JPH07239603A - Electrifier - Google Patents

Abstract

PURPOSE:To provide a contact-system electrifier which applies a proper bias voltage at all times and prevents irregularities in electrification. CONSTITUTION:The electrifier is a magnetic-brush type electrifier 20 being composed so that a magnetic brush 21A is formed onto an electrifying sleeve 22, provided rotably on the outer circumference of a magnetic body 23 with magnetic poles arranged and fixed along the outer circumference to electrify a photosensitive drum 10 by bringing the magnetic brush 21A into contact with the moving photosensitive drum 10 in an oscillating electric field generated by applying the part between the electrifying sleeve 22 and the photosensitive drum 10 an AC bias voltage in which an AC element overlaps a DC element by means of a DC power source 61 and AC power source 63 as constant voltage power sources. In the electrifier 20, the peak-to-peak voltage of the AC element is adjusted according to information on the change width of the current value of the DC element of the bias voltage which is detected by an ammeter 62.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact type charging device incorporated in an image forming apparatus such as an electrophotographic copying machine or an electrostatic recording device for 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, and the charged ions generated at that time are adsorbed to the image forming body to charge. Done.

The corona charger used in such a conventional electrophotographic image forming apparatus can charge the image forming body without mechanically contacting the image forming body, so that the image forming body is damaged during charging. It has the advantage of never. However, since this corona charger uses a high voltage, there is a risk of electric shock or leakage, and ozone generated due to gas discharge is harmful to the human body,
It has the drawback of shortening the life of the image forming body. In addition, the charging potential of the corona charger is unstable because it is strongly affected by temperature and humidity. Furthermore, noise is generated by the high voltage in the corona charger, and it is necessary to obtain a stable charging potential after inputting a high voltage. A certain amount of time is required, which is a major drawback when using an electrophotographic image forming apparatus as a communication terminal or an information processing apparatus.

Many drawbacks of such corona chargers are:
The cause is that the charging is mainly performed by gas discharge.

Therefore, a magnet body is included as a charging device capable of charging the image forming body without causing high-pressure gas discharge like a corona charger and without mechanically damaging the image forming body. As a charging member, magnetic particles are adsorbed on the cylindrical carrier to form a magnetic brush, and the magnetic brush is rubbed against the surface of the image forming body under application of a DC bias voltage to perform charging. A charging device is disclosed in JP-A-59-133569.

However, even when the magnetic brush charging device is used, uniform charging cannot always be obtained.

Therefore, for example, in Japanese Patent Laid-Open Nos. 4-21873 and 4-116674, a magnetic brush for charging an image forming body by applying an AC bias voltage in which a DC voltage is superposed with an AC voltage is applied to the magnetic brush. A charging method has been proposed. In this publication, it is described that the AC bias voltage can be applied and thereby uniform charging can be imparted to the image forming body.

[0008]

However, in the magnetic brush charging method described in the above publication, the magnetic particles as the charging member are fused to the surface of the magnetic particles due to long-term use, or their electric resistance is changed due to environmental changes. Due to a change in voltage, a mounting error in the charging device, and the like, the appropriate values of the peak-to-peak voltage V _PP of the DC component and AC component of the AC bias voltage vary. Therefore, if a constant bias voltage is applied, there is a problem in that charge injection into the image forming body becomes uneven and the charging potential changes or charging unevenness occurs.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and to provide a charging device which prevents uneven charging and always maintains proper charging conditions.

The present invention can be similarly applied to a fur brush brush charging device and a charging device using a conductive roll.

[0011]

The above object is to provide an image forming apparatus for charging the image forming body by applying a bias voltage having an AC component to a charging member in contact with the image forming body. The charging device is characterized in that the voltage of the AC component is adjusted on the basis of the information of the charging variation of the above.

Further, the information is a current value of a DC component of the bias voltage or a charging potential of the image forming body, and the charging is performed by changing the voltage value of the DC component. The charging device characterized in that the electric potential is changed is a preferred embodiment.

[0013]

In the present invention, since the AC component voltage value of the bias voltage applied to the charging member is adjusted based on the information on the charging variation of the image forming body, it is always appropriate to meet the state and environmental conditions of the charging member. An AC bias voltage is applied.

