JPH06230647A - Electrostatic charging device - Google Patents

Abstract

PURPOSE:To provide an electrostatic charging device in which magnetic particles are prevented from adhering to a member to be electrostatically charged and the dielectric breakdown of the member to be electrostatically charged is prevented from being caused and which can perform extremely stable and uniform electrostatic charging. CONSTITUTION:By a non-magnetic conductive electrostatic charging sleeve 22 rotatably disposed on the outer periphery of a magnetic body 23 having an outer periphery where magnetic poles are arranged and fixed so that horizontal magnetic field may be formed in an electrostatic charging area, and a magnetic brush consisting of the layer of the magnetic particles 21 adhering to the outer periphery of the sleeve 22, AC bias electric field is formed between the sleeve 22 and the photosensitive drum 10 so as to electrostatically charge the photosensitive drum 10 in this electrostatic charging device 20. A leveling member 29 is provided at an upstream part of an electrostatic charging area so that one surface of the leading edge and the other surface may come in contact with the photosensitive drum 10 and the magnetic brush, respectively, thereby, the magnetic brush is made stably and accurately to oppose to the moving photosensitive drum 10 without coming into contact with it.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device for an image forming apparatus utilizing an electrostatic transfer process such as an electrophotographic copying machine and an electrostatic recording device.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, a corona charger has generally been used to charge an image forming body which is a member to be charged such as a photosensitive drum. This corona charger applies a high voltage to the discharge wire to generate a strong electric field around the discharge wire to perform gas discharge, and the charged ions generated at that time are adsorbed to the image forming body to charge. Done.

Since the corona charger used in such a conventional image forming apparatus can be charged without mechanical contact with the image forming body, it is said that the image forming body is not damaged during charging. Have advantages. However, since this corona charger uses a high voltage, there is a risk of electric shock or leakage, and ozone generated by gas discharge is harmful to humans, which shortens the life of the image forming body. Had. Further, the charging potential of the corona charger is unstable because it is strongly affected by temperature and humidity. Further, the corona charger generates noise due to high voltage, which causes electrophotographic image formation as a communication terminal or an information processing device. This is a major drawback when using the device.

Many drawbacks of such corona chargers are:
The cause is that gas discharge is involved in charging.

Therefore, a magnet body is included as a charging device capable of charging a charged member without causing a high-pressure gas discharge like a corona charger and without mechanically damaging the charged member. A charging device in which magnetic particles are adsorbed on a cylindrical carrier formed as described above to form a magnetic brush and the surface of the member to be charged is rubbed by the magnetic brush to perform charging is disclosed in Japanese Patent Laid-Open No. 59-133569. No. 4-21873
And JP-A-4-116674.

[0006]

However, even the charging device disclosed in the above publication has a problem in that the member to be charged cannot be charged completely stably and uniformly. That is, in the charging area, the magnetic particles on the surface of the cylindrical magnetic particle carrying carrier are chained along the lines of magnetic force, and charging is performed through the chains. For this reason, there is a problem that magnetic particles adhere to the member to be charged, or the charge is locally excessive, resulting in dielectric breakdown of the member to be charged and uneven charging.

The present invention solves these problems, and provides a charging device capable of performing extremely stable and uniform charging without adhesion of magnetic particles to a member to be charged or dielectric breakdown of the member to be charged. The purpose is to do.

[0008]

The above object is to provide a charging device for forming a magnetic brush on a member to be charged and charging the member under an AC bias, wherein the magnetic brush has a minute gap with the member to be charged. It is achieved by a charging device characterized by facing each other.

The magnetic brush faces the member to be charged under a horizontal magnetic field and is in contact with the leveling member,
The charging device is characterized in that the leveling member is in contact with the member to be charged.

[0010]

In the present invention, since the magnetic brush of the charging device is opposed to the member to be charged with a minute gap,
The member to be charged can be charged with a bias voltage lower than that of the conventional corona discharge.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 is a sectional view showing the outline of the construction of an image forming apparatus using the charging device of the present invention. In the figure, 10
Is a photosensitive drum made of (-) charged OPC which is a member to be charged which rotates in the direction of the arrow (clockwise), and a peripheral portion of which is a charging device 20 and an exposure device in which image light L from an exposure device is incident. Part 12, developing device 30, transfer roller 13, cleaning device 50
Etc. are provided.

