JP3496594B2 - Charging device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, and uniformly coats the surface of a photosensitive member, which is an electrostatic latent image carrier, with a conductive charging brush. The present invention relates to a brush charging device that is charged.

[0002]

2. Description of the Related Art In image forming apparatuses, devices using dry toner occupy the mainstream, and many copying machines, laser printers,
It has been put to practical use as a plain paper facsimile and has made remarkable progress. This image forming apparatus is a device to which an electrophotographic process technology is applied, and visualizes an electrostatic latent image formed on a photoconductor, which is an electrostatic latent image carrier, with toner particles.

In such an image forming apparatus, as a means for uniformly charging the surface of the photoconductor, a corona charging device composed of a wire electrode and a shield electrode has been generally used. However, due to problems such as application of high voltage, low charging efficiency, generation of ozone, and uneven charging due to wire contamination, the utilization of conductive brush charging devices, which is advantageous for cost reduction, has recently been increasing.

The structure and operation of the image forming apparatus using the brush charging device will be described below.

FIG. 4 is a schematic view showing an image forming apparatus having a conventional brush charging device.

The image forming apparatus shown in the figure is an image forming apparatus which visualizes an electrostatic latent image formed on a photoreceptor by an electrophotographic process technique with toner particles, and is a photoreceptor 1 which is an electrostatic latent image carrier. have. The photoconductor 1 rotates in the circumferential direction and conveys a sheet described later.

A charging brush 2 is arranged in contact with the surface of the photoconductor 1. The charging brush 2 is provided with a metal shaft 2 a that is provided parallel to the rotation axis of the photoconductor 1 and rotates.
And a brush having conductivity, which is radially planted on the outer peripheral surface of the shaft 2a. The charging brush 2 is electrically connected to the charging bias supply power source 27, and uniformly charges the surface of the photoconductor 1 by the DC bias voltage supplied from the charging bias supply power source 27.
When the photoconductor 1 is irradiated with the exposure light beam 4 obtained from the exposure optical system 3, the photoconductor 1 is exposed and a predetermined electrostatic latent image is formed.

The developing hopper 9 is provided with a toner supply roller 6 for supplying the toner 10 stirred and conveyed by a toner stirring member 11 whose both ends are rotatably supported to the surface of the developing roller 5. The toner supply roller 6 is composed of a shaft 6a made of metal such as stainless steel and a layer of elastic member such as urethane or silicon formed around the shaft 6a. The shaft 6a has a toner supply bias voltage supply power supply which is a constant voltage power supply. 26 is connected, and a toner supply bias voltage in which direct current or direct current and alternating current are superimposed is applied.

The developing roller 5 comprises a shaft 5a made of metal such as stainless steel as a base material, and an elastic member such as urethane or silicon formed in layers on the outer peripheral surface thereof. A developing bias voltage supply power source 12, which is a constant voltage power source, is connected to the shaft 5a of the developing roller 5.

The toner 10 is a non-magnetic one-component toner, for example, polyester resin with carbon, wax,
The charge control agent and the like are uniformly dispersed.

The developing roller 5 and the toner supplying roller 6 are arranged in contact with each other, and are rotatably supported by shafts 5a and 6a at both ends of the developing hopper 9, respectively. The developing roller 5 and the photoconductor 1 are in contact with each other and rotate in opposite directions, while the developing roller 5
And the toner supply roller 6 come into contact with each other and rotate in the same direction. The developing roller 5 is in contact with the photoconductor 1, and the toner supply roller 6 applies toner to the portion of the photoconductor 1 where the electrostatic latent image is formed by the bias voltage applied by the developing bias voltage power supply 12. When 10 is transferred and attached, the electrostatic latent image is visualized.

The toner stirring member 11 is used as the developing hopper 9.
The toner 10 contained therein is prevented from agglomerating and the toner 10 is conveyed toward the toner supply roller 6.