[0014]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 3 is a schematic sectional view of an image forming apparatus equipped with the charging device of the present invention. In FIG. 3, reference numeral 10 denotes a photoconductor drum that is an image forming body that rotates at a peripheral speed of 240 mm / sec in the direction of the arrow (clockwise). OPC formed by layer order
It is a negatively chargeable photosensitive drum having a photosensitive layer. A charging device 20, a static eliminator 11, an exposure unit 12 on which the image light L enters, a developing device 30, a transfer roller 13, a cleaning device 50 and the like are provided on the peripheral portion thereof. The static eliminator 11 is composed of, for example, an LED array, and is driven by the control of the controller to erase the charging of the frame portion outside the incident area of the image light L on the surface of the photosensitive drum 10. This static eliminator 11 has the same polarity as the charging of the toner used in the developing device 30 by the charging device 20, and the reversal development in which the toner is attached to the portion of the surface of the photoconductor drum 10 where the image light L is incident. In case of, it becomes unnecessary.

Image light L is made incident on the charged surface of the photosensitive drum 10 by a slit exposure device or a laser beam scanner to form an electrostatic latent image, and the electrostatic latent image is charged by the developing device 30 to the photosensitive drum 10. Regular development or reversal development is performed with toner charged to the opposite polarity or the same polarity.

In the developing device 30 of the illustrated example, a magnetic brush composed of a two-component developer in which toner and a magnetic carrier are mixed is formed on the developing sleeve 31 and conveyed in the direction of the arrow to develop the developing sleeve.
31 is the bias voltage of the opposite polarity to the charging of the photosensitive drum 10,
This is a magnetic brush type developing device that develops by applying it for preventing fogging in the case of regular development and for promoting the adhesion of toner to an electrostatic image in the case of reversal development. , The photosensitive drum 10 on the developing sleeve 31
A developer layer that is not in contact with
The AC component may be added to the bias voltage applied to the developing sleeve 31 to perform non-contact development in which the toner flies from the developer layer in the developing area where the developing sleeve 31 is close to the photosensitive drum 10 and adheres to the electrostatic image.

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, its peripheral surface is uniformly charged by a magnetic brush type charging device 20 described later and passes through. On the photoconductor drum 10,
An image is written by the image light L in the exposure unit 12, and an electrostatic latent image corresponding to the image is formed. This electrostatic latent image is developed by the developing device 30, and a toner image is formed on the photosensitive drum 10.

On the other hand, 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 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. By the action of the transfer roller 13 to which a bias voltage is applied from a power source (not shown), 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 nipped by a heat fixing roller and a pressure bonding roller, melted and fixed, and then discharged outside the apparatus. A photoconductor drum 10 that rotates with the toner remaining without being transferred to the recording paper P.
The surface of is scraped off and cleaned by a cleaning device 50 equipped with a blade 51 and the like, and stands by for the next recording.

Before describing the charging device 20 of the present invention,
The particle size of the magnetic particles used as the charging member and the general conditions of the carrier for carrying the magnetic particles will be described.

Generally, when the average particle size (weight average) of the magnetic particles is large, (a) since the state of the ears of the magnetic brush formed on the carrier is rough, even when charged while vibrating by the electric field, The magnetic brush is likely to have unevenness, which causes a problem of uneven charging. To solve this problem, the average particle size of the magnetic particles should be reduced.
It was found that the effect started to appear when the thickness was less than μm, and particularly when the thickness was 150 μm or less, the problem (a) did not occur. However, if the particles are too fine, the photosensitive drum 10
It may adhere to the surface or become easily scattered. These phenomena are related to the strength of the magnetic field acting on the particles and the strength of the magnetization of the particles, but in general,
The average particle size of the particles is remarkably exhibited when it is 30 μm or less. It is preferable that the magnetization intensity is 20 to 200 emu / g.

From the above, the average particle size (weight average) of the magnetic particles is 150 μm or less, particularly preferably 150 μm or less 30 μm.
It is preferably m or more.

Such magnetic particles are the same as in the magnetic carrier particles of the conventional two-component developer as a magnetic material,
Ferromagnetism such as metals such as iron, chromium, nickel and cobalt, or their compounds and alloys such as ferric tetroxide, γ-ferric oxide, chromium dioxide, manganese oxide, ferrite, manganese-copper alloy Body particles or the surface of these magnetic particles is coated with a resin such as styrene resin, vinyl resin, ethylene resin, rosin modified resin, acrylic resin, polyamide resin, epoxy resin, polyester resin, or Particles obtained by making a resin containing magnetic fine particles dispersed therein can be obtained by selecting the particle size by a conventionally known average particle size selecting means.

