JPH086353A - Charging device - Google Patents

Abstract

PURPOSE:To maintain the charging property over a long period, prevent the sticking of a carrier, and prevent the concentration of the charging current to pinholes of a photoreceptor by mixing magnetic grains having multiple characteristics to form a magnetic brush. CONSTITUTION:A conducting magnetic brush is constituted of a nonmagnetic conducting sleeve, a magnet roll 22 incorporated in it, and magnetic conducting grains 23 on the sleeve. The magnetic grains 23 of two kinds A, B having different electric resistance values are mixed for use. The grains A are ferrite grains having the average grain size of 15mum and the volume resistance value of 1X10<7>OMEGAcm, and the magnetic characteristic is the saturation magnetization of 58A.m<2>/kg. The resistance value of the grains B is reduced to 1X10<3>OMEGAcm via the hydrogen reduction on the ferrite surface. When charging is applied with carriers having different resistance value distribution, the macroscopic resistance value is determined by the carrier having a high resistance value, thus the charging current does not concentrically flow into pinholes on a photoreceptor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic apparatus such as a copying machine and a printer, and a process cartridge which can be attached to and detached from the apparatus, and more particularly to an apparatus which charges a photosensitive member with a charging member.

[0002]

BACKGROUND ART Conventionally, a corona charger has been used as a charging device for electrophotography. In recent years, a contact charging device has been put into practical use instead of this. This is low ozone,
Aiming at low power consumption, a roller charging method using a conductive roller as a charging member is particularly preferably used from the viewpoint of stability of charging.

In roller charging, a conductive elastic roller is brought into pressure contact with a member to be charged, and a voltage is applied to the member to charge the member to be charged.

Specifically, since the charging is performed by discharging from the charging member to the body to be charged, the charging is started by applying a voltage higher than a certain threshold voltage. For example, when the charging roller is pressed against the OPC photoconductor with a thickness of 25 μm, the surface potential of the photoconductor begins to rise when a voltage of about 640 V or higher is applied. The surface potential of the photoconductor increases linearly with a slope of 1 with respect to the voltage. Hereinafter, this threshold voltage is defined as the charging start voltage Vth.

That is, in order to obtain the photosensitive member surface potential Vd required for electrophotography, the charging roller requires a DC voltage of Vd + Vth, which is higher than the required DC voltage. The method of applying only the DC voltage to the contact charging member to perform charging by discharging is called DC charging.

However, in DC charging, the resistance value of the contact charging member fluctuates due to environmental fluctuations and the Vth fluctuates when the film thickness changes due to abrasion of the photoconductor, so that the potential of the photoconductor is desired. It was difficult to make it a value.

Therefore, in order to further homogenize the charging, as described in JP-A-63-149669, a desired V
A with a peak-to-peak voltage of 2 x Vth or more in the DC voltage equivalent to d
An AC charging method is used in which a voltage with a superimposed C component is applied to the contact charging member. This is for the purpose of the leveling effect of the potential by AC, and the potential of the charged body converges on Vd which is the center of the peak of the AC voltage, and is not affected by disturbance such as environment.

However, even in such a contact charging device, since the essential charging mechanism uses the discharging phenomenon from the charging member to the photosensitive member, the voltage required for charging as described above. Is required to have a value equal to or higher than the surface potential of the photoconductor, and a small amount of ozone is generated. In addition, when AC charging is performed to make the charging uniform, more ozone is generated, vibration of the charging member and the photoconductor due to the electric field of AC voltage, noise (hereinafter referred to as AC charging sound), and discharge. Deterioration of the surface of the photoconductor due to the phenomenon becomes remarkable, which is a new problem.

Therefore, there has been a demand for charging by directly injecting charges into the photoconductor.

In order to directly inject charges to the surface of the photoconductor, a charging member having a low resistance value is used, and a long charging time is used to charge the charges to the trap level of the charges existing on the surface of the photoconductor. There is.

[0011] However, such resistance of the charging member is assumed that following a low 1 × 10 ³ Ω at charging method, more fully charging the charged charging unit time is long will electrophotographic to perform It had a problem that it was not practical to use. In addition, when a charging member having a low resistance value is actually used, an excessive leak current flows from the contact charging member to scratches, pinholes, etc. generated on the surface of the photoconductor, charging failure in the periphery and expansion of the pinhole, Electric current breakdown of the charging member (hereinafter referred to as pinhole leak) occurs.

In order to prevent the leakage, the resistance value of the charging member needs to be about 1 × 10 ⁴ Ω or more, but the charging member having this resistance value has the property of injecting the charge into the photosensitive member as described above. Will decrease, resulting in a contradiction that charging is not performed.

Therefore, the applicant of the present invention has found a method in which a charge injection layer is provided on the surface of the photoreceptor as disclosed in Japanese Patent Application No. 04-158128, and charges are injected by a contact charging member to the charge injection layer. In this method, by providing a charge injection layer, even a charging member having a resistance value of 1 × 10 ⁴ Ω or more can be sufficiently charged in a short charging time, which causes discharge like other charging methods described above. It is possible to fundamentally solve the problem and the problem that the time required for charging is long.

Specifically, it is effective to use, as the charging member, a large contact nip with the photosensitive member and a magnetic brush roller that can uniformly contact the surface of the photosensitive member and does not remain microscopically charged. , 1 x 10 ⁴ ~ 1 with magnet roll
It is desirable to adopt a magnetic brush-shaped charging member structure that magnetically restrains magnetic particles of medium resistance (hereinafter also referred to as carriers) such as ferrite having a resistance value of × 10 ⁸ Ωcm.