A toner regulating blade 7 is provided in contact with the developing roller 5. This toner regulation blade 7
It is composed of an elastic metal spring plate member 7a and a toner regulating member 7b which contacts the developing roller 5. The toner regulating member 7b is formed by integrally forming an elastic member such as urethane rubber or silicon rubber at one end of the metal spring plate member 7a. The toner regulating blade 7 is screwed to the blade holder 8. This toner regulation blade 7
Is pressing the developing roller 5 with a predetermined linear pressure, and the toner 1 supplied from the toner supply roller 6 to the developing roller 5 is
No. 0 is triboelectrically charged by the toner regulating blade 7, and a thin toner layer is formed on the outer peripheral surface of the developing roller 5.

Sheets 15 are stored in a sheet cassette 14 provided at the starting point of the transport path. The paper 15 is sent one by one from the paper cassette 14 to the transport roller 17 by the paper feed roller 16. Then, the fed-out sheet 15 is nipped by the pair of conveyance rollers 17 and is indicated by the arrow A.
It is conveyed in the direction of the photoconductor 1 indicated by.

In order to make the sheet 15 and the toner image formed on the photoconductor 1 coincide with each other, the registration roller 18 is in contact with the driven roller 19 on the conveying path before reaching the photoconductor 1.
Is provided. Further, a transfer roller 20 is provided so as to be in contact with the photoconductor 1 and rotatably supported.

When the toner image reaches the contact portion between the transfer roller 20 and the photoreceptor 1 as the photoreceptor 1 rotates, the registration roller 18 aligns the toner image with the toner image and the paper 15 also contacts the contact portion. To reach. At this time, when a high voltage from the transfer bias voltage supply power supply 24 is applied to the metal shaft 20a of the transfer roller 20 to apply a charge having a polarity opposite to that of the toner 10 to the back surface of the paper 15, the toner image is transferred onto the paper 15. To be done.

A fixing device 21 including a heat roller 22 having a heat source inside and a pressure roller 23 that nips and conveys the paper 15 together with the heat roller 22 is installed in the subsequent stage of the conveyance path. The sheet 15 to which the toner image is transferred is sent to the fixing device 21 and the toner image is fixed. On the other hand, after the toner image is transferred onto the paper 15, the cleaning blade 25 scrapes off the transfer residual toner, and the static eliminator 13 irradiates the photosensitive drum 1 with light to eliminate the static electricity and prepares for the next process.

[0018]

However, in such a conventional brush charging device, uneven charging of the surface of the photoconductor due to uneven contact of the brush with the photoconductor causes uneven image density in a sweep pattern of the brush. There is a problem that it easily occurs.

As a measure for preventing the sweep-like image density unevenness, a technique of applying a superimposed voltage of a DC voltage and an AC voltage to a brush charging device is widely used. Then, in the technique of applying the superimposed voltage, when the peak-to-peak voltage of the AC voltage is increased, the uneven charging of the surface of the photoconductor due to the uneven contact of the brush is improved.

By generating an oscillating electric field between the brush charging device and the photoconductor, the charges alternately move between the two members, and as the photoconductor passes near the contact portion with the brush, the charge is changed. This is because the amount of movement of is gradually reduced and converges to a constant potential. Therefore, as the peak-to-peak voltage of the AC voltage increases, the amount of movement of the charge increases, and the photoconductor can be charged more uniformly.

However, according to the study by the present inventor, it became clear that when the peak-to-peak voltage exceeds a certain value, spot-like density unevenness occurs in the image. This makes it impossible to substantially uniformly charge the photoconductor.

Therefore, it is an object of the present invention to provide a brush charging device capable of preventing the occurrence of spot-like density unevenness in an image when the peak-to-peak voltage of the applied AC voltage is increased.

[0023]

Means for Solving the Problems In order to solve this problem, a charging device of the present invention is rotated are arranged in contact with the surface of the latent electrostatic image bearing member, the surface of the front Kisei latent image bearing member and charging means for a static-to a predetermined potential, said charging means and is electrically connected to a charging bias power supply for applying a superimposed voltage composed of a DC voltage and an AC voltage to the charging unit, the charging
The DC voltage Vc (V) applied to the means and the AC voltage peak
Relationship between the voltage Vp (V) and Vp <2 × | Vc |
While maintaining the
The charging bias supply voltage is changed so that the voltage
And a control means for controlling the source .

