JPH11231605A - Color image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide such a color image forming device that a sufficient primary transfer is obtainable even by using an intermediate transfer body consisting of an insulator and a semi conductive body. SOLUTION: A color image forming device, which successively and primarily transfers plural toner images formed on an image carrier bodies for each color element onto an intermediate transfer body, and thereby secondarily transfers the superimposed toner image formed on the intermediate transfer body, has a control means for controlling a primary transfer bias so that the transfer potentials at the time of the above-mentioned primary transfers are successively increased for each color element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer and a facsimile, and more particularly to a color image forming apparatus using an intermediate transfer member.

[0002]

2. Description of the Related Art An image forming apparatus, such as a color copying machine, is known in which a toner image of each color component formed on a photoreceptor is temporarily superimposed on an intermediate transfer body and then transferred onto a transfer material. The image forming process of such an image forming apparatus includes:
It is mainly as follows. First, for example, the surface of a belt-shaped photoconductor is charged by a DC electric field such as corona discharge by a main charger. The color-separated optical image of the document is projected onto the surface of the photoreceptor to form an electrostatic latent image. The electrostatic latent image is developed with a first color toner charged to the opposite polarity to the surface of the photoconductor, and a first color toner image is formed on the surface of the photoconductor. The formed first color toner image is primarily transferred to, for example, a belt-shaped intermediate transfer member that is in contact with the photoconductor and is charged with a polarity opposite to that of the toner. After the primary transfer of the first color toner image, the first color toner remaining on the photoreceptor surface is cleaned,
The surface of the photoconductor is neutralized. Thus, the development / primary transfer step for the first color toner is completed. This step is repeated for the toners of the second color or lower, and finally, on the intermediate transfer member, a superimposed toner image is formed in which toner images of the respective color toners necessary to complete the color image are sequentially superimposed. Is done. The superimposed toner image is secondarily transferred to a transfer material charged to a polarity opposite to that of the toner and subjected to a fixing process, whereby a color image is formed on the transfer material.

However, as described above, when the second color toner image and the third color toner image are further superimposed sequentially on the intermediate transfer member on which the first color toner image has been primarily transferred to form a superimposed toner image. Since these toners are all charged to the same polarity, they repel each other, causing image defects such as toner scattering around characters and lines (image bleeding), and The image quality may be degraded due to the occurrence of missing portions due to the reverse transfer of the toner onto the photoconductor. In particular, in the portion where the toners overlap, the potential on the intermediate transfer body side increases each time the primary transfer of each color toner is performed. (The expression of the magnitude of the potential in the present specification does not take the sign into consideration, and the absolute value of the magnitude is not considered. This is based on the increase / decrease.) Therefore, the possibility of occurrence of the above-described image defect further increases.

In order to avoid the above-mentioned image defects,
Japanese Patent Application Laid-Open No. 183276/1990 discloses that “a toner image of each color component formed on a photoconductor is temporarily superimposed on an intermediate transfer medium,
Means for transferring a toner image of each color component from a photoreceptor to the intermediate transfer medium in a full-color copying apparatus of a system for transferring to paper, so that a transfer potential in a final transfer stage is higher than a potential in a transfer stage immediately before; A means for applying a predetermined voltage to the intermediate transfer medium during the transfer to each of the transfer steps. In the full-color copying apparatus, the transfer potential in the final transfer stage (when the last toner image is primarily transferred) at which the transfer efficiency is lowest among the primary transfer of the toner image from the photoreceptor to the intermediate transfer member is increased to increase the transfer efficiency. Is considered to cover the decline.

However, if the same transfer potential as that of the first color is applied in the transfer stage of the second color and the third color in the full color copy mode, especially when an insulator is used as the intermediate transfer body, after the primary transfer stage of each color, Since the potential of the image portion on the intermediate transfer member includes the charging potential of the intermediate transfer member itself in addition to the toner potential, there is an inconvenience that a transfer failure may occur depending on the conditions of the intermediate transfer member.

In the image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 2-183276, the potential of the image portion and the potential of the non-image portion on the intermediate transfer body at the end of the intermediate transfer stage for the second and third colors are determined. , The occurrence of primary transfer dust cannot be sufficiently prevented.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present invention is directed to an image forming apparatus capable of obtaining a sufficient primary transfer even when an intermediate transfer member made of an insulator or a semiconductive material (semiconductor) is used, particularly, a color image forming apparatus. It is a first object to provide an image forming apparatus, and to further effectively prevent the occurrence of transfer dust during primary transfer in such an image forming apparatus.

[0008]

According to the first object of the present invention, a plurality of toner images formed on an image carrier for each color component are sequentially primary-transferred to an intermediate transfer member, respectively, thereby achieving the first object. In a color image forming apparatus in which a superimposed toner image superimposed and formed on an intermediate transfer body is further secondary-transferred to a transfer material, a primary transfer bias is controlled so that a transfer potential at the time of the primary transfer sequentially increases for each color component. The problem is solved by a color image forming apparatus having a control unit that performs the control.

