JP2002023465A - Electrifying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrifying device capable of maintaining uniform electrifying property and always obtaining excellent image quality while preventing the wear of a body to be electrified and the adhesion of toner with simple constitution without causing the increase of the number of parts nor cost rising. SOLUTION: In this electrifying device, the member to be electrified is electrified by using a fine void discharge generated in a fine void formed between the member to be electrified and a charge imparting member by bringing the charge imparting member into contact with the surface of the member to be electrified and applying DC voltage to the charge imparting member. Then, the voltage applied to the charge imparting member is set to be changed from the DC voltage to DC voltage to which AC voltage is applied by detecting or predicting that the member to be electrified is unevenly electrified in the case of electrifying the member to be electrified.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charging device used in an image forming apparatus such as a copying machine or a printer employing an electrophotographic system.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic copying machine or a printer, a surface of a toner image carrier such as a photosensitive drum is charged uniformly to a predetermined potential. Forming an electrostatic latent image on the surface of the toner image carrier by performing image exposure, and developing the electrostatic latent image with a developer to visualize the electrostatic latent image to form a latent image on the toner image carrier. An image is formed on the recording medium by forming a developed image on the recording medium, transferring the developed image to a recording medium such as paper, and fixing the developed image.

In such an image forming apparatus, a charger for charging the toner image carrier by uniformly applying a charge to the surface of the toner image carrier for carrying the toner image,
To remove the charge accumulated on the toner image carrier,
A static eliminator or the like that uniformly applies a charge to a toner image carrier is used. As the charger, two types of charging systems are used, a non-contact charging system such as a scorotron or corotron using corona discharge, and a contact charging system using a charging roll or the like using discharge in a minute gap.

In a charger using corona discharge, a high voltage is applied to a wire stretched opposite to the surface of a toner image carrier to which a charge is to be applied, so that a gap between the toner image carrier and the wire is generated. Generates a corona discharge, and a predetermined charge is applied to the toner image carrier. This type of charger is
Although the charge uniformity is excellent, a large amount of discharge products such as ozone and NOx are generated, so that a processing device is required, and there is a disadvantage that the image forming apparatus tends to be large-sized and costly. Also, dust and toner in the air,
Electrode wires may be contaminated by discharge products, fuser oil, and the like, resulting in non-uniform charging, which may cause image defects.

For this reason, recently, instead of the corona discharge system, the use of a contact charging system in which a charge applying member is brought into direct contact with a toner image carrier to perform charging has been studied. In this contact charging method, a roll-shaped or brush-shaped charge applying member having semiconductivity is arranged so as to be in contact with the surface of the toner image carrier, and a voltage is applied to the charge applying member so that the toner image at the contact portion is contacted. By generating a discharge in the minute gaps on the carrier, a charge is applied to the toner image carrier.
In this method, since corona discharge is not used, the amount of generated ozone is extremely small, and the charge applying member is in contact with the toner image carrier, which is suitable for reducing the size and weight of the apparatus.

However, in such a contact charger, since the charge applying member is brought into contact with the toner image carrier, dirt such as toner, discharge products, dust in the air, and paper dust easily adhere to the charge applying member. However, if such dirt adheres, abnormal discharge or unstable discharge occurs, and there is a problem that image quality defects such as fog are likely to occur. The non-uniform charging due to the adhesion of dirt is particularly remarkable when a DC voltage is applied, and an image quality defect due to the non-uniform charging may be a fatal problem, particularly for an image forming apparatus for high image quality.

[0007] To solve the above-mentioned problem of dirt, Japanese Patent Laid-Open Nos. 5-53413 and 5-2 have been proposed.
No. 65307, JP-A-4-303861, JP-A-6-342236, and JP-B-5-0160.
No. 33 has already been proposed.