As a result, it becomes possible to prevent the occurrence of spot-like density unevenness in the image when the peak-to-peak voltage of the applied AC voltage is increased.

[0025]

DETAILED DESCRIPTION OF THE INVENTION According to a first aspect of the present invention is rotated are arranged in contact with the surface of the latent electrostatic image bearing member, the surface of the pre <br/> Kisei latent image bearing member charging hand of a static-to a predetermined potential
And the step, is the charging unit and electrically connected, the charging bias power supply for applying a superimposed voltage composed of a DC voltage and an AC voltage to the charging unit <br/>, direct current is applied to the charging unit
The voltage Vc (V) and the peak-to-peak voltage Vp (V) of the AC voltage
While maintaining the relationship of Vp <2 × | Vc |
Change the peak-to-peak voltage value of the AC voltage according to the change in pressure.
Control means for controlling the charging bias supply power source so that
Since the charging device is characterized by having a voltage having a polarity opposite to the charging polarity of the electrostatic latent image carrier, the peak-to-peak voltage of the applied AC voltage is increased. In this case, it is possible to prevent the occurrence of spot-like density unevenness in the image.

[0026]

The invention described in claim 2 of the present invention, claim
In the invention described in 1, the DC voltage applied to the charging means is Vc (V), the peak-to-peak voltage of the AC voltage is Vp (V),
A constant T is a charging device having a relationship of Vp = 2 × | Vc | −T, and a voltage having a polarity opposite to the charging polarity of the electrostatic latent image carrier is applied to the charging means even when the value of the DC voltage changes. Since the voltage is not applied, there is an effect that it is possible to prevent the occurrence of spot-like density unevenness in the image when the peak-to-peak voltage of the applied AC voltage is increased. Further, since the peak-to-peak voltage of the AC voltage can be set to be always large, there is an effect that it is possible to prevent the occurrence of sweep density unevenness of an image.

The invention described in claim 3 of the present invention, claim
In the invention described in 2 , the constant T is 0 ≦ T ≦ 200.
It is a charging device , and even if the value of the DC voltage changes, the voltage of the opposite polarity to the charging polarity of the electrostatic latent image carrier is not applied to the charging means. This has the effect of making it possible to prevent the occurrence of spotted density unevenness in the image when raised.
Further, since the peak-to-peak voltage of the AC voltage can be set to be always large, there is an effect that it is possible to prevent the occurrence of sweep density unevenness of an image.

[0029]

Embodiments of the present invention will be described below with reference to FIGS. 1 to 3. In addition, in these drawings, the same members are denoted by the same reference numerals, and duplicate description is omitted.

FIG. 1 is a schematic diagram showing the structure of an image forming apparatus using a brush charging device according to an embodiment of the present invention, and FIG. 2 shows the charging bias voltage of a charging bias supply power source in the image forming apparatus of FIG. Explanatory drawing showing an image evaluation result when the peak-to-peak voltage of a DC voltage and an AC voltage is changed, and FIG. 3 is a schematic view showing a configuration of an image forming apparatus using a brush charging device according to another embodiment of the present invention. It is a figure.

The image forming apparatus shown in FIG. 1 is an image forming apparatus which visualizes an electrostatic latent image formed on a photosensitive member (electrostatic latent image carrier) by toner particles by an electrophotographic process technique. A metal drum such as aluminum is used as a base material, an alumite layer which is an insulator is provided on the outer peripheral surface thereof, and selenium (Se) or an organic photoconductor (hereinafter,
It is called "OPC". ) Or the like is applied to form a thin film in the form of a thin film. Then, the photoconductor 1 rotates in the circumferential direction (in the present embodiment, the clockwise direction of the arrow D) and conveys a sheet to be described later.

