JP2001356569A - Contact electrifying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a contact electrifying member to which AC voltage superimposed on DC voltage is applied free from being soiled by toner or the like and to uniformly electrify the surface of a photoreceptor. SOLUTION: A 1st electrifying brush roller 10 and a 2nd electrifying brush roller 11 are arranged on an upstream side and a downstream side in the rotating direction shown by an arrow A of the photoreceptor 1 rotating in the direction shown by the arrow A, respectively. The DC voltage is applied to the roller 10 and voltage obtained by superimposing the AC voltage on the DC voltage is applied to the roller 11. Since surface potential is previously imparted to the photoreceptor l in DC voltage, the trough of the surface potential is softened and the potential of the surface of the photoreceptor gets stable in the case of applying the voltage obtained by superimposing the AC voltage on the DC voltage. By such constitution, the roller 11 to which the voltage obtained by superimposing the AC voltage on the DC voltage is applied is not soiled by the toner or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a contact charging device for uniformly charging the surface of a photosensitive member using a contact charging member in an electrophotographic image forming apparatus.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus,
A contact charging method in which a member to which a voltage is applied from the outside is brought into contact with a photosensitive member to perform charging is widely used. This contact charging method has the following advantages (1) to (3) as compared with the corona charging method.

(1) The power supply used for the contact charging method is 1 kV, whereas the power supply used for the corona charging method is several kV.
It is a low voltage near kV. (2) Ozone generation is low. (3) The charging potential depends on the applied voltage, and does not depend on the process speed or the capacitance of the photoconductor.

[0004] As a charging method by the contact charging method,
A roller charging system and a brush charging system are known. In the roller charging system, a roller having a multi-layer structure of function separation type has been put into practical use, but this is because there was no other way to obtain the desired elasticity, resistance value, and surface smoothness than using a multi-layer structure. Therefore, it was undeniable that the cost would increase. In addition, even if the surface of the photoconductor is charged by a roller charging method, each part of the surface of the photoconductor is not actually charged uniformly, and spot-like charging unevenness occurs. This is because the electrically conductive member (charged body) to which the voltage is applied and the surface of the photoreceptor (charged body) contacted with the microscopically hardly have an ideally close contact surface due to unevenness on both surfaces. it is conceivable that. Even if an image forming process after exposure is applied to the surface of the photosensitive member in the spot-like uneven charging state, the output image becomes a spot-like black spot image corresponding to the spot-like uneven charging, and a high-quality image cannot be obtained.

[0005] The brush charging system does not need to be multi-layered like the roller charging system, and it is possible to set the elasticity, the resistance value, and the like to desired values with one type of brush.

As a driving method of the contact charging method, there are a method of applying a DC voltage to a contact charging member, a method of applying a DC current, and a method of applying an AC voltage superimposed on the DC voltage.

[0007] The method of applying a DC voltage to the contact charging member is susceptible to the environment, so that the charging stability of the photoreceptor is low and the charging potential is lowered in a low-temperature and low-humidity environment. Specifically, in a solid image (solid image), many white stripes parallel to the longitudinal direction of the photoconductor appear.

Further, the method of applying a direct current to the contact charging member has a problem in that the current is concentrated on a defective portion of the photosensitive drum, and the photosensitive drum and the contact charger are burned.

Further, in the case of applying a method in which an AC voltage is superimposed on a DC voltage to the contact charging member, there is no environmental change such as a DC voltage, and the photosensitive drum or the contact charger is burned like a DC current. Nor. Therefore, as a driving method of the contact charging method, a method in which an AC voltage is superimposed on a DC voltage and applied is effective.

For this reason, as a prior art for charging the surface of a photoreceptor by superimposing an AC voltage on a contact charging member and applying an AC voltage to the contact charging member, JP-A-63-149669 discloses a DC voltage applied to a charged member. A pulsating voltage having a peak-to-peak voltage that is at least twice as high as the charging start voltage is applied to the conductive member to form an oscillating electric field between the charged body and the photosensitive body, thereby uniformly charging the body to be charged. Is disclosed.

However, when the surface of the photoreceptor is charged by this method, the surface potential of the photoreceptor fluctuates at a spatial frequency corresponding to the frequency of the output voltage of the AC power supply and the rotational peripheral speed of the photoreceptor. The change of the surface potential is periodically repeated.

When an electrostatic latent image is formed by exposing a photosensitive member having a surface potential fluctuation, the potential of a portion not exposed (hereinafter, the potential of a portion not exposed is referred to as a dark potential). Of course, the potential of the exposed portion (hereinafter, the potential of the exposed portion is abbreviated to the bright potential) is not slightly different from that of the dark potential. Potential fluctuations remain. Therefore, the difference between the light potential at the portion where the surface potential is high and the light potential at the portion where the surface potential is low affects the toner adhesion amount during development.

For example, when a straight line in the main scanning direction is repeatedly drawn every other line, the line width slightly changes depending on whether the edge portion of the latent image of the other line has a high or low light potential. Therefore, if the spatial frequency of the latent image on the photoreceptor and the spatial frequency of the potential fluctuation of the surface potential due to repeated drawing of a straight line are equal, or if there is a slight difference in the frequency when it is an integral multiple, the line of the adjacent line The position of the thick / narrow width is slightly shifted. As a result, unevenness occurs in a cycle lower than the frequency of the repeated drawing of the straight line. This interference unevenness is generally called moiré, and a high-quality image may not be obtained due to a density difference due to the moiré.

