JP2002207348A - Corona discharge device and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a corona discharge device which is easily exchanged when used as a corona electrifying device, which is hardly broken compared with wires, which decreases the amount of discharge products (ozone and NOx) and which has a long life of the device. SOLUTION: A dielectric materials 2, 3 made of polyimide tapes or the like are stuck to the upper and lower faces of a plate electrode 1 made of molydenum or the like. The width of the dielectric materials 2, 3 is the same as the width of the plate electrode 1 and only the end faces of the plate electrode 1 are exposed to air. The obtained discharge electrode 10 is disposed in a metal casing 11 and high voltage Va is applied on the electrode to generate corona discharge. A grid electrode 12 on which voltage Vg is applied is disposed between the discharge electrode 10 and a photoreceptor 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, and the like. More specifically, the present invention relates to a charging device for charging an image carrier, and an exposure device for exposing the image carrier charged by the charging device. The present invention relates to a corona discharge device that can be used as a charging unit in an image forming apparatus provided with an exposing unit that forms an electrostatic latent image by using a charging unit.

[0002]

2. Description of the Related Art Conventional corona discharge type chargers include a wire discharge type (corotron, scorotron, etc.) and a pin discharge type (pin electrode type,
(Sawtooth electrode type, etc.). The latter has recently come to be used in electrophotographic copying machines, printers and the like due to low ozone generation. In particular, Japanese Unexamined Patent Publication Nos. 63-15272 and 5-45999 disclose a charger having a structure using an electrode plate in which a plurality of sawtooth-shaped electrode portions are provided on one thin plate-shaped member. ing.

One of the problems with the former wire discharge method is to reduce the amount of discharge products, particularly ozone NOx. Ozone has a strong oxidizing power, and if it is in large quantities, there is a concern that it will be harmful to the human body. Therefore, Germany BlueA
According to the ngelMark (BAM) regulations, the emission concentration outside the machine is limited, and an ozone filter is essential for mounting on an actual machine. This causes problems such as an increase in cost, an increase in the complexity of the device, and the necessity of airflow design. Further, there is a problem such as deterioration of parts due to strong oxidizing power of ozone.

[0004] Regarding NOx, it adheres to the photoreceptor as an ammonium salt (ammonium nitrate) and causes an abnormal image. Ammonium nitrate is an insulator at normal temperature and humidity, but absorbs water at high temperature and high humidity and has low resistance. Since the surface resistance is reduced, the latent image is disturbed, causing an abnormal image called image deletion.

[0005] For these reasons, it is desired to reduce ozone and NOx generation in the wire method. Furthermore, since a very thin wire (50 to 100 μm) is used, there are problems that the wire is easily cut at the time of wire replacement and cleaning, the life is short, and handling is difficult. When the wire is cut, the wire must be replaced. However, handling is difficult, and skill is required for the replacement. further,
Since the wire is thin and easily overlooked, it is easily lost when dropped by mistake.

On the other hand, in the case of the latter saw-tooth electrode system, it is known that the amount of ozone generation is small because the radius of curvature of the discharge electrode is small. However, if you discharge for a long time,
As the time elapses, the radius of curvature increases, and the discharge becomes unstable or no longer discharges. Even if it is discharged, the amount of ozone and NOx generated increases, and the effect of the sawtooth is lost. There is also a problem that discharge stops when toner, paper powder, or calcium carbonate adheres to the electrode. Further, when it becomes dirty, it is difficult to clean the electrode because it is a sharp electrode. Further, a problem has been pointed out that the electrode is sharp, so that a serviceman or a user is likely to be injured when replacing the electrode.

Accordingly, the present invention provides a corona discharge device which is easy to replace when used as a corona charger, is less likely to be cut than a wire, reduces the amount of discharge products (ozone, NOx) generated, and has a longer device life. The purpose is to provide. Another object of the present invention is to provide a corona discharge device having a longer life when used as a charger by adopting an electrode configuration that is easy to clean. It is another object of the present invention to provide an image forming apparatus using such a corona discharge device.

[0008]

In order to achieve the above object, a corona discharge device according to claim 1 of the present invention covers a metal electrode with a dielectric and exposes at least a part of the metal electrode to the outside. To form a discharge electrode.