The formation of spherical magnetic particles is
The particle layer formed on the carrier is made uniform, and a high bias voltage can be uniformly applied to the carrier. That is, the fact that the magnetic particles are spherical means that (1) generally, the magnetic particles are easily magnetized and adsorbed in the long axis direction, but due to the spherical shape, the directionality is lost, so that a layer is uniformly formed and local layers are formed. (2) Higher resistance of the magnetic particles is eliminated, and the edge portions seen in conventional particles are eliminated, and electric field concentration on the edge portions is prevented. As a result, even if a high bias voltage is applied to the magnetic particle carrier, it is possible to uniformly discharge the surface of the photoconductor drum 10 and prevent uneven charging.

In the spherical particles having the above effects, the specific resistance of the magnetic particles is 10 ³ Ω · cm or more and 10 ¹² Ω · cm or less, especially 1
It is preferable that conductive magnetic particles are formed so as to be not less than 0 ⁵ Ω · cm and not more than 10 ⁹ Ω · cm. This specific resistance
After tapping in a container having a sectional area of 0.5 cm ^2, multiplied by the packed load the particles on the 1 kg / cm ^2, applying a voltage to the electric field of 1,000 V / cm is generated between the load and a bottom electrode It is a value obtained by reading the current value when, when the specific resistance is low, when a bias voltage is applied to the carrier, electric charges are injected into the magnetic particles, and the magnetic particles are transferred to the surface of the photoconductor drum 10. Are likely to adhere, or the dielectric breakdown of the photosensitive drum 10 due to the bias voltage is likely to occur. If the specific resistance is high, charge injection is not performed and charging is not performed.

Further, it is desirable that the magnetic particles have a small specific gravity and have a suitable maximum magnetization so that the magnetic brush constituted by the magnetic particles can move lightly due to an oscillating electric field and further, external scattering does not occur. Specifically, those having a true specific gravity of 6 or less and a maximum magnetization of 30 to 100 emu / g, especially 40 to 80 emu
It was found that good results were obtained with / g.

In summary, the magnetic particles are spherical so that at least the ratio of the major axis to the minor axis is 3 times or less, there is no protrusion such as needles and edges, and the specific resistance is It is preferably in the range of 10 ⁵ to 10 ⁹ Ω · cm. And, such spherical magnetic particles should be selected as spherical as possible for the magnetic particles, and in the particles of the magnetic fine particle dispersion system, the fine particles of the magnetic material should be used as much as possible, and the spheroidizing treatment should be performed after the formation of the dispersed resin particles. It is manufactured by applying or by forming dispersed resin particles by a spray drying method.

The above are the general conditions for the magnetic particles, and the conditions for the carrier for the magnetic particles for forming the particle layer and charging the photosensitive drum 10 will be described next.

As the carrier for magnetic particles, a conductive carrier capable of applying a bias voltage is used. In particular, a magnet having a plurality of magnetic poles inside a conductive charging sleeve on the surface of which a particle layer is formed. A structure having a body is preferably used. In such a carrier,
The relative rotation with the magnet body causes the particle layer formed on the surface of the conductive charging sleeve to undulate and move in a wave shape, so that new magnetic particles are supplied one after another, and the particle layer on the carrier surface is supplied. Even if there is some non-uniformity in the layer thickness, the effect is sufficiently covered by the above-mentioned wavy undulation so as not to be a practical problem. It is preferable that the surface of the carrier has an average roughness of 5.0 to 30 μm for stable and uniform transfer of magnetic particles.If the carrier is smooth, the carrier cannot be carried sufficiently. A current flows, and in either case uneven charging is likely to occur. Sandblasting is preferably used to achieve the above surface roughness. The diameter of the carrier is preferably 5.0 to 20 mm. With the above diameter, a contact area necessary for charging is secured. If the contact area is unnecessarily large, the charging current will be excessive, and if it is small, uneven charging is likely to occur. If the diameter is small as described above,
Since the magnetic particles are easily scattered or adhered to the photosensitive drum 10 due to the centrifugal force, it is preferable that the linear velocity of the carrier is slow within the following range. It is preferable that the transport speed of the magnetic particles by the rotation of the transport carrier is almost the same as or slower than the moving speed of the photoconductor drum 10. 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.

