JP2002031935A - Image forming device - Google Patents

Abstract

(57) [Problem] To obtain a color image having a sufficient density of each color by controlling a transfer current so as to balance the transfer efficiency of each color and the retransfer rate in an image forming unit after the next color of each color. It is an object of the present invention to provide an image forming apparatus capable of doing this. SOLUTION: A transfer current Ia giving a maximum transfer efficiency;
The transfer current Ib at which high transfer efficiency and low retransfer rate are compatible
There is a relationship of Ia> Ib, and in the example of FIG.
It is around 5 μA and Ib = 11.5 μA. Therefore,
At the first station where retransfer does not occur, the transfer current is
= 17.5 μA to maximize the transfer efficiency, and at the second and subsequent stations where retransfer occurs, Ib = 11.5 μA
In the vicinity of A, high transfer efficiency and low retransfer rate were set. This transfer current is obtained by applying a constant primary transfer bias to Vy = 300 V at the first station and Vm at the second to fourth stations.
= Vc = Vk = 170V.

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 such as a copying machine, a printer, a facsimile, etc., which uses an electrophotographic system, and more particularly to a transfer material for transferring a plurality of color toner images formed on a plurality of image carriers. An image forming apparatus for sequentially transferring images on a transfer material on a belt or superimposing images on an intermediate transfer belt for primary transfer and then collectively transferring the images to a transfer material to obtain a color image on the transfer material About.

[0002]

2. Description of the Related Art Conventionally, there has been known an electrophotographic image forming apparatus for transferring a toner image formed on an image carrier using electrophotography to a transfer material and obtaining an image on the transfer material. In these circumstances, with the progress of the information-oriented society in recent years, needs for color image forming apparatuses are spreading. Further, in order to speed up the color image output, a plurality of image carriers are arranged in a line, and a toner image is sequentially formed on each image carrier, and the toner image is transferred to a transfer material directly or via an intermediate transfer member. Attention has been paid to an in-line type image forming apparatus that transfers images by using a transfer method.

An example of an electrophotographic in-line type full-color image forming apparatus will be briefly described with reference to FIG. 1. The image forming apparatus includes four types of yellow (Y), magenta (M), cyan (C), and black (K). A color image forming section (image forming station) 10Y, 10M, 10C, 10K is provided, and each station has a photosensitive drum 70 as an image carrier.
(70Y to 70K).

In order to form an image, first, the surface of the photosensitive drum 70 (70Y to 70K) at each station is uniformly charged with the primary transfer roller 12 (12Y to 12K), and the laser exposure device 13 (13Y to 13K). Is subjected to image exposure in which the original is color-separated by the above-described method, an electrostatic latent image corresponding to the separated color of the original is formed on the surface of the photosensitive drum 70, and the latent image is
4Y to 14K) to develop using a minus toner to form a toner image of each color on the surface of the photosensitive drum 70,
The toner image of each color on the photosensitive drum 70 is transferred to the intermediate transfer belt 8.
0, the constant voltage power supply 48 (48Y to 48) of the primary transfer power supply.
K) primary transfer roller 5 to which primary transfer bias is applied
4 (54Y to 54K) to superimpose sequentially and perform primary transfer.

Then, a secondary transfer bias is applied from the constant voltage power supply 49 of the secondary transfer power supply to the four color toner images on the intermediate transfer belt 80 onto the transfer material P conveyed to the intermediate transfer belt 80. The secondary transfer is collectively performed by the transfer roller 55, and the transfer material P on which the secondary transfer is completed is conveyed to the fixing device 40, and is pressurized and heated to melt and mix the four color toners and fix the toner on the transfer material P. Thus, a full-color image is formed on the transfer material P.

[0006]

In the above-described image forming apparatus having a plurality of image forming stations, in order to obtain a full-color image having a sufficient density, each of the stations 1 is required.
The primary transfer efficiency of the toner image on the photosensitive drum 70 to the intermediate transfer belt 80 from 0Y to 10K, and the toner image on the intermediate transfer belt such as an image forming station for the second and subsequent colors for the first yellow toner image It is important to balance the retransfer rate of the image forming station after the first color to the photosensitive drum of the image forming station.

