JP3607446B2 - Corona charging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a corona charging device for charging a charge carrier of an image forming apparatus using an electrophotographic process such as an electrostatic copying machine or a laser printer.
[0002]
[Prior art]
FIG. 5 is a schematic diagram of an image forming apparatus using an electrophotographic process. The photoconductor drum 1 is obtained by applying the photoconductor 1b to the surface of the conductor 1a, and rotates in the direction of the arrow in FIG. In the image forming apparatus, an image is formed in the following procedure.
[0003]
1. A corona charging device 2 charges the surface of the photoreceptor 1b to a desired potential.
2. The exposure device 3 exposes the photoconductor 1b to form an electrostatic latent image corresponding to a desired image on the photoconductor 1b.
3. The developing unit 4 develops the electrostatic latent image with toner to form a toner image on the photoreceptor 1b.
4). The transfer unit 5 transfers the toner image on the photoreceptor 1b onto a transfer body 6 such as paper that is transported by a transport unit (not shown).
5. The cleaning unit 7 cleans the toner that is not transferred to the transfer member 6 and remains on the photosensitive member 1b.
6). The transfer body 6 onto which the toner image has been transferred is conveyed to a fixing device (not shown). In the fixing device, the toner is heated and fixed on the transfer member.
Since the photosensitive drum 1 rotates in the direction of the arrow in FIG. 5, a desired image is formed on each of the plurality of transfer members by repeating the above steps 1 to 6.
[0004]
As a charging device in an electrophotographic process, a corona charging device 2 that charges a photoreceptor using corona discharge has been conventionally used. FIG. 6 is a schematic diagram of an example of the corona charging device 2. The discharge wire 25 is made of tungsten and has a wire diameter of 60 [μm]. The discharge wire 25 is stretched in the axial direction x of the photosensitive drum 1 at a position as shown in FIG. 6 (center of the case 22), and a high voltage (about −7 [kV]) is applied. The discharge wire 25 is covered with a charging case 22. The material of the case 22 is stainless steel that is not easily oxidized. Further, there is a grid 23 between the discharge wire 25 and the photosensitive drum 1, and a voltage of about −0.6 [kV] is applied to the grid 23. The grid 23 is a stainless steel plate (plate thickness: 0.1 [mm]) perforated in a mesh shape.
[0005]
In the corona charging device 2 shown in FIG. 6, the photosensitive member 1b is charged as follows. In the vicinity of the stretched wire, a strong electric field is formed, air breakdown occurs, and ions are generated. Some of these ions move due to the electric field between the wire 25 and the conductor 1a immediately below the photoreceptor 1b, and the surface of the photoreceptor is charged. Since charging of the photoreceptor 1b continues until the surface potential becomes substantially equal to the potential applied to the grid 23, the surface potential of the photoreceptor 1b can be controlled by the potential applied to the grid 23.
As a corona charging device other than the corona charging device 2 using the discharge wire 25, there is one using a sawtooth electrode as a discharge electrode (JP-A-8-20210, JP-A-6-301286). FIG. 7 is a schematic view of an example of the corona charging device 20 using the sawtooth electrode 21. The sawtooth electrode 21 has a shape as shown in FIG. 8, and is formed by forming sawtooth 202 with a pitch of 3 mm on a stainless steel plate having a thickness of 0.1 [mm]. 7, the sawtooth electrode 21 is fixed to the support member 24, and a high voltage (−5 [kV] is applied by the power source 8. The corona charging device 20 using the sawtooth electrode 21 (FIG. 7). However, similarly to the corona charging device 2 (FIG. 6) using the discharge wire 25, the material is covered with a charging case 22 made of stainless steel, and a grid 23 is provided between the sawtooth electrode 21 and the photoreceptor 1b. The charging of the photoreceptor 1b by the corona charging device 20 (FIG. 7) using the sawtooth electrode 21 is the same as that when the discharge wire 25 is used, and the sawtooth 202 of the sawtooth electrode 21 is also charged. Corona discharge occurs near the apex.Other corona charging devices have been devised in which the discharge electrode is a needle-like (pin-like) electrode.
