JPH06194928A - Magnetic brush electrostatic charger - Google Patents

Abstract

PURPOSE:To provide the electrostatic charger which can make extremely stable and uniform electrostatic charge without generating ozone and prevent the adhesion of magnetic particles to an image forming body by specifying the characteristics of the magnetic particles forming a magnetic brush. CONSTITUTION:The saturation magnetization sigma of the magnetic particles 21 forming the magnetic brush is 20 to 100emu/g and the average grain size R thereof is 30 to 100mum and 90wt.% of the magnetic particles are confined within a + or -10mum range from the average grain size. Further, sigma and R are 2X10<6=sigmaR<3=4X10<7>. The problem of unequal electrostatic charge is solved simply by reducing the average grain size of carrier particles, but the pickup of the particles to the surface of the image forming body and conversely the easy tendency to splashing are simultaneously liable to arise if the particles are too small. Then, 90wt.% of the magnetic particles are required to be confined within the + or -10mum range from the average grain size. Since an AC bias voltage is supplied from a bias power source 24, vibrations are applied to the charge implantation from the magnetic brush and the extremely stable and uniform electrostatic charge is possible.

Description

Detailed Description of the Invention

[0001]

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

[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.Charging is performed by adsorbing the ions generated at that time to the image forming body. Be seen.

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. In addition, the charging potential of the corona charger is unstable because it is strongly affected by temperature and humidity. Furthermore, it takes several seconds for the corona charger to obtain a stable charging potential after high voltage input. However, this is a major drawback when an electrophotographic image forming apparatus is used as a communication terminal or an information processing apparatus.

Many drawbacks of such corona chargers are:
The cause is that gas discharge is involved in charging.

Therefore, as a charging device capable of charging the image forming body without causing a gas discharge like a corona charger and mechanically damaging the image forming body, a cylindrical body containing a magnet is used. Japanese Patent Application Laid-Open No. 59-133569 discloses a charging device in which magnetic particles are adsorbed on a magnetic brush to form a magnetic brush, and the surface of the image forming body is rubbed with the magnetic brush to perform charging.

[0006]

However, even in the charging device disclosed in Japanese Patent Laid-Open No. 59-133569, the image forming body is not completely stably and uniformly charged, resulting in uneven charging, and further image formation. There is a problem that magnetic particles adhere to the surface of the formed body and impair the image quality.

The present invention solves these problems and can perform extremely stable and uniform charging without generation of ozone, and there is no adhesion of magnetic particles to the image forming body to give an image of good quality. The purpose is to provide a device.

[0008]

[Means for Solving the Problems] In solving the problems, the form of uneven charging is mainly a sweep based on a rigid magnetic brush, weakening the magnetization of each magnetic particle to soften the rigidity of the magnetic brush, And it needs to be dense.

On the other hand, with respect to the adhesion of magnetic particles to the surface of the image forming body, the magnetic particles are prevented from being detached from the charged rubbing surface, that is, the magnetization of the magnetic particles is strengthened to increase the particle volume and the magnetic particle binding force of the charging device is relatively increased. Need to be strengthened.

There are reciprocal factors that prevent the solution of problems.
In the present invention, this reciprocity is eliminated by specifying the relationship between the granular condition of the magnetic particles and the magnetization.

That is, according to the present invention, a magnetic brush made of magnetic particles is formed on a freely rotating cylinder (hereinafter referred to as a sleeve) that encloses the outer circumference of a magnet in which magnetic poles are arranged, and the magnetic brush has a DC component. In a charging device that applies an AC bias voltage and contacts the rotating image forming body surface to give an electric charge by rubbing, the magnetic particles forming the magnetic brush have a saturation magnetization σ of 20 to 100 emu / g, and an average particle diameter. R is 30 ~
90 wt% of the magnetic particles are within 100 μm and within ± 10 μm from the average particle size, and 2 × for σ and R.
The problem of the present invention is solved by a magnetic brush charging device characterized in that 10 ⁶ ≦ σR ³ ≦ 4 × 10 ⁷ .