According to a second aspect of the present invention, in order to achieve the above object, in the corona discharge device of the first aspect, the exposed area of the metal electrode exposed from the dielectric is sufficiently larger than the surface area of the metal electrode. It is characterized by being made smaller.

According to a third aspect of the present invention, in order to achieve the above object, in the corona discharge device according to the first or second aspect, the metal electrode is plate-shaped, and the width of the plate-shaped metal electrode is wide. It is characterized in that the dielectric is attached to a surface.

According to a fourth aspect of the present invention, in order to achieve the above object, in the corona discharge device of the third aspect, the width of the attached dielectric is the same as the width of the plate-shaped metal electrode. It is characterized by the following.

According to a fifth aspect of the present invention, in order to achieve the above object, in the corona discharge device according to the third or fourth aspect, the length L1 in the longitudinal direction of the metal electrode and one of the attached dielectrics are provided. Is characterized in that the length L2 in the longitudinal direction and the length L3 in the other longitudinal direction satisfy the relationship of L2 <L1 or L3 <L1.

According to a sixth aspect of the present invention, in order to achieve the above object, in the corona discharge device according to any one of the third to fifth aspects, the flat portion of the discharge electrode is substantially parallel to the member to be charged. It is characterized by.

According to a seventh aspect of the present invention, in order to achieve the above object, in the corona discharge device according to any one of the first to sixth aspects, a bias applied to the discharge electrode is a constant current control. And

According to an eighth aspect of the present invention, in order to achieve the above object, in the corona discharge device according to any one of the first to seventh aspects, the metal electrode is made of a material having a high melting point such as W, Mo, and Ta. It is characterized by becoming.

An image forming apparatus according to a ninth aspect is characterized in that the corona discharge device according to the first to eighth aspects is used as a charger in order to achieve the above object.

According to a tenth aspect of the present invention, in order to achieve the above object, the corona discharge device according to the first to eighth aspects is used as a static eliminator.

According to the eleventh aspect, in order to achieve the above object, the corona discharge device according to the first to eighth aspects is used as a charge applying device at the time of transfer.

[0019]

Embodiments and examples of the present invention will be described below with reference to the drawings. It is known that the amount of ozone generation increases in proportion to the wire diameter of the corona charger. NOx has a wire diameter of 30 to 100 μm
It is known that in the range, the larger the wire diameter, the larger the amount of generation. This is because the larger the wire diameter, the larger the volume of the discharge region and the greater the probability of ozone and NOx generation reaction. Therefore, the discharge products (ozone, NO
To reduce x), the discharge area may be reduced. However, if the diameter of the discharge wire is simply reduced, it is easy to cut and the maintenance is very difficult. The inventor of the present application has examined these problems, and as a result, it has been found that these problems can be solved by using a discharge electrode having a multilayer structure instead of wires.

FIG. 1 is a front view (A) and perspective views (B to D) showing one example of a discharge electrode used in a corona discharge device of the present invention, and an image using one embodiment of a corona discharge device according to the present invention. It is sectional drawing (E) of the use part of a forming apparatus. For example, dielectric members 2 and 3 made of, for example, a 50 μm-thick polyimide tape are adhered to the upper and lower surfaces of a plate-like electrode 1 made of molybdenum or the like having a thickness of 25 μm and a width of 2 mm. Here, the upper surface and the lower surface are surfaces (two) having a larger area among the six surfaces of the molybdenum plate constituting the plate-shaped electrode 1. Only the part exposed to the atmosphere is important, and the shape of the bulk part of the electrode does not matter. Considering that the number of electrode materials is reduced and unnecessary materials are reduced, and that fabrication is easy, a plate electrode is most preferable.

As shown in the figure, the width of the dielectrics 2 and 3 is the same as the width of the plate-like electrode 1, so that only the end face of the plate-like electrode 1 is exposed to the atmosphere. However, the end portions expose the plate-like electrode 1 so that a voltage can be easily applied. Although the way of exposure is described later, various modes can be adopted as shown in FIG. 1 (B to D).

The discharge electrode 10 having such a structure is used in FIG.
As shown in (E), it is installed in the metal casing 11,
Corona discharge is generated by applying the high voltage Va.
The grid electrode 12 to which the voltage Vg is applied is arranged between the discharge electrode 10 and the photoconductor 13 in order to obtain charging uniformity.
However, depending on the linear velocity, the electrode configuration, and the polarity of the photoreceptor 13, uniform charging may be possible even without the grid electrode 12, and the grid electrode 12 is not essential.

