JP2011107532A - Charging device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charging device using a contact charging system, which reduces charging noise and increases service life thereof. <P>SOLUTION: A surface 2a of a photoreceptor drum 2 is charged by first and second charging rollers 7a, 7b. Each of the rollers 7a, 7b are arranged in contact with the surface 2a of the photoreceptor drum 2. Furthermore, DC voltage is applied to a first charging roller 7a located in the upstream in the rotational direction of the photoreceptor drum 2 and voltage containing AC component is applied to the second charging roller 7b which is located in the downstream. In addition, hardness of the surface of the first charging roller 7a is harder than the hardness of the surface of the second charging roller 7b. Thus, the first charging roller 7a is prevented from becoming dirty by fine powder toner, and the charging noise is prevented in the second charging roller 7b to which AC voltage is applied. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a charging device used for a copying machine, a facsimile, a laser beam printer, a process cartridge and the like using an electrophotographic method or an electrostatic recording method, and an image forming apparatus incorporating such a charging device.

  Conventionally, in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a manufacturing process such as an electrophotographic process including an electrification process or an electrostatic recording process is applied to an image carrier such as a photoreceptor or a dielectric. Execute image formation. As the charging means used in the charging process in this case, a non-contact charging method using a corona discharge phenomenon such as a scorotron charger is generally used. The corona discharge device is effective as a means for uniformly charging the surface of an object to be charged such as an image carrier to a predetermined potential, but has problems such as the need for a high-voltage power supply and the generation of ozone due to corona discharge. is doing.

  For this reason, in recent years, a contact charging method has been used in which a conductive roller-type charging member is brought into direct contact with or close to an image carrier such as a photoconductor, and a charging voltage is applied to the charging member. For example, a conductive rubber roller is brought into contact with a photosensitive drum that is an image carrier, and is driven to rotate along with the rotation of the photosensitive drum. Charge. The contact charging method in which charging is performed by bringing a charging member into direct contact with the image carrier has the following advantages over the non-contact charging method. That is, the applied voltage required to obtain a desired potential on the surface of the object to be charged can be reduced, the amount of ozone generated in the charging process is very small, and the need for an ozone removal filter is eliminated. Therefore, there is an advantage that the configuration of the exhaust system of the apparatus is simplified.

  In the contact charging method, the voltage applied between the charging member and the member to be charged is not only the direct current voltage (DC application method), but also an oscillating voltage (a voltage whose voltage value changes periodically with time) is applied. There is also an AC application method (see Patent Document 1). Such an AC application method can perform uniform charging (static elimination) processing, and is effective for improving image quality. The oscillating voltage is an oscillating voltage component (AC component, AC component) or a superimposed voltage of an AC component and a DC component (voltage corresponding to the target charging potential, DC component). The waveform of the AC component may be a sine wave, a rectangular wave, a triangular wave, or the like, or a rectangular wave voltage formed by periodically turning on and off a DC power supply.

  As a structure using the contact method as described above, there is a structure in which a photosensitive drum (image carrier) is charged by one charging roller (charging member). Such a charging roller includes a cylindrical elastic conductive layer containing conductive carbon in SBR (styrene butadiene rubber) and the like arranged concentrically in order on the outer periphery of the core metal, and a high level for preventing poor charging. It consists of a resistance coating layer and a protective coating layer for preventing sticking to the photosensitive drum. The charging roller is pressed against the photosensitive drum with a predetermined pressure by a spring member, and is driven to rotate as the photosensitive drum rotates. Then, a predetermined charging bias (DC application method or AC application method) is applied through the mandrel, and the outer peripheral surface of the photosensitive drum is caused by discharge between a minute gap generated between the charging roller and the photosensitive drum. It is charged by the contact charging method.

