JP2008145847A - Charging apparatus, image forming apparatus, and charging control method - Google Patents

Abstract

Banding that occurs due to cleaning of a second charging member is prevented in charging of an image carrier by first and second charging members.
During image formation, an applied voltage of a first charging member is set to a voltage whose absolute value is larger than an absolute value of the discharge start voltage Vth, and an applied voltage of a second charging member is an absolute value of the discharge start voltage Vth. A voltage whose absolute value is smaller than the value is set. In non-image formation, the applied voltage of the first charging member is set to a voltage whose absolute value is smaller than the absolute value of the discharge start voltage Vth, and the applied voltage of the second charging member is set to be smaller than the absolute value of the discharge start voltage Vth. The absolute value is set to a voltage having a large absolute value and a larger absolute value than the absolute value of the voltage of the first charging member. This prevents banding while effectively cleaning the second charging member.
[Selection] Figure 5

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, there is known an image forming apparatus including a charging device that charges two rotating members in contact with the photosensitive member in order to uniformly and satisfactorily charge the surface of the rotating photosensitive member. (For example, see Patent Document 1). In the charging device of this image forming apparatus, the charging unevenness when the surface of the photosensitive member is charged by the first charging member upstream of the photosensitive member rotation direction is removed by the second charging member downstream of the photosensitive member rotation direction, The photoreceptor is uniformly charged.

However, in contact-type charging, residual toner or other foreign matters that have passed through the cleaning member of the photosensitive member may adhere to the charging member, the contact charging member may become dirty, and charging unevenness may occur. Therefore, in the charging device described in Patent Document 1, a voltage having an absolute value higher than the absolute value of the discharge start voltage Vth ₁ is applied to the first charging member during image formation, and at least the discharge start is applied to the second charging member. A voltage having an absolute value lower than the absolute value of the voltage Vth ₂ is applied to the second charging member. Further, during cleaning during non-image formation, a voltage having the same polarity as that during image formation and having an absolute value lower than the absolute value of the discharge start voltage Vth ₁ is applied to the first charging member, and image formation is performed on the second charging member. A voltage having a polarity opposite to that of the hour and having an absolute value lower than the absolute value of the discharge start voltage Vth ₂ is applied. This enhances the cleaning effect of the second charging member and stabilizes the surface potential of the photoreceptor.
JP-A-2005-331846.

By the way, the charging device described in Patent Document 1 described above is applied to an image forming apparatus having a transfer belt in contact with the photoconductor to enhance the cleaning effect of the second charging member and stabilize the surface potential of the photoconductor. think of. However, in the charging device described in Patent Document 1 described above, a voltage having an absolute value lower than the absolute value of the discharge start voltage Vth ₂ is applied to the second charging member during cleaning. When such a low voltage is applied to the second charging member, discharge from the second charging member to the photoconductor is not performed, and the surface potential of the photoconductor becomes substantially 0V. For this reason, the photoreceptor in contact with the second charging member is divided into a charged portion and a non-charged portion. As described above, when the charged portion and the non-charged portion are formed on the photosensitive member, the electrostatic attraction force between the photosensitive member and the transfer belt is different. As a result, there is a problem that a speed difference occurs in the transfer belt and banding appears.

  The present invention has been made in view of such circumstances, and an object of the present invention is to prevent banding caused by cleaning of the second charging member during charging of the image carrier by the first and second charging members. A charging device, an image forming apparatus including the charging device, and a charging control method are provided.

  In order to solve the above-described problems, according to the present invention, the control device sets a voltage smaller than the absolute value of the discharge start voltage to the first charging member during cleaning during non-image formation, and the second charging. A voltage having an absolute value larger than the absolute value of the discharge start voltage and a larger absolute value than the voltage applied to the first charging member is set for the member. That is, at the time of cleaning, the magnitude of the absolute value of the applied voltage of the first charging member and the magnitude of the absolute value of the applied voltage of the second charging member is reversed from the magnitude of the absolute value of both applied voltages at the time of image formation. As a result, foreign matters such as residual toner adhering to the second charging member are removed and moved to the image carrier. Therefore, the image carrier can be charged well over a long period of time, and high-quality image formation can be effectively realized.

