JP2001324846A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately control the image density and to stably obtain a high- quality color image by forming a patch on a transfer belt without causing an image defect or the retransfer of the image to an image carrier. SOLUTION: In a printer, four stations PY to PBK are installed along the transfer belt 14, and maximum density control and gradation control are performed for every color by a CPU 25, a RAM 26, a ROM 27 an a test pattern forming means 28 or the like. Transfer power sources 23Y to 23BK are a constant voltage power source capable of generating voltage designated by the CPU 25. Bias optimum to transfer a toner image to transfer material and transfer bias optimum to transfer the patch to the belt 14 are stored in the ROM 27 for every color and in every environment such as high-temperature and high-humidity environment, normal-temperature and normal-humidity environment and low-temperature and low-humidity environment. At the time of forming an image and at the time of controlling the image density, the CPU 25 judges the surrounding environment of the printer by using a temperature and humidity sensor 29, reads out and sets the optimum data of the transfer bias from the ROM 27.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus using an electrophotographic system or the like.

[0002]

2. Description of the Related Art In recent years, with the spread of electrophotographic color image forming apparatuses, there has been an increasing demand for faster image output in addition to a demand for recording quality of color images. In order to meet this demand, several proposals have been made for image forming systems, and among them, there is an image forming system called a tandem type.

In this system, for example, four drum-shaped image carriers are arranged in series along a transfer material conveying member, and four image carriers of yellow, magenta, cyan, and black are arranged on the four image carriers.
A color toner image is formed, this is superimposed on a transfer material conveyed by a transfer material conveying member and transferred, and finally a color image in which four color toner images are superimposed on the transfer material is fixed and transferred. A color image is obtained on a material.

FIG. 7 is a sectional view showing an example of a conventional tandem type color image forming apparatus.

In the apparatus, a transfer belt 14 wrapped around rollers 13a, 13b, 13c and 13d is provided as a transfer material conveying member. The transfer belt 14 rotates in the direction of the arrow, and the transfer material P carried on the transfer belt 14 is moved.
Is transported.

The transfer belt 14 is made of a resin material such as polyvinylidene fluoride (PVdF), polyamide, polyimide, polyethylene terephthalate (PET), or polycarbonate, having a thickness of 50 to 300 μm and a volume resistivity of 10 ^9.
Sheet of about 10 ¹⁶ Ωcm, chloroprene rubber, ethylene-propylene-diene terpolymer (EPD
M), a rubber material such as nitrile butadiene rubber (NBR) or urethane rubber, having a thickness of 0.5 to 2 mm and a volume resistivity of 10
A sheet of about ⁹ to 10 ¹⁶ Ωcm is used.

In some cases, a conductive filler such as carbon, ZnO, SnO ₂ or TiO ₂ is dispersed in these materials to adjust the volume resistivity to about 10 ⁷ to 10 ¹¹ Ωcm.

Along the transfer belt 14, yellow (Y), magenta (M), cyan (C), black (B
k) Image forming units (image forming stations) PY, P
M, PC, and PBk are provided.

In the yellow image forming station PY, an OPC photosensitive drum 1Y rotating in the direction of the arrow in the figure is installed, and around the charging roller 2Y, an exposure device 11Y,
A developing unit 8Y, a transfer roller 4Y, and a cleaner 9Y are arranged. Of these, the photosensitive drum 1Y, the charging roller 2
The Y and the cleaner 9Y are configured in a drum unit 10Y including them, and furthermore, the drum unit 10Y and the developing device 8Y are integrally combined to form a process cartridge detachable from the image forming main body.

The exposure device 11Y is composed of a scanner unit or an LED array for scanning a laser beam with a polygon mirror, and irradiates the photosensitive drum 1Y with a scanning beam (laser beam) modulated based on an image signal. The transfer roller 4Y is connected to a transfer bias power supply 23Y, and transfers the toner image on the photosensitive drum 1Y to the transfer material P conveyed by the transfer belt 14 at a transfer nip formed by the photosensitive drum 1Y and the transfer roller 4Y. Performs electrostatic transfer.

