JP2001188403A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a failure occurring in consequence of carriers accumulated in a space between a charging member disposed corresponding to the surface of a photoreceptor and the surface of the photoreceptor. SOLUTION: This charging device 2 charges the photoreceptor 1 by applying a voltage from a power source 5 on an charging roller 8 being the charging member arranged close to the surface of the photoreceptor 1, by providing an empty space (gap) G. The gap G is made wider than the grain size of the carrier in the two component developer 19 adopted in the developing device 3. As a result, the carrier is prevented from being accumulated in the section of the gap (G).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing device including a charging device for charging a photoconductor by a charging member disposed in close proximity to the photoconductor, wherein a latent image formed on the photoconductor includes at least a toner and a carrier. The present invention relates to an electrophotographic image forming apparatus that develops images by using an image forming method.

[0002]

2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus, as a method of charging a photosensitive member, there is known a contact charging method in which a charging member such as a charging roller is charged in a state of being in contact with the surface of the photosensitive member. Have been. However, in such a contact charging system, since the charging member is in contact with the photoconductor, there is a disadvantage that the surface of the photoconductor is easily damaged when used for a long time. In addition, since the charging member itself is stained by coming into contact with the photoreceptor, if used for a long period of time, the charging characteristics of the soiled portion of the charging member will change, thereby causing abnormal images such as white spots, black spots, and black stripes. There was a drawback that it easily occurred.

Further, since a charging member such as a charging roller contacts the photosensitive member, it is necessary to take care to prevent the charging member from soiling the surface of the photosensitive member. In some cases, there were restrictions on the materials of the parts. In order to solve the problems of the contact charging system and achieve ozone-less, which is the greatest advantage of the contact charging system, recently, a charging member is arranged close to a photosensitive member. A (non-contact) proximity charging method has been proposed (for example, see Japanese Patent Application Laid-Open No. 7-301973).

As the charging member used in the proximity charging type charging device, a member having a medium resistance layer on the surface in order to maintain stable discharge in a minute gap as in the contact charging type charging device. Are suitable. Therefore, a medium-resistance elastic roller used in a conventional contact charging type charging device can be used as a charging member in the proximity charging type charging device. In this proximity charging method,
Since the charging member does not directly contact the surface of the photoreceptor, there is no need to provide elasticity to the surface layer. Therefore, a roller having no elasticity can be used.

In an electrophotographic image forming apparatus, a latent image formed on the surface of a photoreceptor is developed by a developing device into a visible image. However, when the developing device is a two-component developing system, In the developer used there, the particle size is 10
It contains at least a toner having a particle size of about μm and a carrier having a particle size of about 60 μm. Note that, depending on the developer, a lubricant or the like may be externally added.

[0006]

However, since the carrier contained in such a developer is relatively hard, if it adheres to the surface of the photoreceptor, for example, it is necessary to clean the surface of the photoreceptor. In addition, if the cleaning blade rubs the surface between the cleaning blade and the photoconductor, the cleaning blade may be rubbed by the cleaning blade and the photoconductor or the cleaning blade may be easily damaged.

For this reason, in a normal case, control is performed so that the carrier does not adhere to the surface of the photoreceptor. For example, when the power supply is suddenly cut off during image formation, noise may enter the developing bias. As a result, the carrier may adhere to the surface of the photoconductor. In this state, when the power is turned on at the time of next use, the photoconductor on which the carrier is attached rotates and the carrier enters the cleaning blade portion, and in some cases, slips through the cleaning blade. There is a case where a charging member provided with a gap (hereinafter, also referred to as a gap) between the photosensitive member and the photosensitive member is carried to a certain position.

At this time, if the particle size of the carrier is larger than the gap between the charging member and the photosensitive member, the carrier is clogged in the gap. In this case, the bias applied to the charging roller flows into the photoreceptor via the carrier, so that good charging cannot be performed or the surface of the photoreceptor or the charging member is scraped, thereby damaging them. And so on.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has a carrier between a charging member of a charging device and a surface of a photoreceptor provided with a gap with respect to the surface of the photoreceptor. It is intended to prevent a problem caused by clogging.