Inventive constitution and action and effect: First, the particle diameter of the magnetic particles will be described. Generally, when the average particle diameter R of the magnetic particles is large, the state of the ears of the magnetic brush formed on the sleeve becomes rough and the magnetic brush vibrates. Even if the toner is charged while being charged, unevenness is likely to appear on the magnetic brush itself, resulting in uneven charging. To solve this problem, the average particle size of the carrier particles should be reduced, and the effect is obtained with an average particle size of 30 to 100 μm.
Particularly, in the range of 50 to 70 μm, the above problem does 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 and, at the same time, easily scatter.

Therefore, particles having an excessively large or small particle size distribution are removed, and when contaminants such as non-magnetic materials are mixed, the particles are removed by washing, filtering, or an electromagnetic method, and the magnetization of the particles has a particle volume. That is, it is necessary to keep 90 wt% of the magnetic particles within the range of ± 10 μm from the average particle size R, taking into consideration that the particle size changes to the third power and that it fluctuates drastically.

Such a magnetic particle is a metal such as iron, chromium, nickel, cobalt or the like, or a compound or alloy thereof, such as iron tetroxide, γ-oxidation, which is the same as in a conventional magnetic carrier particle. 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, particles obtained by dispersing magnetic fine particles in a resin can be obtained by particle size selection by a conventionally known average particle size selection means.

Further, the shape of the magnetic particles is preferably a sphere having a ratio of the major axis to the minor axis of 2 or less, and an apparent density of ³ to 6 g / cm ^3, more preferably 4 to 5 g / cm ³ .

Forming the magnetic particles in a spherical shape is as follows.
The magnetic particle layer formed on the sleeve becomes uniform, and a high bias voltage can be uniformly applied to the sleeve. That is, the fact that the magnetic particles are spherical is
(1) Generally, magnetic particles are easily magnetized and adsorbed in the long-axis direction, but the spheroidization eliminates the directionality thereof, so that a layer is uniformly formed, and a locally low resistance region or uneven layer thickness is generated. (2) Along with the high resistance of the magnetic particles, there is no edge portion as seen in conventional particles, and the concentration of the electric field on the edge portions does not occur, and as a result, it is high in the sleeve that carries the magnetic particles. Even when a bias voltage is applied, the surface of the image bearing member is uniformly discharged and uneven charging does not occur.

Spherical particles having the above-mentioned effects have a resistivity of 10 ⁴ to 10 ¹⁰ Ω · cm, particularly 10 ⁶ to 10 ⁸ Ω · cm. This resistivity is 0.50 cm ^{2 for} particles
Current after applying a load of 1 kg / cm ² on the packed particles after putting them in a container having a cross-sectional area of 1 and applying a voltage that generates an electric field of 1000 V / cm between the load and the bottom electrode. It is a value obtained by reading the value.If this resistivity is low, when a bias voltage is applied to the sleeve,
The electric charge is injected into the magnetic particles, so that the magnetic particles are easily attached to the surface of the image carrier and the breakdown of the bias voltage easily occurs. If the resistivity is too high, charge injection is not performed and charging is not performed.

The magnetization σ of the magnetic particles whose specifications have been adjusted as described above needs to be 20 to 100 emu / g, preferably 40 to 80 emu / g.

Further, as to the magnetization σ, as described above, for the purpose of the present invention, it is required to have a trade-off between the strength and the strength, and the particle size is related to the cube of the particle size. It is necessary to fit the constraint of 2 × 10 ⁶ ≦ σR ³ ≦ 4 × 10 ⁷ between (μm) and the magnetization σ (emu / g).

The above are the conditions for the magnetic particles, and next, the conditions for the sleeve for forming the particle layer and charging the image carrier will be described.

As the sleeve, a sleeve to which a bias voltage can be applied is used. Particularly, it is preferable that the sleeve has a structure in which a magnet having a plurality of magnetic poles is provided inside the sleeve having a particle layer formed on the surface thereof. In such a sleeve, due to the relative rotation with the rotating magnet, the particle layer formed on the surface of the sleeve becomes wavy and moves, so that new magnetic particles are supplied one after another, and the surface of the sleeve is supplied. Even if there is some unevenness in the layer thickness of the particle layer, its effect is sufficiently covered so that it is not a practical problem due to the corrugation. Further, it is preferable that the transport speed of the magnetic particles due to the rotation of the sleeve or the rotation of the magnet is almost the same as or faster than the moving speed of the image forming body. Further, the transport direction by the rotation of the sleeve is preferably the same direction. The same-direction conveyance is superior to the opposite direction in charging uniformity. But,
It is not limited to them.

