Patent Grant
8195068
This application is based upon and claims the benefit of priority from the corresponding Japanese Patent application No. 2009-008389, filed Jan. 19, 2009, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system. More particularly, the present invention relates to a charging unit that charges a photosensitive member with electricity.
2. Description of the Related Art
Conventionally, in an image forming apparatus that use an electrophotographic process, a charging unit is used to charge the surface of an image-bearing member (photosensitive member). As the charging unit, a noncontact charging unit, such as a scorotron charging unit or a corotron charging unit, is used. In addition, in consideration of the environment, a contact charging unit is also used which has a charging member, such as a charging roller or a charging brush, that is in contact with a photosensitive member, to charge the photosensitive drum. By using the contact charging unit, it is possible to reduce the amount of ozone emission.
For example, in the situation where a charging roller is used as the contact charging unit, an electric charge is directly applied to the surface of the photosensitive member by applying a direct-current voltage of about 1 kilovolt to the charging roller that is in contact with the surface of the photosensitive member thereby charging the surface of the photosensitive member to a predetermined potential. However, because the surface of the charging roller, to which the voltage is applied, and the surface of the photosensitive member with which the charging roller is in contact have microscopic irregularities, spot-like charging unevenness can occur on the surface of the photosensitive member.
As a method for preventing such charging unevenness it is known, for example, to uniformly charge the target surface by applying a pulsating-current voltage having a peak-to-peak voltage that is at least twice the charge-starting voltage to an electrically conductive member (charging member). This forms an oscillating electric field between the target member and the electrically conductive member.
As a method for developing an electrostatic latent image formed on the photosensitive member, a so-called jumping development method is sometimes used. This method can develop the electrostatic latent image on the photosensitive member into a visible image by forming a uniform thin developer (toner) layer on a developing roller that functions as a toner bearing member, which is close to the surface of the electrostatic latent image, and applies a developing bias. A direct-current bias and an alternate-current bias are superposed to the developing roller to thereby electrically attract the toner causing the toner to attach to the electrostatic latent image. This improves the stability of the density of the image and reduces fogging.
In such a jumping development method, where a superposed direct-current bias and alternate-current bias is used for both the charging bias and the developing bias, problems with the unevenness of formed images can occur. This can be caused by the frequency of the alternating-current component of the charging being similar to: the frequency of the developing bias; frequency of an integral multiple thereof; or the frequency that is an integral submultiple thereof.
A possible cause of the problems is that when the alternate-current components of the charging bias and the developing bias are applied at a close range, at the same time, electromagnetic induction or the like occurs in the wires of the high-voltage supply board, the charging unit, and the developing unit. Therefore, the alternate-current component of the developing bias may be (electromagnetically) induced into a charging bias. Likewise, the alternating-current component of the charging bias may be (electromagnetically) induced into the developing bias.
In particular, when the alternating-current component of the developing bias is induced into the charging bias, beats are generated between the original frequency of the alternating-current component of the charging bias itself and the frequency of the induced alternating-current component. If these beats cause a charging unevenness during the formation of the electrostatic latent image, the formed image can look jittery, blurry.
Therefore, a method for preventing interference between the charging bias and the developing bias is proposed. For example, a method (hereinafter referred to as “method A”) for preventing the occurrence of radiant (electrostatic) noise to avoid surrounding electronic devices from malfunctioning is known. This is accomplished by surrounding a contact nipping portion formed between the charging member and the target member with a shield member formed of a grounded electrically conductive member.
Furthermore, a method (hereinafter referred to as “method B”) for preventing charging unevenness due to the close placement of an upstream charging member and a downstream developing unit due to the miniaturization of image forming apparatus and close placement of image forming units is known. The apparatus includes cleaning means having a cleaning member and a grounded conductive support member (a shield member) that supports the cleaning member. The support member is located so as to shield the visual angle between the developing member of the developing means and the charging member that charges the image-bearing member next to the developing member.
However, it is difficult for method A to sufficiently achieve miniaturization because the shield member is located outward, away from the support member (casing) of the charging member. Moreover, to replace the charging unit, for example, it is necessary to remove the charging member together with the casing after the shield member is removed. This can affect the attachment/detachment of the charging unit complicating maintenance work.
