JP3234164B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using electrophotography, such as a copying apparatus, an image recording apparatus, a printer, and a facsimile, and more particularly, to an image forming apparatus in which color mixing during development is eliminated.

[0002]

2. Description of the Related Art FIG. 5 is a schematic diagram of a digital full-color copying machine. The copier has a photosensitive drum 1 as an image carrier, and has four developing units 4Y, 4Y near its periphery.
A developing device 4 having C, 4M, and 4K is arranged.
The developing units 4Y, 4C, 4M, and 4K are yellow,
A contact-type two-component developing method for each color of cyan, magenta and black and using a two-component developer composed of a non-magnetic toner and a magnetic carrier is employed.

[0005] First, a copying machine converts a document image 30 into a CCD 4 image.
1, the analog image signal obtained is amplified to a predetermined level by an amplifier 42, and is converted into an 8-bit digital image signal (0 to 256 gradations) by an A / D converter 43. Next, this digital image signal is converted to a gamma converter (256
Γ-corrected by a conversion unit 45 composed of a byte RAM and performing a digital conversion by a lookup system, and then input to a D / A converter 46. The image signal converted into an analog signal again by the D / A converter 46 is compared with a signal of a predetermined cycle generated from a triangular wave generation circuit 49 by a comparator 47 and pulse width modulated.

The binarized image signal subjected to the pulse width modulation is directly input to a laser drive circuit 50 and used as an on / off control signal for light emission of a laser diode 51. The laser light emitted from the laser diode 51 is scanned in the main scanning direction by a well-known polygon mirror 3a, and passes through an f / θ lens 3b and a reflection mirror 3c.
Irradiation is performed on the photosensitive drum 1 rotating in the direction of the arrow R1, and an electrostatic latent image is formed on the surface of the photosensitive drum 1.

[0005] The photosensitive drum 1 is first uniformly discharged by the exposure device 11 and then uniformly charged negatively by the charger 2. Thereafter, an electrostatic latent image corresponding to the image signal is formed on the surface by receiving the laser light. The developing devices 4Y to 4K for the respective colors of the developing device 4 develop the electrostatic latent image on the photosensitive drum 1 by a well-known reversal developing method, that is, the portion on the photosensitive drum 1 which has been subjected to the static elimination by the laser beam is negatively charged. The developed toner is adhered and developed to be visualized as a toner image. FIG. 6 shows the relationship between the surface potential of the photosensitive drum 1 and the development contrast in such reversal development.

In FIG. 6, Vd is the surface potential of the photosensitive drum 1 negatively charged by the charger 2. V00 is A /
The laser is driven by an image signal of 00H (0 level) digitized by the D converter 43, and the surface potential of the photosensitive drum irradiated with the laser beam, and Vff is similarly FF
This is the surface potential of the photosensitive drum when irradiating laser light in the case of an image signal of H (256 levels). At the time of development, each developing device 4Y-4
A DC voltage of Vdev and an AC voltage (for example, frequency 2 kHz, peak-to-peak voltage 2 kVp
p) is applied, and | Vdev−V
Development corresponding to the development contrast indicated by xx | is performed.

Therefore, if | Vdev-Vff | shown in FIG. 6 is an appropriate contrast potential Vcont and a developing density developed by Vcont is Dmax, generally, 1.2 to 1.8.
It can be seen that the value of Vcont should be set to an appropriate value in order to obtain a desirable image density. In addition,
The fog removal potential Vback in FIG. 6 is for completely removing the fog on the white image portion irradiated with the light amount of 00H.

As shown in FIG. 5, the toner image formed by the negative toner formed on the photosensitive drum 1 by the above-described development is transferred onto a transfer material (generally paper) P carried on a transfer drum 5a by a transfer charger. Transcribed by the action of 5b.
After the transfer, the photosensitive drum 1 is subjected to the above-described image forming process again after the residual toner remaining on the surface is scraped off by the cleaner 6. By repeating this image forming process for each color, a full-color image in which four color toner images are superimposed and transferred on the transfer material P is formed.