Further, the thickness of the particle layer formed on the carrier is preferably such that it is sufficiently scraped off by the regulating means to form a uniform layer. If there are too many magnetic particles on the surface of the carrier in the charging region, the vibration of the magnetic particles will not be sufficiently performed, causing wear and uneven charging of the photoconductor, and overcurrent will easily flow, resulting in a large drive torque of the carrier. There is a drawback that On the other hand, if the amount of the magnetic particles present on the carrier is too small in the charged area, an incomplete contact with the photoconductor drum 10 may occur, causing magnetic particles to adhere to the photoconductor drum 10 or cause uneven charging. Become. As a result of repeated experiments, the preferable amount W of the magnetic particles in the charged region is 10 to 300 mg / cm ² , and particularly preferably 30 to 15
It was found to be 0 mg / cm ² . This abundance is
It is the average value in the charged area of the magnetic brush.

The gap Dsd between the carrier and the photosensitive drum 10 is preferably 0.1 to 5.0 mm. If the surface gap Dsd between the carrier and the photoconductor drum 10, which is the image forming body, becomes too narrower than 0.1 mm, it becomes difficult to form the magnetic brush ears that uniformly charge the surface, and it is sufficient. Since it becomes impossible to supply the magnetic particles to the charging area, stable charging cannot be performed, and when the gap Dsd exceeds 5.0 mm, the particle layer is coarsely formed and charging unevenness easily occurs, and sufficient charging is performed. Will not be obtained. in this way,
When the gap Dsd between the carrier and the photosensitive drum 10 becomes extreme,
On the other hand, the thickness of the particle layer on the carrier cannot be adjusted appropriately, but when the gap Dsd is in the range of 0.1 to 5.0 mm, the particle layer can be formed with an appropriate thickness, The ears of the magnetic brush are also formed uniformly. Further, the carry amount (W) and the gap (Dsd) are 300 ≤ W / Dsd ≤ 3,000 (mg / cm ³ ) in order to uniformly and stably charge at high speed.
It has been confirmed that the above relation must be satisfied, and that when W / Dsd is out of this range, charging becomes non-uniform.

Dsd is considered to be a factor that determines the chain length of magnetic particles. It is considered that the electric resistance corresponding to the chain length corresponds to the ease of charging and the charging speed. On the other hand, W is considered to be a factor that determines the density of chains of magnetic particles. It is believed that increasing the number of chains improves the charging uniformity.
However, it is considered that when the magnetic particles pass through the narrow gap in the charging region, the compressed state of the chains of the magnetic particles is realized. At this time, the chains of the magnetic particles are in contact with each other and are bent, and the photosensitive drum 10 is disturbed while being disturbed.
Are facing each other.

It is considered that this disturbing condition facilitates the transfer of electric charges without causing streaks of charging and is effective for uniform charging. That is, when W / Dsd corresponding to the magnetic particle density is small, the chains of the magnetic particles become coarse and the ratio of disturbance is small, resulting in non-uniform charging. On the other hand, if W / Dsd is too large, the chains of the magnetic particles are not sufficiently formed due to high compression, and the magnetic particles are less disturbed. It is considered that this hinders the free movement of the charges and prevents uniform charging.

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 wt%. 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 image forming body 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.

Next, the charging device 20 of the present invention will be described. FIG. 1 is an enlarged sectional view showing an example of a charging device of the present invention,
FIG. 5 is a graph showing a preferable range of the AC component of the bias voltage applied to the charging sleeve of the charging device. In FIG. 1, 21 is a magnetic particle, 22 is a charging sleeve having a diameter of 15 mm, which is a carrier for the magnetic particle 21 made of non-magnetic and conductive metal such as aluminum, and 23 is fixed inside the charging sleeve 22. It is a cylindrical magnet body provided, and this magnet body 23 is formed on the surface of the charging sleeve 22 as shown in FIG.
It has 6 or 8 magnetic poles that are alternately magnetized so that the S poles and the N poles are approximately 1,000 gauss. The charging sleeve 22 is rotatable with respect to the magnet body 23, and at a position facing the photoconductor drum 10, the charging sleeve 22 is 0.1 to 0.1 in the same direction as the moving direction of the photoconductor drum 10.
It is preferably rotated at a peripheral speed of 1.0 times.

