JPH06180523A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device capable of extremely steady, uniform electrification free of production of ozone and dielectric breakdown of an image forming body. CONSTITUTION:The image forming device is provided with: a nonmagnetic, conductive electrifying roller 22 disposed rotatably around the periphery of a fixed magnet body 23 with magnetic poles arranged along the periphery; and an electrifier 20 which electrifies the photosensitive drum 10 by bringing a magnetic brush, composed of a layer of magnetic particles 21 sticking to the periphery of the electrifying roller 22, into contact with the moving photosensitive drum 10, and by generating a bias electric field between the electrifying roller 22 and the photosensitive drum 10. One of the magnetic poles of the magnet body 23 is positioned opposite a control plate 26. An AC bias voltage is applied from a bias power source 24 to the control plate 26, as an application electrode, which is set so that a gap DB between the front end of the control plate 26 and the electrifying roller 22 is smaller than a gap DP between the electrifying roller 22 and the photosensitive drum 10. Thereby, the extremely steady, uniform electrification free of electrification irregularities can be performed via the magnetic brush 21A on the electrifying roller 22.

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, a corona charger has generally been used for charging an image forming body 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 the image forming body without causing high-pressure gas discharge like a corona charger and without mechanically damaging the image forming body. 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 an image forming body is rubbed with the magnetic brush to perform charging is disclosed in Japanese Patent Laid-Open No. 59-133569. Japanese Patent Laid-Open No. 4-21873 and Japanese Patent Laid-Open No. 4-116674.

[0006]

However, even the charging device disclosed in the above publication has a problem that the image forming body cannot be charged completely stably and uniformly. That is, the magnetic particles on the surface of the cylindrical magnetic particle carrier have become a magnetic brush composed of a large number of chain-shaped ears along the lines of magnetic force, and the magnetic brush has been charged. The bias voltage for this charging is provided through a terminal that comes into contact with the carrier, or
This is done through the bearing of the rotary shaft of the carrier. For this reason, there is a problem that the contact is unstable, the application of the charging bias voltage is sometimes interrupted, the charge is not injected, and the charging is not stably performed. There is also a problem that an overcurrent flows when the substrate is exposed on a part of the photoconductor.

An object of the present invention is to solve these problems and to provide an image forming apparatus capable of performing extremely stable and uniform charging without dielectric breakdown of the image forming body or generation of ozone.

[0008]

An object of the present invention is to charge magnetic particles on a carrier to form a magnetic brush, and to place the magnetic brush on the carrier under an oscillating electric field to charge an image forming body. In the image forming apparatus, a charging bias voltage including an AC component is applied from the applying electrode through the magnetic brush to the carrier.

The gap between the applying electrode and the carrier is smaller than the gap between the carrier and the image forming body, and the applying electrode and the magnetic pole of the magnet body inside the carrier are opposed to each other. The image forming apparatus is a preferred embodiment in which the applying electrode is a magnetic brush regulating unit.

[0010]

In the present invention, the charging bias voltage containing the AC component of the charging device is applied to the carrier by the application electrode also serving as the magnetic brush restricting means through the magnetic brush, so that the charging bias voltage is not interrupted. The electric charge is surely injected into the image forming body through the magnetic brush. Further, even if a part of the base body of the image forming body is exposed, since the current path through the electromagnetic brush is long, overcurrent is prevented from flowing.

[0011]

EXAMPLES Before describing the examples 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) the state of the ears of the magnetic brush formed on the carrier is rough, and therefore, even when charged while vibrating by the electric field, The magnetic brush is likely to have unevenness, which causes a problem of uneven charging. To solve this problem, the average particle size of magnetic particles should be reduced.
It has been found that the effect starts to appear when the thickness is m or less, and particularly when the thickness is 100 μm or less, the problem (a) does not substantially occur. However, if the particles are too fine, they tend to adhere to the surface of the image forming body at the time of charging, or easily 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 size of the particles becomes prominent at 30 μm or less. In addition,
A magnetized strength of 20 to 200 emu / g is preferably used.

