JPH06186821A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device in which magnetic particles are prevented from adhering to an image forming body and stable electrostatic charging is efficiently performed at a low cost. CONSTITUTION:This image forming device is provided with an electrostatic charging device 20 in which a non-magnetic conductive electrostatic charging sleeve 22 rotatably disposed on the outer periphery of a magnetic substance 23 having magnetic poles arranged and fixed on its outer periphery, and a magnetic brush 21A consisting of the layer of the magnetic particles 21 adhering to the outer periphery of the sleeve 22 are brought into contact with a moving photosensitive drum 10, and oscillating electric field is formed between the sleeve 22 and the drum 10, so that the drum 10 is electrostatically charged. Then, the particle size, the resistivity and the magnetic susceptibility of the magnetic particle 21 are 40-60mum, 10<4>-10<10>OMEGA.cm, and 40-80emu/g, respectively, and common to the magnetic particles used for development.

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 conductive magnetic particles are adsorbed on a cylindrical carrier 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 Application Laid-Open No. 2000-242242. 59-133569, JP-A-4-218
No. 73 and Japanese Patent Laid-Open No. 4-116674.

[0006]

In the image forming apparatus using the charging device disclosed in the above publication, the magnetic particles used in the charging device and the developing device are different and two types of magnetic particles are used. Therefore, if magnetic particles adhere to the image forming body during charging, they cannot be collected during development and taken into the developer. Therefore, leakage or discharge of transfer voltage occurs through the conductive magnetic particles during transfer, or image forming body during cleaning. There was a problem such as scratching the photosensitive layer.

The present invention solves these problems and provides an image forming apparatus which does not adhere magnetic particles to an image forming body and can perform highly stable and uniform charging at low cost efficiently. To aim.

[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 particle size of the magnetic particles, the resistivity and the magnetic susceptibility are ⁴⁰ to 60 μm, 10 ⁴ to ¹⁰ 10 Ωcm, respectively.
This is achieved by an image forming apparatus that is common to magnetic particles used for development at 40 to 80 emu / g and that the strength of the oscillating electric field in the developing section is smaller than the strength of the oscillating electric field in the charging section.

[0009]

In the present invention, the particle size, the resistivity and the magnetic susceptibility of the magnetic particles used in the charging device of the image forming apparatus are each 40%.
〜60μm, 10 ⁴ 〜 10 ¹⁰ Ω ・ cm, 40〜80emu / g in common with magnetic particles used for development, the strength of the oscillating electric field for development was made smaller than the strength of the oscillating electric field for charging. The magnetic particles adhered at the time of charging can be collected at the time of development, and the adhesion of the magnetic particles to the image forming body or the dielectric breakdown of the image forming body does not occur.

[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 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, which will be described later, an exposure part 12 on which image light L 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 the image writing device such as a laser beam in the exposure unit 12 to form an electrostatic latent image corresponding to the image.

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

In this embodiment, the magnetic particles used as the carrier of the developer are used in common with the magnetic particles of the charging device 20 and therefore have conductivity, and when a high AC bias voltage is applied, an overcurrent flows to the photosensitive drum 10. The photosensitive layer of the photosensitive drum 10 may be destroyed. Therefore, for example, when an AC bias is applied to both the developing bias power supply and the charging bias power supply, the maximum voltage difference of the applied AC bias voltage is Vc ^{PP in the} charging portion, Vd ^{PP in the} developing portion, the charging sleeve 22 and the photosensitive sleeve 22. The gap between the body drum 10 and Dc, developing sleeve 31
When the gap between the photosensitive drum 10 and the photosensitive drum 10 is Dd, Vc ^PP /
By setting Dc> Vd ^PP / Dd so that the strength of the oscillating electric field for development is smaller than the strength of the oscillating electric field for charging, it is possible to prevent the generation of overcurrent and the destruction of the photosensitive drum 10. Even if Vc ^PP = Vd ^PP, the condition can be satisfied by setting Dc <Dd.

