JP2002049190A - Image forming system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming system capable of stabilizing transfer efficiency by eliminating the fluctuation of the surface potential of an intermediate transfer medium in the case of using a constant-voltage power source as a primary transfer voltage power source. SOLUTION: This image forming device is provided with a latent image carrier 21, plural developing devices 23Y, 23M, 23C and 23K, a primary transfer part R1 transferring a toner image successively developed with different color toner to the intermediate transfer medium 41, a primary transfer bias applying power source 126 for applying bias at the primary transfer part, and a secondary transfer part R2 transferring the full color toner image superposed and transferred on the intermediate transfer medium to recording paper. In the device, the constant-voltage power source is used as the primary transfer bias applying power source, and the electrification potential of a latent image carrier by an electrifying means 22 is fixed at least for each color toner and the gradation of the image is adjusted by adjusting the quantity of exposing light L to an image part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method for an image forming apparatus such as a printer, a facsimile, and a copying machine using an electrophotographic method, and more particularly, to a toner formed on a latent image carrier such as a photosensitive member. The present invention relates to an image forming method of an image forming apparatus having an intermediate transfer medium on which an image is primarily transferred and further secondary-transferring this toner image to a recording medium.

[0002]

2. Description of the Related Art Generally, an image forming apparatus using an electrophotographic technique includes a photosensitive member having a photosensitive layer on an outer peripheral surface as a latent image carrier, and a charging means for uniformly charging the outer peripheral surface of the photosensitive member. An exposure means for selectively exposing the outer peripheral surface uniformly charged by the charging means to form an electrostatic latent image, and applying a toner as a developer to the electrostatic latent image formed by the exposure means. The image forming apparatus includes a developing unit that is charged and applied to form a visible image (toner image), and a transfer device that transfers the toner image developed by the developing unit to a recording medium such as paper.

Conventionally, as a transfer device for transferring a toner image developed on a photoconductor to a recording medium such as paper, a toner image formed on the photoconductor is transferred (primary transfer).
There is known an image forming apparatus including an intermediate transfer medium for further transferring (secondarily transferring) the toner image to a recording medium.

FIG. 8 is a view showing one example of an image forming apparatus provided with such an intermediate transfer medium, wherein FIG.
(B) is a bb partial cross-sectional view in FIG.

In FIG. 8, reference numeral 201 denotes a photoconductor, which has a conductive layer 201a and a photosensitive layer 201b formed on the conductive layer 201a. The conductive layer 201a is grounded.

Reference numeral 202 denotes an intermediate transfer medium, for example, a dielectric (medium resistance layer) having a volume resistance of about 10 ⁷ to 10 ¹⁴ Ωcm.
It is composed of Such an intermediate transfer medium 202 is
It can be prepared by kneading conductive carbon into a synthetic resin or the like.

The intermediate transfer medium 202 contacts the photosensitive member 201 at least at the time of image formation, and the contact portion R1 forms a primary transfer portion. In the primary transfer section R1, a primary transfer roller 203 is disposed inside the intermediate transfer medium 202, and a primary transfer voltage is applied to the intermediate transfer medium 202 by pressing the primary transfer roller 203.

Further, a secondary transfer roller 204 for applying a secondary transfer voltage is pressed against the intermediate transfer medium 202, and the pressed portion forms a secondary transfer portion R2. In the secondary transfer section R2, a backup roller 205 is disposed from inside the intermediate transfer medium 202.

At the time of image formation, first, the photosensitive member 201 and the intermediate transfer medium 202 are driven to rotate, and the photosensitive layer 201b of the photosensitive member 201 is uniformly charged by a charging unit (not shown), and then exposed by an exposing unit (not shown). (Not shown) to form an electrostatic latent image. Next, a toner as a developer is applied to the electrostatic latent image by a developing unit (not shown) to form a visible image (toner image), and this toner image is transferred to the primary transfer unit R1.
Is transferred onto the intermediate transfer medium 202, and then, in the secondary transfer section R2, is transferred to a recording medium such as a sheet supplied to the secondary transfer section R2.

The recording medium onto which the toner image has been transferred passes through a fixing device (not shown), where the toner image is fixed.