[0009]

However, the conventional image forming apparatus employs a developing method in which the developer on the developing sleeves (developer carrying members) of the developing units 4Y to 4K contacts the photosensitive drum 1. The first color, for example, the yellow toner image on the photosensitive drum 1 is changed to the developing device 4C for the second color or later.
When passing through 4K, these developing units 4C to 4K
The developer on the developing sleeve contacts the yellow toner image, and the cyan, magenta, and black toners of the second and subsequent colors adhere to the yellow toner image.
There is a fear that the yellow toner adheres from the photosensitive drum 1 onto the developing sleeve of C to 4K, causing a so-called color mixing phenomenon. The reason will be described with reference to the potential model of FIG.

FIG. 7 schematically shows the surface potentials of the photosensitive drum 1 during the image forming process for the first color and the developing sleeves of the developing units 4C, 4M, and 4K used for the image forming processes for the second and subsequent colors. FIG. The surface potential of the photosensitive drum 1 is changed from V00 to Vff by the exposure of the first color.
Developing devices 4C to 4K used in the image forming process for the second and subsequent colors
The surface potential of the developing sleeve becomes Vss induced by the surface potential of the photosensitive drum.

For this reason, when the image portion of the first color on the surface of the photosensitive drum 1 passes through the developing devices 4C to 4K for the second and subsequent colors, the portion of the image portion on the surface of the photosensitive drum 1 which has a higher potential than Vss. Then, the toner in the developer on the developing sleeves of the developing devices 4C to 4K adheres, and color mixing occurs in the first color toner image on the photosensitive drum 1. In addition, the toner of the first color in a portion having a potential lower than Vss in the image portion on the surface of the photosensitive drum 1 adheres to the developing sleeves of the developing devices 4C to 4K, and the toner of the developing devices 4C to 4K is mixed.

Further, when the toner image of the first color on the photosensitive drum 1 passes through the locations of the developing devices 4C to 4K for the second and subsequent colors, the toner image is rubbed by the developer on the developing sleeves of the developing devices 4C to 4K. There is also a problem that the image is disturbed and streaky traces are generated on the first color toner image and consequently on the multicolor image.
When the second and subsequent developing units, for example, the second developing unit 4C are the first-color developing units, the first-color electrostatic latent image formed on the photosensitive drum 1 is used as the first developing unit. When passing through the portion 4Y, the first color electrostatic latent image is disturbed by the rubbing of the developer on the developing sleeve of the developing device 4Y.
The problem of image streaks occurs.

As a method for solving the above problem, developing units 4Y to 4K are detachably mounted on the photosensitive drum 1,
There is a method in which a developing sleeve of a developing device other than the developing sleeve used for developing the first color image is separated from the photosensitive drum.

However, this method has a problem that the apparatus configuration is complicated and the cost is increased, and the image is disturbed due to a shock at the time of attaching / detaching the developing unit.

An object of the present invention is to provide a method for developing a latent image formed on an image carrier by a plurality of two-component contact developing devices arranged in the vicinity of the periphery of the image carrier. It is an object of the present invention to provide an image forming apparatus capable of obtaining a high-quality multi-color image by preventing image disturbance due to simplicity and low cost.

[0016]

The above object is achieved by an image forming apparatus according to the present invention. In summary, in the present invention, a plurality of developers are arranged in the vicinity of the image carrier along the rotation direction, and each developer has a different color developer composed of a magnetic carrier and a non-magnetic toner. A two-component contact developing device for developing a developer by contacting the image bearing member, forming a latent image on the image bearing member every one rotation of the image bearing member, and forming the latent image on a plurality of developing devices. Develop with the corresponding color developer,
An image forming step of transferring the obtained toner image onto a transfer material is repeated for a plurality of colors during a plurality of rotations of the image carrier, and a multi-color image obtained by superimposing a plurality of color toner images on the transfer material is obtained. In an image forming apparatus for obtaining an image, a developing nip formed by a developer carrier and an image carrier with respect to a developing device other than a developing device used for developing a first color image among a plurality of developing devices. The electrode members are installed on both sides of the image carrier, and during the pre-rotation of the image carrier before forming the image of the first color, the developer carrier of the developing device other than the first color is not rotated,
And a DC bias having a polarity opposite to that of the magnetic carrier of the developer is applied to the electrode member.

According to the present invention, when the image carrier is pre-rotated, 1
The reverse-polarity DC bias applied to the electrode members of the developing units other than the color can be applied even during the image forming process for the first color. When the image carrier after the image forming process of the final color is rotated, the developer carrier of the developing device other than the first color is rotated while the magnetic carrier of the developer is applied to the electrode member of the developing device other than the first color. And a DC bias having the same polarity as.