[0010]

According to the present invention, there is provided a charging device for charging a photosensitive member by applying a voltage to a charging member disposed in close proximity to the surface of the photosensitive member. In an image forming apparatus that develops a latent image formed thereon with a developer containing at least a toner and a carrier, the gap at the closest position between the discharge surface of the charging member and the photoconductor is made larger than the particle size of the carrier. It is a big one.

The voltage applied to the charging member is preferably a voltage obtained by superimposing an AC voltage on a DC voltage. The charging member is preferably a roller having a medium resistance layer outside the conductive substrate. Then, the resistance value of the resistance layer in that the roller body may be a ¹⁰ 4 ^{~10 1 0 Ωcm.}

Further, the roller body may be formed by coating the surface layer of the conductive substrate with a medium-resistance resin or covering the surface layer of the conductive substrate with a medium-resistance conductive tube. The conductive substrate may be a metallic roller member.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a part of an image forming unit and a sheet feeding system of an image forming apparatus according to an embodiment of the present invention.

This image forming apparatus includes a drum-shaped photoreceptor 1 which rotates in the direction of arrow A in an image forming unit, and a charging device 2, a developing device 3, a transfer belt 4, A cleaning device 6 and the like are provided. The charging device 2 applies a voltage from a power source 5 to a charging roller 8, which is a charging member arranged close to the surface of the photoconductor 1 by providing a gap (hereinafter also referred to as a gap) G with the surface of the photoconductor 1. Charge.

On the downstream side of the charging device 2 in the rotation direction of the photoreceptor 1, a laser beam is irradiated onto the surface of the photoreceptor 1 uniformly charged by the charging device 2 to form a latent image. A part 7 is provided. The developing device 3 develops the latent image formed on the photoreceptor 1 by the exposure unit 7 with a developer containing at least a toner and a carrier, and forms the toner image. In this image forming apparatus, the gap G at the closest position between the discharge surface (surface) of the charging roller 8 and the photoconductor 1 is defined as
It is larger than the particle size of the carrier of the developer.

The transfer belt 4 is stretched between rollers 11 and 12 which are rotatably provided at intervals, is rotatable in the direction of arrow B by a drive source (not shown), and is in contact with the photosensitive member 1. A bias roller 14 is provided on the inner surface of the position,
A predetermined transfer bias is applied thereto from a power supply (not shown).

The cleaning device 6 removes residual toner and the like remaining on the surface of the photoreceptor 1 after image transfer by a cleaning blade 18 rubbing the surface of the photoreceptor 1.
A paper feed cassette 15 is provided below the image forming unit, and the transfer paper P stored in the paper feed cassette 15 can be fed by a paper feed roller 16. A registration roller pair 17 is provided on the downstream side of the paper feed roller 16.

In this image forming apparatus, when the image forming operation is started, first, the photosensitive member 1 starts rotating in the direction of arrow A in FIG. Then, the surface of the photoconductor 1 is uniformly charged by the charging roller 8 rotating in the direction of arrow C at the same linear speed as the photoconductor 1. The charged surface is irradiated with laser light from the exposure unit 7, and a latent image corresponding to an image to be formed thereon is formed.

In the developing device 3, a two-component developer 19 composed of a toner and a carrier is agitated, and the carrier is held on the surface of a developing sleeve 20 in a spike state to form a magnetic brush. Then, the carrier in the spike state comes into sliding contact with the surface of the photoconductor 1, and the toner adhering to the carrier adheres to the latent image on the photoconductor 1, whereby the latent image is developed into a toner image. .

On the other hand, the transfer paper P
Is sent out by a paper feed roller 16 rotated at a predetermined timing, is temporarily stopped by a pair of registration rollers 17, and is conveyed to a transfer position where a bias roller 14 is located in synchronization with the toner image on the photoconductor 1. . Therefore,
The toner image on the photoconductor 1 is transferred onto the transfer paper P by an electric field formed by a potential difference between the bias applied to the bias roller 14 and the photoconductor 1.

The transfer paper P on which the toner image has been transferred is conveyed to a fixing device (not shown) provided on the downstream side of the transfer belt 4 where the toner is fixed and discharged to a paper discharge tray or the like. Then, the residual toner remaining on the photoconductor 1 without being transferred to the transfer paper P at the transfer unit is cleaned by the cleaning blade 18 of the cleaning device 6.