Further, as the charge injection layer on the photosensitive member, a material in which conductive fine particles are dispersed in an insulating and translucent binder is preferably used. By contacting this charge injection layer with a charged magnetic brush to which a voltage is applied, the conductive particles exist as if they were infinite floating electrodes on the conductive base of the photoconductor, and these floating electrodes form these. It can be expected that the capacitor will be charged.

Therefore, the voltage applied to the contact charging member and the surface potential of the photosensitive member converge to the same value, and the low voltage charging method can be realized.

[0017]

[Problems to be Solved by the Invention]

(1) However, in the injection charging method, when a magnetic brush is used as the charging member, it is necessary for each magnetic particle forming the magnetic brush to come into contact with each other to form a conductive path, which flows through this conductive path. Although the surface of the photoconductor is charged and charged by the electric charge, when dust or the like is mixed in the magnetic brush, the toner or the like adhering to the surface of the photoconductor due to image formation reaches the magnetic brush without being cleaned. In this case, the conductive path is blocked by these impure insulators, which makes charging difficult.

In practice, even in an image forming apparatus that was initially charged well, as the durable paper feed is performed, toner and the like that could not be completely cleaned during the previous image formation are mixed into the magnetic brush, thereby charging the toner. Deteriorates. Specifically, there is a problem in that the surface of the photosensitive member which has been lowered by exposure cannot be recharged, and in the reversal development type apparatus, fog is caused due to charging failure.

(2) Further, in the direct charge injection charging method, unlike the charging method using discharge, the photosensitive member is not charged unless the photosensitive member and the charging member are in direct contact with each other. It is preferable to be small in order to gain an opportunity to contact with the body. However, if only small particles are used to secure the charging property, the magnetic force of each particle is reduced, and the magnetic restraint of the magnet roll is escaped, causing the conductive magnetic particles (carriers) to be released. The removal of the carrier reduces the nip width of the magnetic brush, which lowers the charging property and causes the various conductive particles that have separated into the developing device to cause various adverse effects such as leakage and reduction of developer tribo. I had a problem.

(3) Further, the occurrence of carrier adhesion is greatly influenced by the surface property of the photoconductor, and when a material having a large surface energy is used as the surface material of the photoconductor, the intermolecular between the carrier and the photoconductor surface is used. There has been a problem that the force is increased and carrier adhesion is accelerated.

(4) Further, in order to efficiently inject the charge into the photosensitive member, it is preferable that the contact charging member has a low resistance value, but a charging member having a volume resistance value of 1 × 10 ⁴ Ωcm or less is used. When a charge such as a pinhole is generated on the photoconductor, the charging current concentrates there and the voltage applied to the charging member drops, causing a linear charging failure. There is a problem that an image is generated and the charging member and the photoconductor are destroyed.

[0022]

According to the present invention, charging is performed by directly injecting an electric charge by bringing a medium-resistance magnetic brush to which a voltage is applied into contact with a photoconductor having a charge injection layer on its surface. The charging device is characterized in that the magnetic brush is formed by mixing magnetic particles having a plurality of characteristics.

Specifically, by mixing particles having different electric resistance values to form a charging magnetic brush, it is possible to prevent pinhole leakage due to the concentration of overcurrent generated on the photoconductor on the photosensitive member, and at the same time, it is preferable. It is possible to maintain good chargeability. As for the electric resistance value required for the carrier, both effects are effective by mixing low resistance particles with good charge injection property to the photoconductor and medium resistance particles effective for preventing pinhole leakage. You will be able to make full use of it.

Regarding the particle size, a conductive path between the particles can be formed when particles of large particle size that have a large magnetic binding force and are advantageous for carrier adhesion and insulating dust such as toner are mixed, so that a charging ghost can be formed. By mixing particles having a small resistance and a small particle size, which suppresses the generation and is excellent in charge injection property, it has become possible to solve the problems peculiar to direct charge injection charging, which have been a conventional problem. Here, the particles having a large particle size magnetically restrain the small particles, and have a great effect on preventing the carrier from separating.

Regarding the magnetic characteristics, when carriers having different electric characteristics and particle diameters are mixed, the carrier having a small particle diameter tends to selectively cause carrier adhesion due to intermolecular force acting between the carrier and the photoconductor. However, it is possible to solve the problem of selective carrier adhesion by using a material having excellent magnetic properties, specifically, one having a large saturation magnetization and a large residual magnetization as such particles.

By mixing and using a plurality of particles having different characteristics as described above, the charging property is maintained by the durability, the carrier is prevented from adhering, the charging current is prevented from concentrating on the pinholes of the photoconductor, It has become possible to simultaneously satisfy the charge injection property and the like.

In the direct charge injection charging method of this method, the charge injection property to the photoconductor can be improved by providing the charge injection layer on the photoconductor surface. By imparting lubricity to and reducing the surface energy, the intermolecular force acting between the carrier and the photoconductor can be weakened, and the carrier adhesion can be greatly improved. In particular, when used as a charging member by mixing particles having a plurality of particle sizes as in the present invention,
Since particles having a smaller particle size have a larger intermolecular force with the photoconductor, lowering the surface energy of the photoconductor surface is particularly effective in preventing carrier adhesion.

As means for imparting lubricity to the surface layer of the photoreceptor, it is effective to disperse lubricant particles of fluorine type, polyolefin type, silicone type or the like in the charge injection layer.

[0029]

EXAMPLES First, the photoconductor used in this example will be described.

