JPH10161386A - Multicolor image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To develop a second toner image without stripping off a first toner image, formed on a photoreceptive drum, at the time of second development and to obtain a bicolor image in which the image quality of the second toner image is fairly high. SOLUTION: Two-component developer or one-component nomnagnetic toner is used for first development using a first developing device. One-component magnetic toner is used for the second development using a second developing device 42, its developing sleeve 423 is held and fixed so as not to be in contact with the photoreceptive drum 1, and a magnet roller 424 having a plurality of magnetic poles is rotatably arranged in the developing sleeve. As the second development, development similar to that obtained by an AC development method can be realized even by a DC development method, by applying a DC voltage between the photoreceptive drum 1 and the developing sleeve 423 while rotating the magnet roller 424.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multicolor image forming apparatus for forming a plurality of images on an image carrier, and more particularly to a multicolor image forming apparatus such as a full-color or multicolor copying machine, a printer, a facsimile, and the like.

[0002]

2. Description of the Related Art In recent years, a plurality of developing units accommodating developers (toners) of different colors have been provided, and one or more recording cycles using electrophotography have been performed on a photosensitive drum as an image carrier. There has been proposed a multicolor image forming apparatus which forms a visible multicolor image on a recording material, collectively transfers the multicolor image onto a recording material, and fixes the multicolor image to obtain a desired multicolor image.

FIG. 3 shows a schematic configuration of such a conventional multicolor image forming apparatus, for example, a digital two-color printer.

The two-color printer shown in FIG. 3 has a photosensitive drum 1 in which a photoconductive layer is formed on a drum-shaped conductive substrate. The photosensitive drum 1 rotates in a steady direction in the direction of the arrow in FIG. A first charging device 21 for uniformly charging the photosensitive drum 1 along the line, a first image exposure 31 by laser irradiation of a laser optical system for forming a first latent image, and developing the first latent image to form a first latent image. A first developing device 41 for forming one toner image, a second charger (recharger) 22 for recharging the photosensitive drum 1, a second charger
By laser irradiation of a laser optical system that forms a latent image of
Image exposure 32, a second developing device 42 for developing a second latent image to form a second toner image (the second toner used in the second developing device is the same as the first toner used in the first developing device) (A different color), a pre-transfer charger 5 for equalizing the transferability of the first and second two-color toner images formed on the photosensitive drum 1 to the recording material, and transferring the two-color toner image to paper or the like. A transfer charger 6 for transferring to a recording material, a cleaner 7 for removing residual toner remaining on the photosensitive drum 1 for transfer, and a pre-exposure device (not shown) for removing electricity from the surface of the photosensitive drum 1 are provided. I have.

Of course, it can be used as a single-color printer, but when forming a multi-color image, as described above, the photosensitive drum 1 is uniformly charged by the chargers 21 and 22, and the image exposure 31 based on a specific color signal is performed. , 32 to form latent images of the first and second colors, and develop them by using developing devices 41 and 42 with corresponding color toners.
The two color toner images are formed in a superimposed manner, and the two color toner images are collectively transferred to a transfer charger 6 on paper or the like. Next, the transfer material is passed through a fixing device (not shown) to fix the toner, thereby forming a two-color permanent print image.

Although light sources for the image exposures 31 and 32 are not described in detail, semiconductor lasers 81 and 82 and LEDs are generally used. For example, the semiconductor lasers 81 and 82 are used.
Is scanned by the polygon scanner 9 on the photosensitive drum 1. In a document reading unit (reader) not shown, for example, R (red), G
The color separation image data of the original is created using a CCD capable of reading three colors (green) and B (blue), and if necessary, signals of two colors of red and black are generated. Then, the first and second image data are created, and the semiconductor laser, the LED, and the like are driven to emit light according to the first and second image data.

The two-color printer is characterized in that toner images of the first and second colors are formed on the photosensitive drum 1 in a superimposed manner to obtain a multicolor toner image. In general, a developing method in which the developing sleeve (developer carrying member) of the second developing device 42 is not in contact with the photosensitive drum 1 for the purpose of preventing the first toner image formed on the photosensitive drum 1 from being peeled off. (Japanese Patent Laid-Open No. 56-1)
44562). According to this, the developing sleeve does not come into contact with the first toner image on the photosensitive drum and mechanically strips the first toner image.

