JPH06250492A - Electrostatic charging device - Google Patents

Abstract

PURPOSE:To execute extremely stable and uniform electrostatic charge without causing the adhesion of magnetic particles on an image forming body and without producing ozone by forming a magnetic brush for the image forming body and executing electrostatic charge under AC bias. CONSTITUTION:A photosensitive drum 10 is previously rotated in the direction shown by an arrow. Simultaneously or a little later than that, a cylinder(electrostatic charging roller) 22 is rotated in the same direction as the direction shown by the arrow at circumferential speed 0.2-2.0 times as high as that of the drum 10. Then, the layer of the magnetic particles 21 stuck and carried on the cylinder 22 is magnetically coupled at an opposed position to the drum 10 on the cylinder 22 by the magnetic flux of a magnet 23 so as to form the magnetic brush 21A. By impressing DC and AC bias voltage between the cylinder 22 and the drum 10, charge is injected on a photosensitive layer 10a through the conductive magnetic particles 21 and the electrostatic charging is executed. In such a case, since the voltage is the bias voltage obtained by superposing an AC component on a DC component, the charge is injected from the magnetic brush 21A associated with the movement of the charge or discharge phenomenon and the efficiency thereof is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device using a magnetic brush for uniformly charging an image forming body in an image forming device such as an electrophotographic copying machine or an electrostatic recording device.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, a corona charger has generally been used 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 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 cylinder 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 JP-A-59-133569 and JP-A-4-133569. 21873, JP 4-116674
It is disclosed in the publication.

[0006]

However, in the charging device disclosed in the above publication, magnetic particles move and adhere to the image forming body at the start and stop of charging, which may damage the image forming body or cause uneven charging. There was a problem of causing it.
Further, when toner is mixed in the charging device, the charging ability is lowered, and there is a problem that magnetic particles are attached.

The present invention solves these problems, and provides a charging device capable of carrying out extremely stable and uniform charging without causing adhesion of magnetic particles on an image forming body, generation of ozone. The purpose is to

[0008]

The above object can be achieved by the following three inventions. According to a first aspect of the present invention, a magnetic brush is charged without directly contacting an image forming body. In a charging device for forming a magnetic brush on an image forming body and charging under an AC bias, the magnetic brush is a thin plate. It is achieved by a charging device characterized in that it contacts the image forming body through the charging device. Here, it is an embodiment of the first invention that the thin plate is preferably (a) an elastic member of 10 to 300 μm and (b) replaceable.

The second and third aspects of the present invention prevent magnetic particles from adhering to the image forming body at the start of charging and at the time of stopping charging.
In a charging device for charging magnetic particles on a carrier to form a magnetic brush and placing the magnetic brush on the carrier under an oscillating electric field to charge an image forming body, a DC voltage is applied at the start of charging, and then an AC voltage is applied. Is achieved by the charging device. In order to prevent the adhesion of magnetic particles when charging is stopped, the magnetic particles are supplied onto the carrier to form a magnetic brush, and the magnetic brush on the carrier is placed under an oscillating electric field to charge the image forming body. In the charging device, when the charging is stopped, the AC voltage is stopped and then the DC voltage is stopped.

[0010]

The second and third aspects of the present invention are characterized in that the order of applying and stopping the bias voltage of the DC component and the AC component bias voltage of the bias voltage of the charging device using the magnetic particles of the image forming apparatus is proper. To do. This is intended to prevent magnetic particles from adhering to the image forming body.

That is, the bias voltage applied between the carrier and the image forming body is sufficient to inject charge into the magnetic brush, and in order to perform uniform charging of the image forming body, an AC component is added to the DC component. A superposed one is preferable. However,
When the direct current component and the alternating current component of the bias voltage are simultaneously started and stopped at the time of starting and stopping charging, a high potential difference is momentarily generated between the image forming body and the magnetic brush, and the magnetic particles move to the image forming body. Adhere to. In order to prevent this, in the present invention, the DC component is applied at the start of charging, and then the AC component is applied, and when the charging is stopped, the application of the AC component is stopped and then the application of the DC component is stopped.