[0018]

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

Embodiment 1 FIG. 1 is a cross-sectional view showing a schematic structure of an embodiment of an image forming apparatus according to the present invention, and shows a color image forming apparatus. The present image forming apparatus has a digital color image reader unit at the top and a digital color image printer unit at the bottom.

In the reader section, the original 30 is placed on an original platen glass 31 and exposed and scanned by an exposure lamp 32, so that a reflected light image from the original 30 is condensed on a full color sensor 34 by a lens 33, and color separation is performed. Obtain an image signal. The color separation image signal is supplied to an amplifier (not shown).
Are processed by a video processing unit (not shown) and sent to a printer unit.

In the printer section, a photosensitive drum 1 serving as an image carrier is rotatably supported in the direction of an arrow. Around the photosensitive drum 1, a pre-exposure lamp 11, a corona charger 2, a laser exposure optical system 3, a potential sensor 12, four developing units 4Y, 4C, 4M, and 4K of different colors, an on-drum light amount detecting unit 13, a transfer device 5, and a cleaning unit 6 are arranged.

In the laser exposure optical system 3, an image signal from a reader section is converted into an optical signal by a laser output section (not shown), and the converted laser light is reflected by a polygon mirror 3a, and is converted into a lens 3b and a mirror. 3c, and is projected on the surface of the photosensitive drum 1.

The printer unit includes a photosensitive drum 1 for forming an image.
Is rotated in the direction of the arrow, and the photosensitive drum 1 after the charge is removed by the pre-exposure lamp 11 is uniformly charged by the charger 2 to irradiate a light image E for each separation color. Form a latent image.

Next, a predetermined developing device is operated to develop the latent image on the photosensitive drum 1 by the well-known reversal development, thereby forming an image using a resin-based toner on the photosensitive drum 1, that is, a toner image. I do. On the other hand, the recording material is conveyed from the recording material cassette 7 to the image transfer unit facing the photosensitive drum 1 via the conveyance system and the transfer device 5, and the toner image formed on the photosensitive drum 1 is recorded in the image transfer unit. Is transferred onto the material.

In the present embodiment, the transfer device 5 includes a transfer drum 5a.
And a transfer charger 5b and an adsorption charger 5c around the transfer charger 5b.
And an attraction roller 5g, an inner charger 5d, an outer charger 5e, and the like opposed thereto. The transfer drum 5a is supported so as to be driven to rotate, and a recording material carrying sheet 5f is integrally extended in a cylindrical shape in a peripheral opening area. As the recording material supporting sheet 5f, a dielectric sheet such as a polycarbonate film is used. The recording material is an adsorption charger 5
Due to the action of c and the suction roller 5g, the sheet is electrostatically adsorbed on the surface of the recording material carrying sheet 5f, and is repeatedly rotated through the image transfer section according to the rotation of the transfer drum 5a.

The toner image on the photosensitive drum 1 is moved to the image transfer section as the photosensitive drum 1 rotates,
The toner image is transferred onto the recording material passing through the image transfer portion by the transfer of the transfer member a by the operation of the transfer charger 5b. In this manner, a desired number of color toner images, in this example, four color toner images are superimposedly transferred onto the recording material on the transfer drum 5a, and a full-color image is formed.

In the case of forming a single-sided full-color image, after the transfer of the four-color toner image onto the recording material is completed, the separation claw 8
a, the recording material is separated from the transfer drum 5a by the action of the separation push-up roller 8b and the separation charger 5h, and the separated recording material is sent to the heat roller fixing device 9 to be fixed, thereby completing the image formation, and the one-sided full-color image Is discharged onto the tray 10. After the transfer, the photosensitive drum 1 is cleaned by a cleaning device 6 to remove the residual toner on the surface, and then subjected to the image forming process again.

When a full-color image is to be formed on both sides of the recording material, the conveyance path switching guide 19 is driven immediately after the recording material has left the fixing device 9, and the recording material is reversed through the conveyance vertical path 20. 21a. Thereafter, by the reverse rotation of the reversing roller 21b, the recording material is retreated in a direction opposite to the direction in which the recording material is fed, with the rear end when fed in as the head,
It is stored in the intermediate tray 22. Thereafter, the recording material is sent from the intermediate tray 22 to the transfer drum 5a, and a full-color image is formed on the other surface of the recording material again by the above-described image forming process, thereby obtaining a recording material of a double-sided full-color image.

