JPH08110672A - Color image forming device - Google Patents

Abstract

PURPOSE: To maintain image quality without color mixture over a long period by making the potential of the nonimage region of an electrostatic latent image carrier equal to that of a developing roll while a nonimage region passes through a nondeveloping roll, when a developing operation is not executed. CONSTITUTION: This device is controlled so that the DC voltage of the developing roll bias of a developing device which does not execute development for the next process is made equal to the surface potential of a photoreceptor drum 1 when the streak of residual toner after a cleaning blade 16 is retreated passes through the developing device. That is, a prescribed developing bias voltage is applied to the developing rolls 3a-6a of respective developing devices 3-6 by DC and AC power sources for a developing bias 20 and 21 and the developing bias voltage applied to the respective developing rolls 3a-6a can be controlled by a control circuit 22 at prescribed timing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus used as an electronic copying machine, an electrostatic recording device, a facsimile, a transmission device, a laser printer, etc., and more particularly to a color image having a plurality of developing devices. The present invention relates to a forming device.

[0002]

2. Description of the Related Art Conventionally, various types of color image forming apparatuses using electrophotography have been proposed and commercialized. As one of such color image forming apparatuses, toner images of a predetermined number of colors are sequentially formed on a photosensitive drum, and the toner images of the respective colors formed on the photosensitive drum are recorded on a recording paper held around the transfer drum. There is one in which a color image is formed by transferring them in the state of being sequentially overlapped. In the case of this type of color image forming apparatus, a charger, an exposure unit, and a plurality of, for example, cyan, magenta, yellow, and black developing units corresponding to respective colors, a transfer unit, and a cleaning unit are provided around a drum-shaped photosensitive member. Place the device. Then, the steps of charging, exposing, developing, transferring, and cleaning are repeated for each rotation of the photosensitive drum, and the toner images of the respective colors formed on the photosensitive drum are sequentially transferred in a state of being superposed on the recording paper, A color image is formed by passing the recording paper through a fixing device.

As another system of the above-mentioned color image forming apparatus, a charging device, an exposing part, a plurality of developing devices for cyan, magenta, yellow and black corresponding to respective colors are provided around a drum-shaped photosensitive member. A transfer unit and a cleaning device are arranged, and toner images of respective colors are sequentially formed on the photosensitive drum in a state of being overlapped with each other, and the toner images of respective colors formed on the photosensitive drum are transferred once. There is also known a one-time transfer method color image forming apparatus that forms a color image by collectively transferring onto a recording paper in a process.

Among the above-mentioned color image forming apparatuses, in the case of an apparatus for sequentially transferring the toner images of respective colors in a state of being superposed on the recording paper held around the former transfer drum, the image is transferred onto the photosensitive drum after transfer. The toner remaining after transfer adheres, but the surface of the photoconductor drum is cleaned by the cleaning device that cleans the remaining toner, and the photoconductor drum again forms a new electrostatic latent image and repeats image formation. I can do it. This color image forming apparatus can perform a cleaning operation by constantly contacting the cleaning device with the photoconductor drum while forming an image with a plurality of cyan, magenta, yellow, and black developing devices, for example. Therefore, it is not necessary to release the cleaning member from the surface of the photosensitive drum for each color.

On the other hand, among the above-mentioned color image forming apparatuses, in the case of an apparatus for sequentially forming toner images of respective colors on the photosensitive drum of the latter one by one for one rotation and transferring the toner images collectively onto a recording sheet, If the cleaning operation is performed by contacting the photoconductor drum during formation, the image created in the previous step will be erased, so the cleaning member is released from contact with the photoconductor drum and is kept in a separated non-contact state. At least after the image formation is completed, the cleaning operation is performed by contacting the surface of the photoconductor drum, and the photoconductor drum again forms an electrostatic latent image so that the image formation can be repeated. Similarly, in the color image forming apparatus,
The developing device that has not been developed needs to be released from contact during image formation and kept in a non-contact state apart from it.

Conventionally, a blade cleaning method is often used as a cleaning means for cleaning the toner remaining on the photosensitive drum because of its simple structure. However, in the latter color image forming apparatus, since the toner images are superposed on the photosensitive drum, toner streaks at the blade edge are left in stripes on the photosensitive drum after the cleaning member is separated. Normally, the separation position of the cleaning blade is set in a non-image area on the photoconductor drum so that a streak of toner accumulation does not occur on the recording paper.

