JPH06118767A - Electrifier - Google Patents

Abstract

PURPOSE:To provide an electrifier capable of extremely, stably, and uniformly performing electrification without producing ozone. CONSTITUTION:As to the electrifier 20 electrifying a photosensitive drum 10 by bringing a magnetic brush consisting of a non-magnetic and conductive cylinder 22 where the external periphery of a magnet 23 arranged with a magnetic pole on its external periphery is rotatably disposed and magnetic particles 21 layer sticking on the external periphery of the cylinder 22 into contact with the photosensitive drum 10 when the drum 10 moves, so that a bias electric field is formed between the cylinder 22 and the photosensitive drum 10; and also an oscillating electric field is used as a bias electric field, and the magnetic brush is formed in a state where the amount of magnetic particles at an electrified area is 10-200mg/cm<2>.

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. Japanese Patent Laid-Open No. 59-133569 discloses a charging device in which magnetic particles are adsorbed on a cylindrical support to form a magnetic brush and the surface of the image forming body is rubbed with the magnetic brush to perform charging. , JP-A-4-21873,
It is disclosed in Japanese Patent Laid-Open No. 4-116674.

[0006]

However, even the charging device disclosed in the above publication has a problem that the image forming body cannot be charged completely stably and uniformly. That is, when the amount of magnetic particles present on the surface of the cylindrical magnetic particle carrier is too large in the developing area, the attraction force and frictional force between the magnetic particle carrier and the magnetic particles are reduced, and the magnetic particles form an image. The magnetic particles adhere to the body, the vibration of the magnetic particles is not sufficiently performed, uneven charging occurs, and the driving torque of the carrier increases. On the other hand, if the amount of the magnetic particles existing on the carrier for carrying the magnetic particles in the developing region is too small, there is a problem in that an imperfect portion is brought into contact with the image forming body to cause uneven charging.

An object of the present invention is to solve these problems and to provide a charging device which does not generate ozone and can perform extremely stable and uniform charging.

[0008]

The above object is to bring a magnetic brush composed of a magnetic particle layer onto a conductive magnetic particle carrier having a magnetic force and being rotatable, in contact with a moving image forming body to carry the carrier. In a charging device for charging the image forming body by forming a bias electric field between the carrier and the image forming body, an oscillating electric field is used as the bias electric field, and the presence of the magnetic particles in the charging region. Amount is 10 ~ 200mg /
This is achieved by a charging device characterized in that the magnetic brush is formed to have a size of cm ² .

[0009]

DETAILED DESCRIPTION OF THE INVENTION The present invention is characterized in that the amount of magnetic particles present in a charged region of an image forming apparatus is limited to an appropriate range. As a result, the amount of magnetic particles present in the charging region is made proper, and uneven charging is prevented. Generally speaking, if the average particle size of the magnetic particles is large, (a) the magnetic brush formed on the carrier has a rough ears, so that the magnetic particles are charged while vibrating. However, unevenness is likely to appear on the magnetic brush, which causes 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 100 μm or less, and the problem (a) did not substantially occur when the average particle size was 70 μm or less. 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 in the conventional magnetic carrier particles as a magnetic substance, such as metals such as iron, chromium, nickel and cobalt, or their compounds and alloys such as iron 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. Spherical particles that exhibit the above effects have conductivity such that the magnetic particles have a resistivity of 10 ³ Ωcm or more, especially 10 ¹² Ωcm or less, especially 10 ⁴ Ωcm or more and 10 ⁹ Ωcm or less. It is preferable that the magnetic particles are formed. The resistivity after tapping putting particles in a container having a sectional area of 0.50 cm ^2, a load of 1 kg / cm ² on packed particles, the 1000V / cm between the load and a bottom electrode It is a value obtained by reading the current value when a voltage that causes an electric field is applied.If this resistivity is low, charges are injected into the magnetic particles when a bias voltage is applied to the carrier, and image formation occurs. The magnetic particles are likely to adhere to the body surface, 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 present invention are
It is desirable that the magnetic brush constituted by it has a small specific gravity and a suitable maximum magnetization so that the magnetic brush moves lightly due to an oscillating electric field and does not cause external scattering. Specifically, it was found that good results can be obtained by using a material having a true specific gravity of 6 or less and a maximum magnetization of 30 to 100 emu / g.

In summary of the above, the magnetic particles are spherical so that the ratio of the major axis to the minor axis is at most 3 times, there is no protrusion 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.

