JP4525293B2 - Color image forming apparatus and color image forming method - Google Patents

Description

  The present invention relates to an electrophotographic color image forming apparatus, and more particularly to a color image forming apparatus having a plurality of image forming means and a color image forming method.

  In an electrophotographic color image forming apparatus, a multi-color image is formed by superimposing single-color toner images of yellow, magenta, cyan, and black. However, toner of other colors is mixed in one single-color toner image. Therefore, there is a problem of color mixing that degrades image quality, and various countermeasures have been proposed as follows.

  In Patent Document 1, the development bias pulse width is devised to prevent the first color toner from being mixed into the second and subsequent developing devices.

  In Patent Document 2, color mixing is prevented by separating and removing the toner of the first developer mixed in the second developer.

  In Patent Document 3, color mixing is prevented by discharging the toner of the first developer mixed in the second developer from the second developer by development.

In Patent Document 4, color mixing is prevented by applying a voltage that presses the toner on the photoconductor toward the photoconductor when passing through the developing device.
JP-A-8-234528 JP-A-9-50162 Japanese Patent Laid-Open No. 11-174771 JP 2002-31174 A

  In recent years, a color image forming apparatus having a plurality of color image modes has been desired as the use of the color image forming apparatus expands or user needs diversify. Such multiple modes include a wide color mode that forms a color image with a wide color gamut and rich color expression, and a standard mode that saves toner even when the color gamut is sacrificed to some extent. is there.

  By the way, a color image forming apparatus capable of forming an image under advanced image quality control, such as color image formation, is often equipped with image density stabilizing means, and the image density stabilizing means. Thus, a stable color image can be formed in terms of color tone, density, and the like.

  In a color image forming apparatus having an image density stabilizing unit, the selection of the plurality of modes is conveniently performed by adjusting the control level of the image density stabilizing unit.

  That is, by adjusting the control level of the image density stabilizing means, the maximum density is adjusted and the color gamut is enlarged or reduced.

  Thus, it has been found that when image density control is performed using the image density stabilizing means, color mixture is likely to occur. This color mixture occurs when a single color toner used for subsequent image formation is mixed into a previously formed single color toner image.

  In all of the conventional color mixing prevention techniques including the techniques described in Patent Documents 1 to 4, the toner that forms the previously formed single color image is mixed into the developing device that forms the toner image later. Therefore, it cannot be used to prevent color mixing caused by selection of a color mode, that is, color mixing that occurs when toner used for subsequent image formation is mixed into a previously formed single color image.

  Accordingly, an object of the present invention is to prevent color mixing that may occur with the expansion of the color gamut.