As means for preventing the occurrence of moire, means for setting the spatial frequency of the fluctuation of the surface potential to be higher than the spatial frequency for repeated drawing of a straight line, and means for setting a lower frequency in an area where moire does not occur. However, in order to adopt such means, it is necessary to increase the frequency of the charged AC power supply in accordance with the increase in resolution of the image forming apparatus, and therefore, the AC current increases, and the capacity of the AC power supply increases. Is required. Further, the charged body may vibrate at the frequency, and the vibration may reach an audible frequency and become a noise source.

In view of such a problem, in order to stabilize the charging potential on the surface of the photosensitive member by the contact charging method, two charging members are provided for charging the photosensitive member in contact with the photosensitive member, and the photosensitive member is charged twice. Several methods have been disclosed.

Japanese Patent Application Laid-Open No. Hei 4-30186 discloses that at least an AC voltage is applied to a first charging means disposed on the upstream side in the rotation direction of a photoconductor, and the rotation direction of the photoconductor is higher than that of the first charging means. The second charging means disposed on the downstream side of
A method of applying a DC voltage or setting a float state is disclosed.

Japanese Patent Application Laid-Open No. 6-95478 discloses that
A voltage in which a DC component is superimposed on an AC component is applied to the first charging means disposed on the upstream side in the rotation direction of the photoconductor, and the first charging means is disposed downstream of the first charging means in the rotation direction of the photoconductor. Further, a method of applying only a DC voltage component to the second charging means is disclosed.

Although a voltage in which an AC voltage is superimposed on a DC voltage is not applied to the charging means, see Japanese Patent Application Laid-Open No. 7-43.
No. 988 discloses that in addition to a main charging device to which a DC voltage is applied, a pre-charging device is provided further upstream in the rotation direction of the photoconductor, and a voltage applied to the main charging device by the pre-charging device. A method of applying a voltage having the same polarity as the above and having a small absolute value is disclosed.

[0019]

However, in Japanese Patent Laid-Open Publication No. Hei 4-30186, even if the frequency of the AC voltage applied to the first charging means is set low enough not to generate charging noise, the second charging Means can alleviate the uniform contact charging performance of the surface to be charged by the first charging means and prevent charging unevenness. However, the first charging means in which an AC voltage is superimposed on a DC voltage and applied is a toner. The effect of preventing uneven charging on the surface of the photoreceptor is lost due to contamination caused by such factors.

[0020] Also, in JP-A-6-95478 public information,
At the time of charging by the first charging unit, minute discharge unevenness occurs on the surface of the photoreceptor, but thereafter, by contacting the second charging unit, the minute unevenness can be eliminated and the charging potential can be stabilized. In addition, the effect of stabilizing the charged potential on the surface of the photoreceptor is lost due to the contamination of the first charging unit to which the DC voltage is applied with the AC voltage by toner or the like.

Also, in Japanese Patent Application Publication No. 7-43988,
The charging voltage on the surface of the photoconductor is increased stepwise along the rotation direction of the photoconductor, so that the charging potential on the surface of the photoconductor can be stabilized, but the voltage applied to the charging member is only a DC voltage. It is susceptible to the environment and has low charging stability.

As another problem, in a charging device using two contact charging members, such as the contact charging device shown in FIG. 11, as shown in a time chart of FIG. When a voltage is simultaneously applied to the first contact charging member, the section 1 on the surface of the photoconductor between the first and second contact charging members is charged by the second contact charging member by the first charging member. The photosensitive member surface potential rises sharply, the current flowing into the photosensitive member becomes an overcurrent, and the surface potential becomes unstable.

An object of the present invention is to provide a contact charging device which is not contaminated by toner and charges the photoreceptor surface uniformly when the surface of the photoreceptor is charged by the contact charging method. .

[0024]

SUMMARY OF THE INVENTION The present invention is used in an electrophotographic image forming apparatus, in which first and second contact charging members are provided along the rotation direction of a photoreceptor, and an electrostatic latent image is formed. In the contact charging device for contacting the first and second contact charging members with the photoreceptor before forming the photoreceptor and charging the surface in a plurality of times, a first charging member located on the upstream side in the rotation direction of the photoreceptor is provided. A DC voltage is applied to the contact charging member, and a voltage obtained by superimposing an AC component on a DC component is applied to a second contact charging member located on the downstream side.

According to the present invention, as shown in the contact charging device of FIG. 1, a DC voltage is applied to the first contact charging member located on the upstream side in the rotation direction of the photoconductor, and the second contact charging member located on the downstream side. A voltage obtained by superimposing an AC voltage on a DC voltage is applied to the member. That is, since the surface potential is applied to the photoreceptor in advance by the DC voltage by the first contact charging member, when a voltage obtained by superimposing the AC voltage on the DC voltage is subsequently applied by the second contact charging member. As a result, the valley of the surface potential of the photoconductor is alleviated, and image fog is reduced. Further, with the above configuration, the second contact charging member applied with the AC voltage superimposed on the DC voltage is not contaminated by toner or the like.

Therefore, compared with a method in which a DC voltage is superimposed on a DC voltage to the first contact charging member and a DC voltage is applied to the second contact charging member, the surface potential of the final photosensitive member is more uniform. It is possible to extend uniform charging and extend the life.

Further, the present invention provides the first and second contact charging members such that the second contact charging member acts only on the area on the photoreceptor which has been charged by the first contact charging member. The operation start timing of the member is set.