Further, the absolute value of the toner image portion potential Vt on the surface of the intermediate transfer member after the primary transfer of the toner image of each color component is smaller than the absolute value of the background portion potential Vb where no toner is attached. Alternatively, even if the absolute value of the toner image portion potential is larger, the control means may control each color component so that the difference is smaller than a predetermined value α, that is, | Vt | − | Vb | ≦ α. The second problem is solved by controlling the primary transfer bias at the time of the primary transfer of the toner image. At this time, the predetermined value α is selected so that the occurrence of transfer dust can be suppressed to an allowable range. In particular, it is advantageous to set α = 200 (V).

Further, it is advantageous to control the primary transfer bias in accordance with the relationship between the image area of the image to be formed and the toner adhesion amount. A color image forming apparatus capable of switching image forming modes according to the type of image to be formed, for example, at least a character mode which is an image forming mode mainly for characters and lines, and an image mainly for solid or halftone In a color image forming apparatus having a photographic mode, which is a forming mode, it is preferable to control the primary transfer bias for each selected image forming mode to switch the transfer potential during the primary transfer. At this time, in the case of the character mode whose main purpose is to form an image having a large amount of toner adherence to the image area such as a character or a line, the photographic mode whose main purpose is to form an image including a lot of solids and halftones such as a photograph is used. It is particularly advantageous to make the primary transfer bias larger than in the case.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. Here, one embodiment of the present invention will be described using an example of an electrophotographic color copying machine (hereinafter, referred to as a “color copying machine”) that is a color image forming apparatus.

FIG. 1 is a schematic configuration diagram of an image forming section of a color copying machine according to the present embodiment. This color copier is
In addition to the image forming unit shown in FIG. 1, the image forming unit includes a color image reading unit (hereinafter, referred to as a color scanner) (not shown), a paper feeding unit, a control unit, and the like.

The color scanner converts color image information of a document into, for example, red, green, and blue (Red, Green, Blue,
(R, G, and B respectively) and read out for each color separation light and convert it into an electrical image signal. Then, based on the R, G, and B color separation image signal intensity levels obtained by the color scanner, a color conversion process is performed by an image processing unit (not shown) to obtain black (hereinafter, referred to as “Bk”), Cyan (C
yan, hereinafter “C”, magenta (Magenta, below “M”), yellow (Yellow, below “Y”)
Is obtained.

The image forming section shown in FIG. 1 includes a photosensitive drum 100 as an image carrier and a charger 2 as a charging unit.
00, a photoconductor cleaning device 300 including a cleaning blade and a fur brush, a writing optical unit (not shown) as an exposure unit, a revolver development unit 400 as a development unit, an intermediate transfer unit 500, a paper transfer unit 600, and a fixing roller pair 701. It is composed of the used fixing unit and the like.

The writing optical unit converts the color image data from the color scanner into an optical signal, and charges the photosensitive drum 100 uniformly charged by the charger 200.
The optical writing corresponding to the image of the original is performed on the surface of the photosensitive drum 100 to form an electrostatic latent image on the photosensitive drum 100. This writing optical unit is, for example, a semiconductor laser as a light source,
It can be composed of a laser emission drive control unit, a polygon mirror and its rotation motor, an f / θ lens, a reflection mirror, and the like.

The photosensitive drum 100 rotates counterclockwise as indicated by the arrow. Around the photoreceptor drum 100, a charging charger 200, a revolver developing unit 400, an intermediate transfer belt 501 as an intermediate transfer member of the intermediate transfer unit 500, and a photoreceptor cleaning device 300
Etc. are arranged.

The revolver developing unit 400 includes a Bk developing device 401 using Bk toner, a C developing device 402 using C toner, an M developing device 403 using M toner, a Y developing device 404 using Y toner, and the entire unit. It consists of a development revolver drive unit that rotates counterclockwise,
A developing device used for development at that time is a photoconductor 100.
Is brought to a position opposed to. Revolver developing unit 4
Each developing unit provided in the developing unit 00 has a developing sleeve serving as a developer carrier that rotates by contacting a spike of the developer with the surface of the photosensitive drum 100 to develop an electrostatic latent image, and pumps the developer. And a developing roller drive unit for rotating the developing roller in a clockwise direction. In the example shown here, the toner in each developing device is charged to a negative polarity by stirring with the ferrite carrier, and a negative DC voltage Vdc ( A developing bias voltage in which an AC voltage Vac (AC component) is superimposed on a DC component) is applied, and the developing sleeve is biased to a predetermined potential with respect to the metal base layer of the photosensitive drum 100.