In order to prevent powder from adhering to the recording material carrying sheet 5f of the transfer drum 5a and to prevent oil from adhering to the recording material, the fur brush 14 facing the recording material carrying sheet with the fur brush 14 interposed therebetween. By the operation of the backup brush 15, the oil removing roller 16 and the backup brush 17, the recording material carrying sheet 5f is cleaned. Such cleaning is performed before or after image formation, and is performed at any time when a jam (paper jam) occurs.

In this embodiment, the gap between the recording material carrying sheet 5f and the photosensitive drum 1 is reduced by operating the eccentric cam 25 at a desired timing and operating the cam follower 5i integrated with the transfer drum 5a. The configuration can be set arbitrarily. For example, during standby or when the power is off, the interval between the transfer drum 5a and the photosensitive drum 1 is increased.

According to the present invention, a special development process is performed during the pre-rotation of the photosensitive drum 1 before performing the above-described four-color image forming process, that is, before performing the first-color image forming process, thereby performing color mixing of images. And so on. Hereinafter, the developing process during the pre-rotation will be described.

During the pre-rotation of the photosensitive drum 1, a plurality of developing devices 4
Developing devices for the second and subsequent colors other than the developing devices used for developing the first color image formation of Y to 4K, for example, the second, third, and fourth cyan and magenta other than the first yellow developing device 4Y , Black developing units 4C, 4M, and 4K
A bias in which the developing sleeve is not rotated and DC is superimposed on DC is applied to the developing sleeve. In this example, −
Apply 350V DC + 2kHz, 2kVpp AC. The surface of the photosensitive drum 1 is uniformly maintained at a predetermined potential, in this example, -470V.

Due to this superimposed bias AC electric field, FIG.
As shown in (a), the developer 62 on the developing sleeve 60 of the developing device for the second and subsequent colors, together with the toner and the magnetic carrier, is located between the photosensitive drum 1 and the developing sleeve 60 (SD).
). At this time, since both the photosensitive drum 1 and the developing sleeve 60 have a convex curved surface at the opposing portion, as shown in FIG.
Is almost linear in the vicinity of the development nip G, but curves outwardly convex toward the outside between S and D.

In the region where the electric field lines h are curved, FIG.
As shown in (b), the developer located at the point a1 on the line of electric force h1 has a vector velocity V1 in the tangential direction of the line of electric force h1 at the point a1. At the next moment, when the developer moves to the point a2 on the line of electric force h2, it has a vector velocity V2 in the tangential direction of the line of electric force h2 at the point a2. Then, the developer moves from the point a2 in the direction of V1 + V2 obtained by combining the velocities V1 and V2 in a vector manner, that is, in the direction outside the vector V2.

As described above, when the developer moves from the developing sleeve 60 to the photosensitive drum 1, the developer always moves outward from the center of the developing nip G. Similarly, when returning from the photosensitive drum 1 to the developing sleeve 60, it always moves away from the center of the developing nip G to the outside. For this reason, while the developer 62 on the developing sleeve 60 repeats the reciprocating movement, it spreads from the inside of the developing nip G to both sides in the circumferential direction of the developing sleeve 60, and when the pre-rotation is completed, FIG.
As shown in (2), the developer 62 that comes into contact with the photosensitive drum 1 almost disappears in the developing nip (the area where the photosensitive drum 1 can be developed with the toner) G between SD.

According to the present invention, as shown in FIG. 4A, electrode members are provided on both sides of the developing nip G so as to be in non-contact with the photosensitive drum 1 and the developing sleeve 60. An electrode member 64 obtained by plating a tungsten wire with gold is provided.

Along with the application of the DC + AC bias to the developing sleeve 60, a DC bias having a polarity opposite to that of the magnetic carrier, -400 V in this example, is applied to the electrode member 64. Then, as shown in FIG. 4B, the developer 62 located outside the development nip G is attracted to the electrode member 64, and the developer 62 is completely out of contact with the photosensitive drum 1. Thereafter, the application of the DC bias to the electrode member 64 is stopped, and the image formation of the first color is performed.

