JP2008145848A - Charger, image forming apparatus, and charge control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To effectively prevent occurrence of vertical stripes in contact charging of an image carrier by constant current control using a charging brush and a charging roller. <P>SOLUTION: A charging brush current Ia<SB>1</SB>(μA) and a charging roller current Ia<SB>2</SB>(μA) are set so as to satisfy ¾Ia<SB>1</SB>¾/5 < ¾Ia<SB>2</SB>¾. Consequently, flowing of unnecessary amount of the charging brush current Ia<SB>1</SB>(μA) can be suppressed. Thereby film reduction on the photosensitive layer 2a of the image carrier 2 which is caused by the flowing of unnecessary amount of the charging brush current Ia<SB>1</SB>(μA) can be suppressed. Thus, the absolute value of potential on the surface of the photosensitive layer 2a of the image carrier 2 can be maintained at a value higher than the absolute value of image writing potential and stable and effective charging can be achieved. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a technical field of a charging device for charging an image carrier, an image forming apparatus including an electrophotographic apparatus including the charging device, and a charge control method for controlling charging of the image carrier.

  In a conventional image forming apparatus including an electrophotographic apparatus such as an electrostatic copying machine, a printer, or a facsimile, the surface of the photosensitive member is uniformly charged by the contact charging device, and then electrostatically applied to the surface of the photosensitive member by the exposure device. A latent image is formed. The electrostatic latent image is developed into a toner image by a developing device, and is transferred to a transfer material such as paper via an intermediate transfer member or directly. Finally, an image is formed by fixing the toner image on the transfer material by a fixing device.

  As a conventional image forming apparatus, in order to uniformly and satisfactorily charge the surface of the rotating photoconductor, primary charging is performed by contacting the photoconductor with a charging brush as a first charging member, and then the photoconductor rotates. There is known an image forming apparatus including a charging device that performs secondary charging by bringing a charging roller, which is a charging roller downstream of a charging brush in a direction, into contact with a photosensitive member (see, for example, Patent Document 1). In the charging device of this image forming apparatus, the photosensitive member is uniformly charged by removing the charging unevenness when the surface of the photosensitive member is charged by the charging brush on the upstream side in the rotating direction of the photosensitive member by the charging roller.

JP-A-2005-215321.

  However, in the charging device described in Patent Document 1, when a charging brush is used, impurities such as toner adhere between a large number of brush hairs and brush hairs when used for a long time. For this reason, the charging efficiency by a charging brush falls. Therefore, conventionally, a voltage higher than the voltage necessary for charging the image carrier for image formation is applied to the charging brush. For this reason, a large amount of current flows between the image carrier and the charging brush, and the photosensitive layer of the image carrier is subjected to electrical stress. As a result, the film thickness of the photosensitive layer of the image carrier is reduced. In that case, the contact state of a large number of bristles of the charging brush with the image carrier varies depending on the bristles. Therefore, the magnitude of the reduction of the film thickness in the image carrier varies depending on the position of the image carrier in the axial direction. In addition, in the charging of the image carrier by double contact charging using a charging brush and a charging roller that are in contact with the image carrier, the film thickness of the photosensitive layer of the image carrier is reduced even when the number of images formed is increased.

  As described above, when the film thickness of the image carrier is reduced and the magnitude of the reduction in the film thickness varies, the unevenness of the primary charging due to the secondary charging of the charging roller is insufficient. Therefore, the absolute value of the surface potential of the photosensitive layer may be lower than the preset absolute value of the image writing potential depending on the axial position of the image carrier. As described above, when the surface potential of the photosensitive layer 2a is lower than the absolute value of the image writing potential, the portion becomes defectively charged and appears as a vertical stripe in the image, resulting in poor image quality.

  The present invention has been made in view of such circumstances, and an object thereof is to effectively prevent vertical stripes in contact charging of an image carrier by constant current control using a charging brush and a charging roller. It is an object to provide a charging device, an image forming apparatus, and a charging control method.

In order to solve the above-described problem, according to the present invention, the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) satisfy | Ia ₁ | / 5 <| Ia ₂ |. It is set. Thereby, it is possible to suppress the flow of the charging brush current Ia ₁ (μA) more than necessary. Therefore, it is possible to suppress the film loss of the photosensitive layer of the image carrier due to the flow of the charging brush current Ia ₁ (μA) more than necessary. In this way, the absolute value of the potential on the surface of the photosensitive layer of the image carrier can be maintained higher than the absolute value of the image writing potential over a long period of time, and stable and favorable charging can be performed. In addition, since the charging roller current Ia ₂ (μA) can be controlled by linking with the charging brush current Ia ₁ (μA), charging failure due to insufficient leveling can be prevented even if the film thickness of the photosensitive layer of the image carrier is reduced. it can. As a result, it is possible to effectively prevent vertical stripes from appearing in the image, so that a high-quality image can be obtained.

When the number of rotations of the image carrier reaches a predetermined number of rotations set in advance, the absolute value of the charging brush current Ia ₁ (μA) is increased. Thus, even if the film thickness of the photosensitive layer of the image carrier is reduced to some extent by long-term use of the image forming apparatus, the absolute value of the surface potential of the photosensitive layer is maintained higher than the absolute value of the image writing potential. Can do. Therefore, stable and good charging can be performed. As a result, vertical streaks can be prevented from appearing in the image in the same manner as described above, and a high quality image can be obtained.