  In addition, when cleaning the second charging member, the surface potential of the image carrier in contact with the second charging member can be kept constant. Thereby, the formation of the charged portion and the non-charged portion in the image carrier is suppressed, and the electrostatic adsorption force between the image carrier and the transfer belt can be made substantially the same. As a result, a speed difference does not occur in the transfer belt, and it is possible to prevent banding and resist misalignment.

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 charging device 3 charges the image carrier 2, and a charging roller (CR) 3a, which is a second charging member, and the image carrier upstream of the charging roller 3a in the rotational direction α. A charging brush (CB) 3d as a first charging member that charges the body 2 is provided. The charging roller 3 a is in contact with the outer peripheral surface of the image carrier 2 and is rotated in a direction β (counterclockwise in FIG. 1) opposite to the rotation direction α of the image carrier 2.

  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, 0.3 parts by weight of sodium trifluoroacetate as a conductive material, zinc oxide (ZnO) with respect to 100 parts by weight of epichlorohydrin rubber (Epichromer CG-102; manufactured by Daiso Corporation) ) 5 parts by weight and 2 parts by weight of 2-mercaptoimidazoline (Axel-22) as a vulcanizing agent are kneaded with a roll mixer, press-molded on the surface of a metal shaft having a diameter of 6 mm, and polished to a diameter of 12 mm. A roller having a conductive rubber elastic member formed on the surface can be obtained.

  The charging brush 3d has a large number of bristles and is supported by a brush support member (not shown). The tips of a large number of brush bristles are in contact with the outer peripheral surface of the image carrier 2. As the charging brush 3d, a known charging brush that has been conventionally used can be used. An example of the charging brush 3d is shown in Table 1.

As shown in Table 1, the brush hair of the brush 1 as an example has 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 image carrier 2), 5 mm in width, and 5 mm in height. This brush 1 is a brush made by Toei Sangyo Co., Ltd.
In the image forming apparatus 1 of this example, the image carrier 2 is first uniformly charged by the charging brush 3d (primary charging) and then uniformly charged by the charging roller 3a (secondary charging).

  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 uniformly primary charged by a charging brush 3Yd, and then secondary charged uniformly 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 transferred to the primary transfer device. It is conveyed toward 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. 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. 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 toner image is formed on the transfer material.

  By the way, in the charging device 3 of the image forming apparatus 1 of this example, the following second charging member cleaning means is provided in order to remove foreign matters adhering to the charging roller 3a as the second charging member. . The second charging member cleaning means controls the charging roller 3a, which is the second charging member, to clean the charging roller 3a by bias cleaning.

FIG. 3 is a block diagram for applying a voltage to the first and second charging members of the charging device.
As shown in FIG. 3, the image forming apparatus 1 of this example performs an image forming operation on the central processing unit (CPU) 9 and the image forming apparatus 1 in order to apply charging voltages to the charging brush 3d and the charging roller 3a, respectively. The image forming operation member (image formation command member) 10 such as an operation key of an operation panel for outputting an operation command (image formation command) or an operation touch key on the operation screen is provided.

  Further, the CPU 9 in this example includes a second charging member cleaning unit 11 for cleaning the charging roller 3a by bias cleaning. The second charging member cleaning means 11 performs bias cleaning by controlling the applied voltages of the charging brush 3d and the charging roller 3a when cleaning the charging roller 3a.

  A specific example of negative charging of the image carrier 2 will be described. In this case, in this example, the discharge start voltages of the charging brush 3d and the charging roller 3a are set to the same discharge start voltage Vth. As the applied voltages of the charging brush 3d and the charging roller 3a, a voltage (−) having an absolute value larger than the absolute value of the (−) discharge start voltage Vth (V) is applied to the charging brush 3d during image formation. Similarly, during image formation, a voltage (−) whose absolute value is smaller than the absolute value of the discharge start voltage Vth (V) of (−) is applied to the charging roller 3a. In this case, the voltage applied to the charging roller 3a can be set to 0V during image formation.