  However, when charging is performed by the charging roller as described above, the high resistance coating layer is easily affected by temperature and humidity, and the resistance value varies greatly depending on the environment. In addition, the resistance value of the high resistance coating layer and the conductive elastic layer tends to increase with time, and this tendency is promoted as the amount of energization increases. Therefore, when charging is performed with one charging roller, it is difficult to ensure stable charging performance. Further, when the photosensitive drum rotates at a high speed, the contact portion between the photosensitive drum and the charging roller tends to become unstable, and charging unevenness may occur. For this reason, a contact charging type charging device using a single charging roller is required to have high reliability, for example, and is difficult to use in a high-end image forming apparatus such as a high-speed copying machine having a large copy volume.

  On the other hand, there is a structure in which charging is performed using two charging rollers (see Patent Document 2). Thus, in the case of the structure described in Patent Document 2 using two charging rollers, a DC voltage is applied to the upstream charging roller, and a superimposed voltage of the DC voltage and the AC voltage is applied to the downstream charging roller. Each is applied. The upstream charging roller uses a metal conductor layer made of aluminum or the like around the core bar, and is charged in a non-contact manner close to the photosensitive drum. On the other hand, the downstream charging roller uses a conductive elastic material around the core bar, and is charged by contacting with the photosensitive drum.

  In the case of such a structure described in Patent Document 2, the photosensitive drum is charged approximately to a desired charging potential by the charging roller upstream of the photosensitive drum, and the uneven charging of the photosensitive drum is corrected by the downstream charging roller. To do. For this reason, uneven charging due to high-speed rotation hardly occurs. In addition, since the current flowing through each charging roller can be reduced, the life of the high resistance coating layer and the conductive elastic layer of the charging roller can be significantly extended. For this reason, the charging performance is stabilized over a long period of time, and it can be used for high-class image forming apparatuses such as high-speed copying machines.

JP-A 63-149669 JP-A-7-110616

  By the way, in a structure in which charging is performed by bringing a charging roller into contact with a photosensitive drum, when a voltage including an AC component is applied, a so-called “charging sound” due to an AC frequency may occur. This charged sound is an abnormal noise generated when a voltage applied to the charging member includes an alternating current component and an oscillating electric field is formed between the charging member and the member to be charged, thereby causing both to vibrate. Such charging noise is known to increase as the peak-to-peak voltage of the AC component increases and as the AC frequency increases. As the AC voltage applied to charge the photosensitive drum as necessary, it is desirable to apply a peak-to-peak voltage that is at least twice the charging start voltage of the photosensitive drum when a DC voltage is applied. For this reason, if the peak-to-peak voltage is lowered in order to suppress the charging noise, it becomes difficult to maintain the uniformity of the charging potential and obtain high image quality. Further, when the process speed of the image forming apparatus is fast, it is necessary to increase the AC frequency. In recent years, there has been a strong demand for higher image quality and higher speed of image forming apparatuses, so it is difficult to reduce the AC frequency.

  Therefore, it is conceivable to reduce the charging noise by reducing the surface hardness of the charging roller. However, if the surface hardness of the charging roller is lowered, the charging roller may be contaminated by a fine toner or an external additive that has passed through a cleaning member for cleaning toner remaining on the photosensitive drum after development, and the service life may be shortened. is there. That is, when the surface hardness of the charging roller is low, the contact width (nip width) between the charging roller and the photoconductor is large, and the fine toner and the external additive are charged at the contact portion (nip portion) between the charging roller and the photoconductor. Easy to adhere to the roller. For this reason, the charging roller is easily contaminated and the life is shortened. In addition, if the charging roller is contaminated, charging failure occurs, and image density unevenness due to the charging failure and toner adhesion (fogging) to a white portion in the image may occur.

  In the case of the structure described in Patent Document 2 above, the upstream charging roller is not in contact with the photosensitive drum. Therefore, as described above, a high voltage power source is required, ozone is generated by corona discharge, and the like. Have the following problems. In order to eliminate such a problem, it is conceivable that the upstream charging roller is of a contact type. However, in the case of the structure described in Patent Document 2, the upstream charging roller is a hard roller in which a metal conductor layer is arranged around a cored bar. For this reason, the nip width with the photosensitive drum cannot be secured, and charging failure may occur, and the contact pressure may increase and the surface of the photosensitive drum may be damaged. For this reason, the structure described in Patent Document 2 cannot employ a structure in which the upstream charging roller is a contact type.

