JPH06138747A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device provided with a magnetic brush where the evolvement of ozone is kept as little as possible, the breakdown of an image forming body is prevented from occurring, the deterioration of magnetic particles is restrained and extremely stable and uniform electrostatic charging is performed. CONSTITUTION:This image forming device is provided with an electrostatic charging device 20 electrostatically charging a photosensitive drum 10 by bringing a non-magnetic conductive electrostatic charging roller 22 obtained by rotatably disposing the outer periphery of a magnetic body 23 having an outer periphery where different magnetic poles are alternately arranged and fixed, and the magnetic brush consisting of the layer of the magnetic particles 21 adhering to the outer periphery of the roller 22 into contact with the moving photosensitive drum 10, and forming bias electric field between the roller 22 and the drum 10. A bias AC current IAC at the electrostatic charging part of the roller 22 is current-controlled to be 20-500muA/cm, and existing amount W of the magnetic particles at the electrostatic charging part is 10-500mg/cm<2>, and is within 300<=W/D<=3000 (mg/cm<3>) when it is assumed that the gap between the roller 2 and the drum 10 is D (cm).

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 such as an electrophotographic copying machine or an electrostatic recording apparatus, which charges an image forming body and utilizes an electrostatic transfer process.

[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, the charging device using the magnetic brush disclosed in the above publication also has some problems. That is, there is a problem that the image forming body cannot be charged completely stably and uniformly. That is, in the transfer area, the magnetic particles on the surface of the cylindrical magnetic particle carrier are chained along the lines of magnetic force and charged through the chains, but the state in the transfer area is stable and uniform. If it does not exist, there is a problem that the charge is locally excessive, resulting in dielectric breakdown of the image forming body and uneven charging. Further, there is a problem that ozone is generated due to discharge within and between chains of the magnetic brush. Furthermore, there is a problem that the magnetic particles are deteriorated by the charging operation, and the charging state is different at the initial stage of copying and after a lapse of time.

The present invention solves these problems, reduces the dielectric breakdown of the image forming body and the generation of ozone, and makes the magnetic particles highly durable so that an extremely stable and uniform charging can be performed. An object is to provide a forming device.

[0008]

The above object is to supply magnetic particles onto a rotating carrier to form a magnetic brush,
In an image forming apparatus in which a magnetic brush on the carrier is placed under an oscillating electric field to contact the image carrier to charge the image carrier, the bias AC current I _AC in the charging unit on the carrier is 20 to 500 μA. / Cm, and the existing amount W of the magnetic particles in the charging section is 10 to 500 mg / cm ^2, which is achieved by the image forming apparatus.

Further, when the gap between the carrier and the image forming body is D (cm), 300 ≦ W / D ≦ 3000 (mg / cm ³ ) is preferable. It is an embodiment.

[0010]

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 of magnetic particles is large,
(B) Since the state of the magnetic brush formed on the carrier is rough, even if it is charged while being vibrated 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 the magnetic particles should be reduced, and as a result of the experiment, the effect begins to appear when the average particle size is 150 μm or less, and in particular when the average particle size is 100 μm or less, the problem of (a) It turns out that no longer occurs. 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 also 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 particles is
Remarkably appears below 30 μm.

From the above, the average particle size of the magnetic particles is
It is preferably 150 μm or less, particularly preferably 100 μm or less and 30 μm or more. The magnetic strength of the magnetic particles is 20
Those of about 200 emu / g are preferably used.

Such a magnetic particle is similar to the magnetic carrier particle of a conventional two-component developer as a magnetic substance,
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 the spherical particles lose their directionality, so that the layers are uniformly formed and local (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.

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, especially 1
It is preferable that conductive magnetic particles are formed so as to be 0 ⁴ Ω · cm or more and 10 ⁹ Ω · cm or less. This resistivity is 0 for particles.
After tapping in a container having a sectional area of 50 cm ^2, a load of 1 kg / cm ² on packed particles, applying a voltage to the electric field of 1,000 V / cm is generated between the load part and a bottom electrode It is a value obtained by reading the current value at the time.If this resistivity is low, when a bias voltage is applied to the carrier, electric charges are injected into the magnetic particles and the magnetic particles adhere to the surface of the image forming body. Or the dielectric breakdown of the image forming body due to the bias voltage is likely to occur. 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 this has a small specific gravity and has a suitable maximum magnetization so that the magnetic brush moves lightly by an oscillating electric field, and external scattering does not occur. 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 at least the ratio of the major axis to the minor axis is 3 times or less, 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% 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 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 particle layer formed on the surface of the conductive carrier moves up and down in a wave shape due to the relative rotation with the magnet body, 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. And, the transport speed of the magnetic particles by the rotation of the transport carrier is almost the same as the moving speed of the image forming body,
It is preferable to be faster than that. 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 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 50.
0 mg / cm ² more preferably from 10-300 mg / cm ^2, particularly preferably found to be 30~150mg / cm ^2. In addition,
This abundance is an average value in the contact area of the magnetic brush. When the abundance W is W <10 mg / cm ² , the charging uniformity cannot be obtained, and the adhesion of magnetic particles was recognized. W> 300m
When it is g / cm ² , the AC current I _AC flowing through the charging unit per unit length of the carrier is increased, and the amount of ozone generated is increased. Further, when W ≧ 500 mg / cm ² , the magnetic particles are clogged in the charging part, and the photoconductor is worn or the conveyance is poor, so that a preferable result cannot be obtained.

