JPH09258621A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the density difference of image caused by the potential unevenness of the surface of a photoreceptor without remarkably increasing a cost. SOLUTION: This device is provided with an electrostatic charging roller 2, a discharging lamp 9 provided on the upstream side in the rotational direction of the photoreceptor 1 with respect to the electrostatic charging roller 2 and a discharging brush 8 discharging the electric charge of a reverse polarity to electrostatically charged electric charge by the roller 2 and provided on the upstream side, and voltage is applied to the roller 2 and the brush 8 form a common power source 10. Thus, even in the case that the potential of the reverse polarity to the electrostatically charged electric charge by the roller 2 is generated at the photoreceptor 1 at the time of transferring the toner image on the photoreceptor 1 on transfer paper P by a transferring device 5, it is discharged by the brush 8, so that the potential unevenness is not generated. Also, the voltage is applied to the roller 2 and the brush 8 from the common power source 10, so that the increase of the cost is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a laser printer and a facsimile.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus, an electrostatic latent image is formed by irradiating light on the surface of a photosensitive member negatively or positively charged by a charging means to eliminate the electric charge. There is a negative / positive developing system in which a toner image is formed by attaching toner charged to the light irradiation portion with the same polarity as the charging potential of the photoconductor.

In this type of image forming apparatus, the toner image on the photoconductor is transferred onto a transfer paper by a transfer means having a bias (or charge) having a polarity opposite to that of the photoconductor and the toner. Further, as a charging means for charging the surface of the photoconductor, a contact charging method using a contact charging member such as a charging roller obtains a desired charging potential as compared with a corona charging method which has been conventionally performed. Therefore, it has come to be used because it has the advantages that the applied voltage required for that purpose can be reduced and that the amount of ozone generated during the charging process is extremely small.

[0004]

However, in the case of the above-mentioned image forming apparatus, the application of the transfer bias of the transfer means generates a potential of the opposite polarity to the potential applied to the photoconductor by the charging means. . The generation of the electric potential of the opposite polarity changes depending on the transfer conditions, but is particularly likely to occur at the portion where the toner image is present or the surface portion of the photoconductor corresponding to the space between the transfer sheets. For example, when the photoconductor is negatively charged, the portion corresponding to the space between the transfer papers on the surface of the photoconductor is likely to be positively charged by applying a positive bias by the transfer unit.

In particular, in the case of a high speed machine having a high processing speed, a large amount of transfer current is required, so that the surface of the photosensitive member is likely to be reversely charged as compared with the low speed machine.
In this way, if the surface of the photoconductor is oppositely charged, the photoconductor normally has a negative effect on the charge removal effect due to light, and therefore the sensitivity is negative. If the charging unit has the charging polarity side by the charging unit, the reverse charging unit by the transfer unit cannot be neutralized by the optical neutralization unit, so that there is a problem in that potential unevenness occurs on the surface of the photoconductor. .

In such a case, the surface portion of the photosensitive member having the potential unevenness proceeds to the charging step and is charged, and an image is formed at that portion, so that a density difference occurs in the formed image. There was a chance that it would end up. The occurrence of this density difference is apt to appear remarkably when the contact charging type charging means is used because the charging ability is inferior to that of the corona discharge type. Therefore, conventionally, the transfer voltage of the transfer means has been controlled so as to be weakened at a portion corresponding to a space between transfer sheets. However, in the case of this method, even though the reverse charging between the transfer papers can be prevented, the reverse charging that occurs during transfer cannot be prevented.
It has not been possible to prevent the potential unevenness as described above.

The present invention has been made in view of the above problems, and it is possible to prevent a difference in image density caused by potential unevenness on the surface of a photoconductor without significantly increasing the cost. With the goal.

[0008]

In order to achieve the above object, the present invention provides a contact charging means that rotates while contacting a rotating photoreceptor, and an upstream side of the contact charging means in the rotation direction of the photoreceptor. Is provided on the upstream side in the rotation direction with respect to the optical charge eliminating means, and a contact charge eliminating means for eliminating charge having a polarity opposite to the charge charged by the contact charging means is provided on the upstream side in the rotation direction. The image forming apparatus is configured by connecting the means to a common power source.

With this configuration, when the surface of the photoconductor charged by the contact charging means is irradiated with light, an electrostatic latent image is formed on the surface, and the electrostatic latent image has the same charge potential as the photoconductor. A toner image is formed by the toner charged in polarity. The toner image is transferred onto a transfer sheet by a transfer means having a bias having a polarity opposite to that of the toner, and at that time, a potential having a polarity opposite to that applied to the photoconductor by the charging means is generated. Is removed by contact removing means for removing charges having a polarity opposite to the charge charged by the contact charging means, and the surface of the photoconductor is removed by light removing means. Therefore, no potential unevenness occurs on the surface of the photoconductor, so that the formed images have no density difference. Further, since a voltage is applied to the contact charging means and the contact charge eliminating means from a common power source,
The increase in cost can be suppressed by the fact that it is not necessary to provide a power source for each.