  In view of such circumstances, the present invention was invented to achieve a structure that can reduce charging noise and extend the life of the apparatus with a contact charging structure.

  The present invention relates to a charged body whose charging surface moves, and a plurality of charging members which are arranged side by side in the moving direction of the charging surface and charge the charging surface by applying a voltage to the charged body. The plurality of charging members have an elastic layer around a cored bar and are disposed in contact with the charging surface. The charging member disposed upstream is applied with a DC voltage, and the charging member disposed downstream in the moving direction of the charging surface is applied with a voltage containing an AC component, and the charging surface of the most upstream charging member is applied. The hardness of the surface that contacts the charging surface is higher than the hardness of the surface that contacts the charging surface of the most downstream charging member.

  According to the present invention, with the structure of the contact charging system, the charging noise can be reduced and the life of the apparatus can be extended. That is, among the plurality of charging members, the most upstream charging member having the highest surface hardness is used as the most upstream charging member having the greatest influence of contamination by fine powder toner or external additives. The life of the entire device can be kept high on average and the life can be extended. Further, since only the direct current voltage is applied to the most upstream charging member having a high surface hardness, no charging noise is generated. On the other hand, the hardness of the surface of the most downstream charging member to which a voltage containing an AC component is applied can be lowered to reduce the generation of charging noise due to the AC voltage. In addition, since any charging member has a structure having an elastic layer around the core metal, even the most upstream charging member having a high surface hardness can secure a nip width with the object to be charged, and charging failure And damage to the charged surface of the member to be charged can be prevented. In addition, since any charging member is brought into contact with the surface of the member to be charged and charged, the applied voltage can be lowered as compared with the non-contact method, and generation of ozone due to corona discharge can be suppressed.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a schematic diagram illustrating a part of a charging device incorporated in the image forming apparatus. (A) is a photosensitive member surface potential graph by charging of the upstream charging roller, and (b) is a photosensitive member surface potential graph when the downstream charging roller is further charged. The figure which shows the relationship between a charging frequency and a sound pressure. FIG. 6 is a schematic schematic view of a part of a charging device according to a second embodiment of the present invention.

<First Embodiment>
A first embodiment of the present invention will be described with reference to FIGS. First, the overall configuration of the image forming apparatus will be briefly described with reference to FIG. The image forming apparatus shown in FIG. 1 is a tandem type full-color copying machine in which four image forming units Py, Pm, Pc, and Pk are arranged in a straight section of the intermediate transfer belt 1. In each of the image forming units Py, Pm, Pc, and Pk, the yellow, magenta, cyan, and black toner images respectively formed on the photosensitive member (photosensitive drum) 2 that is a charged body and also an image carrier are intermediate. The toner images are sequentially superimposed on the transfer belt 1 and primarily transferred. The four-color toner images primarily transferred to the intermediate transfer belt 1 are conveyed to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P on which the four-color toner images are secondarily transferred by the secondary transfer portion T2 is heated and pressurized by the fixing device 3 to fix the toner image on the surface, and then discharged to the outside of the image device. . The fixing device 3 is a roller fixing device configured by pressing a pressure roller 5 on a fixing roller 4 provided with a halogen heater. The toner image electrostatically carried on the recording material P is converted by heat and pressure. It is fixed on the surface of the recording material P.