The object is achieved by the following invention.
1. An image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, a developing unit that develops the electrostatic latent image to form a toner image, and a plurality of image forming units including a primary transfer unit;
Intermediate transfer member,
Secondary transfer means for transferring the toner image transferred onto the intermediate transfer member by the primary transfer means to a recording material; and
Image density detection means for detecting the density of a toner image formed on the image carrier by exposure based on reference density data, and transferred and formed on the intermediate transfer body by primary transfer, and an output of the image density detection means An image density stabilizing means having an image formation control means for controlling image forming conditions based on
In a color image forming apparatus having
The control level by the image density stabilizing means can be selected and set from a plurality of levels, and when the high density control level is set, at least the image forming unit set to the high density control level A color image forming apparatus comprising fog margin control means for setting a fog margin | V0−Vdc | in the developing means of the image forming unit downstream of the image forming unit to be higher than a standard value.
However, V0 is the unexposed portion potential of the image carrier, and Vdc is the potential of the DC component of the developing bias in the developing means. 2. The color image forming apparatus according to item 1, wherein the fog margin control unit sets the fog margin for all of the developing units of the plurality of image forming units.
3. An image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, a developing unit that develops the electrostatic latent image to form a toner image, and a plurality of image forming units including a primary transfer unit;
Intermediate transfer member,
Secondary transfer means for transferring the toner image transferred onto the intermediate transfer member by the primary transfer means to a recording material; and
Image density detection means for detecting the density of a toner image formed on the image carrier by exposure based on reference density data, and transferred and formed on the intermediate transfer body by primary transfer, and an output of the image density detection means An image density stabilizing means having an image formation control means for controlling image forming conditions based on
In a color image forming apparatus having
The control level by the image density stabilizing means can be selected and set from a plurality of levels, and when the high density control level is set, at least the image forming unit set to the high density control level A color image forming apparatus comprising: frequency control means for setting a frequency of an AC component of the developing bias in the developing means of the image forming unit on the downstream side higher than a standard value.
4). 4. The color image forming apparatus according to 3, wherein the frequency control unit sets the frequency for all of the developing units of the plurality of image forming units.
5). 5. The color image forming apparatus according to any one of 1 to 4, wherein the control level is detected by a signal from an operation unit.
6). 3. The image forming apparatus according to 1 or 2, wherein the fog margin control unit controls at least one of a charging unit that charges the image carrier in the image forming unit and a bias power source that outputs the DC component. Color image forming apparatus.
7). 7. The color image forming apparatus according to any one of 1 to 6, wherein the developing unit forms a toner image using a polymerized toner.
8). By performing a toner image forming step of forming a single color toner image on the image carrier and transferring the single color toner image on the image carrier to an intermediate transfer member a plurality of times by a plurality of image forming units , the intermediate transfer A color image forming method for forming a multicolor toner image in which a plurality of single color toner images are superimposed on a body, and transferring the multicolor toner image on the intermediate transfer body to a recording material,
When a patch image is formed on the image carrier, the formed patch image is transferred to the intermediate transfer member, and the density of the patch image on the intermediate transfer member is detected to form an image on a recording material. In the color image forming method for controlling the image forming conditions of
According to the density information of the patch image on the intermediate transfer member, the control level can be selected and set from a plurality of levels, and at least when the high density control level is set, the high density control is performed. A color image forming method, wherein a fog margin | V0−Vdc | in a developing process of an image forming unit downstream of an image forming unit set at a level is set higher than a standard value .
However, V0 is the unexposed portion potential of the image carrier, and Vdc is the potential of the DC component of the developing bias in the developing step . By performing a toner image forming step of forming a single color toner image on the image carrier and transferring the single color toner image on the image carrier to an intermediate transfer member a plurality of times by a plurality of image forming units , the intermediate transfer An image forming method of forming a multicolor toner image in which a plurality of the single color toner images overlap on a body, and transferring the multicolor toner image on the intermediate transfer body to a recording material,
An image when a patch image is formed on the image carrier, the patch image is transferred to the intermediate transfer member, and the density of the patch image on the intermediate transfer member is detected to form an image on a recording material. In the color image forming method for controlling the forming conditions,
According to the density information of the patch image on the intermediate transfer member, the control level can be selected and set from a plurality of levels, and at least when the high density control level is set, the high density control is performed. A color image forming method, wherein the frequency of the AC component of the developing bias in the developing process of the image forming unit downstream of the image forming unit set to the level is set higher than a standard value .
10. 10. The color image forming method according to 8 or 9, wherein the density information is obtained from an operation unit of a color image forming apparatus.
11. 10. The color image forming method according to 8 or 9, wherein the density information is obtained by detecting the density of the patch image on the intermediate transfer member.

  According to the invention of any one of claims 1 to 11, even in the color mode in which the color gamut is expanded, the color mixture is well prevented and a high quality color image is formed.

  According to the seventh aspect of the invention, a high-quality image excellent in image characteristics such as resolution and gradation reproducibility is formed.

<Color image forming apparatus>
FIG. 1 is a schematic diagram showing an example of the overall configuration of a color image forming apparatus according to an embodiment of the present invention.