The basic operation of the copy process of this embodiment is as follows.
When a copy start command is sent from an operation unit (not shown) to a control unit (not shown), the control unit controls the photosensitive drum.
10 starts rotating in the direction of the arrow. As the photosensitive drum 10 rotates, the peripheral surface of the photosensitive drum 10 is uniformly charged by a charging device 20 described later and passes through. An image is written on the photoconductor drum 10 by the image writing device such as a laser beam in the exposure unit 12 to form an electrostatic latent image corresponding to the image.

Inside the developing device 30, there is a two-component developer consisting of magnetic particles and toner, which is common to that used in the charging device 20, and is agitated by agitating screws 33A and 33B, and then outside the magnet roller 32. A magnetic brush of developer is formed by adhering to the outer periphery of the rotating developing sleeve 31, and a predetermined AC bias voltage is applied to the developing sleeve 31,
Non-contact or contact reversal development is performed in the development area facing the photoconductor drum 10 to form a toner image.

From the paper feed cassette 40, the recording papers P are fed one by one by a first paper feed roller 41. The fed recording paper P is sent onto the photosensitive drum 10 by the second paper feed roller 42 which operates in synchronization with the toner image on the photosensitive drum 10. Then, by the action of the transfer roller 13, the toner image on the photoconductor drum 10 is transferred onto the recording paper P and separated from the photoconductor drum 10. The recording paper P on which the toner image has been transferred is sent to a fixing device (not shown) via the conveying means 80, is sandwiched by a heat fixing roller and a pressure bonding roller, is fused and fixed, and is then discharged to the outside of the apparatus. Recording paper P
The surface of the photoconductor drum 10 that has toner remaining without being transferred to the surface of the photoconductor drum 10 is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and stands by for the next image recording.

Before explaining the charging device 20 of the embodiment of the present invention, the particle size of the magnetic particles and the conditions of the carrier will be described.

Generally, when the average particle size (average weight) of the magnetic particles is large, (a) since the state of the magnetic brush formed on the carrier is rough, even when charged while vibrating by the electric field, The magnetic brush is likely to have unevenness, which causes a problem of uneven charging. In order to solve this problem, the average particle size of the magnetic particles should be reduced.
The effect begins to appear below, especially when it becomes 100 μm or less,
It was found that the problem (a) would not substantially occur.
However, if the particles are too fine, they tend to adhere to the surface of the member to be charged during charging, or are likely to scatter. These phenomena are related to the strength of the magnetic field acting on the particles and the strength of the magnetization of the particles thereby, but generally, the average particle diameter of the particles becomes prominent at 30 μm or less. It is preferable that the magnetization intensity is 20 to 200 emu / g.

From the above, the average particle diameter (average weight) of the magnetic particles is 150 μm or less, particularly preferably 100 μm or less 30 μm.
It is preferably m or more.

Such magnetic particles are the same as those used in the magnetic carrier particles of the conventional two-component developer as a magnetic material.
Ferromagnetism such as metals such as iron, chromium, nickel and cobalt, or their compounds and alloys such as ferric tetroxide, γ-ferric oxide, chromium dioxide, manganese oxide, ferrite and manganese-copper alloys. Body particles or the surface of these magnetic particles is coated with a resin such as styrene resin, vinyl resin, ethylene resin, rosin modified resin, acrylic resin, polyamide resin, epoxy resin, polyester resin, or Particles obtained by making a resin containing magnetic fine particles dispersed therein can be obtained by selecting the particle size by a conventionally known average particle size selecting means.