The photoconductor is a negatively charged OPC photoconductor, φ30 m
Five functional layers are provided on an aluminum drum of m.

The first layer is the undercoat layer and the second layer is the second layer in this order from the Al base layer side.
The layer is a positive charge injection preventing layer, the third layer is a charge generating layer, and the fourth layer is a charge transporting layer. In this embodiment, a function-separated type OPC photoreceptor which is usually used is used. Is not limited to the structure of the present invention, single-layer OPC, Z
It is also possible to use a photoreceptor such as nO or selenium amorphous silicon.

The fifth layer is a charge injection layer, which is a feature of the present invention, and is a photo-curable acrylic resin in which SnO ₂ ultrafine particles are dispersed. Specifically, SnO ₂ particles having a particle diameter of about 0.03 μm, which is doped with antimony and has a reduced resistance, are dispersed at a weight ratio of 5: 2 with respect to the resin.

The volume resistance value of the charge injection layer changes depending on the dispersion amount of SnO ₂ which is actually conductive, and the resistance value of the charge injection layer is 1 × 10 ⁸ in order to satisfy the condition that image deletion does not occur. Ωcm
It must be above.

The resistance value of the charge injection layer is obtained by coating the charge injection layer on an insulating sheet and measuring the surface resistance with a high resistance meter 4329A manufactured by HP at an applied voltage of 100V.

The coating solution thus prepared was applied to a thickness of about 3 μm by a suitable dipping coating method to form a charge injection layer.

In this embodiment, the charge injection layer has a volume resistance value of 1
A film having a density of × 10 ¹² Ωcm was used.

Next, the contact charging member will be described.

The conductive magnetic brush is a non-magnetic conductive sleeve,
The magnet roll contained in this, composed of magnetic conductive particles on the sleeve, the magnet roll is fixed,
The sleeve surface is rotated so as to move in the direction opposite to the peripheral speed direction of the photosensitive drum. The magnetic flux density on the sleeve surface at the closest position between the photoconductor and the charging sleeve is 950 gauss, and the spikes of the magnetic brush are regulated by the magnetic blade facing the sleeve to form spikes of about 1 mm. The peripheral speed ratio of the surface of the sleeve to the photosensitive member was such that the sleeve was driven and rotated in the direction opposite to the moving direction of the photosensitive member at a speed of 10% of the speed of the photosensitive member.

In this example, two kinds of magnetic particles A and B having different electric resistance values were mixed and used.

The first particle A is a ferrite particle having an average particle size of 15 μm and a volume resistance value of 1 × 10 ⁷ Ωcm, and its magnetic property is that its saturation magnetization is 58 (A · m ² / kg).

The resistance value of the magnetic particles is measured by measuring a bottom area of 228 mm ²
Fill 2g of carrier into the cylindrical container and pressurize.
Apply a voltage of 100V, calculate from the current flowing through it,
It is defined as normalized.

A second particle B is mixed with this. Particle B
Is mixed with a ferrite whose resistance value has been reduced by hydrogen reduction on the surface. The ferrite particles can reduce the resistance value to some extent by reducing the surface thereof with hydrogen and changing the degree thereof, and in this example, specifically, the resistance value is lowered to 1 × 10 ³ Ωcm. I used the one.

The mixing ratio of the particles A and B was set to 2: 1 in this embodiment, but since this is determined by ensuring the charging property and the pinhole performance of the photoconductor, the particle size of the particles used is The optimum value changes depending on the resistance value and the like.

Here, the effects of the present invention will be described based on the present embodiment.

The anti-pinhole leak performance is specifically shown in FIG.
As shown. When the charging member r having a low volume resistance value is used, the pinhole generated on the photoconductor is shown in FIG.
As described above, the charging current flows intensively through the surface of the charging member. Therefore, not only the potential of the pinhole part but also A
The potential at the point also drops to about 0 V, which is the potential of the photoconductor substrate, and causes a charging failure at the point A. This is because the carrier resistance existing between the pinhole and point A is shown in Fig. 2 (b).
The reason is that there is only a value of 2r. In order to prevent this, it is necessary to set the resistance value of the charging member to 1 × 10 ⁴ Ω or more. On the other hand, in the direct charge injection charging, the charge is directly injected from the carrier surface to the charge injection layer on the surface of the photoconductor, so that the injection property is improved by using a charging member having a low resistance value. It is considered that this is because the lower the resistance value of the carrier, the smaller the injection time constant, and the lower the contact resistance value at the interface between the carrier and the photoconductor.

Therefore, it is difficult to satisfy both the anti-pinhole leak performance and the charge injection property when the carrier is charged by a carrier having a single resistance value distribution as in the prior art.

However, if charging is performed with carriers having different resistance distributions as in the present embodiment, a macro resistance value is generated by a carrier having a high resistance value because a carrier having a low resistance value and a carrier having a medium resistance value are mixed. Since it is determined, the charging current does not flow intensively into the pinhole on the photoconductor.

Specifically, as shown in FIG. 2 (a), the resistance value of the carrier between the pinhole portion and the point A is R + r to R and a medium resistance value which does not drop the potential at the point A. This is because

Further, since the injection time constant is small in the portion where the carrier having a low resistance value and the photosensitive member are in contact with each other,
Since the electric resistance at the interface is small, electric charges are injected onto the photoconductor, and good charging can be realized.