  In FIG. 1, 10Y, 10M, 10C, and 10K are photosensitive members as image carriers, 11Y, 11M, 11C, and 11K are charging units, 12Y, 12M, 12C, and 12K are exposure units, and 13Y, 13M, 13C, and 13K are exposure units. Developing means 14Y, 14M, 14C and 14K are cleaning means for cleaning the surface of the photosensitive member, and 15Y, 15M, 15C and 15K are toner images on the photosensitive members 10Y, 10M, 10C and 10K. Primary transfer means. Reference numeral 20 denotes an intermediate transfer member. The mechanical configuration of the image forming unit Y that forms a yellow toner image, the image forming unit M that forms a magenta toner image, the image forming unit C that forms a cyan toner image, and the image forming unit K that forms black toner is the same. Each image forming unit includes the photoconductor, the charging unit, the exposure unit, the developing unit, the primary transfer unit, and the cleaning unit.

  The charging unit 11Y and the exposure unit 12Y, the charging unit 11M and the exposure unit 12M, the charging unit 11C and the exposure unit 12C, and the charging unit 11K and the exposure unit 12K each form an electrostatic latent image on the image carrier. Configure.

  The image forming units Y, M, C, and K are arranged in the order of Y, M, C, and K with respect to the traveling direction of the intermediate transfer member 20, and the photosensitive members 10Y, 10M, 10C, and 10K are intermediate transfer members. 20 is brought into contact with the extending surface and rotated at the contact point in the same direction as the traveling direction of the intermediate transfer body 20 and at the same linear speed.

  The intermediate transfer member 20 is stretched around a driving roller 21, an earth roller 22, a tension roller 23, and a driven roller 24 to constitute a belt unit.

  The intermediate transfer member 20 travels by rotation of the drive roller 21 by a drive motor (not shown).

  The photoreceptors 10Y, 10M, 10C, and 10K are obtained by forming a photosensitive layer such as a conductive layer, an a-Si layer, or an organic photoreceptor (OPC) on the outer periphery of a cylindrical metal base formed of, for example, an aluminum material. Rotate counterclockwise as indicated by the arrow in FIG. 1 with the conductive layer grounded. In the illustrated embodiment, a negatively chargeable OPC photoreceptor is used.

  As the charging units 11Y, 11M, 11C, and 11K, known charging devices such as a scorotron charging device and a roller charging device dew are used.

  As the exposure means 12Y, 12M, 12C, and 12K, a known exposure device such as a laser exposure device using a semiconductor laser as a light source or an LED array is used to perform dot exposure based on image data.

  As the transfer devices 15Y, 15M, 15C, and 15K, a corona discharge device, a transfer roller, or the like is used.

The intermediate transfer member 20 is an endless belt having a volume resistivity of 10 ⁶ to 10 ¹² Ω · cm. For example, the intermediate transfer member 20 may be an engineering plastic such as modified polyimide, thermosetting polyimide, ethylene tetrafluoroethylene copolymer, polyvinylidene fluoride, and nylon alloy. A semiconductive seamless belt having a thickness of 0.015 to 0.05 mm, in which a conductive material is dispersed.

  A secondary transfer unit 25 includes a transfer roller that can come into contact with and release from the earth roller 22, and retransfers the toner image formed on the intermediate transfer body 20 to the recording material P.

  A cleaning unit 26 having a cleaning blade is provided to face the driven roller 24 with the intermediate transfer member 20 interposed therebetween. After the toner image is transferred to the recording material P, the intermediate transfer member 20 passes through the cleaning means 26, and the toner remaining on the peripheral surface is cleaned by the cleaning blade.

  A registration roller 27 conveys the recording material P toward the secondary transfer unit 25 in synchronization with image formation in the image forming units Y, M, C, and K.

A fixing device 4 heats and fixes the toner image on the recording material P onto which the toner image on the intermediate transfer member 20 has been transferred.
<Developing means>
Next, the developing units 13Y, 13M, 13C, and 13K will be described with reference to FIG. Since these developing means have the same structure, the reference numerals Y, M, C, and K are omitted.