In each of the image forming stations 10Y to 10K, the surface potential of the photosensitive drum 70 and the constant voltage power supply 48 (4
It is important to optimize the primary transfer bias of 8Y to 48K). In particular, the primary transfer current greatly affects the density of each color of a full-color image, and appropriate control is required.

It is an object of the present invention to obtain a color image having a sufficient density of each color by controlling the transfer current so as to balance the transfer efficiency of each color and the retransfer rate of the next color in the image forming section after the next color. It is an object of the present invention to provide an image forming apparatus capable of doing this.

[0009]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
A plurality of image carriers, an intermediate transfer member that rotates along the plurality of image carriers, and a plurality of colors on the plurality of image carriers that face each other with the plurality of image carriers and the intermediate transfer member interposed therebetween. And a plurality of transfer charging means for transferring the toner image to the intermediate transfer body, the transfer current in the transfer charging means at the most upstream part in the rotation direction of the intermediate transfer body among the plurality of transfer charging means. The image forming apparatus is characterized in that the value is larger than the transfer current value in the remaining transfer charging means.

Further, according to the present invention, a plurality of image carriers, a transfer material carrier rotating along the plurality of image carriers, and a plurality of image carriers and the transfer material carrier facing each other with the transfer material carrier interposed therebetween, And a plurality of transfer charging means for transferring a plurality of color toner images on the plurality of image carriers to a transfer material on the transfer material transporting body. An image forming apparatus is characterized in that the transfer current value in the transfer charging means at the most upstream part in the rotation direction of the material conveying member is larger than the transfer current value in the remaining transfer charging means.

According to the present invention, a constant voltage power supply is used as a power supply for the plurality of transfer charging means. One common constant voltage power supply is used as the power supply for the remaining transfer charging means. Or
A constant current power supply is used as a power supply for the plurality of transfer charging units.
One common constant current power supply is used as a power supply for the remaining transfer charging means. The intermediate transfer member is an intermediate transfer belt that rotates endlessly. The transfer material transporter is a transfer belt that rotates endlessly. The transfer charging unit is a transfer roller.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic configuration diagram showing an embodiment of the image forming apparatus of the present invention. This image forming apparatus is configured as a four-drum, intermediate transfer type full-color printer.

As shown in FIG. 1, the present image forming apparatus includes four color image forming sections (image forming stations) of yellow (Y), magenta (M), cyan (C), and black (K).
10Y, 10M, 10C, and 10K, and further includes a transfer device including the intermediate transfer belt 80 and the fixing device 40.

Each of the image forming stations 10Y, 10M,
Reference numerals 10C and 10K denote image forming units, and photosensitive drums (drum-shaped electrophotographic photosensitive members) 70Y, 70M, 70C, and 70K, which are image carriers, are installed so as to be rotatable in the direction of arrow a. The photosensitive drums 70Y, 70
Primary charging rollers 12Y, 12 which uniformly charge the photosensitive drum surface on the outer peripheral surfaces of M, 70C, 70K, respectively.
M, 12C, and 12K are arranged, and laser exposure devices 13Y and 13Y that expose a laser beam modulated in accordance with an image signal to the surface of the photosensitive drum are provided downstream of the photosensitive drum in the rotation direction.
3M, 13C, and 13K further develop the electrostatic latent images of the respective colors on the surface of the photosensitive drum formed by laser exposure using the yellow, magenta, cyan, and black toners of the corresponding colors on the downstream side. Developing units 14Y, 14
M, 14C and 14K are arranged.

Photosensitive drums 70, 70M, 70C, 70K
The primary transfer rollers 54 </ b> Y and 5 </ b> Y forming a primary transfer portion together with the photosensitive drum
4M, 54C and 54K are installed facing each other. The primary transfer rollers 54Y, 54M, 54C, and 54K have constant voltage power supplies 48Y, 48M,
8C and 48K are connected to apply variable primary transfer biases Vy, Vm, Vc and Vk, respectively.

The intermediate transfer belt 80 includes a driving roller 51,
A tension roller 52 and a secondary transfer opposing roller 53 are installed in a state of being stretched over the three rollers, and traverse through the image forming stations 10Y to 10K to form photosensitive drums 70Y to 70K.
The contact is arranged. The intermediate transfer belt 80 is driven to rotate by the driving roller 51 in the direction of arrow b in the figure.