By the way, the carrier contained in the two-component developer 19 does not usually adhere to the surface of the photoreceptor 1,
As described above, for example, when the power supply is suddenly cut off during image formation, noise may enter the developing bias, and the noise may adhere to the surface of the photoconductor 1.

In this state, when the power of the image forming apparatus is turned on at the time of next use, the photosensitive member 1 with the carrier attached thereto rotates in the direction of arrow A in FIG. When it moves and reaches the position of the cleaning device 6, it enters the lower portion of the cleaning blade 18, and further passes through the cleaning blade 18 to reach the position of the charging roller 8.

Here, if the particle size of the carrier is larger than the gap G between the charging roller 8 and the photosensitive member 1,
The carrier is blocked in the gap G. In this case, since the carrier is generally relatively hard, the surface of the photoconductor 1 may be scratched by rubbing. Further, if the carrier is clogged between the charging roller 8 and the photoconductor 1, the bias applied from the power supply 5 to the charging roller 8 flows into the photoconductor 1 via the carrier, so that good charging is achieved. Performance may not be obtained.

However, in the image forming apparatus according to this embodiment, as described above, the gap G at the closest position between the discharge surface (surface) of the charging roller 8 and the photosensitive member 1 is increased by the particle size of the developer carrier. Larger than. Therefore, since the carrier does not become clogged in the gap G, it is possible to prevent the above-described problems from occurring.

By the way, as in the charging device 2 shown in FIG. 1, the gap G in the proximity charging system is closely related to the discharge starting voltage as shown by Paschen's discharge curve in FIG. That is, the DC voltage (D
The charging potential when C bias is applied is approximately (applied voltage)
− (Discharge start voltage). Therefore, if the gap G slightly fluctuates due to eccentricity or runout of the charging roller, the discharge starting voltage changes, and the charging potential fluctuates.

For example, as shown in FIG.
When one end 28a of the charging roller 8 is brought into contact with the surface of the photoreceptor 1 and the other end 28b is provided with insulating butts 22, 22 at both ends of the charging roller 28 so that the gap G gradually widens. The larger the gap G, the higher the discharge starting voltage, and thus the lower the charging potential. As a result, background contamination occurs on the other end 28b where the gap G is wide. As described above, when only a DC voltage is applied to the charging roller, a delicate difference in the gap is directly linked to a difference in the charging potential, so that it is difficult to obtain uniform charging performance.

In the image forming apparatus according to this embodiment, as described above, the carrier or the toner of the developer passes through the gap G between the photosensitive member 1 and the charging roller 8, but the negative / positive (N) / P), the charging polarity of the photosensitive member and the charging characteristic of the toner are the same, but the charging characteristic of the carrier is the opposite polarity. When the carrier passes through the gap, the carrier adheres to the charging roller due to Coulomb force.

When the carrier adheres to the surface of the charging roller in this way, when the voltage applied to the charging roller is only the DC voltage, the unevenness of the charging roller pitch on the image corresponds to the contamination of the charging roller. Can be. Therefore, in addition to the DC voltage, the charging roller has a discharge starting voltage (at the time of application of only the DC voltage) of the set gap.
It is preferable to apply a voltage on which an AC voltage having a peak-to-peak voltage twice or more is superimposed.

As described above, by superimposing an AC voltage having a peak-to-peak voltage that is twice or more the discharge starting voltage on the DC voltage, the discharge from the charging roller to the photosensitive member by the frequency and the reverse discharge are continuously performed. This causes the charging roller to be less likely to generate uneven charging corresponding to the contamination even if the charging roller has dirt. Further, when a voltage obtained by superimposing an AC voltage on such a DC voltage is applied to the charging roller, the charging potential becomes substantially the same as the DC component, so that the charging voltage is superior to the fluctuation of the gap.

By the way, the charging roller 8 shown in FIG.
As shown in FIG.
It is good to make it the roller body which formed. The reason is that
This is because, when the surface of the roller has a low resistance, when a voltage is applied to the charging roller, dielectric breakdown of air occurs and the air leaks immediately.

