JPH0862922A - Image forming device - Google Patents

Abstract

PURPOSE: To provide an image forming device capable of using toner having small grain size without hindrance and fogging, and obtaining highly dense and extremely sharp image recording in a color image forming device forming an image by superposing toners of many colors on an image carrier by plural developing devices. CONSTITUTION: In this image forming device which is provided with plural developing devices performing reversal development by allowing the toner to fly under oscillating electric field, and allows many colors of toner to be superposed on the image carrier 1; the respective developing devices 4 satisfy following conditions. (1) |DC bias voltage E1 impressed on a developer carrier |<=|DC bias voltage E3 impressed on an electrode part |<| the maximum potential of an electostatic latent image on the image carrier | when the electrostatic latent image on the image carrier (1) is developed with toner loaded in the developing device 4, and (2) | the maximum potential of the electrostatic latent image on the image carrier |<=|DC bias voltage E3 impressed on the electrode part$ k when the electrostatic latent image on the image carrier 1 is developed with the toner loaded in the other developing devices 4.

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 an electrophotographic copying / recording apparatus, and more specifically, to an upstream side in the moving direction of the developer carrying body in a space where an image carrying body and a developer carrying body face each other. , A plate-shaped member having an electrode part is installed, and a two-component developer is supplied to the developer transport body, and a plurality of developing devices for performing reversal development by causing toner to fly under an oscillating electric field are provided, and on the image carrier The present invention relates to an image forming apparatus that superimposes multicolor toners.

[0002]

2. Description of the Related Art Various methods have been disclosed for forming a color image in a conventional electrophotographic copying / recording apparatus or the like.

For example, Japanese Patent Application Laid-Open No. 60-76766 discloses a color image recording method in which toner images are superposed on an image carrier by using a two-component developer in which toner and an insulating carrier are mixed. This method is very useful because it has features such that the recording apparatus is compact and low-cost, and that the toner charge control is easy because a two-component developer is used. However, the toner particle size conventionally used in this method is relatively coarse, 10 μm or more, and it is not possible to reproduce the density faithful to fine lines and originals, which is incomplete for obtaining high-quality images. there were. In order to solve this problem, atomization of the toner is indispensable, but it is known that various problems occur when a fine particle toner of 10 μm or less is used. The cause is (1)
Due to atomization, the Van der Waals force becomes relatively larger than the Coulomb force, and it becomes difficult to prevent fogging even by applying a DC bias voltage to the developer transport body. (2)
The atomization makes it difficult to uniformly charge the toner, and the toner may be fogged by a poorly charged toner. Various methods have been proposed to overcome such problems.

For example, in Japanese Unexamined Patent Publication No. 1-94368, a leveling member is set between the central portion of the developing area and a member for controlling the layer thickness of the developer, and the leveling member is provided with a charging polarity of toner particles. Discloses that a DC voltage of opposite polarity is applied as a bias voltage. In this method, since a bias having a polarity opposite to the charging polarity of the toner particles is applied to the leveling member, the toner adheres to the leveling member, which adheres to the image carrier to cause image stains or leveling. When the toner is fused to the member, vertical streaks occur in the image.

For example, in Japanese Patent Laid-Open No. 4-115264, developers of different colors are sequentially applied to an image carrier by a plurality of developing devices to form a visible image of the developers of a plurality of colors, and then the images are transferred. In the image forming apparatus, an oscillating electric field is formed between an electrode body and a developing carrier which are arranged in proximity to and in contact with the developer of the developer carrier and whose tip is located in the developing area. There is disclosed a method in which the developer is dispersedly ejected from the tip of the electrode body and supplied to the image carrier. In this method, since the electrode body for dispersing and flying the developer is also present in the upstream portion of the contact portion with the developer transport body, a fluctuating electric field similar to that in the developing area is formed during transport of the developer, and Will be returned to the upstream side. As a result, the amount of the developer that passes through the electrode body and is conveyed to the developing area is reduced, and the developability is poor, and the developer returned to the upstream side easily causes toner adhesion to the back surface of the electrode body. Is more likely to occur.

For example, in Japanese Unexamined Patent Publication No. 5-346736, a plate-shaped member having an electrode in the upstream portion of the developing area is brought into contact with a developer transport body, and a first vibration is applied between the electrode body and the developer transport body. A method is disclosed in which a second oscillating electric field is formed between the image forming body and the developer carrying body, and an electric field is formed between the image forming body and the electrode body to move the toner to the image forming body. Has been done. In this method, the absolute value of the DC bias voltage applied to the electrode plate is larger than the absolute value of the surface potential of the image forming body, which is good for electrode contamination, but the density of the solid portion of the latent image is There was a defect that it was low or the background portion of the latent image was easily fogged.

[0007]

As described above, the electrodes are installed in the developing space where the image carrier and the developer transport body face each other,
In an image forming apparatus having a plurality of developing devices that convey a two-component developer onto the developer conveying body, perform reversal development by an oscillating electric field, and superimpose multicolor toners on the image bearing body, the developing device is loaded into the developing device. When developing the electrostatic latent image on the image bearing member with the toner, the toner drawn into the developing space may adhere to the electrode section, or the electrostatic latent image on the image bearing member may be damaged by another developing device. When developing another image to develop an image, there is a problem that a toner image on the image carrier previously developed flows backward to the electrode portion and adheres to the electrode portion to stain the recorded image.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above problems and provide an image forming apparatus capable of obtaining a clear recorded image with high density without fog.

