JP2001188401A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device by which a multicolor image is obtained by performing excellent density control by means of accurate density detection by preventing the retransfer of toner to an image carrier when an image pattern for density detection passes through a transfer part on a down stream side. SOLUTION: In forming a continuous image, a transfer material P1 on a transfer belt 12 is positioned at a transfer part TY of yellow which is next to the most upstream transfer part and a transfer material P2 is positioned at a transfer part TC of cyan, and the image pattern GC of cyan transferred to a space between the sheets of paper is positioned at a transfer part TM of magenta, and passes there. Through a magenta toner image is transferred by applying the transfer bias of +600 V at the point of time when the transfer material P2 is positioned at a transfer part TM, the transfer bias is lowered to +200 V for the prevention of the retransfer of the toner or the like when the image pattern GC passes. The transfer bias is similarly lowered from +800 V to +200 V when the image pattern GC of cyan passes the transfer part TY of yellow. Then, the density of the image pattern GC is inspected by a sensor 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an in-line type image forming apparatus such as an electrophotographic copying machine, a facsimile, and a printer for forming a multicolor image with a plurality of photosensitive members.

[0002]

2. Description of the Related Art Conventionally, a plurality of image carriers composed of photoreceptors are provided, toner images of different colors are formed on the respective image carriers, and the images of the respective colors are overlaid and transferred onto the same transfer material. Various image forming apparatuses of an in-line type for forming a multicolor image on a transfer material have been proposed.

In such a multi-color image forming apparatus, a transfer material is electrostatically attracted to a transfer material carrier (transfer belt, transfer drum) and is conveyed to a transfer portion of each image carrier, and a transfer material on each image carrier is transferred. A method in which images are sequentially superimposed and transferred, or an image on each image carrier is superimposed and transferred on an intermediate transfer body (an intermediate transfer belt or an intermediate transfer drum) by a primary transfer unit and then transferred to the intermediate transfer body. A method of batch transfer to a material by a secondary transfer unit is employed.

The density and gradation of the image of each color transferred to each image bearing member are determined by changes in the characteristics of the photosensitive member due to the use environment, long-term use, etc., changes in the T / C ratio of the developer, and use of the developer. Therefore, it is necessary to control the image density in order to always output a high-quality and stable multi-color image with high density.

To control the image density, an image pattern for density detection having a gradation for each color is created, transferred to a transfer material transporting body or an intermediate transfer body, and the image pattern is optically reflected by a reflected light amount sensor. And read based on it.

An image pattern for density detection is used in order to prevent the transfer material from being wasted or stained.
In many cases, the image is transferred to a so-called inter-sheet position between the transfer material conveyed by the transfer material conveyance body or between the transfer material and the next transfer material, or to a position corresponding to this position on the intermediate transfer body.

[0007]

However, in the above-described image forming apparatus of the in-line system, when the image pattern transferred to the transfer material transporting body or the intermediate transfer body at the upstream transfer unit passes through the transfer unit at the downstream position, In some cases, a part of the toner is re-transferred from the image pattern to the image carrier at a position downstream of the toner, resulting in a problem that accurate density detection cannot be performed.

Further, even if retransfer does not occur, the improper application of a transfer bias at the time of transfer may cause a problem that affects the subsequent image formation such as a charged memory (transfer memory) on the image carrier. there were.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent the toner from being re-transferred from the image pattern to the image carrier at the transfer portion when the image pattern for density detection passes through the transfer portion on the downstream side. Provided is an image forming apparatus which enables density detection, and as a result, can obtain a high-quality multi-color image with stable density by good density control, and does not generate a transfer memory on an image carrier. It is.

[0010]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
A transfer material transporter, a plurality of image carriers arranged along the transfer material transporter, and sequentially transported to a plurality of transfer units that are carried on the transfer material transporter and face the plurality of image carriers. Transferring a plurality of toner images of a plurality of colors formed by charging the plurality of image carriers by a primary charger, exposing the image by an exposure unit, and developing by a developing unit onto the transfer material, and sequentially transferring the superposed toner images. And a plurality of transfer charging means to which a transfer bias is applied, which is disposed on the inner side of the transfer material transporter, and which are formed on the plurality of image carriers and do not overlap a plurality of colors on the transfer material transporter. A density control device that detects the density of the transferred density detection image of each color and controls the image density of each color, wherein the density detection image on the transfer material transporting body is the image of the image. Downstream of the transfer section where the The transfer bias applied to the transfer unit of the downstream transfer unit when passing through the transfer unit is changed to have the same polarity as the transfer bias and to reduce the difference from the applied voltage of the primary charger. Image forming apparatus.