Further, the resistance value of the contact charge eliminating means may be smaller than the resistance value of the contact charging means. that way,
When the applied voltage required to obtain a predetermined charging potential is applied to the contact charging means, the voltage is also applied to the contact charge eliminating means to generate a charge having a polarity opposite to that of the charge charged by the contact charging means on the photoconductor. Since the effect of static elimination by the contact static elimination means for eliminating static electricity is enhanced, it is possible to surely eliminate the charges of the opposite polarity on the photoconductor. Therefore, uneven potential is less likely to occur on the surface of the photoconductor.

Further, in the above image forming apparatus, it is preferable to provide means for applying a voltage to the contact charging means and the contact charge removing means at the same timing. By doing so, a uniform and even image can be obtained by the same simple control of the applied voltage as in the case where the contact charge eliminating means is not provided.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an image forming unit of an image forming apparatus together with a voltage control system for explaining an embodiment of the present invention. In this image forming apparatus, a drum-shaped photoconductor 1 that rotates in the direction of arrow A is provided in the approximate center of a device body (not shown), and in the surroundings thereof, in the direction of arrow B while contacting the photoconductor 1. A charging roller 2 which is a rotating contact charging means, an optical writing device 3, and a developing device 4.
A transfer device 5 having a transfer belt 13, a cleaning device 6 having a cleaning blade 7, a static elimination brush 8 which is a contact static elimination unit, and a static elimination lamp 9 which is an optical static elimination unit are arranged.

The charge eliminating lamp 9 is provided on the upstream side of the charging roller 2 in the rotation direction of the photoconductor 1, and the charge eliminating brush 8 is further disposed on the upstream side of the charge elimination lamp 9 in the rotation direction of the photoconductor 1. Is provided. The static elimination brush 8
Has a function of removing a negative charge from the charging roller 2 and a positive charge having a reverse polarity. Then, the charging roller 2 and the charge eliminating brush 8 are connected to a common power source 10.

The transfer device 5 includes a transfer belt 13 wound around a pair of rollers 11 and 12, and the transfer belt 1 thereof.
Bias roller 15 for applying a transfer bias to 3
Etc. The drive-side roller 12 is rotated in the arrow C direction in FIG. 1 by a motor (not shown), whereby the transfer belt 13 is rotated in the arrow E direction. The roller 11 is a conductive driven roller made of metal and simply supports the transfer belt 13 rotatably, and is not electrically directly connected to another conductive member.

On the other hand, the driving-side roller 12 has a function of increasing the grip force when the transfer belt 13 is driven,
The surface is formed of a material such as rubber. The bias roller 15 is arranged so as to contact the inner surface of the transfer belt 13 near the position where the transfer belt 13 contacts the photoconductor 1. The bias roller 15 functions as a contact electrode for applying a charge having a polarity opposite to the charge polarity of the toner on the photoconductor 1 to the transfer belt 13. The bias roller 15 is connected to a high voltage power source 20 that applies a transfer bias to the transfer belt 13.

As the transfer belt 13, it is preferable to use a transfer belt having a small resistance change due to environmental changes. For example, a two-layer structure may be used.
Then, carbon, zinc oxide or the like is used as a conductive material for adjusting the resistance of the transfer belt 13, and an appropriate amount thereof is added, and the material of the transfer belt 13 is, for example, chloroprene rubber, EPDM rubber, silicone rubber, epichlorohydrin. It is advisable to select a material such as rubber that has a low hygroscopicity and a stable resistance value.

A voltage is applied to both the charging roller 2 and the static elimination brush 8 by the controller 30. Then, voltage is applied to the charging roller 2 and the charge eliminating brush 8 by the controller 30 at the same timing. As shown in FIG. 2, the control device 30 includes a central processing unit (CPU) 32 having various judgment and processing functions, a ROM 33 storing each processing program and fixed data, and a RAM 34 which is a data memory storing the processing data. And a microcomputer including an input / output circuit (I / O) 35, a drive circuit 36 for applying a voltage to the charging roller 2 and the charge eliminating brush 8, and the like.