  As the developing unit 13, a developing unit using a two-component developer mainly composed of a carrier and a toner is used, but a two-component developing unit using a small particle size toner is preferable. In addition, a developing unit that performs development by regular development or a unit that performs reversal development can be used as a developing unit. However, a developing bias having the same polarity as the charging of the photosensitive member 10 is applied to the developing sleeve 131 to thereby charge the photosensitive member 10. Reversal development in which development is performed with toner charged to the same polarity is preferable, and in this embodiment, development is performed by reversal development using negatively charged toner.

  For such a small particle size, it is desirable to use a polymerized toner.

  The polymerized toner means a toner obtained by forming a binder resin for toner and forming the toner shape by polymerization of a raw material monomer or prepolymer of the binder resin and subsequent chemical treatment. More specifically, it means a toner obtained through a polymerization reaction such as suspension polymerization or emulsion polymerization and, if necessary, a step of fusing particles between them. In a polymerized toner, a raw material monomer or prepolymer is uniformly dispersed in an aqueous system and then polymerized to produce a toner, so that a toner having a uniform toner particle size distribution and shape can be obtained.

  Specifically, a polymer is produced by emulsion polymerization of a monomer in a liquid of an aqueous medium to which an emulsion is added or a suspension polymerization method, and then an organic solvent, agglomeration is produced. It can manufacture by the method of adding an agent etc. and making it associate. A method of preparing by mixing with a dispersion liquid of a release agent or a colorant necessary for the constitution of the toner at the time of association, or a toner component such as a release agent or a colorant dispersed in a monomer Examples thereof include emulsion polymerization. Here, the association means that a plurality of resin particles and colorant particles are fused.

  As the magnetic particles of the carrier, conventionally known materials such as metals such as iron, ferrite and magnetite, and alloys of these metals with metals such as aluminum and lead are used. Ferrite particles are particularly preferable.

  The carrier is preferably a carrier in which magnetic particles are further coated with a resin, or a so-called resin-dispersed carrier in which magnetic particles are dispersed in a resin. The resin composition for coating is not particularly limited, and for example, olefin resin, styrene resin, styrene-acrylic resin, silicone resin, ester resin, or fluorine-containing polymer resin is used. The resin for constituting the resin-dispersed carrier is not particularly limited, and known resins can be used. For example, a styrene acrylic resin, a polyester resin, a fluorine resin, a phenol resin, or the like can be used. it can. Although the particle diameter of the carrier will be described later, the particle diameter of the carrier is also a value measured by the same method as that of the toner.

  FIG. 2 is a diagram showing a configuration of the developing unit 13.

  In FIG. 2, reference numeral 130 denotes a casing for accommodating a two-component developer composed of toner and carrier. Inside the developing sleeve 131, a magnet roll 132 is provided as a fixed magnetic field generating means.

  A regulating member 133 is disposed to face the developing sleeve 131, regulates the amount of the developer Dev conveyed by the developing sleeve 131, and forms a uniform developer Dev layer on the developing sleeve 131.

  Reference numeral 134 denotes an impeller for transporting the developer Dev by rotation, and 135 and 136 are transport screws for stirring and transporting the developer Dev.

A developing bias in which a DC component from the bias power source Edc and an AC component from the bias power source Eac are superimposed is applied to the developing sleeve 131.
<Color image formation>
A yellow toner image, a magenta toner image, a cyan toner image, and a black toner image are formed on the photoreceptors 10Y, 10M, 10C, and 10K of the image forming portions Y, M, C, and K by charging, exposure, and development, and are primarily transferred. These toner images are transferred to the intermediate transfer member 20 by the means 15Y, 15M, 15C, and 15K, and superimposed. The multicolor toner image formed on the intermediate transfer body 20 by superimposing is transferred to the recording material P conveyed in synchronization with the toner image formation in the image forming portions Y, M, C, K by the secondary transfer means 25. Is done.

The recording material P onto which the multicolor toner image has been transferred passes through the fixing unit 4 and is subjected to fixing processing. The recording material P that has been subjected to fixing processing is discharged outside the apparatus.
<Image density stabilization means>
FIG. 3 is a block diagram of the control system in the embodiment of the present invention.

