JP3131348B2 - 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, such as a copying machine or a laser beam printer, provided with a charging device for performing injection charging by bringing a charging member into contact with a photosensitive member.

[0002]

2. Description of the Related Art Conventionally, a corona charger has been used as a charging device for electrophotography. In recent years, contact charging devices have been put to practical use instead. This is aimed at low ozone and low power, and among them, a roller charging method using a conductive roller as a charging member is particularly preferable in terms of charging stability and widely used.

[0003] In roller charging, a conductive elastic roller is pressed against an object to be charged, and a voltage is applied thereto to charge the object to be charged.

[0004] Specifically, since charging is performed by discharging from a charging member to a member to be charged, charging is started by applying a voltage equal to or higher than a certain threshold voltage. For example, when the charging roller is pressed against an OPC photosensitive member having a thickness of 25 μm, the surface potential of the photosensitive member starts to rise when a voltage of about 640 V or more is applied, and after the rise starts. The surface potential of the photoreceptor linearly increases with a slope of 1 with respect to the applied voltage. This threshold voltage is defined as a charging start voltage _Vth .

[0005] That is, the charging roller in order to obtain a photosensitive member surface potential V _d required for electrophotography (V _d +
V _th ) must be applied. The method of applying only a DC voltage to the contact charging member to perform charging in this manner is referred to as DC charging.

However, in DC charging, the resistance value of the contact charging member fluctuates due to environmental fluctuations and the like, and _Vth fluctuates when the film thickness changes due to shaving of the photoreceptor. It was difficult to make the value.

[0007] Therefore, as disclosed in JP-63-149669 discloses to achieve uniform further charging, the DC voltage corresponding to the desired _{V d, (2 × V th} ) or more peaks An AC charging system in which a voltage obtained by superimposing an AC component having an intermediate voltage is applied to a contact charging member is used. This is for the purpose of the potential leveling effect of AC, and the potential of the charged body converges to _Vd , which is the center of the peak of the AC voltage, and is not affected by disturbances such as the environment. .

However, even in such a contact charging device, the essential charging mechanism uses a discharge phenomenon from the charging member to the photoreceptor, so that the voltage applied to the charging member is as described above. A value higher than the surface potential of the photoconductor is required, and a small amount of ozone is generated. Further, when AC charging is performed for uniform charging, generation of further ozone, A
Vibration and noise (hereinafter referred to as "AC charging noise") of the charging member and the photoreceptor due to the electric field of the C voltage, and the deterioration of the photoreceptor surface due to the discharge become remarkable, which has been a new problem. .

Therefore, as a new charging method, a charging method by directly injecting a charge into a photosensitive member is disclosed in Japanese Patent Application No. 04-158.
No. 128, Japanese Patent Application No. 05-066150, and the like. This charging system applies a voltage to contact conductive members such as a charging roller, a charging brush, and a charging magnetic brush,
This is a method of injecting charges into trap levels on the surface of a photoreceptor to perform contact injection charging (hereinafter referred to as “injection charging”). In this charging method, since the discharge phenomenon is not used, the voltage required for charging is only the desired photoconductor surface potential and no ozone is generated. Furthermore, since no AC voltage is applied, no charging noise is generated, and the charging method is excellent in ozone-less and low-power compared to the roller charging method.

[0010]

However, when the magnetic brush is used as the charging member for the injection charging in the above-mentioned conventional example, there are the following disadvantages.

In the injection charging, the photosensitive member is charged by charge injection by contact between the conductive magnetic particles (hereinafter referred to as "carrier") forming the magnetic brush and the photosensitive member. Therefore, in order to improve the charge injection performance, the magnetic brush is rotated so as to have a relative speed to the rotation speed of the photoconductor. Here, in a conventional magnetic brush, a magnetic pole of a magnet roll is usually arranged at a position closest to a nip in order to hit a carrier ear to a photoconductor.

For this reason, in the conventional apparatus, a magnetic force acts to restrain the carrier in the nip, and the smooth movement of the carrier in the nip is hindered, so that the contact nip becomes non-uniform, and the carrier is exposed to light. There has been a problem that the injection charging ability is reduced due to a reduced chance of contact with the body and charging failure occurs.