FIG. 2 is a side view showing a dimensional configuration of the discharge electrode 10 described above. The length in the longitudinal direction of the plate electrode 1 is L
1. When the length of the dielectric 2 on the upper surface in the longitudinal direction is L2 and the length of the dielectric 3 on the lower surface in the longitudinal direction is L3, a relationship of L1> L2 or L1> L3 is established therebetween. Let it. This facilitates voltage application.

For example, as shown in FIGS. 1B and 1C, the dielectric 2 on the upper surface side is short, and the plate-like electrode 1 is exposed at the end, and the wiring from the power supply is connected to the exposed portion. Connecting.
If the thickness of the plate-like electrode 1 is sufficient in terms of strength, the length relationship may be L1> L2 and L1> L3. That is, as shown in FIG.
3 is shortened so that the electrode portions are exposed on both the upper and lower sides of the plate-like electrode 1. However, when the plate-like electrode 1 is thin, sufficient strength cannot be obtained only at the electrode portion. Therefore, it is better to provide a dielectric on one surface as shown in FIGS. 1B and 1C. .

FIG. 3 is a sectional view showing another example of the discharge electrode of the corona discharge device according to the present invention. In this example, three surfaces except for one side surface of the plate-like electrode 1 are covered with the dielectric 4,
Only one surface is exposed to the atmosphere. FIG.
When used as in (E), when the moving speed of the photoconductor 13 is slow, the discharge current does not need to be so large. In this case, it is not necessary to expose the two surfaces of the plate electrode 1 to the atmosphere, and the configuration shown in FIG. 3 may be used. The plate-like electrode 1 can be formed by coating by dipping or the like and then exposing one surface by polishing or the like. In this case, only one surface needs to be polished, which reduces the number of steps and is advantageous in cost.

FIG. 4 is a sectional view (A) and a perspective view (B, B) showing still another example of the discharge electrode of the corona discharge device according to the present invention.
C). In this example, the periphery of the plate electrode 5 is covered with a dielectric 6 and spot-shaped holes 7 are provided on the side of the dielectric 6 (FIG. 4B), or a thin slit 8 is formed on the side of the dielectric 6.
(FIG. 4C) to expose a very small portion of the plate electrode 5 to the atmosphere. Although not shown, holes and slits may be similarly provided on the surface opposite to the surface on which holes 7 and slits 8 are provided. In these examples, stable charging can be realized by fixing the discharge point.

FIG. 5 is a sectional view (A) and perspective views (B, C, D) showing still another example of the discharge electrode of the corona discharge device according to the present invention. In this example, a cylindrical wire-shaped electrode 15 is covered with a dielectric 16 and spot-shaped holes 17 are provided on the side of the dielectric 16 (FIG. 5B), or a thin slit 18 is formed on the side of the dielectric 16. (FIG. 5C), or alternatively, the slit 18 is extended to the end (FIG. 5D), and only a part of the electrode 15 is exposed to the atmosphere. The number of slits 18 in FIGS. 5C and 5D is determined by the charging ability and the discharge stability, and is not always two. One or three or more may be used.

In the examples shown in FIGS. 4 and 5, no electrodes for applying a voltage are provided, but actually, at the longitudinal ends, as shown in FIGS. 1B to 1D. The electrodes are exposed for voltage application.

FIG. 6A is a view showing the result of observing the discharge point when corona discharge is performed using the discharge electrode of FIG. Approximately 35 discharge lights 20 are arranged in a zigzag on one side. FIG. 6A shows a part of the discharge light 20 of the discharge electrode 1. FIG. 6B shows the discharge electrode 1 of FIG.
Discharge light 2 when a simple plate-like electrode without the dielectrics 2 and 3 (polyimide tape) on the upper and lower surfaces is used for the corona electrode.
It is a figure showing a situation of 0. The discharge points are more densely arranged than in FIG. 6A, and about 60 discharge lights 20 are arranged in a zigzag on one side. However, when the comparison is performed with the same discharge current, it is considered that the discharge light 20 shines more strongly in FIG. 6B and the discharge region is larger. With the electrode configuration shown in FIG. 1, a discharge point is formed as shown in FIG. 7A, but when there is no dielectric 2, 3, the discharge point becomes as shown in FIG. 7B. Therefore, it is considered that light emission is strongly observed. That is, in the case of the discharge electrode having the configuration shown in FIGS. 1 to 5, the discharge area is reduced, and the amount of generated discharge products is reduced.