[0008] On the other hand, not only two colors so far,
2. Description of the Related Art In a copying machine or a printer using an electrophotographic system such as a full-color system, an AC voltage has been used instead of a DC voltage as a developing bias applied to a developing sleeve of a developing device. This is because excess toner excessively attached to the image portion on the photosensitive drum and fogging toner attached to the non-image portion can be pulled back to the developing sleeve side by the action of the AC voltage, and high image quality can be achieved.

[0009]

However, in a two-color copying machine or printer, the second developing device 42
At the time of development, when an AC voltage is applied to the developing sleeve to improve the image quality of the second toner image, the first toner image on the photosensitive drum 1 is
The toner is slightly removed from the toner image,
There is a problem that the toner is mixed into the second developing device 42.

This is because the above-described effect of pulling back the toner occurs because the AC voltage is applied to the developing sleeve of the second developing device, and the first toner on the photosensitive drum 1 is peeled off. It can be called electrical stripping.

In order to prevent this, of course, the second development may be performed using a DC voltage instead of an AC voltage as a developing bias. However, naturally, the image quality of the second toner image is deteriorated. Become.

Alternatively, for example, see Japanese Patent Application Laid-Open No. 2-77767.
As shown in (1), it has been proposed to reduce the stripping as a waveform in which the amount of pullback of the AC component of the developing bias is weakened. Attempts have also been made to make the AC bias frequency higher than that used for normal electrophotographic development, reduce the frequency response of the toner, and weaken the toner pullback to reduce stripping. However, as long as the AC bias is used, it is difficult to eliminate the toner by removing the toner, but it is difficult to eliminate the toner, and the use of the copier for a long period of time inevitably causes the toner to be mixed.

As described above, in the conventional two-color image forming apparatus, it is contradictory to reliably overlap two-color toner images on the photosensitive drum and to improve the image quality of the second toner image. Had been an issue.

Accordingly, an object of the present invention is to develop the second toner image without peeling off the first toner image already formed on the photosensitive drum at the time of the second development, and to develop the second toner image. An object of the present invention is to provide a multicolor image forming apparatus capable of obtaining a two-color image having a sufficiently high image quality.

[0015]

The above object is achieved by a multicolor image forming apparatus according to the present invention. In summary, the present invention has a latent image carrier on which a plurality of electrostatic latent images are sequentially formed, and a plurality of developing units containing developers of different colors, and is formed on the latent image carrier. Developing the formed electrostatic latent image with a developer of a corresponding color at least twice to form a toner image of two or more colors on a latent image carrier by superimposing the toner images of the plurality of colors. In a multicolor image forming apparatus for transferring an image collectively onto a transfer material, a non-magnetic developer is used for development by a first developer among the plurality of developers, while a second and subsequent developers are used. A magnetic developer is used for the second and subsequent developments, and a developer carrier of the second and subsequent developing units is held and fixed in non-contact with the latent image carrier, and a plurality of magnetic poles are provided inside the developer carrier. A magnetic field generating means having a rotatable arrangement, and a DC voltage applied between the latent image carrier and the developer carrier. Is a multi-color image forming apparatus characterized by causing the application to the second and subsequent development.

According to the present invention, the developers of the plurality of developing units can have the same charging polarity and the same charging polarity as the developer carrier. The magnetic developer of the second and subsequent developing units may be a magnetic developer having a high resistance of 10 ¹⁰ Ωcm or more. The magnetic field generating means of the second and subsequent developing devices is a magnet roller having a plurality of magnetic poles, and can have a symmetric magnetic pole pattern. In the developer carrying members of the second and subsequent developing devices, a portion near a position closest to the latent image carrying member has a low resistance, and the other portions have a high resistance. The second and subsequent electrostatic latent images among the plurality of electrostatic latent images are binary digital electrostatic latent images. The development by the first developing device may be a one-component development method using a one-component non-magnetic toner. The development by the second and subsequent developing units is 1
A one-component developing method using a component magnetic toner can be employed.