In this image forming apparatus, when the image forming operation is started, the photoreceptor 1 which is the negatively charged OPC photoreceptor shown in FIG. 1 rotates in the direction of arrow A in FIG. The charging roller 2 to which -1500V is applied is uniformly charged to, for example, -900V. Then, light corresponding to the image of the original is incident on the charged surface from the optical writing device 3, and an electrostatic latent image is formed there. The electrostatic latent image is removed (trimmed) of electrostatic charges in the area outside the size of the transfer paper P used by an eraser (not shown), and the remaining electrostatic latent image is developed by the developing sleeve 16 of the developing device 4. The toner image (visible image) is obtained by performing reversal development with the negatively charged toner supplied by.

On the other hand, the transfer paper in the paper feed cassette (not shown) is fed by a paper feed roller which rotates at a predetermined timing.
The sheets are sent out one by one, temporarily stopped by the pair of registration rollers 14 and adjusted in timing, and then conveyed toward the transfer portion of the transfer device 5 at an accurate timing that matches the toner image on the photoconductor 1. Then, the toner image on the photoconductor 1 is transferred onto the transfer paper P by the positive transfer bias applied from the high voltage power source 20 to the bias roller 15 in contact with the inner surface of the transfer belt 13.

At this time, a positive transfer bias generates a positive potential having a polarity opposite to that of the negative potential applied by the charging roller 2 on the photosensitive member 1. Although this phenomenon changes depending on the transfer conditions, it is particularly likely to occur at the portion where the toner image is present and the surface portion of the photoconductor 1 corresponding to the space between the transfer sheets.
The transfer paper P on which the image is transferred is separated from the photoconductor 1 and conveyed to a fixing device (not shown), where the toner is fixed and then discharged to a discharge tray or the like outside the device.

After the transfer, foreign matters such as residual toner and paper dust remaining on the photoconductor 1 are removed by the cleaning device 6, and the surface of the photoconductor 1 is supplied with -1500V from the power source 10. With the static elimination brush 8,
The positive potential is raised to the negative side. After that, the residual potential (minus side) remaining on the photoconductor 1 is uniformly removed by the static elimination lamp 9 to prepare for the next charging by the charging roller 2.

As described above, in this image forming apparatus, the toner image on the photoconductor 1 developed by the developing device 4 is transferred onto the transfer paper P by the transfer device 5 having a positive bias having a polarity opposite to that of the toner. At this time, a positive potential is generated on the photoconductor 1, but since the positively charged potential is neutralized by the static elimination brush 8, potential unevenness does not occur on the surface of the photoconductor 1. Therefore, it is possible to prevent the density difference from being generated in the image due to the potential unevenness. Further, in this image forming apparatus, since the voltage is applied from the common power source 10 to the charging roller 2 and the static elimination brush 8, it is not necessary to provide a power source to each of them, and thus the cost increase can be suppressed.

In this embodiment, the static elimination brush 8 is also used.
Is smaller than the resistance value of the charging roller 2. As a result, when the applied voltage required to obtain a predetermined charging potential is applied to the charging roller 2, the charge removal brush 8
Just by applying the same applied voltage to the
Since the effect of eliminating the positive charge by the is enhanced, the positive charge on the photoconductor 1 can be eliminated more reliably. Therefore, uneven potential is unlikely to occur on the surface of the photoconductor 1.

[0024]

As described above, according to the present invention, since a voltage is applied to each of the contact charging means and the contact charge removing means by a common power source, only one power source is required, which results in a significant cost increase. Without this, it is possible to prevent a difference in image density due to potential unevenness from occurring on the surface of the photoconductor.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an image forming unit of an image forming apparatus together with a voltage control system for explaining an embodiment of the present invention.

2 is a block diagram showing a configuration of a control device 30 of FIG.

[Explanation of symbols]

 1: Photoconductor 2: Charging roller (contact charging means) 8: Static elimination brush (contact static elimination means) 9: Static elimination lamp (optical static elimination means) 10: Power supply 30: Control device

Front Page Continuation (72) Inventor Takaya Muraishi 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Company (72) Inventor Toshiyuki Uchida 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Company

Claims (3)

[Claims] 1. A contact charging unit that rotates while being in contact with a rotating photosensitive member, a light discharging unit provided upstream of the contact charging unit in the rotation direction of the photosensitive member, and a light discharging unit thereof. Is provided on the upstream side in the rotation direction, a contact charge removing unit for removing charge having a polarity opposite to that of the charge charged by the contact charging unit, and the contact charging unit and the contact charge removing unit are connected to a common power source. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein a resistance value of the contact charge eliminating means is smaller than a resistance value of the contact charging means. 3. The image forming apparatus according to claim 1, further comprising means for applying a voltage to the contact charging means and the contact charge removing means at the same timing.