When the photosensitive member surface between the first contact charging member and the second contact charging member is charged by the second contact charging member without being charged by the first contact charging member, Is that the surface potential of the photoreceptor rises rapidly and the current flowing into the photoreceptor becomes an overcurrent. However, in the present invention, the second surface is always applied only to the surface of the photoreceptor charged by the first contact charging member. Since the contact charging member is charged, the surface potential of the photoconductor can be stabilized without the surface potential becoming unstable.
That is, in the contact charging device of FIG. 1, when charging the photosensitive member, when a voltage is applied to the first contact charging member to charge the surface of the photosensitive member, the charged portion moves by the section 1 and the first contact charging member moves by the section 1. The section 1 is charged by both the first and second contact charging members by applying a voltage to the contact charging device 2 at the same timing when the contact charging member 2 reaches the opposing portion of the contact charging member.

Further, according to the present invention, the surface potential of the photosensitive member obtained by the charging action of only the first contact charging member is equal to or less than the first contact charging member.
And a voltage applied to the first and second contact charging members is set so as to fall within a range of 25% to 100% of a photoconductor surface potential obtained by a charging action of the second contact charging member. And

As the absolute value of the surface potential of the photosensitive member obtained by the charging action of the first contact charging member is increased, fog does not occur, but white spots tend to occur. Even if the absolute value is too small, a white point is generated.

On the other hand, in the present invention, the surface potential of the photosensitive member obtained by operating the first contact charging member as described above is changed to the surface potential of the photosensitive member obtained by both the first and second contact charging members. Is within the range of 25% to 100%, so that there is no image fog nor image defects due to white spots.
Good images can be obtained.

Further, the present invention is characterized in that a DC voltage applied to the first contact charging member is set in a range of -1200V to -500V.

According to the present invention, the absolute value of the DC voltage applied to the first contact charging member is large by setting the voltage applied to the first contact charging member within the range of -1200 to -500 V. White spots that occur when the absolute value is excessively small, and image fogging that occurs when the absolute value is too small, so that a good image without image defects can be obtained. it can.

The present invention also provides a voltage difference Vp between the maximum value and the minimum value of the AC voltage component applied to the second contact charging member.
-p is set in the range of 500V to 1200V.

The potential difference of the AC voltage component is from 500 V to 120
If the voltage is outside the range of 0 V, adverse effects occur. For example, if the potential difference is less than 500 V, white spots are likely to be generated in the image, and 1
When the voltage exceeds 200 V, white spots begin to appear again, and image fog worsens. Therefore, by setting the potential difference within the above range, both white spots and image fog can be suppressed.

In the present invention, the frequency of the AC voltage component applied to the second contact charging member is 200 to 1000 Hz.
Is set in the range of.

According to the present invention, the frequency of the AC voltage component applied to the second contact charging member is 250 to 1000H.
When z is within the range, both the cycle unevenness that occurs when the frequency of the AC voltage component applied to the second contact charging member is low and the charging noise that occurs when the frequency becomes high can be suppressed.

Further, the present invention is characterized in that the DC voltage applied to the first contact charging member and the DC voltage component applied to the second contact charging member are supplied from a common constant voltage power supply. And

According to the present invention, the space and cost of the charging device can be reduced by sharing the power source of the DC voltage supplied to the first and second contact charging members.

Further, the present invention is characterized in that the first and second contact charging members have fixedly disposed brushes, and are fixed brushes for charging the brush by bringing the brush into contact with the surface of the photoreceptor. And

According to the present invention, the size of the charging device can be reduced by using a fixed brush for the first and second contact charging members.

Further, the present invention is characterized in that the first and second contact charging members are rotating brushes which rotate and contact the brush with the surface of the photoreceptor to charge.

According to the present invention, by using a rotating brush for the first and second contact charging members, the contact portion to the photoreceptor is constantly renewed, so that the brush has excellent life characteristics, and And good image quality free of burnout is obtained.

Further, the present invention is characterized in that the rotary brush rotates on a contact surface with the photoconductor in a direction opposite to a moving direction of the photoconductor surface.

According to the present invention, by rotating the rotating brush in a direction (counter direction) opposite to the moving direction of the photosensitive member surface, image fog is reduced and the photosensitive member surface can be uniformly charged.

The present invention is further characterized in that the peripheral speed ratio of the rotating brush to the peripheral speed of the photoreceptor is set within a range of 1.8 to 2.2.

According to the present invention, the ratio of the peripheral speed of the rotating brush to the peripheral speed of the photosensitive member is set in the range of 1.8 to 2.2. Can reduce image fogging and brush strokes that occur when is small, and also can suppress the damage to the brush tips that becomes a problem when the peripheral speed ratio is too large, so that good image quality is obtained, and It is possible to extend the life of the rotating brush.

Further, the present invention is characterized in that the first contact charging member is a plate-shaped member.

According to the present invention, by using a plate-shaped member as the first contact charging member, it is possible to reduce the size of the contact charging device as compared with a device using a fixed brush or a rotating brush.

Further, the present invention is characterized in that the first contact charging member also serves as means for removing residual toner on the surface of the photoreceptor.

According to the present invention, since the first contact charging member also serves as a means for removing the residual toner on the surface of the photoreceptor, there is no need to separately provide a means for removing the residual toner on the surface of the photoreceptor. The size of the image forming apparatus can be reduced.

Further, the present invention is used in an electrophotographic image forming apparatus, which has a contact charging member and contacts the photosensitive member before forming an electrostatic latent image with the contact charging member to form a plurality of surfaces. In the contact charging device that charges the battery in different times, a plurality of the contact charging members are provided along the rotation direction of the photoconductor, and a contact charging member provided on the most downstream side in the rotation direction of the photoconductor has a direct current. A voltage obtained by superimposing an AC component on the component is applied, and a DC voltage is applied to each of the plurality of contact charging members disposed on the upstream side, and the absolute value of the DC voltage applied to each contact charging member is the downstream value. A contact charging device characterized in that the contact charging device is set so as to increase toward the side.