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

Such magnetic particles are the same as 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 formation of spherical magnetic particles is
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, uniform discharge is caused on the surface of the image forming body and charging unevenness does not occur.

In the spherical particles having the above effects, the magnetic particles have a resistivity of 10 ³ Ω · cm or more and 10 ¹² Ω · cm or less, particularly 10 ⁴ Ω · cm or less.
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 easily adhere to the surface of the image forming body, or the dielectric breakdown of the image forming body due to the bias voltage occurs. It can happen 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 by using a material having a true specific gravity of 6 or less and a maximum magnetization of 30 to 100 emu / g.

In summary of the above, the magnetic particles are spherical so that the ratio of the major axis to the minor axis is at most 3 times, there are no protrusions such as needles and edges, and the resistivity is high. 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 image forming body at the time of charging. Under the condition that the toner is mixed with magnetic particles and the toner concentration is adjusted to 1%. When the triboelectrification amount of the toner was the same and the charging polarity was 1 to 20 μC / g, the toner could be prevented 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 image forming body.

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 image forming body will be described.

A conductive carrier that can apply 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 roller 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 roller to undulate and move, so that new magnetic particles are supplied one after another. Therefore, even if the particle layer on the surface of the carrier has some unevenness in the layer thickness, the effect is sufficiently covered so as not to cause a practical problem due to the corrugation. Then, it is preferable that the transport speed of the magnetic particles due to the rotation of the transport carrier is substantially the same as or faster than the moving speed of the image forming body. 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.

The carrier for transporting the magnetic particles is not limited to the one composed of a charging roller having a magnet body which is fixed or rotated inside, and the carrier does not have a charging roller and the magnet body rotates so that N and S alternate. It may be composed only of the magnet body magnetized in the above.

Further, the thickness of the particle layer formed on the carrier is preferably such that it is sufficiently scraped off by the regulation plate 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 image forming body, resulting in adhesion of the magnetic particles to the image forming body and uneven charging. As a result of repeated experiments,
The preferable amount W of the magnetic particles in the charging region is 10 to 30.
A 0 mg / cm ^2, more preferably found to be 30~150mg / cm ^2. The existing amount is an average value in the contact area of the magnetic brush.

The gap D between the carrier and the image forming body is
0.1 mm ≤ D ≤ 10 mm is preferable, more preferably 0.2 mm ≤
D ≦ 5 mm is preferable. The surface gap between the carrier and the image forming body
If it is too narrower than 100 μm, it becomes difficult to form the ears of a magnetic brush that uniformly acts on it, and it becomes impossible to supply sufficient magnetic particles to the charging section, and stable charging is achieved. If it is not performed and the gap exceeds 5,000 μm, the particle layer is coarsely formed and charging unevenness is likely to occur, and the charge injection effect is deteriorated and sufficient charging cannot be obtained. As described above, when the gap between the carrier and the image forming member becomes extremely large, the thickness of the particle layer on the carrier cannot be adjusted appropriately. However, when the gap is in the range of 100 to 5,000 μm, On the other hand, the thickness of the particle layer can be appropriately formed, and it is possible to prevent the occurrence of sweeps due to the rubbing of the magnetic brush. 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 stably at high speed
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 the bent state, the image forming body is rubbed 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.

When the transport amount W is less than 10 mg / cm ² , adhesion of magnetic particles and uneven charging appear, and when it is more than 300 mg / cm ² , abrasion of the photosensitive member and uneven charging appear. I couldn't get it. The preferred range in between is 30
It was ~ 150 mg / cm ² .

Further, when the distance between the image forming body and the magnetic particle carrying carrier is D (cm) under the above carrying amount condition, W
By setting / D to be 300 mg / cm ³ <W / D <3,000 mg / cm ³ , it has been clarified that more preferable uniform charging characteristics without adhesion of magnetic particles or uneven charging can be obtained. When the amount was less than 300 mg / cm ^{3 or} more than 3,000 mg / cm ³ , there was a phenomenon that magnetic particles adhered and uneven charging occurred.