The best mode for carrying out the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram schematically and partially showing an example of an embodiment of an image forming apparatus according to the present invention.

  As shown in FIG. 1, the image forming apparatus 1 of this example is formed as a full-color tandem type image forming apparatus composed of yellow (Y), magenta (M), cyan (C), and black (K). Yes. The image forming apparatus 1 includes, for example, image carriers 2Y, 2M, 2C, and 2K formed of a photosensitive drum, and these image carriers 2Y and 2M, on which electrostatic latent images and toner images are formed for the respective colors. , 2C, 2K, charging devices 3Y for charging the image carriers 2Y, 2M, 2C, 2K, which are sequentially arranged from the upstream side of the rotation direction α of the image carriers 2Y, 2M, 2C, 2K, respectively. , 3M, 3C, 3K, image writing devices 4Y, 4M, 4C, 4K for writing an electrostatic latent image on each image carrier 2Y, 2M, 2C, 2K, on each image carrier 2Y, 2M, 2C, 2K The toner images on the developing devices 5Y, 5M, 5C, and 5K and the image carriers 2Y, 2M, 2C, and 2K that develop the electrostatic latent image with the toner that is the developer of each color to form the toner image are primarily transferred. Cleaning for cleaning the primary transfer devices 6Y, 6M, 6C, 6K and the image carriers 2Y, 2M, 2C, 2K Location 7Y, is equipped with 7M, 7C, and 7K.

  Each image carrier 2Y, 2M, 2C, 2K, each charging device 3Y, 3M, 3C, 3K, each image writing device 4Y, 4M, 4C, 4K, each developing device 5Y, 5M, 5C, 5K, primary transfer device The configurations of 6Y, 6M, 6C, and 6K and the cleaning devices 7Y, 7M, 7C, and 7K are the same for the colors Y, M, C, and K, respectively.

FIG. 2 is a diagram for explaining each device configured in the same manner for each color. In FIG. 2, the description will be made by taking the Y, M, C, and K symbols of each color.
As shown in FIG. 2, the surface layer of the image carrier 2 is formed on a photosensitive layer 2a having a predetermined thickness. As will be described later, an electrostatic latent image is written by the image writing device 4 on the surface of the photosensitive layer 2 a charged by the charging device 3.

  Further, the charging device 3 charges the photosensitive layer 2a of the image carrier 2, and a charging roller (CR) 3a as a second charging member, and the image carrier on the upstream side in the rotation direction α from the charging roller 3a. And a charging brush (CB) 3d which is a first charging member for charging the second photosensitive layer 2a. The charging roller 3a is in contact with the surface of the photosensitive layer 2a of the image carrier 2 and is rotated in the direction β opposite to the rotation direction α of the image carrier 2 (counterclockwise in FIG. 1).

  As the charging roller 3a, a known charging roller that has been conventionally used can be used. That is, the charging roller 3a can be manufactured by a manufacturing method described in, for example, JP-A-10-48916. That is, the charging roller 3a is configured by providing, for example, a cylindrical conductive elastomer layer 3c made of, for example, conductive rubber or the like having a surface inclined resistance layer on a metal shaft 3b made of stainless steel. ing. The conductive elastomer layer 3c is set to have a constant outer diameter and a substantially constant electric resistance along the axial direction.

  As an example of the charging roller 3a having the conductive elastomer layer 3c, 100 parts by weight of epichlorohydrium (Epichromer CG-102; manufactured by Daiso Corporation) is used as a conductive material, such as sodium trifluoroacetate, zinc oxide (ZnO), and a vulcanizing agent. 2 parts by weight of 2-mercaptoimidazoline (Accel-22) is kneaded with a roll mixer, press-molded on the surface of a metal shaft having a diameter of 6 mm, polished to a diameter of 12 mm, and a conductive rubber elastic member is formed on the surface of the shaft. A formed roller was obtained. In that case, the respective addition amounts of sodium trifluoroacetate and zinc oxide (ZnO) were arbitrarily adjusted to obtain five charging rollers 3a having different resistance values. Table 1 shows the resistance values of the conductive elastomer layer 3c of the five charging rollers (1) to (5).

As shown in Table 1, the resistance value of the charging roller (1) is 3.0 × 10 ¹⁰ Ω at a relatively low temperature of 10 ° C., 1.6 × 10 ⁷ Ω at a normal temperature of 23 ° C., and a relatively high temperature of 35 It is 2.4 × 10 ⁶ Ω at ° C. The resistance value of the charging roller (2) is 4.0 × 10 ⁸ Ω at 10 ° C., 3.7 × 10 ⁶ Ω at 23 ° C., and 6.8 × 10 ⁵ Ω at 35 ° C. Further, the resistance value of the charging roller (3) is 6.9 × 10 ⁷ Ω at 10 ° C., 1.5 × 10 ⁶ Ω at 23 ° C., and 3.5 × 10 ⁵ Ω at 35 ° C. Further, the resistance value of the charging roller (4) is 1.4 × 10 ⁷ Ω at 10 ° C., 4.5 × 10 ⁵ Ω at 23 ° C., and 1.1 × 10 ⁵ Ω at 35 ° C. Further, the resistance value of the charging roller (5) is 3.0 × 10 ⁵ Ω at 10 ° C., 6.0 × 10 ⁴ Ω at 23 ° C., and 3.0 × 10 ⁴ Ω at 35 ° C.