In a standby state of the main body of the color copying machine, the revolver developing unit 400 is stopped at a home position where the Bk developing unit 401 is located at a developing position. Starts reading of Bk color image data, and based on this color image data, optical writing with laser light and formation of an electrostatic latent image are started (hereinafter, the electrostatic latent image based on the Bk image data is referred to as “Bk electrostatic latent image”). The same applies to C, M, and Y).
Before the front end of the electrostatic latent image reaches the developing position, the Bk developing sleeve starts rotating and the Bk electrostatic latent image is developed with Bk toner so that development can be performed from the front end of the Bk electrostatic latent image. I do. Thereafter, the developing operation of the Bk electrostatic latent image area is continued, but when the rear end portion of the Bk electrostatic latent image passes the developing position, the revolver is quickly moved until the developing device of the next color comes to the developing position. The developing unit 400 rotates. This is completed at least before the leading end of the electrostatic latent image based on the next image data arrives.

The intermediate transfer unit 500 includes an intermediate transfer belt 5 as an intermediate transfer member stretched over a plurality of rollers.
01 or the like. Around the intermediate transfer belt 501, a pre-transfer charger (hereinafter, referred to as “PTC”) 502 as a pre-transfer charging unit, a paper transfer unit 60
And a secondary transfer bias roller 60 as a secondary transfer charge applying unit.
5. A brush roller 5 serving as a conductive brush member which also serves as an intermediate transfer member static elimination unit and an intermediate transfer unit cleaning unit
14. Lubricant application brush 505 as lubricant application means
Are disposed so as to face each other.

The intermediate transfer belt 501 includes a primary transfer bias roller 507 as a primary transfer charge applying means, a belt driving roller 508, a belt tension roller 509, a secondary transfer opposed roller 510, and a cleaning opposed roller 51.
1 and a belt neutralizing roller 512 as a neutralizing means before primary transfer. Each roller is formed of a conductive material, and each roller other than the primary transfer bias roller 507 is grounded. The primary transfer bias roller 507 includes
A primary transfer power supply 801 controlled by a constant current or a constant voltage supplies a transfer bias controlled to a predetermined magnitude of current or voltage according to the number of superimposed toner images. Further, the intermediate transfer belt 501 is driven by a belt drive roller 508 that is rotationally driven by a drive motor (not shown).
Driven in the direction of the arrow.

As shown in the sectional view of FIG. 2, the intermediate transfer belt 501 is formed of a belt material having a multilayer structure including a surface layer 501a, an intermediate layer 501b, and a base layer 501c. The outer peripheral surface of the belt in contact with the photosensitive drum 100 is the surface layer 501a, and the inner peripheral surface of the belt is the base layer 501.
c. Further, an adhesive layer 501d for bonding the two layers is interposed between the intermediate layer 501b and the base layer 501c.

In a transfer section for transferring the toner image on the photosensitive drum 100 to the intermediate transfer belt 501 (hereinafter, referred to as a “primary transfer section”), the primary transfer bias roller 507 and the belt discharging roller 512 expose the intermediate transfer belt 501 to light. A nip portion having a predetermined width is formed between the photosensitive drum 100 and the intermediate transfer belt 501 by being stretched so as to be pressed against the body drum 100 side. In this nip portion, a grounded belt neutralization brush 513 as a primary transfer unit neutralization means is provided on the inner peripheral surface of the intermediate transfer belt 501.
Is in contact.

As shown in FIG. 3, the nip width Wn of the primary transfer portion and the distance L from the downstream end of the nip portion in the belt moving direction to the contact position of the belt neutralizing brush 513 are:
The settings are made so as to obtain predetermined transfer conditions.

The PTC 502 uniformly charges the toner image on the intermediate transfer belt 501 transferred from the photosensitive drum 100 in the primary transfer section before transferring the toner image to transfer paper as a transfer material. belongs to.

The conductive brush roller 514 serves to remove and clean the intermediate transfer belt 501 after the secondary transfer has been performed, and has the same potential as the surface potential of the intermediate transfer belt 501 after the secondary transfer. A DC voltage having a polarity is applied by a power supply 804 for static elimination. This DC voltage
An AC voltage may be superimposed to increase the charge removal efficiency.
The conductive brush roller 514 can switch between contact with the intermediate transfer belt 501 and separation from the belt by a separation / contact mechanism (not shown).

The brush roller 514 is turned on as follows.
/ OFF controlled. For example, when one full-color image is formed, the brush roller 514 is brought into contact at least until the intermediate transfer belt 501 makes one rotation after the completion of the secondary transfer. In the case of forming a full-color image repeat, after the secondary transfer is completed, the brush roller 514 is brought into contact until the leading end of the next toner image reaches the neutralization position and the cleaning position, respectively. In the case of forming a single monocolor image, the brush roller 514 is brought into contact with the intermediate transfer belt 501 at least until the intermediate transfer belt 501 makes one rotation after the primary transfer. In the case of forming a monochromatic image repeat, after the primary transfer is completed, the brush roller 514 is brought into contact until the leading end of the next toner image reaches the neutralization position and the cleaning position, respectively.

The lubricant application brush 505 is for polishing zinc stearate 506 as a lubricant formed in a plate shape and applying the polished fine particles to the intermediate transfer belt 501. The lubricant application brush 505 is also configured to be able to contact and separate from the intermediate transfer belt 501, and is controlled so as to contact the intermediate transfer belt 501 at a predetermined timing.