Conversely, if the surface of the roller is too high in resistance, it is necessary to apply a high voltage due to a large voltage drop in the resistance layer in order to cause discharge. A high AC voltage of -5 kV or more is required. Therefore, in order to lower the voltage even a little, it is necessary to make the resistive layer thin. However, when a high voltage is applied to the thin resistive layer, dielectric breakdown is apt to occur, and leakage tends to occur. Therefore, in order to maintain a stable discharge, it is necessary to appropriately adjust the resistance and the thickness of the resistance layer. Therefore, the resistance value of the medium resistance layer 24 of the charging roller 8 is specifically 10 ⁴ to 10.
^{1 0} [Omega] cm, preferably may be approximately ¹⁰ 5 ^{~10 9} Ωcm.

When the gap between the photosensitive member and the charging roller is widened, the discharge starting voltage is increased as described above, so that a high voltage is required and the power supply cost is increased.
In addition, when a high voltage is applied, even if the roller surface is covered with a medium-resistance material, abnormal discharge is likely to occur, and it becomes difficult to perform stable discharge and charge the photosensitive member to a target charging potential.

Therefore, it is desirable to set the gap between the charging roller and the photosensitive member to a minimum necessary gap. For this purpose, the deflection of the charging roller (the deflection of the outer peripheral surface of the roller that occurs when the charging roller is rotated once according to the straightness) is as follows.
It is better to be as small as possible. Also, the gap between the charging roller and the photosensitive member needs to be set so that the carrier does not jam in the gap even if the outer diameter of the charging roller is the largest in the variation.

Further, the voltage applied to the charging roller must be such that it can be charged sufficiently even at the maximum gap position where the outer diameter of the charging roller becomes the smallest due to variation. In other words, the larger the deflection of the charging roller is, the higher the voltage applied to the charging roller becomes,
It also becomes difficult to perform stable discharge.

Therefore, in the case of such a proximity charging type charging device, the portion corresponding to the image forming area of the charging roller does not come into contact with the surface of the photoreceptor. It is better to use a hard roller that coats the surface of a metal roller with a medium-resistance resin or coats the surface of the metal roller with a medium-resistance conductive tube than an elastic roller that is used. Good.

Actually, the difference between the maximum value and the minimum value of the outer diameter of a rubber roller used as a charging roller in a contact charging device is about 30 to 100 μm, whereas the hard roller described above is used. In this case, a deflection of about 20 μm is sufficient. Therefore, the cost tends to be high due to a decrease in yield due to tight outer diameter tolerance in the rubber roller, but the surface of the metal roller is coated with a medium resistance resin or coated with a medium resistance conductive tube. With such a roller, it is possible to easily maintain the gap between the charging roller and the photoconductor at a set value while reducing the cost.

[0038]

EXAMPLE Next, in order to confirm the effect of the present invention, the actual
The results of the experiments performed at this time will be described. In addition, the line
Comparative examples described above will also be described.First embodiment  The charging roller used in the experiment was a φ12 mm metal roller
10 on the surface of⁹Spray coat Ωcm polyurethane
And a medium resistance layer was formed. That coat
The thickness of the medium resistance layer was 40 μm,
The difference between the maximum and minimum outer diameter of the roller is 20 μm.
Was.

At both ends (non-image forming area) of the middle resistance layer of the charging roller, as shown in FIG.
m of the insulating film 21 was wound and brought into contact with the photoreceptor 1. As a result, the gap G between the surface of the intermediate resistance layer 24 of the charging roller 8 and the surface of the photoreceptor 1 was about 80 μm. Therefore, the gap G between the surface of the medium resistance layer 24 of the charging roller 8 and the surface of the photosensitive member 1 is at least 60 μm or more even if the outer diameter of the roller when the charging roller 8 is rotated once is considered. Will be.

The particle diameter of the carrier contained in the developer used in the developing device was 50 μm, and the particle diameter of the toner was 9 μm. The charging roller 8 has −70
A peak-to-peak voltage of 3.0 kV and a frequency of 1.
An AC voltage of 5 kHz was applied. Also, in the developing device,
When an AC voltage having a peak-to-peak voltage of 1.0 kV and a frequency of 2.0 kHz was applied to a DC voltage of -500 V and a halftone image was actually output using an image forming apparatus, an image having no problem at first was obtained. Obtained. Further, when 10,000 sheets were continuously fed, a good image was obtained.