The spherical shape of the magnetic particles means that
The particle layer formed on the carrier is made uniform, and a high bias voltage can be uniformly applied to the carrier. That is, the fact that the magnetic particles are spherical means that (1) generally, the magnetic particles are easily magnetized and adsorbed in the long axis direction, but due to the spherical shape, the directionality is lost, so that a layer is uniformly formed and local layers are formed. (2) Higher resistance of the magnetic particles is eliminated, and the edge portions seen in conventional particles are eliminated, and electric field concentration on the edge portions is prevented. As a result, even if a high bias voltage is applied to the magnetic particle carrying carrier, it is possible to uniformly discharge the surface of the member to be charged and prevent uneven charging.

The spherical particles having the above effects have a magnetic particle resistivity of 10 ³ Ω · cm or more and 10 ¹² Ω · cm or less, and particularly 10 ⁴
It is preferable that the conductive magnetic particles are formed so as to be Ω · cm or more and 10 ⁹ Ω · cm or less. This resistivity is 0.50 for particles
After tapping in a container with a cross-sectional area of cm ² ,
A value obtained by applying a load of 1 kg / cm ² on the packed particles and reading the current value when a voltage that generates an electric field of 1,000 V / cm is applied between the load and the bottom electrode. When the resistivity is low, when a bias voltage is applied to the carrier, electric charges are injected into the magnetic particles and the magnetic particles tend to adhere to the surface of the charged member, or the dielectric breakdown of the charged member due to the bias voltage. May occur easily.
If the resistivity is high, charge injection is not performed and charging is not performed.

Further, the magnetic particles used in the present invention are
It is desirable that the magnetic brush constituted by it has a small specific gravity and a suitable maximum magnetization so that the magnetic brush moves lightly due to an oscillating electric field and does not cause external scattering. Specifically, it was found that good results can be obtained when the true specific gravity is 6 or less and the maximum magnetization is 30 to 100 emu / g, particularly 40 to 80 emu / g.

In summary, the magnetic particles are spherical so that at least the ratio of the major axis to the minor axis is 3 times or less, and there are no protrusions such as needle-shaped portions and edge portions, and the resistivity is The appropriate condition is preferably 10 ⁴ Ω · cm or more and 10 ⁹ Ω · cm or less. Then, such spherical magnetic particles should be selected as spherical as possible for the magnetic particles, and in the particles of the magnetic particle dispersion system, the particles of the magnetic material should be used as much as possible.
After the dispersed resin particles are formed, a spheroidizing treatment is performed, or the dispersed resin particles are formed by a spray drying method.

When toner is mixed in the magnetic brush,
Since the toner has a high insulating property, the charging property is lowered and uneven charging occurs. In order to prevent this, it is necessary to lower the charge amount of the toner so that the toner moves to the member to be charged during charging. Under the condition that the toner is mixed with magnetic particles and the toner concentration is adjusted to 1% by weight. When the triboelectrification amount of the toner was the same and the charging polarity was 1 to 20 μC / g, it was possible to prevent the toner from accumulating on the magnetic brush. It is considered that this is because even if the toner is mixed, it adheres to the photoconductor during charging. It was confirmed that when the charge amount of the toner is large, it becomes difficult to separate from the magnetic particles, and when it is small, it becomes difficult to electrically move to the member to be charged.

The above are the conditions for the magnetic particles. Next, the conditions for the carrier for the magnetic particles for forming the particle layer and charging the member to be charged will be described.

A conductive carrier for applying a bias voltage is used as the carrier for the magnetic particles, and in particular, a magnet having a plurality of magnetic poles inside a conductive charging sleeve on the surface of which a particle layer is formed. A structure having a body is preferably used. In such a carrier,
The relative rotation with the magnet body causes the particle layer formed on the surface of the conductive charging sleeve to undulate and move in a wave shape, so that new magnetic particles are supplied one after another, and the particle layer on the carrier surface is supplied. Even if there is some non-uniformity in the layer thickness, the effect is sufficiently covered by the above-mentioned wavy undulation so as not to be a practical problem. The surface of the carrier is preferable to have an average surface roughness of 2 to 15 μm for stable and uniform transfer of the magnetic particles. A current flows, and in either case uneven charging is likely to occur. Sandblasting is preferably used to achieve the above surface roughness. The diameter of the carrier is preferably 5 to 20 mm. With the above diameter, a charging area required for charging is secured. If the charging area is unnecessarily large, the charging current will be excessive, and if it is small, uneven charging is likely to occur. When the diameter is small as described above, magnetic particles are easily scattered or adhered to the member to be charged due to centrifugal force. Therefore, the linear velocity of the carrier is preferably slow within the following range. The transport speed of the magnetic particles due to the rotation of the transport carrier may be almost the same as or faster than the moving speed of the member to be charged, but it is preferable that the transport speed is slow because the scattering of the magnetic particles is reduced. In addition, it is preferable that the transporting carrier is rotated in the same direction. Uniformity of charging is better in the same direction than in the opposite direction. However, it is not limited thereto.