  In the example of FIG. 3, the image stabilization unit includes a control unit CR, an image density detection unit 30 and bias power supplies EdcY, EdcM, EdcC, and EdcK, and includes density information of the patch image detected by the image density detection unit 30. Based on this, the bias power supplies EdcY, EdcM, EdcC, and EdcK are controlled so that the image density becomes a constant control level.

  EdcY is a bias power source of the developing unit 13Y in the image forming unit Y, EdcM is a bias power source of the developing unit 13M in the image forming unit M, EdcC is a bias power source of the developing unit 13C in the image forming unit C, and EdcK is an image forming unit. This is a vice power source for the developing means 13K in K.

  As shown in FIG. 1, reference numeral 30 denotes an image density detecting means which is disposed facing the intermediate transfer member 20 and has a light emitting element made of LED and a light receiving element made of a phototransistor. Detect the density of. The CR controls the charging units 11Y, 11M, 11C, and 11K and the power sources EdcY, EdcM, EdcC, and EdcK based on the output of the image density detection unit 30.

  Image stabilization control is executed before image formation, after image formation, immediately after the main switch of the color image forming apparatus is turned on, every time a predetermined number of images are formed, etc., and stabilizes image density. To be executed.

  In a toner image forming process consisting of charging, exposure and development, a patch image consisting of a solid image is formed on the photoreceptor 10 by inputting image data of a predetermined density value to the exposure means 11 to form and develop an electrostatic latent image. Is formed. This patch image is formed by the image forming means Y, M, C, and K, respectively.

  The patch images transferred onto the intermediate transfer member 20 by the transfer units 15Y, 15M, 15C, and 15K are formed in parallel without overlapping on the intermediate transfer member 20.

  The yellow patch image, the magenta patch image, the cyan patch image, and the black patch image formed on the intermediate transfer member 20 are not transferred when passing through the secondary transfer unit 4, but pass through the image density detection unit 30. The density of each patch image is detected.

  Based on the output of the image density detection means 30, the control means CR controls to maintain the density of the patch image at a predetermined control level, that is, controls the bias power supplies EdcY, EdcM, EdcC and EdcK to obtain predetermined image data. Control is performed so that a base image is formed with a predetermined density.

  For example, when the output of the image density detection means 30 indicates that the density is low, the control means CR performs control to increase the toner adhesion amount and raise the image density by increasing the level Vdc of the DC component of the developing bias. .

  The control unit CR also controls the charging units 11Y, 11M, 11C, and 11K in accordance with the level Vdc of the DC component of the developing bias so that the fog margin | V0−Vdc | becomes constant. V0 is the unexposed portion potential of the photoreceptors 10Y, 10M, 10C, and 10K.

  The fog margin | V0−Vdc | is a parameter that determines the degree of fog. If the fog margin | V0−Vdc | When −Vdc | is large, fogging is difficult to occur, and the fogging level is low.

  To suppress fogging, the fogging margin | V0−Vdc | can be set to a large value, but it is usually set to an optimum value in relation to the material properties such as the photoconductor and developer, environment, and maximum density. It is.

  Control of the charging units 11Y, 11M, 11C, and 11K is provided in each of the image forming units Y, M, C, and K, and potential detection units 31Y, 31M, which detect the surface potentials of the photoreceptors 10Y, 10M, 10C, and 10K. This is performed based on the detection results of 31C and 31K.

  The image density stabilization control is performed independently for each of the image forming unit Y, the image forming unit M, the image forming unit C, and the image forming unit K. By such control, an image having a constant image density is formed with respect to constant image data, and a high-quality color image without fogging is stably formed by the density control.

  The above-described image density stabilizing means maintains the image density constant by controlling the DC component level Vdc of the developing bias. However, the image density is kept constant by other density control means. It is also possible to use image density stabilizing means.