Also, the smooth movement of the carrier is impeded,
If the movement of the carrier in contact with the photoreceptor surface becomes poor,
There is also a problem in that the carrier itself is charged up, charge injection is hindered, and charging failure occurs. Here, the term “charge-up” refers to a state in which the carrier in contact with the surface of the photoconductor charges the photoconductor and stores the opposite charge, thereby reducing the actual applied voltage.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus which prevents a decrease in injection charging ability and prevents charging failure.

[0015]

[0016]

According to the first aspect of the present invention, there is provided an image forming apparatus, comprising:
A rotatable sleeve that forms a nip between the photoconductor and
A magnet roll fixed in the sleeve.
A magnetic brush, and the surface of the photoconductor is
Rotation of the magnetic brush in an image forming apparatus that charges the surface
Magnetic flux density downstream of the nip in the direction
Is larger than the magnetic flux density at the closest position of the nip.
Comprising a magnetic flux density setting means for listening setting, the magnetic flux density setting
Constant unit, said a point of maximum magnetic flux density on the sleeve corresponding to one magnetic pole of the magnet roll, the magnetic flux density is zero on the sleeve adjacent at said maximum magnetic flux density the sleeve rotation direction upstream side to a point a point where, as nip closest position between the photosensitive member in the range of these two points falls, to set the magnetic pole position of the magnet roll
Features.

According to a second aspect of the present invention, there is provided a photoconductor, comprising:
A magnetic brush that forms a nip between
To charge the photoreceptor surface via the magnetic brush
In the image forming apparatus, the rotation direction of the magnetic brush
The magnetic flux density downstream of the nip,
Set higher than the magnetic flux density at the nearest position
Has a magnetic flux density setting means, said magnetic flux density setting means has a magnetic member disposed outside of said sleeve to set the magnetic flux density of the nip near the magnetic member
Features .

The present invention according to claim 3 provides the present invention according to claim 1 or 2
The image forming apparatus according to, the photosensitive member surface layer of the insulating properties of the binder, and characterized in that it is a charge injection layer containing at least conductive particles and the fluororesin particles
I do .

[0019]

According to the present invention, when a magnetic brush is used as the charging member for injection charging, the magnetic flux density on the downstream side in the magnetic brush rotation direction of the nip where the photosensitive member and the magnetic brush are in contact with each other is determined. By making the magnetic flux density larger than the magnetic flux density at the position closest to the nip, the carrier in the nip can be drawn toward the nip exit, and the carrier in the nip can be moved smoothly.

[0020]

Embodiments of the present invention will be described below with reference to the drawings. <Embodiment 1> In the present embodiment, in the injection charging method using magnetic brush charging, the magnetic flux density on the downstream side of the nip in the magnetic brush rotation direction with respect to the nip where the photoconductor and the magnetic brush are in contact with each other, As means for increasing the magnetic flux density at the position closest to the nip, the magnetic pole of the magnet roll forming the magnetic brush is set downstream of the position closest to the nip in the direction of rotation of the magnetic brush.

More specifically, when the magnetic brush is composed of a rotating sleeve and a fixed magnet roll therein, the point of the maximum magnetic flux density on the sleeve of one magnetic pole of the magnet roll is determined. The point where the magnetic flux density on the sleeve adjacent to the point of the maximum magnetic flux density on the upstream side in the sleeve rotation direction becomes zero and the nip closest position to the photoconductor fall within the range of these two points, By setting the magnetic pole position of the magnet roll, the magnetic flux density on the downstream side of the rotation of the magnetic brush is increased.

First, referring to FIG. 1, a first embodiment of the present invention will be described.
A schematic configuration of the image forming apparatus according to the first embodiment will be described.

The image forming apparatus shown in FIG. 1 is a laser beam printer utilizing an electrophotographic process, and has a rotating drum type electrophotographic photosensitive member (hereinafter simply referred to as "photosensitive member") 1 as an image carrier. The photoreceptor 1 is an OPC photoreceptor having a diameter of 30 mm, and is 100 mm / sec in the direction of arrow R1.
At a process speed (peripheral speed).

