JPH06250495A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device in which unevenness on an image is removed, stable electrostatic charging potential is maintained and further the production of ozone is reduced, and which is provided with a means for eliminating the non-uniformity of the electrostatic charging potential caused in the case of performing electrostatic charging by impressing AC voltage. CONSTITUTION:In this image forming device having a part where an electrostatic charging member 5 consisting of a conductive fiber or the aggregate of the conductive fiber comes into contact with a member to be electrostatically charged 1, and electrostatically charging the member 1 by impressing the superposed voltage of AC voltage and DC voltage, and, when it is assumed that the frequency of the AC voltage is (f), the moving speed of the member to be electrostatically charged as the process speed of the image forming device is Vp(mm/s), and the particle diameter of developer in the image forming device is R(mm), the AC frequency (f) is set to a value satisfying f>Vp/2R, or the member 5 is set as the 1st electrostatic charging member and the 2nd and succeeding electrostatic charging members 5B, on which the only the DC voltage is impressed, are provided on the downstream side of the member 5 one or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copying machine, a printer, and other image forming apparatus using an electrophotographic system for charging an electrophotographic photosensitive member, and more particularly, to improvement of charging means mounted in such an image forming apparatus. It is about.

[0002]

2. Description of the Related Art In an apparatus for forming an image by using a so-called electrophotographic method (Carlson process), a corona charging apparatus utilizing a corona discharge phenomenon is generally used to charge an electrophotographic photoreceptor to a desired potential. Has been used for a long time. However, in this method, due to the high voltage required to cause the discharge phenomenon, electrical noise to various peripheral devices or a large amount of ozone generated during discharge causes unpleasantness to the surrounding people. . Therefore, as an alternative to this corona charging device, a method has been proposed in which a voltage is applied between a conductive resin roller or a conductive fiber and a photoconductor to charge the photoconductor. However, when a conductive resin roller is used, for example, when the photosensitive layer in a minute region of the photosensitive member to be charged is peeled off and the conductive substrate such as Al is exposed, the current flowing from the roller is exposed. There was streak-shaped charging unevenness that was concentrated on the part and extended in the axial direction of the photoconductor.

Further, in the case of a charging device using conductive fibers, it can be roughly classified into two types, one for implanting fibers in a band and the one for implanting fibers in a roller, and in either case, Although the streak-like charging unevenness that occurs in the conductive resin roller is eliminated, for example, the charging member has a DC
When a (DC voltage) is applied, that is, when a DC electric field is generated between the charging member and the photoconductor, the charge potential of the photoconductor at high temperature and high humidity increases compared to that at room temperature and normal humidity.
Stable charging characteristics are not obtained. Further, in the charger, the charging potential was lowered from the beginning of use to the time of use, and the variation with time was large, and the charger was not put into practical use.

Therefore, as a method of solving the problem when only DC is applied, a method of superposing an AC voltage on a DC voltage has been proposed. However, even in this method, although the stability of the charging potential is improved for the time being, if consideration is not given to the selection of the frequency of the AC voltage, for example,
There has been a new problem that ripples caused by the applied AC voltage ride on the charging potential, and in some cases, this appears as a defect on the image. Therefore, for example, one of charging members made of a conductive fiber or an aggregate thereof is used, and a superposed voltage of DC voltage and AC voltage is applied to the charging member so as to face each other with a minute gap and a contact point. Consider the case where a member (here, an electrophotographic photosensitive member) is charged.

FIGS. 8A and 8B are schematic diagrams showing the principle of charging when a superposed voltage of DC voltage and AC voltage is applied to the photosensitive member by using a charging member made of conductive fiber.
(A) is an overall view thereof, and (b) is an enlarged view of the vicinity of the contact surface. In FIG. 8, reference numeral 1 is a photoconductor that is a member to be charged, and 5 is a charger that is a charging member.
A shows that A is flocked. As shown in FIG. 8, an arbitrary point A on the photoconductor 1 is a fiber 5A to which a voltage is applied.
When the applied voltage is higher than the discharge start voltage (Vth) determined by the photoconductor 1 and the air gap, the photoconductor 1 is charged by the discharge. start. Then, when the charging potential (Vsp) rises and the difference between the applied voltage (Vap) and (Vsp) becomes equal to (Vth), the discharge is stopped. That is, when the dark decay of the charging potential of the photoconductor is negligible, (Vsp) = (Vap) −
(Vth) holds. After that, the point A moves out to the position (position B) where it comes in contact with the conductive fiber 5A while leaving the discharge-allowing region while maintaining the charged potential. Needless to say, the potential difference between the conductive fiber 5A at the position B and the point A of the photoconductor 1 is (Vth) as described above, and due to this potential difference, the point between the conductive fiber 5A and the photoconductor 1 ( Electric charges are injected (moved) to A), and the charging potential at point (A) is further increased. That is, it can be seen that the charging potential is given by the discharge phenomenon and the charge injection phenomenon.