When toner is mixed in the magnetic brush,
Since the toner has a high insulating property, the charging property is lowered and uneven charging occurs. In order to prevent this, it is necessary to lower the charge amount of the toner so that the toner moves to the image forming body at the time of charging. Under the condition that the toner is mixed with magnetic particles and the toner concentration is adjusted to 1%. When the triboelectrification amount of the toner was the same and the charging polarity was 1 to 20 μC / g, the toner could be prevented from accumulating on the magnetic brush. It is considered that this is because even if the toner is mixed, it adheres to the photoconductor during charging. It was confirmed that when the charge amount of the toner is large, it becomes difficult to separate from the magnetic particles, and when it is small, it becomes difficult to electrically move to the image forming body.

The above are the conditions for the magnetic particles, and then the conditions for the magnetic particle carrier for forming the particle layer and charging the image forming body 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 having a particle layer formed on the surface thereof. 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, causing wear and uneven charging of the photoconductor, and overcurrent will easily flow, resulting in a large drive torque of the carrier. There is a drawback that On the other hand, if the amount of the magnetic particles present on the carrier in the charged area is too small, an incomplete portion is formed in contact with the image forming body, resulting in adhesion of the magnetic particles to the image forming body and uneven charging. As a result of repeated experiments,
The preferable amount W of the magnetic particles in the charging region is 10 to 20.
There 0 mg / cm ^2, particularly preferably found to be 30~150mg / cm ^2. The existing amount is an average value in the contact area of the magnetic brush.

The gap D between the carrier and the image forming body is
100 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. Further, it has been clarified that the most preferable condition exists between the more appropriate transport amount (W) and the gap (D).

To perform charging uniformly and stably at high speed
The condition of 300 ≦ W / D ≦ 3000 (mg / cm ³ ) was important. W
It was confirmed that the charging becomes non-uniform when / D is small and when it is large.

D is considered to be a factor that determines the chain length of magnetic particles. It is considered that the electric resistance corresponding to the chain length corresponds to the easiness of charging and the charging speed. On the other hand, W is considered to be a factor that determines the density of chains of magnetic particles. It is believed that increasing the number of chains improves the charging uniformity.
However, it is considered that the compressed state of the magnetic particle chains is realized when the magnetic particles pass through the narrow gap in the charged region. At this time, the chains of the magnetic particles are in contact with each other and in a bent state, rubbing against the image forming body while being disturbed.

It is considered that this disturbing condition is effective for uniform charge by facilitating charge transfer without causing charge stripes. That is, when W / D corresponding to the density of magnetic particles is small, the chains of the magnetic particles become coarse and the ratio of disturbance is small, resulting in non-uniform charging. When W / D is large, the magnetic particle chains are not sufficiently formed due to high packing, and the magnetic particles are less disturbed. It is considered that this hinders the free movement of the charges and prevents uniform charging.

Further, in the image forming method in which the present charging device is applied to the cleaning, the reversal development is more preferable than the regular development in the development. This is because the toner is easily discharged from the charging device during charging, the discharged toner has the same polarity during reversal development, and is collected by the developing bias in the developing section, so that image fogging can be prevented.

[0024]

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 having a charging device of the present invention. In the figure, 10
Is a photosensitive drum made of (-) charged OPC, which is an image forming body that rotates in the direction of the arrow (clockwise). A charging device 20, an image exposure L from an exposure device, and a developing device 30 which will be described later are provided on the peripheral portion of the photosensitive drum. ,
A transfer roller 13, 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 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 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.

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 a fixing device (not shown) via the conveying means 80, is sandwiched by a heat fixing roller and a pressure bonding roller, is fused and fixed, and is then discharged to the outside of the apparatus. Recording paper P
The surface of the photosensitive drum 10 which has toner remaining without being transferred to and is rotated is scraped off and cleaned by a cleaning device 50 having a blade 51 and the like, and waits 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.
It is adjusted to m and a specific resistance of 10 ⁸ Ω · cm, and the frictional charge amount with the toner is −5 μC / g under the condition that the toner concentration is 1%.

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 by arranging the S pole and the N pole so that the surface of the cylinder is 500 to 1,000 Gauss, 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. Cylinder 22 is a photosensitive drum
It is held in a gap of 0.5 to 1.0 mm at a position facing 10 and rotated 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 body layer 10a covering the surface thereof, and the conductive base material 10b is grounded.

Reference numeral 24 is a bias power source for applying a bias voltage between the cylinder 22 and the conductive base material 10b, and the cylinder 22 is grounded via the bias power source 24.

The bias power source 24 is a power source for supplying an AC bias voltage in which an AC component is superposed on a DC component set to the same value as the voltage to be charged, and the 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. By applying an AC bias, 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.

Reference numeral 25 denotes a casing forming a storage portion 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.
² Particularly preferably, it is adjusted to 30 to 150 mg / cm ² . The gap between the photosensitive drum 10 and the cylinder 22 is connected by a layer of magnetic particles 21 whose thickness is regulated.