According to the present invention, the density detection of the image for density detection is performed on the downstream side of the most downstream transfer section of the plurality of transfer sections. Further, at the time of continuous image formation, the density detection image of each color is transferred to the transfer material transporting body in such a manner that one color is transferred to an area between the transfer materials carried on the transfer material transporting body. You can do so.

Further, the present invention provides an intermediate transfer member, a plurality of image carriers arranged along the intermediate transfer member, charging of the plurality of image carriers by a primary charger, image exposure and development by exposure means. Means for transferring a plurality of color toner images formed by development by means on the intermediate transfer member in a superimposed manner, wherein the intermediate transfer member is a plurality of primary transfer portions in which the intermediate transfer member and the plurality of image carriers are opposed to each other. A plurality of transfer charging means to which a transfer bias disposed inside is applied,
A density control device for forming a plurality of colors on the plurality of image carriers and transferring the plurality of colors to the intermediate transfer body without overlapping, detecting a density of a density detection image of each color, and controlling an image density of each color; In the image forming apparatus, the plurality of color toner images superimposedly transferred on the intermediate transfer body are thereafter collectively transferred to a transfer material, wherein the density detection image is the intermediate transfer image. When passing through the primary transfer portion on the downstream side of the primary transfer portion transferred onto the body, a transfer bias applied to the transfer means of the primary transfer portion on the downstream side has the same polarity as that of the transfer bias and is applied to the primary charger. An image forming apparatus characterized in that the difference from the voltage is changed to be small.

According to the present invention, the density detection of the density detection image is performed on the downstream side of the most downstream primary transfer section of the plurality of primary transfer sections. Further, at the time of forming a continuous image, the density detection image of each color is transferred to an area between a toner image of a plurality of colors transferred onto the intermediate transfer body and a toner of the next plurality of colors in an area. The image can be transferred to the intermediate transfer member.

[0014]

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

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

This image forming apparatus forms an in-line image by forming toner images of different colors on a plurality of photosensitive drums and sequentially transferring the toner images on each photosensitive drum onto a transfer material to obtain a multicolor image on the transfer material. It is configured in a multi-color image forming apparatus of a system.

As shown in FIG. 1, the image forming apparatus includes an endless belt-like transfer material transporting body made of a dielectric material, that is, a transfer belt 12, and the transfer belt 12 includes a driven roller 9,
It is stretched around the drive roller 10 and the tension roller 11, and is rotated in the direction of the arrow in the figure by the drive of the drive roller 10.

A photosensitive drum 1K for black, a photosensitive drum 1C for cyan, a photosensitive drum 1M for magenta, a photosensitive drum 1M for yellow, and a photosensitive drum 1C as image carriers from the upstream side in the transfer material transport direction along the upper path of the transfer belt 12. Drums 1Y are installed in series, and the photosensitive drums 1K, 1C, 1M,
Around primary charging rollers 2K, 2C, and 2Y, respectively.
M, 2K, exposure unit 3K, 3C, 3M, 3Y, developing unit 4
K, 4C, 4M, 4Y, transfer rollers 13K, 13C, 1
3M, 13Y, drum cleaners 6K, 6C, 6M, 6Y
Is installed.

The exposure unit 3 (3K to 3Y) includes the photosensitive drum 1
(1K to 1Y), and the transfer roller 13
(13K to 13Y) are arranged inside the transfer belt 12 so as to face the photosensitive drum 1. Developing unit 4K, 4
Black, cyan, magenta, and yellow developers are stored in C, 4M, and 4Y, respectively.

A transfer material attracting charging brush 14 facing the driven roller 9 with the transfer belt 12 interposed therebetween is provided at the most upstream position of the transfer belt 12 in the transfer material transport direction. Paper device 19, registration roller 15
Is installed. At the most downstream position of the transfer belt 12 in the transfer material transport direction, the drive roller 1
A charger 17 for peeling / discharging is disposed opposite to 0, and a density sensor 16 is disposed near the upstream side thereof. A belt cleaner 18 is provided on a lower track of the transfer belt 12, and a fixing device 7 is provided on an extension of the transfer belt 12 downstream of the transfer material conveyance direction.

To form an image, the photosensitive drum 1K is rotated in the direction of the arrow by an electric motor, and first, the surface of the photosensitive drum 1K is uniformly charged to a desired polarity by the charging roller 2K. In this embodiment, the photosensitive drum 1 is charged by this charging.
The surface potential of K was -600V. Next, the exposure unit 3K
To form an electrostatic latent image corresponding to black on the surface of the photosensitive drum 1K.