Further, the thickness of the particle layer formed on the carrier is preferably such that it is sufficiently scraped off by the regulating means to form a uniform layer. If there are too many magnetic particles on the surface of the carrier in the charging region, the vibration of the magnetic particles will not be sufficiently performed, causing wear and uneven charging of the photoconductor, and overcurrent will easily flow, resulting in a large drive torque of the carrier. There is a drawback that On the other hand, if the amount of the magnetic particles present on the carrier in the charged area is too small, an incomplete portion is formed in contact with the member to be charged, resulting in adhesion of the magnetic particles to the member to be charged and uneven charging. As a result of repeated experiments, the preferable abundance W of magnetic particles in the charged region is
Was 10-300 mg / cm ^2, particularly preferably found to be 30~150mg / cm ^2. The existing amount is an average value in the charged area of the magnetic brush.

The gap D between the carrier and the member to be charged is
Is preferably 100 to 5,000 μm. If the surface gap D between the carrier and the member to be charged becomes too narrower than 100 μm, it becomes difficult to form the magnetic brush ears that uniformly charge the carrier, and sufficient magnetic particles are supplied to the charging area. If the gap D exceeds 5,000 μm, the particle layer is coarsely formed and uneven charging is likely to occur and sufficient charging cannot be obtained. . As described above, when the gap D between the carrier and the member to be charged becomes extreme, the thickness of the particle layer on the carrier cannot be adjusted appropriately, but when the gap D is in the range of 100 to 5,000 μm. On the contrary, the thickness of the particle layer can be appropriately formed, and the ears of the magnetic brush are also formed uniformly. Further, it has been clarified that the most preferable condition exists between the more appropriate transport amount (W) and the gap (D).

To perform charging uniformly and at high speed and stably
The condition of 300 ≤ W / D ≤ 3,000 (mg / cm ³ ) was important. It was confirmed that when W / D is out of this range, charging becomes non-uniform.

D is considered to be a factor that determines the chain length of magnetic particles. It is considered that the electric resistance corresponding to the chain length corresponds to the ease of charging and the charging speed. On the other hand, W is considered to be a factor that determines the density of chains of magnetic particles. It is believed that increasing the number of chains improves the charging uniformity. However, it is considered that when the magnetic particles pass through the narrow gap in the charging region, the compressed state of the chains of the magnetic particles is realized. At this time, the chains of magnetic particles contact each other,
In a bent state, it faces the member to be charged while being disturbed.

It is considered that this disturbing condition is effective for uniform charge by facilitating the transfer of charge without causing charge stripes. That is, when the W / D corresponding to the magnetic particle density is small, the chains of the magnetic particles become coarse and the ratio of disturbance is small, resulting in non-uniform charging. When W / D becomes large,
The chains of the magnetic particles are not well formed due to the high packing and the magnetic particles are less disturbed. It is considered that this hinders the free movement of the charges and prevents uniform charging.