Conventionally, the macro resistance value of the charging member is
It is known that sufficient charging can be performed practically in the range of 1 × 10 ⁵ to 1 × 10 ⁸ Ωcm, but in order to obtain good injection properties, the volume resistance value of the carrier alone is 1 × 10 ⁶ Ωcm or less is desirable. However, the resistance value of the carrier alone is 1 × 10 ⁵
If it is less than Ωcm, a leak image will appear in the drum pinhole, so a good range of use is 1 × 10 ⁵ ~
The range was as narrow as 1 × 10 ⁶ Ωcm.

In the structure of the present embodiment, by mixing carriers having a plurality of specific resistances, a carrier of 1 × 10 ⁵ Ωcm or less which cannot be used due to pinhole leak in the conventional carrier of one kind of specific resistance. Can be used, and the range of carrier resistance and material selection has expanded dramatically.

In the present embodiment, carriers having two kinds of resistance values were mixed, but this does not limit the concept of the present invention. Mixing particles having various resistance value distributions and broad resistance The same effect can be obtained by using a carrier having a value distribution.

When two kinds of carriers having different resistance distributions are mixed as in this embodiment, it is preferable that the mixing ratio of low resistance particles is large in order to improve the charging property. When the mixing ratio of low-value carriers is increased, from a certain mixing ratio, the conductive path (chain) is formed only by the low-resistance carriers, and at this time, pinhole leakage on the photoconductor occurs. Since the effect of suppressing the current concentration of is lost, it is necessary to use particles with low resistance at a mixing ratio of less than this.

Next, since it has been found that there is a preferable relationship between the resistance of the low resistance particles and the mixing ratio, that will be described. Fig. 3 shows the result of confirming an image by mixing ferrite particles having a volume resistance value of 1 × 10 ⁷ Ωcm with low resistance magnetic particles having different volume resistances. From this result, it was found that when the resistance value of the mixed magnetic particles is 1 × 10 ⁵ Ωcm or more, the charging ability is deteriorated after long-term use. It was also found that when the mixing ratio of the low-resistance particles is 60% or more, the low-resistance particles alone form a conductive path, so that leakage occurs in pinholes. It has been found that no pinhole leakage occurs at 60% or less, preferably 50% or less.
Leakage in pinholes was confirmed by mixing low-resistance particles with ferrite particles of other resistance. No leak image was generated if the mixing ratio was 50% or less. Here, the mixing ratio is the weight ratio of the low resistance particles to the total weight of the magnetic particles.

In this embodiment, as particles having different resistance values,
The same ferrite particles with different surface treatments were used, but in addition to these particles, resin and magnetic powder such as magnetite were kneaded to form particles, or conductive particles for adjusting the resistance value. Mixtures of carbon, etc., sintered magnetite, ferrite, or those whose resistance value has been adjusted by reduction treatment, or those whose magnetic particles have been plated to have an appropriate resistance value, etc. Similar effects were obtained when used in combination.

An example in which an image is formed by using the charger having the above structure will be described. The electrophotographic printer shown in FIG. 1 used has a process speed of 100 mm / sec and uses the negatively charged OPC photoreceptor 1 having a diameter of 30 mm and having the charge injection layer described above.

First, charging is performed by bringing the charging brush 2 to which a DC voltage of -700V is applied into contact with and rotating the photosensitive member. As described above, charging is performed by injecting charges from the charging brush into SnO ₂ on the surface of the photoconductor, so the charging brush must contact the entire surface of the photoconductor. Therefore, the contact nip width of the charging brush is 5m.
It was m.

In such a state, the voltage applied to the charging brush and the surface potential of the photosensitive drum change substantially linearly, the existence of the discharge threshold as in the case of using the conventional charging roller is not recognized, and the charge injection is not performed. Was done. The photosensitive drum charged to -700V is then subjected to image exposure at the exposure section.

A laser beam 3 from a laser diode whose intensity is modulated according to an image signal is scanned by a polygon mirror to serve as exposure means, and an electrostatic latent image is formed on the photosensitive drum.

Next, reversal development is performed using a magnetic one-component insulating toner. A non-magnetic sleeve 4 having a diameter of 16 mm and containing a magnet is coated with the above-mentioned negative toner, and while the distance from the surface of the photosensitive member is fixed to 300 μm, the sleeve is rotated at a constant speed with the photosensitive drum to apply a voltage to the sleeve.

The voltage is a DC voltage of -500V and a frequency of 1800H.
z, jumping development is performed between the sleeve and the photosensitive drum by using a rectangular AC voltage with a peak-to-peak voltage of 1600V superimposed.

The image visualized with the toner in this manner is then transferred to the transfer material 6. A transfer roller 5 having a medium resistance is used in the transfer section. In this embodiment, the roller resistance value is 5 × 10
^The transfer was performed by applying a DC voltage of +2000 V using an ⁸ Ω one.

The print image on which the toner image has been transferred onto the transfer material is then fixed by the heat fixing roller 8 and discharged outside the apparatus. Further, the transfer residual toner is scraped off from the photosensitive drum by the cleaning blade 7 to prepare for the next image formation.

Images were output by the printer having the above-mentioned configuration.

As a comparative example of this embodiment, an experiment was conducted using a carrier having a single resistance value distribution, which has a conventional structure.

[Comparative Example 1] The carrier used had an average particle size of 15
The ferrite carrier has a volume resistance of 1 × 10 ⁹ Ωcm and an average particle size of 15 μm and a volume resistance of 1 × 10 ³ Ωcm.

When an image was output with a carrier having a volume resistance value of 1 × 10 ⁹ Ωcm, the resistance value was too high and sufficient charge injection from the carrier to the surface of the photosensitive member was not carried out, resulting in poor charging from the initial stage.