[0007]

However, the above-mentioned prior art has the following problems. That is, in the case of the above-described conventional color image forming apparatus, when the cleaning member is separated from the photoconductor drum and the toner accumulated at the blade edge on the photoconductor drum passes through the developing device, There is a problem in that the toner pool on the photoconductor drum may adhere to the non-developed development roll in the closest position between the developing roll and the photoconductor drum.

More specifically, in the above-described color image forming apparatus, the developer as a developing device is held in a non-contact state without directly contacting the photoconductor drum, and the developer is held between the developing roll and the photoconductor drum by an oscillating electric field. When a developing method that causes the toner to fly is used, the developability is secured by setting the gap between the photoconductor drum and the developing roll to be small. There is a case where the toner is electrically contacted with the other, and a case where the charged toner flies from the photosensitive drum to the developing roll side due to the potential difference between the photosensitive drum and the developing roll. If the former toner pool on the photoconductor drum physically contacts the developing roll, the gap between the photoconductor drum and the developing roll should be set so that the developing device does not disturb the superposed toner image in the high density area of the image area. It is maintained and very unlikely to occur unless a very high toner accumulation is formed on the photoconductor, and the occurrence frequency is low. On the other hand, the latter toner that moves from the photoconductor drum due to the potential difference between the photoconductor drum and the development roll does not depend on the physical height of the toner, but the charge of the toner and the adhesive force with the photoconductor drum cause The amount adhered to is determined.

As a technique capable of solving such a problem,
For example, as disclosed in Japanese Patent Application Laid-Open No. 1-157673, there is one configured to prevent color mixture during non-development by applying a bias voltage in which an AC component and a DC component are superposed to a developing roll. is there. This causes a potential difference in the direction from the developing roll to the photoconductor drum to cause the toner to fly only from the developing roll to the photoconductor drum side, and prevent the toner from adhering to the developing roll from the opposite photoconductor drum. To prevent. In the case of the apparatus according to the above proposal, it is an effective means for the image area at the time of non-development. The toner on the photosensitive drum in the image area retains the electric charge when it is developed by the potential of the photosensitive drum surface and the developing bias, and does not contain the toner of the opposite polarity or the toner of the opposite polarity. Since it is very small even if it is included, the direction of the electric field generated by the bias of the developing roll and the potential of the photosensitive drum can be regulated so that it does not adhere to the developing device from the photosensitive drum.

However, the charges of the residual toner accumulated on the edge of the blade after cleaning are not always the same in polarity. This is because when the toner image is transferred from the photoconductor drum onto the recording paper, a discharge having a polarity opposite to the polarity of the toner is carried out from the back surface of the recording paper, and the charge of the toner image on the photoconductor drum is exchanged. Then, the toner is attached to the paper. At this time, the residual toner after the transfer on the photoconductor drum has a difference in charge due to the history of charge exchange at the time of transfer, and a toner having a reverse polarity is partially generated. Since the toner of the opposite polarity is included in the residual toner accumulated on the edge of the blade after cleaning, naturally the toner of the opposite polarity is also included in the toner pool generated on the photosensitive drum when the blade is separated. . Therefore, as described above, in the developing roll bias in which the direction of the electric field generated by the bias voltage of the developing roll and the potential of the photosensitive drum is set in one direction so as not to adhere to the developing device, the toner having the opposite polarity is used. Is included in the residual toner on the photoconductor drum, the toner having the opposite polarity is attracted toward the developing roll, which causes a problem that the residual toner on the photoconductor drum adheres to the developing roll. .

When the product is continuously used for a long period in that state,
There is a problem in that the toner of different color mixed in the developing device increases from the toner pool generated when the cleaning blade on the photoconductor drum is separated, so that the color appears as dust on the copy and the image quality deteriorates. It was

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art. The object of the present invention is to attach and detach the cleaning means to the surface of the electrostatic latent image carrier at a predetermined timing. By doing so, in the color image forming apparatus that removes the toner remaining on the electrostatic latent image bearing member, it is possible to prevent the residual toner on the electrostatic latent image bearing member from being mixed into the developing unit when the cleaning operation is cancelled. An object of the present invention is to provide an image forming apparatus that can maintain image quality without color mixture for a long period of time.

