JPH096094A - Charging device and image forming apparatus including the same - Google Patents

Abstract

(57) [Abstract] [Purpose] To prevent image defects caused by magnetic particles when a pole position exists in the charging nip where the contact charging member and the photosensitive drum are in contact with each other. [Structure] A magnetic brush 2 is formed by supporting a carrier 23 on the surface of a magnet roll 21 in which N poles and S poles are alternately arranged side by side in the circumferential direction. Then, the magnetic brush 2 is brought into contact with the photosensitive drum 1 and a voltage is applied.
As a result, the magnetic brush 2 is charged by directly injecting electric charges. At this time, the surface roughness at the pole position 24 of the magnet roll 21 is made larger than the surface roughness at the gap 25 between the magnet rolls 21 so that when the pole position exists in the charging nip portion, the pole position is increased. The density of the carrier 23 carried by 24 is increased to improve the transportability to the photosensitive drum 1, and the transport amount of the carrier 23 on the photosensitive drum 1 is kept substantially constant.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device which is provided in an electrophotographic apparatus such as a copying machine or a laser beam printer and which directly charges an object to be charged by injecting it.

[0002]

2. Description of the Related Art A corona charger has been used as a charging device for a conventional electrophotographic apparatus, but in recent years, a contact charging device has been put into practical use in place of the corona charger. This contact charging device uses a charging roller system using a conductive roller as a charging member from the viewpoint of charging stability, and has an advantage that ozone generation can be suppressed low and power consumption can be suppressed low. are doing.

In the charging roller system, a charging roller, which is a conductive elastic roller, is brought into pressure contact with a photosensitive drum, which is a member to be charged, and a voltage is applied to the photosensitive drum to charge the photosensitive drum. Further, since the charging is performed by discharging from the charging roller to the photosensitive drum, the charging becomes possible only by applying the charging voltage equal to or higher than the threshold value, that is, equal to or higher than the charging start voltage Vth. For example, the thickness is 25 μm
When the charging roller is brought into pressure contact with the OPC photosensitive drum, the surface potential of the photosensitive drum starts to rise when a voltage of about 640 V or higher is applied, and thereafter the surface potential is inclined with respect to the applied voltage. 1 increases linearly.

That is, in order to obtain the surface potential Vd of the photosensitive drum required for electrophotographic image formation, it is necessary to apply a DC voltage of (Vd + Vth) or more to the contact charging member such as the charging roller. However, in the method in which only the DC voltage is applied to the charging roller to perform charging, if the resistance value of the charging roller changes due to environmental changes or the photosensitive drum is scraped and the film thickness changes, the charging start voltage Vth is changed.
Fluctuates, it is difficult to set the charging potential of the photosensitive drum to a predetermined value.

Therefore, in order to uniformly charge the photosensitive drum by the contact charging member, as disclosed in Japanese Patent Laid-Open No. 63-149669, an AC charging method is used, and a DC voltage corresponding to the surface potential Vd is used. Is applied with a voltage in which an AC component having a peak-to-peak voltage of 2 × Vth or more is superimposed. This is for obtaining the potential leveling effect due to the AC component, and the surface potential Vd of the photosensitive drum converges to a value at the center between the peaks of the AC voltage and is not affected by disturbances such as the environment. Absent.

However, even in such a contact charging device, the charging of the photosensitive drum actually utilizes the discharging phenomenon from the charging roller to the photosensitive drum. Therefore, the applied voltage required for the charging is applied to the photosensitive drum. Above the surface potential,
A small amount of ozone is generated. In addition, when AC charging is performed for uniform charging, the amount of ozone generated increases, and vibration and noise (hereinafter referred to as "charging sound") between the charging roller and the photosensitive drum due to the electric field of AC voltage are generated. There is a problem in that deterioration of the surface of the photosensitive drum due to generation and discharge becomes significant.

Therefore, as a new charging method, a charging method by directly injecting an electric charge into the photosensitive drum is considered as disclosed in Japanese Patent Laid-Open No. 6-3921.
In this charging method, a voltage is applied to a contact conductive member such as a charging roller, a charging brush, and a charging magnetic brush, and charges are injected into a float electrode on a photosensitive drum having a charge injection layer on the surface to perform contact injection charging. As a charge injection layer, an acrylic resin having SnO ₂ particles made conductive by antimony dope, which is a conductive filler, dispersed on the surface of the photosensitive drum is applied and used. In this direct injection charging method, since the discharge phenomenon is not used, the voltage required for charging is a DC voltage only for the surface potential required by the photosensitive drum, so charging can be performed at a low voltage. Moreover, since no AC voltage is applied, the generation of ozone does not occur and the generation of ozone can be suppressed to a lower level as compared with the charging roller system.