[0006] Japanese Patent Publication No. 3-52058 discloses an idea for the purpose of making the charging potential uniform by the contact charging method of the charging member and the charged member. However, here, the charging member is limited to the one formed of rubber in the shape of a roller or the shape of a pad, and does not mention the one in which conductive fibers are planted. That is, when a DC voltage is applied to the charging member, it is shown that the charging is started from the discharge start voltage as taught by Paschen. Therefore, there is no movement of the charge at the contact point between the charging member and the charged member. It is assumed that there is no charge, and it can be easily understood that all are charged by discharge. Therefore, a relatively large AC voltage value, which is equal to or more than twice the discharge start voltage that is the same as the charge start voltage, is applied between the two, and the charge potential is made uniform by using the discharge phenomenon (suppression of spot-like charge unevenness). ).

Further, regarding the contact charging method of the charging member and the member to be charged as described above, there is an idea of defining the frequency of AC charging for the purpose of removing uneven development, and the like.
4, JP-A-3-100675, JP-A-3-1017.
No. 64 and Japanese Patent Application Laid-Open No. 3-101765, all of them are those using a charging method as described in Japanese Patent Publication No. 3-52058. JP-A-3-100674 and 3-100675
The frequency regulation described in each of the gazettes is to reduce the vibration noise generated by applying an AC voltage and increase the number of discharges in the post-discharge region to converge the unevenness of the charging potential to make the charging potential uniform. It is to improve. In these techniques, the specified frequency range is set to "1000 Hz or less" in Japanese Patent Laid-Open No. 3-100674, and Japanese Patent Laid-Open No. 3-100.
In Japanese Patent No. 675, "1000 Hz or less, 2500 Hz or more, preferably 10 Hz or less, 10000 Hz or more" is set, which is largely different from the frequency application range in the present invention described later.

Japanese Unexamined Patent Publication (Kokai) No. 3-101764 also uses the charging method described in Japanese Examined Patent Publication No. 3-52058, and an image is generated when uneven charging occurs due to the influence of fluctuations in the power source or the like. This unevenness is reduced by defining the frequency of AC charging. Basically, these techniques achieve elimination of image unevenness by sufficiently increasing the number of times charges are exchanged due to a discharge phenomenon and converging unevenness of the charging potential. As described above, the charging mechanism is formed by injecting and discharging electric charge, regardless of whether the charging member is made of the conductive fiber or the charging member made of the resin-based material described in the prior art. If they are aligned, they will be sufficient.

It is quite conceivable that if an AC voltage is applied to the charging system having the above-mentioned mechanism, the AC voltage to be applied will be added to the charging potential due to the injection phenomenon at the contact portion. Of course, there is a possibility that the AC voltage applied to the charging potential may be added due to the discharge phenomenon. In any case, it is clear that it is caused by the applied AC voltage, and as a matter of fact, as will be described later, unevenness appears in the final image in accordance with the process speed and the period that can be calculated from the AC frequency. To summarize the above, for example, a charging member formed of a conductive fiber and a photoconductor are formed into a microscopic void by a discharging phenomenon and an injection phenomenon, and
Further, when charging is performed while having a contact surface, and when one charging member is used and at least the AC voltage is used as the applied voltage, the periodic image defect appearing in the final image is the applied AC voltage. It is due to.

Therefore, at least one other charging member (hereinafter referred to as the second charging member) is installed between the conventional charging member (referred to as the first charging member) and the developing tank.
It is possible to eliminate the ripple of the charging potential due to the AC voltage applied to the first charging member by the second and subsequent charging members. As described above, a method of charging a plurality of charging members while contacting them with the photoconductor is disclosed in Japanese Patent Laid-Open No. 56-912.
53, JP-A-62-143072, JP-A-4-16
It has already been proposed in Japanese Patent No. 867. Among these, the problem described in Japanese Patent Laid-Open No. 56-91253 is damage to the photoconductor, and the cause of this is that the charging member rapidly applies a charging potential. . Therefore, also in the solution which is the gist of the present invention, the DC voltage applied to the first charging member is set as low as 200 (V), and the DC voltage is gradually increased toward the second and third charging members. It is to go. Also, the peak-to-peak voltage of the AC voltage applied simultaneously is 20% of that of the DC voltage.
% Or less, that is, the value applied to the first charging member is 20
The value is as low as 0 × 0.2 = 40 (V) or less. Moreover, it is proposed to apply the AC voltage to the third charging member, which is the last charging member.

Further, the problems referred to in JP-A-62-143072 are exactly the same as the problems described in JP-A-56-91253, and a solution which is the gist of the invention. As the electric resistance value of the first charging member is set to the maximum value and gradually decreased to the second and third values, the first charging member is changed to the photosensitive member in the same manner as in JP-A-56-91253. It is intended to prevent damage to the photoreceptor by suppressing the applied potential to a low level. Japanese Patent Laid-Open No. 4-168
The problem to be solved in Japanese Patent No. 67 is to correct the non-uniformity of the charging potential due to the applied AC voltage. However, the solution to this problem, that is, the gist of the invention, is that in JP-A-4-16867, an AC voltage is applied to both the first charging member and the subsequent members, and the phase of the cycle of the charging potential by each member. This is a proposal to try to correct the nonuniformity of the potential by deviating. Also in this case, it is proposed to apply the AC voltage to the second charging member which is the last charging member.