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

While rotating the photoconductor drum 10 in the direction of the arrow, the cylinder 22 is moved in the same direction as shown by the arrow of 1.2% of the peripheral speed of the photoconductor drum 10.
When rotated at a peripheral speed of up to 2.0 times, the layer of magnetic particles 21 attached and conveyed to the cylinder 22 is magnetically connected at the position facing the photosensitive drum 10 on the cylinder 22 by the magnetic field lines of the magnet 23, which is a kind. In the shape of a brush, a so-called magnetic brush is formed. Then, the magnetic brush 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. Since the AC bias voltage is applied between the cylinder 22 and the photoconductor drum 10, charges are injected onto the photoconductor layer 10a via the conductive magnetic particles 21 to perform charging. In this case, in particular, the bias voltage is the AC bias voltage, so that the efficiency of charge injection from the magnetic brush that accompanies the movement and discharge phenomenon of charges is improved, and extremely stable high-speed 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.

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. 3, the range shaded with vertical lines is the range where dielectric breakdown is likely to occur, the range shaded with 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.

When the transport amount W is less than 10 mg / cm ² , the adhesion of magnetic particles or uneven charging is greater than 200 mg / cm ² , the photoreceptor is worn or uneven charging occurs. I couldn't get it. The preferred range in between was 30-150 mg / cm ² .

Further, when the distance D (cm) between the image forming body and the magnetic particle carrying carrier is further set under the above-mentioned carrying amount condition, W / D
It was clarified that by setting the condition of 300 mg / cm ³ <W / D <3000 mg / cm ³ to obtain more preferable uniform charging characteristics without sticking of magnetic particles or charging unevenness.
When the amount was less than 300 or greater than 3000, the phenomenon of magnetic particle adhesion and uneven charging was observed.

From the above, the preferable condition is that the magnetic brush composed of the magnetic particle layer adhered on the magnetic particle carrying carrier having magnetic force is brought into contact with the moving image forming body so that the magnetic particle carrying carrier and the image forming body are separated from each other. In a charging device for charging an image forming body by forming a bias electric field between them, an oscillating electric field is used as the bias electric field, and the abundance W of magnetic particles in the charging region becomes 10 to 200 mg / cm ^2. When the magnetic brush is formed as described above, and the gap between the magnetic particle carrier and the image forming body is D (cm), 300 ≦ W / D ≦ 3000 is a preferable condition.

In the present embodiment, the charge of the image forming body can be removed by setting only the DC component of the bias voltage to zero. After the image formation, only the AC component is applied to rotate the image forming body to eliminate the charge. After the application of the AC component is stopped at the time when the charge removal of the image forming body is completed, 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 horizontal magnetic field makes it parallel to the tangential direction of the portion facing the photoconductor drum 10, and the tip of the magnetic brush can be separated from the photoconductor drum 10 without adhering magnetic particles to the image forming body.

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.

To this end, the polarity of the DC bias voltage applied to the cylinder 22 when the photosensitive drum 10 is rotated before or after image formation is set high, or the AC voltage is set high.
It is possible to prevent toner from entering by setting conditions in which the toner easily adheres to the photoconductor. 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, 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 attached to the magnet roll to form the magnetic brush. .

[0046]

According to the present invention, since the photoconductor drum is charged by directly injecting electric charges through the magnetic brush, the bias voltage can be lowered and the generation of ozone can be prevented. Further, since an oscillating electric field is formed between the magnetic brush and the photosensitive drum as a bias electric field, and the existing amount W of the magnetic particles in the developing area is set to 10 to 200 mg / cm ² , the magnetic brush vibrates. In particular, it is possible to provide a charging device in which the magnetic brush easily moves in the direction in which the potential difference is particularly large, and which can perform extremely stable and uniform charging without uneven 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 the charging device of the present invention.

FIG. 3 is a charging characteristic diagram when a frequency and a voltage of an AC voltage component are changed.

[Explanation of symbols]

 10 Photoconductor drum 20 Charging device 21 Magnetic particles 22 Cylindrical 23 Magnet 24 Bias power supply 25 Casing 26 Regulator plate 28 Protective resistance

Claims (2)

[Claims] 1. A magnetic brush composed of a magnetic particle layer attached on a magnetic particle carrier having magnetic force is brought into contact with a moving image forming body to form a bias electric field between the carrier and the image forming body. By doing so, in the charging device for charging the image forming body, an oscillating electric field is used as the bias electric field, and the abundance W of the magnetic particles in the charging area is 10 to 200 mg / cm ^2. A charging device characterized by forming a magnetic brush. 2. The charging device according to claim 1, wherein when the gap between the magnetic particle carrier and the image forming body is D (cm), 300 ≦ W / D ≦ 3,000.