[0008]

However, when a magnetic brush in which magnetic particles are directly attached to a magnet roll is used as the contact conductive member and is rotated to charge the surface of the photosensitive drum, the N pole and the S pole are charged. At the pole position of the magnet roll in which the poles and the poles are alternately arranged side by side, the magnetic particles are formed along the direction of the magnetic force lines, so the transportability of the magnetic particles into the charging nip where the photosensitive drum and the magnetic brush come into contact. Becomes worse. Therefore, the density of the magnetic particles in the charging nip portion is smaller than that when the gap between the electrodes is in the charging nip portion.
When the pole position is present in the charging nip portion, it becomes rougher, so that the area of the magnetic particles that can contact the photosensitive drum becomes smaller.

Particularly in the injection charging, the charge is injected from the former to the latter when the magnetic particles come into contact with the photosensitive drum, so that the charging failure occurs in the portion of the surface of the photosensitive drum where the magnetic particles cannot contact. Resulting in. For this reason, when the pole position is in the charging nip portion, the charging property is lower than that between the poles, and there is a problem that uneven charging of the magnet roller in the pole cycle appears in the image.

Therefore, the present invention has been made in order to solve the above problems, and is caused by magnetic particles when a pole position exists in the charging nip portion where the contact charging member and the photosensitive drum are in contact with each other. An object of the present invention is to provide a charging device that prevents image defects.

[0011]

In order to achieve the above object, the charging device according to the present invention carries magnetic particles on the surface of a magnet roll having N poles and S poles alternately arranged in the circumferential direction. To form a contact conductive member, rotate the contact conductive member to slide the surface of the charged body by the magnetic particles, and apply a voltage to the contact conductive member to charge the charged body through the magnetic particles. The charge is directly injected into the surface of the body, and the contact conductive member is provided so as to improve the transportability of the magnetic particles existing at the pole positions of the magnet roll to the charged body. The surface roughness at the pole position is made larger than the surface roughness between the poles of the magnet roll.

[0012] Preferably, the contact conductive member is formed by a magnetic brush.

As an example, the pole surface of the magnet roll is ground by sandblasting.

As an example, the number of poles of the magnet roll is 4 to 40.

Further, the image forming apparatus of the present invention provides the charging device described in any one of the above, the charged body directly injected and charged by the charging apparatus, and the electrostatic latent image formed on the charged body. A developing device that adheres toner to form a toner image,
A transfer charger for transferring the toner image on the charged body to a recording material and a fixing device for fixing the toner image transferred on the recording material are provided.

Further, preferably, the member to be charged has a charge injection layer on the surface, and a voltage is applied to the contact conductive member to directly charge the charge injection layer to be charged.

[0017]

Based on the above construction, while the contact conductive member having magnetic particles carried on the surface of a magnet roll in which N poles and S poles are alternately arranged side by side in the circumferential direction is rotated, the charged body is charged. A voltage is applied while making contact. As a result, the charge is directly injected into the charged body to be charged.
At this time, by making the surface roughness at the pole position of the magnet roll larger than the surface roughness between the poles of the magnet roll, when the pole position exists in the charging nip portion, it is carried at the pole position. The density of the magnetic particles is increased to improve the transportability to the charged body, and the transport amount of the magnetic particles in the charged body is kept substantially constant.

As a result, the charging uniformity of the surface of the member to be charged is improved, and it becomes possible to obtain a high-quality image for a long period of time.

[0019]

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a sectional view showing a contact conductive member of the charging device of the present invention, and FIG. 2 is a schematic configuration view showing an image forming apparatus using the contact conductive member of FIG.

First, the image forming apparatus used in this embodiment will be described.

The image forming apparatus shown in FIG. 2 is a laser beam printer utilizing an electrophotographic process. In the figure,
A rotary drum type electrophotographic photosensitive drum (hereinafter referred to as “photosensitive drum”) 1 as an object to be charged is an OP having a diameter of 30 mm.
The C photosensitive drum is rotationally driven in the clockwise direction at a process speed (peripheral speed) of 100 mm / sec as indicated by an arrow R1 (the photosensitive drum 1 will be described in detail later). A magnetic brush serving as a contact conductive member that contacts the photosensitive drum 1 in a clockwise direction while rotating around the photosensitive drum 1 (the magnetic brush will be described in detail later).
2. A laser beam scanner (not shown) as an exposing means, a reversal developing device 3, a contact transfer device 4, and a cleaning device 6 are arranged in this order. In addition, the above-described image forming apparatus includes four process devices including the photosensitive drum 1, the magnetic brush 2, the reversal developing device 3, and the cleaning device 6,
It is built in the cartridge 20.