The particle diameter of the magnetic particles will be described. Generally, when the average particle diameter of the magnetic particles is large, (a) the state of the ears of the magnetic brush formed on the carrier is rough, so that the electric field causes vibration. Even if the toner is charged while being given, unevenness is likely to appear on the magnetic brush, causing a problem of uneven charging. To solve this problem, the average particle size of the magnetic particles should be reduced,
As a result of the experiment, it was found that the effect began to appear when the average particle size was 150 μm or less, and particularly when the average particle size was 100 μm or less, the problem (a) did 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. It is preferable that the magnetization intensity is 20 to 200 emu / g.

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.

Such magnetic particles are the same as those used in conventional magnetic carrier particles as magnetic materials, such as metals such as iron, chromium, nickel and cobalt, or their compounds and alloys such as ferric tetroxide and γ-oxidation. Ferric iron, chromium dioxide, manganese oxide, ferrite, manganese-copper alloy,
The ferromagnetic particles or the surfaces of these magnetic particles are coated with a resin such as styrene resin, vinyl resin, ethylene resin, rosin-modified resin, acrylic resin, polyamide resin, epoxy resin, polyester resin, etc. Alternatively, the particles obtained by making the resin containing the magnetic fine particles dispersed therein are subjected to particle size selection by a conventionally known average particle size selection 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 that exhibit the above effects have conductivity such that the carrier particles have a resistivity of 10 ³ Ωcm or more and 10 ¹² Ωcm or less, particularly 10 ⁴ Ωcm or more and 10 ⁹ Ωcm or less. It is preferable that the magnetic particles are formed. This resistivity is 1 kg on the packed particles after tapping the particles into a container with a cross-sectional area of 0.50 cm ^2.
/ cm ² load, 1000V / cm between the load and the bottom electrode
Is a value obtained by reading the current value when a voltage that causes the electric field is applied.If this resistivity is low, when a bias voltage is applied to the carrier, electric charges are injected into the magnetic particles, The magnetic particles are likely to adhere to the surface of the formed body, or the breakdown of 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 second and third inventions of the present invention have a small specific gravity so that the magnetic brush constituted by the magnetic particles moves lightly by an oscillating electric field and does not cause external scattering. Those having an appropriate maximum magnetization are desirable. Specifically, the true specific gravity is 6 or less and the maximum magnetization is
It was found that good results were obtained using 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.

In the first aspect of the present invention, since the magnetic brush is in contact with the image forming body through the thin plate, the magnetic particles adhere to the image forming body and the toner used for development is magnetic. Does not mix with the brush. However, in the second and third inventions, since the magnetic particles are in direct contact with the image forming body, if the toner used for the development is mixed in the magnetic brush, the toner has a high insulating property, so that the chargeability is lowered and the toner is charged. It causes unevenness. 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 triboelectric charge amount of the toner was the same as the charge polarity and 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, the toner 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 magnetic particle carrier for forming the particle layer and charging the image forming member will be described.

As the magnetic particle carrier, a conductive carrier capable of applying a bias voltage is used. In particular, a magnet having a plurality of magnetic poles is provided inside a conductive cylinder on the surface of which a particle layer is formed. The structure described above is preferably used. In such a carrier, rotation of the magnet relative to the rotating magnet causes the particle layer formed on the surface of the conductive cylinder to undulate and move in a wavy shape, so that new magnetic particles are supplied one after another. Even if the particle layer on the surface of the carrier has some non-uniformity in the layer thickness, the effect is sufficiently covered by the corrugation so that it does not cause any practical problem. Then, it is preferable that the transport speed of the magnetic particles due to the rotation of the transport carrier or the rotation of the magnet is almost the same as or faster than the moving speed of the image forming body. In addition, it is preferable that the transporting carrier is rotated in the same direction. Uniformity of charging is better in the same direction than in the opposite direction. However, it is not limited thereto.

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, and the photoreceptor will be worn and charging unevenness and overcurrent will flow, and the driving torque of the carrier will be increased. It has the drawback of becoming large. On the contrary, if the amount of the magnetic particles present on the carrier in the charged area is too small, uneven charging and adhesion of the magnetic particles to the image forming body will occur. The preferred amount of magnetic particles in the developing area is 10 to 200 mg /
It was cm ² . This abundance is an average value in the contact area of the magnetic brush.