From the above, the preferable condition is that a magnetic brush composed of a magnetic particle layer of magnetic particles having magnetic force adhered on a carrier is brought into contact with a moving image forming body, and the moving carrier is separated from the image forming body. In the charging device for charging the image forming body by forming the bias electric field on the magnetic field, an oscillating electric field is used as the bias electric field, and the abundance W of the magnetic particles in the charging region is 10 to 300 mg / cm ^2. When a magnetic brush is formed on the magnetic recording medium and the gap between the carrier for carrying the magnetic particles and the image forming body is D (cm), it is preferable that 300 ≦ W / D ≦ 3,000 (mg / cm ³ ). .

(Embodiment) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the outline of the construction of an electrostatic recording apparatus which is an image forming apparatus of the present invention. In the figure,
Reference numeral 10 is an image forming body that rotates in the direction of the arrow (clockwise) (-).
The photosensitive drum is composed of a charging OPC, and a charging device 20, which will be described later, an exposing portion on which image light L from the exposing device is incident, a developing device 30, a transfer roller 13, and a cleaning device 50 are provided on a peripheral portion of the photosensitive drum.
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 image light L of a laser beam from an image writing device or the like, and an electrostatic latent image corresponding to the image is formed.

There is a two-component developer in the developing device 30, which is agitated by the agitating screws 33A and 33B, and then adheres to the outer periphery of the developing sleeve 31 which is outside the magnet roller 32 and rotates to cause the magnetic field of the developer. A brush is formed, a predetermined bias voltage is applied to the developing sleeve 31, and reversal development is performed in the developing area facing the photoconductor drum 10.

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 and is rotated is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and waits for the next recording.

FIG. 2 is a sectional view showing an embodiment of the charging device 20 used in the image forming apparatus of FIG. In the figure,
Reference numeral 21 is a magnetic particle, 22 is a charging roller that is a carrier for the magnetic particles 21 made of non-magnetic and conductive metal such as aluminum, and the surface thereof has an average surface roughness for stable and uniform transfer of the magnetic particles 21. The thickness is preferably 2 to 15 μm. If it is smooth, it cannot be sufficiently conveyed, and if it is too rough, an overcurrent flows from the convex portions on the surface, and uneven charging tends to occur in either case. Sandblasting is preferably used to achieve the above surface roughness. Further, the diameter of the charging roller 22 is preferably 5 to 20 mm. With the above diameter, a contact area necessary for charging is secured. If the contact 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, the magnetic particles 21 are easily scattered or adhered to the photoconductor drum 10 due to the centrifugal force. Therefore, it is preferable to reduce the linear velocity of the charging roller 22. Reference numeral 23 denotes a columnar magnet body fixedly arranged inside the charging roller 22, and this magnet body 23 has an S pole and an N pole so that the surface of the charging roller 22 has 500 to 1,000 gauss as shown in the figure. It is magnetized with poles arranged. Of these magnetic poles, the magnetic pole 23a of the charging portion closest to the photosensitive drum 10 will be referred to as the main magnetic pole. The charging roller 22 is rotatable with respect to the magnet body 23, and is 0.5 to 1.0 mm at a position facing the photoconductor drum 10.
Is held in the gap of the same direction as the moving direction of the photoconductor drum 10.
It can be rotated at a peripheral speed of 1.2 to 2.0 times.

The position of the main magnetic pole 23a of the magnet body 23 closest to the photosensitive drum 10 is the charging roller 22 and the photosensitive drum.
An angle θ formed by the center line of the line connecting the center of the charging roller 22 and the main magnetic pole 23a near the center line connecting the center of the photosensitive drum 10 and the center of the charging roller 22. As a result of the experiment, it was found that it is preferable to be within a range of ± 15 °. Further, one magnetic pole is also provided at a position facing a restriction plate 26 described later.