[0013]

The above-mentioned technical problems are as follows.
In the color image forming apparatus according to the first aspect of the present invention, a charging unit, an exposing unit and a plurality of developing units are arranged around the electrostatic latent image carrier, and the electrostatic latent image carrier is moved a plurality of times. By repeating the steps of rotating, charging, exposing, and developing a plurality of times to form a plurality of images of different colors on the electrostatic latent image carrier in a superimposed manner, a transfer means for collectively transferring to a transfer material is provided. In the color image forming apparatus having a cleaning unit that comes in contact with and separates from the electrostatic latent image carrier, a color image forming apparatus has a control circuit that can arbitrarily change the DC voltage applied to the developing roller of the developing unit, This is achieved by controlling the electric potential of the non-image area of the electrostatic latent image carrier to be equal to the electric potential of the developing roll when at least the non-image area passes through the non-developing roll when the developing operation is not performed.

Further, in the color image forming apparatus according to the second aspect of the present invention, at least the cleaning means is retracted from the electrostatic latent image carrier with respect to the potential of the non-image area on the electrostatic latent image carrier. This is achieved by controlling the DC voltage applied to the developing roll of the developing means so as to have a means for detecting the potential of the above-mentioned portion and to make it equal to the detected potential of the electrostatic latent image carrier.

Further, in the color image forming apparatus according to the third aspect of the present invention, the developing device for performing the control can be achieved at least by a device other than the developing device using the black toner.

[0016]

The color image forming apparatus according to the present invention has the control circuit capable of arbitrarily changing the DC voltage applied to the developing roll of the developing means, and at least when the developing operation for image formation is not performed, the non-image area is formed. Is configured to control the potential of the non-image area of the electrostatic latent image bearing member to be equal to the potential of the developing roll when passing through the non-developing roll. At the time of separation, even when the residual toner cleaned by the cleaning means adheres to the surface of the electrostatic latent image carrier in a streak pattern,
When the surface of the electrostatic latent image carrier to which the residual toner adheres passes the non-developing roll, the potential of the non-image area of the electrostatic latent image carrier becomes equal to the potential of the developing roll. Therefore, in the residual toner adhered to the surface of the electrostatic latent image carrier, even if there is a toner charged to the opposite polarity in addition to the normally charged toner, the non-image area of the electrostatic latent image carrier is Since the electric potential and the electric potential of the developing roll are equal, an electrostatic field does not act in any direction between the non-image area of the electrostatic latent image bearing member and the developing roll, and thus the electrostatic latent image carrier is charged with the opposite polarity. It is possible to prevent toner and the like from electrostatically adhering to the surface of the developing roll. Therefore, it is possible to prevent the residual toner on the electrostatic latent image carrier from being mixed into the developing unit when the cleaning unit is deactivated, and it is possible to maintain image quality without color mixture for a long period of time.

Further, in the color image forming apparatus according to the second aspect of the present invention, at least the cleaning means is retracted from the electrostatic latent image carrier with respect to the potential of the non-image area on the electrostatic latent image carrier. Since it has a means for detecting the potential of the portion, and is configured to control the DC voltage applied to the developing roll of the developing means so as to be equal to the detected potential of the electrostatic latent image carrier, The electric potential of the non-image area of the electrostatic latent image carrier and the electric potential of the developing roll can be accurately controlled to be equal to each other, and the adhesion of the residual toner to the developing roll can be prevented more reliably.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 1 shows an embodiment of an image forming apparatus according to the present invention.

In FIG. 1, reference numeral 1 denotes a photosensitive drum as a cylindrical electrostatic latent image carrier, and the photosensitive drum 1 is rotated at a predetermined rotational speed in the direction of an arrow by a driving device (not shown). It is designed to be rotationally driven by.
Around the photosensitive drum 1, a primary charger 2 composed of one scorotron and four developing devices 3 to 6 are arranged. These developing devices 3 to 6 are composed of, for example, yellow, magenta, cyan, and black developing devices.