In an image forming apparatus having an intermediate transfer medium 202 formed of a uniform resistor as described above, a transfer electric field is generally applied by a primary transfer roller 203 in contact with the back surface of a transfer portion. When distortion or the like is generated on the transfer medium 202 or dust adheres to the primary transfer roller 203, an electric field cannot be partially applied, and the electric field of the transfer unit becomes non-uniform, causing unevenness in the transferred image.

Therefore, as the intermediate transfer medium 202, FIG.
, A conductive layer 202a integrally formed on an insulating base 202c made of a synthetic resin, and a resistive layer 202 integrally formed thereon and pressed against the photoconductor 201.
b is also known, in which case the conductive layer 202a is exposed in a strip shape by removing the resistive layer 202b in a strip shape at the side edge of the intermediate transfer medium 202, An electrode roller contacts the exposed portion to apply a primary rotation voltage. Thus, the conductive layer 2
In the image forming apparatus using the intermediate transfer medium 202 having the second transfer medium 02a, even when the intermediate transfer medium 202 is distorted or dust adheres to the roller of the transfer section, a uniform transfer electric field is applied to the entire transfer section. Therefore, there is an advantage that image unevenness due to transfer is eliminated.

In an image forming apparatus using the intermediate transfer medium 202 having the conductive layer 202a to which the primary transfer voltage is applied, the timing of the primary transfer and the timing of the secondary transfer must be overlapped for speeding up. Therefore, a constant voltage power supply is used as a primary transfer voltage power supply, and a constant current power supply is used as a secondary transfer voltage power supply (Japanese Patent Laid-Open No. 9-16039).
No. 5).

In US Pat. No. 5,243,392, efficient use is made of an intermediate transfer medium of a high-resistance belt having a volume resistivity of about 10 ¹² Ωcm or more and a relaxation time of 0.3 to 200 ms. Has been proposed to perform secondary transfer.

[0015]

When the intermediate transfer medium 202 having a low volume resistivity is used in the configuration as shown in FIGS. 8 and 9, the phenomenon in which toner scatters or comes off between lines occurs remarkably. Although the deterioration of the line image is remarkable and the photosensitive memory is easily generated, these problems can be solved by increasing the volume resistivity of the intermediate transfer medium 202 to some extent.

However, when the resistivity is high, the charge from the photoreceptor is charged on the intermediate transfer medium, and it is difficult to remove the charge, which causes a problem. Specifically, when the surface potential of the photoconductor and the belt surface potential of the intermediate transfer medium become equal to or more than a certain potential difference, discharge occurs, and the intermediate transfer medium receives a negative charge of the photoconductor (when the photoconductor is negatively charged), The charging of the intermediate transfer medium occurs. The photoconductor surface potential V ₀ is
Usually, depends developing characteristics of the developing device, even largely different set values of the photosensitive member surface potential V ₀ which the characteristics change over time from the color or initial use of the developing device or the like. Therefore, the amount of negative charge received by the intermediate transfer medium from the photoconductor varies, and the surface potential of the intermediate transfer medium becomes unstable.

Since constant work is performed on the photosensitive member image portion by controlling the primary transfer with a constant current, there is no problem due to the unstable surface potential of the intermediate transfer medium at least at the moment of the primary transfer.

However, as described above, when a constant voltage power supply is used as the primary transfer voltage power supply, the state in which the surface potential of the intermediate transfer medium is greatly reduced and the shortage of the potential difference from the image portion of the photoreceptor cannot be eliminated. It has been found that a problem that the transfer efficiency is lowered occurs.

Therefore, it is conceivable to increase the primary transfer voltage applied to the intermediate transfer medium. However, if the primary transfer voltage is excessively increased, a problem arises that a photosensitive memory is likely to be generated by the primary transfer voltage.

The present invention has been made in view of such problems of the prior art, and has as its object to eliminate transfer of the surface potential of an intermediate transfer medium when a constant voltage power supply is used as a primary transfer voltage power supply. An image forming method for stabilizing the efficiency may be provided.

