JP3165925B2 - Charging device - Google Patents

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 apparatus such as an electrophotographic copying machine.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, a corona charger is generally 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. Done.

The corona charger used in such a conventional image forming apparatus can be charged without mechanically contacting the image forming body, so that the image forming body is not damaged during charging. Has advantages. However, this corona charger uses a high voltage, so there is a risk of electric shock or leakage, and ozone generated by gas discharge is harmful to the human body, and the life of the image forming body is shortened. Had. Further, the charging potential of the corona charger is unstable because it is strongly affected by temperature and humidity. Further, in the corona charger, it takes about several seconds to obtain a stable charging potential after high voltage input. This is a major drawback when using an electrophotographic image forming apparatus as a communication terminal or an information processing apparatus.

Many disadvantages of such corona chargers are:
This is because charging involves gas discharge.

[0005] Therefore, as a charging device capable of charging an image forming body without performing gas discharge as in a corona charger and without causing mechanical damage to the image forming body, a charging device on a cylinder containing a magnet is used. Japanese Patent Application Laid-Open No. 53-133569 discloses a charging device in which a magnetic brush is formed by adsorbing magnetic particles to the surface of an image forming body by rubbing the magnetic brush with the magnetic brush. .

[0006]

However, even the charging device disclosed in Japanese Patent Application Laid-Open No. 53-133569 has a problem that the image forming body cannot be completely and stably charged uniformly. there were. In this regard, mixing of the toner into the magnetic brush is a serious problem, which is attributed to the fact that the toner is insulative and the toner reduces the chargeability.

An object of the present invention is to solve the above problems and to provide a charging device capable of performing extremely stable and uniform charging without generating ozone.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a magnetic brush comprising a cylinder rotatably disposed around the outer periphery of a magnet having magnetic poles disposed thereon and a magnetic particle layer attached to the cylindrical portion, in a moving direction of the image forming body. The magnetic particles are moved and slid in the same direction or in the opposite direction, and a bias voltage is applied to the magnetic brush to charge the image forming body. Is less than 3 times spherical
The bias voltage applied to the magnetic brush is Pe to a DC component having an absolute value of 500 to 1000 V.
200-3500 as ak-Peak voltage (V _PP )
An AC bias voltage on which an AC component of V is superimposed.
The polarity of the DC component of the bias voltage constitutes the magnetic brush
This is achieved by a charging device characterized in that the polarity of the toner is the same as the polarity of the toner obtained by frictionally charging the magnetic particles and the toner.

[0009]

The present invention is characterized in that the charging polarity of the toner and the magnetic particles in the image forming apparatus is the same as the charging polarity for charging the image forming member. Thereby, the adhesive force between the magnetic particles and the toner is reduced, and the toner mixed into the image forming body is adhered.
Here, the particle size of the magnetic particles will be described. In general, if the average particle size of the magnetic particles is large, (a) the state of the ears of the magnetic brush formed on the carrier is rough, and the magnetic particles are charged while applying vibrations by an electric field. Even so, unevenness is likely to appear on the magnetic brush, causing the problem of uneven charging. In order to solve this problem, the average particle size of the carrier particles may be reduced. As a result of the experiment, the effect starts to appear when the average particle size is 100 μm or less. Has been found to no longer occur. However, if the particles are too fine, they will adhere to the surface of the image carrier at the time of charging, or will be easily scattered. These phenomena are related to the intensity of the magnetic field acting on the particles and the intensity of the magnetization of the particles, but generally, the average particle size of the particles becomes remarkable when it is 30 μm or less. Note that the magnetization intensity is preferably 20 to 200 emu / g.

From the above, the average particle diameter of the magnetic particles is
It is preferably 100 μm or less, particularly preferably 70 μm or less and 30 μm or more.

Such magnetic particles are made of a metal such as iron, chromium, nickel, cobalt or the like, or a compound or alloy thereof, such as triiron tetroxide, γ-oxide, as in the case of conventional magnetic carrier particles. Ferric iron, chromium dioxide, manganese oxide, ferrite, manganese-copper alloy,
The surface of the ferromagnetic particles or the 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, etc. Or by using a resin containing magnetic fine particles dispersed therein to obtain particles obtained by a conventional known average particle size selecting means.