[0012]

Therefore, in order to solve the above-mentioned instability of the charging potential due to the passage of time or environmental changes, a DC voltage and an AC voltage are applied between the charging member and the member to be charged. As described above, when the superposed voltage is applied, the stability of the charging potential is improved. However, as a result of the experiments conducted by the present inventors, a striped pattern orthogonal to the traveling direction of the paper appeared on the image when the frequency of the AC voltage was lower than a certain value. Moreover, this striped pattern appeared at almost the same period as the frequency of the AC voltage, and became more remarkable when the AC voltage was increased. This striped pattern depends on the frequency f of the AC voltage, the moving speed Vp (mm / s) of the charged member as the process speed of the image forming apparatus, and the particle diameter R (mm) of the developer of the image forming apparatus. It was clarified from the results of the experiments by the present inventors that the AC frequency f occurs when f <Vp / 2R.

Further, as described above, the present inventors proceeded diligently with the conventional means in which the second charging member is provided in addition to the first charging member and the correction effect by the second charging member is expected. However, by using a charging member made of a conductive resin and a general organic photoconductor, an AC voltage exceeding twice the discharge starting voltage is obtained.
When the voltage is applied, the AC peak-to-peak voltage is increased, and accordingly, the injected AC voltage is also increased, and naturally the voltage applied to the second charging member that corrects this must also be increased, and It has been found that there is a problem in that the charging potential increases according to the peak-to-peak voltage without being restricted by the DC voltage applied to the first charging member. Therefore, an object of the present invention is to provide an image forming apparatus that eliminates unevenness on an image, maintains a stable charging potential, and further reduces the generation of ozone, and further has at least a portion that comes into contact with a portion to be charged. Another object of the present invention is to provide an image forming apparatus provided with a means for solving the non-uniformity of the charging potential caused when the AC voltage is applied and charging.

[0014]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the first one of the gist thereof is a conductive fiber or an aggregate of the conductive fibers. While having at least a portion where the charging member and the member to be charged are in contact with each other and also having a portion facing each other with a minute gap, an AC voltage and a DC voltage are applied between the charging member and the member to be charged. In an image forming apparatus that applies a superimposed voltage to charge a charged member, the frequency of the AC voltage is f, the moving speed of the charged member as the process speed of the image forming apparatus is Vp (mm / s), and In the image forming apparatus, the AC frequency f is set to a value satisfying f> Vp / 2R, where R (mm) is the particle size of the developer of the image forming apparatus.

A second aspect of the present invention is characterized in that it has at least a portion where a charging member formed by implanting conductive fibers or an aggregate thereof and a member to be charged are in contact with each other, and is minute. In the image forming apparatus for charging a charged member by applying a superposed voltage of AC voltage and DC voltage between the charging member and the charged member while having a portion facing each other in the gap, Is the first charging member, and at least one or more second and subsequent charging members to which only the DC voltage is applied are provided on the downstream side thereof.

In the second aspect of the present invention, the DC voltage applied to the first charging member is equal to the desired charging voltage, and the DC voltage applied to the second and subsequent charging members is the same. Is equal to or higher than the DC voltage applied to the first charging member, or the peak-to-peak voltage of the AC voltage applied to the first charging member is the charged member and the gap. It is smaller than the value of twice the discharge starting voltage determined by the atmosphere inside, or further, the contact area between the first charging member and the charged member causes the contact of the second and subsequent charging members. It is effective that the area is larger.

Further, in the first and second aspects of the present invention, the charging member is made of conductive fibers, or an aggregate of the conductive fibers, which are flocked in a belt shape or a roller shape. In addition, or in addition, the charging member is a conductive fiber or an aggregate of the charging members, which is flocked in a roller shape, and the charging member makes a rotational motion, and the peripheral speed thereof is the charged member. The moving speed of the charged member is not the same, or in addition to this, the charging member is a conductive fiber or an aggregate thereof that is flocked in a strip shape and is parallel to the moving direction of the charged member. It is effective to vibrate in directions other than the above.

[0018]

In the present invention, the potential fluctuation due to the environmental change in the charging means adopted in the present invention, which is formed by the discharge phenomenon and the charge injection phenomenon, is mainly caused by the charge injection phenomenon. That is, the charging means in the present invention is not intended to cause bidirectional movement of charges due to the discharge phenomenon, but rather the phenomenon of charge injection (movement phenomenon) from both the charger and the photoconductor on the contact surface. It is not necessary to set the peak-to-peak voltage of the AC voltage to be applied to be twice the discharge starting voltage or more as in Japanese Patent Publication No. 3-52058 described above. For this reason, the power source of the charger can be made inexpensive, and an arbitrary AC voltage can be adopted. Here, the "AC voltage to be applied" has the same value as the "AC voltage applied between the tip of the electrically conductive fiber as the charging member and the member to be charged".