Assume that the discharge area is proportional to the area of the electrode exposed to the atmosphere. The surface area Sw of a normal wire is represented by L, the wire radius in the major axis direction of the photoconductor 13, the wire radius r,
Assuming that the pi is π, Sw = 2πrL. For example, for a 300 mm long, 60 μm diameter wire, r = 30
× 10 ⁻⁶ m and L = 300 × 10 ⁻³ m, so that Sw =
5.652 × 10 ⁻⁵ m ² .

Assuming that the electrode area exposed to the air in the embodiment of the present invention is S, the effect of the present invention can be expected if S <Sw. In the case of a shape in which a dielectric is attached to both upper and lower surfaces of the electrode (FIG. 1A), 1.68 × 10 ⁻⁵ m ² is satisfied, and S <Sw is satisfied, and the effect of the present invention is expected. Assuming that the amount of ozone and NOx generated is simply proportional to the surface area, in the example of FIG. 1, compared to the conventional 60 μm wire,
It becomes about 1/3 when 1.682 / 5.652 = 0.297. On the other hand, if only a plate electrode having a thickness of 28 μm, a width of 2 mm, and L = 300 mm (when there is no dielectric), S = 2 ×
(28 μm + 2 mm) × L = 4.056 × 10 ⁻³ × L =
1.2168 × 10 ⁻³ m ² , S> Sw,
The effect of the present invention cannot be expected.

It is expected that the smaller the thickness of the electrode, the smaller the discharge area becomes. However, if the electrode is too thin, the current density increases, and the electrode may be melted. In an experiment conducted by the inventor of the present application, it was confirmed that stable discharge was performed up to 200 nm. Also, there is a concern that the electrodes may be worn due to sputtering. Therefore, the electrode is preferably made of a material having plasma resistance, specifically, tungsten (W), molybdenum (Mo), or tantalum (Ta).

Further, according to the embodiment of the present invention, at a constant voltage of -6 kV
When the voltage was applied, it was confirmed that the discharge current varied greatly in the range of 300 μA to 550 μA, and the discharge point was also unstable. This is presumably because the upper and lower dielectrics are charged by ionized ions and the electric field weakens. However, when the applied voltage was controlled so as to keep the current value constant, the discharge point became stable, and the applied voltage was also in the range of 5.9 kV to 6.1 kV.

FIG. 8 is a sectional view showing an example of how the discharge electrode 10 is stretched in the apparatus of the embodiment shown in FIG. The most effective method is to arrange the flat surface substantially parallel to the charging surface as shown in FIG. When arranged as shown in FIG.
The ions generated by the discharge on the side farther from the photoconductor 13 (upper side in the figure) do not go to the photoconductor 13 but all flow into the casing 11. Only the ions formed by the discharge on the side near the photoconductor 13 contribute to the charging, and when the interval between the discharging points is wide, charging unevenness is likely to occur. If the linear speed (moving speed) of the photoconductor 13 is high, a sufficient charging potential cannot be obtained. FIG. 8 (A)
In this arrangement, since the discharge points are staggered, even when the interval between the discharge points is wide, sufficient charge can be obtained with uniform charging.
Therefore, it can be said that the arrangement of the discharge electrodes in FIG. Further, since the configuration of FIG. 8A corresponds to a double wire in the case of a discharge wire, a sufficient charging ability is guaranteed, and it is possible to cope with a high speed.

Of course, since the corona discharge device of the present invention is a discharge device, it can be used not only as a charger in an electrophotographic copying machine or a printer, but also as a device for applying electric charges to paper in a remover or a transfer device. be able to.

Hereinafter, several embodiments will be described. <Example 1> thickness 25 μm, width 2 mm, length 350 mm
A discharge electrode having a 50 μm-thick polyimide tape adhered to the Mo electrode was placed at the center of a casing member having an opening width of 20 mm and a height of 18 mm, thereby forming a charger. A grid electrode used for a normal scorotron charger was disposed between the discharge electrode and the photosensitive member in order to obtain uniform charging. This charger was equipped with a fan having a flow rate of 1000 L / min, a width of 255 mm, a length of 520 mm, and a height of 8
It was placed in a 0 mm acrylic resin container, a voltage was applied using a constant current power supply, and ozone and NOx concentrations were measured at the fan outlet. The measurement was performed at 25.5 ° C. and 62% RH.