  For example, the image density is controlled and maintained constant by controlling the rotation speed of the developing sleeve, and the exposure means 12Y, 12M, 12C, and 12K are controlled by controlling the image data generating means to control the image density. It is possible to use one that keeps constant.

The fog margin | V0−Vdc | is changed and set according to changes in the number of prints and the environment (temperature / humidity), and is set for each of the image forming portions Y, M, C, and K.
<Fog Margin Control (Embodiment 1)>
There are various needs for color images. That is, there is a case where a color image having a wide color gamut and rich in color expression is desired, or a color image which saves toner consumption may be desired. There is a demand for a color image forming apparatus that can select the type of color image, that is, the color mode, according to a command from the apparatus.

In the color image forming apparatus having the image density stabilization means, image density stabilization means includes a plurality of control levels, the image density stabilization control by selecting the control level in accordance with the color mode set by the user Do. As a result, color images having different color gamuts are stably formed.

  However, it has been found that fogging may occur in color image formation when the control level is raised to widen the color gamut. Next, fog that occurs as the control level is raised will be described. In the following description, “downstream” and “upstream” are based on the moving direction of the intermediate transfer body 20.

  When the control level is raised for a certain color among yellow, magenta, cyan, and black, a mixed color in which the downstream color is mixed with the color with the raised control level occurs. That is, color mixing occurs in which the color of the image formed in the image forming unit downstream in the moving direction of the intermediate transfer body 20 is mixed with the color of the image formed in the upstream portion from the image forming portion of the color whose control level is raised.

  By increasing the control level, when the toner image with increased adhesion amount passes through the primary transfer portion, the toner adhering to the photosensitive member as fog is transferred to the intermediate transfer member. Is thought to occur.

  Such fog generated with the expansion of the color gamut is reduced by raising the fog margin | V0−Vdc | in the downstream image forming unit. That is, in the downstream image forming unit, the above-described fog generation can be prevented by performing image formation under the condition that fog is less likely to occur.

  The fog margin control will be described with reference to the drawings.

  For the purpose of expanding the color gamut, the level of the DC component of the developing bias is raised from Vdc1 to Vdc2. Corresponding to raising the level of the DC component, the unexposed portion potential is raised from V01 to V02, and V02 and Vdc2 are set so that the fog margin | V01−Vdc1 | <| V02−Vdc2 |.

  Fog is more likely to occur in magenta and cyan than yellow and in magenta and black than cyan. In color image formation, image forming portions are arranged in the order of yellow, magenta, cyan, and black from the upstream with respect to the intermediate transfer member. Therefore, a magenta, cyan or black color mixture often occurs in a yellow image.

In image formation with an increased control level, the control of the fog margin to raise the fog margin | V0−Vdc | must be performed at least in the image formation of the downstream color of the color with the raised control level . When the image is pulled up, the fog margin may be raised in all the image forming units.

As described above, the fog margin is controlled according to the user's setting. However, the fog margin can be controlled based on the detection signal of the image density detection means 30. That is, the density of the patch image is detected even in the normal image stabilization control, but the change in the density of the patch image due to the change of the color mode is much larger than the change in the normal image stabilization control. Thus, the control device CR when a change of such a large patch image density is detected, and controls the fog margin.
<Frequency control (Embodiment 2)>
In this embodiment, as another means for preventing fogging that may occur when the control level of the image density stabilizing means is changed, a means for controlling the AC component of the developing bias is employed.

  In the image density stabilization means, the control means CR controls the vice power supplies EacY, EacM, EacC, and EacK to increase the frequency of the AC component of the developing bias when a high control level is set. By increasing the frequency, fogging is suppressed, and as a result, even when image formation is performed to increase the toner adhesion amount in the upstream image forming unit, color mixing on the downstream side is prevented. In FIG. 3, a bias power source EacY is a power source that applies an AC component of the developing bias in the image forming unit Y, a bias power source EacM is a power source that applies an AC component of the developing bias in the image forming unit M, and a bias power source EacC is the image forming unit. A power source for applying an AC component of the developing bias in C and a bias power source EacK are power sources for applying an AC component of the developing bias in the image forming unit K.