Here, the charging means and the Japanese Patent Publication No.
9 and FIG. 1 show how the charging means in the technology of Japanese Patent Laid-Open No. 52058 differs in the process of forming the charging potential.
It will be described using 0. First, an area (hereinafter referred to as a pre-discharge area) in which a gap between the charging member (for example, a roller or a brush) and a member to be charged (photoreceptor) becomes small and a discharge occurs between the charging member and the member to be charged according to Paschen's law. In the contact area between the charging member and the charged member, a charging potential having unevenness as shown in FIGS. 9 and 10 is formed on the charged member. However, in Japanese Examined Patent Publication No. 3-52058, charges are exchanged between the charging member and the member to be charged due to the discharge phenomenon in the pre-discharge region, and no charges are exchanged in the contact region. On the other hand, in the charging means of the present invention, charges are exchanged between the charging member and the member to be charged by the charge injection phenomenon in the contact area.

Then, in a region where the charging member and the charged member are separated from each other, discharge again occurs between the charging member and the charged member according to Paschen's law (hereinafter referred to as a post-discharge region). Japanese Patent Publication No. 3-52058 discloses that in this region, the amount of charges exchanged between the charging member and the member to be charged due to discharge is reduced, and the unevenness of the charging potential is canceled to obtain a uniform charging potential. On the other hand, in the charging means of the present invention, this post-discharge does not cause sufficient post-discharge to sufficiently eliminate the unevenness of the charging potential due to the charge injection in the contact region, leaving the unevenness of the charging potential due to the charge injection. Will end up. When the charging potential of the member to be charged is microscopically observed, the unevenness of the potential is formed as shown in FIG. Further, the interval between the irregularities is f for the frequency of the AC voltage and Vp (mm for the process speed of the image forming apparatus of the present invention.
/ S) was Vp / f (mm).

In the present invention using such a charging means, when an image is actually output, a line-shaped image unevenness that is perpendicular to the paper traveling direction appears on the image, and this image unevenness is a process speed. It was found that the photoconductor had a spatial wavelength (Vp / f) determined by the moving speed (Vp) of the photoconductor and the frequency (f) of the alternating voltage. Further, as the peak-to-peak voltage of the AC voltage was increased, streak-like unevenness became more prominent. This fact is that in the charging means of the present invention described above, the unevenness of the charging potential corresponding to the spatial wavelength (Vp / f) on the member to be charged when the frequency f of the alternating voltage and the process speed (Vp) is set to the post discharge. That is, the area where the charging potential is low is developed and becomes streaky unevenness.

In order to solve this problem, the inventors of the present invention have conducted experiments. As a result, the frequency of the AC voltage is f, the moving speed of the charged member as the process speed of the image forming apparatus is Vp (mm / s), Further, when the particle diameter of the developer of the image forming apparatus is R (mm), by setting the AC frequency f to a value satisfying f> Vp / 2R, it is possible to eliminate the unevenness on the image. . This is because the interval (Vp / f) of the unevenness of the charging potential formed on the member to be charged is large when the frequency f of the alternating voltage is low, as described in the description of the charging means in the present invention. The resolution of the apparatus, which is determined by the particle size of the developer in the forming apparatus, sufficiently follows up to form streaky image unevenness on the image. But,
When the frequency f of the alternating voltage increases, the interval between the irregularities of the charging potential decreases, and the particle diameter R of the developer of the image forming apparatus increases.
When the length 1/2 × (Vp / f) value on the member to be charged in the region of low potential in the unevenness of charging potential is smaller than (mm), the developer adheres to the region of low potential. No image unevenness appears on the image.

Further, in the present invention, the non-uniformity of the charging potential generated by the first charging member and caused by the applied AC voltage is corrected by the injection voltage by the DC voltage applied to the second charging member. Is what you are trying to do. Of course, if there is a potential difference larger than the voltage permitting the discharge between the photoconductor and the second charging member, it is naturally possible to correct the potential by the first charging member due to the discharge phenomenon. From the above description, DC and AC are used for the first charging member, which is the main charging system.
It is necessary to apply the superposed voltage of, and it is necessary to separate a minute gap for exciting discharge from the member to be charged. Further, in the second charging system, the non-uniformity of the potential due to the first charging system is to be corrected by DC at least by the injection phenomenon. Therefore, the DC voltage applied to the second charging system is the first. It can be easily understood that it is necessary to be equal to or more than the applied value of the charging system.

According to the experiments conducted by the present inventors, this injection phenomenon has a certain time constant. Naturally, the injection potential Vin after Ti (sec.) After the voltage is applied is Vin.
j. Is increasing as Ti (sec.) Increases. Here, since the time for which injection is allowed is equal to the time during which the charging member and the member to be charged are in contact, the contact time between the second charging member and the photoconductor is relative to the contact time between the first charging member and It is desirable to set a large value. Further, judging from only the above description, the AC peak voltage value applied to the first charging member is not more than twice as large as the AC voltage having the peak-to-peak voltage that is twice or more the discharge starting voltage. It is considered that the correction effect by the DC voltage applied to the second charging member appears exactly the same as when the voltage is applied.

However, as described above, when an AC voltage exceeding twice the discharge start voltage is applied using a charging member made of a conductive fiber and a general organic photoreceptor, as the AC peak-to-peak voltage increases, The DC voltage applied to the second charging member for correcting this also has to be increased, and the charging voltage increases according to the peak-to-peak voltage without being regulated by the DC voltage. It has been found that there is a problem that it cannot be controlled, and as a result, the peak-to-peak voltage of the AC voltage applied to the first charging member is 2 times the discharge start voltage.
It is desirable to set it to a value equal to or less than double the value.