The paper transfer unit 600 includes a secondary transfer belt 60 stretched around three support rollers 602 to 604.
1 and so on. The stretching portion between the supporting rollers 602 and 603 of the secondary transfer belt 601 is
No. 1 can be pressed against the portion located at the secondary transfer facing roller 510. Three support rollers 602
Reference numeral 604 denotes a driving roller that is rotationally driven by a driving unit (not shown), and the driving roller drives the secondary transfer belt 601. The secondary transfer bias roller 605 is disposed so as to sandwich the intermediate transfer belt 501, the transfer paper, and the secondary transfer belt 601 between the secondary transfer bias roller 510 and the secondary transfer opposing roller 510. A transfer bias of a predetermined current is applied by a power supply 802. Further, the supporting roller 602 and the secondary transfer bias roller are arranged so that the secondary transfer belt 601 and the secondary transfer bias roller 605 can take a position where the secondary transfer belt 601 is pressed against the secondary transfer facing roller 510 and a position where the secondary transfer belt 601 is separated from the secondary transfer opposing roller 510. 60
5 is provided. The secondary transfer belt 601 at the separated position is indicated by a two-dot chain line in FIG.

A portion of the secondary transfer belt 601 wrapped around the support roller 603 on the side opposite to the fixing roller is provided with a transfer sheet charge removal unit 606 as a transfer material charge removal unit and a belt charge removal unit as a transfer material carrier charge removal unit. 60
7 are facing each other. A cleaning blade 608 as a transfer material carrier cleaning unit is in contact with a portion of the secondary transfer belt 601 that is wound around the lower support roller 604 in the figure.

The transfer paper discharging charger 606 discharges the charge held on the transfer paper so that the transfer paper can be satisfactorily separated from the secondary transfer belt 601 by the strength of the transfer paper itself. It is. The belt static eliminator 607 includes a secondary transfer bias roller 605.
And removes the charge remaining on the secondary transfer belt 601. In addition, the cleaning blade 6
Reference numeral 08 denotes a device that removes and adheres to the surface of the secondary transfer belt 601 for cleaning.

When the image forming cycle is started in the color copier having the above structure, first, the photosensitive drum 100 is rotated in the counterclockwise direction by the drive motor (not shown), and the intermediate transfer belt 501 is rotated clockwise by the arrow. Is rotated by a driving roller. Intermediate transfer belt 50
With the rotation of 1, the Bk toner image formation, the C toner image formation, the M toner image formation, and the Y toner image formation are performed, and finally Bk, C,
A toner image is formed on the intermediate transfer belt 501 in the order of M and Y.

The formation of the Bk toner image is performed as follows. The charger 200 uniformly charges the photosensitive drum 100 with a negative charge to approximately a predetermined potential by corona discharge.
Then, raster exposure is performed by the writing optical unit based on the Bk color image signal. When this raster image is exposed, the exposed portion of the photosensitive drum 100 that is initially uniformly charged loses charge proportional to the amount of exposure light, and Bk
An electrostatic latent image is formed. Then, when the negatively charged Bk toner on the Bk developing roller comes into contact with the Bk electrostatic latent image,
The toner does not adhere to the portion of the photosensitive drum 100 where the charge remains, and the portion without the charge, that is, the exposed portion,
The Bk toner is attracted, and a Bk toner image similar to the electrostatic latent image is formed. Then, the Bk formed on the photosensitive drum 100
The toner image is transferred onto the surface of the intermediate transfer belt 501 that is driven at a constant speed in contact with the photosensitive drum 100.
Hereinafter, the transfer of the toner image from the photosensitive drum 100 to the intermediate transfer belt 501 is referred to as “belt transfer”.

A small amount of untransferred residual toner remaining on the surface of the photosensitive drum 100 after the belt transfer is cleaned by the photosensitive member cleaning device 300 in preparation for reuse of the photosensitive drum 100.

On the photosensitive drum 100 side, the process proceeds to the C image forming process after the Bk image forming process, and at a predetermined timing, the reading of the C image data by the color scanner is started.
A C electrostatic latent image is formed by laser light writing using image data. Then, after the trailing end of the previous Bk electrostatic latent image has passed and before the leading end of the C electrostatic latent image has reached, the rotating operation of the revolver developing unit is performed, and the C developing device 402 is moved to the developing position. And the C electrostatic latent image is developed with C toner. Thereafter, the development of the C electrostatic latent image area is continued, but when the rear end of the C electrostatic latent image passes, the rotation operation of the revolver developing unit is performed as in the case of the Bk developing device 401, The next M developing device 403 moves to the developing position. This is also completed before the leading end of the next M electrostatic latent image reaches the developing position.

The M and Y image forming steps are as follows.
Each color image data reading, electrostatic latent image formation,
The development operation is the same as the above-described Bk and C steps, and a description thereof will be omitted.