The surface of the photoconductor drum 1 is uniformly charged to a predetermined potential (for example, -650 V) by the primary charger 2, and then a laser light generator including a semiconductor laser and a polygon rotary mirror 7 is built in. An image corresponding to yellow color is exposed by the laser beam 9 emitted from 8 according to image information, and an electrostatic latent image corresponding to yellow color is formed on the surface of the photoconductor drum 1. The electrostatic latent image formed on the photoconductor drum 1 is developed by the developing device 3 for yellow color. At this time, the laser beam 9 for image exposure is written in the image portion on the photoconductor drum 1, and has the same polarity as the surface potential of the photoconductor drum 1 charged by the primary charger 2. With the toner having the electric charge of, the laser light irradiation portion of the photosensitive drum 1 is developed by the reversal development method. Further, the yellow developing device 3 is configured to be in a non-contact state with the photosensitive drum 1 at least after the development is completed,
To prevent the toner image once developed on the photoconductor drum 1 from being scraped before starting the developing process of another color, the rotation of the developing roll 3a is stopped when passing through the image area during non-developing, and any developing roll The bias is applied by the developing bias power source.

Next, as a second color image forming cycle,
The photoconductor drum 1 is recharged by the primary charger 2, is irradiated with the laser beam 9 in the previous step (yellow image forming step), and the surface of the photoconductor drum 1 having the potential lowered again becomes the original one. The potential recovers to near the potential. Then, an image corresponding to magenta color is exposed on the surface of the photoconductor drum 1 by the laser beam 9 emitted from the laser beam generator 8 to form an electrostatic latent image corresponding to magenta color. The electrostatic latent image formed on the photoconductor drum 1 is magenta toner having a charge having the same polarity as the surface potential of the photoconductor drum 1, and magenta developed on the laser light irradiation portion of the photoconductor drum 1. Developed by being supplied by the device 4. The magenta developing device 4 disturbs the toner image on the photoconductor drum 1 developed by the black developing device 3 in the previous step, or develops the toner image already developed on the photoconductor drum 1. The device has been devised so that it does not undergo reverse transfer.

Thereafter, similarly, as the third image forming cycle, the primary charger 2 for the surface of the photosensitive drum 1 is used.
Recharge by laser light 9 from laser light generator 8
Of a cyan image by the developing device 5 for cyan, and as the fourth cycle, recharging by the primary charger 2, irradiation of a black image by the laser beam 9 from the laser beam generator 8, and development for black. Each step of developing a black image by the device 6 is performed. The cyan and black developing devices 5 and 6 disturb the toner images of the respective colors on the photoconductor drum 1 developed by the developing device in the previous step, and the photoconductor drum 1
The same devise as that of the developing device 4 is applied so that the toner image already developed above is not reversely transferred to the developing device side.

In this way, at the time when the black image developing process by the black developing device 6 is completed, yellow, magenta, cyan, and black are formed on the photosensitive drum 1 according to the image irradiated with the laser beam 9. A four-color toner image is formed.

After that, the yellow, magenta, cyan, and black toner images formed on the photosensitive drum 1 are
If necessary, pre-transfer charging by a pre-transfer charging device (not shown) is performed, and after charging or discharging is performed in an optimum state for transferring a toner image, the transfer charging device 11 causes the paper guide 12 to move.
The yellow, magenta, cyan, and black toner images on the photoconductor drum 1 are transferred onto the transfer paper (not shown) that has entered further in one transfer process. After the transfer is completed, the transfer sheet is destaticized by the peeling charger 13, separated from the photosensitive drum 1, and conveyed to a fixing device (not shown) by the conveyor belt 14 to be subjected to a fixing step.

After the transfer process is completed, the residual toner on the photosensitive drum 1 is cleaned by the cleaning blade 16 of the cleaning device 15, and subsequently, the series of image forming processes are completed by receiving the exposure from the charge eliminating exposure device 17. If necessary, the charging process by the primary charger 2 is started again, and all the above-mentioned processes are repeated to output a plurality of continuous prints.

Here, the cleaning device 15 is structured such that the entire device or the cleaning blade 16 can be retracted, and is in non-contact with the photosensitive drum 1 during the formation of the toner images of the four colors. Is in a state,
It is configured so as not to disturb the toner image formed on the photosensitive drum 1. The cleaning blade 16 is
Retraction is performed at the position of the non-image forming area so that streaks of residual toner are not generated on the print in the next process.