[0026]

Embodiments of the present invention will be described below with reference to the drawings. First, FIG. 1 is a schematic explanatory view showing an overall configuration of an image forming apparatus of the present invention, and FIG. 1 (a) is a front view of an embodiment in which a charging member made of a conductive fiber is used alone, and FIG. [Fig. 4] is a front view of a main part of an embodiment using the first and second charging members. The case where the conductive fibers of the charging member are flocked into a roller will be described. First, in FIG. 1, reference numeral 16 is a controller for processing image forming data from a host computer (not shown), and 17 is an engine for receiving a signal for starting image formation from the controller 16 and controlling start of driving the image forming apparatus. The controller. Reference numeral 7 is a cassette of a transfer material such as copy paper, and 8 is a paper feed roller for pulling out the transfer material, which is arranged so as to be conveyed to a registration roller 11 following a series of conveyance rollers 9 and 10.

Reference numeral 1 denotes a photosensitive drum having a photosensitive layer 1a on its surface, which is rotated at a constant speed in the clockwise direction in FIG. 1 by a driving means (not shown). Furthermore, around the photosensitive drum 1,
In FIG. 1A, the charging device 5 mainly includes the conductive fiber 5A in the clockwise direction, and in FIG. 1B, the charging device 5 and 5B, the exposure writing head or exposing device 6, the developing device 2, and the transfer roller. A transfer device 3 and a cleaner 4 are arranged respectively. Next, regarding the developing device 2, 2e is a toner tank equipped with an agitator roller 2a, and 2f is a developing tank, which is a magnet roller 2 for charging the developer.
d, and the toner supply roller 2 from the toner tank 2e.
It has a mixer roller 2c for stirring the developer supplied in b.

As for the cleaner 4, a cleaning blade 4a for scraping off the developer on the surface of the photosensitive drum 1 and a toner screw 4b for sending the scraped developer into a container (not shown) for discarding the scraped developer. A cleaning unit is mainly configured. On the other hand, as a fixing unit for the copy sheet that has passed between the transfer unit 3 and the photoconductor drum 1, the fixing unit 12 is composed of a heat roller 12a having a heater 12c built therein and a pressure roller 12b. For the object, the stack guide 15 is provided via the transport roller 13 and the paper discharge roller 14.
Configured to be sent to.

Next, the operation of the apparatus according to the embodiment of the present invention shown in FIG. 1 will be described with reference to the drawings. First, data relating to image formation from a host computer (not shown) is processed by the controller 16. Then, an image formation start signal is sent to the engine controller 17. From this, the operation proceeds according to a predetermined process. The transfer materials stored in the transfer material cassette 7 are drawn out one by one by the paper feed roller 8 and conveyed by the transport rollers 9, 10.
As a result, the sheet is conveyed to a position before the registration roller 11. The photoconductor 1 rotates at a constant speed by a rotation mechanism (not shown). In this case, in FIG. 1A, the charging roller 5 rotates at a constant speed, for example, in the direction opposite to the photoconductor 1, and in FIG. 1B, the first charging roller 5 and the second charging roller 5 are rotated. 5B also rotates at a constant speed in the direction opposite to that of the photoconductor 1, for example.

The charging roller 5 of FIG. 1 (a) used here is used.
As shown in the schematic explanatory view of FIG. 2, the charging rollers 5 and 5B shown in FIG. 1B each have a conductive charging roller shaft 5c having a radius of about 3 mm, and the amount of carbon dispersed in rayon is adjusted, for example. A fiber whose resistance value is adjusted to a desired value or a conductive fiber cloth 5a in which the aggregate is planted is wound around a rotary shaft. These charging rollers 5 or 5, 5B are connected to a roller driving motor 5b and rotate. The photoconductor 1 used is a conventional organic material type photoconductor (OPC). On the other hand, in the developing unit 2, the toner on the magnet roller 2d is appropriately sent from the toner tank 2e to the developing tank 2f by the supply roller 2b so as to be agitated by the mixer roller 2c so that the toner has a predetermined density. At this time, the toner is charged to have the same polarity as the photoconductor charging potential. When a value close to the charging potential of the photoconductor 1 is applied to the magnet roller 2d, the toner adheres to the portion irradiated by the exposure writing head 6 and is developed.

Next, the transfer material is conveyed by the registration roller 11 at a timing corresponding to the image position of the photosensitive member 1. The transfer material is nipped and conveyed by the photoconductor 1 and the transfer roller 3. At this time, a voltage having a polarity opposite to that of the toner is applied to the transfer roller 3. Therefore, the toner on the photoconductor 1 is transferred onto the transfer material. The toner on the transfer material is nipped by a heat roller 12a and a pressure roller 12b that internally include a heater 12c, and the toner is fused and fixed on the transfer material during this period. The transfer material is sent to the stack guide 15 by the transport roller 13 and the supply roller 14. On the other hand, photoconductor 1
The toner that has not been transferred is scraped off from the photoconductor 1 by the cleaning blade 4a of the cleaning unit 4 and sent to a not shown container for toner disposal by the toner screw 4b. This completes the series of image forming steps. When measuring the charged potential of the photoconductor without forming an image, a potential measuring probe is installed at the position of the developing tank.