[0009]

The above-described object is to install a plate-shaped member having an electrode portion on the upstream side in the moving direction of the developer transport body in the space where the developer transport body and the image carrier face each other. The image forming apparatus has a plurality of developing devices for supplying a two-component developer to a carrier and flying the toner under an oscillating electric field to perform reversal development. The developing device (1) applies a DC bias voltage applied to the developer carrier to the electrostatic latent image on the image carrier with the toner loaded in the developing device. DC bias voltage | <| maximum potential of electrostatic latent image on image carrier |, and (2) when electrostatic latent image on the image carrier is developed by toner loaded in another developing device. , | Maximum potential of electrostatic latent image on image carrier | ≤ | It can be achieved by the | flow bias voltage.

Further, in the non-image area where the electrostatic latent image to be developed by each developing device does not exist on the image carrier, | the maximum value of the surface potential of the image carrier | <| the DC bias voltage applied to the electrode portion It is a preferred embodiment to apply a bias voltage in which a DC bias voltage satisfying the condition | is superimposed on an AC bias voltage.

[0011]

In the present application, a plate-shaped member having an electrode portion is installed in the developing space where the image carrier and the developer transport body face each other, and the electrode portion has a DC bias voltage of the same polarity as the toner or the toner. Is applied with a DC bias voltage having the same polarity as the DC bias voltage, and an AC bias voltage having the DC bias voltage having the same polarity as the toner is applied to the developer carrier.

If the absolute value of the DC bias voltage applied to the electrode portion of the developing device to be developed is equal to or greater than the absolute value of the voltage applied to the developer transport body, the DC electric field tends to press the toner against the sleeve. The converted toner is present on the sleeve side more than on the electrode side, so that toner stains on the electrode part can be removed.Furthermore, the DC bias voltage applied to the electrode part is the absolute value of the maximum potential of the electrostatic latent image on the image carrier. By making the size smaller than that, the fog toner flying to the background portion does not adhere to the image bearing member and is pulled back toward the developer carrying member, so that fogging can be prevented.

When developing the electrostatic latent image on the image bearing member by a developing device of another color, the absolute value of the DC bias voltage applied to the electrode portion is calculated from the absolute value of the maximum potential of the electrostatic latent image on the image bearing member. By also increasing the value, it is possible to prevent the backflow of the toner image formed on the image bearing member to the electrode portion.

Further, in the non-image area, by applying an AC bias voltage to the electrode portion, the toner adhering to the electrode portion can be peeled off, and the absolute value of the absolute value of the surface potential of the image carrier is higher than the maximum value. By increasing the absolute value of the DC bias voltage applied to the electrodes, the DC electric field tends to press the toner toward the image forming body side, and it is possible to prevent contamination more reliably.

[0015]

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

FIG. 1 is a block diagram of a modified DC9028 (manufactured by Konica Corporation) used as the image forming apparatus of this embodiment.
Reference numeral 1 denotes a photosensitive drum, which is an image carrier having an aluminum tube coated with a photoconductor, and is configured to rotate clockwise.

2 is a scorotron charger which is a charging means, and 3
Is an image writing device using a laser beam as an image exposing means, 4A, 4B, 4C and 4D are developing devices each containing a developer of a specific color, 5 is a transfer device, 6 is a cleaning device, and 8 is an image reading device. is there.

Each of the developing devices 4A, 4B, 4C, 4D
Is yellow (Y), magenta (M), cyan (C),
A developer containing black (K) developer is provided, and each developer carrier 41 having a predetermined gap from the photoconductor drum 1 is provided, and the latent image on the photoconductor drum 1 is visualized by non-contact reversal development. It has a mechanism to change. The developer transport body 41 is configured to rotate counterclockwise. The developing device 4
Details of A, 4B, 4C and 4D will be described later.

The transfer belt 5b of the transfer device 5 contains a transfer pole 5a and is kept at a position separated from the surface of the photosensitive drum 1 during image formation, and is transferred after the image formation on the photosensitive drum 1 is completed. Only when transferring to a material, the surface of the photosensitive drum 1 is contacted as shown in the figure.

The cleaning blade 6a and the toner carrying roller 6b of the cleaning device 6 are kept at a position separated from the surface of the photosensitive drum 1 during image formation, and are only shown at the time of cleaning after image transfer as shown in FIG. 1
Is pressed against the surface of. A color image is formed by the image forming apparatus as follows.

First, formation of a multicolor toner image according to the present embodiment starts by reading an image. That is, the halogen lamp 80 of the image reading device 8 illuminates the original document placed on the document table 7, and the reflected light is reflected by the first mirror 81, the second mirror 82, and the third mirror 83, and the lens 84, the dichroic prism. The color is separated into blue, red and green through 85. Blue is CCD 86 for blue, red is CCD 87 for red, green is C for green.
It is read on CD88. An analog signal from the CCD is converted into a digital signal by an A / D conversion board (not shown) and image processed.

When this image-processed digital signal is input to the image writing device 3, the semiconductor laser which is a writing light source (not shown) passes through a collimator lens and a cylindrical lens (not shown), and the drive motor 31
It is rotated and scanned by the rotating polygon mirror 34 rotated by
Mirror via fθ lens 32 and cylindrical lens 33
The light path is bent by 35 and is projected onto the peripheral surface of the photoconductor drum 1 to which uniform charge is applied by the scorotron charger 2 which is a charging means in advance, and main scanning is performed to form a bright line. An electrostatic latent image corresponding to the first color is formed on the peripheral surface of the photosensitive drum 1 by the main scanning by the laser beam and the sub-scanning by the rotation of the photosensitive drum 1.

This electrostatic latent image is reversely developed by the developing device 4A loaded with a two-component developer consisting of yellow (Y) toner and a magnetic carrier, and a toner image is formed on the photosensitive drum 1. The obtained toner image is the photosensitive drum 1.
After passing through the transfer device 5 and the cleaning device 6 which are separated from the photoconductor drum 1 while being held on the surface thereof, the next image forming cycle is started.