[0041]Comparative Example 1  The same as the charging roller used in the first embodiment,
The thickness of the film wound around each end of the layer is 60
A similar experiment was performed with a μm diameter. At this time
Gap between the surface of the medium resistance layer of the charging roller and the surface of the photoconductor
The diameter of the tape was about 50 μm.

An AC voltage having a peak-to-peak voltage of 2.5 kV and a frequency of 1.5 kHz was applied to a DC component of -700 V to the charging roller. Other conditions were exactly the same as in the first embodiment. The experimental results showed that there was no particular problem with the image at the beginning, but when the paper was passed, leakage occurred on about 100 sheets. Therefore, as shown in FIG.
When the photosensitive member 1 was observed, traces a and b as if scraped by the carrier of the developer were observed.

[0043]Comparative Example 2  The charging roller has a surface of 10 mm
⁹Spray coated with Ωcm polyurethane and with medium resistance layer
Using the fixed, DC voltage is fixed at -700V,
It was confirmed that the AC peak-to-peak voltage was gradually increased. What
When this charging roller is used, the discharge starting voltage
Exceeds 1 kV. The experimental results show that the peak-to-peak voltage
Charged to about -200 V at 3.0 kV,
Raises the leak, causing the photoconductor to charge to -700V
I couldn't let it.

[0044]Comparative Example 3  The charging roller has a surface of 10 mm
³Spray polyurethane with Ωcm carbon dispersed
One having a medium resistance layer formed by coating was used. this
Using a charging roller, fix to DC -700V
When the peak-to-peak voltage of the current was gradually increased,
It was charged to about -400 V at a voltage of 0.6 kV,
Raising the upper voltage causes air breakdown and charges at once
Although the potential exceeded -1 kV, charging unevenness occurred on the entire surface,
Uniform charging could not be performed.

[0045]Second embodiment  The charging roller used is shown in Fig. 4 in the center of the roller in the longitudinal direction.
As shown the vertical cross-sectional shape of the part, the metal of φ11.8mm
A roller having a thickness of 100 μm
m nylon tube (press-fit)
Was formed, and the outer diameter of the charging roller was φ12 mm. That
The volume resistance of the charging roller is 10⁶Ωcm.

When the charging roller is rotated once, the deflection of the medium resistance layer 24 formed of a nylon tube is about ± 10% of the thickness of the tube. When the run-out of the tube and the run-out of the outer diameter of the portion of the metal roller before being covered with the tube in one rotation are combined, the outer diameter of the medium resistance layer of the final charging roller becomes a maximum value and a minimum value. The difference was about 22 μm.

In this charging roller, since the adhesive force of the tube forming the medium resistance layer to the metal roller is weak, a method for maintaining a predetermined gap G in order to prevent a load from being applied to the adhesive surface. In this case, an abutting roller 22 as shown in FIG. 7 was employed. Of course, if the bonding with sufficient durability can be achieved, the previous embodiment (see FIG. 5)
Alternatively, a method in which an insulating film is wound around the end of the roller to maintain the gap may be used.

The charging roller in this embodiment has an outer diameter of a medium resistance layer portion of φ12.2 mm, which is pressed by its own weight, and the abutting roller 22 is slightly deformed. Was about 85 μm. Therefore, even when the deflection of the charging roller is considered, the gap G is sufficiently larger than the particle diameter of the carrier of the developer of 60 μm (the particle diameter of the carrier used in the experiment).

In an experiment using this charging roller, an AC voltage having a peak-to-peak voltage of 2.5 kV and a frequency of 1.5 kHz was applied to the charging roller with a DC component of -700 V. As a result, an initially satisfactory image was obtained.
Further, when 10,000 sheets were continuously fed, a good image was obtained.

[0050]Third embodiment  The charging roller used in this embodiment is also outside the metal roller.
And cover it with a 50 μm-thick nylon tube (pressure
In) A medium resistance layer was formed. The volume resistance of the charging roller is
10⁹Ωcm. When the charging roller is rotated once
The deflection of the outer surface of the resistance layer was 15 μm.