  A method for measuring the resistance value of each charging roller (CR) will be described. As shown in FIG. 3, a charging roller (CR) 3 a that is an object to be measured is placed on an aluminum tube 9 used for the photoreceptor 2. Further, the metal shaft 3b of the charging roller (CR) 3a and the aluminum tube 9 are electrically connected by a constant voltage power source 10 (R8340A manufactured by Advantest). Then, a load of 250 g is applied to the metal shafts 3b at both ends of the charging roller (CR) 3a so that the charging roller (CR) 3a is pressed against the aluminum tube 9. Further, the aluminum tube 9 is rotated at a rotation speed of 5 rpm. In this state, a voltage of 160 V is applied from the constant voltage power supply 10 and the current value flowing through the constant voltage power supply 10 is read by the ammeter 10a. Finally, based on the read current value and voltage, the resistance value R is calculated from the equation V = IR (V: voltage, I: current, R: resistance).

  The charging brush 3d is configured as a deck brush having a large number of bristles and fixedly supported by a brush support member (not shown). The brush support member is fixed to the apparatus main body. Therefore, the charging brush 3d is also fixed to the apparatus main body. The tips of a large number of brush hairs are in contact with the surface of the photosensitive layer 2 a of the photoreceptor 2. As the deck brush of the charging brush 3d, a known charging brush that has been conventionally used can be used. Examples of the charging brush 3d are shown in Table 2.

As shown in Table 2, the brush bristles of the brush 1 as an example have a material of 6 nylon, a fineness of 220T / 96F, a density of 240 kf / inch ² , a yarn resistance of 7.1 LogΩ, and a pile length of 5 mm. The overall shape of the brush 1 is 300 mm in length (along the axial direction of the photoreceptor 2), 5 mm in width, and 5 mm in height. As another example, the brush bristles of the brush 2 are made of nylon 6 material, a fineness of 330 T / 48 F, a density of 80 kf / inch ² , a yarn resistance of 9.3 LogΩ, and a pile length of 5 mm. The overall shape of the brush 1 is 300 mm in length (along the axial direction of the photoreceptor 2), 5 mm in width, and 5 mm in height. Both of these brushes 1 and 2 are brushes made by Toei Sangyo Co., Ltd.

In the image forming apparatus 1 of this example, for example, when the image carrier 2 is negatively charged by a DC voltage, first, as shown in FIG. 4, the discharge start voltage between the charging brush 3 d and the image carrier 1. A negative voltage −Va ₁ (V) having an absolute value higher than the absolute value of (−V) is applied to the charging brush 3d. Thereby, the photosensitive layer 2a of the image carrier 2 is negatively charged to the primary charging potential (eg, −800 V) (primary charging) by the charging brush 3d. On the other hand, a negative voltage −Va ₂ (V) or a positive voltage + Va ₂ (V), Va, whose absolute value is lower than the absolute value of the discharge start voltage (−V) between the charging roller 3 a and the image carrier 1. ₂ (V) is applied to the charging roller 3a. As a result, the photosensitive layer 2a of the image carrier 2 is uniformly charged by the charging roller 3a (secondary charging). Thus, in the image forming apparatus 1 of this example, double contact charging (W charging) of the image carrier 2 is performed by the charging brush 3d and the charging roller 3a. As a result, the surface of the photosensitive layer 2a of the image carrier 2 is uniformly charged to an image writing potential (for example, −600 V) set in advance for image writing.

FIG. 5 is a diagram showing the surface potential of the image carrier due to W charging.
As shown in FIG. 5, in the W charging, when the photosensitive layer 2a of the image carrier 2 is primarily charged by the charging brush 3d, the primary charging potential of the photosensitive layer 2a is −800 depending on the position of the image carrier 2 in the axial direction. There is variation between ~ 900V. The cause of this variation is that a large number of bristles of the charging brush 3d are not uniformly pressed against the photosensitive layer 2a of the image carrier 2. The secondary charging potential of the photosensitive layer 2a is equalized by the secondary charging of the photosensitive layer 2a of the image carrier 2 with the charging roller 3a. Regardless of the position in the axial direction, the image writing potential is made substantially uniform at about -600V.

  The image writing device 4 writes an electrostatic latent image on the image carrier 2 charged by the charging device 3 with, for example, a laser beam. The developing device 5 includes a developing roller 5a, a toner supply roller 5b, and a toner layer thickness regulating member 5c. At both ends of the developing roller 5a, developing rollers 5d (only one developing roller 5a is shown in FIG. 2) are provided. These developing rollers 5d constitute a developing roller / image carrier axis distance regulating member. In other words, the distance between the axial center of the developing roller 5 a and the axial center of the image carrier 2 is regulated by the development rollers 5 d coming into contact with the outer peripheral surface of the image carrier 2. Thus, a predetermined development gap is set between the outer peripheral surface of the developing roller 5a and the outer peripheral surface of the image carrier 2.

