JP2002268322A - Multicolor imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a color image and a monochromatic image of high quality, in a tandem type color imaging device. SOLUTION: In a multicolor imaging device, a larger number of imaging stations, having photosensitive bodies as the centers are arranged in the direction of form carrying, and color images formed in respective imaging stations are successively superposed on a form to form one multicolor image at multicolor image formation; only the image forming station placed at the top in the direction of form carrying is used to form a monochromatic image on a form at monochromatic image formation; a bias voltage applying device is provided which applies a developing bias voltage, where an AC component is superposed on a DC component, to a developing device of the imaging station placed at the top; and the bias voltage applying device changes the amplitude of the AC component of the developing bias voltage between multicolor imaging and monochromatic imaging times.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus, and more particularly, to a method in which a developer image developed on a photoconductor such as an electrophotographic color copying machine or a color printer is transferred by a semiconductive transfer belt. The present invention relates to a tandem-type image forming apparatus that forms an image by sequentially performing multiple transfer while transferring a transfer material.

[0002]

2. Description of the Related Art Conventionally, a plurality of developing devices are provided, and a visible image (toner image) having a different color depending on each developing device.
Various color image forming apparatuses have been proposed in which a toner image is formed, and these toner images are finally superimposed and transferred on the same sheet.
Generally, color image forming apparatuses are roughly classified into a single drum type and a multi-stage drum type (tandem type).

FIG. 5 is a diagram schematically showing a main part of a conventional single-drum type color image forming apparatus. In the color image forming apparatus shown in FIG. 1, an initializing charger 2, a recording head for irradiating a laser beam, and four developing units 4a to 4d are provided along the peripheral surface of a single photosensitive drum 1. A semi-conductive drum 5 around which a transfer portion is formed is formed opposite to the photosensitive drum 1. A corona discharger 6 that discharges a transfer current is disposed in the cylinder of the semiconductive drum 5 around which the paper is wound, in the vicinity of the transfer portion.

[0004] The sheet winding semiconductive drum 5 winds a sheet carried in from the lower left as shown by an arrow A in the figure in a counterclockwise direction as shown by an arrow B in the figure, and makes a round. . The photoreceptor drum 1 is rotated at the same peripheral speed as the peripheral speed of the semi-conductive drum 5 around which the paper is wound, in the clockwise direction as indicated by the arrow C in FIG. An image is formed, and the toner image is transferred to a sheet by the discharge current of the corona discharger 6. Next, the paper winding semiconductive drum 5 makes one rotation again, and accordingly, the photosensitive drum 1 forms a C (cyan: greenish blue) toner image, and the corona discharger 6 forms the toner image on the paper. Transfer and stack on top.

[0005] Again, the paper winding semiconductive drum 5
Makes one rotation, and in response, the photosensitive drum 1 forms a Y (yellow: yellow) toner image, which is
Is transferred onto paper and overlaid. Finally, the paper winding semiconductive drum 5 makes one rotation again, and accordingly, the photosensitive drum 1 forms a Bk (black: black) toner image, and the corona discharger 6 transfers the toner image onto the paper. Overlap.

When the transfer (overlapping) of the four color toner images is completed, the wrapping of the sheet is released, and the sheet is conveyed by the conveying belt 7 to the fixing section 8 disposed on the left side. The transferred four-color transfer toner images are heat-fixed to the paper surface.

As described above, the single-drum type color image forming apparatus uses three subtractive primary colors of M (magenta) toner, C (cyan) toner, and Y ( In order to transfer a total of four types of toner, that is, yellow (yellow) toner and black (Bk) toner dedicated to printing of the black portion, the printing (exposure, exposure,
(Recording, developing, and transferring), the printing process is repeated four times for one page of paper, and therefore, the printing process takes a long time.

On the other hand, a tandem-type color image forming apparatus transfers four types of toner sequentially onto a sheet in one process and transfers the same, so that the processing speed is almost four times that of a single drum type. are doing. For this reason, in recent years, the tandem type color image forming apparatus has become mainstream, in combination with the fact that the internal apparatus has been miniaturized and the apparatus has been assembled (unitized) to be relatively inexpensive.