[0017]

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

Embodiment 1 FIG. 1 is a schematic structural view showing a second developing unit in an embodiment of the multicolor image forming apparatus of the present invention. The present invention is characterized by using the second developing device 42 having the configuration shown in FIG. 1. The configuration of the multicolor image forming apparatus itself is basically the same as that of the conventional multicolor image forming apparatus shown in FIG. Are the same. Hereinafter, FIG. 3 will be referred to as needed in the description of the present invention.

As shown in FIG. 1, the second developing device 42
A developing container 421 containing a component magnetic toner is provided, and a developing sleeve 423 is disposed in an opening of the developing container 421 facing the photosensitive drum 1. According to the present invention, the developing sleeve 423 is in non-contact with the photosensitive drum 1. And non-rotatably fixed. The developing sleeve 423 is made of non-magnetic S
It is made of a conductive material such as US. Agitating and conveying means 422a and 422b are provided on the inner side of the developing container 421, and the rotation thereof causes old and new toner in the container 421 to be agitated and mixed, and the toner is conveyed toward the developing sleeve 423.

A magnet roller 424 is disposed inside the developing sleeve 423. According to the present invention, the magnet roller 424 is rotatably installed. The magnet roller 424 is rotated at a high speed in the same direction as the rotation direction of the photosensitive drum 1 (counter direction in the developing nip portion).

In the present embodiment, the magnet roller 424 has a symmetrical magnetic pattern of eight poles, and each pole is about 1000
It has a magnetic flux density of about Gauss at its peak and rotates at a high speed of several tens to several hundreds rps in the same direction as the photosensitive drum 1.

The toner conveyed and supplied to the developing sleeve 423 by the stirring and conveying means 422a and 422b moves on the fixed developing sleeve 423 in the same direction as the photosensitive drum 1 by the magnetic force generated by the rotation of the magnet roller 424. Conveyed. The toner moved on the developing sleeve 423 is regulated by a magnetic regulating member 425 disposed on the approximate top of the developing sleeve 423 and spaced apart therefrom, and is applied to the thin toner layer on the developing sleeve 423. After that, the sheet is conveyed to a developing nip 426 near the closest position to the photosensitive drum 1.

The developing sleeve 423 is arranged so as not to be in contact with the photosensitive drum 1, and the thickness of the toner layer carried on the developing sleeve 423 is set to about 150 μm or less. By arranging, the second developing device 42 can be brought into mechanically non-contact with the photosensitive drum 1.

According to the present invention, when developing the second electrostatic latent image formed on the photosensitive drum 1, the developing sleeve 423 is used.
, A DC bias is applied as a developing bias.

In the present invention, a two-color image is formed by using the above-described developing device as the second developing device 42 of the two-color image forming apparatus shown in FIG. Hereinafter, the two-color image forming process will be described step by step. FIGS. 2A to 2F show the surface potential of the photosensitive drum in each step of image formation according to the present invention.

In the two-color image forming apparatus shown in FIG. 3, the photosensitive drum 1 has a photoconductive layer such as OPC or α-Si, rotates in the direction of the arrow, and the surface thereof is set to, for example, −600 V by the first charger 21. It is uniformly charged (FIG. 2A).
The first semiconductor laser 81 emits a first laser beam (first image exposure) 31 modulated by a first image signal, and the first laser beam 31 is emitted by a polygon scanner (rotating polygon mirror) 9. The light beam is deflected and raster-scanned on the photosensitive drum 1, and the surface of the exposed portion is attenuated to, for example, -100 V to form a first electrostatic latent image (FIG. 2B).

The first latent image is developed by the first developing device 41 and is visualized as a first toner image. In this embodiment,
The first developing device 41 employs a two-component developing method, and uses a two-component developer composed of negatively charged nonmagnetic red toner and magnetic particles (carrier) such as ferrite.
At the time of the first development by the first developing device 4, the developing sleeve is supplied with an AC voltage of, for example, 1600 Hz and 1800 Vpp.
A developing bias on which a DC voltage of 500 V is superimposed is applied, whereby the first latent image is reversely developed (FIG. 2).
(C)). At this time, the first toner image potential drops to about -100 V due to toner charge, and becomes about -200 V.