  Here, the image forming units Py, Pm, Pc, and Pk are configured substantially the same except that the color of toner used in the attached developing device 6 is different from yellow, magenta, cyan, and black. Accordingly, hereinafter, the image forming unit Py will be described as a representative. In the image forming unit Py, a first charging roller 7a and a second charging roller 7b, which are charging members, an exposure device 8, a developing device 6, a primary transfer roller 9, and a cleaning device 10 are arranged around the photosensitive drum 2. . When image formation is performed, the surface 2a (charging surface) of the photosensitive drum 2 is charged to a predetermined potential by the first and second charging rollers 7a and 7b, and the surface of the photosensitive drum 2 charged by the exposure device 8 is charged. An electrostatic latent image is written in 2a. Then, the developing device 6 moves toner to the electrostatic latent image on the photosensitive drum 2, reversely develops the electrostatic latent image, and forms a toner image on the photosensitive drum 2. This toner image is transferred onto the intermediate transfer belt 1 at a primary transfer portion T1 that sandwiches the intermediate transfer belt 1 between the photosensitive drum 2 and the primary transfer roller 9. The transfer residual toner that passes through the primary transfer portion T1 and remains on the surface 2a of the photosensitive drum 2 is removed by the cleaning device 10.

  Next, a charging device incorporated in the image forming apparatus as described above will be described with reference to FIG. In FIG. 2, the photosensitive drum 2 and the first and second charging rollers 7a and 7b are extracted and shown. The photosensitive drum 2 is an organic photoconductor (OPC) having a negative charging property and is driven to rotate in the direction of an arrow R1. The first and second charging rollers 7a and 7b are arranged side by side in the moving direction of the surface 2a (charging surface) of the photosensitive drum 2, that is, in the direction of the arrow R1. Further, the first and second charging rollers 7 a and 7 b are arranged in contact with the surface 2 a of the photosensitive drum 2, respectively. Accordingly, the first and second charging rollers 7a and 7b are driven to rotate in the direction of the arrow R2 as the photosensitive drum 2 rotates.

  In the present embodiment, the surface hardness of the first charging roller 7 a disposed upstream of the photosensitive drum 2 in the rotational direction is set to be higher than the surface hardness of the second charging roller 7 b disposed downstream of the photosensitive drum 2 in the rotational direction. Is also high. Here, the first charging roller 7a corresponds to the most upstream charging member in the moving direction, and the second charging roller 7b corresponds to the most downstream charging member in the moving direction. The surface hardness refers to the hardness of the surface of the charging rollers 7a and 7b that contacts the surface 2a of the photosensitive drum 2. Therefore, in the present embodiment, the first charging roller 7a is a hard roller, and the second charging roller 7b is a soft roller. For this purpose, the first and second charging rollers 7a and 7b are configured as follows.

First, the first charging roller 7a has a three-layer configuration in which a conductive elastic layer 12a, a high resistance coating layer 13a, and a protective coating layer 14a are sequentially laminated from the inner diameter side on the outer periphery of the core metal 11a. It is a conductive roller whose surface layer has an electrical resistance value of about 10 ² to 10 ⁶ Ω. Among these, the high resistance coating layer 13a is provided to prevent current leakage from pinholes and scratches that may be present on the surface 2a of the photosensitive drum 2 and to prevent charging failure to the photosensitive drum 2. . The protective coating layer 14 a is provided to prevent the charging roller 7 a from sticking to the photosensitive drum 2.

  The conductive elastic layer 12a of the first charging roller 7a is made of a conductive rubber tube that has been cured in advance. As such a material, for example, a rubber material such as silicone rubber, acrylonitrile butadiene rubber, urethane rubber, ethylene propylene rubber is used as a base, and a conductive material such as carbon black or metal powder is blended to impart conductivity thereto. is doing. And resistance adjustment is carried out by adjusting the compounding quantity of such a conductive material. Also, for example, the surface hardness of the first charging roller 7a is set to 65 ° Asker C hardness by adjusting the blending amount of carbon black. In addition, it is preferable that this surface hardness shall be Asker C hardness 60 degrees-90 degrees. The reason why the Asker C hardness is 90 ° or less is that if the surface hardness is too high, it becomes difficult to secure a nip portion for charging, and charging failure is likely to occur. That is, in order to secure this nip portion, it is preferable that the Asker C hardness is 90 ° or less. The reason why the Asker C hardness is preferably 60 ° or more will be described later.