Here, the alumite layer is formed to protect the photosensitive receiving layer against electrical breakdown. The photoreceptive layer has a laminated structure of a charge generation layer in which a charge generation substance is dispersed and contained in a binder resin, and a charge transport layer in which a charge transport substance is contained. In the present embodiment, the charge transport layer is laminated on the charge generation layer, and the thickness of these photosensitive receiving layers is 25 μm.

A charging brush 2 is arranged in contact with the surface of the photoconductor 1 which is an electrostatic latent image carrier. The charging brush 2 is provided with a shaft 2a that is provided in parallel with the rotation axis of the photoconductor 1 and that is made of metal and that rotates, and that rotates.
And a brush having a conductive property that is radially implanted. Then, in the present embodiment, the charging brush 2 rotates in the direction of arrow E, which is clockwise.

Here, the brush is made of rayon, acrylic, polypropylene, polyethylene, nylon, ETF.
Materials made of materials such as E are used, and these are electrically conductively treated with carbon, metal or the like. In this embodiment, carbon is kneaded, and the conductive rayon fiber having a thickness of 6 denier has a density of 100,000 fibers / inch ² and an outer diameter of 6 fibers.
The outer diameter is 18 mm by uniformly bristling the metal shaft 2a with a diameter of 18 mm.
It is a 5 mm brush. In addition, the resistance value of the brush is 3 m in contact width instead of the photoconductor 1 as in the actual use condition.
When the aluminum drum was abutted at m and a voltage of 100 V was applied, the value obtained was 10 ⁵ Ω.

Such a charging brush 2 has a charging bias supply power source 2 for applying a superimposed voltage of a DC voltage and an AC voltage.
7 is electrically connected to the charging bias power supply 2
The surface of the photoconductor 1 is uniformly charged by power supply from 7.

When such a photoconductor 1 is irradiated with an exposure light beam 4 obtained from the exposure optical system 3 by subjecting an image signal to a light intensity modulation or a pulse width modulation by a laser drive circuit (not shown), the photoconductor 1 is irradiated. It is exposed to form a predetermined electrostatic latent image.

The toner 10 which is agitated and conveyed by the toner agitating member 11 whose both ends are rotatably supported by the developing hopper 9 is supplied to the surface of the developing roller 5 which is a toner carrier and left without being developed. Toner 1 on the developing roller 5
To scrape 0, a toner supply roller 6 is installed. The toner supply roller 6 is formed by forming a conductive foam on the outer peripheral surface of a shaft 6a made of metal such as stainless steel as a base material, and has a resistance value of 10 ⁶ Ω · cm. A toner supply bias voltage supply power supply 26, which is a constant voltage power supply, is connected to the shaft 6a of the toner supply roller 6, and the toner supply bias voltage is applied.
In the present embodiment, the nip width between the toner supply roller 6 and the developing roller 5 is 2 mm. In the present embodiment, the developing roller 5 and the toner supply roller 6 both rotate in the counterclockwise directions of arrow B and arrow C.

The developing roller 5 is intended a metal shaft 5a of stainless steel or the like of the single-layer silicon rubber was formed is a conductive elastic member on its outer circumferential surface as a base material, the resistance value, for example 10 ⁶ It is Ω · cm. A developing bias voltage supply power source 12, which is a constant voltage power source, is connected to the shaft 5a of the developing roller 5.

The rubber hardness of the developing roller 5 is preferably in the range of 30 to 60 degrees from the viewpoint of preventing creep and dents, and the surface roughness is such that the smoother the surface, the thinner the toner layer. Can be made uniform in the formation of 1 μm Rz
7 μm Rz is preferred. The developing roller 5 in the present embodiment has a rubber hardness of 60 degrees and a surface roughness of 2 μmR.
It is z.

The toner 10 is a non-magnetic one-component toner, for example, polyester resin with carbon, wax,
The charge control agent and the like are uniformly dispersed.