According to the present invention, a DC voltage is superimposed on an AC voltage to be applied to the contact charging member located at the most downstream side in the rotation direction of the photoreceptor, and the photoreceptor is charged before the surface is charged. Since the surface potential is applied to the photoreceptor by a DC voltage from a plurality of contact charging members located on the upstream side in the rotational direction of the photoreceptor, the valley of the surface potential of the photoreceptor is reduced, and image fog is reduced. In addition, since the pre-charging by the contact charging member on the upstream side is divided into a plurality of steps and performed in a stepwise manner, the surface potential of the photoreceptor rises rapidly and the current flowing into the photoreceptor is excessive, as in the case where the applied voltage is raised at a stretch. It becomes possible to further stabilize the surface potential of the photoconductor without causing the current to become unstable.

Further, by arranging a plurality of contact charging members to which a plurality of DC voltages are applied on the upstream side, the toner of the contact charging member to which the AC voltage is applied while being superimposed on the DC voltage located at the most downstream side is applied. Contamination can be prevented,
It is possible to extend the life of uniform charging of the photoconductor surface potential.

[0055]

FIG. 2 is a diagram showing a configuration of an image forming apparatus 40 using a contact charging device 2 according to an embodiment of the present invention. The image forming apparatus 40 includes the photosensitive drum 1
(Hereinafter, abbreviated as photoreceptor 1), around which a contact charging device 2, an exposure device 3, a developing device 4, a transfer roller 6, and a cleaning device 8 are arranged in this order.

In the image forming apparatus 40, the photosensitive member 1
Plays a role as an image carrier, and rotates in the direction of arrow A about a rotation axis L1 extending in a direction perpendicular to the paper surface of FIG. The contact charging device 2 contacts the photosensitive member 1 and charges the surface of the photosensitive member. The exposure device 3 irradiates light to form an electrostatic latent image on the surface of the photoconductor. The developing device 4 has a developing roller 5 that rotates in the direction of the arrow B about the rotation axis, applies toner to the surface of the photoconductor on which the electrostatic latent image is formed, and forms a toner image on the surface of the photoconductor. . The transfer roller 6 allows a sheet of paper 7 as a transfer material to pass between the toner image on the photoreceptor surface and the transfer roller 6 to transfer the toner image onto the sheet of paper 7. The cleaning device 8 has a cleaning blade 9 and removes toner remaining on the surface of the photoconductor by the cleaning blade 9.

Next, the image forming operation will be described. First, the pre-charging and the main charging are performed so that the surface of the photoreceptor 1 rotating in the direction of arrow A has a predetermined charging potential by the first and second charging brush rollers 10 and 11 of the contact charging device 2. Is carried out, and then the exposure device 2
The exposure according to the image information is performed by this to form an electrostatic latent image.

Next, the photoreceptor 1 on which the electrostatic latent image is formed is developed by the developing device 3 to form a toner image on the surface, and the toner image is transferred between the photoreceptor 1 and the transfer roller 5. Is transferred to a sheet conveyed so as to pass through. On the other hand, the residual toner is removed from the photoreceptor 1 after the transfer by the cleaning blade 8 of the cleaning device 7, and the photoreceptor 1 waits for the next image forming process.

FIG. 3 shows the image forming apparatus 40 according to the present invention.
FIG. 2 is a diagram illustrating a contact charging device 2 that charges a photoconductor surface. The configuration will be described with reference to the contact charging device 2 of FIG. The contact charging device 2 includes a charging brush roller 10 as a first contact charging member, a charging brush roller 11 as a second contact charging member, a constant voltage power supply 12, and an AC voltage power supply 13.
including.

A predetermined DC voltage is applied to the first charging brush roller 10 from a constant voltage power supply 12. A voltage obtained by superimposing an AC component on a DC component is supplied to the second charging brush roller 11 by a constant voltage power supply 12 and an AC voltage power supply 13.
More applied. As described above, the DC voltage applied to the first charging brush roller 10 and the second charging brush roller 1
By using a configuration in which the DC component of the superimposed voltage applied to 1 is applied from the common constant voltage power supply 12, space saving and cost reduction can be achieved as compared with a configuration in which a constant voltage power supply is separately provided.

The first and second charging brush rollers 1
Reference numerals 0 and 11 denote rotating brushes that rotate in contact with the photoconductor 1. The rotary type is superior to the fixed type in that the contact portion to the photoreceptor 1 is constantly updated by the rotating action, so that the brush itself has excellent life characteristics. The rotation direction of the first and second charging brush rollers 10 and 11 is a direction (counter direction C) which is opposite to the rotation direction A of the surface of the photoconductor at the contact surface with the photoconductor 1.
Is set to By rotating the brush in the counter direction C with respect to the rotation direction A of the photoconductor, the surface potential of the photoconductor 1 can be uniformly charged. As a result, image fog is reduced. Each of the first and second charging brush rollers 10, 11 is made of nylon, rayon, or the like, which is made conductive by adding carbon to a conductive support shaft made of iron, copper, stainless steel, aluminum, or the like. It is a configuration in which a cloth in which brush hairs are planted is wound.

Hereinafter, the contact charging device 2 will be described in more detail. As described above, the contact charging device 2 of the present embodiment applies a direct current to the first charging brush roller 10 located on the upstream side of the
, And thereafter, a voltage in which an AC component is superimposed on a DC component is applied to the second charging brush roller 11 located on the downstream side.