The gap between the carrier and the image forming body is 10
0 to 5000 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 against it, and sufficient magnetic particles will be supplied to the charging section. If it becomes impossible to perform stable charging, and if the gap exceeds 5000 μm, the particle layer will be rough and uneven charging will occur easily. Will not be obtained. As described above, when 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 when the gap is in the range of 100 to 5000 μm, Thus, the thickness of the particle layer can be appropriately formed, and it is possible to prevent the generation of sweeps due to the rubbing of the magnetic brush.

[0023]

【Example】

(First Embodiment) A second 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 equipped with the charging device of the present invention.

In the figure, reference numeral 10 is a photosensitive drum made of (-) charged OPC which is an image forming body which rotates in the direction of the arrow (clockwise). A peripheral portion of the photosensitive drum 14 is a discharging lamp 14, and a charging device 20 which will be described later. Image exposure L from exposure device, developing device 30, transfer roller
13, a cleaning device 50 and the like 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 discharged by the discharging lamp 14, and after the discharged portion has passed the cleaning, it is uniformly charged by the charging device 20 described later. Charging is performed only in a certain area including the image area. An image is written on the photosensitive drum 10 by, for example, a laser beam L 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, and is a magnetic brush of the developer. Forming and developing sleeve
A predetermined bias voltage is applied to 31 and the photosensitive drum
Reverse development is performed in the development area facing 10.

From the paper feed cassette 40, the recording papers P are fed one by one by a first paper feed roller 41. The fed recording paper P is sent onto the photosensitive drum 10 by the second paper feed roller 42 which operates in synchronization with the toner image on the photosensitive drum 10. Then, by the action of the transfer roller 13, the toner image on the photoconductor drum 10 is transferred onto the recording paper P and separated from the photoconductor drum 10. The recording paper P on which the toner image has been transferred is sent to the fixing device 81 via the conveying means 80, is nipped by the heat fixing roller and the pressure bonding roller, melted and fixed, and then discharged outside the device. A photoconductor drum 10 that rotates with the toner remaining without being transferred to the recording paper P.
The surface of (1) is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and after the rear end of the charging area is photo-electrified, the rotation of the photosensitive drum 10 stops and stands by for the next copying.

FIG. 2 is a sectional view showing an embodiment of the charging device 20 used in the second and third inventions of the electrostatic recording apparatus of FIG. In the figure, 21 is a magnetic particle,
Spherical ferrite particles coated 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 adjusted to have a spherical shape with a particle size of 50 μm and a specific resistance of 10 ⁸ Ω · cm.
It is 5 μC / g.

Reference numeral 22 is a cylinder which is a carrier for the magnetic particles 21 formed of non-magnetic and conductive metal such as aluminum, and 23 is a columnar magnet arranged inside the cylinder 22. As shown in FIG. 3, the S pole and the N pole are magnetized so that the cylindrical surface has 700 gauss on the peripheral surface, and the cylinder 22 is rotatable with respect to the fixed magnet 23. Further, the magnet 23 may rotate as a magnetic pole having an equal pole. The cylinder 22 is at a position facing the photoconductor drum 10 and is 0.2 to 2.0 in the same direction as the moving direction of the photoconductor drum 10.
It is rotated at double the peripheral speed. When the magnet 23 is fixed, the position of the main magnetic pole of the magnet 23 closest to the photosensitive drum 10 is the center line connecting the center of the photosensitive drum 10 and the center of the charging roller 22, the center of the charging roller 22, and the main magnetic pole. It is desirable that the angle θ formed by the straight line connecting the lines is in the range of 0 ° ≦ θ ≦ 15 ° on the upstream side.

