JP3362293B2 - Image forming device - Google Patents

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 equipped with a contact type charging device incorporated in an image forming apparatus such as an electrophotographic copying machine or an electrostatic recording apparatus 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 to charge a charged 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, the corona charger causes noise due to high voltage. This is a major drawback when using a photographic image forming apparatus.

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

Therefore, as a charging device capable of charging an image forming body without performing gas discharge by applying a high voltage like a corona charger, a fur brush or a conductive elastic roll is brought into contact with the image forming body for charging. There is known a charging device adapted to perform. In addition, JP-A-59-133569 discloses that
As a charging member, magnetic particles are adsorbed on a cylindrical carrier containing a magnetic body to form a magnetic brush, and this magnetic brush rubs the surface of the image forming body while applying a DC bias voltage to charge the image forming body. A charging device adapted to do so is disclosed.

A contact charging device such as the fur brush charging device, a charging device using a conductive elastic roll, or a magnetic brush charging device does not discharge a gas by applying a high voltage, and an image of a charged object at a relatively low voltage. The formed body can be charged and is superior to the corona charging device. However, when the above-mentioned contact charging device was used, uniform charging was not always obtained.

Therefore, for example, Japanese Patent Laid-Open No. 60-220587,
JP-A-63-149668, JP-A-4-116674,
A contact charging method has been proposed in which a contact charging member such as a fur brush, a conductive elastic roll, or a magnetic brush is applied with an AC bias voltage in which an AC voltage is superimposed on a DC voltage to charge an image forming body. In these publications, 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 contact charging method described in each of the above-mentioned publications, toner or the like is fused to the surface of the contact charging member due to long-term use, and the electric resistance of the contact charging member increases. was there. Therefore, the peak-peak voltage V of the AC voltage of the AC bias voltage is
_It was necessary to set _PP gradually higher. However, if the peak-to-peak voltage V _PP is increased, the output voltage of the power supply becomes unstable and the output voltage becomes unstable, and uneven charging occurs, or the high voltage causes a breakdown that destroys the photoreceptor layer of the image forming body. Further, there is a problem in that the fur brush, which is a contact charging member, burns due to a high voltage, and the magnetic particles forming the magnetic brush adhere to the image forming body.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an image forming apparatus provided with a contact type charging device for preventing uneven charging and breakdown.

[0010]

The above-mentioned object is to bring a charging member of magnetic particles into contact with a photoconductor so that an AC component is superposed on a DC component.
In an image forming apparatus that charges by applying a folded AC bias voltage, the control unit initially sets the peak of the AC component.
・ Bias with gradually increasing peak voltage from low value
Apply a voltage to detect the current value of the DC component,
Initial value of peak-to-peak voltage value when voltage reaches saturation point
Is measured, and based on the initial value of the peak / peak voltage value,
The initial setting value of the peak / peak voltage of the applied AC component and
Determine the specified value, and directly perform the image formation before the image formation.
The peak / peak voltage value at which the current value of the current component reaches the saturation point
Of the AC component applied during image formation.
Set the peak voltage and the set peak / peak voltage value.
Compared with the specified value, the peak and peak voltage values are specified
When it is less than the value, the image forming operation is continued and the peak peak
Image formation is stopped when the voltage value exceeds the specified value,
This is achieved by an image forming apparatus characterized by performing control .

The image forming apparatus is characterized in that the specified value is 1.5 to 2 times the initial setting value of the AC voltage value, and the charging member is magnetic particles.

[0012]

In the contact type charging device of the present invention, the AC voltage value of the AC bias voltage applied to the charging member in order to provide the charging uniformity is variable depending on the charging condition, and when the value exceeds the specified value, the image Since the forming operation is stopped, image formation is not performed while the variable AC bias voltage is unnecessarily high.

[0013]

【Example】

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

FIG. 1 is a schematic sectional view of an image forming apparatus of the present invention. In FIG. 1, 10 is the peripheral speed 24 in the direction of the arrow (clockwise).
A photosensitive drum, which is an image forming body and a charged body, that rotates at 0 mm / sec, and includes an OPC photosensitive layer formed by sequentially providing an undercoat layer, a charge generation layer, and a charge transport layer on a conductive base material made of aluminum or the like. It is a negatively chargeable photoconductor drum. 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 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 same polarity as the charging of the photoconductor 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 contact charging device 20 described later and passes through. Photoconductor drum 10
On the upper part, an image is written by the image light L in the exposure section 12, and an electrostatic latent image corresponding to the image is formed. This electrostatic latent image is developed by the developing device 30, and the photosensitive drum 10
A toner image is formed on the top.