FIG. 4A shows the first charging brush roller 1.
It shows the surface potential of the photoconductor when the charging is performed using only the second charging brush roller 11 without performing charging (pre-charging) by 0. FIG. 4B shows the photoconductor surface potential when both the first and second charging brush rollers 10 and 11 are used.

A comparison between FIG. 4A and FIG. 4B shows that the valley of the surface potential is reduced when the AC voltage is applied on the DC voltage. For this reason, image fog is reduced.

Further, as compared with the prior art configuration in which the superimposed voltage is applied first and then the DC voltage is applied later, the superimposed voltage is applied to the second charging brush roller 11, so that contamination by toner or the like is caused. It can prevent fog and brushes from being emitted over a long period of use.

In the contact charging device 2, the photosensitive member 1
The second charging brush roller 11 that is located downstream of the rotation direction A only in the area of the photoreceptor surface that has been charged by the first charging brush roller 10 that is positioned upstream in the rotation direction A
The operation start timing is set so that. That is, the first and second charging brush rollers 1
Instead of applying the voltages 0 and 11 simultaneously, the second charging brush roller is activated when the photosensitive member charged by the first charging brush roller 10 reaches the facing portion of the second charging brush roller 11. The voltage application is started to 11.

Specifically, as shown in FIG. 2, the contact charging device 2 applies a voltage between the constant voltage power supply 12 and the first charging brush roller 10 to the first charging brush roller 10. A switch 30 for turning on or off the voltage is provided, and an AC voltage power supply 13 and a second charging brush roller 11 are provided.
A switch 31 for turning ON or OFF the voltage to be applied to the second charging brush roller 11 is provided between them. The switches 30 and 31 are opened and closed by applying a voltage to the second charging brush roller 11 when the photosensitive member charged by the first charging brush roller 10 reaches the opposing portion of the second charging brush roller 11. It is controlled to be applied.

With reference to the contact charging device 2 shown in FIG. 2 and the time chart shown in FIG. 5, the operation of charging the photosensitive member 1 will be described. The photoconductor 1 rotates in the direction of arrow A, and first, at time t1.
Then, the switch 30 of the first charging brush roller 10 is turned on, and a DC voltage is applied to the first charging brush roller 10. Thereafter, the switch 31 of the second charging brush roller 11 is turned on at time t2 when the portion of the photoreceptor surface charged by the first charging brush roller 10 reaches the opposing portion of the second charging brush roller 11. Then, a voltage obtained by superimposing a DC voltage and an AC voltage is applied to the second charging brush roller 11. When the charging operation is completed, the time t
At 3, the switches 30 and 31 are turned off at the same time. The time (t2-t1) for delaying the timing of starting the voltage application is determined by the distance between the two charging brush rollers and the peripheral speed of the photoconductor.

As described above, without applying a voltage to the first and second charging brush rollers 10 and 11 at the same time, the photosensitive member portion charged by the first charging brush roller 10
At the time when the charging brush roller 11 reaches the facing portion of
The first and second charging brush rollers 10 and 11 are configured such that a voltage is applied to the charging brush roller 11 of the first and second charging brush rollers.
Eliminates the problem of applying voltage to
It is possible to uniformly and stably charge the entire photoconductor.

Tables 1 to 5 show the experimental results when the frequency of the DC voltage, AC voltage or AC component applied to the contact charging device 2 was changed.

Table 1 shows that the target photosensitive member surface potential obtained by the operation of both the first and second charging brush rollers 10 and 11 in the contact charging device 2 is -750V, Of the fogging and white spots by changing the pre-charging value and examining how much potential is preferably applied to the photoreceptor surface in pre-charging performed only by the charging brush roller 10 of FIG. It is. In addition, to the second charging brush roller 11, a superimposed voltage having a voltage difference Vp-p between the maximum value and the minimum value of the AC component of 800 V and a frequency of 800 Hz was applied to a DC voltage of -750 V. The peripheral speed ratio between the photosensitive member 1 and the first and second charging brush rollers 10, 11 is 2.

[0072]

[Table 1]

From Table 1, it can be seen that as the pre-charging by the first charging brush roller 10 is increased, fogging does not occur. On the other hand, it can be seen that white spots are likely to appear in the image when the pre-charging potential is too low or too high. Thus, the surface potential of the photoconductor applied by the first charging brush roller 10 is 25% to 100% of the surface potential of the photoconductor after the main charging.
It is preferable to set so as to fall within the range of, more specifically, 80 to 95%.

Table 2 shows that in the contact charging device 2, the voltage applied to the first charging brush roller 10 was changed.
It is a result of examining the presence or absence of fog and white spots. The second charging brush roller 11 has a DC voltage of -750V.
The voltage difference Vp-p between the maximum value and the minimum value of the AC component is 800
A superimposed voltage with a frequency of 800 Hz was applied at V. Photoconductor 1
And the peripheral speed ratio between the first and second charging brush rollers 10 and 11 is 2.

[0075]

[Table 2]

From Table 2, it can be seen that as the voltage applied to the first charging brush roller 10 is increased, fogging does not occur. On the other hand, if the applied voltage is too low or too high,
It can be seen that white spots tend to appear in the image. Than this,
The voltage applied to the first charging brush roller 10 is −12.
In the range of 00 to -500 V, and more specifically -800 to
It is preferable to set so as to fall within the range of -1100V.