The formation of the magnetic particles in a spherical shape is as follows.
The effect that the particle layer formed on the carrier becomes uniform and a high bias voltage can be uniformly applied to the carrier is also provided. That is, the fact that the magnetic particles are spherical is 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 layer is formed uniformly and the local layer is formed. (2) With the increase in the resistance of the magnetic particles, the edge portion as seen in conventional particles is eliminated, and the concentration of the electric field on the edge portion is reduced. As a result, even when a high bias voltage is applied to the magnetic particle carrier, an effect is obtained in that the surface of the image carrier is uniformly discharged and uneven charging does not occur. The spherical particles exhibiting the above effects have a carrier particle resistivity of 10 ⁵ Ωcm.
As described above, it is particularly preferable to form magnetic particles having a chargeability of 10 ¹² Ω · cm or less. This resistivity reduces the particle to 0.
After tapping in a container having a sectional area of 50 cm ^2, a load of 1 kg / cm ² on packed particles, when a voltage is applied to the field of 1000V / cm is generated between the load and a bottom electrode Is the value obtained by reading the current value of
When the resistivity is low, when a bias voltage is applied to the carrier, electric charges are injected into the magnetic particles, so that the magnetic particles easily adhere to the surface of the image carrier, or the breakdown of the bias voltage easily occurs. Or On the other hand, if the resistivity is high, charge injection is not performed and charging is not performed.

In view of the above, the magnetic particles are spherical so that the ratio of the major axis to the minor axis is at least three times or less, there are no projections such as needles or edges, and the resistivity is low. 10 ⁶ Ω
It is an appropriate condition that the thickness be not less than cm and preferably not more than 10 ¹⁰ Ω · cm. For such spherical magnetic particles, select magnetic particles as spherical as possible.For particles of a magnetic fine particle dispersion system, use magnetic particles as much as possible, and perform spheroidizing treatment after forming dispersed resin particles. It is manufactured by applying or having dispersed resin particles by a spray drying method.

In order for the toner and the magnetic particles to move to the image carrier during charging, a low toner charge amount is required.
When the toner was mixed with the magnetic particles and the amount of triboelectric charge was adjusted to 3 to 20 μC / g under the condition of adjusting the toner concentration to 1%, accumulation of the toner on the magnetic brush could be prevented. It was found that when the charge amount of the toner is large, it is difficult to separate from the magnetic particles, and when the charge amount is small, it does not electrically move to the image carrier.
This is considered to be due in part to the generation of opposite polarity toner.

The conditions for the magnetic particles have been described above. Next, the conditions for the magnetic particle carrier for forming a particle layer and charging the image carrier will be described.

As the magnetic particle carrier, a carrier to which a bias voltage can be applied is used. In particular, a carrier having a plurality of magnetic poles is provided inside a sleeve having a surface on which a particle layer is formed. Are preferably used. In such a carrier, since the particle layer formed on the surface of the sleeve moves undulatingly due to relative rotation with the rotating magnet body, new magnetic particles are supplied one after another, Even if there is some non-uniformity in the thickness of the particle layer on the sleeve surface, the effect is sufficiently covered by the wavy undulation so as not to cause a practical problem. The transport speed of the magnetic particles due to the rotation of the sleeve or the rotation of the magnet body is preferably almost the same as or faster than the moving speed of the image carrier. Further, the conveying direction by the rotation of the sleeve is preferably the same. The same direction has better charging uniformity than the opposite direction. However, it is not limited to them.

The thickness of the particle layer formed on the carrier is preferably such that it is sufficiently scraped off by the regulating plate to form a uniform layer. The gap is preferably 100 to 5000 μm. If the surface gap between the carrier and the image carrier becomes too narrower than 100 μm, it becomes difficult to form the magnetic brush ears that uniformly act on the surface, and supply sufficient magnetic particles to the charging unit. In addition, stable charging cannot be performed, and if the gap exceeds 5000 μm, the particle layer is formed coarsely and charging unevenness easily occurs, and the charge injection effect is reduced and sufficient charging is performed. Can not be obtained.
As described above, when the gap between the carrier and the image carrier is extremely large, the thickness of the particle layer on the carrier cannot be appropriately adjusted.However, when the gap is in the range of 100 to 5000 μm, on the other hand, Thus, the particle layer can be formed to have an appropriate thickness. This is because it is possible to prevent the occurrence of sweeping lines due to sliding of the magnetic brush.

In the image forming method to which the present charging device is applied, reversal development is more preferable in development. This is because the toner discharged from the charging device has the same polarity during reversal development, so that fogging of the image can be prevented.

[0019]

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 configuration of an electrophotographic recording apparatus provided with the charging device of the present invention.