[0021]

According to the image forming method of the present invention which achieves the above object, the surface is uniformly charged by a charging means while rotating, and selectively discharged by an exposure means to form an electrostatic latent image. A latent image carrier to be formed; a plurality of developing units for selectively applying different color toners to the surface of the latent image carrier to convert the latent image into a visible image; Primary transfer section for transferring the toner image developed by the above to the intermediate transfer medium, a primary transfer bias application power supply for applying a bias in the primary transfer section, and all the colors transferred by being superimposed and transferred on the intermediate transfer medium In an image forming apparatus having a secondary transfer unit for transferring a toner image onto recording paper, a constant voltage power supply is used as the primary transfer bias application power supply, and the charging potential of the latent image carrier by the charging unit is low. Both are fixed for each color toners, the tone adjustment of the image is characterized in that performed by adjusting the exposure amount of the image portion.

In this case, the intermediate transfer medium includes a conductive layer,
It is preferable that the first transfer bias is constituted by a multilayer body having a resistive layer integrally formed on the conductive layer and to which the toner is transferred, and to which a primary transfer bias is applied via the conductive layer.

It is desirable that a constant current power supply be used as a secondary transfer bias application power supply for applying a bias in the secondary transfer section.

The charging potential of the latent image carrier by the charging means may be fixed to all color toners.

Further, the charging potential of the latent image carrier by the charging means differs for each color of the color toner, and the transfer order of the toner image transferred to the intermediate transfer medium is lower than the charging potential of the latent image carrier by the charging means. The color toner may be set so as to be higher in color from the color toner.

In the present invention, a constant voltage power supply is used as a primary transfer bias application power supply, the charging potential of the latent image carrier by the charging means is fixed at least for each color toner, and the gradation of the image is adjusted. Is performed by adjusting the exposure amount to the image portion, so that the fluctuation of the surface potential on the intermediate transfer medium during the transfer of the toner images of different colors to the intermediate transfer medium in order can be greatly reduced, and the intermediate transfer of the toner image can be performed. The transfer efficiency to the medium can be stabilized, and a reliable image forming apparatus can be realized without increasing the cost of the apparatus.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An overall configuration of an example of a printer of an image forming apparatus using an electrophotographic method to which the image forming method of the present invention is applied will be described below.

FIG. 1 is a diagram showing one embodiment of an image forming apparatus to which the image forming method of the present invention is applied. Also,
FIG. 2 is a block diagram showing an electrical configuration of the image forming apparatus of FIG. This image forming apparatus forms a full-color image by superimposing four color toners of yellow (Y), cyan (C), magenta (M), and black (K), or uses only black (K) toner. To form a monochrome image. In this image forming apparatus, when an image signal is provided from an external device such as a host computer to the main controller 11 of the control unit 1, the engine controller 12 functions as an image forming unit in response to a command from the main controller 11. An image corresponding to the image signal is formed on the sheet S by controlling each unit of the unit E.

In the engine section E, a toner image can be formed on the photosensitive member 21 of the image carrier unit 2. That is, the image bearing special unit 2 includes a photoconductor 21 rotatable in the direction of the arrow in FIG. 1, and further includes a charging roller 22 as a charging unit around the photoconductor 21 along the rotation direction. Developing units 23Y, 23C, 2
3M, 23K, and the cleaning unit 24 are arranged, respectively. The charging roller 22 is applied with a high voltage from the charging bias generating unit 121 and contacts the outer peripheral surface of the photoconductor 21 to uniformly charge the outer peripheral surface. Photoconductor 21
Is, as shown in FIG. 3, a conductive layer 21a and a conductive layer 2a.
1a formed on the photosensitive layer 21a.

Then, a laser beam L is emitted from the exposure unit 3 toward the outer peripheral surface of the photoconductor 21 charged by the charging roller 22. The exposure unit 3 is configured as shown in FIG.
Is electrically connected to the image signal switching section 122, and scans and exposes the laser beam L onto the photoconductor 21 in accordance with the image signal given through the image signal switching section 122, An electrostatic latent image corresponding to the image signal is formed on 21. For example, the CPU 1 of the engine controller 12
When the image signal switching unit 122 is electrically connected to the patch creation module 124 based on the command from the patch 23, the patch image signal output from the patch creation module 124 is given to the exposure unit 3 to form a patch latent image. Is done.
On the other hand, the image signal switching unit 122
Is electrically connected to the CPU 111, the laser beam L is scanned and exposed on the photosensitive member 21 in accordance with an image signal given from an external device such as a host computer via the interface 112, and an image is formed on the photosensitive member 21. An electrostatic latent image corresponding to the signal is formed.