The positions of the two magnetic poles located in the charging portion of the charging sleeve 22 facing the photoconductor drum 10 are θ ₁ = 5 to 30 ° for the magnetic poles upstream from the closest position to the image forming body. It is preferable to set. Further, the magnetic pole on the downstream side of the charging portion is preferably set to θ ₂ = 10 to 40 ° because the magnetic brush on the outlet side is separated in a state of forming a uniform layer. Furthermore, it is preferable that θ ₂ > θ ₁ .

Reference numeral 25 denotes a casing forming a storage portion for the magnetic particles 21, and the charging sleeve 22 is provided in the casing 25.
And a magnet body 23 are arranged, and a regulation plate 26 is provided at the outlet of the casing 25 to regulate the thickness of the layer of magnetic particles 21 attached to the charging sleeve 22 and carried out. Regulation plate 26
The gap between the tip of the magnetic particles 21 and the charging sleeve 22 is 10 to 300 mg / cm ² particularly preferably 30 to 150 m when the transport amount of the magnetic particles 21, that is, the existing amount of the magnetic particles 21 on the charging sleeve 22 in the charging region.
Adjusted to g / cm ² . 27 is a stirrer for stirring the magnetic particles 21 to make them uniform, 28 is a scraping member for scraping the magnetic particles 21 from the charging sleeve 22, and the magnetic particles 21 are continuously stirred by the scraping member 28 and the stirrer 27. It is mixed and always kept in a uniform state.

Further, the gap Dsd where the charging sleeve 22 faces the photoconductor drum 10 can be set in the range of 0.1 to 5.0 mm, and when it is narrower than this range, the durability of the photoconductor drum 10 and the like is rapidly deteriorated. As a result, it becomes difficult to form the magnetic brush 21A composed of the magnetic particles 21 that appropriately rub the photosensitive drum 10, and when the magnetic brush 21A becomes wider, the magnetic brush 21A contacts the photosensitive drum 10 uniformly. Therefore, it becomes difficult to uniformly charge the photoconductor drum 10. A gap Dsd between the charging sleeve 22 and the photosensitive drum 10 is connected by a conductive magnetic brush 21A whose thickness is regulated.

Not limited to the example shown in FIG. 1, the magnet body 23 has N and S magnetic poles at equal positions in the circumferential direction and rotates in the direction opposite to the conveying direction of the magnetic particles 21. The magnet 22 may be stationary or may rotate in the opposite direction to the magnet body 23. Further, the above-described rotation directions of the charging sleeve 22 and the magnet body 23 may be such that the conveying direction of the magnetic brush 21A at the position where the charging sleeve 22 faces the photoconductor drum 10 is opposite to the moving direction of the photoconductor drum 10. Good. However, the photosensitive drum 10
Is preferable in terms of the uniformity of charging, the reducibility of the magnetic brush 21A passing through the sliding position of the photoconductor drum 10 into the container 25, and the durability of the photoconductor drum 10 and the like. The conveying direction is the same as the moving direction of the photosensitive drum 10, and the conveying speed is 0.1% of the moving speed of the photosensitive drum 10.
It is preferably ˜1.0 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 numerals 61 and 63 denote bias power supplies for applying an AC bias voltage in which an AC component is superimposed on a DC component between the charging sleeve 22 and the conductive base material 10b, 61 is a DC power supply, and 62 is a DC component current. Ammeter to detect value, 63 is AC power supply, 70
Is a CPU of the control unit, 71 is a ROM that stores data used when controlling the output voltage of the AC power supply 63, 72 is an analog / digital converter (A / D converter), and 73 is a digital / analog converter ( D / A converter). The AC bias voltage from the bias power sources 61 and 63 is applied to the charging sleeve 22 via the protection resistor R. The bias power sources 61 and 63 are constant voltage power sources.

