JPH06186819A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device capable of extremely efficient, stable and uniform electrification free of dielectric breakdown of an image forming body, sticking of magnetic particles, and production of ozone. CONSTITUTION:The image forming device is provided with an electrifier 20 which electrifies a photosensitive drum 10 by bringing a nonmagnetic and conductive electrifying roller 22 disposed so that the outer circumference of a magnet body 23 is rotatably arranged and fixed with magnetic poles in the outer circumference thereof into contact with a magnetic brush 21A consisting of a layer of magnetic particles 21 sticking to the periphery of the electrifying roller 22 corresponding to the movement of the photosensitive drum 10, and thereby generating an oscillating electric field between the electrifying sleeve 22 and the photosensitive drum 10. For the generation of an oscillating electric field, the electrifier 20 applies an AC bias voltage having the same polarity as electrification polarity and a high impulse.

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]

In the charging device disclosed in the above publication, the bias voltage for charging is as shown in FIG.
The AC component of a symmetrical rectangular wave or sine wave as shown in FIG. 3 is superposed on the DC component, and the charging of the image forming body consists of a combination of charge transfer and discharge. Therefore, an AC component having a high amplitude is required for efficient charging. For this reason, there is a problem in that the generation of ozone cannot be completely eliminated, and the adhesion of magnetic particles to the image forming body and the dielectric breakdown of the image forming body occur.

The present invention solves these problems, and an image forming apparatus capable of efficiently performing extremely stable and uniform instantaneous charging without adhesion of magnetic particles to the image forming body or dielectric breakdown of the image forming body. The purpose is to provide.

[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 forming apparatus, the waveform of the bias voltage that forms the oscillating electric field has an impulse of a voltage having a high polarity that is the same as the charging polarity.

[0009]

In the present invention, the waveform of the bias voltage forming the oscillating electric field of the charging device of the image forming apparatus is set to have an impulse of a high voltage having the same polarity as the charging polarity. Is done.

[0010]

Embodiments 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 a photosensitive drum made of (-) charged OPC which is an image forming body rotating in an arrow (clockwise) direction, and image light L from a charging device 20 and an exposure device, which will be described later, is incident on the peripheral portion of the photosensitive drum. Exposure unit 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 photoconductor drum 10 rotates, its peripheral surface is uniformly charged by a charging device 20A, which will be 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.

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 circumference of the developing sleeve 31 which is outside the magnet roller 32 and is magnetized by the developer. Forming a brush, developing sleeve 31
A predetermined bias voltage is applied to the photosensitive drum 10
Inverse development is performed in the development area opposite to, and a toner image is formed.

From the paper feed cassette 40, 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) 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. 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 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 due to them, but in general, it appears that the average particle diameter of the particles is not more than 30 μm, and further 15 μm or less. become. 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 and 15 μm or more, particularly preferably 1
It is preferably 00 μm or less and 30 μm or more.

Such magnetic particles are used as magnetic materials in the same manner as in magnetic carrier particles of conventional two-component developers.
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, manganese-copper alloy 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.

The spherical particles having the above-mentioned effects have a magnetic particle 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, 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 is no protrusion such as a needle-shaped portion or an edge portion, 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 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 for applying a bias voltage is used as the carrier for magnetic particles, and in particular, a magnet having a plurality of magnetic poles inside a conductive charging roller having a particle layer formed on the surface thereof. 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. The surface of the carrier is preferably an average roughness of 2 to 15 μm for stable and uniform transfer of the magnetic particles. If the surface is smooth, the carrier cannot be carried sufficiently.
If it is too rough, an overcurrent will flow from the convex portion of the surface, and uneven charging tends to occur in either case. 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 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. Further, when the diameter is small as described above, the linear velocity of the carrier is preferably slow within the following range because magnetic particles are easily scattered or adhered to the image forming body due to centrifugal force. The transportation speed of the magnetic particles due to the rotation of the transportation carrier is preferably almost the same as or faster than the movement 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, and 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 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 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 magnetic particles in the charged region is 10
A to 300 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 becomes too narrower than 100 μm, it will be difficult to form the magnetic brush ears that have uniform charging action against it.
Also, it becomes impossible to supply sufficient magnetic particles to the charging section, stable charging is not performed, and the gap becomes 5,
If it exceeds 000 μm, the particle layer is coarsely formed and charging unevenness is likely to occur, and the charge injection effect is deteriorated so that sufficient charging cannot be obtained. Thus, if the gap between the carrier and the image-forming body becomes extreme, the thickness of the particle layer on the carrier cannot be adjusted appropriately, but if 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 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 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.

Incidentally, 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
It was 30 to 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 conditions, W
It was revealed that by setting / D to 300 mg / cm ³ <W / D <3,000 mg / cm ³ , more preferable uniform charging characteristics without sticking of magnetic particles or charging unevenness can be obtained. When the amount was less than 300 mg / cm ³ or the amount was more than 3,000 mg / cm ^3, phenomena such as adhesion of magnetic particles and uneven charging were observed.

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. By forming a bias electric field in
In a charging device for charging an image forming body, an oscillating electric field is used as a bias electric field, and a magnetic brush is formed so that the abundance W of magnetic particles in the charging area is 10 to 300 mg / cm ^2, and When the gap between the carrier for carrying particles and the image forming body is D (cm), 300 ≦ W / D ≦ 3,000 (mg / c
m ³ ) is a preferable condition.