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 explaining the contact type charging device 20 using the above magnetic brush, the particle size of magnetic particles as a charging member used therein and general conditions of a carrier for conveying magnetic particles will be described.

Generally, when the average particle diameter (weight average) of the magnetic particles is large, (a) the state of the ears of the magnetic brush formed on the carrier is rough, so that 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 diameter (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 used as a magnetic material in the same manner as in the magnetic carrier particles of the conventional two-component developer.
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 and manganese-copper alloys. 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 spherical shape of the 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.

For 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.

Furthermore, the magnetic particles used in the contact type charging device are such that the magnetic brush constituted by them moves lightly by the oscillating electric field, and further, external scattering does not occur.
A material having a small specific gravity and an appropriate maximum magnetization is desirable. Specifically, the true specific gravity is 6 or less and the maximum magnetization is 30 to 100e.
It was found that good results can be obtained by using mu / g, especially 40 to 80 emu / 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 are no protrusions 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.

A conductive carrier for applying a bias voltage is used as a carrier for magnetic particles, and 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.

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 100 to 400 mg / cm ² , and particularly preferably 200 to
It was found to be 300 mg / cm ² . The existing amount is an 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 D between the carrier and the photoconductor drum 10, which is an image forming body, becomes too narrower than 0.1 mm, it becomes difficult to form the magnetic brush ears that uniformly act on it, and it is sufficient. When the magnetic particles cannot be supplied 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 the contact state is insufficient. Uneven charging easily occurs and sufficient charging cannot be obtained. In this way, if the gap Dsd between the carrier and the photosensitive drum 10 becomes extremely extreme, the thickness of the particle layer on the carrier cannot be adjusted appropriately, but the gap Dsd becomes smaller.
In the range of 0.1 to 5.0 mm, the thickness of the particle layer can be appropriately formed, and the ears of the magnetic brush are also formed uniformly. Further, the conveyance amount (W) and the gap (Dsd) have a relationship of 2,000 ≦ W / Dsd ≦ 6,000 (mg / cm ³ ) in order to maintain a good contact state and to perform charging uniformly and stably at high speed. It is necessary to satisfy the requirement, and it was confirmed that the charging becomes non-uniform when W / Dsd is out of this range.

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.
Is facing and in contact with.

It is considered that this disturbing condition is effective for uniform charge by facilitating the transfer of charges without causing charging stripes. 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.

Next, the charging device 20 used in the present invention will be described. FIG. 2 is an enlarged sectional view showing an example of the charging device used in the present invention. In FIG. 2, 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. As shown in the figure, this magnet body 23 is a cylindrical magnet body having 6 or 8 magnetic poles alternately magnetized in the peripheral edge so that the surface of the charging sleeve 22 has 500 to 1,000 gauss. have. The charging sleeve 22 is rotatable with respect to the magnet body 23, and can be rotated at a peripheral speed of 0.1 to 1.0 times in the same direction as the moving direction of the photoconductor drum 10 at a position facing the photoconductor drum 10. preferable.

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 of 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 100 to 400 mg / cm ² particularly preferably 200 to 30 when the amount of the magnetic particles 21 conveyed, that is, the amount of the magnetic particles 21 present on the charging sleeve 22 in the charging region.
It is adjusted to be 0 mg / 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.

The gap Dsd in which the charging sleeve 22 faces the photoconductor drum 10 can be set in the range of 0.1 to 5.0 mm, and if it becomes narrower than this range, the photoconductor drum 10 in contact with the magnetic brush 10 will be described. And the like, the durability of the photosensitive drum 10 is rapidly deteriorated, and it becomes difficult to form the magnetic brush 21A composed of the magnetic particles 21 that appropriately rub the photosensitive drum 10. It becomes difficult to contact 10 uniformly, and thus to uniformly charge the photosensitive drum 10. Charging sleeve 22 and photoconductor drum 10
The gap Dsd between is a conductive magnetic brush 21 whose thickness is regulated.
Connected with A.

Not limited to the example shown in FIG. 2, 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 material layer 10a covering the surface thereof, and the conductive base material 10b is grounded.

Reference numerals 61 and 63 are bias power supplies for applying an AC bias voltage in which a DC component is superimposed on the AC 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 memory for storing 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.