  On the other hand, when the charging roller 3a is cleaned during non-image formation, a voltage (−) having a smaller absolute value than the discharge start voltage Vth (V) or a voltage of 0 V is applied to the charging brush 3d. Similarly, when the charging roller 3a is cleaned, a voltage (−) having an absolute value larger than the discharge start voltage Vth (V) is applied to the charging roller 3a.

  In this way, the magnitude relationship between the absolute values of the applied voltages of the charging brush 3d and the charging roller 3a is reversed between when the image is formed and when the charging roller 3a is cleaned. In the image forming apparatus 1 of this example, this voltage control of the charging roller 3a for cleaning the charging roller 3a is performed when the image forming apparatus 1 is not forming an image.

FIG. 4 is a diagram showing a flow for cleaning by voltage control of the charging roller.
As shown in FIG. 4, when an image formation command is issued in step S1, the image carrier 2 starts to be driven in step S2. Next, in step S3, a voltage greater than the absolute value of the discharge start voltage Vth (V) is applied to the charging brush (first charging member) 3d in absolute value, and in step S4, the charging roller (second charging member) 3a. A voltage having an absolute value equal to or less than the absolute value of the discharge start voltage Vth (V) is applied. At this time, the discharge start voltage Vth (V) is a voltage (−). As a result, the image carrier 2 is primarily charged by the charging brush 3d and then secondarily charged by the charging roller 3a to be uniformly charged.

  Next, an image forming operation starts in step S5. When the image forming operation is completed in step S6, the voltage of the charging brush 3d is turned off in step S7, or the voltage is stepped down to a voltage smaller in absolute value than the absolute value of the discharge start voltage Vth (V). Next, in step S8, the voltage of the charging roller 3a is boosted to a voltage (−) that is larger in absolute value than the absolute value of the discharge start voltage Vth (V). That is, the respective voltages of the charging brush 3d and the charging roller 3a are controlled in reverse to the respective voltages of the charging brush 3d and the charging roller 3a for image formation by absolute values. In that case, it is desirable to apply the voltage so that the voltage of the charging roller 3a is equal to or higher than the absolute value of the developing bias (−). Then, foreign matter such as residual toner attached to the charging roller 3a moves to and adheres to the image carrier 2 by electrostatic attraction force. As a result, the foreign matter adhering to the image carrier 2 is removed by the cleaning blade 7 a of the cleaning device 7 of the image carrier 2 and collected by the collection unit of the cleaning device 7.

  When the cleaning of the charging roller 3a by boosting the voltage with the absolute value of the charging roller 3a in step S8 is completed, it is determined in step S9 whether there is a next image formation command. For example, when it is determined that there is a next image formation command as in the case of a predetermined number of continuous image formations, the process proceeds to step S3, and each process after step 3 is performed.

  If it is determined in step S9 that there is no next image formation command, the voltage of the charging brush 3d is turned off in step S10 (if the voltage of the charging brush 3d is turned off in step S7, this step 10). Is omitted). Next, after the voltage of the charging roller 3a is also turned off in step S11, the driving of the image carrier 2 is stopped in step S12. Thereby, all image formation by the image formation command in step S1 is completed.

FIG. 5 is a diagram showing an example of the applied potential at the time of charging bias switching for cleaning when the image carrier is negatively charged.
With the applied potential at the time of charge bias switching in this example, when the applied voltage of the charging brush 3d as the first charging member is at the time of image formation (or when image formation is possible), as shown by the solid line in FIG. It is set to −1500 V, which is larger than the low discharge start voltage Vth (V). Further, the voltage applied to the charging roller 3a, which is the second charging member, is set to approximately 0 V, which is smaller in absolute value than the absolute value of the discharge start voltage Vth (V), as indicated by a dotted line in FIG. When non-image formation is completed after image formation is completed (or when image formation is not possible), the applied voltage of the charging brush 3d is set to 0 V, which is smaller than the absolute value of the discharge start voltage Vth (V) at the snow position, and then the charging roller The applied voltage 3a is set to -1200 V, which has an absolute value larger than the absolute value of the discharge start voltage Vth (V). Then, cleaning of the charging roller 3a is started, and the surface of the image carrier 2 is charged to -1200V.