The photosensitive drum 1 is recharged by the scorotron charger 2, and then a digital signal corresponding to the second color is input to the image writing device 3.
Writing on the surface of the photosensitive drum 1 is performed in the same manner as in the case of the first color signal described above, and an electrostatic latent image is formed. The electrostatic latent image is a developing device 4B loaded with a two-component developer including a magenta (M) toner and a magnetic carrier as a second color.
Reverse development is performed by. This magenta (M) toner image is also formed on the previously formed yellow (Y) toner image.

4C is a developing device having a two-component developer composed of cyan (C) toner and a magnetic carrier, and a cyan (C) toner image is formed on the surface of the photosensitive drum 1 in the same manner as the first and second colors. Form. Further, 4D is a developing device having a two-component developer consisting of black toner and a magnetic carrier,
A black toner image is formed on the surface of the photoconductor drum 1 by the same processing as that for the color described above.

The multicolor toner image thus formed on the peripheral surface of the photosensitive drum 1 is sent from the paper feeding device 9 by applying a high voltage having a polarity opposite to that of the toner to the transfer pole 5a of the transfer device 5. It is configured to be transferred to a transfer material.

That is, the transfer material stored in the paper feeding device 9 is fed to the transfer device 5 at the same timing as the image formation of the photosensitive drum 1 by carrying out the uppermost one sheet by the rotation of the paper feeding roller 10. To be done.

The transfer material having undergone the image transfer in this way is separated from the photosensitive drum 1 along the transfer belt 5b of the transfer device 5. The transfer material separated from the transfer belt 5b by a paper removing electrode (not shown) is conveyed to a fixing device 12 by a conveying belt 11, and the fixing roller 12a of the fixing device 12 is conveyed.
After the image is welded by the sheet, it is ejected to the sheet ejection tray 13 via the sheet ejection roller 12b.

On the other hand, the photosensitive drum 1 which has been transferred to the transfer material is further rotated to bring the blade 6a of the cleaning device 6 and the toner carrying roller 6b into a pressure contact state,
The residual toner is removed, and after the end, the blade 6a is separated, and a little later, the toner conveying roller 6b is removed.
To start a new image forming process.

Next, the developing devices of 4A to 4D described above will be described in detail. The developing devices 4A to 4D have the same configuration, and will be denoted by reference numeral 4 below.
FIG. 2 is a schematic sectional view of the developing device 4 used in the present invention. Reference numeral 41 in FIG. 2 denotes a developer carrier which is rotatably supported by a sand blasting treatment of aluminum, stainless steel or the like, which is subjected to a rough surface treatment of 1-2 μm according to JIS 10 point average roughness (JIS-B0610), 43 Is a stirrer for stirring the developer 42 to make the components uniform, 44 is a supply roller for supplying the developer 42 to the developer transport body 41, and 46 is the thickness of the developer layer on the developer transport body 41. Reference numeral 47 denotes a developer transport restricting rod, and 47 denotes a scraper for scraping off the consumed developer 42 on the developer transport body 41. Reference numeral 45 is a plate-shaped member having an electrode portion, and is composed of a plate-shaped member 45a and an electrode portion 45b. In addition,
Details of the plate member 45 having the electrode portion will be described later.

The developer carrier 41 has a DC bias power source E1.
A bias voltage capable of superimposing an alternating current on a direct current is applied by the alternating current bias power source E2 via the protection resistor R1. Further, a bias voltage capable of superimposing an alternating current on a direct current is applied to the electrode portion 45b by a direct current bias power source E3 and an alternating current bias power source E4 via a protective resistor R2.

In FIG. 3, a bias in which an AC bias voltage is superimposed on a DC bias voltage is applied to the developer carrier 41, and a DC bias voltage is applied to the electrode portion 45b. 6 is an enlarged view of a developing space A of the developing device 4 when developing the electrostatic latent image of FIG. Electrode part 45b
Is installed closer to the developer transport body 41 than the photoconductor drum 1, so that the strength of the first oscillating electric field 48 becomes larger than the strength of the second oscillating electric field 49. The alternating voltage components of both oscillating electric fields have the same phase because they are generated by the same AC power source E2. The first oscillating electric field 48 causes the toner to fly and generate a toner cloud. The toner cloud flies to the electrostatic latent image on the photosensitive drum 1 by the second oscillating electric field 49 having the same phase as the first oscillating electric field 48, and development is performed.

In FIG. 4, the developer transport body in the full color mode of the embodiment of the image forming apparatus of the present invention is shown.
41, the timing of applying a bias voltage to the electrode portion 45
The timing of applying the bias to b will be described in detail.

FIG. 4A shows the timing of applying a bias voltage to the electrode portion 45b, and T1 indicates the time when the electrostatic latent image on the photosensitive drum 1 is developed by the toner loaded in the developing device ( (At the time of development), T2 is for developing an electrostatic latent image on the photoconductor drum 1 by the toner loaded in another development device (at the time of development of another color), and T3 is for each development device on the photoconductor drum 1. When there is no electrostatic latent image to be developed (non-image area), the electrostatic latent image on the photosensitive drum 1 is developed by yellow (Y), magenta (M), cyan (C), black ( It can be seen that development is performed in the order of K).

FIG. 4B shows the timing of applying a bias to the developer transport body 41. The solid line represents the DC bias component, and the broken line represents the AC bias component. During development of another color, it is preferable to stop the rotation of the developer transport body, stop the AC bias component applied to the developer transport body, and apply only the DC bias component in order to prevent image contamination in the non-image area. It is a usage form.