The photosensitive drum 1 is a recharger (second charger)
22, the potential of the first toner image on the photosensitive drum 1 is lowered (FIG. 2D). At this time, the first non-image portion potential on the photosensitive drum 1 also slightly decreases. The potential after recharging is about -650 V in the first non-image area and about -500 V in the first image area (first toner image).

The second semiconductor laser 82 emits a second laser beam (second image exposure) modulated by a second image signal.
Inject 32. The second laser beam 32 is deflected by the polygon scanner 9, performs raster scanning on the photosensitive drum 1, and attenuates the exposure portion potential to, for example, -100 V to form a second electrostatic latent image (FIG. E)).

Thereafter, the second developing device 42 uses a black negatively charged one-component magnetic toner to form a developing sleeve 423.
A DC bias of, for example, -550 V is applied to
Is developed in reverse and visualized as a second toner image (FIG. 2 (F)).

In the present invention, the use of the second developing device 42 having the above-described structure and the execution of the two-color image forming process ensure that the first toner image is not peeled off during the second development, It is possible to realize two-color image formation in which a sufficiently high image quality of the toner image is achieved.

That is, in the present invention, since the second development is of the one-component non-contact development type as in the conventional example, the first toner image is prevented from being mechanically peeled off. Further, the developing sleeve 423 of the developing device 42 is used for the second development.
DC development method with DC bias applied to
The first toner image is prevented from being electrically stripped. That is, in FIG. 2F, the DC bias of the second development = −550 V is lower than −500 V of the toner layer potential of the first image portion after recharging, and the direction in which the first toner is pressed against the photosensitive drum. Therefore, the first toner is not electrically stripped off. Further, since a two-component developer composed of a non-magnetic toner and a magnetic carrier is used in the first development, the present invention provides a photosensitive drum 1 in the second development.
No magnetic pullback acts on the first nonmagnetic toner above.

As described above, in the present invention, the second developing device is in a state in which there is no mechanical, electrical or magnetic pull-back action with respect to the first toner image with respect to stripping of the first toner. At the time of development, stripping of the first toner can be eliminated.

The sufficiently high image quality of the second toner image is as follows. That is, in the present invention, since the DC development method is used for the second development, as described above, the image quality of the second toner image becomes inferior as it is as described above. this is,
This is because there is no pullback of toner like AC development,
Also, it does not work periodically.

In the present invention, as described above, the magnet roller 424 is disposed inside the developing sleeve 423,
The magnetic force can cause a pull-back action only to the second toner of the magnetic toner. Further, since the developing sleeve 423 is fixed and the magnet roller 424 is rotated at a high speed, the second toner can be periodically returned to the second toner. Of course, the rotation of the magnet roller 424 also has a role of moving and transporting the second toner on the developing sleeve 423, as described above.

As described above, in the present invention, with respect to the improvement of the image quality of the second toner image, the magnetic pull-back is periodically applied only to the second toner image, and this is combined with the DC development described above. The flying state of the two toners can be made similar to that of the conventional AC development, and a sufficiently high quality second toner image can be obtained as in the AC development.

Hereinafter, experimental examples of the present invention will be described.

As the photosensitive drum 1, an OPC photosensitive member having a sensitivity around 700 nm is used, and charging is performed by a first charger 2
A scorotron charger is used for both the first and second chargers 22,
The potential relationship due to the corona charging is shown in FIGS.
The same as shown in FIG. The image exposure by the laser beam uses a visible light laser beam of 680 nm for both the first and second image exposures 31 and 32, and the first and second electrostatic latent images are formed on the photosensitive drum 1 as digital latent images of 600 dpi. Formed.

Document reading units (readers) for generating two-color image data are R (red), G (green), B
(Blue) CCD consisting of 3 lines, read the original and Bk (black) by the well-known inking process (UCR process)
Data was generated and the remaining color components were appropriately allocated to R and Bk. Here, a warm color tone component such as yellow or red is added to the red image data, and a cool color component such as blue is added to the black image data.

The first developing device 41 is a two-component developing system as described above. As is well known, the two-component developer used for this is formed by applying a resin coating or the like to magnetic particles such as ferrite and providing a carrier having a high resistance of about 10 ⁷ to 10 ¹² Ωcm and a carrier having a particle size of about 30 to 100 μm. 5-12μ
About m of non-magnetic toner is mixed in about several weight%. The charge of the toner is imparted by friction with the carrier particles.
It has about 0 μC / g.