  Then, toner (not shown) as a developer is supplied onto the developing roller 5a by the toner supply roller 5b, and the toner on the developing roller 5a is regulated in its thickness by the toner layer thickness regulating member 5c and the image is carried. The electrostatic latent image on the image carrier 2 is developed in a non-contact jumping manner with the conveyed toner, and a toner image is formed on the image carrier 2.

The primary transfer device 6 includes a primary transfer roller 6a, and the toner image on the image carrier 2 is transferred to an intermediate transfer medium 8 formed of, for example, an intermediate transfer belt by the primary transfer roller 6a.
The cleaning device 7 has a cleaning blade 7a made of an elastic material such as rubber. The cleaning blade 7a is attached to the apparatus main body via a blade support member (not shown). The cleaning blade 7 a is always in pressure contact with the outer peripheral surface of the image carrier 2. The image carrier 2 is cleaned by the cleaning blade 7a, and the transfer residual toner on the image carrier 2 is removed and collected by a toner collection unit (not shown).

  Further, the image forming apparatus 1 in this example is not shown in the figure, but is secondarily transferred to a known secondary transfer device that transfers the toner image transferred to the intermediate transfer medium 8 to a transfer material such as paper, and to a transfer material. At least a known fixing device that heats and presses and fixes the toner image.

  Next, an image forming operation of the image forming apparatus 1 of this example will be described with reference to FIG. In this case, the constituent members provided for the respective colors will be described with reference numerals of the corresponding colors Y, M, C, and K attached to the constituent members.

  In FIG. 1, a yellow (Y) image carrier 2Y is primarily charged by a charging brush 3Yd, and then uniformly secondary charged by a charging roller 3Ya. Next, after an electrostatic latent image is written on the image carrier 2Y by the image writing device 4Y, the electrostatic latent image is developed by the toner conveyed by the developing roller 5Ya in the developing device 5Y. Thus, a yellow (Y) toner image is formed on the image carrier 2Y. In the same manner for the other colors M, C, and K, toner images of corresponding colors are sequentially formed on the respective image carriers 2M, 2C, and 2K with a predetermined time delay.

  The yellow (Y) toner image on the image carrier 2Y is primarily transferred onto the intermediate transfer medium 8 by the primary transfer device 6Y, and the yellow (Y) toner image on the intermediate transfer medium 8 is magenta (M ) To the primary transfer device 6M. Then, the magenta (M) toner image on the image carrier 2M is primary-transferred to the predetermined position on the intermediate transfer medium 8 with the yellow (Y) toner image by the primary transfer device 6M. Further, the toner image on the intermediate transfer medium 8 in which the colors of yellow (Y) and magenta (M) are superimposed is conveyed toward the primary transfer device 6C of cyan (C). Then, the cyan (C) toner image on the image carrier 2C is primary-transferred at the predetermined position on the intermediate transfer medium 8 with the yellow (Y) and magenta (M) toner images by the primary transfer device 6C. Is done. Further, the toner image on the intermediate transfer medium 8 on which the colors of yellow (Y), magenta (M), and cyan (C) are superimposed is conveyed toward the primary transfer device 6K of black (K). Then, the black (K) toner image on the image carrier 2K is transferred to the yellow (Y), magenta (M), and cyan (C) toner images and colors at predetermined positions on the intermediate transfer medium 8 by the primary transfer device 6C. Overlaid and primary transferred. Thus, a full-color toner image is formed on the intermediate transfer medium 8.

  Further, the full-color toner image on the intermediate transfer medium 8 is secondarily transferred to a transfer material such as paper by a secondary transfer device, and the full-color toner image on the transfer material is heated and pressure-fixed by a fixing device. . Thus, a full color image is formed on the transfer material.

By the way, when the charging brush 3d is used as the first charging member, impurities such as toner adhere to the brush hair as described above, and the charging efficiency by the charging brush 3d is reduced. Therefore, conventionally, a voltage higher than the voltage necessary for charging the image carrier 2 for image formation is applied to the charging brush 3d. For this reason, a large amount of current flows between the image carrier 2 and the charging brush 3d, and the photosensitive layer 2a of the image carrier 2 is subjected to electrical stress. Thereby, the film thickness of the photosensitive layer 2a of the image carrier 2 is reduced. In this case, the contact state of a large number of brush hairs of the charging brush 3d with the image carrier 2 varies depending on the brush hairs. Therefore, the magnitude of the reduction of the film thickness in the photosensitive layer 2a varies depending on the position of the image carrier 2 in the axial direction.
Further, in the charging of the image carrier 2 by double contact charging using the charging brush 3d and the charging roller 3a that are in contact with the image carrier 2, the film thickness of the photosensitive layer 2a of the image carrier 2 increases the number of images formed. As it decreases.

  FIG. 6 is a diagram showing the relationship between the number of image formations obtained by experiments to be described later and the film thickness of the photosensitive layer 2a using four types of photosensitive drums as the image carrier 2. In FIG. As shown in FIG. 6, the film thickness of the photosensitive layer 2 a decreases as the number of image formations increases in any of the four types of photosensitive drums. The magnitude of the film thickness reduction in the photosensitive layer 2a varies depending on the position of the image carrier 2 in the axial direction.