Further, the thickness of the particle layer formed on the sleeve is preferably such that it is sufficiently scraped off by the regulating plate to form a uniform layer, and the gap between the sleeve and the image forming body is 100. ˜5000 μm is preferred. This can contribute to preventing sweeping due to rubbing of the magnetic brush. If the surface gap between the sleeve and the image forming body becomes narrower than 100 μm, it becomes difficult to form the magnetic brush spikes that uniformly act on the surface, and it is also possible to supply sufficient magnetic particles to the charging section. If it disappears, stable charging will not be performed, and if the gap becomes much larger than 5000 μm, the particle layer will be rough and uneven charging will occur easily, and the charge injection effect will decrease and sufficient charging will be obtained. Will not be. In this way, if the gap between the sleeve and the image forming body deviates from the preferable range, the thickness of the particle layer on the carrier cannot be properly maintained.

[0023]

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 equipped with the charging device of the present invention.

In the figure, reference numeral 10 designates a photosensitive drum which is an image forming body which rotates in the direction of an arrow (clockwise direction), and a peripheral portion of which is a charging device 20, a static eliminator 12, and an image exposure L from an exposing device, A developing device 30, a 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 transmitted 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 the charging device 20 described later and passes through the static eliminator 12. The static eliminator 12 is controlled by the control unit so that the frame portion outside the image region is, for example, L
Irradiate with ED to remove electricity. However, the static eliminator 12 becomes unnecessary in the reversal development described later. An image is written on the photosensitive drum 10 by a scanning exposure device that scans an original image (not shown), an image writing device, or the like, for example, a laser beam L, 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
Development takes place in the development area opposite 10. Here, regular development is performed when a latent image is formed using a general scanning optical system, and reversal development is normally performed when image exposure with a laser beam is performed. The electrostatic latent image on the photosensitive drum 10 is developed by the developing device 30 and becomes a visible toner image.

The recording paper P is fed from the paper feed cassette 40 one by one by the first paper feed roller 41. The fed recording paper P is transferred to the photosensitive drum 10 by the second paper feed roller 42 which operates in synchronization with the toner image on the photosensitive drum 10.
Dispatched on. 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 roller, is fused and fixed, and is 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 the sheet is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and stands by for the next copying.

FIG. 2 is a sectional view showing an embodiment of the charging device 20 of the present invention used in the electrostatic recording apparatus of FIG. In the figure, 21 is a magnetic particle, and spherical ferrite particles coated so as to have conductivity are used. In addition, conductive magnetic resin particles obtained by pulverizing the magnetic particles and a resin as main components after thermal smelting can also be used. The outer shape is a true sphere and the particle size is 50μ to ensure good charging.
Adjusted to m and specific resistance of 10 ⁸ Ω · cm.

Reference numeral 22 is a conductive sleeve made of non-magnetic metal, 23 is a columnar magnet rod (roll) arranged inside the conductive sleeve 22, and this magnet 23 has an S pole and an N pole in the figure. The conductive sleeve 22 is magnetized to be arranged, and is rotatable with respect to the fixed magnet 23. Further, the magnet 23 may be a magnetic pole having an equal pole, or may be rotatable. The conductive sleeve 22 is rotated at a position facing the photosensitive drum 10 in the same direction as the moving direction of the photosensitive drum 10 at a peripheral speed of 1.2 to 2.0 times.

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 the conductive sleeve 22 and the conductive base material 10b.
A bias power source for applying a bias voltage between the conductive sleeve 22 and the conductive sleeve 22 is grounded via the bias power source 24.

The bias power source 24 is a power source for supplying an AC bias voltage in which an AC component is superposed on a DC component set to the same value as the voltage to be charged. The AC bias voltage varies depending on the size of the gap between the conductive sleeve 22 and the photoconductor drum 10, the charging voltage charged on the photoconductor drum 10, and the like, but the gap is held between 0.1 to 5 mm and charged. To the DC component of 500 to 1000V, which is almost the same as the power voltage, Peak-Peak
By supplying an AC bias voltage in which an AC component of 200 to 3500V is superimposed as the voltage (Vp-p), a preferable charging condition could be obtained.

The bias power source 24 performs constant voltage control for DC components and constant current control for AC components.