A conductive magnetic brush 2 as a contact charging member is disposed above the photoreceptor 1. The conductive magnetic brush 2 is configured by attaching a carrier 23 to the non-magnetic rotatable electrode sleeve 21 by the magnetic force of a fixed magnet roll 22 so that the carrier 23 comes into contact with the surface of the photoconductor 1. . This sleeve 21 is
In order to increase the charging capability of the photosensitive member 1, the photosensitive drum 1 is driven and rotated at a peripheral speed of 100 mm / sec so that the moving direction on the nip surface is opposite to the moving direction on the surface of the photoreceptor 1 (in the direction of arrow R21). The magnetic brush 2 has a charging bias application power source S
1, a DC charging bias of -700 V is applied, whereby the surface (outer peripheral surface) of the photoreceptor 1 as a charged surface is uniformly charged to approximately -700 V.

The surface of the photosensitive member 1 is intensity-modulated in accordance with a time-series electric digital pixel signal of target image information outputted from a laser beam scanner (not shown) including a laser diode, a polygon mirror and the like. The scanning exposure L is performed by the laser beam thus formed, and an electrostatic latent image corresponding to the target image information is formed on the surface of the photoconductor 1. The electrostatic latent image is developed as a toner image by a reversal developing device 3 using a magnetic one-component insulating toner. Reference numeral 3a denotes a non-magnetic developing sleeve having a diameter of 16 mm and enclosing the magnet 3b. To apply a developing bias voltage to the developing sleeve 3a from the developing bias power source S2. The voltage was a DC voltage of -500 V and a frequency of 18
Jumping development is performed between the developing sleeve 3a and the photoreceptor 1 by using a superimposed rectangular AC voltage of 00Hz and a peak-to-peak voltage of 1600V.

On the other hand, a transfer material P as a recording material is supplied from a paper supply unit (not shown), and the medium 1 is transferred to the photosensitive member 1 with a predetermined pressing force as a contact transfer means. It is introduced at a predetermined timing into a pressure nip (transfer portion) T with the roller 4. A predetermined transfer bias voltage is applied to the transfer roller 4 from a transfer bias application power source S3. The transfer roller 4 has a roller resistance value of 5 × 10 ^{8 in} this embodiment.
The transfer was performed by applying a DC voltage of +2000 V using a Ω-type.

The transfer material P introduced into the transfer portion T is conveyed by nipping the transfer portion T, and the toner image formed and carried on the surface of the photoreceptor 1 on its surface side is sequentially reduced by electrostatic force and pressing force. Is transcribed.

The transfer material P to which the toner image has been transferred is separated from the surface of the photoreceptor 1 and introduced into a fixing device 5 such as a heat fixing system, where the toner image is fixed and the image is formed as an image product (print) outside the device. Is discharged to

After the transfer of the toner image onto the transfer material P, the surface of the photoreceptor 1 is cleaned by the cleaning device 6 after removing the adhered contaminants such as residual toner.
It is repeatedly provided for image formation.

In the image forming apparatus of this embodiment, the cartridge 20 includes four process devices, namely, the photosensitive member 1, the contact charging member 2, the developing device 3, and the cleaning device 6, and collectively operates the image forming apparatus main body. Although it is a cartridge type device which can be freely attached and detached, it is not limited to this.

Next, the photosensitive member 1 of this embodiment will be described.

The photoreceptor 1 is a negatively charged OPC photoreceptor, in which the following first to fifth functional layers are provided in order from the bottom on an aluminum drum base having a diameter of 30 mm.

The first layer is a subbing layer, which is provided to smooth defects of the aluminum substrate (aluminum drum) and to prevent the occurrence of moire due to the reflection of laser exposure, and has a thickness of about 20 μm. Layer.