[0006]
The corona charging device 20 using the sawtooth electrode 21 has the advantages of small size and low ozone generation compared to the charging device 2 using the discharge wire 25. In the sawtooth electrode 21, the directivity of corona discharge is high, and the ion flow toward the charging case 22 is smaller than the ion flow toward the grid 23. For this reason, the width of the charging device 20 (the width in the y direction of the opening of the charging case 22 on the photosensitive member side) can be reduced. This is important for downsizing of the entire image forming apparatus. Further, since the directivity of corona discharge is high, the charging efficiency of the photoreceptor 1b is increased, and the current flowing through the corona charging device 20 can be reduced. As a result, the amount of ozone generated is reduced.
[0007]
The corona charging device disclosed in Japanese Patent Application Laid-Open No. 5-173403 generates a corona discharge by applying a high voltage to a stretched wire to charge the photoconductor. A groove is formed in the covering material so that the wire body is exposed from the groove. Thus, it is described that the generation point of corona discharge is limited to the inside of the groove, and the generation amount of ozone is reduced.
[0008]
Japanese Patent Application Laid-Open No. 6-301268 discloses a charging device that generates corona discharge from a sawtooth discharge electrode mounted on an insulating substrate, and the tip of the discharge electrode starts from the end of the insulating substrate. It is described that the protrusion is provided as a feature. Further, it is said that the generation of ozone is small, the handling is prepared, and the characteristics of the discharge electrode can be stabilized.
[0009]
JP-A-8-202120 discloses a corona charging device provided with a sawtooth electrode plate and a control grid for controlling corona discharge from the sawtooth electrode. When the value is Dp, and the value Dg is the distance between the apex of the sawtooth electrode and the control grid, the value of Dp is 1 mm or more and 4 mm or less, and Dg is 2 times or less of Dp. ing. As a result, it is described that discharges from adjacent sawtooth electrodes do not overlap and charging unevenness according to the pitch of the sawtooth electrodes hardly occurs, uniform charging is obtained, and controllability of the control grid is improved.
[0010]
[Problems to be solved by the invention]
As described above, the corona charging device using the sawtooth electrode has the advantages as described above as compared with the device using the wire. However, although the voltage applied to the sawtooth electrode is smaller than that in the case of a wire, it is still a high voltage of about −5 [kV]. This is because of the stability of discharge. In other words, in a corona charging device that uses a sawtooth electrode, the discharge conditions at the top of the sawtooth (distance from the grid, radius of curvature of the top, etc.) differ slightly for each sawtooth, so when a low voltage is applied In some cases, discharge occurs at each apex or does not occur. In addition, when a low voltage is applied, deterioration of the sawtooth electrode becomes a problem. That is, when the charging device is used for a long time, the surface of the apex portion deteriorates. This deterioration causes a peak where the discharge is less likely to occur. Therefore, when a low voltage is applied, some of the peak may stop discharging.
[0011]
As described above, when a vertex where no discharge occurs is generated in the saw blade, the photosensitive member is not charged immediately below the vertex. As a result, this portion becomes an uncharged portion and causes image defects. For this reason, in a corona charging device using a conventional sawtooth electrode, in order to ensure the stability of such a discharge (in order to prevent the occurrence of a sawtooth that does not cause a discharge), the voltage applied to the electrode is set. It was supported by increasing the size. However, in order to cope with a large voltage, a power source capable of dealing with a high voltage is required, which causes an increase in the cost of the entire image forming apparatus. In addition, in corona charging using a conventional sawtooth electrode, there is a method in which the tip angle of the sawtooth electrode is an acute angle (10 ° or less) in order to ensure the stability of discharge. When the tip angle is set to an acute angle in this way, there are problems that it is difficult to shape the tip of the electrode, or that the tip of the electrode is easily deformed by an external force due to insufficient strength.