Photosensitive drums 70, 70M, 70C, 70K
Downstream of the primary transfer rollers 54Y, 54M, 54C, and 54K, the drum cleaners 16Y, 16M, 16C, 1
6K is installed. Further, a belt cleaner 33 is disposed at the position of the driving roller 51 of the intermediate transfer belt 80.

The image forming operation of the image forming apparatus configured as described above will be described by taking the yellow image forming station 10Y as an example.

The photosensitive drum 70Y of the yellow station 10Y has a photoconductive layer formed on the surface of an aluminum cylindrical body, and the surface thereof is uniformly reduced by the primary transfer roller 12Y during the rotation in the direction of arrow a. Charged (charge potential = −650 V), and then the laser exposure device 13
Image exposure is performed by Y (surface potential after exposure = −2).
50V), an electrostatic latent image corresponding to the yellow image component of the document is formed on the surface of the photosensitive drum 70Y. This latent image is
The developing unit 14Y develops the image using the negatively charged yellow toner, and the latent image is visualized as a yellow toner image. The obtained yellow toner image is primarily transferred onto the intermediate transfer belt 80 by applying a primary transfer bias from the primary transfer power supply 48Y to the primary transfer roller 54Y. The transfer residual toner adhered to the surface of the photosensitive drum 70Y after the transfer is removed by the cleaner 16Y, and is used for the next image formation.

The above image forming operation is performed at predetermined timing in each of the image forming stations 10Y to 10K, and the toner images on the photosensitive drums 70Y to 70K are sequentially transferred onto the intermediate transfer belt 80 at the respective primary transfer sections. The primary transfer is performed again.

In the case of the full color mode, the toner images are sequentially transferred to the intermediate transfer belt 80 in the order of yellow, magenta, cyan, and black. Are transferred in the same order as above.

Thereafter, as the intermediate transfer belt 80 rotates in the direction of arrow b, the four color toner images on the intermediate transfer belt 80 are interposed between the grounded secondary transfer opposing roller 53 and the intermediate transfer belt 8. The secondary transfer roller 55 is moved to the contacted secondary transfer section, and is supplied to the secondary transfer roller 55 by the paper feed roller 20 at a predetermined timing. By applying a secondary transfer bias from the power supply 49, the secondary transfer is performed collectively.

Transfer material P on which four color toner images are secondarily transferred
Is transferred to a fixing device 40, where it is pressurized and heated to melt and mix the four color toners and fix the same on the transfer material P, thus forming a full-color image on the transfer material P. Meanwhile, 2
The transfer residual toner remaining on the surface of the intermediate transfer belt 80 after the completion of the next transfer is removed by the belt cleaner 33.

In this embodiment, the photosensitive drums 70 (70Y to 70Y)
70K), a negative bias OPC drum having a diameter of 30.6 mm is used, and a charging bias in which an AC component is superimposed on a DC component is applied to the charging roller 12 (12Y to 12K). The surface was uniformly charged to about -650V. The exposure unit 13 (13Y to 13K) has a wavelength of 760
and a polygon mirror that scans the photosensitive drum 70 with a laser beam. The surface potential of the photosensitive drum 70 is reduced to -250 V in an exposed portion, and the electrostatic potential is set as an image portion. Form a latent image.

The black developing device 14K is a developing device of a jumping development system using a magnetic toner (magnetic one-component developer), and has a sleeve-shaped developer carrying member including a roller-shaped fixed magnetic member (magnet roller). A magnetic toner having a particle diameter of 6 μm is carried on the (developing sleeve), and the thickness of the toner layer is regulated by an elastic blade with the rotation of the developing sleeve. The toner is conveyed to the developing section facing the photosensitive drum, and a DC component applied to the developing sleeve is applied. The toner on the developing sleeve jumps to the electrostatic latent image on the photosensitive drum and adheres to the exposed portion of the latent image by the developing bias in which the AC component is superimposed on the latent image, and the latent image is reversely developed.