The developing device 8Y includes a developing sleeve 5Y, a developer applying roller 6Y, and a developer applying blade 7Y, and includes a non-magnetic one-component developer (non-magnetic toner).
3Y is housed.

As the transfer roller 4Y, for example, an EPD in which a metal core is adjusted to a volume resistivity of 10 ⁵ to 10 ⁸ Ωcm.
A roller covered with an elastic body such as M, urethane rubber, or NBR can be used.

The other magenta, cyan and black stations PM, PC and PBk are also yellow stations P
As shown by the reference numerals with M, C, and Bk added to the components corresponding to the components of Y, the yellow station P
It has the same configuration as Y.

When the image forming operation is started, the photosensitive drums 1Y to 1Bk, the transfer belt 14, and the like start rotating at a predetermined process speed. The photosensitive drum 1Y is uniformly charged to a negative polarity by the charging roller 2Y, and is subsequently irradiated with a scanning beam from the exposure device 11Y to form an electrostatic latent image according to image information on the surface.

The non-magnetic toner 3Y in the developing unit 8Y is supplied and carried by a coating roller 6Y on a rotating developing sleeve 5Y, and is then regulated by a coating blade 7Y to be charged to a negative polarity. To a predetermined toner layer thickness. And the developing sleeve 5Y
Is conveyed to the developing section facing the photosensitive drum 1Y.

The latent image on the photosensitive drum 1Y is
When the rotation of Y reaches the developing section, the developing sleeve 5Y
The image is reversely developed by the upper negatively charged toner 3Y and is visualized as a yellow toner image.

On the other hand, the transfer material P loaded on the transfer material cassette 15 is taken out by the half-moon feeding roller 16,
After being separated one by one by a transfer material separating roller 17, the sheet is conveyed to a registration roller 19 by a conveying roller 18. Then, the toner is supplied to the transfer belt 14 by the registration roller 19 in synchronization with the toner image on the photosensitive drum 1Y.

The supplied transfer material P is applied to the roller 13a or the attraction roller 2 opposed thereto with the transfer belt 14 interposed therebetween.
0, is electrostatically attracted to the surface of the transfer belt 14, and is conveyed to the transfer nip with the transfer roller 4Y as the transfer belt 14 rotates.
The yellow toner image on the photosensitive drum 1Y is transferred to the transfer material P by the transfer roller 4Y to which a positive transfer bias is applied by 3Y.

In synchronization with the transfer of the transfer material P by the transfer belt 14, the photosensitive drums 1M, 1C,
The formation of the toner image on Bk and the transfer to the transfer material P are performed, and a color image in which four color toner images of yellow, magenta, cyan, and black are superimposed and transferred on the transfer material P is obtained. The transfer material P on which the toner images of the four colors are superimposedly transferred is separated from the transfer belt 14 and sent to the fixing device 21, where the toner image is melted and fixed by heating and pressing, and output to the transfer material P. A full-color permanent image is formed as an image.

The toner remaining on the photosensitive drums 1Y to 1Bk is cleaned by drum cleaners 9Y to 9Bk such as fur brushes and blade means. The toner adhered to the transfer belt 14 is cleaned by a belt cleaner 22 such as a fur brush or a blade.

In general, in the above-described color image forming apparatus, if the image density fluctuates due to a change in the environment in which it is used, the number of prints, and other conditions, the original color tone cannot be obtained in the color image. Therefore, a toner image (patch) for density detection of each color is experimentally formed on the transfer belt 14, and the density is detected by the density sensor 24, and the density is determined based on image forming conditions such as an exposure amount and a developing bias. By performing feedback and performing image density control, a color image having a stable color tone is obtained.