Furthermore, due to the gap between the shield member and the casing, the space surrounded by the shield member can widen in the periphery of the portion between the charging unit to which the charging bias is applied and the image-bearing member. Thus, there is the possibility that an electric field, of the developing bias, can leak across the shield member (leaked electric field), which can affect the charging bias.
Furthermore, with method B, the support member (shield member) only covers a part of the casing of the cleaning member or the charging-roller cover (casing), so that the leaked electric field cannot be sufficiently prevented, which can affect the charging bias.
An advantage of the present invention is to provide a compact charging unit that is easily detachable and easy to maintain and can reduce the influence of the developing bias on the charging bias, as well as an image forming apparatus.
A charging unit according to an embodiment of the present invention includes a charging member, an insulating casing surrounding the charging member, and a metal shield member surrounding the casing; and the metal shield member is located in close contact with an outer peripheral surface of the insulating casing.
An image forming apparatus according to another embodiment of the present invention includes a charging unit; an image-bearing member that is charged by the charging unit; an exposing unit that irradiates light onto the image-bearing member, that is charged by the charging unit to form an electrostatic latent image; and a developing unit that applies a developing bias including an alternating-current component to develop the electrostatic latent image. The charging unit includes a charging member; an insulating casing surrounding the charging member; a metal shield member surrounding the casing; and the metal shield member is located in close contact with an outer peripheral surface of the insulating casing.
Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.
In the accompanying drawings:
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
The image forming units Pa to Pd include photosensitive drums (image-bearing member) 1a, 1b, 1c, and 1d, respectively, that carry visual color images (toner images). After the toner images that are formed on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8, in sequence in an overlapping manner (primary transfer), moved by a driving unit (not shown), next to the image forming units Pa to Pd, while rotating clockwise in
The paper P, to which the toner images are transferred, are stored in a paper cassette 16 at the lower part of the apparatus 100. The paper P is fed to the secondary transfer roller 9 via a paper feed roller 12a and a register roller pair 12b. The intermediate transfer belt 8 is made of a dielectric resin sheet; for example, an endless belt having ends that are lap joined into an endless form or a seamless belt can be used. A belt cleaning unit 19 for removing toner that remains on the surface of the intermediate transfer belt 8 is positioned upstream of the image forming unit Pa in the direction the intermediate transfer belt 8 moves.
Now, the image forming units Pa to Pd will be described. The units include around and below the rotatable photosensitive drums 1a to 1d, charging units 2a, 2b, 2c, and 2d, for charging the photosensitive drums 1a to 1d, an exposure unit (exposing unit) 4 for exposing the photosensitive drums 1a to 1d to irradiating light corresponding to the image information, and a developing unit 3a, 3b, 3c, and 3d for forming toner images on the photosensitive drums 1a to 1d, and cleaning units 5a, 5b, 5c, and 5d for removing developer (toner) remaining on the photosensitive drums 1a to 1d, respectively.
When the start signal is initiated by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging units 2a to 2d. Next, the photosensitive drums 1a to 1d are irradiated with light by the exposure unit 4, forming electrostatic latent images corresponding to an image signal on the individual photosensitive drums 1a to 1d. The developing units 3a to 3d are each equipped with a developing roller 27 (toner bearing member, see
Then, while an electric field having a predetermined transfer voltage is applied to the transfer rollers 6a to 6d, the yellow, cyan, magenta, and black toner images on the photosensitive drums 1a to 1d are primary-transferred onto the intermediate transfer belt 8. These four-color images are formed using a predetermined positional relationship for forming a full-color image. Thereafter, toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by cleaning units 5a to 5d in preparation for the subsequent formation of new electrostatic latent images.
The intermediate transfer belt 8 is stretched over a driven roller 10, a driving roller 11, and a tension roller 20. When the intermediate transfer belt 8 begins its clockwise rotation due to the rotation of the driving roller 11, the paper P is transported from the register roller pair 12b to a secondary transfer nip between the intermediate transfer belt 8 and the secondary transfer roller 9, located next to the intermediate transfer belt 8, at a predetermined timing. A full-color image is transferred onto the paper P at the secondary transfer nip. The paper P, on which the full color toner image has been transferred, is transported to the fixing unit 7.