Reference numeral 25 denotes a casing forming a storage portion for the magnetic particles 21, the charging roller 22 and the magnet body 23 are arranged in the casing 25, and the outlet of the casing 25 is a magnetic brush regulating means. A regulation plate 26 also serving as an application electrode is provided to regulate the thickness of the layer of magnetic particles 21 attached to and discharged from the charging roller 22, and the charging roller 22 is charged through the magnetic particles 21. Apply a bias voltage. The gap DB between the tip of the regulation plate 26 and the charging roller 22 is such that the amount of conveyance of the magnetic particles 21, that is, the amount of the magnetic particles 21 present on the charging roller 22 in the charging area is 10 to 300 mg / cm ^2, particularly preferably 30 to 150 mg / cm ^2. It is adjusted to be ² and smaller than the gap DP between the charging roller 22 and the photosensitive drum 10, that is, it is set so that DB <DP.
As a result, the gap DP between the charging roller 22 and the photosensitive drum 10
Are connected by a magnetic brush of magnetic particles 21 whose thickness is regulated.

Further, this preferable gap ratio is DB / DP =
It is 0.6 to 0.9.

When this value is large, the voltage is not sufficiently applied to the charging roller, and the magnetic particles are easily clogged at the charging portion. On the other hand, if this value is small, the magnetic particles conveyed to the charging section are reduced, and uneven charging occurs.

The photosensitive drum 10 comprises a conductive base material 10b and a photosensitive material 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 roller 22 and the conductive base material 10b. The bias voltage is applied to the charging roller 22 from the regulation plate 26 via the magnetic particles 21 via the protective resistance R. Is being applied.

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 DP between the charging roller 22 and the photosensitive drum 10. , The gap DP is 0.1 to 5 depending on the size of the photosensitive drum 10 and the charging voltage for charging the photosensitive drum 10.
When held between mm, it is almost the same as the voltage to be charged −
The peak-to-peak voltage (V
By supplying an AC bias voltage superposed with an AC component of ( _PP ) 200 to 3,500 V (frequency 0.3 to 10 KHz) via the protective resistance 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. 27 is an agitator composed of a rotating body having a plate-like member for correcting the bias of the magnetic particles 21 around the axis.

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

When the charging roller 22 is rotated in the same direction as the arrow at a peripheral speed of 0.2 to 2.0 times the peripheral speed of the photosensitive drum 10 while rotating the photosensitive drum 10 in the direction indicated by the arrow, the charge roller 22 adheres to the charging roller 22. The layer of the magnetic particles 21 to be conveyed is magnetically connected by a magnetic force line of the magnet body 23 at a position where the regulation plate 26 on the charging roller 22 and the photosensitive drum 10 face each other to form a kind of brush, which is a so-called brush shape. The magnetic brush 21A is formed. Then, the magnetic brush 21A is conveyed in the rotation direction of the charging roller 22 and comes into contact with and slides on the photosensitive layer 10a of the photosensitive drum 10. Since the AC bias voltage is applied between the charging roller 22 and the photoconductor drum 10 via the regulation plate 26, it is reliably applied to the charging roller 22 via the conductive magnetic particles 21, and the An electric charge is injected onto the photoconductor layer 10a through the magnetic brush 21A to perform charging. In this case, in particular, one of the magnetic poles of the magnet body 23 is opposed to the regulation plate 26, and an oscillating electric field is formed by applying an AC bias through the regulation plate 26, and the main magnetic pole 23a is located at the center of the charging area. Particularly preferably within the range of ± 15 °, the upstream side (θ> 0) is installed (θ> 0) and the chains of the magnetic brushes 21A connected to the chain of the magnetic particles 21 are laid down, so that charge injection from the magnetic brush is ensured and efficiency is improved. Improve,
In addition, the charging area can be widened, and extremely stable high speed and uniform charging can be performed.