A laser beam scanner having a laser diode, a polygon mirror, etc. outputs scanning exposure light L by a laser beam whose intensity is modulated corresponding to a time-series electric digital pixel signal of image information, so that the surface of the photosensitive drum 1 is covered. An electrostatic latent image corresponding to target image information is formed on the charged surface. The reversal developing device 3 contains a magnet 3b in a non-magnetic developing sleeve 3a having a diameter of 16 mm, and is arranged at a distance of 300 μm from the photosensitive drum 1. The developing bias power source S2 is connected to the developing sleeve 3a, and the developing sleeve 3a to which the developing bias voltage is applied is rotated at the same speed as the photosensitive drum 1 to coat the magnetic one-component insulating toner (negative toner). ,
The electrostatic latent image on the photosensitive drum 1 is developed to form a toner image. In this case, a voltage in which a DC voltage of -500V and a rectangular AC voltage having a frequency of 1800Hz and a peak-to-peak voltage of 1600V are superimposed is used, and jumping development is performed between the developing sleeve 3a and the photosensitive drum 1.

On the other hand, the transfer roller 4 having a medium resistance (the resistance value is, for example, 5 × 10 ⁸ Ω) as the contact transfer device 4 is arranged so as to come into contact with the photosensitive drum 1 with a predetermined pressing force, and is + 2000V. Is connected to the transfer bias power source S3 for applying the DC voltage of. The transfer material P as a recording material supplied from a paper supply unit (not shown) is introduced at a predetermined timing into a pressure contact nip part (transfer part) T where the photosensitive drum 1 and the transfer roller 4 come into contact with each other. Has been done. That is, the transfer material P supplied from the paper feeding unit is temporarily stopped by a timing roller (not shown) so that the transfer surface thereof faces the transfer unit T in synchronization with the toner image forming surface of the photosensitive drum 1. It has become. Then, when the transfer material P is conveyed to the transfer portion T, the toner image is transferred from the surface of the photosensitive drum 1 by the electrostatic force and the pressing force.

After the transfer material P is separated from the photosensitive drum 1, the transfer material P is introduced into a fixing device 5 such as a heat fixing system to fix a toner image, and is discharged outside the device as an image formed product (print paper). .

Further, adhered contaminants such as residual toner remain on the photosensitive drum 1 after the toner image is transferred onto the transfer material P. The cleaning device 6 removes the attached contaminants and cleans the photosensitive drum 1. As a result, the photosensitive drum 1
Are repeatedly used for image formation.

Next, the photosensitive drum 1 will be described.

The photosensitive drum 1 is formed by providing five functional layers on a drum base made of aluminum. The first layer is
An undercoating layer that is a conductive layer having a thickness of about 20 μm is provided to smooth defects such as the aluminum drum base and to prevent moire due to reflection of laser exposure.

The second layer is a positive charge injection preventing layer which is a medium resistance layer having a thickness of about 1 μm, and the positive charges injected from the aluminum drum substrate cancel the negative charges charged on the surface of the photosensitive drum 1. The resistance is adjusted to about 10 ⁶ Ωcm by the amylan resin and methoxymethylated nylon.

The third layer is a charge generation layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin, and positive and negative charge pairs are generated by laser exposure.

The fourth layer is a charge transport layer that does not move the negative charges charged on the surface of the photosensitive drum 1 but transports only the positive charges generated in the charge generation layer to the surface of the photosensitive drum 1, and is a polycarbonate resin. P with hydrazone dispersed in
Type semiconductor.

The fifth layer is a photo-curable acrylic resin containing Sn.
This is a charge injection layer in which ultrafine particles of O ₂ are dispersed. Specifically, antimony-doped SnO ₂ particles with a resistance of about 0.03 μm are added to an acrylic resin in an amount of 7%.
It is a dispersion of 0 wt%. Further, in the case of the magnet roll rotating system as in this embodiment, since the spikes of the magnetic particles (carrier) 23 are harder than those in the sleeve system (the explanation is omitted), Teflon should be dispersed by 26% to improve the sliding. Makes the carrier 23 easy to move.

Next, the magnetic brush shown in FIG. 1 will be described.