2A and 2B, the schematic illustrations of FIGS. 2A and 2B will be explained again. When the conductive fibers 5A are planted in a roller shape, as shown in FIG.
Since the belt-like fiber cloth 5a having a narrow width, in which the non-flocked region 5D exists around the flocked region, is wound around the power supply shaft 5C, the winding interval 5 without the conductive fiber is inevitable.
E will occur. If the roller 5 and the photoconductor 1 rotate at the same speed (relative speed is 0) here, the roller 5
Since the same surface of the photoconductor 1 and the photoconductor 1 always face each other,
The points on the surface of the photoconductor facing the winding interval 5E cannot be charged, resulting in uneven charging. Therefore, when the conductive fiber 5A is formed into a roller shape, it is desirable that the peripheral speeds of the roller 5 and the photoconductor 1 be different values (the relative speed is not 0). For example, in FIG. 3, the rotational directions of the two are opposite to each other so that the difference between the peripheral velocities of the two is as large as possible. Note that FIG. 3 is an explanatory diagram showing an example of the dimensional relationship between the photoconductor 1 and the charging roller 5.

Further, when the conductive fibers 5A are planted in a flat shape as shown in FIG. 4, the structure is simple as compared with the case where the conductive fibers 5A are rotated in a roller shape, but the same portion of the fibers 5A is always the photosensitive member. Since the fibers 5A are in contact with each other, the fibers 5A are abraded, or the developer is attached to the tips of the fibers 5A, so that the charging failure occurs at those parts. Therefore, as shown in FIG. 4, it is desirable to vibrate the photoconductor 1 so that it is perpendicular to the rotation direction. That is, FIG. 4 is a schematic explanatory diagram showing a state in which the charging member 5 formed by planting the conductive fibers 5A in a flat shape is brought into contact with the photoconductor 1 to be used, and the charging member 5 rotates the photoconductor 1. In the direction perpendicular to the direction, the arrow S
As indicated by, the surface of the photoconductor 1 is vibrated. In FIG. 1B, the case where both the first and second chargers are roller-shaped is described as an example, but the effect of the present invention is not limited to this, and a plurality of belt-shaped chargers are used. It goes without saying that it is also exhibited in combination with a charger and a roller-shaped object.

Next, the moving speed Vp of the photosensitive member 1 of the image forming apparatus in this embodiment is 52.4 mm / s, and the particle diameter R of the developer loaded in the developing device 2 is 15 (μm). In such a configuration, the voltage condition applied to the charging roller 5 is as follows: DC voltage VDc = -650V, AC voltage Vp-p = 900V, frequency = 100
Hz, frequency condition: f <Vp / 2R DC voltage VDc = -550V, AC voltage Vp-p = 1500V, frequency = 100
Hz, frequency condition: f <Vp / 2R DC voltage VDc = -650V, AC voltage Vp-p = 900V, frequency = 2000
Hz, frequency condition: f> Vp / 2R DC voltage VDc = -550V, AC voltage Vp-p = 1500V, frequency = 2000
Hz, frequency condition: f> Vp / 2R Only DC voltage applied: VDc = -1050V was set, and image evaluation and charge potential measurement were performed.

(A) Result of measurement of charging potential The charging potential was measured under the above conditions (1) to (3). As a result, the change over time in the charging potential was almost eliminated compared to when only the DC voltage under the conditions was applied.
A uniform charging potential of 0 V was observed. Also, the change in charging potential due to environmental changes was significantly reduced compared to the case where only DC voltage was applied. The expression "uniform charging potential" is used here, because the spatial resolution of the probe for measuring the charging potential (3 mm) is such that when the frequency of the AC voltage is 100 Hz, the unevenness of the charging potential on the photoconductor is used. Spatial wavelength of (5
2.4 / 100 = 0.52 mm) is too large, and the output value is a value obtained by averaging the unevenness of the charging potential.
This is because the potential is apparently uniform.

Therefore, at the time of measuring the charging potential, the value of current flowing into the photosensitive drum was measured. The current value observed here showed a Sin waveform centered on 0 as shown in FIG. As shown in FIG. 6, this current flows through the brush 5 / comprising capacitors C ₁ , C ₂ , C ₃ and resistors R ₁ , R ₂ , R _3.
C of each part in the equivalent circuit between the contact interface B / photoreceptor 1
It is considered that this is an AC injection current flowing through the component, and the size of the unevenness of the charging potential formed on the photoconductor 1 by the AC injection current can be obtained by obtaining the capacitance C ₃ of the portion of the photoconductor 1.