The exposing unit 3 (3K to 3Y) is composed of a laser generator, and generates a laser beam corresponding to the original by driving with an image signal of an original image from an external device (not shown). The laser beam 8 (8K to 8Y) passes through the photosensitive drum 1 (1K to 1Y) via the polygon mirror and the reflection mirror.
Y), an electrostatic latent image corresponding to the original image is formed on the surface of the photosensitive drum 1 as a laser irradiation unit.

The electrostatic latent image on the photosensitive drum 1K is developed by a developing unit 4K using a black developer, and is visualized as a black toner image. Black, cyan,
The magenta and yellow developers are applied to the photosensitive drum 1 respectively.
And black, cyan, magenta, and yellow having the same polarity as the charged polarity on the surface of the magnetic carrier and a magnetic carrier. The electrostatic latent image on each of the photosensitive drums 1 (1K to 1Y) is developed by the reversal developing method with the negative toner adhered thereto.

On the other hand, the transfer material is fed to the transfer belt 12 via the paper feeding device 19 and the registration roller 15, and the transfer material is stuck on the transfer belt 12 by the action of the suction charging brush 14 to which the suction bias is applied. It is held by electroadsorption. The transfer material held on the transfer belt 12 is conveyed to a transfer section facing the photosensitive drum 1K with the rotation of the transfer belt 12, and is transferred by a transfer roller 13K to which a positive transfer bias of a polarity opposite to that of the toner is applied. The black toner image on the photosensitive drum 1K is transferred onto the transfer material. A drive start signal of the registration roller 15, that is, a registration adjustment transfer material conveyance roller, is used as a reference for matching the timing of the transfer of the transfer material and the formation of the toner image on the photosensitive drum 1K.

The same steps as those for forming the toner image on the black photosensitive drum 1K and transferring the toner image to the transfer material are carried out as follows.
Cyan, magenta, and yellow photosensitive drums 1C, 1M,
By performing the same for 1Y, a full-color image in which four color toner images of black, cyan, magenta, and yellow are superimposed on the same transfer material is formed.

The transfer bias applied at the time of transferring the toner image is such that the surface potential of the transfer material is increased by the charge of the toner transferred onto the transfer material.
In the present embodiment, for example, when the potential difference between the photosensitive drum 1 and the transfer material becomes 500 V or less, there is a risk of re-transfer of the toner from the transfer material to the photosensitive drum. + 200V, + 400V,
The voltage is set and applied as +600 V, +800 V.

The transfer material onto which the four color toner images have been transferred is separated from the transfer belt 12 via the peeling / discharging charger 17 and then sent to the fixing device 7 to fix the toner images, thereby forming a full-color image. finish.

On the other hand, each of the photosensitive drums 1 (1K-1
In the case of Y), the transfer residual toner remaining on the surface is removed by the drum cleaner 6 (6K to 6Y) to prepare for the next image formation of the transfer material. After the transfer material is separated, the transfer belt 12 is prepared for the next image formation of the transfer material by removing the adhered matter such as toner and paper powder adhered to the surface by the belt cleaner 18.

Next, the details of the image density control according to the present invention will be described with reference to FIG. In the image forming apparatus of the present embodiment, image patterns (patch marks) of black, cyan, magenta, and yellow for density detection are incorporated in a ROM of a density control device (not shown). In order to control the density, first, a latent image of the image pattern for density detection of each color is formed on the photosensitive drums 1K to 1Y of the corresponding color, and is developed by the developing units 4K to 4Y to form the photosensitive drum 1K.
1Y, an image pattern of each color of black, cyan, magenta, and yellow composed of a toner image is formed, transferred directly onto the transfer belt 12 without overlapping each other, and the density of the image pattern of each color is determined by the density. It is detected by the sensor 16.

The transfer of the image pattern for density detection is as follows.
During the continuous image formation, the transfer is performed between the transfer materials on the transfer belt 12, that is, between the transfer materials. Normal image formation is performed in which a toner image is formed on 1Y by a normal image forming process, and the toner images of the respective colors are superimposed and transferred.

The ROM of the density control device contains lower limit set values Kmin, Cmin, Mmin, Ymin and upper limit set values KMax, CMax, MMax, YMax for the densities of black, cyan, magenta, and yellow image patterns, respectively. ing. The CPU of the density control device determines the densities K1, C1, M1, and Y1 of the detected black, cyan, magenta, and yellow image patterns as lower limit set values Kmin, Cmin,
Mmin, Ymin and upper limit set values KMax, CMax, MMax, YMax
Is determined, and if there is a color that is not within the range, a command to start density control of the color is issued.