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

While rotating the photosensitive drum 10 in the direction of the arrow, the charging sleeve 22 is rotated in the same direction as the arrow at a peripheral speed of 0.1 to 1.0 times the peripheral speed of the photosensitive drum 10.
The layer thickness of the magnetic particles 21 adhered to and conveyed by the magnetic particles 22 is regulated by the regulation plate 26, and at the same time, the magnetic particles 21 are positioned on the charging sleeve 22 at a position facing the photosensitive drum 10 by the magnetic lines of force of the magnet body 23. The magnetically connected chains form a kind of brush, forming a so-called magnetic brush 21A. Then, the magnetic brush 21A is conveyed in the rotating direction of the charging sleeve 22 and contacts and slides on the photosensitive layer 10a of the photosensitive drum 10. Since an oscillating electric field due to the AC bias voltage is formed between the charging sleeve 22 and the photoconductor drum 10, the charge is smoothly injected onto the photoconductor layer 10a via the magnetic brush 21A, and the charge is uniformly applied. High-speed charging is performed.

The AC component of the bias voltage in this case is shown in FIG.
It is preferable to set the white area shown in (1) to a stable charging. In FIG. 8, a shaded area in the vertical line is a range where dielectric breakdown is likely to occur, a shaded area is a range in which charging unevenness is likely to occur, and a low-frequency region in which dotted dots are shaded has a low frequency. Therefore, this is the range where uneven charging occurs.
The waveform of the AC component is not limited to a sine wave, and may be a rectangular wave, a triangular wave, or the like.

Further, the peak-to-peak voltage (V _PP ) of the AC component, which is the voltage of the AC component, and the absolute value (|
The relationship of V _S |) is shown in FIG. In FIG. 4, the horizontal axis shows the peak-to-peak voltage (V _PP ) of the AC component of the AC bias voltage, and the vertical axis shows the absolute value (│V _S │) of the charging potential of the photosensitive drum 10. Absolute value of charging potential (| V _S |) as peak-peak voltage V _PP increases
Becomes larger, and the charging potential V _S is the value of the DC component of the bias voltage V _{DC at} the threshold (V _PP ) th with a constant peak-peak voltage.
Saturates at a value equal to and peak voltage V
There is a characteristic that the charging potential V _S does not change even if _PP is increased. The electric resistance of the magnetic particles 21 varies depending on environmental conditions and is high at low temperature and low humidity and low at high temperature and high humidity. Therefore, the characteristic curve is located on the right side as indicated by a solid line (a) in low temperature and low humidity, and is located on the left side as indicated by a dashed line (b) in high temperature and high humidity. the threshold (V _PP) th voltage at low temperature and low humidity (V _PP) th
a, (V _PP ) thb at high temperature and high humidity. As a result of the experiment, the preferable charging condition is the peak of the AC component under each environmental condition.
It has been found that the peak voltage V _PP can be obtained by setting the range of 0.8 to 1.5 times the (V _PP ) th. If the peak-peak voltage V _PP is smaller than this, uneven charging and magnetic particle adhesion are large, and if the peak-peak voltage V _PP is larger than 2 × (V _PP ) th, breakdown easily occurs.

The charging potential V _S is the current value I of the DC component.
It became clear that it is proportional to _DC . That is, the current value I _{DC of the} DC component increases as the peak-to-peak voltage V _PP , which is the AC component voltage, increases, but when it becomes (V _PP ) th or more, I _DC changes so as to saturate. That is, the current value I _DC of the DC component with respect to the change of the AC component voltage is shown in FIG.
It changes as shown in.

If the peak-to-peak voltage V _PP is lower than the threshold value (V _PP ) th, uneven charging occurs as described above,
When the current value I _DC of the DC component of the bias voltage during charging is measured, it fluctuates as shown in FIG. 6, and the peak-peak voltage V
The fluctuation range is larger as _PP is lower than (V _PP ) th. FIG. 7 shows the relationship between the fluctuation width and the peak-peak voltage (V _PP ) th. That is, the fluctuation range of the I _DC can be used as information on the charging variation.

The above charging characteristics are not limited to the temperature / humidity characteristics of the magnetic particles. When examined in more detail, they vary depending on the following factors.