FIG. 6 shows a schematic configuration of a color image forming apparatus to which the tandem system is applied. In FIG. 6, a drum-shaped photoreceptor 1 rotates in a direction indicated by an arrow in the figure, and a charging roll brush 2, a developing device 4, and a cleaning device 10 are disposed therearound. The charging roll brush 2 is a contact charging member constituting a charging device for uniformly charging the surface of the photoconductor 1. The surface of the photoreceptor between the charging roll brush 2 and the developing device 4 is irradiated with a laser beam 3 from a not-shown writing optical system to form an electrostatic latent image on the photoreceptor 1.

[0010] The four image forming stations Pa to Pd centering on the photosensitive member 1 are arranged side by side along a transfer conveyance belt 5 which is a transfer material conveyance means.
The transfer conveyance belt 5 is in contact with the photoconductor 1 between the developing device 4 and the cleaning device 10 of each image forming station, and a transfer bias is applied to the surface (back surface) of the transfer conveyance belt 5 opposite to the photoconductor 1. The transfer brush 6 for performing the transfer is disposed. Each of the image forming stations Pa to Pd has the same configuration except that the toner color inside the developing device 4 is different.

In the color image forming apparatus having the structure shown in FIG. 6, an image forming operation is performed as follows. First,
In each of the image forming stations Pa to Pd, the photoconductor 1
Is charged by the charging roll brush 2 rotating in the counter direction, and then an electrostatic latent image corresponding to the image of each color to be formed is formed by the laser beam 3 in the exposure section. Next, the latent image is developed by the developing device 4 to form a toner image.

The developing devices 4 are C (cyan) and M, respectively.
In a developing device that performs development with (magenta), Y (yellow), and K (black) toners, toner images of respective colors formed on four photoconductors 1 are superimposed on paper. The paper is sent out from a tray 11 by a paper feed roller (not shown), temporarily stopped by a pair of registration rollers 12, and sent to the transfer conveyance belt 5 at the same time as the image formation on the photoconductor 1. The sheet is conveyed onto the transfer conveyance belt 5, and the transfer of the toner images of each color is performed at a contact position (transfer portion) with each photoconductor 1.

The toner image on the photosensitive member 1 is transferred onto paper by an electric field formed by a transfer bias applied to the transfer brush 6 and a potential difference between the photosensitive member 1 and the transfer bias. Then, the sheet on which the toner images of the four colors are superimposed after passing through the four transfer units is transported to the fixing device 8, where the toner is fixed, and is discharged to a discharge unit (not shown). Further, the residual toner remaining on each photoconductor 1 without being transferred by the transfer unit is collected by the cleaning device 10.

By the way, in such a tandem type multi-color image forming apparatus, even when a single-color image is formed, since the paper comes into contact with another photoreceptor, unnecessary color toner remaining on the photoreceptor is generated. There is a problem that the toner adheres to the paper and degrades image quality. There is also a problem that unnecessary contact of the transfer belt shortens the life of the photoconductor and the transfer conveyance belt.

A typical prior art for solving such a problem is disclosed in JP-A-6-258914, JP-A-10-198121, and the like. In other words, during the formation of a single-color image, the paper transport belt is separated from the photoreceptor that is not involved in image formation, thereby preventing toner of unnecessary colors from being mixed, and at the same time, abrasion of both due to the contact between the paper transport belt and the photoreceptor. Has been avoided.

[0016]

The image forming stations of each color have a desirable arrangement order because a slight difference in image quality occurs depending on the arrangement order. In particular, black, which is frequently used as a single color, has a high concealing effect on other colors, so when forming an image at the most downstream, each color image formed so far is concealed and becomes a darkened image. In a full-color image requiring a delicate balance, it is highly necessary to form an image at the uppermost stream in order to obtain high image quality.

On the other hand, in the tandem system, the untransferred toner once transferred onto the transfer material at the image forming station on the upstream side is robbed at the station on the downstream side, causing a reverse transfer phenomenon, and a good color image is obtained. Can not be. That is,
The toner image transferred in the image forming station of the first color in the uppermost stream is reversely transferred to the photoreceptor side at the subsequent transfer in the image forming stations of the second to fourth colors.
The toner of the color image forming station is reverse-transferred in the third and fourth color image forming stations, and the toner transferred in the third color image forming station is reverse-transferred in the fourth color image forming station. For this reason, after the transfer of the fourth color toner image is completed, the toner adhesion amount of the first color toner image to the transfer material P is reduced by several tens of percent compared to the initial adhesion amount.