The cleaning of the photosensitive drum 1 is
The process is performed in a cycle after the transfer is completed, and after the transfer of the immediately preceding image is completed, the photosensitive drum 1 is brought into contact with the photosensitive drum 1, and the image to be created next is retracted before reaching the cleaning device 15. Also,
For cleaning the surface of the photosensitive drum 1, the time for one rotation of the photosensitive drum may be provided. In the former case, it is possible to increase the apparent copy speed, but since the image forming cycle is entered when the cleaning device 15 is retracted / contacted, it causes a speed fluctuation in the photosensitive drum 1. On the other hand, in the latter case, the effect of the retreat / contact of the cleaning device 15 is small, but since a cleaning cycle needs to be separately provided, the apparent copy speed decreases.

By the way, in this embodiment, when the streaks of the residual toner after the cleaning blade 16 retreats passes through the developing device which is not developing in the next step, the DC voltage of the developing roll bias of the developing device is changed. The surface potential of the photosensitive drum 1 is controlled to be equal to the surface potential. That is, in this embodiment, as shown in FIG. 1, the developing rolls 3a to 6a of the developing devices 3 to 6 are provided with a DC power source 20 (-500Vor-650V) for developing bias and an AC power source 2.
1 (8 KHz, 1.6 kV _PP ) allows a predetermined developing bias voltage to be applied, and
The developing bias voltage applied to the developing rolls 3a to 6a is controlled by the control circuit 22 at a predetermined timing.

FIG. 2 shows the sequence of one embodiment of the present invention. In this embodiment, the timing at which the developing roll bias DC is changed to the three-stage setting of the bias during image formation, the bias during non-image formation, and the bias during non-image formation is shown. Further, this is an example in which the developing devices 3 to 5 other than the black toner in which color mixing is less likely to be a problem are controlled.

In the color image forming apparatus according to the present embodiment having the above-described structure, the residual toner on the photosensitive drum at the time of releasing the cleaning operation is prevented from being mixed into the developing means and the color mixing is performed for a long time as follows. It has become possible to maintain high image quality. That is, in the color image forming apparatus, as shown in FIG. 2, the drum motor is driven to rotate the photosensitive drum 1, and the surface of the photosensitive drum 1 is driven by the primary charger 2 to a predetermined potential (for example, (-650V) is uniformly charged.
Next, as shown in FIG. 1, the surface of the photosensitive drum 1 is exposed with an image corresponding to black by a laser beam 9 emitted from a laser beam generator 8 according to image information,
An electrostatic latent image corresponding to yellow is formed on the surface of the photosensitive drum 1. The electrostatic latent image formed on the photoconductor drum 1 is developed by the developing device 3 for yellow color.

A cleaning blade 16 of a cleaning device 15 for cleaning the surface of the photosensitive drum 1 prior to exposing the photosensitive drum 1 to a black image.
Is to be retracted from the surface of the photosensitive drum 1 as shown in FIG. Therefore, the photosensitive drum 1
As the cleaning blade 16 retreats, the toner T that was previously removed remains in a streak shape on the surface of the.

By the way, in this embodiment, as shown in FIG. 2, when the streaky toner adhering to the surface of the photosensitive drum 1 passes through the developing devices 3-5, the developing devices 3-5
The developing rolls 3a to 5a of No. 5 are configured to apply a voltage (-650 V) equal to the potential of the non-image area of the photosensitive drum 1.

Thereafter, as shown in FIG. 2, the same operation is repeated for the magenta, cyan, and black toner images.

Thus, the toner pool T generated on the photosensitive drum 1 due to the withdrawal of the cleaning blade 16 does not move to the developing roll side when passing the portion where the developing roll and the photosensitive drum 1 are close to each other. This is because, when a toner image is formed on the photoconductor drum 1, toner powder charged to almost the same polarity moves in the direction from the developing devices 3 to 5 side toward the photoconductor drum 1 under the force of the electric field. , A toner image is formed in proportion to the magnitude of the electrostatic latent image potential of the photosensitive drum 1. The toner on the photoconductor drum 1 formed in this way has a slight change in the charge of the toner itself, but does not lose the charge or change such that the polarity is further inverted. Therefore, the photosensitive drum 1
In order to prevent the toner from scattering from above, the developing roll bias may be determined so that an electric field can be formed in the direction in which the toner does not fly from the photosensitive drum 1.