The magnetic brush 2 includes magnetic particles (carriers) 23 on a magnet roll 21 rotatable by a cored bar 22.
(Refer to FIG. 2), the surface roughness of the pole position of the magnet roll 21 (range in which the magnetic force in the direction perpendicular to the surface of the magnet roll is 50% or more of the peak value) 24 Between poles (between pole positions) 2
The surface roughness is larger than that of No. 5. This improves the transportability of the carrier 23 (see FIG. 2) at the pole position 24 to the charging nip portion where the photosensitive drum 1 and the magnetic brush 2 contact each other.

The magnet roll 21 has, for example, eight poles of equal poles, in which N poles and S poles are alternately arranged side by side in the circumferential direction, and the diameter is 1
The length is 8 mm and the length in the longitudinal direction is 230 mm.
A conductive adhesive layer 26 is formed on the outer peripheral surface of the magnet roll 21.
A conductive tape (copper tape having a thickness of 50 μm in this embodiment) 27 is adhered to and covered with the magnetic brush to make the surface attached to the magnetic brush conductive.

The surface roughness of the portion (pole position) of the magnet roll 21 that contacts the pole is formed to be larger than the surface roughness of the gap 25 between the poles by sandblasting. As a means for increasing the surface roughness of the pole position 24, the sandblasting treatment is used in consideration of the stability of the surface roughness, but iron rasp, sandpaper or the like may be used instead. In addition, the pole position 24 may be coated with a mixture of particles in a conductive binder. The average surface roughness at the pole position 24 was about 5 μm, and the average surface roughness at the gap 25 was about 2 μm.

As for the average surface roughness of the magnet roll 21, if this roughness is smaller than 5% of the average particle diameter of the carrier 23, the carrier 23 on the surface of the magnet roll 21.
Slips, and the transportability of the carrier 23 to the charging nip portion deteriorates. Further, when the carrier 23 exceeds 100% of the average particle diameter, the carry amount does not change. Therefore, the range of the average surface roughness of the magnet roll 21 is preferably 5 to 100% of the average particle size of the carrier 23.
When the number of poles of the magnet roll 21 is less than 4,
The difference in magnetic restraining force on the carrier 23 is greatly different between the pole position 24 and the gap 25, and the gap 25 is likely to be attached to the photosensitive drum 1. Further, if the number of poles is too large, the leakage magnetic flux is reduced, so that the layer thickness of the carrier 23 is reduced and it is difficult to form the charging nip portion. As a result, the charging property is deteriorated. From the above, the number of poles of the magnet roll 21 is preferably 4 to 40 poles, and it is preferable that the pole gaps 25 are as uniform as possible and have the same magnetic force.

The magnetic flux density at the magnetic pole surface position of the magnet roll 21 was 0.1 T (tesla). The magnetic flux density is 0.
03T or more is preferable.

The carrier 23 of this embodiment has an average particle size of 30 μm.
m, maximum magnetization 60 Am ² / kg, density 2.2 g / cm
² Medium resistance ferrite carrier is used. The gap from the magnet roll 21 to the surface of the photosensitive drum 1 is maintained at 500 μm by mounting rollers on both ends of the magnet roll 21. When the carrying amount on the magnet roll 21 is set to 20 g, the entire charging nip width including the carrier pool is about 5 mm. The carrier resistance in this charging nip width is 5 × when a DC voltage of 100 V is applied.
It was 10 ⁶ Ω.

The peripheral speed ratio between the magnetic brush 2 and the photosensitive drum 1 is defined by the following equation.

Peripheral speed ratio (%) = {(magnetic brush peripheral speed-photosensitive drum peripheral speed) / photosensitive drum peripheral speed} × 100 The peripheral speed of the magnetic brush 2 is the rotational direction of the photosensitive drum 1 and the rotational direction of the magnetic brush 2. If and are in the same direction, the value is negative. Considering the opportunity of contact between the magnetic brush 2 and the photosensitive drum 1, it is desirable that the absolute value of the peripheral speed ratio is 100% or more, but -100% is the state in which the magnetic brush 2 is stopped. Magnetic brush 2 located at the nip
Where the surface of the photosensitive drum 1 and the surface of the photosensitive drum 1 do not sufficiently contact with each other, charging failure occurs, and the location of contact failure appears in the image as it is. Further, in the forward rotation, if an attempt is made to obtain the same peripheral speed ratio as in the reverse rotation, the rotational speed of the magnetic brush 2 becomes high, which causes a disadvantage that the carrier 23 is scattered. Sets the peripheral speed ratio to -200%. When a DC bias voltage of -700V was applied from the charging bias applying power source S1 to the magnetic brush 2, the outer peripheral surface of the photosensitive drum 1 was uniformly charged to approximately -700V.