According to the present invention, the developing units 4C to 4C for the second and subsequent colors are thus obtained until the pre-rotation of the photosensitive drum 1 is completed.
The developer 62 on the K developing sleeve 60 is removed from the developing nip G and is kept out of contact with the photosensitive drum 1.
The first color image formation is started. Therefore, when the first color image is formed, even if the first color toner image formed on the photosensitive drum 1 passes through the location of the developing unit for the second and subsequent colors, the photosensitive drum using the developer on the developing sleeve 60 for the second and subsequent colors 1 mixed color of the first color toner image, or 2
It is possible to prevent the occurrence of image disturbance due to the mixed color of the developers of the developing devices of the second and subsequent colors and the rubbing of the first color toner image by the magnetic carriers of the second and subsequent colors of the developing agent.

Further, for example, the second developing device 4C
Is the first color developing device, the first developing device 4Y
, Electrode members 64 are provided on both sides of the developing nip,
Similarly, a DC bias is applied during the pre-rotation of the photosensitive drum 1 to attract the developer on the developing sleeve 60 of the developing device 4Y to the electrode member 64, thereby forming the photosensitive drum 1 on the photosensitive drum 1 with the magnetic carrier of the developer. The first color electrostatic latent image can be prevented from being disturbed, and the resulting image can be prevented from being disturbed.

Second Embodiment According to the first embodiment, during the formation of the first color image, it is possible to substantially eliminate the color mixture and the disturbance of the image due to the magnetic carrier.

However, when the electrostatic latent image or the developed toner image on the photosensitive drum 1 passes through the developing sleeve 60 of the developing device for the second and subsequent colors as shown in FIG. Is charged to a polarity opposite to that of the magnetic carrier, and although slightly, the developer is attracted to the surface of the photosensitive drum 1 and comes into contact again, which may cause color mixing or image disturbance.

Therefore, in this embodiment, a DC bias having a polarity opposite to that of the magnetic carrier, that is, -400 V in this embodiment, is applied to the electrode member 64 even during the first color image forming process following the pre-rotation. The developer 62 on the developing sleeve 60 of the developing device for the second and subsequent colors during the image forming process for the first color
Is maintained in a non-contact position from the photosensitive drum 1 more reliably. Therefore, according to the present embodiment, it is possible to more effectively prevent color mixture and image disorder.

Embodiment 3 This embodiment provides a method for stably obtaining the effect of preventing color mixing and image disturbance of Embodiments 1 and 2 for a longer period of time.

As shown in FIG. 4B, during the pre-rotation of the image forming step and during the image formation, the developer 62 attracted to the electrode member 64 applies a DC bias to the electrode member 64. Is turned off, and the developing sleeve 60 is rotated to return to the state before the DC bias application shown in FIG.

At this time, although slightly, the developer 62 may remain attached to the electrode member 64 without separating. This is repeated many times, and when the amount of the developer attached to the electrode member 64 increases, the force of the electrode member 64 to attract the developer decreases, and the developer cannot be moved from the photosensitive drum 1 to a non-contact position.

In the present embodiment, in order to prevent this, at the time of post-rotation of the photosensitive drum 1 after the completion of the image forming process of the fourth color, that is, after the completion of the image forming process of the fourth color, the developing devices of the second and subsequent colors are rotated. While the developing sleeve 60 is being rotated, a DC bias having the same polarity as the magnetic carrier of the developer, in this embodiment, +2
00V was applied. As a result, the developer adhered to the electrode member 64 is repelled by the electrode member 64, and the electrode member 6
4 and the performance of the electrode member 64 is maintained.

Therefore, according to the present embodiment, it is possible to stably obtain the effect of preventing color mixing and image disturbance of an image for a longer period.