  When the film thickness of the photosensitive layer 2a is reduced and the magnitude of the reduction in the film thickness varies, the potential of the surface of the photosensitive layer 2a due to secondary charging of the charging roller 3a is changed to the axial position of the image carrier 2. Depending on the case, the absolute value may be lower than the preset absolute value of the image writing potential (eg, -600 V). As described above, when the surface potential of the photosensitive layer 2a is lower than the absolute value of the image writing potential, the portion becomes defectively charged and appears as a vertical stripe in the image, resulting in poor image quality.

Therefore, in the charging device 3 of this example, the first charging member current (charging brush current) Ia ₁ (μA) flowing between the image carrier 2 and the charging brush 3d, the image carrier 2 and the charging roller 3a The image carrier 2 is charged by constant current control in which a second charging member current (charging roller current) Ia ₂ (μA) flowing between them has a predetermined relationship. That is, in the charging device 3 of this example, the absolute value of the charging brush current Ia ₁ (μA) is set to be smaller than the absolute value of the charging roller current Ia ₂ (μA) (| Ia ₁ | / 5 <| Ia ₂ |). In the present invention, the charging brush current Ia ₁ (μA) is defined as positive (+) when flowing from the image carrier 2 to the charging brush 3 d, and negative (−) when flowing from the charging brush 3 d to the image carrier 2. Define. In the present invention, the charging roller current Ia ₂ (μA) is defined as positive (+) when flowing from the charging roller 3a to the image carrier 2, and negative (−) when flowing from the image carrier 2 to the charging roller 3a. ). In the charging device 3 of this example, the voltage Va ₁ (V) applied to the charging brush 3d and the voltage Va ₂ (V) applied to the charging roller 3a are controlled, and thus the charging brush current Ia ₁ ( μA) and charging roller current Ia ₂ (μA) are set.

FIG. 7 is a block diagram for setting the charging brush current and the charging roller current.
As shown in FIG. 7, the image forming apparatus 1 of this example includes a central processing unit (CPU) 11 for applying charging voltages to the charging brush 3d and the charging roller 3a and controlling the applied voltages. ing. The CPU 11 is a CPU that controls the entire image forming apparatus 1 in order to form an image. The CPU 11 has an image forming operation member (image forming command) such as an operation key of an operation panel for outputting an operation command (image forming command) or an operation touch key on an operation screen for causing the image forming apparatus 1 to perform an image forming operation. Member) 12, a first ammeter 13 that measures charging brush current Ia ₁ (μA), a second ammeter 14 that measures charging roller current Ia ₂ (μA), and the number of times of image formation by image forming apparatus 1 are counted. An image carrier rotation number counter 15 is connected to each other.

When the image forming command signal is input from the image forming operation member 12, the CPU 11 causes the charging brush 3 d and the charging roller 3 a to perform the voltage Va ₁ (V) and the image forming apparatus 1, respectively. A voltage Va ₂ (V) is applied. On the other hand, when the image forming command signal is input, the CPU 11 charges the image carrier 2 which is one of the image forming operations of the image forming apparatus 1, so that the voltage Va _{1 is} applied to the charging brush 3 d and the charging roller 3 a, respectively. (V) and voltage Va ₂ (V) are applied. The CPU 11 receives a measurement signal of the charging brush current Ia ₁ (μA) from the first ammeter 13 and a measurement signal of the charging brush current Ia ₂ (μA) from the second ammeter 14. The CPU 11 then applies the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) to the charging brush 3d and the charging roller 3a so that | Ia ₁ | / 5 <| Ia ₂ | The applied voltage -Va ₁ (V) and voltage Va ₂ (-V or V) are controlled.

Further, in the image forming apparatus 1 of this example, the number of rotations of the image carrier 2 of the image forming apparatus 1 is counted by the image carrier rotation number counter 15 as the number of rotations of the image carrier increases. The count value signal is input to the CPU 11. When the number of rotations of the image carrier 2 of the image forming apparatus 1 increases due to use for a long period of time or the like and reaches a predetermined number of rotations set in advance, that is, when the number of image formations reaches a predetermined number of images formed in advance. However, it is determined that the film thickness reduction of the photosensitive layer 2a of the image carrier 2 has reached a predetermined amount, and the absolute value of the charging brush current Ia ₁ (μA) is increased by a certain amount. That is, when the number of rotations of the image carrier 2 reaches the specified number of rotations, that is, when the number of image formations reaches the specified number, the charging brush current Ia ₁ (μA) is preset in advance whose absolute value is higher than the absolute value of the current value. The applied voltage of the charging brush 3d is controlled so that the obtained value is obtained.

FIG. 8 is a diagram showing a flow for controlling the charging brush current in accordance with the number of rotations of the image carrier.
As shown in FIG. 8, in order to control the charging brush current Ia ₁ (μA) in accordance with the number of rotations of the image carrier 2, an image forming command is issued in step S1. Then, the image carrier 2 starts to be driven in step S2, and the number of rotations of the image carrier 2 is counted by the image carrier rotation number counter 15 in step S3. Next, in step S4, it is determined whether or not the accumulated count value of the number of rotations of the image carrier 2 has reached a specified number of rotations. If it is determined that the accumulated count value of the rotation number of the image carrier 2 has not reached the specified rotation number, image formation is started in step S5.