In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 5-53413, a photosensitive drum, a charging roller provided on the surface thereof so as to be able to contact and separate therefrom, and a surface of the photosensitive member charged by the charging roller are exposed. Exposure means, a developing device for turning a latent image formed by the exposure into a visible image, transfer means for transferring the formed visible image to paper, and a surface of the photosensitive drum after the transfer by the transfer means is completed And a cleaning device that cleans the photosensitive drum with a cleaning member. After the photosensitive drum starts rotating, a position corresponding to the cleaning member on the surface of the photosensitive drum is located at a driving start end thereof. Roller pressing timing adjusting means for pressing the charging roller against the photosensitive drum from the separated state when the charging roller is rotated and moved to a position corresponding to the charging roller. It is obtained by configured to provide.

Further, the cleaning device for a charging roller according to the above-mentioned Japanese Patent Application Laid-Open No. 5-265307 is a cleaning device for a charging roller for removing foreign matter adhering to the surface of the charging roller. A cleaning member disposed in contact with the charging roller and the photoconductor, and a driving unit that slides the cleaning member in a longitudinal direction of the charging roller. It is configured so that

Further, the charging member according to the above-mentioned Japanese Patent Application Laid-Open No. Hei 4-303861 is characterized in that the charging member which is charged by contacting the member to be charged contains a layered solid lubricant at least in a layer in contact with the member to be charged. It is composed.

Furthermore, the image forming apparatus according to the above-mentioned Japanese Patent Application Laid-Open No. 6-342236 discloses an image carrier, a charging roller which rotates in contact with the image carrier, and a lubricant applied to the charging roller. The linear velocity of the charging roller can be changed with respect to the linear velocity of the image carrier in the image forming apparatus provided with.

Further, the charging device according to the above Japanese Patent Publication No. 5-016033 includes a member to be charged and a member to contact the member to be charged in the longitudinal direction of the member to be charged in order to charge the member. A charging member to which a voltage is applied, wherein the charging member comprises: a conductive base to which a voltage is applied; and a first resistor provided closer to the member to be charged than the conductive base. And a second resistance layer having a larger volume resistivity than the first resistance layer provided closer to the object to be charged than the first resistance layer. And the voltage has a peak-to-peak voltage that is at least twice the charging start voltage of the member to be charged.

[0013]

However, the prior art described above has the following problems. That is, JP-A-5-53413 and JP-A-5-53413 described above.
Japanese Patent Publication No. 265307 discloses a method in which a charging roller is provided so as to be able to contact and separate from the surface of a photosensitive drum, and an existing cleaning member is used, or a cleaning means for removing dirt on the surface of a charging member such as a charging roller is disclosed. However, in the case of these technologies, a new contact / separation mechanism for bringing the charging roller into contact with / separating from the surface of the photoreceptor drum and a cleaning means for removing dirt on the surface of the charging member such as the charging roller are required. In addition to an increase in points and cost, there is a problem that the configuration is complicated.

The above-mentioned JP-A-4-303861 and JP-A-6-342236 disclose methods for incorporating a lubricant into the surface of the charge-applying member. It has the effect of reducing the amount of toner adhering to the toner, and it is possible to suppress uneven charging due to adhesion of dirt to some extent, but it is difficult to maintain the effect for a long period of time, and the lubricant is There is also a problem that it affects the charging property and it is difficult to perform a predetermined uniform charging.

Furthermore, Japanese Patent Publication No. 5-016033 discloses a method of applying a voltage having a peak-to-peak voltage that is at least twice the charging start voltage of a member to be charged.
In this method, the charging efficiency is low, and the surface of the member to be charged is subjected to more discharge than necessary, thereby easily deteriorating the surface of the member. ing.

On the other hand, as described above, in the method of applying a DC voltage to the charge applying member, an abnormal discharge or an unstable discharge is caused when dirt such as toner, discharge products, dust in the air, and paper dust adheres. This causes a problem that image quality defects such as fog are easily caused.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a simple configuration without increasing the number of parts and cost. An object of the present invention is to provide a charging device which can maintain a uniform charging property and always obtain good image quality while preventing wear of a member to be charged, fixing of toner, and the like.