The electrostatic latent image formed in this manner is
Is developed with toner. That is, in this embodiment, as the developing unit 23, a developing unit 23Y for yellow, a developing unit 23C for cyan, a developing unit 23M for magenta, and a developing unit 23K for black are arranged in this order.
Are arranged along. These developing units 23Y and 23
C, 23M, and 23K are configured to be freely movable toward and away from the photoreceptor 21, and in response to a command from the engine controller 12, the four developing units 23Y, 23M,
One of the developing units 23C and 23K selectively selects the photosensitive member 2
1, and a high voltage is applied to a developing roller 25 of a developing device by a developing bias generating unit 125 to apply a toner of a selected color to the surface of the photoconductor 21 to cause the photoconductor 2
1 to make the electrostatic latent image visible.

The toner image developed by the developing unit 23 is primarily transferred onto the intermediate transfer belt 41 of the transfer unit 4 in a primary transfer region R1 located between the black developing unit 23K and the cleaning unit 24. The transfer unit 4
Will be described later in detail.

Further, from the primary transfer region R1 in the circumferential direction (FIG. 1)
(In the direction of the arrow in FIG. 3), a cleaning unit 24 is disposed, and scrapes off toner remaining on the outer peripheral surface of the photoconductor 21 after the primary transfer.

Next, the configuration of the transfer unit 4 will be described. In this embodiment, the transfer unit 4 includes a roller 4
2 to 47; an intermediate transfer belt 41 stretched over the rollers 42 to 47; and a secondary transfer roller 48 for secondary transferring the intermediate toner image transferred to the intermediate transfer belt 41 to the sheet S. ing.

The intermediate transfer belt 41 has a conductive layer 41a integrally formed on an insulating base 41c made of a synthetic resin as shown in a cross section in FIG. 3, similarly to the conventional example described with reference to FIG. And a resistance layer 41b integrally formed thereon and pressed against the photoreceptor 21. The resistance layer 4b is provided at a side edge of the intermediate transfer belt 41.
1b is removed in a strip shape to expose the conductive layer 41a in a strip shape, and the primary transfer voltage is applied from the primary transfer bias generator 126 by the electrode roller 50 contacting the exposed portion. When the color image is to be transferred to the sheet S, the primary transfer backup roller 42 is displaced to the solid line position, and the intermediate transfer belt 41 is pressed against the photosensitive member 21, and each color formed on the photosensitive member 21 is pressed. The toner image is transferred onto the intermediate transfer belt 41 by the primary transfer voltage applied to the conductive layer 41a of the intermediate transfer belt 41, and the photoconductor 21 and the intermediate transfer belt 41 are driven to circulate to transfer the toner images of each color to the intermediate transfer belt 41. A color image is formed by superimposing and transferring the sheet on the sheet, and the sheet S is taken out from the cassette 61, the manual feed tray 62, or an additional cassette (not shown) by the sheet supply unit 63 of the sheet supply / discharge unit 6, and the secondary transfer area is formed. Transfer to R2. Then, the secondary transfer roller 48 is displaced to the solid line position and pressed against the secondary transfer backup roller 45 from the back surface side of the sheet S, and a secondary transfer voltage is applied from the secondary transfer bias generator 129, The color image is secondarily transferred to the sheet S to obtain a full-color image. When a monochrome image is transferred to the sheet S, only a black toner image is formed on the photoconductor 21 and transferred onto the intermediate transfer belt 41, and is transferred to the secondary transfer region R2 in the same manner as in the case of a color image. The monochrome image is obtained by transferring the image onto the conveyed sheet S.

After the secondary transfer, the toner remaining on the outer peripheral surface of the intermediate transfer belt 41 is removed by the belt cleaner 49. This belt cleaner 4
Reference numeral 9 denotes an intermediate transfer belt which is disposed opposite to the roller 46 with the intermediate transfer belt 41 interposed therebetween. At an appropriate timing, the cleaner blade comes into contact with the intermediate transfer belt 41 to scrape the toner remaining on the outer peripheral surface thereof. Drop.