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

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

Generally, when the average particle size (average weight) of the magnetic particles is large, (a) since the state of the magnetic brush formed on the carrier is rough, even when charged while vibrating by the electric field, The magnetic brush is likely to have unevenness, which causes a problem of uneven charging. In order to solve this problem, the average particle size of the magnetic particles should be reduced.
The effect begins to appear below, especially when it becomes 100 μm or less,
It was found that the problem (a) would not substantially occur.
However, if the particles are too fine, they tend to adhere to the surface of the 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 diameter of the particles becomes prominent at 30 μm or less. Incidentally, the strength of magnetization is preferably used 20 ~ 200emu / g,
In the present invention, good results were obtained using an average particle size (average weight) of 40 to 60 μm and a magnetization intensity of 40 to 80 emu / g.

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 effects have a magnetic particle resistivity of 10 ³ Ω · cm or more and 10 ¹² Ω · cm or less, particularly 10 ⁶ Ω · cm or less.
It is preferable that the conductive magnetic particles are formed so as to be Ω · cm or more and 10 ¹⁰ 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 when the true specific gravity is 6 or less and the maximum magnetization is 30 to 100 emu / g, particularly 40 to 80 emu / g.

In summary, the magnetic particles are spherical so that at least the ratio of the major axis to the minor axis is 3 times or less, there is no protrusion such as a needle-shaped portion or an edge portion, and the resistivity is The appropriate condition is preferably 10 ⁶ Ω · cm or more and 10 ¹⁰ Ω · cm or less. And, such spherical magnetic particles should be selected as spherical as possible for the magnetic particles, and in the particles of the magnetic fine particle dispersion system, the fine particles of the magnetic material should be used as much as possible, and the spheroidizing treatment should be performed after the formation of the dispersed resin particles. It is manufactured by applying or by forming dispersed resin particles 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.

As the carrier for magnetic particles, a conductive carrier capable of applying a bias voltage is used. In particular, a magnet having a plurality of magnetic poles inside a conductive charging sleeve on the surface of which a particle layer is formed. A structure having a body is preferably used. In such a carrier,
The relative rotation with the magnet body causes the particle layer formed on the surface of the conductive charging sleeve to undulate and move in a wave shape, so that new magnetic particles are supplied one after another, and the particle layer on the carrier surface is supplied. Even if there is some non-uniformity in the layer thickness, the effect is sufficiently covered by the above-mentioned wavy undulation so as not to be a practical problem. The surface of the carrier is preferable to have an average surface roughness of 2 to 15 μm for stable and uniform transfer of the magnetic particles. A current flows, and in either case uneven charging is likely to occur. Sandblasting is preferably used to achieve the above surface roughness. The diameter of the carrier is preferably 5 to 20 mm. With the above diameter, a 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 magnetic particles is not limited to the one composed of a charging sleeve having a magnet body which is fixed or rotated inside, and the magnet body rotates without a charging sleeve, 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 Dc between the carrier and the image forming body
Is preferably 0.1 mm ≦ Dc ≦ 10 mm, more preferably 0.2 m
It is preferable that m≤Dc≤5 mm. If the surface gap Dc 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.
When the gap Dc exceeds 5,000 μm, the particle layer is coarsely formed and charging unevenness easily occurs, and the charge injection effect is deteriorated and sufficient charging cannot be obtained. As described above, when the gap Dc between the carrier and the image forming body becomes extreme, the thickness of the particle layer on the carrier cannot be adjusted appropriately, but when the gap Dc 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 the generation of sweeps due to the rubbing of the magnetic brush can be prevented. In addition, more appropriate transport amount (W) and gap (D
It became clear that the most favorable conditions exist between c) and.