The Bk, C, M, and Y toner images sequentially formed on the photosensitive drum 100 are transferred onto the intermediate transfer belt 501 while being sequentially aligned on the same surface. As a result, a toner image (superimposed toner image) in which a maximum of four colors are superimposed is formed on the intermediate transfer belt 501. The superimposed toner image on the intermediate transfer belt 501 is a PTC5
After being uniformly charged at 02, they are collectively transferred to transfer paper in the next secondary transfer step.

At the time when the image forming operation is started, the transfer paper is fed from a paper feed unit such as a transfer paper cassette (not shown) or a manual feed tray, and stands by at a nip of a pair of registration rollers (not shown). Then, the secondary transfer facing roller 51
When the leading end of the toner image on the intermediate transfer belt 501 approaches the secondary transfer portion where the nip is formed by the zero and secondary transfer bias rollers, the registration is performed so that the leading end of the transfer paper coincides with the leading end of the toner image. The roller pair is driven, and registration of the transfer sheet with the toner image is performed. Then, the transfer paper is superimposed on the toner image on the intermediate transfer belt 501 and passes through the secondary transfer portion. At this time, the four-color superimposed toner image on the intermediate transfer belt 501 is collectively transferred onto the transfer paper by the transfer bias formed by the transfer bias applied to the secondary transfer bias roller. And
When passing through a portion facing a transfer sheet static elimination charger 606 disposed downstream of the secondary transfer portion in the secondary transfer belt moving direction, the transfer sheet is neutralized, and the intermediate transfer belt 501 is removed.
From the fixing roller pair 701. Then, the toner image is fused and fixed at the nip portion of the fixing roller pair 701, and sent out of the apparatus main body by a discharge roller pair (not shown).

On the other hand, the surface of the intermediate transfer belt 501 after the transfer of the toner image onto the transfer paper is applied to a brush roller 51 pressed against the intermediate transfer belt 501 by a contact / separation mechanism (not shown).
4 and the cleaning is performed.

Here, in the case of a repeat copy, the operation of the color scanner and the image formation on the photosensitive drum 100 follow the image formation process of the first sheet of the fourth color (Y).
At a predetermined timing, the process proceeds to the image forming process for the first color (Bk) of the second sheet. Further, the intermediate transfer belt 501 transfers the second Bk toner image to the area of the front surface cleaned by the brush roller 514, following the batch transfer process of the first four-color superimposed toner image onto the transfer paper. Is transferred to the belt. After that, the operation is the same as that of the first sheet.

The above is an example of obtaining a four-color full-color copy. In the case of three-color copy or two-color copy, the same operation as described above is performed for the designated color and the number of times.

In the apparatus described here as an example, the brush roller 514 in which the charge removing means and the cleaning means for the intermediate transfer belt are integrated is used. Of course, the charge removing means and the cleaning means for the intermediate transfer belt are conventionally used. May be provided individually.

Also, the intermediate transfer belt 501 after the secondary transfer
The condition for removing the charge by the brush roller 514 may be switched according to the surface potential of the intermediate transfer belt 501 so that the charge on the intermediate transfer belt 501 can be removed uniformly. For example, the intermediate transfer belt 501
The magnitude of the DC component of the output voltage of the power supply 804 is switched in accordance with the number of times the toner image is superimposed on the power supply.

As described above, the primary transfer bias roller 5
To 07, a transfer bias controlled to a predetermined magnitude of current or voltage is supplied from a primary transfer power supply 801 controlled by a constant current or a constant voltage. At this time, the potential of the surface of the intermediate transfer member after the first color transfer during the second color transfer, the potential of the surface of the intermediate transfer member after the second color transfer during the third color transfer, etc. By applying a large primary transfer bias for the primary transfer of the next color in consideration of the above, it is possible to prevent a transfer failure from occurring even when the insulator is used as an intermediate transfer body, and to achieve a more efficient primary transfer. Transfer becomes possible. For example, during full-color copying, the transfer potential of the first color from the image bearing member to the intermediate transfer member is
Assuming that V1 and the transfer potential of the second color are V2, and the transfer potentials of the third and fourth colors are V3 and V4, respectively, at the time of the primary transfer operation from the image carrier to the intermediate transfer body, for example, if V1 = 1 kV, then V2 , V3 are 1.2 kV and 1.4 kV, respectively, so that the transfer potential of each transfer step is higher than the transfer potential of the immediately preceding transfer step (that is, V1 <V2 <V3 <V4).

In what is called primary transfer dust, the absolute value of the toner image portion potential Vt on the intermediate transfer member is larger than the absolute value of the background portion potential (non-image portion potential where no toner is attached) Vb of the intermediate transfer member. Larger ones tend to occur (eg, FIG. 4a). on the other hand,
If the absolute value of the background portion potential is larger than the absolute value of the toner image portion potential, naturally the primary transfer dust is less likely to occur (for example, FIG. 4B). However, in order to suppress the occurrence of the primary transfer dust, it is not always necessary to make the absolute value of the background portion potential larger than the absolute value of the toner image portion potential. Even if the absolute value of the toner image portion potential is larger, As long as the difference between the toner image portion potential and the background portion potential is made smaller than the predetermined value α (for example, FIG. 4C), the primary transfer dust can be efficiently suppressed. Therefore, when the condition capable of suppressing the primary transfer dust is represented by a mathematical expression, | Vt | − | Vb | ≦ α. In the prior art, it was difficult to sufficiently suppress transfer dust due to a large potential difference between Vt and Vb. The color image forming apparatus according to the present invention has control means for setting the potential difference between Vt and Vb to a value smaller than α, so that the occurrence of transfer dust can be suppressed.