In this embodiment, the contact area S between the photosensitive member and the brush is S.
Is 220 × 5.8 mm ² , and the relative permittivity ε _r of the photoconductor is 3.1.
3 and the film thickness d is 20 μm. When the peak value of the current value is io and the frequency of the applied voltage is f, the fluctuation range ΔV of the charging potential is

[Equation 1] It is considered that the actual charging potential is −550V ± ΔV / 2. When the ΔV / 2 value under the above conditions was calculated as described above, the Vsp value under each condition was as follows. Condition: Vsp≈−550 ± 70V Condition: Vsp≈−550 ± 320V Condition: Vsp≈−550 ± 60V Condition: Vsp≈−550 ± 250V

(B) Image Evaluation Results Next, image evaluation was carried out under the above applied voltage conditions. In addition,
As a pattern to be printed, in order to see the stability of the charging potential before exposure, a pattern that was entirely white was used. In the image forming apparatus used in this embodiment, a charging potential of about −550 V is preferable for making an output image white, and a charging potential higher than this causes the carrier of the developer to rise. Further, it is known that the developing is carried out to a density which can be visually recognized at a charging potential lower than this, for example, -500V. The results of image evaluation using such an image forming apparatus will be described below. For comparison, an image to which only a DC voltage was applied, in which unevenness of the charging potential due to an AC voltage did not occur, was also evaluated. When only DC is applied. : Almost no unevenness is observed on the image.

Condition: Black streak (BL) -like image unevenness appeared on the image as shown in FIG. 7 at right angles to the traveling direction of the paper. Considering the value of the charging potential described in the item (a), the minimum charging potential is − (550−70) = − 480V. This area width is 52.4 (mm / s) / (100 (H
z) × 2) = 262 μm, which is larger than the minimum line width of 15 μm that can be developed with the developer (particle size 15 μm) used in this example, and is a line width that can be sufficiently developed. Therefore, it is considered that such black streaks BL appeared. The interval between the black streaks BL coincided with the interval (Vp / f) between the unevenness of the charging potential formed on the photoconductor. Condition: The same image unevenness appeared under these conditions. However, at this time, the black streak BL described in FIG. 7 is more intense, and as can be seen from the result of (a),
The potential of the image uneven portion is-(550-320) =-
It is understood that this is because the voltage is 230 V, which is lower than that under the condition. That is, at the frequency of this condition, when the peak-to-peak voltage of the AC voltage is increased, the unevenness of the charging potential becomes more remarkable.

Condition: Under this condition, the unevenness on the image was the same as the condition where only the DC voltage was applied. Considering the result of (a), the minimum charging potential is-(550-6
0) = − 490 V, and the charging potential has dropped to the level at which development is performed. However, when the frequency of the applied voltage at this time is 2 kHz, the area width in which the charging potential drops is 52.4.
(Mm / s) / (2000 (Hz) × 2) = 13.1 μm
Is. This value is the value of the developer (particle size 15) used in this example.
It is smaller than the minimum line width of 15 μm that can be developed by (μm), and it is shown that the unevenness of the charging potential due to the AC voltage does not affect the image under this condition. Condition: The peak-to-peak voltage of the AC voltage is made higher than that of the condition, and the minimum charging potential under this condition is-(550-250) =-300 V from the result of (a), and the charging potential drops to a level where sufficient development is possible. ing. However, even under this condition, an image similar to that when only the DC voltage is applied is obtained, and it can be seen that the unevenness of the charging potential due to the AC voltage does not affect the image by satisfying the conditions of the present invention.

Next, an embodiment of the present invention using a plurality of charging members will be described. The charging member used was one in which conductive fibers were planted in a roller shape, and the first charging member had a DC voltage of −550 (V) and a peak-to-peak voltage of 1050.
A superposed voltage of an AC voltage of (V) is applied, only the DC voltage is applied to the second charging member, and its value is set to −550 (V), which is the same value as that of the first charging member. Here, a value of 574 (V) determined by Paschen's discharge law is used as the discharge start voltage. The charging potential in this case is
It is obtained by following the order like the charging process as follows. Charge potential V1 V1 =-(550 + 1050/2) -Vth = (1075-57) due to the discharge phenomenon in the first charging member
4) =-501 (V) Charging potential V2 after receiving the injected charge at the contact portion of the first charging member V2 = V1 ± ΔV = −501 ± ΔV where the charging potential due to the AC voltage The ripple of Δ
It was set to V.

On the other hand, as the constitution of the charging member 5 used,
As described above with reference to FIG. 2, a charging member 5 having brush-shaped fibers (trade name: REC) in which conductive carbon is dispersed in rayon is pressed into the photosensitive member 1 with a pressure of 1 mm to charge it. In this case, when the peak-to-peak voltage of the AC voltage is applied at 1050 (V) as in the above condition, the maximum value of the injection voltage (ΔV) at the contact portion is 65 (V). Was confirmed. That is, V2 = −501 ± 65 (V) under the above conditions. Here, when the image is developed with the charged potential of V2 and the image is output, the image is developed at the minimum value of V2 depending on the development bias value, and on the final image, as shown in FIG. The black streak BL corresponding to appears. In fact, when a general value of −350 (V) is set as the developing bias value, the above-mentioned minimum value of V2 = −436 (V),
The black streaks BL corresponding to this appear at a density of a level that can be visually discriminated.

On the other hand, in the case of the present invention in which the second charging member is provided, in the charging area of the second charging member, the potential V2 given by the first charging member is maintained. To reach. A DC voltage (-550) is applied to the second charging member.
(V)) is applied, so the minimum value with the previous V2 −
Between 436 (V), 550-436 = 114
A potential difference (V) is generated, and the potential difference causes a charge to be injected from the charging member at the contact portion between the second charging member and the photoconductor to raise the minimum charging potential of the photoconductor. As a result, the black streak BL caused by the AC voltage on the final image, which appears when only the first charging member is used, is eliminated.