FIG. 9 shows the measurement results. The horizontal axis is the discharge current,
The vertical axis represents the ozone concentration (FIG. 9A) and the NOx concentration (FIG. 9B). For comparison, the results for a 60 μm diameter wire are also shown. From the figure, it is confirmed that the electrode according to the example of the present invention has the effect of reducing ozone by about 60% and NOx by about 20%. In comparison of the surface area, it was predicted to be about 1/3, but ozone was as expected. N
Ox has not been reduced as expected. It is thought that NOx cannot be discussed simply by the size of the discharge region, because the oxidation reaction by ozone is involved.

<Embodiment 2> For comparison, FIG. 10 shows the results obtained when the electrode of the configuration of Embodiment 1 was used without attaching a polyimide tape to the electrode. Measurement is 23 ° C, 80% RH
Went under. As in FIG. 6, the diameter is 60 μm for comparison.
The measurement results for the wires are also shown. In the absence of polyimide, the amount of ozone generated was greater than the 60 μm wire, and N
It can be seen that Ox is equivalent. From this, it is understood that a simple plate electrode has no effect, and it is important to attach polyimide to the upper and lower surfaces of the electrode.

Example 3 In the structure of Example 1, a continuous discharge was performed for 100 hours under constant current control (500 μA). Initially, 65 discharge points were found on the entire electrode, but 42 discharge points were found at the end of the experiment. Had become. When the electrode surface was wiped with absorbent cotton impregnated with ethanol, the number of discharge points returned to the original value and became 64. This is considered to be due to the fact that the discharge products attached to the electrode surface by the continuous discharge were wiped off and removed, and the initial state was attained. On the other hand, when the discharge electrode was Al, the electrode was completely melted after 30 hours, and the experiment was impossible. The discharge light also had a slightly yellowish color. Therefore, it is understood that the life is greatly affected by the electrode material. If Mo is used, discharge can be performed for 100 hours or more by using cleaning together, and uniform charging can be realized.

Example 4 The discharge device of Example 1 was mounted on a modified machine of Imagio MF4550 (trademark) manufactured by the present applicant (Ricoh Co., Ltd.) and used as a charger of an image forming apparatus. Charging was performed at a constant current of 500 μA, and a halftone image was output by adjusting the developing bias. When the image unevenness was observed, a uniform halftone image was obtained, and it was confirmed that the charging was uniform. Was done. Next, the same image output was performed with the wire applied voltage being constant at a constant voltage of −6 kV, and a black raindrop-like abnormal image was generated.
This is presumably because the discharge point was unstable and uncharged portions occurred.

<Embodiment 5> FIG. 11 shows the measurement results of the charging potential with respect to the linear velocity of the photoconductor when the electrodes are arranged as shown in FIGS. The grid applied voltage is -900V. In the case of the configuration shown in FIG. 8A, the charging potential is about-even at the linear speed of the photosensitive member of 400 mm / s.
In contrast to the voltage of 800 V, in the arrangement of FIG.
It can be seen that the battery was charged only up to 50 V, and the charging ability was insufficient. When an image was taken out by the apparatus of the fourth embodiment, in the arrangement of FIG. 8B, at a linear speed of 400 mm / s, a black raindrop-shaped abnormal image was seen in the image, and there was an uncharged area. Was confirmed. On the other hand, in the arrangement of FIG. 8A, a uniform halftone image was obtained.

[0042]

As described above, since the corona discharge device according to the present invention has a structure in which the discharge electrode is covered with the dielectric and partially exposed, the discharge area is minimized, and the discharge product ( Ozone and NOx) are very small.

In the corona discharge device according to the third aspect, as described above, the metal electrode is plate-shaped, and the dielectric is attached to two wide surfaces of the metal electrode. In addition, a desired effect can be obtained with a small amount of electrode material.

In the corona discharge device according to the fifth aspect, as described above, the length of the electrode in the longitudinal direction is longer than the length of the dielectric in the longitudinal direction.
There is an effect that it is easy to secure the electrodes for applying the voltage, and the discharge current can flow reliably.