The developing roller 5 and the toner supplying roller 6 are arranged in contact with each other, and are rotatably supported by shafts 5a and 6a at both ends of the developing hopper 9, respectively. As shown, the developing roller 5 and the photoconductor 1
, And the developing roller 5 and the toner supplying roller 6 are in contact with each other and rotate in the same direction (as indicated by the counterclockwise arrow C). The rotation direction of the photoconductor 1 indicated by the arrow D, which is clockwise with respect to the rotation direction of the developing roller 5 indicated and the rotation direction of the toner supply roller 6 indicated by the arrow B). Therefore, the developing roller 5 and the toner supply roller 6 are in a frictional contact state.

The developing roller 5 is in contact with or close to the photosensitive member 1, and the developing bias voltage power source 12
When the toner 10 is transferred and adhered to the portion of the photoconductor 1 where the electrostatic latent image is formed by the applied developing bias voltage, the electrostatic latent image is visualized.

The toner stirring member 11 draws a circular locus along with the rotation of the toner supply roller 6 to prevent the toner 10 contained in the developing hopper 9 from aggregating and convey the toner 10 to the toner supply roller 6. To do.

A toner regulating blade 7 is provided in contact with the developing roller 5. This toner regulation blade 7
It is composed of a metal spring plate member 7a having elasticity such as a stainless plate or a phosphor bronze plate, and a toner regulating member 7b which contacts the developing roller 5. The toner regulating member 7b is formed by integrally forming an elastic member such as urethane rubber having a rubber hardness of 60 degrees at one end of the metal spring plate member 7a. The toner regulating blade 7 is screwed to the blade holder 8. The toner regulating blade 7 presses the developing roller 5 at a linear pressure of 80 g / cm, and the toner 10 supplied from the toner supplying roller 6 to the developing roller 5 is pressed.
Is triboelectrically charged by the toner regulating blade 7, and the developing roller 5
A thin toner layer is formed on the outer peripheral surface of the. In addition,
In the non-magnetic one-component contact developing method as in this embodiment, the toner layer on the developing roller 5 is 0.4 to 0.6 mg.
The range of / cm ² is preferable.

The photoconductor 1, the developing roller 5, the toner supplying roller 6 and the toner regulating blade 7 are arranged so as to be in constant contact with each other at their respective contact portions.

The paper 15 is stored in the paper cassette 14 provided at the starting point of the transport path. The paper 15 is fed to a paper cassette 14 by a half-moon shaped paper feed roller 16.
Are sent one by one to the transport roller 17. Then, the fed-out recording sheet 15, which is a recording sheet, is nipped by a pair of conveyance rollers 17 and conveyed in the direction of the photoconductor 1 indicated by arrow A.

The sheet 15 is temporarily stopped and waited for the sheet 1
5 and the toner image formed on the photoconductor 1 are made to coincide with each other, the driven roller 1 is provided on the conveyance path before reaching the photoconductor 1.
A registration roller 18 is provided so as to be in contact with 9.
Further, a transfer roller 20 is provided so as to be in contact with the photoconductor 1 and rotatably supported. The transfer roller 20 is formed by forming a conductive foam around a shaft 20a made of metal such as stainless steel, and has a resistance value of 10 ⁷ Ω · cm. A transfer bias voltage supply power supply 24, which is a constant current power supply, is connected to the shaft 20a.

When the toner image reaches the contact portion between the transfer roller 20 and the photoreceptor 1 as the photoreceptor 1 rotates, the registration roller 18 aligns the toner image with the toner image and the paper 15 also contacts the contact portion. To reach. At this time, when a high voltage from the transfer bias voltage supply power supply 24 is applied to the metal shaft 20a of the transfer roller 20 to apply a charge having a polarity opposite to that of the toner 10 to the back surface of the paper 15, the toner image on the photoconductor 1 is formed. It is transferred onto the paper 15.

