated value acquired regarding the second image forming section. The image forming apparatus according to any one of items 1 to 3. A plurality of the image forming units are provided along the moving direction of the transfer-receiving body, and the plurality of image forming units comprise a first image forming unit and the first image-forming unit in the moving direction of the transfer-receiving body. and a second image forming section arranged downstream of the image forming section, wherein the transfer section of the first image forming section and the transfer section of the second image forming section form the transfer material on the transferred material. It is possible to sequentially transfer toner images,
The obtaining unit is configured to perform the transfer in the transfer unit of the first image forming unit based on the information regarding the print pixel area obtained from the image data regarding the toner image formed by the first image forming unit. acquiring the integrated value of the index value corresponding to the amount of toner of the toner image transferred onto the body and passing through the transfer portion of the second image forming portion;
19. The image forming apparatus according to any one of claims 1 to 18, wherein the control section performs the control regarding the cleaning operation in the second image forming section based on the integrated value acquired regarding the second image forming section. The image forming apparatus according to any one of items 1 to 3. 21. The controller according to any one of claims 1 to 20, wherein in the cleaning operation, the controller controls to form an electric field between the image carrier and the charging member in a direction opposite to that during image formation. 1. The image forming apparatus according to item 1 or 1. The information relating to the print pixel area is information relating to the emission time of the light emitted to the image carrier corresponding to the toner image formed on the image carrier by an exposure device for irradiating the image carrier with light. The image forming apparatus according to any one of claims 1 to 21, characterized by: 23. The image forming method according to any one of claims 1 to 22, wherein toner remaining on the image bearing member without being transferred onto the transfer receiving member during image formation is recovered by the developing member. Device. 24. The image forming apparatus according to any one of claims 1 to 23, wherein the toner is a one-component developer.

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