  As described above, the magnitude relationship between the applied voltages of the charging brush 3d and the charging roller 3a is opposite between the time of image formation and the time of cleaning. Further, even when the charging roller 3a is cleaned, the surface of the image carrier 2 is charged to -1200 V, which is larger in absolute value than the absolute value of the discharge start voltage Vth (V). As described above, in the charging device 3 of this example, the surface potential of the image carrier 2 is maintained at a certain value or more in absolute value even when the charging roller 3a is cleaned.

In the charging device of this example, the bias cleaning of the charging roller 3a is applied to the tandem type full-color image forming apparatus shown in FIG.
FIG. 6 shows a case where bias cleaning is performed according to the flow shown in FIG. 4 with a time difference for each charging roller between each color paper (after the completion of image writing of each color) in the tandem type image forming apparatus shown in FIG. It is a timing chart. The applied voltages shown in FIG. 6 are applied to the charging brushes 3Yd, 3Md, 3Cd, 3Kd, which are the first charging members of the respective colors, and the charging rollers 3Ya, 3Ma, 3Ca, 3Ka, which are the second charging members, respectively. The applied voltage (-V) similar to the applied voltage shown in FIG.

  As shown in FIG. 6, at the time of image formation, the charging brushes 3Yd, 3Md, 3Cd, and 3Kd, which are the first charging members, have an absolute value that is greater than the absolute value of the discharge start voltage Vth (V) as described above. At the same time, a voltage that is an absolute value equal to or less than the absolute value of the discharge start voltage Vth (V) is applied to each of the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka that is the second charging member. Thereby, image formation of each color is started sequentially with a predetermined time lag. When the yellow (Y) image formation is completed, the process for the yellow (Y) charging brush 3Yd in step S7 shown in FIG. 4, that is, the voltage applied to the charging brush 3Yd is equal to the discharge start voltage Vth (V). The voltage is stepped down to a voltage that is smaller than the absolute value. Next, in step S8, the process for the yellow (Y) charging roller 3Ya, that is, the voltage applied to the charging roller 3Ya is boosted to a voltage (−) that is larger in absolute value than the absolute value of the discharge start voltage Vth (V). Thus, bias cleaning of the yellow (Y) charging roller 3Ya is started.

  When the cleaning of the charging roller 3Ya is completed, if there is a next image formation command, the charging brush 3Yd again has the absolute value equal to or greater than the absolute value of the discharge start voltage Vth (V) as shown in FIG. A voltage is applied. Further, the same voltage as described above which is an absolute value below the absolute value of the discharge start voltage Vth (V) is again applied to the charging roller 3Ya. As a result, the image carrier 2Y is uniformly charged as described above for the next image formation. On the other hand, when there is no next image formation command and the image formation is completed, the applied voltages of the charging brush 3Yd and the charging roller 3Ya are both turned off, and no voltage is applied to these charging members. Therefore, charging for image formation on the surface of the yellow (Y) image carrier 2Y is not performed.

  Subsequently, when the time corresponding to the distance between the yellow (Y) image carrier 2Y and the adjacent magenta (M) image carrier 2M has elapsed, the magenta (M) image formation is completed. Then, the voltage applied to the charging brush 3Yd of magenta (M) is stepped down to a voltage smaller in absolute value than the absolute value of the discharge start voltage Vth (V), as in the case of the above-mentioned yellow (Y). Next, the voltage applied to the charging roller 3Ma of magenta (M) is boosted to a voltage (−) that is larger in absolute value than the absolute value of the discharge start voltage Vth (V). Thereby, bias cleaning of the charging roller 3Ma of magenta (M) is started.

  When the cleaning of the charging roller 3Ma is completed, as in the case of yellow (Y), if there is a next image formation command, the charging brush 3Md again has an absolute value equal to or greater than the absolute value of the discharge start voltage Vth (V). The same voltage as above is applied. Further, the same voltage as described above which is an absolute value below the absolute value of the discharge start voltage Vth (V) is again applied to the charging roller 3Ma. As a result, the image carrier 2M is uniformly charged as described above for the next image formation. On the other hand, when there is no next image formation command and image formation is completed completely, the applied voltages of the charging brush 3Md and the charging roller 3Ma are both turned off, and no voltage is applied to these charging members. Therefore, charging for image formation on the surface of the image carrier 2M of magenta (M) is not performed.