[Comparative Example 2] When a carrier having a volume resistance value of 1 × 10 ³ Ωcm was used, good charging was performed without causing charging failure, but the charging current was concentrated on the defects existing on the photoconductor. However, a pinhole was formed and a horizontal black line was generated due to poor charging. When the image is further output, carrier adhesion occurs due to an electric force centering on the pinhole, and finally the pinhole expands, causing damage to the photoconductor and the charging member.

These comparative examples are remarkable phenomena which occur when the resistance value of the charging member is high or low.
With the carrier of the constitution of this embodiment, 1 × 10 ⁹ Ωcm and 1 × 10 ³ Ωcm
By mixing 2: 1 with a resistance value of cm, the volume resistance value of the mixed carrier was 5 × 10 ⁸ Ωcm.

This corresponds to the case where a high resistance value and a low resistance value are connected in series, and since the macro resistance value is dominated by the higher resistance value, pinhole leakage does not occur. Further, when a carrier having a low resistance value comes into contact with the surface of the photoconductor, the time constant of charge injection is sufficiently small because the volume resistance value of the carrier itself is low, and furthermore, the contact resistance at the interface is also small, so that the charge is strongly applied to the photoconductor. Can be injected.

Therefore, in the structure of this embodiment, unlike the case where the charging member is formed by the specific resistance of one kind of material, by mixing a plurality of kinds of materials, the pinhole leak prevention and the chargeability are compatible at a good level. I can do it now.

By using the charging member of this embodiment,
When the resistance value of the charge injection layer of the photoconductor was in the range of 1 × 10 ⁸ to 1 × 10 ¹⁵ Ωcm, no image deletion occurred and good chargeability could be obtained.

Further, the type of the photoconductor is not limited to the OPC, and the charging member of this embodiment can be used even in the silicon drum in which the resistance value of the surface silicon carbide is 5 × 10 ¹³ Ωcm. , Good charge injection could be performed. Specifically, the drum surface was able to be charged to 480 V with an applied voltage of 500 V.

Further, by directly performing the charge injection charging in this manner, it is possible to completely eliminate the generation of ozone and the deterioration of the surface of the photoconductor, which have been conventionally caused by the discharge, and this effect is durable. It can be maintained regardless of the paper.

[Second Embodiment] The present embodiment is characterized in that the magnetic particles constituting the charging magnetic brush are formed by mixing particles having different particle diameters.

When electric charges are moved by using discharge like the conventional direct charging, the electric charges are moved and charged even if a dischargeable gap is formed between the carriers and the photoconductor.

However, in the direct charge injection charging, the charge on the photosensitive member is caused by movement of the charge in the conductive path between the carriers.
Further, it is necessary to directly contact the carrier with the charge injection layer on the surface of the photoreceptor to inject charges. Therefore, when insulative dust such as toner is mixed in the carrier during durable paper feeding, or when the carrier surface is deteriorated due to toner fusion or the like and the resistance value of that part increases, the conductive path is cut off. As a result, a microscopically-charged residual area is formed on the photoconductor.

In such a situation, the poorly charged area becomes a black spot in the electrophotographic process of reversal development, and in the macro, the portion where the potential is attenuated by image exposure during the previous image formation becomes black. Charged positive ghost occurs.

To suppress this, it is effective to reduce the average particle size in order to increase the chances of contact between the carriers and the carrier and the photoconductor, but in this case, the magnetic binding force of each particle is increased. However, there is a problem in that the carrier is detached from the photosensitive member because of the decrease in the temperature.

The present embodiment is characterized in that a carrier having a plurality of peaks in the particle size distribution is used in order to solve the occurrence of charging failure due to the mixing of insulating dust and the like and carrier adhesion.

In this embodiment, the average particle diameter is 15 μm and the volume resistance value is 1
Ferrite carriers of × 10 ⁶ Ωcm and magnetite particles of 10 μm in average particle size and 1 × 10 ⁴ Ωcm in volume resistance were mixed and used at a weight ratio of 10: 1.

As described above, when particles having different particle diameters are used, even if an insulating material such as toner or paper powder is mixed in the carrier during durable paper feeding or the like, conduction between the carrier and between the carrier and the drum is cut off. As shown in FIG. 4, the small particle size carriers form an electrically conductive path between the large particle size carriers, and the conduction is maintained, whereby the charging failure can be prevented.

Also, the presence of the small particle size carrier between the carrier and the photosensitive drum serves to substantially increase the contact nip between the carrier and the photosensitive member, so that the charging property can be further improved.

Furthermore, by mixing the large particle size carrier and the small particle size carrier, the small particle size carrier is magnetically and physically bound to the large particle size carrier, and carrier adhesion is less likely to occur.

At this time, as shown in the first embodiment, even if the volume resistance value of one particle is low, the resistance value of the entire carrier is determined by the particles having a high volume resistance value, and the resistance value to the pinhole of the photoconductor is increased. Since the property can be maintained, it is preferable that the resistance value of the carrier on the small particle size side forming the conductive path is smaller than that on the large particle size side.

Actually, in the above-mentioned particle constitution except for the magnetic particles, when a durable paper feed was carried out by a printer in the same manner as described in Example 1, a good chargeability was obtained even when 5,000 sheets of images were formed. Was given.

Observation of the carrier after passing 5,000 sheets with an electron microscope shows that the toner is mixed, but the conductive carrier having a small particle size, which is a feature of the structure of the present embodiment, enters between the large-diameter carriers and is electrically conductive. It was confirmed that the route was maintained. Further, since the small-diameter carrier improves the fluidity of the entire carrier, and the itself functions as a cushion and reduces the shearing force between the carriers, the toner is not fused to the large-diameter carrier. Almost never happened.