The diameter of the cylinder 22, which is a carrier, is 5 to 20 mmφ.
It is preferably within the range. With the above diameter, a contact area necessary for charging is secured. If the contact area is larger than necessary, the charging current becomes too large, and if it is small, uneven charging is likely to occur. When the diameter is small as described above, the linear velocity of the carrier is preferably slowed down because the magnetic particles are easily scattered or adhered to the image forming body due to centrifugal force.
The surface of the cylinder 22 preferably has an average surface roughness of 2 to 15 μm in order to stably and uniformly convey the magnetic particles. If it is smooth, it cannot be sufficiently conveyed, and if it is too rough, an overcurrent flows from the convex portions on the surface, and uneven charging tends to occur in either case. Sandblasting is preferably used to achieve the above surface roughness.

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 denotes a bias power source for applying a bias voltage between the cylinder 22 and the conductive base material 10b, and the cylinder 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 the cylinder 22 and the photosensitive drum 10
Depending on the size of the gap between the photoconductor drum 10 and the charging voltage charged on the photosensitive drum 10, the gap is maintained between 0.1 and 5 mm, and a DC component of -500 to -1000V, which is almost the same as the voltage to be charged, is generated. By supplying an AC bias voltage on which an AC component of 200 to 3500 V is superimposed as a peak-to-peak voltage (Vp-p) through the protective resistor 28, a preferable charging condition can be obtained. When only the DC bias voltage is applied without applying the AC bias voltage, the photosensitive drum 10 is not charged. By applying an AC bias voltage, an oscillating electric field can be formed and uniform charging can be obtained.

The bias power source 24 performs constant voltage control for the DC component and constant current control for the AC component, and the control unit (not shown) independently turns on and off the DC component and the AC component.
You can do it. Further, it is preferable that the ON / OFF operation continuously changes the applied voltage, not instantaneously. Specifically, it is preferable that the time is about 1 to 500 msec in order to prevent the magnetic particles from adhering to the image forming body.

Reference numeral 25 denotes a casing forming a storage part for the magnetic particles 21, the cylinder 22 and the magnet 23 are arranged in the casing 25, and a regulation plate 26 is provided at the outlet of the casing 25. The thickness of the layer of magnetic particles 21 attached to the cylinder 22 and carried out is regulated. The gap between the regulation plate 26 and the cylinder 22 is such that the transport amount of the magnetic particles 21, that is, the existing amount of the magnetic particles on the cylinder 22 in the developing region is 10 to 200 mg / cm.
Adjusted to ² . The existing amount is an average value of the contact area of the magnetic brush. Photoconductor drum 10 and cylinder
The gap with 22 is connected by 21 layers of magnetic particles whose thickness is regulated. The stirring plate 27 is a rotating body having a plate-shaped member that corrects the bias of the magnetic particles 21 around the axis.

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

The photosensitive drum 10 is rotated in the direction of the arrow in advance, and then the cylinder 22 is rotated at the peripheral speed of 0.2 to 2.0 times the peripheral speed of the photosensitive drum 10 in the same direction or at the same time with a slight delay. Then, the layer of the magnetic particles 21 attached and conveyed to the cylinder 22 will be a photosensitive drum on the cylinder 22 due to the magnetic lines of force of the magnet 23.
Magnetically connected at a position facing 10 to form a kind of brush-like, so-called magnetic brush 21A is formed. Then, the magnetic brush 21A is conveyed in the rotating direction of the cylinder 22 and comes into contact with and slides on the photosensitive layer 10a of the photosensitive drum 10. The rubbing operation of the magnetic brush is preferably performed before the DC component of the charging bias is applied. When a voltage is applied to the cylinder 22 and a potential difference is generated between the cylinder 22 and the photoconductor drum 10, magnetic particles tend to adhere to the photoconductor drum 10.

At this time, the cylinder 22 is rotated in advance and the magnetic brush rubs against it, so that the adhesion of magnetic particles can be prevented.
It is considered that the transfer of electric charges is facilitated and the potential difference is reduced, so that the adhesion of magnetic particles is reduced.

At the start of charging, the cylinder 22 and the photosensitive drum 10
3, the DC component is first applied following the rotation of the cylinder 22, and after the DC component reaches a predetermined value, immediately or after a short time d ₁ elapses. The AC component is increased from 0V to a predetermined peak-to-peak voltage.
By shifting the application timing of the AC component in this way, it was possible to prevent the magnetic particles from moving and adhering to the photosensitive drum 10 at the start of charging. It is preferable that the rising time t _D1 accompanying the ON operation of the DC component and the rising time t _A1 accompanying the ON operation of the AC component shown in FIG. 3 are ₁ to 500 msec.