In the figure, reference numeral 10 denotes a (-) charged OPC photosensitive drum which is an image forming body which rotates in the direction of the arrow (clockwise).
In the peripheral portion, a charging device 20 described later, image exposure L from an exposure device, a developing device 30, a transfer roller 13, and a 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 photosensitive drum is controlled by the control unit.
10 starts rotating in the direction of the arrow. As the photoreceptor drum 10 rotates, its peripheral surface is uniformly charged and passed by a charging device 20 described later. 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.

A two-component developer is contained in the developing device 30 and is stirred by the stirring screws 33A and 33B, and then adheres to the outer periphery of the rotating developing sleeve 31 outside the magnet roller 32 and adheres to the outer periphery of the rotating developing sleeve 31. Forming a developing sleeve
A predetermined bias voltage is applied to 31 and the photosensitive drum
Reversal development is performed in the development area facing 10.

From the paper feed cassette 40, the recording paper P is fed out one by one by a first paper feed roller 41. The fed recording paper P is sent out onto the photosensitive drum 10 by a second paper feed roller 42 that operates in synchronization with the toner image on the photosensitive drum 10. Then, the toner image on the photosensitive drum 10 is transferred onto the recording paper P by the operation of the transfer roller 13 and separated from the photosensitive drum 10. The recording paper P to which the toner image has been transferred is sent to a fixing device (not shown) via a conveying means 80, is pinched by a heat fixing roller and a pressure roller, is melt-fixed, and is discharged out of the device. Recording paper P
The surface of the photoreceptor drum 10 that rotates with the toner remaining without being transferred to the surface is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and waits for the next copy.

FIG. 2 is a sectional view showing an embodiment of the charging device 20 of the present invention used in the electrostatic recording apparatus shown in FIG. In the figure, reference numeral 21 denotes magnetic particles, and spherical ferrite particles coated to have conductivity are used. In addition, conductive magnetic resin particles obtained by pulverizing magnetic particles and a resin as main components after thermal smelting can also be used. In order to perform good charging, the external shape is a true sphere and the particle size is 50μ
m, and the specific resistance was adjusted to 10 ⁸ Ω · cm, and the triboelectric charge amount with the toner was -5 μC / g under the condition of a toner concentration of 1%.

22 is a conductive cylinder formed of a non-magnetic metal,
Reference numeral 23 denotes a columnar magnet rod (roll) disposed inside the conductive cylinder 22. The magnet 23 is magnetized so as to arrange an S pole and an N pole in the figure.
Is rotatable with respect to the fixed magnet 23. Further, the magnet 23 may be rotated as an equipolar arrangement magnetic pole. The conductive cylinder 22 is rotated at a position facing the photosensitive drum 10 at a circumferential speed of 1.2 to 2.0 times in the same direction as the moving direction of the photosensitive drum 10.

The photosensitive drum 10 comprises a conductive base material 10b and a photosensitive body layer 10a covering the surface thereof, and the conductive base material 10b is grounded.

Reference numeral 24 denotes a bias power supply for applying a bias voltage between the conductive cylinder 22 and the conductive base 10b. The conductive cylinder 22 is grounded via the bias power supply 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 a voltage to be charged. The bias power supply 24 is connected to the conductive cylinder 22 and the photosensitive drum via a protection resistor 28. The gap is maintained between 0.1 mm and 5 mm, and the DC component of -500 V to -1000 V, which is approximately the same as the voltage to be charged, although it varies depending on the size of the gap between the photoconductor 10 and the charging voltage for charging the photosensitive drum 10. The Peak-Peak voltage (V
By supplying an AC bias voltage in which an AC component of 200 to 3500 V was superimposed as pp), favorable charging conditions could be obtained. By applying an AC bias, charging without unevenness can be obtained.

The bias power supply 24 performs constant voltage control for the DC component and constant current control for the AC component.

Reference numeral 25 denotes a casing forming a storage portion for the magnetic particles 21. The conductive cylinder 22 and the magnet 23 are disposed in the casing 25. A regulating plate 26 is provided at an outlet of the casing 25. The thickness of the layer of magnetic particles 21 adhered to the conductive cylinder 22 and carried out is regulated, and the gap between the photosensitive drum 10 and the conductive cylinder 22 is a magnetic layer whose thickness is regulated. The particles are connected by 21 layers.