The paper P is then heated and pressed as the paper P passes through a nipping portion (fixing nipping portion) of a fixing roller pair 13. The toner image is thereby fixed to the surface of the paper P to form a full-color image. The direction in which the paper P having the full-color image is fed, is divided by a diverging unit 14 that allows paper to be fed in a plurality of directions. In a situation where an image is formed only on one side of the paper P, the paper P exits to an output tray 17 by an output roller pair 15.
On the other hand, in the situation where images are formed on both sides of the paper P, part of the paper P that has passed through the fixing unit 7 temporarily protrudes from the output roller pair 15 to the outside of the apparatus 100. Thereafter, the paper P is fed in a reverse direction by reversely rotating the output roller pair 15. The paper is fed to a paper transport path 18 by the diverging unit 14 and fed again to the secondary transfer nip, with the image surface being reversed. The next image that is formed on the intermediate transfer belt 8 is then transferred to the surface of the paper P that does not bear an image, by the secondary transfer roller 9. The paper P is then fed to the fixing unit 7, where the toner image is fixed, and then to the output tray 17.
Around the photosensitive drum 1a, the charging unit 2a, the developing unit 3a, and the cleaning unit 5a are positioned along the rotational direction of the photosensitive drum 1a (counterclockwise in
As shown in
The charging roller 21 is in pressure contact with the photosensitive drum 1a, at a predetermined nip pressure, and rotates with the photosensitive drum 1a. The charging roller 21 is preferably of a solid type, such as a conductive rubber roller having a surface layer made of, for example, epichlorohydrin rubber with a resistance of 105 to 106Ω and a surface roughness of RZ=10 μm; however, the roller may be of any type and is not limited thereto. For example, the charging roller 21 may be of a sponge type wherein a foam rubber roller is provided that is covered with a tube, a conductive brush roller, or the like.
As shown in
As shown in
The surface of the developing roller 27 consists of a sleeve having a uniformly conductive material such as aluminum, stainless steel, a conductive resin coating, or the like. As shown in
The magnetic roller 29 includes a rotatable cylinder consisting of a nonmagnetic metallic material and a plurality of stationary magnets inside the rotatable cylinder. A gap is provided between the plurality of stationary magnets and the rotatable cylinder. The magnets cause a magnetic brush having the developer to be generated on the surface of the magnetic roller 29. The thickness of the magnetic brush is regulated by a regulating blade (not shown). As shown in
The third power source 49 is electrically connected to a ground that is common to the second power source 47 and can apply a second developing bias, that is superposed on the first developing bias, to the magnetic roller 29. This allows the first and second developing biases to be separately set, preventing the combined waveform of the bias formed between the developing roller 27 and the magnetic roller 29 from being influenced by the second developing bias of the third power source 49, thus improving the developing performance.
However, the method for applying the first and second developing biases to the developing roller 27 and the magnetic roller 29 is not limited thereto. Thus, the second and third power sources 47 and 49 can be electrically connected to different grounds.
As shown in
The developer forms a magnetic brush on the magnetic roller 29. The thickness of the magnetic brush is regulated by the regulating blade (not shown). The magnetic brush comes into contact with or is near to the developing roller 27 at a predetermined thickness to form a toner layer (thin toner layer) on the developing roller 27 with a potential difference being created between the magnetic roller 29 and the developing roller 27 due to the first and second developing biases.
Because of the potential difference between the developing roller 27 and the photosensitive drum 1a due to the first developing bias and surface potential of the photosensitive drum 1a, the toner located on the developing roller 27 is attracted to the photosensitive drum 1a. A toner image is thereby created on the photosensitive drum 1a. The toner remaining on the developing roller 27, after the development step, is collected by the magnetic brush on the magnetic roller 29 and also due to the potential difference between the developing roller 27 and the magnetic roller 29. The collected toner is dropped to the developer in the developing unit 3a with the carrier, due to the magnetic field produced by the magnets in the magnetic roller 29. The toner is stirred and then transported by the stirring transport screws 25, thus, circulation of the toner is boosted.