That is, the first charging voltage Va ₁ (−V) controlled so as to be the charging brush current Ia ₁ (μA) set at that time is applied to the charging brush 3d. The charging brush 3d performs primary charging of the photosensitive layer 2a of the image carrier 2 as one of image forming operations. At this time, the charging roller current Ia ₂ (μA) is set to satisfy | Ia ₁ | / 5 <| Ia ₂ | with respect to the charging brush current Ia ₁ (μA). Therefore, the charging voltage Va ₁ (−V) controlled to be the charging roller current Ia ₂ (μA) is applied to the charging roller 3a. The charging roller 3a performs a secondary charging operation of the photosensitive layer 2a of the image carrier 2 as one of the image forming operations. When the image formation is completed in step S6, the charging control of the photosensitive layer 2a of the image carrier 2 by the current control of the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) is ended.

If it is determined in step S4 that the accumulated count value of the number of rotations of the image carrier 2 has reached the specified number of rotations, the charging brush current Ia ₁ (μA) is increased by a predetermined amount in step S7. That is, the first charging voltage −Va ₁ (V) is newly controlled so that the charging brush current Ia ₁ (μA) is increased by a predetermined amount. Further, the charging roller current Ia ₂ (μA) is newly set so as to satisfy | Ia ₁ | / 5 <| Ia ₂ | with respect to the charging brush current Ia ₁ (μA) increased by a predetermined amount. Then, the charging voltage Va ₂ (−V or V) of the charging roller 3a is newly controlled so that the charging roller current Ia ₂ (μA) is newly set. Next, the process proceeds to step S5 described above, and each process after step S5 is performed. During image formation in step S5, a newly controlled voltage -Va ₁ (V) is applied to the charging brush 3d, and a newly controlled voltage Va ₂ (-V or V) is applied to the charging roller 3a. ) Is applied. In step S6, image formation is completed, and charging control of the photosensitive layer 2a of the image carrier 2 by the above-described current control is completed.

According to the image forming apparatus 1 of this example, the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are set so as to satisfy | Ia ₁ | / 5 <| Ia ₂ |. . Thereby, it is possible to suppress the flow of the charging brush current Ia ₁ (μA) more than necessary. Therefore, the film loss of the photosensitive layer 2a of the image carrier 2 due to the flow of the charging brush current Ia ₁ (μA) more than necessary can be suppressed. In this way, the absolute value of the surface potential of the photosensitive layer 2a can be maintained higher than the absolute value of the image writing potential over a long period of time, and stable and good charging can be performed. Further, since the charging roller current Ia ₂ (μA) can be controlled by linking with the charging brush current Ia ₁ (μA), even if the film thickness of the photosensitive layer 2a of the image carrier 2 is reduced, charging failure due to insufficient leveling. Can be prevented. As a result, vertical streaks can be prevented from appearing in the image, and a high-quality image can be obtained.

When the number of rotations of the image carrier 2 reaches a predetermined number of rotations set in advance, the absolute value of the charging brush current Ia ₁ (μA) is increased. Thus, even if the film thickness of the photosensitive layer 2a of the image carrier 2 is reduced to some extent by the long-term use of the image forming apparatus 1, the absolute value of the surface potential of the photosensitive layer 2a is made to be larger than the absolute value of the image writing potential. Can be kept high. Therefore, stable and good charging can be performed. As a result, vertical streaks can be prevented from appearing in the image in the same manner as described above, and a high quality image can be obtained.

Next, an experiment for confirming the effect obtained by the present invention was conducted.
(Experimental device)
As the experimental apparatus, a commercially available laser color printer LP9000C manufactured by Seiko Epson Corporation was used. In that case, the charging unit of the LP9000C photoconductor unit was modified so that a charging brush 3d as a first charging member and a charging roller 3a as a second charging member could be attached. As the charging brush 3d, the brushes 1 and 2 shown in Table 2 were used, and as the charging roller 3a, the charging rollers (3) and (4) shown in Table 1 given as an example were used. Further, a voltage can be applied and controlled from the outside to the charging brush 3d and the charging roller 3a. Voltage was applied to the charging brush 3d and the charging roller 3a so that the charging of the image carrier 2 by the charging brush 3d and the charging roller 3a was performed by constant current control. Therefore, two ammeters 13 and 14 are provided as shown in FIG. Other photoreceptor drum (image carrier), cleaning blade, optical writing device, developing device (including genuine toner), transfer device (including intermediate transfer belt), and fixing device are all LP9000C. Using. As shown in Table 3, the experiments were carried out using Examples 1 to 6 and Comparative Examples 1 and 2 using the image carrier 2 having a photosensitive layer thickness (OPC thickness) of 26 μm, and a photosensitive layer thickness (OPC). This was carried out for Examples 7 to 10 and Comparative Examples 3 to 6 using the image carrier 2 having a film thickness of 15 μm.

(Image formation test)
The image formation test is the same in all experiments of Examples 1 to 10 and Comparative Examples 1 to 6. That is, 100K monochrome halftone images are continuously formed on A4 size plain paper in an environment of 23 ° C. (room temperature). Each image formed is visually confirmed, and when there is no vertical stripe, it is determined that the charge is excellent or good. When the vertical stripe is recognized, the charge is poor or slightly poor. Judged.