As described above, by using a large-diameter carrier and a small-diameter carrier as a charging member by mixing them, carrier contamination due to durable paper feeding, which has been a problem in the related art,
It has become possible to dramatically solve charging defects.

Comparative Example 1 Average particle diameter 15 μm, volume resistance value 1
× 10 ⁶ Ωcm Ferrite carrier alone The charging member was initially uniformly charged and good images were obtained.
After passing 00 sheets, a charging failure occurred and became a charging ghost in reversal development.

[Comparative Example 2] Average particle diameter 15 μm, volume resistance value 1
× 10 ⁶ ferrite carrier [Omega] cm, an average particle diameter of 10 [mu] m, a volume resistivity 1 × 10 ⁶ wt ferrite carrier [Omega] cm ratio of 10:
Mixed at a ratio of 1. A charge ghost occurs when 2000 sheets are passed.

The charging ghost was evaluated by outputting a solid white image after solid black, and the fogging after one round of the photosensitive drum after solid black was caused by the fact that complete charging was not performed by a Macbeth densitometer (RD-1255 manufactured by Macbeth Co.). And was used as an index of charging performance. In Comparative Examples 1 and 2, it was confirmed that the fog increased as the durable paper passed.

When the surface of the carrier was observed under an electron microscope in this state, it was confirmed that fine powder of toner was mixed in the carrier, and when the paper was further passed, the toner and the like were fused to the surface of the carrier, and It was found that it hindered the transfer of electric charges of.

In the present embodiment, two kinds of carriers having different particle sizes, that is, large particle size and small particle size, are mixed for convenience, but three or more kinds of particles can be mixed. Further, by using the powder having a broad particle size distribution having the particle size ranges of both the large diameter and the small diameter described in this embodiment, the effect of preventing carrier adhesion and ensuring the charging property can be obtained. It is possible.

[Third Embodiment] In this embodiment, either or both of saturation magnetization and remanent magnetization of the magnetic particles forming a peak on the small particle size side of the particle size distribution form a peak on the large particle size side. It is characterized in that it is larger than that of the magnetic particles.

When a magnetic brush is formed by mixing small-sized particles and large-sized particles, the small particles are detached from the magnetic brush at the time of image formation because the magnetic binding force is weak, and the small particles are separated. When mixed in the developing device, the developability is reduced,
The detachment from the brush may reduce the charging uniformity.

As an index showing the magnetic binding force, whichever is larger, the value of the saturation magnetization showing the magnetization in the magnetic flux or the value of the residual magnetization showing the degree of magnetic orientation, is effective for carrier adhesion. It is preferable that either or both of these satisfy the above relationship.

Concretely, a hard ferrite having a large residual magnetization or a carrier using a rare earth metal having a large saturation magnetization is used as a carrier on the small grain size side, or these metal powders having a large magnetization are dispersed and prepared. It is possible to use a resin carrier or the like.

Unlike the first embodiment, the charging device charges the carrier by directly attaching the carrier to the magnet roll of the other pole and driving the magnet roll to rotate.
In the combination of a fixed magnet and a rotating sleeve, the magnetic flux density decreases with the distance between the magnet roll and the sleeve surface, which tends to cause carrier adhesion. Can be improved. The magnet roll used in this example has a magnetic flux pattern of 15 mm in diameter and 8 poles with equal poles, and the maximum magnetic flux density on the roll surface is 1500 gauss. The height of the spikes of the carrier was regulated to 1.5 mm by the magnetic blade, and the gap between the magnet roll and the photoconductor was set to 500 μm by the roller.

[Experimental Example 1] The particle size is 15 μm and the saturation magnetization is 58 Am.
^At a ratio of 10: 1 to ² / Kg (emu / g) ferrite carrier,
A ferrite carrier with a particle size of 10 μm and saturation magnetization of 60 Am ² / Kg is mixed. The electrification deteriorates after passing 5000 sheets. There was almost no change in the carrier mixing ratio for 5000 sheets.

[Experimental Example 2] Saturation magnetization was 58 Am with a particle size of 15 μm.
^The particle size is 1μ at a ratio of 10: 1 to ² / Kg ferrite carrier.
Mix ferrite carrier with saturation magnetization of 60Am2 / Kg at m.
Good electrification up to 10,000 sheets. However, the mixing ratio of the carrier on 5,000 sheets was reduced to 20: 1, and fog was observed due to deterioration in developability.

[Experimental Example 3] The particle size is 15 μm and the saturation magnetization is 58 Am.
^The particle size is 1μ at a ratio of 10: 1 to ² / Kg ferrite carrier.
saturation magnetization m is a mixture of ferrite carrier of 77Am ² / Kg.
Good electrification up to 10,000 sheets. There was almost no change in the carrier mixing ratio for 5000 sheets. Therefore, no fog was observed due to a decrease in developability.

The saturation magnetization here is (1 / 4π) × 10 ⁷ A / m (1
0k Oersted) This is a measurement value under a magnetic field.

From these experimental examples, when the saturation magnetization on the small particle size side of the magnetic particles is around 60 Am ² / Kg which is the same as the large particle size, it is not preferable in terms of carrier adhesion unless the particle size is larger than 10 μm. There now (experimental example 1 / example 2), the adhesion of the saturation magnetization of the small particle diameter side even particle size 1μm by a 77Am ² / Kg can be prevented. As a result, the charging performance was significantly improved.