In the vicinity of the roller 43, a patch sensor PS for detecting the density of a patch image formed on the outer peripheral surface of the intermediate transfer belt 41 is arranged, and a reference position of the intermediate transfer belt 41 is detected. A reading sensor RS for synchronization is provided.

Returning to FIG. 1, the description of the configuration of the engine unit E will be continued. The sheet S to which the toner image has been transferred by the transfer unit 4 is disposed downstream of the secondary transfer area R2 along a predetermined paper feed path (two-dot chain line) by the paper feed unit 63 of the paper feed / discharge unit 6. The toner image on the sheet S conveyed to the provided fixing unit 5 is fixed on the sheet S.
Then, the sheet S is further conveyed to the sheet discharging unit 64 along the sheet feeding path 630.

The paper discharge section 64 has two paper discharge paths 641
a, 641b, one discharge path 641a extends from the fixing unit 5 to the standard discharge tray, and the other discharge path 641b is substantially parallel to the discharge path 641a.
It extends between the re-feeding section 66 and the multi-bin unit. Three pairs of rollers 642 to 644 are provided along the paper discharge paths 641a and 641b, and the fixed sheet S is discharged toward the standard paper discharge tray or the multi-bin unit side, or the other side. In order to form an image, the sheet is conveyed to the re-feeding unit 66 side.

As shown in FIG. 1, the re-feeding section 66
The sheet S reversely conveyed from the sheet discharging unit 64 as described above is fed along the re-feeding path 664 (two-point line) to the sheet feeding unit 63.
And the three re-feeding roller pairs 6 arranged along the re-feeding path 664.
61 to 663. As described above, the paper discharge unit 6
4 is returned to the gate roller pair 637 along the re-feed path 664 so that the sheet S
3, the non-image forming surface of the sheet S is the intermediate transfer belt 4
1 and the image can be secondarily transferred to the surface.

In FIG. 2, reference numeral 113 denotes an interface 112 from an external device such as a host computer.
Is an image memory provided in the main controller 11 for storing an image given through
Represents control data for controlling the engine unit E and the CPU 1
23 for temporarily storing the operation result and the like in
M, and reference numeral 128 denotes a ROM that stores an arithmetic program and the like performed by the CPU 123.

Here, in the above image forming apparatus,
A primary transfer bias generator 126 for applying a primary transfer voltage to the intermediate transfer belt 41 in the primary transfer section R1 is composed of a constant voltage power supply, and applies a secondary transfer voltage to the secondary transfer roller 48 in the secondary transfer region R2. Next transfer bias generator 129
Is composed of a constant current power supply.

The resistance layer 41b of the intermediate transfer belt 41
Has a volume resistivity of 1.5 when a primary transfer voltage of 250 V is applied.
× 10 ¹² Ωcm (23 ° C., 65% RH).

Using such an apparatus, the photosensitive member 21 is negatively charged, and the developing units 23Y, 23C, 23M, 23K
The case of reversal development with a negatively charged one-component non-magnetic toner will now be considered.

First, when the charging bias applied from the charging bias generator 121 to the charging roller 22 was changed, the change in the surface potential of the intermediate transfer belt 41 was checked each time the number of times of primary transfer was repeated (number of rotations). This surface potential is the potential of the non-image portion. At this time, the primary transfer bias generator 1
26, the voltage applied to the conductive layer 41a of the intermediate transfer belt 41 is fixed at +300 V, and the temperature and humidity are 23
° C, 65% RH. The results are shown in FIGS.
Shown in In the figure, the surface potential of the intermediate transfer belt 41 is indicated by “medium medium surface potential”, and the cumulative number of primary transfers is indicated by “medium medium circumference”.