Yellow developing unit 14Y, magenta developing unit 1
The 4M cyan developing unit 14C is a non-magnetic toner (non-magnetic 1
Component developer), a polymerized toner having a core / shell structure with a particle diameter of 6 μm containing wax as a non-magnetic toner is coated on the surface of the developing sleeve by an application roller. With the rotation of the developing sleeve, the thickness of the toner layer is regulated by an elastic blade, and the toner layer is conveyed to the developing unit facing the photosensitive drum, and is transferred onto the photosensitive drums 70Y, 70M, and 70C in the same manner as the black developing device 14K. Jumping to the electrostatic latent image and reversal development.

Primary transfer roller 54 (54Y to 54K)
Is formed by coating a conductive rubber layer of EPDM over a length of 310 mm on a core metal having a diameter of 8 mm to form a diameter of 16 mm. Each core metal is connected to a high voltage power supply 48 (48Y to 48K) via a power supply spring. It is connected. The roller hardness of the primary transfer roller 54 is 35 ° in Asker C,
The resistance value is determined under the condition that the primary transfer roller is brought into contact with an aluminum cylinder having a diameter of 30 mm which is driven to rotate at a peripheral speed of 24 mm / sec with a load of 500 g at both ends, and 50 V is applied between the cylinder and the primary transfer roller. Measure 1 × 1
0 ⁶ Ω.

The secondary transfer roller 55 is formed by covering a core metal having a diameter of 8 mm with a urethane-based conductive rubber layer over a length of 310 mm in a longitudinal direction to form a roller having a diameter of 17 mm. The resistance value measured by the same method as that of the primary transfer roller is 1 × 10 ⁷ Ω. This 2
The core of the next transfer roller 55 is also connected to the high voltage power supply 49 via a power supply spring.

The driving roller 51, the tension roller 52,
Each of the secondary transfer opposing rollers 53 is made of an aluminum conductive roller having a diameter of 32 mm, and a metal core portion is grounded via a power supply spring.

The intermediate transfer belt 80 is a single-layer seamless endless belt made of polyimide resin whose resistance is adjusted by carbon dispersion, and has a thickness of 75 μm, a circumference of 1115 mm, and a width of 310 mm perpendicular to the circumferential direction. are doing. In accordance with JIS-K6911, in order to obtain good contact between the electrode and the belt surface, conductive rubber was used as the electrode, and the volume resistance of the intermediate transfer belt was measured using an R8340 ultrahigh resistance meter manufactured by Advantest. When the rate ρv and the surface resistivity ρs are measured, ρ is obtained when 100 V is applied for 10 seconds.
v = 5 × 10 ⁸ Ωcm and ρs> 1 × 10 ¹³ Ω / □ were obtained. The same result is obtained when ρs is measured on either the front or back surface of the belt.

The tension of the intermediate transfer belt 80 stretched over the three rollers 51, 52, 53 is 6 kgf.
The distance between the driving roller 51 and the tension roller 52 is 50
0 mm, and each image forming station 10 (10Y-
The primary transfer section composed of a 10K) photosensitive drum 70 (70Y to 70K) and a primary transfer roller 54 (54Y to 54K) is equally spaced on the intermediate transfer belt 80 between the rollers 51 and 52. ing. Each of the primary transfer rollers 54 is lifted by a spring having a load of 500 gf provided at both ends thereof.
It is in contact with the back surface of the intermediate transfer belt 80 with a force obtained by subtracting g.

In this image forming apparatus, the maximum size of the transfer material that can be used is A3. The process speed is 117
mm / sec. By setting the primary transfer biases Vy to Vk at +300 V and the secondary transfer bias at +2.3 kV, good transferability on plain paper can be obtained for all colors.

Next, the relationship between the transfer current, the transfer efficiency, and the retransfer rate in one image forming station of the image forming apparatus will be described. Transfer current and transfer efficiency,
The relationship with the retransfer rate is as shown in FIG.

The transfer efficiency indicates the ratio of the amount of toner transferred from the solid image to the intermediate transfer belt 80 with respect to the total amount of toner in the solid image developed on the photosensitive drum 70.
The retransfer rate indicates the ratio of the amount of toner in which the solid image is reverse-transferred to the photosensitive drum at the next station on the downstream side with respect to the total amount of toner of the solid image transferred on the intermediate transfer belt 80 in a certain station. Therefore, in the first color yellow station 10 </ b> Y existing at the most upstream part in the moving direction of the intermediate transfer belt, it is not necessary to consider retransfer.