In the density control, the detected densities K1 to Y1 of the image pattern are generally set to P1, the lower limit set values Kmin, Cmin, M
min, Ymin is Pmin, upper limit set values KMax, CMax, MMax,
When YMax is PMax and P1 ≧ PMax, the exposure unit 3
A process (down process) is performed to reduce the density of the corresponding color by reducing the exposure amount of the corresponding color exposure device of (3K to 3Y). When Pmin ≧ P1, the corresponding color is used. A process (up process) for increasing the density of the color by increasing the exposure amount of the exposing device is performed.

After performing the above-described density control, the process returns to the density detection again, an image pattern of the color subjected to the density control is formed on the transfer belt 12, and the density is detected by the sensor 16.
A check is made to determine whether the density of the color whose density was inappropriate due to the density control has become appropriate. This operation is repeated if necessary for the color of which the density is inappropriate, and after the density of all four colors is made appropriate, the density control ends.

On the other hand, as described above, the image formation on the transfer material is continued in parallel with the image formation of the image pattern and the density control based on the density detection. enter. Then, when the image formation on the transfer material proceeds and reaches a preset number of prints, the above-described density control below the density detection is performed again. Thereafter, this is repeated.

In the cyan, magenta, and yellow transfer portions of the second and subsequent colors downstream of the uppermost black transfer portion of the first color, the surface potential of the transfer material sequentially increases due to the toner on the transfer material. Therefore, the photosensitive drum 1C,
As described above, a sufficiently high transfer bias of, for example, +400 V, +600 V, and +800 V is applied to the transfer rollers 13C, 13M, and 13Y in order to transfer the toner images on 1M and 1Y. However, in this case, when the image pattern transferred onto the transfer belt 12 passes through these transfer portions, an electrostatic memory (transfer memory) is formed on the photosensitive layers of the photosensitive drums 1C to 1Y due to excessive positive charging.
Occurs.

According to the study made by the present inventor, if the voltage is reduced while the transfer bias is kept positive when the image pattern passes, the transfer bias has a polarity that attracts the toner to the transfer belt 12. In addition, since the voltage difference from the surface potential of the photosensitive drum by the primary charger (the voltage applied to the primary charger) is small, 1C to 1Y are applied to the photosensitive drum.
It has been found that the image pattern is not disturbed by the discharge to the toner or the like and the toner is not retransferred to the photosensitive drum or the transfer memory is generated on the photosensitive drum.

Therefore, in the present invention, when the image pattern on the transfer belt 12 passes through the cyan, magenta, and yellow transfer portions which are not the uppermost stream, the transfer rollers 13C
The transfer bias was applied to about 13Y with a small restriction. This transfer bias may be limited to the same transfer bias value in all the transfer sections through which the image pattern passes, may be different for each transfer section, or may be changed to a different transfer bias value for each color of the image pattern. You may.

In this case, the photosensitive drum 1 and the transfer belt 12
Is less than about 800 V, it is possible to reduce the possibility of occurrence of transfer memory.
If the voltage is 0 V or higher, the possibility of occurrence of a noticeable transfer memory increases.

In this embodiment, the transfer bias when passing the image pattern is limited to +200 V in all of the cyan, magenta, and yellow transfer portions. According to the transfer bias, the potential difference of the transfer belt 12 can be stopped at 800 V with respect to the surface potential of the photosensitive drum 1 of -600 V, and the transfer memory can be prevented.

In this case, if the surface of the photosensitive drum temporally deteriorates, the relationship between the charging voltage of the primary charger and the surface potential of the photosensitive drum is broken. Therefore, the applied voltage of the primary charger is used as the surface potential of the photosensitive drum. Instead, it is preferable to install a potential sensor (not shown) to measure the surface potential of the photosensitive drum, and to determine a limited transfer bias when the image pattern passes through the measured surface potential.

The above-described prevention of retransfer will be further described with reference to FIG. FIG. 3 is a conceptual diagram showing an arrangement of a transfer material and an image pattern for density detection during continuous image formation. The transfer material is continuously carried on the transfer belt 12, and the transfer portions T of the respective colors are transferred.
K, TC, TM, and TY are sequentially conveyed. Now, the transfer material P1 on the transfer belt 12 is located at the yellow transfer portion TY.
The transfer material P2 is located at the cyan transfer portion TC, and the cyan image pattern GC transferred from the photosensitive drum 1C at the cyan transfer portion TC is located at the magenta transfer portion TM between the sheets of the transfer materials P1 and P2. ing.