As shown in FIG. 8, a light-emitting element 241 such as an LED and a light-receiving element 242 such as a photodiode and CdS are incorporated in a holder 243, as shown in FIG. The density sensor 24 includes a light emitting element 24
1 irradiates the patch T on the transfer belt 14 with light, and receives the reflected light from the patch T by the light receiving element 242 to measure the density of the patch T.

[0023]

By the way, the transfer bias for transferring a patch (toner image for density detection) onto a transfer belt has the same size as that for transferring an output image toner image to a transfer material. When the bias is applied, the capacitance is increased by the amount of the transfer material when transferring the patch, so a larger transfer current flows and abnormal discharge occurs between the photosensitive drum and the transfer belt. It will be easier.

When abnormal discharge occurs, there arises a problem of defective image of the patch such that the patch is not transferred to the transfer belt or a discharge mark is formed on the transferred patch.

In the tandem type color image forming apparatus, after the patches are transferred to the transfer belt at the transfer positions of the four colors, the density of each patch is measured by the density sensor 24 downstream of the station for the fourth color. Therefore, a problem associated with retransfer also occurs.

That is, for example, the patches of the first color
When passing through the transfer position of the fourth color, a part of the toner forming the patch of the first color is transferred to the photosensitive drums 1M, 1C, and 1D.
A phenomenon called retransfer that shifts to Bk occurs, and when measured by the density sensor 24, the patch becomes a very low density patch, and the patch density is erroneously measured low. As a result, there is a problem that a high-quality color image cannot be obtained by performing inappropriate density control.

Accordingly, an object of the present invention is to form a patch on a transfer belt satisfactorily without causing image defects or retransfer to an image carrier, thereby enabling accurate image density control. An object of the present invention is to provide an image forming apparatus capable of stably obtaining a high-quality color image.

[0028]

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, a transfer material carrying member for carrying a transfer material and sequentially carrying the plurality of image carriers to the plurality of image carriers, and the carried transfer of a plurality of color toner images formed on the plurality of image carriers; A plurality of transfer means for sequentially superimposing and transferring the toner images onto the material, the toner image transferred to the transfer material is fixed to form an output image, and the density of a plurality of colors is experimentally tested on the plurality of image carriers. Forming a toner image for detection, transferring the toner image without overlapping the transfer material transporting member, detecting the density of the toner image for density detection transferred to the transfer material transporting member for each color, and outputting the output. In an image forming apparatus having an image control mode for controlling an image forming condition of an image, a transfer condition for each color when a toner image for an output image is transferred to the transfer material, and a density detection condition for the transfer material conveying member. Each color when transferring a toner image A transfer condition of an image forming apparatus which is characterized in that by changing not the same.

According to the present invention, the transfer of the toner image for the output image to the transfer material and the transfer of the toner image for density detection to the transfer material conveying member are both performed by a transfer bias of a constant voltage. The constant voltage value of the transfer bias is changed. Alternatively, the transfer of the toner image for the output image to the transfer material and the transfer of the toner image for density detection to the transfer material transport member are both performed by a constant current transfer bias, and the constant current value of the transfer bias is used. To change. Alternatively, the transfer of the toner image for the output image to the transfer material is performed by a transfer bias of a constant current, and the transfer of the toner image for density detection to the transfer material conveying member is performed by a transfer bias of a constant voltage.

Further, at the time of non-image formation, a constant current or constant voltage transfer bias is applied to at least one of the plurality of transfer means, and a voltage value or a current value generated at this time is measured. The transfer condition of the toner image for the output image to the transfer material and the transfer condition of the toner image for density detection to the transfer material conveying member may be changed.
Measure one or both of the temperature and humidity of the atmosphere in which the image forming apparatus body is placed, and transfer the toner image for the output image to the transfer material according to the measurement result,
The transfer condition of the toner image for density detection to the transfer material conveying member may be changed.