The experimental conditions common to Examples 1 to 10 and Comparative Examples 1 to 6 are that the process speed is 210 mm / sec and the peripheral speed ratio between the peripheral speed of the charging roller and the peripheral speed of the photosensitive member is 1. Further, the development application voltage is also a superimposed voltage of the DC voltage and the AC voltage, and the DC voltage V _DC = −200 V, the AC voltage V _PP = 1400 V, the AC voltage frequency f = 3.0 kHz rectangular wave (50% duty) Furthermore, the transfer application voltage is + 200V.

The individual experimental conditions of Examples 1 to 10 and Comparative Examples 1 to 6 will be described.
(Example 1)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (3). The voltage Va ₂ (V) applied to the charging roller 3a was set to a positive DC voltage of +80 V, which was satisfactory when image density adjustment was performed by voltage control. Then, the charging roller 3a + 80V is applied to, a current Ia ₁ (μA) current meter 13 which flows from the image carrier 2 to the charging brush 3d, also charging current Ia ₁ flowing from the roller 3a in the image carrier 2 (.mu.A ) Were monitored by ammeters 14 respectively. Furthermore, the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) give the charging brush 3d a negative DC voltage −Va ₁ (V) where | Ia ₂ |> | Ia ₁ | / 5. Applied. In this case, in Example 1, Ia ₁ was +66 μA and Ia ₂ was +30 μA.

(Example 2)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (3). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the first embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 2, Ia ₁ was +59 μA and Ia ₂ was +25 μA.

(Example 3)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (4). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the first embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 3, Ia ₁ was +54 μA and Ia ₂ was +20 μA.

Example 4
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (4). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the first embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 4, Ia ₁ was +49 μA and Ia ₂ was +15 μA.

(Example 5)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (3). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the first embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 5, Ia ₁ was +46 μA and Ia ₂ was +12 μA.

(Example 6)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (3). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the first embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 6, Ia ₁ was +44 μA and Ia ₂ was +10 μA.

(Comparative Example 1)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (4). Then, +80 V is applied to the charging roller 3a, and the negative DC voltage − in which the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are | Ia ₂ | <| Ia ₁ | / 5 Va ₁ (V) was applied to the charging brush 3d. In this case, in Comparative Example 1, Ia ₁ was +40 μA and Ia ₂ was +7 μA.

(Comparative Example 2)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (4). Then, +80 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are set to | Ia ₂ | <| Ia ₁ | / 5 as in Comparative Example 1. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 5, Ia ₁ was +35 μA and Ia ₂ was +4 μA.

(Example 7)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (3). The voltage Va ₂ (V) applied to the charging roller 3a was set to a positive DC voltage of + 150V, which was good after image density adjustment by voltage control. Then, the charging roller 3a + 150 V by applying, in current Ia ₁ (μA) current meter 13 which flows from the image carrier 2 to the charging brush 3d, also charging current Ia ₁ flowing from the roller 3a in the image carrier 2 (.mu.A ) Were monitored by ammeters 14 respectively. Furthermore, the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) give the charging brush 3d a negative DC voltage −Va ₁ (V) where | Ia ₂ |> | Ia ₁ | / 5. Applied. In that case, in Example 7, Ia ₁ was +122 μA and Ia ₂ was +40 μA.

(Example 8)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (3). Then, +150 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the seventh embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 8, Ia ₁ was +118 μA and Ia ₂ was +35 μA.

Example 9
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (4). Then, +150 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the seventh embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 9, Ia ₁ was +112 μA and Ia ₂ was +30 μA.

(Example 10)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (4). Then, +150 V is applied to the charging roller 3a, and the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) are expressed as | Ia ₂ |> | Ia ₁ | / 5 as in the seventh embodiment. The negative DC voltage −Va ₁ (V) is applied to the charging brush 3d. In that case, in Example 10, Ia ₁ was +106 μA and Ia ₂ was +25 μA.

(Comparative Example 3)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (3). Then, + 150V is applied to the charging roller 3a, and the negative DC voltage − in which the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) become | Ia ₂ | = | Ia ₁ | / 5 Va ₁ (V) was applied to the charging brush 3d. In this case, in Comparative Example 3, Ia ₁ was +100 μA and Ia ₂ was +20 μA.

(Comparative Example 4)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (3). Then, + 150V is applied to the charging roller 3a, and the negative DC voltage − in which the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) become | Ia ₂ | <| Ia ₁ | / 5− Va ₁ (V) was applied to the charging brush 3d. In that case, in Comparative Example 4, Ia ₁ was +95 μA and Ia ₂ was +15 μA.

(Comparative Example 5)
As shown in Table 3, the charging brush 3d used the brush 2, and the charging roller 3a used the charging roller (4). Then, + 150V is applied to the charging roller 3a, and the negative DC voltage − in which the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) become | Ia ₂ | <| Ia ₁ | / 5− Va ₁ (V) was applied to the charging brush 3d. In this case, in Comparative Example 5, Ia ₁ was +90 μA and Ia ₂ was +10 μA.