The second layer is a positive charge injection preventing layer, which functions to prevent the positive charge injected from the aluminum substrate from canceling out the negative charge charged on the surface of the photoreceptor 1, and is provided with an amylan resin and methoxymethylated This is a medium resistance layer having a thickness of about 1 μm, the resistance of which is adjusted to about 10 ⁶ Ωcm by nylon.

The third layer is a charge generation layer, which is a layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin, and generates a positive / negative charge pair by receiving laser exposure.

The fourth layer is a charge transporting layer in which hydrazone is dispersed in a polycarbonate resin, and is a P-type conductor. Therefore, the negative charges charged on the surface of the photoreceptor 1 cannot move through this layer, and only the positive charges generated in the charge generation layer can be transported to the surface of the photoreceptor 1.

The fifth layer is a charge injection layer, which is a coating layer of a material in which ultrafine particles of SnO ₂ are dispersed in a photocurable acrylic resin and has a thickness of about 3 μm.

As a result, the resistance on the surface of the photoreceptor was reduced to 1 × 10 ¹¹ Ωcm, compared with 1 × 10 ¹⁵ Ωcm for the charge transport layer as the fourth layer.

Next, the photosensitive member 1 and the contact charging member 2
The principle of charging by using is described.

The present invention relates to a medium-resistance contact charging member for injecting electric charge into the surface of a photoreceptor having a medium-resistance surface resistance. In this embodiment, the electric charge is applied to the trap potential of the photoreceptor surface material. Instead of injecting, the charge is performed by charging the conductive particles of the charge injection layer.

Specifically, as shown in FIG. 2, the charge transport layer 11 is made of a dielectric material, and the aluminum base 14 and the charge injection layer 1
This is based on the theory that the contact charging member 2 charges a minute capacitor using the conductive particles 12 in the electrode 3 as both electrode plates. At this time, the conductive particles 12 are electrically independent of each other and form a kind of minute float electrode.
For this reason, macroscopically, the surface of the photoconductor 1 appears to be charged and charged to a uniform potential, but in reality, there are situations in which a myriad of minutely charged SnO ₂ covers the surface of the photoconductor 1. It has become. For this reason, even if image exposure is performed by a laser, each SnO ₂ particle is electrically independent, so that an electrostatic latent image can be held.

Next, FIG. 3 shows a magnetic pole brush charger used in this embodiment.

The magnetic brush 2 used in this embodiment
Is a non-magnetic rotatable sleeve 21 of φ16 mm,
Fixed magnet roll 22 with a longitudinal length of 230 mm
The carrier 23 adheres by the magnetic force of the above.

The carrier 23 desirably has a resistance of about 10 ⁵ to 10 ⁸ Ω for use as a charging member.
If the resistance of the carrier 23 is too low, if a pinhole is formed on the photoreceptor 1, the potential of the sleeve 21 drops, resulting in poor horizontal line charging. Conversely, if the resistance of the carrier 23 is too high, no charge is injected into the carrier 23 this time, resulting in poor charging. Further, the particle size of the carrier 23 is desirably small to some extent in order to keep the contact with the photoreceptor 1 densely.
m or less is good. However, if the particle size is too small, it is difficult to hold the particles with a magnetic force. Therefore, in order to reduce the particle size, it is necessary to use a material having a large maximum magnetization. Examples of the carriers 23 that can be actually used include a carrier obtained by kneading a polystyrene resin, magnetite, and conductive carbon for adjusting a resistance value to form particles, a magnetite carrier, a ferrite carrier, and the like. Here, from the viewpoint of the resistance value and the maximum magnetization, a ferrite carrier having an average particle diameter of 30 μm, a maximum magnetization of 60 Am ² / kg, and a density of 2.2 g / cm ³ is used. When the gap between the sleeve 21 and the photoconductor 1 is 500 μm and the amount of the carrier on the sleeve 21 is 10 g, the carrier 23 forms a nip having a width of about 2 mm between the sleeve 21 and the photoconductor 1,
Further, since the magnetic brush 2 is rotated with a peripheral speed difference with respect to the photoreceptor 1, a carrier reservoir K1 is formed about 3 mm wide upstream of the sleeve rotation direction, and the entire charging nip is about 5 mm wide.
mm. Here, the carrier resistance at this nip width was 5 × 10 ⁶ Ω when DC 100 V was applied.