The two-component developer is conveyed by rotation of the developing sleeve while being held by a magnet roller fixedly arranged in the developing sleeve. The developing sleeve has a surface subjected to sandblasting using alundum beads in order to have a developer transport property. The transport amount of the two-component developer by the developing sleeve is about ²⁰ to 100 mg / cm ² .

The second development was performed by applying an AC bias to the developing sleeve while the developer was in contact with the photosensitive drum at the developing nip. As the AC bias, a known one such as a sine wave, a rectangular wave, a duty (Duty) wave, and a blank pulse wave can be used. In this experiment, the frequency was 2.0 kHz, the peak-to-peak voltage was 2.0, and the like.
A kV rectangular wave was used.

The second developing device 42 is a non-contact developing system using a one-component magnetic toner, as described above.

The magnetic toner used for the second development is insulating and is charged by friction with the non-magnetic developing sleeve 423. The amount of triboelectric charge of the toner includes the prescription and particle size of the toner, the addition of a charge control agent, the surface properties of the developing sleeve 423, the distance between the regulating member 425 and the developing three 423, and the strength of toner packing (toner density). And so on. In this experiment, a styrene-acrylic magnetic toner having an average particle diameter of 8 μm was used, and silica was externally added to impart fluidity.

The developing sleeve 423 was used by subjecting a surface of a sleeve base made of non-magnetic stainless steel to blasting with about 400 glass beads. The regulating member 425 is made of magnetic stainless steel and has a thickness of 1.02 mm. The gap between the regulating member 425 and the developing sleeve 423 was set to 200 μm. At this time, the triboelectric charge amount of the toner is about -15 to -20.
μC / g and the applied amount was about 0.8 to 1.2 mg / cm ² .

The magnet roller 424 is about 800 to 1
It was symmetrically magnetized to eight poles with a magnetic force of 200 Gauss. The magnetic toner on the developing sleeve 423 moves and is conveyed on the developing sleeve by the rotation of the magnet roller while being raised by the magnetic force. The rotation speed of the magnet roller 424 was set to 150 rps.

The gap at the closest position between the developing sleeve 423 and the photosensitive drum 1 was 150 μm. The gap between the photosensitive drum and the developing sleeve is generally about 250 to 300 μm in one-component development. However, in the present invention, since the developing method of the second development is DC development and the density is hardly obtained, this is not preferable. The density of the second development was ensured by narrowing the gap to 150 μm and increasing the electric field intensity. The raised developer is brought into non-contact with the photosensitive drum.
Ears of about 20 μm were actually measured, and it was confirmed that they were not in contact.

Under the above conditions, a two-color image forming experiment was conducted in which a first toner image having an image ratio of 6% was formed by the first development and a second toner image having an image ratio of 6% was formed by the second development. Repeated. As a result, it was confirmed that even after printing 10,000 sheets, the first toner was not mixed into the second developing device due to stripping, and a two-color image having a sufficient image quality of the second toner image was obtained.

Example 2 In this example, the first development was the non-contact development method using a one-component non-magnetic toner in Example 1. Others were the same as Example 1.

When the first development is a two-component development system as in the first embodiment, since a developer mixed with a magnetic carrier is used, the carrier adheres to the photosensitive drum 1 and at the time of the second development, In some cases, the carrier may be attracted by the magnet roller 424 in the developing sleeve 423 of the second developing device 42. In this embodiment, since the non-magnetic one-component toner is used for the first development, such a problem can be avoided.

In one-component non-magnetic development, an elastic blade is used as a regulating member, which is brought into contact with a developing sleeve, and the toner is charged by frictional charging between the elastic blade and the toner. For example, the elastic blade is made of silicone rubber, and this is put on a developing sleeve made of SUS or the like in an amount of 15 to 20 g.
Counter contact at a linear pressure of / cm. Thus, the frictional charge amount of -20 to-30 .mu.C / g of toner, the coating amount and 0.4~0.8mg / cm ² approximately. At this time, a conventional developing bias may be used.