In the reverse transfer phenomenon, when toner adheres to the photosensitive member from the developing roller in accordance with the image potential of the photosensitive member (drum or belt) in the image forming process, the toner always has a uniform negative charge. Not necessarily
Some have a weak negative polarity and some have a positive charge. On the other hand, a discharge occurs in the step of peeling off from the photoreceptor after the transfer, and some of them have a positive polarity later. It is said that these are phenomena caused by returning to the photoconductor little by little each time a positive-polarity transfer charge is received in a subsequent transfer step.

For this reason, in the image forming station on the upstream side, in order to obtain an appropriate image quality at the time of forming a full-color image in which a delicate balance is required, an image forming station in which a large amount of toner adheres in anticipation of the reverse transfer phenomenon. The conditions are set. Therefore, when such a method is applied to the conventional technology, when the paper transport belt is separated from the photoreceptor that is not involved in image formation during monochromatic image formation, mixing of unnecessary color toner and the paper transport belt and the photoreceptor are not performed. Instead of preventing wear, several tens% of the toner to be lost by the reverse transfer phenomenon remains attached to the paper, which causes a problem that the image density becomes high.

The present invention has been made in view of such circumstances, and forms a black image at the most upstream image forming station, and separates the other image forming stations from the transfer / transport belt during black image formation. In addition, by changing the developing bias voltage to the developing device of the most upstream image forming station between the time of forming a multi-color (full color) image and the time of forming a single-color image, a good image can be formed in both multi-color and single-color images. An object of the present invention is to provide a multicolor image forming apparatus capable of obtaining a quality image.

[0021]

According to the present invention, a plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and when forming a multi-color image, a color image formed by each image forming station is formed. To form one multi-color image by sequentially superimposing on a sheet, and forming a single-color image on the sheet using only the image forming station located at the most upstream position in the sheet conveying direction when forming a single-color image. A bias voltage applying device for applying a developing bias voltage in which an AC component is superimposed on a DC component with respect to a developing device of an image forming station at the most upstream position, wherein the bias voltage applying device is used when forming a multicolor image. An object of the present invention is to provide a multi-color image forming apparatus characterized in that the amplitude of the AC component of the developing bias voltage is changed when a single-color image is formed.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As one of the developing techniques of an electrophotographic apparatus, a developing bias voltage in which a large-amplitude AC voltage is superimposed on a DC voltage of an appropriate level is applied to a developing device, and the bias voltage is applied to a developing space in a developing space. There is a method of improving the image quality by vibrating the toner to create a state where the toner easily moves to the electrostatic latent image on the photoconductor.

Although the detailed specification of the developing bias voltage varies depending on the model to which it is applied, an AC voltage having an amplitude of 1 km to several kilovolts and a frequency of 1 km to several kilohertz is superimposed on a DC voltage of 0 to several hundred volts. The applied bias voltage is applied to the developing device.

[0024] This creates a state in which the toner in the developer adsorbed and held on the developing roller is activated by the vibration effect caused by the application of the AC voltage, and the state easily shifts to the electrostatic latent image on the photosensitive member. When the amplitude (peak voltage width) Vp-p of the AC voltage is increased as shown in FIG. 7, the amount of toner transferred to the photoconductor in the developing unit is increased, and the image density is increased as shown in FIG. Because you do.

That is, the DC component of the developing bias voltage
Since the surface potential of the uniformly charged photoreceptor and the normal charging polarity of the toner are the same, the toner which has been triggered to be separated from the carrier by the application of the AC voltage component to the developing bias voltage is not charged with the developing bias voltage. It does not adhere to a portion having a higher potential than the DC component, but is exposed and adheres to a portion having a lower potential than the DC component of the developing bias voltage, thereby performing reversal development. The present invention has been made focusing on this principle.