Referring to FIG. 5, the timing of applying the bias voltage to the developer carrier 41 and the timing of applying the bias to the electrode portion 45b in the monochrome mode of the embodiment of the image forming apparatus of the present invention will be described in detail. To do.

FIG. 5A shows the timing of applying a bias voltage to the electrode portion 45b, and the electrostatic latent image on the photosensitive drum 1 is developed only in black (K). T1 is for developing the electrostatic latent image on the photoconductor drum 1 by the black toner loaded in the developing device (during development), and T2 is for another color developing device (other color) when the black toner is being developed. During development), T
Reference numeral 3 represents a non-image area where there is no electrostatic latent image to be developed by the black developing device on the photosensitive drum 1.

FIG. 5B shows the timing of applying a bias to the developer transport body 41. The solid line represents the DC bias component, and the broken line represents the AC bias component. In developing devices and non-image areas other than black,
In order to prevent image stains, it is a preferable usage mode to stop the rotation of the developer transport body, and the AC bias component applied to the developer transport body 41 is stopped, and only the DC bias component is applied.

FIG. 4A, FIG. 5A and FIG.
5B, the deviation of the bias voltage application timing in the AC bias voltage of FIG. 5B represents the time difference depending on the position of the developing device.

In the non-image area indicated by T3, the toner adhering to the electrodes is discharged toward the photoconductor, so that the AC bias voltage is applied to the electrode portions 45b to peel off the toner adhering to the electrode portions 45b. By making the absolute value of the DC bias voltage applied to the electrodes larger than the maximum absolute value of the surface potential of the photoconductor drum 1, the DC electric field is in the direction of pressing the toner toward the photoconductor 1 side, and more reliably. It can prevent dirt. Further, the surface potential of the photosensitive drum 1 is preferably lowered by performing exposure, lowering the charging grid, or stopping charging in order to strengthen the DC electric field that presses the toner onto the photosensitive drum 1.

When the plate-like member 45 having the electrode portion is set in the developing space A, the angle θ (see FIG. 3) between the opposing position of the developer carrier 41 and the photosensitive drum 1 and the tip of the electrode portion is: Considering the developability, it is preferable that θ is large, but if it is too large, the effect of installing the electrode in the developing space A is lost, so θ is preferably an angle of 0 to 20 °. If the gap between the photosensitive drum 1 and the developer transport body 41 is small, the applied bias voltage can be reduced, but if it is too small, it becomes difficult to install the plate electrode 45 having the electrode portion, so 0.2 to 1.0. mm is preferred. Further, the distance between the developer transport body 41 and the electrode portion 45b is equal to the distance between the developer transport body 41 and the photosensitive drum 1.
The gap is preferably 0.2 to 0.6 times. If the ratio of the moving speed Vs (mm / sec) of the developer transport body 41 and the moving speed Vp (mm / sec) of the photosensitive drum 1 is large, the developability is improved, but if it is too large, the edge effect at the trailing edge of the image is produced. Therefore, the range of Vs / Vp = 0.5 to 4.0 is preferable. If the amount of the developer transported by the developer transport body 41 is large, the developing property is improved, but if it is too large, the carrier is attached to the photosensitive drum 1 due to the spikes, so that the amount is 5 mg / cm ² to 40 mg / cm ^2. It is preferably included in the range of ² .

The waveform of the AC voltage component is not limited to a rectangular wave, and may be a sine wave or a triangular wave. The higher the voltage value, the more the toner vibrates and the developability improves. However, if the voltage value is too high, the lightning strike phenomenon on the photosensitive drum 1 due to the dielectric breakdown and the fogging on the non-image area become remarkable. Therefore, the applied AC bias voltage (VACp-p) is preferably in the range of 200V to 2kHz. When the frequency is low, it is advantageous for the developability and gradation, but when it is too low, the toner previously developed on the photosensitive drum 1 is pulled back to the developing device and causes color mixing. Therefore, the frequency is preferably in the range of 500Hz to 20kHz. The occurrence of fogging is prevented by controlling the DC voltage component, and the dielectric breakdown is provided by providing an insulating or semi-insulating coating on the surface of the developer transport body 41, and by insulating the carrier of the developer. Can be prevented.

FIG. 6 shows the shape and arrangement of the plate-like member 45 having the electrode portion that can be used in the present invention. The plate-shaped member 45 having the electrode portion is provided to form an oscillating electric field that generates a toner cloud in the developing space A as described above. In the present application, the entire electrode portion 45b is arranged so as to be located on the downstream side in the rotation direction of the developer transport body 41 from the closest contact point P between the developer transport body 41 and the plate-shaped member 45a. With this configuration, the toner cloud does not occur on the upstream side of the closest contact P, and the toner concentration of the developer on the downstream side can be kept uniform. Figure 6
When the photosensitive drum 1 side in FIG. 6A is formed of an insulating layer as shown in FIG. 6B, it is possible to prevent current leakage to the photosensitive drum 1 and maintain a high voltage. It is preferable because it becomes possible. Also in FIG. 6A, the electrode portion
It is desirable to coat the exposed portion of 45b with an insulating layer of about 10 to 200 μm.

The plate-like member 45a is made of an insulating material having a thickness of about 50 μm to 400 μm. For example, polyethylene terephthalate, polystyrene, polypropylene, polycarbonate, polybutylene terephthalate, phenol resin, polyimide resin, polyamideimide resin, epoxy resin Polyether-based resins and the like are preferable, and it is more preferable to use glass fibers, carbon fibers, and the like in which the insulator is reinforced. Then, the electrode portion 45b is made of copper foil having a width of about 100 μm to 2000 μm in the developer conveying direction, and is supported by the plate member 45a.