Here, the transfer current means the intermediate transfer belt 8
0 means a current flowing in a contact area (transfer nip) of a primary transfer portion where the photosensitive drum 70 and the primary transfer roller 54 are in contact with each other.
As long as there is no current interference between the tension rollers, it matches the current flowing in each of the constant voltage power supplies 48 (48Y to 48K), and therefore the transfer roller 54 (54Y to 54K).
In this embodiment, the solid image on each photosensitive drum 70 has a triboelectric charge amount (tribo) of −26 μC / g and a ride amount of 0.62 mg / cm ² .

In general, the transfer efficiency shows a profile having a peak value near a certain transfer current, while the re-transfer rate shows a monotonic increase with respect to the transfer current, and the transfer current Ia giving the maximum transfer efficiency is high. There is a relationship of Ia> Ib with the transfer current Ib at which the transfer efficiency and the low re-transfer rate are well compatible.

In the present image forming apparatus, the maximum transfer efficiency is Ia
= 17.5 μA, high transfer efficiency,
Good compatibility with a low retransfer rate can be obtained with a transfer current near Ib = 11.5 μA. Then, at the first yellow station 10Y where retransfer does not occur, the transfer current is set to Ia = 17.
It is assumed that the current is about 5 μA, and the second and subsequent magenta, cyan,
At each black station 10M, 10C, 10K,
The transfer current was a current around Ib = 11.5 μA.

In this embodiment, by setting the primary transfer bias of the constant voltage power supply 48Y to Vy = 300V,
a = 17.5 μA is transferred to the constant voltage power supply 48.
The primary transfer bias of M, 48C and 48K is set to Vm = Vc =
By setting Vk = 170V, Ib = 11.5
A transfer current around μA was obtained. Thereby, the first color is transferred at the maximum transfer efficiency, the second and subsequent colors are transferred at a high transfer efficiency,
In addition, retransfer is kept low during the transfer of the second and subsequent colors,
A full-color image having a sufficient density of each color could be output.

In the above, the absolute value of the transfer current differs depending on the characteristics of the components of the apparatus such as the toner, the photosensitive drum, the intermediate transfer belt and the transfer roller. The required lower limit of the transfer efficiency, the upper limit of the retransfer rate, and the balance thereof differ depending on the level required by the apparatus and the user. For example, when importance is also placed on the image density of a line image or a high-order color image such as a secondary color, it is necessary to set a transfer current value in consideration of the transfer efficiency and the transfer current of the re-transfer rate. Also, in a so-called cleaner-less image forming apparatus in which the transfer residual toner remaining on the photosensitive drum is collected and reused in the developing device again, there is a concern that a color change due to mixing of other color toner into each developing device. Therefore, it is necessary to set the transfer current so as to minimize the retransfer rate.

The present invention does not specify the absolute value of the transfer current at each image forming station or the difference of the transfer current between different image forming stations, but it satisfies the unique requirements of each apparatus. Needless to say, the above-described setting of the transfer current value is effective for obtaining a full-color image having a sufficient density.

As described above, according to this embodiment,
Since the transfer current of the first image forming station is made larger than the transfer current of the second and subsequent image forming stations, the primary transfer efficiency of each color and the retransfer rate of the next color of each color in the image forming section are balanced. It is possible,
A full-color image having a sufficient image density of each color can be obtained.

Embodiment 2 FIG. 3 is a schematic diagram showing another embodiment of the image forming apparatus of the present invention.

In this embodiment, in the image forming apparatus of the first embodiment shown in FIG. 1, among the constant voltage power supplies 48Y to 48K for applying the primary transfer bias, the power supplies 48M to 48C after the second image forming station are used. As shown in FIG. 3, the power supply was simplified by being combined into a single common constant voltage power supply 48X.

The other structure of the image forming apparatus of the present embodiment is basically the same as that of the image forming apparatus of the first embodiment shown in FIG. 1, and the same reference numerals in FIG. The elements of

In this embodiment, the same primary transfer bias Vy = 300 V as in the first embodiment is applied to the primary transfer roller 54Y by the constant voltage power supply 48Y.
The same primary transfer bias Vx (Vm, Vm) as in the first embodiment is supplied to the M, 54C, and 54K by the common constant voltage power supply 48X.
c, Vk) = 170 V was applied in parallel.