[0018]

That is, according to the first aspect of the present invention, the charging member is brought into contact with the surface of the charging member by applying a DC voltage to the charging member. In a charging device that charges a member to be charged by using a minute gap discharge generated in a minute gap formed between the charge applying member and the charge providing member, when the member to be charged is charged, the charging of the member to be charged is performed. The charging device is characterized in that the charging device detects or predicts that it is becoming non-uniform, and changes the voltage applied to the charge applying member from a DC voltage to a DC voltage to which an AC voltage is applied. When the voltage applied to the charge applying member is immediately changed from a DC voltage to a DC voltage to which an AC voltage is applied, the change is performed in a non-image portion such as between sheets so that the effect does not appear on an image. It is desirable.

Further, according to the present invention, when the member to be charged is charged, it is detected or predicted that the charge of the member to be charged is becoming non-uniform, and the charged member is applied to the member to be charged. The charging device according to claim 1, wherein the voltage to be applied is gradually changed from a DC voltage to a DC voltage to which an AC voltage is applied.

Further, according to a third aspect of the present invention, the desired charging potential of the member to be charged is Vpr, and the charging start voltage is V.
th, when the amplitude of the AC component of the DC voltage on which the AC voltage is superimposed is Vpp and the value of the DC component is Vdc, the applied voltage Vdc when the DC voltage is first applied and charged is approximately (Vp
r + Vth), Vpp is set to approximately 0, image output is started, and when charging the member to be charged, Vd is detected or predicted to be non-uniformly charged.
3. The charging device according to claim 2, wherein c is gradually changed from substantially (Vpr + Vth) to substantially Vpr, and Vpp is gradually changed from substantially zero to substantially (Vth × 2).

Still further, according to the present invention, the charging becomes non-uniform during image output by observing the discharge current waveform during charging and judging the magnitude of the peak current value. The charging device according to claim 1, wherein the charging device detects the presence of the charging device.

Further, according to a fifth aspect of the present invention, the member to be charged is an image carrier for forming an image, and the density and / or pattern of the image to be formed on the image carrier is determined in advance. The charging device according to claim 1, wherein a time during which an image is formed when the charging becomes non-uniform is estimated.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 2 shows an image forming apparatus to which the charging device according to one embodiment of the present invention is applied.

In FIG. 2, reference numeral 1 denotes a charging device such as a charging roll using the principle of minute gap discharge, 2 denotes a photosensitive drum,
Is a discharge current observation means, 4 is a high voltage power supply control means, 5 is a high voltage power supply capable of outputting only a DC voltage to a DC voltage superimposed with an AC voltage, 6 is an exposure device, 7 is a developing device, 8 is a transfer device, and 9 is a paper. Reference numeral 10 denotes a fixing device, and 11 denotes a pixel counter.

The photosensitive drum 2 uses a photosensitive member made of OPC (organic photoconductor) or the like. The photosensitive drum 2 is driven by a driving means (not shown) at a predetermined rotational speed in the direction of the arrow. Has become. As described below, the surface of the photosensitive drum 2 is uniformly charged to a predetermined potential by a charging roll 1 as a charging device, and is then subjected to image exposure by an exposure device 6 according to image information. , An electrostatic latent image is formed.

The charging roll 1 is formed by coating a surface of a metal shaft member with an elastic layer of synthetic rubber or the like having a controlled resistance value to a predetermined thickness. It is arranged to be. The charging roll 1 is basically formed by applying a predetermined DC voltage to a metal shaft member to generate a minute gap formed in a minute gap formed between the photosensitive drum 2 and the charging roll 1. The surface of the photosensitive drum 2 is charged to a predetermined potential by using discharge.