In the above description, the plate member 45 having the electrode portion used in the developing device 4 of the image forming apparatus of the present application is described with reference to FIG. 6, but a plate member having a structure derived from this is described. Of course, the same can be applied to the image forming apparatus of the present invention.

In the developing device 4 used in the image forming apparatus of the present application, a preferable toner is a resin such as styrene resin, vinyl resin, acrylic resin, polyamide resin, silicone resin, polyester resin, fluororesin or epoxy resin. It can be prepared by adding a coloring component such as a color pigment or a color dye, a charge control agent and the like, and by the same method as a conventionally known method for producing toner particles. This is obtained by selecting particles having an average particle size of 20 μm or less, preferably 5 to 10 μm, by a conventionally known particle size selection method. The charge amount of the toner depends on the particle size, but is preferably 3 to 300 μC / g, and more preferably 10 to 50 μC / g.

The carrier may be a metal such as iron, chromium, nickel, cobalt, zinc or copper, or a compound or alloy thereof such as a ferromagnetic substance such as γ-ferric oxide, chromium dioxide, manganese oxide or ferrite. Or spherical particles of paramagnetic material, or the surfaces of these magnetic material particles are spherically coated with a resin such as styrene resin, vinyl resin, ethylene resin, acrylic resin, polyamide resin, polyester, or magnetic material. Particles obtained by forming spherical particles of a resin or fatty acid wax containing fine particles dispersed therein are used. Those having an average particle size of 70 μm or less, preferably about 30 to 50 μm are suitably used. If the average particle diameter is larger than this, unevenness appears in the toner image and the toner density becomes low, so that problems such as high-density development occur. If it is smaller than this, the photosensitive drum 1 together with the toner occurs. There is a problem of flying to the electrostatic latent image above. Further, it is more preferable that the carrier is spherical because the stirring property and the transport property of the toner and the carrier are improved and the charge controllability of the toner is also improved. Coating the carrier particles with a resin or the like has the effect of increasing the bias voltage applied to the developer transport body 41. Therefore, even if a high bias voltage is applied to the developer transport body, the photosensitive drum 1 is not discharged and the image is not disturbed. The carrier having a resistivity of 10 ⁸ Ωcm or more, preferably 10 ¹³ Ωcm or more, is preferable. This resistivity was measured by placing particles in a container having a cross-section of 0.5 cm ² and tapping them, then applying a load of 1 kg / cm ² on the packed particles, and applying an electric field of 1000 V / cm between the load and the bottom electrode. It is a value obtained by reading the current value when the generated voltage is applied. When this resistance value is low, the developer transport body 41 is
When a bias voltage is applied to the carrier, electric charges are injected into the carrier, carrier particles adhere to the photosensitive drum, and a breakdown of the bias voltage easily occurs.

As described above, the two-component developer in which the spherical carrier and the toner are mixed is preferably used in the image forming apparatus of the present application. If necessary, a fluidizing agent for increasing the fluidity of particles or a fluidizing agent may be used. A cleaning agent for cleaning the surface of the image forming body can be mixed with the toner. As a fluidizing agent, colloidal silica, silicon varnish, metal soap,
A nonionic surface active agent or the like can be used, and as the cleaning agent, a surface active agent such as fatty acid metal salt, organic group-substituted silicon, or fluorine can be used.

Example 1 Experimental machine: DC0928 (manufactured by Konica Corp.) modified machine Developer (42): Carrier has an apparent density of 2.45 g / cm ³ , a volume average particle diameter of 45 μm, and a saturation magnetization of 60 emu / g. A spherical magnetic carrier coated with resin on ferrite particles is used.The toner is produced by kneading 100 parts by weight of styrene-acrylic resin and 10 parts by weight of a pigment with a necessary amount of a charge control agent, and is obtained by a pulverization and granulation method. Weight average particle size 7
Non-magnetic toner of μm with a toner ratio of 10w to carrier
600 g of each color developer was prepared so that t% and the average toner charge amount measured by the development separation method (measured by the development separation method) were −20 μC / g.

Photoconductor (1): OPC photoconductor drum (φ18
0 mm) Peripheral speed 140 mm / sec Developer carrier (41): Roughened surface of stainless steel by sand blasting (φ20 mm), rotation speed is 300 rpm Plate-shaped member (45) with electrode part: reinforced by glass fiber Copper foil with a width of 0.5 mm is formed by etching on epoxy resin (0.1 mm thick). 10 μm in the conductive part
A thick insulating coating was applied. The setting was made as shown in FIG. 6A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photosensitive drum 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² black Maximum potential of the electrostatic latent image on the photoconductor drum 1 at the developing position: -700V, -800V Bias voltage applied to the electrode body 45b: T1 (during development) DC bias voltage: Variable T2 (during other color development) DC bias voltage : -900V T3 (non-image area) DC bias voltage: -900V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -600V AC bias voltage 8kHz, monochrome mode under the condition of 800V or more Then, one copy was made, and the amount of adhered toner (primary adhered amount) on the photosensitive drum 1 at that time was examined. The transfer device 5 and the cleaning device 6 are set so as to be kept at positions separated from the photosensitive drum 1. The primary adhesion amount was measured by peeling off the toner image of the solid portion with a 10 cm ² tape after the machine was stopped. The result at that time is shown in FIG. The solid line shows the result when the maximum potential of the electrostatic latent image on the photosensitive drum 1 is -700V, and the dotted line shows the result when it is -800V. It was found that the primary adhesion amount drastically decreases when the maximum potential of the electrostatic latent image on the photosensitive drum 1 is exceeded.

(Example 2) Experimental machine: DC9028 (manufactured by Konica Corporation) remodeling machine Developer (42): apparent density of carrier is 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600 g of each color developer is prepared so that the value becomes −20 μC / g.