[0031]

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

Embodiment 1 FIG. 1 is a schematic structural view showing an embodiment of the image forming apparatus of the present invention, and shows a tandem type full-color printer. In FIG. 1, those having the same configuration and operation as those of the conventional printer shown in FIG.
The description is omitted.

As shown in FIG. 1, the printer has a CPU 25 for controlling the printer. The CPU 25 includes a RAM 26 used as a working memory,
ROM that stores programs and various data to be executed
27 and a test pattern generating means 28 are connected. The test pattern generator 28 may be mounted in a video controller (not shown).

A temperature / humidity sensor 29 is installed at an appropriate place of the printer in FIG. 1, for example, above the paper feed cassette 15 to measure the temperature / humidity of the atmosphere of the environment where the printer is placed. How to use the temperature and humidity sensor 29 will be described later.

The image density control performed in the present invention will be described. The image density control includes maximum density control (Dmax control) for adjusting the maximum density of an image to a predetermined density, and gradation control for adjusting the tone characteristics of the image to the predetermined characteristics. Dmax control is performed as follows.

First, the CPU 25 determines appropriate timings such as turning on the power of the color printer main body or the elapsed time since the power was turned on, the number of prints, instructions from the host and the user, and the like.
Is detected, the CPU 25 starts the Dmax control. Next, the CPU 25 reads the development bias of each color for Dmax control and the target density for Dmax control from the ROM 27. Thereafter, the CPU 25 starts the initial operation of the printer main body and charges the photosensitive drums 1Y to 1Bk with a predetermined charging bias.

Next, the CPU 25 sends the image data of the patch generated from the test pattern generating means 28 to the exposure device 11Y, exposes the photosensitive drum 1Y by the exposure device 11Y, and places the image on the photosensitive drum 1Y along the rotation direction. A latent image of five patches PY1 to PY5 is formed by the same image data. The developing device 8Y applies these latent images to the PY1 patch for the development bias VY1, the PY2 patch for the VY2,
The patch of PY3 is developed by VY3, and the patch of PY4 is developed by VY4.

The patches PY1 to PY5 of the yellow toner image thus formed on the photosensitive drum 1Y are transferred to the transfer belt 14 by applying a transfer bias to the transfer roller 4Y from the power source 23Y.

In the same manner as for yellow, magenta, cyan, and black, as shown in FIG.
Black patches PY1 to PY5, PM1 to PM5, P
C1 to PC5 and PBk1 to PBk5 are transferred to the transfer belt 14
Are formed in a straight line in the longitudinal direction.

Next, these patches PY1 to PY5, PM1 to PY5 are detected by the density sensor 24 as shown in FIG.
The concentrations of M5, PC1 to PC5, and PBk1 to PBk5 were measured, and the measured concentration values DY1 to DY5 and DM1 to D
M5, DC1 to DC5, DBk1 to DBk5 in RAM2
Write to 6. After the density measurement, the patches PY1 to PBk5
Is removed by the cleaner 22 of the transfer belt 14.

When the patch density measurement is completed, the CPU 25
Then, from the densities of the patches stored in the RAM 26, the developing bias necessary to obtain a predetermined Dmax is calculated. For example, the latent images of the yellow density detection patches PY1 to PY5 are separated by different developing biases VY1 to VY1.
When developed at Y5, the density D measured by the density sensor 24 is obtained.
Y1 to DY5 are as shown in FIG. Development bias VY1
To VY5, the control target density DTY must be DY1 to DY5
Is set in advance based on the environmental data calculated from the measurement data of the temperature and humidity sensor 29 so as to fall within the section of.

FIG. 3 shows that the developing bias VYT necessary to obtain the control target density DTY includes the patch densities DY3 and DY4 sandwiching DTY and the corresponding developing bias VY.
3, two points (DY3, VY3) on the coordinates created by VY4,
It can be obtained by linear interpolation between (DY4, VY4).

That is, VTY = {(VY4-VY3) / (DY4-DY3)}
× (DTY−DY3) + VY3.