Reference numeral 25 denotes a casing forming a storage portion for the magnetic particles 21, and the conductive sleeve is provided in the casing 25.
22 and magnet 23 are arranged, and also casing 25
A restriction plate 26 is provided at the outlet of the conductive sleeve 22.
The thickness of the magnetic particle 21 layer adhered to and discharged from the photosensitive drum 10 is regulated, and the gap between the photosensitive drum 10 and the conductive sleeve 22 is connected by the magnetic particle 21 layer whose thickness is regulated. .

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

When the conductive sleeve 22 is rotated in the same direction as the arrow at a peripheral speed of 1.2 to 2.0 times the peripheral speed of the photosensitive drum 10 while rotating the photosensitive drum 10 in the arrow direction, the conductive sleeve 22 adheres to the conductive sleeve 22. 21 layers of magnetic particles being conveyed are magnets 23
Magnetic lines of magnetic force at the position facing the photoconductor drum 10 on the conductive sleeve 22 to form a kind of brush.
A so-called magnetic brush is formed. Then, the magnetic brush is conveyed in the rotating direction of the conductive sleeve 22 and is transferred to the photosensitive drum.
The photoconductor layer 10a of 10 is contacted and rubbed. Conductive sleeve 22
Since the AC bias voltage is applied between the photoconductor drum 10 and the photoconductor drum 10, the photoconductor layer passes through the conductive magnetic particles 21.
Electric charges are injected into 10a to be charged. In this case, since the AC bias voltage is used as the bias voltage in particular, 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-shaped member that corrects the bias of the magnetic particles 21 around the axis.

In the above embodiment, the sleeve 2
The results of changing the frequency and voltage of the AC voltage component applied to the 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. It is a preferable range where 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.

In the present embodiment, non-charging can be performed by setting the DC component of the bias voltage to zero. Further, when the NS direction of the magnetic pole of the magnet 23 is rotated so as to be parallel to the tangent line of the facing portion of the photosensitive drum 10, the ears of the magnetic brush are parallel to the tangential direction of the facing portion of the photosensitive drum 10 due to the horizontal magnetic field. Since the tip of the magnetic brush is separated from the photoconductor drum 10, it can be uncharged.

The magnetic particles 21 of the toner remaining on the surface of the photoconductor drum 10 without being cleaned due to long-term use.
If the amount of toner mixed in the layer increases, the resistance of the magnetic brush increases and the charging efficiency is impaired. Therefore, it is necessary to prevent toner from mixing. To this end, magnetic particles are designed to be triboelectrically charged with the toner, and a collecting roller to which a voltage for generating an electric field for attracting the toner is applied is provided in the charging device 20 so that the mixed toner can be adsorbed and collected in an electric field. . Further, when the polarity of the DC bias voltage applied to the conductive sleeve 22 is the same as the toner charging, the toner easily adheres to the photoconductor during charging, and the toner mixing can be prevented. Particularly 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 polarity is the same as the toner polarity in the developing device, and therefore a very suitable combination that does not appear as fogging in the image during development. Becomes

[0042]

According to the present invention, since the charges are directly injected into the photosensitive drum, the bias voltage can be lowered, ozone is prevented from being generated, and the bias voltage is an AC bias voltage. It is possible to provide a charging device capable of performing uniform charging.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing 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 a 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 Photoconductor drum 20 Charging device 21 Magnetic particles 22 Conductive sleeve 23 Magnet 24 Bias power supply 25 Casing 26 Regulator plate 28 Protective resistance

 ─────────────────────────────────────────────────── ─── 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 (1)

[Claims] 1. A magnetic brush made of magnetic particles is formed on a freely rotating cylinder that encloses the outer circumference of a magnet in which magnetic poles are arranged, and an AC bias voltage having a DC component is applied to the magnetic brush to rotate the magnetic brush. In a charging device that gives electric charges by contacting and rubbing against the surface of an image forming body, the magnetic particles forming the magnetic brush have saturation magnetization σ of 20 to 100 emu / g, average particle size R of 30 to 100 μm, and average particle size. 90% by weight of the magnetic particles are within a range of ± 10 μm from the diameter, and further, 2 × 10 ⁶ ≦ σR ³ ≦ 4 × 10 ⁷ with respect to σ and R described above.