  In the same manner as described above, bias cleaning of the cyan (C) charging roller 3Ca and the black (K) charging roller 3Ka is sequentially performed. Thereafter, even when there is a next image formation command and when there is no next image formation command, the application voltages of the charging brushes 3Cd and 3Kd and the charging rollers 3Ca and 3Ka are controlled in the same manner as described above. . When the image formation is completely completed, the image carriers 2Y, 2M, 2C, and 2K for all colors are stopped and the image formation is completed.

  According to the image forming apparatus 1 of this example, a voltage having an absolute value larger than the absolute value of the discharge start voltage Vth is applied to the charging brushes 3Yd, 3Md, 3Cd, and 3Kd at the time of image formation for each color. The charging roller 3Ya, 3Ma, 3Ca, 3Ka is applied with a voltage whose absolute value is smaller than the absolute value of the discharge start voltage Vth and whose absolute value is smaller than the voltage applied to the charging brushes 3Yd, 3Md, 3Cd, 3Kd. Further, for each color, when the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka are cleaned at the time of non-image formation of the color, the charging brushes 3Yd, 3Md, 3Cd, and 3Kd have an absolute value that is greater than the absolute value of the discharge start voltage Vth. A voltage to which a small voltage is applied and whose absolute value is larger than the absolute value of the discharge start voltage Vth to the charging rollers 3Ya, 3Ma, 3Ca, 3Ka and whose absolute value is larger than the applied voltage to the charging brushes 3Yd, 3Md, 3Cd, 3Kd Is applied. That is, at the time of cleaning, the magnitude of the absolute value of the applied voltage of each charging brush 3Yd, 3Md, 3Cd, 3Kd and the absolute value of the applied voltage of each charging roller 3Ya, 3Ma, 3Ca, 3Ka is determined as the applied voltage during image formation. The absolute value is reversed. As a result, foreign matters such as residual toner adhering to the charging roller 3a can be removed and moved to the corresponding image carriers 2Y, 2M, 2C, 2k. As a result, the image carriers 2Y, 2M, 2C, and 2K can be charged satisfactorily over a long period of time, and high-quality image formation can be effectively realized.

  Moreover, the surface potentials of the image carriers 2Y, 2M, 2C, and 2K that are in contact with the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka, respectively, are kept constant during the cleaning of the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka. Can be held. As a result, the formation of charged portions and non-charged portions in each image carrier 2Y, 2M, 2C, 2K is suppressed, and electrostatic adsorption between each image carrier 2Y, 2M, 2C, 2K and the intermediate transfer belt 8 is suppressed. The power is almost the same. As a result, no speed difference occurs in the intermediate transfer belt 8, and banding and registration deviation can be prevented.

FIG. 7 is a timing chart when bias cleaning is performed according to the flow shown in FIG. 4 after the image formation of all colors in the tandem type image forming apparatus shown in FIG.
In the bias cleaning of the charging roller in the example shown in FIG. 6 described above, the bias cleaning of the charging roller for each color is sequentially performed at the end of image formation for each color (end of image writing). On the other hand, in the image forming apparatus 1 of this example, the charging rollers 3Ya, 3Ma, 3Ca, 3Ka are subjected to bias cleaning for all colors at the end of image formation for all colors (end of image writing). , 3Ma, 3Ca, 3Ka are simultaneously subjected to bias cleaning.

  That is, as shown in FIG. 7, in the image forming apparatus 1 of this example, at the time of image formation (or when image formation is possible), as described above, the charging brushes 3Yd, 3Md, 3Cd, and 3Kd, which are the first charging members, A voltage that is greater than the absolute value of the discharge start voltage Vth (V) is applied in absolute value, and the discharge start voltage Vth (V) of the absolute value is applied to each of the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka that is the second charging member. A voltage less than the absolute value is applied. Thereby, image formation of each color is sequentially started for each color.