Examples of toner that can be used in the present invention include toners having external additive particles, such as silica, titanium oxide, strontium titanate, alumina, or the like, embedded in the surface thereof so as to partially protrude and toner in which external additive particles are electrostatically attached on the surface thereof. Examples of carriers include magnetite carriers, such as Mn-ferrite, Mn—Mg ferrite, Cu—Zn ferrite, and resin carriers in which magnetic substances are dispersed in the resin. Also, carrier having a treated surface can be used and its surface still maintains an appropriate resistance within a range in which the resistance is not increased beyond even after the treatment (on its surface).
As shown in
As described above, the charging bias is applied from the first power source 45 to the charging roller 21, and the first and second developing biases are applied from the second and third power sources 47 and 49 to the developing roller 27 and the magnetic roller 29, respectively. However, if the charging bias and the first and second developing biases are applied near each other at the same time, it is possible that the alternate-current components of the first and second developing biases will interfere with the charging bias, due to electromagnetic induction or the like of the wires in the high-voltage power board (not shown), the charging unit 2a, and the developing unit 3a themselves.
The interference caused by the first and second developing biases with the charging bias causes a blurred image, thus causing a reduction in image quality. Therefore, a metal shield member 43 is provided around the casing 41 of the charging unit 2a, as described below.
As shown in
As shown in
As shown in
The end of the metal shield member 43 on the front side of the apparatus main body, in the longitudinal direction (the left side in
By way of example, a voltage, having a direct-current component Vdc1 of 500 V, an alternating-current component Vpp1 of 1.2 kV, and a frequency of 2 kHz, is applied to the charging roller 21 from the direct-current power source 45a and the alternate-current power source 45b of the first power source 45. A voltage, having a direct-current component Vdc2 of 70 V, an alternating-current component Vpp2 of 1.5 kV, and a frequency of 4.7 kHz, is applied to the developing roller 27 from the direct-current power source 47a and the alternate-current power source 47b of the second power source 47.
A voltage, having a direct-current component Vdc3 of 370 V, an alternating-current component Vpp3 of 2.3 kV, and a frequency of 4.7 kHz, is applied to the magnetic roller 29 from the direct-current power source 49a and the alternate-current power source 49b of the third power source 49. Accordingly, the difference between the charging bias Vpp1 applied to the charging roller 21 and the first developing bias Vpp2 applied to the developing roller 27 is set at 0.3 kV. The difference between the charging bias Vpp1 applied to the charging roller 21 and the second developing bias Vpp3 applied to the magnetic roller 29 is set at 1.1 kV.
In this embodiment, the difference between Vpp1 and Vpp3 is greater than the difference between Vpp1 and Vpp2; therefore, it is possible that the second developing bias applied to the magnetic roller 29 will exert a greater influence on the charging bias than the first developing bias applied to the developing roller 27. However, when image formation was performed with such bias setting, image degradation, such as blurriness or unevenness, did not occur.
That is, in this embodiment, even if the difference in Vpp between the charging bias and the developing bias is as great as 1.1 kV, interference of the developing bias with the charging bias can be prevented, thus allowing image degradation, such as blurriness or unevenness, to be sufficiently prevented. Moreover, image degradation due to development failure did not occur.
Since the outer periphery of the casing 41 is surrounded by the metal shield member 43, and the metal shield member 43 is in close contact with the outer peripheral surface of the casing 41, as described above, they can be assembled and disassembled as one piece. This can improve the detachability and maintenance of the charging unit 2a and can decrease the size of the charging unit 2a.
Since the periphery of the casing 41 is surrounded by the metal shield member 43, the area of the opposing portion N of the charging roller 21 and the photosensitive drum 1a (see
In addition, since the metal shield member 43 is in close contact with the outer peripheral surface of the casing 41, the metal shield member 43 can be located close to the opposing portion N. This can reduce the space around the opposing portion N, that is, by the charging bias, that is not surrounded by the metal shield member 43.
Accordingly, this can prevent the first and second developing biases from interfering with the charging bias across the shield member 41 and the alternating-current component of the electric field from being induced into the charging bias. This can prevent the first and second developing biases from interfering with the charging bias and influencing the charging bias, thus stabilizing the charging of the photosensitive drum 1a.