Also in this embodiment, the abutment roller 22 described with reference to FIG. 7 was used so that the gap G was 80 μm. The charging roller has a DC component of -700V,
An AC voltage having a peak-to-peak voltage of 3.0 kV and a frequency of 1.5 kHz was applied. As a result, an initially satisfactory image was obtained. Further, when 10,000 sheets were continuously fed, a good image was obtained.

[0052]Fourth embodiment  The charging roller used in this embodiment is located outside the metal roller.
Is covered with a 100 μm-thick PAE tube (pressure
In) A medium resistance layer was formed. The volume resistance of the charging roller is
10⁵Ωcm. When the charging roller is rotated once
The run-out on the outer surface of the resistance layer was 18 μm.

Also in this embodiment, the abutment roller 22 described with reference to FIG. 7 was used so that the gap G was 85 μm. The charging roller has a DC component of -700V,
An AC voltage having a peak-to-peak voltage of 2.5 kV and a frequency of 1.5 kHz was applied. As a result, an initially satisfactory image was obtained. Further, when 10,000 sheets were continuously fed, a good image was obtained.

[0054]Comparative Example 4  With the charging roller described in the fourth embodiment, the abutting roller 22 is used.
And changing the outer diameter to make the gap G 40 μm
went. At this time, the charging roller is supplied with a -700 V DC component.
The peak-to-peak voltage of 2.0 kV and the frequency of 1.5 kHz
When applying a streaming voltage, an image with no problem initially appears
Obtained. However, when continuous paper was passed, 8
Vertical streaks appeared in the halftone after 1,000 continuous passes.
Therefore, when the charging roller was observed,
In some places, toner sticking with a width of about 1 mm is seen.
And the surface of the photoconductor corresponding to that part is slightly scraped.
Was raw.

[0055]Fifth embodiment  The charging roller is a charging roller used in a normal contact charging device.
Manufactured using hydrin rubber like a roller
Was. In other words, a 3 mm thick cover
A conductive elastic roller is formed from phosphor rubber, and the conductive elastic
Films are applied to both ends of the flexible roller in the same manner as in the first embodiment.
Wrap the gap between the conductive elastic roller and the surface of the photoreceptor.
The gap was 80 μm. However, the conductive elastic roller is
Tighter tolerances to reduce the outside diameter run-out in one rotation of the roller
Since the thickness is about 20 μm, the yield is low and the cost is high.
became.

In the conductive elastic roller portion, when the charging roller is pressed against the photosensitive member due to the elasticity of the rubber, the film sinks into the rubber, so that a film having a thickness of 120 μm is used to maintain the gap of 80 μm. Needed. An AC voltage having a peak-to-peak voltage of 2.5 kV and a frequency of 1.5 kHz was applied to the charging roller with a DC component of -700 V. As a result, an initially satisfactory image was obtained. Further, when 10,000 sheets were continuously fed, a good image was obtained.

[0057]Comparative Example 5  The hydrin rubber used in the fifth embodiment is replaced with a conductive elastic roller.
Used in normal contact charging with the charging roller used in the section
When using the one with the same processing accuracy as the one
The deflection of the outer diameter in one rotation of the roller is close to 80 μm,
The center of the roller in the longitudinal direction was bulging
The gap G with the photoconductor at the center is about 40 μm
It was time. Therefore, the same voltage as in the fifth embodiment is used.
Was applied and the evaluation was performed.
Vertical stripes appeared in the halftone. So, the charging roller
Was observed, the central part in the longitudinal direction of the charging roller,
Occurrence of toner fixation with a width of about 1 mm was observed.
The surface of the photoreceptor corresponding to the image was slightly scraped.

[0058]Comparative Example 6  The charging roller of Comparative Example 3 was exposed at the center in the longitudinal direction.
160μm thick so that the gap with the body is 80μm
When the gap is made with the film of m
White spots occur due to abnormal discharge at both ends of the
No uniform charging performance was obtained.