Conventionally, when the pole position of the magnet roll reaches the charging nip portion, the carrier ears are formed along the direction of the lines of magnetic force at the pole position, so that the carrier density is lower than between the poles. Therefore, when the pole position reaches the charging nip portion, a portion where the carrier and the photosensitive drum do not come into contact with each other occurs, and a charging failure occurs in this portion, and the charging failure appears as an uneven charging in a polar cycle in an image. There was a problem. Therefore, there is a method of roughening the surface of the magnet roll as a method of increasing the carry amount of the carrier. However, if the entire surface of the magnet roll is roughened uniformly, the carry amount of the carrier similarly increases at the pole positions and between the poles. Therefore, it was not possible to eliminate the uneven charging of the polar cycle.

Therefore, by increasing the surface roughness of the pole position 24 as compared with the surface roughness of the pole gap 25 as in the present embodiment, the carry amount at the pole position 24 is increased and the pole position 24 is charged at the charging nip. By setting the density of the carrier 23 when reaching the inside of the unit to be approximately the same as that of the gap 25, the carrier 23 existing in the charging nip portion is brought into uniform contact with the photosensitive drum 1.
As a result, the surface of the photosensitive drum 1 can be uniformly charged, and a good image can be obtained for a long time.

In the image forming apparatus of the above-described embodiment, the four process equipments including the photosensitive drum 1, the magnetic brush 2, the reversal developing device 3 and the cleaning device 6 are connected to the cartridge 2
The cartridge 20 is built into the image forming apparatus main body so that it can be attached to and detached from the main body of the image forming apparatus in a detachable and replaceable manner, but the present invention is not limited to this.

Further, in the above embodiment, the OPC photosensitive drum provided with the charge injection layer is used as the charged body, but the present invention is not limited to this, and the charged body has an injection point a on its surface.
Even if a photosensitive drum such as -Si or a-Se is used, the same effect as that of the above embodiment can be obtained.

[0046]

As described above, according to the present invention,
Since the contact conductive member has the surface roughness at the pole positions of the magnet roll in which the N poles and the S poles are alternately arranged side by side in the circumferential direction larger than the surface roughness between the poles, the pole positions exist in the charging nip portion. At this time, the transportability of the magnetic particles adhering to the pole positions to the body to be charged can be improved, and the transport amount within the charging nip portion can be maintained at the same level as when the gap between the electrodes reaches the charging nip portion.

As a result, the charging uniformity on the surface of the member to be charged is improved, and a high-quality image can be obtained for a long period of time. In addition, since the contact conductive member is configured to be directly injected and charged by the contact conductive member, the voltage applied to the contact conductive member can be lowered, and at the same time, generation of ozone, generation of charging noise due to AC charging, and discharge caused by discharge. It is possible to reduce the deterioration of the surface of the charged body.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of a magnet roll used in a charging device according to the present invention.

FIG. 2 is a schematic configuration diagram showing an example of an image forming apparatus provided with a charging device of the present invention.

[Explanation of Codes] 1 charged body (photosensitive drum) 2 contact conductive member (magnetic brush) 21 magnet roll 23 magnetic particles (carrier) 24 pole position 25 pole gap

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Harumi Ishiyama 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Takeo Yamamoto 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Within the corporation

Claims (6)

[Claims] 1. A contact conductive member is formed by supporting magnetic particles on the surface of a magnet roll in which N poles and S poles are alternately arranged side by side in the circumferential direction, and the contact conductive member is rotated to generate the contact conductive member. A charging device for slidingly contacting a surface of an object to be charged and applying a voltage to the contact conductive member to directly inject an electric charge to the surface of the object to be charged via the magnetic particles, wherein the contact conductive member The surface roughness of the pole position is made larger than the surface roughness between the poles of the magnet roll so as to improve the transportability of the magnetic particles existing at the pole position of the magnet roll to the charged body. Characteristic charging device. 2. The charging device according to claim 1, wherein the contact conductive member is formed of a magnetic brush. 3. The charging device according to claim 1, wherein the pole surface of the magnet roll is ground by sandblasting. 4. The number of poles of the magnet roll is 4 to 4
The charging device according to claim 1, wherein the charging device has 0 poles. 5. The charging device according to claim 1, a charged body directly injected and charged by the charging device, and a toner on an electrostatic latent image formed on the charged body. A developing device that adheres to form a toner image, a transfer charger that transfers the toner image on the body to be charged onto a recording material, and a fixing device that fixes the toner image transferred on the recording material An image forming apparatus characterized by the above. 6. The charged body has a charge injection layer on its surface, and is charged by directly injecting charge from the contact conductive member into the charge injection layer. Image forming apparatus.