Furthermore, in this embodiment, since the metal shield member 43 has a cross sectional U-shape having the opening at the opposing portion N between the charging roller 21 and the photosensitive drum 1a, and the second end 43a of the metal shield member 43 inclines toward the charging roller 21, the area by the charging bias, which is not surrounded by the metal shield member 43, is reduced. This reduces the influence of electric fields that leak from the first and second developing biases, on the charging bias.
The end of the U-shape of the metal shield member 43 that is adjacent to the cleaning unit 5a (see
Here, although the first end 41a of the U-shape of the casing 41 is also inclined toward the charging roller 21, with the second end 43a of the metal shield member 43, only the second end 43a of the metal shield member 43 may be inclined toward the charging roller 21. In such a case, for example, the first end 41a of the casing 41 may not be inclined and can be shorter than the second end 43a of the metal shield member 43, and only the second end 43a, which protrudes toward the photosensitive drum 1a more than the first end 41a, may be inclined.
In an embodiment, the metal shield member 43 has a flat-spring grounding portion 43b. This further ensures grounding, thus further preventing any influence from electric fields from the first and second developing biases on the charging bias. However, the method for the grounding is not particularly limiting; all that is necessary is that the metal shield member 43 is grounded. The grounding portion 43b may either be integrally formed with the metal shield member 43 or be separately formed and thereafter be mounted to the metal shield member 43.
In an embodiment, an image forming apparatus 100 is provided that includes the charging unit 2a, the photosensitive drum 1a, the exposure unit 4, and the developing unit 3a. Pursuant to the present invention, an image, in which unevenness in charging is prevented, can be formed.
In an embodiment, the developing unit 3a includes a developing roller 27 that faces the photosensitive drum 1a without contact therewith. The developing unit 3a includes a magnetic roller 29 that forms a thin toner layer on the developing roller 27 using a magnetic brush, and second and third power sources 47 and 49 that apply first and second developing biases each formed of direct-current and alternating-current components to the developing roller 27 and the magnetic roller 29, respectively. Therefore, the charging can be stabilized even when the first developing bias and the second developing bias is prone to exert an influence on the charging bias.
Methods for using the developing unit 3a are not limited to this embodiment; development can be performed using only the developing roller 27 without using the magnetic roller 29. In such a situation, the developing roller 27 having a sleeve consisted of, for example, a magnetic material, may be used. A magnetic brush may be formed on the developing roller 27. Toner may be electrically attracted from the magnetic brush onto the photosensitive drum 1a.
Although the photosensitive drum 1a, the charging unit 2a, and the developing unit 3a located in the image forming unit Pa have been described, the charging units 2b to 2d and the developing units 3b to 3d located in the image forming units Pb to Pd, respectively, have substantially the same configurations and offer the same operational advantages.
For example, in the above-described embodiment, a belt cleaning unit 19 equipped with the belt cleaning roller 19a is used as a means for cleaning the intermediate transfer belt 8. However, a belt cleaning unit 19 equipped with a belt cleaning blade, instead of the belt cleaning roller 19a, may be used.
Although an embodiment uses a charge cleaning roller 23, this charge cleaning roller 23 is not required, and may not be used. Although in an embodiment, a contact charging unit 2a having the charging roller 21 is used as a charging unit, a noncontact charging unit, such as a scorotron charging unit or a corotron charging unit, may be used.
The present invention can be applied to various image forming apparatuses that charge the surface of the photosensitive member using a charging unit. Such apparatus include digital multi-functional peripherals, copying machines, such as tandem color copying machines and analog monochrome copying machines, facsimile machines, and laser printers.
It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-008389 | Jan 2009 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4851960 | Nakamura et al. | Jul 1989 | A |
| 5870657 | Nagame et al. | Feb 1999 | A |
| 7035567 | Gila et al. | Apr 2006 | B2 |
| 20060104681 | Hanashi | May 2006 | A1 |
| Number | Date | Country |
|---|---|---|
| 63149669 | Jun 1988 | JP |
| 06-175472 | Jun 1994 | JP |
| 2006-139155 | Jun 2006 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20100183334 A1 | Jul 2010 | US |