Similarly, the CPU 25 also obtains the developing biases VTM, VTC, and VTBk for other colors, writes these values into the RAM 26, and uses these developing biases for the subsequent image formation.

When the developing bias is set by the Dmax control, the gradation control is continuously performed as follows.

First, the CPU 25 starts the initial operation of the main body of the image forming apparatus and sets the photosensitive drums 1Y to 1Bk.
Each is charged with a predetermined charging bias. Next CPU
25 sends image data of a patch for gradation control generated from the test pattern generating means 28 to the exposure device 11Y, and forms latent images of seven patches PY1 to PY7 on the photosensitive drum 1Y along the rotation direction. . Here patch PY1
As for PY7, image data SY1 to SY7 are set at predetermined intervals. These latent images are developed by the developing unit 8Y and the developing bias VT set by the Dmax control described above.
Develop each with Y.

The patches PY1 to PY7 of the yellow toner image thus formed on the photosensitive drum 1Y are transferred onto the transfer belt 14 by applying a transfer bias to the transfer roller 4Y. Similarly to the transfer belt 14 for yellow, magenta, cyan, and black
On top, patches PY1 to PY7, PM1 to PM7, and PC1 for tone control of yellow, magenta, cyan, and black are provided.
PC7, PBk1 to PBk7 are formed.

Next, the density sensor 24 detects these patches PY1 to PY7, PM1 to PM7, PC1 to PC7, P
The concentrations of Bk1 to PBk7 were measured, and the measured concentration values DY1 to DY7, DM1 to DM7, DC1 to DC7, D
Bk1 to DBk7 are written to the RAM 26. After the density measurement, the patches PY1 to PBk7 are cleaned and removed by the cleaner 22 of the transfer belt 14.

When the patch density measurement is completed, the CPU 25
Then, by interpolating between the density of each patch and the image data stored in the RAM 26 using a polynomial or the like, the tone characteristics of the printer are obtained.

Normally, the gradation characteristics of an electrophotographic printer are as shown in FIG. Therefore, a gradation correction characteristic for adjusting between the image data input to the printer engine and the data to be sent to the exposure device is used as a look-up table so that the gradation characteristic is linear or predetermined based on this characteristic. And save it in RAM 26,
In the subsequent image formation, gradation correction is performed using this lookup table.

A color image having a stable color tone can be obtained by the developing bias and the look-up table obtained as described above.

Although the Dmax control and the gradation control are performed successively here, they may be performed at different timings. If either characteristic is stable, only one of them is performed. Is also good.

The printer of the present invention generally performs image density control as described above.

In this embodiment, the transfer bias power supply 2
As 3Y to 23Bk, a constant voltage power supply capable of generating a predetermined voltage according to an instruction from the CPU 25 is used.
In the ROM 27, biases T1Y to T1Bk optimal for transferring a toner image for an output image onto a transfer material and a toner image (patch) for density detection on the transfer belt 14 are provided.
Transfer biases T2Y to T2B optimal for direct transfer of
k is stored for each environment such as high temperature and high humidity, normal temperature and normal humidity, and low temperature and low humidity. At the time of normal image formation or image density control on the transfer material, the CPU 25 determines the environment in which the printer is placed using the temperature and humidity sensor 29, and reads the transfer biases T1Y to T1Bk, T from the ROM 27.
The optimum data of 2Y to T2Bk is read and set.

Next, after explaining the mechanism of occurrence of retransfer, the transfer biases T1Y to T1Bk, T2Y to T
2Bk will be described.

For example, the first color yellow station P
The yellow toner image transferred to the transfer belt 14 in Y is
When the transfer belt 14 moves to the second color magenta station PM, it comes into contact with the photosensitive drum 1M. At this time, the negative surface potential of the photosensitive drum 1M and
Due to the potential difference between the transfer roller 4M and the portion of the transfer belt 14 to which the positive transfer bias is applied, discharge may occur in the transfer nip as shown in FIG.