Therefore, the multicolor image forming apparatus of the present invention
A multi-color image forming apparatus that forms a single-color image on a sheet by using only the image forming station located at the most upstream position in the sheet transport direction when forming a single-color image. A bias voltage applying device for applying a developing bias voltage in which an alternating current component is superimposed on a direct current component, wherein the bias voltage applying device changes the amplitude of the alternating current component of the developing bias voltage during multicolor image formation and single color image formation. It is characterized by the following.

In the present invention, it is preferable that the bias voltage applying device sets the amplitude of the AC component of the developing bias voltage at the time of forming a multicolor image to be larger than that at the time of forming a monochromatic image.

Further, the image forming apparatus of the present invention applies a developing bias voltage in which an AC component is superimposed on a DC component at the time of forming a multi-color image to the developing device of the most upstream image forming station, and applies a DC bias at the time of forming a monochromatic image. A bias voltage application device for applying a development bias voltage consisting of only components may be provided. It is preferable that the most upstream image forming station is for black toner.

The paper in the present invention includes a sheet-shaped recording medium made of paper or resin, and a commercially available copy paper or OHP film can be used for this.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the embodiments shown in the drawings. This does not limit the present invention. First Embodiment FIG. 1 is a configuration explanatory view showing a schematic configuration of a tandem type color image forming apparatus of a first embodiment. In FIG. 1, drum-shaped photoconductors 6B, 6C, 6M, and 6Y rotate in the direction of arrow X in the figure, and chargers 1B, 1C, 1M, and 1 surround each of them.
Y, developing devices 3B, 3C, 3M, 3Y, and cleaning devices 5B, 5C, 5M, 5Y are arranged, respectively.

The chargers 1B, 1C, 1M, and 1Y uniformly charge the surfaces of the photoconductors 6B, 6C, 6M, and 6Y, respectively. Chargers 1B, 1C, 1M, 1Y and developing devices 3B, 3
Laser light from the writing optical systems 2B, 2C, 2M, and 2Y is irradiated on the photoconductor surface between C, 3M, and 3Y, respectively, and electrostatic latent images are formed on the photoconductors 6B, 6C, 6M, and 6Y, respectively. It has become.

The photosensitive members 6B, 6C, 6
Four image forming stations 20B, 20C, 20M, and 20Y centered on M and 6Y, respectively, are juxtaposed along the transfer and transport belt 27 of the paper transport unit 30.

The paper transport unit 30 includes a transfer transport belt 27, rollers 20, 21 and transfer rollers 4B, 4C,
4M and 4Y, a toner density sensor 30, and a cleaning device 14. The transfer / conveying belt 27 is a developing device 3 of each image forming station when a four-color image is formed.
B, 3C, 3M, 3Y and cleaning devices 5B, 5C,
The photoconductors 6B, 6C, 6M and 6Y are in contact with the photoconductors 5M and 5Y, respectively. Transfer rollers 4B, 4C, 4M, 4
Y is provided for applying a transfer bias to the opposite side of the photoconductor from the transfer conveyance belt 27.

Further, the transfer / conveying belt 27 is
21. The toner density sensor 10 includes a light emitting element 12 and a light receiving element 13 and is provided below the transfer / conveying belt 27 together with the cleaning device 14 to perform image formation before actual image formation on the sheet P or on a predetermined number of sheets P. After the operation, the toner image is formed directly on the transfer / conveyance belt 27 to check the amount of adhered toner and to clean the adhered toner. The image forming stations 20B, 20C, 20M, and 20Y are the developing devices 3
The toner colors inside B, 3C, 3M, and 3Y are black, cyan, magenta, and yellow, respectively.

A developing bias voltage is applied to the developing device 3B of the image forming station 20B by a DC power supply 31 and an AC power supply 32. The DC power supply 31 has a variable output voltage, and the AC power supply 3
Reference numeral 2 denotes a variable output voltage amplitude and frequency.

In the color image forming apparatus shown in FIG. 1, at the time of forming a monochrome (black) image, the paper transport unit 30 is rotated in the direction of arrow A by a drive mechanism (not shown) as shown in FIG. As a result, only the photoconductor 6B keeps contact with the transfer conveyance belt 27, and the other photoconductors 6C, 6M, 6Y
Is designed to be separated from the transfer conveyance belt 27.