1) Factors of variation on the charging device side are lot differences in magnetic particle characteristics, temperature and humidity characteristics, toner mixture, setting conditions of the charging device (gap between charging sleeve and image forming body, charging sleeve and regulation plate). Gap, magnetic pole angle, etc.) 2) Factors of variation on the image forming body side are the thickness of the image forming body,
There are temperature and humidity characteristics and fatigue characteristics.

Due to this fluctuation factor, the proper peak-to-peak voltage value (V _PP ) constantly fluctuates. In order to correct this, the fluctuation range of I _DC is measured immediately before the start of copying in the image forming apparatus or after every predetermined number of copies, and the peak / peak voltage V _{PP at which} the fluctuation range of I _DC becomes substantially zero is detected. It is necessary to control the charging potential V _S to always be an appropriate value by controlling the AC brush to apply the peak-to-peak voltage V _PP to the magnetic brush.

Instead of the I _DC fluctuation range, the fluctuation range may be divided by the absolute value of I _DC . Further, as a criterion for making the fluctuation range zero, a value at which the inflection point or the fluctuation range magnitude with respect to V _PP of the fluctuation range becomes equal to or less than a specified value is adopted.

To satisfy this, the applied peak-to-peak voltage V _PP is gradually changed from a low value to a large value,
At that time, the current value I _DC detected by the ammeter 62 is A /
CP converted to digital value by D converter 72
Enter in U70. Based on this current value, the CPU 70 detects the fluctuation range and compares it with the fluctuation reference value stored as data in the ROM 71, and the peak / peak voltage value when the fluctuation range becomes lower than the fluctuation reference value. Detect V _PP . The detected value is determined as the V _PP to be applied, and the CPU 70
Outputs a control signal. This control signal is converted to an analog value by the D / A converter 73 and then sent to the AC power supply 63, and the AC power supply 63 outputs a proper peak-to-peak voltage V _PP of the AC component. As a result, the charging potential V _S of the photosensitive drum 10 can always be maintained at an appropriate value.

Further, as shown in FIG. 4, since the charging potential of the photosensitive drum 10 is determined by the voltage value V _DC of the DC component of the bias voltage, the charging value can be changed by changing the voltage value V _DC. . Therefore, in the image forming apparatus that needs to change the charging potential, the charging potential can be easily changed by changing the voltage value V _DC without complicating the apparatus.

As the magnetic particles 21 in the above-mentioned embodiment, spherical ferrite particles coated so as to have conductivity were used. In addition, conductive magnetic resin particles obtained by pulverizing the magnetic particles and a resin as main components after thermal smelting can also be used. In order to perform good charging, the outer shape is adjusted to a spherical shape with a particle size of 50 μm and a specific resistance of 10 ⁸ Ω · cm, and the amount of frictional charge with the toner is −5.0 μC when the toner concentration is 1% by weight.
/ G.

When the charging is stopped, the charging device 20 of this embodiment is used.
It is preferable to remove the charge from the photoconductor drum 10 by using.
The static elimination can be performed by setting only the DC component of the bias voltage to zero. After the image formation, only the AC component is applied to rotate the photoconductor drum 10 to eliminate the charge on the photoconductor drum 10. When the charge removal of the photoconductor drum 10 is finished, the application of the AC component is also stopped. Then charging sleeve
22 and the rotation of the photosensitive drum 10 are stopped. At the start of charging, the voltages are applied in the reverse order.

(Embodiment 2) FIG. 2 is an enlarged sectional view showing another embodiment of the charging device of the present invention. In this embodiment, the information on the charging variation is obtained from the fluctuation range of the charging potential. The same parts as those of the charging device 20 of FIG. 1 are denoted by the same reference numerals and detailed description thereof will be omitted. In the figure, 64 is an electrometer provided downstream of the charging device 20 for detecting the charging potential of the photoconductor drum 10, and 74 is an A / D converter for analog / digital converting the output of the electrometer 64.

Immediately before the start of copying in the image forming apparatus or every time a predetermined number of copies are made, the peak of the AC component of the bias voltage
The peak voltage V _PP is gradually changed from a low value to a large value. The output signal of the electrometer 64 at that time is the A / D converter 74.
After being converted into a digital value by, it is input to the CPU 70. The CPU 70 detects the fluctuation range of the charging potential and compares it with the potential fluctuation reference value stored as data in the ROM 71. Then, the peak-to-peak voltage V _PP of the AC component when the fluctuation width becomes equal to or less than the reference value is detected. Determines that V _PP to apply a peak-to-peak voltage V _PP of this time, the control signal is outputted from the CPU 70.
This control signal is converted into an analog value by the D / A converter 73 and then sent to the AC power supply 63 to adjust the peak-to-peak voltage V _PP of the AC component in accordance with the change in the charging condition, so that the photoconductor drum 10 is always operated. It is possible to maintain the charging potential V _{s of the above} at an appropriate value.