The positive charge e ⁺ and the negative charge e ⁻ generated by this discharge move by being pulled by the negative potential on the surface of the photosensitive drum 1M and the positive potential applied to the transfer belt 14, respectively. ⁺ Is photosensitive drum 1
M and the negative charge e ⁻ reaches the yellow toner on the transfer belt 14. However, some positive charges e ⁺
It reaches the negative toner T ^{- -} middle yellow toner T to move to the photosensitive drum 1M thereby charged to the positive polarity opposite. As a result, the positively charged toner T ^{+ of the} opposite polarity is transferred to the surface of the photosensitive drum 1M having a negative potential, and the yellow toner is retransferred to the photosensitive drum 1M.

Actually, when the transfer bias is increased,
From the fact that the amount of retransfer increases, it is considered that the discharge in the transfer nip by the mechanism described above may have occurred.

Now, assuming that the capacitance of the transfer belt 14 is CB, the capacitance of the transfer material is CP, and the total capacitance of the transfer belt and the transfer material is CB + P, CB + P = CB · CP / Since it can be expressed as (CB + CP), the relationship of CB + P <CB always holds.

That is, the amount of charge required to transfer a toner image is the same in both cases of transferring a toner image onto a transfer material and transferring a toner image onto a transfer belt if the amount of toner is the same. Therefore, it can be seen that when the toner image is directly transferred to the transfer belt, a voltage lower than that when the toner image is transferred to the transfer material is required because the capacitance is larger.

Actually, the optimum transfer bias in a normal temperature and normal humidity (J / J) environment is as shown in Table 1.

[0062]

[Table 1]

As shown in Table 1, the reason why the transfer bias for image formation for transferring a toner image to a transfer material is gradually increased is that, in image formation on a transfer material, the transfer material is charged at each station. This is because it is necessary to overlap toner of two or more colors in order to improve the image quality and to form a full-color image.

On the other hand, since the transfer belt is a self-decay type belt having a resistance value at which the charge applied to the surface rapidly attenuates under normal temperature and normal humidity, no charge-up occurs. The transfer bias for controlling the image density to be transferred to the four colors may be the same for the four colors, thereby preventing the occurrence of retransfer.

The optimum transfer bias in a low temperature and low humidity (L / L) environment is as shown in Table 2.

[0066]

[Table 2]

As shown in Table 2, in a low-temperature and low-humidity environment, the resistance value of the transfer belt increases, and the surface of the transfer belt is slightly charged up. By increasing the value, good transferability was obtained.

Further, the optimum transfer bias in a high temperature and high humidity (H / H) environment is as shown in Table 3.

[0069]

[Table 3]

As shown in Table 3, in a high-temperature and high-humidity environment, it is not necessary to consider the charge-up of the transfer material and the transfer belt. Thus, the same four colors are used for image formation and image density control. By setting the bias, good transferability and prevention of retransfer were achieved.

Therefore, in this embodiment, the above data is stored in the ROM 27. Then, the temperature and humidity of the atmosphere of the environment where the printer is placed are measured by the temperature and humidity sensor 29 shown in FIG. Is appropriately selected and used.

Therefore, according to the present embodiment, it is possible to form a patch appropriately and perform accurate image density control without causing image defects even during image density control.

Even if the transfer bias value of each color is set to the optimum value, retransfer may occur slightly during image formation. In this case, the color balance on the final image including the retransfer may be included. Is correct, a good image can be obtained.
According to the present invention, the optimum bias value can be independently selected at the time of image formation and at the time of transfer of a patch in image density control, so that the amount of retransfer at the time of patch transfer is substantially equal to that of image formation. The transfer bias can be set. Therefore, there is an advantage that the image density can be controlled in a system in which retransfer is slightly present.

Embodiment 2 FIG. 6 is a block diagram showing another embodiment of the image forming apparatus of the present invention.