In such a configuration, the color image forming operation is performed as follows. First, in each of the image forming stations 20B, 20C, 20M, and 20Y, the photoconductors 6B, 6C, 6M, and 6Y are connected to the chargers 1B, 1C, and 1Y.
M, 1Y, and then the optical systems 2B, 2C, 2
An electrostatic latent image corresponding to an image of each color to be created is formed by the laser light from M and 2Y.

Next, the latent images are developed by the developing devices 3B, 3C, 3M, and 3Y to form toner images. Developing device 3
B, 3C, 3M, and 3Y are B (black) and C, respectively.
(Cyan), M (Magenta), and Y (Yellow) toners are developed, and the toner images of each color formed on the four photoconductors 6B, 6C, 6M, and 6Y are superimposed on the paper P.

The sheet P is sent out from a tray (not shown) in the direction of the arrow Y, and the sheet P held on the transfer / conveyance belt 27 is conveyed to a contact position (transfer section) of the photoconductors 6B, 6C, 6M, 6Y. The transfer of each color toner image is performed.

The four transfer rollers 4B, 4C, 4
The sheet on which the toner images of four colors are superimposed after passing through M and 4Y is conveyed to the fixing device 28, where the toner is fixed, and is discharged to a discharge unit (not shown).

The photosensitive member 6B,
Residual toner remaining in 6C, 6M, 6Y is collected by cleaning devices 5B, 5C, 5M, 5Y, respectively.
In FIG. 1, the image forming stations 20C, 20M, 2
0Y is arranged in the order of C (cyan), M (magenta), and Y (yellow) from the upstream side to the downstream side in the paper transport direction, but is not limited to this order, and the color order is arbitrarily set. Is what is done.

Next, the monochrome image forming operation is performed as follows. First, the paper transport unit 30 rotates to the position shown in FIG. Then, in the image forming station 20B, the photoconductor 6B is charged by the charger 1B, and then an electrostatic latent image corresponding to a black image to be formed is formed by the laser beam from the optical system 2B. Next, the developing device 3B
Develops with black toner.

A black toner image is transferred to the sheet P conveyed by the transfer conveying belt 27 at a contact position (transfer portion) with the photosensitive member 6B. The sheet P on which the transfer is completed is conveyed to a fixing device (FIG. 1), and is discharged after being fixed. Further, the toner remaining on the photoreceptor 6B is
Collected by B.

Here, in the first embodiment, the developing bias voltage applied to the developing device 3B is set so that the amplitude of the AC component is different between when a color image is formed and when a single-color image is formed. Increasing the amplitude (peak voltage width) of the AC component increases the frequency of toner detachment from the carrier, so that the amount of toner adhering to the photoconductor can be increased. Conversely, if the amplitude (peak voltage width) of the AC component is reduced, In addition, the frequency of toner detachment from the carrier is reduced, so that the amount of toner adhering to the photoconductor can be reduced.

That is, as the amplitude Vp-p of the AC component increases, the amount of toner adhering from the developing device to the photosensitive member increases as shown in FIG. 7, and the paper P as shown in FIG.
The upper image density also increases. In the present invention, an attempt is made to adjust the amount of toner adhering to the photoreceptor, that is, the image density, by utilizing this characteristic. In the first embodiment, the amplitude Vpp of the DC component and the AC component of the developing bias voltage is set as shown in Table 1.

[0046]

[Table 1]

As a result, good quality images could be obtained both when forming a color image and when forming a monochromatic image.

Second Embodiment FIGS. 3 and 4 show the structure of the second embodiment, and correspond to FIGS. 1 and 2, respectively. And the second embodiment is the first embodiment.
In the embodiment, the AC component of the developing bias voltage (AC power supply 32)
Is added to the switch 33 for removing the image at the time of monochrome image formation, and the other configuration and operation are the same as those of the first embodiment.

From the knowledge on the frequency of toner detachment from the carrier depending on the magnitude of the amplitude of the AC component of the developing bias voltage, if the developing bias voltage is made only of the DC component, the developing bias voltage is applied to the photosensitive member more than when the superimposed voltage is applied. Focusing on the fact that the toner adhesion amount can be reduced, in the second embodiment, the amplitudes Vp-p of the DC component and the AC component of the developing bias voltage are shown in Table 2.
(The AC component is not superimposed at the time of monochromatic image formation).