  A power source S1 is connected to the core metal 11a of the first charging roller 7a. Then, a charging bias voltage, which is a DC voltage, is applied to the metal core 11a by the power source S1. Further, both end portions of the metal core 11a are rotatably held by the bearing members 15a, respectively, and are urged toward the photosensitive drum 2 by the pressing springs 16a. The first charging roller 7a is pressed against the surface 2a of the photosensitive drum 2 with a predetermined pressing force.

Next, the second charging roller 7b is configured in the same manner as the first charging roller 7a. That is, a conductive elastic layer 12b, a high-resistance coating layer 13b for preventing charging failure, and a protective coating layer 14b for preventing adhesion to the photosensitive drum 2 are sequentially formed on the outer periphery of the cored bar 11b from the inner diameter side. A three-layer structure is formed. Such a second charging roller 7b is a conductive roller having an electric resistance value of about 10 ⁶ to 10 ⁸ Ω on the surface layer.

  Further, the conductive elastic layer 12b of the second charging roller 7b is made of ethylene propylene diene rubber (EPDM), urethane rubber, nitrile rubber (NBR), silicone rubber, isoprene rubber (IR) or the like, carbon black or metal oxide. It is made by dispersing conductive materials such as Moreover, you may use what foamed such a rubber material. And resistance adjustment is carried out by adjusting the compounding quantity of such a conductive material. Note that the resistance may be adjusted using an ion conductive material without dispersing the conductive substance. In any case, the surface hardness of the second charging roller 7b is set to 35 ° Asker C hardness. In addition, it is preferable that this surface hardness shall be Asker C hardness 25 degrees-55 degrees. The Asker C hardness of 25 ° or more is because if the surface hardness is too low, the shape of the roller is not stable. Largely received, it becomes easy to cause poor charging. That is, in order to make this nip width appropriate, it is preferable that the Asker C hardness is 25 ° or more. The reason why the Asker C hardness is preferably 55 ° or less will be described later.

  A power source S2 is connected to the cored bar 11b of the second charging roller 7b. Then, a charging bias voltage, which is a voltage including an AC component, is applied to the core metal 11b by the power source S2. Further, both end portions of the metal core 11b are rotatably held by the bearing members 15b and are urged toward the photosensitive drum 2 by the pressing springs 16b. The second charging roller 7b is pressed against the surface 2a of the photosensitive drum 2 with a predetermined pressing force.

The charging device of the present embodiment configured as described above charges the surface 2a of the photosensitive drum 2 as follows. First, the surface 2a of the photosensitive drum 2 is exposed by, for example, a pre-exposure device (not shown) to remove residual charges. The first charging roller 7a is charged to a predetermined potential at the position of the first charging roller 7a. Specifically, a DC voltage of 10 ² to 10 ⁴ V is applied from the power source S1 to the core 11a of the first charging roller 7a (DC application method). As a result, a current of approximately 10 ⁻⁵ to 10 ⁻³ A flows, and the surface 2 a of the photosensitive drum 2 is charged to a potential V 1 that is substantially close to the target potential V 0 as shown in FIG.

  The surface potential of the photosensitive drum 2 after passing through the first charging roller 7a has a partial potential drop P as shown in FIG. The surface 2a of the photosensitive drum 2 having such a non-uniform potential distribution is then leveled by the second charging roller 7b, and the surface potential of the target potential V0 is obtained as shown in FIG. Charged. That is, an oscillating voltage obtained by superimposing an AC voltage on a DC voltage is applied to the core 11b of the second charging roller 7b by the power source S2. At this time, in the portion P having a low potential on the surface 2a of the photosensitive drum 2 shown in FIG. 3A, the potential difference from the second charging roller 7b is large, so that the charge is intensively injected. Therefore, by appropriately setting the voltage applied to the second charging roller 7b, the nonuniformity of the charging potential V1 of the surface 2a of the photosensitive drum 2 as shown in FIG. The surface 2a of the photosensitive drum 2 can be charged to a uniform desired surface potential V0 as shown in FIG. 3A and 3B show the photosensitive member surface potential with respect to the position of the photosensitive drum 2 in the scanning direction (rotating axis direction).