Photoconductor (1): OPC photoconductor drum (φ18
0mm) Peripheral speed is 140mm / sec Developer carrier (41): Roughened surface of stainless steel by sandblasting (φ20mm), rotation speed is 300rpm Plate-like member with electrode part (45): Reinforced by glass fiber Copper foil with a width of 0.5 mm is formed by etching on epoxy resin (0.1 mm thick). An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photosensitive drum 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² black Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the developing position: -800V Bias voltage applied to the electrode body 45b: T1 (during development) DC bias voltage: Variable T2 (during other color development) DC bias voltage: -900V T3 (non-image area) DC bias voltage: -900V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V -600V AC bias voltage 8kH, 800V One copy was made, and the contamination of the electrode portion 45b at that time was examined. Contamination of the electrode part 45b can be observed with a microscope after stopping the machine (× 25
0). The results at that time are shown in Table 1.

[0055]

[Table 1]

Table 1 is a table showing the relationship between the DC bias voltage and the toner stain on the electrode portion when the DC bias voltage applied to the electrode portion is changed at T1 (during development).

[0057] Table in 1, a ○ the toner per electrode unit 45B1mm ² is 10 or less, △ 11 per 1 mm ² to 50
, X represents 51 to 100, and xx represents 101 or more. When the DC bias voltage applied to the electrode portion 45b is set to be equal to or higher than the DC bias voltage applied to the developer transport body 41, the electrode portion 45b is less contaminated.

Example 3 Experimental machine: DC9028 (manufactured by Konica Corp.) modified machine Developer (42): apparent density of carrier is 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, 600 g of a yellow developer and a black developer were prepared so that the average toner charge amount measured by the development separation method was −20 μC / g.

Photoconductor (1): OPC photoconductor drum (φ18
0mm) Peripheral speed is 140mm / sec Developer carrier (41): Roughened surface of stainless steel by sandblasting (φ20mm), rotation speed is 300rpm Plate-like member with electrode part (45): Reinforced by glass fiber Copper foil with a width of 0.5 mm is formed by etching on epoxy resin (0.1 mm thick). An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between developer transport body 41 and photosensitive drum 1: 0.5 mm Closest distance between developer transport body 41 and electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² each Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the developing position: -900V, -800V (when developing yellow (Y), the value at the yellow (Y) position, when developing black (K)) Bias voltage applied to the electrode body 45b (black (K) position is shown): T1 (during development) DC bias voltage: -700V T2 (during other color development) DC bias voltage: Variable (black) -900V (yellow) ) T3 (non-image area) DC bias voltage: -900V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V AC bias voltage 8kHz, 800V In full color mode Make one copy, then yellow It was examined toner stains black with the electrode portion 45b of the (Y). The developing device 4 operated only yellow (Y) and black (K). That is, a yellow toner image is previously formed on the photosensitive drum 1,
The DC bias voltage applied to the black electrode portion 45b was made variable. The electrode part 45b was stained by visual observation and microscopic observation (× 250) after the machine was stopped. The results at that time are shown in Table 2.

[0061]

[Table 2]

Table 2 shows the results when the DC bias voltage applied to the electrode portion was changed at T2 (during other color development).
6 is a table showing a relationship between a DC bias voltage and a stain caused by another color toner attached to an electrode portion.

[0063] Table in the 2 and ○ the toner per electrode unit 45B1mm ² is 5 or less, △ 6 per 1 mm ² to 20
, X indicates 21 to 50, and xx indicates 51 or more. If the DC bias voltage applied to the electrode portion 45b is made higher than the maximum potential of the electrostatic latent image on the photosensitive drum 1, the electrode portion 45b
Less dirt on the.

Example 4 Experimental machine: DC9028 (manufactured by Konica Corporation) modified machine Developer (42): apparent density of 2.45 g / cm ^{3 for} carrier,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600g of developer of each color is prepared so that is -20μC / g.

Photoconductor (1): OPC photoconductor drum (φ180 mm), peripheral speed is 140 mm / sec Developer carrier (41): Stainless steel is roughened by sandblasting (φ20 mm), rotation speed is 300 rpm The plate-like member (45) having: a copper foil with a width of 0.05 mm is formed on the epoxy resin (thickness 0.1 mm) reinforced by glass fiber by etching. 10 μm in the conductive part
A thick insulating coating was applied. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photosensitive drum 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² each Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the developing position: -800V -850V (value at the yellow (Y) position when developing yellow (Y), black (when developing black (K)) (K) indicates the value at the position) Bias voltage applied to the electrode body 45b: T1 (during development) DC bias voltage: -600V T2 (during other color development) DC bias voltage: -900V T3 (non-image area) DC bias Voltage: Variable AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V AC bias voltage 8kHz, 800V 100 copies are made in full color mode under the above conditions. The contamination of the electrode part 45b was examined. The developing device 4 operated only yellow (Y) and black (K). The electrode part 45b was stained by microscopic observation (× 250) after the machine was stopped. The results at that time are shown in Table 3.
Shown in

[0067]

[Table 3]

Table 3 is a table showing the relationship between the DC bias voltage and the toner stain on the electrode portion when the DC bias voltage applied to the electrode portion is changed in T3 (non-image area).

[0069] Table in the 3, a ○ the toner per electrode unit 45B1mm ² is 5 or less, △ 6 per 1 mm ² to 20
X, 21 to 50, x x 51 to 100 x x x
Indicates 101 or more. When the DC bias voltage applied to the electrode portion 45b is made higher than the maximum potential of the electrostatic latent image on the photosensitive drum 1 (the maximum absolute value of the surface potential of the photosensitive drum 1 because it is a non-image area). , The electrode portion 45b
Less dirt on the.