The transfer material P2 has +4 on the transfer roller 13C.
A cyan toner image is transferred from the photosensitive drum 1C by applying a transfer bias of 00V, and a yellow toner image is transferred from the photosensitive drum 1Y to the transfer material P1 by applying a + 800V transfer bias to the transfer roller 13Y. Is done.

At this time, in the magenta transfer portion TM, when the previous transfer material P1 is located, the magenta toner image is transferred from the photosensitive drum 1M to the transfer material P1.
Although a transfer bias of +600 V was applied to the transfer roller 13M, since the cyan image pattern GC has passed, the transfer bias is reduced to + 200V. Thereby, retransfer of the toner from the cyan image pattern GC to the photosensitive drum 1M and occurrence of a transfer memory of the photosensitive drum 1M can be prevented.

Similarly, when the cyan image pattern GC passes through the yellow transfer portion TY, the transfer roller 13Y
+200 V transfer bias applied to
To prevent the transfer of the toner from the cyan image pattern GC to the photosensitive drum 1Y and the occurrence of a transfer memory of the photosensitive drum 1Y.

In this way, the density of the cyan image pattern GC that has passed through the magenta transfer portion TM and the yellow transfer portion TY reaches the density sensor 16 and is transferred to the photosensitive drum by the transfer portions TM and TY. Since there is no re-transfer of the toner, the image density does not change and the sensor 16 can detect an accurate density. The image pattern GC received the density detection by the sensor 16 is transferred to the transfer belt 1.
As the roller 2 rotates, it passes through the belt cleaner 18, where it is cleaned and removed from the transfer belt 12.

Similarly, in the transfer section TM of magenta,
Between the transfer material P1 on the transfer belt 12 and the transfer material separated from the previous transfer belt 12, the photosensitive drum 1M
The transfer bias applied to the transfer roller 13Y is reduced to +200 V when the magenta image pattern GM transferred from the printer passes through the yellow transfer portion TY. Thus, the density of the magenta image pattern GM can be prevented from being changed, and the accurate density can be detected by the sensor 16.

In the black transfer portion TK, the transfer material P
The same applies to the black image pattern Gk transferred from the photosensitive drum 1K between the sheet 2 and the next transfer material carried on the transfer belt 12, and the image pattern GK is a cyan transfer portion TC and a magenta transfer portion TM. , Yellow transfer part TY
, When the transfer rollers 13C, 13M, 1
The transfer bias is applied to 3Y with the transfer bias reduced to + 200V.

In the yellow transfer portion TY, the density of the yellow image pattern GY transferred from the photosensitive drum 1Y onto the transfer belt 12 between the sheets of the transfer material is immediately detected by the sensor 16 without passing through another transfer portion. This is not the case for the image pattern GY with respect to taking measures to reduce the transfer bias when passing through the transfer section. In addition, since the image pattern does not pass through the uppermost black transfer portion TK, no measure is taken to lower the transfer bias when passing through the transfer portion.

In the above embodiment, the image density control by detecting the density of the image pattern is performed by controlling the transfer belt 1 during image formation.
Although the image pattern is formed between the sheets of the transfer material on the sheet 2, the transfer can be performed even when the transfer material is not being conveyed, such as when the image forming apparatus is started. When the apparatus is in a star state, image formation cannot be performed until the temperature of the fixing unit 7 ends, but the density detection / image density control of the image pattern without using the fixing unit is performed until the temperature of the fixing unit ends. It can be done using time.

In this case, since no transfer material is carried on the transfer belt 12, only the image pattern is transferred to the transfer belt 12.
As described above, when the image pattern passes through the transfer portion on the downstream side, the transfer bias at the transfer portion may be controlled to be small.

Further, the density detection / image density control of the image pattern can be performed when a transfer failure occurs or as shown in FIG. 2 for each specific number of prints.

Although the density sensor 16 is provided only at one location in the moving direction of the transfer belt 12, it can be installed at a plurality of locations in the longitudinal direction to simultaneously detect image patterns of different gradations. .

The image density was controlled by changing the exposure amount of the image on the photosensitive drum, but finally the image density on the transfer material was controlled by the developing bias of the developing device and the charging potential of the photosensitive drum. Anything that can be applied can be applied.