In this case, the peak-to-peak voltage (V _PP ) of the AC component of the bias voltage and the absolute value (│V
The relationship of _S |) is shown in FIG. The horizontal axis represents the peak-to-peak voltage (V _PP ) of the AC component of the bias voltage, and the vertical axis represents the absolute value (│V _S │) of the charged potential of the photosensitive drum 10. As the peak-peak voltage V _PP increases, the absolute value (| V _S |) of the charging potential increases, and the charging potential V _S increases.
Is saturated at a value where the peak-peak voltage is a constant threshold value (V _PP ) th and is approximately equal to the DC component value V _DC of the bias voltage. Even if the peak-peak voltage V _PP is further increased, the charging potential V _S remains It has the characteristic that it hardly changes. Magnetic particles 21
Although the electric resistance of the toner also changes depending on the environmental conditions, the electric resistance increases as the toner is fused to the surfaces of the magnetic particles 21 as it is used. Therefore, the characteristic curve is shown on the left side as shown by the solid line in the case of new magnetic particles in the early stage of use, and on the right side as shown by the dotted line in the case of magnetic particles used for a long time (b). Will be located in.
Therefore, the threshold (V _PP ) th also changes, and in the initial (a), (V
_{The value of PP} ) th gradually increases as it is used, and approaches (V _PP ) thB.

As a preferable charging condition for contact charging, the peak-to-peak voltage V _PP of the AC component is 0.8 × (V _PP ) th ≦ V _PP ≦ 1.5 × (V _PP ) th under various environmental conditions as a result of experiments. It turned out that it can be obtained. If the peak-peak voltage V _PP is smaller than this, uneven charging and the adhesion of magnetic particles are large, and breakdown is likely to occur at the peak-peak voltage V _PP larger than 2 × (V _PP ) th.

Further, it has been clarified that the charging potential V _S in the contact charging is proportional to the current value I _DC of the DC component.
That is, the current value I _{DC of the} DC component increases as the peak-to-peak voltage V _PP of the AC component increases, but (V _PP ) t
When it becomes more than h, I _DC becomes saturated. That is,
As shown in FIG. 5, the current value I _DC of the DC component shows almost the same change as that of FIG. 4 with respect to the change of the AC component.

In the contact type charging device of the image forming apparatus of the present invention, the voltage value of the DC component is set to a predetermined V value at the time of manufacturing and shipping.
_{A DC} voltage is applied, and a bias voltage in which the peak-to-peak voltage (V _PP ) of the AC component is gradually increased from a low value is applied, and the current value I _DC changing at that time is detected by the ammeter 62. The detected current value I _DC is converted into a digital value by the A / D converter 72 and then input to the CPU 70. CPU70
Is V _PP when this current value I _DC reaches the saturation point (V _PP ).
This is stored in the memory 71 as th and this (V _PP ) th
The value of 1.5 to 2 times is calculated and stored in the memory 71 as a specified value.

[0046] When the copying is performed, the V _PP when the current value I _DC to gradually increase the V _PP from a low value before the copy start has reached a saturation point and (V _PP) th, using the bias voltage To form an image. The CPU 70 determines (V _PP ) th as described above each time an image is formed, and the CPU 70 outputs a control signal based on the determined value. 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 the determined peak-to-peak voltage V _PP of the AC component. Also, C
PU70 along with determining the V _PP, reads the specified value of the stored in the memory _{71 (1.5~2) × (V PP} ) th compared with this. Although determined (V _PP) th is the specified value between a smaller continues image formation, when determined (V _PP) th is greater than the prescribed value, sends a stop signal to the image forming operation control unit CPU70 is not shown Then, the image forming operation is stopped and the abnormality of the charging device is displayed on the display section of the operation panel (not shown). A flowchart of this operation is shown in FIG.

By this operation, a value lower than the specified value is always applied to the AC component V _PP, so that the photosensitive drum 10
The charging potential V _S is maintained constant, the AC power supply 63 is not output over, and an excessive peak-to-peak voltage V _PP is not applied.

Spherical ferrite particles coated so as to have conductivity were used as the magnetic particles 21 in the above-mentioned embodiment. 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. 3 is an enlarged sectional view showing another embodiment of the contact type charging device used in the present invention. The same parts as those of the charging device 20 of FIG. 2 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 72 is an A / D converter for analog / digital converting the output of the electrometer 64.

In the charging device of this image forming apparatus, the voltage value of the DC component is set to a predetermined V _DC at the time of manufacture and shipment, and a bias voltage is applied by gradually increasing the peak-to-peak voltage (V _PP ) of the AC component from a low value. The charging potential V _S which changes at that time is detected by the electrometer 64. The detected analog value of the charging potential V _S is converted into a digital value by the A / D converter 72 and then input to the CPU 70. The CPU 70 sets V _PP when this charging potential V _S reaches the saturation point to (V _PP ) t
It is stored in the memory 71 as h, and 1. of this (V _PP ) th.
A value of 5 to 2 times is calculated and stored in the memory 71 as a specified value.