  In the case of this embodiment, since the first charging roller 7a almost charges the surface 2a of the photosensitive drum 2, the current flowing through the second charging roller 7b is charged by one charging roller. Compared with the case where it does, it is less. Therefore, the life of the conductive elastic layer 12b and the high resistance coating layer 13b of the second charging roller 7b can be significantly extended, and the charging device can be stabilized for a long time.

  Next, a description will be given of a point that the surface hardness of the first charging roller 7a is preferably set to 60 ° or more Asker C hardness. First, as described above, in the contact-type charging method, the contact-type charging member (charging roller) is contaminated by the fine toner and the external additive that have passed through the cleaning device 10 (FIG. 1). In addition, image density unevenness due to charging failure and toner adhesion (fogging) to white portions in the image are likely to occur. This problem is particularly noticeable when a soft roller having a low surface hardness and a wide nip width with the photosensitive drum 2 is used.

  For this reason, in the present embodiment, the first charging roller 7a, which is a hard roller having a high surface hardness and is difficult to get dirty, is disposed upstream of the influence of contamination by the fine toner and the external additive. For this reason, the lifetime of the first charging roller 7a can be extended. Here, the relationship between the Asker C hardness of the upstream first charging roller 7a and the charging roller contamination is shown in Table 1. In Table 1, the Asker C hardness is changed from 35 ° to 65 °, and the durable life of each is determined until the first or second charging roller 7a, 7b has an image defect due to contamination. (A4 conversion).

  In the image forming apparatus according to the present embodiment, the life of the OPC photosensitive drum 2 is equivalent to 50,000 sheets image formation (A4 conversion), and therefore the life required for the charging roller is 50,000 sheets equivalent to that. As can be seen from Table 1, if the first charging roller 7a has an Asker C hardness of 60 ° or more, the life can be increased to 50000 sheets or more, which is equal to or longer than the life of the photosensitive drum 2. For this reason, it can be seen that the life of the charging roller does not affect the life of the entire apparatus. Therefore, it is preferable to use a charging roller having a surface hardness of 60 ° or more as the Asker C hardness as the upstream first charging roller 7a that is greatly affected by contamination by fine powder toner or external additives.

  When a soft roller having the same surface hardness (Asker C hardness 35 °) as that of the above-described second charging roller 7b is used as the upstream first charging roller 7a, for example, the life is 30000 sheets. The life of the entire apparatus is shortened. On the other hand, when the surface hardness of the first charging roller 7a is set to an Asker C hardness of 65 ° or more, the life is 54,000 sheets or more, which exceeds the life of the photosensitive drum. However, as described above, the Asker C hardness is preferably 90 ° or less.

  Next, a description will be given of a point that the surface hardness of the second charging roller 7b is preferably set to 55 ° or less Asker C hardness. First, as described above, when an AC application type and contact charging type charging roller is used as the charging means of the photosensitive drum 2, a so-called "charging noise" may be generated due to the AC frequency. According to the experiments of the present inventors, there is a correlation between the sound pressure of the charging sound and the Asker C hardness of the charging roller, and the sound pressure of the charging sound can be lowered by reducing the Asker C hardness. I understood. This is because the vibration energy of the charging roller decreases as the Asker C hardness decreases. Table 2 shows the relationship between Asker C hardness and charging sound pressure when the frequency of the oscillating voltage applied to the charging roller is 1500 Hz. FIG. 4 shows the relationship between the charging frequency and the sound pressure when the Asker C hardness of the charging roller is 65 ° and 55 °.

  According to the study of the present inventor, it has been found that if the charging sound pressure is 50 dB or less, the charging sound is not harsh in a normal printing operation. As is apparent from Table 1 and FIG. 4, if the Asker C hardness is 55 ° or less, the charging sound pressure can be 50 dB or less up to a charging frequency of 1500 Hz. Therefore, it is preferable to use a charging roller having a surface hardness of 55 ° or less as the second charging roller 7b downstream to which AC is applied.