[0048]

As described above, according to the present invention,
A plurality of two-component contact developing devices are arranged along the vicinity of the image carrier, forming a latent image for each rotation of the image carrier, developing the latent image by the developing device, and transferring the obtained toner image onto a transfer material. The image forming step of transferring the image is repeated a plurality of times during the plurality of rotations of the image carrier, and when a plurality of color images are obtained on the transfer material, other than the developing device used for developing the first color image formation For the developing device, electrode members are installed on both sides of the developing nip formed by the developer carrier and the image carrier, and during the pre-rotation of the image carrier before forming the first color image, Without rotating the developer carrying members of the developing units other than the first color,
A bias in which AC is superimposed on C is applied, and a DC bias having a polarity opposite to that of the magnetic carrier of the developer is applied to the electrode member so that the developer on the developer carrier of the developing device other than the first color is placed in the developing nip. In addition, the image carrier is attracted to the electrode member and brought out of contact with the image carrier, so that color mixing of the image and disturbance of the image due to the magnetic carrier can be prevented easily and at a low cost when forming the first color image, and high-quality multi-color can be prevented. Images can be obtained.

In addition, a DC bias of the opposite polarity applied to the electrode members of the developing units other than the first color during the pre-rotation of the image carrier is
By applying the voltage during the first color image forming process, it is possible to more reliably prevent the color mixture of the image and the disturbance of the image at the time of forming the first color image. Further, during the post-rotation of the image carrier after performing the image forming process for the final color, the developer is applied to the electrode members of the developer other than the first color while rotating the developer carrier of the developer other than the first color. By applying a DC bias having the same polarity as that of the magnetic carrier, the performance of attracting the developer by the electrode member can be maintained, and the effect of preventing color mixing and disturbance of an image can be stably obtained for a longer period.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a schematic configuration of an embodiment of an image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing that a developer is removed from a developing nip by applying a bias to a developing sleeve in a developing process performed during pre-rotation in the present invention.

FIG. 3 is an explanatory view showing a mechanism of removing from a developing nip by a bias to a developing sleeve in FIG. 3;

FIG. 4 is an explanatory diagram showing that a developer comes into non-contact with a photosensitive drum by application of a bias to an electrode member in a developing process performed during pre-rotation.

FIG. 5 is an explanatory diagram illustrating a schematic configuration of a conventional image forming apparatus.

FIG. 6 is a potential model diagram illustrating development in the image forming apparatus of FIG. 5;

FIG. 7 is a potential model diagram for explaining the reason of the color mixing phenomenon occurring in the development of FIG. 5;

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 2 corona charger 3 laser exposure optical system 4Y to 4K developing device 5a transfer drum 60 developing sleeve 62 developer 64 electrode member

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kazuhiro Hasegawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-61-26064 (JP, A) JP-A-8 JP-A-334951 (JP, A) JP-A-8-286476 (JP, A) JP-A-8-69184 (JP, A) JP-A-5-346736 (JP, A) (58) Fields investigated (Int. . ^7, DB name) G03G 13/01 G03G 15/01 - 15/01 117 G03G 15/06 - 15/06 102 G03G 15/08 - 15/095

Claims (3)

(57) [Claims] A plurality of developing devices are arranged in the vicinity of an image carrier along a rotation direction thereof, and each developing device has a developer of a different color composed of a magnetic carrier and a non-magnetic toner. A two-component contact developing device that contacts and develops an image carrier, forms a latent image on the image carrier for each rotation of the image carrier, and develops the latent image into a corresponding color of a plurality of developing devices. The image forming step of developing the toner image on the transfer material and developing the toner image on the transfer material is repeated for a plurality of colors during a plurality of rotations of the image carrier, so that a plurality of color toners are formed on the transfer material. In an image forming apparatus for obtaining a multicolor image in which images are superimposed, a developer carrying member and an image carrying member for a developing device other than a developing device used for developing an image of a first color out of a plurality of developing devices. Electrode members are installed on both sides of the development nip formed by
During the pre-rotation of the image carrier before forming the first color image,
Applying a bias in which AC is superimposed on DC and applying a DC bias having a polarity opposite to that of the magnetic carrier of the developer to the electrode member without rotating the developer carrying member of the developing device other than the first color. Characteristic image forming apparatus. 2. The image forming apparatus according to claim 1, wherein a DC bias of a reverse polarity applied to the electrode members of the developing units other than the first color is applied during the pre-rotation of the image carrier during the image forming process of the first color. 3. When the image carrier after the image forming step of the final color is rotated, the developer carrier of the developing device other than the first color is rotated and the electrode member of the developing device other than the first color is rotated. 3. The image forming apparatus according to claim 1, wherein a DC bias having the same polarity as that of the magnetic carrier of the developer is applied.