Further, in order to prevent the adhesion of magnetic particles,
Before the charging, the charge of the image forming body is set to about 0 in advance by the static elimination lamp 14.
It is preferable to eliminate the charge to V.

As described above, by applying the DC and AC bias voltages between the cylinder 22 and the photoconductor drum 10, the charge is injected onto the photoconductor layer 10a via the conductive magnetic particles 21 to be charged. Done. In this case, since the bias voltage is a bias voltage in which an alternating current component is superimposed on a direct current component, it is possible to inject charges from the magnetic brush that accompany the movement of charges and the discharge phenomenon and improve its efficiency, which is extremely stable. It is possible to perform uniform charging at high speed.

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

In FIG. 8, 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. In addition, in FIG. 8, the low frequency region of 300 Hz or less, which is shaded,
This is the range where uneven charging occurs due to the low frequency.

(Second Embodiment) The configuration and operation of this embodiment showing the third embodiment of the present invention are exactly the same as those of the first embodiment, and therefore detailed description thereof will be omitted. In the second embodiment, a unique method is used to stop the application of the AC bias voltage when the charging is stopped.

That is, when the charging is stopped, as shown in FIG. 3, first, the peak-to-peak voltage of the AC component is lowered to about 0V, and immediately or after a short time d _{2 is} passed, the DC component is lowered to 0V. The rotation of the cylinder 22 is stopped. In this way, the AC component between the magnetic brush and the photoconductor drum 10 is eliminated and then the DC component is reduced. Therefore, it is possible to prevent the magnetic particles from moving and adhering to the photoconductor drum 10 when the charging bias is stopped. It was The fall times t _A2 and t _D2 associated with the OFF operation shown in FIG. 3 are preferably 1 to 500 msec. Further, d ₂ is preferably 0 to 500 msec.

Further, in order to prevent the magnetic particles from adhering, it is preferable that the charge of the image forming body is previously discharged to about 0 V by the discharging lamp 14 even after the charging bias is stopped. By doing so, it is possible to prevent a potential difference from being generated between the photosensitive member which is the image forming body and the cylinder 22, and to prevent the magnetic particles from being attached to the image forming body.

After the charging bias is stopped, the rubbing operation of the magnetic brush is stopped. This stop is performed before or simultaneously with the stop of the photosensitive drum 10.

In the same manner as in the first and second embodiments, without using the static elimination lamp 14, the image forming body potential is charged by applying the direct current component of the bias voltage and then applying the alternating current component, or stopping the alternating current component. This can be done by stopping the DC component, but the static elimination lamp
The use of 14 can reliably prevent magnetic particles from adhering to the image forming body.

When the cylinder 22 is retracted or the magnetic poles of the magnet 23 are rotated so that the NS direction is parallel to the tangent line of the facing portion of the photoconductor drum 10, the ears of the magnetic brush move to the photoconductor drum 10 by the horizontal magnetic field. Since the magnetic brush is parallel to the tangential direction of the facing portion and the tip of the magnetic brush is separated from the photoconductor drum 10, it can be brought out of contact with the image forming body. As a result, it is possible to prevent the photoreceptor from being deteriorated or changed when it is left.

It is desirable that the operation of separating the magnetic brush from the photoconductor is performed after the application of the DC and AC biases is stopped. It is possible to prevent the phenomenon in which the magnetic particles move to the photoconductor when a potential difference occurs between the photoconductor and the cylinder 22.

The ON / OFF of the bias voltage at the time of starting / stopping the rotation of the cylinder 22 is delayed at the start,
In the case of stopping, it is preferable to do it early. The start / stop of the rotation of the photoconductor drum 10 is not particularly limited, but the start of the rotation of the photoconductor drum 10 starts earlier than the start of the rotation of the cylinder 22 of the charging device, and the start of the rotation stops late when the charging is stopped to shorten the charging time. It is preferable because When the image forming apparatus causes a paper jam or the like to start / stop the operation, it is preferable to perform the same as above.