[0052] When the copying is performed, the V _PP when the charge potential V _S to gradually increase the V _PP from a low value before the copy start has reached a saturation point and (V _PP) th, using the bias voltage To form an image. The CPU 70 determines (V _PP ) th as described above each time an image is formed. A control signal is output from the CPU 70 based on this determined value. 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 the determined peak-to-peak voltage V _PP of the AC component. Also,
CPU70 is thereby determines a V _PP, reads the specified value of the stored in the memory _{71 (1.5~2) × (V PP} ) th compared with this. Image formation is continued while the determined (V _PP ) th is smaller than the specified value, but the determined (V _PP ) th is determined.
Is greater than the specified value, the CPU 70 sends a stop signal to the image forming operation unit (not shown) to stop the image forming operation, and displays an abnormality of the charging device on the display unit of the operation panel (not shown).

By this operation, a value lower than the specified value is always applied to the AC component V _PP, so that the photosensitive drum 10
The charging potential V _s is maintained constant, the AC power supply 63 is not output over, and a bias voltage having an excessive peak-peak voltage V _PP is not applied.

In each of the embodiments described above, the peak-to-peak voltage V _PP of the bias voltage is changed to obtain the DC current value I _DC or V _PP when the charging potential V _S reaches the saturation point, and this is used. Although the AC voltage value and the specified value were set, in addition to this, a standard patch was provided at one corner of the document scanning surface, and the latent image of the standard patch was formed outside the image area of the image forming body by an optical system that optically scans the document. , The density of the standard patch image obtained by developing this was measured, and V when the density reached the saturation point
It is also possible to find the _PP and set the AC voltage value and the specified value by this.

Although all of the above-mentioned embodiments show the magnetic brush charging device using the magnetic brush as the charging member, other contact charging devices such as a fur brush charging device and a charging device using a conductive elastic roll. Can also be used.

[0056]

According to the contact type charging device of the present invention,
When the charging member's resistance changes due to a long use period, the peak / peak voltage of the AC component of the bias voltage is changed accordingly to keep the charging potential of the charged object constant. When the voltage becomes higher than the specified value, the image forming operation is stopped and the abnormality of the charging device is displayed. Therefore, the peak-to-peak voltage does not become higher than the specified value, and the photoreceptor of the charged body due to the high voltage It provides an image forming apparatus with excellent performance such as stable bias voltage supply and no uneven charging because it does not force a bias power supply to generate an undesired breakdown. can do.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing an image forming apparatus of the present invention.

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

FIG. 3 is an enlarged cross-sectional view showing another embodiment of the charging device of the device of the present invention.

FIG. 4 is a graph showing the relationship between the peak-to-peak voltage of the AC component of the bias voltage and the charging potential.

FIG. 5 is a graph showing the relationship between the peak-to-peak voltage of the AC component of the bias voltage and the DC current value.

FIG. 6 is a flowchart showing an operation of the device of the present invention when setting an AC voltage.

[Explanation of symbols]

10 Photoconductor drum (body to be charged) 20 Charging device 21 Magnetic particles (charging member) 21A magnetic brush 22 Charging sleeve (carrier) 23 Magnet body 61 DC power supply 62 ammeter 63 AC power supply 64 electrometer 70 CPU 71 memory R protection resistance

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Toru Komatsu 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (72) Inventor Hiroyuki Nomori 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (56) ) References JP-A-4-138467 (JP, A) JP-A-5-323765 (JP, A) JP-A-5-119569 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB) Name) G03G 13/02 G03G 15/02

Claims (2)

(57) [Claims] 1. In an image forming apparatus for charging by charging a charging member of magnetic particles to a photoconductor and applying an AC bias voltage in which an AC component is superimposed on a DC component, charging is performed by an initial peak of the AC component. Peak voltage value
Apply a bias voltage that gradually increases from a low value.
Detects the current value of the DC component and reaches the saturation point.
The initial value of the peak-to-peak voltage value is measured, and the alternating current applied based on the initial value of the peak-to-peak voltage value.
Determine the initial setting value and specified value of the peak / peak voltage of the component
Constant, and the current value of the DC component at the time of image formation before the image formation
Measure the peak-to-peak voltage value that reaches the saturation point and
Set the peak / peak voltage of the AC component that is applied during formation
And set the peak / peak voltage value to the specified value.
By comparison, when the peak / peak voltage value is less than the specified value, image formation
Image formation when the operation is continued and the peak / peak voltage value is higher than the specified value.
An image forming apparatus characterized by stopping and controlling . 2. The specified value is the peak peak of the AC component.
The image forming apparatus according to claim 1, characterized in that a 1.5 to 2 times the initial set value of over click voltage value.