As described above, the peak is generated only by the electrometer 64.
Although the peak voltage V _PP can be adjusted,
The current value I _DC detected by the ammeter 62 is also fed back to the CPU 70 via the A / D converter 72 and combined with the feedback by the electrometer 64 to more reliably control the peak / peak voltage V _PP. it can. For example, first, by comparing the fluctuation range of the output signal of the ammeter 62 with the current fluctuation reference value, the peak
Control the peak voltage V _PP , which in turn results in photoreceptor drum 1
The charged potential of 0 is detected by the electrometer 64, and the output signal is compared with the reference value for potential fluctuation. If the difference is within the allowable error, the current fluctuation standard is used if the difference exceeds the allowable error. by controlling the peak-to-peak voltage V _PP to reset the value, it is possible to control the more reliable the peak-to-peak voltage V _PP.

The charging method of the present invention is suitable for a magnetic brush charging device, but is not limited to this, and can be used for potential stabilization control of roller charging and fab brush charging.
Then, these V _PP settings may be made each time image formation is performed, or by setting each V _P for every specific number of prints.
It is possible to prevent the instability of the charging condition due to the variation of (V _PP ) th during continuous printing.

[0060]

According to the charging method of the present invention, even if the resistance of the magnetic brush or the charging condition of the image forming member changes due to changes in conditions such as the environment and the period of use, the bias voltage is used as information on the charging variation. Since the fluctuation range of the current value of the DC component or the fluctuation range of the charging potential is used and the peak / peak voltage of the AC component of the bias voltage is changed by these fluctuation ranges, the environment and other conditions may change. It is possible to provide a charging device using a magnetic brush that constantly changes the bias voltage, which tends to become unstable, to an appropriate value, and does not cause magnetic particles to adhere to the photoconductor or uneven charging.

[Brief description of drawings]

FIG. 1 is an enlarged cross-sectional view showing an embodiment of a charging device of the present invention.

FIG. 2 is an enlarged cross-sectional view showing another embodiment of the charging device of the present invention.

FIG. 3 is a schematic sectional view showing an image forming apparatus provided with a charging device of the present invention.

FIG. 4 is a graph showing a relationship between an AC component voltage of a bias voltage and a charging potential.

FIG. 5 is a graph showing a relationship between an AC component voltage of a bias voltage and a DC current value.

FIG. 6 is a graph showing the relationship between the AC component voltage of the bias voltage and the fluctuation range of the charging potential or the DC current value.

FIG. 7 is a graph showing the relationship between the AC component voltage of the bias voltage and the fluctuation range.

FIG. 8 is a diagram showing a preferable range of an AC component of a bias voltage.

[Explanation of symbols]

 10 Photoconductor drum (image forming body) 20 Charging device 21 Magnetic particles 21A Magnetic brush 22 Charging sleeve (conveying carrier) 23 Magnet body 61 DC power source 62 Ammeter 63 AC power source 64 Potentiometer 70 CPU 71 ROM R Protection resistance

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukie Hososhizawa, Konica stock company, 2970 Ishikawa-cho, Hachioji-shi, Tokyo (72) Inventor Toru Komatsu 2970 Ishikawa-machi, Hachioji-shi, Tokyo Konica stock (72) invention Noriyuki Hiroshi Nomori 2970 Ishikawa-cho, Hachioji City, Tokyo Konica Stock Company

Claims (4)

[Claims] 1. An image forming apparatus for charging an image forming body by applying a bias voltage having an AC component to a charging member which is in contact with the image forming body. A charging device characterized by adjusting the voltage of a component. 2. The charging device according to claim 1, wherein the information is a current value of a DC component of the bias voltage. 3. The charging device according to claim 1, wherein the information is a charging potential of the image forming body. 4. The charging potential is changed by changing the voltage value of the DC component.
Charging device.