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

Dc 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 the magnetic particles are in contact with each other, and in a bent state, they are rubbing against the image forming body 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 W / Dc corresponding to the density of the magnetic particles is small, the chains of the magnetic particles become coarse and the ratio of disturbance is small, resulting in non-uniform charging. When W / Dc is large, the chains of the magnetic particles are not sufficiently formed due to 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, further, under the above-mentioned carrying amount condition, when the distance between the image forming body and the magnetic particle carrying carrier is Dc (cm),
By setting the W / Dc to 300 mg / cm ³ <W / Dc <3,000 mg / cm ³ , it has been clarified that more preferable uniform charging characteristics without adhesion of magnetic particles and 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 particle carrier and the image forming body is Dc (cm), 300 ≦ W / Dc ≦ 3,000 (mg / cm
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 sleeve that is a carrier for the magnetic particles 21 made of a non-magnetic and conductive metal such as aluminum, and 23 is a columnar magnet body fixedly arranged inside the charging sleeve 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 sleeve 22 has 500 to 1,000 gausses as shown in FIG. Of these magnetic poles, the magnetic pole of the charging section closest to the photoconductor drum 10
23a will be referred to as the main magnetic pole.

The position of the main magnetic pole 23a of the magnet body 23 closest to the photosensitive drum 10 is the position of the charging area where the charging sleeve 22 and the photosensitive drum 10 are closest to each other, that is, the center of the photosensitive drum 10 is charged. The angle θ between the center line of the charging sleeve 22 and the center line of the straight line connecting the center of the charging sleeve 22 and the main magnetic pole 23a is −15 ° as a result of the experiment.
It has been found that it is preferable to be in the range of ≦ θ ≦ 15 °, preferably the upstream side θ> 0.

The charging sleeve 22 is rotatable with respect to the magnet body 23, and has a gap D at a position facing the photosensitive drum 10.
c is held at 0.5 to 1.0 mm, and is 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 sleeve 22 is provided inside the casing 25.
And the magnet body 23 are arranged. 26 is a casing 25
A regulating plate which is a means for regulating the amount of the magnetic particles 21 attached to and conveyed by the charging sleeve 22 provided at the outlet of the rotating body, and 27 is a rotating body having a plate-like member for correcting the bias of the magnetic particles 21 around the shaft. A stirring member composed of 28, magnetic particles from the charging sleeve 22
The magnetic particles 21 are constantly stirred and mixed by the scraping member 28 and the stirring member 27 by the scraping member that scrapes off the magnetic particles 21 and are always kept in a uniform state. The regulation plate 26 and the charging sleeve
A gap between the magnetic particles 21 and the gap 22 is, that is, the amount of the magnetic particles 21 present on the charging sleeve 22 in the charging region is 10 to 30.
0 mg / cm ^2, particularly preferably adjusted to be 30~150mg / cm ^2. In the charging area, the photosensitive drum 10 and charging sleeve
The gap Dc with 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 sleeve 22 and the conductive base material 10b, and the charging sleeve 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 superimposed on a DC component set to the same value as the voltage to be charged, and a gap Dc between the charging sleeve 22 and the photosensitive drum 10. , The gap Dc is 0.1 to
When held at 5 mm, it is almost the same as the voltage to be charged −50
The peak value voltage (V
c ^PP) 200~3,500V, by supplying through a protective resistor R AC bias voltage obtained by superposing an AC component of frequency 0.3～10KHz, it was possible to obtain a preferable charging conditions.
The bias power source 24 performs constant voltage control for the DC component and constant current control for the AC component.

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

While rotating the photosensitive drum 10 in the direction of the arrow, the charging sleeve 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.
The layer of magnetic particles 21 adhered to and conveyed by 22 is magnetically connected to the photosensitive drum 10 on the charging sleeve 22 by magnetic lines of force of the magnet body 23 to form a kind of brush shape by magnetically connecting in a chain. The magnetic brush 21A is formed. Then, the magnetic brush 21A is conveyed in the rotation direction of the charging sleeve 22, reaches the charging area, and is appropriately compressed, so that the photosensitive layer of the photosensitive drum 10 is formed.
Contact 10a and rub it. Charging sleeve 22 and photoconductor drum
Since the AC bias voltage is applied between 10 and 10, charging is performed by injecting charges onto the photoconductor layer 10a via the conductive magnetic particles 21. In this case, since the main magnetic pole 23a is installed within ± 15 ° of the center of the charging area, preferably θ> 0, the tips of the magnetic brush 21A lie down to expand the charging area and a stable oscillating electric field is formed to improve charging efficiency. To do.