  When a hard roller having the same surface hardness (Asker C hardness 65 °) as the first charging roller 7a described above is used as the downstream second charging roller 7b, for example, an irritating charging sound is generated. . On the other hand, when the surface hardness of the second charging roller 7b is set to 35 ° or less Asker C hardness, the charging sound can be reduced to a level that is hardly noticed. That is, since the noise level excluding the charging sound of the image forming apparatus is about 35 dB (chain line in FIG. 4), if the Asker C hardness is 35 ° or less, the noise level is close to the apparatus noise level and is charged to the extent that it does not matter. It can be seen that the sound can be reduced. However, as described above, the Asker C hardness is preferably 25 ° or more.

  According to this embodiment, the structure of the contact charging system can reduce charging noise and extend the life of the apparatus. That is, since the upstream first charging roller 7a having the highest influence of contamination due to the fine powder toner and the external additive has a high surface hardness, the first charging roller 7a is difficult to be stained, and the entire apparatus as a whole. The life can be kept high on average and the life can be extended. Further, since only the DC voltage is applied to the first charging roller having a high surface hardness, no charging sound is generated. On the other hand, it is possible to reduce the surface hardness of the downstream second charging roller 7b to which a voltage containing an AC component is applied, thereby reducing the generation of charging noise due to the AC voltage. Each of the charging rollers 7a and 7b has a structure having elastic layers 12a and 12b around the core bars 11a and 11b. For this reason, even the upstream first charging roller 7a having a high surface hardness can ensure a nip width with the photosensitive drum 2, and can prevent charging failure and damage to the surface 2a of the photosensitive drum 2. In addition, since any of the charging rollers 7a and 7b is brought into contact with the surface 2a of the photosensitive drum 2 to be charged, the applied voltage can be lowered compared to the non-contact method, and generation of ozone due to corona discharge can be suppressed.

<Second Embodiment>
A second embodiment of the present invention will be described with reference to FIG. Note that the basic configuration of the image forming apparatus of the present embodiment is the same as that of the first embodiment described above. Accordingly, elements having the same functions and configurations as those of the first embodiment are denoted by the same reference numerals, and detailed description and illustration thereof are omitted. In the case of the present embodiment, as shown in FIG. 5, a contact-type third charging roller 7c is arranged between the first charging roller 7a and the second charging roller 7b in the rotation direction of the photosensitive drum 2. is doing. Although some members are not shown in FIG. 5, the configuration other than the portion related to the third charging roller 7c is the same as that in FIG.

The third charging roller 7c has a three-layer structure in which a conductive elastic layer 12c, a high resistance coating layer 13c, and a protective coating layer 14c are sequentially laminated from the inner diameter side on the outer periphery of the core metal 11c. Sex roller. Further, the surface layer has an electric resistance value of about 10 ² to 10 ⁶ Ω. The conductive elastic layer 12c is made of the same material as the first charging roller 7a, and the surface hardness of the third charging roller 7c contacting the surface 2a of the photosensitive drum 2 (surface hardness) is also the first. The surface hardness of the charging roller 7a is the same. Therefore, the third charging roller 7c is a hard roller like the first charging roller 7a. Note that the surface hardness of the third charging roller 7c may be lower than the surface hardness of the first charging roller 7a because it is less affected by contamination with fine toner than the first charging roller 7a. It is preferable to make it higher than the surface hardness of the second charging roller 7b.

  A power source S3 is connected to the cored bar 11c of the third charging roller 7c. Then, a charging bias voltage, which is a DC voltage, is applied to the cored bar 11c by the power source S3. Further, both end portions of the core metal 11c are rotatably held by the bearing members 15c, respectively, and are urged toward the photosensitive drum 2 by the pressing springs 16c. The third charging roller 7c is pressed against the surface 2a of the photosensitive drum 2 with a predetermined pressing force.