In the present embodiment, an example in which the photosensitive drum 100 is used as an image carrier has been described. However, the present invention can be applied to an image carrier having another shape. For example, the present invention can be applied to an apparatus using a photosensitive belt that is stretched between two rollers and moves endlessly.

Further, the present invention can be applied to a device using an intermediate transfer member such as an intermediate transfer drum having a shape other than the belt as the intermediate transfer member. The electrical characteristics (volume resistivity, surface resistivity, etc.), thickness, and structure (single-layer, two-layer,
Various suitable materials can be selected and employed depending on the image forming conditions and the like.

The present invention can be applied not only to a device using a primary transfer bias roller as in the present embodiment, but also to a device employing a primary transfer charge applying device having another shape as in the present embodiment. it can. At this time, if the discharge position is on the downstream side of the neutralization position by the primary transfer neutralization means such as the neutralization brush in the moving direction of the intermediate transfer belt, the primary transfer charge may be applied within the nip of the primary transfer portion.

As the secondary transfer charge applying means, a member having another shape such as a blade or a brush may be used instead of the secondary transfer bias roller. Here, the case where the charging potential of the photoconductor drum 100 is negative and the developing device adopting the reversal development method using the two-component type developer is described. The present invention is not limited to those having a negative potential, and can be similarly applied to those using a one-component developer or those employing a regular development method.

[0049]

Next, an embodiment in which the present invention is applied to the above color copying machine will be described.

The intermediate transfer belt 501 has a thickness of 0.1
Using an intermediate transfer belt having a width of 5 mm, a width of 368 mm, and an inner peripheral length of 565 mm, the moving speed of the intermediate transfer belt 501 is set to 200.
mm / sec. Surface layer 50 of intermediate transfer belt 501
1a is formed of an insulating layer having a thickness of about 1 μm,
01b is made of PVDF (polyvinylidene fluoride) and has a thickness of 7
The base layer 501c is formed of an insulating layer of about 5 μm (volume resistivity: about 10 ¹³ Ωcm), and the base layer 501c is a medium-resistance layer of about 75 μm (volume resistivity: 10 ⁸ ) made of PVDF and titanium oxide.
To 10 ¹¹ Ωcm). The volume resistivity of the entire intermediate transfer belt formed of such a material is 10 ⁷ to 10 ¹⁴ Ωc.
m. (The above volume resistivity values are in accordance with JIS K 691
Using the measurement method described in 1, the voltage of 100 V
It was measured by applying for 0 seconds. The surface resistivity of the surface of the intermediate transfer belt 501 on the side of the surface layer 501a is measured using a resistance measurement device “Hirester IP” manufactured by Yuka Denshi.
Was 10 ⁷ to 10 ¹⁴ Ωcm. The surface resistivity can be measured by using the above-mentioned resistance measuring instrument, JIS
It can also be measured by the surface resistance measurement method described in K6911. Also, the primary transfer bias roller 50
7 was a nickel-plated metal roller, a metal roller was used as the ground roller 512, and a metal roller or a conductive resin roller was used as the other rollers.
For example, the primary transfer bias roller 507
1 kV was applied to the primary transfer of the Bk toner image, and a DC bias was applied so that the transfer bias gradually increased for the primary transfer of the second and subsequent color toner images.

The nip width Wn of the primary transfer portion was set to 15 mm, and the distance L from the downstream end of the nip portion in the belt moving direction to the contact position of the belt neutralizing brush 513 was set to 7 mm (see FIG. 3). Further, as the belt neutralizing brush 513, a conductive brush having carbon-containing resin fibers implanted was used.

As the PTC 502, a charger with a grid electrode was used. For this PTC 502, PTC
The power supply 803 applied a DC bias voltage having the same polarity as the charging polarity of the toner image on the intermediate transfer belt 501.
More specifically, a constant current controlled DC voltage of -500 μA is applied to the main wire 502a of the PTC 502,
A DC voltage set within the range of 0 to -3 kV was applied to the grid electrode 502b.

The intermediate transfer belt 5 after the secondary transfer
As the brush roller 514 for removing and cleaning 01, a brush having conductive brush fibers implanted on a metal shaft was used. An intermediate transfer belt 50 before static elimination is attached to this shaft.
DC voltage (positive voltage) having a polarity opposite to the surface potential of No. 1 was applied. The conductive brush roller 51 to which the predetermined voltage is applied
By using No. 4, even when the intermediate transfer belt 501 having a multilayer structure including the intermediate layer 501b having a high resistance as in the present embodiment is used, the phenomenon that the charge once discharged appears again on the surface is not accompanied. In addition, the neutralization of the intermediate transfer belt 501 after the secondary transfer can be favorably performed.