From the above, by making the saturation magnetization or remanent magnetization of the magnetic particles on the small particle size side larger than that of the large particle size, it is possible to prevent detachment of the small particle size from the magnetic brush. It is possible to prevent a decrease in chargeability and a decrease in developability that accompany it.

[Fourth Embodiment] In the present embodiment, either the saturation magnetization or the residual magnetization of the magnetic particles forming the peak on the small particle size side of the particle size distribution, or both of them form the peak on the large particle size side. Is smaller than that of the magnetic particles and has a resistance value of 1 × 10 ⁰ to 1 × 10 ⁵ Ωcm.

Carrier adhesion to the drum, which has been a problem of the prior art, was more remarkable when conductive particles were used than particles of medium resistance due to charge induction. Therefore, if the resistance value of the small particle size is reduced, the particles adhere to the drum, and the charging property is deteriorated. In this example, the magnetic restraining force of the small-sized particles was improved so that the resistance of the small-sized particles could be lowered.

The constitution of the magnetic particles of this embodiment is as follows:
Ferrite carrier with a saturation magnetization of 58Am ² / Kg and a resistance of 1 × 10 ⁶ Ωcm at 15μm at a ratio of 10: 1, with a particle size of 1μm and a saturation magnetization of 77Am ² / Kg and a resistance of 1 × 10 ³ Ωcm. Of ferrite carrier. When the image was printed and evaluated on the printer,
Fogging was good at less than 2% after passing 20,000 sheets.

[Comparative Example 1] Saturation magnetization was 58 Am with a particle size of 15 μm.
² / Kg, 10 × on a ferrite carrier with a resistance of 1 × 10 ⁶ Ωcm:
A ratio of 1 was mixed with a ferrite carrier having a grain size of 1 μm, a saturation magnetization of 58 Am ² / Kg, and a resistance value of 1 × 10 ³ Ωcm, and the image was evaluated. % Was exceeded. When this phenomenon was observed, it was observed that particles having a low resistance and a small particle size adhered to the drum during image formation.

[Comparative Example 2] Saturation magnetization was 58 Am with a particle size of 15 μm.
² / Kg, 10 × on a ferrite carrier with a resistance of 1 × 10 ⁶ Ωcm:
1 ratio, particle size saturation magnetization 1μm is 77Am ² / Kg, and resistance by mixing ferrite carrier 1 × 10 ⁶ [Omega] cm, were evaluated image reproduction, fog at 10,000 number of fed sheets 3 Exceeded%. When observing this phenomenon, particles of small particle size with low resistance hardly adhered to the drum during image formation, but the toner mixed with the brush during image formation hindered the conductive path of the carrier. I was able to observe it.

In the present embodiment, as a carrier on the small grain size side, hard ferrite having a large residual magnetization is hydrogen-reduced to reduce the resistance value to about 1 × 10 ⁴ Ωcm or less, or a rare earth metal having a large saturation magnetization is used. It is possible to use the carrier used, or a resin carrier prepared by dispersing these metal powders having large magnetization.

From the above, the saturation magnetization or remanent magnetization on the small particle size side of the magnetic particles is made larger than that of the large particle size,
Moreover, by setting the resistance value of the small particle size to 1 × 10 ⁰ to 1 × 10 ⁵ Ωcm, it is possible to prevent the small particle size from adhering to the drum, and to reduce the charging property and developability during the durability. Can be prevented.

[Fifth Embodiment] In this embodiment, the surface energy of the charge injection layer located on the surface of the photoconductor is lowered, and particularly, the magnetic particles having a small particle size are generated by the intermolecular force between the photoconductor and the photoconductor. Teflon particles were dispersed as lubricant particles in order to suppress the adherence of detached carriers. In this example, the particle size of 0.3 μm
PTFE particles (Teflon, trade name, manufactured by DuPont) were dispersed by 30% in weight ratio with the binder.

Usually, when Teflon particles or the like are dispersed in the charge transport layer in order to impart lubricity to the photoconductor, the charge transport layer has a large film thickness of about 20 μm, which may scatter image exposure, so that a large amount of it may occur. However, unlike the case of the charge transport layer, the charge injection layer is formed as a thin film with a thickness of 2 to 3 μm, so there is no need to worry about light scattering.
It is possible to disperse up to about%.

In this embodiment, since Teflon particles are dispersed as lubricant particles in the charge injection layer, the surface energy of the charge injection layer is lowered and the releasability of the particles is improved. As compared with the case where the carrier was not dispersed, the small-diameter carrier was remarkably less attached to the surface of the photoconductor.

Specifically, a magnetic brush in which 15 μm ferrite and 1 μm magnetite were mixed at a ratio of 20: 1 as magnetic particles was combined with a drum in which lubricant particles were not dispersed, and 1,000 sheets of images were passed after the paper was passed. The ratio of 1 μm magnetite was reduced to 1000: 1, and fog was also increased due to deterioration of the charging property. However, when 1000 images were passed by combining the same mixed carrier with the above-mentioned drum in which Teflon was dispersed, the chargeability was good and there was almost no change in the particle ratio.

[0116]

As described above, an electrophotographic apparatus for charging a photoreceptor having a charge injection layer with a medium resistance magnetic brush to which a DC voltage corresponding to the required photoreceptor surface potential is applied is brought into contact. In the charging device of, the medium resistance magnetic brush is
A magnetic particle whose resistance value is adjusted is bound by a magnet.By mixing multiple types of particles, each of which has different electrical, magnetic, or particle sizes, a single particle can be obtained. You can now obtain various effects that were not possible.