The gap D between the carrier and the image forming body is
100 to 5,000 μm is preferable. If the surface gap between the carrier and the image forming body becomes too narrower than 100 μm, it will be difficult to form the magnetic brush ears that have a uniform charging action, and sufficient magnetic particles will be supplied to the charging section. It becomes impossible to do stable charging.
If the gap is much larger than 5,000 μm, the particle layer is roughly formed and charging unevenness is apt to occur, and the charge injection effect is deteriorated so that sufficient charging cannot be obtained. As described above, when the gap between the carrier and the image forming body becomes extremely large, the thickness of the particle layer on the carrier cannot be adjusted appropriately. However, when the gap is 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 amount (W) and the gap (D).

To charge uniformly and stably at high speed
It was important to meet the condition of 300 ≦ W / D ≦ 3,000 (mg / cm ³ ). If the W / D was less than 300mg / cm ³ and 3,
It was confirmed that when the amount was more than 000 mg / cm ^3, the adhesion and charging of magnetic particles became 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 charges without causing charging 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.

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 charging device of the present invention charges a grounded image forming body by applying a bias voltage from a carrier to the image forming body via a magnetic brush. It is an AC bias voltage in which an AC component is superposed on a DC component set to a value almost the same as the voltage to be charged. The AC bias voltage varies depending on the gap D between the carrier and the image forming body and the charging voltage charged on the image forming body, but the peak-to-peak voltage ( 200~3500V as V _PP)
It is a preferable charging condition to supply an AC bias voltage superposed with the AC component of. By the way, in the magnetic brush charging, the electric resistance of the magnetic particles itself changes due to long-term use, and also changes due to toner adhesion. Further, the applied voltage causes electric discharge within or between the chains of the magnetic brush to generate ozone.

The AC component applied here has a function of accelerating the injection of electric charges of the DC component into the image forming body, but when it is excessive, the ozone generation amount increases. The inventors of the present invention have found that it is appropriate from a number of experiments that the voltage of the AC component is applied by constant current control, and the AC current I _AC is between 20 and 500 μA / cm per unit charging width. Was required as a necessary condition. Here, the alternating current I _AC is an alternating current flowing through the charging unit per unit length in the axial direction of the charging roller which is the carrier. AC current I _AC is I _AC <20μA / cm
If so, the charging uniformity of the image forming body cannot be obtained, and I _AC >
It is recognized that when it is 500 μA / cm, the amount of ozone generated sharply increases and accelerates the deterioration of magnetic particles.

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 so that the space between the carrier and the image forming body is increased. In the charging device for charging the image forming body by forming a bias electric field on the oscillating electric field, an oscillating electric field is added to the bias electric field in addition to the direct current component and a constant current controlled AC current I _AC of 20 to 500 μA / cm. With use,
By forming the magnetic brush so that the existing amount W of the magnetic particles in the charging region is 10 to 500 mg / cm ² , uniform charging with a small ozone generation amount is performed. Further, when the gap between the carrier for carrying magnetic particles and the image forming body is D (cm), 300 ≦
It is a preferable condition that W / D ≦ 3,000 (mg / cm ³ ). Thereby, the density of the magnetic brush suitable for charging can be maintained.

(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) (-).
A photoconductor drum composed of a charging OPC, and a charging device 20 described below, an exposure part on which image light IL from the exposure device is incident, a developing device 30, a transfer roller 13, and a cleaning device on a peripheral portion thereof.
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, for example, the image light IL 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.

The recording paper P is fed from the paper feed cassette 40 one by one by the 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, and a spherical ferrite particle coated so as to have conductivity is used as the magnetic particle 21. 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. The outer shape is a true sphere and the particle size is 50μ to ensure good charging.
m and a specific resistance of 10 ⁸ Ω · cm, and the triboelectric charge amount with the toner is −5 μC / g under the condition that the toner concentration is 1%. twenty two
Is a charging roller, which is a carrier for the magnetic particles 21 formed of a non-magnetic and conductive metal such as aluminum, 23
Is a columnar magnet body fixedly arranged inside the charging roller 22, and this magnet body 23 has a charging roller on the periphery as shown in the figure.
It is magnetized by alternately arranging S poles and N poles so that the surface has 500 to 1,000 Gauss.

The charging roller 22 has a diameter of 5 to 30 mmφ and is rotatable with respect to the magnet body 23. The charging roller 22 is held in a gap of 0.5 to 1.0 mm at a position facing the photoconductor drum 10 and the photoconductor drum.
It can be rotated at the peripheral speed of 1.2 to 2.0 times in the same direction as the moving direction of 10. The linear velocity of the photosensitive drum 10 can be applied up to 50 to 5000 mm / sec.