FIG. 1 is a sectional view showing an embodiment of the charging device (charging device 20 used in the image forming apparatus of FIG. 2) of the present invention. In the figure, 21 is a magnetic particle, 22 is a magnetic particle made of a non-magnetic and conductive metal such as aluminum.
21 is a charging sleeve which is a carrier, 23 is a charging sleeve 22
2 is a columnar magnet body that is fixedly arranged in the inside of the charging body. The magnet body 23 is formed on the periphery of the surface of the charging sleeve 22 as shown in FIG.
It is magnetized by arranging the S pole and the N pole so that it becomes 500 to 1,000 Gauss. The charging sleeve 22 has a diameter of 5 to 30 mm and is rotatable with respect to the magnet body 23.
It is held in a gap of 0.5 to 1.0 mm at a position facing 10 and rotated in the same direction as the moving direction of the photosensitive drum 10 at a peripheral speed of 1.2 to 2.0 times.

The positions of the two different magnetic poles of the magnet body 23 closest to the photoconductor drum 10 are the closest positions of the charging sleeve 22 and the photoconductor drum 10, that is, the photoconductor drum 10.
The angles θ1 and θ2, which are on both sides of the center line connecting the center of the charging sleeve 22 and the center of the charging sleeve 22 and the center line of the straight line connecting the center of the charging sleeve 22 and the magnetic poles, are 5 ° to 45 °, respectively, as a result of the experiment. It has been found that good results are obtained in the range of °, preferably 30 °. It is not necessary to limit the polarity of the two different magnetic poles to either N or S. The angles θ1 and θ2 are more preferably θ1 ≧ θ2. By doing so, charging can be performed in a state where a strong magnetic field is formed in the downstream portion, and this is a preferable configuration for preventing the adhesion of magnetic particles.

As a result, the direction of the magnetic lines of force in the charging region is parallel to the tangential direction of the photosensitive drum 10. This magnetic field is called a horizontal magnetic field.

Reference numeral 25 denotes a casing which is a container for forming a storage portion for the magnetic particles 21, and the casing 25 is made of an insulating material, and the charging sleeve 22 is provided inside the casing 25.
And the magnet body 23 are arranged. 26 is a casing 25
A regulating plate which is a means for regulating the amount of the magnetic particles 21 attached to and conveyed by the charging sleeve 22 provided at the outlet of the rotating body, and 27 is a rotating body having a plate-like member for correcting the bias of the magnetic particles 21 around the shaft. A stirring member composed of 28, magnetic particles from the charging sleeve 22
The magnetic particles 21 are constantly stirred and mixed by the scraping member 28 and the stirring member 27 by the scraping member that scrapes off the magnetic particles 21 and are always kept in a uniform state. The regulation plate 26 and the charging sleeve
A gap between the magnetic particles 21 and the gap 22 is, that is, the amount of the magnetic particles 21 present on the charging sleeve 22 in the charging region is 10 to 30.
0 mg / cm ^2, particularly preferably adjusted to be 30~150mg / cm ^2. Reference numeral 29 is a leveling member that is provided upstream of the charging region so as to press the layer of magnetic particles 21 toward the charging sleeve 22, and the tip thereof is in contact with the peripheral surface of the photosensitive drum 10. And the surface of the charging sleeve 22 are not scratched and have excellent wear resistance, and even if a high bias voltage is applied to the charging sleeve 22, a thickness that does not cause dielectric breakdown of the photosensitive drum 10 due to the bias voltage. An insulating elastic material having a thickness of about 50 to 300 μm, for example, one made of urethane rubber used as the cleaning blade 51 or a mylar thin plate is preferably used. However, the material is not limited to one made of an elastic material such as rubber, but may be made of a material having higher rigidity. One surface of the tip end of the leveling member 29 contacts the surface of the photoconductor drum 10, and the other surface of the tip end contacts the magnetic brush composed of the magnetic particles 21, and the tip end, the charging sleeve 22, and the photoconductor drum 10 are connected. The angle from the closest position (hereinafter referred to as the closest position), that is, the straight line connecting the tip of the leveling member 29 and the center of the charging sleeve 22 and the center connecting the center of the photosensitive drum 10 and the center of the charging sleeve 22. The angle β with the line is smaller than θ1 and θ1>β> 0.
Is preferred. Moreover, the tip of the leveling member 29 is preferably 1 to 3 mm from the closest position. This leveling member
29 will limit the charged area to 4-5 mm.
By the leveling member 29, the layer of the magnetic particles 21 is further leveled immediately before the charging area, and the unevenness of stripes that is likely to occur due to clogging of the regulation plate 26 is eliminated, and a uniform thin layer is conveyed to the charging area. The magnetic brush of the magnetic particles 21 has its ears suppressed by the leveling member 29 and is formed tangentially on the peripheral surface of the photoconductor drum 10 by the horizontal magnetic field, while the gap D between the photoconductor drum 10 and the charging sleeve 22 is Within the above range, the layer of the magnetic brush is 20 from the peripheral surface of the photosensitive drum 10 from its tip.
It is kept contactless at ~ 300 μm apart.