  Then, in the image forming apparatus 1 of this example, each image formation (primary transfer) of yellow (Y), magenta (M), and cyan (C) is finished except black (K) which is the last color. However, bias cleaning of these colors is not performed. When the last black (K) image formation (primary transfer) is completed, the bias cleaning of the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka for all the colors by the processes in steps S7 and S8 of the flow shown in FIG. Is called.

When the simultaneous bias cleaning of the charging rollers 3Ya, 3Ma, 3Ca, 3Ka for all colors is completed, if there is a next image formation command, image formation and simultaneous bias cleaning are performed again. Further, when the simultaneous bias cleaning of all the colors is completed and there is no next image formation command and the image formation is completely completed, all the charging brushes 3Yd, 3Md, 3Cd, 3Kd and all the charging rollers 3Ya, 3Ma are obtained. , 3Ca, 3Ka are both turned off, and no voltage is applied to these charging members. Therefore, charging for image formation is not performed on the image carriers 2Y, 2M, 2C, and 2K of all colors. Then, the image carriers 2Y, 2M, 2C, and 2K for all colors are stopped and image formation is completed.
Also with the image forming apparatus 1 of this example, the same effect as the example shown in FIG. 6 described above can be obtained.

  The bias cleaning of the charging rollers 3Ya, 3Ma, 3Ca, and 3Ka shown in FIGS. 6 and 7 is not limited to the tandem type image forming apparatus 1 shown in FIG. The present invention can also be applied to a four-cycle full-color image forming apparatus in which one image carrier rotates once for each transfer. This four-cycle full-color image forming apparatus can achieve the same effects as the example shown in FIG.

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 charging brush shown in Table 1 was used, and as the charging roller 3a, the charging roller mentioned as an example was used. Further, a voltage can be applied and controlled from the outside to the charging brush 3d and the charging roller 3a. At this time, the discharge start voltage Vth of the charging brush 3d and the charging roller 3a was both -570V. Further, the LP9000C was modified so that the bias cleaning of the charging roller 3a for each color shown in FIG. 6 can be performed. Other photoreceptor drums, cleaning blades, optical writing devices, developing devices (including genuine toner), transfer devices (including intermediate transfer belts), and fixing devices were all LP9000C. The experiment was conducted for Examples 1 to 5 and Comparative Examples 1 to 4 shown in Table 2.

The experimental conditions common to Examples 1 to 5 and Comparative Examples 1 to 4 are that the charging voltage applied to the charging brush 3d as the first charging member is a DC voltage of −1500 V and is applied to the charging roller 3a as the second charging member. The charging voltage is a DC voltage of -50V. Further, 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.

(Image formation test)
The image formation test is the same in all experiments of Examples 1 to 5 and Comparative Examples 1 to 4. That is, two monochrome halftone images are continuously formed on A4 size plain paper in an environment of 23 ° C. (room temperature). Each image formed was visually confirmed, and it was determined that charging was good when there was no banding, and it was determined that charging was poor when banding was observed.
The individual experimental conditions of Examples 1 to 5 and Comparative Examples 1 to 4 will be described. These individual experimental conditions are shown in Table 2.

(Example 1)
As shown in Table 2, a DC voltage (DC) of −1400 V, which is larger in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -812V.

(Example 2)
As shown in Table 2, a DC voltage (DC) of -1200 V, which is larger in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -620V.

(Example 3)
As shown in Table 2, a DC voltage (DC) of −1000 V, which is larger in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. At this time, the surface potential of the image bearing member was -401V.

Example 4
As shown in Table 2, a DC voltage (DC) of −800 V, which is larger in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3 a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -198V.

(Example 5)
As shown in Table 2, a DC voltage (DC) of −650 V, which is larger in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3 a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -55V.

(Comparative Example 1)
As shown in Table 2, a DC voltage (DC) of −500 V, which is smaller in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -17V.

(Comparative Example 2)
As shown in Table 2, a DC voltage (DC) of −400 V, which is smaller in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. The image carrier surface potential at this time was -15V.

(Comparative Example 3)
As shown in Table 2, a DC voltage (DC) of −200 V, which is smaller in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V ) A direct current voltage (DC) of 0 V, which is smaller in absolute value, was applied. At this time, the surface potential of the image carrier was -8V.