In this embodiment, the constant current measuring circuits 30Y to 30Y-3 are connected to the constant voltage power supplies 23Y to 23Bk, which are transfer bias power supplies.
0Bk is attached, the current when the transfer bias is applied is measured, and the measured value can be taken into the CPU 25 by connection means (not shown). Other mechanical configurations of the present embodiment are basically the same as those of the image forming apparatus shown in FIG. 1, and the same reference numerals in FIG. 6 as those shown in FIG. 1 indicate the same members.

When a color printer such as the one in this embodiment is made compatible with double-sided printing, the resistance of the transfer material after single-sided printing usually increases. In addition, the types of transfer materials are very large, and the resistance values vary accordingly. Generally, in order to obtain good transferability, it is necessary to supply a constant current value regardless of the type and environment of the transfer material.

In the case of this embodiment, specifically, the transfer of the toner image (toner image for the output image) to the transfer material is 12 μA for all four colors, and the patch (the toner image for density detection) on the transfer belt 14 is used. The transfer of ()) required 10 μA for all four colors.

Therefore, in this embodiment, the optimum transfer current I1 (= 1) for transferring the toner image onto the transfer material in the ROM 27 is used.
2 .mu.A) and an optimum transfer current I2 (= 10 .mu.A) for directly transferring the toner image to the transfer belt. The CPU 25 reads the transfer currents I1 and I2 from the ROM 27 at the time of forming an image on the transfer material or at the time of controlling the image density, and transfers the transfer current I1 at the time of transferring the toner image onto the transfer material.
Is maintained from the measurement data of the current measuring circuits 30Y to 30Bk so that the transfer current I2 is maintained during the transfer of the toner image onto the transfer belt 14 so that the constant voltage power supply 23Y is maintained.
A voltage adjustment of ２３23 Bk is performed. This method is very effective even when the resistance of the transfer belt itself fluctuates due to a rise in temperature or deterioration due to use.

However, when the resistance fluctuation of the transfer belt is small, it is not always necessary to apply a constant current transfer bias for the transfer of the toner image onto the transfer belt. May be used. In this case, the transfer bias power supplies 23Y to 23Bk
A constant current power supply and a constant voltage power supply are prepared and switched as appropriate, instead of using a combination of the current measurement circuits 30Y to 30Bk, so that a constant current bias is applied when transferring to a transfer material, and a toner is applied to the transfer belt. When transferring an image, a constant voltage bias may be used.

Further, a transfer bias is applied in advance to the station of each color at the time of non-sheet passing, a transfer voltage at a predetermined current value is measured, and a transfer voltage at the time of sheet passing is determined based on the result. The present invention can be applied to such a case.

That is, for example, the voltage measurement is performed at the station of the first color, and the constant current value at this time is 10 μA. Then, with respect to the measured voltage VTS, the transfer bias VT1 of the first station at the time of paper passing = VT
S × α Transfer bias at the time of image density control VT2 = VTS × β It is possible to set an optimum transfer bias by setting α = 3, β = 1, and the like. The transfer bias of the second and subsequent stations may be obtained by calculation from the VTS or the transfer bias of the first station, and the same control as that of the first station is performed for the second to fourth stations during non-sheet passing. Thus, an optimum transfer bias value may be obtained.

With the above arrangement, even when controlling the image density, the patch is appropriately formed without causing image defects and retransfer, and accurate image density control can be performed.

Although the embodiments of the image forming apparatus of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made without departing from the scope of the present invention. . For example, the transfer member is not a transfer roller,
Other devices such as a transfer blade, a transfer brush, and a corona charger may be used. The color order of image formation may be arbitrary. The image density control method may be other than the embodiment.

The present invention can also be applied to color misregistration control in which four color toner images are formed on a transfer belt and their positions are detected to match the write start positions of the four colors. Needless to say.