At this time, first, the charging roll 1
A DC voltage of about -1 kV is applied by the high-voltage power supply 5, and the surface potential of the photosensitive drum 2 is charged to about -400V. Next, the surface of the photosensitive drum 2 uniformly charged to the predetermined potential is exposed to the exposure device 6 as described above.
Thereby exposing the image portion to form an electrostatic latent image.

The electrostatic latent image formed on the surface of the photosensitive drum 2 is developed (reversed development) by a developer (toner) of the developing device 7 to become a toner image. The toner image developed on the photosensitive drum 2 is fed from a paper feed tray 12 by a transfer roll 8 as a transfer device, and is transferred by a registration roll 13 at a predetermined timing to a transfer position on the surface of the photosensitive drum 2. Is transferred onto the sheet 9 conveyed to the printer. Paper 9 on which the unfixed toner image is transferred
Is transported to the fixing device 10, the unfixed toner image is fixed on the paper 9 by heat and pressure by the fixing device 10, and is discharged onto a discharge tray (not shown), thereby completing the image forming process.

The transfer efficiency when the toner image formed on the photosensitive drum 2 is transferred onto the paper 9 by using a spherical toner or the like as a developer (toner) used in the developing device 7. As a result, the amount of toner remaining on the photosensitive drum 2 after transfer can be reduced, and a cleaning device for removing the toner and the like remaining on the surface of the photosensitive drum 2 can be omitted. System.

In the image forming apparatus having such a configuration, an image is output by the above-described electrophotographic process. However, normally, 100% toner is not transferred to the sheet 9 in the transfer step, and the Even if the transfer efficiency is improved by using the toner, the toner of about several% to 10% remains on the surface of the photosensitive drum 2 as transfer residual toner, returns to the charging roll 1 as it is, and partially Attaches to surface. Since ordinary toner is electrically substantially an insulator, electric charges are likely to accumulate and cause uneven discharge with the photosensitive drum 2 as shown in FIG.

FIGS. 3A and 3B show a comparison between a normal discharge current waveform and a discharge current waveform when an uneven discharge occurs. When an uneven discharge occurs, FIG.
As shown in the figure, it can be seen that the peak current value is 3 to 5 times larger than that of the normal state. During the print output, the peak current value is monitored by the discharge current observation means 3, and the discharge current observation means 3 detects when a discharge current waveform in which the peak current value exceeds a certain threshold (for example, 1 mA) is observed. Judge as abnormal. As the discharge current observation means 3, for example, 1 kΩ connected between the photosensitive drum 2 and the ground
A current detecting means including a resistance element of about a degree, an A / D converter for converting the feedback signal into digital data, and the like are used. By monitoring the discharge current waveform by such means, the degree of contamination due to residual toner can be detected or predicted.

Also, depending on the image pattern to be output,
Contamination due to residual toner can be predicted. In an image in which a high-density image is continuous in the process direction, a vertical line of a ruled line of a table, and the like, stain due to residual toner is likely to occur.
Thus, for example, the pixel counter 11 may use a unit length in the longitudinal direction of the charging roll 1 (for example, 5 mm).
Is counted from the start of image output, and when the number of accumulated pixels exceeds a certain number (for example, 10000), it is considered that the charging roll 1 is stained.

As described above, the contamination of the charging roll 1 is detected or predicted, and the high-voltage power supply control means 4 controls the high-voltage power supply 5 to change the voltage applied to the charging roll 1 from a DC voltage to a DC voltage obtained by superimposing an AC voltage. In addition, as shown in FIG. 1, the temperature is gradually changed to prevent uneven charging due to dirt.