Table 3 shows that the contact charging device 2 changes the voltage difference Vp-p between the maximum value and the minimum value of the AC component of the superimposed voltage applied to the second charging brush roller 11 so that fog and white This is the result of examining the existence of points. In addition,
A DC voltage of -1100 V was applied to the first charging brush roller 10, the DC component of the second charging brush roller 11 was -750V, and the frequency was 800 Hz. Also,
Photoconductor 1 and first and second charging brush rollers 10, 1
The peripheral speed ratio of 1 is 2.

[0078]

[Table 3]

From Table 3, it can be seen that when the voltage difference is less than 500 V, white spots tend to appear in the image, and when it exceeds 1200 V, white spots begin to appear again and image fog worsens. Accordingly, it is preferable that the Vp-p voltage of the AC component applied to the second charging brush roller 11 is set to be in a range of 500 to 1200 V, and more specifically, in a range of 1000 to 1100 V.

Table 4 shows the results obtained by changing the frequency of the AC component of the superimposed voltage applied to the second charging brush roller 11 in the contact charging device 2 and examining the presence or absence of cycle unevenness and charging noise. Note that a DC voltage of -1100 V is applied to the first charging brush roller 10, the DC component of the second charging brush roller 11 is -750V, and V
The peak-to-peak voltage was 800V. The peripheral speed ratio between the photosensitive member 1 and the first and second charging brush rollers 10, 11 is 2.

[0081]

[Table 4]

From Table 4, when the frequency is 100 Hz or less, cycle unevenness (pitch unevenness of the charging brush) is likely to occur.
It can be seen that at a frequency of 1100 Hz or more, a charging noise is likely to be generated. Thus, the second charging brush roller 11
The frequency of the AC component applied to the
It is preferable that the frequency range is set so as to fall within the range of, more specifically, 300 to 800 Hz.

Table 5 shows that the photosensitive members 1 of the first and second charging brush rollers 10 and 11 in the contact charging device 2 were used.
5 shows the result of examining the presence or absence of fog, fogging, and fogging after aging by changing the peripheral speed ratio with respect to. The rotating direction of the rotating brush is the counter direction C. In addition, the first charging brush roller 10 has −110
0 V DC voltage is applied to the second charging brush roller 11
The DC component was -750 V, the Vp-p voltage of the AC component was 800 V, and the frequency was 800 Hz.

[0084]

[Table 5]

From Table 5, it can be seen that the ratio of the peripheral speed of the first and second charging brush rollers 10, 11 to the peripheral speed of the photosensitive member 1 is 1.
When the relative speed of less than 8 is not sufficient, the occurrence of image defects is remarkable, and as the relative speed increases,
The image quality is improved at 1.8 or more and less than 2.2, but 2.2
In the above, the first and second charging brush rollers 10, 11
Is accelerated, so that image fogging occurs after the printing test. Accordingly, the peripheral speed ratio of the first and second charging brush rollers 10 and 11 to the photosensitive member 1 is 1.8 to 2.0.
It is preferable to set the value so as to fall within the range of 2, more specifically, the range of 2 to 2.1.

FIG. 6 is a view showing a contact charging device 50 according to a second embodiment of the present invention. The contact charging device 50 of the present embodiment includes a fixed brush 1 serving as a first contact charging member.
4. It includes a fixed brush 15 as a second contact charging member, a constant voltage power supply 12, and an AC voltage power supply 13. Members common to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. The fixed brushes 14 and 15 are fixedly arranged on the photoconductor 1, and the ends of the brushes are in contact with the photoconductor 1.
As described above, since the first and second contact charging members are not the rotary brush type but the fixed brush type, the contact charging device can be downsized as compared with the rotary brush type because a rotating mechanism is not required. It is.

FIG. 7 is a view showing a contact charging device 60 according to a third embodiment of the present invention. The contact charging device 60 of the present embodiment includes a blade-type charging member 16 as a first contact charging member, a charging brush roller 11 as a second contact charging member, a constant voltage power supply 12, and an AC voltage power supply 13.
Members common to the first embodiment are denoted by the same reference numerals, and description thereof will be omitted. Blade type charging member 16
Is a rectangular and plate-shaped member having conductivity and elasticity, which is curved in a direction opposite to the rotation direction A and elastically folds on the photoconductor 1. Thus, the rotation direction A of the photoconductor 1
The first contact charging member disposed on the upstream side is a blade type, and the second contact charging member disposed on the downstream side is a rotary brush type. The size of the device can be reduced.

FIG. 8 is a view showing a contact charging device 70 according to a fourth embodiment of the present invention. The contact charging device 70 of the present embodiment includes a blade-type charging member 16 as a first contact charging member, a fixed brush 15 as a second contact charging member,
A constant voltage power supply 12 and an AC voltage power supply 13 are included. Also,
Members common to the first, second, and third embodiments are denoted by the same reference numerals, and description thereof is omitted. in this way,
The first contact charging member disposed on the upstream side in the rotation direction A of the photoreceptor 1 is a blade type, and the second contact charging member disposed on the downstream side is a fixed brush type. The size of the device can be reduced. Further, since the blade-type charging member 16 is curved in a direction opposite to the rotation direction A and resiliently folds on the photoconductor 1, the toner can be efficiently removed. Therefore, the cleaning blade 9 of the cleaning device 8 and the blade-type charging member 16 can be used in common, and with such a configuration, the entire image forming apparatus can be reduced in size.

FIG. 9 is a view showing a contact charging device 80 according to a fifth embodiment of the present invention. In the contact charging device 80 of the present embodiment, first and second charging brush rollers 20 and 21 are disposed in order from the upstream side in the rotation direction A of the photoconductor 1, and the third contact charging member is provided on the most downstream side. Is provided. In addition, it includes a constant voltage power supply 23 and an AC voltage power supply 24 for applying a voltage to the charging brush rollers 20, 21, 22. The first, second, and third charging brush rollers rotate in the counter direction C with respect to the rotation direction A of the photoconductor 1.