Further, the image forming apparatus has been described by way of example in which a plurality of toner images on a plurality of photosensitive drums are directly superimposed and transferred onto a transfer material. Arranged along an intermediate transfer body such as a belt, a plurality of toner images are superimposed on the intermediate transfer body and primary transferred,
Thereafter, the present invention can be applied to an intermediate transfer type image forming apparatus that performs secondary transfer collectively on a transfer material. The image pattern for density detection is formed by transferring the image pattern on the intermediate transfer member, and the transfer bias may be controlled when the image pattern passes through the transfer portion on the downstream side.

[0055]

As described above, according to the present invention,
When the image pattern for density detection passes through the transfer section on the downstream side, the transfer bias is applied with the same polarity as this and limited to a voltage value such that the potential difference from the applied voltage of the primary charger is reduced. The transfer unit prevents the toner from being re-transferred from the image pattern to the image carrier. As a result, the density of the image pattern is accurately detected without changing, and the density is stable with high quality by good density control. A multicolor image can be obtained. Further, no transfer memory is generated on the image carrier when the image pattern passes.

[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 a flowchart illustrating image density control based on image pattern density detection during continuous image formation in the image forming apparatus of FIG. 1;

FIG. 3 is a conceptual diagram showing an arrangement of a transfer material and an image pattern for density detection during continuous image formation in the image forming apparatus of FIG. 1;

[Explanation of symbols]

 1K-1Y Photosensitive drum 4K-4Y Developing device 12 Transfer belt 13K-13Y Transfer charger 16 Density sensor 17 Belt cleaner GK-GY Image pattern P1, P2 Transfer material TK-TY Transfer unit

Claims (6)

[Claims] 1. A transfer material transporter, a plurality of image carriers arranged along the transfer material transporter, and a plurality of transferers carried on the transfer material transporter and facing the plurality of image carriers. A plurality of color toner images formed by charging the plurality of image carriers by a primary charger, exposing an image by an exposure unit, and developing by a developing unit are superimposed and sequentially transferred to a transfer material sequentially conveyed to the unit. A plurality of transfer charging means disposed inside the transfer material transporter of the plurality of transfer units and to which a transfer bias is applied; and a plurality of colors formed on the plurality of image carriers and provided on the transfer material transporter. And a density control device that detects the density of the image for density detection of each color transferred without overlapping, and controls the image density of each color. The image has transferred the image When passing through the transfer unit on the downstream side of the image transfer unit, the transfer bias applied to the transfer unit of the transfer unit on the downstream side has the same polarity as this, and the difference between the transfer bias and the voltage applied to the primary charger is reduced. An image forming apparatus characterized by having been changed to: 2. The image forming apparatus according to claim 1, wherein the density detection of the density detection image is performed on a downstream side of a most downstream transfer unit among the plurality of transfer units. 3. A method for transferring a density detection image of each color to a transfer material carried on the transfer material transporting body by transferring one color to an area between the transfer materials during the continuous image formation. The image forming apparatus according to claim 1, wherein the image is transferred to a body. 4. An intermediate transfer member, a plurality of image carriers arranged along the intermediate transfer member, and charging of the plurality of image carriers by a primary charger, image exposure by exposure means, and development by development means. The intermediate transfer member and the plurality of image carriers are arranged inside the intermediate transfer member of a plurality of primary transfer portions opposed to each other, in which the formed plurality of color toner images are sequentially superimposed and transferred on the intermediate transfer member. A plurality of transfer charging means to which a transfer bias is applied, and detecting a density of a density detection image of each color formed on the plurality of image carriers and transferred to the intermediate transfer body without overlapping a plurality of colors. do it,
A density control device for controlling the image density of each color, wherein the toner images of a plurality of colors transferred by being superimposed on the intermediate transfer body are thereafter collectively transferred to a transfer material; When the detection image passes through the primary transfer portion downstream of the primary transfer portion that has transferred the image onto the intermediate transfer member, a transfer bias applied to the transfer means of the primary transfer portion on the downstream side is set to An image forming apparatus having the same polarity as that of the image forming apparatus and having a smaller difference from an applied voltage of the primary charger. 5. The image forming apparatus according to claim 4, wherein the density detection of the density detection image is performed on a downstream side of a lowermost one of the plurality of primary transfer units. 6. A continuous image forming method, wherein one color is transferred to an area between a toner image of a plurality of colors transferred onto the intermediate transfer member and a toner of the next plurality of colors in an area. The image forming apparatus according to claim 4, wherein an image for detection is transferred to the intermediate transfer body.