Next, the configuration of the magnetic pole position of the magnet roll 22, which is a feature of this embodiment, will be described.

As shown in FIG. 4, the arrangement of the magnetic poles of the magnet roll 22 used in the present embodiment is of four equal poles, and the magnetic flux density on the sleeve 21 at the peak position is 9.5 ×. 10 ⁻² T. Here, in this embodiment, four equal poles are used.
The number of magnetic poles may be any number, and need not be equal, and does not limit the content of the present invention.

Since the magnetic poles of the magnet roll 22 in this embodiment are four equal poles, the point between the point A of the maximum magnetic flux density of the magnetic pole of interest and the point B of the maximum magnetic flux density of the adjacent magnetic pole on the upstream side in the sleeve rotation direction. (Between AB) is a magnet roll 22
90 ° apart from each other as an angle (angle AOB) with respect to the center O of In addition, since the magnetic flux density is zero, the point C adjacent to the point A on the upstream side in the sleeve rotation direction is equal to A−
It is just in the middle between the B magnetic poles. Therefore, the angle range (between A and C, angle AOC) of the point where the magnetic flux density becomes zero from the point of the maximum magnetic flux density of the magnetic pole is 45 ° in this embodiment. Here, when the nip closest position N is brought into this angular range, the magnetic flux density on the downstream side becomes larger than the upstream side on the nip, and the magnetic force in the nip exit direction is applied to the carrier 23 in the nip. Is the working angle range.

Therefore, the nip closest position N between the sleeve 21 and the photosensitive member 1 is set within this angle range of 45 °. In this embodiment, as shown in FIG. 3, the point A of the maximum magnetic flux density of the noted magnetic pole of the magnet roll 22 is set at 15 ° downstream of the nip closest position N in the electrode sleeve rotation direction, and the nip closest position is set. The position N was set to fall within this angle range.

Image formation was performed between the apparatus set as described above and the conventional apparatus, and compared.

In the conventional device, the magnetic pole of the magnet roll is located at the position closest to the nip between the electrode sleeve and the photosensitive member, so that a magnetic force acts to restrain the carrier in the nip. Stagnation occurred.
As a result, the contact nip becomes non-uniform, and the chance of contact between the carrier and the photoreceptor is reduced, so that the injection charging ability is reduced and poor charging occurs. In addition, the carrier itself was charged up, hindering the injection of charges, which also resulted in poor charging.

Next, an angle range of 45 ° according to the present invention.
The image formation was performed when the magnetic pole was set beyond the limit. Actually, the magnetic pole (point A) of the magnet roll 22 is set downstream of the 60 ° nip closest position N in the sleeve rotation direction. As a result, the magnetic pole on the upstream side (point B) approached near the entrance of the nip, and the magnetic flux density on the entrance side became larger than the magnetic flux density on the exit side of the nip. The force of pulling back to the inlet side acted, which caused the carrier 23 to stay in the nip, which also resulted in poor charging.

On the other hand, in the configuration of the present embodiment in which the magnetic pole position of the magnet roll 22 is arranged at 15 ° downstream of the nip closest position N in the electrode sleeve rotation direction, the carrier 23 in the nip is moved in the nip exit direction. Since the attracting magnetic force acts, the carrier 23 can move smoothly in the nip, the number of times of contact between the photoconductor 1 and the carrier 23 that has not been charged up increases, and a good charging without defective charging occurs. Images can now be obtained. In addition, since the movement of the carrier 23 in the charging nip is improved, the nip uniformity can be improved, and the charging uniformity in the sleeve longitudinal direction is improved.