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 carrying the magnetic particle 21 made of non-magnetic and conductive metal such as aluminum, and 23 is a columnar magnet body fixedly arranged inside the charging roller 22. As shown in FIG. 2, the magnet body 23 is magnetized by arranging the S pole and the N pole so that the surface of the charging roller 22 has 500 to 1,000 gausses as shown in FIG. Of these magnetic poles, the magnetic pole 23a of the charging portion closest to the photoconductor drum 10 will be referred to as a main magnetic pole.

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.
The position of the charging area closest to the position of 10, that is, the center line of the straight line connecting the center of the charging roller 22 and the main magnetic pole 23a on both sides of the center line connecting the center of the photosensitive drum 10 and the center of the charging roller 22. The angle θ formed with is −15 ° ≦ θ ≦
It has been found that it is preferable to be in the range of 15 °, preferably the upstream side θ> 0.

The charging roller 22 is rotatable with respect to the magnet body 23, and the gap at the position facing the photosensitive drum 10 is zero.
It is held at 5 to 1.0 mm and rotated in the same direction as the moving direction of the photosensitive drum 10 at a peripheral speed of 0.2 to 2.0 times.

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 roller 22 and the magnet body 23 are arranged inside the casing 25. There is. Further, 26 is a regulation plate which is a means for regulating the amount of the magnetic particles 21 attached to and conveyed by the charging roller 22 provided at the outlet of the casing 25, and 27 is the magnetic particles 21.
, A stirring member composed of a rotating body having a plate-shaped member around the axis for correcting the bias, 28 is a scraping member for scraping the magnetic particles 21 from the charging roller 22, and the magnetic particles 21 are the scraping members.
It is constantly stirred and mixed by the stirring member 28 and the stirring member 27, and is always kept in a uniform state. The gap between the regulation plate 26 and the charging roller 22 is the carrying amount of the magnetic particles 21, that is, the existing amount of the magnetic particles 21 on the charging roller 22 in the charging region is 10 to 300 mg / cm ² ,
Particularly preferably, it is adjusted to 30 to 150 mg / cm ² .
In the charging area, the gap between the photosensitive drum 10 and the charging roller 22 is connected by the magnetic brush 21A of the magnetic particles 21 whose thickness is regulated.

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, and the charging roller 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 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, and the like, when the gap is held at 0.1 to 5 mm, it is almost the same as the voltage to be charged -500 V to-
Peak voltage (V _PP ) 200 to 1,000 V DC component
Protecting the AC bias voltage in which an AC component having an impulse of 2 to 5 times the DC component Vd having the same polarity as the charging polarity as shown in FIG. 3A is superimposed on an AC of 3,500 V and a frequency of 0.3 to 10 KHz. By supplying through the resistor R, it was possible to obtain a preferable charging condition. If it is 5 times or more, a reverse current flows at the time of charging and damages the image forming body. If it is more than twice, the non-uniformity of charging cannot be solved. The impulse is at the rising edge when the charging polarity is applied (see FIG.
(A)) or after rising (Fig. 3 (b)), the waveforms that it has are available. The bias power source 24 performs constant voltage control for the DC component and constant current control for the AC component.
Further, not only the one having a rectangular wave impulse (solid line) but also the one having a sine wave impulse (broken line) may be used.

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 indicated by 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 toner is attached to the charging roller 22. The layer of the magnetic particles 21 conveyed is magnetically connected by a magnetic force line of the magnet body 23 at a position facing the photoconductor drum 10 on the charging roller 22 to form a kind of brush shape, which is a so-called magnetic brush 21A. It is formed. And this magnetic brush 21
A is conveyed in the rotation direction of the charging roller 22, reaches the charging area, is appropriately compressed, and contacts and rubs against 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, at the same time as the charging by injecting the charge onto the photoconductor layer 10a through the conductive magnetic particles 21, the impulse voltage is applied. Efficient instantaneous charging is also performed by the discharge of. In this case, the main pole 23
Since a is set in the range of ± 15 ° in the center of the charging area, particularly θ> 0, the tips of the magnetic brush 21A lie down, the charging area is expanded, a stable oscillating electric field is formed, and the charging efficiency is improved.

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. 4, 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. 4, the low frequency region shaded with dots is a range in which 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 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 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. 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 20 is used as a cleaning device, reversal development is preferable to regular development in developing. Because the toner is easily discharged from the charging device 20 during charging, and the discharged toner is
This is because the polarity becomes the same during reversal development, and it 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.

[0050]

According to the present invention, a magnetic brush having an image forming body formed on a carrier is used to directly inject charges under an oscillating electric field by an AC bias voltage having an impulse having a high polarity. Instantaneous charging by electric discharge makes it possible to extremely reduce the generation of ozone, and also provides highly efficient and stable uniform charging without magnetic particles adhering to the image forming body or dielectric breakdown of the image forming body. It is possible to provide an image forming apparatus that can perform the operations.

[Brief description of drawings]

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

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

FIG. 3 is a diagram showing a bias voltage waveform for charging according to the present invention.

FIG. 4 is a charging characteristic diagram when a frequency and a voltage of an AC voltage component are changed.

FIG. 5 is a diagram showing a conventional bias voltage waveform for charging.

[Explanation of symbols]

 10 Photoconductor drum (image forming body) 20 Charging device 21 Magnetic particles 21A Magnetic brush 22 Charging roller (conveying carrier) 23 Magnet body 24 Bias power supply 25 Casing 26 Regulation plate R Protective resistance

Front page continued (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) Inventor Nomori Hiroyuki Konica Co., Ltd. 2970 Ishikawa-cho, Hachioji-shi, Tokyo

Claims (1)

[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. The image forming apparatus is characterized in that the waveform of the bias voltage to be formed has a high-voltage impulse having the same polarity as the charging polarity.