FIG. 4A shows that the charging bias is -1200.
When V is applied, the photoconductor surface potential V ₀ becomes −600 V. Before the primary transfer, the medium medium potential was 300 V. However, once the primary transfer was performed, it was 260 V, twice, 245 V, three times, 240 V, and 238, four times.
The intermediate medium surface potential decreases to V. This is the photosensitive member 21
This is because the negatively charged charges on the surface of the photoconductor 21 are discharged and accumulated on the surface of the intermediate transfer belt 41 in accordance with the potential difference between the surface potential of the intermediate transfer belt 41 and the surface potential of the intermediate transfer belt 41.

FIG. 4B shows that the charging bias is -1000.
FIG. 5 is a similar diagram when V and −1400 V are applied, and the photoconductor surface potential V ₀ becomes −400 V and −800 V, respectively. Before the primary transfer, the medium medium surface potential was 300 V. 287V, 233 each when performed once
When the voltage is applied twice, the voltage decreases to 282 V and 209 V, respectively. When the voltage is applied three times, the voltage decreases to 280 V and 200 V. When the voltage is applied four times, the voltage decreases to 280 V and 196 V.

From the above results, the width of change of the surface potential of the intermediate transfer belt 41 depending on the number of times of primary transfer differs depending on the charging bias to the photosensitive member 21. If the surface potential of the intermediate transfer belt 41 greatly fluctuates, the primary transfer efficiency becomes unstable. Therefore, in the above-described image forming apparatus, it is desirable that the charging bias is not changed as much as possible.

FIG. 5 shows the result of examining the amount of negative charge on the intermediate transfer belt 41 during the first round of transfer when the potential difference between the photosensitive member surface potential V ₀ and the intermediate medium surface potential changes. FIG. 5 shows that the surface potential of the intermediate transfer belt 41 fluctuates and the potential difference between the surface potential V ₀ of the photoreceptor 21 fluctuates from the result in the second and subsequent rounds. Then, it can be seen that the amount of negative charge on the intermediate transfer belt 41 changes in proportion to it. When the amount of negative charge on the intermediate transfer belt 41 changes, the transfer efficiency of the toner image transferred from the photoconductor 21 to the intermediate transfer belt 41 by the constant primary transfer voltage also changes.

In the above-described image forming apparatus, conventionally, in order to adjust the gradation of an image to be formed, as shown in a schematic diagram of FIG. Is changed to adjust the amount of toner adhering to the image portion. In FIG. 6A, when the charging bias is increased to increase the photosensitive member charging potential from -500 V to -800 V, the potential distribution of the image portion and the non-image changes from the solid line to the broken line, and the developing bias of -300 V in the drawing. Since the area surrounded by the dashed line below becomes smaller, the developing bias -300
The amount of toner adhering to a potential portion having an absolute value lower than V decreases and becomes thinner. Conversely, the charging potential of the photoconductor is -800V
When the voltage is lowered to -500 V, the amount of toner adhering to the image portion increases and becomes darker.

However, as described above, when such gradation adjustment is performed by changing the charging bias, the variation width of the surface potential of the intermediate transfer belt 41 becomes large, and the primary transfer efficiency becomes unstable. Therefore, it is desirable not to adjust the gradation of the image by adjusting the charging bias.

Therefore, in the present invention, as shown in FIG. 6B, the gradation of the image is adjusted by adjusting the exposure amount of the image portion without changing the charging potential by changing the charging bias. To do it. In FIG. 6B, the exposure amount of the image portion is reduced to raise the exposure portion potential of the photoconductor from -50 V to -200.
When the voltage is increased to V, the potential distribution of the image portion and the non-image changes from the solid line to the broken line, and the area surrounded by the broken line of −300 V or less in FIG.
The amount of toner adhering to a potential portion whose absolute value is lower than 300 V is reduced and becomes thinner. Conversely, when the exposure amount is increased and the potential of the exposed portion of the photoreceptor is reduced from -200 V to -50 V, the amount of toner adhering to the image portion increases and becomes darker.