Example 5 Experimental machine: DC9028 (manufactured by Konica Corp.) modified machine Developer (42): apparent density of carrier is 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600g of developer of each color is prepared so that is -20μC / g.

Photoconductor (1): OPC photoconductor drum (φ18
0mm) Peripheral speed is 140mm / sec Developer carrier (41): Rough surface treatment (φ20mm) of stainless steel by sandblasting, rotation speed is 300rpm Plate-like member with electrode part (45): Reinforced by glass fiber Copper foil with a width of 0.5 mm is formed by etching on epoxy resin (0.1 mm thick). An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between developer transport body 41 and photosensitive drum 1: 0.5 mm Closest distance between developer transport body 41 and electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² each Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the developing position: -800 V (value at the yellow (Y) position when developing yellow (Y), black (K) when developing black (K)) Bias voltage applied to electrode body 45b: T1 (during development) DC bias voltage: -700V T2 (during other color development) DC bias voltage: -900V T3 (non-image area) DC bias voltage: -900V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V AC bias voltage 8kHz, 800V Image formation is performed in full color mode under the above conditions.
It was transferred to plain paper by corona discharge and passed through a fixing device 12 consisting of a fixing roller 12a having a surface temperature of 140 ° C. to be fixed. The obtained image had no fog, had a high density, and was extremely clear. At this time, the electrode portion 45b was not stained with toner. Subsequently, 50,000 sheets were copied, but even the recorded image at the end had no fog as in the initial recorded image, and it was possible to obtain an extremely clear image with high density. Further, no stain was found on the electrode portion 45b even after the end of 50,000 copies in the full color mode.

(Comparative Example 1) Experimental machine: DC9028 (manufactured by Konica Corporation) modified machine Developer (42): apparent density of carrier is 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600g of developer of each color is prepared so that is -20μC / g.

Photoconductor (1): OPC photoconductor drum (φ18
0mm) Peripheral speed is 140mm / sec Developer carrier (41): Rough surface treatment (φ20mm) of stainless steel by sandblasting, rotation speed is 300rpm Plate-like member with electrode part (45): Reinforced by glass fiber Copper foil with a width of 0.5 mm is formed by etching on epoxy resin (0.1 mm thick). An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photoconductor 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode part 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² Each development Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the position: -800 V (value at the yellow (Y) position when developing yellow (Y), black (K) position when developing black (K)) Bias voltage applied to the electrode body 45b: during development (T1) DC bias voltage: -600V during other color development (T2) DC bias voltage: -900V non-image area (T3) DC bias voltage:- 700V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V AC bias voltage 8kHz, 800V Image formation is performed in full color mode under the above conditions.
It was transferred onto plain paper by corona discharge, and passed through a fixing device 12 consisting of a fixing roller 21a having a surface temperature of 140 ° C. to be fixed. The obtained image had no fog, had a high density, and was extremely clear. However, the electrode part 45
Contamination due to toner was found in b. When 50,000 sheets were subsequently copied, as the copy progressed compared to the initial recorded image, the recorded image became conspicuous.
After the completion of 50,000 copies, the electrode portion 45b was soiled with the developer.

(Comparative Example 2) Experimental machine: DC9028 (manufactured by Konica Corp.) modified machine Developer (42): carrier apparent density 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600g of developer of each color is prepared so that is -20μC / g.

Developer carrier (41): stainless steel is roughened by sandblasting (φ20 mm), rotation speed is 30
Plate-shaped member (45) with 0 rpm electrode: A 0.5 mm wide copper foil was formed by etching on epoxy resin (0.1 mm thick) reinforced by glass fiber. An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photoconductor 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² Each development Maximum potential of the electrostatic latent image on the photoconductor drum 1 at the position: -800 V (when developing yellow (Y), the value at the yellow (Y) position; when developing black (K), the (K) position) Bias voltage applied to the electrode body 35b: during development (T1) DC bias voltage: -900V during other color development (T2) DC bias voltage: -900V non-image area (T3) DC bias voltage: -900V AC bias voltage: 8 kHz, 800 V Bias voltage applied to the developer carrier 41: DC bias voltage -700 V AC bias voltage 8 kHz, 800 V Image formation is performed in the full color mode under the above conditions.
It was transferred to plain paper by corona discharge and passed through a fixing device 12 consisting of a fixing roller 12a having a surface temperature of 140 ° C. to be fixed. Fogging was observed in the obtained image, and the density was too low to be considered a good image. At this time, the electrode part 45
No stain due to the developer of b was observed. Subsequently, 50,000 copies were made in the full color mode, but only the same recorded image as the initial recorded image could be obtained.

(Comparative Example 3) Experimental machine: DC9028 (manufactured by Konica Corp.) modified machine Developer (42): apparent density of carrier is 2.45 g / cm ³ ,
Uses spherical magnetic carrier resin coated with ferrite particles with volume average particle diameter of 45 μm and saturation magnetization of 60 emu / g. Toner is 100 parts by weight of styrene-acrylic resin,
A non-magnetic toner having a weight average particle size of 7 μm, which is produced by pulverizing and granulating the mixture, is prepared by kneading 10 parts by weight of a pigment with a required amount of a charge control agent, and the toner ratio is 1 to the carrier.
0 wt%, average toner charge (measured by development separation method)
600g of developer of each color is prepared so that is -20μC / g.

Developer carrier (41): Stainless steel roughened by sandblasting (φ20 mm), rotation speed 30
Plate-shaped member (45) with 0 rpm electrode: A 0.5 mm wide copper foil was formed by etching on epoxy resin (0.1 mm thick) reinforced by glass fiber. An insulating film with a thickness of 10 μm was applied to the conductive part. The setting was made as shown in FIG. 4A, and θ was set to 0 °.