If the gap is wide, it becomes difficult to charge at a low voltage, and if it is too close, adhesion of magnetic particles and non-uniformity of charging due to overcurrent become remarkable.
~ 300 μm.

The photosensitive drum 10 comprises a conductive base material 10b and a photosensitive body layer 10a covering the surface thereof, and the conductive base material 10b is grounded.

Reference numeral 24 is a bias power source for applying a bias voltage between the charging sleeve 22 and the conductive base material 10b, and the charging sleeve 22 is grounded via the bias power source 24.

The bias power source 24 is a power source for supplying an AC bias voltage in which an AC component is superposed on a DC component set to the same value as the voltage to be charged, and a gap D between the charging sleeve 22 and the photoconductor drum 10. , The gap D is 0.1 to 5 m, although it depends on the size of the photosensitive drum 10 and the charging voltage for charging the photosensitive drum 10.
-500V, which is almost the same as the voltage to be charged when held at m
Voltage between peak value (V _PP ) 400 for DC component of -1,000V
By supplying an AC bias voltage in which an AC component of ˜7,000 V and a frequency of 0.3 to 10 KHz is superimposed to the charging sleeve 22 through the protective resistor R, a preferable charging condition could be obtained. The bias power source 24 performs constant voltage control for the DC component and constant current control for the AC component.

Next, the operation of the charging device 20 described above will be described.

When the charging sleeve 22 is rotated in the same direction as indicated by the arrow while rotating the photosensitive drum 10 at a peripheral speed of 0.2 to 2.0 times, preferably 0.3 to 0.7 times the peripheral speed of the photosensitive drum 10. , Magnetic particles attached to and transported by the charging sleeve 22
The layer 21 is magnetically connected in a chain shape at a position facing the photosensitive drum 10 on the charging sleeve 22 by the magnetic force lines of the magnet body 23 to form a kind of brush shape, so that a so-called magnetic brush is formed. The magnetic brush is conveyed in the rotating direction of the charging sleeve 22, reaches the charging area, and becomes parallel to the peripheral surface of the photosensitive drum 10 by the horizontal magnetic field, and the magnetic brush layer is separated from the photosensitive layer 10a by 20 to 300 .mu.m as described above. Are facing each other. Since the AC bias voltage is applied between the charging sleeve 22 and the photoconductor drum 10, the gas is discharged in the minute gap between the conductive magnetic particles 21 and the photoconductor layer 10a so that the charging is performed. Is done. In this case, in particular, forming an oscillating electric field by applying an AC bias voltage,
The magnetic poles of the magnet body 23 are arranged so as to form a horizontal magnetic field in the charging region, and the leveling member 29 is provided in the upstream portion of the closest position between the charging sleeve 22 and the photosensitive drum 10, so that the magnetic particles 21 are chained. Due to the horizontal magnetic field, the connected magnetic brushes are parallel to the tangential direction of the peripheral surface of the photoconductor drum 10 and can be stably and accurately provided with a minute gap so as to be in non-contact, and the magnetic particles adhere to the photoconductor drum 10. It has become possible to prevent charging, and to perform extremely stable high speed and uniform charging with low bias voltage.

The results of changing the frequency and voltage of the AC voltage component applied to the charging sleeve 22 in the above embodiment are shown in FIG.