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

In FIG. 3, the range shaded 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.

Next, the developing device 30 will be described.

Reference numeral 34 is a bias power source for applying a bias voltage to the developing sleeve 31, and the developing sleeve 31 is grounded via the bias power source 34.

The bias power source 34 is a power source for supplying an AC bias voltage obtained by superimposing an AC component on a DC component which is set to a value slightly lower than the charging voltage. The bias power source 34 forms a gap Dd between the developing sleeve 31 and the photosensitive drum 10. The gap Dd is 0.1 to 5 mm, although it depends on the size and the developing conditions for developing the photosensitive drum 10.
When held at, the DC voltage component of -500V to -1,000V, which is 50 to 200V lower than the voltage to be charged, causes the peak value voltage (V
d ^PP) 200~3,500V, by supplying through a protective resistor R AC bias voltage obtained by superposing an AC component of frequency 0.3～10KHz, it was possible to obtain a preferred development conditions.
The bias power source 24 performs constant voltage control for the DC component and constant current control for the AC component.

Incidentally, it is preferable that Vd ^PP be smaller than Vc ^PP / Dc because both conditions of charging and development are satisfied. This preferable condition is that the ratio is 2 to 5 times.

As the magnetic particles 21 of the above embodiment, spherical ferrite particles coated so as to have conductivity were 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, resistivity and magnetic susceptibility are 40-60 μm and 10 ⁴ respectively.
It is adjusted to -10 ¹⁰ Ω · cm and 40 to 80 emu / g and is common to the magnetic particles used for development, and the triboelectric charge amount with the toner is -10 μC / g under the condition that the toner concentration is 5%.

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. To this end, the polarity of the DC bias voltage applied to the charging sleeve 22 at the time of rotation of the photosensitive drum 10 before or after image formation is set high,
Alternatively, it is possible to prevent the toner from mixing by setting a high AC voltage and setting a condition that the toner is likely to adhere to the photosensitive drum 10. In particular, when the photosensitive drum 10 has the same polarity as the toner, such as in an image forming apparatus that performs reversal development, the toner polarity in the developing device 30 is the same as that of the toner. It does not appear as a fog in the image and is a very suitable combination.

[0054]

According to the present invention, since the image forming body is charged by the magnetic brush composed of the above-mentioned magnetic particles on the carrier, the electric charge is directly injected under the oscillating electric field by the AC bias voltage, so that ozone is generated. It is possible to carry out charging with an extremely small amount. In addition, since the magnetic particles used for charging and development are shared, the cost is low, and even if the magnetic particles attached by charging are collected during development, the development characteristics do not change, and the adhesion of magnetic particles to the image forming body and image formation It is possible to provide an image forming apparatus capable of performing efficient, stable, and uniform charging without body dielectric breakdown.

[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 charging characteristic diagram when a frequency and a voltage of an AC voltage component are changed.

[Explanation of symbols]

 10 Photosensitive drum (image forming body) 20 Charging device 21 Magnetic particles 21A Magnetic brush 22 Charging sleeve (conveying carrier) 23 Magnet body 24 Bias power supply 30 Developing device 31 Developing sleeve 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 (2)

[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. Particle size, resistivity and magnetic susceptibility are each 40 ~
An image forming apparatus characterized in that it has a particle size of 60 μm, 10 ⁴ to 10 ¹⁰ Ω · cm, and 40 to 80 emu / g and is common to magnetic particles used for development in the developing section. 2. The image forming apparatus according to claim 1, wherein the oscillating electric field strength of the developing section is smaller than the oscillating electric field strength of the charging section.