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

The bias power supply 24 is a power supply 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 the gap between the charging roller 22 and the photosensitive drum 10 is Although it depends on the size, the charging voltage for charging the photosensitive drum 10, etc., the gap is maintained between 0.1 and 5 mm, and is the same as the voltage to be charged through the protective resistor 28, that is, -500 V to -1,000 V DC component. In addition, the bias power source 24 applies a constant voltage for the DC component so that an AC bias voltage in which an AC component of 200 to 3,500 V is superimposed as a peak-to-peak voltage (Vp-p) is applied to the charging roller 22 through the protection resistor 28. As for the control, the alternating current component is subjected to constant current control so that the alternating current I _AC is in the range of 20 to 500 μA / cm.

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 a regulation plate 26 is provided at the outlet of the casing 25. Therefore, the thickness of the layer of magnetic particles 21 attached to the charging roller 22 and carried out is regulated. The gap between the regulation plate 26 and the charging roller 22 is the amount of conveyance of the magnetic particles 21, that is, the magnetic particles 21 on the charging roller 22 in the charging area.
Abundance W of 10 to 500 mg / cm ² particularly preferably 30 to 150 mg /
Adjusted to be cm ² , the photoconductor drum 10 and the charging roller 2
The gap between 2 and 2 is connected by a magnetic brush of magnetic particles 21 whose thickness is regulated. Reference numeral 27 denotes a stirrer, which is a rotating body having a plate-like member for correcting the bias of the magnetic particles 21 around the shaft. Further, a leveling plate 29 made of an insulating elastic member is provided at the position of the magnetic pole on the upstream side of the charging area so as to press the layer of magnetic particles 21 toward the charging roller 22. The layer of the magnetic particles 21 is further leveled just before the charging area by the insulating leveling plate 29 having elasticity which is made of urethane rubber, and the streak unevenness that is likely to occur due to clogging of the regulation plate 26 is eliminated and a uniform thin layer is formed. It is conveyed to the charging area and charged in the charging area.

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 1.2 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 facing the photosensitive drum 10 on the charging roller 22 to form a kind of brush shape, forming a so-called magnetic brush. To be done. Then, this magnetic brush 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, charges are injected onto the photoconductor layer 10a via the conductive magnetic particles 21 to perform charging. In this case, in particular, by forming an oscillating electric field for applying an AC bias by the constant current-controlled AC current I _AC , charges are efficiently injected from the magnetic brush in the charging region, while the ozone generation amount is small. As a result, the charging area can be expanded to improve the efficiency of charge injection from the magnetic brush, and extremely stable high speed and uniform charging can be performed.

In the above embodiment, 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.

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 image forming body to eliminate the charge on the photosensitive drum 10.

The magnetic particles 21 of the toner remaining on the surface of the photosensitive drum 10 without being cleaned due to long-term use
There is a case where the amount of the particles mixed in the layer is increased, the resistance of the magnetic brush is increased, and the charging efficiency is deteriorated. To this end, the polarity of the DC bias voltage applied to the charging roller 22 at the time of rotation of the photosensitive drum 10 before or after image formation is set to be high, or the AC voltage is set to be high so that toner adheres to the photosensitive drum 10. Toner mixing can be prevented by setting easy conditions. Especially when the charging polarity of the photoconductor drum 10 is the same as that of the toner, as in the image forming apparatus that performs reversal development, a developing device.
Since it has the same polarity as that of the toner in 30, the contamination by the toner is less likely to occur, and it does not appear as a fog in the image at the time of development, resulting in an extremely suitable combination.

The carrier for the magnetic brush has a magnet 23 inside.
Not limited to the configuration of the charging roller 22 having the above, the internal magnet body 23 may rotate, or the charging roller 22 may not be provided and only the magnet body 23 may be configured.

[0048]

According to the present invention, since the image forming body is charged by directly injecting the electric charge through the magnetic brush formed on the carrier, the bias voltage can be lowered and the bias AC current can be controlled by the current. By doing so, it is possible to provide an image forming apparatus capable of reducing the ozone generation amount, suppressing deterioration of the magnetic particles and enhancing durability, and performing 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 20 Charging device 21 Magnetic particles 22 Charging roller 23 Magnet body 24 Bias power supply 25 Casing 26 Regulation plate 28 Protective resistance 29 Leveling plate

 ─────────────────────────────────────────────────── ─── 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 (2)

[Claims] 1. Magnetic particles are supplied onto a rotating carrier to form a magnetic brush, and the magnetic brush on the carrier is placed under an oscillating electric field and brought into contact with an image forming body to charge the image forming body. In the image forming apparatus, the bias AC current I _AC in the charging portion on the carrier is 20 to 500 μA / cm, and the abundance W of magnetic particles in the charging portion is 10 to 10 μA / cm.
An image forming apparatus characterized by being 500 mg / cm ² . 2. The image according to claim 1, wherein when the gap between the carrier and the image forming body is D (cm), 300 ≦ W / D ≦ 3000 (mg / cm ³ ). Forming equipment.