As shown in FIG. 1, in an image forming apparatus provided with four color developing units 23Y, 23C, 23M, and 23K to form a full-color image by superimposing four color toners, Since the characteristics of the toner are different, in practice, the photoconductor surface potential V ₀ depends on the color of the toner.
Have to be different. In the above embodiment, in order to form an optimal toner image on the photoconductor 21, when a yellow toner image is formed by the developing unit 23 </ b> Y, the charging bias is set to −1400 V and the photoconductor surface potential V _{0 is set} to −. 80
In the case where a magenta toner image is formed at 0 V by the developing unit 23M, the charging bias is set to -1300 V and the photoconductor surface potential V ₀ is set to -700 V. When the cyan toner image is formed by the developing unit 23C, the charging bias is set. When the photosensitive member surface potential V _{0 is set} to −600 V and the developing device 23K forms a black toner image, the charging bias is set to −1.
The photoreceptor surface potential V ₀ which in the 100V it is desirable to -500 V.

FIG. 7 is a diagram showing the result of examining the relationship between the order of forming toner images of different colors and the change in the surface potential of the medium when the charging bias must be changed depending on the color of the toner to be developed. It is. Temperature and humidity are 23 ℃,
It was 65% RH.

In FIG. 7, the order of increasing the order is the order of increasing charging bias, that is, the first cycle is a -1400V yellow toner image, the second cycle is a -1300V magenta toner image, and the third cycle is -1200V. The cyan toner image and the fourth cycle are cases where a -1100 V black toner image is formed. The smaller order means the order in which the charging bias is smaller, that is, the first cycle is the -1100 V black toner image and the second cycle is the second cycle. The eye is a cyan toner image of -1200V, the third lap is a magenta toner image of -1300V, and the fourth lap is -1400.
This is a case where a yellow toner image of V is formed.

As is apparent from FIG. 7, when the primary transfer is performed in the order of increasing charging bias, the surface potential of the medium is 300 V when the primary transfer is performed once and 235 V when the primary transfer is performed twice.
When the primary transfer is performed in ascending order of the charging bias, the voltage is lowered to 194 V when the transfer is performed three times and to 170 V when the charge transfer is performed four times.
74V for 3 times, 245V for 4 times, 214V for 4 times
When the primary transfer is performed in the order of apparently smaller charging bias, the decrease in the surface potential of the intermediate medium is smaller, and the primary transfer can be performed more stably and reliably.

As described above, the image forming method of the present invention
A constant primary transfer voltage is applied from a constant voltage power supply of the primary transfer bias generator 126, and the charging potential of the photoconductor 21 by the charging roller 22 is fixed at least for the color toner image of each color, and Is adjusted by adjusting the amount of exposure to the image portion. For this purpose, the charging bias from the charging bias generating section 121 is adjusted to adjust the gradation of the toner image to be visualized. Instead, the intensity of the laser beam L from the exposure unit 3 that is intensity-modulated in accordance with the image signal provided via the interface 112 can be adjusted for gradation adjustment.

Further, according to the image forming method of the present invention, the developing units 23Y, 23C,
23M and 23K are selected and brought into contact with each other to perform the developing operation. To this end, the engine controller 12 selects the four developing units 23Y, 23M, 23C and 23K in the order of the smaller charging bias as described above. To contact the photoconductor 21.

Although the image forming method of the present invention has been described based on the embodiments, the present invention is not limited to these embodiments, and various modifications are possible.

[0060]

As is clear from the above description, according to the image forming method of the present invention, a constant voltage power supply is used as a primary transfer bias application power supply, and the charging potential of the latent image carrier by the charging means is at least each color. Is fixed for the color toner, and the gradation of the image is adjusted by adjusting the exposure amount to the image portion. Therefore, the toner images of different colors are sequentially transferred to the intermediate transfer medium while being transferred to the intermediate transfer medium. The fluctuation of the surface potential can be greatly reduced, the transfer efficiency of the toner image to the intermediate transfer medium can be stabilized, and a reliable image forming apparatus can be realized without increasing the cost of the apparatus.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment of an image forming apparatus to which an image forming method according to the present invention is applied.

FIG. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus of FIG. 1;

FIG. 3 is a cross-sectional view illustrating a layer configuration of an intermediate transfer belt and a photoconductor.

FIG. 4 is a diagram illustrating a result of examining a change in a surface potential of an intermediate transfer belt every time the number of times of primary transfer is increased when a charging bias is changed.