[0059]

As described above, according to the present invention, the following effects can be obtained. According to the image forming apparatus of the first aspect, the gap at the closest position between the discharge surface of the charging member and the photoconductor is made larger than the particle size of the carrier of the developer. Since the carrier and the toner adhered to the carrier are not pinched, the carrier does not damage or stain the photosensitive member or the charging member. Therefore, stable charging performance can be maintained over a long period of time, and a good image can be formed.

According to the image forming apparatus of the present invention, since a voltage obtained by superimposing a DC voltage on an AC voltage is applied to the charging member, uniform charging performance can be obtained even if the gap is larger than the carrier particle size. And a good image can be formed.

According to the image forming apparatus of the third aspect, the charging member is a roller body having a medium resistance layer outside the conductive substrate. Since the resistance value is set to 10 ⁴ to 10 ¹⁰ Ωcm, there is no danger of leakage and the discharge is stabilized. Thereby,
Uniform charging performance can be obtained, and good images can be formed.

According to the image forming apparatus of the fifth and sixth aspects, since the accuracy of the outer diameter of the roller can be increased without particularly complicated steps, the gap is set to a target value at low cost. be able to. Further, since the accuracy of the outer diameter of the roller can be increased, the amount of runout of the surface per rotation of the charging roller is reduced, so that it is not necessary to set the gap more than necessary when setting the gap. For,
Abnormal discharge is less likely to occur.

According to the image forming apparatus of the present invention, since the conductive base is a metal roller member, the accuracy of the outer diameter of the roller can be relatively easily increased as compared with the case where the elastic layer is provided.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating a part of an image forming unit and a sheet feeding system of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a discharge curve of Paschen.

FIG. 3 is a front view showing a charging roller and a photoconductor used for confirming a relationship between a gap between the charging roller and the photoconductor and a discharge starting voltage.

FIG. 4 is a longitudinal sectional view showing a charging roller having a medium resistance layer formed outside a conductive substrate.

FIG. 5 is a front view showing an example of a charging roller in which insulating films are wound around both ends of a middle resistance layer.

FIG. 6 is a front view for explaining a state in which a carrier makes shaving marks on both surfaces of the charging roller and the photoconductor when a gap between the charging roller and the photoconductor is smaller than a particle diameter of a carrier.

FIG. 7 is a front view showing an example of a charging roller together with a photoconductor in which a gap is formed between the charging roller and the photoconductor by providing abutting rollers at both ends.

[Explanation of symbols]

 1: Photoconductor 2: Charging device 5: Power supply 8, 28, 38: Charging roller (charging member) 19: Two-component developer 23: Conductive substrate 24: Medium resistance layer

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H003 AA12 BB11 CC05 EE12 EE19 3J103 AA02 AA13 AA18 AA20 AA21 AA53 AA69 AA72 BA31 BA41 BA46 EA05 EA08 EA09 FA02 FA06 FA07 FA09 FA14 FA18 GA02 GA52 GA57 GA11 HA60 HA04 HA05 HA48 HA53 HA54

Claims (7)

[Claims] A charging device configured to apply a voltage to a charging member disposed close to a surface of the photoconductor to charge the photoconductor, and to form a latent image formed on the photoconductor at least by toner and carrier. An image forming apparatus that develops with a developer containing the carrier, wherein a gap between a discharge surface of the charging member and a photoconductor is larger than a particle diameter of the carrier. 2. The image forming apparatus according to claim 1, wherein the voltage applied to the charging member is a voltage obtained by superimposing an AC voltage on a DC voltage. 3. The charging member according to claim 1, wherein the charging member is a roller having a medium resistance layer outside a conductive substrate.
Or the image forming apparatus according to 2. 4. The resistance value of the medium resistance layer of the roller body is 10
The image forming apparatus according to claim 3, wherein the thickness is ⁴ to 10 ¹⁰ Ωcm. 5. The image forming apparatus according to claim 3, wherein the roller body is formed by coating a medium resistance resin on a surface layer of the conductive substrate. 6. The image forming apparatus according to claim 3, wherein the roller body is formed by covering a surface layer of the conductive substrate with a conductive tube having a medium resistance. 7. The image forming apparatus according to claim 3, wherein the conductive base is a metallic roller member.