A fixing device 21 including a heat roller 22 having a heat source inside and a pressure roller 23 for nipping and conveying the paper 15 together with the heat roller 22 is installed at the subsequent stage of the conveying path. Therefore, the paper 15 onto which the toner image has been transferred is sent to the fixing device 21, where the toner image is fixed by pressure and heat by the nip rotation of the heat roller 22 and the pressure roller 23. On the other hand, after the toner image is transferred onto the paper 15, the cleaning blade 25 scrapes off the transfer residual toner, and the static eliminator 13 irradiates the photosensitive drum 1 with light to eliminate the static electricity and prepares for the next process.

The image forming apparatus shown in FIG. 3 is, in addition to the image forming apparatus described with reference to FIG. 1, temperature / humidity detecting means 28 for detecting the temperature and humidity in the image forming apparatus, and the number of printed sheets of paper 15. Printed sheet number detecting means 29, charging potential detecting means 30 for detecting the charging potential of the surface of the photoconductor 1, paper 1
Image density detecting means 31 for detecting the density of the image printed on the image 5, and these detecting means 28, 29, 30, 31
The charging bias voltage for the charging brush 2 by the charging bias supply power supply 27, the developing bias voltage for the developing roller 5 by the developing bias voltage supply power supply 12, and the transfer bias voltage for the transfer roller 20 by the transfer bias voltage supply power supply 24 are changed based on the signal A CPU (control means) 32 is provided to correct the image.

[0052]

EXAMPLES Next, examples of the present invention using the image forming apparatus of FIGS. 1 and 3 described above will be described.

(Embodiment 1) In the electrophotographic apparatus using the brush charging device shown in FIG. 1, the DC voltage Vc of the charging bias voltage and the peak-to-peak voltage Vp of the AC voltage in the charging bias supply power source 27 are changed. Image evaluation was performed.

Here, the frequency f of the AC voltage of the charging bias voltage is 800 Hz, the process speed which is the peripheral speed of the photosensitive member 1 is 124 mm / sec, and the peripheral speed of the charging brush 2 is 1.
61 mm / sec, the peripheral speed of the developing roller 5 is 217 mm
/ Sec, the peripheral speed of the toner supply roller 6 is 10 ⁸ mm /
set to sec.

FIG. 2 shows the image evaluation results when the DC voltage Vc of the charging bias voltage and the peak-to-peak voltage Vp of the AC voltage are changed. In FIG. 2, the horizontal axis represents the peak-to-peak voltage Vp of the AC voltage of the charging bias voltage, and the vertical axis represents the DC voltage Vc of the charging bias voltage. Further, “⊚” indicates an image of good image quality, “Δ” indicates an image in which spotted density unevenness is slightly generated, and “x” indicates an image in which spotted density unevenness is significantly generated over the entire image. “O” indicates an image in which the brush-like density unevenness of the brush is slightly generated, and “□” indicates an image in which the brush-like density unevenness of the brush is significantly generated over the entire surface of the image.

As is apparent from FIG. 2, when the peak-to-peak voltage Vp of the AC voltage exceeds twice the DC voltage Vc, spot-like density unevenness occurs in the image. When the peak-to-peak voltage Vp of the AC voltage exceeds twice the DC voltage Vc, the voltage applied to the charging brush 2 instantaneously becomes positive, and at this time, the surface of the negative charging type photoreceptor 1 becomes positive. Be charged. As a result, it is considered that the charges are not uniformly transferred between the photoconductor 1 and the charging brush 2 due to the AC voltage, and the charging potential is locally nonuniform, so that spot-like density unevenness occurs.

Further, the brush-like density unevenness of the brush is caused by the peak-to-peak voltage Vp of the AC voltage regardless of the value of the DC voltage Vc.
Occurs at 1.0 kV or less. This is because when the peak-to-peak voltage Vp of the AC voltage is 1.0 kV or less, the potential difference between the photoconductor 1 and the charging brush 2 does not satisfy the discharge start voltage according to Paschen's discharge theory, and the electric charge between the two members alternates. It is thought that this is not done.

As described above, the superimposed voltage of the DC voltage and the AC voltage is applied to the charging brush 2 arranged in contact with the photoconductor 1, the DC voltage is Vc (V), and the peak voltage of the AC voltage is Vp (V). Then, by having the relationship of Vp <2 × | Vc |, it is possible to prevent the spotted density unevenness of the image.