As the secondary transfer bias roller 605, a roller having a surface layer made of conductive sponge or conductive rubber and a core layer made of metal or conductive resin is used. A transfer bias controlled by a constant current was applied. Also, the secondary transfer belt 6
As 01, a belt material formed of PVDF and having a thickness of 100 μm and a volume resistivity of 10 ¹³ Ωcm was used.

The transfer paper static elimination charger 606 may be operated by an AC power supply or an AC + DC power supply (not shown).
Using a discharger to which a voltage is applied, a belt neutralization charger 6
As 07, a discharger to which only an AC voltage or an AC + DC voltage was applied by a power supply (not shown) was used. Further, the cleaning blade 608 is brought into contact with a portion of the secondary transfer belt 601 that is wrapped around the support roller 604 by a counter method.

Control conditions for primary transfer bias (transfer conditions)
The following two were used for comparison. First, as the transfer condition a, when transferring the Bk toner of the first color,
V1 = 1 kV, V2 = 1.2 kV at the time of C toner transfer of the second color,
V3 = 1.4 kV during transfer of M toner of the third color, Y of the fourth color
V4 = 1.6 kV was applied to the primary transfer bias roller 507 during toner transfer. As the transfer condition b, the first color Bk
V1 = 1kV during toner transfer, V2 during second color C toner transfer
= 2 kV, V3 = 3 kV during the transfer of the third color M toner, and V4 = 4 kV during the transfer of the fourth color Y toner to the primary transfer bias roller 507.

When a DC bias is applied to the primary transfer bias roller 507 under the above respective transfer conditions, the potential of the toner image portion on the intermediate transfer belt 501 after the primary transfer from the first color to the fourth color is performed. Vt and the non-image portion potential Vb where no toner is applied are shown in FIGS. 5 and 6 (that is, after Bk toner transfer, after C toner transfer, after M toner transfer,
Vt and Vb after Y toner transfer are plotted). FIG. 7 shows the potential difference (| Vt | − | Vb |) between the toner image portion and the non-image portion on the surface of the intermediate transfer belt 501 after the primary transfer from the first color to the fourth color under both transfer conditions. ). 5 and 6 that the absolute value of the potential of the surface of the intermediate transfer member increases with the progress of the primary transfer stage (that is, each time the primary transfer of the toner image of each color is performed). The degree of the increase depends on the transfer conditions.
As can be seen from FIG. 7, in either transfer condition,
Up to the primary transfer stage of the C toner image of the second color, the absolute value of the toner image portion potential is smaller than the absolute value of the non-image portion potential, so that the occurrence of primary transfer dust can be sufficiently suppressed. However, under the transfer condition a, the relationship between the toner image portion potential and the non-image portion potential is reversed after the primary transfer of the third color M toner image, and the absolute value of the toner image portion potential is non- It becomes larger than the absolute value of the image portion potential. Therefore, in order to suppress the occurrence of the primary transfer dust when performing four-color full-color copying, the primary transfer bias for the primary transfer from the third color onward is set such that the potential difference therebetween is smaller than a predetermined value α. Must be controlled as follows. On the other hand, in the case of the transfer condition b, the absolute value of the non-image portion potential can be made larger than the absolute value of the toner image portion potential even after the primary transfer of the third color. Even after the transfer, the potential difference can be made substantially zero. Therefore, controlling the primary transfer bias in accordance with the transfer condition b can suppress the primary transfer dust more advantageously than using the transfer condition a in that the potential difference can be reduced. Further, in the case of four-color full-color copying as discussed in the present embodiment, it can be seen that the transfer condition b is more advantageous as the control condition for reducing the primary transfer dust than the transfer condition a.

If the transfer potential is increased to suppress the primary transfer dust, the surface potential of the intermediate transfer member is increased, so that there is a possibility that the inconvenience of imposing a burden on the charge removing section may occur. is there. However, such an inconvenience can be avoided by controlling the transfer potential according to the image to be formed according to the advantageous configuration of the present invention. For example, when the image to be formed has a large amount of toner adherence to the image area and is severe against transfer dust, such as characters and lines, the transfer potential is positively changed more aggressively. It is preferable to suppress the transfer dust.

FIG. 8 shows the potential difference (| Vt |) on the surface of the intermediate transfer member.
-| Vb |) and a graph showing the relationship between the occurrence of transfer dust and the like. The vertical axis is the rank evaluation value for the occurrence of transfer dust. As the value approaches 5, the transfer dust decreases. That is, a good image is obtained. When the rank evaluation value is 3 or more, the occurrence of transfer dust is at an acceptable level. Therefore,
The value of α described above can be 200 (V). That is, the condition that can suppress the primary transfer dust is represented by the following equation: | Vt | − | Vb | ≦ 200 (V).