In FIG. 3, the range shaded with vertical lines is the range where dielectric breakdown is likely to occur, the range shaded with diagonal lines is the range where uneven charging is likely to occur, and the range not shaded is stable. This is a preferable range in which electrostatic charging is obtained. As is clear from the figure, the preferable range changes slightly depending on the change of the AC voltage component. The waveform of the AC voltage component is not limited to a sine wave, and may be a rectangular wave or a triangular wave. Further, in FIG. 3, the low frequency region shaded with dots is a range where uneven charging occurs due to the low frequency.

As the magnetic particles 21 in the above-mentioned embodiment, spherical ferrite particles coated so as to have conductivity are used. In addition, conductive magnetic resin particles obtained by pulverizing the magnetic particles and a resin as main components after thermal smelting can also be used. In order to perform good charging, the outer shape is adjusted to a spherical shape with a particle size of 50 μm and a specific resistance of 10 ⁸ Ω · cm. The frictional charge amount with the toner is -5 μC / toner concentration 1% by weight.
It is g.

It is also possible to use the charging device 20 of this embodiment to eliminate the charge on the photosensitive drum 10. The static elimination can be performed by setting only the DC component of the bias voltage to zero. After the image formation, only the AC component is applied to rotate the member to be charged, so that the photosensitive drum 10 is discharged. When the charge removal of the photoconductor drum 10 is finished, the application of the AC component is also stopped.

In the image forming method in which the charging device 20 is used as a cleaning device, reversal development is preferable to regular development in developing. This is because even if toner is discharged from the charging device 20 at the time of charging, the discharged toner has the same polarity during reversal development and is collected by the developing bias in the developing section, thus preventing image fogging. .

Incidentally, the magnetic particles 21 of the toner remaining on the surface of the photosensitive drum 10 without being cleaned due to long-term use.
In many cases, the magnetic brush 21A is highly mixed with the layer and the resistance of the magnetic brush 21A is increased, so that the charging efficiency is deteriorated. To this end, the polarity of the DC bias voltage applied to the charging sleeve 22 at the time of rotation of the photosensitive drum 10 before or after image formation is set high,
Alternatively, it is possible to prevent the toner from mixing by setting a high AC voltage and setting a condition that the toner is likely to adhere to the photosensitive drum 10. In particular, when the photosensitive drum 10 has the same polarity as the toner, such as in an image forming apparatus that performs reversal development, the toner polarity in the developing device 30 is the same as that of the toner. It does not appear as a fog in the image and is a very suitable combination.

[0048]

According to the present invention, the magnetic brush is opposed to the member to be charged in a non-contact manner, and the charging method by gas discharge between the minute gaps is adopted. Therefore, compared with the conventional charging method by corona discharge. It can be charged with a very low bias voltage, there is no adhesion of magnetic particles to the member to be charged, the occurrence of dielectric breakdown of the member to be charged is prevented, the ozone generation is extremely stable, high speed and uniform A charging device capable of charging can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an embodiment of a charging device of the present invention.

FIG. 2 is a sectional view showing the outline of the configuration of an image forming apparatus using the charging device of the present invention.

FIG. 3 is a charging characteristic diagram when a frequency and a voltage of an AC voltage component are changed.

[Explanation of symbols]

 10 Photosensitive drum (charged member) 20 Charging device 21 Magnetic particles 22 Charging sleeve (transport carrier) 23 Magnet body 24 Bias power supply 25 Casing 26 Regulation plate 29 Leveling member R Protective resistance

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masakazu Fukuchi 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (72) Inventor Shizuo Morita 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (72) Invention Noriyuki Hiroshi Nomori 2970 Ishikawa-cho, Hachioji City, Tokyo Konica Stock Company

Claims (4)

[Claims] 1. A charging device for forming a magnetic brush on a member to be charged and charging the member under an AC bias, wherein the magnetic brush is opposed to the member to be charged with a minute gap. apparatus. 2. The charging device according to claim 1, wherein the magnetic brush faces the member to be charged under a horizontal magnetic field. 3. The charging device according to claim 1, wherein the magnetic brush is in contact with a leveling member. 4. The charging device according to claim 1, wherein the leveling member is in contact with the member to be charged.