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

When the conductive cylinder 22 is rotated in the direction indicated by the arrow at a peripheral speed of 1.2 to 2.0 times the peripheral speed of the photosensitive drum 10 while rotating the photosensitive drum 10 in the direction indicated by the arrow, the conductive cylinder 22 The layer of magnetic particles 21 attached and conveyed is magnetically connected at a position facing the photosensitive drum 10 on the conductive cylinder 22 by lines of magnetic force of the magnet 22 to form a kind of brush, forming a so-called magnetic brush. . And the magnetic brush is a conductive cylinder 22
Of the photosensitive drum 10
And rub. Since the AC bias voltage is applied between the conductive cylinder 22 and the photosensitive drum 10, charges are injected into the photosensitive layer 10a through the conductive magnetic particles 21 to perform charging. In this case, in particular, since the bias voltage is set to the AC bias voltage, the charge injection from the magnetic brush is vibrated, and extremely stable and uniform charging can be performed. The stirring plate 27 is a rotating body having a plate member for correcting the bias of the magnetic particles 21 around the axis.

In the above embodiment, the conductive cylinder
FIG. 3 shows the result of changing the frequency and the voltage of the AC voltage component applied to 22.

In FIG. 3, the range shaded by a vertical line is a range where dielectric breakdown is likely to occur, the range shaded by a diagonal line is a range where charge unevenness is likely to occur, and the range without shade is stable. This is a preferred range in which charging can be obtained. As is apparent from the figure, the preferable range slightly changes due to the change in the AC voltage component. The waveform of the AC voltage component is not limited to a sine wave, but may be a rectangular wave or a triangular wave. In FIG. 3, the low-frequency region shaded in a scattered manner is a range in which charging unevenness occurs due to a low frequency.

In this embodiment, non-charging can be performed by setting the DC component of the bias voltage to zero. When the NS direction of the magnetic pole of the magnet 22 is rotated so as to be parallel to the tangent to the opposing portion of the photoreceptor drum 10, the spikes of the magnetic brush are parallel to the tangential direction of the opposing portion to the photoreceptor drum 10 by a horizontal magnetic field. Thus, the tip of the magnetic brush is separated from the photosensitive drum 10, so that the magnetic brush can be uncharged.

The toner magnetic particles 21 remaining on the surface of the photoreceptor drum 10 after being used for a long time without being cleaned.
In some cases, the mixing into the layer increases, the resistance of the magnetic brush increases, and the charging efficiency is impaired.

For this purpose, the polarity of the DC bias voltage applied to the conductive cylinder 22 during the pre-rotation of the photosensitive drum 10 before image formation is set high, or the AC voltage is set high.
Conditions for easily attaching the toner to the photoreceptor can be set to prevent the toner from being mixed. In particular, when the charging polarity of the photoconductor drum 10 is the same as that of the toner, as in an image forming apparatus that performs reversal development, the toner is hardly contaminated because it has the same polarity as the toner in the developing unit, and the image is fogged during development. , Which is a very suitable combination.

[0039]

According to the present invention, the charge is directly injected into the photosensitive drum, so that the bias voltage can be reduced, the generation of ozone is prevented, and the bias voltage is set to the AC bias voltage, so that it is extremely stable. Thus, a charging device capable of performing uniform charging can be provided.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an outline of a configuration of an electrostatic recording device including 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 charging characteristic diagram when the frequency and voltage of an AC voltage component are changed.

[Explanation of symbols]

 10 Photoreceptor drum 20 Charging device 21 Magnetic particles 22 Conductive cylinder 23 Magnet 24 Bias power supply 25 Casing 26 Regulator plate 28 Protection resistor

Claims (2)

(57) [Claims] A magnetic brush comprising a cylinder having a magnetic pole and a magnetic particle layer attached to the cylindrical portion rotatably arranged on the outer periphery of the magnet, and a magnetic brush made of a magnetic particle layer attached to the cylindrical portion; In a charging device for charging the image forming body by applying a bias voltage to the magnetic brush, the magnetic particles have a spherical magnetic shape having a ratio of a major axis to a minor axis of 3 times or less.
A particle, a bias voltage applied to the magnetic brush
Is Peak- to the DC component whose absolute value is 500-1000V.
Peak voltage (V _PP ) of 200-3500 V
An AC bias voltage on which a flow component is superimposed.
The polarity of the DC component of the magnetic brush is
A toner having the same polarity as the toner obtained by frictionally charging the conductive particles and the toner. 2. The toner according to claim 1, wherein the toner has a triboelectric charge amount of 1
2. The method according to claim 1, wherein the percentage is 3 to 20 .mu.C / g.
Charging device.