In the corona discharge device according to the sixth aspect, as described above, since the flat portion of the discharge electrode is substantially parallel to the member to be charged, the charging efficiency is improved in addition to the common effect. effective.

In the corona discharge device according to the seventh aspect, as described above, the bias applied to the discharge electrode is a constant current control. Is obtained.

In the corona discharge device according to claim 8, as described above, the electrode material is made of a high melting point material (W, M).
o, Ta, etc.), in addition to the above-mentioned common effects, there is an effect that the electrodes are not deteriorated even if the discharge is performed for a long time and a stable discharge is maintained.

In the image forming apparatus according to the ninth to eleventh aspects, the corona discharge device of the present invention described above is used as a charger, a static eliminator, or a charge applying device at the time of transfer. A common effect can be obtained.

[Brief description of the drawings]

FIG. 1 is a front view (A) showing an example of a discharge electrode used in a corona discharge device of the present invention, a perspective view (B to D), and an image forming apparatus using an embodiment of a corona discharge device according to the present invention. It is sectional drawing (E) of a use part.

FIG. 2 is a side view showing a dimensional configuration of a discharge electrode in the example of FIG.

FIG. 3 is a sectional view showing another example of the discharge electrode of the corona discharge device according to the present invention.

FIG. 4 is a sectional view (A) and perspective views (B, C) showing still another example of the discharge electrode of the corona discharge device according to the present invention.

FIG. 5 is a sectional view (A) and perspective views (B, C, and C) showing still another example of the discharge electrode of the corona discharge device according to the present invention.
D).

6 is a diagram showing the results of observing discharge points when corona discharge is performed using the discharge electrodes of FIG.

FIG. 7 is a view showing discharge electrodes and discharge light corresponding to the discharge state of FIG. 6;

8 is a cross-sectional view showing an example of how discharge electrodes are stretched in the apparatus of the embodiment shown in FIG. 1;

FIG. 9 is a diagram showing the measurement results of the ozone concentration (A) and the NОx concentration (B) of the example using the corona discharge device according to the present invention.

10 is an ozone concentration (A) in the case of using an electrode without attaching a polyimide tape to the electrode in the configuration of the embodiment of FIG. 9;
FIG. 7 is a diagram showing measurement results of N and NОx concentration (B).

FIG. 11 shows the configuration of the embodiment shown in FIG.
FIG. 9 is a diagram showing the results of measuring the charging potential with respect to the linear velocity of the photoconductor in the cases of (A) and (B).

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 plate electrode 2, 3 dielectric 4 dielectric 5 plate electrode 6 dielectric 7 hole 8 slit 10 discharge electrode 11 metal casing 12 grid electrode 13 photoreceptor 15 wire electrode 16 dielectric 17 hole 18 slit 20 discharge light

Claims (11)

[Claims] 1. A corona discharge device, wherein a metal electrode is covered with a dielectric, and at least a part of the metal electrode is exposed to the outside to form a discharge electrode. 2. The corona discharge device according to claim 1, wherein an exposed area of the metal electrode exposed from the dielectric is made sufficiently smaller than a surface area of the metal electrode. 3. The corona discharge device according to claim 1, wherein the metal electrode is plate-shaped, and the dielectric is attached to two wide surfaces of the plate-shaped metal electrode. Corona discharge device. 4. The corona discharge device according to claim 3, wherein the width of the attached dielectric is the same as the width of the plate-shaped metal electrode. 5. The corona discharge device according to claim 3, wherein a length L1 in the longitudinal direction of the metal electrode, a length L2 in one longitudinal direction of the attached dielectric material, and a length in the other longitudinal direction. A corona discharge device wherein L3 satisfies the relationship of L2 <L1 or L3 <L1. 6. The corona discharge device according to claim 3, wherein a plane portion of said discharge electrode is substantially parallel to a member to be charged. 7. The corona discharge device according to claim 1, wherein a bias applied to the discharge electrode is a constant current control. 8. The corona discharge device according to claim 1, wherein said metal electrode is made of a material having a high melting point such as W, Mo, and Ta. 9. An image forming apparatus using the corona discharge device according to claim 1 as a charger. 10. An image forming apparatus using the corona discharge device according to claim 1 as a static eliminator. 11. An image forming apparatus, wherein the corona discharge device according to claim 1 is used as a charge applying device during transfer.