A plurality of different DC voltages can be taken out from the constant voltage power supply 23. A DC voltage -V1 is applied to the first charging brush roller 20 by the constant voltage power supply 23, and the second charging brush roller 21 is a DC voltage -V
2 is applied. A voltage obtained by superimposing a DC voltage and an AC voltage by a constant voltage power supply 23 and an AC voltage power supply 24 is applied to the third charging brush roller 22. Of the two pre-charging charging brush rollers, the charging brush roller is disposed downstream of the DC voltage -V1 applied to the first charging brush roller 20 disposed upstream in the rotation direction A of the photoconductor 1. The direct voltage -V2 applied to the second charging brush roller 21 is set higher. In this embodiment, a DC voltage of -800 V is applied to the first charging brush roller 20, a DC voltage of -1100V is applied to the second charging brush roller 21, and a third charging brush roller 22, which is a main charging step, is applied. An AC voltage having a Vp-p voltage of 800 V was superimposed on a DC voltage of -900 V.

FIG. 8 is a time chart showing the timing of voltage application in the configuration of the contact charging device 80 shown in FIG. First, time t1 is set by the charging brush roller 20.
To charge the photoreceptor surface. Next, the charging brush roller 2
0, a voltage is applied to the charging brush roller 21 at time t2 when the surface of the photoreceptor surface that has been charged by the charging operation reaches the opposing portion to the charging brush roller 21, and these two pre-charging charging brush rollers are further charged. At time t3 when the portion of the photoreceptor surface that has been pre-charged at 20 and 21 reaches the portion facing the second charging brush roller 22, the second charging brush roller 22
Will be applied to the superimposed voltage. When the operation is completed, at time t4, the voltages applied to the first, second, and third charging brush rollers 20, 21, 22 are simultaneously cut off.

Since the contact charging device 80 performs the pre-charging by the first and second charging brush rollers 20 and 21 in a stepwise manner, the surface potential of the photoreceptor is changed as if the applied voltage was raised at a stretch. Rises rapidly, and the current flowing into the photoconductor 1 becomes an overcurrent, so that the surface potential is not unstable, and the surface potential of the photoconductor can be further stabilized. Further, by increasing the number of chargers, it is possible to prevent the toner from contaminating the charging member on which the most downstream AC component is superimposed, so that a longer and more stable charging is possible.

[0093]

As described above, according to the present invention, a DC voltage is applied to the first contact charging member provided on the photosensitive member on the upstream side in the rotational direction, and the second contact member provided on the downstream side is provided on the photosensitive member. Since the surface of the photoconductor is charged by superimposing an AC voltage on the DC voltage on the contact charging member and applying a voltage to the photoconductor surface, the valley of the photoconductor surface potential is reduced, image fog is reduced, and the DC voltage is changed to the AC voltage. Is not contaminated by toner or the like, so that uniform charging of the surface of the photoconductor can be extended.

Further, according to the present invention, only the area of the surface of the photoreceptor charged by the first contact charging member is required
Since the operation timing of the first and second contact charging devices is set so as to be charged by the contact charging member, the surface potential of the photoconductor can be stabilized.

According to the present invention, the surface potential of the photosensitive member obtained by the charging operation of the first contact charging member is changed to the surface potential of the photosensitive member obtained by the charging operation of both the first and second contact charging members. Since it is set within the range of 25 to 100%, there is no image defect due to image fog and white spots,
Good images can be obtained.

According to the present invention, since the voltage applied to the first contact charging member is set in the range of -1200 to -500 V, a good image free from image fog and image defects due to white spots is obtained. be able to.

According to the present invention, the voltage difference between the maximum value and the minimum value of the AC voltage component applied to the second contact charging member is 5
Since the voltage is set in the range of 00 to 1200 V, a good image free of image defects due to image fog and white spots can be obtained.

According to the present invention, the frequency of the AC voltage component applied to the second contact charging device is 250 to 1000H.
Since it is set within the range of z, cycle unevenness and charging noise can be suppressed.

Further, according to the present invention, since the power source of the DC voltage supplied to the first and second contact charging members is shared,
The space and cost of the charging device can be reduced.

According to the present invention, since the fixed brush is used for the first and second contact charging members, the size of the charging device can be reduced.

Further, according to the present invention, since the rotating brush is used for the first and second contact charging members, the life characteristics of the brush are excellent, and a good image without brush stroke can be obtained. Further, according to the present invention, since the rotating brush is rotated in a direction opposite to the moving direction of the photoconductor surface, image fog can be reduced and the photoconductor surface can be uniformly charged.

According to the present invention, the ratio of the peripheral speed of the rotating brush to the peripheral speed of the photosensitive member is set in the range of 1.8 to 2.2, so that image fogging and brush stroke can be suppressed. Damage to the brush tips can be suppressed.

Further, according to the present invention, since a plate-like member is used as the first contact charging member, the size of the charging device can be reduced.

Further, according to the present invention, since the first contact charging member also serves as a means for removing the residual toner on the surface of the photoreceptor, the size of the image forming apparatus can be reduced.