As described above, when the primary transfer bias in the second and subsequent stations may have the same value, the primary transfer bias may be applied in parallel by a single constant voltage power supply, thereby reducing the cost of the apparatus. Is realized.

According to this embodiment, it is also possible to balance the primary transfer efficiency of each color and the retransfer rate of each color in the image forming section after the next color, and to obtain a full-color image having a sufficient image density of each color. .

Embodiment 3 FIG. 4 is a schematic structural view showing still another embodiment of the image forming apparatus of the present invention.

This embodiment is different from the image forming apparatus of the first embodiment shown in FIG.
, The constant current power supply 50 (50Y
-50K) was used.

In this embodiment, the constant current power supply 50Y
A primary transfer bias is applied to the next transfer roller 54Y by a constant current control of a transfer current Iy = 17.5 μA, and a constant current power supply 5
0M, 50C, 50K, and the primary transfer rollers 54M, 5M.
4C and 54K, the transfer current Im = Ic = Ik =
A primary transfer bias was applied at a constant current control of 11.5 μA.

As described above, by using the constant current power supply as the primary transfer power supply for each image forming station, the transfer current can be stabilized. When the intermediate transfer belt has a higher resistance and has a characteristic of gradually charging up at the time of transfer at each image forming station, and also at the time of continuous printing, when the charging potential and exposure potential of the photosensitive drum at each station changes, or when the image forming Even when the charging potential and the exposure potential of the photosensitive drum differ from station to station, stable transfer current control is maintained.

In the above, the constant current power supplies 50 are supplied to the transfer rollers 54Y to 54K of the first to fourth stations.
Although Y to 50K are provided, the constant current power supplies 50M to 50K of the second and subsequent stations having the same primary transfer bias value may be combined into a single constant current power supply according to the second embodiment. Cost reduction is realized.

According to this embodiment, it is also possible to balance the primary transfer efficiency of each color and the retransfer rate of each color in the image forming section after the next color, and to obtain a full-color image having a sufficient image density of each color. .

Embodiment 4 FIG. 5 is a schematic structural view showing still another embodiment of the image forming apparatus of the present invention.

The image forming apparatus of the present embodiment is a modification of the image forming apparatus of the third embodiment shown in FIG. 4 of the intermediate transfer type to an image forming apparatus of the multiple transfer type.

In this embodiment, instead of the intermediate transfer belt 80 of the image forming apparatus shown in FIG. P is carried on the transfer belt 90 and conveyed, and the toner image of each color on the photosensitive drum 10 is transferred onto the transfer material P at a transfer section facing the photosensitive drum 70 (7Y to 70K) of each of the image forming stations 10Y to 10K. Then, multiple transfer is performed by the transfer roller 54 (54Y to 54K) to which a transfer bias is applied from the constant current power supply 50 (50Y to 50K). Thereafter, the transfer material P is separated from the transfer belt 90 and conveyed to the fixing device 40, where the toner images of four colors are pressed and heated to form a full-color fixed image.

The transfer belt 90 is made of the same material and the same size as the intermediate transfer belt 80, but has a higher resistance value adjusted by carbon dispersion in order to have an action of electrostatically attracting the transfer material P. are doing. When the volume resistivity ρv and the surface resistivity ρs of the transfer belt 90 are measured under the conditions of application of 100 V and 10 seconds by the measurement method described in Example 1, ρv = 2 × 10 ¹⁵ Ωcm, ρs> 1 × 10 ¹³ Ω / □
Was obtained. The same result is obtained when ρs is measured on either the front or back surface of the belt.

In this embodiment, as in the third embodiment, the transfer current Iy = 1 is applied to the transfer roller 54Y by the constant current power supply 50Y.
A transfer bias is applied under the constant current control of 7.5 μA, and the transfer roller 54 is controlled by the constant current power supplies 50M, 50C and 50K.
M, 54C, and 54K respectively have a transfer current Im = Ic =
The transfer bias was applied under the constant current control of Ik = 11.5 μA.