(Comparative Example 6)
As shown in Table 3, the charging brush 3d used the brush 1, and the charging roller 3a used the charging roller (4). Then, + 150V is applied to the charging roller 3a, and the negative DC voltage − in which the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) become | Ia ₂ | <| Ia ₁ | / 5− Va ₁ (V) was applied to the charging brush 3d. In this case, in Comparative Example 6, Ia ₁ was +84 μA and Ia ₂ was +18 μA.

  The experimental results are shown in Table 3 and FIG. As can be seen from Table 3 and FIG. 9, when the OPC film thickness was 26 μm, all of Examples 1 to 5 of the present invention were excellent and good in Example 6. That is, in Examples 1 to 6, no vertical streak occurred and good charging was obtained. On the other hand, in Comparative Examples 1 and 2, vertical streaks occurred and the charging was defective.

Further, when the OPC film thickness was 15 μm, all of Examples 7 to 10 of the present invention were excellent. That is, in Examples 7 to 10, no vertical streak occurred and good charging was obtained. On the other hand, in Comparative Example 3, some vertical streaks were generated and the charging was slightly poor. In Comparative Examples 4 to 6, vertical streaks were generated and charging was poor.
Thereby, it was confirmed that the intended effect can be obtained by the present invention.

In the image forming apparatus 1 of the above example, the present invention is applied to an image forming apparatus in which the image carrier 2 is negatively charged by the charging device 3 during image formation. However, the present invention is not limited to this, and can also be applied to an image forming apparatus in which the image carrier 2 is positively charged by the charging device 3 during image formation. In that case, the magnitude relationship in absolute value between the charging brush current Ia ₁ (μA) and the charging roller current Ia ₂ (μA) at the time of image formation, that is, the relationship of | Ia ₂ |> | Ia ₁ | / 5 is This is the same as the case where the image carrier 2 is negatively charged. The above-described effects of the present invention can be obtained even when the image carrier 2 is positively charged.

  In each of the image forming apparatuses 1 of the above-described examples, the transfer belt of the present invention is secondarily transferred to the transfer material such as paper by transferring the transferred toner image from the image carrier 2 to the toner image. The intermediate transfer belt 8 is applied. However, the transfer belt of the present invention is not limited to this, and can also be applied to a transfer belt that conveys a transfer material such as paper to which the toner image of the image carrier 2 is transferred.

1 is a diagram schematically and partially showing an example of an embodiment of an image forming apparatus according to the present invention. It is a figure explaining each apparatus comprised by the same color of the image forming apparatus shown in FIG. It is a figure explaining the measuring method of the resistance value of a charging roller. It is a figure explaining the voltage application by the constant current control to the 1st and 2nd charging member. It is a figure which shows the charging potential in each position of the axial direction of an image carrier. It is a figure which shows the relationship between the number of image formation, and the film thickness of the photosensitive layer of an image carrier. It is a block diagram for the voltage application to the 1st and 2nd charging member. It is a figure which shows the flow for controlling the electric current of the 1st charging member at the time of film thickness reduction of the photosensitive layer of an image carrier. It is a figure which shows an experimental result.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Image carrier, 3 ... Charging device, 3a ... Charging roller (second charging member), 3d ... Charging brush (first charging member)

Claims (4)

A charging brush that is rotatably provided and contacts the image carrier, and charges the surface of the image carrier;
A charging roller that contacts the image carrier downstream of the charging brush in the rotational direction of the image carrier and charges the surface of the image carrier;
A controller for controlling each voltage applied to the charging brush and the charging roller,
The control device is configured such that one-fifth of the absolute value of the charging brush current flowing between the image carrier and the charging brush is an absolute value of the charging roller current flowing between the image carrier and the charging roller. A charging device, wherein the charging device is a control device that controls voltages applied to the charging brush and the charging roller so as to be smaller. An image carrier that is rotatably provided to form an electrostatic latent image, a charging device that charges the image carrier, a writing device that writes an electrostatic latent image on the image carrier, and the image carrier At least a developing device for developing the electrostatic latent image on the image carrier to form a developer image on the image carrier, and a transfer device for transferring the developer image on the image carrier to a transfer belt,
The image forming apparatus according to claim 1, wherein the charging apparatus is the charging apparatus according to claim 1. Charging the surface of the image carrier with a charging brush that is rotatably provided and abuts the image carrier;
Charging the surface of the image carrier with a charging roller that contacts the image carrier downstream of the charging brush in the rotational direction of the image carrier;
The charging of the image carrier by the charging brush and the charging roller is performed by constant current control, respectively.
In the charging control by the constant current control, a step of measuring a charging brush current flowing between the image carrier and the charging brush, and a charging roller current flowing between the image carrier and the charging roller are measured. And using the charging brush current and the charging roller current, the charging brush and the charging brush are charged so that one fifth of the absolute value of the charging brush current is smaller than the absolute value of the charging roller current. And a step of controlling a voltage applied to each of the rollers. And a step of counting the number of rotations of the image carrier.
When the count value of the number of rotations of the image carrier reaches a preset specified number of rotations, the absolute value of the charging brush current is set to a value that is set in advance and higher than the absolute value of the current charging brush current. The charging control method according to claim 3, wherein:

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