[0012]
The first object of the present invention is to increase the stability of discharge, which is a problem in a corona charging device using a sawtooth electrode, and the stability of discharge without significantly reducing the tip angle of the sawtooth electrode. The second purpose is to ensure the above.
[0013]
[Means for Solving the Problems]
(1) In a corona charging device (20) for generating a corona discharge by applying a voltage to a sawtooth electrode (21) and charging a movable object to be charged (1b),
In the sawtooth electrode (21), the sawtooth (202) is covered (203) with an insulating material except for its apex portion, and the apex portion is uncovered and is covered with the insulating material. The corona charging device is characterized in that the unexpanded portion has a radius of 0.1 [mm] or more in terms of the radius from the tip of the saw tooth. In addition, in order to make an understanding easy, the code | symbol of the corresponding element of the Example shown in drawing and mentioned later is added to the parenthesis for reference.
[0014]
According to this, the electric field between the serrated electrode (21) and the opposing conductor (1a, 23) is concentrated on the apex portion where there is no coating, and corona discharge is likely to occur at the apex portion, and the serrated electrode (21). Even if the voltage applied between the electrode and the opposing conductor (1a, 23) is relatively low, corona discharge is likely to occur at the apex, and each of the conventional voltages (−5 KV) applied to the sawtooth electrode (21). Corona discharge is generated stably at the top of the sawtooth, and the directionality and stability of the discharge are high.
[0015]
When the charging device is used for a long time, the apex portion is worn, but the spread of the portion of the saw blade not covered with the insulating material is 0.1 [mm] or more in radius from the tip of the saw blade. Corona discharge is stably generated during a long period of use in which the exposed portion is worn down to the position of the insulating material.
[0016]
(2) The size of the portion of the apex portion of the sawtooth electrode (21) that is not covered with the insulating material is within 1 [mm] in radius from the tip of the sawtooth. If the area that is not covered with the insulating material is widened, the electric field concentration at the tip of the saw blade will be weakened, and the directivity and stability of the discharge will be reduced, but the exposed portion will be within 1 mm in radius from the tip of the saw blade. Therefore, the electric field concentration at the tip of the saw tooth is strong and the directivity and stability of corona discharge is high.
[0017]
(3) The tip angle of the sawtooth electrode (21) is greater than 10 °. As a result, the shape of the sawtooth tip can be easily processed and the strength is high, so that the electrode tip is not easily deformed by an external force, and the processing and handling are facilitated. When the tip angle increases, the electric field concentration at the tip of the saw blade generally becomes weaker, but the tip portion of the saw blade is exposed and the other part is covered with an insulating material, which strengthens the electric field concentration. High nature.
[0018]
Other objects and features of the present invention will become apparent from the following description of embodiments with reference to the drawings.
[0019]
【Example】
-1st Example-
FIG. 2 shows a corona charging device 20 according to a first embodiment of the present invention, and a sawtooth electrode 21 of the corona charging device 20 is shown in FIG. The corona charging device 20 of the first embodiment shown in FIGS. 2 and 1 is different from the conventional corona charging device (for example, FIGS. 7 and 8) except for the apex portion of the sawtooth 202 of the sawtooth electrode 21. The electrode 21 is covered with an insulating film 203.
[0020]
Referring to FIG. 1, the sawtooth electrode 21 is obtained by processing a stainless steel plate (thickness 0.1 [mm]) into a shape in which a sawtooth 202 protrudes from a thin plate-like stem 201. The apex pitch Dp of the sawtooth 202 is 3 [mm], and the protruding length of the sawtooth 202 is 3 [mm]. The tip (vertex) of the sawtooth 202 is processed so that the radius of curvature is 20 [μm]. The material of the sawtooth electrode 21 may be any conductive material capable of corona discharge, but the above-described stainless steel plate is desirable in terms of ozone resistance and discharge wear resistance. The plate thickness may be any plate thickness as long as corona discharge is possible, but about 0.1 [mm] is desirable in terms of ease of manufacture, ease of corona discharge, and the like. . Further, the pitch Dp of the saw teeth 202, the protrusion length Dg of the saw teeth 202, the tip shape, etc. are not limited to the above values.