Further, according to the present invention, the DC voltage is superimposed on the contact charging member located on the most downstream side in the rotation direction of the photosensitive member, and the AC voltage is applied to the contact charging member. The surface potential is applied to the photoreceptor with a DC voltage by a plurality of contact charging members located on the upstream side in the rotational direction of the photoreceptor, so that the valley of the surface potential of the photoreceptor is relaxed and image fog can be reduced. it can. Further, the preliminary charging by the contact charging member on the upstream side is performed stepwise in a plurality of steps, so that the photoconductor surface potential can be stabilized. Furthermore, since a contact charging device to which a plurality of DC voltages are applied is arranged on the upstream side, contamination by the toner of the contact charging member, which is applied by superimposing an AC voltage on the DC voltage located on the most downstream side, is prevented. Can be prevented. Therefore, uniform charging of the photoconductor surface potential can be extended.

[Brief description of the drawings]

FIG. 1 is a diagram showing a contact charging device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an image forming apparatus 40 using a contact charging device 2 according to an embodiment of the present invention.

FIG. 3 is a diagram showing a contact charging device 2 according to an embodiment of the present invention.

FIG. 4 shows a charging potential on a photosensitive member surface by the contact charging device 2 shown in FIG.

FIG. 5 is a time chart showing operation timings of the contact charging device 2 shown in FIG.

FIG. 6 shows a contact charging device 5 according to a second embodiment of the present invention.
FIG.

FIG. 7 shows a contact charging device 6 according to a third embodiment of the present invention.
FIG.

FIG. 8 shows a contact charging device 7 according to a fourth embodiment of the present invention.
FIG.

FIG. 9 shows a contact charging device 8 according to a fifth embodiment of the present invention.
FIG.

FIG. 10 is a time chart showing operation timing of the contact charging device 80 of FIG.

FIG. 11 is a view showing a contact charging device according to the related art.

FIG. 12 is a time chart for simultaneously applying a voltage to two contact charging members.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2,50,60,70,80 Contact charging device 10,11,20,21,22 Charging brush roller 12,23 Constant voltage power supply 13,24 AC voltage power supply 14,15 Fixed brush 16 Blade type charging member 30 , 31 switch

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Sakita 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Inside Sharp Corporation (72) Inventor Hiroo Naoi 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka F term in reference (reference) 2H003 AA12 BB11 CC04 CC06 DD03 DD16 EE11 2H027 DA02 DA03 DA06 DA50 DD09 EA01 EA18 ED02 ED03 EE02 EE03 ZA01

Claims (14)

[Claims] An image forming apparatus of an electrophotographic type, wherein first and second contact charging members are provided along a rotation direction of a photosensitive member, and the first and second contact charging members are provided on the photosensitive member before forming an electrostatic latent image. In a contact charging device for bringing the first and second contact charging members into contact with each other and charging the surface thereof a plurality of times, a DC voltage is applied to the first contact charging member located on the upstream side in the rotation direction of the photoconductor. And a voltage in which an AC component is superimposed on a DC component is applied to a second contact charging member located on the downstream side. 2. The operation of the first and second contact charging members so that the second contact charging member acts only on the area on the photoreceptor that has been charged by the first contact charging member. 2. The contact charging device according to claim 1, wherein a start timing is set. 3. The photoconductor surface potential obtained by the charging operation of only the first contact charging member is 25 to 100% of the photoconductor surface potential obtained by the charging operation of the first and second contact charging members. 2. The contact charging device according to claim 1, wherein a voltage applied to the first and second contact charging members is set so as to fall within the range. 4. The contact charging device according to claim 1, wherein a DC voltage applied to said first contact charging member is set within a range of -1200 V to -500 V. 5. A voltage difference Vp-p between a maximum value and a minimum value of an AC voltage component applied to the second contact charging member is 500.
The contact charging device according to claim 1, wherein the voltage is set within a range of V to 1200V. 6. The contact charging device according to claim 1, wherein a frequency of an AC voltage component applied to the second contact charging member is set in a range of 200 to 1000 Hz. 7. A DC voltage applied to the first contact charging member and a DC voltage component applied to the second contact charging member are supplied from a common constant voltage power supply. Item 7. The contact charging device according to Item 1. 8. The first and second contact charging members,
2. The contact charging device according to claim 1, further comprising a fixedly arranged brush, wherein the fixed brush is configured to charge the brush by contacting the surface of the photoreceptor. 9. The first and second contact charging members,
2. The contact charging device according to claim 1, wherein the rotating brush is a rotating brush that contacts the surface of the photoreceptor while rotating and charges the brush. 10. The contact charging device according to claim 9, wherein the rotating brush rotates on a contact surface with the photoconductor in a direction opposite to a moving direction of the photoconductor surface. 11. The contact charging device according to claim 9, wherein a ratio of a peripheral speed of the rotating brush to a peripheral speed of the photoconductor is set in a range of 1.8 to 2.2. 12. The contact charging device according to claim 1, wherein the first contact charging member is a plate-shaped member. 13. The contact charging device according to claim 12, wherein the first contact charging member also serves as a unit for removing residual toner on the surface of the photoreceptor. 14. An electrophotographic image forming apparatus having a contact charging member, wherein the contact charging member is brought into contact with a photosensitive member before forming an electrostatic latent image, and the surface thereof is divided into plural times. In the contact charging device, a plurality of the contact charging members are provided along the rotation direction of the photoconductor, and the contact charging member provided at the most downstream side in the rotation direction of the photoconductor has a direct current component and an alternating current component. A voltage in which the components are superimposed is applied, and a DC voltage is applied to each of the plurality of contact charging members disposed on the upstream side, and an absolute value of the DC voltage applied to each contact charging member is directed toward the downstream side. A contact charging device characterized in that the contact charging device is set so as to become larger as the size of the contact charging device increases.