[0085]

As described above, according to the present invention,
In a tandem-type color image forming apparatus, a toner image for density control can be appropriately formed on a transfer belt without causing image defects and retransfer, and accurate image density control is performed to always achieve high image density. High quality color images can be obtained.

[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 illustrating a state of a patch formed on a transfer belt of the image forming apparatus of FIG. 1;

FIG. 3 is a conceptual diagram illustrating a method of calculating a developing bias in Dmax control performed in the image forming apparatus of FIG. 1;

FIG. 4 is a diagram illustrating gradation characteristics possessed by the image forming apparatus.

FIG. 5 is a schematic diagram showing a state of discharge in a transfer nip between a photosensitive drum and a transfer belt.

FIG. 6 is a schematic configuration diagram showing another embodiment of the image forming apparatus of the present invention.

FIG. 7 is a schematic diagram illustrating a conventional image forming apparatus.

FIG. 8 is a schematic diagram illustrating a density sensor installed in the image forming apparatus of FIG. 7;

[Explanation of symbols]

 1Y-1Bk Photosensitive drum 2Y-2Bk Charging roller 4Y-4Bk Transfer roller 8Y-8Bk Developing device 14 Transfer belt 23Y-23Bk Transfer bias power supply 25 CPU 26 RAM 27 ROM 28 Test pattern generating means 30Y-30Bk Current measuring circuit

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H027 DA01 DA10 DA11 DA13 DA14 DE02 DE09 EA02 EA03 EA05 EB04 EC03 EC06 ED24 2H030 AA01 AA02 AB02 AD16 BB13 BB23 BB34 BB54 BB56 2H032 AA05 AA15 BA18 CA01 CA14 CA15

Claims (6)

[Claims] A plurality of image carriers, a transfer material carrying member for carrying the transfer material and sequentially carrying the plurality of image carriers to the plurality of image carriers, and a plurality of color toner images formed on the plurality of image carriers. A plurality of transfer means for sequentially superimposing and transferring the transferred transfer material, wherein the toner image transferred to the transfer material is fixed to form an output image, and a test image is formed on the plurality of image carriers. A toner image for density detection of a plurality of colors is formed on the transfer material conveying member, and the toner image is transferred without being superimposed on the transfer material conveying member. The density of the toner image for density detection transferred to the transfer material conveying member is detected for each color. An image forming apparatus having an image control mode for controlling an image forming condition of the output image; a transfer condition for each color when a toner image for an output image is transferred to the transfer material; The toner image for density detection to And a transfer condition for each color when,
An image forming apparatus, wherein the image forming apparatuses are not identical but are changed. 2. The transfer of a toner image for an output image to the transfer material and the transfer of a toner image for density detection to the transfer material conveying member are both performed by a transfer bias of a constant voltage. The image forming apparatus according to claim 1, wherein the constant voltage value is changed. 3. The transfer of a toner image for an output image to the transfer material and the transfer of a toner image for density detection to the transfer material conveying member are both performed by a constant current transfer bias. The image forming apparatus according to claim 1, wherein the constant current value is changed. 4. The transfer of a toner image for an output image to the transfer material is performed by a constant current transfer bias, and the transfer of the density detection toner image to the transfer material transport member is performed by a constant voltage transfer bias. The image forming apparatus according to claim 1. 5. When a non-image is formed, a transfer bias of a constant current or a constant voltage is applied to at least one of the plurality of transfer units, and a voltage value or a current value generated at this time is measured. The transfer condition of a toner image for an output image onto the transfer material and the transfer condition of a toner image for density detection on the transfer material conveying member are changed accordingly.
5. The image forming apparatus according to any one of items 4. 6. A condition for transferring a toner image for an output image onto the transfer material according to a result of measurement of one or both of the temperature and the humidity of an atmosphere in which an image forming apparatus main body is placed. The image forming apparatus according to claim 1, wherein a condition for transferring a toner image for density detection to the transfer material conveying member is changed.