[0021]
The sawtooth electrode 21 is covered with an insulating film 203 except for the apex of the sawtooth 202. In the corona charging device 21 shown in FIGS. 1 and 2, the insulating film 203 is a fluororesin. As the material for the insulating film 203, any material may be used as long as it can ensure insulation, such as silicone resin, ceramic, and epoxy resin.
[0022]
In the first embodiment described above, the coating 203 was formed by applying a fluorine resin to the stainless steel plate by spraying. At this time, spraying was performed while masking the apex portion so as not to adhere to the apex portion of the sawtooth 202. The portion not covered with the insulating material at the apex portion is a portion within a radius of 1 [mm] from the tip. The method of applying the insulating material to the sawtooth electrode 21 may also be other than the method described above. Moreover, the film thickness of application | coating should just be a suitable film thickness, if it is a grade which can ensure insulation.
[0023]
The sawtooth electrode 21 thus produced was supported by an insulating support member (material: ABS) 24 and fixed to the charging case 22 as shown in FIG. A voltage (−3 [kV]) is applied to the sawtooth electrode 21 by the power supply 8. The sawtooth electrode 21 is inside a U-shaped charging case 22 (material: stainless steel plate). The charging case 22 is for preventing the outside of the case (excluding the photoreceptor 1b) from being charged. A grid 23 is disposed in the charging case opening between the sawtooth electrode 21 and the photosensitive drum 1. The grid 23 is obtained by etching a stainless steel plate (plate thickness: 0.1 [mm]) to form a mesh at a pitch of 1 [mm]. A voltage (−0.6 [kV]) is applied between the charging case 22 and the grid 23 by the power source 9.
[0024]
In addition, in the corona charging device 20 shown in FIGS. 1 and 2, the distance from the apex of the sawtooth electrode 21 to the grid 23 (electrode / grid distance Deg) is 8 mm, and the opening width Wo of the charging case 22 is 12 mm. [Mm]. The opening width Wo is narrower than the opening width of the conventional example (for example, FIGS. 6 and 7). As shown in FIG. 3, in the first embodiment (FIG. 3 (a)), the electric field concentration at the apex (tip) of the saw blade 202 is strong (the electric field gradient is high). Not only does corona discharge easily occur, but also the spread of the electric field in the x direction (the width direction of the charging device) is narrow (high directivity), so that the photoreceptor 1b can be effectively charged even if the opening width Wo is reduced. It is. In the conventional example, as shown in FIG. 3 (b), the electric field lines are formed to the vicinity of the base of the saw blade 202, and the electric field concentration at the apex (tip) of the saw blade 202 is weak (the electric field gradient is low). Since the spread of the electric field in the x direction (width direction x of the charging device) is large (low directivity), not only the applied voltage has to be made relatively high, but also the opening width Wo is increased to the photosensitive member 1b. It is necessary to widen the area of corona discharge.
[0025]
-Second Example-
FIG. 4 shows a sawtooth electrode 21 according to a second embodiment of the present invention. The configuration other than the sawtooth electrode 21 and the applied voltage are all the same as those of the charging device 20 (FIG. 2) of the first embodiment. In the second embodiment, as shown in FIG. 4, the tip angle of the sawtooth electrode 21 is 20 °. The tip angle of the sawtooth electrode 2 does not have to be the above-mentioned value (20 °) and may be any value exceeding 10 ° as long as corona discharge is possible.
[0026]
【The invention's effect】
In the corona charging device of the present invention, since the sawtooth electrode is used, the unique advantage when the sawtooth electrode as pointed out in the section of the prior art is used is inherited as it is. That is, there are advantages such as small size and low ozone generation compared to the corona charging device using a discharge wire.