For comparison, only the first charger is used as the charging member, and the voltage applied to this is DC =
-625 (V), AC (voltage between peaks) = 900
(V) and frequency = 800 Hz, the final image has periodic black streaks BL due to the AC voltage as shown in FIG.
Was appearing. On the other hand, the pushing pressure is made stronger than the first charging member between the developing tank and the first charging member,
A second charging member having a large contact area with the photoconductor is installed, and DC = the same value as that applied to the first charging member.
It was confirmed that when −625 (V) was applied, black streaks BL on the final image as shown in FIG. 7 were reduced at least to the extent that they could not be visually confirmed.

[0045]

As described above, when the charging method adopted in the present invention is used, the frequency of the AC voltage is f, and the moving speed of the charged member as the process speed of the image forming apparatus is Vp.
(Mm / s), and the particle size of the developer of the image forming apparatus is R
(Mm), by setting the AC frequency f to a value satisfying f> Vp / 2R, it is possible to eliminate unevenness on the image. Further, by applying the DC / AC superimposed voltage, it is possible to suppress the temporal change of the charging potential and the environmental change, and it is possible to suppress the generation of ozone by the direct charging method.
Further, by providing the second and subsequent charging members to which only the DC voltage is applied on the downstream side of the first charging member, it becomes possible to eliminate the image unevenness caused by the AC voltage appearing on the final image. is there.

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram showing an overall configuration of an embodiment of an image forming apparatus of the present invention.

FIG. 2 is a schematic explanatory diagram showing a configuration of a charging roller used in the present invention.

FIG. 3 is an explanatory diagram showing an example of a dimensional relationship between a photosensitive member and a charging roller used in the embodiment.

FIG. 4 is a schematic explanatory diagram showing a body condition of a charging member in which hair is formed in a flat shape.

FIG. 5 is a curve showing an example of a current waveform.

FIG. 6 is an equivalent circuit diagram of a charging roller (brush) / contact interface / photoreceptor.

FIG. 7 is a schematic diagram of an output image having image unevenness.

FIG. 8 is a schematic diagram showing a charging principle when a DC / AC voltage is superimposed by using conductive fibers.

FIG. 9 is a schematic view showing an example of a conventional charging potential forming process.

FIG. 10 is a schematic diagram showing the principle of the charging potential forming process in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photoconductor 2 Developing device 3 Transfer device 4 Cleaner 5 Charging device (first charging member) 5A Conductive fiber 5a Conductive fiber cloth 5B Second charging member 5c Charging device shaft 6 Exposure device 7 Transfer material cassette 8, 9, 10 , 13 paper feed roller 11 registration roller 12 fixing device 14 paper ejection roller 15 stack guide 16 controller 17 engine controller

Claims (8)

[Claims] 1. An electrically conductive fiber, or at least a portion where a charging member formed by flocking an aggregate thereof and a charged member are in contact with each other, and also having a portion facing each other with a minute void, In an image forming apparatus for applying a superimposed voltage of an alternating voltage and a direct current voltage between the charging member and the charged member to charge the charged member, the frequency of the alternating voltage is f, and as a process speed of the image forming apparatus. The moving speed of the charged member of Vp (mm /
s), and the particle size of the developer of the image forming apparatus is R (m
m), the AC frequency f is set to a value satisfying f> Vp / 2R. 2. A conductive fiber, or at least a portion where a charging member formed by implanting the aggregate thereof and a charged member are in contact with each other, and also having a portion facing each other with a minute void, In an image forming apparatus for charging a charged member by applying a superimposed voltage of an AC voltage and a DC voltage between the charging member and the charged member, the charging member is a first charging member, and a DC voltage is provided downstream of the charging member. An image forming apparatus comprising at least one charging member after the second, to which only a voltage is applied. 3. The DC voltage applied to the first charging member is equal to the desired charging voltage, and the DC voltage applied to the second and subsequent charging members is the DC voltage applied to the first charging member. The image forming apparatus according to claim 2, wherein the voltage is equal to or higher than the voltage. 4. The peak-to-peak voltage of the AC voltage applied to the first charging member is smaller than twice the discharge start voltage determined by the member to be charged and the atmosphere in the gap. The image forming apparatus according to claim 2. 5. The image forming apparatus according to claim 2, wherein the contact area of the second and subsequent charging members is larger than the contact area of the first charging member and the charged member. 6. The image forming apparatus according to claim 1, wherein the charging member is formed of conductive fibers or an aggregate of the conductive fibers, which are flocked into a band shape or a roller shape. 7. The charging member is one in which conductive fibers or an aggregate thereof are planted in a roller shape, the charging member makes a rotary motion, and the peripheral speed thereof is that of the charged member. It is not the same with respect to the moving speed.
6. The image forming apparatus according to item 6. 8. The charging member is made of conductive fibers or an aggregate thereof that are flocked in a band shape and vibrates in a direction that is not parallel to the movement direction of the charged member. The image forming apparatus according to claim 1.