A voltage of a primary transfer bias applied to a primary transfer charge applying means such as a primary transfer bias roller 507;
The values of the current and the like are not limited to those shown in the present embodiment, but can be set to appropriate values according to various image forming conditions.

[0061]

According to the first aspect of the present invention, in the color image forming apparatus, the primary transfer bias is such that the transfer potential when the toner image of each color component is primarily transferred from the image carrier to the intermediate transfer member is sequentially increased for each color. Therefore, the transfer of the toner image from the image carrier to the intermediate transfer member can be performed without causing transfer failure even when the insulator is used as the intermediate transfer member.

The color image forming apparatus according to claim 2 is
Further, since the primary transfer bias is controlled so that the toner image portion potential and the background portion potential on the surface of the intermediate transfer member after the primary transfer of the toner images of the respective color components satisfy a predetermined relationship, the intermediate transfer member is moved from the image carrier to the intermediate transfer member. The transfer dust generated at the time of the primary transfer to the toner can be more effectively and efficiently prevented.

Further, in the color image forming apparatus according to the third and fourth aspects, the primary transfer bias is controlled for each image forming mode, that is, according to the relationship between the image area of the image to be formed and the toner adhesion amount. Therefore, the above-described effect can be obtained without imposing a burden on the charge eliminating unit.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a color copying machine as one embodiment of the present invention.

FIG. 2 is a sectional view of an intermediate transfer belt used in the color copying machine shown in FIG.

FIG. 3 is an enlarged view of a primary transfer section of the color copying machine shown in FIG.

FIG. 4 is a diagram illustrating a relationship between a potential Vt of a toner image portion on the surface of an intermediate transfer member and a potential Vb of a non-image portion where no toner is applied. a is an example of a potential relationship in which primary transfer dust tends to occur, and b and c are potential relationships in which primary transfer dust can be suppressed.

FIG. 5 shows transfer conditions a (V1 = 1 kV, V2 = 1.2 kV, V3 =
(1.4 kV, V4 = 1.6 kV), intermediate transfer member surface potential (toner image portion potential Vt and background portion potential Vb) after execution of primary transfer for each color as the primary transfer step progresses
6 is a graph showing a change in the graph.

FIG. 6 shows transfer conditions b (V1 = 1 kV, V2 = 2 kV, V3 = 3 kV,
6 is a graph similar to FIG. 5 in the case of (V4 = 4 kV).

FIG. 7 is a graph showing a change in a potential difference (| Vt | − | Vb |) between a toner image portion and a background portion on the surface of an intermediate transfer member as the primary transfer step progresses under transfer conditions a and b. .

FIG. 8: Surface potential difference between transfer dust and intermediate transfer body (| Vt | − | Vb
It is a graph showing the relationship with |). The vertical axis shows the result of the rank evaluation. The closer the value is to 5, the less the dust, and the closer to 0, the more the dust.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 photoconductor drum 200 charging charger 300 photoconductor cleaning device 400 revolver developing unit 500 intermediate transfer unit 501 intermediate transfer belt 501a surface layer of intermediate transfer belt 501b intermediate layer of intermediate transfer belt 501c base layer of intermediate transfer belt 502 pre-transfer charger (PTC) ) 502a PTC main wire 502b Grid electrode 507 Primary transfer bias roller 512 Ground roller 513 Belt neutralizing brush 514 Conductive brush roller 600 Paper transfer unit 601 Secondary transfer belt 604 Support roller 605 Secondary transfer bias roller 606 Transfer paper neutralizing charger 607 Belt neutralization charger 608 Cleaning blade 701 Fixing roller pair 803 Power supply for PTC

Claims (4)

[Claims] 1. A plurality of toner images formed on an image carrier for each color component are sequentially primary-transferred to an intermediate transfer body, respectively.
Accordingly, in a color image forming apparatus which further secondary-transfers a superimposed toner image superimposedly formed on the intermediate transfer body to a transfer material, the primary transfer is performed such that the transfer potential at the time of the primary transfer sequentially increases for each color component. A color image forming apparatus comprising a control unit for controlling a bias. 2. The control means according to claim 1, wherein after the primary transfer of the toner image of each color component, the potential Vt of the toner image portion on the surface of the intermediate transfer member and the potential Vb of the background portion where no toner is attached are | Vt |-| Vb. 2. The color image forming apparatus according to claim 1, wherein a primary transfer bias at the time of primary transfer of the toner image of each color component is controlled so as to satisfy a relationship of | ≦ 200 (V). 3. The image processing apparatus according to claim 1, further comprising at least a character mode, which is an image forming mode mainly for characters and lines, and a photograph mode, which is an image forming mode mainly for solids and halftones. 3. The color image forming apparatus according to claim 1, wherein the control unit controls a primary transfer bias according to a selected image forming mode to switch a transfer potential at the time of primary transfer. 4. The color image forming apparatus according to claim 3, wherein the control unit increases the primary transfer bias in the character mode than in the photograph mode.