As described above, in the present embodiment, in the injection charging method using magnetic brush charging, the magnetic brush 2 of the charging member is constituted by the rotating electrode sleeve 21 and the fixed magnet roll 22 therein. The electrode sleeve 2 of one magnetic pole with the magnet roll 22
1, a point at which the magnetic flux density on the electrode sleeve 21 adjacent to the point of the maximum magnetic flux density on the upstream side in the sleeve rotation direction becomes zero, By setting the magnetic pole position of the magnet roll 22 so that the position N closest to the nip is located, the carrier 23 in the nip can move smoothly, and the photosensitive member 1 is charged up. The number of times of contact with the carrier 23 is increased, and it is possible to obtain a good image free from poor charging. Further, since the movement of the carrier 23 in the charging nip is improved, the nip uniformity is improved, and the uniformity of the charging in the longitudinal direction of the sleeve can be improved. <Embodiment 2> In this embodiment, in the injection charging method using magnetic brush charging, the magnetic flux density on the downstream side in the magnetic brush rotation direction of the nip where the photoreceptor and the magnetic brush are in contact is determined by the magnetic flux at the position closest to the nip. As a means for increasing the density, a member made of a magnetic material is provided outside the sleeve on the downstream side in the magnetic brush rotation direction from the nearest position of the nip.

FIG. 5 shows the configuration of the magnetic brush charger used in this embodiment. The image forming apparatus used in this embodiment is the same as that of the first embodiment except for the configuration of the magnetic brush charger. Therefore, the image forming process and the principle of charging are the same as in the first embodiment.

The magnetic brush 2 used in this embodiment
Is a nonmagnetic φ12 mm rotatable sleeve 21,
The carrier 23 is attached by the magnetic force of the fixed magnet roll 22.

The magnetic poles of the magnet roll 22 have four equal poles, one of which faces the nip closest position N.

At a position 1.5 mm away from the surface of the photosensitive member 1 and the surface of the sleeve on the downstream side of the nip between the magnetic brush 2 and the photosensitive member 1 in the direction of rotation of the magnetic brush, a φ3 mm characteristic of the present embodiment is provided. Magnet 24 extending in the longitudinal direction
Is installed. This magnet 24 allows the sleeve 2
The magnetic flux of the inside magnet roll 22 is concentrated, and the magnetic flux density on the sleeve surface at this position is 9.0 × 10 ⁻² T,
It is 1.5 times as large as the case without the magnet 24. As a result, the magnetic flux density near the nip exit becomes larger than the magnetic flux density at the nip nearest position N, and a magnetic force in the nip exit direction acts on the carrier 23 in the nip. Here, in this embodiment, a round magnet is used, but the shape may be any shape such as a square or a triangle. The same effect can be obtained by using iron, nickel or the like as long as the material is a magnetic material in which magnetic flux is concentrated.

An image was formed between the thus set image and the conventional image, and compared.

In the conventional device, there is no difference between the magnetic flux density near the nip exit and the closest position of the nip between the sleeve and the photosensitive member. Therefore, the fluidity of the carrier in the nip is poor and the contact nip becomes uneven. Poor contact between the carrier and the photoreceptor reduced the injection charging ability, resulting in poor charging. Further, as the diameter of the sleeve has been reduced, the internal magnet roll has become smaller, the magnetic binding force for holding the carrier has become weaker, and the carrier has moved away from the magnetic brush.

On the other hand, in the configuration of the present embodiment in which the magnet 22 is provided outside the sleeve 21 and the magnetic flux density at the nip exit is higher than the magnetic flux density at the nip closest position N as in the present embodiment, Since a magnetic force acts to draw the carrier 23 in the nip toward the nip exit, the carrier 23 in the nip can be transported smoothly, and the number of times of contact between the photoconductor 1 and the carrier 23 that has not been charged up is reduced. As a result, a good image free from poor charging can be obtained. In addition, since the movement of the carrier 23 in the charging nip was improved, the nip uniformity was improved, and the charging uniformity in the sleeve longitudinal direction was improved. Further, the magnet 24 and the electrode sleeve 21 at the nip exit are provided.
Since the carrier 23 is held by the magnetic field at K2 between the magnetic brush 2 and the magnetic sleeve 2, the carrier 23 does not have to be separated from the magnetic brush 2 even if the electrode sleeve 21 is reduced in diameter and the magnetic binding force of the internal magnet roll 22 is reduced. is there.