  Also in the frequency control, it is a necessary condition that the frequency control described above is performed in the image forming unit downstream from the image forming unit that has raised the control level of the image density stabilizing unit. The frequency control described above may be performed.

A color image forming apparatus shown in FIG. 1 and having the following configuration and conditions was used.
Photoconductor: negatively charged OPC photoconductor, drum diameter: φ60 mm
Non-image part potential of photoconductor: -250 V to -700 V variable Exposure: Laser scanning exposure using a semiconductor laser, power: 300 μW
Development: Two-component development method Toner density control: Toner density control using a toner density sensor that detects the toner density by detecting the permeability of the developer Development bias: Vdc: -200 V to -700 V variable, Vac: 0. 5 Vp-p to 2/0 Vp-p variable,
・ Development bias AC component frequency: 2 kHz to 7 kHz variable ・ Developer: Two-component developer having emulsion polymerization toner and resin coating carrier ・ Intermediate transfer member: Seamless semiconductive resin belt (Material: polyimide, surface resistivity: 10 ¹¹ Ω / □, volume resistivity: 10 ⁸ Ω · cm)
・ Primary transfer means: firing roller (diameter: φ20mm, resistance: 10 ⁶ Ω)
Secondary transfer means: firing roller (diameter: φ30mm, resistance: 10 ⁷ Ω)
Image density detection means: a reflective sensor comprising an LED and a phototransistor (Example 1)
The yellow toner adhesion amount, fog margin | V0−Vdc |, and the frequency of the AC component of the developing bias in the patch image formed on the intermediate transfer member were set as shown in Table 1 to form an image on the recording material. .

  As a result, as shown by the color mixing level in the solid image in Table 1, a sufficiently satisfactory fog level image was formed. Note that the fog margin control shown in Table 1 was performed for all image forming portions of Y, M, C, and K.

(Example 2)
An image was formed on a recording material by setting the yellow toner adhesion amount, fog margin | V0−Vdc |, and the frequency of the AC component of the developing bias in the patch image formed on the intermediate transfer member as shown in Table 2. .

As shown in Tables 1 and 2, fogging was sufficiently suppressed and a good image was formed even when the toner adhesion amount was changed.
(Comparative example)
As shown in Table 3, the image formation was performed with the fog margin | V0−Vdc | and the frequency of the AC component of the developing bias being constant while changing the toner adhesion amount.

  As a result, a good image was formed at a toner adhesion amount of 4.0, but at a toner adhesion amount of 4.5, a satisfactory level of fogging was observed. Conspicuous fogging was observed by visual observation.

  The results in Tables 1 to 3 are the results of an image formation test in which 100,000 sheets were printed at 30 ° C. and a high temperature and high humidity of 80% relative humidity.

1 is a diagram illustrating an example of an overall configuration of a color image forming apparatus according to an embodiment of the present invention. It is a figure which shows the structure of the image development means in embodiment of this invention. It is a block diagram of a control system in an embodiment of the present invention. It is a figure explaining control of a fog margin.

Explanation of symbols

10, 10Y, 10M, 10C, 10K photoconductor 13, 13Y, 13M, 13C, 13K developing means CR control means Edc, EdcY, EdcM, EdcC, EdcK, EacY, EacM, EacC, EacK Bias power supply

Claims (11)