FIG. 5 is a diagram showing a result of examining a negative charge amount charged on the intermediate transfer belt when a potential difference between a photosensitive member surface potential and the intermediate transfer belt changes.

FIG. 6 is a diagram for explaining conventional tone adjustment of an image and tone adjustment of an image of the present invention.

FIG. 7 is a diagram showing a result of examining a relationship between a toner image forming order and a change in an intermediate-medium surface potential when a charging bias must be changed depending on a color of toner to be developed.

FIG. 8 is a diagram illustrating an example of an image forming apparatus provided with an intermediate transfer medium.

FIG. 9 is a cross-sectional view illustrating a layer configuration of an intermediate transfer medium and a photoconductor in a modified example of FIG.

[Explanation of symbols]

 E: engine unit S: sheet L: laser beam R1: primary transfer area R2: secondary transfer area PS: patch sensor RS: synchronization reading sensor 1: control unit 2: image carrier unit 3: exposure unit 4: transfer Unit 5: Fixing unit 6: Paper supply / discharge unit 11: Main controller 12: Engine controller 21: Photoconductor 21a ... Conductive layer 21b ... Photosensitive layer 22: Charging roller 23: Developing unit 23Y: Yellow developing unit 23C: Cyan Developing device 23M Magenta developing device 23K Black developing device 24 Cleaning section 25 Developing roller 41 Intermediate transfer belt 41a Conductive layer 41b Resistive layer 41c Insulating substrate 42 Primary transfer backup rollers 43, 44 Roller 45: Secondary transfer backup roller 46, 47 ... Roller 48: Secondary transfer roller Roller 49 belt cleaner 50 electrode roller 61 cassette 62 manual feed tray 63 paper feeding unit 64 paper discharging unit 66 refeeding unit 111 CPU 112 interface 113 image memory 121 charging bias generator 122 ... Image signal switching unit 123 ... CPU 124 ... Patch creation module 125 ... Development bias generation unit 126 ... Primary transfer bias generation unit 127 ... RAM 128 ... ROM 129 ... Secondary transfer bias generation unit 630 ... Sheet feeding path 637 ... Gate roller pair 641a, 641b: paper discharge path 642 to 644: roller pair 661 to 663: re-feed roller pair 664: re-feed path

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat ゛ (Reference) G03G 15/16 103 F Term (Reference) 2H027 EA01 EA02 EA03 EB03 EB04 EC20 ED03 ED04 ED24 2H030 AA03 AD01 AD02 AD19 BB13 BB23 BB42 BB54 2H032 AA05 AA15 BA09 CA02 CA12 CA15 2H076 DA06

Claims (5)

[Claims] 1. A latent image carrier on which a surface is uniformly charged by a charging unit while rotating and selectively discharged by an exposure unit to form an electrostatic latent image, and a surface of the latent image carrier is A plurality of developing units that selectively apply different color toners to make the latent image visible, and a primary transfer unit that sequentially transfers toner images developed by different color toners to an intermediate transfer medium; An image forming apparatus having a primary transfer bias application power supply for applying a bias in a primary transfer section, and a secondary transfer section for transferring a full-color toner image superimposed and transferred on an intermediate transfer medium onto recording paper In the apparatus, a constant voltage power supply is used as the primary transfer bias application power supply, and a charging potential of the latent image carrier by the charging unit is fixed at least for color toners of each color. Image forming method, which comprises carrying out by adjusting the exposure amount of the image portion. 2. The intermediate transfer medium is composed of a multilayer body having a conductive layer and a resistive layer integrally formed on the conductive layer and onto which toner is transferred. 2. The image forming method according to claim 1, wherein the voltage is applied through the conductive layer. 3. The image forming method according to claim 1, wherein a constant current power supply is used as a secondary transfer bias application power supply for applying a bias in the secondary transfer section. 4. The apparatus according to claim 1, wherein a charging potential of the latent image carrier by the charging unit is fixed to be the same for all color toners. Image forming method. 5. The charging potential of the latent image carrier by the charging unit is different for each color of the color toner, and the transfer order of the toner image to be transferred to the intermediate transfer medium is changed by the charging unit. 4. The image forming method according to claim 1, wherein the charging potential is set so as to be higher in color toner in order from lower color toner.