(Embodiment 2) In the image forming apparatus, it is general that the image density and the gradation are largely changed due to a change with time or an environmental change.

Therefore, as shown in FIG. 3, the temperature / humidity detecting means 28, the number of printed sheets detecting means 29, and the charging potential detecting means 3 are detected.
0, the charging bias voltage, the developing bias voltage, and the transfer bias voltage can be changed based on the signal from the image density detecting means 31. That is, in the brush charging device, the charging potential of the photoconductor 1 can be corrected and the image density can be corrected by changing the DC voltage Vc.

However, with respect to the change over time, for example, due to the influence of the change in the resistance value of the charging brush 2 due to the adhesion of foreign matter and the decrease in the film thickness of the photosensitive layer of the photoconductor 1 due to abrasion, the uniform charging state is obtained in the initial state. This is because even with the obtained value of the peak-to-peak voltage Vp, charging unevenness may occur over time.

Here, a charging bias voltage was controlled by the charging potential detecting means 30 so that the charging potential of the photoconductor 1 was always constant, and a durability test of 20,000 sheets was conducted. The frequency f of the AC voltage and the peripheral speed of each member are the same as in the first embodiment.

The evaluation results are shown in (Table 1) and (Table 2) below.
Shown in. (Table 1) shows that the peak-to-peak voltage Vp is constant and only the DC voltage Vc is changed, and (Table 2) shows the peak-to-peak voltage Vp.
And the evaluation results when the DC voltage Vc is changed. In (Table 1) and (Table 2), “⊚” indicates an image of good image quality, “∘” indicates an image in which brush uneven density is slightly generated, and “□” indicates brush uneven density. An image in which unevenness is remarkable is shown.

[0064]

[Table 1]

[0065]

[Table 2]

As is clear from (Table 1) and (Table 2), when the value of the DC voltage Vc changes by changing the value of the peak-to-peak voltage of the AC voltage Vp according to the change of the DC voltage Vc. However, it is possible to prevent the occurrence of brush-like density unevenness of the brush.

Further, in the present embodiment, the control for the fluctuation of the charging potential of the photosensitive member 1 is performed, but in the case of the control for the fluctuation of the charging potential, the DC voltage Vc
The peak-to-peak voltage Vp is reduced to the DC voltage V
It is possible to exceed 2 times c. Even in this case,
By changing the value of the peak-to-peak voltage Vp according to the change of the DC voltage Vc, it is possible to prevent the spot-like density unevenness.

In Table 2, a value obtained by subtracting the constant 100 from twice the DC voltage Vc is defined as the AC voltage Vp. In this way, if the relationship of Vp = 2 × | Vc | −T (T is a constant) is satisfied, it is possible to easily prevent sweep density unevenness and spot-like density unevenness of the brush.

If the value of the constant T is too large, the discharge start voltage of the Paschen discharge theory cannot be satisfied.
200 or less is preferable. Further, when the constant T is a negative value, the AC voltage Vp exceeds twice the DC voltage Vc, so that 0 ≦ T ≦
It suffices if there are 200 relationships.

(Embodiment 3) Next, in the image forming apparatus shown in FIG. 1, the frequency f of the AC voltage of the charging bias voltage is
The image evaluation was performed by changing the peripheral speed of the charging brush 2.

Here, the contact width between the photoconductor 1 and the charging brush 2 is 3 mm, and the DC voltage Vc of the charging bias voltage is -60.
The peak-to-peak voltage Vp of 0 V and the AC voltage was set to 1.1 kV. The peripheral speed of the photoconductor 1, the peripheral speed of the developing roller 5, and the peripheral speed of the toner supply roller 6 are the same as those in the first embodiment.

The evaluation results are shown in (Table 3) below. In Table 3, “⊚” indicates an image with good image quality, “Δ” indicates an image with a slight periodic density unevenness, and “x” indicates an image with a remarkable periodic density unevenness. .