Specifically, by mixing particles of medium resistance with particles of low resistance, the macro resistance value of the charging member is set to the medium resistance region, and the charging current is concentrated in the pinholes on the photoconductor. While preventing, it became possible to improve the charging performance by violently injecting charges into the photoconductor from the low resistance particles.

By using magnetic particles having a small particle size and having a plurality of particle size peaks as a charging member, the conductive path is maintained even when an insulator such as toner is mixed. It has become possible to prevent the deterioration of the charging property due to durable paper passing. At this time, the large particle size restrains the small particle size so that the carrier adhesion can be minimized.

Further, by using materials having large saturation magnetization and remanent magnetization and different magnetic characteristics for particles having a small particle diameter that easily cause carrier adhesion, selective carrier adhesion is prevented, and durability of the charging member is improved. Can be improved.

Further, by imparting lubricity to the charge injection layer, specifically, by dispersing Teflon particles having a small surface energy, carrier adhesion that remarkably occurs when a magnetic brush is used as a charging member is achieved. Can be prevented.

In particular, when a plurality of types of carriers are mixed as in the present invention, and particles having electric, particle size distribution, and magnetic characteristics that easily cause carrier adhesion are used, the charge is By dispersing lubricant particles such as Teflon in the injection layer, a greater effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram illustrating a configuration of an electrophotographic printer according to the present invention.

FIG. 2 is a conceptual diagram showing a pinhole leak prevention mechanism based on the configuration of the present invention.

FIG. 3 is a diagram showing chargeability when low resistance particles are added to magnetic particles.

FIG. 4 is a diagram showing a conductive path when toner is mixed in a charging member composed of magnetic particles having different particle diameters.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Contact charging member 3 Exposure means 4 Developing device 5 Transfer roller 6 Transfer material 7 Cleaning blade 8 Thermal fixing roller 21 Blade for regulating the carrier layer thickness of the contact charging member 22 Fixed magnet roll 23 Conductive magnetic particles (carrier)

Claims (21)

[Claims] 1. A contact charging member using conductive magnetic particles for a photoreceptor, the surface of which is a charge injection layer made of a material having a resistance value of 1 × 10 ⁸ to 1 × 10 ¹⁵ Ωcm. A charging device for charging by applying a voltage and bringing them into contact with each other, wherein the conductive magnetic particles are a mixture of a plurality of types of particles having different characteristics. 2. The charging device according to claim 1, wherein the magnetic particles are a mixture of plural kinds of particles having different electric resistance values. 3. The magnetic particles are at least 1 × 10 ⁰ -1.
The charging device according to claim 2, comprising a particle group having a volume resistance value of × 10 ⁵ Ωcm and a particle group having a volume resistance value of 1 × 10 ⁵ or more. 4. 1 × 10 ⁰ ~1 × 10 ⁵ charging apparatus according to claim 3, wherein the weight occupied particles in the range of Ωcm is less than half of the total magnetic particle weight constituting the charging member. 5. The charging device according to claim 2, wherein the particle size distribution of the magnetic particles has at least two peaks. 6. The charging device according to claim 5, wherein at least one peak of the particle size distribution of the magnetic particles is 30 μm or less. 7. The volume resistance value of at least one particle group forming a peak on the large particle size side of at least one particle group forming the small particle size side of the peak of the particle size distribution of the magnetic particles among the magnetic particles. The charging device according to claim 5, wherein the charging device is lower than the charging device. 8. The volume resistance value of at least one of the particles forming the small particle size side of the peak of the particle size distribution of the magnetic particles is 1 × 10 ⁰ to 1 × 10 ⁵ Ωcm. The charging device according to claim 5. 9. The charging device according to claim 1, wherein the magnetic particles are composed of a plurality of types of particles having different magnetic properties. 10. The charging device according to claim 9, wherein the particle size distribution of the magnetic particles has at least two peaks. 11. The charging device according to claim 10, wherein at least one peak of the particle size distribution of the magnetic particles is 30 μm or less. 12. Among the above magnetic particles, one of the saturation magnetization and the residual magnetization of a magnetic particle forming a peak on the small particle size side of the particle size distribution, or both of the magnetic particles forming a peak on the large particle size side. The charging device according to claim 10, wherein the charging device is larger than that. 13. The magnetic particles according to claim 1, wherein at least one of the particle groups forming the small particle size side of the peak of the particle size distribution of the magnetic particles has a volume resistance value of 1 × 10 ⁰ to 1 × 10 ⁵ Ωcm. The charging device according to claim 12. 14. The charging device according to claim 1, wherein the charge injection layer is formed by dispersing conductive fine particles in a light-transmitting and insulating binder. 15. The charging device according to claim 14, wherein the conductive fine particles are SnO ₂ . 16. The charging device according to claim 1, wherein a lubricant powder is dispersed in the charge injection layer. 17. The charging device according to claim 16, wherein the lubricant powder is any one of a fluorine resin, a polyolefin resin, and a silicone resin. 18. The charging device according to claim 1, wherein the contact charging member moves with a peripheral speed difference with respect to the body to be charged. 19. The charging device according to claim 1, wherein the charging device is used in an image forming device for performing reversal development, and the image forming device has a contact transfer means. 20. The charging device according to claim 1, wherein the magnetic particles are a mixture of at least ferrite and magnetite. 21. The charging device according to claim 1, wherein at least one peak of the particle size distribution of the magnetic particles is 30 μm or less and is magnetite.