However, the charge of the toner pool generated on the photosensitive drum 1 due to the separation of the cleaning blade 16 has a wide charge distribution unlike the toner charge immediately after the development. This is for the following reasons. That is, the residual toner after the transfer on the photoconductor drum 1 has a difference in charge due to the history of charge exchange at the time of transfer, and toner of opposite polarity is partially generated. Since the toner of the opposite polarity is included in the residual toner accumulated on the edge of the blade 16 after cleaning, naturally, the toner pool generated on the photosensitive drum 1 when the blade 16 is separated also has the opposite polarity. It will contain toner. As a result, when the toner pool attached to the photosensitive drum 1 passes near the developing roll to which a bias that is displaced to the photosensitive drum 1 is applied, the toner is attached to the developing roll.

On the other hand, as described above, when the streaks of the residual toner after the cleaning blade 16 retreats passes through the developing devices 3 to 5 in which the developing process of the next step is not performed, the developing devices 3 to 5 concerned. By controlling the DC voltage of the developing roll bias of No. 5 to be equal to the surface potential of the photoconductor drum 1, when the toner pool attached on the photoconductor drum 1 passes near the developing roll, the toner pool is collected. It is possible to prevent the reverse polarity toner inside from adhering to the developing roll.

FIG. 3 shows the result of the test by the actual machine.
In this result, the weight of the residual toner on the photosensitive drum 1 before and after passing through the developing roll was measured, and the reduction amount of the residual toner was compared when the potential of the photosensitive drum 1 in the residual toner portion was −650V. The gap between the developing roll and the surface of the photosensitive drum 1 is about 20.
It is set to 0 μm, and the height of the toner pool generated on the photosensitive drum 1 is 50 μm at the maximum, and it has been confirmed that the toner pool is a height that does not physically contact. As a result, when the developing roll bias DC has a value close to the surface potential of the photosensitive drum 1, it is found that the residual toner is less reduced and does not easily mix in the developing device.

In the above embodiment, the DC voltage value of the developing roller is set to the preset potential of the photosensitive drum due to charging, but in order to surely match it, it is shown in FIG. So that the cleaning blade 16
Of the residual toner generated when the toner is separated from the photosensitive drum 1
This can be achieved by detecting the potential of No. 2 with the electrometer 30 immediately before passing through the developing device and controlling the developing roll bias DC.

The other structure and operation are the same as those of the above-mentioned embodiment, and the description thereof will be omitted.

[0041]

According to the present invention, which has the above-described structure and operation, the toner remaining on the image carrier is removed by bringing the cleaning means into and out of contact with the surface of the image carrier at a predetermined timing. In the image forming apparatus, it is possible to provide the image forming apparatus which can prevent the residual toner on the image carrier from being mixed into the developing device when the cleaning operation is released, and can maintain the image quality without color mixture for a long period of time.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a color image forming apparatus according to the present invention.

FIG. 2 is a timing chart showing the operation of an embodiment of the color image forming apparatus according to the present invention.

FIG. 3 is a graph showing effects of an embodiment of the color image forming apparatus according to the present invention.

FIG. 4 is a block diagram showing another embodiment of the color image forming apparatus according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Photoconductor drum, 3-6 developing device, 3a-6a developing roll, 20 developing bias DC power supply, 21 developing bias AC power supply, 22 control circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nagakazu Okegawa 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (3)

[Claims] 1. A charging unit, an exposing unit, and a plurality of developing units are arranged around an electrostatic latent image carrier, and the electrostatic latent image carrier is rotated a plurality of times to charge, expose, and develop each process. A plurality of times, to form a plurality of images of different colors on the electrostatic latent image bearing member in a superimposed manner, and then collectively transferring the image onto a transfer material, the electrostatic latent image bearing member A color image forming apparatus having a cleaning unit for contact and separation has a control circuit capable of arbitrarily changing a DC voltage applied to a developing roll of the developing unit, and at least a non-image when a developing operation for image formation is not performed. A color image forming apparatus, wherein the potential of the non-image area of the electrostatic latent image carrier is controlled to be equal to the potential of the developing roller when the area passes through the non-developing roller. 2. The electrostatic latent image carrier is provided with a means for detecting at least a potential of a portion of the non-image area on the electrostatic latent image carrier where the cleaning means has retreated. 2. The color image forming apparatus according to claim 1, wherein the DC voltage applied to the developing roll of the developing means is controlled so as to be equal to the potential of the latent image carrier. 3. The image forming apparatus according to claim 1, wherein the developing device that performs the control is other than a developing device that uses at least black toner.