At this time, the voltage applied to the charging roll 1 is such that a desired charging potential of the photosensitive drum 2 is Vpr, a charging start voltage is Vth (for example, 500 V), and an AC component of a DC voltage on which an AC voltage is superimposed. When the amplitude is Vpp and the value of the DC component is Vdc, the applied voltage Vdc when the DC voltage is first applied and charged is substantially (Vpr + Vth ≒ -1k).
V), Vpp was set to approximately 0 (without applying an AC voltage), image output was started, and when the photosensitive drum 2 was charged, it was detected or predicted that charging was becoming uneven. From the time point, the DC voltage Vdc is substantially reduced to (Vpr + Vt
h) to approximately Vpr, and Vpp from approximately 0 to approximately (Vth × 2
(≒ 1 kV) gradually. As described above, the reason why the voltage is gradually changed from the DC voltage to the DC voltage to which the AC voltage is applied is to prevent the influence due to the change from the DC voltage to the DC voltage to which the AC voltage is applied from appearing on the image quality. The time for gradually changing the DC voltage to the DC voltage to which the AC voltage is applied is appropriately set according to the process speed and the like.

As described above, the contamination of the charging roll 1 is detected or predicted, and the high-voltage power supply control means 4 controls the high-voltage power supply 5 to change the voltage applied to the charging roll 1 from a DC voltage to a DC voltage obtained by superimposing an AC voltage. By gradually changing the voltage as shown in FIG. 1, it is possible to prevent foreign substances such as toner attached to the surface of the photosensitive drum 2 from being charged. Uniform discharge can be prevented from occurring, and image defects such as fog can be prevented from occurring.
Moreover, as described above, since the AC voltage is not always applied to the charging roll 1 from the beginning of the image output, the charging efficiency is reduced as in the case where the AC voltage is constantly applied to the charging roll 1, When the photosensitive drum 2 receives an excessive discharge, the surface of the photosensitive drum 2 is deteriorated, and it is possible to prevent the amount of wear of the photosensitive drum 2 from increasing and the toner from sticking.

Foreign matter such as toner adhered to the surface of the charging roll 1 is neutralized by applying an AC voltage.
The toner is easily separated from the surface of the charging roll 1 and is transferred to the surface of the photosensitive drum 2 and is collected by the developing device 7 or transferred onto the sheet 9, so that the surface of the photosensitive drum 1 is gradually removed. Will be removed.

Further, in the above image forming apparatus, after a series of image forming steps is completed, when the next image forming step is started, the application may be immediately switched to the application of only the DC voltage. 3, the discharge current may be monitored, and if no discharge current exceeding a predetermined threshold is observed, the application may be switched to the application of only the DC voltage.

As described above, according to the present invention, it is detected or predicted during image output that charging is becoming uneven, and the voltage applied to the charger is gradually changed from DC to DC with AC superimposed. By doing so, it is possible to stably output high-quality images while minimizing generation of ozone and deterioration of the photoconductor due to AC discharge.

Next, in order to confirm the effects of the present invention, the inventor prototyped an image forming apparatus as shown in FIG. 2 and performed a print test for actually forming an image.

That is, in the image forming apparatus using the charging roll 1, the surface of the charging roll 1 is not stained at the beginning of the image output, but when a large number of sheets 9 are output,
The surface of the charging roll 1 is gradually contaminated by the transfer residual toner. For this reason, at the beginning of the image output, the image is uniformly charged and a good image is obtained. However, when the surface of the charging roll 1 becomes contaminated, the discharge becomes abnormal and the charging starts to be uneven. When the discharge becomes abnormal, a large current flows transiently. When the abnormal discharge is detected even once, for example, by the discharge current observation means 3, it is determined to be abnormal, and the high voltage power supply control means 4 and the high voltage power supply 5 change the voltage applied to the charging roll 1 from a normal DC to a DC Until gradually changed.

In an actual print test, an abnormal current is detected during the output of the tenth sheet, and the applied voltage is changed to a voltage obtained by superimposing AC on DC until output of the tenth sheet is completed.
Output was continued continuously up to the 20th sheet. FIG. 1 shows the waveform of the applied voltage at this time.

When a print test was performed using the charging device of the present invention as described above, it was confirmed that a good image having a uniform charging potential and no image quality defects was obtained. Also, when the applied voltage was changed to a voltage in which alternating current was superimposed on direct current, no boundary was recognized in the image.