Also in this embodiment, a magnetic one-component toner is used for the second development, and the developing sleeve 423 of the second developing device 42 is held and fixed in a non-contact manner with the photosensitive drum 1. The photosensitive roller 1 and the developing sleeve 4 are rotated by rotating a magnet roller 424 having
Since the second development is performed by applying a DC voltage between the first toner image and the second toner image 23, the first toner image obtained in the first development is not peeled off during the second development, and the image quality of the second toner image is sufficient. Two
Color images can be obtained.

Embodiment 3 This embodiment is characterized in that the material of the developing sleeve 423 of the second developing unit 42 in the first embodiment is partially changed. That is, the developing nip portion 4 of the developing sleeve 423
26 is the same conductive SU as in the first embodiment.
S was used, but the material of the other parts of the developing sleeve 423 was a high-resistance insulator.

This is because, during the second development, the flying of the second toner is made closer to the flying of the toner by the AC developing method. Such a developing sleeve 42 of which material is partially different
3, when the same DC developing bias as in Example 1 is applied, the magnetic pullback force of the toner in the direction of the developing sleeve 423 near the developing nip portion 426 is smaller than that of the toner.
The method of electrically discharging toner in the direction of the photosensitive drum 1 by applying a DC bias is strong.

However, since the insulating portion of the developing sleeve is located downstream of the developing nip portion, the applied DC
Most of the bias voltage is divided into its insulating portion, and a lower voltage than in the first embodiment is applied to the air gap portion on the photosensitive drum 1 side where the toner flies. It is considered that the magnetic pull-back force in the direction is relatively increased, so that the toner can be pulled back more strongly, and the toner takes a behavior similar to AC development.

As a result, it was confirmed that according to this embodiment, the image quality of the second toner image can be improved as compared with the first embodiment.

Embodiment 4 This embodiment is different from Embodiment 1 in that the method of forming the second electrostatic latent image is changed. That is, in this embodiment, the second electrostatic latent image is formed as a binary digital latent image.

The DC developing method has a binary characteristic in which development is performed above a certain threshold value and is not performed below the threshold value. Therefore, a latent image to be subjected to DC development is converted into a binary latent image. An image is formed and its resolution is set to, for example, 600 dpi.
It is known that high image quality can be achieved by high resolution.

However, the DC developing method generally has a problem that it is difficult to reproduce a halftone of an image. However, in the present invention,
In the DC development method of the second development, pseudo AC development can be realized by rotating the magnet roller 424 in the development sleeve 423 of the second development device 42, so that the problem of difficulty in reproducing halftones does not occur.

That is, according to the present embodiment, the second latent image is
A digital latent image of a value is developed, and the development is performed by combining DC development with development similar to the AC development method. Therefore, high image quality can be achieved by DC development of a binary latent image, and high image quality can be achieved by pseudo AC development. And a higher-quality two-color image can be formed.

Therefore, the second latent image is formed by a conventionally known pulse width modulation (PWM) method so as to impart gradation to the image.
Is not formed so as to have 256 gradations in one pixel by using a well-known error diffusion method, a dither method, or the like.
The number of gradations was reduced to about 6, so that they were formed.

In each of the above embodiments, the rotation direction of the magnet roller 424 of the second developing device 42 is opposite to that of the photosensitive drum 1 at the developing nip 426, and the direction of toner transport is the photosensitive drum 1 at the developing nip. Although the direction is the same as that of the drum, the present invention is not limited to this, and the rotation direction of the magnet roller and the toner conveyance direction may be reversed.

Further, a negatively charged OPC photosensitive member is used as the photosensitive drum 1, and a first developing device 41 and a second developing device 42 are used.
The first latent image and the second latent image on the photosensitive drum 1 were reversely developed using a negative polarity toner to form a first toner image and a second toner image. May be used. In this case, it is common to employ positive charging of the photosensitive drum, for example, because the amount of generated ozone can be reduced. For this reason, the first development and the second development require:
A positive polarity toner may be used.

Before the transfer of the two-color toner image on the photosensitive drum, the charge amounts of the two-color toners were made uniform by the pre-transfer charger 5.
By employing a scorotron charger, the charge amounts of the two color toners can be controlled and made uniform by applying an AC or DC voltage.