As a result, even in the multiple transfer method, the transfer current of the first station is made larger than the transfer current of the second and subsequent stations, so that the primary transfer efficiency of each color and the image formation unit of the next color of the respective colors and thereafter are formed. The retransfer rate can be balanced, and a full-color image having a sufficient image density of each color can be obtained.

In the above, the constant current power supplies 50 are supplied to the transfer rollers 54Y to 54K of the first to fourth stations.
Y to 50K are provided, but the constant current power supplies 50M to 50K of the second and subsequent stations having the same primary transfer bias value are used.
The devices may be combined into a single constant current power supply, and the cost of the device can be reduced.

Also in the multiple transfer system, the transfer power supply can be a constant voltage power supply as in the first and second embodiments.
By making the transfer current of the first station larger than the transfer current of the second and subsequent stations, the primary transfer efficiency of each color and the retransfer rate in the image forming unit for the next color and the following colors are balanced, and the image of each color is A full-color image having a sufficient density can be obtained.

[0063]

As described above, according to the present invention,
Transfers multi-color toner images on multiple image carriers at multiple image forming stations to an intermediate transfer member, or transfers multiple color toner images on multiple image carriers to a transfer material on a transfer material carrier The transfer current of the first station to the second
By making the transfer current larger than that of the subsequent stations, the primary transfer efficiency of each color and the retransfer rate of each color in the subsequent stations can be balanced to obtain a full-color image with sufficient image density of each color. .

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of an image forming apparatus of the present invention.

FIG. 2 is an explanatory diagram showing a relationship between a transfer current, a transfer efficiency, and a retransfer rate in an image forming station of the apparatus of FIG.

FIG. 3 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.

FIG. 4 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.

FIG. 5 is a schematic configuration diagram showing still another embodiment of the image forming apparatus of the present invention.

[Explanation of symbols]

 10Y to 10K Image forming station 48Y to 48K Constant voltage power supply 50Y to 50K Constant current power supply 54Y to 54K Transfer roller 70Y to 70K Photosensitive drum 80 Intermediate transfer belt 90 Transfer belt

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H027 EA03 EB04 EC20 ED24 EE07 EF09 ZA01 2H030 AA06 AB02 AD16 BB23 BB42 BB44 BB54 2H032 AA05 BA09 BA18 BA23 CA01 CA13

Claims (9)

[Claims] A plurality of image carriers, an intermediate transfer member rotating along the plurality of image carriers, and the plurality of image carriers opposed to the plurality of image carriers with the intermediate transfer member interposed therebetween. An image forming apparatus having a plurality of transfer charging means for transferring a plurality of color toner images on a body to the intermediate transfer body, wherein a transfer of a most upstream part of the intermediate transfer body in the rotation direction of the plurality of transfer charging means is performed. An image forming apparatus, wherein a transfer current value in the charging unit is larger than a transfer current value in the remaining transfer charging unit. 2. A plurality of image carriers, a transfer material carrier rotating along the plurality of image carriers, and the plurality of image carriers opposed to each other across the plurality of image carriers and the transfer material carrier. A plurality of transfer charging means for transferring a plurality of color toner images on an image carrier onto a transfer material on the transfer material transport body, wherein the transfer material transport member of the plurality of transfer charge means Transfer current value in the transfer charging means at the most upstream part in the rotation direction of
An image forming apparatus, wherein the transfer current value is larger than the transfer current value of the remaining transfer charging means. 3. The image forming apparatus according to claim 1, wherein a constant voltage power supply is used as a power supply for the plurality of transfer charging units. 4. The image forming apparatus according to claim 3, wherein one common constant voltage power supply is used as a power supply for the remaining transfer charging means. 5. The image forming apparatus according to claim 1, wherein a constant current power supply is used as a power supply for the plurality of transfer charging units. 6. The image forming apparatus according to claim 5, wherein one common constant current power supply is used as a power supply for the remaining transfer charging means. 7. The image forming apparatus according to claim 1, wherein the intermediate transfer member is an intermediate transfer belt that rotates endlessly. 8. The image forming apparatus according to claim 2, wherein said transfer material transporting body is a transfer belt which rotates endlessly. 9. The image forming apparatus according to claim 1, wherein the transfer charging unit is a transfer roller.