Further, a method of estimating a stain due to the residual toner on the basis of the output image pattern was attempted. The cumulative number of pixels per 5 mm in the length of the charger in the longitudinal direction was counted from the start of image output, and when the cumulative number of pixels exceeded 10,000 during the output of the tenth sheet, it was considered that the charger was stained, and ten sheets were counted. By the end of outputting the eyes, as in FIG.
The applied voltage was changed to a voltage in which alternating current was superimposed on direct current. 2
Output was continued continuously up to the 0th sheet, but it was confirmed that a good image having a uniform charging potential and no image quality defects was obtained.

In this embodiment, the present invention is applied to a cleaner-less process in which the charging device is easily contaminated.
Since the charging device is considerably contaminated by an external additive of the toner, the same effect can be obtained.

[0046]

As described above, unlike the present invention, a simple structure can be used to prevent abrasion of a member to be charged and fixation of toner while preventing the increase in the number of parts and cost. It is possible to provide a charging device which can maintain good charging performance and can always obtain good image quality.

[Brief description of the drawings]

FIG. 1 is a waveform diagram showing a voltage applied to a charging device according to Embodiment 1 of the present invention.

FIG. 2 is a configuration diagram illustrating an image forming apparatus to which the charging device according to the first embodiment of the present invention is applied;

3 (a) and 3 (b) show Embodiment 1 of the present invention.
FIG. 6 is a waveform chart showing waveforms of a discharge current flowing through the charging device when a DC voltage is applied to the charging device according to the first embodiment.

FIG. 4 is a schematic view showing an abnormal discharge when a foreign substance adheres to the charging device.

[Explanation of symbols]

1: charging roll (charging device), 2: photosensitive drum, 3:
Discharge current observing means, 4: high voltage power control means, 5: high voltage power supply, 6: exposure apparatus, 7: developing apparatus, 8: transfer apparatus, 9:
Paper, 10: fixing device, 11: pixel counter.

Claims (5)

[Claims] 1. A charge generating member is brought into contact with a surface of a member to be charged, and a direct current voltage is applied to the member to generate a charge in a minute gap formed between the member to be charged and the member to be charged. In a charging device that charges a member to be charged by using a minute gap discharge, when the member to be charged is charged, it is detected or predicted that the charging of the member to be charged is becoming non-uniform, A charging device, wherein a voltage applied to the applying member is changed from a DC voltage to a DC voltage to which an AC voltage is applied. 2. A method for detecting or predicting that charging of a member to be charged is becoming uneven when charging the member to be charged, and changing a voltage applied to the member to be charged from a DC voltage to an AC voltage. 2. The charging device according to claim 1, wherein the voltage is gradually changed to the applied DC voltage. 3. A desired charging potential of the member to be charged is Vp
r, the charging start voltage is Vth, the amplitude of the AC component of the DC voltage on which the AC voltage is superimposed is Vpp, and the value of the DC component is Vdc. The applied voltage Vdc when the DC voltage is first applied and charged.
Is set to approximately (Vpr + Vth), Vpp is set to approximately 0, and image output is started. When charging or non-uniformity is detected or predicted when charging the member to be charged, Vdc is substantially reduced. From (Vpr + Vth) to approximately Vpr,
3. The charging device according to claim 2, wherein Vpp is gradually changed from approximately 0 to approximately (Vth.times.2). 4. The method according to claim 1, wherein a discharge current waveform during charging is observed, and the magnitude of the peak current value is determined to detect that charging is becoming non-uniform during image output. The charging device according to claim 1. 5. An image forming apparatus according to claim 1, wherein the member to be charged is an image carrier for forming an image, and the density and / or pattern of the image to be formed on the image carrier is determined in advance so that charging becomes uneven. 2. The time of the prediction is predicted.
The charging device according to any one of claims 1 to 3.