(Comparative Example 4)
As shown in Table 2, a DC voltage (DC) of 0 V, which is smaller in absolute value than the discharge start voltage Vth (V), is applied to the charging roller 3a during bias cleaning, and the discharge start voltage Vth (V) is applied to the charging brush 3d. A smaller direct current voltage (DC) of 0 V was applied. At this time, the surface potential of the image carrier was −0V.

The experimental results are shown in Table 2. As is clear from Table 2, in Examples 1 to 5 of the present invention, banding did not occur and good charging was obtained. On the other hand, in Comparative Examples 1 to 4, banding occurred and a charging failure occurred.
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 between the applied voltage of the charging roller 3a and the applied voltage of the charging brush 3d at the time of image formation and the absolute value of the applied voltage of the charging roller 3a and the applied voltage of the charging brush 3d at the time of bias cleaning. Are the same as in 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.

  The first charging member is not limited to the charging brush 3d, and other charging members such as a charging roller or a charging brush roller can be used. Further, the second charging member is not limited to the charging roller 3a, and other charging members such as a charging brush or a charging brush roller can be used.

  Further, in each of the charging devices 3 of the above-described examples, the discharge start voltage of the charging roller 3a and the discharge start voltage of the charging brush 3d are set to be the same, but the discharge start voltage is the same as that of the charging roller 3a and the charging brush. It can be different from 3d.

  Further, in each of the image forming apparatuses 1 of the above-described examples, the transfer belt of the present invention is transferred onto the transfer material such as paper by secondary transfer of the toner image on which the toner image is primarily transferred from the image carrier 2. 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 block diagram for the voltage application to the 1st and 2nd charging member of a charging device. It is a figure which shows the flow for the cleaning by the voltage control of a charging roller. It is a figure which shows the applied voltage to the 1st and 2nd charging member at the time of image formation and cleaning. 6 is a timing chart when bias cleaning is performed between sheets of different colors. 6 is a timing chart when bias cleaning is performed after image formation for all colors is completed.

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 (6)

A first charging member which is rotatably provided and contacts the image carrier, and charges the surface of the image carrier;
A second charging member that contacts the image carrier downstream of the first charging member in the rotation direction of the image carrier and charges the surface of the image carrier;
A control device for controlling voltages applied to the first charging member and the second charging member,
The control device sets an applied voltage having an absolute value larger than an absolute value of the discharge start voltage to the first charging member and forms an absolute value of the second charging member based on an absolute value of the discharge start voltage during image formation. An applied voltage that is smaller and smaller in absolute value than the applied voltage to the first charging member is set, and a voltage smaller than the absolute value of the discharge start voltage is set in the first charging member during non-image formation. 2. A charging device, wherein an applied voltage having an absolute value larger than an absolute value of a discharge start voltage and a larger absolute value than an applied voltage to the first charging member is set for the two charging members. The charging device according to claim 1, wherein the first charging member is a charging brush, a charging roller, or a charging brush roller. 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 device is a charging device according to claim 1. The image carrier, the charging device, the writing device, the developing device, and the transfer device are arranged in tandem for each of a plurality of colors, and the transfer belt is in contact with the image carrier of each color. The image forming apparatus according to claim 3, wherein the image forming apparatus is disposed as a single image. The developing device is disposed in the vicinity of the image carrier for each of a plurality of colors, and the developer image on the image carrier developed for each color is transferred to a multi-cycle transfer belt. The image forming apparatus according to claim 3. Charging the surface of the image carrier with a first charging member provided rotatably and in contact with the image carrier;
Charging the surface of the image carrier with a second charging member that contacts the image carrier downstream of the first charging member in the rotational direction of the image carrier;
At the time of image formation, a voltage having an absolute value larger than the absolute value of the discharge start voltage is applied to the first charging member, and the absolute value is smaller than the absolute value of the discharge start voltage to the second charging member. Apply a voltage whose absolute value is smaller than the voltage to the member,
During non-image formation, a voltage smaller than the absolute value of the discharge start voltage is applied to the first charging member, and the absolute value is larger than the absolute value of the discharge start voltage and applied to the first charging member. A charging control method characterized by applying a voltage having an absolute value larger than the applied voltage.

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