An image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, a developing unit that develops the electrostatic latent image to form a toner image, and a plurality of image forming units including a primary transfer unit;
Intermediate transfer member,
Secondary transfer means for transferring the toner image transferred onto the intermediate transfer member by the primary transfer means to a recording material; and
Image density detection means for detecting the density of a toner image formed on the image carrier by exposure based on reference density data, and transferred and formed on the intermediate transfer body by primary transfer, and an output of the image density detection means An image density stabilizing means having an image formation control means for controlling image forming conditions based on
In a color image forming apparatus having
The control level by the image density stabilizing means can be selected and set from a plurality of levels, and when the high density control level is set, at least the image forming unit set to the high density control level A color image forming apparatus comprising fog margin control means for setting a fog margin | V0−Vdc | in the developing means of the image forming unit downstream of the image forming unit to be higher than a standard value.
However, V0 is the unexposed portion potential of the image carrier, and Vdc is the potential of the DC component of the developing bias in the developing means.   The color image forming apparatus according to claim 1, wherein the fog margin control unit sets the fog margin for all of the developing units of the plurality of image forming units. An image carrier, a latent image forming unit that forms an electrostatic latent image on the image carrier, a developing unit that develops the electrostatic latent image to form a toner image, and a plurality of image forming units including a primary transfer unit;
Intermediate transfer member,
Secondary transfer means for transferring the toner image transferred onto the intermediate transfer member by the primary transfer means to a recording material; and
Image density detection means for detecting the density of a toner image formed on the image carrier by exposure based on reference density data, and transferred and formed on the intermediate transfer body by primary transfer, and an output of the image density detection means An image density stabilizing means having an image formation control means for controlling image forming conditions based on
In a color image forming apparatus having
The control level by the image density stabilizing means can be selected and set from a plurality of levels, and when the high density control level is set, at least the image forming unit set to the high density control level A color image forming apparatus comprising: frequency control means for setting a frequency of an AC component of the developing bias in the developing means of the image forming unit on the downstream side higher than a standard value.   The color image forming apparatus according to claim 3, wherein the frequency control unit sets the frequency for all of the developing units of the plurality of image forming units.   The color image forming apparatus according to claim 1, wherein the control level is detected by a signal from an operation unit.   3. The fog margin control unit controls at least one of a charging unit that charges the image carrier in the image forming unit and a bias power source that outputs the DC component. A color image forming apparatus described in 1.   The color image forming apparatus according to claim 1, wherein the developing unit forms a toner image using a polymerized toner. By performing a toner image forming step of forming a single color toner image on the image carrier and transferring the single color toner image on the image carrier to an intermediate transfer member a plurality of times by a plurality of image forming units , the intermediate transfer A color image forming method for forming a multicolor toner image in which a plurality of single color toner images are superimposed on a body, and transferring the multicolor toner image on the intermediate transfer body to a recording material,
When a patch image is formed on the image carrier, the formed patch image is transferred to the intermediate transfer member, and the density of the patch image on the intermediate transfer member is detected to form an image on a recording material. In the color image forming method for controlling the image forming conditions of
According to the density information of the patch image on the intermediate transfer member, the control level can be selected and set from a plurality of levels, and at least when the high density control level is set, the high density control is performed. A color image forming method, wherein a fog margin | V0−Vdc | in a developing process of an image forming unit downstream of an image forming unit set at a level is set higher than a standard value .
However, V0 is the unexposed portion potential of the image carrier, and Vdc is the potential of the DC component of the developing bias in the developing step . By performing a toner image forming step of forming a single color toner image on the image carrier and transferring the single color toner image on the image carrier to an intermediate transfer member a plurality of times by a plurality of image forming units , the intermediate transfer An image forming method of forming a multicolor toner image in which a plurality of the single color toner images overlap on a body, and transferring the multicolor toner image on the intermediate transfer body to a recording material,
An image when a patch image is formed on the image carrier, the patch image is transferred to the intermediate transfer member, and the density of the patch image on the intermediate transfer member is detected to form an image on a recording material. In the color image forming method for controlling the forming conditions,
According to the density information of the patch image on the intermediate transfer member, the control level can be selected and set from a plurality of levels, and at least when the high density control level is set, the high density control is performed. A color image forming method, wherein the frequency of the AC component of the developing bias in the developing process of the image forming unit downstream of the image forming unit set to the level is set higher than a standard value .   The color image forming method according to claim 8, wherein the density information is obtained from an operation unit of a color image forming apparatus.   The color image forming method according to claim 8, wherein the density information is obtained by detecting a density of the patch image on the intermediate transfer member.

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