When the potential of the photosensitive drum 1 is attenuated by using the above-described pre-transfer charging together with the exposure for static elimination, etc.
The transfer / separation step of the color toner image can be made effective. In addition, for the pre-exposure before the charge is finally removed from the photosensitive drum 1, it is necessary to appropriately set the wavelength, intensity, and the like of the light used to prevent ghosting of the previous image.

In any case, it can be easily inferred by those skilled in the art that the above-described exposure for static elimination and the like can be used in combination with the present invention. It is easily understood by those skilled in the art that the technical idea of the present invention is not substantially affected.

In the two-color image forming method, a so-called negative-negative recharging method in which the charging polarity of the toner and the charging polarity of the photosensitive drum are the same has been described. In addition, a so-called negative-positive process (Japanese Patent Application Laid-Open No. 55-137538) and a three-value process (Japanese Patent Application Laid-Open No.
81855), which are a combination of regular development and reversal development. In these image forming methods, when the AC bias is used for the second development, the first toner is pulled back. However, when the second development is performed by the DC development method as in the present invention, the first toner is prevented from being pulled back. However, the image quality of the second toner image can be improved, which is similarly effective.

[0068]

As described above, according to the present invention, the first
A non-magnetic developer is used for the first development by the developing device, while
A magnetic developer is used for the second development by the second developing device, and the developer carrying member of the second developing device is held in non-contact with the image carrier,
Fixed, magnetic field generating means having a plurality of magnetic poles is rotatably arranged inside the developer carrier, and a DC voltage is applied between the image carrier and the developer carrier while rotating the magnetic field generating means. Then, since the second development was performed, during the second development, without removing the first toner image by the first development,
A second toner image of sufficient image quality can be obtained, and a high-quality two-color image can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a second developing device installed in a multicolor image forming apparatus of the present invention.

FIG. 2 is a schematic diagram showing a surface potential of a photosensitive drum in each step of image formation according to the present invention.

FIG. 3 is a schematic configuration diagram showing a conventional multicolor image forming apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 21, 22 Charger 31, 32 Image exposure 41 1st developing device 42 2nd developing device 423 Developing sleeve 424 Magnet roller 425 Control member

Claims (8)

[Claims] 1. A latent image carrier on which a plurality of electrostatic latent images are sequentially formed, and a plurality of developing devices containing developers of mutually different colors. The step of developing the electro-latent image with the developer of the corresponding color is performed at least twice, and a plurality of toner images of two or more colors are superimposed on the latent image carrier, and the toner images of the plurality of colors are collectively collected. In the multi-color image forming apparatus, the non-magnetic developer is used for development by the first developer among the plurality of developers, while the second developer is used for the second and subsequent developers. A magnetic developer is used for subsequent development, and the developer carrier of the second and subsequent developing units is held and fixed in non-contact with the latent image carrier, and a magnetic field having a plurality of magnetic poles inside the developer carrier The generator is rotatably arranged, and a DC voltage is applied between the latent image carrier and the developer carrier. A multicolor image forming apparatus, which performs second and subsequent development. 2. The multicolor image forming apparatus according to claim 1, wherein said plurality of developers have the same charge polarity as the developer and have the same charge polarity as the developer carrier. 3. The magnetic developer of the second and subsequent developing devices,
3. The multicolor image forming apparatus according to claim 1, wherein the magnetic developer has a high resistance of 10 ¹⁰ Ωcm or more. 4. The multicolor image forming apparatus according to claim 1, wherein the magnetic field generating means of the second and subsequent developing units is a magnet roller having a plurality of magnetic poles and has a symmetric magnetic pole pattern. 5. The developer carrier of the second and subsequent developing devices has a low resistance in a portion near a position closest to the latent image carrier and a high resistance in other portions. And 3 or 4 multicolor image forming apparatuses. 6. The electrostatic latent image of a second or subsequent one of the plurality of electrostatic latent images is a binary digital electrostatic latent image.
2, 3, 4 or 5 multicolor image forming apparatuses. 7. The development by the first developing device is a one-component development method using a one-component non-magnetic toner.
2, 3, 4, 5, or 6 multicolor image forming apparatuses. 8. The development by the second and subsequent developing units is performed by
8. The multicolor image forming apparatus according to claim 1, wherein the apparatus is a one-component developing system using a component magnetic toner.