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.

For this purpose, the bias voltage applied to the cylinder 22 to such an extent that magnetic particles do not adhere to the photoreceptor before the image formation, during the image formation, or during the preliminary rotation of the photoreceptor drum 10 after the image formation is charged by only the DC component. The voltage of the DC component is set lower than that of the charging time, or the voltage of the AC component is set to be lower than that of the charging when the DC voltage is the same as when charging, and the condition that toner easily adheres to the photoconductor is set to prevent toner mixing. It can be prevented from increasing. In particular, when the charging polarity of the photosensitive drum 10 is the same as that of the toner as in an image forming apparatus that performs reversal development, the toner polarity is the same as the toner polarity in the developing device, so that contamination with toner is less likely to occur, and the image during development It does not appear as a fog and is a very suitable combination.

Further, the charging device 20 described above also serves as a cleaning device for cleaning the residual toner on the photoconductor drum 10 by making the DC component of the output bias voltage the same as the charging polarity of the residual adhered toner. You can also do it. Further, the voltage of the DC component of the bias power source 24 can be changed so that it can be used as a device for controlling the temperature and humidity of the photoconductor and the potential fluctuation due to repetition to be constant.

Further, in the present invention, the magnet is contained in the cylinder as the magnetic particle carrier, but the cylinder may be omitted and the magnetic particles may be directly adhered to the magnet roll to form the magnetic brush. .

(Third Embodiment) In the second and third inventions of the present invention described in the first and second embodiments, the magnetic particles adhere to the image forming body at the start and stop of charging. The magnetic brush 21 of the charging device 20 according to the first aspect of the present invention described in the third embodiment will be described below.
A has a structure in which it does not come into direct contact with the photoconductor and prevents toner from adhering to the image forming body. FIGS. 4, 5, 6 and 7 show the charging device of this embodiment. The thin plate 29 interposed between the magnetic brush 21A and the photosensitive drum 10 is 10 to 30.
A thin elastic film having a thickness of 0 μm and having a medium to high resistance (10 ¹² Ω · cm or less) of 10 ⁵ Ω · cm or more is preferably used. This thin plate 29 can also be brought into sliding contact with the photosensitive drum.
10 It is necessary to be a shielding member that does not damage the surface and does not cause dielectric breakdown due to charging, which has excellent wear resistance, and is used, for example, as a blade for cleaning having the above-mentioned conductivity. Urethane rubber and mylar thin plate mixed with carbon are preferably used.

In the charging device of this embodiment, the description of the previous embodiment will be applied as it is, except for the newly provided thin plate 29, and will therefore be omitted. In the embodiment shown in FIG. 4, one end 291A of the thin plate 29A is attached to the casing 25 or the regulating plate 26 portion of the charging device 20A which corresponds to the upstream side of the photoconductor drum 10 which rotates in opposition.
The thin plate 29A is interposed between the magnetic brush 21A and the photosensitive drum 10 to form a contact surface with a uniform pressure by the magnetic brush. , Is in sliding contact with the surface of the photoconductor. With this configuration, magnetic particles are prevented from adhering to the surface of the photoconductor, the breakdown of the photoconductor that is the charged body is prevented, and uniform charging is performed. Further, by interposing the thin plate 29A, it is possible to prevent foreign matter from mixing with the magnetic particles in the charging device 20.

In the embodiment shown in FIG. 5, both ends 291B and 292B of the thin plate 29B are provided in the opening portions on both sides of the casing 25 (one is the regulating plate 26 in this embodiment) so that the thin plate 29B covers the opening portion of the charging device 20B. The magnetic particles are attached so that the magnetic particles do not leak from the charging device 20 to the outside. Also in this embodiment, the thin plate 29B is interposed between the magnetic brush 21A and the photoconductor drum 10, forms a contact surface with a uniform pressure by the magnetic brush, and slides the photoconductor surface. With such a structure, the magnetic particles are prevented from adhering to the surface of the photoconductor and the breakdown is prevented, and uniform charging is performed.