[0050]

[Table 2]

As a result, images of good quality could be obtained both when forming a color image and when forming a single-color image.

[0052]

According to the present invention, an image is formed only at the most upstream image forming station of the sheet conveying belt at the time of monochromatic image formation, and the most upstream image development is performed at each of the multicolor image formation and the single color image formation. Since the amplitude (peak voltage width) of the AC component of the developing bias voltage applied to the apparatus is changed, the difference in image density between when forming a multicolor image and when forming a single-color image can be eliminated.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing the operation of the first embodiment.

FIG. 3 is a diagram corresponding to FIG. 1 of a second embodiment.

FIG. 4 is a diagram corresponding to FIG. 2 of a second embodiment.

FIG. 5 is a configuration explanatory view showing a conventional example.

FIG. 6 is a configuration explanatory view showing another conventional example.

FIG. 7 is a graph showing the amount of toner adhering to a photosensitive member with respect to an AC bias voltage according to the present invention.

FIG. 8 is a graph showing an image density of a sheet with respect to an AC bias voltage according to the present invention.

[Explanation of symbols]

 1B Charger 1C Charger 1M Charger 1Y Charger 2B Optical System 2C Optical System 2M Optical System 2Y Optical System 3B Developing Device 3C Developing Device 3M Developing Device 3Y Developing Device 4B Transfer Roller 4C Transfer Roller 4M Transfer Roller 4Y Transfer Roller 5B Cleaning Device 5C Cleaning device 5M Cleaning device 5Y Cleaning device 6B Photoconductor 6C Photoconductor 6M Photoconductor 6Y Photoconductor 10 Toner density sensor 12 Light emitting element 13 Light receiving element 14 Cleaning device 20B Image forming station 20C Image forming station 20M Image forming station 20Y Image forming Station 20 Roller 21 Roller 27 Transfer conveyor belt 28 Fixing device 30 Paper transport unit 31 DC power supply 32 AC power supply

Continued on the front page (72) Inventor Eiji Saiko 22-22, Nagaikecho, Abeno-ku, Osaka-shi, Japan Inside Sharp Corporation (72) Inventor Osamu Morikuni 22-22, Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp shares In-company (72) Inventor Shoji Nakamura 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation (72) Inventor Masahiro Tsuji 22-22 Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Corporation F Terms (reference) 2H027 EA05 EB04 ED09 ED24 EE07 FA28 2H030 AA02 AB02 AD07 AD17 BB02 BB23 BB34 BB53 2H073 AA02 BA04 BA13 BA21 CA22

Claims (4)

[Claims] A plurality of image forming stations centering on a photoreceptor are arranged along a sheet conveying direction, and when forming a multi-color image, color images formed by the respective image forming stations are sequentially superimposed on a sheet. A multi-color image forming apparatus for forming one multi-color image and forming a single-color image on a sheet using only an image forming station located at the most upstream position in the sheet conveying direction when forming a single-color image, comprising: A bias voltage applying device for applying a developing bias voltage in which an ac component is superimposed on a dc component to a developing device of the image forming station, wherein the bias voltage applying device is configured to apply the developing bias for forming a multicolor image and for forming a monochromatic image. A multicolor image forming apparatus characterized by changing the amplitude of an AC component of a voltage. 2. The multicolor image forming apparatus according to claim 1, wherein the bias voltage applying device sets the amplitude of the AC component of the developing bias voltage at the time of forming the multicolor image to be larger than that at the time of forming the single color image. 3. A plurality of image forming stations centering on a photoreceptor are provided along a sheet conveying direction, and when forming a multi-color image, color images formed by each image forming station are sequentially superimposed on a sheet. A multi-color image forming apparatus for forming one image and forming a single-color image on a sheet using only an image forming station located at the most upstream position in the sheet transport direction when forming a single-color image, comprising: A multi-color image forming apparatus including a bias voltage applying device for applying a developing bias voltage in which an alternating current component is superimposed on a direct current component when forming a multicolor image to a developing device of a station, and applying a developing bias voltage including only a direct current component when forming a single color image; Color image forming apparatus. 4. The multicolor image forming apparatus according to claim 1, wherein the most upstream image forming station is for a black toner.