In the above embodiments, the charging roller
The results of changing the frequency and voltage of the AC voltage component applied to 22 are shown in FIG.

In FIG. 3, the range shaded by vertical lines is the range where dielectric breakdown is likely to occur, the range shaded by 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.

Spherical ferrite particles coated so as to have conductivity were used as the magnetic particles 21 in the above-mentioned embodiment. 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 spherical and the particle size is adjusted to 50 μm, the specific resistance is 10 ³ Ω · cm, and the frictional charge amount with the toner is −5 μC / g under the condition that the toner concentration is 1%. .

It is also possible to remove the charge from the photoconductor drum 10 by using the charging device 20 of this embodiment. 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 image forming body to eliminate the charge on the photosensitive drum 10. When the charge removal of the photoconductor drum 10 is completed, the application of the AC component is stopped, and the magnetic pole of the magnet body 23 is rotated so as to be parallel to the tangent line of the facing portion of the photoconductor drum 10. Is parallel to the tangential direction of the portion facing the photoconductor drum 10 due to the horizontal magnetic field, and the magnetic particles 21 are transferred to the photoconductor drum.
The tip of the magnetic brush 21A can be separated from the photoconductor drum 10 without adhering to the ten circumferential surfaces.

Further, in the image forming method in which the charging device is used as a cleaning device, reversal development is preferable to regular development in developing. This is because the toner is easily discharged from the charging device during charging, the discharged toner has the same polarity during reversal development, and is collected by the developing bias in the developing section, so that image fogging can be prevented.

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. This includes the charging roller when the photosensitive drum 10 rotates before or after image formation.
If the polarity of the DC bias voltage applied to 22 is set high or the AC voltage is set high, the toner is
It is possible to prevent the toner from being mixed by setting conditions in which the toner easily adheres. 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.

As the charging roller, the one in which the magnet body is contained in the sleeve is used, but the charging roller is not limited to this, and may be composed of only the magnet body.

[0053]

According to the present invention, the image forming body is charged by directly injecting the electric charge through the magnetic brush formed on the carrier, so that the bias voltage can be lowered and the generation of ozone can be prevented. Further, one magnetic pole of the magnet body inside the carrier is made to face the restricting means of the magnetic brush, and the gap between the tip of the restricting means and the carrier is set smaller than the gap between the carrier and the image forming body. Since the AC bias voltage is applied by using the means as the application electrode, the magnetic particles in the vicinity of the regulating means are compressed, and the wide regulating means can surely apply the bias voltage to the carrier by way of the conductive magnetic particles. It is possible to provide an image forming apparatus in which electric charges are injected into the image forming body through the above magnetic brush to perform extremely stable and uniform charging without uneven charging.

[Brief description of drawings]

FIG. 1 is a sectional view showing the outline of the configuration of an image forming apparatus of the present invention.

FIG. 2 is a cross-sectional view showing an embodiment of the charging device of FIG.

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

[Explanation of symbols] 10 Photoconductor drum (image forming body) 20 Charging device 21 Magnetic particles 22 Charging roller (conveying carrier) 23 Magnet body 24 Bias power supply 25 Casing 26 Regulation plate (regulation means) 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 (5)

[Claims] 1. An image forming apparatus for charging an image forming body by supplying magnetic particles onto a carrier to form a magnetic brush, and placing the magnetic brush on the carrier under an oscillating electric field, which comprises an AC component. An image forming apparatus characterized in that a charging bias voltage is applied from an applying electrode through the magnetic brush to the carrier. 2. The image forming apparatus according to claim 1, wherein a gap between the application electrode and the carrier is smaller than a gap between the carrier and the image forming body. 3. The image forming apparatus according to claim 1, wherein the applying electrode and the magnetic pole of the magnet body inside the carrier are opposed to each other. 4. The image forming apparatus according to claim 1, wherein the applying electrode is a magnetic brush restricting unit. 5. The image forming apparatus according to claim 4, wherein a ratio of the gap is 0.6 to 0.9.