[0027]
However, in a corona charging device using a conventional sawtooth electrode, it is necessary to apply a high voltage in order to ensure the stability of discharge. In other words, it is necessary to apply a high voltage in order to prevent the occurrence of a vertex from which no discharge occurs due to a slight difference in the conditions of the sawtooth electrode or deterioration of the electrode tip.
[0028]
In the corona charging device of the present invention, since the sawtooth electrode is covered with an insulating material except for the tip of the sawtooth, the electric field lines are concentrated more in the vicinity of each vertex of the sawtooth electrode, and a strong electric field is generated. Even if the electric field in the vicinity of the apex is reduced due to the slight difference in the conditions of the sawtooth electrode and the deterioration of the electrode tip, the discharge does not stop. Therefore, it is possible to reduce the voltage applied to the electrode as compared with a corona charging device using a conventional sawtooth electrode. Compared to conventional corona charging devices that use serrated electrodes, even when the voltage applied to the electrodes is reduced, the stability of the discharge is ensured, resulting in the occurrence of uncharged portions of the photoconductor that cause abnormal images. do not do.
[0029]
The size of the portion of the tip portion of the sawtooth electrode that is not covered with the insulating material is set to be within 1 [mm] in radius from the tip portion. According to the experiments by the inventors, it has been found that when the size of the portion not covered with the insulating material is within this range, the stability of the discharge is more easily secured. That is, the concentration of electric lines of force increases at the apex portion of the sawtooth electrode, and the effect of covering the peripheral portion with the insulating material increases, and the stability of discharge is ensured even at a lower applied voltage.
[0030]
By making the tip angle of the sawtooth electrode larger than 10 °, conventional problems such as difficulty in shape processing and easy deformation are improved. Except for the serrated tip portion of the serrated electrode, it is covered with an insulating material, so that even if the tip angle is increased, a strong electric field that requires corona discharge is formed at the electrode tip.
[Brief description of the drawings]
FIG. 1 is an enlarged plan view of a sawtooth electrode 21 shown in FIG.
FIG. 2 is a cross-sectional view of the first embodiment of the present invention.
3A is a plan view schematically showing electric lines of force between the serrated electrode 21 and the grid 23 shown in FIG. 1, and FIG. 3B is an electric diagram formed by the conventional serrated electrode 21. FIG. It is a top view which shows a force line.
FIG. 4 is an enlarged plan view of a sawtooth electrode 21 used in the second embodiment of the present invention.
FIG. 5 is a block diagram showing a cross section of a conventional image forming apparatus using a charging device 2 using a discharge wire.
6 is an enlarged cross-sectional view of only the periphery of the charging device 2 in FIG. 5;
FIG. 7 is a cross-sectional view of a conventional image forming apparatus using a charging device 20 using sawtooth electrodes.
8 is an enlarged plan view of the sawtooth electrode 21 shown in FIG.
[Explanation of symbols]
1: Photoconductor drum 1a: Conductor 1b: Photoconductor 2: Charging device using discharge wire 3: Exposure device 4: Developer 5: Transfer device 6: Transfer device 7: Cleaning means 20: Charging device 21 using sawtooth electrode : Sawtooth electrode 22: Charging case 23: Grid 24: Support member 25: Discharge wire 201: Sawtooth electrode stem 202: Sawtooth electrode sawtooth 203: Insulating film

Claims (3)

In a corona charging device that generates a corona discharge by applying a voltage to a sawtooth electrode and charges a movable object to be charged,
In the sawtooth electrode, the sawtooth is covered with an insulating material except for its apex portion, the apex portion is not covered, and the spread of the portion not covered with the insulating material extends from the tip of the sawtooth. A corona charging device having a radius of 0.1 mm or more. 2. The corona charging according to claim 1, wherein a size of a portion of the apex portion of the sawtooth electrode that is not covered with an insulating material is within 1 [mm] in radius from the tip of the sawtooth. apparatus. The corona charging device according to claim 1, wherein a tip angle of the sawtooth electrode is larger than 10 °.

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