As described above, in the present embodiment, in the injection charging method using the magnetic brush charging, the magnetic flux density on the downstream side in the magnetic brush rotation direction of the nip where the photosensitive member 1 and the magnetic brush 2 are in contact with each other is increased. As a means, by installing a member made of a magnetic material outside the sleeve on the downstream side of the nip nearest position N in the magnetic brush rotation direction, the carrier 23 in the nip can move smoothly, and The number of times of contact between the body 1 and the carrier 23 that has not been charged up has increased, and it has become possible to obtain a good image free from poor charging. In addition, since the movement of the carrier 23 in the charging nip is improved, the nip uniformity is improved, and the uniformity of charging in the longitudinal direction of the sleeve can be improved.

Further, in the present embodiment, the sleeve 21 is made smaller by reducing the diameter of the sleeve 21 with the downsizing of the apparatus.
The reduction in the magnetic binding force of the internal magnet 22 can be compensated for by placing the magnet 24 outside the sleeve 21.

[0063]

As described above, according to the present invention,
When a magnetic brush is used as the charging member for injection charging, the magnetic flux density at the downstream side of the nip in the magnetic brush rotation direction with respect to the nip where the photoconductor and the magnetic brush are in contact By setting the density higher than the density, the chance of contact between the carrier and the surface of the photoreceptor can be increased, and the injection charging ability can be improved.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment.

FIG. 2 is a schematic diagram showing the principle of injection charging of the magnetic brush charger in the first embodiment.

FIG. 3 is a schematic view of a magnetic brush charger according to the first embodiment.

FIG. 4 is a diagram showing a magnetic pole arrangement of the magnet roll according to the first embodiment.

FIG. 5 is a schematic view of a magnetic brush charger according to a second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoreceptor (charged body) 2 Magnetic brush (magnetic brush charger) 3 Developing device 4 Transfer roller 5 Fixing device 6 Cleaning device 11 Charge transport layer 12 Conductive particles 13 Charge injection layer 14 Aluminum base 21 Sleeve 22 Magnet roll 23 Carrier 24 Magnet N Nip closest position P Transfer material

────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-6-161208 (JP, A) JP-A-4-338782 (JP, A) JP-A-5-94858 (JP, U) (58) Survey Field (Int.Cl. ⁷ , DB name) G03G 15/02 G03G 15/09

Claims (3)

(57) [Claims] (1)Between the photoreceptor and the surface of the photoreceptor
A rotatable sleeve forming a nip and within the sleeve
A magnetic brush having a fixed magnet roll is provided.
And charging the surface of the photoreceptor via the magnetic brush.
In an image forming apparatus, Downstream of the nip in the direction of rotation of the magnetic brush
At the closest position of the nip
Equipped with magnetic flux density setting means to set higher than magnetic flux density
e,  The magnetic flux densitySetting meansIs one of the magnet rolls
Point of the maximum magnetic flux density on the sleeve corresponding to the magnetic pole of
And the point of the maximum magnetic flux density at the upstream side in the sleeve rotation direction.
At which the magnetic flux density on the adjacent sleeve becomes zero,
Within the range of these two points, the nip closest to the photoconductor
Adjust the magnetic pole position of the magnet roll so that the
Setting, characterized byImage forming device. (2)Between the photoreceptor and the surface of the photoreceptor
A magnetic brush forming a nip;
In the image forming apparatus to charge the surface of the photoreceptor, Downstream of the nip in the direction of rotation of the magnetic brush
At the closest position of the nip
Equipped with magnetic flux density setting means for setting larger than magnetic flux density
And  The magnetic flux density setting means is disposed outside the sleeve.
A magnetic member, near the nip by the magnetic member.
Setting the magnetic flux density ofImage forming device. Wherein the surface layer of the photosensitive member, in the insulating property of the binder, a charge injection layer containing at least conductive particles and fluororesin particles, the image according to claim 1 or 2, characterized in that Forming equipment.