In the embodiment shown in FIGS. 4 and 5, due to long-term use, adhesion of toner or the like occurs on the thin plates 29A and 29B.
The surface of the photoconductor is contaminated and the photosensitivity of the photoconductor is deteriorated. FIG. 6 and FIG. 7 to be described next solve this problem.

In FIG. 6, two winding cores 291C and 292C are provided with the opening of the charging device 20C sandwiched therebetween, and a thin plate 29C is wound around one of the two winding cores 291C and 292C, for example, and the other winding core 292.
It is configured to wind with power on C, core 291
An appropriate back tension is applied to the C side, and the winding speed on the winding core 292C side is extremely reduced or intermittently performed. With this configuration, the thin plate 29C
Even if toner or the like adheres to the photoconductor side of the thin plate 29C during charging via the sheet, the thin plate 29C portion that has become dirty due to the adhesion is wound up on the winding core 292C side and is in a new clean state.
Since C is supplied from the core 292C in an appropriate tension state,
The surface of the photoconductor is slidably contacted with a uniform pressure of the magnetic brush, and the surface of the photoconductor is not contaminated, and uniform charging is performed. Also in this case, the adhesion of magnetic particles to the surface of the photoconductor and the breakdown of the photoconductor are prevented, as in the previous embodiment.

In the embodiment shown in FIG. 7, two winding cores 291D and 292D are provided so as to sandwich the opening of the charging device 20D and two winding cores are provided.
A thin plate 29D passed between 291D and 292D is interposed between the magnetic brush 21A and the photosensitive drum 10 with an appropriate tension, and is reciprocated appropriately. Thin plate for thin plate 29D
The cleaning blade 293D for cleaning the toner and the adhered matter adhered to the surface of the 29D is brought into contact with the cleaning blade 2
The toner swept off by 93D is designed to fall into the toner collecting section 294D. Thin plate constructed in this way
By appropriately reciprocating the 29D, the surface of the thin plate 29D is cleaned by removing the toner, and the thin plate 29D forms a contact surface with the photoconductor drum 10 with a uniform pressure of the magnetic brush, and the surface of the magnetic particles of the photoconductor is formed. Uniform charging is performed without adhesion to the surface or breakdown of the photoconductor.

[0063]

EFFECT OF THE INVENTION The charging device according to the present invention generates almost no ozone, and magnetic particles move and adhere to the image forming body not only during charging but also during charging start and stop, thereby damaging the image forming body and uneven charging. Therefore, it is possible to provide a charging device capable of performing stable and uniform charging without causing a charging.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an outline of a configuration of an image forming apparatus provided with a charging device of the present invention.

FIG. 2 is a sectional view showing an embodiment of the charging device of the present invention.

FIG. 3 is a time table showing the operation of the present invention.

FIG. 4 is a cross-sectional view showing another embodiment A of the charging device of the present invention.

FIG. 5 is a sectional view showing another embodiment B of the charging device of the present invention.

FIG. 6 is a cross-sectional view showing another embodiment C of the charging device of the present invention.

FIG. 7 is a sectional view showing another embodiment D of the charging device of the present invention.

FIG. 8 is a charging characteristic diagram when the frequency and voltage of an AC voltage component are changed.

[Explanation of symbols]

 10 Photosensitive drum 20, 20A, 20B, 20C, 20D Charging device 21 Magnetic particles 22 Cylinder (charging roller) 23 Magnet 24 Bias power supply 25 Casing 26 Regulator plate 28 Protective resistance 29, 29A, 29B, 29C, 29D Thin 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 (5)

[Claims] 1. A charging device which forms a magnetic brush on an image forming body and charges under an AC bias, wherein the magnetic brush contacts the image forming body through a thin plate. 2. The charging device according to claim 1, wherein the thin plate is an elastic member having a thickness of 10 to 300 μm. 3. The charging device according to claim 1, wherein the thin plate is replaceable. 4. A charging device 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. A charging device characterized by applying an alternating voltage after the application. 5. In a charging device for charging magnetic particles on a carrier to form a magnetic brush and placing the magnetic brush on the carrier under an oscillating electric field to charge an image forming body, an AC voltage is applied when charging is stopped. A charging device characterized in that the DC voltage is stopped after the charging is stopped.