[0073]

[Table 3]

From the results of (Table 3), when the peripheral speed of the charging brush 2 is 161 mm / sec, the periodical density unevenness becomes inconspicuous at f = 95 Hz and does not occur at f = 100 Hz. Further, the peripheral speed of the charging brush 2 is 2
In the case of 51 mm / sec, periodical density unevenness became inconspicuous at f = 125 Hz and did not occur at f = 130 Hz. In this way, the photoconductor 1 and the charging brush 2
Since the appropriate value of the frequency f of the charging bias voltage differs depending on the difference in the relative speed v at the contact portion with, the following facts have been revealed.

That is, if the cycle of the AC voltage is 1 or more while the photoconductor 1 passes through the contact width 1 with the charging brush 2,
Cyclic charging unevenness occurs on the surface of the photoconductor 1 and density unevenness occurs. Therefore, in order to prevent uneven density, v / l <
It is necessary to satisfy the relationship of f. In the present embodiment, the contact width between the photoconductor 1 and the charging brush 2 is 3 mm.
= 85 mm / sec, the proper value of frequency f is 95
It becomes more than Hz.

As described above, the frequency of the charging bias voltage is f (Hz), the relative speed at the contact portion between the photoconductor 1 and the charging brush 2 is v (mm / sec), and the nip between the developing roller and the toner supply roller is If the width is 1 (mm), f>
By having the relationship of v / l, it is possible to prevent periodical density unevenness due to the frequency of the AC voltage.

[0077]

As described above, according to the present invention, since the voltage having the opposite polarity to the charging polarity of the electrostatic latent image carrier is not applied to the brush, the peak-to-peak voltage of the applied AC voltage is eliminated. It is possible to obtain an effective effect that it is possible to prevent the occurrence of spotted density unevenness in the image when the value is raised.

If the value of the peak-to-peak voltage of the AC voltage is changed according to the change of the DC voltage applied to the charging brush, the peak-to-peak voltage of the AC voltage can be always set to a large value. It is possible to obtain an effective effect that it is possible to prevent the occurrence of uneven density density.

If the AC voltage is applied to the conductive brush for one cycle or more while the electrostatic latent image bearing member passes through the contact portion with the brush, the surface of the electrostatic latent image bearing member caused by the AC voltage is applied. The effective effect that it becomes possible to prevent the periodic charging unevenness is obtained.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration of an image forming apparatus using a brush charging device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing image evaluation results when the peak-to-peak voltage of the charging bias voltage and the AC voltage of the charging bias supply power source in the image forming apparatus of FIG. 1 is changed.

FIG. 3 is a schematic diagram showing a configuration of an image forming apparatus using a brush charging device according to another embodiment of the present invention.

FIG. 4 is a schematic view showing an image forming apparatus having a conventional brush charging device.

[Explanation of symbols]

1 Photoconductor (electrostatic latent image carrier) 2 charging brush 2a shaft 28 Temperature / humidity detection means 29 printed sheet number detection means 30 Charge potential detecting means 31 image density detection means 32 CPU (control means)

Claims (3)

(57) [Claims] 1. A charging unit which is arranged in contact with the surface of the electrostatic latent image carrier to rotate and charges the surface of the electrostatic latent image carrier to a predetermined potential, and electrically connected to the charging unit. A charging bias supply power source for applying a superimposed voltage composed of a DC voltage and an AC voltage to the charging means, and a DC voltage Vc (V) and a peak-to-peak voltage Vp (V) of the AC voltage applied to the charging means. Between Vp <2 × | Vc
And a control unit that controls the charging bias supply power source so as to change the value of the peak-to-peak voltage of the AC voltage according to the change of the DC voltage while maintaining the relationship of |. 2. A DC voltage applied to the charging means is Vc
(V), peak-to-peak voltage of AC voltage is Vp (V), constant T
Then, the charging device according to claim 1, having a relationship of Vp = 2 × | Vc | −T. 3. The charging device according to claim 2, wherein the constant T is 0 ≦ T ≦ 200.