In the present embodiment, the surface 2a of the photosensitive drum 2 charged to a predetermined potential by the first charging roller 7a is further charged at the position of the third charging roller 7c. Specifically, a DC voltage of 10 ² to 2 × 10 ³ V is applied from the power source S3 to the core 11c of the third charging roller 7c (DC application method). As a result, a current of approximately 10 ^{−5 to} 2 × 10 ⁻⁴ A flows, and the surface 2 a of the photosensitive drum 2 is charged to a potential that is closer to the target potential than in the case of FIG. In the present embodiment, a DC voltage of 10 ² to 2 × 10 ³ V is applied to the first charging roller 7a. As a result, a current of approximately 10 ⁻⁵ to 2 × 10 ⁻⁴ A flows through the first charging roller 7a. Note that the voltage applied to the third charging roller 7c may be the same as the voltage applied to the first charging roller 7a. In any case, the surface 2a of the photosensitive drum 2 is charged to the potential V1 close to the target potential by the first charging roller 7a and the third charging roller 7c. The power source S3 and the power source S1 (see FIG. 2) may be shared.

  In the case of this embodiment, the surface 2a of the photosensitive drum 2 is charged to the potential V1 close to the target potential by the charging rollers 7a and 7c that apply a DC voltage among the three charging rollers 7a, 7b, and 7c. The applied voltage shared by the charging rollers 7a and 7c can be further reduced. As a result, the life of the apparatus can be further extended. In addition, since the first and third charging rollers 7a and 7c, which are hard rollers having high surface hardness and are not easily contaminated, are disposed upstream of the influence of contamination by fine toner and external additives, the life of the entire apparatus is increased. High on average and long life can be achieved. Further, since a DC voltage is applied to the first and third charging rollers 7a and 7c, no charging noise is generated from both the charging rollers 7a and 7c. Other functions and effects are the same as those of the first embodiment.

  In each of the above-described embodiments, the structure using the intermediate transfer belt has been described. However, in the present invention, the recording material is conveyed by the recording material conveyance belt, and the image is directly transferred from each photosensitive drum to the recording material. It can also be applied to structures. In addition to the tandem structure described above, the present invention can also be applied to a one-drum structure. That is, a developing device having a plurality of colors is arranged around one photosensitive drum, and one rotation is performed for each color, and in the case of four colors, a total of four times of image formation is performed by one photosensitive drum. Applicable.

2... Photosensitive drum (charged body, image carrier), 2a... Surface (charged surface), 7a... First charging roller, 7b. Third charging roller, 11a, 11b, 11c ... cored bar, 12a, 12b, 12c ... conductive elastic layer (elastic layer)

Claims (4)

And a plurality of charging members that are arranged side by side in the moving direction of the charging surface and charge the charging surface by applying a voltage between the charging surface and the charging surface. In the charging device,
The plurality of charging members have an elastic layer around a cored bar and are disposed in contact with the charging surface,
Among the charging members, the charging member arranged at the most upstream side in the moving direction of the charging surface is applied with a DC voltage, and the charging member arranged at the most downstream side in the moving direction of the charging surface is a voltage containing an AC component. Is applied,
The charging device according to claim 1, wherein a hardness of a surface of the most upstream charging member contacting the charging surface is higher than a hardness of a surface contacting the charging surface of the most downstream charging member.   The hardness of the surface of the most upstream charging member that contacts the charging surface is Asker C hardness of 60 ° or more, and the hardness of the surface of the most downstream charging member that contacts the charging surface is Asker C hardness of 55 ° or less. The charging device according to claim 1, wherein:   The hardness of the surface of the most upstream charging member that contacts the charging surface is Asker C hardness of 90 ° or less, and the hardness of the surface of the most downstream charging member that contacts the charging surface is Asker C hardness of 25 ° or more. The charging device according to claim 2, wherein: In an image forming apparatus comprising: an image carrier; and a charging member that charges the surface of the image carrier.
4. The charging device according to claim 1, wherein the image carrier is the member to be charged, a plurality of the charging members are disposed, and the charging device including the image carrier and the charging members. An image forming apparatus comprising the charging device according to claim 1.

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