Gap between the developer transport body 41 and the photoconductor 1: 0.5 mm The closest distance between the developer transport body 41 and the electrode portion 45b: 0.25 mm Vs / Vp = 2.24 Developer transport amount: 10 mg / cm ² Each development Maximum potential of the electrostatic latent image on the photosensitive drum 1 at the position: -800 V (value at the yellow (Y) position when developing yellow (Y), black (K) position when developing black (K)) Bias voltage applied to the electrode body 35b: during development (T1) DC bias voltage: -700V during other color development (T2) DC bias voltage: -700V non-image area (T3) DC bias voltage:- 700V AC bias voltage: 8kHz, 800V Bias voltage applied to the developer carrier 41: DC bias voltage -700V AC bias voltage 8kHz, 800V Image formation is performed in full color mode under the above conditions.
It was transferred to plain paper by corona discharge and passed through a fixing device 12 consisting of a fixing roller 12a having a surface temperature of 140 ° C. to be fixed. No fogging was observed in the obtained image, but the density was slightly low. At this time, the electrode part was slightly stained with the toner of another color. When 50,000 sheets were subsequently copied, as the copy progressed compared to the initial recorded image, the recorded image became conspicuous. At the end of 50,000 copies, the electrode portion 45b was soiled with another color toner.

In the embodiments of the present invention, a modified DC9028 machine is used as an image forming apparatus, but the present invention can also be applied to an image forming apparatus having a similar configuration. That is, it is needless to say that it can be similarly used for an apparatus that forms an image by writing an image on a photosensitive drum or a photosensitive belt as an image carrier, such as a copying machine, a printer, a facsimile, or a composite machine of these. .

The image forming apparatus of the present invention is suitable for a two-component developer composed of carrier and toner.
A one-component developer can also be used with almost the same structure.

[0084]

As described above, in the image forming apparatus of the present invention, the electrode portion 45b is formed in the non-image area during development, development of another color.
By changing the voltage applied to
It is possible to use a toner having a particle size of μm or less without any trouble, and it is possible to obtain an extremely clear recorded image with high density without fog.

[Brief description of drawings]

FIG. 1 is a DC used in an embodiment of an image forming apparatus of the present invention.
It is a block diagram which shows a 9028 (made by Konica Corporation) remodeling machine.

FIG. 2 is a schematic cross-sectional view showing an embodiment of a developing device used in the image forming apparatus of the present invention.

FIG. 3 is an enlarged view of a developing space of the developing device.

FIG. 4 is a time chart showing the timing of the bias voltage applied to the electrode section used in the full color mode of the embodiment of the image forming apparatus of the present invention and the timing of the bias voltage applied to the developer transport body.

FIG. 5 is a timing chart of the bias voltage applied to the electrode section used in the monochrome mode of the embodiment of the image forming apparatus of the present invention,
And a time chart showing the timing of the bias voltage applied to the developer transport body.

FIG. 6 is a principle configuration diagram showing an embodiment of a plate-shaped member having an electrode portion used in the developing device of the image forming apparatus of the present invention.

FIG. 7 is a graph showing the toner adhesion amount (primary adhesion amount) developed on the photosensitive drum when the DC bias voltage applied to the electrode portion is changed at T1 (during development).

[Explanation of symbols]

1 photoconductor drum (image carrier) 2 scorotron charger 3 image writing device 4, 4A, 4B, 4C, 4D developing device 5 transfer device 6 cleaning device 7 platen 8 image reading device 9 paper feeding device 12 fixing device 41 developing device Agent carrier 42 Developer 43 Stirrer 44 Supply roller 45 Plate-shaped member 45a having electrode part 45a Plate-shaped member 45b Electrode part 46 Developer transfer regulation rod 47 Scraper 48 First oscillating electric field 49 Second oscillating electric field A Development space E1 DC bias power source (developer carrier side) E2 AC bias power source (developer carrier side) E3 DC bias power source (electrode part side) E4 AC bias power source (electrode part side) T1 Image bearing by toner loaded in developing device When developing the electrostatic latent image on the body (at the time of development) T2 When developing the electrostatic latent image on the image bearing member by toner loaded in another developing device (When developing another color) T3: Where there is no electrostatic latent image to be developed by each developing device on the image bearing member (non-image area)

Front page continued (72) Inventor Masayasu Onodera 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (72) Inventor Yotaro Sato 2970 Ishikawa-cho, Hachioji, Tokyo Konica stock company (72) Inventor Hiroyuki Nomori 2970 Ishikawa-cho, Hachioji City, Tokyo Konica Stock Company

Claims (2)

[Claims] 1. A plate-shaped member having an electrode portion is installed on an upstream side in a moving direction of the developer transport body in a space where the developer transport body and the image bearing body face each other, and a two-component developer is placed in the developer transport body. In an image forming apparatus that has a plurality of developing devices that supply and fly toner under an oscillating electric field to perform reversal development, and the multicolor toners are superposed on the image carrier, each of the developing devices is
(1) When developing an electrostatic latent image on the image carrier with the toner loaded in the developing device, | DC bias voltage applied to the developer carrier | ≦ | DC bias voltage applied to the electrode section | <| Is the highest potential of the electrostatic latent image on the image carrier, and (2) when the electrostatic latent image on the image carrier is developed by toner loaded in another developing device, | An image forming apparatus characterized in that the maximum potential of an electrostatic latent image on a carrier | ≦ | a direct current bias voltage applied to an electrode section |. 2. In a non-image area where there is no electrostatic latent image to be developed by each developing device on the image carrier, | maximum surface potential of the image carrier | <| DC bias applied to the electrode section The image forming apparatus according to claim 1, wherein the voltage is |, and an AC bias voltage is superimposed.